Surface Characterization of 5M NaOH Treated Ti-Ta Alloys Exposed to Simulated Body Fluid

Ti-Ta biocompatible alloys are considered for biomedical applications and therefore it is necessary to clarify the influence of tantalum on the alloys in terms of microstructure, mechanical properties, and corrosion resistance [1–3] and to enhance their properties as biomaterials. Alkali treatment using NaOH aqueous solutions is an effective surface treatment method to improve the bioactivity and osteoinduction of metallic biomaterials. It was found that alkali treatment with an appropriate concentration of NaOH yields a significant improvement in biocompatibility and bioactivity of high purity Ta deposited by plasma spraying [4]. With the alkali treatment of NaOH, an osteogenic layer of sodium titanate is generated on which apatite can be easily deposited and immobilized. The influence of Ta content on the microstructure of Ti-xTa alloys manufactured using an induction furnace under protective atmosphere was studied. Moreover, the influence of alkali treatment by surface functionalization using 5 M NaOH was performed to study the osteogenic potential of these alloys and the best behavior was observed for Ti-25%Ta. After surface treatment functionalization, the samples were immersed in Ringer’s solution to improve biocompatibility and an apatite film was detected on the surface of the samples. The Ti-xTa alloys with different Ta contents (5%, 15%, and 25% Ta, respectively) were prepared using an induction furnace under protective atmosphere. The materials were prepared with a diameter of 20 mm and a length of 30 mm. Heat treatment was performed at 1000 °C followed by cooling in air to avoid segregation. ASTM E3-11 (2017) was used to prepare the samples for metallography prior microstructural investigations. After etching using Kroll´s reagent, the samples were imaged using SEM secondary electron signal at high vacuum mode at 15 kV (Zeiss Sigma 300 VP, Carl Zeiss, Jena, Germany). Energy dispersive X-ray (EDS) was also employed to reveal the chemical composition of the alloys. To improve the osteoinduction, a superficial treatment was employed by immersing the samples in 100 ml of 5M NaOH at 60 °C for 24h and its effect, followed by drying and exposure to Ringer’s solution for 5 days at 37 °C. After exposure, the samples were investigated by SEM and EDS (Quanta FEG 250, FEI, Hillsboro, OR, USA). As it can be observed from Figure 1, the micro-structure of Ti-xTa samples changed with the addition of Ta. The Ti-5%Ta alloy presents a microstructure characterized by α´ lamellar structure and equiaxed primary α in a matrix of transformed β, while the Ti-25%Ta alloy presents a lamellar α´ structure in the β phase as well with fine acicular α´´. Microstructure of Ti-5%Ta, Ti-15%Ta and Ti-25%Ta before NaOH treatment. Microstructure of a) Ti-5%Ta, b) Ti-15%Ta, c) Ti-25%Ta after NaOH treatment and exposure to Ringer’s solution at 80k magnification Microstructure of a) Ti-25%Ta after NaOH treatment and exposure to Ringer’s solution at 40k magnification, b) EDS spectrum on selected area and c) elemental mapping on selected area on Ti-25%Ta After NaOH treatment, a functionalized film was developed on the surface of Ti-xTa samples as shown in Figure 2. The films present an increased roughness compared to the non-treated samples. A higher amount of Ta within the alloys (in our case 25%Ta) leads to a reduced number of cracks which might enhance the biocompatibility of this alloy. The EDS analysis presented in Figure 3 reveal the presence of O, Na, Ti and Ta which might be attributed to the formation of sodium titanate and sodium tantalate combined with Ti and Ta oxides. After exposure to Ringer’s solution, P and Ca are present as well. Since all the films presented similar composition with slightly variations of O, Ti and Ta, only the results for Ti-25%Ta are presented. The formation of the apatite film on the surface of the Ti-xTa alloys is presented in Figure 4. Formation of the functionalized film on the surface of the alloys. In our study, after surface treatment functionalization by using 5M NaOH new films were developed on the surface of Ti-xTa which might lead to an improved biocompatibility due to the increased roughness of the new developed films and the presence of P and Ca which might stimulate the formation of apatite on the surface of samples. Among these alloys, Ti-25%Ta exhibited a more uniform and reduced number of cracks.