Conductive Textiles for Signal Sensing and Technical Applications

Abstract

Conductive textiles have found notable applications as electrodes and sensors capable of detecting biosignals like the electrocardiogram (ECG), electrogastrogram (EGG), electroencephalogram (EEG), and electromyogram (EMG), etc; other applications include electromagnetic shielding, supercapacitors, and soft robotics. There are several classes of materials that impart conductivity, including polymers, metals, and non-metals. The most significant materials are Polypyrrole (PPy), Polyaniline (PANI), Poly(3,4-ethylenedioxythiophene) (PEDOT), carbon, and metallic nanoparticles. The processes of making conductive textiles include various deposition methods, polymerization, coating, and printing. The parameters, such as conductivity and electromagnetic shielding, are prerequisites that set the benchmark for the performance of conductive textile materials. This review paper focuses on the raw materials that are used for conductive textiles, various approaches that impart conductivity, the fabrication of conductive materials, testing methods of electrical parameters, and key technical applications, challenges, and future potential.

1. Introduction

Textiles with functional properties such as antimicrobial efficacy, water repellency, and fire retardancy have been known for decades. Similarly, when conductive properties are imparted to textiles, their applications go beyond many known traditional uses. Conductive textiles can be used to devise electrodes and sensors to capture signals, shield against electromagnetic waves, and harvest energy [1]. The earliest known use of wearable conductive textiles dates back to 1883 when conductive headbands were worn by the “La Farandole” ballet dancers [2]. Since then, their application areas have gradually expanded to different domains thanks to the advances in material science and engineering and information technology [3]. Conductive textiles have been recognized profoundly during the last two decades due to some outstanding characteristics, such as their flexibility, lightweight structure, wide range of conductivity, adaptability with the human body, adequate mechanical and chemical durability, and expanded life cycle [4,5].

An increasing number of research studies reported in the scientific literature illustrate the significant interest in conductive textiles [6]. Several books have been published reviewing conductive materials and the recent applications of conductive textiles [7,8]. Still, a combined review on conductive materials and fabrication processes of conductive textiles remains as a literature gap. The aim of this review is to provide a comprehensive understanding of the key conductive materials and pathways for developing and testing conductive textiles. Some important technical applications of conductive textiles, such as electromagnetic shielding, supercapacitors, soft robotics, and e-textiles are overviewed.

2. Materials Used to Impart Conductivity on Textiles

Textiles can be given conductive features by using three types of materials: (a) conductive polymers, (b) conductive metals, and (c) conductive non-metals. These are explored in detail in the following subsections.

2.1. Conductive Polymers

Electroconductive polymers are organic polymers that retain conductivity over a wide range of magnitudes based on the doping mechanism and bond-conjugated system in their polymeric structures. These polymers combine the beneficial characteristics of typical polymers and metals [9]. The most used conductive polymers are Polypyrrole (PPy), Polyaniline (PANI), and Poly(3,4-ethylenedioxythiophene) (PEDOT), and their chemical structures are given in Figure 1 with descriptions afterward.

2.1.1. Polypyrrole (PPy)

PPy is one of the most extensively studied conducting polymers for many commercial applications due to their excellent thermal stability, good electrical conductivity, relative ease of synthesis, and environmental stability [10,13]. Three different methods of polymerization of pyrrole are usually followed to obtain PP; These methods are: (i) chemical polymerization in solution, (ii) chemical vapor deposition (CVD), and (iii) electrochemical polymerization [14].

Electroconductive PPy can be prepared in various solutions such as water, diethyl ether, and acetonitrile (ACN). PPy powder prepared in H₂O exhibits the largest capacitance value, approximately 355 Fg⁻¹ [15]. The preparation of conducting membranes by chemical polymerization of PPy in the pores of the macroporous membrane in a similar way was also reported. The conductivities of the order of 10⁻³ Scm⁻¹ and 10⁻² Scm⁻¹ were obtained, indicating that thinner composites with a more homogenous distribution of PPy throughout the sample performed better compared to the thicker composites [16]. Chemical polymerization of PPy on the surface of a porous graphite fiber matrix was done to prepare composite electrodes for supercapacitors. A specific capacitance of about 400 Fg⁻¹ and a coulombic efficiency of 96–99% were obtained in the presence of ferric nitrate (Fe(NO₃)₃.H₂O), along with indicating that the dipping method is suitable for the modification of carbon-based porous graphite fibers to improve the performance of electrodes used in supercapacitors [17].

Single-walled carbon nanotube (SWCNT)-PPy nanocomposite electrodes were fabricated to improve the specific capacitance of the supercapacitor. A high specific capacitance was reported compared to the pure PPy and the SWCNTs. The SWCNT-PPy nanocomposite electrodes show a maximum specific capacitance of 265 Fg⁻¹ [18]. Similarly, to improve the conductivity of the PPy compared to its pure form, an SWCNT-PPy was prepared by in situ chemical polymerization [19]. PPy-coated multi-walled carbon nanotubes (MWCNTs) prepared by the in situ chemical polymerization of pyrrole showed improved conductivity compared to pure PPy. The room temperature electrical conductivity of the MWCNT-PPy nanotubes synthesized by the chemical method using FeCl₃ was measured to be 1.50–2.40 Scm⁻¹, which is higher than that of PPy and neat MWCNT. The conductivity decreases with the increase of the feeding mass ratio of PPy to MWCNTs [20].

2.1.2. Polyaniline (PANI)

PANI has become an influential resource for conductive textiles because of its facile manufacturing techniques, biocompatibility [21], porous nature [22], thermal stability up to 420 °C [23], catalytic behavior [24], distinct adaptation methods for acquiring diversified behavior, various colorful forms and their interconversion [25], and its transparent medium [26]. Although the application of PANI was initially for use as writing ink, it was first acknowledged by Marcel Jozefowicz with his research group (researchers of the School of Industrial Physics and Chemistry of the City of Paris) as an electroconductive material during the late 1960s [7]. The polymer molecules of PANI are incorporated with poly-conjugated chains derived from reversible phenyl rings and nitrogen-based groups. The profound delocalization, polarization, and one-dimensional characteristics of $\mathsf{\pi }$-electron clouds of poly-conjugated networks have led to the outstanding structural, optical, and electroconductive dispositions of PANI.

Numerous versatile polymerization techniques, such as oxidative polymerization, direct and inverse emulsion polymerization, interfacial polymerization, solution polymerization, seeding polymerization, metathesis polymerization, and self-assembling polymerization, have been adapted to synthesize PANI [27]. Customarily, three forms of PANI, exhibited in Figure 2, can prevail, thereby relying on their extent of oxidation and degree of protonation. These are:

(a).

Leucoemeraldine (C₆H₄NH)_n—100% reduction level

(b).

Emeraldine ([C₆H₄NH]₂[C₆H₄N]₂)_n—50% oxidation, 50% reduction

(c).

Pernigraniline (C₆H₄N)_n—100% oxidation level

Figure 2. Chemical structure of different forms of PANI, adapted from [28] following Creative Common Licence.

Figure 2. Chemical structure of different forms of PANI, adapted from [28] following Creative Common Licence.

Only the Emeraldine form of PANI can be applied directly to the textiles to impart conductivity among these three categories; however, the pristine emeraldine retains a lower conductivity level and limited processability [25].

Though the potential growth of PANI-based conductive textiles has escalated drastically in recent years, the inherent toxic nature and lower conductivity of PANI than metallic particles, even compared to other intrinsic conductive polymers (PEDOT, PPy) are still the major handicaps for commercializing PANI-based conductive materials. Notable research projects related to ascertaining the environmentally friendly and cost-effective synthesis process of PANI-based conductive textiles have been reported [29,30].

2.1.3. Poly(3,4-ethylenedioxythiophene) (PEDOT)

PEDOT demonstrates an intrinsic electroconductive polymer that incorporates two linked groups of benzene rings where two carbon atoms of each are substituted by oxygen atoms and a sulfur-based pentagonal thiophene ring. PEDOT can be easily synthesized from a 3,4-ethylenedioxythiophene (EDOT) monomer through oxidative [31] and electrochemical [32] vapor phase polymerization (VPP) techniques [33], but it cannot be executed commercially due to its lower dispersion in water. As a result, a template polymer poly (styrene sulfonate) (PSS) is utilized during the synthesis of PEDOT, which yields a water-dispersible and transparent film-like polymer that can be conveniently processable [34].

Uniform dispersion in water, convenient methods to adapt electrical properties, high flexibility, excellent transparency (up to 95%), stability against humid air and high temperature are some of the features that allow PEDOT:PSS to add a new dimension over the other intrinsic polymers in terms of diversified practical applications of conductive textiles, such as solar cells [35], electricity storage devices [36], electrochromic devices [37], electrochemical transistors [38], OLED [39], patch antenna [40], supercapacitors [41], actuators [42], and strain sensors [43]. Several variants of PEDOT:PSS dispersions are available according to their molecular weight under the trade name of Clevios and Orgacon given by Heraeus Epurio and Agfa Gevaert, respectively; however, the corrosiveness and unfavorable impacts of PSS cause impediments to the expedition of conductive textiles approach towards the green industry. Studies on the eco-friendly application of conductive PEDOT films on textiles have been reported. Lock et al. experimented with a customized vacuum chamber for the eco-friendly deposition of PEDOT films on the substrates, thereby resulting in conductive substrates that exhibit a noteworthy conductivity of 105 Scm⁻¹, including an 84% transmittance of visible light [44]. Brooke et al. applied the VPP technique to synthesize eco-friendly PEDOT: Tosylate films coated with conductive electrodes [45]. Later, Vacuum vapor phase polymerization (VVPP) was acquainted with feasible scale-up production of PEDOT-coated conductive textiles with significant conductivity (1485 Scm⁻¹) [46].

