###### Abstract  
Rescue operations in mountain areas, lasting many hours, pose a mental and physical burden on the rescuer's body. In addition, they cause thermal discomfort associated with variable energy expenditures and the high variability of weather conditions. One of the solutions for improving the comfort of the work of mountain rescuers is clothing with an active heating function. This paper discusses the technology of manufacturing flexible heating inserts with steel thread as a heating element. In order to assess the durability and safety of the applied heating inserts, they were subjected to simulated conditions of use. Multiple washing (25 cycles) and bending (9000 cycles) as well as exposure to artificial acidic sweat did not cause a significant change in the electrical power and resistance of the heating inserts. In order to assess the effectiveness of the inserts, they were tested within a system of clothing fabrics on an \"artificial skin model\". Supplying the heating insert with a voltage of 4 V increased the thermal resistance of the fabric system, incorporating the insert by approx. 40%. Due to their high flexibility, durability, and heating efficiency, the developed heating inserts are a major step towards the development of electrical heated clothing. heating insert; electrical heated clothing; clothing for mountain rescuers; smart textiles; electronic textiles; wearable electronics; heating efficiency +
Footnote †: journal: _Sci._  
0000-0001-6000-0001-6000][PERSON]  
0000-0002-000  
etc. Such clothing generates data and sends it by wire or wirelessly to a computing unit responsible for further processing of the acquired data. Such a mode of operation leads to obtaining a response or feedback from the user [1]. The latest generation of smart textiles and garments recognizes and responds to stimuli or the environment. It includes, among others, thermoelectric clothing or clothing that monitors the user's health [2; 3; 4]. Active and smart clothing also includes heated clothing if it adapts the heating system to the user's needs. Heating systems used in textiles are based on metals (mainly chromium, copper, or nickel) or polymers. Such textiles are made by integrating metal wires into the fabric or yarn or by coating individual fibers or fabrics with metals [5; 6; 7]. If the flexibility of the textile material is of the essence, heating can be based on polymeric materials. Conductive polymer yarns can be incorporated in textiles using knitting, weaving, or embroidery techniques. Multilayer composite structures are obtained by coating and lamination. This solution is used in jackets, gloves, inserts, undersuits, insulated vests, and socks. For instance, heated insoles constitute a source of heat inside footwear, being set by means of wireless remote control. Insulated fabric prevents heat transfer through the footwear soles [3]. Textiles may also contain heating elements made of carbon fiber, which are characterized by good thermal efficiency and generate heat quickly and evenly. A carbon heating element enables the generation of far infrared radiation, and it can, additionally, perform a therapeutic function in physiotherapy treatments. When combined with a low-voltage DC battery, its service life is up to 100,000 h. An additional advantage of such heating elements is their small size, allowing their placement or use when they are carried inside garment pockets [8]. To date, heating panels used in clothing have been based on electric or infrared heating [9]. Many researchers have conducted research and produced prototypes of clothing protecting against cold. In 2010, [PERSON] et al. [10] investigated the effects of cold protective clothing on user comfort. Their study was carried out on three winter military uniforms used in different decades. According to the subjects tested, the new system, compared to the other two, provided the highest feeling of warmth and better comfort of use during physical exertion.