How Can Smart, Conductive Textiles Be Used?

How Can Smart, Conductive Textiles Be Used?

Recent advances in intelligent textiles, device miniaturization and the advent of the Internet of Things (IoT) have led to an increase in demand for innovative means of incorporating electrical features into textile-based materials such as conductive fibers, fabrics, yarns, and even finished goods.

Study: Progress in Flexible Electronic Textile for Heating Application: A Critical Review. 

A recent study presented in a special 'Nanotechnologies in Textiles' issue of the Materials journal offered useful insights into this exciting, ever-evolving area and detailed some of the possibilities, balances, and challenges offered via the current wave of smart textiles.

Most notably, the study looked at the assembly processes involved in smart textiles' manufacture, investigating the need to strike a careful balance between product ergonomics, versatility, integration potential, heating properties and energy efficiency. It also sheds considerable light on the importance of conductive fibers to these devices.

 (a) Polymeric, (b) metallic, and (c) carbon-based electroactive materials for e-textiles. (d) Electrical conductivity vs. Young's modulus of different electroactive fibers based on CNTs (blue diamonds), carbon fibers (gray stars), ICPs (red circles), blends of conjugated and insulating polymers (orange/white circles), graphene (yellow triangles), nanocomposites of CB (blue/white diamonds), CNTs or graphene embedded in an insulating polymer matrix and (green/white circles) coatings of textile fibers with ICPs, CNTs, or graphene. Image Credit: Repon, Md. R et al., Materials

Textiles offer excellent potential for flexible electronic devices. These typically offer exceptional deformability, robust moisture resistance, and a general soft feel and level of comfort that makes them appealing options for consumers looking for comfortable, aesthetically pleasing wearable technologies.

The key concept behind electronic textiles (e-textiles) is that this type of device is able to provide useful information to facilitate an appropriate response.

For example, an e-textile could be able to sense and respond to specific thermal, mechanical, chemical, electrical, magnetic, or optical stimuli and prompt some sort of action in response, potentially heating a garment or adjusting its properties.

Existing types of smart textiles are typically classified into three generations. The first generation—passive e-textiles—features sensors able to track or stimulate environmental changes; the second generation—active e-textiles—features textiles containing sensors and actuators able to detect and actuate a part of their environment. The third generation—sophisticated e-textiles—can feel and appropriately respond to external stimuli.

Central to the usefulness and applicability of all these generations of smart textiles is an array of conductive fibers, yarns, fabrics, and textiles. These electrically conductive threads are often designed to incorporate specifically conductive materials such as metal wires, metallic yarns, or even conductive polymers.

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 (A). Electrically conductive coatings for fibre-based e-textiles; (B). (a) Silver nanowire coated knitted wool fabrics for wearable electronic applications, SEM images of conductive knitting fabric after coating and washing with distilled water under (b) 60x and (c) 1000x magnifications. Image Credit: Repon, Md. R et al., Materials

The properties of these versatile conductive fibers essentially provide the foundation of smart textiles' functionality. Using advanced manufacturing methods, it has been possible to produce conductive fibers in staple or filament lengths, integrating these with conventional fibers and adding functionality to wearables and garments without impacting flexibility or wearer comfort.

Typical production methods have thus far included electroless plating, sputtering, evaporative deposition, and conductive polymer coating. New developments are ongoing, however, particularly looking at the development of conductive textiles by printing patterns on fabrics with conductive ink.

The authors of the research anticipate a boom in the production of smart textiles and their applications in the coming years, driven in part by ongoing advances in the fields of textiles, electronics, materials, polymers, and information technology.

The potential application areas for conductive materials are vast, including advanced wearable health and fitness monitoring devices, smart protective clothing able to intelligently respond to external risks such as heat or moisture, and intelligent fashion able to help dynamically manage user comfort.

(A). I–V curve and (b) Joule heating performances of silicone-coated M-textile. (c) Time–temperature curve at a at a constant voltage of 3 V for silicone-coated M-textile. (d) Temperature stability of the silicone-coated M-textile in heating/cooling cycles; (B). SEM images of CBPFs and GCBPFs ((a,b) CNC-coated bamboo pulp fabric; (c,d) G/CNC-coated fabric with lowest thermal conductivity; (e,f) G/CNC-coated fabric with highest thermal conductivity). Image Credit: Repon, Md. R et al., Materials

From heat pads used for pain management through to the reduction of inflammation in joint injuries, the application of heated materials and textiles has a long history of use in therapeutic applications. To this end, one significant development highlighted by the authors lies in the area of carbon-based heating textiles and textile-based heaters, which employ Joule's law to generate heat by passing an electrical current through a material.