Though being one of the most promising conductive polymers, the employment of PEDOT in conductive textiles is limited to research projects and narrow fields of practical application due to its relatively lower conductivity for the proper optimization of PEDOT in wearable electronics; unstable conductivity for any change of temperature, humidity, bending and washing cycles [47]; and maintaining the precise process control parameters during the in situ polymerization of EDOT monomer [7].

Notable works are ongoing to deal with these challenges of PEDOT [48,49]. Until now, the commercial aspects of PEDOT have been fully established in the applications of capacitors, printed wiring boards, packaging films, photographic films, electronic films, and electroluminescent lamps [7].

2.2. Conductive Metals

Metals, such as copper, silver, and steel, are the most electroconductive materials, and are extensively used in smart textile applications such as copper yarns and silver-coated fibers [1,50]. Silver (Ag), gold (Au), copper (Cu), titanium dioxide (TiO₂), zinc oxide (ZnO), nanolayered clay, and other organic and inorganic nanoparticles and their nanocomposites are applied to textile materials to pass on different multifunctional properties [51]. The conductivity of metallic-nanoparticle-coated threads is low because their small size leads to many interparticle junctions [52].

A few metals and metal-derived oxides used in textile finishing are discussed here.

2.2.1. Silver Nanoparticles (AgNPs)

AgNPs are known to be antibacterial, and their therapeutic properties have been proven against a broad range of microorganisms that can cause diseases. Ag can be synthesized through various methods ranging from simple chemical reduction processes to photochemical, biochemical, sonochemical, and Tollens’ reagent methods [51]. The deposition of nanowire films around threads can be achieved through dip-coating [52]. Chemically metallized Ag threads are used to produce textiles with antistatic properties or to shield against electromagnetic waves. Ag-coated fibers are used to improve the behavior of smart textiles [50].

2.2.2. Gold Nanoparticles (AuNPs)

AuNPs are widely known for their optical and biological properties. They could be used for highly sensitive diagnostic assays, thermal ablation, radiotherapy enhancement, and drug and gene delivery. They are mainly prepared via the chemical reduction method using gold salts such as hydrogen tetrachloroaurate (HAuCl₄) as a precursor and citrate as a reducing agent [51]. Au nanoparticles chemically synthesized by citrate reduction can be immobilized onto chitosan-treated soybean knitted fabric via the exhaustion method [53]. A gold coating with solution deposition on knitted fabric is highly conductive, stretchable, wearable, and washable [54].

2.2.3. Copper Nanoparticles (CuNPs)

CuNPs have heat transfer properties such as high thermal conductivity, low preparation cost, antibacterial efficiencies, catalytic activity, and electrical and optical properties. They are produced by the chemical reduction method [51]. The surface of the textile fabric can be covered with a layer of densely packed Cu particles after ELD [55]. Cu-coated textiles can show a bactericidal effect [56]. The antibacterial property is important because electromagnetic interference (EMI) shielding fabrics are excellent media for microorganism growth [57].

2.2.4. Metal Oxide Nanoparticles (MONPs)

MONPs are renowned for their environmental remediation and electronic applications. The particles can generate charge carriers through energy absorption. TiO₂ and ZnO are widely used in textile finishing [51]. Nano-iron oxide, nano-copper oxide, nano-cobalt oxide, and nano-manganese oxide in cotton fabrics can be created for high antibacterial efficiency. In addition, UV protection and flame retardation can be developed by in situ preparation of nanometal oxides [58].

2.3. Conductive Non-Metals

Carbon-based materials (CNTs, graphene, carbon fibers, carbon aerogels) play a fundamental role in this type of conductive textiles. These materials are characterized by high electroconductivity (10⁴ Scm⁻¹), superior strength, environmental stability, and a cost-effective manufacturing process that exposes these as promising materials for wearable electronics [59]. Many research studies have demonstrated the remarkable features of these non-metallic conductive textiles [60,61]. In recent years, nanocomposites have been intensified for their mechanical, electrical, and thermal performances, where carbon, such as carbon nanofiber, CNTs, graphene, and graphite, are used as a filler [62]. CNTs and graphene have been given special attention due to their unique features, such as their high mechanical stiffness, tensile strength, and exceptional thermal and electrical conductivity [63,64]. Researchers are continuing to discover new dimensions of carbon nanomaterials. After that, a more versatile material, graphene, was discovered and focused on flexibility, higher electron conductivity, and increasing the scope of applications [65].

2.3.1. Carbon Nanotube (CNTs)

CNTs are unique nanostructures theoretically considered one-dimensional (1D) quantum wires. The basic building block of the CNTs is very long cylindrical single-walled carbon nanotubes (SWCNTs) that are one atom in wall thickness and ten atoms in circumference. CNTs are the seamless cylindrical shape of graphene sheets. Iijima (a Japanese physicist and inventor) first observed the multi-walled carbon nanotubes (MWCNTs), which were remarkable for their unique structure and electrical properties [66]. After two years, Iijima and his team discovered SWCNTs, which were supposed to be an important material for their theoretical experiment [67]. It was predicted in 1992 that a historical breakthrough would happen in 1996 after successfully synthesizing bundles of aligned single-wall carbon nanotubes with very small diameters [68].

CNTs have many exceptional physical properties. CNTs are 100 times more potent than the highest grades of carbon steel, and their higher tensile modulus can almost be stretched five times the original length [69,70]; moreover, CNTs show extraordinary electronic properties such as 10²–10⁶ Scm⁻¹ for SWCNTs and 10³–10⁵ Scm⁻¹ for MWCNTs [71], which perform better than copper when transporting electrons long distances without interruption [72,73]. Consequently, it is considered the ideal nanotube because of its mechanical and electrical characteristics. Given this, many potential applications have been proposed, such as H₂ storage media [74,75], batteries [76], solar cells [77], transistors [78,79], diodes [80,81], and sensors [82,83].

CNTs have a high aspect ratio that makes them suitable for reinforced materials. CNTs can make pure CNT fiber or reinforce polymeric, ceramic, and metallic fibers. CNT fibers have been processed from aqueous dispersions and dispersions in strong acids, drawn from CNT forests, or pulled from a CNT CVD reactor in the form of an aerogel fiber.

CNTs have shown a great range of variety in applications in the last couple of decades of research. However, with the potential of CNTs, it has gained limited success in the marketplace, especially due to issues in processing and scaleup. There are also intrinsic challenges at the nanoscale (such as assembly, the role of interfaces, and contacts), which also affect the applications’ progress. Instead of these challenges, a few applications of nanotubes are available commercially.

2.3.2. Graphene

Graphene is a two-dimensional (2D) monolayer [84] ultra-thin carbon film [85] with a hexagonal structure [86] that has become the center of attraction for researchers due to its exceptional electrical conductivity, as well as its thermal, mechanical, optical, and electrochemical properties [87]; moreover, some extra mentionable intrinsic properties added extra advantages when embedded into microstructures, which is crucial for promoting its potentiality in practical uses [88,89,90]. As a consequence, the development of graphene-based 3D aerogels (GBAs), 2D membranes (GBMs), and 1D fibers (GBFs) have progressed. Three-dimensional GBAs have achieved the current world record as the lightest material, having a density of 0.16 mg cm⁻³ [91]. Two-dimensional GBMs can be produced by CVD or infiltration processes for energy storage and conversion. However, one-dimensional GBFs are more applicable than GBFs and GBMs for their electrical and mechanical properties. GBFs are flexible and fabricable; therefore, they can be used in any form and in multifactional applications. Graphene can be produced in monolayer and multi-layer forms based on the application. The rapid acceptance of graphene as a substance of interest lies in its actual availability in the preparation technique and perhaps mostly because monolayer and multi-layer graphene and graphene oxide (GO) have several versatile properties.

Graphene can be chemically modified to obtain inexpensive materials such as graphene oxide (GO) [92], reduced graphene oxide (rGO) [93], and their derivatives (fGO, frGO and mG) on a large scale. This chemical modification is done to get fibers, films, porous frameworks, or modified or hybridized forms to diversify their function. Graphene-based materials such as GO and rGO have great prospects for use as a material for eco-friendly wearable e-textiles [41,94,95,96,97]. The negative charge in GO helps to generate and facilitate their association with the fibers/fabrics’ functional groups [98]. Thus, it helps with the better fixation of textile fabrics and of acquiring flexible, washable, and durable wearable e-textiles [99].

As graphene was an extraordinary material for future technologies, great emphasis was given to bulk production [84]. Various bulk production methods have been approached, including the ‘Hummers’ method for large-scale graphene synthesis [100]. A chemical [101] or thermal [102] reduction process is performed to procure reduced GO. There are also other methods to produce GO by ion implantation, CVD [103], mechanical exfoliation, chemical exfoliation, electrochemical exfoliation, liquid phase exfoliation, and epitaxial growth on silicon carbide (SiC) [104,105], but many of these are not popular. Graphene can be formed with other materials containing special characteristics to get a unique composite material. Nanocomposites such as polymer, active carbon, metal, metal oxide, and carbon fiber are employed with graphene to get exceptional properties. Polymer nanocomposites are generally fabricated using GO and rGO as fillers. Graphene-based nanocomposites can be used as potential applications for energy storage and conversion [106,107,108], electromagnetic interference shielding [109,110], and sensors [111]. Activated carbon with graphene is advantageous for its wide range of availability and relatively good performance due to its large surface area. Graphene/activated carbon can be used as an electrode (cathode) in Li-ion batteries [112], high-performance supercapacitors [113,114], and sensors. Metals such as Pt, Al, Pd, Co, Si, Mg, Cu, Au, Fe, and Ce have been embodied into graphene as a composite for supercapacitor sensors. Graphene/metal oxide composites are used as capacitive materials for supercapacitors due to their higher density of energy [115].

