Flexible Coated Conductive Textiles as Ohmic Heaters in Car Seats

Abstract

Pleasant interior conditions within cars, as well as a comfortable feeling, are primary needs of car drivers to perform vital body functions during driving. In a cooler outer environment, a warming system within seats can quite feasibly maintain a realistic thermal balance in the body. An inbuilt heating system in car seats can not only provide adequate heat to the driver but is also relevant to minimizing the energy consumption within the vehicle interior. In order to evenly distribute the heat over the body contact area of the automotive seats, conductive textiles are proposed. In fact, these textiles behave as a semiconductor (as an electrical conductor and also creating resistance, which in turn creates heat). Flexible textile ohmic heaters present great advantages due to their ability to bend, stretch, and stitch. These properties make them valuable to ensure uniform heaters for irregular geometries. The present review highlights the use of different textile-based ohmic heaters, their fabrication methods, range of heating, and durability. Moreover, this review also focuses on the comparative mechanical performance and comfort properties of the presented fabrics (used in car seats). This study is beneficial for future trends of minimizing energy consumption and providing an effective way to reduce the carbon emissions and air pollution produced by vehicles.

1. Introduction

Recent decades have seen a variety of research in the area of lowering energy consumption due to environmental concerns [1]. The electrification of the vehicle fleet is regarded as a promising strategy for lowering greenhouse gas emissions, energy demand, air pollution, and dependence on oil, among other cutting-edge techniques to decarbonize urban transport [2]. Heated seats in vehicles are one of the primary electric energy consumers in many parts of the world where winters can be relatively cold and long. As a result, the research and development of novel materials and methods that could be more energy efficient has increased [3].

Electric vehicles (EVs) in the transportation sector have the advantages of negligible harmful emissions, low engine noise, improved propulsion efficiency, and the potential for carbon footprint reduction [4]. As a result, most cars, including those that are less expensive, now come with heated car seats as standard [5]. Engine waste heat and electrical resistances are both employed for heating. The subjective comfort sensation is the key problem concern. This varies from person to person, and management is frequently influenced by passengers’ choices. Maintaining the ideal comfort levels for all passengers is a challenge because air is often the medium that transfers heat [6]. Among most potential applications such as weather forecasting, monitoring systems, bio sensors, and electronic sensors [7], the use of conductive fabrics as an ohmic heater in car sears is quite a novel approach. The size conformation, long durability, lightweight property are the key components of conductive textiles which make them an excellent solution in various realizations of platforms for use as ohmic heaters [8].

Resistive heating is the most useful approach adopted in utilizing automobile seats as heaters. A carrier material (a hybrid of textile and foam) is filled with sinusoidal loop-shaped wires between its layers in a seat-heater produced by FOAM-AD. A heating wire is connected to the carriers in a tailored pattern to provide an ideal heating system in the vehicle seat considering the specifications provided by the customer [9]. The wiring pattern is shown in Figure 1.

The wire might be sewn to the continuous non-woven base before the finished product is trimmed into the appropriate shape for a heating mat using automated manufacturing embroidery machinery. For instance, a thin nonwoven layer could be added to a structure made of a narrow sponge shielded with adhesive, the heating element introduced, and the residual adhesive on the mat covered. Another option is to use an automated series of machines where the glue is sprayed upon a non-woven bottom of a pre-defined structure, followed by the placement of the heating wire in the appropriate shape. The multistage method for producing a single mat, the insufficient endurance of anchoring the heating element to the mat foundation, and the inadequate robustness of certain mats for mechanical stress such as loading or abrasion are, nevertheless, the most frequent problems brought up by manufacturers. Wearable thermal technology that utilizes metal wire heating systems has limiting heating regions, poor elasticity, and oxidative deterioration and delivers inefficient heating. It also has a short lifespan due to wire breakage [9,10].

Wearable heating systems must be mechanically durable, lightweight, and able to withstand stretching and bending without obstructing the wearer’s or the garment’s potential to conform to body curvature. The propensity to expand would improve the device’s ability to contour to the body. Stretchable devices must undergo a minimal recurring strain of approximately 15–20% to remain flexible and fit [11]. Graphene inks, carbon-black [12], and blends of silver nanoparticle and laser lowered graphene oxide (GO), along with amino functionalized graphite and carbon nanotubes, have all been used to create flexible imprinted textile heaters. Printed heaters have used tiny carbon heating elements between silver conductors to get around the large sheet resistance of carbon sheets [13]. GNP-only inks are ideal for wearable heating applications, and rigid substrates have been employed in printed heating systems because of their high resistance [14]. The effectiveness of the heat pumps during tensile extensions had not traditionally been evaluated, notwithstanding flex testing. Moreover, it has been demonstrated that commercially available carbon-inks are not sufficiently serviceable against wrinkling and electro–thermal effects [15]. Polymer composites compounds that make up nanofillers have benefits including being lightweight, resistance to corrosion, simple processing methods, degradability, chemical stability, tuneable electrical properties, and lower production costs for electric heater applications [16]. Graphene-based heaters frequently exhibit fast heating rates and uniform temperature delivery [17]. However, because graphene-based heaters generally possess a high sheet resistance, they demand substantial voltage levels for applications which require extreme heat, which could pose an injury risk. As a result, a low resistance heater that operates at lower voltages is preferred [17]. As a result of their minimal sheet resistance, metal nanostructure heating systems could be operated at lower driving voltage levels. The incorporation of electro-conductive features into the fabric structure results in a flexible textile which does not compromise the level of comfort, maintenance, and breathability of the clothing. Electrical heating items, in general, increase the temperature through integrated heating elements. Metal-coated fibers or fabrics are frequently utilized to heat textiles for heat control using various techniques [18].

