An anti-swelling chitosan-based hydrogel driven by balance of hydrophilic segment and hydrophobic segment strategy for underwater detection of human motion

Hydrogel-based flexible strain sensors have gained rapid development. However, most of traditional hydrogels were prone to undergo swelling behavior underwater as solvent molecules continuously penetrated, which limited their application scenarios. To make up this deficiency, a chitosan-based anti-swelling hydrogel cross-linked by ionic liquid are presented. The hydrogel is consisted of chitosan (CS) and copolymer of acrylic acid (AA), 2-hydroxyethyl methacrylate (HEMA), butyl acrylate (BA) and dimethylaminoethyl methacrylate maleate (DMAEMA-MA). The P(HEMA-BA-AA)/ILs/CS hydrogel exhibited excellent anti-swelling properties driven by balance of hydrophilic segment and hydrophobic segment strategy (equilibrium swelling rate of −3.79 % in water for 100 days). The ionic liquid was introduced into the hydrogel system as cross-linking agent, which could effectively prevent the loss of conductivity of the hydrogel after soaking in water. The synergy effect chain entanglement, H-bonds and electrostatic interactions endowed the hydrogel with excellent mechanical property. Surprisingly, the modulus and toughness of the hydrogel were significantly boosted after a week of submersion. Thus, this anti-swelling hydrogel was employed as wireless strain sensor, which implemented underwater motion monitoring and safety alarms. It is expected that the anti-swelling hydrogel driven by balance of hydrophilic and hydrophobic effects would inspire the development of underwater sensors.