Preparation of Lignin-Based Hydrogels and Their Application in Flexible Strain Sensing

Lignin has attracted increasing attention as a sustainable raw material for flexible electronic applications owing to its abundance, renewability, diverse functional groups, and tunable chemical structure. In this work, sodium lignosulfonate (SL), tannic acid (TA), and ferric chloride (FeCl3) were employed to synthesize lignin-based conductive hydrogels through a facile one-pot polymerization approach. The resulting hydrogels formed a three-dimensional conductive network via free radical polymerization and cross-linking, exhibiting outstanding mechanical and electrical properties. Under the optimized conditions (SL: 0.004 g, TA: 0.004 g, FeCl3: 0.08 g), the hydrogel displayed a maximum strain of 1116%, a stress of 0.027 MPa, and an electrical conductivity of 2.66 S·m−1. Furthermore, the hydrogels demonstrated excellent water retention ability, maintaining more than 70% water content under both ambient and elevated temperatures. After the introduction of glycerol, the water retention further improved to over 80% at 60 °C and approximately 95% at −20 °C, confirming remarkable stability across a wide temperature range. As a strain sensor, the hydrogel exhibited a high gauge factor of 11.242 at 920% strain, along with rapid response (1836 ms) and recovery (1838 ms) times under 10% strain. The resistance signal remained stable after 200 stretching cycles, and the electrical conductivity was maintained from −20 to 60 °C. Overall, the lignin-based conductive hydrogel combines high flexibility, durability, and environmental adaptability, offering great promise for applications in wearable electronics and structural health monitoring.