생체활성 물질의 코팅에 의한 임플란트용 지르코니아 기판의 표면 개질

Abstract

Zirconia ceramics have excellent mechanical properties such as corrosion resistance, abrasion resistance, and chemical stability, so they have used for hip implants, bearings, and grinding balls. In addition, it has excellent light transmittance, biocompatability, and a similar color with a tooth, so it used as a dental implant. In the past, titanium-based materials were mainly used as medical implants because they had excellent mechanical properties and easy machining ability, despites of their low bioactivity. Among zirconia-based ceramics, tetragonal stabilized zirconia containing 3 mol% of yttria (3 mol% yttria-stabilized tetragonal zirconia polycrystals, hereinafter referred to as 3Y-TZP) is used as the main material for dental ceramic implant. Due to its aesthetics, it is emerging as an alternative material for titanium implant. 3Y-TZP is an attractive biomaterial because of its high biocompatibility and relatively high fracture strength. In particular, after the introduction of the CAD/CAM in the manufacture of ceramic implant, it has been mainly used for prosthetic restorations where aesthetics and strength are required. Also, 3Y-TZP has been studied as a ceramic restoration and osseointegration materials. In the case of simply machined 3Y-TZP implant product, its osseointegration reaction in body is similar to that of titanium, but when surface modification is performed, osseointegration performance of 3Y-TZP is superior to titanium implant. 3Y-TZP implants without metallic color are bioinert and biocompatible, and do not interfere with the growth of osteoblasts, indicating better osseointegration relatively. In this study, to improve the surface roughness of zirconia implants, we modified the 3Y-TZP surface with bioactive materials using sol-gel and room temperature spray coating methods, for examples, hydroxyapatite, wollastonite, and biocomposite of hydroxyapatite and forsterite. Firstly, hydroxyapatite as a bioactive material was deposited on 3Y-TZP substrate by sol-gel method in order to improve the surface bioactivity and roughness of zirconia substrate. In this experiment, we investigated the effect of microstructural and biological changes on the coating condition through the enhancement of surface roughness and bioactivity of zirconia substrate. Secondly, we studied the wollastonite surface modification on 3Y-TZP substrate by room temperature spraying process. After the pre-treatment of commercial wollastonite powder by calcination, we fabricated the coating surface on substrate with various thickness by the control of deposition cycle. We also finally investigated the biological properties of modified 3Y-TZP substrate through an in vitro experiment. Finally, we modified 3Y-TZP surface using the biocomposite powder of wollastonite and forsterite by room temperature spray coating and investigated the mixed effect of two typed powders on the evolution of surface microstructure, i.e, coating thickness and roughness, and biological interaction during the in vitro test in SBF solution. comparing the bioactivity improvement through the observation of dissolution and re-precipitation on specimen surface. From the above experimental study, we arrived at meaningful conclusions as follows; In the first experiment, the surface microstructure and deposition thickness of coated HA layers are dependent on the coating cycles. With increasing the coating cycles, HA coating thickness also increased in proportion to coating cycle, but the surface roughness of coating layer gradually decreased due to the dense arrangement of HA particles. The maximum surface roughness (Ra; 0.25 μm) of HA coating layer was obtained at minimum coating thickness among the samples. From in vitro test in SBF solution, slight dissolution of HA coatings was observed, reduced the surface roughness of coating layer. In general, HA coatings contributed to roughness improvement of 3Y-TZP surface, and conclusively, the bioactivity of 3Y-TZP substrate. Secondly, bioactive wollastonite coating layers were uniformly deposited on 3Y-TZP substrates with various surface microstructure and coating thickness, depending on the deposition cycle or the type of starting wollastonite powder. A net pattern on coating layer of 3Y-TZP substrate was evolved as the wollastonite coating layer thickened. Also, surface roughness of coated wollastonite layer was increased with coating thickness. Maximum surface roughness (Ra = 0.45 µm) was obtained at the coating layer fabricated by 40 deposition cycles using wollastonite powder calcined at 900 °C. From in vitro test in SBF solution at pH 7.4, we observed severe dissolution at initial stage of immersion on wollastonite coatings. In addition, new precipitated hydroxyapatite particles were covered on wollastonite coating surface after 5 days of immersion. Conclusively, the wollastonite coating on zirconia substrate by the room temperature spraying process could substantially improve the bioactivity of 3Y-TZP substrate. Finally, we confirmed the bioactivity improvement of 3Y-TZP substrate by the surface modification with hydroxyapatite+forsterite composite powder using a room temperature spray coating, From the comparative investigation of surface microstructure with pure hydroxyapatite coating, thin coating layer was fabricated by a room temperature spray coating. Hydroxyapatite coating had maximum thickness of 26 µm for 40 deposition cycles, but, thin coating of 10.1 µm was obtained from composite powder at same processing condition. From the result of in vitro test, dissolution by SBF solution, and then new precipitation of hydroxyapatite were faster at hydroxyapatitre coating layer than that of composite powder.