Abstract
Electric heating garment can improve the thermal comfort for people living and working in cold environment. Compared with passive heating materials, electrical heating shows dominant advantages on reusability, controlled temperature, safety and so on. This review article systematically introduced the material preparation, electric-thermal properties, advantages and disadvantages of the existing flexible heating elements, and elaborated the research and application progress of smart garments in detail, providing reference for the research of flexible heating elements and smart garments. And the existing challenges and the possible future perspectives were also discussed.
Keywords
Introduction
With the improvement of people's living standards and their increasing attention to their own health, smart garments attract the researcher’s attention, becoming a research point. Smart garments can monitor the physiological and environmental parameters of the human body and make reasonable responses according to the needs of the human body, thus improving the quality of people's life [1,2]. Electric heating garment (EHG) is an important part of smart garments. EHG can warm people actively for living and working in a low-temperature environment, improve the thermal comfort of the wearer, and effectively relieve the pain of patients with rheumatic diseases [3–6]. Heating clothing can be divided into solar heating clothing, chemical heating clothing, phase change material heating clothing and electric heating clothing. The solar heating clothing absorb visible light and near-infrared radiation from solar radiation by the solar thermal storage materials, then reflect human thermal radiation. The chemical heating clothing provides thermal energy by chemical reactions; the phase change material keeps warm by energy storage. Whereas the heating temperature of the solar, chemical, phase changed materials is not controllable and the application is restricted. It is difficult to meet the thermal needs for long-term and low-temperature environment. The emergence of solar thin-film batteries and new lithium batteries with high energy density provides conditions for the development of EHG with light weight and good thermal effect [7].
EHG is a kind of clothing that uses electric energy to drive the heating element inside the clothing to provide thermal energy actively, where the electric energy is environmental-friendly. The heating theory of the heating element is Joule heating, which generates heat by electrical current passing through a heat-generating conductor. Light weight, mechanical flexibility and durability are the necessary conditions of the heating element. The electric heating clothing on the market get a wide varieties, from head to feet. Whereas the qualities are uneven, durability and stretch-proof are the key point of the application. As shown in Figure 1, academic publications with regard to electric heating elements are undergoing growth from these decades. Although the number of the publication is growing, electric heating area has not received enough attention, and the key points are not solved for industry. Moreover, only few articles about heating garments were published.
Number of publications sorted by year with the key words ‘‘electric heating garment’’ and ‘‘electric heating element’ indexed in the Web of Science in Dec. 2019.
EHG is generally composed of power supply, flexible heating element, clothing, safety protection device, temperature measurement and temperature control module. The flexible heating element is a key component of EHG. Its performance affects the safety and comfort of heating clothing directly, which heating principle is Joule effect, the heating power (
Flexible heating element
Flexible heating element is an important component to provide energy for EHG, which is composed of conductive material and substrate material. Conductive materials includes metallic and non-metallic materials. Metal materials include metal fiber, metal coated yarn and metal nanowires and so on. Nonmetallic materials include carbon fibers, carbon nanomaterials, and conductive polymers (polypyrrole, polythiophene, polyaniline, and polyethylene dioxophene thiophene:polystyrene sulfonate etc.). The substrate material of the flexible heating element determines its flexibility, stability and safety. Flexible heating elements are mainly divided into flexible heating fabric, flexible heating film and multi-functional flexible heating elements.
Flexible heating fabric
Fabric has a good air permeability and is easy to integrate into clothing, so integrating conductive material into fabric becomes an important branch of flexible heating element. Metal wire, metal coating, metal nanowires, conductive polymer coating, carbon fiber, carbon nanotubes, graphene and other conductive materials can be used in the preparation of flexible heating fabrics.
Metal heating fabric
Fabrication process of coating [14] Copyright © 2016, American Chemical Society
Li et al. [15] used solution dipping method to deposit Cu with fluorinated-decyl polyhedral oligomeric silsesquioxane (F-POSS) and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) onto the cotton fabric which had a low sheet resistance of 0.33Ω·sq−1, which could heat to 135°C by applied voltage of 1V. The twisting, folding and bending behaviors led the sample resistance increasing slightly, which indicated F/Cu/PAH film adhered with cotton fabric tightly.
Ali Hamdani et al. [17] used silver-plated yarn and elastic filament to knit plain, rib and interlock rib heating fabrics, and studied the heating properties of the three fabrics. At a voltage of 9V, the maximum equilibrium temperatures of plain, rib and interlock rib heating fabrics were 83.7°C, 69.3°C and 107.8°C, respectively. Obviously, the interlock rib structure is more suitable for the heating clothing. The temperature only changed no more than 1°C when the knitted fabric stretch range was in 100%. Li et al. [18] focused on the electromechanical properties of conductive yarns and conductive knitting stitches are given in terms of length-related resistance and contact resistance. The electromechanical properties of conductive yarns, two overlapped conductive yarns, and conductive knitted fabrics under unidirectional extension were investigated by the three experiments. The experimental results demonstrate the equivalent resistance could be modeled by superposition of length-related resistance and contact resistance, then the corresponding empirical equations are established which had a high coefficient of determination (0.99, 0.96, 0.94) and low standard errors (0.43, 0.077, 0.18) of the fitting curves for experiment I, II, and III, respectively.
Li et al. [19] used an electrothermal model to study the static and dynamic behavior of conductive fabric with silver yarn. The calculated coefficient of determination is greater than 0.98, and the fit standard error is smaller than 1.11. So the analytical equation could accurately model the electro-thermal characteristics of the thermal fabrics under an applied electric current and compute the temperature at a certain time. Li et al. [20] evaluated the thermal-electrical stability of heating element with silver yarn. The results showed a larger linear density made the silver coated yarns better electrical resistance stability in the oven ageing experiment. The better conductive properties made the samples obtain higher temperature at the same voltage. The temperature is proportional to the power in the heating process. Kexia et al. [21] studied the resistance values and temperature of electro-thermal knitted conductive fabric (EKCF) with wool and silver yarns. The experiment results showed that when the EKCFs were subjected to certain voltages over time, the resistance values of the resistance area increased over a short time and then gradually decreased, and the temperature gradually increased in the first 1000 s and tended toward stability after a certain period of time. In this study, the double-needle-bed (1 × 1 rib) knitted structure with a voltage of 2.4V was most suitable for heating element.