The potential application of graphene can be for manufacturing high-capacity batteries [116,117], actuators [118,119,120], supercapacitors [121,122], more efficient solar cells [123,124], corrosion prevention [125], circuit boards [126], and medicinal technologies such as the point-of-care detection of diseases [127], in field-effect transistors [85], transparent electrodes [88,128], energy storage materials [129], composites [130], chemical and biosensing [131,132], and many other areas; so many scientists are focusing on the revolutionary nature of graphene and how it could be commercially utilized into everyday life.

3. Fabrication Processes of Conductive Textiles

Fine metal wires can be combined with textile structures during textile processes like spinning, knitting, and weaving, but it may come at the sacrifice of flexibility and the tactile properties of traditional textiles [133]; however, the common alternatives to them are: modifying textile surface by depositing a conductive layer with the help of additional finishing processes such as a deposition [134]; coatings such as dip coating, spray coating, rod coating, or roller coating on the textile substrate (fiber, yarns, fabrics) [135]; and printing textiles with conductive materials [136]. The choice of the process depends on the process parameters and properties such as flexibility, stability, and degree of conductivity of the final products [59]. The common approaches are described in the following subsections.

3.1. Deposition Method

The deposition is an effective way of applying a thin coating on textile substrates. Metals or non-metals and intrinsically conductive polymers (ICPs) can be deposited on a suitable substrate by following three steps: (a) scatter or vapor deposition, (b) employing sol precursor, and (c) sintering the substrate to start a sol-to-gel transformation [137].

3.1.1. Vapor Deposition

Highly pure and high-performing solid materials, such as metal, nanotubes, and ICP, are deposited as thin films of one or several layers [138]. The deposition of thin perovskites is a thin film based on organo-inorganic materials, such as CH₃NH₃PbX₃ (X = I−, Br−, Cl−), which has fascinating optical and electronic properties [139]. A film of solid and liquid through vapor deposition without solvent has been investigated. In vapor deposition, vapor can be condensed by two processes: (a) physical condensation or (b) chemical reaction; furthermore, depending on these two processes, vapor deposition is classified by: (i) physical vapor deposition (PVD) and (ii) chemical vapor deposition (CVD) [140].

• Physical Vapor Deposition (PVD)

PVD was developed to address the capacity to create a layer with continuous electrical conductivity and the ability to create an element with a specific geometry and dimensions, which is not possible in conventional layering technology such as printing, dipping, etc. [141]. PVD is a process of applying a fine coating of conductive materials on the textile substrate by the vaporization process. PVD comprises four steps: (a) production of vapor phase by evaporation of target materials, (b) transportation of the vapor to the substrate, (c) reaction between the metal atoms and the appropriate reactive gas, and (d) condensing vapor on the surface of the substrate [51,142,143]. PVD can be deposited in almost all materials, such as pure metal, metal, and organic material mixtures such as glasses, alloys, compounds, and layer systems [144]. Silva et al. studied the PVD of aluminium on bare Kapton and when it was coated with PVC/PU [145]. The PVC-coated substrate was the best among the samples for making flexible electronics. Depending on the deposition technique, PVD can be classified by evaporation techniques (vacuum thermal evaporation, electron beam evaporation, laser beam evaporation, pulsed electron deposition, arc evaporation, molecular beam epitaxy, ion plating evaporation) [146] and the sputtering technique [147,148,149]. A silver coating was deposited on cotton fabric using PVD (vacuum thermal evaporation) with a Flexicoat 850 coating apparatus to impart conductivity [150]. The same process was used to deposit a nano-coating of Al and Zn on cotton fabric, with Zn coating having a greater EMI shielding performance than Al-coated cotton fabric [151].

Sputtering is a non-thermal physical vapor deposition technique where molecules move from the surface of the material through the knocking of high energy particles. Molecules that emerge from the target material by the shell firing of high energy particles on the material are condensed on the surface of the substrate to deposit a thin film of the target material [152]. Park et al. investigated DC magnetron sputtering of an Ni-Fe/Cu multi-layer (500 nm Ni-Fe/500 nm Cu) thin film to impart conductivity and led to an EMI shielding effect in the range of 0.7 GHz–6 GHz, which is a better EMI shielding effect than a pure Ni-Fe and Cu layer [153]. Sputtering deposition of Cu metal is used to make ECG electrodes for biomonitoring smart wear [154]. Wang et al. reported the very first PVD of amorphous carbon-coated 3D NiCo₂O₄ on carbon cloth to make a binder-free flexible electrode with high dimensional and cyclic stability (535.47 mAhm⁻¹ at 500 mA g⁻¹ for 100 cycles) capacity and electrochemical performances [155]. A better electrochemical performance was found for NiCo₂O₄/NiO/C composites on carbon cloth obtained by magnetron sputtering PVD of NiCo₂O₄ nanowires/NiO nanoflakes with cyclic stability (1051 mAh g⁻¹ for 100 cycles at 100 mA g⁻¹) [156].

2.

Chemical Vapor Deposition (CVD)

CVD refers to the segregation or chemical reactions of gaseous reactants near a heated substrate surface in an activated situation (heat, light, plasma), which will finally form a stable solid product [157]. Its history goes back to 1893 when De Lodyguine (an electrical engineer and inventor) deposited tungsten onto carbon lamp filaments through the reduction of tungsten hexachloride (WCl₆) by hydrogen (H₂). This initiated the pedestal of the industrial utilization of CVD [158]. Typically, the CVD method is applied to deposit conductive polymers on the textile substrate (fiber, yarn, fabric) [159]. The fundamental concept of CVD is a chemical reaction that forms a layer on the substrate from the vapor of the reagents, leaving the by-products in a volatile form.

Researchers experiment with different kinds of materials by using the CVD method to make textile materials conductive. FeCl₃-coated cotton yarns were exposed to pyrrole vapor, thereby producing a coated layer of PPy on cotton yarns [160]. The CVD method is adopted for PPy deposition in various fabrics such as viscose, cupro, lyocell, nylon, PET, etc. [161,162,163].

The CVD process is a recognized technique for making highly conductive polymeric layers on different substrates [164]. To get high conductivity and a uniform polymeric layer on a flexible and rigid substrate, a novel method called Oxidative Chemical Vapor Deposition (OCVD) was implemented [165]. OCVD is a solvent-free technique that results in more homogenous, thin, and highly conductive polymer layers on different substrates. The steps for OCVD of PEDOT include: (i) First, fiber penetration with oxidant (FeCl₃) solution and sub-drying; (ii) then, oxidant-enriched fibers were exposed to EDOT monomer vapor; and last, (iii) PEDOT-coated fiber doping. It is reported that the CVD technique could successfully coat PET fabrics with PEDOT, but PEDOT-coated PET fabrics showed reduced conductivity due to poor fiber contact [166]. PEDOT-coated PET yarn fibers with good electrical and mechanical characteristics were developed, which could be converted into woven structures with good electrical properties. Furthermore, viscose yarn was tested in both the CVD and OCVD processes. Although the OCVD process found a high conductivity level of 14.2 Scm⁻¹, fewer mechanical properties were obtained at 15 wt% oxidant concentration on viscose yarn [167].

3.1.2. Layer-by-Layer Deposition (LbLD)

LbLD is a new and simple method of developing a thin film on the substrate at normal temperature and pressure. Alternate deposition occurs using a poly-ion layer on solid substrates by dipping into an oppositely charged solution and washing between each depositing layer. The alternation of the surface charge results in polyelectrolytic layers on the solid. Charged particles are bound by the electrostatic and Van der Waals bond [168,169,170,171]. The LbLD process has already been applied to impart conductivity to textile materials in many applications [172,173]. Electromagnetic interference shielding of cotton fabric by layer-by-layer (LbL) assembly of positive and negative charge MWCNTs and nickel ferrite (NiFe₂O₄) nanoparticles with a PDMS covering to keep the layer stable was reported [169]. A cationic surfactant is employed to create positively charged MWCNT, and EDTA is used to functionalize NiFe₂O₄, which is controlled by the ionization of both catalysts [169]. The vacuum-assisted spray-LbLD process can deposit MWCNT without any binder, thereby producing a high degree of binding sites and resulting in better sensing capabilities [174]. Poly(ethylenimine)/CNTs and ammonium polyphosphate layer nanocomposite coating were placed LbL on cotton fabrics to generate superior hydrophobic conductive cotton fabric, and PDMS coating imparted an identical energy density throughout the surface [175]. LbL deposited Chitosan–graphene cation and PSS anion layers one after the other on cotton fabric, resulting in an electrical conductivity of 1.67 × 10³ Sm⁻¹ for 10 layers [176]. V₂O₅-nanostructured supercapacitors show low energy density and electrical conductivity. Deposition of graphene layers between V₂O₅-coated MWCNT films prevents congregation and improves the energy density (96 Whkg⁻¹). The graphene layer acts as a barrier and improves the durability and capacitance of the device [177]. An LbLD process featuring a pad-dry-cure method for developing an electrically conductive Ag nanoparticles–carboxymethyl cellulose composite was reported [178].

