Lignin-containing hydrogels: Transforming a low-value byproduct to absorbents, wound dressings and strain sensors

Lignin is the most abundant renewable source of aromatic macromolecules. Its valorization towards functional materials can be a key component of the transition toward a more sustainable circular economy. There has been increasing interest in developing lignin-containing hydrogels for the fabrication of advanced devices such as absorbents, wound dressings, and strain sensors. Multiple covalent and non-covalent interactions existing between lignin and the matrix polymers endow hydrogels with various functionalities which are explored in these devices. Significant progress has been made in the field in the past three years or so. It is important to provide a systematic analysis of the current state of lignin-containing hydrogels, and identify existing challenges and future trends in the field. Herein, we first present a brief introduction to the hydrogels, followed by a discussion of lignin and its typical applications. Next, we focus on the preparation of lignin-containing hydrogels using different forms of lignin, including lignin, lignin nanoparticles, Ag-Lignin nanoparticles, and lignin-metal ions. This is followed by a detailed analysis of applications of lignin in absorbents, wound dressings, and strain sensors. Finally, we offer our perspectives on the challenges and opportunities in lignin-containing hydrogels, in support of efforts to advance lignin valorization.