Wetting and plasma-protein adsorption studies using surfaces with a hydrophobicity gradient

Wetting gradients on silica were prepared according to the technique developed by Elwing et al. (J. Colloid Interface Sci., 119 (1987) 1). The contact angle, determined by the Wilhelmy-plate method, showed an S-shaped dependence on the position along the gradient. The advancing water contact angle was 0° on the hydrophilic end and 97±3° on the hydrophobic end. The receding water contact angles were ∼40° smaller in the wetting transition region and ∼20° smaller on the hydrophobic side of the gradient. Total internal reflection fluorescence (TIRF) was used to study the adsorption of three fluorescein-labelled proteins: human serum albumin, IgG and fibrinogen. The gradient surface exposed to a single protein solution resulted in an amount of adsorbed protein on the hydrophobic end that approximately corresponded to a monolayer. As a rule, lower adsorption was found on the hydrophilic end. When the proteins were adsorbed from a mixture, the surface concentration of albumin was generally not affected, the fibrinogen adsorption decreased by 40–50% and the IgG adsorption decreased to 10% of the adsorption measured from a single protein solution. The so-called Vroman effect (for example, transient IgG adsorption maximum in adsorption kinetics) was found on the hydrophobic side of the gradient. The desorption experiments were carried out with a nonionic polymeric surfactant, an ethylene oxide-propylene oxide copolymer: EO100-PO70-EO100. The adsorption affinity of albumin to the wetting transition region, as measured by surfactant induced desorption, was extraordinarily high.