Conductive polymer heating fabric
Generally, the structure of conductive polymers are composed of a polymer chain structure and a non-bonding anion or cation. Therefore, the conductive polymer not only has the characteristics of metal (high conductivity) and semiconductor (p- and n-type) brought by doping, but also has the characteristics of diversified molecular design structure, which is processed easily. Conductive polymers provide a new research direction for the preparation of flexible heating elements. Some researchers obtained the conductive polymer coating on the fiber surface by in-situ polymerization, electrochemistry and impregnation, which was used in the preparation of heating elements. However, the conductive polymers, such as polypyrrole (PPy), has poor mechanical properties and poor stability under ambient conditions, which restricts its commercial applications.
Hao et al. [24] prepared PPy conducting cotton fabric by in situ polymerization. The surface resistivity of PPy coating fabric (3 × 3 cm) was 303 Ωsq−1. At the applied voltage of 3/6/7. 5/9V, the maximum equilibrium temperature can reach to 28/56/76/83°C. The power density of 9V was only 2.9 W/m2, which was associated with maximum linear equilibrium temperature. The PPy coating fabric can work well under bending condition. Xie et al. [25] discussed the ratio of pyrrole monomer (Py) and oxidant FeCl3 by in situ polymerization. When the concentration ratio of Py: FeCl3 was 2:1, the surface resistivity of PPy coating fabric was 0.37 Ωcm−1. At the applied voltage of 5V, PPy cotton fabric reached 168.3°C in 3min, and its tensile strength reached 58 MPa, which was far higher than that of pure cotton fabric (8.9 MPa). Lv et al. [26] combined in situ polymerization and interfacial polymerization method to coat a thin and compact PPy layer on cotton, silk, wool and polyester fabric, the surface resistivity was less than 10 Ωsq−1. At the voltage of 6 V, the temperature of the PPy coated fabric could rise to 100°C. The results of the washable test showed that the conductivity of the PPy coated fabric remained basically unchanged after being washed in dichloromethane for 20 times. Meanwhile, the resistance was unchanged under the deformation of flatting, curving and twisting.
Because the cotton has hydroxyl groups which could combine with PPy easily, most researchers use cotton as a substrate for the preparation of conductive fabrics. But as a result of the limitation of cotton fabric's own characteristics, such as its flexibility is poor, some researchers also such choose polyester fabric, such as PET, as substrate. However, the fabric surface is smooth, which will affect the coating property, so the researchers select different dopant, or modifying substrate to improve the electrical conductivity of the polyester fabric. Kaynak and Håkansson [27] prepared PPy coating PET - Lycra fabric by chemical polymerization, FeCl3 as oxidant, anthraquinone - 2 - sulfonic acid (AQSA) and naphthalene sulfonic acid (NSA) as the dopant. The surface resistivity of coated fabric was 150 ∼ 500 Ωsq−1. The results showed that the polymerization time was too long, the polymerization at room temperature and the polymerization bath ratio were too large lead to the polymerization of pyrrole which formed the coating layer unevenly on the fabric surface. The resistance of the prepared conductive fabric increased by 10% after 72 hours due to the gradual oxidation and degradation of PPy at a voltage of 24V, which caused the loss of conjugation of the polymer backbone. The experiment also explored two ways of fastening wires to conductive fabrics, one was sewing copper wires to both sides of the conductive fabric with polyester yarn, and the other was fastening copper wires to both sides of the conductive fabric with metal fasteners. After testing, the way of fixing the wire with metal fastening is more effective.
Tavanai and Kaynak [28] conducted alkali reduction treatment on PET fabric that etched the fabric surface to modify the contact property. After treatment, the fiber surface was rough and the rubbing fastness decreased, making PPy easier to deposit on the fabric surface. When alkali reduction last for 30 min, the PET fabrics resistance decreased to 352Ω. At the applied voltage of 35 V, it could heat to 100°C in 10 min. The resistance of the coated fabric decreases with the increase of the voltage. Wang et al. [29] fabricated a highly conductive and hydrophobic textiles by depositing in situ polymerized polypyrrole (PPy) modified MXene sheets onto poly(ethylene terephthalate) textiles followed by a silicone coating. This fabric exhibited an excellent moderate voltage-driven Joule heating performance, the electrical conductivity of ∼1000 Sm−1. The sample (4 × 1 cm2) reached to 57 and 79°C at the voltage of 3V and 4V, respectively. According to the experiments, the fabric showed a stable equilibrium temperature around 57°C during 3600 s, and no significant change of temperature after 50cycles. A considerable number of researches have been carried out on PPy in order to improve its stability, mechanical properties and processability by the formulation of nanocomposites [30,31].
Moraes et al. [34] pretreated polyamide 66(PA66) fabric with plasma, and then coated it with 1–5 layers of glycerin and PEDOT:PSS, which was packaged and adapted into clothing. The coating fabric with 5 layers of glycerin and PEDOT: PSS had the highest stability. It could be heated up to 38°C at the voltage of 7.5V, the current density of 0.3A g−1. The experimental results showed that the target heating temperature of the coating fabric can be adjusted according to the applied voltage and the coating layer number of glycerin and PEDOT: PSS. Although uniformity of coating thickness needs to be improved, the heating effect was similar as that of CNTs coated fabrics at the same voltage. However, the heating performance was not well by 4 times heating, which had the same reason, oxidative degradation, with PPy conductive fabric. Gong et al. [35] adopted the spray drying method to spray 5 wt % of PEDOT: PSS onto polyester fabric (2/4/6/8layers sprayed), as shown in Figure 3. The influence of spray quantity to the conductive performance and surface morphology was studied in this research. After spraying 8 layers PEDOT:PSS, the fabric surface resistance fell to 12.10 Ω/□, which could be heated to 56.2°C in 100 s at the voltage of 7 V. The infrared images showed that the temperature distribution of the coated fabric was even, illustrated the PEDOT: PSS coating was uniform. The rubbing and washing behaviors affected the resistance slightly, the reasons were as following, polar hydrophilic groups and roughness of the fabric surface increased by the plasma treatment which led to a good adhesion between fabric and PEDOT:PSS.
Illustration of fabrication of PEDOT:PSS-coated PET fabrics through spray-coating [35]. Copyright © 2018 Elsevier B.V. All rights reserved.
Carbon-based heating fabric
Carbon-based conductive materials, such as graphene, graphite powder, carbon black and carbon nanotubes, are generally attached to the surface of ordinary fibers in the form of nanoparticles to form conductive fibers, which has a wide range of sources. Especially the invention of graphene and CNTs, which has effectively improved their conductive properties. Carbon fiber is a kind of high-strength and high-modulus fiber with more than 90% carbon content.
