Systematic Review of Conductive Inks for E-textiles: Formulation,Printing Methods,Challenges,and Opportunities

Abstract

The development of e-textiles for various sensing and health monitoring applications has remained difficult because the printed electronic inks must be highly stable on textile substrates, yet flexible enough to adapt to the human contour. Despite the adverse challenges, significant progress has been made over the last few years in the formulation and application of conductive inks for e-textiles. This mini systematic review, therefore, presents the recent advancements in the formulation and application of conductive inks for e-textiles in concert with the clarion call for sustainable textiles. The Scopus database was used to identify relevant publications within the field for effective analysis. After scrutinizing the bulk of publications, a total of 30 studies comprising articles and conference papers were selected and structured in 3 themes, namely, formulation of conductive inks for e-textiles, fabrication and printing methods, and challenges of fabricating conductive inks on textiles. Findings show that significant progress has been made toward formulating silver nanowire inks with exceptional conductivity and flexibility, stabilization of carbon nanotubes and graphene inks with consistent electrical properties, and high substantivity to different textile substrates without pretreatment. Subsequently, deposition techniques such as screen printing, ultrasonic spray coating, syringe-based deposition systems, and direct-write printing are being used for the full realization of printing conductive inks for e-textiles, and are highlighted in this paper. In general, technological advancements have increased the capabilities of e-textile devices made with conductive inks.

Introduction

In the dynamic realm of wearable technology, electronic textiles have emerged as a groundbreaking trend, seamlessly bridging the domains of fashion and technology. ¹ These pioneering textiles hold the promise of revolutionizing how we interact with our clothing, infusing garments with electronic capabilities that were once confined to traditional devices. At the heart of the realization of electronic textiles lies the development of conductive inks, a pivotal factor in enabling the advancement of these textiles. These conductive inks can be printed or applied to textiles, giving rise to flexible, stretchable, and washable electronic circuits and sensors. Nevertheless, the formulation of conductive inks that harmonize with textile manufacturing processes presents formidable technical hurdles. ² These inks must seamlessly blend high electrical conductivity with impeccable mechanical attributes, strong adhesion, and enduring wash durability on fabric substrates,^2,3 which is quite challenging to achieve.

As a result, the journey toward unlocking the full potential of conductive inks for e-textiles has been striking a delicate balance between preserving textile properties while achieving optimal conductivity, durability, and adhesion. ⁴ As the demand for wearable electronics continues to surge, dedicated researchers and industry experts have steadfastly endeavored to surmount these obstacles and redefine the possibilities of e-textiles. ⁵ The formulation of conductive inks is a significant factor impacting the performance and adaptability of e-textiles. Advanced development of novel materials for conductive inks has sought to maintain electrical conductivity while being compatible with textile surfaces. These compositions frequently incorporate carefully balanced conductive nanoparticles, polymers, and additives to assure conductivity, flexibility, and durability. ⁴ Printing methods represent another important dimension in the evolution of e-textiles, with diverse integration techniques such as screen printing, inkjet printing, and 3D printing as powerful tools for precisely depositing conductive inks onto textile surfaces.^6,7

E-textile adoption faces major challenges in retaining the textile’s breathability, durability, flexibility, stretchability, appearance, washability, and establishing a balance between electrical performance and environmental sustainability following electronic integration. Integrated devices and materials must be durable enough to survive the lifetime of their intended use while providing consistent electrical performance. ⁷ Studies have however reported on the fabrication and unique properties of these conductive inks. Herein, this study acknowledges review studies conducted on specific areas such as the preparation of conductive inks for electrochemical biosensors and sensors, ⁸ graphene conductive inks for flexible electronics,^9,10 conductive inks for printed electronics on textiles ⁴ and carbon based conductive inks. ¹¹

In this systematic mini-review, we aim to consolidate the knowledge on the recent advancements in addressing the multifaceted challenges associated with conductive inks for e-textiles, which is often scattered across different journals, and provide a concise source of references for researchers, engineers, and industry. Research into conductive inks over the years has explored the fusion of material science, nanotechnology, and textile engineering to develop inks that not only deliver exceptional conductivity but also seamlessly integrate with the intrinsic qualities of textiles. Although significant progress has been made in conductive ink research over the last decade, a more concerted effort is still crucial for realizing the full potential of printed e-textiles. We are moving toward an exciting future where clothing transcends being just a piece of fabric and transforms into a dynamic canvas for interaction and creativity, thanks to the increased collaboration and proof of synergy across several disciplines. With an emphasis on its uses in the field of e-textiles, this thorough assessment traces the future trajectory of conductive ink development and integration and offers an overview of recent breakthroughs as well as a list of enduring difficulties.

Methodology

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines established by Liberati et al. ¹² served as a guide for the study. After adhering to the identification, screening, eligibility, and included steps outlined in PRISMA (Figure 1), the applicable research or publications on recent developments to overcome the difficulties of conductive inks for e-textiles were retrieved.

Figure 1.

Systematic flowchart to retrieve relevant documents using preferred reporting items for systematic reviews and meta-analyses (PRISMA).

The relevant search terms (“printed electronics” OR “metal inks” OR “conductive inks” AND “problems” OR “challenges” AND “textiles”) were key in carrying out the search. On November 3, 2023, a search within the Scopus database was conducted, producing initial search results of sixty-one publications. The latter comprised varying source types, languages, and document types, among other features, after our initial search. Then, to focus our results throughout the Screening phase, we used filters. Only English-language journal articles published, including conference papers, were included in the results after proper filtration. This limited the search results to 45 documents that included article publications and conference papers, with no other restrictions. The abstracts were then examined in the Eligibility phase by our team to make sure they complied with the requirements for inclusion in our study. After a critical analysis, 15 documents were excluded due to their unrelatedness and inaccessibility that is, irrelevant and non-full text. Finally, for our evaluation procedure, a total of 30 documents were used in the Included step. These sources were properly mentioned, and they served as the foundation for our in-depth examination of the included studies. The Endnote reference software was used in this study to effectively extract and account for all references used in the writing of the paper.

