An Anti-Swellable Hydrogel Strain Sensor for Underwater Motion Detection

Hydrogel-based wearable electronic devices have received increasing attention. However, the construction of underwater strain sensors remains a significant challenge because of the swelling of hydrogels in an aquatic environment. This work presents the fabrication of an anti-swellable hydrogel composed of polyvinyl alcohol (PVA), a copolymer of [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA) and 2-hydroxyethyl methacrylate. Interestingly, facile switch of the SBMA moiety from neutral to positively charged status at a low pH value leads to reduced osmotic pressure of the hydrogel for electrostatic repulsion-driven elimination of water molecules and anti-swelling. The resulting anti-swellable hydrogel exhibits high toughness (518 kJ m(-3)) and compressive modulus (8.12 Mpa), ionic conductivity (up to 4.58 S m(-1)), and anti-swelling behavior (equilibrium swelling ratio of 9% in water for 30 days). An underwater strain sensor based on this anti-swellable hydrogel is further developed to monitor the movements of underwater sports. High sensitivity is achieved to identify multidirectional motions, including raising the head, swinging the arm, bending the elbow, knee and finger. Therefore, this study offers a facile strategy to generate hydrogel-based sensors that can be adopted in an underwater environment as well as expands the potential applications of wearable electronic devices.