Ilanchezhiyan [39] prepared conductive cotton fabric with CNTs by dipping and coating, which use single-walled carbon nanotubes (SWCNTs) as dispersant to combine CNTs with cotton fabric. First, 0.5 mg mL−1 SWCNTs were dispersed in water containing 10 mg mL−1 sodium dodecyl sulfide (SDS) as a dispersant. After 1 hour of ultrasonic dispersion, SWCNTs were coated on cotton fabric and dried at 120°C for 10 minutes. After 10 cycles, the adhesion of CNTs on cotton fabric increased. The surface resistance of fabric decreased from 190 kΩ to 5 kΩ. Due to the adhesion to the increased number of CNTs on the surface of the fabric, more conductive paths were produced and the conductive performance was improved. It could be heated to 96°Cat the applied voltage of 40V. Rahman and Mieno [40] coated cotton fabric with multi-wall carbon nanotubes (MWCNTs) by impregnation-drying coating technology to enhance the thermal properties of the fabric and improve the thermal conductivity. First, MWCNTs were treated with a citric acid plasma, which resulted in a large number of carboxyl groups adhering on the surface. The pure cotton fabric and thread were impregnated into 0.25wt%–1.0wt% MWCNTs solution and dried at room temperature. After repeated impregnation, the resistance was reduced by 3 orders of magnitude. The thermal conductivity of pure cotton fabric was 0.027w mK−1, and that of treated conductive fabric was 0.045w mK−1. At the power density of 0.1 w cm−2, MWCNTs can be heated to 80°C. After standard washing for 30 minutes, MWCNTs are still attached to the fabric surface. They can also be used as wires to light the LED.
Alvira Ayoub et al. [41] mixed organic material with CNTs. The protein can disperse CNTs to avoid the reunion phenomenon. The organic/CNTs was vapor deposited on the surface of polyester-cotton plain woven fabric to decrease resistance to 15 ∼ 20 Ω, which had a high thermal stability. The CNT ink prepared by this method could be used for printing technology. It was a convenient way to prepare the conductive lines. Sadi et al. [42] developed the CNT ink which could be printed directly on the cotton fabric. After 3 times printing, the resistivity was 50.75 Ωsq−1, which reached to the maximum equilibrium temperature of 100.9°C by 30s at the voltage of 8V. After 5 and 10 heating cycles, the resistance was stable and only increased 0.51 Ωsq−1, respectively. When the sample was put on the wrist, there was only 0.2°C changed under bending and relaxing during heating.
Wang et al. [43] modified PET plain fabric with polydiallyl dimethyl ammonium chloride (PDDA), and then coated it with Ag/RGO by chemical reduction. The results showed that dense silver particles and reduced graphene oxide (RGO) sheets were deposited on the surface of the modified PET fabric. Compared with the coated fabric without PDDA modification, the deposit amount of the modified fabric was larger, so the resistance rate is low, only 0.173 Ωsq−1. At the applied voltage of 4V, the conductive fabric could heat to 69.6 °C in 30s. After washing, it still could heat to 59.7°C. The heating fabric also has hydrophobic, antistatic and electromagnetic shielding functions. Wang et al. [38] adopted an improved Hummer method to prepare graphene sheet, which was dissolved to prepare the conductive PET non-woven fabric vapor deposition method, shown in Figure 4(b). This method was based on the vapor system with hydride and acetic acid, then the multifunctional reduction graphene oxide/polyester (RGO/PET) fabric was prepared through the adsorption filtration and reduction method, which surface resistivity is 24.7 Ωsq−1 by this method. The resistance had no obvious change after 100 times bending and 10 times fold. It could be heated to 50°C in 3 s rapidly at the applied voltages of 6 V. The sample existed the excellent properties on tensile sensing and capacitance.
Mingwei et al. [44] applied graphene/polyurethane (PU) composite ink to spray the cotton woven fabric, which was a simple preparation method, as shown in Figure 5(a). First, graphene/PU composite inks were prepared, and graphene and PU were mixed by ultrasound (600 W, 40 kHz) at a weight ratio of 3:2 for 120 min to obtain a uniform ink. The parameters of the spray gun (spray pressure was 0.5 mpa, liquid flow was 0.5 ml cm2, spray distance was 50 cm) were determined through pre-test. Graphene/PU ink solutions were sprayed on the plain fabric, which thickness was 0.28mm and area density was 160g cm2, by different weight ratios (1:100, 5:100, 10:100) to form a double-layer structure. The resulting fabric was dried at 120°C for 5 min. The fabric sprayed with 5 layers of GO/PU had the most significant joule heating performance. At the applied voltage of 12V, the maximum equilibrium temperature could be up the 162.6°C with the highest heating rate of 8.4°C s−1. Hyelim et al. [45] used impregnation and hot pressing method (shown in Figure 5(b)) to prepare graphene/waterborne polyurethane (WPU) solution of 8wt% graphene to be coated onto para aramid knitted fabric. The durability of flexible fabric was increased by WPU, and the conductivity the fabric was improved by graphene. The surface resistance of 7.5 x104 Ωsq−1 and the capacitance of 89.4 pF were obtained by dip coated 5 times and hot pressing at 140 °C. At the applied voltage of 50 V, the maximum equilibrium temperature could be up to 58.4°Cin 20mins, and maintain for 60mins.
(a) Schematic diagrams of the fabric heaters fabrication process with spray method [44] Copyright © 2018 Elsevier B.V. All rights reserved; (b) Illustration of the fabrication process for graphene/WPU by impregnation and hot pressing method [45]Copyright © 2019, Springer Nature; (c) Schematic illustration of the fabrication process of GFs. (I) HI reduction; (II) thermal annealing and mechanical press; (III,IV) twist [46] Copyright © 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim; (d) Scheme to the preparation of highly conductive SOGS-PVA nanocomposites by hot drawing method [47] Copyright © 2018, American Chemical Society.
In addition to spraying and impregnating method, some researchers also prepared graphene films into graphene fibers. Wang et al. [46] prepared graphene fibers by twist-spinning of thermally annealed graphene films, as shown in Figure 5(c). The fracture strain increased by 70% with the toughness of 22.45 mj m−1. The ultrahigh temperature thermal treatment can repair the structural defects of graphene, and the conductivity can be increased to 6 × 105 S m−1. Therefore, graphene fibers can provide stable electro-thermal performance under deformation conditions such as bending and stretching. Due to the high conductivity, the heater made by graphene fibers had an ultra-fast electric heating response, which can be heated to 424°C at the voltage of 5V with a heating rate of 571°C s−1. In particular, these fibers could be woven into electrical heating fabrics because of its excellent mechanical and electro-thermal properties. An embroidery fabric was made from the graphene fibers which could dry a drop of water at the voltage of 3 V, and be heated to 400°C at the voltage of 4.7 V in 100 s. The button battery could provide enough energy to reach to the heating temperature person required in 2s. The thermal stability of the fabric under the conditions of human motion was studied in this research, such as bending and torsion. There was no significant change of the heating temperature after 1000 times bending.
