Papers by Author: Dae Joon Kim

Abstract: The initial osteoblast like cell response to bioactive nano-sized hydroxyapatite (HAp) and bioinert zirconia was evaluated with the cell morphology by SEM and cell adhesion proteins by fluorescence microscopy. Surface roughness also measured by a confocal laser microscopy. The surface roughness and topography was almost identical among specimens. The nano-sized HAp specimens showed better initial cell adhesion and activity than bioinert zirconia ceramics.

Abstract: Two-step sintering was performed to prepare hydroxyapatite (HAp) with a high density and a fine microstructure utilizing hydrothermally synthesized nano powders. Its effectiveness was dependent on starting chemicals for precursors of HAp, powder processing condition, initial temperature in the two-step sintering, heating rate to the initial temperature, and holding time at final temperature. HAp powder, hydrothermally treated after aging of precipitate of Ca(OH)2 and H3PO4 for 48 h, led to HAp having a high phase stability at elevated temperatures, a high density, and a fine microstructure after two-step sintering with heating rate of 10°C/min to 1275°C.

Abstract: The sliding wear of four different compositions of novel low temperature degradation-free zirconia/alumina (LTD-free Z/A) composites were characterized in a ceramicceramic point contact pair. The wear tests were performed by a pin-on-disk type wear tester in a linear reciprocal sliding motion with a point contact in both dry and bovine serum lubricated conditions at room temperature. For the dry sliding wear tests, AZ-2 (20 vol% (Y,Nb,Ce)-TZP/ 80 vol% Al2O3) showed the best wear resistance among four kinds of LTD-free Z/A composites. For the bovine serum lubricated sliding wear tests, wear was too little to be measured for all kinds of Z/A composites. These novel LTD-free Z/A composites having excellent wear resistance demonstrated a potential as the alternative materials for the ceramic- ceramic contact pairs of femoral head and acetabular liner in total hip replacement.

Abstract: Nanosized hydroxyapatite(HAp) was synthesized hydrothermally and influence of processing parameters, such as reaction temperature and time, pH, and Ca/P ratio, on the phase stability of HAp was evaluated after pressureless sintering at 1250°C in air. The highest stability was achieved from the powder obtained at a processing condition for the lowest solubility of Ca2+, which corresponds to temperature close to 200°C, time <4h, pH>9.0, and Ca/P = 1.67. The stability of nanosized HAp in HAp/3Y-TZP composites was slightly inferior to commercial one, but the involvement of hydrothermally synthesized HAp improved sintered density of the composites and stability of t-ZrO2 phase in the composites.

Abstract: To combine the bioinert titanium with the biological active coating of hydroxyapatite [HA, Ca10(PO4)6(OH)2] a new pulsed laser deposition(PLD) method was applied. The initial osteoblast-like MC3T3-E1 cell response to this thin HA coated(<1μm) surface was compared with non-coated surfaces. Cell responses on four different pure titanium surfaces, such as smooth titanium, HA coated smooth titanium, rough titanium and HA coated rough titanium, were compared. Cell morphology and attachment were examined by scanning electron microscopy after 6, 24 and 72 hrs of culture, cell proliferation by MTS assay after 3, 7 and 14 days. The differentiation of the cells was analyzed by real time PCR up to 21 days. The cells were well spread and attached on all surfaces and showed more irregular shapes on the HA coated specimens. They proliferated continuously on all specimens up to 14 days. Smooth surface specimens showed better cell proliferation than rough ones after 7 days, and the HA coated specimens demonstrated slightly greater MTS activity than the non-coated groups after 14 days. The surface topography affected cell proliferation initially however; there was no significant difference afterwards. Thin HA coating influenced the expression of some genes related with cell differentiation.

