Introduction: Titanium alloy, especially Ti6Al4V alloy is widely used in medical prosthesis and orthopedic and dental implants. However, the implants failure generally occurs between the implant and the underlying bone surface because of poor osseointegration. To optimize the surface of Ti alloys for enhancing interaction of orthopedic implant and bone, the ordered TiO2-array film with nano-characteristic is anodized in electrolyte containing fluoride ions at an anodic voltage[1],[2]. In anodization, many parameters are key factors for the fabrication of nanotubes, including anodizing voltage, duration time, and water content. The aim of this paper is to provide a systematic approach to grow highly-ordered TiO2 nanotubes-array on Ti alloy by orthogonal design of experiments (DOE).
Materials and Methods: Ti6Al4V alloy with 1mm thickness was washed with acetone, ethanol and distilled water by ultrasonic. The anodizing electrolyte was prepared by mixing ethylene glycol (EG), H2O and 0.25 wt% NH4F in a glass beaker at room temperature. The electrochemical anodization was performed in a two-electrode system with Ti alloy as the anode and plumbum foil as the cathode under an anodizing voltage.
For experimental design, the orthogonal experimental design is used. The different levels for anodizing voltage (20, 30, 40, 50V), duration time (2, 3, 4, 5h) and H2O content (2, 3, 4, 5 wt%) in the electrolyte were determined, respectively. After anodization, the as-anodized TiO2 nanotubes-array was rinsed with deionized water and then ultrasonicated in ethanol for 1 min. The morphology of the TiO2 nanotubes-array on Ti6Al4V was observed using field emission scanning electron microscope (FESEM). Finally, the dimensions of TiO2 nanotubes were measured using ImageJ software.
Results and Discussion: From the SEM micrographs, we observe that the small nanopores have been grown on the surface of Ti6Al4V at 20V, 2.0 wt% H2O for 2h. This porous layer is initiated during the first stage of growth, also called compact oxide layer. The nanograss is formed at 20V because the long TiO2 nanotubes are bundled at the top parts. With the increase of water content from 2 wt% to 5 wt%, the neat and distinct TiO2 nanotube is anodized with the clear surface at 30V. At the same time, the ripples are observed on the side wall of tubular from the sectional view. At 40V, the serious collapse and disintegrate of the top part of TiO2 nanotubes are formed in EG electrolyte with 2 wt% H2O for 4h and 3 wt% H2O for 5h. At 50V, the large diameter and the long neat nanotubes are formed with honeycomb-like structure.
The range analysis of orthogonal experimental design indicates that the voltage is the key factor in fabricating the dimension (including the mean and SD of the nanotubes diameter) of the anodized nanotubes. The sequence of influential factors is voltage > duration time > H2O content. The variance analysis indicates that the statistical significant difference is existed for the voltage (p < 0.05). The voltage is linearly correlated with the mean diameter. The result indicates that the optimal level for the water content and duration time is 4 wt% H2O and 3h, respectively.
Conclusion: Based on the method of orthogonal experimental design, the highly-ordered TiO2 nanotubes-array is successfully fabricated in EG electrolyte containing 0.25 wt% NH4F. The voltage is the key factor in fabricating the dimension of the TiO2 nanotubes. The optimal level for the water content and duration time is 4 wt% H2O and 3h, respectively.
This work is financially supported by the the China Scholarship Council (CSC) for State Scholarship Fund No. [2014]3012, National Natural Science Foundation of China (3127008, 11120101001, 61227902). the 111 Project (B13003), International Joint Research Center of Aerospace Biotechnology and Medical Engineering, Ministry of Science and Technology of China, Specialized Research Fund for the Doctoral Program of Higher Education, and National High Technology Research and Development Program of China (863 program, 2011AA02A102).
References:
[1] Regonini, D., C. R. Bowen, et al. A review of growth mechanism, structure and crystallinity of anodized TiO2 nanotubes. Materials Science & Engineering R-Reports. 2013, 74(12): 377-406.
[2] Minagar, S., J. Wang, et al. Cell response of anodized nanotubes on titanium and titanium alloys. Journal of Biomedical Materials Research Part A. 2013, 101(9): 2726-2739.