Advances in chitosan-based hydrogels: Evolution from covalently crosslinked systems to ionotropically crosslinked superabsorbents

Among the various physical forms of chitosan, viz. films, fibres, nanoparticles, gels, sponges, solution, etc., gels hold the most commercial importance. They are widely used in many areas, ranging from biomedicine to agriculture. This review discusses the recent advances in the chitosan-based hydrogels while highlighting its evolution from covalently crosslinked systems to ionotropically crosslinked super-absorbents. Distinctions into first, second and third generations of chitosan-based hydrogels are drawn based on the nature of crosslinks, environmental stability, mechanical strength and toxicity. While the first generation comprises chemically and metal-coordination crosslinked hydrogels, which were among the earliest to be identified, the second generation comprises of those physically crosslinked. Although the latter lack strength and stability, they are biocompatible in nature. The third generation, on the other hand, comprises of dense physical crosslinks that contribute to the strength and stability, without compromising on the biocompatibility.