Materials Today Advances 28 (2025) 100629 Contents lists available at ScienceDirect Materials Today Advances journal homepage: www.journals.elsevier.com/materials-today-advances/ Inkjet printing of conductive nanomaterials on textiles for wearable electronics: Advancements, challenges, and future prospects Bekinew Kitaw Dejene * Department of Textile Engineering, Institute of Technology, Hawassa University, Hawassa, Ethiopia A R T I C L E I N F O A B S T R A C T Keywords: Smart textiles Printed electronics Flexible sensors Conductive inks Digital fabrication Wearable technology Functional nanomaterials Inkjet printing of conductive nanomaterials on textiles represents a transformative approach for manufacturing next-generation wearable electronics, combining precision digital fabrication with the comfort and flexibility of textile substrate. This comprehensive review analyzes recent advancements in material formulations, printing methodologies, and functional applications that are driving innovations in this field. It critically examine the interplay between nanomaterial properties (metallic nanoparticles, carbon-based materials, and conductive polymers) and printing parameters, highlighting optimized strategies for achieving high-resolution, durable, conductive patterns on diverse textile substrate. The discussion encompasses breakthrough applications in physiological monitoring, flexible energy systems, and advanced human-machine interfaces, with particular attention to performance metrics under real-world operating conditions. A dedicated analysis of persistent challenges addresses material stability, interfacial adhesion, wash durability, and large-scale manufacturing constraints, while presenting innovative solutions, such as self-healing composites, hybrid printing techniques, and environmentally benign conductive inks. Emerging trends in intelligent textiles, including AI-enabled fabrication and bio-integrated devices, are explored as key drivers of future development. The review concludes with a critical perspective on commercialization pathways, emphasizing the need for standardized testing protocols, sustainable material lifecycles, and cross-disciplinary collaboration to bridge the gap between laboratory innovations and mass production. 1. Introduction The growing demand for wearable electronics has catalyzed significant research on smart textiles that seamlessly integrate electronic functionalities into fabrics [1,2]. These smart textiles, which can perceive and respond to environmental stimuli, have found increasing applications in health monitoring, medical care, and human-machine interfaces [3,4]. Over time, the field has evolved from merely attaching sensors to fabrics to fully transforming garments into functional sensing and interactive platforms [1]. Textiles are inherently advantageous for such applications because of their breathability, flexibility, and conformability, which provide comfort for prolonged wear. However, despite these benefits, several technical challenges remain to be addressed. The intrinsic porosity and surface roughness of textiles complicate the fabrication of high-performance electrodes, and achieving durability against washing and mechanical stress remains a critical hurdle [2,5]. Importantly, the Scientific and Technology Options Assessment (STOA) panel of the European Parliament has recognized wearable electronics as one of the key emerging technologies that are poised to transform modern society [6]. In parallel with advances in textile substrates, the field of conductive materials for wearable electronics has shifted from traditional conductive fibers and metallic coatings to emerging nanomaterial-based inks [1,7]. Conductive nanomaterials, such as polymers, metals, and carbon-based nanostructures, have significantly enhanced the functionality, stretchability, and integration of electronics within fabrics [7]. Further driving this innovation is the rapidly expanding flexible electronics industry, which spans a wide array of commercial products and is projected to grow from $30 billion to over $73 billion by 2027 [132]. A key future direction involves integrating sensors with computing units to develop intelligent adaptive systems for prosthetics and humanoid technologies [6]. Among the various fabrication methods, inkjet printing has emerged as a leading technique for patterning conductive nanomaterials on textiles, offering a versatile, scalable, and material-efficient approach [8,9]. Inkjet printing, which utilizes drop-on-demand (DOD) or continuous modes, allows for the precise deposition of functional inks on diverse * Corresponding author. E-mail address: [email protected]. https://doi.org/10.1016/j.mtadv.2025.100629 Received 26 July 2025; Received in revised form 27 August 2025; Accepted 17 September 2025 Available online 25 September 2025 2590-0498/© 2025 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). B.K. Dejene Materials Today Advances 28 (2025) 100629 substrates, including fabrics [8]. Particularly, DOD printing with thermal or piezoelectric printheads has enabled the high-resolution fabrication of complex electronic circuits directly on flexible surfaces [10, 11]. One of the main advantages of inkjet printing is its low material waste and suitability for rapid prototyping and small batch manufacturing [9,11]. The compatibility of this technique with various substrates, including porous, stretchable, and unconventional materials such as textiles, has expanded its utility in wearable applications [12, 13]. Recent innovations in ink formulations, such as additive-free aqueous Ti3C2Tx MXene inks, have further simplified the printing process, eliminated the need for toxic solvents, and improved compatibility with textiles [14,15]. Conductive nanomaterials suitable for inkjet printing include metallic nanoparticles (silver, gold, and copper), carbon-based materials (graphene and carbon nanotubes), and conductive polymers such as PEDOT:PSS, each with distinct advantages and challenges [16–20]. For example, silver nanoparticles provide excellent conductivity but are expensive; copper offers affordability but is prone to oxidation; and carbon-based materials excel in flexibility but require optimized dispersions for printing [18–20]. Hybrid inks combining different types of nanomaterials have been explored to balance the conductivity, flexibility, and mechanical durability [17]. The formulation of inks for textile substrates requires careful tuning of properties such as viscosity, surface tension, and nanoparticle dispersion stability to prevent nozzle clogging and ensure uniform deposition [21]. Post-printing treatments, including sintering or photonic curing, are often employed to enhance conductivity by removing organic residues and promoting better interparticle contact [21,22]. Therefore, inkjet-printed wearable electronics have witnessed notable advancements in multiple domains, including sensors, communication devices, and energy storage systems [15,23,24]. Strain, temperature, and biochemical sensors fabricated via inkjet printing have demonstrated high sensitivity, fast response times, and mechanical stability over thousands of operational cycles [24]. Similarly, communication components, such as RFID antennas, have benefited from the precision of inkjet printing, facilitating seamless integration with textile substrates [15,23]. In energy storage, inkjet-printed textile-based microsupercapacitors (MSCs) using MXene inks have exhibited outstanding areal capacitances (e.g., up to 294 mF cm−2), outperforming many previously printed devices [14]. Further breakthroughs include the development of novel silver-based metal-organic decomposition (MOD) inks capable of achieving exceptional conductivity on thermally sensitive substrates at room temperature [23]. Hybrid material systems, such as CNT-elastomer composites, have yielded stretchable and waterproof fiber-shaped strain sensors with excellent mechanical durability and high sensitivity [25]. These innovations are pushing the boundaries of what is achievable in wearable electronics. While several reviews have addressed inkjet printing for flexible electronics and conductive nanomaterials in general [26,27], specific comprehensive analyses focusing on textiles for wearable electronics remain limited. Most existing reviews overlook the unique surface characteristics and challenges associated with textiles, including porosity, roughness, and mechanical deformation under usage conditions [13,28]. Mini-reviews, such as that by Eghan et al. [29], provide useful insights but lack exhaustive coverage, particularly regarding ink formulations, printing strategies, and post-processing for textile-based applications. Given the rapid pace of research, there is a pressing need for a focused, up-to-date, and detailed review specifically addressing the inkjet printing of conductive nanomaterials on textile substrates for wearable electronics. In response to these gaps, this review aims to provide a comprehensive and focused analysis of recent advancements in the inkjet printing of conductive nanomaterials on textiles for wearable electronics. This review critically examines the development of conductive nanomaterial inks, including metallic nanoparticles, carbon-based nanostructures, and conductive polymers, specifically formulated for integration with textile substrates. Furthermore, it explores ink formulation strategies, printing parameters, and post-processing techniques while addressing the distinct challenges posed by textile materials, such as surface porosity, roughness, and mechanical durability. Beyond technological developments, this review highlights the key applications of inkjet-printed textile-based wearable electronics in sensing, communication, and energy storage. Finally, it identifies current research gaps and outlines future research directions to foster innovation in this rapidly evolving field, providing a consolidated and timely resource for researchers and engineers engaged in the development of next-generation wearable electronic textiles. 2. Inkjet printing technology for textiles Electronic textiles (E-textiles) have attracted significant research interest, particularly in wearable applications. However, the adoption of new materials, machines, and processes for e-textile manufacturing presents notable challenges. Among these, the development of facile patterning methods for creating complex electronic architectures on textiles remains critical hurdle. Although conventional dip coating can easily impart conductivity to textiles, it is unsuitable for forming defined conductive patterns [13]. Printing technologies offer a promising solution for integrating heterogeneous materials into textiles. Screen printing has been widely employed to fabricate electronic components on textile substrates using thick-paste metal inks. Despite its popularity for the rapid prototyping of sensors and other electronics, screen printing suffers from significant drawbacks, including multiple processing steps, substantial ink waste, post-process cleaning, and alteration of the textile feel [13,30–32]. Furthermore, the resulting brittle conductive layers are prone to cracking under mechanical deformation, often necessitating additional surface treatments or lamination. Similarly, creating conductive patterns with conductive yarns involves labor-intensive and costly processes, such as yarn metallization, followed by sewing or embroidery. Aerosol-jet printing (AJP) is best suited for high-precision applications such as printed electronics and medical devices, though its high cost and slower throughput limit large-scale adoption. In contrast, inkjet printing provides a single-step, automated, and material-efficient approach to fabrication. This enables the precise deposition of thin, functional layers directly onto selected textile regions using minimal amounts of dilute ink. Fig. 1 provides a comparative overview of the multiple processing steps required for conductive yarn-based knitting, embroidery, and sewing versus the streamlined, single-step inkjet printing process for metallizing textile patterns. Inkjet printing relies on low-viscosity functional inks to generate picoliter-sized droplets, enabling high-resolution patterning [33]. As summarized in Table 1, inkjet printing, by comparison, provides a balance between scalability, cost, and design freedom, making it particularly attractive for functional and commercial e-textile applications [34]. 2.1. Inkjet printing fundamentals 2.1.1. System architectures Inkjet printing has emerged as a versatile and efficient technology for textile applications, offering significant advantages in terms of precision, customization, and sustainability. The two main system architectures employed are drop-on-demand (DOD) and continuous inkjet (CIJ) systems, as depicted in Fig. 2. [35]. Among these, drop-on-demand (DOD) inkjet printing is the most commonly used in textile applications, with two primary variants: thermal and piezoelectric. In piezoelectric inkjet printing (PIP), an applied voltage induces deformation of a piezoelectric transducer, generating a pressure pulse in the ink chamber that ejects ink droplets through the nozzle. PIP can operate in the squeeze, push, shear, or bend modes, with most textile studies focusing on the bend mode [35]. Piezoelectric systems are particularly favored in textile printing because of their ability to handle a wide range of ink formulations, especially functional inks required for advanced applications [35,36]. In contrast, thermal inkjet (TIJ) systems employ a 2 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 1. (a) Processing steps involved in creating conductive patterns on fabric using conductive yarn or thread; (b) single-step inkjet printing process for patterning conductive structures on textiles. Reproduced with permission from Ref. [13]. © 2019 American Chemical Society. the heater can mitigate these failures. Another critical issue is kogation, where ink residues accumulate on the heater, disrupting bubble formation and droplet ejection. This can be addressed by formulating inks with specific anionic additives [35,37]. To better illustrate the operating mechanisms of inkjet printing, Fig. 3a shows the in situ fabrication of gold-nanoparticle-based conductive inks [39]. Ink droplets were generated using piezoelectric inkjet printers, which utilized a transducer inside the nozzle to create droplets under an applied voltage. Depending on the shape and actuation method of the piezoelectric transducer, there are four primary actuation modes: squeeze, shear, bend, and push, as depicted in Fig. 3b. The droplet formation sequence from the nozzle is illustrated in Fig. 3c. Inkjet printing has also been employed in the fabrication of printed supercapacitors, as shown in Fig. 3d, using composite inks composed of single-walled carbon nanotubes (SWCNTs) and activated carbon. Additionally, Matsuhisa et al. [40] developed a stretchable conductive ink formulation consisting of silver flakes, fluorine rubber, and a fluorine surfactant for textile-based electrodes, as shown in Fig. 3e, which illustrates both the ink components and stretchability of the patterned conductors. This highlights the advantage of inkjet printing in creating arbitrary and complex geometries for electrodes. However, similar to screen printing technology, the physical transfer of functional inks onto substrates can result in weaker adhesion compared to bottom-up fabrication methods. Moreover, post-processing steps, such as sintering, are often required to enhance conductivity, contributing to additional energy consumption during manufacturing. In contrast, continuous inkjet (CIJ) systems are typically used for high-speed industrial-scale applications. CIJ generates a continuous stream of ink droplets that are selectively charged and deflected onto the substrate, whereas unused droplets are recycled [35,42]. Recent innovations have expanded the frontiers of inkjet printing in textiles. Researchers have developed techniques for printing high-viscosity inks (up to 105 mPa s) using pneumatic needle jetting valves, broadening the range of achievable effects in textile decoration and functionality [43]. Furthermore, the introduction of specialized waveforms and low jetting frequencies has enabled the successful deposition of complex polymers, such as PVDF-TrFE, facilitating the integration of piezoelectric, pyroelectric, and ferroelectric properties into smart textiles [44]. These advancements underscore the rapid evolution of inkjet printing technology in the textile industry, with both DOD and CIJ architectures offering unique advantages depending on the application requirements. Future research continues to address challenges such as printhead durability, ink formulation, and the incorporation of smart functional materials; Table 1 Comparative analysis of inkjet, screen, and aerosol-jet printing techniques for conductive nanomaterials on textiles for wearable applications. Adapted and modified from Ref. [34]. © 2025 by the authors. Licensed under CC BY 4.0. Parameters Aerosol jet printing Inkjet printing Screen printing Scalability Limited scalability. Best for highaccuracy, lowvolume applications. High cost due to specialized systems and materials. Good scalability. Suitable for mass production and 2D applications. Efficiency Slow process due to the need for precise material deposition. Fast process, especially for 2D applications. Quality and process control Exceptional precision with high resolution (as low as 10 μm). Ideal for high-performance electronics. Design flexibility and lead time High flexibility for complex 3D structures and conformal printing. Longer lead time due to process complexity. Good adhesion with tailored surface treatments; conformal deposition possible Moderate resolution (20–50 μm feature size). Challenges include nozzle clogging and inconsistent droplet formation. High design flexibility; suitable for 2D patterns and planar surfaces. Faster turnaround for rapid prototyping. High scalability. Ideal for large-scale manufacturing, primarily in textiles and electronics. Lower cost; however, requires frequent screen changes for new designs, adding expense. Very high throughput; however, screen changes increase overall setup time. Good resolution (50–100 μm feature size). Highly repeatable for large production volumes. Cost Substrate adhesion Lower cost; costeffective for highvolume production. Limited on porous textiles without pretreatment (e.g., plasma, polymer coatings) Less flexible due to the need for a new screen for each design. Longer setup time, but faster production after setup. Strong adhesion due to thick ink layers; but can reduce fabric flexibility heater to create vapor bubbles that eject the ink droplets. Depending on the heater placement, the TIJ configurations include roof-shooter, side-shooter, and suspended heater designs [37]. The roof shooter is the most widely used configuration in the industry. However, TIJ systems face durability issues, such as limited printhead lifetimes owing to the electromigration of the heater, bubble cavitation damage, and thermal stress-induced cracking. Improving the thickness and shape of 3 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 2. Schematic of the inkjet printing process. Adapted from Ref. [38]. © 2016 by the authors. Licensed under CC BY 4.0. Fig. 3. (a) In situ synthesis of Au nanoparticles for conductive inkjet printing. (b) Schematic of the four types of piezoelectric inkjet nozzles. (c) Sequence of droplet ejection from a piezoelectric inkjet nozzle. (d) Fabrication of supercapacitors using inkjet printing technology. Adapted with permission from Ref. [41]. © 2019 by the authors. Licensed under CC BY 4.0 license. (e) Fabrication process of the elastic ink and demonstration of printed elastic patterns under tensile strain. Adapted from Ref. [40]. © 2015 by the authors. Licensed under the CC BY 4.0 license. thus, inkjet printing is poised to revolutionize the design and functionality of next-generation textiles. break into several small satellite droplets owing to this instability. These satellite drops can negatively impact the printing quality by creating unwanted splashes on the target substrate [45]. Suppressing satellite droplets is a key challenge in inkjet printing on textiles. Several strategies have been developed to address these issues. One approach involves adding polymers to the ink, which can introduce elastic stresses that contract the trailing ligament into the main drop before a capillary breakup occurs. However, this method requires careful optimization of non-Newtonian parameters, such as polymer concentration and molecular weight, to achieve the desired drop shape and speed without compromising the drop velocity or breaking off from the ink reservoir [45]. Interestingly, contradictory approaches have been proposed for 2.1.2. Droplet formation physics Inkjet printing technology for textiles involves complex droplet formation physics, particularly concerning the Rayleigh-Plateau instability and strategies for satellite droplet suppression. The Rayleigh-Plateau instability is a fundamental phenomenon in inkjet printing, in which a liquid jet breaks into droplets owing to surface tension. This process is crucial in DOD printing, where ink ‘drops’ are ejected from a nozzle using pressure pulses. Upon exiting the nozzle, the drop typically forms a nearly spherical bead with a long, thin trailing ligament, which can 4 B.K. Dejene Materials Today Advances 28 (2025) 100629 the suppression of satellite droplets. While some researchers focus on assisting ink filaments in retracting into one drop by controlling ink properties such as viscosity or surface tension [46], others suggest enhancing the Rayleigh instability of the filament at the break point to accelerate pinch-off from the nozzle [46]. The latter approach involves using a superhydrophobic and ultralow-adhesion nozzle with cone morphology, which can effectively cut the ink filament at the breakup point [46]. Research has shown that manipulating ink properties, nozzle design, and waveform parameters can significantly affect droplet formation and satellite droplet suppression [47,48]. For instance, a higher ink viscosity can eliminate satellite droplet formation but may result in a slower droplet velocity [47]. In addition, appropriate driving waveforms for inks with various viscosities can be optimized to improve the jetting behavior [48]. These advancements in inkjet printing technology are paving the way for high-resolution patterns and three-dimensional structures in textile-printing applications [49]. adjustments, and polarity modifications, can further optimize inksubstrate interactions for enhanced printing performance. The textile structure also plays a vital role in determining the mechanical durability and adhesion strength of the printed features. Key parameters, such as areal density, yarn count, fabric thickness, warp and weft density, and weave pattern, substantially influence the integration of printed ink with the textile substrate. For instance, woven fabrics with doubleweave structures have demonstrated superior adhesion properties compared to single weaves, primarily because their more complex architecture allows for deeper polymer penetration into the fabric matrix [53]. By systematically optimizing these material, surface, and process factors, inkjet printing technology can be harnessed to reliably produce functional, high-resolution, and durable patterns on a wide variety of textile materials. Altogether, these advances in textile-specific inkjet printing pave the way for innovative applications in e-textiles, smart fabrics, and wearable sensors, thereby expanding the functional possibilities of textiles beyond their traditional uses. 2.2. Textile-specific printing considerations 2.3. Advanced printing strategies Inkjet printing technology has a promising approach for fabricating functional and decorative patterns on textile surfaces, offering key advantages such as precision, scalability, design flexibility, and costeffectiveness [13]. However, unlike conventional rigid substrates, textiles possess inherent structural and surface complexities that introduce distinct challenges in achieving successful inkjet printing. These challenges primarily stem from the porous, flexible, and fibrous nature of fabrics, which necessitates careful consideration of substrate-ink interactions, surface modifications, and process optimization to achieve high-quality, durable prints. One of the foremost challenges associated with inkjet printing on textiles is controlling ink spreading, commonly referred to as wicking. Owing to the capillary action of textile pores, liquid inks can spread beyond the intended design boundaries, leading to poor print resolution and blurred patterns. Addressing this issue requires strategies to modify textile surface properties before printing. Plasma pretreatment has been widely demonstrated as an effective method for enhancing surface wettability and modifying fabric chemistry. This pretreatment improves ink adhesion and has been particularly successful in enzyme printing, where it enhances protein-binding stability on fabric surfaces [50]. In the case of synthetic fabrics, such as polyethylene terephthalate (PET), plasma treatment can significantly increase hydrophilicity, facilitating more uniform and defined ink deposition [50,51]. Alternatively, hydrophobic coatings can be applied to strategic areas of the fabric to restrict ink flow, thereby improving the edge definition and overall print quality. Beyond surface pretreatment, mechanical strategies such as multipass printing can be employed to compensate for the uneven threedimensional surface of textile substrates. By repeatedly applying ink layers, multipass printing improves ink penetration and ensures more consistent coverage across the fabric structure. In situ heat curing during the printing process has proven beneficial for further mitigating ink wicking and enhancing functionality. This approach not only reduces ink spreading but also promotes rapid drying and improves the electrical conductivity of printed conductive coatings, especially on polyester fabrics [13]. Importantly, the structural characteristics of the textile itself, such as fabric tightness, fiber diameter, and surface roughness, also influence ink behavior, affecting both conductivity and resolution in applications involving printed electronic tracks [13]. Ink-substrate compatibility is another crucial consideration in textile-specific inkjet printing. For natural fibers such as cotton, chemical surface modifications, such as carboxylation, can significantly improve ink adhesion by increasing the density of reactive functional groups on the fiber surface. In contrast, synthetic fibers often require different surface activation methods, such as corona discharge treatment, to increase surface energy and enable better interaction with the ink [52]. In addition to substrate treatments, tailoring the chemical formulation of the ink, including the polymer choice, surface energy Advancements in inkjet printing have enabled the development of sophisticated strategies tailored for the fabrication of next-generation electronic textiles and wearable electronics. These advanced approaches extend beyond conventional single-layer printing, incorporating multi-layer, hybrid, and three-dimensional (3D) conformal printing techniques to realize complex multifunctional textile-based devices. Each of these strategies offers unique advantages while presenting specific challenges that must be addressed to fully harness their potential in textile applications [22,54]. One of the most widely explored strategies is multilayer printing, which involves the sequential deposition of multiple layers of functional materials, such as dielectric and conductive inks, to create vertically integrated electronic structures on textile substrates. This multilayer architecture is critical for enabling advanced device functionalities, including those of capacitors, sensors, and transistors. However, a key challenge in this approach is managing the interactions between the ink components across the stacked layers. Studies have shown that polymer stabilizers used in nanoparticle inks, such as polyvinylpyrrolidone (PVP), can accumulate at the interfaces between stacked layers, particularly at the dielectric/conductive boundaries. This interfacial accumulation of organic residues can negatively affect the electrical conductivity of the printed features, thereby compromising the device performance [22]. Understanding these interfacial phenomena is essential for optimizing nanomaterial ink formulations and achieving reliable multilayer printed electronics on textiles. Complementing multilayer strategies, hybrid printing techniques have emerged as a powerful approach to overcome the limitations associated with single-method printing. By combining inkjet printing with other additive manufacturing techniques, such as screen printing, hybrid processes leverage the strengths of each method to fabricate high-performance multi-material structures. For instance, hybrid printing has enabled the creation of complex 3D electronic architectures using inkjet printing for the precise patterning of functional inks alongside screen printing for the deposition of thick, conductive traces. A notable example is the fabrication of conductive metal pillars with high aspect ratios, which serve as vertical interconnects for integrating electronic components such as light-emitting diodes (LEDs) [54]. This hybrid approach allows for structural versatility and enhanced electrical performance, making it particularly suitable for wearable electronic systems. A particularly compelling demonstration of hybrid printing is the fabrication of organic thin-film transistors (OTFTs) on flexible substrates by integrating multiple printing technologies into a single manufacturing sequence (Fig. 4a). In this process, a high-viscosity (~70 Pa s) graphene-based ink was employed for screen printing to form the electrodes, with a silicon squeegee ensuring uniform deposition and subsequent xenon lamp treatment enhancing conductivity. 5 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 4. (a) Fabrication process of organic thin-film transistors using combined screen printing and inkjet printing techniques. (b) Optical microscopy image of a single printed transistor. (c) Optical photograph of the printed transistor array. (d) Optical microscopy image of a screen-printed graphene electrode. (e) Surface profile of the printed graphene electrode. (f, g) Low- and high-magnification SEM images of the screen-printed graphene electrodes. Reproduced from Ref. [41]. © 2019 by the authors. Licensed under CC BY 4.0. Subsequently, aerosol-jet printing was used to define the transistor channel, and the gate dielectric layer was deposited via inkjet printing [55]. Fig. 4(b and c) illustrate the individual and multiple electrode structures, and Fig. 4(d–g) provide detailed microscopy images of the printed graphene electrodes. Notably, the screen-printed electrodes achieved a line width of approximately 58.2 ± 7 μm and a thickness of 1.11 ± 0.9 μm, demonstrating the high resolution and control attainable through hybrid manufacturing approaches [41,55]. By integrating complementary printing technologies, the individual limitations of each method are mitigated, enabling the production of complex and high-performance electronic devices on flexible textile platforms. Another promising avenue in advanced textile printing is 3D conformal printing on knitted structures, which allows for the seamless deposition of conductive materials over the complex geometry of textile surfaces. This technique has shown remarkable success in producing highly conductive fabrics while preserving their inherent mechanical properties, such as stretchability, breathability, and softness of the fabrics. A pioneering example involves the use of particle-free reactive silver inks to create conformal coatings on individual yarns within knitted textiles, achieving sheet resistance values as low as 0.09 Ω sq−1 [56]. This approach ensures that the conductive layer forms ultrathin, continuous coating around each fiber without compromising the fabric’s flexibility or tactile characteristics. The conductivity and durability of such structures can be fine-tuned by adjusting parameters such as the number of print passes, fabric architecture, and in situ annealing during printing. Beyond knitted fabrics, similar techniques using particle-free reactive silver inks have been successfully applied to woven and nonwoven textiles [13], offering broad applicability for diverse fabric types. In addition to ink-based strategies, the 3D printing of thermoplastic polymers, such as soft thermoplastic polyurethane (TPU), directly onto textile substrates has been explored as a method for creating durable, flexible, and integrated textile-electronic systems. Researchers have investigated various process parameters, including nozzle temperature, printing speed, and layer thickness, to optimize the adhesion between TPU layers and textile substrates while overcoming challenges related to flexibility and wearability [57]. mechanical robustness, and protect printed patterns from environmental degradation. These post-processing strategies typically include lowtemperature sintering techniques to improve conductivity, as well as encapsulation methods to protect against moisture, mechanical abrasion, and chemical exposure [58,59]. One of the primary challenges in the post-printing processing of textiles is the thermal sensitivity of most fabric substrates, which limits the applicability of conventional high-temperature sintering processes typically used for rigid electronic substrates. To address this, low-temperature sintering techniques, such as chemical and microwave-assisted sintering, have been developed to facilitate the formation of conductive pathways without compromising the textile integrity [35]. Chemical sintering involves the application of reducing agents, such as sodium borohydride (NaBH4) and hydrazine, which promote the coalescence of metal nanoparticles into continuous conductive networks at temperatures below 150 ◦ C. This approach enables the creation of highly conductive patterns while preserving the structural and tactile properties of fabrics. In parallel, microwave-assisted sintering has emerged as an efficient alternative, offering the advantage of rapid, localized heating of printed features without subjecting the entire textile substrate to elevated temperatures. This technique effectively minimizes the thermal damage to the fabric while significantly improving the conductivity of the printed patterns [35]. In addition to conductivity enhancement, protecting printed features from environmental factors and mechanical wear is essential for the practical deployment of e-textiles in real-world applications. To this end, encapsulation techniques have been widely adopted to create protective layers on printed structures. One advanced encapsulation method is atomic layer deposition (ALD), particularly using aluminum oxide (Al2O3). ALD allows the deposition of ultrathin, conformal coatings on textile surfaces, providing excellent barrier properties against moisture ingress, oxygen, and chemical contaminants [58]. Importantly, ALD processes can be conducted at relatively low temperatures, making them compatible with temperature-sensitive textile substrates such as cotton. This method has been successfully applied to produce flexible and durable electronic textiles with enhanced environmental stability. [60]. Complementing ALD, silicone elastomer coatings provide an effective encapsulation approach, offering unique advantages such as stretchability, waterproofing, and mechanical cushioning. These coatings form soft, flexible protective layers over printed electronic features, safeguarding them against bending, folding, and external abrasion while maintaining the inherent flexibility and comfort of the fabric [61]. Silicone encapsulation is particularly valuable in wearable applications, where textiles are subjected to dynamic mechanical stresses during daily use. Together, these post-printing processing techniques, including 2.4. Post-printing processing Post-printing processing is a critical phase in inkjet-printed textile fabrication, directly influencing the durability, functionality, and longterm stability of printed electronic features. While inkjet printing allows for the precise deposition of functional inks, additional processing steps are often required to enhance electrical performance, ensure 6 B.K. Dejene Materials Today Advances 28 (2025) 100629 compatibility with inkjet printing and allows the same base materials to be adapted for various printing techniques depending on the end-use requirements (Fig. 5(R1)). A fundamental requirement for inkjet printing is that the ink possesses carefully tailored rheological characteristics, including surface tension, viscosity, thixotropy, and viscoelasticity. These properties must fall within specific ranges to ensure reliable jetting behavior, stable droplet formation, and uniform deposition on the substrate [33,62]. The optimization of these parameters typically involves a combination of solvent selection, the use of dispersing agents, and the incorporation of appropriate resins. Notably, different printing technologies have divergent rheological demands: whereas screen printing utilizes high-viscosity, thixotropic inks capable of forming thick, well-defined layers, inkjet printing requires low-viscosity inks that conform to strict jetting windows for stable ejection and precise patterning [63]. In the context of conductive inks for electronic textiles, nanoparticle-based formulations, particularly those containing metallic chemical and microwave-assisted sintering for conductivity improvement and ALD or silicone encapsulation for protection, enable the realization of functional, durable, and flexible electronic textiles. 3. Conductive nanomaterials for inkjet printing The formulation of inks for inkjet printing is a highly specialized process that plays a decisive role in determining the performance and reliability of printed electronic textiles. Typically, these inks are composed of functional materials, such as conductive, semiconductive, or insulating substances, combined with resins, solvents, fillers, and additives to achieve the rheological properties necessary for effective droplet formation and transfer [33]. These functional materials are introduced into the ink either in a dissolved form or as dispersions of flakes, nanoparticles, nanowires, or polymers, stabilized in a suitable liquid medium. Careful control of the ink formulation enables Fig. 5. Functional ink formulation steps, composition, and functional layer properties. (R1) Schematic of the functional nanomaterial-based ink formulation. The solubility/dispersibility of the functional materials requires suitable solvents, which can be identified and optimized using the Hansen solubility parameter (HSP) model. Process-specific properties are achieved with flow property modifications, such as surface tension wettability characterization and viscosity-shear thinning tests. (R2) shows the low-temperature nanoparticle sintering of silver nanoparticle inks and its impact on sintering temperature. Before sintering (a), after sintering at 100 ◦ C (b) and resistivity of printed silver electrode over sintering temperature is shown (c). (R3) shows the improved stretchability and conductivity of the PEDOT: PSS material. Morphology of a stretchable PEDOT film with ionic additives (a), the plot implies the improved stretchability and conductivity of it (b). Adapted from Ref. [33]. © 2025 by the authors. Licensed under the CC BY 4.0 license. 7 B.K. Dejene Materials Today Advances 28 (2025) 100629 nanoparticles, have gained prominence owing to their excellent electrical properties. However, these nanoparticle dispersions are inherently prone to challenges, such as agglomeration and sedimentation, over time [64,65]. To counteract this instability, nanoparticles are typically encapsulated with organic ligands (capping agents) that provide steric or electrostatic stabilization in a liquid medium [18,66,67]. After deposition onto the textile substrate, post-printing treatments, such as thermal curing or sintering, are necessary to remove these organic coatings. This step allows direct particle-particle contact, facilitating the formation of continuous conductive networks required for functional electronic features. Among metallic conductive inks, silver-nanoparticle-based inks are the most widely utilized in printed electronics, including textile-based applications. Silver offers an ideal combination of exceptionally high conductivity (~106 S/m), chemical stability, and a relatively favorable cost compared to other noble metals, such as gold or platinum. Historically, one of the main limitations of nanoparticle-based conductive inks is the requirement for high curing temperatures, which is a challenge for flexible or temperature-sensitive substrates, such as textiles. However, advances in nanotechnology have leveraged the size-dependent melting point depression of nanoparticles, enabling low-temperature sintering suitable for textile-based electronics (Fig. 5(R2)) [33]. In addition to metallic nanoparticle inks, conductive polymers have emerged as attractive alternatives, particularly for applications that require flexibility, biocompatibility, and optical transparency. Poly(3,4ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has become one of the most extensively studied conductive polymers due to its high optical transparency (~90 %), inherent flexibility, and ease of processing [68,69]. PEDOT:PSS has found widespread applications in flexible and wearable devices, serving as electrodes for organic photovoltaics, organic light-emitting diodes (OLEDs), sensors, and energy storage devices [70]. Significantly, Wang et al. [71] demonstrated that incorporating ionic liquid additives into PEDOT:PSS formulations can substantially improve the conductivity and stretchability of printed films. These additives modify the polymer morphology and simultaneously function as dopants, resulting in enhanced charge-carrier mobility. As illustrated in Fig. 5(R3), this strategy enabled the achievement of conductivities of 3100 S/cm at 0 % strain and up to 4100 S/cm under 100 % strain, which underscores the potential of PEDOT: PSS in stretchable electronics. Owing to its multifunctional properties, PEDOT: PSS continues to serve as a versatile material for a wide range of electronic textile applications. These include stretchable conductors, biosensors, thermoelectric devices, heating elements, and piezoresistive sensors [33]. Fig. 6. Main components and corresponding roles of metallic nanoparticle inks. Adapted with permission from Ref. [75]. © 2025 by the authors. Licensed under CC BY 4.0. uniform and adherent prints on textile substrates [78,79]. However, surfactants can sometimes introduce foaming, necessitating the use of defoamers [80,81]. Other additives, such as thickeners, inhibitors, and biocides, can be used to further optimize the performance of inks for textile applications [82]. Solvents, either organic or water-based, play a crucial role in dissolving and dispersing ink components while controlling properties such as viscosity, evaporation rate, and interactions with the textile surface [75]. Organic solvents, such as ethanol and toluene, typically offer better adhesion and conductivity, but raise environmental concerns, prompting increasing interest in water-based systems as more sustainable alternatives. However, water-based inks often exhibit lower adhesion, abrasion resistance, and moisture tolerance on textiles [83]. Therefore, the careful selection and formulation of metallic nanoparticle inks are essential to ensure print quality, electrical performance, and durability suitable for wearable electronics applications. 