Tannic-Acid-Stabilized Black Phosphorus as an Interfacial Nano Reinforcement in Ion-Rich Covalent–Ionic Double-Network Hydrogels With Combined Photothermal and Sensing Properties

Abstract Conductive hydrogels for underwater strain sensing require a balance between mechanical properties, low relative mass change during aqueous exposure, and retained charge transport. Herein, an ion-rich P(AM-co-AA)-SA-TABP double-network hydrogel was prepared through straightforward aqueous polymerization followed by sequential CaCl2 and LiBr soaking. Tannic acid improved the aqueous dispersion of black phosphorus and provided phenolic hydroxyl groups for interfacial hydrogen bonding with the polymer chains, enabling TABP to act as a nanoscale reinforcing filler within the hydrogel network. The optimized hydrogel exhibited a tensile strength of 677 kPa, a fracture strain of 350%, and an initial conductivity of 1.92 S·m–1. After 21 days of immersion, the hydrogel exhibited relative mass changes (Δm/m0) of approximately 53% in deionized water and 48% in normal saline while maintaining its macroscopic integrity. Furthermore, it retained a conductivity of 1.23 S·m–1 following this 21-day water exposure. The hydrogel sensor showed gauge factors of 0.70 and 2.67, response and recovery times of 189 and 257 ms, respectively, and maintained distinguishable periodic resistance signals during a 3000 s cyclic tensile test. It further detected joint movements under air and underwater conditions and enabled simple Morse-code input. TABP also introduced an additional photothermal response. These results demonstrate a straightforward strategy for combining mechanical robustness, a lower relative mass change with retained macroscopic integrity, and retained sensing functionality after water exposure.