Results and Discussion

A careful analysis of the included studies revealed publications within this area to be fluctuating with relatively low outputs as evident from Figure 2(a). There was however a rise in the number of published papers in 2019, 2021, and the current year 2023. It is worth noting that the three publications in 2017 highlighted precursor inks deposited using ultrasonic spray coating techniques to form silver layers on substrates with effective performance, ¹³ the possibility of inkjet printing graphene-based conductive patterns pre-treated on the textile material ¹⁴ and the potential for depositing polymer composites with conductive nanomaterials for wearable electronics. ¹⁵ Furthermore, as shown in Figure 2(b), the leading country contributing to this field or the country of origin of the published documents within the scope of the study is China, closely followed by the UK and the US. Significantly, studies that originated from China broadly covered the fabrication of conductive inks using different chemical substances on fibers and fabric substrates and via different printing methods to ensure the deposited inks exhibited unique and effective performance for e-textiles.

Figure 2.

(a) Publication trends in the field of conductive inks for e-textiles and (b) distribution of the top contributing countries.

Main themes of the included studies

In striking out a balance through cutting-edge research, scholars have shifted their attention to the numerous challenges of e-textiles and conductive inks, providing solutions to the problems posed by consumers and pushing the scope forward through research findings. The included 30 publications were grouped under three thematic areas after reading the abstracts and findings of the documents, that is, formulating conductive inks for e-textiles, fabrication, and printing methods, and the challenges of fabricating conductive inks on textiles (Figure 3).

Figure 3.

Main themes from the selected studies

Formulating Conductive Inks forE-Textiles

The utilization of the appropriate materials in their right quantities is vital to form stable conductive inks with unique properties for application on a textile substrate. Conductive ink formulation, especially for e-textiles, involves careful consideration of conductivity, flexibility, adhesion, printability, and durability to meet the demands of e-textile applications. In agreement with to Mei ¹⁶ and Boumegnane et al. ⁴ a novel method using printing techniques and cutting-edge conductive inks to fabricate electronic products is one of the world’s most dynamic technologies. Scholars have identified the formulation of the conductive inks as the greatest challenge, as this determines not only the convenience of the materials being deposited but also the final comprehensive product. In this case, metallic particles or precursors are used in place of the ink pigments to provide electrical conductivity to the printed patterns. However, recent studies have addressed issues with adhesion, conductivity, environmental friendliness, stability, and dispersion, among others,^4,16 which are the essence of this review.

Other studies conducted by Liang et al. ¹⁷ stated that the limited dispersibility of carbon nanotubes (CNTs) in most solvents and possible cytotoxicity are limiting the usage of these materials in bio-systems, although CNTs have a tremendous deal of potential for flexible electronics. As shown in Figure 4(a)–(d), experimental procedures described a hybrid ink formulation made of natural Silk Sericin-CNT (SSCNT). The results showed that the SSCNT dispersion had a high colloidal stability, which demonstrated sericin’s good performance in facilitating the dispersion of CNTs in water. The sericin-modified-CNTs exhibit good dispersity in water, as seen by the ability of droplets of the as-prepared SSCNT ink to spread quickly in water. Drops of a sonicated CNT–water mixture lacking sericin soon sank into the liquid, producing observable precipitates. The transmission electron microscopy (TEM) image demonstrates that prepared SSCNT has a larger degree of dispersion. Surprisingly, the SSCNT dispersion is quite stable and can be kept in storage for months without developing any visible precipitates. Thus, the consistent and dense network of CNTs ensures the SSCNT film’s good conductivity. Additionally, it was determined that the electrical conductivity of the SSCNT film was 42.1 ± 1.8 S·cm⁻¹, showing that the conductivity of the SSCNT ink is adequate for the majority of wearable applications, as shown in Figure 4(e)–(j). Sericin modification using a biocompatibility test alleviated the biocompatibility problem. Human glioblastoma cells (U87), human umbilical vein endothelial cells (HUVEC), and adenocarcinomic human alveolar basal epithelial cells (A549) implanted in Petri dishes were used to incubate living cells to confirm SSCNT. In addition to a Blank group, the Petri dishes had sericin (SS group), CNTs (CNT group), and SSCNTs (SSCNT group) applied before the incubation. After incubation, the cells were stained to distinguish between live and dead cells. The findings demonstrate that U87 cells, like A549 cells and HUVEC cells, maintained their normal morphology in the presence of SSCNT after 3 days. Additionally, sericin and sericin-CNT hybrids promoted the proliferation of U87 cells, indicating their long-term cytocompatibility, making sericin a more secure option for creating CNT ink for printing electronics.

Figure 4.

(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 usage.

Another study by Ahmed et al. ¹⁸ looked at creating very stable graphene-based (graphene nanoplatelets) conductive inks that could address a significant difficulty of electrical property consistency over the storage period in the development of wearable electronics. Thus, two cyclohexanones [(CH2)5CO)] with either ethylene glycol [(CH2OH)2] (CEG ink) or cyclohexanone [(CH2)5CO] with terpineol (CT ink) binary solvent mixes of organic chemicals were synthesized and used to formulate two graphene-based inks. The Hansen solubility parameters (HPs) were used to develop binary solvents. According to published research,^19,20 the HPs of a solvent are D = 18 MPa1/2, P = 10 MPa1/2, and H = 7 MPa1/2 in order to establish a stable graphene flake dispersion. Experimental procedures show that the Ra between graphene and a binary solvent based on CEG is 1.44 MPa1/2 and that the Ra between graphene and a binary solvent based on CT is 2.44 MPa1/2. Strong interactions between graphene and the solvents are indicated by the decreased value of Ra in both solvents. The two inks inherit great solubility and stability as a result. Findings on the electrical conductivity show that the sheet resistance increased for both solvent-based inks, where CT-based inks had higher sheet resistance values as compared to CEG-based inks (Figure 5).

