Surfactant-Free Preparation of Biodegradable Hydrogel Microspheres for Protein Release

The preparation of biodegradable hydrogel microspheres in the absence of surfactants was carried out by a two-step procedure which involved the formation of non-crosslinked microspheres from gelatin based on its inherent gelation nature at low temperatures and the subsequent glutaraldehyde (GA) crosslinking. The size of the microspheres was controlled in the range of 3 to 100 μm by changing the concentration of gelatin or GA, the emulsification method, and the crosslinking time. Neutral aqueous solutions of proteins with different isoelectric points (IEPs) and molecular weights (Mws) were infused into freeze-dried hydrogel microspheres to produce protein-incorporated gelatin microspheres. In vitro protein release from the microspheres depended on the protein's IEP but not on the Mw. The incorporated basic proteins with IEPs > 7.0 were released initially from the acidic gelatin microspheres, followed by no substantial release, whereas a larger initial release of the incorporated acidic proteins with IEPs < 7.0 was observed. The basic gelatin microspheres exhibited an opposite relationship between proteins IEP and protein release. Noncharged dextran rapidly diffused out of acidic gelatin microspheres, irrespective of the Mw. These findings indicate that an ionic interaction with gelatin constituted hydrogel microspheres prevented oppositely charged protein from being released from gelatin under in vitro non-degradation conditions.