Neella et al. [48] fabricated a single step approach for electrically conductive textile for large scale production of reduced graphene oxide (RGO) nanosheets coated cotton cloth based films for heat generation application. The thickness of heating fabric is only 200 μm, and the maximum equilibrium temperature reached to 52°C by 1min at the voltage of 40 V. Yang et al. [47] designed slightly oxidizing graphene sheets and hot drawing nanocomposites to form a uniform, dense and highly aligned graphene network, which fabrication process was shown in Figure 5(d). The achieved conductivity of the nanocomposites was 25S m−1 with 6.25wt% of graphene. By the voltage of 10, 15 and 20V, the maximum equilibrium temperature went to 37.2, 43.4 and 53.6°C.
The materials, preparation and heating properties of heating fabrics are summarized in Table 1. As it shown, a great number of preparation method are used for preparing the conductive and heating fabrics, but the resistivity or conductivity has a clear difference which leads to different the maximum equilibrium temperature. It is easy to know that whatever preparation method is used, a controller resistance is difficult to achieve.
Summary of materials, preparation and properties of heating fabrics.
Flexible heating film
Recently, polymer composites gets a continuous development because of its excellent mechanical properties and simple preparation method. As a kind of reinforcing material, conductive polymer combined with the polyurethane matrix which shows an excellent electrical and mechanical properties [49]. This is a good and facile way to improve stability and safety of the conductive and thermal materials. Obviously, the advantages of electro-thermal film are thin, flexible, fast heating rate, and good heating uniformity.
Carbon-based filling materials
In order to improve the stability and safety of the heating materials, the flexible heating films are prepared by conductive metal wire or conductive nanomaterials being packaged or mixed by polymers, such as (polyethylene terephthalate, polydimethylsiloxane, thermoplastic urethanes, polyimide, polyisophthaloyl metaphenylene diamine, polymethyl methacrylate).
Luo et al. [52] synthesized a high-performance electro-thermal film by a novel layer-by-layer method, as shown in Figure 6(a). Electro-thermal films were prepared by pouring CNT solution into the designed mold and subsequent curing of thermoplastic polyurethane (TPU) applied on the CNT layer. To improve the electro-thermal property, increasing the CNT content is feasible. By the voltage of 10V, the maximum equilibrium temperature of the film could reach ∼140°C in 100∼150s. Ning et al. [53] employed a facile and low-cost method, as shown in Figure 6(b), free from CNT pre-dispersion for fabricating super-aligned carbon nanotube (SACNT)/Polyimide (PI) composite film which combines the advantages of both SACNT and PI. Compared with PI film, SACNT/PI film had better strength, Young modulus, conductive and heat conduction properties.
(a) Schematic diagram of CTF fabrication procedure by layer-by-layer method [52] Copyright © 2016 Elsevier Ltd. All rights reserved;(b)Schematic illustration of the fabrication process of SACNT/PI composite film [53] Copyright © 2018 Elsevier Ltd. All rights reserved;(c)Fabrication procedures of graphene-based heating chips [54] Copyright © 2018 Elsevier B.V. All rights reserved.
The advantages of this method include larger CNT content and improved dispersion. It is an easy-operating and low-cost process for industrial production. The film was thin enough and thermal uniform. Because of the excellent conductive property, the thermal respond time was very short, only ∼5s. According the test, the film could maintain a stable temperature, the standard deviation was less than 5.2°C. The heating rate ranged from 3.3°C s−1 to 118°C s−1. Wang et al. [55] fabricated MWCNT/poly(m-phenylene isophthalamide) (PMIA) nanocomposite films by a dissolution-dispersing and thermal-forming method. The film with 7.0wt% MWCNTs could reach to 130°C in 30s under voltage of 12V, the heating rate ranged from 1.0∼8.2°C s−1.
Metallic based filling materials
Metal material, such as Ag, Cu, et al, has a good electric-thermal property. Thus the metallic based filling materials are used to mix with polymer as a heating element.
AgNWs is a kind of common material as the filling material for preparing heating film, which has a fast thermal response rate and an excellent electro-thermal performance. Ji et al. [58] studied the thermal response characteristics of transparent AgNWs/PEDOT:PSS composite conductive film. Based on the thermodynamic analysis, the influence of substrate heat capacity, heat transfer coefficient between air and heater, the thermal response which was effected by the resistance and size of AgNWs thin film was studied in this research. AgNWs films with a suitable surface resistance are formed on glass or PET substrate. The transparent AgNWs/PEDOT:PSS film heater based on 2mm glass can be heated to 60°C within 100s and 90°C within 300s at the voltage of 6V. The power consumption of the heater was only 179°Ccm2w−1, which had a uniform temperature distribution and a good stability on reusing. Hong et al. [59] inset AgNWs to polydimethylsiloxane (PDMS) films. As shown in Figure 7, under the condition of over 60% strain and other simulated human motion, the conductive film had an excellent electrical conductivity, which could generate joule heating with a fast thermal response. The resistance only changed a little when the film was heating. Li et al. [60] embedded AgNWs in a new type of heat-resistant polyacrylate to prepare transparent and flexible heating film. The resistivity of this heating film was 25 Ωsq−1, the transmission rate was 86.4% under 550 nm monochromatic light irradiation. This heating film had a corrosion resistance as well. It also can be heated to 230°C at the applied voltage of 13V with a fast thermal response rate. The flexible curvature diameter of the film heater was 10mm, after 3000 times bending, the maximum equilibrium temperature only decreased 3°C.
