Multifunctional Antiswelling Hydrogel for Underwater Sensing Achieved through Ultrafast Protocatechualdehyde-Fe ³⁺ Autocatalytic Gelation

Hydrogel-based sensors typically exhibit pronounced swelling underwater, leading to the accelerated deterioration of mechanical properties and signal distortion, thereby limiting their applicability. In this study, we developed an antiswelling hydrogel comprising <i>N</i>, <i>N</i>'-methylenebis (acrylamide)-cross-linked poly(acrylic acid) (PAAc) and polylysine (PLL) in a water/glycerol (Gl) binary solvent system via a protocatechualdehyde (PA)-Fe³⁺ autocatalytic system, inspired by natural catechol/metal-ion coordination and solvent-exchange strategies. The PA-Fe³⁺ system rapidly initiates AAc polymerization at ambient temperature, while Gl's viscosity regulates heat transfer and enhances reaction efficiency. Upon underwater immersion, Gl is replaced with water molecules, triggering network reconstruction through ion complexation and electrostatic interactions to form a dense spatial structure. The antiswelling performance is tunable by adjusting PA/Fe³⁺ ratios and Gl content. Remarkably, the hydrogel maintains stable mechanical properties and sensing functions in both air and underwater environments. It enables both airborne motion sensing (e.g., speech recognition) and underwater strain sensing (e.g., Morse code detection). This study provides a facile strategy for the design of conductive hydrogel sensors with antiswelling performance for long-term underwater applications.