The current review focuses on the use of various textile-based ohmic heating systems, their fabrication techniques, heating range, and reliability. Furthermore, the review focuses on the mechanical efficiency and comfort features of the described fabrics (used in car seats). This study is useful for future trends in reducing energy consumption, which is an effective strategy for lowering the air pollution and carbon emissions caused by vehicles.

2. Principle of Electrical Resistance Heating in Textiles

Heat is produced when an electric current flows through any conductive material. The electric current that passes through the resistor produces heat depending on its level in an electric resistance heater. However, according to a fundamental concept of Ohm’s law, the voltage and current in the circuit limit the amount of current that can flow [19]. Resistors with specific values can be designed and produced in a variety of sizes and shapes within a fabric. The placement of the resistors on the fabric seems to have no effect on the resistor’s value. The value of the resistance is unaffected by the location of the resistors on the fabric. Conductive pieces can be arranged in parallel and series, regardless of the fabric’s area or surface, to create electrical networks inside or on top of the fabric structure.

The different types of conductive materials provide different ranges of resistance values, that can range from a few ohms to many kilo ohms. Actually, the linear resistance of conductive tracks determines the resistance value of the electrical networks constructed within the fabric structure. Careful attention must be given to the formation and insertion of these conductive tracks in textile-based heating systems, as the manufacturing process could harm them and result in a loss of conductivity [20,21]. Conductive elements can also be arranged in parallel and series to construct electrical network connections inside/over the fabric structure independent of the fabric’s area and surface. Resistance values can range from very few ohms to several kilo ohms which depends on the type of conductive element. The values of resistance for fabric-manufactured electrical systems are described by the linear resistance of conducting yarns/tracks. In the heating systems based on textile materials, the manner in which conductive tracks are created and inserted is crucial since the production process might damage the conducting paths and lower their conductance level [22]. Moreover, the conducting lines used in wearable technology need to be robust enough to prevent breaking and offer effective heating while also being flexible and light enough to bend [23]. Every square centimetre of fabric will release heat in the form of many watts of power, depending on the conductance level of the resistors. Higher power dissipation can be obtained by scaling up the resistor in the fabric area; however, high power dissipation necessitates an increased level of power supply. The electrical systems must be made larger for efficient heat dissipation on a fine surface of textiles. Evidently, one of the key considerations in designing an effective heating system is the efficiency level in removing the heat owing to the electrical core network on the fabric [24]. In rechargeable batteries-based portable electronics, specifying the necessary wattage value instead of the rated voltage allows for a greater design flexibility in the heating system. In order to calculate the required amount of distance for including conductive threads or tracks and, in turn, the measurements of textile-based heating systems, it may be more flexible to assess the watts value with a specified electrical resistance of the conductor [24].

3. Metal-Coated Electrically Conductive Heating Textiles

Electrically conductive fabrics and yarns are the key components of heat-producing textiles. The incorporation and coating of conductive material (metals and carbon-based) are used to fabricate electrically conductive textiles. The choice of metal-coating strategy for the manufacturing of conductive fabrics is critical in terms of processing parameters that tend to be associated with economic attractiveness and determine the reliability, flexibility, physiological, and electrical properties of end e-textiles [25]. In particular, there are two methods for producing conductive textiles. The first method involves weaving, knitting, or embroidering conductive fibres, threads, or yarns into fabrics. Nevertheless, these techniques are highly complex, and because the fabric is hard and inflexible due to the implanted electrical fibres or threads, it may not be comfortable to wear [26]. The typical method entails coating the surfaces of fabrics with robust electrically conducting materials (conductive nanoparticles, metals, and conductive metals, etc.) using a variety of coating techniques in order to give the textiles their electric properties throughout the finishing process. The incorporation of different metals on surface of the fabric (a process known as metallization) has drawn considerable interest among the various techniques employed to attain conductivity for their distinctive properties in offering multifunctional performance, including EMI shielding, antimicrobial activities, fungicidal activity, electrical properties, and ohmic heating [9], in addition to a lustrous metallic visual appeal for aesthetic reasons. The most popular techniques for metallizing textiles can be subdivided as follows (Figure 2) [27]:

(i) Metallic inks are printed on surfaces using a lithographic technique, inkjet, screen printing, and stencils;

(ii) Deposition (electrochemical, chemical and physical vapour, electroless);

(iii) Spray coating (thermal, plasma-assisted cold, etc.).

The appropriate method is determined by several parameters, including the form and type of textile material, operating temperature, metal type, and end application, which is also related to the required thickness of the layer of conductive material [27].