Figure 5.

Sheet resistance against the storage time duration of CEG and CT graphene conductive ink printed e-textile. ¹⁸

A key factor in defining the rheological characteristics of graphene conductive inks is viscosity. According to Hernandez et al., ²¹ an optimal viscosity range for inkjet printing is between 8 and 15 mPa s. This criterion improves conductive ink flow without problems that could lead to leaking, drying out, or coagulation. After 2 months of storage, the viscosity of both conductive inks barely decreases, but their values are still between 9 and 10 mPa s, indicating compatibility with inkjet printing. ¹⁸ The newly created CEG and CT graphene electro-conductive inks’ stored zeta potential values and both showed negative findings. The final zeta potential values for CEG = −45 mV and CT = −42 mV suggest that the inks are in the good stability range. The newly created conductive inks’ absorption wavelength was determined using the K/S values. CEG ink doesn’t change after two months of storage, while CT indicates a tiny decline in K/S value. The small variations in K/S values show that the inks show little to no agglomeration.

Aside from the stability of conductive inks, adhesion problems between the textile substrate and the fabricated conductive inks, especially graphene-based have been explored and solutions provided. Chen et al. ²² found that a longer ultrasonication duration helps to lower the size of graphene oxide (GO) flake when they looked into the impact of GO flake size and thermal reduction on electric resistance. The cotton yarn coated with GO had better conductivity because of the prolonged reduction action. The GO-coated cotton yarn was also able to attain a lower resistance in less time by reduction at a higher temperature. Furthermore, low resistance stability and poor adhesion remained a critical problem of conductive inks when printed on a substrate, a situation which affects its application for stable and stretchable fabrics. In a study conducted by Jiang et al., ²³ a thermoplastic polyurethane (TPU)-based multi-walled carbon nanotube (TMWCNT) conductive ink was fabricated, suitable for the production of screen-printed fabric electrodes (SPFEs) with excellent conductive and mechanical performance. The percentage growth of MWCNTs from 1 to 5 wt%, and the sheet resistance of TMWCNT/polyamide FEs rapidly reduced, reaching a value of 2.689 kX/sq. This is so that the charge transfer impedance can be reduced by the increased MWCNT concentration, which makes it easier for conductive channels to form in the screen-printed conductive layer. The resistance did not, however, continue to decline at a 6 wt% concentration increase. This is because conductive TMWCNT ink, which has a MWCNT concentration of 6 wt%, has a high viscosity (23.2 Pa.s at shear rate of 1 s⁻¹ and 250° C) and is therefore not suitable for screen printing. Results show that conductive ink and fabric substrates adhere to TMWCNT very well, which the TPU matrix in the ink is responsible for due to molecular chains and a lot of functional groups, like amino and carbonyl groups. This is demonstrated by the Scotch tape test findings, where 10 tests showed no noticeable change in the overall surface morphology of FEs. Good interfacial adhesion between TMWCNT conductive ink and polyamide fabric was also demonstrated in the cross-sectional picture of the FE, which showed no obvious delamination of the conductive TMWCNT layer from the fabric substrate.

The issues of the porous nature of the structure and high surface roughness have affected the realization of printing conductive patterns for effective performance. To overcome this, an investigation conducted by Hong et al. ²⁴ fabricated a UV curable nano-silver conductive ink, a low-cost and environmentally friendly method that cures quickly at a lower temperature for fabrics. The experimental process aimed at tackling the issues of poor adhesion and low electrical conductivity caused by the porous structure and rough surface of fabric substrates. Nano-silver flakes, isopropylthioxanthone (ITX), BYK-333, BYK-A555, KH-560, and aliphatic polyurethane acrylate (PUA) were used in the formulation of the UV curable nano-silver conductive ink. Six distinct UV curable solutions for conductive inks were created using the monomers PUA, TPGDA, and TMPTA together with photo initiators (1173, 907, ITX, and 1-hydroxycyclohexyl-phenyl), defoamers (BYK-333 and BYK-A555), and adhesion promoters (KH-560). The six distinct UV-curable conductive inks with varied nano-silver loading were created by adding various amounts of nano-silver flakes to each solution. After that, carbonate-coated nylon woven fabrics were directly screen-printed with these conductive inks. Investigating the influence of the type and concentration of photo initiators, it was evident that the UV curable ink with photo initiator TPO and 907 had the fastest and slowest curing speed respectively. The screen printing of carbon ink was evaluated on a variety of fabric substrates, including twill nylon woven fabric, plain weave nylon fabric, carbonate-coated nylon fabric, and knitted fabric. Ink diffusion, penetration, and uniformity during printing were greatly impacted by the pore size and regular texture structure of the materials. The quality of the printed conductive patterns was improved by smaller holes and smoother surfaces. In order to achieve the best print resolution and quality while guaranteeing conductivity, carbonate-coated nylon fabric with micrometer-level pores allowed for excellent ink control, crisp pattern edges, well-defined conductive lines, and uniform surfaces. The cross-section of the conductive pattern screen-printed with CI-60 wt% demonstrates the silver layer’s continuous, thick structure, demonstrating the uniform dispersion of nano-silver flakes on the surface of woven fabrics. Additionally, subjected to after UV cure for 20 s, the obtained electrical resistivity is as low as 4.04 ± 0.76 × 10⁻⁵ Ω cm at 60 wt% silver loading, which has a positive economic impact. This shows that the electrical resistance of the conductive patterns dramatically decreases when the nano-silver loading rises from 30 to 50 wt%. This might be because the electrical conductivity of printed patterns has significantly improved due to the increase in nano-silver loading, which expands the contact area between nano-silver flakes. The other components in the conductive ink are significantly reduced when the silver loading is too high, and the nano-silver cannot be completely and evenly dispersed, which affects the conductive channel formed by the nano-silver, and the electrical resistance of the conductive patterns will rise. Moreover, the average electrical resistance of knitted fabric is around 106 Ω, while those of printed patterns on twill, plain weave, and carbonate-coated fabrics are 18.22 ± 2.09, 5.89 ± 0.83, and 5.04 ± 0.61 Ω, respectively. This highlights the ratio of nano-silver flakes for the conductive line printed on carbonate-coated woven fabric to be the highest. This conclusion explains why the conductive patterns printed on coated woven fabric exhibit the lowest electrical resistance.