Abstract: A hydrothermal method has been used to produce nanoparticles of 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP) with high fracture toughness. The effects of reaction temperature and pH on crystallite size and sintered density, and the influence of Nb2O5 addition into 3Y-TZP on toughness have been investigated. The particle size increased with increasing the processing parameters and the influence of temperature was more significant than pH. The density of nanosized 3Y-TZP increased with increasing pH up to 9 and then decreased abruptly with further increase. The density increased slightly with the reaction temperature up to 140°C and then changed little with increasing temperature. The toughness increased in proportional to the Nb2O5 content. After sintering for 1 h at 1270°C, the highest relative density of 98% was obtained from nanoparticles of about 17 nm, prepared at 200°C and pH 9, and the toughness of 3Y-TZP was increased from 2.5 to 8.3 MPam1/2 by addition of 1.2 mol% Nb2O5.

Abstract: This study was designed to evaluate mechanical properties and the marginal fitness of glass infiltrated alumina core fabricated from aqueous alumina tape. Utilizing the automatic Dr. Blade model (DP-150 of Japan JinChungjungGi manufacturer) fabricated the slurry into 0.5 ㎜ thickness aqueous-based alumina tape. The coefficient of thermal expansion and biaxial flexural strength of alumina composite produced from alumina tape were investigated. Three upper central resin incisors were prepared with 90o, 110o, 135o shoulder margin for all-ceramic crown. Individual tooth model’s impressions were taken 15times each to make epoxy resin die. Cores were made of alumina tape to each and every dies. Crown setting was done on the epoxy resin dies. The specimens were evaluated for measuring for marginal gaps of glass infiltrated alumina core fabricated from aqueous alumina tapes under 180 magnifications with Kan Scope(Sometech Vision, Korea) was done. The retained measurements were analyzed with Turkey’s Studentized Range Test for marginal fitness of each specimen. Coefficient of thermal expansion of alumina tape was 7.5x10-6/°C, and biaxial flexural strength was observed to be 498±32MPa. The marginal fits of alumina cores made of alumina tapes showed the least marginal gap of 41.5 ㎛ in the 110o shoulder margin, and increasingly with 135o, 90o shoulder margin. Marginal fitness should be better in angles larger than 90o.

Abstract: The effect of hydrothermal reaction temperature and pH on crystallite size, phase stability, and sintered density of hydroxyapatite (HAp) has been investigated. The crystallite size increased with the increase in temperature but was not significantly affected by pH. The decomposition of HAp into tricalcium phosphate was observed at 1250°C and the extent of decomposition decreased with increase in pH. The influence of temperature and pH on the crystallite size and the decomposition was related to the decrease in solubility of Ca2+. The phase stability of HAp, prepared hydrothermally, in HAp/zirconia composites was higher than that of commercial HAp due to its high sintered density.

Abstract: The sliding wear behaviors of three different compositions of novel low temperature degradation-free zirconia/alumina (LTD-free Z/A) composites were examined in a ceramicceramic contact pair. The wear tests were performed by using a pin-on-disk type wear tester in a linear reciprocal sliding motion with a line contact in both dry and bovine serum lubricated conditions at room temperature. From the results of dry sliding wear tests, Z/A#1 ((5.3Y,4.6Nb)-TZP/80 vol% Al2O3) showed the best wear resistance among three kinds of LTD-free Z/A composites. For the bovine serum lubricated sliding wear tests, wear was too little to be measured for all kinds of Z/A composites. These novel LTD-free Z/A composites having excellent wear resistance demonstrated a potential as the alternative materials for the ceramicceramic contact pairs of femoral head and acetabular liner in total hip replacement.

Abstract: The osseointegration rate of a new zirconia/alumina composite ceramic implant material, (Y,Nb)-TZP/ alumina, was compared with conventional titanium implant in the rabbit model. Four different groups of threaded implant were compared after 4 and 6weeks healing period. The bone implant contact ratio and bone volume was evaluated by undecalcified histologic sections. The surface of each implant group was observed under SEM. Within the limitations of this study, Zr/Al composite ceramic implants demonstrated favorable bone response compared to titanium implant. This material demonstrated a possibility of an alternative to titanium dental implant. Further studies are needed to confirm this result.