4. Metal-Coating Techniques

4.1. Electroless Plating

Electroless plating is an established controllable method in which single metallic nanoparticles such as silver (Ag), copper (Cu) [28,29], nickel (Ni) [29,30], gold (Au) [30], palladium (Pd) [31], platinum (Pt) [32], and or poly-alloys such as Ni-W, Cu-Ni, and others are chemically deposited onto non-conductive (dielectric) materials (plastic, printed circuit boards, ceramics, glass, and textiles) from an aqueous solution to develop a conductive layer of metals. Figure 3 shows the deposition of a conductive layer of copper and silver particles and the deposition of nickel over the textile structure. Furthermore, the electroless-plating process can be applied to multiple kinds (synthetic and natural) and forms of textiles, such as fibres, yarns, and plain fabrics (non-woven, woven, or knitted) [33]. According to Zhang et al. [34], non-diffusive copper (II) imprinting with a viscous fluid Cu (OH)₂ solution followed by Cu deposition on fabric by utilising various reducing agents is able to produce well-defined and extremely well-conducting patterns on cotton. Wills et al. [35] produced a precise structure of silver NPs to function as a catalyst on cotton using ink-jet and micro-dispensing printers, followed by electroless plating of copper (patent pending). Wu et al. [36] developed a method for the fabrication of a stainless-steel stencil mask with the required design on top of the fabric to create textured electrodes for light-emitting e-textiles (signalisation, entertainment, and fashion, etc.). According to Hu et al. [1], screen printing a dopamine surface modification followed by electroless silver plating allowed for the development of a precisely articulated Ag conducting network on PES fabrics [35,37].

4.2. Electroplating

Electroplating is the process of coating a thin metal layer of reduced metal with the help of an electric current on a conductive substrate. Prior to plating, the substrate should be cleaned to eliminate contamination, bringing about effective attachment of the metal particles to the surface. In general, there are three main phases during cathodic metal deposition: (i) ion migration, (ii) intercalation of adsorbed metal ions, and (iii) electron transfer [38]. The electrochemical deposition has several advantages such as controlling the thickness and morphologies of the deposited metal, and controlling the parameters of plating (time, temperature, and concentration). Additionally, the process only requires low-cost equipment, which makes the process economical and easy [39]. The main drawback during electroplating is that the targeted substrate being plated must already be electrically conductive enough for current to flow through it. Hence, there is a need to activate the textile or polymers prior to electroplating, as they typically function as natural insulators. The non-conductive substrate is normally activated by the making of conductive layers (in a pre-treatment step) on a material surface, utilizing various metals, for example, Ag [40,41], Cu [33,42], Ni [43], Mxenes [44], Ni–Co selenide nanowires [45], NiMoO₄/CoMoO₄ nanorods [46], LaMnO₃/MnO nanoarrays, ZnO nanostructures stored on a Ni–CuNi conductive layer [47], and LiMn₂O₄ [48], conductive polymers [49], or carbon-based materials [50]. Figure 4 shows the flow of the electroplating deposition process. The substrate is activated by a metallic nanolayer of Ni₃Se₂ onto the PES texture, utilizing a three-cathode framework [51].

Cu is one of the most mind-blowing metals for the pre-treatment of material substrates, since it usually covers minor imperfections. As a result of electrodeposition, it is fairly lethargic, has a negligible cost and a high declaration viability, and is significantly conductive [52]. Utilizing a Cu-polythiophene composite film as a seed layer for direct electrodeposition, bringing about a stretchy texture with a high electrical dependability for observation, Ali et al. investigated the in situ deposition of Cu particles on cotton fabric [53] due to its potential to shield electromagnetic interference. They discovered that 100- and 150-plunge textures with a thick, uniform, and permeated organization of conductive Cu particles on a superficial level displayed the highest level of protection, 10 dB. Zhao et al. utilized flexible polyaniline/polyethylene terephthalate (PANI/PET) conductive textiles made by chemically polymerizing PANI on PET textiles and electrochemically depositing Cu crystals for potential e-textile applications [54]. Cu was recently deposited using a new technology called hydrogen evolution-assisted electroplating, which improved deposition rates compared to the conventional electroplating method [55]. Cheng et al. reported the use of in situ hydrothermal growth of Cu NPs, with a polydopamine-pre-treated PET knitted fabric as a template. During the long-term cycle test, the resistance-strain behaviour was extremely stable. The planned framework showed its appropriateness for use as a wearable strain sensor, for example, to recognize ongoing biceps brachii motion, shank walking, subtle body deformation, muscle movements during swallowing, etc., as well as electric heaters available for use in thermal therapy.

Azam et al. fabricated a more compact layer of copper deposition over the textile fabrics. As shown in Figure 5, the textile cotton–nylon blended fabrics were activated with copper nanoparticles and subsequently copper plating was performed [56]. Electroplating was performed with power rated at 24 V/2 A with a consistent current flow of 1.9 ± 0.1 A. The conductive copper rod was attached at the anode, while the copper-particle-coated fabric (substrate being plated) was attached at the cathode. By applying a voltage difference across the fabric while measuring the temperature change at its surface, the fabrics’ heating properties were examined [56].

SEM images of fabrics are depicted, showing the more dense and uniform deposition of Cu-NPs prior to further copper plating. The final copper-plated fabric is depicted, showing the thicker layer of Cu with extra electroplating and plated copper morphologies showing the orbicular structure similar to a crocodile-skin surface [56].

