Preparation and characterization of uniform-sized chitosan microspheres containing insulin by membrane emulsification and a two-step solidification process.

Chitosan microsphere has important application in controlled release of protein and peptide drug, because it shows excellent mucoadhesive and permeation enhancing effect across the biological surfaces. In the conventional preparation methods of chitosan microsphere, the W/O emulsion was usually prepared by mechanical stirring method, and then the droplets were solidified by glutaraldehyde. There existed limitation and shortage such as broad size distribution, de-activity of bio-drug and difficulty in drug release because protein and peptide drug have the same amino group as chitosan. In this study, we established a method to prepare uniform-sized microsphere, and solve above problems by combining a special membrane emulsification technique and a step-wise crosslinking method. That is, the chitosan/acetic acid aqueous solution was pressed through the uniform pores of a porous glass membrane into a paraffin/petroleum ether mixture containing PO-500 emulsifier, to form a W/O emulsion with uniform droplet size. Then, the uniform droplets were solidified by a two-step crosslinking method. At the first step, tripolyphosphate (TPP) solution was dropped gradually in the emulsion, TPP diffused into the droplet to crosslink chitosan by an ionic linkage, generating a microgel. At the second step, an adequate amount of glutaraldehyde was added. The solidification conditions of the two-step process were optimized by investigating the effects of solidification conditions on morphology of microspheres, encapsulation efficiency (EE), drug activity and release profile in vitro. The suitable preparative conditions were determined as follows: pH value of aqueous phase and TPP solution was 3.5-4.0, the molar ratio of amino group of chitosan to aldehyde group of glutaraldehyde was 1:1 and the crosslinking time of glutaraldehyde was 60 min.