## 2 Materials and Methods
### Materials
#### 2.1.2 Preparation of Samples of Heating Inserts  
Heating inserts were made on a knitted fleece fabric (B1) by automatic lockstitch sewing a in a \"one wave\" pattern using a CNC Template RPAS 1200 mm \(\times\) 900 mm (Richpeace company, Shenzhen, China) sewing machine. The technological parameters were as follows: working speed--500 RPM; stitch length--0.25 mm; stitch type--lockstitch; bottom thread tension--high; and top thread tension--low.  
To prepare a heating insert, a Bekinox VN/HT 14.2.9 stainless steel fiber heating thread with a thickness of 0.3 mm and an electrical resistance of approx. 30 \(\Omega\)/m, applied in heatable textiles (Bekaert, Zwevegem, Belgium), was used as a heating element. The thread was characterized by a fiber diameter of 12 to 14 \(\upmu\)m and the highest durability. Thin, flexible copper-clad steel wires with a diameter of 1 mm and an electrical resistance of 0.18 \(\Omega\) were used to connect the heating thread with the built-in system. The heating thread was connected to electrical wires using copper sleeves with a diameter of 1.5 mm.  
Two variants of heating inserts were produced. One variant was made by sewing a heating thread onto the knitted fleece fabric by the quilting method using the CNC Template RPAS (the insert marked with the symbol A, Figure 1). The heating thread (silver) was a needle thread, and the polyester thread (red) was used as a hook thread. In the second variant, the heating thread was sewn on in the same way, but, in addition, the heating thread was secured with an openwork knitted fabric (the insert marked with the symbol B, Figure 2). The knitted fleece fabric (B1) with a sewn-in heating thread was glued together with the openwork knitted fabric using a polyurethane adhesive for fabrics (sold in sheets). This also prevented a short circuit between two opposite sections of the heating thread. The developed heating inserts were characterized by high flexibility (Figure 3).  
In order to characterize the heating inserts, their basic parameters, such as thickness, weight, and surface area, were determined. The characteristics of the heating inserts are presented in Table 2.  
The produced heating inserts are a compromise between size limitations and heat flux felt in a comfortable way by the user. The heating effects depend on the total power of the clothing and, therefore, on the power of a single insert, with a specific resistance at a given supply voltage applied. Real-life tests involving end users allowed the authors to determine the maximum required power to be provided by the clothing at a level of approx. 4 W per insert at a supply voltage of 5.5 V, which is safe and does not cause unpleasant tingling.  
The inserts could not be too large due to the fact that steel thread limits the flexibility of clothing, which is unacceptable in professional garments intended for mountain rescuers. On the other hand, a small size of heating insert results in a high temperature increase, which may be uncomfortable or even dangerous for the user. Hence, an insert area of 165 cm\({}^{2}\) was found as the optimal size (for the heating thread resistance of 30 \(\Omega\)/m and the supply voltage of 5.5 V).