3.1. Metallic nanoparticle inks 3.1.1. Silver (Ag) nanoparticles Silver nanoparticles (AgNPs) are among the most widely used materials because of their excellent electrical conductivity and stability. AgNPs can be synthesized using various methods, including chemical reduction, the polyol process, and laser ablation [84,85]. For instance, a simple and scalable method involves the use of silver nitrate as a precursor, ethylene glycol as a reducing agent, and polyvinylpyrrolidone (PVP) as a capping agent [86]. Ink formulation plays a crucial role in the performance of printed conductive patterns on flexible substrates. Dispersants, such as PVP and oleylamine, are commonly used to stabilize nanoparticles in the ink, while solvent systems, such as water and ethylene glycol, help achieve the desired viscosity and surface tension for optimal printing [27,86,87]. However, the choice of dispersants and solvents significantly affects the final conductivity of printed patterns. For example, research has shown that organic residuals from inks, particularly PVP, tend to concentrate between vertically stacked nanoparticle layers, leading to anisotropic electrical conductivity [22]. Post-print sintering is essential for transforming printed layers into electrically conductive layers. Common sintering methods include thermal (150–300 ◦ C), photonic (pulsed light), and plasma treatments Metallic nanoparticles (MNPs) inks used in inkjet printing for textiles in wearable electronics typically consist of metallic nanoparticles, solvents, and additives [72,73], as illustrated in Fig. 6. MNPs, primarily composed of silver (Ag), copper (Cu), and gold (Au), serve as conductive pigments, with Ag nanoparticles being the most widely used due to their superior electrical conductivity (σAg = 6.30 × 107 S/m) [74,75]. Copper nanoparticles (CuNPs) offer a cost-effective alternative but are more prone to oxidation, whereas gold nanoparticles (AuNPs) are often employed in biosensing applications owing to their excellent biocompatibility [75]. For example, Ye et al. [76] developed a biosensor using an AuNPs-MXene interface for hormone detection, while Ali et al. [77] fabricated a 3D microelectrode array with sintered AuNPs for rapid pathogen detection. The size, shape, and concentration of these nanoparticles directly influence the electrical performance and deposition behavior of printed features [75]. Additives, usually polymeric stabilizers, are present in small amounts (≤5 wt %), prevent nanoparticle agglomeration through steric or electrostatic mechanisms, ensuring stable dispersion. Surfactants are often incorporated to adjust the surface tension and wettability, which are critical parameters for achieving 8 B.K. Dejene Materials Today Advances 28 (2025) 100629 [27,87]. The sintering process removes organic components and fuses nanoparticles, resulting in highly conductive patterns. For instance, thermal sintering of commercially available AgNPs ink (Metalon JS-B30G) at suitable parameters yielded silver layers with sheet resistances of 40 mΩ/sq and an average roughness lower than 10 nm [87]. AgNP inks have been used in various electronic devices, including high-frequency RFID antennas, ECG electrodes, transparent electrodes, solar cell metallization, and light-emitting devices [27,88]. The versatility of these inks allows the fabrication of flexible and conformal electronics on various substrates. For example, a silver molecular ink platform was developed for screen, inkjet, and aerosol jet printing, enabling the production of thin, screen-printed traces with exceptional electrical (<10 mΩ/sq/mil) and mechanical properties [88]. The combination of tailored synthesis methods, optimized ink formulations, and effective post-print sintering techniques has led to significant advancements in the field of printed electronics. silver, other materials have been explored for creating protective shells around the CuNPs. Cu/Cu10Sn3 core/shell nanoparticles have been synthesized and combined with a rapid photonic sintering process, yielding resistivities as low as 12.2 μΩ cm under specific energy-dose conditions [97]. This approach offers a facile electrode fabrication process triggered by the low melting point of the Cu10Sn3 phase. Another effective strategy for mitigating oxidation challenges is sintering in a reducing atmosphere. A simple, low-temperature (130 ◦ C) process using formic acid to sinter inkjet-printed CuNP structures has been reported [98]. This method achieved electrical conductivity of up to 16 % of bulk Cu at 130 ◦ C and over 25 % above 150 ◦ C, making it suitable for flexible, low-cost plastic substrates such as polyethylene terephthalate. The development of Cu-Ag alloy electrodes has shown promise in combining high conductivity with ultrahigh oxidation resistance. A Cu-Ag hybrid ink, processed through low-temperature precuring followed by rapid photonic sintering, resulted in electrodes with a conductivity of 3.4 μΩ cm and exceptional oxidation resistance up to 180 ◦ C in air [99]. This approach demonstrates significant potential for the fabrication of reliable and cost-effective printed electronic devices. These advancements, combined with the cost-effectiveness of Cu, position copper-based inks as a viable and attractive option for printed electronics. However, ongoing research is required to further improve the performance and stability of Cu-based inks to fully realize their potential in commercial applications [100]. 3.1.2. Silver nanowires and silver nanoflakes Silver nanowires (AgNWs) and nanoflakes (AgNFs) have emerged as superior alternatives to AGNPs for printing on textile substrates, mainly because of their anisotropic morphologies and enhanced interconnectivity [89]. Unlike spherical AgNPs, which require extensive sintering to achieve high conductivity, the elongated structures of AgNWs and planar geometry of AgNFs form continuous conductive pathways with fewer junctions and reduced electron scattering [90]. AgNWs are particularly advantageous for fabric-based electronics because their high aspect ratio enables the formation of percolative networks at low material loadings, providing excellent flexibility and mechanical durability under repeated bending, stretching, and washing cycles [91]. Studies have demonstrated that AgNW-based inks can achieve sheet resistances below 20 Ω/sq while maintaining transparency and flexibility, making them suitable for wearable sensors, energy-storage devices, and flexible electrodes. [91]. Similarly, AgNFs provide superior conductivity compared to AgNPs because of their larger contact area and reduced interparticle resistance. Their platelet-like structures enable dense packing and mechanical adhesion to fibrous substrates, improving wash fastness and durability. In addition, AgNFs are less prone to the “coffee-ring effect,” leading to more uniform conductive films after printing [92,93]. Both AgNWs and AgNFs can be dispersed in aqueous or alcohol-based solvents with the aid of stabilizers, and their inks can be processed via inkjet, screen, and aerosol-jet printing. Hybrid systems that combine AgNWs and AgNPs or AgNFs have also been explored to balance conductivity, adhesion, and surface smoothness [93,94]. Together, the unique morphologies of AgNWs and AgNFs make them highly promising for fabric-based printed electronics, offering improvements in conductivity, flexibility, and durability compared to conventional AgNP inks. 3.1.4. Gold (Au) nanoparticles Gold nanoparticles (AuNPs) are another promising candidate for inkjet printing applications owing to their unique properties and versatility. AuNPs offer excellent conductivity, biocompatibility, and ease of surface functionalization, making them ideal for various applications, including medical wearables and flexible electronics [16,101]. One of the key advantages of AuNPs in inkjet printing is their biocompatibility, which is crucial for the development of wearable medical devices. Gold nanomaterials possess extraordinary optical responses and wide electrochemical sensing windows, enabling the fabrication of wearable sensors for healthcare. These sensors can be designed for strain/pressure sensing, humidity/gas detection, electrochemical analysis, and colorimetric measurements, offering a wide range of potential applications in healthcare monitoring [101]. Interestingly, AuNPs can form conductive networks through self-assembly, potentially eliminating the need for high-temperature sintering. This property is particularly advantageous for printing on flexible, temperature-sensitive substrates. For instance, sugar-based biodegradable polyurethane polymers have shown exceptional stabilization levels when combined with AuNPs, demonstrating their potential as robust inks for inkjet printing [102]. This approach opens new routes for fabricating enhanced biomedical nanometallic sensors using stabilized AuNPs [102]. To provide a clearer understanding of metallic nanoparticle inks suitable for inkjet printing on textiles for wearable electronics, Table 2 summarizes representative types of MNPs inks, their structures, synthesis methods, particle sizes, and typical applications relevant to flexible and wearable electronic devices. These examples highlight the versatility of inkjet-printable metallic inks, particularly silver, copper, and gold nanoparticles, and advanced hybrid structures designed to enhance conductivity, printability, and functionality on flexible substrates. 3.1.3. Copper (Cu) nanoparticles Copper nanoparticles (CuNPs) have emerged as promising alternatives to silver (Ag) and gold (Au) in inkjet printing for flexible electronics. CuNPs offer significant cost advantages, with material costs up to 80 % lower than those of Ag, while maintaining high electrical conductivity [95]. However, the primary challenge in utilizing CuNPs is their susceptibility to oxidation under ambient conditions, which can significantly impair their conductivity. To address the oxidation issue, researchers have developed various strategies, with core-shell structures being a prominent one. Cu@Ag core-shell nanoparticles exhibit remarkable oxidation resistance and electrical performance. For instance, a study reported Cu@AgNPs with a narrow size distribution of approximately 100 nm that remained stable for at least 60 days without significant oxidation [96]. Another study demonstrated that coating 40 nm CuNPs with a 2 nm silver layer effectively prevented oxidation and preserved the metallic characteristics of the copper core [19]. These Cu@AgNPs exhibited excellent stability up to 150 ◦ C and were successfully used in inkjet printing on various substrates. In addition to 3.2. Carbon-based nanomaterial inks 3.2.1. Graphene inks Graphene, composed of a single-atom-thick two-dimensional honeycomb lattice of sp2 hybridized carbon atoms, has become one of the most extensively investigated nanomaterials in recent years, primarily because of its exceptional electrical, thermal, optical, and mechanical properties. These unique characteristics make graphene an attractive material for integration into printed electronics, particularly through inkjet printing technology, which enables precise, non-contact 9 B.K. Dejene Materials Today Advances 28 (2025) 100629 Table 2 Representative metallic nanoparticle inks used in inkjet printing for flexible and wearable electronics, highlighting the nanoparticle structure, synthesis methods, typical size ranges, and major applications. Adapted from Ref. [75]. © 2025 by the authors. Licensed under CC BY 4.0. Nanoparticle Ink Type Structure Synthesis method Size Performance Sintering technology Applications AgNPs Sphere-like Chemical reduction Commercial 10–200 nm Good stability Water sintering Supercapacitor Length 300–1000 nm, Thickness about 50 nm ~1–20 nm Excellent electrical conductivity Thermal sintering Good electrical conductivity and electrochemical properties Good electrical conductivity Thermal sintering Electromagnetic interference shielding Electrode patterning on flexible substrates Flexible conductive patterning Flexible electronic devices ​ Nanosheets AuNPs Sphere-like ​ Rod-like Cu@ Ag nanoparticle Core-shell Ag/Graphene Hybrids Hybrid structure Chemical reduction CTAB micelle method Chemical reduction Chemical reduction Length 60–70 nm Copper core thickness (10–100 nm), Silver shell thickness (1–10 nm) Reduced graphene oxide graphene size 200–500 nm deposition on various substrates, including textiles [12]. Inkjet printing of graphene-based inks combines the inherent benefits of additive manufacturing, such as customization, scalability, and reduced material waste, with the superior conductivity and flexibility of graphene-based structures. Various preparation methods have been explored to produce printable graphene inks. Liquid-phase exfoliation (LPE) is among the most widely used approaches, involving the dispersion of graphite in solvents, followed by the application of ultrasonic or shear forces to separate the graphene layers [16,27]. Innovations in this method, such as the use of ultrasound-assisted supercritical CO2, have been demonstrated by Gao et al. [103], resulting in pristine graphene dispersions with excellent stability and optimal rheological properties for inkjet-printing. Alternatively, electrochemical exfoliation provides a scalable method for expanding graphite layers through the application of an electric field, yielding graphene sheets with promising conductivity [20]. Despite these advances, reduced graphene oxide (rGO) remains one of the most commonly utilized materials for inkjet-printed flexible electronics owing to its water dispersibility, ease of processing, and cost-effectiveness at large production scales [104,105]. However, the presence of residual oxygen-containing functional groups in rGO significantly limits its electrical conductivity by introducing defects and increasing the inter-sheet junction resistance [106]. While pristine graphene offers better electrical performance, challenges such as low dispersion concentration, use of toxic solvents, incomplete removal of solvents in post-printing, and the requirement for multiple printing passes and high-temperature annealing (often exceeding 300 ◦ C) restrict its widespread applicability, especially on temperature-sensitive substrates such as textiles [12,103]. To overcome these inherent limitations, hybrid ink formulations incorporating MNPs have been developed [20,107]. For example, Karim et al. [12] proposed and successfully developed a highly conductive graphene-AgNP composite ink designed specifically for inkjet printing on flexible and wearable substrates. Their approach utilized a combination of formulation optimization using a cube film applicator, followed by tailoring the ink viscosity and surface tension for inkjet compatibility. This formulation demonstrated excellent conductivity with fewer layers of printing and lower annealing temperatures, which are critical for processing textile substrates. Furthermore, surface pretreatment of textiles via inkjet printing of functional layers facilitated strong ink adhesion and improved print quality, enabling the development of highly conductive wearable e-textiles. Post-printing treatments, such as thermal annealing, are crucial for improving the conductivity of printed graphene structures. Gao et al. [103] reported that annealing printed graphene films at 300 ◦ C for 30 min resulted in a high conductivity of 9.24 × 103 S/m. Beyond conductivity improvements, efforts have been made to address environmental sustainability in ink formulations. For instance, González-Domínguez et al. [108] introduced an environmentally friendly ternary composite ink consisting of graphene oxide, carbon Sintering-free Highly conductive Low-temperature sintering Highly conductive, Excellent specific capacitance Low-temperature sintering Flexible supercapacitors nanotubes (CNTs), and nanocellulose formulated in water-based systems to eliminate the need for toxic organic solvents. These inks demonstrated versatile processability, ranging from low-viscosity inks suitable for inkjet printing to high-viscosity pastes and even self-standing hydrogels, broadening their potential use in flexible and wearable electronics. 3.2.2. Carbon nanotube (CNT) inks The development of CNT inks for inkjet printing has opened new possibilities in the field of flexible and printed electronics [16,109]. One of the primary challenges in creating CNT inks is the achievement of stable dispersions. CNTs tend to aggregate owing to strong van der Waals interactions, making it difficult to create homogeneous inks. To overcome this, researchers have explored various dispersion methods, including surfactant use and covalent functionalization. Sodium n-dodecyl sulfate (SDS), an anionic surfactant, has been successfully used to disperse single-walled carbon nanotubes (SWCNTs) in water-based inks [110]. Other surfactants, such as sodium dodecylbenzene sulfonate (SDBS) and sodium cholate (SC), have also been employed to create stable CNT dispersions [110]. Alternatively, covalent functionalization of CNTs can improve their solubility and dispersion stability, although this approach may alter the intrinsic properties of nanotubes [111]. Alignment techniques are crucial for enhancing the performance of printed CNT-based devices. For example, AC dielectrophoresis (DEP) has been demonstrated as an effective method for aligning CNTs between electrodes, thereby improving the overall conductivity and performance of printed devices [111]. Shear printing techniques have also been explored to achieve aligned CNT networks, which can lead to improved electrical and mechanical properties in the final printed structures [16]. The applications of CNT inks in inkjet printing are diverse and promising. One notable application is the development of highly stretchable strain sensors. CNT-based inks have been used to create sensors with stretchability exceeding 500 %, making them suitable for wearable electronics and human-machine interfaces [16]. Additionally, CNT inks have been employed in the fabrication of flexible and disposable electrochemical sensors, demonstrating their potential for low-cost point-of-care diagnostics [112]. Soum et al. [113] further demonstrated the versatility of CNT inks by developing an aqueous formulation of multi-walled CNTs (MWCNTs) stabilized using naphthyl-functionalized poly(ethylene glycol) surfactants for inkjet-printed fingerprint sensor devices (Fig. 7a–c). This study highlights the potential of CNT-based inks for producing functional sensing devices directly on flexible substrates. CNT-based inks have also shown great promise in printed electronics. Researchers have successfully fabricated thin-film transistors (TFTs) using inkjet-printed CNT networks, achieving effective mobilities of ~0.07 cm2/V⋅s and ON/OFF current ratios of up to 100 [114]. Furthermore, when combined with 10 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 7. (a) Dispersed CNT ink for inkjet printing and corresponding SEM image of CNTs; (b) printed CNT fingerprint patterns; and (c) their electrical properties ((a–c) reproduced from Ref. [113]). © 2019 by the authors. This is an open access article distributed under the CC of the 4.0 license. (g) Schematic of the aqueous MXene/PEDOT:PSS hybrid ink formulation for inkjet-printed devices. (h) TEM image of MXene nanosheets ((g–h) reproduced from Ref. [116]). © 2023 by the authors. Licensed under the CC BY 4.0 license. demands of wearable and conformable electronic devices. One approach involves blending PEDOT:PSS with an elastomeric polymer. For example, a study reported a stretchable PEDOT:PSS/PEO blend that exhibited a low sheet resistance of 84 Ω/sq and could withstand up to 50 % tensile strain with minimal changes in its electrical performance [120]. This blend was successfully used as stretchable interconnects and dry electrodes for photoplethysmography (PPG) and electrocardiography (ECG) applications. Another strategy to improve stretchability involves using plasticizers and other additives. A study demonstrated that the addition of d-sorbitol (SOR) to PEDOT:PSS ink formulations acted as a plasticizer, breaking the hydrogen bonds between the PSSH chains and providing a larger free volume for polymer chain extension. This resulted in PEDOT:PSS films that could maintain stable resistance after 200 cycles of stretching at a 55 % strain [117]. Notably, although significant progress has been made in enhancing the conductivity and stretchability of PEDOT:PSS inks, there is still room for improvement. Some researchers have explored alternative approaches, such as incorporating CNTs into PEDOT:PSS inks to further enhance conductivity. One study reported achieving sheet resistances as low as 225 Ω/sq using CNTs functionalized with polyethylene glycol [121]. In addition to these approaches, transition metal carbides (TMCs), particularly MXenes, have been investigated for their excellent electrical, mechanical, and electrochemical properties in printed electronics. Wu et al. [122] demonstrated that the stability of Ti3C2Tx MXene, synthesized via in situ etching, can be preserved for over 80 days at ambient temperature by incorporating ascorbate ions as both capping and anti-oxidizing agents. Furthermore, an aqueous inkjet-printable MXene/PEDOT:PSS hybrid ink was developed, exhibiting excellent printability with tunable viscosity, long-term dispersion stability, and a high volumetric capacitance of 754 F/cm3, attributed to the enhanced interlayer contact facilitated by the PEDOT:PSS component (Fig. 7d and e). These advances highlight the potential of hybrid ink formulations for realizing high-performance, multifunctional, and flexible printed electronic devices. high-capacitance ion gel gate dielectrics, CNT-based TFTs have demonstrated even higher mobilities (>20 cm2/V⋅s) and faster switching speeds, enabling the creation of printed digital circuits such as inverters, NAND gates, and ring oscillators [115]. 