Highly stretchable and transparent heater. a) Schematic illustration of the stretchable and transparent heater composed of Ag NW percolation network on PDMS film. b,c) Pseudocolor image at room temperature (left) and infrared camera thermal image (right) of a Ag NW/PDMS stretchable and transparent heater operating at 60°C with (b) no strain and (c) at 60% strain condition [59]. Copyright © 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim
Zhang et al. [61] reported a new method for preparing AgNWs film heaters, which used RGO sheet as a protective coating layer. The results showed that RGO coating was well combined with AgNWs film and played a protective role. The film heater had a good optical transparency, electrical conductivity, oxidation resistance and thermal stability. The film was prepared under 700°C. After annealing, the surface resistivity was 27 Ωsq−1, which could be heated to 150°C at the voltage of 10V. He et al. [62] prepared a AgNW/polymethyl methacrylate (PMMA) composite film by synthesis and in situ transfer method. The film exhibited excellent conductivity, high figure of merit and strong adhension of percolation network to substrate. The sheet resistance of 0.13∼1.4 Ωsq−1 of the film was achieved by adjusting the AgNW density. The maximum equilibrium temperature could reach ∼130°C in 10∼30s under applied voltage of 3V. When the applied voltage increased to 5V, the maximum equilibrium temperature reached 97°C. Cai et al. [63] prepared AgNW/TPU film with Hydroxylpropyl methyl cellulose (HPMC), which was easy to prepared and made the AgNW dispersed evenly. At the applied voltage of 3, 6, 9 and 12V, the the maximum equilibrium temperature reached 23, 38, 59 and 77°C. By the applied voltage of 9V, the fill had no significant change on thermal stability after 600 cycles. However, under applied voltage of 12V, the film deformed by 83 cycles.
Kim et al. [64] used transfer printing to prepare white copper (CuNi) micromesh on glass and polymer substrates. The basic parameters of the film was as following: the thickness of CuNi layer was 150 nm with a 2 mm line-width and line-spacings of 100, 200 and 300 mm, the surface resistivity were 16.2, 38.4 and 50.9 Ωsq−1. The property of the CuNi film was similar as the commercial indium tin oxide (ITO) film. At the voltage of 9V, the power density of the samples (25 mm×25 mm) with 100, 200 and 300 mm line-space were 2.72, 1.69, and 1.15 W, respectively. At a voltage of 9V, the sample with 100 line-space could be heated to 225°C in 60s. The heating film had a good thermal stability after heating 20 times. The heating property was also stable under stretching and bending, hence, this film can be used as the material of wearable heating element.
Hybrid filling materials
Only one kind of conductive filling materials may have shortages for heating film, such as poor conductivity and thermal stability. So more and more researchers try to use variety of conductive material together to make up for the shortage of single materials.
Jana et al. [65] used a simple in-suit chemical oxidative polymerization method to synthesize Polypyrrole-Single Walled Carbon Nanotube (PPY-SWCNT) composite. PPy grew along the surface of the SWCNTs to form a more ordered molecular structure with increased crystallinity. The electrical conductivity was 26.2Scm−1. Li et al. [66] fabricated graphene and aligned silver nanowire (G-AgNW) hybrid structure film by thermal evaporation of silver on the aligned electrospun nanofiber templates and subsequent transfer of monolayer graphene onto the AgNWs. At an input power approximately 0.31 W/cm2, the maximum equilibrium temperature of the film could reach 96°C, which the electric resistivity was 8.2 Ωsq−1. Meanwhile, the films showed excellent electromechanical stability under cyclic bending because the graphene anchoring on the top surface of ASNWs could share tensile stress and serve as local conducting pathways at break-points even if small cracks were generated. The film could withstand 1000 cycles bending at a radius of 150 μm with only a 110% increase in resistivity. He et al. [67] fabricated a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and silver nanowire (AgNWs) composite ink to print pattern on poly(ethylene terephthalate) (PET) substrate by screen printing. The printing parameters - mesh number, printing layer, mass ratio of PEDOT:PSS to AgNWs and pattern shape - were discussed in this research. AgNWs were prepared by a hydrothermal method and the conductive ink was prepared with dimethyl sulfoxide (DMSO) and PEDOT:PSS. At the applied voltage of 10, 20, 30, 40 and 50V, the maximum equilibrium temperature of the conductive film printed was 29.1, 40.1, 62.3, 73.3 and 92.4°C, respectively. The pattern of hexagon could reach to 99°C in 20s at the voltage of 12V, however the PET substrate melted under this temperature.
Other filling materials
Polymer film has the advantages of insulation, but its low thermal conductivity, prone to trap heat, result in membrane damage or danger such as high temperature burned circuit, in addition to the research of polymer film add thermal conductive performance good carbon-based materials, and the researchers by joining other heat conduction performance of the material, to improve the defects.
Hao et al. [68] prepared a SiC/polyurethane (PU) film to improve the thermal conductivity. Firstly, the nano SiC particles were modified by silane coupling agent, which could help SiC particles dispersing into PU solvent evenly. Then this modified SiC/PU solution was coated on the silver wire of 0.05mm diameter stuck on the glass. The SiC/PU film with 10wt% SiC had the best thermal stability and mechanical properties, and showed an excellent performance of thermal conductivity. The heating temperature of the heating film increased 28.4°C in 3s. In this way, aging or short circuit of heating wire can be effectively prevented.
The materials, preparation method and heating properties of heating films are listed in Table 2. Compared with the heating fabrics, the heating temperature is much higher because polymers could bear a high temperature than the fabrics. Similar with the heating fabrics, a controller resistance is hard to get. Whereas the permeability of the polymer film is a defect which may affect the wearing comfort which should be improved in the futureS. And for all the researches, the relation between temperature and resistance under different load voltage was not investigated which was a limitation of the heating element research.
Summary of materials, preparation and properties of heating films.
Multi-function flexible heating element
Some flexible heating elements sensing, thermos-chromic and other functions except for heating function, which could be called multi-functional heating element.
Bai et al. [69] proposed a new method for preparing soft heating fabrics with temperature-sensitive functions. A kind of temperature-sensitive fine copper wire was arranged on the fabric surface in S shape, and then integrated into two pieces of adhesive interlining through a simple thermal bonding method, and its thermoelectric and mechanical properties were studied. The parameters of the adhesive interlining was: the gram weight was 30g/m2, the coefficient of thermal conductivity was 0.204 W mK−1. The spacing of copper wire was 0.5cm that resistance of this heating element was 5.849Ω. The experiment showed that the resistance is linearly correlated with the temperature. When the temperature rose from 30°C to 100°C, the resistance change rate was 24%. The maximum equilibrium temperature can reach 70°C at the voltage of 8V. This research also simulated the power consumption of heating element in a low temperature environment, where 5.69W and 4.82W were required to reach the preset temperature of 40°C under −20 and −10°C environment.
