Lignin-Densified Conductive Gelatin Hydrogels for Tough and Sensitive Tactile Sensing

Conductive hydrogels with high mechanical robustness and electrical performance are highly desirable for wearable sensing applications; however, simultaneously achieving mechanical reinforcement and reliable conductivity in protein-based hydrogels remains a significant challenge. Herein, a lignin-densified conductive gelatin hydrogel was developed through the synergistic integration of betaine (BT)-stabilized liquid metal (LM) nanodroplets and sodium lignosulfonate (LS). Uniformly dispersed LM nanodroplets established conductive pathways within the gelatin matrix, while LS induced network densification through hydrogen bonding and electrostatic interactions, thereby reinforcing the hydrogel structure and enhancing ionic conductivity. The optimized LM5@BT5-GL40%-48h hydrogel exhibited a tensile strength of 1.51 ± 0.05 MPa and a toughness of 2.42 ± 0.41 MJ/m3 (n = 3), together with a conductivity of 0.88 S/m. Moreover, the hydrogel maintained a clearly distinguishable strain-sensing response throughout 3300 stretching–releasing cycles. As a wearable strain sensor, it demonstrated high sensitivity with GF values of 1.5 ± 0.1 at 0–100% strain and 3.9 ± 0.2 at 100–150% strain (n = 3), and enabled monitoring of both large-scale body motions and subtle physiological activities, including facial expressions, swallowing, speech, and handwriting. This work highlights the effectiveness of lignin-induced network densification for simultaneously enhancing the mechanical and sensing performance of protein-based conductive hydrogels and provides a sustainable strategy for developing multifunctional soft materials for wearable sensing.