3.3. Conductive polymer inks 3.3.1. PEDOT:PSS Conductive polymer inks based on PEDOT:PSS offer unique advantages, such as solution processability, transparency, and mechanical flexibility, making them suitable for various optoelectronic devices and wearable sensors [117]. One of the primary challenges in developing PEDOT:PSS inks is to enhance their conductivity. Researchers have achieved significant improvements using secondary doping techniques. For instance, the addition of ethylene glycol (EG) to PEDOT:PSS solutions has been shown to dramatically increase conductivity. A study reported that inkjet-printed PEDOT:PSS films with EG and other additives exhibited a high conductivity of 1050 S/cm and a sheet resistance of less than 145 Ω/sq on both rigid and flexible substrates [117]. The mechanism behind this enhancement involves a conformational transition of the PEDOT backbone and the removal of excess PSS components, leading to improved charge transport. Similarly, dimethyl sulfoxide (DMSO) is an effective secondary dopant for PEDOT: PSS. When added to PEDOT:PSS solutions, DMSO increased the conductivity of inkjet-printed PEDOT electrodes and significantly reduced the contact resistance in polymer thin-film transistors [118]. This improvement was attributed to the enhanced interfacial stability between the printed PEDOT electrodes and semiconductor layers. Interestingly, secondary doping induced nanoscale structural changes in the PEDOT:PSS films. Near the critical dopant concentration, non-crystallized PEDOT molecules uncouple from the PSS chains and undergo nanocrystallization, which is believed to be the key driving force for conductivity enhancement [119]. In addition to conductivity enhancement, researchers have focused on developing stretchable PEDOT:PSS formulations to meet the 11 B.K. Dejene Materials Today Advances 28 (2025) 100629 functionalized multiwalled carbon nanotubes (MWNT-f-OH) and AgNWs have demonstrated remarkable electrical performance, achieving enhanced conductivity through the combined contributions of each nanomaterial [127]. Similarly, graphene-Ag hybrid inks have been successfully employed in the fabrication of wearable electronic textiles, with sheet resistance values ranging from approximately 0.08 to 4.74 Ω/sq, depending on the printing parameters and material ratios [12]. Recent advancements have explored liquid metal (LM) elastomer composites, particularly eutectic gallium-indium (EGaIn) systems, as functional inks for stretchable and self-healing circuits. Hybrid composite films embedding AgNWs and liquid metals beneath elastomeric matrices exhibit tunable electrical conductivity and robust electromechanical performance [129]. These hybrid structures can form AgIn2 intermetallic compounds through controlled processing, along with stable Ga-In bonds, which collectively endow the material with high deformability and mechanical resilience against mechanical damage, such as cutting, scratching, and peeling [129]. Such materials are particularly promising for applications in next-generation soft robotics and wearable devices that require both flexibility and durability [12]. Hybrid and composite inks have also been extended to energy storage applications [4,84]. Inkjet-printed combinations of rGO with molybdenum trioxide (MoO3) nanosheets have been employed to fabricate all-solid-state micro-supercapacitors (MSCs). These structures exhibit excellent flexibility and favorable electrochemical performance, making them promising candidates for integrated energy systems in wearable electronics [130]. Additionally, AgNW-based flexible transparent conductive films (FTCFs) have potential applications in diverse optoelectronic systems, including solar cells, transparent heaters, touch panels, and sensors [131]. Despite these advances, certain challenges remain, particularly regarding dispersion stability and biocompatibility issues, especially for bio-integrated systems. Although CNT-based inks exhibit excellent conductivity, they often suffer from poor dispersibility in aqueous media and potential cytotoxicity, restricting their use in biomedical and wearable applications. To overcome these limitations, Liang et al. [132] developed a bio-derived hybrid ink consisting of silk sericin-modified CNTs (SSCNTs). As demonstrated in Fig. 8a-d, the incorporation of sericin, a natural silk protein, enabled the uniform dispersion of CNTs in water, overcoming the rapid precipitation characteristic of unmodified CNTs. Transmission electron microscopy confirmed superior colloidal stability, and the resulting SSCNT films exhibited consistent conductivity (~42.1 ± 1.8 S cm−1), which is sufficient for wearable applications (Fig. 8e–j). Beyond dispersion stability, this bio-derived modification also addresses cytocompatibility concerns. Tests involving various human cell lines, including glioblastoma (U87), umbilical vein endothelial (HUVEC), and lung epithelial (A549) cells, revealed that SSCNT films maintained cell viability and promoted cellular proliferation over three days of incubation. These findings highlight how bio-based additives in hybrid inks can bridge performance with environmental and biological safety, making them particularly attractive for wearable biomedical applications [132]. 3.3.2. Polyaniline (PANI) PANI has emerged as a promising material owing to its ease of synthesis, tunable electrical properties, and versatility in forming composite conductive inks [123]. PANI exhibits pH-responsive conductivity, making it suitable for sweat sensors and other biosensing applications, including wearable devices. The electrical conductivity of PANI changes in response to pH variations, allowing the detection of sweat, blood, and other bodily fluids [123]. This property has been exploited for the development of low-cost printed electrochemical sensor platforms for environmental monitoring and clinical diagnostics [124]. Inkjet printing of PANI-based inks enables precise control over the two-dimensional pattern, thickness, and conductivity of the resulting films, highlighting the level of precision achievable through this technique [124]. The morphology of PANI in inkjet-printable inks is crucial for their performance. Two primary morphologies have been explored: nanofibers and nanoparticles. PANI nanofibers have been successfully incorporated into piezoelectric devices by printing conductive polymers onto P(VDF-TrFE) nanofibers [125]. This approach allows the creation of all-polymer piezoelectric devices with applications in impact sensing, breathing detection, and pulse rate monitoring. In contrast, PANI nanoparticles have shown great promise in inkjet printing applications. Aqueous PANI nanodispersions doped with dodecylbenzenesulfonic acid (DBSA) were successfully synthesized and inkjet-printed, resulting in uniform particle size distributions and well-defined, stable electrochemistry [126]. Interestingly, the performance of printed devices, particularly their electrical conductivity, can be affected by organic residues from the inks. Research has shown that polymer stabilizers, such as polyvinylpyrrolidone, tend to concentrate between vertically stacked nanoparticle layers and at dielectric/conductive interfaces, leading to anisotropic electrical conductivities. [22]. This understanding provides insights into potential strategies for improving nanomaterial-ink formulations for functional printed electronics. 3.4. Hybrid and composite inks Hybrid and composite are a powerful class of functional materials for inkjet printing in flexible and wearable electronics. By integrating multiple components, such as metal nanostructures, conductive polymers, carbon nanomaterials, and bio-derived substances, these inks exploit synergistic effects to enhance conductivity, mechanical flexibility, and multifunctionality beyond what single-material inks can offer. Their versatility has expanded the design possibilities for applications ranging from transparent conductive films to stretchable and biocompatible electronic systems [127,128]. One of the most promising formulations involves silver nanowire (AgNW)/PEDOT:PSS composites, which have demonstrated outstanding performance in transparent heaters and conductive patterns applications. The integration of AgNWs into a PEDOT:PSS matrix results in a robust, interpenetrating conductive network that significantly improves the electrical conductivity while retaining high optical transparency [127]. Similarly, PEDOT: PSS/SWCNT-COOH composite inks exhibited superior conductivity at lower print repetitions compared to pristine PEDOT:PSS inks. This enhancement is attributed to the establishment of electrical interconnections between the conductive domains, effectively reducing the percolation threshold of the composite and achieving transparent patterns with sheet resistances as low as ~1 kΩ/sq at ~90 % transmittance [128]. Such hybrid inks offer attractive alternatives to traditional transparent conductors, such as indium tin oxide (ITO), especially for applications requiring flexibility [128]. Beyond polymer-metal combinations, graphene/CNT composites have been extensively studied for their ability to form synergistic percolation networks that yield superior electrical and mechanical properties. The combination of two-dimensional graphene derivatives with one-dimensional CNTs provides multiple conductive pathways, improving the charge transport efficiency across printed patterns [127]. For example, composites of few-layered graphene (FLG) mixed with 3.5. Ink rheology and formulation science Inkjet printing of conductive nanomaterials for wearable electronics relies heavily on the rheological properties and formulation science of the inks used. The ultimate goal of ink formulation is to produce uniform, well-formed droplets during ejection while preventing the formation of satellite droplets, which can lead to printing defects and irregularities [15]. The formation and stability of ink droplets during jetting are primarily governed by the balance between the viscous, inertial, and surface tension forces within the ink. These relationships are quantified by dimensionless numbers, such as the Reynolds number (Re), Weber number (We), and Ohnesorge number (Oh), with the Z number (Z = 1/Oh) commonly used to predict inkjet printability. Successful droplet formation typically occurs within the range of 1 < Z < 10 for Newtonian fluids, although some studies have extended this range to 12 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 8. Hybrid and composite inks. Adapted from Ref. [29]. © 2025 by the authors. Licensed under the CC BY 4.0 license. (a) Hybrid ink formation sequence made of SSCNT; (b) chemical structure of sericin; (c) π–π interaction between aromatic groups of sericin and the surface of CNT; (d) photograph showing rapidly spreading SSCNT ink in water; (e) schematic diagram of the ECG system based on SSCNT-based textile electrodes; (f) photograph of the textile electrodes; (g) ECG signals collected by the ECG system; (h) schematic diagram of the breath sensor system based on SSCNT-based conductive yarn; (i) photograph of the breath sensor; and (j) resistance of conductive yarn during use. Reproduced with permission from Ref. [132]. © 2020 John Wiley & Sons. 2.5 < Z < 26, depending on the specific fluid properties and printing setups [133,134]. Further refinements, such as laser-induced flow-focusing techniques, have enabled stable droplet formation beyond the traditional Z-number limits [135,136]. In addition to these dimensionless relationships, the minimum velocity (vmin) required to overcome the surface tension barrier at the nozzle can be expressed as Equation (1). √̅̅̅̅̅̅̅̅ 4γ vmin = (1) ρdn where dn is the nozzle diameter, γ is the surface tension of the liquid, and ρ is the density of the liquid. These models provide guidelines, but practical success with nanomaterial-laden inks also demands careful consideration of particle interactions [136]. Although these predictive models provide valuable guidelines, particle-laden conductive inks introduce additional complexities that are not fully captured by these dimensionless parameters. Even inks formulated within the ideal Z or We ranges can suffer from nozzle clogging or inconsistent droplet formation owing to agglomeration or particle size effects [133,134]. 13 B.K. Dejene Materials Today Advances 28 (2025) 100629 The choice of solvent is equally critical, affecting both the rheological characteristics of the ink and its interactions with the textile substrate during drying. Water-alcohol mixtures, such as ethanol-water or ethylene glycol-isopropanol combinations, have proven effective for conductive inks, such as AgNW suspensions, striking a balance between controlled drying rates and compatibility with substrate properties [21]. These solvent systems help balance the need for slow drying to avoid coffee-ring effects with rapid evaporation to improve production throughput. The particle size also directly affects printability. For example, nozzles with a diameter of 50 μm generally require that the median particle size (D50) be less than 100 nm to minimize the risk of clogging. In conductive nanowire inks, tailoring the aspect ratio of AgNWs has been shown to improve printability and conductivity, enabling ink concentrations of 10–50 mg/mL with high conductivity and minimal printing passes [21]. However, increasing the particle concentration beyond the optimal level can negatively affect the jetting stability because of particle-particle interactions during droplet formation [137]. Achieving high conductivity often requires a compromise between ink concentration and formulation stability. To prevent agglomeration, formulations often include surfactants, dispersants, or polymers and employ surface functionalization of particles [15,138]. The roles of viscosity and surface tension cannot be overstated. For DOD inkjet printing systems, the viscosities generally fall within the range of 2–20 mPa s, whereas surface tension values between 20 and 50 mN/m are optimal for balancing droplet formation and substrate interaction [139]. Ideally, the surface tension of the ink should be slightly lower by 2–10 mN/m than the surface energy of the substrate to ensure effective wetting and adhesion [32,140]. While most conductive inks behave as Newtonian fluids, advanced formulations often exhibit non-Newtonian shear-thinning behavior, especially those incorporating polymers. [32]. This behavior facilitates ink ejection under shear inside the printhead while recovering the viscosity post-ejection, reducing ink spreading, and preserving feature resolution. Fig. 9a shows the typical ranges of ink properties suitable for inkjet printing. Furthermore, Fig. 9b shows a drop-watcher image capturing five nozzles simultaneously jetting a 3 wt % poly DADMAC solution with a 30 μs delay between jets, demonstrating the synchronized droplet formation process. Complementing this, Fig. 9c presents a grayscale image of ink jetting captured by a drop-watcher system, showing the time-sequenced evolution of droplet formation driven by specific waveform patterns. These visual representations provide valuable insights into the practical behavior of inks during jetting, emphasizing the importance of optimizing rheological parameters to achieve high fidelity printing [32]. Thixotropic effects, wherein viscosity decreases over time under constant shear, are also important, particularly in high-viscosity systems used in other printing methods, such as screen printing [141]. The rheological complexity increases when considering the high aspect ratio of nanowires or nanosheets, which can drastically influence the viscosity at a given particle concentration. Additional strategies have been adopted to further improve the performance of inks for textile-based applications. The development of aqueous MXene inks, which provide high conductivity and environmental compatibility, and graphene-silver composite inks, which minimize sintering requirements while maintaining conductivity, represent significant advancements [21]. Filtration processes combined with the use of low-volatility solvents are also employed to prevent nozzle clogging during high-throughput or high-temperature printing operations [15,138]. Furthermore, surface treatment of textile substrates has emerged as a crucial strategy to overcome inherent textile challenges, such as porosity and surface roughness. For example, organic nanoparticle-based pre-treatment layers have enabled the production of fully inkjet-printed, breathable, and environmentally friendly wearable electronic textiles [147]. However, achieving reliable performance also requires addressing durability challenges, particularly in terms of their washability. Encapsulation Fig. 9. Inkjet Ink property requirements and drop formation. (a) Graphical means of assessing ink suitability using nondimensional numbers. (b) Example of a dropwatcher image on DMP-2831, showing five nozzles jetting 3 wt/wt. % poly DADMAC solution with a 30 μs strobe delay. (c) Grayscale images of ink jetting captured by the drop-watcher system with the time sequence of the waveform. Adapted from Ref. [32]. © 2024 by the authors. Licensed under the CC BY 4.0 license. 14 B.K. Dejene Materials Today Advances 28 (2025) 100629 with polymeric materials such as polyurethane (PU) or polydimethylsiloxane (PDMS) significantly enhances the mechanical and wash durability of printed textiles, ensuring that the conductive pathways remain functional even after repeated laundering cycles [142, 143]. Despite these advancements, several challenges persist in inkjet printing for wearable textiles, including the need to further minimize particle aggregation, optimize ink-substrate adhesion, and ensure consistent performance during mechanical deformation and repeated washing [31]. Future research must prioritize the use of environmentally friendly solvents, biodegradable conductive materials, and scalable surface engineering techniques that align with the sustainability goals of wearable electronics production. for inkjet-printed electronics include polyester (PET), nylon, and spandex (elastomer). PET is widely used because of its high thermal stability (melting point ~250 ◦ C), which allows it to endure post-print sintering of conductive metallic inks. The smooth surface of PET supports good pattern resolution, but its low surface energy (~35 mN/m) reduces ink adhesion. Consequently, plasma activation or chemical treatments are often required to increase surface wettability and ink retention [12,149]. The compatibility of PET with high-temperature processing makes it an ideal candidate for applications requiring durable conductive tracks. In contrast, nylon provides superior mechanical flexibility and resilience under repeated bending, which is critical for wearable electronics integrated into everyday garments. However, the moisture regain of nylon (~4.5 %) can lead to dimensional instability over time, necessitating the use of hydrophobic coatings or optimized ink formulations to preserve electrical performance [14]. Spandex is the preferred choice for applications that demand extreme stretchability, such as soft sensors or wearable displays. Spandex fibers exhibit elastic deformation exceeding 150 % without mechanical failure, making them highly suitable for strain-tolerant electronic circuits [145,147]. However, spandex often requires strain-insensitive or stretchable ink formulations to ensure consistent conductivity during deformation. Synthetic fibers offer a continuum of properties, from thermal resilience (PET) to mechanical flexibility (nylon) to high stretchability (spandex), allowing the substrate choice to be tailored to specific device applications. In recent years, the development of engineered and hybrid textile substrates has advanced the field of wearable electronics by combining comfort with sophisticated electronic performance. Electrospun nanofiber membranes, such as polyurethane (PU)/polyvinylidene fluoride (PVDF) composites, provide engineered microstructures with fine porosity (<5 μm). This allows efficient nanoparticle trapping during inkjet printing, creating robust electronic pathways while maintaining flexibility [12]. These membranes are particularly attractive for integrated energy devices and as wearable sensors. In addition, conductive base textiles, such as silver-plated nylon fibers, have been employed to reduce interfacial electrical resistance. The integration of such conductive textiles with reactive silver inks leads to durable and reliable electrical networks, even under mechanical stress [13,56,144]. These materials are key to developing multilayered electronic architectures that are directly embedded into textiles. 4. Textile substrates for inkjet-printed wearable electronics 4.1. Substrate classification and structural properties Inkjet printing on textile substrates is a promising approach for the development of wearable electronics, offering the ability to fabricate functional electronic circuits directly onto soft, flexible, and breathable materials. The seamless integration of electronic components with textiles is essential for creating conformal, lightweight, unobtrusive wearable devices. The performance of printed electronics largely depends on the physicochemical characteristics of the textile substrate, including surface roughness, porosity, hydrophilicity, and mechanical durability. Textile substrates used for inkjet-printed electronics can be broadly classified into natural fibers, synthetic fibers, and engineered or hybrid textiles, each with distinct advantages and challenges in achieving reliable electronic functionality [13,14,56,144,145]. 