On-Skin Printable Conductive Granular Hydrogels for Human-Machine Interface

Over the past decades, conductive hydrogels have been widely utilized as strain sensors for their tissue-like soft modulus and strain-sensitive properties. However, their poor adhesiveness and air permeability have resulted in performance issues such as delamination from skin surfaces and inflammatory responses, thereby restricting their application in wearable devices. In this study, we present a novel approach to create on-skin printable conductive granules composed of a hyaluronic acid core and a polymerized catecholamine shell. The multifunctional shell layer significantly enhances the cohesion of printed structures, improves adhesiveness to both skin surfaces and hydrophobic substrates (e.g., polystyrene) even under wet conditions, and demonstrates self-doped ionic conductivity via comproportionation reactions. Moreover, the excellent injectability of this granular hydrogel enables its use as on-tissue printable strain sensors for human-machine interfaces, including applications such as operating robotic arms and in virtual reality environments. In a virtual reality demonstration, the hydrogel strain sensors enable interactive human-machine interface (iHMI) by real-time monitoring of both large deformations, such as finger bending to control the motion of a virtual avatar, and small changes, like vibrations from a vibrator, with feedback mechanisms activating an LED and vibrator when the avatar performs a designated motion. This novel strategy for fabricating conductive granular hydrogels for strain sensors paves a new path for the design of hydrogels in wearable bioelectronics.