Anti-swelling and adhesive γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogels for underwater wearable sensors

Unnecessary swelling and lack of interfacial adhesion in underwater environments will lead to distortion of sensing signal based on the conductive hydrogel. However, achieving the anti-swelling and underwater adhesive capabilities of conductive hydrogels is still challenging. Based on the strategies of the Hofmeister effect and catechol chemistry, polyglutamic acid (γ-PGA) and polyvinyl alcohol (PVA) were used as the hydrogel matrix, and poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) and tannic acid (TA) were performed as a conductive filler and modifier, respectively. The γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogel was successfully prepared by freezing-crystallization (FC γ-PGA/PVA/PEDOT:PSS/TA). The resultant composite hydrogel reveals high anti-swelling and underwater adhesive properties, the equilibrium water absorption rate is 13.0 ± 0.9 %, and the underwater adhesive strength to human skin is up to 11.2 kPa. In addition, the composite hydrogel shows high tensile mechanical strength (0.9 ± 0.1 MPa) at break and tensile breaking elongation (1495.5 ± 15.5 %), and its electrical conductivity is 0.94 ± 0.05 S·m−1. Most importantly, the FC γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogel can maintain long-term underwater mechanical and sensing stability. Underwater strain sensors of the composite hydrogel can be employed for motion signal detection (breathing, drinking, finger bending) and underwater communication (Morse code). The research provides ideas for designing conductive hydrogel sensors with anti-swelling and underwater adhesion, which can be utilized for long-term underwater applications.