Fabrication of a high strength, swelling resistance, and conductive hydrogel for flexible underwater sensor via ketalization

Conductive hydrogels have been widely applied in biomedical and wearable electronic devices due to outstanding physical properties such as electrical conductivity, excellent flexibility, and biocompatibility. However, the poor swelling resistance of conventional conductive hydrogels would lead to a significant decrease in their electrical conductivity and mechanical strength in wet or underwater environments, which largely limited their application. Here, a high strength, swelling resistance, and conductive hydrogel was prepared via the formation of ketals. First, ketalization between hydroxyethyl methacrylate (HEMA) and polyvinylpyrrolidone (PVP) was conducted using acid as the catalyst. Then, the hydrogel was fabricated through the polymerization of HEMA in the presence of polypyrrole (PPy). The resulting hydrogel showed high mechanical strength (the tensile strength was 557.7 kPa, and the breaking elongation was 451.4 %, the toughness was 1730.7 kJ/m 3 ), good conductivity (0.78 S/m) and swelling resistance (the equilibrium swelling rate was 21.1 %). In addition, the hydrogel displayed excellent stability and high sensitivity as an underwater strain sensor. It was also found that the hydrogel indicated voltage piezoresistivity, photo deformation performance, charge storage properties, and high sensitivity for heavy metal ion detection. Thus, the obtained hydrogel had a huge application potential for multifunctional flexible underwater sensors.