Gralczyk et al. ¹⁵ used materials such as graphene nanoplatelets, silver nanoparticles/flakes, multi-walled carbon nanotubes, and polymethyl methacrylate (PMMA) as the polymer matrix to investigate the potential of using conductive polymer composites with graphene, silver nanoparticles, and carbon nanotubes for wearable electronics applications. Graphene nanoplatelets were deagglomerated using ultrasonication with water for 240 min to prepare the graphene ink. Surfactants adhered to the surface of the graphene flakes during the sonification process for a functionalized phase, preventing the reagglomeration of graphene with surfactants. Using a magnetic blade mixer, 8 wt% of polymethyl methacrylate granulate (PMMA) was simultaneously dissolved in butyl carbitol acetate (solvent). By grinding and rolling 10% graphene flakes with 90% PMMA solution, finer (12 μm) agglomerates were produced. For carbon nanotube ink, a similar procedure was used, in which 4% multi-walled carbon nanotubes were combined with 8% PMMA solution and homogenized. While silver ink was marketed by ITME17, silver-graphene paste and silver-carbon nanotube paste were created by mixing 5% graphene and 1% multi-walled carbon nanotubes, respectively, into the silver paste. Three inks—graphene, CNT, and silver-graphene—were arrived at, printed on polyethylene terephthalate (PET) and cotton fabric, and evaluated for conductivity and flexibility. The findings showed that whereas graphene and carbon nanotube composites had greater resistivity but better flexibility, silver-based composites had the lowest resistivities and poor mechanical characteristics on textiles. Additionally, layered composites with a top covering of graphene/CNT and a bottom layer of silver offered acceptable conductivity, but the mechanical flexibility is not encouraging, necessitating further research. The overall performance on PET films was superior to that directly on fabrics.

In another study, Wang et al. ²⁵ highlighted a generalized and environmentally friendly method in manufacturing highly conductive aqueous inks by combining organic salts generated from biomass, such as succinic acid–chitosan (SA–chitosan) and sebacic acid–chitosan (SEA–chitosan), with silver nanowires (AgNWs). This solved the typical water-based inks’ poor stability and conductivity. When AgNW aqueous solution and SA-chitosan aqueous solution were combined directly, as shown in Figure 6(a), a water-based conductive ink was created. These biomass-derived SA–chitosan organic salts can be easily employed as polymer binders for water-based coatings. The SA–chitosan/AgNW inks were used as the precursor solutions and dip-coating or drop-casting techniques were used to create the conductive films or coatings on various substrates, including polyimides and textiles, as illustrated in Figure 6(b). The findings show that the created chitosan and SA complex can stop the quick crystallization of chitosan and SA, which leads to the development of a homogeneous, smooth organic salt coating on the substrate. Water-borne SA–chitosan/AgNW inks can achieve great thermal and chemical resistance by cross-linking SA and chitosan by amidation reactions on the post-treated organic salt film (Figure 6(c)), which makes them perfect as conductive coatings. Again, the dispersion of AgNWs in the polymer matrix with average lengths of 20 and 130 μm demonstrates uniform dispersion in the SA/chitosan polymer matrix, with no overt aggregation observed for any of the composite films with AgNW concentrations ranging from 20 to 80 wt%. When the AgNW content is 20 wt%, the surface of the composite coating of conductive network of AgNWs forms becomes more densely packed when the AgNW content is 80 wt%, encouraging conductivity. The calculated electromagnetic interference (EMI) shielding performance of the conductive SA−chitosan/AgNW composite coatings with a thickness between 10 and 15 μm at the X-band reveals that a value of 73.3 dB in relation to an increased AgNW content of 80 wt% exhibits an efficient blocking value of 99.9999% of EM waves. In a further study to address poor durability and high production costs, Li et al. ²⁶ fabricated an additive-free conductive ink using a 100% precursor titanium aluminum carbide (MAX)/MXene approach. This improved the viscosity in a controlled manner and further chemical cross-linked was performed for effective high performance suitable for wearables and use in extreme conditions. This proved the potential for use for multifunctional, durable, and robust e-textiles for various applications. Still on improving the performance of strain-sensing e-textiles, a recent study conducted by Tian et al. ²⁷ fabricated highly effective composite conductive inks using different binders (Figure 7). Here, the fabricated carbon black/high elastic transparent glue composite ink showed dynamic stability, wide strain working range and high sensitivity when printed on a commercial stretchable textile substrate. The authors conclude that the use of binders in the fabrication of conductive inks could potentially influence their performance and durability on substrates.

Figure 6.

Schematic illustration of the fabrication procedures for water-based (a) conductive ink and (b) conductive composite coatings on PI and textiles and (c) the cross-linking reaction for the SA−chitosan organic salt.

Figure 7.