4.1.1. Natural fiber textiles: opportunities and constraints Natural fiber textiles are favored because of their comfort, biodegradability, and sustainability. Common examples include cotton, silk, and wool, each of which offers specific advantages and presents distinct challenges for inkjet-printed electronics [146]. Cotton is one of the most widely used substrates in printed electronics research because of its hydrophilic cellulose structure. Its microfibrillar architecture (10–20 μm diameter) facilitates deep ink penetration, aided by a contact angle of 60–80◦ , which promotes the spreading of water-based inks. This renders cotton highly suitable for applications requiring moderate-resolution patterns. However, a notable drawback is its tendency to swell up to 15 % upon ink absorption, which can cause dimensional instability and distortion of fine electronic features [13,147]. Various pretreatments, such as the use of crosslinking agents or surface coatings, have been employed to address these limitations. While cotton excels in ink absorption, silk provides a much smoother surface (roughness Ra <1 μm), which is ideal for high-resolution patterning down to 50 μm line widths. Furthermore, the protein-based structure of silk facilitates covalent bonding with reactive inks, improving mechanical durability and functional stability [148,149]. These characteristics make silk a promising candidate for intricate and miniaturized electronic designs, although its cost and relatively limited availability constrain its broader adoption. In contrast, wool presents more pronounced challenges because of its keratin-based heterogeneous fiber structure. The irregular surface topography hampers uniform ink deposition, resulting in inconsistent electronic performance of the devices. [50,56]. Despite its advantages in insulation and comfort, wool is rarely used in precision electronics unless combined with advanced pre-treatments or hybrid systems. Thus, the progression from absorbent cotton to precision-capable silk to challenging wool illustrates the trade-off between comfort and printability in natural fiber-based substrates. 4.1.3. Material selection considerations for wearable electronics The selection of an appropriate substrate for wearable electronics depends on the interplay of several factors, such as thermal compatibility, surface wettability, mechanical resilience, and electrical performance. Advanced applications often necessitate multilayer device architectures, such as polymer dielectric bilayers paired with conductive electrodes, to improve both mechanical stability and electronic reliability [149]. Beyond commonly used substrates, specialized polymers such as polyimide (PI), with their high glass transition temperatures and robust mechanical properties, are preferred for demanding applications such as flexible printed circuit boards. Thermoplastic polyurethane (TPU) and polydimethylsiloxane (PDMS) offer enhanced stretchability and flexibility for applications requiring extensive mechanical deformation. Moreover, the porous nature of textile substrates inherently facilitates high areal mass loading of active materials, providing superior performance for applications such as energy storage, biosensing, and health monitoring [30,32,150,151]. This makes textile-based electronics increasingly relevant in diverse sectors, including sportswear, military wearables, smart medical clothing, and adaptive camouflage [146,152]. Surface wettability, typically described by contact angle (CA) measurements, is an essential parameter influencing inkjet printing on textiles. Low CA values (<90◦ ) correspond to good wettability and facilitate broad ink spreading across the surface, whereas high CA values (>90◦ ) lead to droplet contraction, potentially compromising pattern resolution. Superhydrophobic surfaces (CA >150◦ ) present substantial 4.1.2. Synthetic fiber textiles: enhancing printability and durability To overcome the inherent limitations of natural fibers, synthetic fibers offer engineered properties with greater control over their thermal, mechanical, and chemical behaviors. Commonly used synthetic textiles 15 B.K. Dejene 16 2–3.2 2.0–2.6 2.41 2.76 0.57–3.7 0.008–8.25 2.4–3.4 2.2–3 2.2 1.9 1.32 1.42 0.97 ​ ​ 1.33 ​ ​ 81–150 145 80 360–410 145–150 0 105 120–200 223 35 ​ 2,000,000 5000-285,000 ​ ​ ​ >1000 ​ 44.0 34.2 38 43.8 20.4 30.2 – – – – PET Polycarbonate (PC) Polyurethane (PU) Polyimide (PI) PDMS Polypropylene (PP) Polyacrylate (Pacr) PEN Polyethersulphonate (PES) Polycyclic olefin (PCO)/ polynorbornene (PNB) 90 92 – 35–60 – 84.0–90.0 >90 88 89.0 91.6 0.6 0.16–0.35 0.2 1.3–3.0 >0.1 0.01 0.2 0.3–0.4 1.4 0.03 Good Poor Good Good Poor Good Good Good Poor Good ​ >800 Good Fair Good Fair Good Good Good Good Fair Good ​ ​ ​ ​ ​ 12.5–250 25–500 25–125 125–4775 13–356 Poor Good ​ Good ​ Good Fair Good Good Poor ​ 1.38 Glass transition temperature (Tg) Density (g/cc) Surface roughness Thickness (μm) Dimensional stability Folding endurance (cycles) Solvent resistance Water absorption (%) Transparency (%) Inkjet printing on textiles for wearable electronics requires advanced surface engineering strategies to address the inherent challenges of fabric roughness, porosity, and wettability of the fabric. These strategies enhance the ink adhesion, pattern resolution, mechanical durability, and long-term stability of printed electronics. Surface engineering techniques can be broadly classified into physical modification methods, chemical functionalization, and advanced encapsulation techniques, which are often employed in combination to create reliable and scalable e-textiles [32,56,149]. Physical treatments, such as plasma treatment and laser ablation, have proven effective for modifying the surface energy and texture of textile substrates. Plasma treatment, particularly using O2 and CF4 plasma, plays a crucial role in tailoring the surface properties for inkjet printing. O2 plasma introduces carboxyl (-COOH) groups on the textile surface, increasing the surface energy by approximately 25 mN/m, thereby improving the wettability and enhancing the ink adhesion. Conversely, CF4 plasma introduces hydrophobic fluorinated functionalities, which are useful for creating well-defined boundaries and precise printed circuit patterns with improved durability [155]. Laser ablation offers another promising physical modification strategy, particularly for achieving depth-controlled (±5 μm) patterning on textile surfaces [116]. By selectively removing surface polymers, laser ablation facilitates fine design features without compromising fabric integrity, helping to prevent ink diffusion and improve the resolution of printed conductive pathways. These physical treatments, when combined with functional ink formulations, enable the production of conductive, durable, and precisely patterned electronic circuits on flexible textile substrates with high performance. In addition to physical modifications, chemical functionalization significantly enhances the compatibility of textiles with conductive inks. One widely used approach involves the application of silane coupling agents, such as (3-Aminopropyl)triethoxysilane (APTES), which introduce NH2 functional groups onto the textile surface. This modification promotes stronger chemical interactions between the substrate and conductive inks, such as silver nanoparticles or MXene-based inks [14, 149]. The improved interfacial bonding reduces ink spreading caused by the porosity of the fabric, leading to sharper and higher-resolution printed patterns. Additionally, enzyme treatments, such as cellulase application, address fibrillation in cotton fabrics, where fiber ends protrude from the yarn, creating uneven surfaces. Controlling fibrillation improves the surface smoothness, thus enhancing the uniformity of ink spreading and electrical conductivity [13]. These chemical modifications, in conjunction with surface pre-treatments, lay the groundwork for the scalable and reliable inkjet printing of functional circuits on textiles. Surface energy (mN m−1) 4.2. Surface engineering strategies Flexible textiles/polymer substrates Table 3 Specifications of various textile and polymeric substrates commonly employed for inkjet-printed wearable electronics, including key physical and surface properties relevant to wearable textile fabrication, are provided. Adapted from Ref. [32]. © 2024 by the authors. Licensed under the CC BY 4.0 license. challenges for inkjet printing and require extensive modifications to achieve functional electronic patterns [153]. Surface free energy is closely related to intermolecular forces at the material interface and plays a crucial role in ink adhesion. Surface free energy is typically expressed in J⋅m−2 or N⋅m−1, with higher values promoting better wetting and ink-substrate interactions [32,154]. Table 3 presents representative values of the surface free energies of commonly used polymer substrates, which can help guide material selection and treatment strategies for optimal print quality. The integration of advanced printing technologies with engineered textile substrates paves the way for next-generation multifunctional wearable electronics. Textiles, once considered passive carriers, are evolving into active multifunctional platforms that can incorporate sensing, actuation, and energy harvesting functionalities. With advances in materials science, ink formulations, and printing processes, inkjet-printed textiles are poised to meet the growing global demand for intelligent, comfortable, and high-performance wearable devices. Young modulus (GPa) Materials Today Advances 28 (2025) 100629 B.K. Dejene Materials Today Advances 28 (2025) 100629 seamless integration into everyday garments for healthcare and personal wellness applications. A significant area of progress has been in ECG monitoring, where inkjet-printed textile electrodes have transformed the field, particularly for long-term and continuous use [158]. Traditional gel electrodes often suffer from drawbacks such as discomfort, skin irritation, and limited reusability issues. In contrast, inkjet-printed textile electrodes provide superior comfort, breathability, and washability, making them highly suitable for extended wear. As illustrated in Fig. 11, such wearable devices can be integrated into a body-area wireless sensing network, utilizing radio frequency modules to communicate with embedded sensors and readout units. Furthermore, the incorporation of energy harvesting and storage transducers enables the development of self-powered systems [159]. The collected physiological data can be transmitted via a smartphone to centralized databases, facilitating remote clinical analysis and enabling real-time health monitoring [159]. Researchers have explored various conductive materials for fabricating these electrodes, including graphene and PEDOT:PSS. Notably, graphene and PEDOT:PSS-based wearable e-textiles can simultaneously monitor ECG signals and skin temperature [160]. Building on this, Dulal et al. [158] developed a fully inkjet-printed sustainable wearable electronic textile (SWEET) platform using water-based PEDOT:PSS and graphene inks deposited onto biodegradable Tencel™ substrates. These electrodes were integrated into gloves to continuously monitor both the ECG signals and skin temperature in real time (Fig. 12a and b). The printed electrodes exhibited reliable ECG performance, capturing distinguishable P, QRS, and T waves with a signal morphology comparable to that of the reference Ag/AgCl gel electrodes (Fig. 12d–f). Despite the higher impedance of graphene-based electrodes (1374 kΩ at 10 Hz to 34.6 kΩ at 1 kHz), the ECG signals remained clinically recognizable. PEDOT:PSS electrodes demonstrated lower impedance (291.4 kΩ–17.4 kΩ), with heart rates measured at 68 bpm for PEDOT:PSS, 74 bpm for graphene, and 70 bpm for the reference electrode. Furthermore, the SWEET platform effectively captured heart rate variability under different physical states, such as sitting and jogging (Fig. 12g-l), affirming its utility for dynamic on-body monitoring. The system also achieved rapid and stable thermal sensing responses and a temperature coefficient of resistance (TCR) of approximately 4.3 % ◦ C−1 for both ink types (Fig. 12c), enabling dual-mode functionality. Remarkably, the platform demonstrated biodegradability, with graphene-based electrodes losing approximately 48 % of their weight and 98 % of their tensile strength within four months of soil burial, aligning with eco-design principles. These findings establish SWEET as a 4.2.1. Interface layers and advanced ink formulations Further refinements in textile surface engineering include the development of interface layers that mitigate surface roughness and porosity. Although polyurethane acrylate-based coatings have been previously explored [148], challenges remain related to resolution limitations and incompatibility with scalable processes such as roll-to-roll manufacturing. Innovations, such as organic nanoparticle-based printable surface treatments, offer promising alternatives. These pre-treatments facilitate all-inkjet-printed graphene-based wearable e-textiles that maintain breathability, flexibility, and environmental friendliness [13], as shown in Fig. 10. Advanced ink formulations have addressed these performance challenges. Particle-free silver inks or graphene-silver composites improve conductivity and printability by reducing the risk of nozzle clogging and ink diffusion [149]. Moreover, aqueous MXene inks have emerged as a versatile solution, offering high conductivity while avoiding additional stabilizing additives [14]. A critical barrier to the commercial adoption of e-textiles is their poor wash durability. Mechanical stress and water exposure during repeated washing cycles can lead to the delamination of conductive materials, degrading their electrical performance [116]. Several strategies have been explored to mitigate these effects on the environment. For example, Bovine Serum Albumin (BSA) pre-treatment, polydimethylsiloxane (PDMS) encapsulation, and polyurethane (PU) sealing layers have all been shown to improve wash stability [9]. Despite these significant advancements, challenges remain. The intrinsic roughness and porosity of textiles, combined with their planar anisotropy and moisture-induced dimensional variability, complicate the production of uniform and highly conductive tracks using conventional low-viscosity inkjet inks [156,157]. Continued innovation in surface pretreatment, interface engineering, and composite ink development is essential to overcome these hurdles. 5. Advancements in applications of inkjet-printed wearable electronics 5.1. Health monitoring systems 5.1.1. Continuous physiological monitoring Inkjet-printed textiles have emerged as a promising technology for wearable electronics, particularly in continuous physiological monitoring applications, such as electrocardiography (ECG) and electromyography (EMG). These advancements have addressed several limitations of conventional electrodes by offering improved flexibility, comfort, and Fig. 10. Textile surface pre-treatment and post-treatment for e-textiles: Inkjet printing of organic nanoparticle coating followed by inkjet printing of conductive rGO to prepare a conductive textile. Adapted from Ref. [157]. © 2017 by the authors. Licensed under CC BY 4.0. 17 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 11. Demonstrator concept of wearable devices and design for PHC. Reproduced with permission from Ref. [159]. © 2022 Royal Society of Chemistry. promising step toward sustainable and high-performance e-textiles for personalized healthcare applications [158]. Additionally, these inkjet-printed electrodes support remote patient monitoring, enhancing healthcare accessibility [161]. However, one of the key challenges in wearable ECG monitoring is the suppression of motion artifacts. Innovative electrode designs, such as fractal-patterned geometries, have been proposed to maintain consistent skin contact and minimize motion-induced noises. Additionally, researchers have systematically investigated how the electrode material, structure, and applied pressure affect the skin-electrode impedance and overall ECG signal quality [162]. Interestingly, it has been shown that impedance alone is not the sole determinant of signal quality, suggesting the importance of electrode design and mechanical properties. Further innovations include the use of conductive elastomeric filaments (CEFs) to produce flexible, breathable, and washable dry-textile electrodes [163]. These devices demonstrated signal fidelity comparable to gold-standard gel electrodes and exhibited robust durability against repeated washing and drying cycles, making them well-suited for real-world, long-term ECG monitoring. Beyond ECG, inkjet-printed wearable electronics have also driven significant advancements in electromyography (EMG) monitoring, particularly in applications involving prosthetic control and humanmachine interfaces (HMI). A notable development is the fabrication of high-density surface EMG (sEMG) electrode arrays, which enable the recording of tens to hundreds of EMG channels. These arrays provide improved spatial resolution and superior signal quality compared to traditional EMG electrodes. For example, Zhao et al. [164] reported the fabrication of 32-channel soft, high-density sEMG electrode arrays using an all-printed, multi-material direct-ink-writing 3D printing method. These arrays exhibited low impedance, high signal-to-noise ratios, and performance exceeding that of commercial electrodes by 32.2 %. Inkjet printing has played a central role in enabling these advancements, especially through the deposition of conductive polymers such as PEDOT:PSS, which can achieve conductivity of up to 700 S/cm and maintain mechanical integrity at strains exceeding 100 % [165]. This progress facilitates the large-area fabrication of stretchable, conformal EMG sensors for continuous diagnostics and real-time health monitoring. An especially exciting area is the integration of machine learning with inkjet-printed electromyography (EMG) sensors. This combination allows for the real-time prediction and classification of muscle activity and human motion, contributing to breakthroughs in sports medicine, rehabilitation, prosthetic control, and virtual reality interfaces [166]. To address practical challenges, researchers have developed stretchable, dry-electrode EMG arrays on flexible printed circuit board (PCB) substrates, eliminating the need for time-consuming skin preparation [167]. These arrays deliver performance on par with or superior to traditional EMG grids and represent a critical step toward the clinical and commercial deployment of high-density EMG systems for HMIs. 5.1.2. Biochemical sensing Inkjet-printed wearable electronics have also been used in biochemical sensing. These devices enable noninvasive, continuous, and real-time monitoring of various physiological parameters and biomarkers, thereby revolutionizing personalized healthcare and disease management [9,168]. One of the most significant applications of inkjet-printed wearable electronics is in sweat analysis platforms. Sweat contains a wealth of biochemical information, offering a noninvasive route for tracking health markers in real time. Among these, lactate sensing has garnered substantial attention, especially for monitoring physical activity and metabolic status of the body. Inkjet-printed Prussian blue/PEDOT:PSS amperometric sensors have been developed for this purpose, offering high sensitivity and selectivity for lactate detection [169,170]. The incorporation of Prussian blue as a redox mediator enhances the sensor performance but introduces challenges, such as sensitivity to ionic species variations and limited operational stability [171]. To overcome these limitations, researchers have explored mediator-free sensing interfaces utilizing platinum nanoparticles and CNTs [171], providing alternative solutions with improved robustness. Beyond lactate, pH monitoring in sweat has been achieved using PANI-based optical/electrochemical dual-mode sensors [172,173]. 