PEO와 RF-sputtering을 이용하여 Sr-HA코팅된 Ti-35Nb-xTa합금의 표면특성과 생체적합성

Abstract

In this study, a new titanium alloy (Ti-35Nb-xTa) was prepared by adding elements such as tantalum (Ta) and niobium (Nb) to improve the biocompatibility and surface properties of the titanium alloy (Ti-6Al-4V). And then, after forming ceramic coatings such as hydroxyapatite (HA) and Sr-HA on the Ti-35Nb-xTa alloy surface using plasma electrolytic oxidation (PEO) and RF-sputtering, the biocompatibility and surface properties of the alloy was investigated by using various experimental techniques. The Ti-35Nb-xTa (Ta = 0, 3, 7, and 15 wt.%) ternary alloys were fabricated by using arc melting furnace. The prepared Ti-35Nb-xTa alloy was homogenized in an Ar gas atmosphere and 1050℃ for 1 hour, and then quenched in 0℃ water. Surface treatment was carried out in a 0.15M calcium acetate monohydrate + 0.02M calcium glycerophosphate electrolyte through a plasma electrolytic oxidation (PEO) at DC 280V for 3 min. Also, a small amount of 0.0075M strontium acetate, which is a bio-functional materials, was added to the electrolyte to increase bioactivity. After that, Sr was coated on the PEO-treated surface in electrolyte containing of Ca and P by using RF-magnetron sputtering. The surface and mechanical properties of the Ti–35Nb–xTa alloys were analyzed by optical microscopy, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction spectroscopy, and nanoindenter. In addition, biocompatibility was analyzed by using surface roughness test, wettability test, SBF formation, and cell culture test. The results are as follows;

1. In the Ti-35Nb-xTa alloy, as the Ta content increased, the martensite structure decreased. The peaks of the α and α＂ phases, which are martensitic crystal structures, mainly decreased as the Ta content increased, whereas the β-phase peaks of the equiaxed structure increased. In addition, indentation hardness and elasticity modulus decreased with increasing Ta content.

2. As a result of the PEO treatment of Ti-35Nb-xTa alloy in different electrolytes, the overall surface was porous with irregular pores. In the case of PEO treatment in the both electrolytes, the porosity and pore size of the alloy surface increased, as the content of Ta increased, but the number of pores decreased. In addition, Nb2O5, Ta2O5, TiO2, anatase, and HA were observed on the surface of the alloy after the PEO treatment in both electrolytes. However, in the case of PEO treatment performed in an electrolyte containing Ca, P, and Sr ions, the XRD peak of HA shifted to the left side.

3. There was no significant difference in the surface morphology of the Sr-coated Ti-35Nb-xTa alloy through the sputtering process after the PEO treatment in the electrolyte containing Ca and P ions, but Sr elements were detected on the surface from EDS analysis. After the sputtering process using the Sr target, XRD peaks of Nb2O5, Ta2O5, TiO2, Sr, anatase, and HA appeared on the surface, and the peak of HA shifted to the left side after the sputtering process.

4. After all surface treatment, surface roughness of the Ti-35Nb-xTa alloy did not show a significant difference, but the surface roughness decreased as the Ta content increased, whereas the Ti-35Nb-15Ta alloy increased.

5. As a result of wettability test of etched and surface-treated Ti-35Nb-xTa alloy, the contact angle was increased in the case of the etched alloy, as the Ta content increased. Also, the contact angle was decreased as a whole compared to the etched alloy after surface treatment. And then, as the Ta content of the alloy increased, the contact angle was decreased.

6. Initially, the hydroxyapatite was nucleated on the whole surface of the oxide film including anatase and HA crystal phase, and grew mainly around the pores with coating time. In addition, when Sr elements were added on the alloy surface during the surface treatment, Sr element was also detected in the hydroxyapatite, and the growth rate of hydroxyapatite increased.

7. As the Ta content increased, cell adhesion and proliferation were well improved, and when Sr was contained in the oxide film through surface treatment, Sr element affected the proliferation of cells, and filopodia of MC3T3-E1 cells was observed around the small pores and in places with a short distance between pores.

In conclusion, the developed Ti-35Nb-xTa alloy is non-toxic and has a low elasticity modulus compared to the Ti alloy used an generally implant material. Especially, a large surface area can be obtained by forming a porous oxide film through the PEO process, and biocompatibility can be improved by doping in the oxide film with a bio-functional elements. Therefore, it is thought that the Ti-35Nb-xTa alloy surface-modified with Sr-HA coating will shorten the healing time through improvement of osseointegration between bone and implant after clinical surgery.