지르코니아 세라믹 임플란트의 소결 물성 및 생체활성 향상

Abstract

In the past, titanium-based materials with excellent mechanical properties and easy processing have been widely used as implant materials. Recently, the improvement of mechanical and optical properties in the fields of ceramics-based materials have been successfully developed by an advanced processing technology. Zirconia ceramics are emerging as dental implant materials due to their superior optical and mechanical properties as well as excellent biophysical properties. Among them, 3Y-TZP(3 mol% yttria-stabilized tetragonal zirconia polycrystals) ceramics have excellent mechanical, biological and optical properties and are widely used as structural materials or biomaterials. In spite of high biocompatibility of zirconia implant, low bioactivity(bioinertness) is pointed out as a disadvantage for wide applications in dental and medical fields. In this study, three kinds of experiment were performed to improve the bioactivity and sintered properties of the zirconia implant. To enhance the bioactivity of bioinert zirconia substrate, two bioactive ceramic materials were coated on zirconia substrate by sol-gel/spin method. By the surface coating of two silicate bioceramic materials(wollastonite, diopside), it is expected that the bioactivity of zirconia substrate will be effectively improved. Finally, sintered properties and microstructural chacteristics of zirconia implants were analyzed for the optimization of processing parameters during ceramic injection molding and post-HIP(Hot isostatic pressing) processing. From the first experiment, wollastonite layers were coated on zirconia substrates by sol-gel method to improve the bioactivity of 3Y-TZP, a bioinert material. Wollastonite-coated surface had a porous microstructure with high surface roughness. Deposition area and layer thickness were proportionally increased with the number of repeated coating cycles. Dense and thin coating layer was obtained on the zirconia substrate by one or two coating cycles. On the contrary, porous and thick coating layers were formed on zirconia substrates from 3-4 coating cycles. Thickness of wollastonite-coated layer was increased from 1.7 μm to 12.7 μm with increasing the coating cycle. Also, surface roughness of wollastonite-coated layer was increased from 0.14 μm to 0.59 μm with coating cycles. From an in vitro test by the immersion of wollastonite-coated specimen into SBF solution for 14 days, surface dissolution and precipitation of hydroxyapatite particles on wollastonite-coated layer were observed after 3 days immersion. After 7 days immersion, all surfaces were covered with precipitated hydroxyapatite particles, indicating that wollastonite-coated layer through the sol-gel coating was significantly improved the bioactivity of zirconia substrate. From the second experiment, diopside ceramics, an excellent bioactive silicate ceramics, was coated on zirconia substrate by sol-gel/spin method. Also, bioactivity enhancement of zirconia substrate was evaluated through an in vitro test in SBF solution. With increasng the repeated coating cycle from 1 to 4 cycles, coating thickness of diopside layer on substrate was increased from 0.9 μm to 23.3 μm. From the results of in vitro test, diopside dissolution on coated surface was observed in the beginning of in vitro test, and thereafter, new hydroxyapatite particles were gradually precipitated on coated surface of zirconia substrate, indicating the improvement of the bioactivity of bioinert zirconia substrate, Diopside sol composition and coating thickness had great effects on the bioactivity of coating layer through the variation of microstructural parameter and chemical composition of coated layer. In last experiment, sintered properties and structural defects of commercial zirconia implants fabricated by injection molding were analyzed and evaluated to optimize the processing parameters. Sintered and microstructural parameters, for examples, sintered density, grain size and phase composition of zirconia implants fabricated by injection molding were dependent on the fixtute size and implant type. With increasing the fixture diameter in 2-body zirconia ceramic implant, sintered density was increased, but grain size was decreased. Maximum sintered density of 99.2 % and minimum grain size of 0.3-0.4 μm were obtained from large-scaled 2-body sample (diameter 7.5 mm). In the case 1-body ceramic implant, high sintered density of 99.5 % was obtained, but it had a little monoclinic phase and wide grain size distribution of 0.3-0.7 μm, influencing a factor of poor quality of implant.