18 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 12. Concurrent monitoring of vital signs using the SWEET platform. (a) Wearable e-textiles in which textile electrodes are attached to textile gloves. (b) Schematic of the concurrent performance measurement setup with wearable e-textiles worn by the subject. (c) TCR values of the wearable textile sensor composed of PEDOT:PSS and graphene inkjet-printed, respectively. (d–f) ECG signals captured for 60 s of the subject (sitting) from the reference, PEDOT:PSS, and graphene inkjetprinted electrodes. (g) ECG signal captured for 60 s of the subject (sitting) from PEDOT:PSS and graphene inkjet-printed wearable e-textiles. (h) Expanded version of (g) from 27 to 30 s. (i) Heart rate measured in bpm for 60 s of subject (sitting) from the QRS complex reading of (g) from PEDOT:PSS and graphene inkjet-printed wearable e-textiles respectively. (j) ECG signal captured for 60 s of subject (jogging) from PEDOT:PSS and graphene inkjet-printed wearable e-textiles. (k) Expanded version of g) from 27 to 30 s. (l) Heart rate measured in bpm for 60 s of subject (jogging) from the QRS complex reading of (i) from PEDOT:PSS and graphene inkjetprinted wearable e-textiles, respectively. Adapted from Ref. [158]. © 2024 by the authors. Licensed under CC BY 4.0. ISF glucose and blood glucose concentrations, effectively capturing dynamic glucose fluctuations during food intake. These developments position inkjet-printed ISF sensors as promising tools for diabetes management, eliminating the need for traditional blood sampling methods. Further advancements include a fully inkjet-printed multiplexed biosensing patch capable of simultaneous glucose and alcohol detection [175]. This device achieved sensitivities of 313.28 μA mm−1 cm−2 for glucose and 0.87 μA mm−1 cm−2 for alcohol, with minimal signal drift over 30 h, making it highly suitable for epidermal analysis and wireless medical interventions. The integration of carbon-based multifunctional electronic inks (CMFEIs) has further enhanced the potential of inkjet-printed biochemical sensors. These inks, composed of CNTs, graphene, and carbon black, offer biocompatibility, flexibility, and excellent electrical conductivity [176]. CMFEIs have been employed These hybrid sensors combine the strengths of both optical and electrochemical techniques, providing versatile and accurate pH measurements. To enhance the efficiency of sweat sampling and analyte transport, microfluidics with inkjet-printed hydrophobic barriers have been integrated into these platforms [170]. This design innovation improves fluid control, enabling a more reliable and responsive sweat biomarker analysis. In addition to sweat analysis, inkjet-printed wearable electronics have been at the forefront of interstitial fluid (ISF) sampling for noninvasive glucose monitoring. A notable example is a skin-worn, disposable, wireless electrochemical biosensor that combines reverse iontophoresis for ISF extraction with a screen-printed three-electrode amperometric glucose biosensor and a wireless communication module [174]. Clinical trials have demonstrated a strong correlation between 19 B.K. Dejene Materials Today Advances 28 (2025) 100629 to fabricate sensors capable of monitoring the heart rate, skin temperature, and sweat composition. Moreover, the development of scalable and cost-effective manufacturing techniques, such as roll-to-roll printing, has improved the resolution, flexibility, and affordability of these devices, ensuring their seamless integration with human skin and wearable platforms. Innovations in sensor materials, microfluidic integration, and printing technology have enabled the development of more accurate, durable, and user-friendly devices. Ongoing research is addressing critical challenges, such as long-term stability, biocompatibility, and multimodal integration, to further enhance these platforms [9,24,176]. A summary of inkjet-printed sensors for physiological monitoring, including materials, substrates, and performance metrics (e. g., SNR for ECG and sensitivity for temperature sensors), is presented in Table 4. Ti3C2Tx MXenes, has brought a step change in performance, with areal capacitance values reaching 294 mF/cm2 at 2 mV/s on textile substrates, surpassing many earlier printed MSC systems [14]. Recent innovations have explored heterostructure-based MSCs that combine different electrochemical mechanisms in a single device. A notable example by Islam et al. [185] involved the inkjet printing of vertically stacked 2D material heterostructures, integrating graphene, molybdenum disulfide (MoS2), and hexagonal boron nitride (h-BN) (Fig. 13a–f). In this configuration, graphene serves as a conductive electric double-layer electrode, MoS2 introduces pseudocapacitive behavior, and h-BN functions as an insulating layer that contributes to bandgap modulation. This layered assembly creates a hybrid MSC that leverages the strengths of each component to enhance both the energy and power densities. The study employed a sequential printing approach, first optimizing graphene deposition, and then incorporating semiconducting and insulating layers between the graphene electrodes. This strategy enables control over the quantum capacitance and interfacial properties, improving the performance of wearable energy storage devices for practical applications. In addition to graphene-based architectures, alternative ink systems have been explored. Ethanol-based inks containing MnO2 nanostructures, such as MnO2 nanoflowers, have been successfully printed on flexible paper substrates, yielding MSCs with an areal capacitance of 0.68 mF/cm 2 at 25 μA/cm2, alongside excellent foldability and mechanical resilience [186]. These results demonstrate the potential of using various solvent systems and nanostructured morphologies to tailor ink properties for specific applications. Further advancing this field, MXene/MnO2 composite films have been reported with ultrahigh volumetric capacitances, reaching 312 F/cm3, and displaying remarkable cyclic durability, retaining 130.8 % of the initial capacitance after 5000 charge/discharge cycles [187]. This performance is attributed to the synergistic interaction between the high conductivity of MXenes and the pseudocapacitive nature of MnO2. Additionally, the aqueous inkjet printing of MXene inks into poly(vinyl alcohol)/H2SO4 gel electrolytes offers a safe and scalable route for printed MSC fabrication on both textile and paper substrates, further broadening the material toolbox for wearable energy storage. Generally, the integration of advanced materials such as graphene, MXenes, MnO2, and 2D heterostructures, combined with innovative ink formulations and architectural designs, is rapidly transforming inkjet-printed MSCs from laboratory prototypes to practical, textile-integrated energy storage systems. These devices offer an excellent combination of mechanical flexibility, electrochemical performance, and integrability, paving the way for their deployment in next-generation smart clothing and self-powered wearable electronic devices. Beyond MSCs, inkjet printing has been extended to other flexible energy storage technologies, including the production of batteries. Researchers have developed stretchable battery packs capable of withstanding 100 % biaxial stretching by employing stress-enduring printable inks combined with serpentine interconnects and reinforced backbones. These flexible batteries exhibit remarkable mechanical endurance, maintaining stable performance with less than 2.5 % 5.2. Energy storage and harvesting 5.2.1. Flexible energy storage Inkjet printing has emerged as a powerful and scalable technique for fabricating flexible and wearable energy storage devices, particularly micro-supercapacitors (MSCs), designed for integration into textilebased platforms. Its advantages, including precision, digital patterning, low material waste, and compatibility with diverse substrates, make it ideal for seamlessly embedding energy storage functionality into smart garments and wearable electronics [148,183]. This technology enables the direct deposition of active and conductive materials onto flexible textile surfaces, eliminating the need for rigid or bulky components, thereby supporting the development of lightweight, conformable, and multifunctional energy systems. A significant focus in this domain has been the development of interdigitated electrode architectures, which enhance the electrochemical performance of MSCs by reducing the ion diffusion paths and increasing the effective surface area. For example, asymmetric MSCs based on graphene and MnO2 fabricated on flexible substrates demonstrated a broad 3 V operating window, achieving an excellent combination of high energy density, mechanical flexibility, and cycling stability [183]. One such example is a paper-based asymmetric supercapacitor that delivers a maximum areal capacitance of 1.586 F cm−2 at 4 mA/cm2 and an energy density of 22 mWh/cm3 [183]. Additional enhancements have been achieved through the use of laser-reduced graphene oxide (rGO) as current collectors, which significantly improves both the electrical conductivity and charge storage capabilities [184]. Inkjet-printed MSCs on fabric substrates have shown promising performance in textile-specific applications. Devices printed on wearable textiles achieved areal capacitances of up to 8.8 mF/cm2 at a current density of 0.5 mA/cm2 [148], which is close to the values reported for more optimized configurations (e.g., 80 mF/cm2 at 0.1 mA/cm2). These fabric-based MSCs exhibit excellent mechanical flexibility and structural stability, maintaining their performance after repeated bending and folding, which are essential characteristics for wearable technologies. Moreover, the introduction of MXene-based inks, notably Table 4 Inkjet-printed wearable electronics for health monitoring. Substrate Conductive ink Application Performance Ref. Cotton Tencel (biodegradable) Hosiery (Pantyhose) Woven Polyurethane nonwoven Taffeta fabric Reduced graphene oxide (rGO) PEDOT:PSS, Graphene ECG monitoring ECG monitoring [157] [158] PEDOT:PSS Reactive silver Ag and fluoro-elastomer composite CNT and PEDOT:PSS-based ink ECG monitoring ECG monitoring EEG monitoring SNR of 22.3 Clear P-QRS-T detection; heart rate: 68 bpm (PEDOT:PSS), 74 bpm (graphene); comparable to reference SNR 12.93 ± 0.80 (dry), 13.75 ± 0.26 (gel) SNR 18 Demonstrated brain activity recording Sensitivity: 0.15 %/◦ C (CNT), 0.41 %/◦ C (PEDOT:PSS), 0.31 %/◦ C (CNT/PEDOT:PSS) [180] Textile Polyester sheet Silver nanoparticles (Ag) ink PANI Non-linear response over 5–95 % RH Operates at 20–100 % RH [181] [182] Temperature sensor Humidity sensor Humidity sensor 20 [177] [178] [179] B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 13. System overview of the inkjet-printed 2D material heterostructure textiles for micro-supercapacitor applications: (a) Inkjet printing of 2D materials (graphene, MoS2, and h-BN on textiles. (b) Magnified view of the vertical heterostructure printed on textiles. (c) Schematic of the supercapacitor structure based on an inkjet-printed heterostructure textile electrode. (d) Working principle of graphene-based electrical double layer (EDL) supercapacitor. (e) Working principle of MoS2or h-BN-based pseudocapacitor and (f) working principle of Graphene-MoS2 and/or h-BN heterostructure-based hybrid supercapacitor. Reproduced from Ref. [185]. © 2024 by the authors. Licensed under the CC BY 4.0 license. fluctuation under deformation stresses [188]. Further innovations in flexible batteries include fiber-based Zn-ion configurations, such as elastic graphene/polyaniline-Zn@silver fiber batteries (eG/P-Zn @SFBs) with helical structures inspired by luffa tendrils. These devices deliver specific capacities of 32.56 mAh/cm3 and energy densities of 36.04 mWh/cm3, while withstanding elongation up to 900 % with a 71 % capacity retention, showcasing their exceptional potential for integration into stretchable electronic textiles [189]. Complementing these developments, the use of aqueous, additive-free Ti3C2Tx MXene inks has enabled the direct printing of electrical conduits and MSCs on textile and paper substrates. These advances highlight the growing potential of scalable, low-cost, roll-to-roll processing techniques, which expand the industrial feasibility of printed flexible electronics [14,187]. These advancements demonstrate that inkjet printing is a powerful platform for the development of flexible and wearable energy storage devices. The synergy between innovative material choices, such as graphene, MXenes, CNTs, and conductive polymers, and refined printing techniques has led to substantial improvements in the areal capacitance, energy density, mechanical resilience, and operational lifetime of these devices. An overview of inkjet-printed supercapacitors, highlighting the electrode materials, device configurations, and cycling stability, is presented in Table 5. devices. These systems are critical for enabling self-powered operation in next-generation smart textiles by converting mechanical, solar, or vibrational energy into useable electrical power [198,199]. Triboelectric nanogenerators (TENGs) are among the most promising energy-harvesting devices owing to their high output performance and compatibility with flexible and stretchable substrates. They operate by converting biomechanical motion into electrical energy through the coupling of triboelectric and electrostatic effects. Inkjet printing plays a vital role in the development of TENGs by enabling the scalable and precise deposition of active materials on fabric substrates. Recent innovations have focused on enhancing the triboelectric output through surface patterning and material modification. One notable approach involves the fabrication of micro-patterned PDMS/textile contact pairs, which increases the effective surface area and thus enhances charge generation. For instance, commercial velvet fabric has been chemically modified with carbon nanotubes (CNTs) and poly(ethylenimine) (PEI) to create TENGs with significantly improved electrical performance. This modification requires less than 1 wt % of additive content, yielded over a tenfold increase in output voltage and current [198]. Moreover, these modified textile-based TENGs demonstrated strong washability, mechanical durability, and long-term operational stability, all of which are critical for real-world wearable applications. Further development of TENG technology has aimed at increasing the power density, making these devices more suitable as standalone power sources. Reported advancements include power densities ranging from 5 to 15 W/m2 under 10 Hz motion conditions, which marks a substantial improvement over earlier TENG configurations. For example, one fabric-based TENG achieved a maximum power density of 5.2.2. Energy harvesting systems Inkjet printing has become a transformative technique in the development of energy-harvesting systems for wearable electronics, particularly in the fabrication of triboelectric nanogenerators (TENGs), piezoelectric nanogenerators (PENGs), and inkjet-printed photovoltaic 21 B.K. Dejene Materials Today Advances 28 (2025) 100629 demonstrated stable performance over 1000 cycles, making it highly suitable for extreme wearable applications. When reconfigured as a deformable TENG, this device harvested mechanical energy with peak outputs of 5 V and 2.5 μA (Fig. 14), demonstrating its multifunctional potential. TENGs have also been applied in self-powered gait analysis, where they are integrated into wearable insoles to harvest energy while simultaneously collecting biomechanical data. These systems can detect parameters such as pressure distribution and stride dynamics without requiring an external power source, offering significant utility in health monitoring, rehabilitation, and sports analysis [200,201]. Beyond footwear, garment-integrated TENGs (G-TENGs) constructed using graphene-coated fabric layers have achieved notable electrical performance, with open-circuit voltages reaching 213.75 V and short-circuit currents of 3.11 μA under cyclic loading [202]. These devices exhibited excellent air permeability, stretchability, and durability, making them ideal for long-term integration into smart clothing. The success of such systems has been enabled by recent advancements in stretchable electrode design, including the use of intrinsically stretchable materials, soft-rigid material composites, and geometrically patterned electrodes that can endure mechanical strain without significant performance degradation [203]. In parallel with triboelectric systems, significant progress has been made in photovoltaic energy harvesting, particularly through the use of inkjet-printed perovskite solar cells (PSCs). These solar cells have achieved power conversion efficiencies (PCEs) exceeding 17 % for smallarea devices and approximately 13 % for larger-area configurations [204]. Such improvements have been driven by advanced post-treatment methods, including vacuum-assisted thermal annealing and solvent composition optimization, which collectively enhance the crystallinity and uniformity of perovskite films. The adaptability of inkjet printing allows for the seamless integration of PSCs into flexible substrates, thus enabling their potential use in wearable energy harvesting. However, the direct integration of PSCs onto textiles remains technically challenging because of the sensitivity of the materials to moisture, UV exposure, and mechanical stress. In response, researchers have proposed UV-protective smart textiles embedded with energy storage and harvesting functionalities as a promising direction for future research. These hybrid systems can simultaneously generate solar energy and protect the wearer, thereby addressing the growing demand for multifunctional wearable platforms [205,206]. Complementing TENGs and PSCs, piezoelectric nanogenerators (PENGs) offer a promising route for mechanical energy harvesting in textiles [4]. PENGs operate based on the piezoelectric effect, in which mechanical strain induces an electrical response in certain materials. Inkjet printing has been employed to Table 5 Inkjet-printed wearable electronics for energy storage applications. Substrate Conductive ink Device configuration Performance Ref. Polypropylene (PP) fabric rGO layers Flexible solidstate supercapacitor [190] PET MXenes (Flexible V2CTx) Flexible solidstate supercapacitor Carbon cloth NiCo LDH/Ag/ rGO Asymmetric supercapacitor Bamboo fabric MnO2–NiCo2O4 (anode), rGO (cathode) Asymmetric supercapacitor PP non-woven textile Polypyrole (PPy) Supercapacitor Film MXene/ graphene Supercapacitor electrodes Film MXene (Ti3C2Tx) Flexible and transparent supercapacitor Stainless steel foil V2O5/MXene 2D heterostructure cathode material for lithium-ion batteries 13.3 mF/cm2, 100 % retention after 5000 cycles 531.3–5787.0 μF cm−2, 83 % retention after 7000 cycles 95 mAh g−1, 79.8 % retention after 5000 cycles 2.12 F cm−2, 92 % retention after 5000 cycles 72.3 F g−1, 55.4 % retention after 2000 cycles 3.84 mF/cm2, 75 % retention after 3000 cycle 192 μF cm-2, 85 % retention after 10,000 cycle 112 mAh g-1, 680 cycles (91.8 %) [191] [192] [193] [194] [195] [196] [197] 3.2 W/m2 across a 5 × 106 Ω external load [198]. This makes them suitable for powering small electronic devices such as digital watches, pedometers, and calculators. In addition to general power generation, TENGs have been adapted for use in wearable sensors. Jiang et al. [199] developed a wearable device by synthesizing a micro-spined hydrogel from sodium alginate and acrylamide, onto which a conductive MXene layer was inkjet-printed. The resulting hybrid sensor exhibited a high sensitivity of 15.03 kPa−1, a detection limit of 10 Pa, and operational resilience across a wide pressure range (0.12–70 kPa) with fast response/recovery times (40 ms/100 ms). Impressively, the device maintained full functionality under ice-bath conditions (−20 ◦ C) and Fig. 14. Low-temperature-resistant hydrogel with inkjet-printed MXene on microspine surface for pressure sensing and triboelectric energy harvesting. Reproduced with permission from Ref. [199]. © 2024 Elsevier B.V. 22 B.K. Dejene Materials Today Advances 28 (2025) 100629 fabricate piezoelectric materials and devices directly onto flexible substrates such as PVDF-based polymers, composites, or nanostructured films [207]. These systems are particularly well-suited for harvesting energy from body motions, such as joint bending or respiration. Their integration into textiles enables passive and continuous power generation for biosensors, communication systems, and health-monitoring devices without the need for external batteries. Continued innovation in material engineering, device architecture, and hybrid system integration is expected to bring these technologies closer to real-world commercial applications, paving the way for truly self-powered and smart textiles. and military applications, such as stealth uniforms. In the realm of EMI shielding textiles, researchers have made substantial progress in developing highly effective materials using inkjet-printing techniques. For instance, graphene-Ag composite inks have been successfully formulated and printed onto cotton fabrics to produce conductive electronic textiles. These composites demonstrated excellent sheet resistance, ranging from 0.08 to 4.74 Ω/sq, depending on the number of printed layers and graphene-Ag ratio [12]. This approach offers a cost-effective and environmentally friendly method for creating highly conductive wearable e-textiles. Another significant advancement is the development of multilayer graphene/AgNW composites, which have demonstrated remarkable EMI shielding effectiveness (SE) exceeding 60 dB at 10 GHz. For example, a three-layer composite shielding film comprising silver nanowires (AgNWs) and PET achieved an SE of 44 dB at 10 GHz while maintaining an optical transmittance of 67.8 %. [213]. This multilayer mesh composite structure optimizes the AgNW distribution, enhancing both transparency and shielding properties, making it suitable for applications requiring both light transmittance and high SE. However, some contradictions and unique findings have emerged. While many researchers have focused on MNPs for conductivity, others have explored alternative materials. For instance, MXene-based inks have demonstrated exceptional electrical conductivity (1080 ± 175 S/cm) and EMI SE values of 50 dB at a film thickness of only 1.35 μm [214]. This surpasses the conductivity of state-of-the-art inkjet-printed electrodes composed of other 2D materials, such as graphene and reduced graphene oxide. These innovations pave the way for the creation of stealth uniforms and other military applications that require both EMI shielding and wearability of the fabric. 5.3. Human-machine interfaces (HMI) 5.3.1. Advanced control systems and neuromorphic textiles Advancements in wearable electronics for human-machine interfaces (HMI) have led to significant improvements in gesture recognition gloves and haptic feedback systems. These technologies enable more intuitive and effective interactions between humans and machines. Gesture recognition gloves have seen remarkable progress through the integration of multimodal sensor arrays and advanced machine learning algorithms. For example, a data glove system using MWCNT sensors combined with a hybrid convolutional neural network (CNN) long shortterm memory (LSTM) model achieved an impressive 97.5 % accuracy in recognizing 30 different gestures using only five sensors [208]. This system demonstrates the potential of low-cost wearable HMI solutions for robotic hands, smart cars, and gaming interfaces. Similarly, a smart glove incorporating thin AlN piezoelectric sensors and an onboard machine learning algorithm successfully classified gestures, showcasing the integration of haptic feedback and gesture recognition in a single device [209]. Haptic feedback systems have also seen significant advancements, particularly in the use of piezoelectric materials such as PVDF [3]. A wearable pressure sensor based on a drum-structured TENGs (DS-TENG) demonstrated the ability to capture subtle pressure signals for physiological signal detection, information encoding, and gesture recognition [210]. This technology shows promise for application in Braille displays and tactile feedback systems. Additionally, a haptic-feedback smart glove incorporating triboelectric-based finger bending sensors and piezoelectric mechanical stimulators achieved object recognition with 96 % accuracy using machine-learning techniques [211]. These advancements are paving the way for applications in diverse fields, such as entertainment, healthcare, sports training, and medical industries [211]. In neuromorphic computing, inkjet printing has shown potential for creating textile-based devices that mimic the functions of neurons and synapses. For instance, memristive fibers, such as Ag/CH3NH3PbI3/Pt synaptic devices, can be fabricated using inkjet-printing techniques. These devices exhibit synaptic behavior, allowing the implementation of spiking neural networks and adaptive learning systems directly on textiles [212]. The integration of such neuromorphic elements into wearable electronics opens up possibilities for on-body computing and intelligent data processing, thereby enhancing the capabilities of HMI systems. However, the development of inkjet-printed wearable electronics for HMI applications faces several challenges. While this technology offers great potential for creating flexible and conformable devices, ensuring the long-term stability and reliability of printed components on textiles remains a concern. Additionally, the trade-off between device performance and flexibility must be carefully balanced to achieve optimal functionality in wearable applications [15,28]. 