Investigation of the Swelling Response and Loading of Ionic Microgels with Drugs and Proteins: The Dependence on Cross-Link Density

Abstract

The pH and NaCl induced swelling response and drug and protein loading of poly(methacrylic acid-co-acrylic acid) microgels (4−10 μm diameter) were measured as a function of cross-link density. The swelling ratio (Q) of the microgels increased linearly from 2 to 12 when the mole fraction of cross-linking monomer decreased from 0.25 to 0.10 (at pH's > 5.3). In the presence of 5 M NaCl (at pH's > 5.3), microgels with cross-linking feed ratios of 0.25 and 0.10 swelled to only 80% and 60% of their maximum volume measured at low ionic strength, respectively. To determine the average pore size in the different cross-linking density microgels (feed ratios = 0.25, 0.20, 0.15, and 0.10), we measured the size cutoffs for the uptake of different sized proteins. On the basis of these size exclusion experiments, we calculated the number of monomers between cross-links in each of these gels to be 6.5, 9.5, 12.5, and 16.5, respectively. These values were used in our theoretical modeling of the network swelling (modified ...