The biological performance of materials is mostly determined by protein adsorption at the biomaterials surface which can be, e.g. affected by surface nanostructures. The nanostructures can, thereby, influence the assembly as well as the orientation of adsorbed proteins. For the class of polymeric (bio)materials a reproducible and well-characterized nanostructure is the ordered chain folded surface of a polyethylene single crystal (PE-SC).
The aim of the current study was to control the protein adsorption by a nanostructured surface. For this, we tested the hypothesis that the trinodal-rod-shaped protein human plasma fibrinogen (HPF) adsorbs on the (001) surface of PE-SCs along specific crystallographic directions.
PE-SC were prepared by isothermal crystallization in diluted solution. The PE-SC surfaces were characterized by atomic force microscopy before and after HPF adsorption at different concentrations and pH values. At a physiological pH of 7.4, HPF assemblies consisting of several connected HPF molecules, e.g. fibrils, fibril networks or sponge-like structures, were observed at the PE-SC surface. These assemblies features no preferred orientations. The nonoriented connected protein assemblies were explained by increased protein-protein interactions and impaired surface diffusion. However, at an increased pH of 9.2 spherical-shaped and trinodal-shaped single HPF molecules as well as agglomerates and a dense layer were observed. The presence of single HPF molecules was explained by decreased protein-protein interactions due the increased pH. The trinodal-rod-shaped HPF molecules showed a preferred orientation along the crystallographic  and  directions on the nanostructured PE-SC surface. This was explained by an increased amount of intermolecular bonds along crystallographic directions with increased surface atom density.
The current study established that HPF molecules can align on chemically homogeneous surface topographies one order of magnitude smaller than the dimension of the protein. This deepens the understanding of how to control the protein assembly and orientation on nanostructured surfaces.