Research on Flexible and Superhydrophobic Temperature Sensor Based on Laser-Induced Graphene

Body temperature is a crucial indicator of human health, and flexible temperature sensors can achieve long-term, remote, and real-time temperature monitoring, which is of great significance for bedside monitoring and early warning of diseases in elderly households. However, the development of most temperature monitoring devices is limited by their high rigidity, large volume, cumbersome preparation process, uncomfortable wearing, and low linearity. To address these issues, this paper proposes a high-precision assembly method for a flexible and superhydrophobic temperature sensor based on patterned laser-induced graphene (LIG). The temperature sensor can be portable integrated into masks and band-aids to achieve the monitoring of breathing frequency and skin temperature in various parts of the body. Firstly, the sensor is designed as a curved snake-shaped structure to ensure a large area of sensing in relative space, while also providing good resistance to bending deformation. It can be conformal attached to the human skin, allowing the sensor to adjust to the body's dynamic curve changes. Secondly, a facile and scalable laser direct writing process is used to prepare a 3D porous graphene foam from the carbon-containing polyimide (PI) film, and the high-performance flexible temperature sensor is obtained through the adjustment of the laser parameter. Finally, immersing the sensor into hydrophobic silica (Hf-SiO2) suspension and dried to obtain a flexible and superhydrophobic temperature sensor with moisture resistance. The one-step prepared temperature sensor is small and light (30 mm×5.5 mm×0.06 mm), which guarantees wearable comfort. The micro-appearance characteristics of the LIG flexible temperature sensor show a large number of three-dimensional stacking discharge micron/nano-porous structures, which are the result of numerous gas products released during the process of laser rapid light and thermal effects. The performance test results show that the temperature sensor has good stability, repetitiveness, and linearity within a detection range of 25～75℃. The sensitivity curve shows the negative temperature coefficient (TCR=−6.534×10-4/℃) and high goodness of fit of 0.991 84. The small-range test close to the skin temperature of the sensor shows a resolution of the 0.1℃. In addition, research on the influence of external factors on the temperature performance of sensors shows that wearable temperature sensors have good bending capacity, and the use of Hf-SiO2 packaging layer can effectively isolate the high humidity in the environment without sacrificing temperature sensing performance. Finally, the temperature sensor shows the real-time monitoring ability of the human body's breathing frequency and the changes in human body temperature in the actual application demonstration. The following conclusions can be obtained through experimental data analysis: (1) Laser direct-writing technology can make a flexible temperature sensor with arbitrary shapes in one step in one step. The small serpentine-shaped structure design effectively improves the bending change and wearable comfort. (2) LIG flexible sensors have high sensitivity, wide detection range, high accuracy, and good stability over a wide temperature range. (3) The introduction of the Hf-SiO2 packaging layer effectively isolates the impact of high humidity of the environment, while not sacrificing the temperature sensing performance. (4) With adhesive, the sensor can be portable integrated with masks and band-aids to achieve real-time monitoring of respiratory rate and body temperature. © 2023 China Machine Press. All rights reserved.