4.3. Printing of Metallic Ink

The ability to deposit conducting materials on various textile surfaces at room temperature, the capacity for obtaining well-defined conductive visuals or designs, the reduction in the use of conductive inks, the transfer of the complete method to the industrial sector, the possibility of mass production, and cost efficiency are just a few advantages which printing has above other techniques that provide electrical properties. Silver and gold are two of the metals that are most abundantly observed in metallic conducting inks [57]. Silver-based inks progressively oxidise on the substrate, generating a tiny conducting oxide coating that over time has no impact on the conductance of the ink. As a result, several companies (DycoTec, Henkel, DuPont, NovaCentrix, and others) have developed and marketed silver-based inks, such as the Intexar-TM fitness able to monitor a T-shirt with Wi-Fi interconnection (DuPont). On the other hand, a high silver loading (greater than 20 wt%) is needed for metal-based inks [41], so the key challenge is to obtain a less expensive but super effective conductive metal to substitute the silver nanoparticles [58].

4.4. Vapour-Deposition Techniques

Vapour deposition is an atomistic deposition method in which the coating materials are vapourised and condensed in a vacuum onto the selected substrate, producing a thin film from virtually all inorganic (compounds, metals, alloys, and mixtures), as well as certain organic materials [59]. The depositing components are evaporated and condensed in a vacuum onto the chosen surface. The two most common solvent-free vacuum deposition techniques are chemical vapour deposition (CVD), in which the surfaces of coated materials and volatile substrates react, and physical vapour deposition (PVD), during which sputtering and evaporation take place. Both CVD and PVD have opened up new possibilities for textile material modification, and they represent an intriguing prospect for the development of e-textiles. The transport of precursors into the reactor, the diffusion of precursors to the surface, the adsorption of precursors on the surface, surface reactions such as the decomposition of precursors on the surface and integration into a solid layer, and the desorption and conjugation of molecular by-products into the gaseous state are all process steps in CVD [60]. The temperature has a significant impact on the reaction mechanism that proceeds when the precursor gases pass over through the heated surface layer. This solid phase is then deposited across the substrate surfaces. The use of substrates that are mainly composed of metals and pure gases instead of their pricey, complex, and frequently toxic chemicals gives PVD significant advantages above other deposition techniques (including CVD), including good quality, efficiency, and excellent biocompatibility. PVD also has the capacity to generate composite coatings. Pulsed-laser deposition, evaporative deposition, physical vapour deposition involving electron beams, sputter deposition, and cathodic arc deposition are among the PVD types employed for textile alteration. The most popular technique for depositing thin, long-lasting layers of metals (Ag, Al, Pt, Ni, Cu, Sn, Ti), metal oxides (Fe₂O₃, WO₃, TiO₂, ZnO), or non-metallic derivatives (silicon, graphite, ceramic) over a variety of kinds and patterns of flat textile materials is magnetron sputtering [61]. Figure 6 depicts the basic pathway of the magnetron-sputtering method [62].

Huang et al. [63] presented the ohmic heating potential, electrical conductivity, and UV-resistance of a nano-Cu film deposited on a polyester fabric surface by using the magnetron-sputtering method with pure Ag and Cu targets in the presence of argon gas. Chu et al. [64] established conducting PP fibres that are simple to work with and long-lasting. These fibres are ideal for joule heating, EMI shielding, and susceptibility to antimicrobial activity, among other purposes. Meng et al. [65] used low-temperature plasma and RF magnetron-sputtering technologies to manufacture copper coatings on polyester surfaces, and they examined how the ambient environment affected the conductance of those coatings. The findings indicated that Cu films were broken and sample stability was lost after 60 days, but the ohmic heating of the samples barely improved with temperature, the intensity of repeated washing, or moisture levels, which are mainly affected by the structural characteristics of substrates. Furthermore, high conductivity ohmic heated fabrics are developed through a variety of two-step processes that incorporate magnetron sputtering and pre-treatment. He et al. [62] fabricated a highly conductive cotton fabric through in situ polymerisation of PPy, followed by direct current magnetron sputtering of a silver thin film. The obtained Ag/PPy-coated cotton fabrics deposited with a sputtering power of 200 W for 25 min seemed to have the maximum electrical conductivity with an average resistive heating of 89 °C. It also had an excellent hydrophobicity, heat resistance, electromechanical effectiveness, and washing durability [62].

4.5. Dip-Coating Techniques

Dip-coating (or pad-coating) is relatively simple, low-cost, and scalable; thus, it is a widely used fabrication technique used to apply conductive (nano) materials onto fibres, yarns, or fabrics, as it does not require any special or expensive equipment [66]. The entire process can be continuous, as presented in Figure 7a, or discontinuous (Figure 7b). It involves the immersion of the fibrous substrate for a short period into the respective aqueous-based coating solution, which, in the case of conductive textiles’ processing, contains different types of conductive compounds, i.e., electro conductive polymers, carbon-based structures, inorganic metallic (nano)structures, or their combinations [67]. Thereby, the conductive compound is deposited on both sides, over the whole surface [68].