Thermo-chromic materials usually used in non-luminous displays, sensors, military camouflage and accessories areas, which changes color by temperature changing triggered by electrical heating. Robertson et al. [70] combined thermos-chromic textiles with a heating circuit, then color changed by the heating circuit. Two different techniques are used to design and prepare heating elements. Copper heat-sinks were used in combination with surface mount devices (SMD technology) which act as resistors enabling heating of the copper. The second technique used Track resistors, consisting of distributed elements, long narrow metal tracks deposited on both rigid and flexible substrates. A series of circuits were designed to optimize the heating performance. The maximum equilibrium temperature and heating rate were controlled by adjusting the applied voltage. The colorless pigment of microcapsules was coated on the substrate by screen printing, and the color change was achieved by controlling the heating temperature at the voltage of 4∼7V. When the temperature reached 47°C, the color change was obvious. Chen and Huang [71] wove the stainless steel conductive yarn into the thermal-chromic household textiles, and the temperature changed by electrical heating. The heating temperature was controlled by the applied voltage, and the discoloration effect can be achieved within 30s at the applied voltage of 6V. Huang et al. [72] developed a simple and low cost approach to prepare an electrically triggered color-changing fabric with a two layered integrated structure. A cotton fabric with PE layer was the substrate, and PPy coating was prepared on the side of cotton fabric by in situ polymerization to make the cotton fabric with conductive heating function. The thermal-chromic ink was printed on the PE layer by screen printing to make PE layer with color-change function, as shown in Figure 8. This new smart fabric had a good discoloration performance controlled by power and was wearable.
Photographs of the textile (3.6 cm × 3.5 cm) showing smart colorchanging effect [72]. Copyright © The Royal Society of Chemistry 2016.
Electric heating clothing
The above heating elements are belong to the active electronic element which could be integrated with smart clothes to make people feel warm and more comfortable [73,74]. Thermal clothes has won a large proportion of clothing market over the last few decades. There are variety of electric hearting garments in the market, that the CNT or silver plated yarns are the materials for the heating element. On the opening ceremony of the 2018 winter Olympics, the US team put on the heating garment made by Ralph Lauren made [75] which was made of the conductive ink. At the closing ceremony, the performers of “Beijing 8 minutes” wore costumes with graphene heating film as heating element [76], which were continuously heated for 4 minutes at −20°C. It can be sure that seniors, the people who work or live in the cold environment, are eager for the warm garment. Due to the improvement of the materials and technology, people have high requirements on the comfort, durability, lightness and safety of clothes.
Structure and classification of electric heating garments
EHG generally includes clothing, heating element, temperature control device and the power supply. The following three parts are integrated in the clothing, where the heating element is connected with temperature control device [77], both of them are getting energy from power supply. The temperature control device includes temperature sensors and controller, which temperature sensor detects the interior temperature around the heating element, the controller regulates the heating temperature with the power supply. EHG includes heating clothing, heating gloves, heating belt, heating knee pads, heating eye patches, heating socks, heating shoes, and so on.
For the heating clothes designing, how to integrate the heating element into clothes to make human comfortable is quite important. The electronic system must provide an additional value for clothing instead of causing discomfort [78]. Some researches showed how that the optimal temperature of human body skin surface in the state of rest is 32.10°C–34.30°C. If the temperature changes more than 4.5°C, human will feel discomfort. The purpose of the heating garment is to create a comfort microclimate for human. The size, property and position of the heating elements will affect the comfort property of the clothes, such as thermal conductivity, humidity, safety.
Electric heating clothing for warming
There are many parts of the body that need to be heated, and EHG can cover almost all the parts, such as gloves, shoes, knee pads, waist guards, underwear, and jackets. In 1988, Scott [79] evaluated the technology and physiological functions of EHG. The following requirements were made for heating elements: (1) Heating elements should be flexible that can be bent and folded because certain parts of the body such as fingers or knees. (2) The applied voltage must be lower than the safety voltage of 6, 12 or 24V. (3) The heating medium must be insulated to prevent a short circuit in contact between different parts of the garment. (4) The entire garment shall be cleanable (by washing or dry-cleaning). (5) The operating temperature of the clothing should be appropriate to ensure that the wearer's skin temperature does not exceed 40–45°C to prevent pain or burn. (6) The current passing through the heating element shall be kept below the safety current passing through the human body. (7) Power controller should be added, which can adjust the temperature to maintain the comfort of human body in various external environmental conditions. (8) The heating elements shall not affect the free movement of the human body. In a word, the above suggestions are instructive for the heating elements and EHG. Haisman [80] arranged the physiological assessment and user experience experiment of various types of EHG (including overalls, gloves, gloves, socks and the insole) by the military facilities. The survey showed that the users in extremely cold conditions may experience the thermal comfort and the flexibility of movement from EHG, but durability of the battery was the shortcoming of EHG.
In the same year of 1988, Orban and Lewis [81] applied for the patent of heating gloves, which were based on woven fabrics coated with conductive metal and heated by electricity. The conductive fabric was cut into the shape of the front and back of the glove, then stitched together only at the fingertips with wires, which were combined in the middle of the two insulating layers. The wires are drawn from the root of the palm. Zhang et al. [82] fabricated PEDOT:PSS layer of 1.5 μm coating on pineapple fiber fabric by an improved vapor deposition method without affecting the fabric feel, weight, and permeability. The surface resistance was 61Ω/□, which could be heated to 45°C at the applied voltage of 6V. Cutting and stitching the conductive fabric did not affect the resistance, so it was feasible to prepare the fabric suiting for any parts of the body, such as the hands and joints. The conductive fabric was stitched to the finger of the glove, the structure and circuit of the glove were shown in Figure 9(a). When 3V was applied, the finger part was heated to 29.1°C, while the palm without the conductive fabric was only 22.5°C.
Cheng et al. [83] prepared Copper nanowire (CuNWs) fibers by dipping method for heating elements. The yarns made by CuNWs fibers of 12 were woven into the fabric (l = 8 cm, r = 0.5 mm, Rn = 2.5Ωm−1), both sides connected with copper wire as electrode, the integration method was shown in Figure 9(b). At the dc voltage, the maximum equilibrium temperature was up in 20s, and the temperature increased when the voltage went up. At the voltage of 3V, this CuNWs heating element could be heated to 57 °C. Under 100 times repeated heating, the maximum equilibrium temperature remained unchanged. When 40% tension was applied, the resistance increased by only 8% and the temperature dropped from 53°C to 46°Cat the voltage of 2V. The fiber not only had an excellent heating property, but also a good mechanical property on resisting the impacts from bending, torsion and stretching. After 1000 tensile cyclic strain (50%), bending radius (1mm) and the distortion (50 per meter circle), the resistance of the fiber respectively increased by only 5%, 2% and 2.5%. Therefore, these yarns could be woven into a wearable heating fabric, which was integrated with the microcontroller unit to control the heating using an android phone program, which could be applied to knee pad for physical therapy and to jackets for warming.