Conductive, carbon-based materials have long been regarded as a useful source of heat generation, and work is ongoing into the development of conductive heating textiles.

Conductive materials based on carbon are a good source of heat generation, with previous studies highlighting the potential of carbon nanotubes, graphene, and graphite powder being attached to fiber surfaces to make these conductive.

While there is certainly work to be done and additional studies to be undertaken—not least in ensuring that smart textiles maintain their conductive properties after repeated use, washing, and exposure to harsh environmental conditions—the field of smart textiles based on conductive fibers shows great promise.

It is only a matter of time before these innovative, flexible, and highly beneficial materials are entered into large-scale production and available on a commercial basis. 

Sources

Repon, Md. R., and Daiva Mikučionienė. 2021. "Progress in Flexible Electronic Textile for Heating Application: A Critical Review" Materials 14, no. 21: 6540. https://www.mdpi.com/1996-1944/14/21/6540

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Integration of Conductive Materials with Textile Structures, an Overview

Smart materials are incorporated into the textile structure by different technologies; embroidering [90], knitting [91], weaving [92], spinning [93], braiding [94], coating [66], printing [84], plating [95] and chemicals that provide specific features such as controlled hydrophobic behavior [4]. The techniques of integrating a conductive material in/onto a textile structure can be categorized based on the form of the starting conductive material they use. The starting conductive materials can be conductive compounds, fibers, yarns, or sheets. The integrating techniques of these starting conductive materials are therefore different, we will present each of them in next sections.

4.1. Integration of Conductive Compounds

Conductive polymers and inks can be incorporated on textile materials by an in-situ polymerization of monomers on the textile substrate or by applying the conductive polymers and inks onto a textile substrate surface. In general, to produce a required e-textiles different approaches can be taken, such as adding the monomer, polymer or ink into a polymer solution during fiber spinning, during the coating/dyeing of textile substrates (fibers, yarns, fabrics) and/or in a printing stage on textile fabrics and garments.

In electrospinning, smart textiles can be also produced by adding sensor materials to a polymer spinning solution in nanotechnology level and microencapsulation and/or electrospinning technology. A process to encapsulate tiny particles or droplets into wall materials is quickly becoming a well-used technology for use in smart textiles [38]. Research is going into modifying fiber surfaces, such as grafting materials onto fibers to create multi-functional, responsive, and adaptive fibers, in order to tailor a hybrid nanolayer of polymer film that will afford several functions and properties through nanotechnology [96]. Printing is a common method to deposit a conductive layer on flexible fabrics and or garments fabric. Direct-write printing is defined as an additive manufacturing method in which the deposited patterns directly follow a pre-designed layout without utilizing masks or subsequent etching processes. Direct-write printing can deposit and pattern different thin film materials necessary for the fabrication of components and systems such as those found in electronic devices, sensors, and other systems. In the development of conductive textiles via printing, the conductive compound can be deposited or transferred to the textile substrate such as in screen and transfer printing, respectively. Inkjet printing and 3D printing can be potentially used to inject conductive materials over the surface of the textile fabrics layer by layer using a nozzle.

4.1.1. Fiber Spinning

Conductive components can be integrated into the textile structure at the fiber spinning stage by adding a conductive component into the polymer solution which are then extruded together to produce conductive fibers or filament. In [97], a conductive PANI fiber was reported using modified carbon black materials as conductive fillers via the wet-spinning process. The conductivity and tensile strength of the fiber were improved after annealing. Liu et al. developed electrically conductive composite fiber from a blend of PEDOT:PSS and PANI using a conventional wet-spinning process having a diameter of 30–60 µm. They reported that the electrical conductivity of the composite fiber increased as the content of PEDOT:PSS increased, where the highest conductivity was 5.0 S/cm when the PEDOT–PSS content was 1.83 wt % [98]. The PEDOT:PSS content does not show any relation to fiber mechanical performance, but it did cause an increase in surface roughness. Radzuan et al. also reported a PPy reinforced carbon fiber developed by the melt-spinning process [99]. The electrical conductivity of the fiber was 0.56 to 3.66 S/cm based on the die configurations. Zheng et al. used the wet-spinning method to fabricate hybrid microfibers composed of hyaluronic acid and multi-walled carbon nanotubes. The obtained hybrid microfibers presented excellent tensile properties with Young’s modulus of 9.04 ± 1.13 GPa and tensile strength of 130.25 ± 10.78 MPa, and excellent flexibility and stability [93]. Åkerfeldt et al. also used the melt spinning technique to produce a fully textile piezoelectric strain sensor, consisting of bi-component fiber yarns of β-crystalline poly(vinylidene fluoride) sheath and conductive high-density polyethylene/carbon black core as insertions in a woven textile, with conductive PEDOT:PSS coatings developed for textile applications [100]. The absence of the binder leads to one order of magnitude less in the surface resistivity, 12.3 Ω/square. However, the surface resistivity increases more upon abrasion when compared against fabric coated with binder added to the solution. Schematic illustration of the wet and melt spinning are shown in .