After depositing the conductive solution onto the fibrous material, the drying phase is carried out at temperatures of 110 °C up to 180 °C for a short time period (3 to 15 min), using hot air or infra-red (IR) heating. This process is also known as a curing, during which the excess solvent evaporates and the conductive (nano)compounds fix/crosslink permanently to the textile’s surface, ensuring a stable coating [69]. In fact, the dip/dry cycles can be repeated several times, with the aim of increasing the amount of deposited conductive material (the so-called multi-layered dip-coating method). Moreover, the drying temperature versus time should be considered to prevent damage of the basic textile material [70]. The main disadvantage of the dip-coating procedure is the variance in the smoothness/roughness of the textile’s surface and the uniformity of the applied conductive layer(s), affecting the variation in electrical resistance [71]. The thickness of the layers cannot be fully controlled, since it depends on the surface morphology and the tension of the textile substrate, as well as on processing parameters such as time, temperature, withdrawal speed, compound concentration, composition of the coating bath, etc. [72]. As mentioned above, numerous electro conductive compounds can be applied to fibres/yarns/fabrics by employing the dip-coating technique. In the following sub-sections, the most used materials are presented, as well as different applications [73].

5. Carbon-Based Heating Textiles

Carbon-based conductive materials are an excellent source of heat generation. Graphite powder, carbon nanotubes (CNTs), and graphene are typically adhered to the fibre surface to render it conductive. It is also feasible to develop conductive heating textiles by immersing and depositing carbon nanotubes in cotton while utilising recycled carbon fibre as a dispersion method [74]. In order to enhance the fabric’s thermal properties, MWCNTs are deposited on cotton using impregnation-drying coating techniques, and a rise in specific heat has been reported. Figure 8 shows time–temperature curves for elastic MXene-decorated textiles with interlaced conducting nanostructures for an integral Joule heater with varied heat resistance and voltage. Cotton fabrics coated with reduced graphene oxide (rGO) nanosheet films are being used to manufacture conductive fabrics for heat-generation applications [75]. Wearable electronics, smart devices for personal heating and cooling, or entire personal thermal management (PTM) electronics can be produced by embedding graphene in textiles. Breathability and stretchability can be easily increased by using high conductivity graphene fibres with excellent heating at low energies [76,77,78]. Figure 9 shows IR pictures of a polymer/graphene-treated textile-based multi-layered heating element with aramid fabric along with the surface temperatures of triple and quadruple layers at various power levels [78,79].

6. Design of Heating Fabrics Seats and Thermo-Electric Response

Heated textiles can be made in two ways: one by fabricating a piece of fabric and inserting an electronic framework into it, and the other by producing a yarn with electronic properties and developing textile products. Metallic wires were first utilized in textile clothing during World War II. Then, instead of metallic wires, more sophisticated conductive yarns with textile yarn characteristics began to be developed. Heating products can be made from knitted, non-woven, woven, and embroidered fabrics. Due to their high resistance, non-woven fabric heating components are only occasionally used. Woven fabrics exhibit significantly less resistance when compared to knitted textile fibres in the same geometrically heated area due to their structural differences [81]. Multifilament carbon-blended stainless-steel fibres with a mild resistance are being developed for appropriate heat-generating applications. According to reports, this particular variety of electro-conductive fibre is exceptionally inflexible and fragile in bending, which renders it inappropriate for textile applications [82].

The standard fabric architecture, including plain, twill-weave patterns or single jersey, interweave, etc., with a standard fabric GSM of 250 g/m², could be specified for flexible heating inside a textile structure. The fibre composition influences the system’s washability and heating behaviour. Wool fabric, for example, has a higher heating performance than cotton fabric. As a consequence, the thermal efficiency of a textile-based electrical heating network appears to be lower than that of woollen materials if cotton is employed as the basis fabric for the electrical transmission system. As a result, selecting an appropriate fibre composition has a significant impact on the heating efficiency of the system. Overall, the area demanding heat must be enclosed with a flexible surface for efficient thermal effectiveness as a result of delivery via convection and conduction mechanisms [83]. Thus, textile-based electrical heating systems offer a major advantage in sustaining the human body’s essential necessary heat levels by permitting greater contact with the flexible architecture and reducing heat loss. In addition, recent research on textile-based electrothermal networks has attempted to develop a product that can control and monitor body temperature in response to environmental change, particularly for outdoor sports such as skiing and snowboarding. The Ag-coated nylon yarns were stitched over the polyester base-fabric in parallel connection that can provide a parallel power supply for the fabrication of electrically heating fabrics. The subsequent resistance of the design is lower when connected in parallel, allowing it to generate huge heating effects. An additional weave was provided in serial fashion to connect with the power supply [84].

A further experimental process of the study was carried out at Ghent University’s Centre for Textile Science and Engineering for the fabrication of heating mats. They produced distance fabrics with an electrically conducting heating source that could be produced in a single manufacturing method on a warp knitting machine. The design process clearly described the key parameters required for the calculation process, including the full length of the heating mats, the quantity of heat sources, and their length, as well as the exact trajectory. The desired heating mats were 50 cm by 30 cm in size. The current study included three design steps: (i) preparing the movement patterns of heating wires, (ii) estimating the resistance required to calculate the approximate power of the entire mat, and (iii) preparing and testing samples. The detailed methodologies for these steps are described below. At the moment, the sources of power used in cars typically provide a voltage of 12 V, while those used in large trucks provide a voltage of 24 V. Heating element samples of carbon fabric yarn with the placement of the thermocouple for central temperature measurements inside the samples are shown in Figure 10 [84].

