Optimization of Anodization Parameters in Ti-30Ta Alloy

: The current metallic biomaterial still presents failures associated with the bulk alloy and the interface of material / human body. In previous studies, titanium alloy with tantalum showed the elastic modulus decrease in comparison with that of commercially pure (cp) titanium. In this study, surface modiﬁcation on Ti-30Ta alloy was investigated. Titanium and tantalum were melted, homogenized, cold-worked by a rotary swaging process and solubilized. The anodization process was performed in electrolyte contained glycerol + NH 4 F 0.25% at 30 V using seven di ﬀ erent durations—4 h, 5 h, 6 h, 7 h, 8 h, 9 h, and 10 h and annealed at 530 ◦ C for 1 h. The surface topography was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) measurements, X-ray di ﬀ raction analysis (XRD), and contact angle. From the results, we conclude the time of anodization process inﬂuences the shape and morphology of the anodized layer. The 5 h-anodization process produced a smooth and porous surface. The 4-, 6-, 7-, 8-, 9-, and 10-h conditions showed nanotubes morphology. All surfaces are hydrophilic ( < 90 ◦ ). Likewise, all the investigated conditions present anatase phase. So, this surface modiﬁcation presents potential for biomedical application. However, more work needs to be done to better understand the inﬂuence of time on the anodization process.


Introduction
Metallic medical devices have been used to replace parts of the body due to suitable mechanical properties and biocompatibility. Despite, the current material in use, including stainless steels, Co-Cr based alloys, titanium, and its alloys still present failures associated with the bulk of the alloy and the interface of material/human body. Recently, the cost of correction of those failures is high and it inspires studies for a new solution [1][2][3][4][5].
The Ti-30Ta wt.% alloy was processed according to our previous studies [19]. Briefly, titanium and tantalum pure (99.99%) (Sorocaba, Brasil) were melt in a high purity argon atmosphere mixing, both purchased from Sigma-Aldrich. The obtained ingot was homogenized in a vacuum at 1000 • C for 24 h. The alloy was then cold-worked by a rotary swaging process and solubilized at 950 • C for 2 h followed by water-cooling. The bars were cut into discs of 6 mm in diameter and 3 mm in thickness [22,23].

Surface Treatment
The anodization process was described on previous studies [27]. Summarily, dual electrode system with platinum (counter electrode) and Ti-30Ta alloy (working electrode) fixed 15 mm apart for anodization process. The electrodes were then suspended in an electrolyte contained glycerol and NH 4 F 0.25%. The electrodes were connected to a power supply (Fisher Scientific FB300 Electrophoresis) and the voltage applied was 30 V at room temperature. They were investigated at seven different durations : 4, 5, 6, 7, 8, 9, and 10 h. Following the anodization process, the Ti-30Ta alloy substrate was rinsed in isopropyl alcohol and dried by compressed air. All anodized substrates were further annealed in an oxygen ambient furnace at 530 • C, with a ramping rate of 5 • C/min for 1 h [27]. Each experiment was reconfirmed on at least three different substrates.

Characterization of Substrates
The surface topography of seven different durations: 4,5,6,7,8,9, and 10 h was characterized by scanning electron microscopy (SEM) imaging (JEOL JSM 6100) (Carl Zeiss, Guaratingueta, Brazil) with 10 mm working distance and 15 keV. Prior to imaging, all substrates were coated with a 15 nm layer of gold. In order to obtain cross-sectional images, the surface was mechanically scratched with metallic tool and imaged by tilting the chamber to 70 • and changing the working distance to 4.6 mm. Atomic force microscopy (AFM) (Shimadzu, Guaratinguetá, Brazil) measurements were performed using a Dimension Icon microscope (Shimadzu, Guaratinguetá, Brazil) from Shimadzu, operating in non-contact mode. The root mean square (RMS) roughness was measured in a 2 × 2 µm 2 area. The wettability of the substrate surfaces was investigated using a sessile drop method (2 mL) with a contact angle goniometer (Kruss DSA 10) (Kruss, Guaratinguetá, Brazil), equipped with video capture. The contact angle was measured through the liquid, where a liquid-vapor interface meets a solid surface. The resulting images at the water-substrate interface were fit using the circle fitting profile [28]. The crystallinity of the substrates was investigated by X-ray diffraction analysis (XRD), using a X' Pert Philips PMD (Philips, São Carlos, Brazil) with a Panalytical X'celerator detector using a Monochromatic CuKα radiation (Philips, São Carlos, Brazil) (λ = 0.1544 nm) applied at 20 mA and 40 kV, with a step size of 0.02 • , and a collecting time of 1.6 s.

