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Chitin is produced by a number of living organisms in lower plant and animal kingdoms, and possesses natural biocompatibility. However, its dissolution is very difficult in the most common solvents. In the present work, chitin was dissolved completely in 8 wt% NaOH/4 wt% urea aqueous solution via the freezing/thawing method to prepare transparent solutions without derivatization, as supported by 13C NMR. Subsequently, hydrogels were prepared directly from the chitin solution in the NaOH/urea aqueous system, for the first time, maintaining the attractive structure and properties of chitin. Two kinds of chitin hydrogels were obtained through physical or chemical cross-linking by using epichlorohydrin (ECH) in the NaOH/urea aqueous system. The rheological measurements revealed that the formation of chitin hydrogels by chemical cross-linking was a rapid gelation process (within 1 min), leading to perfect hydrogels. The chitin hydrogels exhibited a uniformly porous morphology, low crystallinity, good mechanical strength, and high swelling ratio. Furthermore, the results of a 293T cell viability assay indicated their excellent biocompatibility and safety. The chitin hydrogels may find wide use in bio-applications, as a result of the more stable structure and better compatibility of chitin than that of its derivatives, such as chitosan.
Chang et al. (Sat,) studied this question.