3.1.3. Electrochemical Deposition

The electrochemical deposition, or the electrodeposition process, is used to deposit (composite/single) thick, stiff coats of conductive materials onto textiles, especially metal particles, with the help of an electric field [179]. In this process, metal ions turn into solid metal and deposit on the cathode surface [180]. Electrochemical deposition has become so popular due to its versatile application for the deposition of different kinds of materials such as metal, metal alloys, composites, nanocoatings [181], ceramic and organo-ceramic materials [182], semiconductors (chalcogenide and oxides) [183], nanocrystalline and nanophase metallic materials [184], graphene-reinforced metal matrices [185], intrinsic conductive polymers (PANI, PPy, PEDOT, PEDOT: PSS) [186], carbon-based materials and their composites (CNTs, graphene, carbon/metal) [187], MnO₂–carbon-based materials [188], metal oxides [189], and so on.

Nickel was electrochemically deposited on linen and nylon fabric to produce wearable radar-visible fabrics. Electrodeposition assembly consisted of an electrolyte (nickel sulphate hexahydrate, nickel chloride, Boric acid) cathode and anode. The fabric was connected to the cathode by a platinum strip, and another platinum strip was used as an anode. Electrochemical deposition was done at 4 V for 15 min [190]. The Box–Behnken technique of electroless plating of nickel is reported [191]; they noticed that the deposition amount depended on the time and current applied, and Ni was electrodeposited on the Cu nanowires network to produce a transparent conductive film [192]. A process of targeted electrochemical deposition to produce microelectromechanical devices by employing a non-conductive mask between the anode and cathode and regulating the electrolyte with a pump was proposed [193]. Electroless plating on stretchable fabric aids in electrochemical deposition by providing conductivity to the fabric. Later, the electrochemical deposition of metal on every fiber with conductive material makes the fabric stable and provides optimum conductivity [194]. A simple one-step electrochemical deposition method to produce a metal-organic framework and PPy composite capacitor electrode with the help of dopamine with better conductivity than a virgin metal-organic framework was presented [195]. MnO₂-deposited capacitors have low electrical conductivity and have been investigated in many ways to increase this. MnO₂ is electrochemically deposited on CNTs to produce pseudocapacitors [196]. The electrochemical deposition of MnO₂ and PPy composite on carbon cloth produced a flexible electrode for supercapacitors with a capacitance of 325 Fg⁻¹ and MnO₂-coated CNTs deposited on flexible graphene nanosheets produced electrodes for supercapacitors with a capacitance of 442.9 F/g [197]. MnO₂ deposited on activated carbon paper improves its capacitance (485.4 F/g) [198]. Kim et al. prepared a flexible photo sensor that deposits ZnO on Ni-Cu-Ni-coated PET fabric and copper deposited on silane molecules, which were polymerized with 2-(methacryloyloxy) ethyl trimethylammonium chloride (METAC)-coated recycled PET nanofibers [199]. Rosa-Ortiz et al. proposed a method of electrochemical deposition with the help of Hydrogen Evolution that improves the speed of copper coating and is directly fused to the wearable fabric, which eliminates the soldering machine [200].

3.1.4. Electroless Deposition

Electroless is an elegant and multipurpose process of conductive metal deposition on any surface. Apart from other deposition processes, electroless is an autocatalytic chemical reaction and is done without electrical energy. It can deposit both thickly and thinly coated films quickly and effectively. It can deposit metals such as silver [201], aluminum, copper [202], nickel, and iron, which are uniformly and smoothly deposited on the surface of the textile substrate [51,203]. The main principle uses a reducing agent in a solution for the chemical reduction of metal ions and deposits them on the substrate’s surface, which imparts electrical conductivity to the textile substrate. The chemical reaction involved in this process is as follows [204]:

S = Sⁿ⁺ + ne⁻

Mⁿ⁺ + ne⁻ = M

Electroless-deposition-produced conductive textiles, such as conductive woven cellulose fabric [205], PET fabric [206], and others, have a wide range of applications. Silver is deposited with the use of ultrasonics to create a shielding-effective PET fabric [207] treatment with silver nitrate and sodium hydroxide at 130 °C for 1 h, which increases the surface activity and eliminates the need for a catalyst that produces ethylene as a reducing agent, while ammonia treatment stabilizes the conductive layer [208]. Stretchable conductors are prepared by depositing silver through electroless plating on polyurethane filaments [209]. PVC is added to thermoplastic urethane during electroless plating, making the silver layer more adhesive and higher in conductivity.

The electroless deposition of Cu imparts a variety of features to the textile substrates. Cu is seeded with Ag and coated on cellulosic fabric to make it conductive for the manufacture of a light-emitting diode [205]. Lin et al. made ultra-stretchable conductors that deposit Cu on polydopamine-coated cotton fabric with a Pd²⁺ catalyst [210]. Guo et al. employed AgNO₃ instead of PdCl₂ to deposit Cu on PET textiles, resulting in a more uniform metal distribution and denser layer [211], and hydrazine monohydrate (80 wt%) treatment at 60 °C improved the interaction between PET and the Cu layer [212]. The addition of 3-mercaptopropyltriethoxysilane to PET textiles during a deposition improves the washing performance [213]. Paquin et al. described a three-step copper deposition on cotton using Pd or Ag as a catalyst [214]. Cotton fabric can also be activated by Ag and Cu nanoparticle deposition before the electroless plating of Cu [215]. Cu has been electroless deposited on cotton fabric using (NH₄)₂PdCl₄ as a catalyst to create a highly conductive fabric [216]. Laser treatment on cotton fabric improves abrasion resistance [217].

3.2. In Situ Polymerization

In recent years, in situ polymerization has become a popular technology for creating conductive nanocomposite because of its varied and advantageous approach to polymerization that directly uses 90 mixtures of monomer and nanoparticles with various additives. Grothe et al. highlighted the differences between in situ polymerization and other surface modification approaches, such as nanoparticle fusing, and the benefits of in situ polymerization over other methods [218]. Due to the intense interaction between two side-by-side polymer chains, most conductive polymers are thermally unstable and not soluble in different solvents. However, in situ polymerization can polymerize in various ways depending on the conductive polymer for different applications, such as in situ chemical polymerization, in situ electrochemical polymerization, in situ vapor phase polymerization, and in situ polymerization in a supercritical fluid [219].

Liu et al. mentioned that in situ polymerization can reduce the impedance of the electrolyte interface and bring interfacial compatibility and stability to solid-state lithium batteries, thereby simplifying the process of polymerization [220]. Highly electrically conductive and thermally stable graphene/polyaniline (GN/PANI) nanocomposite was synthesized by in situ polymerization [221].

In situ polymerization, according to Park et al., aids in the efficient dispersion of carbon SWNTs in polymer composites in the presence of sonication, which was previously challenging due to the non-reactive surface of SWNTs [17]. The resulting nanocomposite is electrically conductive and optically transparent, with improved mechanical characteristics and thermal stability at low concentrations (0.1% of vol). Hong et al. used ferric p-toluenesulfonic acid (FepTS) and FeCl₃ as oxidants to investigate in situ polymerization of PEDOT on nylon 6, PET, and poly (trimethylene terephthalate) (PTT) fabrics and compared different properties of PEDOT/nylon-6, which has the best conductivity of all the nanocomposites; however, PTT/PEDOT is the most stable [222].

3.3. Coating

A textile conductive coating is a material layer that adheres to a textile structure (fiber, yarn, or fabrics). These coatings are used to make the textile material conductive or to add functionality to the textiles. Coating methods are industrial technologies used to adapt the properties of textiles to those required for technical and specialty applications. The variety of methods allows for good adaption to end-use requirements [223].

3.3.1. Dip Coating

Dip coating is a simple and cost-effective process commonly used in various industrial fields for depositing coating material onto any substrate, along with metallic and ceramic polymer film and textile materials. The process could be interpreted as depositing aqueous liquid phase coating solutions on the surface of any substrate [224,225,226]. Conductive materials are typically dissolved in solutions that are immediately deposited on the surface of the substrate, after which, to obtain the dry film, the sedimentary conductive wet coating has to be evaporated. The technique associates submerging the textile material in the solution of the conductive coating materials, ensuring that the textile material is completely penetrated and then withdrawn from the solution materials. Notably, this supposedly easy process of constructing film through dip coating comprises complex chemical and physical multi-variable norms. The thickness and morphology of depositing thin conductive films were determined by various criteria such as dipping time, withdrawal speed, dipping cycles, substrate (textile material) surface, density, viscosity, surface tension, and evaporation of the conductive coating solution [227,228,229]. Various modified dip coating techniques, such as solution dip coating, sol-gel dip coating, multi-layer dip coating, and vacuum-assisted dip coating, are significant to manufacture deposited films on the surface of textile materials.

Solution dip coating is the most straightforward technique of forming a film on textile materials’ surfaces and is typically used in increasing production. Since it is the most commonly used method in textile manufacturing, dip coating is often referred to as starch finishing or sizing. Solution dip is a user-friendly process to operate and can increase the efficiency of production. However, on the opposite, the solution of dipping the uniformity of the coating is weak and the bonding state is relatively poor [229]. Cotton fabric containing high electrical conductivity has been confirmed to have been established by a two-step dipping and coating. The cotton fabric was immersed in an acetone solution and rinsed properly with deionized water after drying in an oven. Then, GO was coated onto the fabric surface by dipping the modified cotton fabric into GO dispersed solution for two cycles. After that, a padding process was conducted to remove the excess material using a pair of rollers and high vacuum pressure [230]. The dipping process is shown in Figure 3.

