*Review* **Textile-Integrated Thermocouples for Temperature Measurement**

### **Waleri Root, Thomas Bechtold \* and Tung Pham**

Research Institute for Textile Chemistry/Physics, University of Innsbruck, Hoechsterstrasse 73, 6850 Dornbirn, Austria; waleri.root@uibk.ac.at (W.R.); tung.pham@uibk.ac.at (T.P.)

**\*** Correspondence: thomas.bechtold@uibk.ac.at

Received: 9 December 2019; Accepted: 28 January 2020; Published: 31 January 2020

**Abstract:** The integration of conductive materials in textiles is key for detecting temperature in the wearer ´s environment. When integrating sensors into textiles, properties such as their flexibility, handle, and stretch must stay unaffected by the functionalization. Conductive materials are difficult to integrate into textiles, since wires are stiff, and coatings show low adhesion. This work shows that various substrates such as cotton, cellulose, polymeric, carbon, and optical fiber-based textiles are used as support materials for temperature sensors. Suitable measurement principles for use in textiles are based on resistance changes, optical interferences (fiber Bragg grating), or thermoelectric effects. This review deals with developments in the construction of temperature sensors and the production of thermocouples for use in textiles. The operating principle of thermocouples is based on temperature gradients building up between a heated and a cold junction of two conductors, which is converted to a voltage output signal. This work also summarizes integration methods for thermocouples and other temperature-sensing techniques as well as the manufacture of conductive materials in textiles. In addition, textile thermocouples are emphasized as suitable and indispensable elements in sensor concepts for smart textiles.

**Keywords:** textiles; temperature sensor; conductivity; coatings; deposition; thermocouple