Schematic illustration of composite conductive ink design, the fabrication process, and potential application scenarios for printed strain-sensing e-textiles.

Due to their innate stiffness and brittleness, inorganic semiconductors have few practical applications. Based on this, Yuan et al. ²⁸ developed a unique method for creating flexible semiconductor ink from copper sulfide (CuS) nanoparticles and silver selenide (Ag₂Se) nanowires, which produced printable inks suitable for various surfaces. Ag₂Se nanowires and CuS nanoparticles are synthesized as part of the ink preparation process. SeO₂ and β-cyclodextrin are combined with deionized water to create a solution, which is then combined with ascorbic acid in another deionized water solution to create Ag₂Se nanowires. The result is centrifuged, rinsed with water and ethanol, and then aged in ethanol following a 4 h reaction. Se nanowires are dissolved in ethylene glycol for the manufacture of Ag₂Se nanowires, to which AgNO₃ is then gradually added while being stirred, followed by centrifugation and washing. CuCl₂ and Na₂S solutions are combined to create CuS nanoparticles, which are then washed and dried after being heated at 140°C for 12 h in an autoclave. Findings show that the printed ink film is morphologically consistent, stable, compatible with a variety of substrates with good adhesion, and ideal for printing. The film also exhibits outstanding mechanical durability and can survive 100,000 bends without affecting its electrical performance. With the positives drawn from the experimental works reported in studies on good adhesion, stability, conductivity, flexibility, adhesion, printability, and durability, these fabricated conductive inks are suitable for application to the surface of textile materials. Depending on the viscosity and type or nature of the conductive inks, different printing methods are adopted, as carefully discussed in the next section.

Fabrication and Printing Methods

Electronic textiles are an emerging field that integrates electronics with textiles and are sometimes known as smart textiles. By embedding electronic components and functionality into materials, they produce wearable technology that seamlessly integrates into our daily lives. If wearable electronic consumer products are to be commercialized on a large scale, it is imperative to enhance flexible circuit manufacturing technology. Numerous techniques, including knitting and crocheting, weaving, braiding, coating or laminating, chemical processing, and printing, can be used to create electronic textiles (e-textiles). ²⁹ According to studies, printing and fabrication techniques are crucial to the development of e-textiles. Marchal et al. ¹³ assert that the practicality of these novel electronic technologies depends on having access to creative functional materials like metal inks and well-thought-out deposition procedures. Therefore, conductive inks are key for printing, spraying, and coating flexible electrical circuits on fabrics, which are subsequently used to produce wearable electronic products. ³⁰ However, printing is an approach that shows promise because it is affordable, scalable, and allows for quick design customization utilizing digital printing technology. ³¹ Additionally, Cummins and Desmulliez ³² and Khan et al. ³³ highlight that varieties of printing techniques have been suggested for printed electronics as summarized in Table 1.

Table 1.

Comparisons of various printing technologies/methods.

Printing method	Benefits	Drawback
Inkjet printing	Simple design, high printing resolution.Environmentally friendly	Cannot manufacturebatch-to-batch
Screen printing	Print thick layers, good conductivity	Not precisely able to controlthe resolution
Gravure printingor flexography	Mass production	Low resolution
Lithography	High accuracy	It is a complicated process, time-consumingand vulnerable to the outside environment

Subsequently, due to its huge volume deposition, affordability, and a range of other characteristics, researchers have examined the advantages of silver, which possesses the highest electric conductivity of all metals. ¹³ To incorporate electrical circuit components onto temperature-sensitive substrates, Marchal et al. ¹³ used an ultrasonic spray coating method to deposit conductive metal features onto flexible materials. Metal organic decomposition (MOD) inks must be produced and used in the presence of amine ligands to prevent Ag₂O precipitation and guarantee that all silver ions are coordinated. It was possible to boost adhesion at low temperatures between 70°C and 120°C without the need for curing stages with the ink created by the screen-printing technique and remarkable conductivity results were also generated. The outcome shows that this novel combination of nanoparticle-free Ag-inks with the ultrasonic spray coating method has potential for manufacturing 3D objects, high-throughput deposition of highly conductive silver features on heat-sensitive substrates, and a significant advancement in printable electronics. Silver nanowires with dimensions of 8–15 m and 60–100 nm in diameter were produced via screen printing. The silver nanowires were synthesized as the conducting phase for conductive inks, which also included hydrochloric acid, guar gum, polyaniline, and other additives. ³⁴ Using AgNW inks to print a circuit schematic on a clean woven fabric and the multimeter’s diode position as the power source, two 0.06 W LEDs and a 1 K resistor circuit were made practicable. It was clear that the LEDs lit up correctly, and the fabric’s functionality was unaffected by bending.

Another printing method, screen-printing, is the primary method for fabricating and printing the conductive interconnects on the surfaces of polyethylene terephthalate (PET) and cotton fabric. Gralczyk et al. ¹⁵ explored the possibilities of polymer composites with conductive nanomaterials in creating e-textiles. The study found that pastes of multiple functional groups with silver as a conductive phase in the presence of carbon nanotubes or graphene yield the best results. Similarly, Liu et al. ³⁵ spray coated and fabricated screen-printed dye-sensitized solar cells (DSSCs) on Kapton and standard woven polyester cotton fabrics to achieve power conversion efficiencies (PCEs) of 7.03% and 2.78%, respectively at a low temperature to obtain photovoltaic textiles with improved PV efficiency. The screen design ensures that the interface layer is only printed where subsequent layers are required, thereby maintaining the fabric’s flexibility and maximizing breathability, and is carried at a 0.8–1 mm print gap with a 6 kg squeegee pressure setting. The Kapton-based devices showed good printed layer quality and uniformity due to the high chemical and temperature resistance of the polyimide Kapton film, but the interface-coated fabric-based devices suffered from interface deformation after printing the functional layer on top during the annealing stage. In a different approach, screen printing was combined with standard micro-fabrication processes to deposit conductive inks directly and ensure localized micro-circuit patterns on the fabric substrate. ³⁶ The results show that this approach could help improve reliability and reduce the form factor of printed e-textiles.

