10.3.2　Temperature sensor devices

Human skin can not only sense mechanical stimuli，but also perceive changes in the temperature of human body and the external environment.Abnormal changes in human body temperature usually mean changes in health.Therefore，designing a e-skin device with temperature sensing function has important application for detecting abnormal body temperature caused by heatstroke and infection.

At present，the commercial temperature sensors are mainly based on the thermal resistance effect of metal-based or ceramic-based semiconductor materials.These materials are difficult to be widely used in the field of flexible e-skin due to their rigidity.Therefore，it is necessary to design the structure of these materials to make them have better stretchability.For example，metal thermal resistance materials can be designed as a serpentine curved structure or a mesh structure，so that it has good stretchability and can be used in the design and preparation of flexible e-skins.In addition，non-metal thermal resistance materials，such as carbon nanotubes，graphene，and polymer semiconductor materials，can also be used as temperature sensing materials for e-skin.According to the changing trend of resistivity with temperature，the materials can be divided into two categories：the first type of thermal resistance material has an increase in resistivity with increasing temperature，and the second type of thermal resistance material has a decrease in resistivity with increasing temperature.Both types of material can be used in the design of temperature-sensing e-skin.For example，Wang et al.prepared a temperature sensor using a network structure formed by copper nanowires（the first type of thermal resistance material）.The sensitivity of the device in the range of 25℃ to 48℃ is 0.7 Ω·℃-1.Cho et al.used reduced graphene oxide（the second type of thermal resistance material）to prepare a temperature sensor.By adjusting the degree of reduction of graphene，temperature-sensing devices having higher sensitivity and a wider temperature detection range were obtained.(https://www.daowen.com)

In addition to the thermal resistance effect，the thermoelectric effect and the pyroelectric effect can also be used to design temperature-sensing e-skins.Different from temperature sensors based on the thermal resistance effect，devices based on these two principles can work without power supply.The materials having thermoelectric effect can generate electric current under the temperature difference（Seebeck effect），and detect the temperature by detecting the current.For example，Zhu et al.used porous polyurethane and PEDOT：PSS to prepare a temperature sensor with detection accuracy of less than 0.1℃.This device can be used to detect the temperature distribution of hand.The materials with pyroelectric effect will generate instantaneous voltage due to electrical polarization when the temperature changes.Therefore，the temperature can be detected by detecting the change in voltage.For example，Ko et al.prepared a temperature sensor using a composite material of reduced graphene oxide and poly（vinylidene fluoride），and obtained a higher sensitivity（changes 2.93％per degree Celsius）and a wider temperature detection window（0-100℃）.