Highly sensitive strain sensors based on PVA hydrogels with a conductive surface layer of graphene

The sensitivity of hydrogel-based strain sensors is generally lower than that of other elastomer-based strain sensors due to the mismatch between the change in electrical conductivity and hydrogel deformation. Therefore, the fabrication of highly sensitive hydrogel strain sensors is still a challenge. Here, a highly sensitive strain sensor based on poly (vinyl alcohol) (PVA) hydrogel was fabricated by constructing a tightly adhered graphene conductive layer on the surface of PVA hydrogel. The hydrogen bonding interaction between tannic acid (TA) and PVA as well as the strong π-π interaction between TA and graphene were utilized. Utilizing the stronger hydrogen bonding between TA and PVA, the surface layer crosslinking density of PVA hydrogel was greatly enhanced. Relying on the π-π interaction between TA and graphene, the conductive layer of graphene is firmly attached to the surface layer of the PVA hydrogel. Compared to the strain sensor with graphene uniformly dispersed in a PVA hydrogel matrix, the sensitivity was improved from 0.98 to 8.6. The high strain sensitivity allowed a lower detection limit of strain as low as 0.1%. The hydrogel has good tensile properties (maximum tensile stress of 0.38 MPa) and good fatigue resistance. It has excellent durability (over 1000 cycles) and fast response (227 ms).