주입성형에 의한 지르코니아 세라믹 임플란트의 제조 및 슬러리 코팅에 의한 표면 개질

Abstract

As the demands for long-term use increase in application of implants, the patients want new types of dental implants and artificial bones, which concurrently have high fracture toughness and surface roughness, superior aesthetics, excellent bioactivity and biocompatibility, and optimal bone bonding strength. Currently, most dental zirconia implants are fabricated by mechanical machining. However, mechanical machining produces many cracks on the surface of zirconia compacts, and it causes a decrease in mechanical properties and reliability of the sintered zirconia implants. In order to improve the mechanical and biological reliability of zirconia implant after implantation in body, it is necessary to enhance both of fracture toughness and bioactivity simultaneously. Fracture toughness and long-term stability of zirconia-based implants in body can be improved by the removal of surface cracks using a non-machining process during the shaping of implants. Also, the bioactivity of zirconia implant can improve by the surface modification and it induces to bioactive layer from bio-inert surface of zirconia, driving high bone bonding ability and superior cell adhesion. In this study, we firstly tried to fabricate a zirconia implant free of surface cracks by slip casting and post-sintering. Secondly, we made a purpose to improve the surface bioactivity of zirconia implant by surface modification via slurry coating of zirconia and hydroxyapatite powders, leading high surface roughness and outstanding bioactivity. To achieve the zirconia implant without surface cracks, we prepared zirconia green body by slip casting and investigated the effects of powder characteristics on the slip properties, green compacts and final sintered body. From the experiment, we established a processing condition for the preparation of optimal zirconia slip, that is, pH 12, 1 wt% dispersant, and 65 wt% solid loading. As starting materials, we used three types of commercial powder with a particle size of 27～36 nm. After slip casting, specimens were sintered for 2 hours at 1450 °C. All samples showed high sintered density of 6.05～6.08 g/cm3 (maximum 99.5%) and grain size of 350～700 nm, depending on the powder properties. Two kinds of zirconia powder are suitable for producing dental implant by slip casting and posting-sintering because final sintered implants have high density with small tetragonal grains and superior surface properties with high surface roughness and free of surface cracks. On the contrary, a remaining powder is inadequate for zirconia implant production by the formation of large cubic grains and low sintered density. Secondary, we modified zirconia substrate to enhance the surface roughness using a slurry coating of zirconia powder. Two types of zirconia slurry for coating were prepared by mixing of zirconia powder (large or small particle size) and distilled water with processing condition of pH 12 and 1 wt% dispersant. Coating layer on porous zirconia substrate was obtained by spin coating using an optimal slurry. To control the coating thickness and surface roughness, coating process was repeated to 8 times. After drying the coating layer, specimens were finally sintered at 1500 °C for 2 hours. All coated specimens show high sintered density and homogeneous coating layer. As the coating process was repeated, coating thickness gradually increased from 0.7 to 1.7 μm for slurry coating with small particles. In the case of slurry coating with large particles, thick coating layer was obtained, indicating from 0.9 to 2.0 μm according to coating times. Maximum surface roughness was obtained from 4 times coating specimen and indicated to the value of Ra 0.36 μm for the slurry coating of large particles and Ra 0.27 μm for small particles. Finally, the surface modification of zirconia substrate was performed by slurry coating using a hydroxyapatite powder, improving the surface bioactivity of zirconia substrate. Hydroxyapatite slurry for spin coating was prepared by mixing of commercial hydroxyapatite powder and distilled water. In the preparation of hydroxyapatite slurry, processing parameters, for examples, solid loading, pH range and dispersant content were controlled for the obtainment of optimal slurry. Slurry coating was performed on porous zirconia substrate by the same processing of zirconia slurry coating. Final hydroxyapatite coating layers on zirconia substrate after sintering at 1500°C for 2 hours, shows homogeneous microstructure and uniform layer thickness, depending on starting hydroxyapatite particle size. Surface roughness of hydroxyapatite-coated zirconia substrate was higher than that of uncoated substrate. Also, hydroxyapatite coatings fabricated from the slurry with small particles were more effective to enhance the surface roughness than those of large particles. Of the hydroxyapatite-coated layers, 4 times coated layer shows the maximum surface roughness of Ra 0.49 μm for the slurry coating of small particles and Ra 0.35 μm for large particles. From in vitro test in SBF solution, we confirmed that the bioactivity of hydroxyapatite-coated layer was enhanced by the improvement of surface roughness, the formation of low wetting angle and superior bioactivity of hydroxyapatite, compared to pure zirconia substrate.