Development and characterization of advanced thin coatings on nanostructured titanium for biomedical applications

Resumen

UltraFine-Grained Commercially Pure Titanium (UFG CP Ti) has arisen as one of the best candidates for the fabrication of implantable devices due to the superior biocompatibility of CP Ti with respect to the other metallic systems used in the biomedical field, and to the enhanced mechanical properties provided by the refined microstructure obtained through Severe Plastic Deformation (SPD) processes. However, the bioinert character of CP Ti makes necessary to improve the osteoconductivity of the surface in order to ensure a rapid and successful osseointegration. Surface modification approaches including physical and/or chemical roughening and the fabrication of bioactive coatings have been shown to improve the cell adhesion and proliferation and osteoconductivity of CG and UFG CP Ti. In particular, the Plasma Electrolytic Oxidation (PEO) technique is one of the most promising approaches to fabricate thin TiO2-based Ca- and P-containing coatings. These type of coatings provide a suitable combination of surface topography and chemistry for applications that require osteoconductive properties. However, there is a relative lack of knowledge regarding the PEO treatment of UFG CP Ti in comparison with the significant body of research on the PEO treatment of CG CP Ti. The present Thesis is focused on the surface modification of UFG CP Ti to improve its osteoconductivity. The effect of the surface topography of chemically etched UFG CP Ti on the cell adhesion and proliferation was assessed. It was observed that the cell proliferation was dependent on the characteristics of the surface topography. The main body of the Thesis consisted on the fabrication of PEO coatings on both CG and UFG CP Ti using three different electrolytes, one of them aimed to produce Ca- and P-containing coatings. The main objective of this study was twofold: First, to compare the resulting corrosion resistance, cell-surface interaction, and mechanical properties of the three PEO treatments; and second, to compare the applicability of the PEO treatment and the coatings obtained on both CG and UFG CP Ti. The UFG microstructure was found to influence the coatings growth rate, the discharge generation of each treatment and the microstructure of the layers in intimate contact with the substrate. At the same time, the latter was found to cause significant differences in the corrosion behaviour between the CG CP Ti-coated and the UFG CP Ti-coated samples. The mechanical performance of the coatings was strongly dependent on the microstructure of the outer layers of the coatings. The Ca- and P-containing coating fabricated for 120 s on the UFG CP Ti presented the highest celular proliferation rate and metabolic activity showing potential for osteoinductive and osteoconductive capabilities. In order to improve the mechanical performance of the coatings an alternative approach to the common methods for hardening of PEO coatings was proposed.