Thermosensitive hydrogel of poly (N-isopropylacrylamide)-co-poly (2-hydroxyethyl methacrylate) (polyNIPAAm-co-polyHEMA) containing bovine serum albumin (BSA) was prepared by inverse suspension polymerization. The effect of molar ratio of NIPAAm to HEMA and surrounding temperature on release rate of BSA was investigated. The release rate for the hydrogel with molar ratio of 5:5 was faster than that of 7:3. The release rate at 308K was faster than that at 293K for each molar ratio. These results were responsible for the difference in the swelling ratio and hydrophilicity of the hydrogels.

The kinetic model for BSA release was constructed based on the diffusion equation for sphere and mass balance between the sphere and well-mixed water phase. The values of effective diffusion coefficient in the hydrogels were estimated by fitting the calculated results to the experimental data for each experimental condition. The model explained well the effect of molar ratio of NIPAAm to HEMA and surrounding temperature on release rate of BSA. The value of effective diffusion coefficient at 308K was more than two times larger than that at 293K. This result suggested that the hydrogel at surrounding temperature higher than the lower critical solution temperature (LCST) was hydrophobic and the interaction between the hydrogel and BSA was weak. All the estimated values were lower than that of diffusion coefficient in water calculated by the Stokes-Einstein equation, so that the values estimated in this study were reasonable. The model also explained the effect of hydrogel size on BSA release rate.