Results and Discussion
The nano topography has high influence on the success of metallic implantable device [11,26,[28][29][30][31]. Thus, we investigate the formation of nanotubular structure on the surface of Ti-30Ta alloy controlling the parameters of anodization. The anodizing process is an electrochemical reaction. The name "anodize" comes from the fact that the substrate will be the anode. The substrate is immersed in an electrolyte solution with another material that will be the cathode. The current passes through the acidic solution, hydrogen from the cathode is released and oxygen from the anode surface is released as well. The reaction of these elements forms a film on the surface of the substrate.
The anodization process was performed in an electrolyte solution containing NH 4 F 0.25% + glycerol [32] and applied voltage at 30 V to at least three substrates with good repeatability. Seven different conditions on variation of time were investigated (Figures 1 and 2). In this way, nanotubular surface was obtained with different diameters and lengths that could lead to different cellular responses [20,27,[33][34][35][36].
SEM images (Figures 1 and 2) show the Ti-30Ta alloy surface after anodization process for 4, 5, or 6 h. The annealing process was performed at 530 • C for 1 h.
In Figure 1a-c SEM micrographs of the surface of the Ti-30Ta alloy after anodization process for 4, 5, and 6 h at different magnifications were depicted. Figure 1(4h-a,6h-c) present a precursor geometric formation of pores/nanotube with nonhomogeneous morphology and cracks. According to Zhou (2005) TiO 2 oxides are observed on the surface of titanium and its alloys, but it was not found by the authors for Ti-30Ta alloy, with the passive layer of Ta 2 O 5 being more stable [16]. A circular formation for the 5and 6-h conditions and nanotubes in 4 h can be observed in higher magnification.
Also, the surface of 5-h anodization ( Figure 1(5h-b)) presents smooth surface. Therefore, the presence of pores and nanotubes is confirmed for the 4-and 6-h conditions, which does not occur with the 5-h anodization (Figure 1(5h-a-c)). The 5-h anodization does not allow the formation of nanotubes.
Increasing the anodization time at the same voltage and electrolyte leads to nanotubular formation with similar morphologies and lengths [37]. Increasing the anodization time at the same voltage and electrolyte leads to nanotubular formation with similar morphologies and lengths [37].  The software ImageJ was used to determine the diameter and length of the nanotubes formed on the surface of the Ti-30Ta alloy after anodizing at 30 V for 4,5,6,7, 8, 9, and 10 h. For the nanotube length values, the order of lowest to highest increase differs from that followed by the standard diameter being: 4 h-7.13 ± 1.57 µm, 6 h-6.88 ± 1.73 µm, 9 h-6.54 ± 2.09 µm, 7 h-6.87 ± 1.68 µm, 8 h-7.25 ± 0.97 µm, and 10 h-8.61 ± 1.36 µm. According to the literature, the increase in layer thickness is related to the anodizing time [39][40][41][42]. With the increase of the oxide layer thickness, it was possible to dissolve and form the compact oxide layer and the formation of nanotubes. The nanotube can mimic the shape of alveolar bone ( Table 1).
X-ray diffraction analysis were applied to assess the phase changes that occurred during all conditions of the anodizing process. Figure 3 shows the obtained spectra after anodizing process. All investigated conditions showed anatase phase with the greatest peaked around 25 • , the exception is the control group. The anatase phase provides an improvement in the cellular response required in biomedical applications [33,[46][47][48]. The software ImageJ was used to determine the diameter and length of the nanotubes formed on the surface of the Ti-30Ta alloy after anodizing at 30 V for 4,5,6,7, 8, 9, and 10 h.  [39][40][41][42]. With the increase of the oxide layer thickness, it was possible to dissolve and form the compact oxide layer and the formation of nanotubes. The nanotube can mimic the shape of alveolar bone ( Table 1).
The ideal condition was obtained within 9 h (Figure 2 (9h-b)). These results show nanotubes in the dimensions of 57.21 ± 3.27 nm in diameter and 6.54 ± 2.09 µm in length. Studies with pure titanium show that similar dimensions provide an increase in functionality [38,[43][44][45] and hemocompatibility [33,36]. Table 1. Diameter and length measurements of the nanotubes formed on the surface of the Ti-30Ta alloy a SEM images of Ti-30Ta alloy surface after anodization process in NH4F 0.25% + glycerol at 30 V for 4, 5, 6, 7, 8, 9, and 10 h. The annealing process was performed at 530 °C (5 °C/min) for 1 h.