Low-voltage equipment was utilised (having a maximum tendency to supply 12 V), and the power was rated at 40 W. The dimensions of the samples were maintained at 10 × 10 cm (Figure 11) [86]. Multiple strategies were used during each testing step in order to maximise thermal efficiency. In the very first step, the heating accuracy, as well as the lowest and highest temperatures within each sample, was taken into consideration when evaluating five patterns in the form of samples. The range of temperature noted for each sample is shown in Figure 11. The pictures were recorded by an infrared camera associated with the FLIR Research IR software program [86].

Azam et al. [56] proposed a simple method for producing electrically conductive multifunctional textiles through embedding them with copper oxide nanoparticles followed by electroplating with copper. The surface and ageing characteristics of a fabric coated with copper nanoparticles were examined. Furthermore, ageing property issues were addressed in a very simple manner, and different functional properties were investigated. The effects of electroplating variables (acid concentration) on the fabric stretch rate were also investigated. The fabric coated in copper oxide nanoparticles was used as a substrate. In 1 L of distilled water, 250 g of copper (II) sulphate pentahydrate (CuSO₄.5H₂O) was solubilized. Then, gradually, 70 g of 95% conc. H₂SO₄ was added. The results indicated that the electrical power source had a rating of 24 V/2 A. A continuous voltage with current of 1.9 V ± 0.1 A was maintained across the electroplating procedure. The cathode was joined to the conductive fabric (covered with copper nanoparticles), which was used to deposit Cu metal, whereas the anode was coupled to a Cu plate that functioned as a reservoir of Cu metal. The solution was made to circulate continuously in the electroplating chamber. The heating characteristics of the fabrics treated with Cu nanoparticles and fabric samples electroplated with Cu were assessed (Figure 11). The conducting fabric samples displayed the highest surface temperatures of 127, 108, and 94.8 °C for copper-plated fabric samples (S3, S6, and S9) and 72.6 °C for fabrics treated solely with copper oxide NPs at 5 V and with a time interval of 1 min, respectively. As a result, the developed fabrics performed well in terms of heat supply. These fabrics are suitable for thermal applications in cold environments [87].

Zinc oxide (ZnO) nano-whiskers (NWs) were incorporated into the carbon fibre (CF) surface in order to evaluate changes in mechanical properties and ohmic heating for carbon-fabric-reinforced–green-epoxy composites. The ZnO-NWs have been hydrothermally synthesized onto the CF surface in order to create laminated composites that possess various proportions of carbon weave sheets. The electrical energy performance, highest temperature, and temperature-response rate were computed by a three-part analysis of the temperature versus time curve to explore the resistive heating performance of the laminated composites. Later, the impacts of ohmic heating behaviour on the structural collapse and mechanical properties of the composite materials were examined using dynamic mechanical analysis and X-ray micro computed tomography [88].

A test setup with a power source and an IR camera (865Testo UK) was used to observe the heating properties of laminated composites (Figure 12). The electrodes were kept 7 cm apart by having the power supply fastened to CF at two of the composite’s edges. The IR camera recorded the temperature in the centre of each composite during the heating process while being placed 10 cm above the surface. Prior to further analysis, all samples, identified as CH₁, WH₁, CH2, WH2, WH3, and CH3, were continuously heated for 10 h at 3 V to study heat ageing.

The carbon nanotube (CNT) layer on the nylon yarn makes it an excellent conductor of electricity and heat. The goal of this research was to coat the CNT with Nylon 6/Spandex yarn to produce electrical conductivity. SEM analysis was used to investigate the physical properties of CNTs. The electrical properties of yarns coated with CNTs in stretched and relaxed states were explored. The conductive yarns revealed a poor resistance when subjected to a high stretch load. Moreover, the heating efficiency of the yarns coated with CNTs was investigated by applying a variety of voltages to the yarn’s surface. The maximum temperature was around 280 °C when the voltage was 5 V for 60 min [89]. Nylon6/Spandex yarns were treated with a sodium hydroxide solution containing CTAB as the dispersing agent. Then, 1 g/LCTAB and 5 g/L NaOH were combined to produce the solution at room temperature. The prepared solution was then dipped into Nylon6/Spandex yarn, and the M: L (material to liquor) ratio was set at 1:40. For 1 h, the entire assembly was heated to 60 °C. Following that, the yarn was thoroughly rinsed with distilled water and dried. In the second stage, 1 g of CNT was mixed with 0.5 g/L of deionized water dye solution. The mixture was sonicated in a (BANDLIN, SONOPULS) sonicator for 60 min [90]. Every hour, the solution’s conductivity was tested, and its pH was also adjusted (by adding acetic acid dropwise). The pH and electrical conductivity of the solution were measured continuously until constant conductivity values were attained. As a result, the pH was adjusted to 6.5 using acetic acid. The alkali-treated yarn was then immersed and placed in a CNT dispersion for half an hour at room temperature before drying at 50 °C. Dipping in solution and drying were performed indefinitely for 5, 10, 15, and 20 cycles, respectively, to improve CNT loading. Finally, the yarns were post-cured in an oven at 180 °C for an hour. Figure 13 depicts the entire process involved in the application of CNTs to the yarns. The production and passage of charge during CNT development stimulates the additional electrons. Electrostatic interaction is the most likely type of interaction between polyamide and CNTs since the majority of polyamides are positively charged [91].

Table 1. Various car seat covers and their heating response.