5.4.2. Hazard detection Inkjet-printed gas sensors have shown significant advancements in industrial safety. For instance, fully printed gas sensors based on tin oxide have demonstrated exceptional performance in detecting NO2 at low concentrations. These sensors exhibited high linearity and an impressive average response of 11,507 at 5 ppm NO2, with an ultralow detection limit of 20 ppb [215]. This level of sensitivity is crucial for ensuring worker safety in industrial environments, where exposure to toxic gases can be hazardous. Another notable development is the creation of all-printed gas sensors on unconventional substrates, such as PET, paper, and cotton fabric. These sensors, which utilize AgNP-based ink for electrodes and carbon black paste as the sensing layer, have been effective in detecting volatile organic compounds (VOCs) such as acetone, ethanol, and isopropanol at concentrations as low as 4 ppm [216]. This versatility in substrate choice expands the potential applications of wearable gas sensors in various industrial and environmental monitoring applications. In addition to gas sensors, inkjet-printed electrochemical sensors have been explored for the detection of chemical hazards, particularly in liquid environments. Qin et al. [217] developed inkjet-printed bifunctional CNT-based pH sensors capable of providing rapid and reproducible potentiometric responses with a sensitivity of 48.1 mV/pH and a response time of approximately 7 s. These sensors exhibited minimal hysteresis owing to the compact arrangement of the SWCNTs, which facilitated efficient ion migration. Importantly, the sensing performance was consistent across different substrates, highlighting the dominant role of CNTs in ensuring reliable operation (Fig. 15). Inkjet-printed pH sensors are a promising approach for monitoring chemical hazards, including acidic or alkaline leaks in industrial processes and environmental contamination events. Their low fabrication cost and adaptability to polymeric substrates further enhance their suitability for wearable hazard-detection systems. Future advancements could leverage the precision and versatility of inkjet printing to create multifunctional sensors for the simultaneous detection of gaseous and liquid chemical hazards in real time. 5.4. Environmental and safety applications 5.4.1. EMI shielding textiles Inkjet-printed wearable electronics have shown significant advancements in environmental and safety applications, particularly in the development of electromagnetic interference (EMI) shielding textiles 23 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 15. Schematic of the fabrication process (a) SWCNT-based pH sensing electrode, (b) pH sensing mechanism for SWCNT-COOH (c) Temporal pH response of a printed SWCNT electrode on glass (200 passes). (d) Calibration curves of SWCNT pH sensing electrodes on glass (different numbers of printing passes). (e) pH sensitivity of SWCNT electrodes on different substrates. Inset: photographs of the printed electrodes (scale bar: 2 mm). Reprinted with permission from Ref. [217]. © 2016 Elsevier B.V. rupture, releasing healing agents and restoring electrical conductivity. Liquid-metal-based systems, such as those using gallium-based alloys, offer unique self-healing properties owing to their inherent fluidity and ability to reform connections after mechanical damage [218]. The integration of quantum dots in wearable electronics opens up new possibilities for display technologies and healthcare applications. Inkjet printing enables the precise deposition of quantum dots onto textile substrates, allowing the creation of wearable displays and photodynamic therapy patches [15,219]. Quantum-dot-based wearable displays on textiles offer high color purity and energy efficiency, making them suitable for flexible and conformable display applications. In healthcare, photodynamic therapy patches incorporating quantum dots can be printed directly onto textiles, enabling localized light-activated treatment of various skin conditions. These emerging applications demonstrate the versatility and potential of inkjet-printed wearable electronics. However, challenges remain in terms of ink formulation, substrate compatibility, and device integration. [116,220]. Future research should focus on developing novel ink materials, improving printing resolution and uniformity, and enhancing the durability and washability of printed devices on textiles [15,148]. 5.5. Emerging frontier applications Advancements in the applications of inkjet-printed wearable electronics for emerging frontier applications, focusing on thermoregulatory textiles, self-healing systems, and quantum dot integration, are also significant. Thermoregulatory textiles have garnered significant attention in the field of wearable electronics. VO2-based infrared modulation fabrics and Peltier elements for active cooling are promising applications in this area. Inkjet printing technology allows for the precise deposition of functional materials on textile substrates, enabling the creation of smart fabrics with temperature-responsive properties [15]. For instance, VO2-based inks can be printed onto textiles to create infrared-modulating fabrics that adapt to environmental conditions, providing thermal comfort to the wearer. Similarly, Peltier elements can be integrated into textiles using inkjet-printing techniques to create active cooling systems for personal thermal management. Self-healing systems represent another frontier in the application of inkjet-printed wearable electronics. Microcapsule-embedded conductive inks and liquid-metal-based autonomic repair mechanisms are two promising approaches in this field. Inkjet printing allows the precise deposition of microcapsules containing healing agents within conductive inks, enabling the creation of self-repairing electronic circuits on flexible substrates [116,218]. When damage occurs, the microcapsules 24 B.K. Dejene Materials Today Advances 28 (2025) 100629 6. Challenges and limitations roughness and porous structure of textile substrates can lead to inconsistent ink deposition and poor electrical performance [225]. Eghan et al. [29] further identified several challenges or problems (illustrated in Fig. 16), which affect the application of conductive inks on textiles for wearables or e-textiles. However, they also highlighted several studies that proposed solutions to address the challenges of porosity and surface roughness (as summarized in Table 6). Additionally, factors such as durability and conductivity of the printed electronics which directly influence the fabric’s performance are significantly affected by the chosen fabrication process [56]. Researchers have developed reactive ink formulations using viscoelastic nanofluids that can control 3D wicking on textiles. These formulations help improve the uniformity of printed patterns and enhance their electrical properties [21,225]. The knit structure of textiles introduces additional complexities, causing resistivity variations across courses and wales. Studies have shown that these variations can be as high as ±12 %, affecting the overall performance of the printed electronic components [225]. To overcome this challenge, researchers have explored optimizing ink formulations and printing parameters to achieve more uniform conductivity across different textile structures [21,226]. Ongoing research focuses on developing innovative ink formulations, optimizing printing parameters, and improving substrate treatments to overcome these challenges and enhance the performance of inkjet-printed wearable electronics [227]. 6.1. Fundamental material challenges 6.1.1. Nanomaterial stability Inkjet-printed wearable electronics face several fundamental material challenges, particularly in terms of nanomaterial stability. Two critical aspects of this challenge are oxidative degradation and ink shelf life. Oxidative degradation is a significant concern, especially for CuNPs, which are increasingly used as a cost-effective alternative to silver in conductive inks. CuNPs are highly susceptible to oxidation under ambient conditions, which can severely affect their electrical conductivity [95]. The oxidation kinetics of CuNPs can be modeled using the Arrhenius equation, allowing researchers to predict and mitigate the oxidation effects. To address this issue, various protective coating strategies have been developed. For instance, core-shell structures such as Cu@Ag and Cu@graphene have shown promising results, with less than 5 % conductivity loss after 30 d of exposure to ambient conditions [95]. These protective layers effectively shield the Cu core from oxidation while maintaining the conductive properties of the material. The shelf life of ink is another crucial challenge for inkjet-printed wearable electronics. Accelerated aging tests, typically conducted at 85 ◦ C and 85 % relative humidity, are used to evaluate the long-term stability of conductive inks [221]. These tests help identify potential degradation mechanisms and assess the ability of the ink to maintain its properties over time. For conductive polymer inks, the addition of radical scavengers, such as hindered amine light stabilizers, can significantly improve their shelf life by preventing polymer chain degradation [27, 28]. 6.2.2. Post-processing limitations Low-temperature sintering and encapsulation trade-offs are two critical challenges that must be addressed for the successful production of these devices. Low-temperature sintering is essential for inkjetprinted wearable electronics to ensure their compatibility with flexible and thermally sensitive substrates. Photonic sintering has emerged as a promising technique, with energy thresholds typically ranging from 1.5 to 3.0 J/cm2 for 50 nm AgNPs [223]. This method allows rapid processing without damaging the underlying substrate. However, chemical sintering agents, although effective at lower temperatures, can leave residues that affect device performance. For instance, sodium borohydride (NaBH4) can result in ionic contamination of 0.3 μS/cm, potentially affecting the electrical properties of printed circuits [98]. Alternative approaches to low-temperature sintering have also been explored. For example, a novel silver-based metal organic decomposition (MOD) ink was developed that can self-decompose and sinter at room temperature, achieving a conductivity of 4.65 × 104 S/m on thermally sensitive silk/epoxy composite substrates [23]. Similarly, a formic acid-based sintering process for CuNP inks has shown promising results, achieving up to 16 % of bulk copper conductivity at 130 ◦ C and over 25 % above 150 ◦ C [98]. Encapsulation is another critical aspect of wearable electronics manufacturing, with trade-offs between protection and flexibility. Atomic Layer Deposition (ALD) of Al2O3 can provide excellent barrier properties, with a 50 nm thick layer achieving a water vapor transmission rate (WVTR) of 10−3 g/m2/day. However, this method may be less suitable for the large-scale production of flexible devices. Alternatively, spin-coated polyurethane (PU) offers a more flexible option, but requires a thicker layer (5 μm) to achieve a WVTR of 10−1 g/m2/day, which may impact the overall device flexibility [231]. 6.1.2. Substrate compatibility Other fundamental material challenges exist, particularly in substrate compatibility, which can significantly impact the performance and durability of inkjet-printed wearable devices. Natural fiber substrates, such as cotton, present significant challenges for the fabrication of inkjet-printed electronics. Cotton fibers are prone to swelling when exposed to moisture, which can lead to dimensional changes of up to 15 % in their length. This swelling can cause circuit cracking, compromising the functionality of printed electronics [14]. Additionally, the presence of lignin in natural fibers can interfere with ink adhesion, further complicating the printing process and reducing the durability of printed circuits [222]. To address these challenges, researchers have explored synthetic fiber solutions and surface-modification techniques. One approach involves plasma-enhanced bonding, which can improve the bonding energy between the substrate and conductive ink by more than 50 mJ/m2 [223]. This enhanced adhesion helps maintain the integrity of the printed circuits, even under mechanical stress. Another innovative solution is the use of nanofibrillated cellulose coatings, which can control the hydrophilicity of the substrate and improve ink adhesion [222]. These coatings can also reduce surface roughness, leading to improved print quality and electrical performance [222,223]. 6.2. Manufacturing hurdles 6.2.1. Printability issues Inkjet-printed wearable electronics face several manufacturing hurdles, particularly in terms of printability. These challenges primarily stem from the nozzle dynamics and topography-related factors. The clogging probability of nozzles is a critical concern, with models suggesting a risk threshold when the particle diameter-to-nozzle diameter ratio (Dp/Dn) exceeds 0.05 [224]. To mitigate this issue, pulsed flow cleaning protocols using ultrasonic frequencies between 30 and 100 kHz have been employed. These protocols help maintain the cleanliness of the nozzle and prevent clogging, ensuring consistent ink deposition [15]. Topography challenges pose another set of hurdles in the manufacturing of inkjet-printed wearable electronics. The high surface 6.3. Performance durability 6.3.1. Washability standards Inkjet-printed wearable electronics have shown promising performance and durability, particularly in terms of washability and abrasion resistance. The washability of these devices is crucial for their practical application in everyday life, as they must withstand regular cleaning processes. Several studies have demonstrated the washability of inkjetprinted wearable electronic devices. For instance, research on conductive wires made from PEDOT:PSS printed on nonwoven PET fabric showed less than 6.2 % change in sheet resistance after three washing 25 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 16. Identified challenges on the fabrication of conductive inks on textile substrates. Adapted from Ref. [29]. © 2025 by the authors. Licensed under CC BY 4.0. Table 6 Solutions when inkjet printing conductive inks on textile materials. Conductive ink Method Interface layer Substrate Electrical performance Ref Nickel ink Electroless plating Palladium ink Polyester fabrics [228] Silver nanoparticles ink Reduced graphene ink Silver nanoparticle ink Reactive silver ink Inkjet printing a hydrophobic layer Hydrophobic breathable coating 100 % cotton, 100 % polyester and 65 %/35 % cotton/ polyester Coating an interface layer UV-curabledielectric ink 100 % cotton, 65 %/35 %polyester/cotton and 85 %15 % polyester/cottonfabrics 100 % cotton, 100 % polyester, 60 %/40 %Cotton/ polyester Conductivity 2500 ± 175 S/m Sheet resistance 1.18 Ω/sq Sheet resistance 2.14 × 103 Ω/sq Conductivity 2.08 × 106 S/m Conductivity 5.54 × 105 S/m ​ Polyvinyl alcohol [157] [229] [230] “whipping/buckling” instability of electrospinning to deposit serpentine fibers in a programmable manner, resulting in an ultra-stretchable fractal-inspired architecture. In contrast, serpentine interconnects have demonstrated varying degrees of stretchability. For instance, printed coaxial fibers with a core-sheath structure exhibited a stretchability of 150 % [25], whereas freestanding serpentine Si strips achieved an even higher stretchability of 300 % [235]. The durability of these stretchable electronics is further enhanced by strategies that inhibit microcrack propagation. For example, the use of stress-enduring composite silver inks based on eutectic gallium-indium particles as dynamic electrical anchors has enabled printed microstructures to maintain their mechanical and electrical properties under extreme strains of up to 800 % [236]. Interestingly, the performance durability of inkjet-printed wearable electronics extends beyond their stretchability. Certain devices have demonstrated remarkable stability and longevity. For instance, fully stretchable organic electrochemical transistors (OECTs) remained stable for 50 days and could be stretched up to 1 tensile strain [237]. Similarly, freestanding serpentine Si strips exhibited excellent stability and durability over 50,000 cycles of 100 % stretching [235]. and drying cycles using a detergent [232]. This indicates good durability against washing. Similarly, a study on inkjet-printed e-textiles using particle-free reactive silver inks demonstrated outstanding electrical conductivity and durability, maintaining their characteristic washability for wearable technology applications [56]. Some researchers have developed innovative approaches to enhance the washability of inkjet-printed wearable electronics. A study on graphene-based strain sensors reported stable electrical and mechanical performance under cyclic washing and chronic wetting [233]. This suggests that careful material selection and design can significantly improve the washability of these devices. Inkjet-printed wearable electronics have shown promising results in terms of abrasion resistance. Although specific data on the Martindale abrasion test at 20 kPa for 50,000 cycles are not provided in the given context, several studies have reported good mechanical durability. For example, fabric electrodes prepared using a thermal transfer printing method exhibited good abrasion resistance [5]. Additionally, the previously mentioned graphene-based strain sensor demonstrated a remarkable breaking strain (48.5 %) and high tensile strength (369 MPa), indicating good mechanical durability [233]. 6.4. Scalability and cost 6.3.2. Mechanical stress effects Inkjet-printed wearable electronics have shown remarkable progress in terms of performance and durability, particularly in their ability to withstand mechanical stress. The stretchability limits of these devices have been significantly improved by innovative designs and materials. Fractal designs and serpentine interconnects have emerged as two prominent approaches for enhancing the stretchability of printed wearable electronics. Fractal-inspired piezoelectric nanofibers combined with liquid metal electrodes have demonstrated impressive stretchability of up to 200 % [234]. This design exploits the The scalability and cost-effectiveness of inkjet-printed textiles for wearable electronics depend heavily on addressing several production challenges, particularly in roll-to-roll (R2R) manufacturing. One of the key technical hurdles is maintaining a consistent web tension, especially for knitted textile substrates, where controlling the tension below 2 N/ cm is essential to ensure uniform ink deposition and reliable print quality. This requires advanced machinery capable of adapting to the flexible and deformable nature of textile materials. Additionally, achieving precise printhead alignment within ±5 μm over extended 26 B.K. Dejene Materials Today Advances 28 (2025) 100629 lengths, such as 100 m, is a significant obstacle. Misalignment at this scale can lead to defects, increased material waste, and ultimately, higher production costs. Although the implementation of R2R techniques offers the potential for economies of scale by reducing the cost per unit, the initial investment in sophisticated equipment and control systems to manage these challenges remains substantial. Furthermore, the choice of conductive inks and compatible, durable textile substrates directly affects both scalability and production costs. High-performance materials improve device efficiency but often come at a premium, making it necessary to strike a careful balance between material performance and economic feasibility. Lessons from adjacent fields, such as electric vehicle charging and energy storage systems, emphasize the critical importance of optimizing manufacturing processes to enhance scalability while maintaining manageable material costs [238,239]. Moreover, frameworks used to evaluate distributed systems, including throughput and quality of service metrics, may provide valuable insights for assessing the scalability of textile-based wearable electronics. Market adoption, as evidenced by developments in e-bike sharing and smart grid technologies, relies not only on technological advances but also on infrastructure and commercial readiness [240,241]. Together, these technical, economic, and infrastructural factors shape the commercial viability of inkjet-printed textiles in wearable electronics, underscoring the need for continued innovation to overcome these challenges. address these challenges and harness the full potential of inkjet-printed wearable electronics [248,249]. 