High-performance lignin-based hydrogel strain sensors for human motion monitoring

Flexible strain sensors hold considerable potential for human-machine interaction and health monitoring, yet conventional single-network systems often suffer from brittleness, limited durability, and insufficient multifunctionality. Herein, a multifunctional polyacrylamide (PAAm)/polyvinyl alcohol (PVA) composite hydrogel incorporating tannic acid-modified lignin (TA@AL) is developed via a simple one-pot polymerization followed by Fe³⁺ post-impregnation. A hierarchical triple-network structure is constructed, consisting of (i) a covalent PAAm backbone, (ii) a physically interpenetrated PVA network stabilized by hydrogen bonding, and (iii) a dynamic metal-phenol coordination network between TA@AL and Fe³⁺. Owing to this synergistic architecture, the hydrogel exhibits a high tensile strength of 115 kPa, a fracture strain of ∼900%, and a toughness of 0.45 MJ m^-3, together with robust adhesion and high ionic conductivity (0.75 S m^-1). Density functional theory (DFTD3) calculations further reveal the coordination energetics between FeCl₃ and different polymer ligands, demonstrating that multidentate coordination in the PAAm/PVA/TA system provides strong yet dynamic binding, which underpins efficient energy dissipation and mechanical robustness. Benefiting from the coupled mechanical and electrical properties, the hydrogel functions as a high-performance strain sensor with high sensitivity, fast response, and excellent cycling stability, enabling accurate detection of both large-amplitude joint motions and subtle physiological signals such as swallowing and handwriting. This work not only offers an effective strategy for lignin valorization but also provides a molecular-to-macroscopic design framework for high-strength, conductive, and multifunctional hydrogels for wearable and biomedical applications.