Textiles	Depositing Element	Ohmic Heat Temperature Range (°C)	Voltage (V)	Reference
Cotton nylon	AgNPs	34–98	1–5	[89]
Elastomers	AgNWs	40	0.5–0.1	[89]
Poly (ethylene terephthalate) (PET)	Cu-Ni NWs	20–106	3–15	[90]
PET–Cu braided fabric	Cu filament	89	5	[91]
Cotton	Copper	118	10	[92]
Nylon	AgNWs	30–140	10	[38]
Polyurethane (PU)	CuNWs	46–102	7	[80]
Polystyrene film	AgFDs	52.3–180	4	[93]
Polyamide 6	CuNWs	70	1.8	[94]
Cotton	AgNPs	36.5–118.7	0.5–2.0	[88]

provides an overview of the most recent studies.

Michael Weiss et al. invented a seat heater based on flexible carbon conductive fabrics. They employed a textile sheet made up of carbon electrically conductive fibers as heating elements. As a result of its design, this technology guarantees a very homogenous distribution of thermal output, in addition to the advantage of greater mechanical comfort. It is possible to warm seats extremely uniformly with the aid of these well-known seat warmers, resulting in a pleasant temperature and eliminating any noticeable temperature variations in the seat. Despite having a set thermal capacity, these well-known seat heaters cannot rapidly reach this condition given the provided value of the seat’s thermal capacity due to the heaters’ total power requirements. Furthermore, a “quick-heating eld” is created by the unique characteristic for heating a specific portion of the sheet-type heating element with a higher heating density. This causes the thermal energy to focus on a small section of the entire seat, resulting in faster warming in that area. Therefore, the warming process will be felt by the passenger much more rapidly, for example, within the first few seconds after turning on the heater. The lower thermal output from the adjacent seat areas will not be bothersome. The following Figure 14A,B show a description of the designed seat heaters where the carbon-based conductive fabrics are connected with heating elements and electrodes [95].

Figure 14A provides an illustration of a seat heater 60 featuring a pre-designed sheet-type heating element 62. This type of heating element is made up of electrodes 64A through 64G which are woven in the longitudinal direction of the sheet-type heating element 62, spaced apart from and parallel to one another. The sheet-type heating element 62 has a groove 66 between the two outer electrodes 64A and 64G for electric separation. This groove runs transverse to the electrodes. In addition, the electrodes 64C and 64E are separated in the area of the groove 66 to create additional electrodes. The seat heater 60 can be operated in such a way that a spatial concentration of thermal output is produced thanks to the electric separation of the sheet-type heating element 62 based on the groove 66 and separation of the electrodes 64C and 64E. A sheet-type heating element 72 composed of woven carbon fibre filaments, with one main electrode positioned on each of their opposing longitudinal sides, is included in the seat heater 70 seen in Figure 15B. A quick heating element 74, which affects only a small area of the entire surface, is attached to the centre of the sheet-type heating element 72 using traditional strand technology. Two additional electrodes 76A and 76B connect to the rapid heating element 74, in which one electrically insulated heating region runs directly on the sheet-type heating element 72 in a very straightforward strand running configuration. This allows for both a solid integration of the two sheet-type heating components 72 and 74, as well as a sample design. Activation of the quick heating element 74 via the additional electrodes 76A and 76B is independent of the overall heating mode that uses the main electrodes 18A and 18B. While the lines to the additional electrodes 18A and 18B are labelled 24A and 24B, the lines to the main electrodes 13A and 13B are marked as 22A and 22B [95].

In order to evenly distribute the heat over the body-contact area of the automotive seats, Vinko et al. [96] planned to apply the heating coatings to the bottom of the textile over the whole sitting area of the seat. They studied the fabrication and properties of the heating fabric material covered with carbon single-walled nanotubes based on a composite coating using screen printing. The effects of various formulations and additions, including silver nanoparticles and highly electrically conductive polyaniline fibres, were investigated. To enhance the material’s antibacterial properties, silver was added. Three different fabric types: leather, cotton canvas, and synthetic leather, that are frequently used in the manufacture of automotive seats were coated with composite materials. The thickness (number) of the coatings allowed for customization of the electrical conductivity and, as a result, the heating characteristics. As a function of the tensile strain that certain materials are typically subjected to during usage, the electrical characteristics were measured. Additionally investigated were the heating rate and temperature consistency across the entire heated area.

Table 2. Properties of the selected materials used in the study: A—longitudinal and B—transversal direction.

	Material	Thickness (mm)	Surface Mass (g/m2)	Ultimate Tensile Strength (MPa)		Ultimate Tensile Strength (MPa)
				A	B	A	B
1	Artificial leather	0.91	680.0	27.3	70.9	5.6	4.2
2	Leather	0.98	640.1	56.7	49.7	20.8	20.5
3	Cotton canvas	0.43	320.0	14.8	15.1	18.9	34.2

provides a summary of the materials’ characteristics, and Figure 15 shows optical micrographs of the top surfaces [96].

The homogeneity of the surface temperature is one of the crucial factors in determining the potential practical application of this type of heating textile. The temperature gradients on the leather, cotton canvas and artificial leather sample’s surface during the heating process were analysed using a calibrated IR camera. The heating response of all the samples observed with IR camera are showed in Figure 15d,e,f tracked the samples’ gradual heating using the surface IR pictures (Figure 15). The sample began to heat rapidly near the electrical contacts during the initial stage of heating, peak temperatures began to enter the sample’s interior during the intermediate stage, and by the time the heating process was complete, the temperature in the heated area was more-or-less uniform (within a few °C). The study results indicate the importance of using large electrodes that span the entire sample’s edges for optimal performance [96].