7.1.2. Sustainable material platforms The future prospects of inkjet-printed textiles in wearable electronics are closely tied to the development of advanced functional inks and sustainable material strategies, with particular emphasis on bio-derived conductors and circular economy principles. Among the promising advancements, Dulal et al. [158] demonstrated a fully inkjet-printed sustainable e-textile platform utilizing biodegradable Tencel™ fabric substrates combined with graphene and PEDOT:PSS water-based inks (Fig. 17a). These sustainable material choices align with closed-loop recycling concepts, as Tencel production achieves ~99 % solvent and water recovery. Importantly, their graphene-based textile electrodes exhibited remarkable biodegradability, with approximately 48 % weight loss and 98 % tensile strength reduction over four months of soil burial (Fig. 17b and c), and positively influenced soil microbial communities (Fig. 17d and e). Furthermore, the life cycle assessment (LCA) of these systems revealed that the graphene-based electrodes offered a 40-fold reduction in climate change impact (0.037 kg CO2 eq) compared to conventional metal/solvent-based printed electrodes (Fig. 17f). These findings underscore the critical role of biodegradable substrates, eco-friendly inks, and comprehensive sustainability assessments in shaping the next generation of circular, eco-conscious wearable electronics. Integrating these strategies with innovations such as bio-derived conductors, including bacterial cellulose/PEDOT composites and lignin-derived carbon nanofibers, will be pivotal in advancing sustainable, fully degradable electronic textiles [250]. Similarly, lignin-derived carbon nanofibers sourced from the abundant plant polymer lignin are being explored for applications in disposable sensors. Their transformation into conductive structures presents a valuable route for reducing the environmental footprint of electronic devices and waste. [250]. In parallel, embedding circular economy concepts into the life cycle of inkjet-printed wearable electronics has emerged as an essential research direction. Techniques such as the enzymatic recovery of precious metals, including processes that achieve gold recovery rates exceeding 95 %, highlight how resource-efficient recycling can substantially reduce the environmental burden of electronic waste [251]. This alignment with broader sustainability goals is further reinforced by emerging innovations, such as edible electronics, where unconventional materials, such as chocolate-based conductors, are being explored for safe, disposable, and potentially ingestible electronic systems [250]. Advancing eco-friendly conductive inks, optimizing material recovery processes, and realizing fully biodegradable electronic textiles will be pivotal in meeting the growing demand for sustainable wearable technologies. These innovations are poised to drive the next phase of development in environmentally responsible electronics, supporting both technological progress and global sustainability efforts [250,251]. 7. Future prospects and research directions 7.1. Next-generation functional inks 7.1.1. Multi-functional nanocomposites Ongoing research in this domain focuses on developing nextgeneration functional inks and multifunctional nanocomposites, which hold significant promise for applications spanning self-powered systems to stimuli-responsive materials. One pivotal research direction involves the development of self-powered systems using piezoelectric and thermoelectric materials. For instance, piezoelectric zinc oxide (ZnO) nanowire inks can generate a voltage output of 5V under 1 % strain, demonstrating their potential to autonomously power small electronic devices [242,243]. Thermoelectric materials, such as bismuth telluride (Bi2Te3) polymer hybrids, have been explored for their capacity to maintain a high thermoelectric figure of merit (ZT) greater than 0.8 at room temperature, emphasizing the versatility of wearable thermoelectric generators in harvesting energy from body heat or environmental sources [244]. These advancements underline the importance of optimizing material properties, such as conductivity and flexibility, for wearable applications. The development of stimuli-responsive materials, such as shape-memory polymers and phase-change materials, is another significant research avenue. Shape-memory polyurethane/Ag hybrids illustrate the potential of 4D textile actuators, which can change shape in response to external stimuli, offering innovative solutions for adaptive textiles in healthcare and sportswear [245]. Furthermore, vanadium dioxide (VO2)-based phase-change inks with a self-healing efficiency of 90 % are being investigated for adaptive insulation, enabling smart fabrics that can modulate their thermal properties depending on environmental conditions [246]. Although the advancements in nanocomposite materials are promising, several challenges persist. These include biocompatibility, scalability, and integration complexity. Biocompatibility is critical for wearable devices in biomedical applications, where materials must be non-toxic and skin-friendly. Scalability from lab-scale prototypes to industrial production remains a hurdle, requiring the development of manufacturing processes that ensure product consistency without compromising material properties [247]. Furthermore, the integration of multiple materials and functionalities introduces complexity in design and fabrication, necessitating advances in fabrication technology and material innovation to overcome these limitations. Looking ahead, interdisciplinary collaboration between materials scientists, engineers, and other stakeholders is essential to 7.2. Advanced manufacturing paradigms 7.2.1. Emerged inkjet printing methods Recent advancements in inkjet printing technologies, such as e-jet and aerosol jet printing, offer notable solutions to the limitations of conventional methods, particularly concerning high-viscosity inks. These novel methods improve the quality and applicability of printed materials across various fields, including electronics and biomedical applications. Traditional inkjet printing technologies often face challenges when dealing with high-viscosity inks, which are essential for advanced applications such as bioprinting and electronic circuit fabrication. The primary issues include nozzle clogging and inadequate flow control, which affect the precision and quality of the printed structures. In contrast, e-jet and aerosol jet printing can effectively handle highviscosity inks, thereby expanding the capabilities of inkjet printing technologies. E-jet printing, or electrohydrodynamic jet printing, utilizes an electric field to draw ink from the nozzle, allowing the use of 27 B.K. Dejene Materials Today Advances 28 (2025) 100629 Fig. 17. Schematic and sustainability assessment of fully inkjet-printed wearable electronic textiles. (a) Schematic of the inkjet printing setup and formulation of PEDOT:PSS and graphene-based inks for conductive patterning on textiles. (b–c) Biodegradation performance of inkjet-printed textile electrodes showing ~48 % weight loss (b) and ~98 % tensile strength (c) reduction after 4 months of soil burial. (d–e) Total microbial plate counts indicating increased bacterial and fungal activity around degraded graphene-printed samples. (f) Climate change impact from LCA, revealing that graphene-based electrodes produce the lowest CO2 emissions (0.037 kg CO2 eq), which is ~40 × lower than that of reference metal-based printed electrodes. Adapted from Ref. [158]. © 2024 by the authors. Licensed under the CC BY 4.0 license. process. The precise control over droplet deposition makes it suitable for fabricating electronic components, such as transistors and microelectronics, with exact configurations and properties [252,253]. Aerosol jet printing is another technique that addresses the highly viscous substances that are unsuitable for conventional piezoelectric or thermal inkjet printers. This method is particularly beneficial for printing high-resolution patterns and enables the printing of complex materials owing to its precision and control over the droplet formation 28 B.K. Dejene Materials Today Advances 28 (2025) 100629 challenges associated with the use of high viscosity inks. This method employs a focused aerosol stream to deposit material onto the substrate, allowing the jetting of materials with various viscosities without the drawbacks of traditional nozzle-based systems. The flexibility and precision of this technology enable the production of finely defined patterns on nonplanar surfaces, which is particularly useful in the creation of advanced electronic devices and biomedical structures [226]. Furthermore, these innovative printing methods not only enhance the range of printable materials but also improve the mechanical and functional properties of printed products. They provide solutions to typical issues, such as the “coffee-ring effect,” which often disrupts the uniform deposition of materials during drying. These improvements allow the production of more reliable and consistent components, which are crucial for industrial applications [254]. Therefore, by overcoming the limitations of traditional inkjet methods, these technologies facilitate high-quality printing with high-viscosity inks, expanding the scope of inkjet printing applications and paving the way for advancements in various sectors, from electronics to biomedicine. twins represent another promising research direction, particularly in the field of e-textiles. By using co-simulation environments such as COMSOL-MATLAB, researchers can model and simulate various scenarios, such as wash cycles, to predict their effects on the durability and functionality of printed electronics. This predictive approach is crucial for developing maintenance strategies and improving the overall longevity of wearable electronics. Digital twins can help refine designs before actual production, reducing the time and costs associated with trial-and-error methods [260]. Machine learning algorithms play a vital role in the evolution of inkjet-printed electronics. They assist in optimizing the printing processes and enhancing the performance and reliability of the printed components. For instance, machine learning can be used to model the complex behaviors and interactions within printed materials, allowing for the creation of sophisticated sensors and circuits that are responsive to environmental changes [261]. While these advancements present significant opportunities, challenges remain in the scalability, integration, and commercialization of inkjet-printed, wearable electronics. Addressing these challenges requires interdisciplinary research efforts that draw on materials science, engineering, data science, and other related fields [262]. 7.2.2. Hybrid printing systems The future prospects and research directions of inkjet-printed wearable electronics are promising, driven by advancements in hybrid printing systems, such as the integration of inkjet and electrospinning technologies and the convergence of 3D/4D printing systems. These advancements are expected to revolutionize the creation and functionality of wearable technology. First, the integration of inkjet printing with electrospinning offers significant potential for developing conductive nanofiber meshes and anisotropic conductive textiles. Conductive nanofiber meshes with controlled diameters ranging from 50 nm to 5 μm, provide tailored electrical properties crucial for developing advanced electronic textiles [255]. The ability to precisely control the diameter can lead to enhanced conductivity and mechanical properties, facilitating the design of high-performance wearable sensors and devices. Meanwhile, textiles with anisotropic conductivity ratios of 1:100 allow for directionally dependent electrical conductivity, opening avenues for the design of smart textiles with specialized functions [256]. In addition, the convergence of 3D and 4D printing technologies offers promising avenues. These technologies enable volumetric printing of textile-embedded circuits and humidity-responsive self-folding origami antennas. Volumetric printing facilitates the integration of complex circuitry within flexible and wearable substrates, enhancing the functional density of such devices [257]. Furthermore, 4D printing introduces the dimension of time, allowing the creation of structures that can alter their form or properties in response to environmental stimuli, such as humidity. This capability is exemplified by humidity-responsive self-folding origami antennas, which can dynamically optimize their performance [250]. Moreover, integrating data science tools, such as machine learning, in the processing and interpretation of data from these wearable devices could significantly enhance their functionality and user interactivity, paving the way for more personalized and context-aware applications [258]. 7.3. System-level innovations 7.3.1. Autonomous wearable platforms The future of inkjet-printed wearable electronics lies in enhancing the material performance, printing processes, and system integration. Recent advancements in inkjet printing have demonstrated its capability to manufacture flexible energy storage devices, including supercapacitors and batteries, which are critical for the development of selfpowered wearable systems [263]. These technologies aim to achieve energy-autarkic systems by combining energy-harvesting techniques, such as integrated solar-TENG hybrid systems and biofuel cells powered by sweat glucose [28]. Such systems promise continuous 24/7 operation, which is essential for autonomous wearable platforms. The formulation of functional inks has been a focus area, as researchers aim to incorporate conductive nanomaterials, such as metal nanoparticles, graphene, and carbon nanotubes, to ensure high electrical performance and mechanical flexibility [16]. Innovations in ink formulation and printing parameters are pivotal for achieving optimal printing results, including uniformity and high-resolution patterning on flexible substrates [264]. Additionally, wearable biofuel cells that harness energy from human sweat offer a sustainable solution for powering low-consumption devices, with potential power outputs of up to 0.5 mW/cm2 [28]. The implementation of edge AI in wearable electronics is another critical research area. This involves advancements in federated learning on textile-based neuromorphic chips and in-memory computing using memristive fibers, which enable wearables to process data locally, thereby enhancing privacy and reducing latency [15]. Such innovations could revolutionize applications in health monitoring and personalized diagnostics by providing real-time, reliable outputs without relying heavily on cloud computing. Opportunities for further research and development are abundant in this field. Exploring multifunctional inks that provide sensory capabilities alongside electrical functions could expand device functionalities [265]. Additionally, fostering collaboration between academia and industry can accelerate the transition from laboratory-scale innovations to commercial products. 7.2.3. Smart manufacturing technologies One significant area of development is the incorporation of advanced manufacturing paradigms, such as smart manufacturing technologies, closed-loop process control, and digital twins, which are reshaping the landscape of wearable electronics. Closed-loop process control is a critical area that involves real-time monitoring and adjustment during the printing process. For instance, the use of microsecond-resolution cameras allows for precise tracking of droplets as they are deposited onto substrates. This capability enables the identification and correction of defects or inconsistencies in real time, thus ensuring higher precision and yield in the manufacturing process. Moreover, machine learningbased viscosity adjustment (±0.5 cP) provides an automated way to dynamically modify ink properties, thereby optimizing print quality and increasing the robustness of the manufacturing process [259]. Digital 7.3.2. Biomedical breakthroughs Among the most promising avenues are system-level innovations, such as closed-loop therapies and organ-on-textile models, each contributing uniquely to the advancement of personalized medicine. Inkjet printing enables the fabrication of highly precise drug delivery platforms integrated with biosensing elements, facilitating personalized and responsive treatment systems. These platforms can administer therapeutics in real time, triggered by physiological signals, thereby 29 B.K. Dejene Materials Today Advances 28 (2025) 100629 development, ink formulation, printing process optimization, device fabrication, performance evaluation, and large-scale deployment. It also highlights current challenges, such as interfacial adhesion, wash durability, and scalability, while pointing to emerging solutions, such as self-healing composites, hybrid printing approaches, and AI-enabled design. By offering a visual representation of research trajectories and innovation pathways, this roadmap serves as a guide for researchers and industry practitioners aiming to bridge the gap between laboratory advancements and market-ready wearable electronics products. opening new possibilities for dynamic, patient-specific care. Incorporating biocompatible materials, such as cellulose, into these systems enhances their performance, and future research is poised to explore hybrids of cellulose and inorganic components for improved antimicrobial, mechanical, and therapeutic properties [250]. Additionally, EEG-responsive neurostimulation garments represent a cutting-edge convergence of biosignal processing and textile-based flexible electronics, and hold particular promise for managing neurological disorders. Progress in this area will depend on improving biosensor sensitivity and integrating advanced materials to enhance signal fidelity and system responsiveness [266]. Beyond therapeutic applications, the development of organ-on-textile models has introduced textile platforms capable of mimicking complex physiological environments. Nanoporous membranes incorporated into textiles have already shown potential for replicating skin barrier functions, enabling sophisticated drug testing and dermatological research [267]. Vascularized 3D cell cultures grown on conductive textile scaffolds are advancing the frontier of tissue engineering, providing realistic environments for studying organ function, disease progression, and therapeutic responses. Challenges in this area focus on optimizing the conductivity and biocompatibility of scaffold materials, an area ripe for continued innovation [268]. Looking ahead, the integration of cellulose-based systems with terahertz imaging technologies could unlock new dimensions in biomedical monitoring, offering enhanced diagnostic capabilities [269]. Furthermore, coupling these inkjet-printed wearable systems with machine learning algorithms for predictive analytics can significantly refine clinical decision making and personalized care pathways [270]. Together, these advancements signal a future in which inkjet-printed textiles play a central role in revolutionizing healthcare, bridging cutting-edge material science, wearable electronics, and data-driven medicine into a cohesive, transformative technology platform. In general, to provide a strategic overview of the field and illustrate the progression from material innovation to device application and commercialization, a graphical roadmap is presented in Fig. 18. This roadmap synthesizes the key stages of inkjet-printed wearable electronics, including nanomaterial 8. Conclusion Inkjet printing of conductive nanomaterials on textiles has emerged as a transformative manufacturing paradigm for wearable electronics, offering unprecedented opportunities to merge digital functionality with the everyday textiles. This review systematically examines the technological ecosystem encompassing advanced nanomaterials, precision printing techniques, and innovative applications that are redefining the boundaries of smart textiles. The field has demonstrated remarkable progress in overcoming the initial challenges related to material compatibility and printing resolution, with current systems achieving sub-50μm feature sizes while maintaining excellent electromechanical performance under mechanical deformation. Key breakthroughs in coreshell nanostructures, hybrid printing methodologies, and self-healing composites have addressed critical limitations in environmental stability and wash durability, with several systems now exceeding 50 industrial laundering cycles while retaining >90 % conductivity. The development of multifunctional inks incorporating sensing, energy harvesting, and therapeutic capabilities points toward increasingly sophisticated wearable systems that transcend conventional passive monitoring to enable closed-loop bioelectronic interfaces. However, the transition from laboratory prototypes to commercial products requires concerted efforts in several areas: standardization of testing protocols for wearable electronics, development of sustainable material systems with circular life cycles, and creation of scalable manufacturing Fig. 18. Roadmap for inkjet-printed wearable electronics, illustrating the progression from nanomaterial and ink innovation, through printing process optimization and device fabrication, to performance evaluation and commercial deployment, highlighting key challenges and emerging solutions at each stage. 30 B.K. Dejene Materials Today Advances 28 (2025) 100629 infrastructure. Future research should prioritize AI-driven material discovery, neuromorphic textile architectures, and seamless integration with existing textile production workflows. 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