Strategy of Fabricating Flexible Strain Sensor via Layer-by-Layer Assembly of Conductive Hydrogels

Conductive hydrogels are widely used in flexible strain sensors for their excellent performance. However, problems, such as the incompatibility of the conductive structure and flexible domain and the poor interfacial adhesion between the conductive materials and the soft substrates, limit their application in large deformation sensing response. Layer-by-layer (LbL) assembly technology provides a strategy to control and design the composite structure. On the basis of the LbL assembly technique, a flexible strain sensor with a "hydrogels–conductive materials–hydrogels" hierarchical sandwich structure was prepared in this work. Poly(vinyl alcohol) (PVA)/polyvinylpyrrolidone (PVP) was used as a raw material to prepare hydrogel skeleton materials, and conductive materials were prepared by the combination of doped polyaniline (PANI) acid and MXenes (Ti3C2Tx). Introduction of MXene materials into flexible sensors could significantly improve the sensitivity (K). When the MXene contents are 5%, the Kmax of the flexible sensor reaches 5.88. This typical sandwich-like structure strain sensor could withstand various forms of deformation (∼1000% strain) and stress (bending, twisting, folding, compression, stretching), which has potential application in human-activity monitoring, such as finger bending, wrist bending, heeling pressing, phonation recognition, etc.