关键词: 可穿戴, 导电聚合物, MXene, 电加热元件, 可靠性

Abstract: Wearable electric heating elements with high heating efficiency, adjustable heating temperature and long heating time are the key components of electric heating garments, and are widely used in personal heating, sports training and local heat therapy. In recent years, the industrialization of wearable electric heating elements has been accelerated because the preparation and performance of wearable electric heating elements with different materials have been researched in-depth. Such reliability problems as nontoxicity, breathability, flexibility, weight, safety temperature and lifecycle of wearable electric heating elements is commonly ignored.
The wearable electric heating elements including metal materials, carbon materials, conductive polymers and MXene can be classified into one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) wearable electric heating elements. The 1D wearable electric heating elements can be prepared by multiple methods such as preparing metal wires directly with metal materials, composite heating lines, yarns with coating metal particles,and wet spinning with conductive materials. Although the 1D wearable electric heating elements have excellent spinning performance, the overall resistance can be changed because of fabric tightness, density of warp and weft, content of conductive yarn and the fabric strain. The preparation methods of 2D wearable electric heating elements include coating, soaking, chemical deposition and printing, and the heating performance of elements is related to the thickness and uniformity of the conductive layer. The 3D wearable electric heating elements can be prepared by two methods, including building the aerogel structure with the conductive layer obtained by depositing and building the aerogel structure directly with conductive materials. Although the favourable thermal insulation effect is shown in the 3D wearable electric heating elements, the overall resistance is significantly affected by the produced strain when the element is stressed. Metal 1D wearable electric heating elements and carbon nanotube 2D wearable electric heating elements are commonly used in industrial production. According to the modified conductive materials, the preparation of wearable electric heating elements with electromagnetic shielding, human infrared radiation and sensing is becoming a current research hotspot. However, it is easily overlooked that after the conductive materials are treated with different processes, the chemical residues in the elements may be harmful to human health.
The safety voltage of human body is 36 V, and the minimum temperature for low-temperature scald which is caused by direct contact of human skin with heat source is 45 ℃. The scald risk is increased because the pain nerve response becomes dull in the hot and humid environment. Therefore, it is important to limit the applied voltage and maximum temperature of the elements and to ensure the breathability and comfortability of the elements. The excessive heating temperature is caused by uneven resistance distribution or unreasonable design of wearable electric heating elements, and the heating temperature shock is caused by the poor washability and wear-resisting property. The wearable electric heating elements are encapsulated by polymer in order to improve the washability and wear-resisting property, which has a series of negative effects on the elements, such as reduced flexibility, poor breathability and increased weight.
Wearable electric heating elements have become one of the important ways to regulate human thermal comfort. Although increasingly in-depth research has been carried out on electric heating materials, the reliability of the prepared wearable electric heating elements still needs to be improved. Therefore, it is still a difficult point in this field to meet the requirements of safety and washability of components, as well as flexibility, air permeability and lightness. The industrialization of wearable electric heating elements can be accelerated by the in-depth research on the encapsulation process.

Key words: wearable, conductive polymer, MXene, heating element, reliability