*5.1. Wearable Heaters*

Wearable heaters also record temperature profiles as a function of time and can be used in many applications e.g., thermotherapy. In many cases, a combination of heating device and temperature sensor is implement with the aim to control heat generation and to avoid over temperature.

Wearable heaters, which are manufactured from Ag nanofibers (AgNF) on polyethylenterephthalat (PET) and polyimide (PI) by electrospinning, can be affixed to the skin. Heaters were connected at both ends by Cu wires, while the current was applied from the power supply for heat generation. The AgNW (nanowire) heater on the PI substrate shows a considerably stable temperature of 42 ◦C during a stretching test up to 90%. The use of SiO2 as a passivation layer on AgNW heaters can retard Ag oxidation and allow the detection of temperature up to 250 ◦C [79].

Wearable and stretchable heaters were made from PEDOT:PSS, polyurethane, and reduced graphene oxide films, which can be applied in thermotherapy. They imparted an electrical conductivity of 18.2 Scm−<sup>1</sup> and withstood elongation up to 530%. The temperature distribution of composite films was measured in the middle when voltage was applied by two copper wires [80]. Heaters were also manufactured from Ag NWs (nanowires), PEDOT:PSS, and PET materials, which withstood a temperature of 120 ◦C [81]. Stretchable heaters were also fabricated from graphene fiber (GF). The GFs were embroidered into cotton fabric and withstood finger bending and wrist movement. The temperature was recorded by an infrared camera [82].

Flexible and stretchable heaters were manufactured from carbon nanotubes (CNT), copper foil, and silicon elastomers [83]. Flexible and stretchable heaters were constructed from copper-coated polyacrylonitrile fibers, which can operate at temperature up to 328 ◦C. These heaters were manufactured from copper-coated fibers by electroplating on glass substrates [84]. Flexible heaters were manufactured from nylon-coated fabric, which was coated with Ag NWs and rubber shape memory polymer during dip-dry and spray coating. Bending, rolling, gripping, and rubbing did not show any damage of the heaters [85].

Stretchable and conductive heaters were manufactured from poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) and sodium dodecyl sulfate on cotton and polyurethane fabrics by dip coating. The temperature changes were investigated with a digital thermometer while IR images were recorded with an infrared camera [86]. Stretchable heaters were used in thermotherapy, which were produced from styrene–butadiene–styrene and Ag NW substrates. These substrates formed a mesh by thermal welding and heat treatment [87].

In thermotherapy, stretchable heaters could increase the blood flow near the wrist. The heaters were manufactured from kirigami–aluminum paper, thin elastomers of silicon polymer, and polyethylene terephthalate films, and these could be stretched to 400% at a temperature of 40 ◦C [88]. Stretchable heaters were also manufactured from copper wire/alumina/polyimide composites. These composites showed a high visible light transmittance up to 91.4% and reached temperatures up to 300 ◦C. They withstood 100 stretching and relaxation cycles at 30% strain [89]. Stretchable and wearable heaters were manufactured from CuZr and poly(dimethylsiloxane) (PDMS), which could be used at 70% elongation. They were used as portable patch units on human hands and reached temperatures up to 50 ◦C [90]. Stretchable heaters produced from Ag nanowires and polydimethylsiloxane (PDMS) substrates were used to heat human skin. A constant temperature of 50 ◦C could be observed up to 40% strain [91].

Temperature measurements were conducted by conductive substrates in textiles, which formed sensors and flexible electronic structures. Flexible electronic circuits were made by coating 35 nm Cr substrates by photolithography and 25 nm Al2O3 substrates by atomic layer deposition on Kapton E materials. Electronic circuits were integrated through the commercial weaving process integrated in textiles. They formed woven temperature sensors, which operated in the range of 20 to 100 ◦C [92]. Flexible and conductive polyester fabrics were manufactured from ploly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS), 15 wt % graphite, and dimethyl sulfoxide mixtures by coating. These fabrics were used as thermoelectric (TE) textiles, which measured temperatures up to 398 K and showed a power of 0.025 μWm−1K−<sup>2</sup> [93].

Bimodal sensors were used to detect temperature and pressure simultaneously by making use of a piezo-thermoresistive organic conductor and a dialectic substrate. The dielectric substrate was composed of poly(vinylidenefluoride-trifluoro-ethylene) and BaTiO3 nanoparticles. When the human finger pressed on the bimodal sensor, a pressure of up to 0.03 <sup>N</sup>/mm<sup>2</sup> and a temperature of up to 35 ◦C were measured [94].

### *5.2. Sensor Integration in Textiles*

Figure 9 shows eight possible application areas, where the integration of sensors in textiles is of interest. The temperature detection already has been investigated in functional garments, sport garments, the automobile industry, medical institutions, security packaging, and the fashion industry. The future seamless compatibility of sensors with textiles will increase their wearing comfort and lead to prototypes, which can be produced on an industrial scale.

