Piezoionic Skin Sensors for Wearable Applications

Abstract

Piezoionic skin sensors are one kind of artificial electrical skin that can output sensing signals in response to external strain or stress stimulus with merits of flexibility, lightness, scalability, and high sensitivity. They have been emerging as an important platform in artificial intelligence, such as in smart healthcare, bionic robotics, and microelectromechanical systems. Piezoionic sensors are typically composed of an electrolyte laminated with symmetric electrodes and are based on ion migration and redistribution under a gradient strain or stress field. However, existing challenges significantly impede the sensing performance of piezoionic sensors, including the low electromechanical coupling efficiency of the electrode materials, instability of electrolyte materials, and strain-induced interface separation of sensor interfaces. In recent years, our group and collaborators have made attempts addressing the as-mentioned critical challenges in order to achieve flexible piezoionic sensors with satisfying performance for wearable smart applications. First, for the electromechanical coupling efficiency of electrode materials, we have developed various electrode materials with highly efficient ion storage and transfer, such as graphdiyne, quinone composites, and graphitic carbon nitride. These materials present superior electrical and mechanical properties with enhanced electromechanical coupling efficiency. Second, in order to improve the stability of electrolytes, especially in an air environment, we have developed ionogel electrolytes instead of conventional hydrogel electrolytes. Ionogels contain highly stable ionic liquids, which effectively improve the air stability of sensor electrolytes, and the sensing properties of devices are preserved even after several months. Third, with regard to sensor interface separation, we have engineered stable material interfaces for piezoionic sensors with elaborate structures. The as-designed tree-root-inspired interfaces show high mechanical stability under various flexible conditions, and the piezoionic sensors display negligible performance deterioration under thousands of bending cycles in an ambient environment. Finally, we have obtained flexible piezoionic sensors and studied their practical applications, such as wearable electronics, health monitoring, and smart detections. For example, we have realized the accurate detection of blood pressure based on an out-of-plane piezoionic mechanism. This innovative technique completely avoids the cuff issue that commercial sphygmomanometers have. Moreover, we have developed multifinger-touch piezoionic sensor arrays for effective braille recognition, which have the potential to eliminate communication barriers with sight-impaired people. Human voices can be easily differentiated by detecting vocal-cord vibrations based on captured sensing signals with obviously different patterns. This smart technique is promising for extended and applied use in virtual reality technology. Lastly, a perspective on existing challenges of piezoionic sensors is highlighted to set a clear direction for future research, including low-cost material synthesis, the mass production of flexible sensors, and healthcare sensor products.