Further experimental procedures conducted by Khirotdin et al. ³⁷ investigated the feasibility of a syringe-based deposition system for printing micro-conductive ink tracks on fabric. Using a Smartrac Web RFID tag antenna as a model for printing at a speed of 7.00 mm/s and a deposition height of 0.59 mm, the study showed that the syringe-based deposition system is capable of printing micro-conductive ink on a fabric substrate by properly controlling its printing parameters. Again, it was found that curing significantly affects the conductivity of the ink tracks positively. Salam et al. ³⁸ investigated the fabrication process of electronic screen stencil-printing of conductive silver inks on a heat transfer polymer (HTP) substrate using a newly developed roll-to-roll pilot line for flexible hybrid electronics. At a slow print speed of 30 mm/s using a 50° low-angle squeegee, the results showed that the average electrical resistance values of the printed silver on the HTP substrate are 17.3% lower than those on the polyethylene terephthalate (PET) substrate. Via the direct-write printing approach, Kajenski et al. ³⁹ printed meta-surfaces on two different nylon fabrics coated with polyurethane on the reverse side for mechanical reinforcement and to prevent the ink from bleeding through to the other side. CM127-48 and PE876 inks combined were used to fabricate a dual-band meta-surface with a rejection at 4.8 GHz and a bandpass at 2.6 GHz. At room temperature, the printed ink samples stored in a drawer for a year demonstrated better performance and stability compared to PE876 ink. Votzke et al. ⁴⁰ fabricated fully stretchable, multi-layer active circuit arrays in silicone using liquid metal paste interconnects via stencil printing in which power and data wires are repurposed as strain sensors to estimate substrate deformation. The authors demonstrated a 3 × 3 active circuit array that self-senses relative voltage supply at each node to estimate deformation and consumes less than 10 mA per node. The fabricated stretchable circuit was shown to be highly scalable and could support the inclusion of additional sensors or actuators into the stretchable printed circuits. Mehdie et al. ⁴¹ combined screen printing with a vacuum dryer for patterning the AgNPs network onto flexible Bombyx mori silk fibroin (BSF) film. Conductive AgNPs-based patterns were printed over Si wafer through a screen-printing technique in the presence of 30 wt% glycerol as an additive. The AgNP pattern was successfully printed with a recorded sheet resistance at 49 (Ω/sq) during 1000 bending cycles and could light up 8 LEDs when connected to a 9 V battery.

Aside from printing on fabrics, conductive inks can be printed on fiber substrates for effective performance. The viability of extruding or dispenser printing electronics directly into carbon fiber composites was investigated by Idris et al. ⁴² Printing silver electrodes on carbon fiber weaves and running an electrical current through them result in a large-area heater. Scalable solid-state microchips were produced using the pol–gel technique by Lopes et al. ⁴³ and then incorporated into soft-matter and flexible printed electronics. When circuits made of physically cross-linked block copolymer substrate, silver liquid metal composite ink, and digitally printed microchips are subjected to solvent vapor, a polymer gel transition results. In a recent study, Zhang et al. ⁴⁴ experimented by printing conductive inks on 1D stretchable fibers using rotatory inkjet-printing technology. The results showed that this printing method achieved a “high precision and customizable micromachining on ultra-low diameter fiber” with effective strain-insensitive and mechanical stability.

Even with the recurring use of screen printing, ultrasonic spray coating, and inkjet printing amongst others to deposit conductive inks onto the surface of textile materials, certain challenges or factors further limit the functionality of these conductive inks. These challenges are discussed in the next section to provide great insight.

Challenges of Fabricating Conductive Inks on Textiles

The continuous efforts to produce futuristic smart textile wearables for health monitoring and sensing applications have influenced advancements toward the use of conductive or metallic inks. These are to replace the traditional and conventional rigid material electrodes for wearable technology. However, the adoption of these conductive inks for printed electronics toward varying applications in e-textiles has presented several challenges and problems which tend to influence its functionality and performance on textiles. Here, studies have identified several challenges or problems (carefully illustrated in Figure 8), which affect the application of conductive inks on textiles for wearables or e-textiles.

Figure 8.

Identified challenges on the fabrication of conductive inks on textile substrates.

The issues of deformation and transiency of conductive inks remain a critical problem. ⁴⁵ Conductive inks when applied via printing onto the surface of textile, for example, experience deformation when these materials are subjected to continuous bending. This practice of exposing these conductive inks to extreme mechanical bending actions will lead to breaks in the conductive pattern and hence have a direct influence on the electrical resistance and stable electrical performance of the inks on the textile. Also, having a low initial resistance and minimal increase in resistance because of tensile deformation has proved to be very challenging in forming flexible conductive paths with conductive inks on textile substrates. ⁴⁶ Maintaining a well-functioning conductive ink on textiles is very challenging due to the recurring mechanical stress textile materials go through ⁴⁷ when worn on the human body. Alternatively, textiles undergo frequent washing cycles and hence having a controlled transiency (dissolution in water) with conductive inks has proved to be a problem. These conductive inks when subjected to water and mechanical stress during washing show deformation and the degradation of their conductive electrodes which limits the transmission of sensing and electrical performance for varying applications. This thus draws on the need to produce a stabilized conductive ink with good electrical resistance to washing cycles and deformation experienced during bending. For example, a study by Ahmed et al. ¹⁸ on the use of graphene for electroconductive inks on textiles stressed the need for these materials to have good electrical properties without breaking away when subjected to environmental stimuli. Additionally, the strain applied to a textile printed with conductive inks presents challenges. The issue of resistance to strain by stretchable electrical interconnects as opined by Votzke et al. ⁴⁰ leads to severe defects in signaling and delivery of power for the various electrical components.

