Preparation of Double-Layer Hydrogels for Skin Contact and Environmental Strain Sensors

Currently, most hydrogel sensors are single-layer structures, and the current may be potentially harmful to humans. Finding a suitable isolation layer is the key to solving this problem. However, the mismatch of mechanical properties between the isolation layer and the hydrogel sensor may affect the sensing accuracy. In this regard, the two-layer hydrogel sensor consists of a nonconductive layer and a conductive layer. The nonconductive layer is adhesive and tough, which protects the body from the leakage current. We designed and synthesized a nonconductive layer consisting of polyacrylamide (PAM) and PNIPAM for direct skin contact and a conductive layer consisting of PAM, polyethylenimine (PEI), and lithium chloride for conducting current and sensing strain. Sodium caseinate was added to the nonconductive layer to enhance adhesion. The hydrogel has a two-layer structure with a large difference in electrical conductivity. When a voltage is applied, the two-layer hydrogel protects the skin from irritation and damage. More importantly, the mechanical properties of the two layers are close to each other, enabling the bilayer hydrogel to detect the strain effectively. The bilayer hydrogel sensor also has a sensitive response over a wide temperature range. This unique strategy provides valuable inspiration for the development of fast-response and skin-protected hydrogel strain sensors.