Improved biological performance of Ti implants due to surface modification by micro-arc oxidation
Introduction
Pure titanium (Ti) and titanium alloys are frequently used as dental and orthopedic implant materials because of their excellent mechanical strength, chemical stability, and biocompatibility [1]. The biocompatibility of titanium is closely related to the properties of the surface oxide layer, in terms of its structure, morphology and composition. Various physical and chemical treatments of the Ti surface have been proposed with a view to obtaining the most biocompatible implant surface. Among the techniques, which have been found to be beneficial to the biological performance of the implants, are increasing the surface roughness, the oxidation of Ti to form a TiO2 layer on the surface. The incorporation of Ca or P ions into the surface layer, and the validity of these results has been confirmed by several different researchers [2], [3], [4], [5]. The most widely used commercial techniques are sandblasting, acid-etching, and the plasma spraying of hydroxyapatite [6], [7].
Recently, an electrochemical procedure for modifying the Ti surface was proposed, which has since attracted much attention. By applying a positive voltage to a Ti specimen immersed in an electrolyte, anodic oxidation (or anodizing) of Ti occurs to form a TiO2 layer on the surface. When the applied voltage is increased to a certain point, a micro-arc occurs as a result of the dielectric breakdown of the TiO2 layer. At the moment that the dielectric breakdown occurs, Ti ions in the implant and OH ions in the electrolyte move in opposite directions very quickly to form TiO2 again. This process is generally referred to as micro-arc oxidation (MAO) or plasma electrolysis [8]. The newly formed TiO2 layer is both porous and firmly adhered to the substrate, which is beneficial for the biological performance of the implants. Another advantage of this MAO process is the possibility of incorporating Ca and P ions into the surface layer, by controlling the composition and concentration of the electrolyte [9], [10]. The incorporated Ca and P ions were even crystallized into hydroxyapatite or other calcium phosphates by a hydrothermal treatment [10], [11]. Recent studies on the biological response of Ti implants demonstrated that the MAO process constitutes one of the best methods of modifying the implant surface [12], [13], [14], [15], [16], [17]. However, further research is necessary for the complete characterization of the oxide layer and also for the identification of the optimum conditions for the MAO process.
In this study, we formed TiO2 layers with different thicknesses and roughnesses on the Ti surface, by controlling the applied voltage used in the MAO process. The phase, composition and morphology of the oxide layer were monitored with respect to the applied voltage. The biological properties of the layers were evaluated by in vitro tests, in terms of the proliferation and differentiation of certain cell lines. Preliminary in vivo tests were also carried out to confirm the results obtained during the in vitro tests.
Section snippets
Micro-arc oxidation (MAO)
Commercially available pure Ti (CP-Ti, Grade 2, Ka-Hee Metal Industry Co., Seoul, Korea), machined into disks with dimensions of 12 mm (diameter)×1 mm (thickness), was used as the substrate. These disks were ground using 400-grit SiC sandpaper and cleaned ultrasonically in acetone, ethanol and de-ionized water. MAO of the specimen was carried out in an aqueous electrolyte, by applying a pulsed DC field to the specimen. The frequency and duty of the pulsed DC power were 660 Hz and 10%,
Morphology of oxide layer
The surface morphologies of the Ti after MAO using different treatment conditions are shown in Figs. 1(A)–(F). Before the oxidation treatment, only the machining grooves were observed on the surface. When a pulsed DC field of 190 V was applied, a porous oxide layer began to be formed, as shown in Fig. 1(A). When the field was lower than 190 V, this layer was very thin and uniform without any porosity. As the voltage increased to 230 V, the whole layer became porous as shown in Fig. 1(B), but was
Discussion
The purpose of this study was to improve the biocompatibility of Ti implants by modifying the composition and morphology of the implant surface. MAO is a simple, controllable, and cost-effective method of forming a porous TiO2 layer on the implant surface. Moreover, MAO provides an ideal method of producing an oxide layer on the surface of implants which have a complex shape. The properties of the oxide layer, such as its thickness, microstructure, roughness, and concentrations of Ca and P are
Conclusion
The MAO treatment of Ti, by bringing about positive physical and chemical changes to the Ti surface, had a beneficial effect on the biocompatibility of the Ti implant. Increasing the MAO voltage increased the thickness and roughness of the oxide layer, as well as the concentrations of Ca and P ions in the oxide layer. As a result of these changes, the ALP activity of the cells increased, while the cell proliferation rate decreased. The in vivo tests showed a considerable increase in removal
Acknowledgments
This work was supported by a grant from the Korea Health 21 R&D Project, Ministry of Health and Welfare, Republic of Korea (02-PJ3-PG6-EV11-0002).
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