Skin-Attachable Physical Sensors Using Single-Crystalline III-N Piezoelectric Thin Film for Personal Health and Safety Monitoring

Abstract

Abstract Body: Wearable electronics are increasingly important in healthcare applications for monitoring of the daily physical condition and diagnosis of an early stage of disorders. In particular, the wearable sensors on human skin can be utilized in real-time continuous monitoring systems because they provide comfortable wearing and consume less power. Pulse detection is one of the options and critical for high-risk groups of cardiovascular diseases such as angina, acute coronary syndrome, and myocardial infarction among the real-time monitoring of several physical parameters. Moreover, the pulse is easily detected from specific points on the human body such as carotid, temporal, fingertip, dorsal, and posterior tibial artery using skin-attachable sensor [Chen, et al. High durable, biocompatible and flexible piezoelectric pulse sensor using III-N thin film, Adv. Funct. Mater. 29, 1903162 (2019)]. Accurate sensing of the eyelid and eyeball movement is another new application. Brain disorders (e.g. Alzheimer's, Parkinson’s disease, and stroke) are accompanied by eye-related abnormal motions as early symptoms such as slow movements and the long interval between eye blinking. The diseases can be diagnosed in an early stage from the detection of unusual eye motions before they are seriously progressed. For both applications, the sensors should be light and small to attach on the human body, e.g. temple area. Recently developed thin-film piezoelectric skin-attachable sensor based on lead zirconate titanate (PZT) and ZnO inherently possess weakness for the wearable healthcare sensors. PZT contains lead, which is very harmful to the human body and environment by poisoning. A skin-attachable sensor should be containing toxic-free elements. ZnO sensor shows low sensitivity and resolution from the limitation of material, which demands additional signal processing for amplification of exact signal and minimization of the noise level. The benefit of non-toxicity is canceled by low sensitivity and extra work. As an alternative sensing element, III-N thin films are very promising to detect the physical motion of skin sensing due to their advantages of high sensitivity and durability, rapid response time, non-toxic nature on humans, lightweight, and low power consumption. Especially, single-crystalline gallium nitride (GaN) and aluminum-gallium nitride (AlGaN) piezoelectric thin films have been reported to show excellent output voltage, indicating outstanding sensitivity, rapid response time by a high electromechanical coupling factor, chemical and mechanical long-term stability, high thermal resistance, and excellent biocompatibility [Chen et al. Biocompatible and sustainable power supply for self-powered wearable and implantable electronics using flexible III-nitride thin-film-based piezoelectric generator, Nano Energy 57, 670 (2019)]. In this study, we developed and demonstrated skin-attachable GaN and AlGaN sensors for the detection of eye-lid motion to monitor the eye blinking and eyeball movements. Flexible III-N thin films were designed by layer transfer method from the rigid silicon substrate. They were analyzed by X-ray diffraction to reveal its single-crystalline quality without second phase or defects. The single sensor was applied to the temple area of the face for sensing of eye blinking and simple eyeball motion. Eye blinking is conducted with three different levels. Then, the multiple sensors were utilized with appropriate distance on the identical region to detect the transverse, longitudinal, diagonal, and rotational eyeball motions, which explain the abnormal eye movements. All the results from each sensor are distinguishable, which indicates that the sensors generated different values of output signals from each position with varied stimulates.