Anti-swelling hydrogels based on surfactant–polymer interactions for underwater sensing with excellent mechanical properties

When hydrogels are in environments with high water content, they usually undergo swelling, leading to a degradation of mechanical properties, which severely limits their application in underwater sensing. To solve this problem, we synthesized an anti-swelling, tough, double-crosslinked, dual-network hydrogel, P(AA-SMA)-CMC-Na, for underwater sensing. Due to the hydrophobic association and electrostatic interactions between the cationic surfactant N-Hexadecyltrimethylammonium chloride (CTAC) and poly (acrylate-methacrylate octadecyl ester) [P(AA-SMA)], as well as the dense network structure brought about by double cross-linking and the incorporation of CMC-Na, the hydrogel has excellent anti-swelling properties and mechanical properties (large stretchability of 955%, tensile strength of 252 kPa). Notably, after seven days of immersion in water (pH = 3, 5) and saline, stress and strain were elevated compared to the original hydrogel (strain up to 1460% and stress up to 805 kPa after immersion). The dynamic hydrophobic association of CTAC with SMA and the large number of hydrogen bonds within the hydrogel contributed to the excellent self-healing efficiency of the hydrogel (90%). The hydrogel was fabricated into a flexible strain sensor that showed keen sensitivity in the sensing range of 0–400% (GF values of 1.55, 2.87, and 5.03 for the ranges of 0–200%, 200–300% and 300–400%, respectively). It could accurately detect real-time signals of human movement in air, underwater, and seawater. Therefore, the P(AA-SMA)-CMC-Na hydrogel can meet the needs of underwater sensing and have great potential for applications in electronic skin, personal health detection, and flexible wearable electronics.