Prevention of Protein Adsorption by Tethered Poly(ethylene oxide) Layers: Experiments and Single-Chain Mean-Field Analysis Timothy McPherson, † Argaw Kidane, † Igal Szleifer,* ,‡,§ and Kinam Park* ,†,| School of Pharmacy and Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received June 25, 1997. In Final Form: October 24, 1997 X 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/cm 2 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. Introduction Surface-induced thrombosis, which is one of the major problems in the clinical application of blood-contacting materials, begins with adsorption of plasma proteins to the surface. 1,2 Preventing protein adsorption should thus improve biocompatibility. It is known that protein adsorption may be significantly decreased by surface modification with hydrophilic polymers, such as poly- (ethylene oxide) (PEO). 3-9 It has been generally thought that the prevention of protein adsorption is due to the steric repulsion by the surface-grafted (or tethered) PEO chains. 10,11 The steric repulsion picture emerged from the view that the grafted polymer layer forms a brush on the surface. However, the usual grafting densities and molecular weights of the PEO used for improved biocom- patibility are such that the polymer layer is not in the so-called “brush” regime. 12,13 In most cases the surfaces of interest are hydrophobic, and therefore, one should expect the PEO to be attracted to the surface. Although the ability of grafted PEO chains to reduce protein adsorption has been observed frequently, its theoretical analysis has been difficult for a few reasons. First of all, theoretical analysis on the behavior of the tethered PEO chains requires the information on the surface density of the grafted PEO chains. In most of the * To whom correspondence should be addressed. † School of Pharmacy. ‡ Department of Chemistry. § Telephone: (765) 494-5255. Fax: (765) 494-0239. E-mail: igal@shemesh.chem.purdue.edu. | Telephone: (765) 494-7759. Fax: (765) 496-1903. E-mail: esp@omni.cc.purdue.edu. X Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Horbett, T. A.; Brash, J. L. Proteins at interfaces. In Proteins at Interfaces. Physicochemical and Biochemical Studies; Brash, J. L., Horbett, T. A. Eds.; American Chemical Society: Washington, DC, 1987; pp 1-33. (2) Horbett, T. A. Principles underlying the role of adsorbed plasma proteins in blood interactions with foreign materials. Cardiovasc. Pathol. 1993, 2, 137S-148S. (3) Han, D. K.; Jeong, S. Y.; Kim, Y. H.; Min, B. G.; Cho, H. I. 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