A multifunctional ionic-conductive hydrogel sensor with zwitterionic polymer/Al³⁺ crosslinking for wearable and underwater motion monitoring

In the realm of underwater sensing technologies, developing flexible sensors with robust mechanical properties, stable conductivity, strong adhesion, and antifouling capabilities remains a critical challenge. Herein, we report the design and fabrication of a multifunctional ion-conducting hydrogel sensor, P(SBMA-co-AA)/Al³⁺, constructed via photoinitiated polymerization of zwitterionic N-(3-sulfopropyl)-N-(methacryloxyethyl)-N, N-dimethylammonium betaine (SBMA) and acrylic acid (AA), crosslinked with Al³⁺. This hydrogel sensor integrates multiple interaction mechanisms, including coordination bonds, hydrogen bonds and electrostatic interactions between Al³⁺ and carboxyl groups, sulfonate groups, endowing it with outstanding mechanical properties, namely a tensile strain of up to 1490 %, a stress of 517 kPa and a strong adhesion of 155 kPa. The zwitterionic SBMA component enhances hydrophilicity, enabling superior antifouling properties with reduced protein adsorption and effective inhibition of bacterial adhesion. The ion-conductive network, facilitated by mobile ions within the hydrogel, exhibits a conductivity of 0.23 S m⁻¹ and sensitive strain-responsive behavior, with a gauge factor of 0.8–1.6 across a wide strain range. Notably, the sensor demonstrates reliable motion monitoring in both air and underwater environments, accurately detecting human joint movements and marine animal motions through real-time resistance changes. In addition, we use hydrogels as underwater contact communicators, transmitting information through Morse code, which is important in the event of an emergency for underwater divers. This multifunctional hydrogel sensor addresses key limitations of conventional aqueous sensors, offering a promising platform for applications in underwater safety monitoring and marine ecological research.