Hydrogels based on cellulose and chitin: fabrication, properties, and applications

This review is focused on the fabrication, properties, and applications of hydrogels prepared from two of the most abundant biopolymers on earth, cellulose and chitin. The review emphasizes the latest developments in hydrogel preparation (including solvent systems, cross-linker types, and preparation methods, which determine the greenness of the process) using these biocompatible and biodegradable biopolymers. The preparation of both physical (without covalent cross-links) and chemical (with covalent cross-links) hydrogels via dissolution/gelation is discussed. Additionally, formation of injectable thermoset and/or pH sensitive hydrogels from aqueous solutions of derivatives (chitosan, methyl cellulose, and hydroxypropylmethyl cellulose) with or without a cross-linker are discussed. This review also compares the design parameters for different applications of various pure and composite hydrogels based on cellulose, chitin, or chitosan, including applications as controlled and targeted drug delivery systems, improved tissue engineering scaffolds, wound dressings, water purification sorbents, and others.