A conductive hydrogel with excellent self-adhesion, sensitivity, and stability for wearable strain sensors to monitor human motion

Abstract

Conductive hydrogels are widely used in areas such as electronic skin and wearable sensors. Their excellent self-adhesion, sensitivity, and stability ensure that flexible sensors can be used for long-term and stable motion detection. However, developing polyacrylic acid-based hydrogels that simultaneously exhibit the above-mentioned excellent properties remains a challenge. Here, we proposed a biomimetic strategy, grafting levodopamine (L-DOPA) onto chitosan via amidation reaction, followed by the introduction of acrylic acid (AA) and ZnCl2 to prepare PAA/CS-DOPA-Zn2+ hydrogels. By adjusting the content of CS-DOPA in the hydrogel, which exhibited excellent adhesive (~30 kPa), tensile strain (~1100%), and mechanical properties (maximum stress up to 164 kPa). In addition, strain sensors assembled from PAA/CS-DOPA-Zn2+ hydrogels exhibited a high sensitivity (gauge factor (GF) of 25.18), which could precisely monitor subtle movements such as changes in human joint angles and velocities, frowning, swallowing, and other low-amplitude motions. Furthermore, these sensors demonstrated enduring cyclic stability, reproducibility and excellent electrical conductivity (0.88 S/m). This work provided a simple and environmentally friendly strategy for developing polyacrylic acid-based hydrogels with excellent self-adhesive and sensing properties.