Sustainable copper-doped lignin-derived nanobiochar for high-performance hydrogel strain sensors and self-healing flexible supercapacitors.

Hydrogel-based strain sensors (HSSs) hold great promise for flexible and wearable electronics owing to their low modulus, excellent biocompatibility, and strong adhesiveness. However, conventional HSSs typically rely on costly, nonrenewable, and poorly biocompatible conductive fillers. To address this challenge, this study proposes a "hydrothermal coordination‑carbonization" strategy. By doping copper ions into industrial by-product lignosulfonate (LS) molecules to regulate their aggregate structure, highly conductive copper nanoparticle-doped hydroxylated LS-derived nanobiochar (Cu@HLSC) is successfully prepared. Using Cu@HLSC as a green and sustainable conductive filler, a multifunctional composite hydrogel (Cu@HLSC@PAM) is constructed with polyacrylamide (PAM) via a one-pot free radical polymerization method. The resulting hydrogel exhibits outstanding mechanical properties (maximum stress of 66.9 kPa and fracture strain of 792%), high electrical conductivity (0.71 S m-1), a broad strain-sensing range, and a high gauge factor (GF up to 4.36). In addition, the hydrogel displays strong adhesiveness and excellent biocompatibility, enabling stable attachment to various substrates and human skin. Taking advantage of its remarkable strain-sensing capability, the hydrogel can be employed for the real-time monitoring of various human movements (e.g., finger and knee bending, swallowing). Furthermore, by loading activated Cu@HLSC into the PAM hydrogel to serve as an electrode material, an all-hydrogel solid-state supercapacitor was constructed, delivering an areal capacitance of 18.4 mF cm-2 and excellent self-healing performance (capacitance retention >95% after healing). This study provides new insights into the high-value utilization of lignin and the development of multifunctional, self-powered flexible electronic devices.