Kayacan et al. [84] prepared the stainless steel wire heating plate into clothing, and evaluated the thermal performance of the body model in the cold environment. The experiment showed NI-MH battery was suit for supplying power in low temperature environment. At the environment of 0°C, four –layer heating plate could be heated to 30 °C with NI-MH battery, but the heating time was short. The more layer number of the heating plate, the more energy consumption and the shorter heating time was. The heating plate of three-layers could obtain maximum heating energy of 120J. Li et al. [85] used a sheet resistance method for a two dimensional system of conductive fabrics to model the resistance of the complex resistive network simply in term of sheet resistances and dimensions with silver yarns. The experimental results demonstrated that the resistance of conductive fabrics could be calculated with a reasonable margin (∼12%) before any prototype manufacturing, which provides an alternative solution to the existing numerical methods that require high computation cost, as well as assists fashion designers without working knowledge of electronics to produce a smart garment prototype rapidly at low cost to meet the high demand of the market. Havelka et al. [86] also designed heating system based on embroidery with hybrid threads which could be used in winter clothes especially for elderly or people with impaired thermoregulatory capabilities. Consider the safety, routine clothing maintenance, battery placement, electronics interconnection and durability, this study gave a practical example and demonstrated the use of hybrid threads that are applies by embroidering the heating structure directly onto a suitable textile backing.
With the continuous development and expansion of heating elements, carbon fiber is mostly used as material in commercial heating garment at the present stage. Yang [87] summarized carbon fiber based heating garment. In addition to a good thermal performance, carbon fiber heating garment had a far infrared emissivity as high as 0.95. It had good heating performance and high safety under the low voltage 1.5–12V. It can use battery or mobile power supply to provide electric energy. It was especially suitable for special operation protective clothing under extremely cold environment, such as outdoor clothing of soldiers, high-altitude mountaineering clothing, cold storage overalls, submarine overalls, etc. Generally, the heating temperature of EHG was around 40°C to make people comfortable. In the theory of traditional Chinese medicine, hot compress therapy occupies an important position. It can achieve the effect of heat therapy at about 55°C, which could dilate blood vessels, relax muscles and bones, promote blood circulation and remove stasis, and relax muscles, tendons and ligaments. Therefore, carbon fiber based heating fabric was applied to specific body parts, such as the waist, knee, shoulder, etc., which can improve the blood circulation of the human body and achieve the effect of hot compress therapy.
Carbon based heating elements are divided into three types according to the preparation methods. (1) the superfine carbon powder was mixed with the superfine carbon fiber powder to form a conductive coating, and then coated on the plastic film or fabric as a heating layer; (2) carbon fiber paper was used as the heating element; (3) the carbon fiber based fabric as the heating element. The paper also analyzes the power supply of carbon fiber based heating element in detail, and puts forward five requirements: (1) the power supply voltage should be lower than the safety voltage of human body; (2) the power supply shall have a sufficient capacitance to ensure that the electric heating lasts for more than 2 hours; (3) the power supply should be small in size, light in weight and easy to carry; (4) the power supply should have a long power life that can be reused for more than 100 times; (5) the power supply shall have a low cost.
Functional electric heating clothing
On the basis of thermal heating function, the EHG can also achieve medical therapy, sensing, or color change function to become a functional EHG that could satisfy people’s variety requirement.
Some researches integrated the far-infrared materials, traditional Chinese medicine in the EHG that make the heating clothing with a medical therapy effect. Wang et al. [88] combined carbon fiber heating elements with traditional Chinese medicine to prepare knee pads that can be heated, which can not only be used for daily exercise protection, but also for physical therapy, effectively preventing and alleviating the pain of rheumatism or rheumatoid arthritis. Jiangrang [89] prepared heated knee pad with nanometer carbonite graphene, which could not only heat evenly and rapidly, but also achieve the effect of far-infrared physiotherapy. Quangeng [90] combined nanocomposite inorganic far-infrared cloth with resistance wire to prepare a portable far-infrared therapeutic instrument with low cost and flexible operation. Bhat et al. [91] integrated polypyrrole cotton fabric into clothing and used batteries of 9V to make the wearer feel warm. Compared with traditional heating pads, PPy fabric had a better air permeability and anti-electromagnetic interference, as well as functions as gas and temperature sensors, which can be applied to the military or the elderly.
During the last decade, there has been an increasing interest in integrating electronic capabilities and components with textile materials and equipment used by the soldier. Sahin et al. [5] researched smart textiles for soldier. Smart technologies aim to help soldiers fulfill the assignment with lighter load and less equipment. Heating function is one important part for the smart clothes, which could widens the operating temperature range and imporves the protection against cold. Tampere University and University of Lapland [92] in Finland designed some prototypes of the heating garment which could be used in arctic environment in northern countries. Dokuz Eylul University Engineering Faculty [93] designed the heating clothes which integrated with sensors, textile-based heating elements, measurement control unit, humidity sensors, and small batteries. The sensors could sense the skin temperature. The cold weather condition cost a great deal of energy to warm the body.
Functional heating clothing is not only suitable for heating [94], but also can be applied in military and medical fields. Combined with sensing, communication, power transmission and Internet technology, it can realize health monitoring, physiological feedback, interaction and other functions during the wearing process. But there are still two important problems to be solved: (1) the connection of wires should be secret and should not affect the wearing; (2) the power supply should be portable and rechargeable for recycling.
Smart heating garment
Smart heating garment (SHG)is a kind of clothing that can meet user needs, with user interface, mobile application, and temperature adjustment system [95,96], so that users have a better use experience. In 2015, Google announced Project Jacquard, which combines touch-screen technology with a jacket that allows users to experience a variety of mobile services, such as music and navigation, kicking off the combination of electrically heated clothing and smart electronics.
