Technical lignin to hydrogels: An Eclectic review on suitability, synthesis, applications, challenges and future prospects

The present manuscript reviews the suitability of technical lignin's for synthesizing hydrogels, showcases its present applications, challenges, and future directions. This review starts with bibliometric study which reveals drastic increase in publications since last five years reflecting the growing interest. Description of the physico-chemical properties of technical lignin's (kraft, soda, lignosulfonate and organosolv) has been included along with a note on their production conditions and annual availability. The suitability of technical lignin's for synthesizing hydrogels have been examined. It was observed that soda lignin provides superior mechanical properties, organosolv lignin rendered improved water retention ability, while lignosulfonate was mostly used in electronics sector. The prime reason behind the difference is the uniqueness of the functional groups present in each of the technical lignin's. Further, different synthesis techniques have been reviewed and can be broadly classified into physical, chemical, and biological processes. Physical hydrogels are mainly formed due to hydrogen bonds , ionic, hydrophobic and van der Waals interaction. Chemically synthesized hydrogels were mainly cross-linked through ester bonds with the help of cross-linking agents and free radical polymerization in the presence of initiators. Biological mediated processes have been scarcely studied with only a few articles available on laccases/peroxidases catalysed polymerization and whole cell (fungal) mediated process. Present applications of lignin hydrogels have been elaborated in four major sectors: Biomedical (wound healing, anti-microbial coatings, tissue engineering, sustained drug release), Agriculture (sustained water absorption, slow release of pesticides/fertilizers), Environment (heavy metal adsorption, dye removal, inhibiting spontaneous combustion of coal) and Electronics (supercapacitors). • Interpenetration network and copolymerization are often used in hydrogel synthesis. • Lignin hydrogel possess 2.75 folds higher fracture energy than human skin. • Organosolv lignin hydrogels are reported to deliver Bisoprolol fumarate effectively. • Lignosulfonate hydrogel has been used in synthesis of supercapacitors . • Major bottleneck of the process is complex structure of lignin.