Grafting with hydrophilic polymer chains to prepare protein-resistant surfaces

Different ways of grafting poly(ethylene glycol) (PEG) chains to solid polyethylene were compared with respect to grafting density and efficiency in preventing fibrinogen adsorption. Covalent grafting of PEG was performed by attaching a nucleophilic PEG derivative to electrophilic surface groups or by binding electrophilic PEG to nucleophilic groups at the solid surface. Two adsorption procedures were also used. In the first of these an ethylene oxide-propylene oxide (EO-PO) block copolymer was adsorbed at unmodified, hydrophobic polyethylene. In the second procedure the surface was made carboxyl-functional by free-radical grafting of tiglic acid and then exposed to a solution of a positively charged copolymer consisting of PEG chains grafted to poly(ethylene imine) (PEI). According to ESCA measurements, all four routes gave proper PEG grafting densities and the difference in the ratio of C-C-O carbon (from PEG) to C-C-C carbon (from the underlying surface) was relatively small. There was a substantial difference in efficiency in fibrinogen rejection, however. Whereas surface modification with the PEG-PEI graft copolymer gave the lowest, treatment with the EO-PO block copolymer gave the highest amount of protein adsorption. The good effect of the PEG-PEI layer is believed to be related to the large entropy loss associated with protein adsorption on top of this copolymer which is known to be loosely bound in a loops-and-trains configuration. The limited effect of the EO-PO block copolymer may be due to the fact that this polymer is not entirely hydrophilic at the temperature used. Another contributing factor may be that the EO-PO block copolymer, unlike the PEG-PEI graft copolymer, is not irreversibly bound to the surface and may therefore be exchanged by fibrinogen.