Skin-Contactable and Antifreezing Strain Sensors Based on Bilayer Hydrogels

Currently, most hydrogel sensors require an isolation layer to prevent the current from damaging the skin. However, the mismatch of the mechanical property between the isolation layer and the hydrogel sensor may affect the accuracy of the sensing. Herein, a bilayer hydrogel sensor consisted of a nonconductive layer and a conductive layer, which were prepared through twice freeze–thawing methods. The nonconductive layer which could directly come in contact with the skin was composed of polyvinyl alcohol (PVA) and glycerin (GL), and the conductive layer for conducting current and perceiving strain was composed of PVA, GL, and polyaniline (PAni). Unlike ions, PAni was difficult to diffuse in the hydrogels, which had two layers and exhibited significant difference in conductivity. When the voltage was applied, the bilayer hydrogel could protect the skin from irritation and injury. More importantly, the mechanical property of the two layers was close, enabling the bilayer hydrogel to detect strain effectively. Compared with a single-layer hydrogel sensor, the response and recovery time of the bilayer hydrogel were reduced by 14.8 and 8%, and the accuracy was improved by 32.1%. This unique strategy provides novel inspiration for the development of fast responsive and skin-protective hydrogel strain sensors.