Fabrication of a Surface Adhesion Layer for Hydrogel Sensors via Photografting

Hydrogel-based flexible strain sensors have attracted most researchers' attention because of promising applications of wearable devices. However, preparation of reliable adhesive hydrogel strain sensors is necessary and remains challenging. This research reported a method and materials for imparting adhesion property to the surface of hydrogel prepared by photopolymerization of 2-(dimethylamino)ethyl methacrylate, acrylamide, NaCl, and a cross-linker. The polymer of 2-ethylhexyl acrylate (2-EHA) is well-known as the source of pressure-sensitive adhesives. Because of the existence of tertiary amines in the hydrogel matrix, 2-EHA was successfully photografted on the hydrogel surface through hydrogen abstraction photopolymerization. After photografting the layer of the adhesive, the hydrogel remains highly transparent and exhibits robust adhesion to various substrates including skin, glass, polytetrafluoroethylene, wood, and stone. Furthermore, the water retention capacity of the hydrogel is improved because of the hydrophobicity of the layer, thereby improving the stability and durability of the sensor for long-term usage. The hydrogel added with NaCl exhibits good strain sensitivity, stability, and low hysteresis as a wearable flexible strain sensor for human motion detection. This research may provide a facile strategy for fabricating hydrogels with adhesive performance to meet the requirements of wearable strain sensors.