# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 3 Fiber-Based Sensors
### Temperature Sensing Fibers  
Temperature sensors provide important information about the dynamics of physical, chemical, and biological phenomena [25]. These sensors are continually being developed for healthcare monitoring, industrial processing, and military support. In the healthcare field, temperature is essential information on diagnosis for infections, inflammatory diseases, and diabetic complications [125]. In particular, diabetic patients are insensitive to temperature changes, due to poor blood circulation and temperature monitoring, wearable devices can detect their inflammation more quickly [126]. Existing IR imaging systems for temperature measurement have a long equilibration time, due to large thermal mass, making real-time monitoring difficult. As another example, patch-type temperature sensors are not very flexible and bendable, which are difficult to incorporate into clothes. The fiber-based temperature sensors are flexible enough to be woven into a fabric for measuring the temperature of the whole body. They typically consist of a heat-sensitive material in contact with the electrode based on the (1) thermoresistive and (2) thermoelectrics mechanism. Temperature sensing fibers can be simply synthesized by dip-coating commercial threads with thermoresistors that have a positive or negative temperature coefficient, such as a polypyrrole [127; 128], CNT [129], PEDOT: PSS [130; 131], and RGO [132]. The polypyrrole-coated threads were reported to have good linearity, and electrical response retains 18.75% for nine cycles between 65 \({}^{\circ}\)C and room temperature [133]. The PEDOT:PSS-coated cotton threads showed high linearity in a wide range of \(-\)50 \({}^{\circ}\)C to 80 \({}^{\circ}\)C with high sensitivity [134]. RGO is easily dispersed in a solution and has better thermal conductivity than other carbon materials, so it is widely used as a temperature sensor material. The RGO-coated threads woven in a textile structure showed excellent sensitivity and near-constant resistance under bending and compression cycles [135]. This flexible textile could transmit temperature data wirelessly via self-powered Radio Frequency Identification (RFID) or low-powered Bluetooth. [PERSON] et al. fabricated freestanding single RGO-based fiber via wet-spinning, which showed a fast response time of 7 s and a great recovery time of 20 s (Figure 5a,b) [38]. The fiber integrated into the forearm band or undershirt measured the temperature response, while directly in contact with the skin, and it did not show changes in resistance, due to body movements. Although dip-coated and wet-spun fibers have the advantages of being quick and simple, their temperature resolution is not so high. As shown in Figure 5c, the temperature sensing fiber composed of a chalcogenide semiconductor Ge\({}_{17}\)As\({}_{23}\)Se\({}_{14}\)Te\({}_{46}\) (GAST) with two metallic electrodes (Sn\({}_{96}\)Ag\({}_{4}\)) was fabricated through the TDP [25]. The electronic mobility gap of the amorphous GAST semiconductor was small, so it exhibited a high electrical response with a small temperature change. The 8 \(\times\) 8 fiber array with separation distance placed on curved objects reconstructed differential thermal maps in two situations: Heated locally with the touch of a finger and cooled with a piece of ice (Figure 5d). With the TDP, [PERSON] et al. fabricated fiber based on thermoelectric (TE) materials, p-type Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\), and n-type Bi\({}_{2}\)Se\({}_{3}\)[52]. The thermoelectric effect refers to a phenomenon in which an electromotive force is generated due to a temperature difference, or a temperature difference is generated by applying an electric current. In the former, the temperature can be monitored inversely through the electromotive force.