Encapsulation of hydrogel sensors

De/hydration is unavoidable for unencapsulated hydrogels, which would weaken their physical performance during applications. Herein, we report a surface-encapsulating method that can chemically make an elastic coating at the surface of hydrogel to prevent it from de/hydration in air and water. The water loss is less than 6 wt% in weight by keeping the surface-encapsulated hydrogel in air at 28 °C for 20 days, and its weight doesn't increase by immersing it in water for 60 days. The adhesion energy between hydrogel and the coating reaches 2000 J/m2, so that it can keep the structural integrity in response to repeated stretching, compression, bending, twisting, and knotting. This method doesn't require any templates and special devices, which can make surface encapsulation for hydrogels that have irregular shapes such as tubes, spheres, and wrinkles. The encapsulated elastic coating is of long-term (180 days) stability without interfacial failure in air, and in acidic, alkaline, and salt solutions, which enables the hydrogel capable of long-term sensitivity as a strain sensor in air and water systems, showing great potential for signal transmission under water.