Sol-gel technology is a relatively novel method for endowing unique properties upon textiles and is affiliated with decreased ecological impacts [231]. Various functional characteristics such as antibacterial function [232], UV radiation protection [233], dye fastness [234], anti-wrinkle finishing [235], super-hydrophobicity [236], and biomolecule immobilization have been introduced by this method. MWCNTs for conductive purposes and methyltrimethoxy silane for hydrophobicity purposes were coated by the sol-gel process on cotton fabric [237]. The process involves Sol preparation, which is a mixture of methanol, distilled water, and MWCNTs of definite concentration and quantity. Following this, the Sol was mixed well by a magnetic mixer. Then, the cotton fabric was added to the Sol. After checking several parameters, the ultimate result was 40 kΩ cm⁻² [237].

To improve the uniformity characteristics or thickness of the single dip coating film, the researchers investigated the multi-layer dip coating. A multi-layer dip coating was studied at a thickness of 90–100 nm and electrical resistance of <400 Ω mm⁻¹ to increase the conductivity of the PEDOT:PSS-coated fibers, and the density and surface tension of the conductive coating solution were varied by altering the mixture ratio, by adding a fluorosurfactant, or by changing the withdrawing speed to achieve a significant, thick PEDOT:PSS layer [238]. Pu et al. organized a two-step dip coating method to establish Ag nanowire networks on the surface of PET materials [239]. The PET substrates were dipped into conductive Ag nanowire solutions and then dried; afterward, the substrate was coordinated vertically to operate the second-step dip coating. The Ag nanowire-coated PET substrates with an order-enhanced analog crisscross structure were prepared. Repeating the multi-layered dip coating method could increase the thickness, improve uniformity, and manufacture the coatings with desired features. Multi-layered dip coating of various coating solutions mainly achieves functional superposition by several deposited films. The fabrication is facile, and it is easy to achieve the multifunctional synergies of each deposited layer. Furthermore, this multi-layer dip coating process benefits broadening the applications of fibrous materials, such as supercapacitors, lithium-ion batteries, and electrode plates.

The vacuum-assisted dip coating method has been introduced to maintain the desired coverage of nanoparticles on the substrate surface with minimal defects, including metallic and ceramic fibers. Liu et al. demonstrated a vacuum-assisted LbL assembly technique to construct electrically conductive substances on textiles to develop multifunctional and flexible textiles with superb EMI shielding performances, super-hydrophobicity, and a highly sensitive humidity response [240]. Cotton fabric was coated with a GO nanosheet via the vacuum filtration deposition method [29]. The woven cotton fabric was treated with sodium hydroxide solution. Vacuum filtration deposition was used to disperse the GO nanosheet aqueous solution into the cotton fabric surface to fabricate GO–cotton fabric. Refluxed graphene and polyvinylidene fluoride (PVDF) were mixed and dissolved in absolute ethanol. This mixture was then forcibly deposited into the cotton fabric via a vacuum filtration deposition process, as shown in Figure 4 [29,241].

The spray coating technique uses a spraying device (found on compressed air, such as with an airbrush or spray gun) to deposit conductive materials on the textile surface by the breeze. Any substrate can be coated by this simple, economic, fast, and scalable process, whether it be round or flat, or flexible or rigid [242]. Recently, it has been published that the spray coating approach that applies PEDOT:PSS conductive materials containing 5 wt% dimethyl sulfoxide (DMSO) made highly conductive and multifunctional PET fabrics. PET fabrics were coated with conductive PEDOT:PSS solutions that contain 5 wt% DMSO via spraying conductive material onto the surface of pre-treated PET fabrics [243].

3.3.2. Rod Coating

The rod coating process is easy and inexpensive to penetrate the conductive material on the textile substrate in a continuous and controlled manner (see Figure 5). This technique can affect the uniformity and condition of penetrated thin conductive thin coat by key parameters such as the rod’s size, solvents, and conductive solution’s viscosity. The concentration of the coating is influenced by the groove size in the wire-wound rod and depends on the rod’s diameter. The solution flows through the grooves in the wire-wound rod, resulting in a thin film deposition at room temperature [242]. Zhang et al. reported a simple dry-Mayer-rod-coating method that fabricates the conductive sheath/core-structured graphite/silk fibers and indicates their application as wearable strain sensors [244]. A clip is used to align the bundle of silk fiber at one end of the flat plate. Then, graphite was put on the fixed end of the silk fiber and graphite flakes, which were then moved by a Mayer rod pressed onto the fibers [242].

3.3.3. Roller Coating

The Spandex multifilament yarn was coated with thermoplastic polyurethane/carbon nanotube (TPU/CNT) conductive polymer composite (CPC) using the roller coating technique. The conductive mixture was placed into a bath; three rolls pushed the yarn. It was coated when it rolled over the second roll, which was explicitly immersed in the coating bath to pick up the conductive material; next, a glass tube entered with hot air moving through the glass, thereby acting as a drying oven, as shown in Figure 6 [245].

Each coating process has different supremacies that lead to different results. The advantages and disadvantages of various coating processes are shown in

Table 1. Advantages and disadvantages of different kinds of conductive coating processes.

Processes	Material	Coating Parameters	Advantages	Disadvantages	Ref.
Directly solution dip coating	Silver/graphene oxide/cotton fabric	Dipping cycle: 2	user-friendly process	weak uniformity	[230]
		Material to liquor ratio: 1:40	high efficiency rate	poor bonding state
			impressive conductive nature with the least surface resistance of 2.71 ohm/sq.
		Gas Pressure: 0.8 Pa
Sol-gel dip coating	Organo-silicon/graphene/PET fabrics/polypropylene fabrics	Padding rate: 1 m/min	coated at nearly room temperature	more complex dynamic method	[246]
		Squeezing roll pressure: 15 kg/cm		harsh chemical reaction on fibrous material
		Drying temperature: 105 °C
		Drying time: 20 min
Multi-layered dip coating	AgNWs/cotton fabrics	Dipping Cycles:1–4	good uniformity	relative low efficiency	[247]
			multi-layer structure coating
		Dipping time: 1 min	high electrical conductivity (2416.46 Sm−1)
		Temperature: 50 °C	eco friendly
Vacuum-assited dip coating	AgNW/MXene/silk fabric	Dipping Cycles: 10–50	wide range of substrates	complicated dip coating devices	[240]
			facilitate coating solution deposition
Spray coating	PEDOT:PSS/PET fabrics	Drying temperature: 130	excellent control over the conductive thickness	isolated droplets	[243]
			suitable for the deposition of all types of conductive materials	Non-uniform surface
		Liquid flow rate: 2.5 mL/min	large scale production
		Air pressure: 3 bars
Rod coating	Graphite/silk fiber	Coating cycles: 10 times	relatively low-cost and highly efficient fabrication of GSF fibers	ineffective with high viscosity liquid.	[244]
Roller coating	Thermoplastic Polyurethane (TPU)/CNT/Spandex Multifilament	Inlet temperature: 200 ± 5 °C	simple and cost-effective process	ineffective with low viscose liquids	[245]
		Dropping temperature: 150 ± 5 °C	continuous process		[248]
		Roller speed: 0.45 m/min	higher speed

.

3.4. Printing

The printing process is a versatile manufacturing method that enhances and facilitates the field of flexible sensors by providing cost-effective processing routes [249]. Screen and stencil are the two broadly used printing methods for coating conductive materials to different substrates, including textiles [250].

Table 2. Advantages and disadvantages of different kinds of conductive printing processes.

Process	Materials	Printing Parameters	Advantages	Disadvantages	Ref.
Screen printing	CNT/cotton fabrics	Mesh Density: 300 Drying temperature: 60 °C	cheap good efficient process of forming thick Good electrical conductivity with a surface resistance of 50.75 Ω/sq	required high viscosity and low volatile liquid	[251]
Inkjet printing	Reactive silver ink/cotton fabric/PET fabric/wool fabric	Bitmap:100 dpi Resolution: 100 steps per inch in the x and y axes Volume of released drops: ca. 30 nil. Sintering temperature: 90 °C	no masking required low processing temperature based on the drop in demand high resolution printing required low viscosity achieved surface resistance range 0.155–0.235 Ω/sq	limited printing speed	[252]
3D printing	Ninja flex filament/PEDOT:PSS/Polyester fabric	Extrusion temperature: 230° C Build Plate Temperature: 30 °C Printing Speed: 800 mm/min	no need for molds, dyes, lithographic masks high efficiency rate	limited printing material low printing resolution poor functionalities	[253,254]

presents a comparison of the different printing processes used for fabricating conductive textiles.

3.4.1. Screen Printing

The first evolution of screen printing dates back to the beginning of the twentieth century. At the time of printing, there was no actual loss of coating solution. The method is shown in Figure 7 and includes a screen of woven material (i.e., synthetic fiber or steel mesh) that has been attached to a tensioned frame. The pattern is generated by filling the screen with an emulsion that is impermeable to the coating solution in regions where no printing can occur. The region of the printed pattern remains open (without emulsion). The screen is then packed with a coating solution and brought close to the substrate. The squeegee is forced to the screen, bringing it into touch with the textile substrate and then linearly drawn across the screen, pushing the conductive coating solution through the open regions onto the textile substrate and thus reproducing the pattern [255].