Subsequently, poor adhesion and environmental impacts of conductive inks for e-textiles comprise another challenge. ²⁵ Here, the stability of these inks is affected due to their weak interactions with the inherent structure of the textile materials. This phenomenon, as opined by Mamun et al., ⁴⁸ results in poor adhesion between the two components (textile substrate and conductive inks) for varying applications. The issue of adhesion is improved by the addition of polymer binders, ⁴⁹ which leads to a more stable environmental, adhesion, and mechanical performance of the conductive inks. ⁵⁰ Due to the addition of the polymer binders, its dispersion process according to Wang et al. ²⁵ requires the use of organic solvents which are toxic, hence leading to safety and environmental impacts. This implies that such toxic materials could be found in wastewater from washing cycles which could typically have a negative effect on the eco-system.

The characteristics of a textile substrate such as its fibrous and porous nature have been identified to be another influential challenge for integrating conductive inks with the textile structure. Depositing these electrically conductive inks on the porous surface of the textile material as solid patterns ³⁸ and, how these inks can penetrate and spread evenly in the yarns of the fabric, ⁵¹ present a major challenge. Conductive inks need to be in a viscose state (ideally for inkjet printers) to ensure a smooth application and bonding with the fiber content of the textile substrate. This viscose nature coupled with the fibrous and porous substrates of the textiles leads to ink diffusion. ⁵² This problem is caused by the capillary effects of the viscose nature of the inks when they interact with the fibrous substrates, a phenomenon that affects the effective provision of “satisfactory electrical percolation networks.” ⁵³ Other fabric features such as high surface roughness and pores affect the formulation of a continuous conductive ink layer on the textile.^23,24,54 This results in serious deformation, hence, breaks in electrical conductivity when the textile is stretched or bent. Due to the porous and rough surface nature of textiles, it becomes relatively difficult to print highly conductive inks in a continuous manner on such surfaces. ¹⁴ Also, the continuous changes of the fiber structure affected by surrounding water molecules, according to Chauraya et al., ⁵⁵ further lead to a difficulty in producing conductive paths in a uniform manner using inkjet inks of low viscosity. Alternatively, the high surface roughness of the textile substrate leads to low resistance stability and poor adhesion of the electrodes in the conductive inks. With the formation of poor adhesion with the textile substrate, this issue is further worsened upon subsequent washing cycles and bending. Furthermore, the porosity and high surface roughness of the textiles according to Hong et al. ²⁴ affects the printing of conductive inks for microwave passive devices in high resolution on textile substrates. This, however, affects the precision and sharpness of the prints on the textile substrates. However, Yang-Pei-Qi and Yi ⁵⁴ outlined varying studies that have developed solutions to overcome porosity and roughness when adopting inkjet printing of conductive inks on textile materials (as shown in Table 2). Other factors such as durability and conductivity of the printed electronics which influence the fabric’s properties are affected by the fabrication process. ⁵⁶

Table 2.

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	Conductivity 2500 ± 175 S/m	57
Silver nanoparticles ink Reduced graphene ink	Inkjet printing a hydrophobic layer	Hydrophobic breathable coating	100% cotton, 100% polyester and 65%/35% cotton/polyester	Sheet resistance 1.18 Ω/sq Sheet resistance 2.14 × 103 Ω/sq	14
Silver nanoparticle ink	Coating an interface layer	UV-curabledielectric ink	100% cotton, 65%/35%polyester/cotton and 85%15% polyester/cottonfabrics	Conductivity 2.08 × 106 S/m	58
Reactive silver ink		Polyvinyl alcohol	100% cotton, 100 % polyester, 60%/40%Cotton/polyester	Conductivity 5.54 × 105 S/m	59,60

Lastly, temperature settings are required to produce the necessary conductive paths that bond well with the textile substrate. However, forming highly conductive paths requires high curing temperatures. The poor thermal stability of the textile substrates which limits the high thermal curing temperatures required for fabricating conductive inks on textiles has proved to be very challenging. ²⁴ The use of high curing temperatures causes a reduction in the strength of the textile which results in poor thermal stability. This phenomenon affects the scalable fabrication of highly conductive inks on fibrous substrates for e-textiles.

Interconnection of Electronics

The effective integration of electronic components into a textile substrate plays a critical role in the performance of electronic textiles (e-textiles). These flexible or rigid electronic components are interconnected with conductive inks to ensure effective signal delivery and workability. Aside from the requirement of lower curing temperatures for conductive inks, ⁶¹ integrating these electronic components into conductive inks poses critical challenges that tend to limit their full potential. For example, effective gripping or bonding of electronic components to conductive inks is a major problem. The durability is further affected by the effect of temperature and humidity in the environment, which tends to weaken the interconnection.^62,63 Other challenges such as unstable contact resistance, poor mechanical properties, and low conductivity influence the integration of electronic components to conductive inks for effective functionality.^64,65

Implication of Findings

The use of conductive inks on textile substrates provides a promising approach to achieving the integration of electronics for applications like health monitoring. Relevant findings from the study have uncovered the appropriate materials used in fabrication, the techniques used and the challenges of depositing highly conductive inks on textile substrate. Importantly, the implications are that the influence of washing cycles and continuous bending of textile materials containing conductive inks could affect the electrical conductivity or resistance of the inks for effective performance and applications. Since these wearables are intended for use by humans who subject these e-textiles to frequent washing and deformation, establishing care-label information would aid in informing users on their appropriate care. Studies on improving the deformation and transiency of conductive inks for e-textiles will widen their applications. To obtain high conductive inks on textile substrates, excessively high-temperature treatments are required for curing, which typically affects the mechanical properties of the base fiber/fabric. Hence, the appropriate temperature settings on different textile materials and UV-curable techniques of nano silver flake inks with photo initiators should be investigated through research and development investments to streamline the production of e-textiles. This will help push for a more practical realization of scalable fabrication of highly conductive inks on textiles and their ability to resist washing effects.

