Effect of charged groups on the adsorption and penetration of proteins onto and into carboxymethylated poly(HEMA) hydrogels

A range of carboxymethylated poly(hydroxyethyl methacrylate) (CM-PHEMA) hydrogels with varying degrees of carboxymethylation was synthesized for a systematic study of the effects of ionized groups (‘charge’) on the uptake by hydrogel matrices of the proteins, lysozyme and human serum albumin (HSA). Using a radiolabel-tracer technique, X-ray photoelectron spectroscopy, and laser scanning confocal microscopy, we attempted to differentiate between protein molecules that were irreversibly adsorbed onto the hydrogel surface and those that penetrated into the hydrogel matrix. The effective pore size of the CM-PHEMA hydrogels was modelled and compared with the known molecular dimensions of the two proteins. The effects of the presence of varying amounts of ionized groups in the hydrogel matrix differed for the two proteins. For lysozyme, increased uptake was observed at higher carboxymethylation; this is interpreted as resulting from a combination of electrostatic attraction and increasing ease of penetration of the protein into the more porous hydrogel matrix. For HSA, on the other hand, the uptake was primarily by surface adsorption, with little diffusive penetration into the matrix.