Fabrication of Surface-Modified Hydrogels with Polyion Complex for Controlled Release

Surface-modified hydrogels with a polyion complex composed of poly(vinylamine) (PVAm) and poly(acrylic acid) (PAAc) were prepared in order to control the release of drug molecules without a volume change of the hydrogel. We first prepared a poly( N -vinylacetamide)- co -poly( N -vinylformamide) (poly(NVA- co -NVF)) hydrogel, and then used a hydrolysis reaction to produce a cationic PVAm layer on the surface of the hydrogel. The polymerization of AAc to the surface-cationized hydrogel resulted in a hydrogel that possesses a polyion complex (PIC) of PVAm and PAAc only on the surface. This surface-PIC hydrogel (sPIC gel) could suppress the release of a model drug (Fluorescein isothiocyanate labeled Dextran, M w = 9500) under neutral pH conditions because of the tight PIC surface layer, and repeatedly controlled the drug release against the pH conditions depending on the formation and dissociation of PIC. Controlled release was achieved without a large volume change, because the PIC layer was thin enough to maintain the original size of the hydrogel. Furthermore, the sPIC gel retained a larger amount of model drug as compared to the PIC gel, which possesses the polyion complex from the surface to the inside of the hydrogel. Consequently, the surface-modified hydrogel with PIC, that is sPIC gel, is useful for controlled drug delivery systems that require a constant volume and large drug loading.