Controlled release of proteins from dextran hydrogels

Dextran hydrogels were investigated as matrices for the controlled release of proteins. The hydrogels were prepared by a free radical polymerization of aqueous solutions of glycidyl methacrylate derivatized dextran (dex-GMA). The release of three model proteins (lysozyme, BSA and IgG) from hydrogels varying in water content and degree of GMA-substitution was studied. The release rate was dependent on the size of the proteins and the equilibrium water content of the gels. It was shown that the release of the proteins was independent of the degree of GMA substitution of gels at high equilibrium water contents. On the other hand, the release was strongly affected by the degree of GMA substitution of the gels at low water contents. Some of these gels did not show any significant protein release, which suggests that the hydrogel mesh size was smaller than the protein diameter. In hydrogels where no screening occurred, the diffusion of the proteins could be effectively described by the free volume theory. Hydrogel mesh sizes were estimated from swelling data using the Flory-Rehner theory. This approach, however, resulted in an underestimation of the actual hydrogel mesh as derived from release experiments. Possible explanations for this discrepancy are discussed.