*4.2. Aspects of Aging*

The information about the working property of textile sensors, conductive materials, and protective clothing over their entire lifetime still remains underreported in the literature. For firefighters' protective garments, the aging of materials under environmental conditions reduced due to the low shear resistance even after a short period of time. The mechanical strength of textiles was reduced by up to 80% before a damage was detected visually [68]. The thermal degradation in aramid/basophil firefighter cloth occurred before the optical change was detected. After a convective heat of 80 kW/m<sup>2</sup> and a radiant heat exposure of 40 kW/m2, the mechanical properties of fabrics in a tensile test decreased by 40% and 60%, respectively (660 N) [69].

The advantages of thin flexible and electric coatings are the good conductivity and the low impact on textile properties such as their handle, flexibility, and density. Possible problems with coatings are their corrosion and insufficient adhesion between the textile and the coating substrate [70]. Polyester fabrics were coated with polypyrrole during incubation in saline substrate for up to two weeks at 37 ◦C. They exhibited a resistance in the range of 10<sup>3</sup> to 10<sup>4</sup> Ω/square. It was observed that the decrease in electrical conductivity was related to the oxygen uptake during incubation and due to cracking of the coating [71].

Figure 8 shows the results of a literature search on sensor aging containing three different search concepts. The concepts are highlighted with green for "thermal aging of sensors in textiles", blue for "functional aging of sensors in textiles" and red for "aging of temperature sensors in textiles". There are gaps in the literature dynamics of all concepts in the last 29 years, which do not provide a general concept for sensor aging in textiles. During the period of 2000–2019, the total number of references for the concepts of thermal aging of sensors in textiles, functional aging of sensors in textiles, and aging of temperature sensors in textiles were two, five, and four respectively, which indicates low scientific interest in degradation and aging in temperature sensors in textiles.

**Figure 8.** The effects of aging on sensors in textiles.

### *4.3. Aspects of Life Cycle of Conductive Textiles and their Regulation*

Electrically conductive textiles will gain more importance for mass consumer applications. Thus, a new kind of waste will be formed. The market of smart textiles and wearable electronics is estimated to grow from \$20 billion in 2015 to \$70 billion in 2025 [72], which emphasizes their importance for the mass consumer application. According to the European Commission in 2017, the high potential of wearables on the European market was reported in the orientation paper about smart wearables [73].

The waste difficulties of e-textiles can be overcome by implementing an appropriate eco-design strategy, which include e-textile labeling and the use of compatibility standards [74]. The impact of new waste could cause toxicological stress on human health, the ecosystem, resources, land use, and water use. These negative impacts can be reduced through the life cycle assessment at an early stage of the development, which assesses the potential environmental impact of products and identifies solutions for preventing pollution and decreasing the resource consumption [75].

As an example for a toxicological assessment of a surface of a modified textile, the coating of polyester and cotton fabrics with nano-metal oxides such as CuO and ZnO was studied. Fabrics treated with water and ethanol showed a release of CuO and ZnO nanoparticles up to subtoxic concentrations of 1 μg/mL in A549 cells. At a low concentration up to 10 μg/mL, there was no acute toxicity observed in lung epithelial and macrophage cells compared to an exposure of 100 μg/Ml [76].

Besides the toxicological evaluation, the production of e-textiles in industrial processes has to comply with the legal requirements of European Eco-design, which describe the development of energy-related goods. Future goods design and sustainable material managemen<sup>t</sup> can be related to the U.S. Environmental Protection Agency, which regulates the life cycle of products during their manufacture [77]. The use of metals for conductive substrates in textiles should be regarded as a metal finishing process, which is conducted by the industry. Consequently, the industry is bound by the laws of regulation for metal finishing such as the Resource Conservation and Recovery Act, the Clean Air Act, and the Clean Water Act (CWA). The CWA includes the Effluent Guidelines and Standards for Metal Finishing and the Effluent Guidelines and Standards for Electroplating. These guidelines and standards are mandatory for facilities dealing with electroplating, coating techniques, electroless plating, printed circuit board production, chemical etching, and milling. The standards determine the concentration of pollutants in wastewater from the above-mentioned processes, which are described in milligrams per m<sup>3</sup> [78].

### **5. Temperature Sensors and E-Textiles**
