Chitosan microgel: Effect of cross-linking density on pH-dependent release

Chitosan microgels were prepared by a spray-drying method using glutaraldehyde (GA) as a cross-linker. Two kinds of microgels, so-called soft microgel and hard microgel, were prepared using a chitosan to GA ratio of 1: 0.08, and 1: 0.67, respectively. The surfaces of hard microgels were more even that those of soft microgels. The swelling ratio, a measure of degree of swelling, of the soft microgel was pH-sensitive, and it decreased from 1,765% to 1,230%, when the pH increased from 4.0 to 9.0. The deprotonation of amino groups of chitosan could account for the decrease in swelling ratio. The swelling ratio of hard microgels was almost invariable in response to pH change (4.0 to 9.0), and it was much less than that of soft microgels, possibly due to the high cross-linking density. The degree of release from the soft microgels decreased from 65% to 12% when the pH of medium increased from 4.0 to 6.0. FITC-dextran would readily diffuse out of the microgel, possibly due to the larger meshes of the microgels in a strong acidic condition (e.g., pH 4.0). However, the degree of release increased from 12% to 82% when the pH of medium increased from 6.0 to 9.0. In this pH range, the fluorescence dye is believed to be released mainly by a squeezing-out.