A Flexible Strain Sensor with High Electrical Conductivity, Rapid Electrothermal Response, and High Strain Sensitivity for Real-Time Monitoring of Human Joint Movement

Here, a flexible film, called PPA film, is designed as a multifunctional sensor with high electrical conductivity, rapid electrothermal response, and high strain sensitivity. The film contains polyvinyl alcohol (PVA), graphene nanosheets (GNs), nanocellulose crystals (CNCs), polydopamine (PDA), and silver nanoparticles (Ag NPs), symbolized as PPA film. The CNCs enhance the dispersion of the GNs during film formation; the stacked GNs change heat conduction and resistance during film bending; PDA serves as a linking agent; Ag NPs improve the conductivity (i.e., 2.65 x 103 S/m) of the film during electrothermal response measurement. A flexible strain sensor designed from PPA films is mounted on the skin surface of human joints to monitor human movement in real time. The sensor exhibited good stability and cycling durability, i.e., the resistance variation remained stable during the bend-release repetitions for 600 cycles at 1.2 % strain. The flexible sensors exhibited highly sensitive to bending strain, i.e., 30.5 of the gauge factor (GF), and can be applied to monitor bending motions of human joints, such as finger, wrist, elbow, and knee joints.In addition, the PPA films exhibit very high joule heat generation ability and demonstrate its applicability as a joint or stomach warmer and sensor. The as-designed flexible sensor is a promising candidate for application in environmentally friendly and multifunctional wearable devices.