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 the 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, it 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 satisfactory sensitivity (gauge factor (GF) of 1.14 (<300%), 2.92 (300–700%), 7.34 (700–850%), and 25.18 (850–970%)), and 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 cycling stability, reproducibility and good electrical conductivity (0.88 S m⁻¹). This work provided a simple and environmentally friendly strategy for developing polyacrylic acid-based hydrogels with excellent self-adhesive and sensing properties.