Hydrogel Interface

Abstract

In recent decades, soft materials have attracted considerable attention and enabled new applications in the fields of wearable devices, stretchable electronics, soft robots and mobile health. Humans interact with machines through a user interface, namely human-machine interface (HMI). Machine is hard and dry, and conducts electricity through electrons, while human body is soft and wet, and conducts electricity through ions. When machine meets life, the mechanical, chemical and electrical discrepancies hinder the further development of soft devices. Hydrogel, as a typical emerging material, is ideal to bridge the gap between machine and life, due to their soft, stretchable, and ionic nature. Integration of hydrogels with diverse materials gives new innovations and enables new functions. This thesis focuses on diverse hydrogel interfaces in both theoretical and engineering perspectives. First, we apply hydrogel to electrophysiology, one of the earliest and most widely used HMI, where hydrogel serves as both stretchable and transparent sensors and ionic conductors. By rationalizing the unique benefits of hydrogel, we develop a new wireless, wearable, stretchable and real-time health monitoring system. The transmission model of electrolyte is extended and validated for hydrogels. Second, we use hydrogel-metal interface to enable new functions as a sensing platform for the next generation of wearable devices. As an example, we develop a totally soft and stretchable temperature sensor based on electrical double layer at the electronic conductor-ionic conductor interface. Third, we study hydrogel adhesion with diverse materials. Instant and tough, as well as reversible noncovalent adhesion is demonstrated. The adhesion detaches in response to a cue, such as a change in pH or temperature. Based on the same principle, hydrogel can be used as a bio-adhesive, potentially serving as a sealant. We hope these studies of hydrogel interface can build a foundation for the next generation of soft, stretchable and wearable ionic devices, so that machine and life can be seamlessly bridged and interfaced.