Anti-swelling and highly sensitive hydrogel strain sensor for underwater communication and remote robotic operations

For conventional hydrogels utilized in underwater applications, mechanical degradation and signal instability remain major challenges. To solve these problems, we developed hydrogel composites (PSG-Zr4+-CNT) which integrate high toughness, anti-swelling properties, and high sensitivity, by establishing a densely cross-linked network strengthened by zirconium ion (Zr4+) coordination. Polyacrylic acid, sodium alginate, and gelatin were used as the matrix, and carbon nanotubes (CNTs) was incorporated simultaneously to enhance the anti-swelling capacity and electrical conductivity. The hydrogel composite exhibited a high tensile strength of up to 202 kPa, a fracture strain of approximately 632 %, and a stable electrical conductivity. Importantly, it maintained structural integrity and sensing functionality over extended periods in various harsh aqueous environments, including concentrated salt solutions and a broad pH range (3–8.5). With these attributes, the fabricated hydrogel sensors have been successfully applied for underwater motion monitoring, Morse code communication, and remote robotic operations for artifact retrieval. This work provides an effective strategy to address the practical challenges of hydrogel sensor applications in extremely harsh underwater environments.