Solute transport analysis in pH-responsive, complexing hydrogels of poly(methacrylic acid-g-ethylene glycol)

We report on the preparation and properties of hydrogels of poly(methacrylic acid-g-ethylene glycol) that exhibit pH-responsive swelling behavior due to the reversible formation/dissociation of interpolymer complexes. Because of their nature, these materials may be useful in drug delivery applications. In this work, we studied the diffusional behavior of three solutes of varying molecular size in the complexing hydrogels as a function of solution pH. The ability of these gels to control the solute diffusion rates was strongly dependent on the molecular size of the solute and the environmental pH. The diffusion coefficients for solutes were calculated as a function of pH and were lower in acidic than neutral or basic media due to the formation of interpolymer complexes in the gels. However, the ratio of the solute radius to the network mesh size also was a significant factor in the overall behavior of these gels. The diffusion coefficient of the smallest solute, proxyphylline, studied only changed by a factor of five between the complexed and uncomplexed state. However, for the largest solute, FITC-dextran, which has a molecular radius ten times greater than proxyphylline, the diffusion coefficients of the drugs in complexed and uncomplexed gels varied by almost two orders of magnitude. These results are explained in terms of mesh size characteristics of the gels.