GRAFTING WITH HYDROPHILIC POLYMER-CHAINS TO PREPARE PROTEIN-RESISTANTSURFACES

Different ways of grafting poly(ethylene glycol) (PEG) chains to solid polyethylene were compared with respect to grafting density and effic iency in preventing fibrinogen adsorption. Covalent grafting of PEG wa s performed by attaching a nucleophilic PEG derivative to electrophili c surface groups or by binding electrophilic PEG to nucleophilic group s at the solid surface. Two adsorption procedures were also used. In t he first of these an ethylene oxide-propylene oxide (EO-PO) block copo lymer was adsorbed at unmodified, hydrophobic polyethylene. In the sec ond procedure the surface was made carboxyl-functional by free-radical grafting of tiglic acid and then exposed to a solution of a positivel y charged copolymer consisting of PEG chains grafted to poly(ethylene imine) (PEI). According to ESCA measurements, all four routes gave pro per PEG grafting densities and the difference in the ratio of C-C-O ca rbon (from PEG) to C-C-C carbon (from the underlying surface) was rela tively small. There was a substantial difference in efficiency in fibr inogen rejection, however. Whereas surface modification with the PEG-P EI graft copolymer gave the lowest, treatment with the EO-PO block cop olymer gave the highest amount of protein adsorption. The good effect of the PEG-PEI layer is believed to be related to the large entropy lo ss associated with protein adsorption on top of this copolymer which i s known to be loosely bound in a loops-and-trains configuration. The l imited effect of the EO-PO block copolymer may be due to the fact that this polymer is not entirely hydrophilic at the temperature used. Ano ther 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.