Screen printing is cheap and is highly efficient in providing thick conductive film [256]. Conductive silver-based inks can be applied to various woven and non-woven textile substances using screen printing to track various vital signs [257]. Sadi et al. have developed a multifunctional weft-knitted cotton fabric through screen printing of CNT ink for wearable electronic devices, smart displays, and cold weather conditioners [251]. There are some difficulties and disadvantages of screen-printed conductive textiles, such as low abrasion resistance and drying out of the ink on the screen, which destroys the design of the screen [249]. The screen printing method is not ideal for single output compared to the direct write printing method [258].

3.4.2. Inkjet Printing

Digital or direct printing technology is a way of producing low cost and high quantity conductive materials [259]. Inkjet printing is a way of writing patterns directly to substrates. Inkjet printing is a simple term characterized by dye drops as ink is poured into specific positions to form a printed design on a textile substrate with a specified resolution [260]. In essence, the procedure involves the ejection of a fixed quantity of ink in a liquid-filled chamber in response to applying an external voltage. This abrupt reduction produces a shock wave in the liquid, which induces a drop in the liquid to eject from the nozzle [261]. The expelled drop comes under gravity and air resistance gained in motion, and the surface tension helps slow it along the surface [262]. Then, the drop will dry by solvent evaporation.

Kim et al. developed a knit structure printed with silver inks by inkjet printing, which had a resistance of 0.09 Ω/sq [263]. Low production costs, no masking, non-contact viability, lower material consumption, low temperature, and quick inkjet printing processing have gained interest in cost-effective manufacturing processes [264]. The design can be easily changed as no mask is used [261]. The major advantage of the inkjet printing process is that the conductive material can be deposited only in the required area on the substrate [265].

3.4.3. 3D Printing

Three-dimensional printing has gained major importance worldwide for its application in textiles and fashion [266]. Three-dimensional printing techniques have a profound impact on conductive textiles development with multifunctional purposes [267]. Nanocomposites with CNTs or graphene bases possess 3D conductive polymers [268]. The solvent-cast 3D printing method has been characterized by the application of scaffold microstructures (conductive nanocomposite) for its various properties, such as thickness, diameter, and interfilament spacing [269]. Three-dimensional printing of metallic substances enables it to be used for forging stainless steel electrodes with the help of a suitable laser melting machine and can also perform as a pH sensor [270]. Solvent-evaporation-assisted direct-wire (DW) 3D printing techniques have generated biocompatible polymers comprised of polylactic acid (PLA) and polyethylene adipate (PEA) with barium titanate nanoparticles [268]. The Desktop 3D printing method has also enabled the use of CNTs or graphene as construction materials with electrical and thermal conductivity properties [271]. Boron nitride/polyvinyl alcohol has generated a higher thermal conductivity (1.56 and 2.22) than PVA fabrics and cotton fabrics when fabricated by 3D printing techniques, respectively [272]. The textile substrate can accommodate conductors and antennas with the 3D direct-wire (DW) dispensing method, which can provide more accurate data [273]. As no dye is required in this direct-wire (DW) method, the geometric shapes can be conveniently altered [268].

4. Testing for Conductive Textiles

The electrical properties of conductive textile materials require proper characterization based on their applications. They are often characterized through determining the resistance, conductivity, surface resistance or sheet resistance, and EMS (electromagnetic shielding) of the textile substrates. These tests are usually performed following standards or by slightly modifying the standards [134].

4.1. Conductivity Testing

The conductivity of textile substrates is mostly measured under a DC power source. Nevertheless, it was inspected that PPy-coated poly-aramid fabrics showed increased conductivity under an AC power source due to an electron hopping charge transport mechanism rather than electron conduction [274,275,276]. Given this, it is evident that the electrical characteristics may be different for AC-based applications.

The conductivity of polymer coatings is generally determined using the four-probe technique according to the ASTM D-4496 standard. According to the method, four electrical contact terminals are placed in a line at an equal distance on a polymer film. The polymer film is set up on an insulating surface. In this process, a constant current is employed between two outer terminals from a current source, and the voltage is measured between two inner terminals. Conductivity in this method can be calculated from constant current (I, ampere) applied to the outer terminals and voltage drop across the inner points (V, volt). With sample thickness (t), we can measure conductivity following Equation (1). [134].

$$\mathrm{Conductivity}, \sigma =\frac{\mathit{ln}2}{\pi t}\frac{I}{V}$$

(1)

The resistance of a fiber or yarn of a specific length can be measured using the four-probe method, two-probe method, multimeters, or by applying the principle of division measurement with a known resistance and unknown resistance (sample). The resistivity value is generally expressed as the resistance per unit length—often in Ωcm⁻¹ or Ωm⁻¹.

According to the two-probe method, two contact points are used. The resistance is calculated by the measured voltage and measured current. The current applied is constant, but a voltage drop over the ammeter limits the measurement’s accuracy. In 2010, it was observed that the four-probe technique is suitable for inspecting intrinsic electronic transport properties (behavior of electron transport such as hopping or conduction based on material type), such as metallic nanowires, given that these nanowires may have contact resistance, which cannot be avoided in the two-probe method [277]. Moreover, the four-probe technique is more accurate when the resistance value is small as contact resistance is eradicated.

Surface resistivity, often expressed as square resistance, is the resistance between two points of a fabric or coated surface. It is written as Ω per square (Ω/sq), and the value is not dependent on the distance between contact points [134]. The AATCC (American Association of Textile Chemists and Colorists) test method 76 is generally exercised to measure the surface resistivity of conductive fabric or surface. The BS 6254:1984 and ASTM D257-07 test methods can also determine sheet resistance. According to the AATCC test 76, two rectangular-shaped copper plates are placed on a fabric specimen. The resistance (R, resistance) between two copper electrodes is measured, or the current (I, ampere) is determined by applying DC voltage (V, volts). The width (W) of the sample and the distance between electrodes (D) are also measured. The surface resistivity can be calculated using Equation (2). [134].

$$Rs\left(\Omega /m\right)=\frac{W}{D}·R=\frac{W}{D}·\frac{V}{I}$$

(2)

The instruments used for these electrical measurements are usually multimeters, voltmeters, and picometers.

4.2. Electromagnetic Shielding (EMS) Testing

The EMS effectiveness test is a measurement that determines how a textile substrate performs against electromagnetic interference by weakening it. There is no standardized method to test the EMS of conductive textiles to this day. The following properties or parameters of the textile shielding materials used to manufacture textile assemblies or products for EMI shields are very important to determine [278]:

-

Reflection coefficient, which depicts the amount of reflected EM wave under an impedance discontinuity in the transmission medium;

-

Absorption coefficient, which signifies a parameter on how much energy from an EMR (electromagnetic radiation) wave a material can absorb;

-

Transmission coefficient, which indicates the quantity of EMR penetration through a shield.

Determining the effectiveness of EMI shielding or, more distinctly, determining the attenuation of an EM shield can be very complex as the methods of determining shielding vary with the particularity of application. To figure out the performance of the planer shielding structure, a wide range of standards have been adopted [279]. Generally used techniques for testing shielding strength for plane materials incorporate open-field, coaxial transmission lines, and shielded room tests [280,281].

4.2.1. Open-Field Test Method

The Open-Field Test method, also known as the free-space method, is more suitable for flexible, thin textile substrates. This method is used on a finished product or a finished electronic assembly. The test set-up involves mounting a device at 30 m from a receiving antenna and recording radiated wave emissions. The test requires a spacious open field (approximately 900 m² area) and no metal or conductive object should be present between the sample and antenna [282].

The measurement procedure of this method contains [282]:

• Calibration measurement of the transmitted signal without material (P0);
• Measurements of the transmitted signal as a function of incident angle with material (P1);
• The ratio of P1/P0 is the power transmittance required to determine complex permittivity.

4.2.2. Shielded Box Method

This method is widely implemented for comparative studies of different shield materials [278]. The test involves a metal box with a port to set samples. A transmitting antenna is placed outside a box, and the receiving antenna is set inside the box. The transmitter sends the signal, and the receiver receives the signal through an open port or a sample fitted over the port. The standard for shielded box methods frequently mentioned is MIL-STD-285-1956—for the frequency range between 100 kHz to 10 GHz [283]. The measurement ranges in this method are given below [284]:

• Low range—9 kHz to 20 MHz—applicable for magnetic components (H);
• Resonant range—20 MHz to 300 MHz—applicable for electrical components (E);
• High range—300 MHz-18 GHz (100 GHz)—in case of plane wave power (P).

The main limitation of this method is the difficulty of achieving proper electrical contact between the shielded box and the specimen [278].

4.2.3. Shielded Room Method

The shielded room method is the most delicate, and it was developed to overcome the limitations of the shielded box method. The working principle of this method is the same as the shielded box method, but each component is isolated from the others in separate shielded rooms. An anechoic chamber is used to isolate two antennas; moreover, the specimen size is marginally enlarged by up to 2.5 sqm.

