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

This is the authors' abstract. We don't add key points for this paper.

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, CaCl 2 , and carboxyl-functionalized multiwalled carbon nanotubes (fMWCNTs), for underwater sensing. NaCl is associated with salting-out-assisted PVA chain ordering, whereas CaCl 2 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 ∼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 retained a cyclic resistance response over 1,000 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°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.