Microstructures of Anisotropic pH-Responsive Chitosan Hydrogels prepared by Meniscus Splitting Method

Polysaccharides have been expected to be a useful material in wide fields of food packaging, drug delivery, cosmetics, etc., toward a sustainable society. In addition, numerous natural polymers, like chitosan and cellulose, have also demonstrated pH-responsive swelling behavior. However, the hierarchical structures have not been used for materials under physiological conditions. In this study, to reorganize the polymeric systems, aqueous mixtures of chitosan as viscous fluid are used through the meniscus splitting method, for preparation of a chitosan membrane. To understand the factors on the specific deposition in the meniscus splitting, the nucleus position is analyzed thoroughly verifying the effect of the cell width. Furthermore, the dried chitosan membrane shows anisotropic pH-responses as a hydrogel because of the oriented structures in submicron scale.<br/>To clarify the characteristic anisotropy in ion- or pH-sensitive hydrogels, the pH-dependence of three-dimensional swelling ratios and the cyclic pH-changes were validated. Especially, the dried membrane was directly immersed into each buffered solution in a wide range of pH (pH2.2–pH8.0). Under the acidic conditions, the protonation allowed the water molecules to penetrate the interspace not only among the fibers but also among the chains in fibers. As a result, some of the crosslinking points by the hydrogen bond should be dissolved. Besides, toward the use of the membrane in the physiological pH condition, like pH7.4, the repeatability of swelling/deswelling as the hydrogel was validated. In the pH cyclic changes between pH8.0 and pH2.2, the gels showed no reversibility. Through a repeat test of stepwise pH-changes between pH8.0 and pH2.2, it was clarified that the sample continued swelling and redispersed into the aqueous solution, suggesting that the chitosan chain had irreversibly dissolved crosslinking points. In the pH2.2, the protonation of the chitosan chain would induce the dissolution of the hydrogen bonds between the hydroxyl groups and exchange of the citric acid. Interestingly, in the cyclic pH changes between pH8.0 and pH5.0, the chitosan gels had reversibility both in the <i>Y</i>- and the <i>Z</i>-directions. This result suggests that the water molecules can go in and out of the interval of the oriented structure without dissolution of the crosslinking points. The membrane would have a hierarchical architecture: nanofiber as a molecular bundle, and microfibers as a nanofiber bundle. In the presentation, to clarify the nano-/micro anisotropic structure in DRY/WET states, the surface and cross-section positions of the chitosan membrane will be discussed.