Hao et al. [97] invented a flexible heating fabric system. The flexible heating fabric system comprises a flexible heating fabric and a temperature control system. Flexible heating fabric was prepared by waving conductive yarns (conductive metallic yarns or metallic coating yarns) into fabric to achieve heating function. Conductive yarns distributed in warp and weft direction, which could adjust the resistance by different quantity of the conductive yarns. The temperature control system consists of smart phone, single-chip microcomputer system, temperature sensor with micro-electronic switch and bluetooth communication module. The signal of digital temperature sensor could be read by the single-chip microcomputer directly. The heating fabric system can control the temperature by adjusting the resistance, and had the advantages of softness, good wearability, temperature control and preparing easily. Roh et al. [98] developed a conductive yarn blended with ultra-fine polyurethane coated copper wire and polyester filament, and embroidered the conductive yarn into the fabric. This method had obvious design advantages of fabric size, heating area, temperature monitoring and power consumption. Because of the convenience of the embroidery method, the heating fabric was customized, and the heating clothing was highly comfortable to wear. The heating rate of the fabric was fast, the heating temperature was uniform and had a temperature sensing function. Regardless of climatic conditions, the heated clothing can meet the thermal comfort requirements of the wearer. Lee et al. [99] designed a smart temperature-controlled jacket with a heating system installed on the front and back of the jacket, with the control switch and mobile application as the interface to control the heating system. This temperature-controlled smart jacket can provide a comfortable temperature environment for human body, which was developed by the textile tactile sensors and fiber integrated temperature control system.
Shenzhen Smart Clothing Technology Co., Ltd [100] developed smart constant temperature heating clothing with AI and sensing technology, as shown in Figure 10. SHC was prepared with far-infrared physical therapy function due to nanometer graphene fiber and carbon fiber as heating materials, and the heating temperature can be adjusted through APP. The resistance of the heating 3.5 Ω, eating temperature range from 35°C to 65°C at the input voltage of 5V, rated power of 5 ∼ 10W. The heating fabric with graphene fibers can be washed for 100 times, the heating fabric with carbon fiber can be washed 30 times. This company also developed a graphene heat fabric which had high thermal conductivity, ultra-thin, bending resistance and other technical characteristics. The resistance of the heating 1.8 Ω, eating temperature range from 35°C to 65°C at the input voltage of 5V, rated power of 5 ∼ 7W. It can be washed 50 times without affecting the heating performance.
Intelligent thermostatic heating garment and heating infrared images [100].
Development tendency of flexible heating element and EHG
With the development of material science and electronic technology, it will be the trend of future development to realize the real smart of flexible heating element integrated sensor and electronic circuit. The current heating element materials have their own defects, such as the rigidity of the wire resulting in weaving difficulties and poor wearable performance; the high cost of carbon fiber resulting industrialization difficulty; the preparation of the conductive polymer resulting the resistance precision difficulty; the oxidation of the metallic coating fibers resulting the short serving time. Hence, it is necessary to combine multi-materials to make up the defect of single materials. In addition to the research on the existing heating materials, the multi-functional, lightweight, safety and stability of heating elements become the key points of the current researches. In order to make the flexible heating elements more fitly and securely, how to fabricate smart clothing, how to arrange circuit, and how to integrate controlling module with heating elements are also the hotpots in the future research. Therefore, to make heating clothing intelligent, suitable for a variety of situations, satisfied for a variety of requirements, where can develop the great potential of SHG.
However, at present, there is no special testing equipment for the thermal performance test of heating elements or heating clothing. Most researches only test thermal performance by thermal infrared camera or infrared temperature gun, yet these ways are not accurate for heating element or heating clothing. They could only detect one or several pots temperature, not a whole sample, which do not demonstrate the temperature distribution either. Hao et al. [101] developed a Fabrics thermal performance apparatus (FTPA), which includes three parts: infrared image acquisition components, signal measurement with temperature control box and data analysis & processing module. The FTPA can realize the dynamic real-time measurement of the surface temperature of the fabric, and can measure the heating performance of the heating fabric and the thermal conductivity of the fabric, as shown in Figure 11. It has the advantages of high accuracy, fast measurement speed, simple operation and comprehensive data information. In the future, corresponding measurement standards should be developed for testing the thermal property of heating elements and clothing that could promote the development of heating clothing area.
(a)Fabrics thermal performance apparatus (FTPA), (b) schematic diagram of the FTPA [100].
Power consumption, thermal comfort, and theory of the heating transfer in the microclimate of the EHG are some other points that should be studied to improve the EHG. However, there is nearly no researches on these aspects. Wenfang et al. [102] compared the cold protective performance of traditional cold weather clothing, electrically heated garment (EHG) and a chemically heated garment (CHG). The test results showed EHG exhibited a significantly higher effective heating power and thermal insulation compared to the CHG only under the low air velocity (p<0.05). The EHG could improve human wear thermal comfort in cold environment. EHGs presented a number of advantageous features over the CHGs in terms of weight and reusability. This research just compared the power and thermal insulation of the heating clothes, the uniform standard still need to be research to regulate the production of the heating garment.
Conclusion
This paper classified the heating elements into three types: flexible heating fabric, flexible heating film and multi-functional flexible heating element. The conductive materials, fabrication method, and thermal performance and other properties were elaborated in detail. The usual conductive materials of the flexible heating element are: metallic conductive materials, conductive polymer materials and carbon-based conductive materials. In order to integrate the flexible heating element to the clothing, the applied voltage should be controlled below the human body safety voltage of 12V, and the heating element can be heated to the maximum equilibrium temperature in a short time. The defects of single conductive material have led researchers to modify the material or combine multiple materials to improve the bonding strength with the substrate and conductivity. EHG is also classified in terms of functions: thermal heating clothing, functional EHG and SHC. The advantages and disadvantages are summarized, and the improvement suggestions are put forward as well.
Some other challenges will be solved by researchers. For heating element, influence of mechanical properties on electro-thermal properties, wash resistance, the repeatability of heating, the accuracy of temperature controlling are some key points to be explored further. For EHG, firstly, the temperature control systems inside EHGs are not well designed and the temperature cannot change smartly according to current necessity of different body parts [103]. Secondly, the thermal uniformity of the heating elements should be improved. According to the current researches, it’s difficult to control the heating temperature evenly. Thirdly, low power consumption is another point for the heating elements, especially in cold environment. Meanwhile, the size and weight of the battery should be decreased for convenience.
With the development of science and technology, people's demands for heating clothing are no longer only for aesthetics or warm. Lightening, multi-function and artificial intelligence have become the future development direction of EHG. Therefore, understanding the demands of target consumers and combining the research of EHG with new materials and electronic technology can help the manufacturers produce the better products and gain a larger market.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Ministry of Science and Technology National Key R & D Program “Technology Winter Olympics” Key Special Project 2019YFF0302100, Subproject 5, Research and preparation and application of key technologies of low-temperature resistance and protection clothing for winter sports 2019YFF0302105; the Natural Science Foundation of Tianjin (Grant No.18JCYBJC18500); the Postdoctoral Science Foundation of China (Grant No. 2016M591390); and the China National Textile And Apparel Council (Grant No. 2017060).
