Hydrogel ionic sensory systems

When interacting with the dynamic world, creatures always outdo their electronic counterparts because of their sophisticated sensory systems. From direct perception of the world via soft tissue to intelligent information processing, immensely varied biological movements of organisms mostly rely on accurate ion transport. Imitating the flexible structure of biological systems and the functional mechanism based on ion transport would essentially solve the unmet needs of modern sensing technology. Simultaneously, the establishment of ionic sensor systems would effectively narrow the gap between the biological systems and the artificial equipment to promote seamless interaction of both sides, which could bring a revolution in advanced operations and medical rehabilitation. The outstanding properties of hydrogels enable them to be used as ion conductors like biological tissues for the development of ionic sensory systems. In this review, we introduce the basic principles of ion transport in hydrogels, and discuss the five key elements of hydrogel ionic sensory systems: ionic sensors, ionic transporters, ionic processors, ionic effectors, and ionic power sources, in terms of their structures, working mechanisms and applications. Finally, we suggest future directions and potential challenges of hydrogel-based ionic sensory systems.