**Figure 9.** Eight areas for sensor integration in textiles in November 2019.

### *5.3. Body Sweat*/*Moisture and Heat Transfer in Textiles*

Besides sweat, water content influences the wearer´s comfort in textiles. The presence of water in textiles increased the mass and reduced the heat transfer in sport and protective clothing [95]. Textiles with high water vapor permeability can transfer moisture from the skin through the textile into the environment, which continuously keeps the human body in thermal equilibrium.

Therefore, the transmission of water vapor was recorded as a function of air temperature and relative humidity in polytetrafluoroethyle (PTFE) laminated with nylon fabric, woven cotton fabric, polyester fabric (laminated with polyurethane), and hybrid PTFE membranes. The transmission of water vapor was high at high air temperature and low relative humidity [96].

In addition to the body motion, health condition can be monitored by using biocompatible and stretchable carbon nanotube-based electrodes (CNTs), which are used to detect sweat [97]. Sweat also can be detected by a wearable colorimetric pH sensor, which provides information on the metabolic state and activity of a patient. The collection of sweat in T-shirts was investigated on textile biosensors in health managemen<sup>t</sup> [98].

Figure 10 shows the increase in the literature on concepts, which are related to thermal effects and energy generation. The concepts are highlighted with green for "thermal insulation in textiles", yellow for "heat transfer in textiles", blue for "textiles exposed to temperature", and red for "energy harvesting in textiles". Energy harvesting in textiles is a new fast growing field. Its role will be significant with the development of miniaturized temperature sensors that seamlessly adapt to textiles.

**Figure 10.** Temperature measurement in textiles and their use for energy generation.

The thermoelectric effect also can be used to generate electrical energy from temperature differences between a human body and the environment.

As an example, the heat of the human body was used to power a flexible thermoelectric glass fabric, which was formed from eight thermocouples consisting of Bi2Te3 and Sb2Te3 films. It indicated an output voltage of 28 mWg−<sup>1</sup> (ΔT = 50 K) [99]. The temperature of the human body was detected by polyethylene (PE) and polyethylene oxide (PEO) substrates, which were melt mixed with 40 wt % Ni microparticles. The PEO/PE matrix treated with 40 wt % Ni showed sensitivity as temperature sensors of 0.3 V/◦C in the range of 35 to 42 ◦C compared to 50 wt % [100]. The skin temperature was measured by an embedded wire sensor, which was composed of aluminum carbon epoxy composites. These composites detected a higher skin temperature compared to multiple thermistors [101].

### **6. Outlook and Future Perspectives**

The coating of textiles with metals is a key technology for the miniaturization of low weight textile thermocouples. The metal coating follows the structure of the textile and covers its surface with a thin conductive metallic layer. The advantages of thin conductive coatings are the ability to form different geometries on small surfaces and provide a better flexibility compared to thicker substrates. Combining the conductivity of metal coatings with a fabric's flexibility, light weight, and stretch can provide substantial progress in miniaturized textile thermocouple construction. The combination of textiles with low weight thermocouples will improve the sustainability of the assembly.

Using a thermocouple is a simple way to measure temperature in textiles. Conductive thin-coated textiles can be used for a thermocouple construction, which measures temperature based on an electrical signal. There is a growing demand for miniaturized temperature measuring methods in textiles in the near future.

Besides the functionality of a device, material costs will also determine the selection of conductive parts. The use of silver as a conducive material for the manufacture of wearable heaters can be explained by its high electrical conductivity of 6.3 × 10<sup>7</sup> Sm−<sup>1</sup> compared to that of copper, which is 5.9 × 10<sup>7</sup> Sm−1. Despite the lower cost of copper (\$6.7/kg) compared to silver (\$510/kg), immediate oxide layer formation on the copper surface makes its application di fficult. Conductive PEDOT/PSS substrates (2 × 10<sup>4</sup> Sm−1) are very expensive (\$167,000/kg) and may not be suitable for the large-scale production of flexible substrates [102].

The durability of conductive textile thermocouples during wearing under di fferent weather conditions is still underreported in the current literature. The influence of use and wear conditions on the durability of textile thermocouples is due to e ffects of moisture and low or elevated temperature. Additionally, the abrasion and mechanical deformation of conductive textile thermocouples increase the rate of degradation, which is often due to limited adhesion between the textile and conductor materials. The future scientific work should focus on the loss conductivity of textile thermocouples during aging and in situ mechanical deformation. Comprehensive scientific work is required to optimize the design, lifetime, and miniaturization of textile thermocouples. This work must include the life cycle assessment of conductive textile thermocouples to prevent hazardous waste, reduce production costs, and provide appropriate strategies for their recycling.

**Author Contributions:** The authors contributed equally to the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the CORNET framework in the project Ambitex –"Textile integrated sensors for monitoring of ambient parameters" (FFG 855282), NanoStretch "Stretchable conductive textiles based on nanostructured templates" (FFG 865927), and TCCV (FFG 860474).

**Conflicts of Interest:** The authors declare no conflict of interest.