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4.1.3. Plating

In this overview, plating is a process of adding a layer of metal components on the surface of textile materials. Before plating, the substrate must be cleaned to remove impurities, which offer assistance for effective attachment of the metal particles to the surface of the substrate [105]. There are different ways of plating, mostly categorized as electroplating or electroless plating.

In electroplating, metals are plated on a surface of the conductive fabric by using electric current. A clean substrate can be immersed in a solution of metal plating particles and an electric current is applied to make the metals deposited to the surface. Hence, this way works only for conductive surfaces as otherwise the current cannot flow. The conventional textiles are not electrically conductive, therefore, it is not possible to use the electroplating, unless the material is prior conductive. However, this way can be used to improve the conductivity and stability of conductive textiles using metals particles. As an example of electroplated textiles, a carbon fiber electroplated with nickel for thermoelectric energy harvesting application was explored in [106]. A schematic representation of copper electroplating to a prior conductive cotton fabric is shown in a.

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Electroless plating is a chemical process to create metal coatings on various textile materials by an autocatalytic reaction, a chemical reduction of metal cations in a liquid bath. Electroless plating depends on chemical reaction to coat the metals on the surface of a material rather than using electric current. In this way, the plating is first performed by cleaning unnecessary components and impurities with chemical cleansers that are able to remove oils and other corrosive elements from the textile, then dip the substrate into an aqueous solution and add anti-oxidation chemicals. The schematic representation of electroless plating is shown in b.

Among the ways of plating, the electroless plating is more convenient for the traditional textile fabrics as it gives a high friction and corrosion resistance to the resulting conductive textiles, and can be used for non-conductive textile. Because of these, a lot of electroless plated conductive textiles have been developed and reported. For instance, Kumar and Thilagavathi developed a copper plated polyester fabric by using this technique. They reported that the plated polyester fabric provides good electrical conductivity with 300 kΩ/sq surface resistance [107]. Ma et al. used electroless silver plating to develop a conductive cotton/spandex blended fabric having robust electrical conductivity, 15.7 S/m [108]. The resultant fabric has high flexibility and stretchability due to the presence of spandex which could make it suitable as a strain sensor while obtaining an anti-bacterial property due to the presence of the silver. Therefore, this silver electroless plated textile could have a good potential prospect for wearable textiles. In addition, Root et al. reported copper electroless plated cellulose-based woven lyocell fabrics [109]. The resultant fabric was subjected to cyclic tensile tests; the resistance of the coated fabric (19 × 1.5 cm2) dropped from 13.2 to 3.7 Ω at 2.2% elongation. This work could attribute to a better understanding of conductive copper coating on textiles and their applicability as strain sensors. The schematic representation of the copper deposition and the electrical response to stretching of the fabric are shown in .

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4.1.4. Screen Printing

As a feasible practical solution, screen printing is one of the most efficient and cost-effective methods of creating conductive patterns on different textile substrates. The screen-printing process consists of printing a viscous conductive paste through a patterned stencil followed by a curing process depending on the substrate and the property of the conductive compound used. It is among the highly recommended methods of printing since it can simplify the fabrication [53]. Therefore, screen printing is the widely used printing techniques [110] to realize textile electronics as one can easily deposit a pattern of conductive paste onto fabric to form a flexible strong and suitably thick functional layer after curing. The printing technique seems to be equal with dip-coating, but actually it is different. Printing is a structured application of the conductive components on a selected or localized area whereas, dip-coating is unstructured application of conductive materials to the textile material similar to the conventional dyeing of textiles.

Screen printing has powerful potential for manufacturing wearable electronics [111]. This technique can be used to apply conductive polymers and metallic and electrolyte inks onto textile substrates. For instance, screen-printed silver ink on cotton and polyester for ECG electrode [112], dispersion of carbon nanotubes on cotton and polyester fabrics [113], and PEDOT:PSS-based conductive polymer on cotton fabric [114] have been reported.

