Multi-Interaction Conductive Double-Network Polyelectrolyte Hydrogel with High Stretchability, Self-Adhesion, and Tunable Transparency for Bioelectronic Sensing and Information Encryption

Polyelectrolyte hydrogels, combining the conductivity of polyelectrolytes with the flexibility of hydrogels, become a popular candidate for flexible sensors, soft robotics, and electronic skins due to their remarkable stability in electrical conductivity. However, their mechanical properties and adhesive strength are limited because of their single-type bonding interaction. This work introduces a double-networked (DN) polyelectrolyte hydrogel formed through polymer chain entanglements, chemical crosslinking, and the incorporation of multiple strong and/or weak bonding interactions. The first network is a chemically crosslinked polyacrylamide (PAAm). The second network consists of polyelectrolytes (poly(diallyldimethylammonium chloride) (PDDA) and poly(methacrylic acid sodium salt) (PMAANa)), which form numerous weak and/or strong ionic bonds. Weak and/or strong hydrogen bonds are present within and between two networks. The mechanical properties and adhesive strength of the polyelectrolyte DN hydrogel can be tailored through modulating the content of PAAm, polyelectrolytes, and co-solvents. The optimal compositions have a tensile modulus of 10.8 kPa, tensile fracture strain of 1000%, and adhesive strength of 37.8 kPa. The hydrogel sensors are successfully applied to flexible electrodes for various devices for detecting human motion, handwriting recognition, and continuous monitoring of electrophysiological signals. The distinctive solvent-adjustable transparency of the gel also allows for its utilization in information encryption and decryption.