## 4 Conclusions  
The study presents the developed heating inserts incorporating textile materials and a conductive thread. The results indicate that the inserts are characterized by high durability in conditions simulating real use. The exposure of the inserts to multiple low-temperature water washings (25 cycles), as well as to multiple bending cycles (9000 cycles) and artificial sweat, did not cause a significant change to the electrical power and resistance of the inserts. The operation of the inserts after exposure to these factors remained normal. Organoleptic observations of the heating inserts after multiple washing or bending and twisting cycles showed no damage to the knitted fabric or heating thread. However, heating insert A  
Figure 12: Test results for the system of materials with heating insert B (system: U1 + insert B + Z4 + W2 at \(-\)30 \({}^{\circ}\)C: (**a**) temperature changes between the layers of the system depending on the power of the heating insert, (**b**) a graph of the dependence of the thermal resistance of the system with the heating insert and that of the power of the “skin model” measuring plate on the power of the heating insert. turned out to be insufficiently resistant to washing, as, during this process, the heating thread was locally pulled up to form loops. Hence, the conclusion is that the heating element must be secured with an additional material against mechanical damage, as in the case of insert B. The conducted research revealed the need to use an additional layer protecting the heating thread in the construction of the heating insert. Tests of protective properties, determining the level of protection against cold, demonstrated that the thermal resistance of the developed heating inserts (type B) in a system with underwear fabric does not change significantly after repeated washing. The process of the multiple bending and twisting of the inserts (9000 cycles) also did not reduce the thermal resistance of the system of materials with the heating insert. Maintaining thermal resistance at a comparable level after the impact of mechanical factors will contribute to easier modeling of the insert's thermal properties. The above proves a high resistance of the heating thread in wear and tear tests (washing, bending) and the high quality of the inserts. The temperature between the knitted underwear fabric and the warming material incorporating the selected heating insert with an openwork knitted fabric--at the site of insert placement at its highest power, i.e., 3.94 W--did not exceed 43 \({}^{\circ}\)C, which meets the requirements of PN-EN 60335-2-17 (max. temp. 50\({}^{\circ}\)C). It can, therefore, be concluded that the heating insert used should not cause burns to the user's skin. Taking this into account, the developed heating inserts as safe for use in contact with humans and can be incorporated in clothing. The wetting of heating insert B with artificial sweat during its operation did not damage it or interfere with its functioning. The electrical power of the new heating inserts was comparable to the power of the inserts that were wetted with artificial sweat and dried. The conducted research resulted in a positive evaluation of the developed inserts and confirmed their applicability according to the intended use. It was concluded that the materials used, and, in particular, the heating thread, were sufficiently resistant to artificial sweat, simulating the harmful effects of human sweat (due to the various types of salts contained in the sweat, the connections of the inserts may have been damaged). Conceptualization, S.K. and A.G.; methodology, S.K. and A.G.; investigation, S.K., A.G., R.K. and B.P. ; writing--original draft preparation, S.K., A.G., B.M. and P.M.; writing--review and editing, S.K., A.D., R.K. and B.P. ; visualization, A.G. and S.K. ; supervision, G.B. ; project administration, G.B. All authors have read and agreed to the published version of the manuscript.