High strength chitosan hydrogels prepared from NaOH/urea aqueous solutions: the role of thermal gelling

Upon heating and subsequent solvent displacement with pure water, the cold chitosan solution with aqueous alkaline/urea as solvent readily transforms into a hydrogel which is substantially stronger than traditional chitosan hydrogels regenerated from acidic solutions. In this work, we have systematically studied the effects of the treatment parameters in this two-step basic route and found that thermal gelation was the crucial step that dictated the structure and properties of the final gels. We hypothesized that the primary network formed in the thermal gelation step served as a template for the deposition of chitosan chains during solvent displacement, leading to a homogenous and compact structure. The primary network also provides crystalline seeds to facilitate the crystallization of the chitosan chains, leading to higher degree of crystallinity. This study provided a guideline for the preparation of chitosan hydrogels with high mechanical properties which is very meaningful to relevant research and applications.