The adsorption of lysozyme and bovine serum albumin (BSA) onto the surface of a hydrogel polymer has been characterized by neutron reflection and spectroscopic ellipsometry. The polymeric hydrogel was synthesized by copolymerizing methacrylate monomers bearing dodecyl chains and phosphorylcholine (PC) groups. The polymer surface was formed by dip-coating a thin layer of the polymer onto the polished surface of silicon oxide. While ellipsometric measurements at the solid-water interface showed that the polymer film could be represented by a uniform layer of 55 ( 5 Å, the subsequent measurements from neutron reflection suggested that the structural profile of the polymeric film was better described by a three layer model: the inner layer of 30 ( 3 Å mixed with less than 20% water, the middle layer of 20 ( 5 Å mixed with approximately 40% water, and the outermost layer of 25 ( 3 Å mixed with approximately 85% water. The neutron results thus suggest the uneven swelling of the polymer film along the direction normal to the interface. The adsorption of lysozyme and BSA onto the polymer surface was measured over a wide protein concentration range. It was found that the amount of proteins adsorbed on the coated polymer surface was substantially lower than that at the bare silica-water interface under the same solution condition. Thus, the amount of BSA adsorbed onto the polymer-coated surface at pH 5 and at 0.05 g dm -3 was found to be less than 0.5 mg m -2 , as compared with 2 mg m -2 obtained at the bare silica-water interface. In addition, the amount of lysozyme adsorbed onto the coated surface at pH 7 and at 1 g dm -3 was also found to be about 0.5 mg m -2 , as compared with 3.5 mg m -2 at the silica-water interface. The reduction in protein adsorption was attributed to the presence of a PC layer preferably formed on the outer surface of the coated polymer film.

We have studied the adsorption of a number of model proteins onto the surface of a cross-linkable hydrogel polymer incorporated with phosphorylcholine (PC) groups and dodecyl chains (PC 100B). The structure of the coated thin polymer film was determined by neutron reflection combined with spectroscopic ellipsometry. No measurable change in the thickness of the polymer film was detected within the experimental time scale of minutes when immersed in water, showing a fast water solubilization process. The polymer film at the solid−water interface was modeled using a single layer of 51 ± 3 Å with 40 ± 5% water, suggesting a uniform distribution of water across the polymer film. This film structure is in sharp contrast with the uneven swelling of the film formed from a different hydrogel polymer (PC 100A) which had a similar molar ratio of dodecyl chains and PC groups but did not contain any silyl groups as cross-linkers. The results hence suggest that the uniform structure of the PC 100B film is rendered by the formation of the silyl cross-linking network. The effectiveness of the PC 100B film at reducing protein adsorption under different solution conditions was subsequently characterized. Both neutron reflection and spectroscopic ellipsometry showed substantial reduction in protein adsorption on PC 100B. At the bulk protein concentration around 1 g dm-3 the surface excess was found to be less than 1 mg m-2 for lysozyme and fibrinogen at pH 7 and BSA at pH 5, while under the same solution conditions, the surface excess at the hydrophilic silicon oxide−water interface was 3.6 ± 0.3 mg m-2 for lysozyme, 6.0 ± 0.3 mg m-2 for fibrinogen, and 2.5 ± 0.3 mg m-2 for BSA. Despite the structural difference between the two coated polymer films, the residual level of protein adsorption was found to be comparable between the two PC polymer surfaces. The insensitivity of spectroscopic ellipsometry to the presence of a diffuse protein layer on the surface of the coated polymer films is also discussed.

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