Functional lignin hydrogels for biosensors and biomedical therapy

Lignin, the second most abundant natural polymer after cellulose, has emerged as a promising renewable resource for developing functional biomaterials. Due to its aromatic structure and abundance of phenolic, hydroxyl, and methoxy groups, lignin exhibits intrinsic antioxidant, UV-blocking, antimicrobial, and biocompatible properties, making it an attractive candidate for hydrogel fabrication. This review provides a comprehensive overview of lignin-based hydrogel, focusing on their structural characteristics, extraction methods, and strategies used for hydrogel preparation, including crosslinking copolymerization, graft polymerization, interpenetrating polymer networks, and controlled polymerization techniques such as ATRP and RAFT. Particular emphasis is placed on recent advances in the application of lignin-based hydrogels in biosensing and biomedical fields, including wearable strain and pressure sensors, drug delivery systems, wound healing, and tissue engineering. The multifunctional properties of lignin contribute to enhanced mechanical strength, electrical conductivity, UV protection, and controlled drug release, enabling the design of smart and sustainable hydrogel systems. Despite these advantages, several challenges remain that limit large-scale translation, including lignin heterogeneity, limited solubility, variability in hydrogel performance, and potential impurities from industrial extraction processes. Addressing this limitation through improved lignin purification, chemical modification, and standardized synthesis approaches will be essential for advancing lignin-based hydrogels towards practical biomedical and sensing applications.