Diffusion and release of solutes in pluronic-g-poly(acrylic acid) hydrogels

Pluronic-PAA, a thermogelling copolymer composed of side chains of poly(acrylic acid) (PAA) grafted onto a backbone of Pluronic copolymer, is of interest as a vehicle for the controlled release of compounds. The diffusion of various electrochemically active solutes (ferricyanide, the redox protein cytochrome c, and a range of ferrocene derivatives) with varying sizes and hydrophobicities has been examined in solutions and hydrogels of Pluronic-PAA. Release of the solutes from the hydrogel into aqueous media has also been examined. Small hydrophilic molecules diffused freely through the hydrogel, but diffusion of more hydrophobic species was retarded due to their association with hydrophobic micellar aggregates formed on gelation of Pluronic-PAA. The rate of release of solutes from the hydrogel into aqueous media also decreased with increasing hydrophobicity. Constant-rate release was observed over a considerable time period for hydrophobic materials. Cytochrome c exhibited reduced diffusion coefficients at elevated temperatures due to obstruction by the hydrogel network. Release of the protein from the hydrogel was significantly affected by ionic strength, suggesting that electrostatic interactions were an important factor in the mobility of cytochrome c in Pluronic-PAA hydrogels.