Prevention of protein adsorption by the surface-grafted poly(ethylene oxide) (PEO) chains has been well-known. We have examined the mechanisms of how the grafted PEO prevents protein adsorption. PEO−poly(propylene oxide)−PEO (PEO−PPO−PEO) triblock copolymers were used to graft PEO to the trichlorovinylsilane (TCVS)-modified glass by γ-irradiation. The surface density of the PEO chains was varied up to 60 pmol/cm2 and the number of the ethylene oxide (EO) units of the PEO segment was varied from 75 to 128. The adsorption of lysozyme and fibrinogen to the PEO-grafted glass was examined using radiolabeled proteins. The surface protein concentration decreased as the surface density of the grafted PEO increased, but surface protein concentration never reached zero. The experimental data were compared with the predictions by the single-chain mean-field theory. There was very good agreement between the predictions of the theory and the experimental observations. It was found that the mechanism for prevention of protein adsorption by the grafted PEO chains in the hydrophobic surfaces was due to the blocking by the PEO segments of the adsorbing sites of the proteins. The mechanism of the grafted chains to prevent protein adsorption was shown to depend upon the interactions of the surface with the segments of the grafted polymers. Surfaces that did not attract the polymer segments present effective kinetic barriers but were not very good for equilibrium prevention. On the other hand, hydrophobic surfaces, such as the ones used in the experimental work, were very effective for reducing the equilibrium amount of proteins adsorbed. It was found that the most important parameter in preventing protein adsorption by grafted polymers is the surface density of the grafted polymer. The polymer molecular weight, or the chain length, was found to have a weak effect.

Poly(ethylene oxide) (PEO) has been frequently used to modify biomaterial surfaces for improved biocompatibility. We have used PEO-polybutadiene-PEO triblock copolymer to graft PEO to biomaterials by gamma-irradiation for a total radiation dose of 1 Mrad. The molecular weight of PEO in the block copolymer was 5000. In vitro study showed that fibrinogen adsorption to Silastic, polyethylene, and glass was reduced by 70 to approximately 95% by PEO grafting. On the other hand, the reduction of fibrinogen adsorption was only 30% on expanded polytetrafluoroethylene (e-PTFE). In vitro platelet adhesion study showed that almost no platelets could adhere to PEO-coated Silastic, polyethylene, and glass, while numerous platelet aggregates were found on the ePTFE. The platelet adhesion in vitro corresponded to the fibrinogen adsorption. When the PEO-grafted surfaces were tested ex vivo using a series shunt in a canine model, the effect of the grafted PEO was not noticeable. Platelet deposition on ePTFE was reduced by PEO grafting from 8170 +/- 1030 to 5100 +/- 460 platelets 10(-3) microm2, but numerous thrombi were still present on the PEO-grafted surface. The numbers of platelets cumulated on Silastic, polyethylene, and glass were 100 +/- 80, 169 +/- 35, and 24 +/- 22 platelets 10(-3) microm2, respectively. This is about 35% reduction in platelet deposition by PEO grafting. While the numbers of deposited platelets were small, the decreases were not as large as those expected from the in vitro study. This may be due to a number of reasons which have to be clarified in future studies, but it appears that in vitro platelet adhesion and fibrinogen adsorption studies may not be a valuable predictor for the in vivo or ex vivo behavior of the PEO-grafted surfaces.

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