7. Future Prospective of Conductive Textiles in the Automotive Industry

Electrically conductive textiles are constantly being brought into the automotive field. The future trend is the use of textile-based sensors, enabling non-driving activities integrated in the car interior and supporting a richer user experience. Automobile designers are focusing on the integration of lightweight and multifunctional smart textiles in car interiors. For instance, the demand of electrically conductive textiles is increasing to shape new technologies such as automated driving, entertaining in-car user interfaces, and creating unprecedented opportunities for designing further comfort [97]. Moreover, the new designs of car interiors will be integrated with modern technologies that assist the user in driving, such as navigation systems or voice assistant systems, where the user interface is essential to the way people perceive the driving experience. Scholars consider that the integration of smart-textile-based materials and the use of an adequate smart suit (providing a lot of information on the driver) will transform the car into a living space rather than just a means of transportation [98]. The conductive textile-based sensors will indicate the level of thermal comfort, the sense of sound and suffocation in the cabin, the level of concentration of the driver, reduced awareness, and much more. Therefore, there is an intense need to revisit the car interior and design a single system that will support the blend of both interaction design and interior design into a seamless union [99]. This new trend requires novel means for the applications of e-textiles (interactions seamlessly embedded within car fabrics or leather surfaces). Moreover, the novelties in car interiors are inclining to propose the e-textile input interfaces as tactile screenless means of controlling multimedia, windows, and A/C inside the car). We hope that further research will be conducted to overcome the highlighted gaps between the e-textiles and human–vehicle interaction (HVI) areas. Furthermore, the present gaps will inspire researchers to use electrically conductive textiles and develop fully automated vehicles (driving tasks will be removed from drivers, and all tasks will be non-driving tasks). Several techniques, along with their limitations and benefits, are described below in

Table 3. Advantages and disadvantages of conductive fabrics used for ohmic heating.

	Fabrication	Materials	Advantages	Disadvantages	Refs.
1	Metal wire heaters	Copper, silver, steel wires, etc.	Uniform heating and greater heating efficiency	Poor flexibility, rigid oxidative corrosion, and not comfortable	[9] [10] [10]
2	Electroless plating on textile	Silver copper, nickel, palladium, etc.	Uniform deposition, good flexibility, comfortable, and able to apply over complex structures	Oxidation, effluent treatments, and poor durability	[34] [37] [35]
3	Electroplating of metals	Copper, silver, and combinations of metals	High conductive and good ohmic resistance	Less rigid and less flexible	[100]
4	Printing of metallic inks over textiles	Conductive polymers and carbon-based inks	Comfortable, well-defined conductive visuals, lightweight, and cost efficient	Less heat generating capacity and poor durability	[14] [58]
5	Vapour-deposition techniques	Metals, alloys, and mixtures	Produces a thin film of coating	Lack of pressure, highly volatile, and nonpyrophoric	[60]
6	Metal particles coated	Copper, silver	Uniform coating, flexible structure, comfortable, light weight, and greater heating	Oxidation of copper particles, particles eroding, and less durability	[56]
7	Carbon-based conductive materials	Graphite powder, carbon nanotubes (CNTs), and graphene	High specific surface area, high conductivity, and high conversion of efficiency	Complex preparation process and high cost	[92]
8	Graphene sheets	Silver coated graphene	Fast heating and uniform temperature delivery	High temperature but risk to damage skin	[17]
9	Activated carbons	Copper nanoparticles	High porosity, high specific surface area, desired surface functionalization, and heat-resistance	Less durability and less ohmic conductivity	[101]
10	Polymer composites compounds	Carbon fillers	Lightweight, resistance to corrosion, degradability, chemical stability, and lower production costs for electric heater	Low ductility, complex manufacturing process, low stiffness, and low fatigue resistance	[16]

.

8. Conclusions

The electrification of the vehicle is a current focus of research, in the area of lowering energy consumption due to environmental concerns. In order to evenly distribute the heat over the body contact area of automotive seats and to create pleasant interior conditions within cars, as well as provide comfort, conductive textiles are most effective. These textiles behave as a semiconductor (as an electrical conductor and also creating resistance, which in turn creates heat). Flexible textile ohmic heaters present great advantages due to their ability to bend, stretch, and stitch. Different heating systems fabricated for car seats have been explained in this study. Metal-coated textiles should be used on any product that is to heat the human body. As a result of their minimal sheet resistance, metal nanostructure heating systems could be operated at lower driving voltage levels. Electrical heating items, in general, increase the temperature through integrated heating elements. The current review has focused on the use of various textile-based ohmic heating systems, their fabrication techniques, heating range, and reliability. Furthermore, this study is useful for future trends in reducing energy consumption, which is an effective strategy for lowering the air pollution and carbon emissions caused by vehicles. Moreover, another promising effector technology lies in the washability concept of wearable textile-based heating systems. These car-seat fabrications are washable and reusable. This study is beneficial for future trends in minimizing energy consumption, reporting an effective way to reduce the carbon emissions and air pollution produced by vehicles.