Anodization
Nanotube Diameter Nanotube Length   We can observe in Figures 4 and 5, the surface topography obtained by atomic force microscopy (AFM). The data was processed with the Shimadzu SPM software (Scanning Probe Microscopy) offline V.3.31 (Guaratinguetá, Brazil). The substrates subjected to anodizing for 4, 5 and 6 h ( Figure 4) presented a surface with irregular topography and some differences in the topography occurred for 7, 8, 9, and 10 h ( Figure 5) which can be attributed to the formation of nanotubes on the substrate surface after the anodizing process. The condition of 10 h shows a greater change in the surface topography. X-ray diffraction analysis were applied to assess the phase changes that occurred during all conditions of the anodizing process. Figure 3 shows the obtained spectra after anodizing process. All investigated conditions showed anatase phase with the greatest peaked around 25°, the exception is the control group. The anatase phase provides an improvement in the cellular response required in biomedical applications [33,[46][47][48]. The human tissue features variations in which they promote specific protein adsorption and direct chemical interactions through the cellular behavior. Thus, these nanotube topographic characteristics (Figures 4 and 5) allow a promising interface due to an increase in the nanometric scale roughness of the material surface [49].     The human tissue features variations in which they promote specific protein adsorption and direct chemical interactions through the cellular behavior. Thus, these nanotube topographic characteristics (Figures 4 and 5) allow a promising interface due to an increase in the nanometric scale roughness of the material surface [49].
The values of mean roughness (Ra) were obtained analyzing substrates in AFM. The values follows: 4 h-121. 28 Figure 6). The proximity of the observed values in the anodized substrate for 4, 5, 6, 8, and 9 h. The 7-h anodization presented the highest roughness and 10 h, the lowest one. The roughness values influence the strength of cellular adhesion on the surface of the implanted biomaterial [34].
In addition, the wettability of the biomaterial can influence the cell response along with the topographic and roughness surface characteristics. In biomedical applications, wettability is a desirable condition since it promotes increase in cellular interaction [28,50]. Previous studies already related to the surface energy of a biomaterial with cellular functionality, such as protein adsorption, platelet adhesion and activation that leads to blood clotting and bacterial adhesion [51][52][53][54].
According to contact angle results, almost all surfaces are hydrophilic (<90 •  (Figure 6). The proximity of the observed values in the anodized substrate for 4, 5, 6, 8, and 9 h. The 7-h anodization presented the highest roughness and 10 h, the lowest one. The roughness values influence the strength of cellular adhesion on the surface of the implanted biomaterial [34]. In addition, the wettability of the biomaterial can influence the cell response along with the topographic and roughness surface characteristics. In biomedical applications, wettability is a desirable condition since it promotes increase in cellular interaction [28,50]. Previous studies already related to the surface energy of a biomaterial with cellular functionality, such as protein adsorption, platelet adhesion and activation that leads to blood clotting and bacterial adhesion [51][52][53][54].
According to contact angle results, almost all surfaces are hydrophilic (<90°

Conclusions
The search for an ideal metallic biomaterial has been a challenge. Implantable devices for orthopedic and dental application must present elastic modulus near to the bone (E = 10-30 GPa) and nanoscale surface topographies to improve the interface response between material/human body. This study investigated the formation of nanotubular structure on the surface of Ti-30Ta alloy with elastic modulus (E = 69 GPa). From the results obtained so far, it was concluded that time variation of anodization process influences on growth of pore and nanotube structure. The 5-h anodization process produced a smooth surface. However, the presence of pores and nanotubes is confirmed by

Conclusions
The search for an ideal metallic biomaterial has been a challenge. Implantable devices for orthopedic and dental application must present elastic modulus near to the bone (E = 10 − 30 GPa) and nanoscale surface topographies to improve the interface response between material/human body. This study investigated the formation of nanotubular structure on the surface of Ti-30Ta alloy with elastic modulus (E = 69 GPa). From the results obtained so far, it was concluded that time variation of anodization process influences on growth of pore and nanotube structure. The 5-h anodization process produced a smooth surface. However, the presence of pores and nanotubes is confirmed by the 4-, 6-, 7-, 8-, 9-, and 10-h anodization process. According to the contact angle results, all surfaces are hydrophilic (<90 • ). All the investigated conditions showed anatase phase with greatest peaked around 25 • . So, this surface modification presents potential for biomedical application. Therefore, more work needs to be done to understand the anodization process.

Conflicts of Interest:
The authors declare no conflict of interest.