4.2.4. Coaxial Transmission Line Method (Transverse Electromagnetic Cell Method)

This method is the most popular and widely used as it is more suitable for measuring small-sized and thin conductive textile samples and extended frequency ranges [285]. One of the main advantages of this method is that the results produced in different labs are comparable. The ASTM D4935 test method is the most preferred method for measuring EMSE of textile materials (planar materials) because of a frequency range of 30 MHz to 1.5 GHz operating in a far-field EM wave as well as a plain wave [278]. The measurements can be executed at a particular frequency range using a modulated signal generator, crystal detector, and tweaked amplifier [281]. The set-up usually consists of coaxial adaptors, signal attenuators, and network analyzers [286]. The net shielding effectiveness originated from absorption and reflection is determined for far-field values. Near-field values may also be measured for electric (E) and magnetic (H) sources from far-field values determined prior [282]. This method has a positive side in that it does not require the thickness measurement of the sample.

A dual TEM (transverse electromagnetic wave) cell method is also used, which allows for measuring the electric and magnetic polarizabilities of test specimens simultaneously [278].

5. Application of Conductive Textiles

Electrically conductive textiles are used in various fields, such as the medical, sports, soft robotics, security, and military industries, and as wearable displays [287]. Textile-based electrical components include electrodes, sensors, wearable antennas, wearable displays, actuators, and heating devices [288].

5.1. As Electronic Textiles (E-Textiles) for Bio-Sensing and Health Care

People have been investigating the medical applications of electricity in clothing, such as corsets and belts, as early as the 1850s [289]. The first wearable computer was imagined in 1955 by Edward Thorp to predict roulette, terminating in a cooperative endeavor at M.I.T. with Claude Shannon in 1960–1961 [290]. E-textiles, in which electronic functions are integrated more closely within the textile, evolved from wearable computing in the early 1990s. A “Wearable Motherboard” (smart shirt) was developed to monitor the vital signs of humans in an unobtrusive manner, which opened up a new border in healthcare and telemedicine [291]. Scientists conducted additional research on wearable textiles to measure blood pressure, blood oxygen saturation, EEG, EOG, periodic leg movement, core body temperature, and skin temperature in the 2000s [292,293,294,295,296]. A spirometry, a simple test used to help diagnose and monitor certain lung conditions by measuring how much air people can breathe out in one forced breath by pinching noses with a clip and holding a mouthpiece in their mouth, is an uncomfortable condition for the users. An e-textile-based wearable spirometer, Spiro Vest, was demonstrated to gain more comfort than estimated lung behavior from torso–girth movements [297]. E-textiles-based stretch sensors were installed as elbow supports to maintain an ideal setting, and a knitted shirt was used to simulate a daily use setting to prevent caregiver injury in health and medical science [298]. Based on built-in graphene-clad conductive textiles, elastic bands, and lithium batteries for power, a fully wearable smart medical garment comfortably worn around the wrists or neck was reported for ECG monitoring [299]. Sleeping disorders are a major health danger issue in high-paced modern society. A smart bedsheet using a triboelectric nanogenerator (TENG) with pressure sensitivity, large scalability, and washability for real-time and self-powered sleep behavior monitoring was reported [300]. Waterproof fiber-based strain sensors with a high gauge factor and outstanding stability are necessary for smart textiles, wearable devices, and biomedical electronics. To monitor various human motions (e.g., phonation, pulse, finger bending, and walking), this kind of strain sensor was invented and designed through a combination of PU yarn, multi-layer graphene nanosheets (GNSs)/thin gold film (Au)/GNSs, and a PDMS wrapping layer [301]. Recently, a textile magneto-elastic generator (MEG) was invented that could work as a self-powered sensor for continuous respiratory monitoring with heavy perspiration without any encapsulation [302].

Smart fabric sensors (SFS) are produced by modifying a fabric extrinsically or intrinsically for the sensor properties of SFSs. This can be done in various ways, such as the e-textiles fabrication method, via connectors or electrodes or textile circuitry and elements, encapsulation techniques, and coating. SFS sensors can be categorized as pressure and force sensors, fabric strain sensors, optical fabric sensors, fabric sensors for detecting chemicals and gases, and some parameters such as temperature- and humidity-sensitive fabrics [303]. Sensor development, fabric-based ECG signals, skin–electrode interface improvement, sensing shirts, and belts for sleep measurement were investigated. Different parameters, including movement, posture, and temperature, were reviewed and compared to the innovations in the textile field [304]. Figure 8 shows the construction of an ECG detecting smart garment as an example. The ECG sensor patches (5) were placed inside the undershirt (“1. 2.” outside and inside faces for men or the bra strap “3. 4.” for women). (6) Cross-sectional view of the textile electrode (hitoe^®). The cloth of polyester nanofiber (8) was coated by electroconductive polymer PEDOT-PSS (7, blue circles), and its surface was directly placed on the skin (9). The back side of the electroconductive textile was fixed on the waterproof layer (12) and the cloth of the underwear (12). ECG signals were conducted through the textile electrodes (5. 10. 11) to electroconductive yarn (15) and a snap hook button (14); then, these ECG electrodes and the lead wire were connected to the ECG transmitter (13).

The electrodes in e-textiles were used specifically to determine biomedical features such as oxygen, moisture, contaminants, or salinity. Different fabrication techniques can be used [306]. The application of smart textiles in the physiological field, such as blood glucose level, blood pressure, respiration rate, body temperature, physical activity monitoring, and rehabilitation, was discussed [307]. Electronic, pneumatic, and electropneumatic transducers were studied to measure the interface pressure. The avoidance of sickness can be attained with this type of soft sensor. The use of wrist wearable instruments and a central monitoring system were mentioned to enhance the speed of assessing a patient’s condition. The use of ambulatory devices was investigated for mobility and improved healthcare [308].

Humans stretch their bodies to their limits during sports activities or leisure-type amateur training. To get a better result and stretch the personal limit results in muscular and thermophysiological stresses, the garment does not add any further stress; it should, if possible, help the athletes sit, get a better result, and prevent sports injuries [309]. A device named “The Intelligent Knee Sleeve” (IKS) (see Figure 9) was used in football and other sports training to avoid disabling injuries during movements involving rapid deceleration, quick changes of direction, and sudden landings. The IKS consists of a simple, elastic sleeve incorporating disposable PPy-coated nylon-lycra fabric, integrated with appropriate electronic circuitry (3 V) sensor placed over the kneecap, and provides feedback as audio to alert the desired flexion limit [310]. During exercise, a major problem is water loss, including hypersensitivity, headache, dizziness, convulsions, vomiting, increased body temperature and heart rate, increased perceived power, decreased mental function, and delayed gastric emptying [311].

A device was invented to detect increased pH levels using polyamide/lycra blended fabric and a pH sensor for sweat collection and real-time pH monitoring up to 0.2 pH units in 2008 [312]. To prevent knee injury and determine the user’s bending angle and tensile deformation, the PU/PEDOT:PSS-based knitted textile sensor integrated with a wireless transmitter that was used as a knee sleeve was demonstrated [313]. A pre-treatment process of the surface of organic nanoparticle-based inkjet printable textile was reported, which enabled all inkjet-printed graphene-based wearable e-textiles to be breathable, comfortable, and environmentally friendly, and measured the CG signal [262]. Using multiscale disordered porous polyurethane (MPPU) via microfluid spinning, self-sensing and self-cooling properties were invented. It could reduce the temperature by around 2.5 degrees Celsius between skin and apparel compared to cotton [314].

5.2. Soft Robotics

Soft robotics is a branch of robotics that involves constructing robot bodies and actuators using extremely soft or expansible materials [315]. Future soft actuation technologies could utilize conductive textiles as electrode materials of sensing transducers that can be resistive or piezoresistive with a wide variety of sensing functionality, including shape reconstruction, bending angle, direction, contraction length, or circumference [316]. Conductive yarns demonstrate electric actuation by changing the distance of twisted fibers by electromagnetic or electrostatic force [317]. A PVC gel-based actuator comprised of a PVC inner core and insulated PVC shell was designed. A pair of identical fibers that were loosely twisted together got closer together and resulted in the elongation of yarn when voltage was applied to the two fibers’ inner cores. A textile-type and a yarn-type actuator were constructed, and a maximum contraction strain of 53% and an expansion ratio of 1.4% were achieved, respectively [318]. A planer DE (Dielectric Elastomer) actuator, bending DE actuator, and EA (electro-adhesive) actuator have been developed and tested. Under 9 kV and 6 kV, relative expansion rates of 16.4% and 5% were achieved for the planer and bending DE actuators, respectively. Additionally, under 5 kV, the shear adhesive of 0.14 kPa was obtained for the EA actuator. Furthermore, a crawling robot driven by the above actuators has been developed, traveling 18 mm distance in 3 min [319]. An octopus-inspired continuum robot arm was constructed based on a conductive textile material called Electrolycra (Mindsets Ltd., Saffron Walden, UK) [320]. A bending sensor was developed for a robotic glove in which a dielectric silicone was employed as the sensing material, which was placed between conductive knitted electrodes manufactured from silver-coated yarn.

Additionally, the sensor mat was set parallel to the Wales structure, which was sounded with extra silicone [321]. Atalay et al. designed a new shape-sensing system based on electroconductive yarn material integrated into a soft manipulator with pneumatic actuation, which is suitable for implementation in miniature surgical instruments [321]. Mehmet et al. built an artificial muscle with actuating and sensing functions [322]. The bimorph tendril-like fiber was created by thermally pulling cyclic olefin co-polymer elastomer and polyethylene. As a resistance strain sensor, a coating of conductive nanowire mesh was applied to the fiber. The fiber contracted as it was heated, and the electrode’s resistance monitored the contraction and elongation based on piezoresistive feedback.