Furthermore, the dissolution of polymer binders with hazardous organic solvents has major environmental and safety implications, despite improving the conductivity and adhesive properties of conductive inks for e-textiles. This emphasizes the need for the development of eco-friendly binders (such as polymers such as succinic acid–chitosan or sebacic acid–chitosan) that are non-toxic and eco-friendly, promote recyclability, and have low environmental impacts to achieve both conductivity and good adhesion properties on textile substrates. Adopting these eco-friendly material chemicals will help reduce carbon footprint and toxic wastewater generation. These will collectively contribute to Sustainable Development Goals 3 (Good health and Well-being), 9 (Industry, Innovation, and Infrastructure), and 12 (Responsible Consumption and Production). For example, silk protein-CNT (SSCNT) hybrid inks and thermoplastic polyurethane (TPU)-multiwalled CNT inks have emerged as promising solutions to allow effective dispersion in biocompatible solvents. These innovative inks not only exhibit high stability and conductivity but also address concerns related to cytotoxicity. This breakthrough enables the utilization of CNTs in e-textile applications, particularly as electrodes, while ensuring compatibility with biological environments.

Again, fabricating appropriate conductive ink compositions and the deposition techniques for manufacturing highly conductive paths for e-textiles is achievable with novel materials such as graphene and other nanoparticles. These materials could be coupled with organic salt with high conductive properties in appropriate stoichiometric ratios will improve the washing fastness, breathability, and flexibility of textiles and ensure consistent and reliable performance in printed electronics over time. Optimization techniques such as aerosol jet printing, screen and inkjet printing can be investigated for the deposition of synthesized conductive inks for printing high-resolution patterns on textile substrates via the appropriate parameters. This success will improve the quest for large-scale production of conductive inks on textile substrates for e-textiles. Importantly, impacts of ink viscosity, nozzle size, and printing speeds which can affect the deposition and eventual performance of the conductive inks on the textile substrates can be investigated for large-scale production.

In addition to these advancements, researchers are actively working on flexible ink formulations using inorganic semiconductor nanowires and nanoparticles, including materials like silver selenide and copper sulfide. Printed films produced from these formulations exhibit mechanical durability, making them suitable for applications that require flexibility and robustness. These innovations collectively represent significant strides in the development of stable, biocompatible, and high-performance conductive inks for e-textile applications, promising a bright future for wearable technology and beyond.

Key Future Research Directions

With relevant studies conducted within the field, this mini-review paper proposes certain key areas for future research to advance the full potential of applying conductive inks on different textile materials for effective performance. Firstly, to help foster user-friendly and more sustainable e-textiles, future studies could advance experimental procedures for using silk proteins to disperse carbon nanotubes for effective stability and biocompatibility. This is coupled with continuous efforts in the fabrication of new conductive inks that promote durability, washability, and good adhesion, suitable for use on the textile surface. Secondly, drawing away from conventional printing methods, further experimental works on the application of inkjet and direct writing printing methods where conductive inks are directly deposited on different textile surfaces to form conductive patterns without any challenges are needed. These will provide empirical knowledge on how to optimize effectively the conductive inks, material structure, and print speed for good conductive performance, adhesion, and durability. The effective integration of sensors in clothing will enhance the performance, recording, and monitoring of vital signs from the human body. Future studies on effective interconnection strategies of conductive printed patterns and the integration of sensors on clothing will advance the sensing and communication ability for e-textiles. Lastly, utilizing the appropriate post-treatment processes after the deposition of conductive inks on textile surfaces will aid in improving durability and conductivity, which is ideal for future research directions. These research directions will aid in advancing the effective application or disposition of conductive inks on textile surfaces with good functional and performance properties.

Conclusion

E-textile technology improvements have been greatly aided by the invention of conductive inks. As a result of employing silk proteins or specially designed solvents to improve dispersion, stable carbon nanotubes and graphene-based inks with predictable electrical characteristics have made great strides. Chitosan polymers generated from biomass are increasingly used as binders in silver nanowire inks, which provide great conductivity and flexibility. Excellent fabric adherence has been made possible by interface engineering techniques such as UV-curable nano-silver inks and thermoplastic polyurethane carbon nanotube inks. Overall, these developments have increased the functionality of textile-based printed electronics. The novel conductive inks demonstrate notable increases in mechanical resilience, wash durability, and reliable performance during fabric deformation in e-textile sensors, circuits, and devices. Additionally, bio-derived polymers have improved the biocompatibility of carbon nanotube inks, allowing for safer integration with wearable devices.

To properly satisfy the intricate requirements of e-textile systems, however, further work needs to be done. Further research is required to discover methods to reduce cracking and increase the working lifetimes of printed conductive traces. Investigation into scalable fabrication methods and environmentally friendly ink compounds for industrial textile manufacture is still needed. Additionally, there is much room for creativity in the customization of conductive inks to various natural and synthetic fabric kinds. Conclusively, conductive inks are a fascinating field that is advancing e-textile technologies. Even though substantial advancements have been made, the future potential of printed intelligent textiles will only be realized through interdisciplinary cooperation involving materials science, textile engineering, and chemistry. The most recent developments in conductive ink offer a solid foundation, but moving forward, it is important to continue focusing on e-textile electronics’ resilience, environmental impact, and accessibility.