Cationic Poly(2-hydroxyethyl methacrylate) Antiswelling Hydrogel Sensor for Underwater Human Motion Detection and Communication

Conductive hydrogels have been widely studied in the field of wearable electronic devices due to their notable flexibility and ductility. However, their serious swelling phenomenon in aqueous environments restricts their underwater applications. In this study, we constructed a cationic poly(2-hydroxyethyl methacrylate) antiswelling hydrogel (named as P(HEMA-co-DDA)), which was synthesized on the basis of preparing the PHEMA hydrogel by introducing a cationic monomer dimethyldiallylammonium chloride (DDA). The cross-linker structure and ionic characteristics of PDDA enhanced the cross-linking density of the hydrogel, resulting in good mechanical properties (toughness: 288 kJ/m3) and high electrical conductivity (3.05 S/m). Additionally, the positively charged PDDA chains generated electrostatic repulsion and hydrophobic PHEMA with the alkyl chain against infiltrating water molecules, thereby reducing the swelling ratio and contributing to the antiswelling effect. Thanks to these unique properties, the hydrogel was integrated into a strain sensor and then immersed in pure water, where it was able to rapidly, accurately, and repeatedly monitor joint movements under water, including those of the neck, wrists, elbows, knees, and fingers. Moreover, the hydrogel-based sensor can serve as a sensing module for underwater wireless communication devices, enabling the transmission of information via Morse code under water. Overall, this study presents a straightforward strategy for the preparation of cationic PHEMA hydrogels and demonstrates the potential applications of such hydrogel-based flexible sensors in wearable electronic devices for underwater human motion sensing and communication.