A polypyrrole-dopamine/poly(vinyl alcohol) anisotropic hydrogel for strain sensor and bioelectrodes

Flexible hydrogel-based wearable sensors are of interest for monitoring human movement and electrophysiological signal detection. The high mechanical properties, high conductivity, and sensing sensitivity are the main performance parameters. Inspired by the structure of human muscles, ligaments, and other tissues, a polypyrrole-dopamine/poly(vinyl alcohol) (DA-PPy/PVA) hydrogel with a bionic anisotropic structure is prepared in this study using a directed freeze–thaw strategy. The introduction of DA-PPy nanoparticles can significantly improve the mechanics, adhesion, and conductivity due to the high water dispersibility. Moreover, they can act as physical cross-linking sites to further improve the mechanical properties. The anisotropic structure would influence many properties (mechanical, conductivity, resistance to crack extension, etc.) of DA-PPy/PVA hydrogel. In addition, the prepared DA-PPy/PVA hydrogel exhibits a wide range of strain responses, moreover, its relative resistance change (ΔR/R0) strain curve maintains a line linear within 400% strain. The oriented structure would influence the sensing sensitivity, the gauge factor of anisotropic DA-PPy/PVA in parallel direction is 3.00. In addition, the anisotropic DA-PPy/PVA can effectively motor the physiological electrical signals as a bioelectrode. For example, the acquired electrocardiogram (ECG) and electromyography (EMG) signals not only are clearer than those of the commercial electrodes but show a higher signal-to-noise ratio. It is conceivable that the bionic anisotropic DA-PPy/PVA hydrogels provide a feasible material to construct flexible wearable sensors for monitoring human movement and electrophysiological signals.