Furthermore, a humanoid limb was fitted with sensor-fitted fiber actuators. When heated with a hot gun, the fiber actuators contracted, causing the artificial limb to bend, and the resistance change of the electrodes was used to determine the bending angle of the limb [317]. A highly stretchable and durable conductive knitted fabric was developed and applied to the covering of a paper–fabric composite-based actuator, which has the potential as the electronic skin of soft robots. The electrical resistance of the fabric was sustained under a maximum average membrane strain of 300% in 3D deformation. The fabric was capable of withstanding more than 1,200,000 cycles of loading under 20% tensile strain and 10,000 abrasion cycles [323].

5.3. Other E-textiles Applications

The demand for electronic yarn or e-yarn is increasing day by day. A prototype to produce e-yarn—as can be seen in Figure 10—using the encapsulation process was reported to be incorporated into the fabric via knitting and weaving machinery that can meet the demand for electronic textile material [324].

Due to their softness, breathability, and biocompatibility, wearable e-textiles have attracted increasing appeal, making them long-lasting and wearable in the long run. A multifunctional integration system with thermal insulation, Joule heating, and particulate matter (PM) filter features was reported using conductive nanowires [325]. Several companies, including Google, which is developing capacitive touching textiles in its Google Jacquard project, have recognized the potential of electronic wearable textiles. This project enables a seamless and dependable wearable computing concept that can assist customers in performing daily tasks, such as answering phone calls without interrupting an ongoing activity [326].

5.4. EM Shielding Textiles

The rapid expansion of electronic technologies and the vast spread of advanced electrical components in automation, communications, space, computations, and other sectors have introduced the adverse effects of electromagnetic interference (EMI)—mainly in the radio frequency ranging from 10⁴ to 10¹² Hz. Many international radiation regulatory bodies, such as the Australian Radiation Protection and Nuclear Safety Agency Standard (ARPANSA) and International Commission on Non-ionizing Radiation Protection (ICNIRP), have set the safety limit for a specific absorption rate of radiation within 2 WKg⁻¹ of human tissue [327,328]. Wearable EM shielding has become a crucial demand to prevent adverse biological effects, such as raising human eye temperature [329], damaging the human tissues, leading to childhood cancer [330], and even causing hazardous effects on the human reproduction system [331] from EM radiation generated from electrical devices.

A distinguished aspect of conductive textiles is that they can interact with electromagnetic radiation (EMR) by absorbing and deflecting EM waves [278]. This feature facilitates the employment of conductive textiles in EM shielding devices very solemnly. CNT/PANI-coated fabrics were developed, which exhibited a low sheet resistance (20.1 ± 1.7 Ω/sq) along with a moderate range of EMI SE of 23 dB over the frequency range of 4–6 GHz (C-band) and possessed high EMSE retention against various treatments such as wash, sonication, acidic, and alkali [332]. Zhu et al. reported eco-friendly EM shielding textiles consisting of SWCNT, biomass-derived chitosan, and glucaric acid coatings, which retained a convenient EMI SE of 30 dB and better durability than Ag or Ag/polymer composite-film-coated textiles [333]. Yin et al. described multi-layered PANI/MXene/CF fabrics with appropriate EMI shielding properties (26 dB) included with high specific shielding effectiveness (135.5 dB·cm³/g) [334]. In another study, they coated 1D PANI NW/2D MXene nanosheets/carbon fiber fabrics with PDS, resulting in an outstanding increase in EMI shielding performance (SE = 35.3 dB, SSE = 63.8 dB·cm³/g) even at a very low thickness (0.376 mm) with a convenient self-cleaning and air permeability performance, which would provide great access to commercial EMI shielding textiles [335]. Jia et al. demonstrated a machine washable, highly conductive (1227 Sm⁻¹) EMI shielding textile prepared with PU/AgNWs, which exhibited an excellent SE of 63.9 dB—even at a lower density (0.34 gcm⁻³) and thickness (0.6 mm) compared to other metallic shielding textiles based on AgNW, copper nanowire (CuNW), and CNTs—and it could be integrated even in harsh conditions with long-term EMI shielding durability [336]. Wang et al. reported a water-resistant, breathable, mechanically robust PDMS-coated nickel ferrite/MWCNTs/cotton fabrics with a superior EM SE of 84.5 dB in the X-band and an outstanding EMI SE retention (above 90%) against washing cycles [169]. AgNW/Fe₃O₄/PDMS coating was integrated on cotton fabrics, having an outstanding EMI SE of 100.9 dB, which could withstand extreme acidic (pH = 2) and alkali (pH = 12) conditions and retain about 96% and 99% of SE retention, respectively [337]. An ultrahigh SE of 106 dB was evaluated for AgNW/PU/carbon fiber fabrics that retained 95.8 dB of SE even after 100 peeling cycles [338].

Shielding effectiveness (SE) is the total shielding through absorption (SE_a), reflection (SE_r), and multiple reflections (SE_m). SE due to multiple reflections (SE_m) is neglected for practical applications when the SE due to absorption is higher than 10 dB [339].

5.5. Supercapacitor

With the rising demand for fast charging as well as high energy storing devices and the growing concern for human comfort in the wearable smart textiles industry, textile-based supercapacitors are making a revolutionary step due to some of their unique features, such as their extreme wearability, flexibility, comfort, durability, effective electrochemical redox performance, and cost-effective production process [340].

The energy storage performance of supercapacitors is mainly dependent on selecting the electrode and electrolyte materials; moreover, the development of supercapacitors concerning flexibility with comfort and a combination of supercapacitors with textile fabric is resulting in a revolution [341,342,343]. Consequently, a large amount of research has been done to develop advanced electrode materials for flexible supercapacitors with suitable structural properties with deformation, stretching, bending, and washability [344,345].

Song et al. reported textile-based electrodes for supercapacitors composed of PANI/G/HCL through a simple dipping and drying process of graphene on fabrics followed by in situ polymerization of aniline, which exhibited an impressive areal specific capacitance of 1601 mF cm⁻² at the current density of 1 mA cm⁻². In contrast, over 75% of initial area-specific capacitance was preserved after 10,000 cycles [346]. The deposition of rGO and SnO₂ on textile fabric via supersonic spraying showed the highest specific capacitance of 1008 mF cm⁻² at a current loading of 1.5 mA cm⁻², with a capacitance retention of 93% after 10,000 cycles [347].

Lu et al. designed a flexible electrode with a high specific surface area provided by metal–organic frameworks (MOFs) and flexibility provided by CNTs [348]. The performance of that supercapacitor showed a stable energy supply under cyclic deformations and even wearable conditions. Mechanically strengthened ultralong MnO₂ nanowire composites were developed as a stretchable electrode that showed a specific capacitance of 227.2 mF cm⁻² and can be stretched up to 500% without degradation of electrochemical performance. Around 98% of the initial capacitance after 10,000 stretch-and-release cycles was performed under 400% tensile strain [349].

6. Challenges and Future Outlook

During the past two decades, conductive textiles have been a promising segment of textile engineering and have drawn significant research interest. Though outstanding progress has been achieved in this segment through the relentless effort of researchers, there are still some challenges that hinder the commercialization of conductive textiles on a large scale. Most of the textile materials are not conductive, and it is a daunting challenge to make them conductive by incorporating conductive materials within them. There are several challenges to conductive textiles, including connectivity, textiles and conductive materials and their fabrication, and wear of the materials [350]. Besides these challenges, the designer faces some challenges during the prosperity of conductive textiles, such as mechanical environment, washability of the textiles, power supplies in the circuit, commercialization, and cost of the conductive materials [5]. Durability is one of the major concerns for conductive textiles because textiles are prone to wear abrasion, bending of the coated materials, and stretching [351]. The mechanical characteristics of textiles sensors resist being flexible and permeable to air and water [249]. The mass production of conductive textiles is challenging due to the self-energy generation form and the high number of elements [288]. The textile sensor is one of the widespread areas of conductive textiles in which the conventional concept of attaching the solid-state electrical components with textile materials has been turned into flexible e-textiles capable of sensing various phenomena. Until now, one of the major challenges in this regard is maintaining the efficiency of these textile sensors compared to the solid components [352]. The challenge with textile connectors is maintaining the connectivity eminently through the complete textile surface during the assembling stage of developing wearable e-textiles [353]. The textile composite electrode has drawn eminent attention to converting conventional batteries and supercapacitors into flexible energy storage devices. Some challenges are still present to pave the way for the commercialized production of these flexible electrodes. These possess relatively higher resistance, reduced mechanical strength of yarn-like electrodes, and lower energy density than rigid-state batteries and supercapacitors [354]. Higher resistance is mainly due to textiles’ porous structures and lower intrinsic conductive nature. Textile yarn-like electrodes seem more vulnerable to mechanical stress, resulting in more yarn breakage during the fabrication process than conventional yarn [355]. This indicates a challenge to maintaining the high speed of industrial machinery during the fabrication process of these electrodes. Future researchers should focus on these challenges of conductive textiles to pave the way for large-scale commercial production of these materials.

7. Conclusions

This review has highlighted various fabrication methods for conductive textiles with detailed information about conductive polymers. Research should be carried out to eliminate the disadvantages of various coating and printing methods of imparting conductivity. Graphene has become the center of fascination for researchers for its extraordinary properties. Smart textiles with electrical conductivity and signal-sensing capabilities are gaining more and more attention due to their various applications. E-textiles face many challenges, such as washability, power supply, product development, and commercialization, from users’ perspective. There is a huge research gap in this aspect to investigate post-usage problems and the development of solutions.