Dual-salt-induced polyionic hydrogels with controlled swelling for self-healable underwater strain sensors

초록

Although ionic hydrogels are promising soft conductors for underwater strain sensing and human-machine interfaces, prolonged exposure to aqueous or saline environments can cause these hydrogels to exhibit swelling, ion leakage, conductivity drift, and loss of sensing repeatability. In this study, we present a dual-salt hydrogel referred to as PGNCM, which is composed of poly(vinyl alcohol) (PVA), glycerol, NaCl, CaCl2, and carboxyl-functionalized multiwalled carbon nanotubes (fMWCNTs), for underwater sensing. NaCl is associated with salting-out-assisted PVA chain ordering, whereas CaCl2 introduces divalent ion-mediated interactions within the PVA/fMWCNT network. This dual-salt environment reduces swelling while preserving mechanical robustness, electrical conductivity, and self-healing. PGNCM exhibited a swelling ratio of similar to 54 % after 48 h in artificial seawater, retaining 87 % of its initial conductivity after self-healing. As a strain sensor, it shows a linear response over 0-200 % strain (GF = 1.27, R-2 = 0.997) and retains a cyclic resistance response over 1000 cycles at 100 % strain. The sensing response is retained even after two days of immersion in deionized water and seawater, and it remains stable over 5-40 degrees C. A single-sensor element enables underwater Morse code communication and robotic-fish locomotion tracking under direct aqueous exposure, demonstrating the potential of the dual-salt freeze-thaw strategy for hydrogel-based underwater sensing applications.

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