A lot of research effort is put into obtaining textile electronic components with smaller dimensions and with improved performance. It has been reported that electronic textiles like antennas have been screen printed on a polyester fabric and also transmission lines for RF and microwave systems have been screen printed on cotton [115]. Roshni et al. used the technique to develop an E-shaped microstrip patch antenna designed on polyester fabric for WiMAX applications [53]. Since the fabricated antenna was thin, flexible and water resistant, it can be easily integrated into any textile structure and garments which like this are able to sense and communicate data in a non-intrusive way. Liu et al. reported screen printed dye-sensitized solar cells (DSSCs) on woven polyester/cotton and Kapton fabrics for wearable energy harvesting applications. The screen-printed DSSCs on Kapton and polyester/cotton fabrics gave a photovoltaic efficiency of 7.03% and 2.78%, respectively [56]. Tseghai et al. exploited the screen printing of PEDOT:PSS- polydimethylsiloxane to develop a conductive cotton fabric [84]. A wide range of sheet resistance, i.e., 24.8 to 90.8 kΩ/sq was achieved by varying the proportion of the conductive polymer and elastomer. The schematic illustration of a typical flat screen printing is shown in together with sheet resistance results.

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4.1.5. Spray-Coating

In spray-coating, a spray of conductive particles or droplets are deposited to a textile substrate. A screen mesh or frame can be used to apply the conductive components at required locations. This technique has also extensively investigated in textile applications. For instance, Li et al. used the spray-coating to develop a textile based organic solar cells [116]. A power conversion efficiency of 0.4% was achieved. Arumugam et al. [117] also spray-coated silver single nanowire, zinc oxide nanoparticle, poly (3-hexylthiophene): indene-C60 bisadduct and PEDOT:PSS layer-by-layer on a woven polyester/cotton substrate for a solar cells as shown in . A power conversion efficiency of 0.1% was achieved.

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4.1.6. Transfer Printing

In transfer printing, the required design is first printed on a non-textile substrate called print master and then transferred by a separate process to a textile fabric or garment upon the application of heat and pressure. This route would be chosen if direct printing on the fabric is not suitable. In most cases, such difficulties may arise from the rough surface of the textile fabric or the migration of the conductive component with solvent to undesired parts of the fabric due to the wicking effect. The particles can be transferred from the pre-printed master to the textile by sublimation, melt, film release and wet transfer.

The sublimation transfer is suitable for volatile compounds that can be preferentially adsorbed in a vapor phase by the textile material from a print master during heating. Though this method is commercially the most important of the transfer-printing methods, it is not well employed for the development of conductive textiles as volatile conductive compounds able to sublime during heating are not commercialized yet. The melt transfer can be used to print designs to a fabric with compounds that are able to melt on to the fabric in contact with the print master. This method is also not convenient for conductive polymers as they do not have a melting point. Metal conductive inks could be possible but commercial textiles could discompose before the melting point of the metal particles is reached. In the wet transfer, the design is transferred from print master to a moistened textile under a carefully controlled contact pressure. The conductive particles could then transfer by diffusion through the aqueous medium. However, the method is not used to any significant extent at the present time. The film release transfer printing is a little bit similar to melt transfer except the design is held in an ink layer which is transferred to the textile from a release paper using heat and pressure. Adhesion forces stronger than between the conductive film and the paper in the print master are developed between the conductive particles and the textile substrate. Therefore, this method can be potentially used to develop conductive textiles as the heating weakens the adhesion between the conductive film and the paper but not to melt the conductive components in the film.

The film release transfer printing is already commonly used to develop conductive textiles. For instance, Maheshwari et al. used the film release transfer printing of silver nanowire conductive ink to textile fabric surface e-textile [118]. The resultant sheet resistance of the conductive fabric was small, i.e., 3 Ω/sq, and had a light weight and more mechanical flexibility than other conductive fabrics. Shin et al. also used transfer printing to develop a textile-based flexible circuit for a wristwatch that is shown in [119]. The transfer-printed textile circuit showed no change of resistance after folding while its equivalent screen printed circuit changed from 0.73 Ω/cm to 12.85 Ω/cm. Therefore, this is a promising integrating technique for any conductive particle in the form of a film to produce flexible and lightweight conductive textiles for different wearable applications.

