Modified lignin-induced composite hydrogels with good mechanical properties, adhesion, and UV resistance for strain sensors

The flexibility and functionality of conductive hydrogels are crucial to applications under various scenarios and complex environments. Due to single functionality and weakness in mechanical properties, the application of conductive hydrogels in flexible wearable devices remains a challenge. In this study, lignin-graft-poly(acrylic acid) (LPAA) was incorporated into acrylamide (AM)/sodium chloride (NaCl) solution to fabricate composite conductive hydrogels. NaCl enhanced the conductivity of hydrogels. The hydrogen-bond crosslinking network was formed by adding LPAA into the hydrogel, which enhanced the mechanical properties of the hydrogel (the tensile strength was 96 kPa and the compressive strength was 0.54 MPa). With LPAA added, the adhesion of hydrogels was improved to 11.3 kPa on glass, 14.2 kPa on plastic, and 17.3 kPa on metal. LPAA also provided excellent UV shielding ability (99.95%) for hydrogels while maintaining good transparency. The conductive hydrogels with good sensitivity (Gauge Factor = 2.51, 100%–500%) and stability can be used as strain sensors for monitoring physical activity. It is expected to be a candidate material for future flexible wearable electronic devices.