A flexible and ultra-highly sensitive tactile sensor based on Mg-doped ZnO nanorods for human vital signs and activity monitoring

Development of high-performance nano-ZnO based piezoelectric tactile sensors with outstanding sensitivity, good linearity, flexibility, durability and biocompatibility has a great significance for smart robotics, human healthcare devices, smart sensors and electronic skins. A flexible piezoelectric pressure sensor with a sandwich structure was fabricated by using PI/Cu film as top and bottom electrodes, Mg-doped ZnO nanorods (MZO NRs) as sensing layer and polydimethylsiloxane polymer as charge coupling layer. The MZO NRs were synthesized by a one-step hydrothermal method and their structure, morphology and defect states were characterized by using scanning electron microscopy, atomic force microscope and x-ray photoelectron spectroscopy. The piezoelectric properties of the flexible tactile sensors based on MZO NRs hydrothermally synthesized from different Mg2+/Zn2+ molar ratios were investigated comparatively by evaluating their force response actions. The results indicate that the sensors exhibit a linear response in the range of 0-1 N. The device based on MZO NRs can achieve an ultra-high sensitivity of 3.1 V N-1 (308.5 mV kpa(-1)) due to higher piezoelectric coefficient of MZO NRs, which is about five times higher than that of the undoped ZnO NRs sensor. Moreover, the sensor shows excellent stability and durability by loading 2500 cycles of pressing. It is demonstrated that the sensor can used to accurately monitor human motion status and vital signs in real time, such as facial motion, joint movement, wrist pulse, cervical artery, and heartbeat.