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4.1.7. Inkjet Printing

Inkjet printing is a widely used direct-write deposition tool that has rapidly migrated to electronics fabrication in recent years. It is a key printing technique that has not been widely applied to wearable textiles fabrication. In inkjet printing, images and structures are built up in a droplet-by-droplet fashion. The user may make a change to the jetting parameters or to the ink. The technique of inkjet printing structures can be an advantageous manufacturing technique as the functional component can be created within minutes of finalizing the design; the finish is aesthetic and has excellent resolution; it requires minimal material consumption and as no mask is required there is the flexibility to change the design regularly. The process is an additive process that does not require environmentally harmful etching chemicals while minimizing the amount of waste produced and has a high degree of reproducibility as the droplet production allows a user to treat the droplets as building blocks. It is possible to print the ink directly on to the fabric, but Chauraya et al., 2013 argued that the pattern would dissipate into the textile and cannot produce a continuous conducting track without many layers being printed due to the high solvent content (~85%) of the inks required to ensure inkjet printability [115]. Inkjet printing requires the use of a special liquid, usually referred to as ink, which contains the smallest possible electrically conductive particles (their dimensions are usually counted in tens of nanometers at the most). For the stability of such a suspension in time, each of the conductive particles (mostly silver or gold) is covered with a protective organic layer. Carbon nanotube and graphene inks are also used but typically have lower conductivities than metallic inks [120].

Al-naiemy et al. used the inkjet printing technique to develop a microstrip antenna based on nano-silver inkjet material [121]. The developed antenna operated more efficiently than its identical antenna made by screen printing at 2.44 GHz. This indicates that the integration of microstrip antennas, electronic circuits, and sensors to the panels of photovoltaic cells using inkjet printing is considered as a successful and promising design approach for the future. Inkjet printing of sol-gel derived tungsten inks on glass and transparent conductive tungsten oxide and functionality of these transparent WO3 layers were successfully demonstrated in an electrochromic device [122]. He et al. reported fully printed humidity sensors from graphene oxide and few-layered black phosphorus flakes dispersion printed silver nanoparticle electrodes via inkjet printing [123]. The sensor can give an electrical response from 11% to 97% relative humidity. In addition, the capacitance sensitivity was also high in both the graphene oxide (4.45 × 104 times) and the black phosphorus (5.08 × 103 times) sensor at 10 Hz operation frequency. Weremczuk et al. also used this technique to produce a textile-based humidity sensor that has satisfying metrological parameters [24]. This work demonstrated a prospective opportunity of integration with smart wearable electronics used for making medical applications. The photographic image of the humidity sensor and some results are shown in .

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Fabrication of Conductive Fabrics Based on SWCNTs, MWCNTs and Graphene and Their Applications: A Review

Conductive fabrics are usually made from various substrates, such as cotton [ 90 ], polyester [ 91 ], wool [ 92 ], and nylon [ 93 ], using numerous techniques, such as embroidery [ 94 ], knitting [ 95 ], spinning [ 96 ], coating [ 90 ], printing [ 97 ], dipping and drying, drop casting [ 98 ], and others. To make fabrics electrically conductive, there are usually two approaches: one approach is to incorporate conductive fillers, such as metal nanoparticles and carbon-based materials [ 99 ], graphene and carbon nanotubes, into the fabric. The second approach is to coat the fabric with a conductive polymer, such as PEDOT:PSS, which contains little or no metal. This review focuses on the fabrication of conductive fabrics from carbon-based materials.

In another study, SWCNT ink was printed on a stretchable substrate using inkjet printing technique [ 103 ]. It was found that the sheet resistance of the conductive substrate depended on the number of coating layers and decreased as the number of layers increased. The minimum sheet resistance was 19.08 Ω/□ for the five-layer coating, and this value changed slightly after the sample was stretched. Zhang et al. [ 104 ] fabricated conductive cotton nylon with SWCNTs by immersion drying and then modified the fabric by plasma. The results showed that the sheet resistance of the modified fabric 2.0 k Ω/□ was lower than the sheet resistance of the unmodified fabric 4.9 k Ω/□, which was attributed to the increase in the surface roughness of the fabric. SWCNTs dispersed in dodecylbenzene sulfonic acid, sodium salt, were applied to polyester fabric by the coating dry cure method [ 105 ]. Before the coating process, the fabric was treated with plasma using different working gases and different treatment times. The results showed that this treatment led to an improvement in the antistatic properties of the polyester fabric. It was found that the antistatic property increased with increasing plasma treatment and then decreased. The effect of SWCNTs as absorbers of UV light for cotton fabrics was investigated in the study published by Mahmoudifard and Safi [ 106 ] and compared with ZnO andabsorbers. It was found that SWCNTs absorbed UV light with a high UPF value compared to ZnO and TiO2 absorbers. In another interesting study [ 107 ], a piezoresistive stretchable sensor based on SWCNTs and fabrics was fabricated, the joint movements of children were measured and compared with a rectangular sensor. It was found that the stretchable sensor had the same effect as the rectangular sensor with electrical resistance in the range of 280 Ω and 290 Ω. A flexible and stable supercapacitor w