Controlled Dual Functionalization of Lignin Generates Enhanced Properties in Gelatin-Based Hydrogels

Uncontrolled side reactions during lignin isolation lead to a loss of chemical functionalities. Functionalization steps are thus usually performed on isolated lignin to improve reactivity and/or miscibility toward other materials. Even with these additional steps, potential high-performance applications are generally substantially curtailed. Here, we use aldehyde-assisted fractionation with two aldehydes containing other functionalities to introduce multiple functionalities on the lignin in a controlled way and in a single step from lignocellulosic biomass. The quantity of the various functionalities is easily tuned, imparting to the lignin different reactivity and solubility properties. These bifunctional lignins are employed to fabricate novel covalently bonded lignin–gelatin-based hydrogels by simple mixing of reagents at room temperature without the addition of initiators for soft tissue engineering. These hydrogels show enhanced and controllable mechanical (up to 134% in storage modulus and +107% in tensile strength compared to gelatin alone), adhesive, temperature-resistant, and self-healing properties at 37 °C and pH 7.4 only when a bifunctional lignin is used. This work demonstrates the importance and potential of controlled multifunctionalization of lignin, opening the way for its use in high-performance materials.