Comparative Assessment of Change in Surface Properties and Biocompatibility of Ti6Al4V ELI and Ti13Nb13Zr Alloys after Titanium Dioxide Nanotube (TNT) Growth

Titanium dioxide nanotubes (TNTs) have drawn significant attention due to their prospective applications in osseo-integrative biomedical implants. Understanding the influence of alloy composition and phase distribution on TNT morphology, particularly nanotube length, is essential for optimizing these materials for biomedical applications. In this study, we synthesized TNTs on two titanium alloys, Ti6Al4V ELI and Ti13Nb13Zr, to investigate how their distinct microstructures impact nanotube formation. Electron backscatter diffraction (EBSD) analysis revealed unique grain structures and phase orientations. The microstructure of the Ti6Al4V ELI depicted an equiaxed grain structure parallel to the (0001) α-plane while that of the Ti13Nb13Zr showed the needle-like α-phase in the β-phase matrix. These microstructural differences were hypothesized to enhance ion diffusion and selective dissolution, promoting longer nanotube formation. This work shows that Ti13Nb13Zr synthesized nanotubes with an average length of 11.33 µm, significantly exceeding the 1.76 µm length observed in Ti6Al4V ELI. The higher aspect ratio in Ti13Nb13Zr is attributed to its β-phase matrix, which facilitated ion migration during anodization. Electrochemical analyses, including Tafel polarization and impedance spectroscopy, confirmed the superior dissolution resistance of Ti13Nb13Zr, supporting extended nanotube growth. The results pinpoint that the microstructure of Ti13Nb13Zr is more appropriate for synthesizing longer TNTs based on improved ion diffusion and selective phase dissolution. These properties make this alloy a good candidate for its use in areas that need high amounts of surface area. Higher surface area facilitates more cell adhesion and thus an enhanced biocompatibility in biomedical implants. Furthermore, other factors that govern cell attachment are the hydrophilic nature and surface roughness of the implant. Thus, to assess the biomedical responses of both the alloys contact angle measurements and surface roughness properties were measured before and after TNT growth. Contact angle analysis proved the surfaces to be hydrophilic for all the specimens. However, TNTs on Ti13Nb13Zr showed super hydrophilic behaviour with a contact angle of 5 degrees.
Moreover, an in-vitro mineralization assay was performed to assess the effect of TNTs grown on both alloys on facilitating the adsorption of hydroxyapatite crystals from simulated body fluid (SBF) which is the first step towards making them suitable for osteoconductive implants. It has been observed that the formation of nanocrystals with a hydroxyapatite (HAp) structure on the top edges of the 10 nm-thick nanotube walls was identified after 7 days of immersion in SBF. The nano-sized HAp phases nucleated and grew, and the formation kinetics of the HAp phase were accelerated with the development of the nanostructured surface features. These nanocrystals, formed in 14 days, caused the exponential growth of the HAp layer thicker than 5 µm in cross-section over the TNT layer. It is hypothesized that the TNTs on Ti13Nb13Zr which had higher surface roughness (Ra) due to biphasic phase of α and β enhanced more HAp coating as compared to that of TNTs on Ti6Al4V ELI. These alloys with and without TNT growth are presently being assessed for cytocompatibility with L929 fibroblasts and Saos-2 osteoblast-like cells. Further, studies will assess for mineralization studies using Alizarin red S staining technique to assess calcium Phosphate secretion from Saos-2 cells in the presence of these alloys with and without TNT growth. This research work aims at providing a new approach using electrochemical techniques to determine how the microstructures and phases affect the nanostructuring of surfaces and in turn affect biomedical responses and their suitability for use as potential biomedical implants.