Lignin-Based Conductive Hydrogels with Plasticity, Recyclability, and Self-Adhesion as Flexible Strain Sensors for Human Motion Monitoring

Conductive hydrogels possessing conductivity, flexibility, and biocompatibility have garnered considerable attention in recent years for their applications in flexible wearable devices. However, most reported conductive hydrogels are mainly elastic hydrogel substrates with chemically cross-linked networks, poor shape adaptability, and irreversible electromechanical properties after molding, thereby limiting their prospective utility in flexible electronics. In this study, we fabricate multifunctional lignin-gelatin-polypyrrole (LGP) hydrogels with plasticity, recyclability, strong adhesion, and biocompatibility via a straightforward methodology employing gelatin, polypyrrole, and sodium lignosulfonate. The resultant LGP hydrogel is interlinked by dynamic noncovalent bonds, yielding remarkable plasticity and recyclability, and could be manipulated by hand to fashion diverse shapes. Additionally, the LGP hydrogel displays substantial adhesion (23.88 kPa to pig skin) and maintains strong adhesion to wide substrates. The LGP hydrogel strain sensor demonstrates high sensitivity (GF = 6.08) and rapid response (107 ms), providing a stable resistive signal output for both large (25–200%) and small (1–5%) strains across diverse operating conditions. Moreover, the LGP hydrogel can be seamlessly integrated as a flexible, wearable strain sensor to facilitate real-time monitoring of human physiological activities.