A High-Strength, Anti-Swelling Sodium Alginate/Polyacrylamide Hydrogel Strain Sensor for Underwater Motion Monitoring and Information Transmission

Recently, conductive hydrogels have gained extensive applications in flexible wearable electronics and have garnered considerable attention. However, their inherent swelling behaviour and limited mechanical strength have hindered their further development. In this study, a polyacrylamide/sodium alginate (PAM/SA, PS)-based hydrogel with high mechanical strength and anti-swelling properties was prepared by combining mechanical stretching–drying pretreatment with a bimetallic ion (Li+/multivalent metal ion) post-soaking strategy. Among multivalent metal ions (Ca2+, Al3+, and Zr4+), the Al3+-crosslinked hydrogel (PS-Al3+) demonstrated outstanding overall performance. It exhibited excellent mechanical properties, with tensile strength, elongation at break, and impact strength reaching 9.71 MPa, 993.53%, and 75 MJ/m3, respectively. Its dense network structure also gave it excellent anti-swelling properties (swelling ratio of 14%). As a strain sensor, the PS-Al3+ hydrogel displayed good conductivity (1.33 S/m), high sensitivity (GF = 2.25), fast response (response time of 403 ms), and negligible hysteresis (recovery time of 407 ms). Benefiting from its exceptional resistance to expansion, the material’s sensor response signals in underwater environments are highly consistent with those in air. Furthermore, this sensor has been successfully applied to swimming motion monitoring and data transmission in underwater environments. This study proposes a novel, low-cost, and simple approach for developing flexible sensors suitable for underwater environments.