Biological behavior of titanium processed by severe plastic deformation

Abstract

Hydrostatic Extrusion (HE) was successfully applied to achieve grain refinement in titanium Grade 2. Cellular response (WST-1 assay) and protein adsorption tests (colloidal gold method) were performed to investigate the effects of severe plastic deformation on the biocompatibility of titanium. STEM observations were performed in order to characterize the microstructure of the deformed metal followed by correlation with it properties. This also made it possible to better understand the phenomena occurring at the surface. The surface characteristics, including wettability, surface energy and X-ray photoelectron spectroscopy (XPS), were used to determine those factors responsible for the increase in the biocompatibility of the extruded samples. The nanostructure had beneficial effect on proliferation and attachment of SaOS-2 cells. Grain refinement affected protein adsorption behavior. Non-specific adsorption represented by albumin was favored as a result of the high density of non-equilibrium defects. After the HE process, the biocompatibility of the titanium increased due to microstructural changes affecting the properties of the native passive oxide layer. Grain refinement promoted the formation of charged sites on the surface which improved protein adsorption and increased the amount of hydroxyl (OH−) groups. Surface free energy measurements revealed that this was attributed to a high acid-base component, γAB. Indirect XPS studies demonstrated that, after the HE process, the oxide layer on the titanium surface was thinner, which contributed to greater homogeneity and better corrosion resistance. The most possible explanations for the improved biocompatibility of the nanostructured materials are also discussed in this paper.