Papers by Author: Kyung Sik Oh

Abstract: Adhesion characteristics of base-coated single crystal brackets for orthodontics were investigated to evaluate the feasibility for the ceramic bracket applications. Sapphire single crystal ingot were machined, abraded and spray-coated with slurries containing 50 wt% of flux and 40µm, 80µm and 150µm of granules. Maximum adhesion to the base surface of the brackets was observed when the size of granule in the slurries was 80µm. It may be due to higher bonding area of adhesive to granule and to base as a result of interlocking structure which has moderate thickness and uniform distribution of granules. The one-way analysis of variance indicated that there were not significant differences in bond strength measurements between the once and twice coating condition in 80µm and 150µm granule groups. But ARI values exhibited large differences between once and twice coating groups and showed different median or ranges. Adhesive remnant of adhesive resin was minimized when the slurry of 80µm granule was coated once. Reduction in adhesive remnant is likely due to the higher bonding of adhesive resin to granules. No teeth and bracket damages were found in any teeth de-bonded by shear force. The base-coated brackets with higher strength (12.9 MPa) and no adhesive remnant were newly proposed for the orthodontics applications.

Abstract: Magnetite (Fe3O4) particles were prepared for biomedical application through three different routes. Each route had the difference in the selection of pH controlling agents. Ammonia, Urea and combined use of ammonia and urea were tested in the synthesis through co-precipitation. With the combined use of urea and ammonia, the size of particles could be reduced as small as 10 nm while effectively suppressing the agglomeration among particles. The surface charge measured at physiological condition explained the good dispersion behavior of nano-magnetite particles. The superparamagnetism observed in nano magnetite particles was expected to be useful for biomedical application in the respect of magnetic localization and dispersion.

Abstract: To overcome the difficulty of preparing the stoichiometric single phase calcium phosphate materials, ultrasonic spray pyrolysis was used to fabricate various calcium phosphates. Effect of the initial Ca/P ratios and pyrolysis temperature were investigated in preparation of single phase materials. Also thermal stability of the obtained powders was estimated with the heat treatment in the air. Crystallinity of the powders was analyzed using XRD and FT-IR. The HAp and β-TCP powders with single phase could be obtained from their stoichiometric Ca/P ratios of 1.50 and 1.67, respectively. These single phase powders with good crystallinity and stoichiometry were stable with respect to heat treatment up to 1000oC. The particle size of these powders would be controlled by the concentration of starting solution.

Abstract: To improve the potential of osteogenic repair, we developed macroporous biphasic hydroxyapatite-tricalcium phosphate (HA-TCP) ceramic and evaluated its efficiency as a scaffold for tissue engineered bone regeneration, which allows for appropriate cellular attachment and proliferation with osteogenic differentiation by evaluating ectopic bone formation ability after the implantation of cell-matrix construct in the skid mice subcutaneous pouches for 3 weeks. The macroporous biphasic HA-TCP ceramic matrix, with an average porosity of 86% and 200 µm mean pore size, provided favorable conditions for the attachment of cultured bone marrow derived osteoblastic cells along its inner surfaces in a filed up pattern and the active proliferation of them. The implanted cell-matrix constructs in the subcutaneous pouches induced favorable ectopic bone formation without any remarkable inflammatory reactions. These findings suggest that the biphasic HA-TCP ceramic matrix with macroporous structure has excellent biocompatibility, and that it allows for favorable cellular attachment with the acceleration of cellular proliferation and osteogenic differentiation support as well. Thus, with the controlled biodegradability, the biphasic HA-TCP ceramic may be a promising scaffold for tissue engineered bone regeneration technology.

Abstract: Effect of the starting compostion was studied in bone cement containg coarse b-tricalcium phosphate (b-TCP) granules which was very dense and round. With respect to the mixing ratio between b-tricalcium phosphate and monocalcium monophophate (T:M), the properties such as setting time, density and compressive strength were measured. The properties of bone cement prepared from normal powdery b-TCP was strongly dependent on the initial mixing ratio (T:M). Though the compressive strength as well as density was maximum at T:M = 6:4, small departure of compostions from T:M=6:4 resulted in drastic decrease of compresive strength. On the contrary, in the specimens from granular b-TCP, compressive strength was much less deependent on the initial mixing ratio. Range of optimum compressive strength covered from T:M=6:4 to 8:2. Therefore, granular groups provided more degree of freedom to control other properties such as setting time while maintaing its compressive strength.

Abstract: b-Ca3(PO4)2 (TCP) based cement combined with polyphosphate was investigated. In the bone cement composed of b-TCP, monocalcium monophosphate (MCPM) and calcium sulfate hemihydrate (CSH), effect of the amount of setting agent on workability, setting time, temperature rise and compressive strength was evaluated. The polyphosphate, which was selected as a growth enhancer, could be introduced without affecting the properties of cement. The polyphosphate containing cement was introduced at the calvarial defect in a rabbit. After three months, the defect was covered with newly formed bone, in contrast with the case of polyphosphate free bone cement. These results suggest that the bone cement containing inorganic polyphosphate can be used as effective bone filler with considerable potential of bone regeneration for bony defects.

Abstract: Ag doped Hydroxyapatites (Ag-HAp) was prepared through either ion exchange or coprecipitation to compare the durability of antimicrobial effect. In case of ion exchanged Ag-HAp, the microbials reproliferated after 100 h, in spite of the rapid reduction of E.Coli during the initial moment. On the contrary, coprecipitated Ag-HAp effectively suppressed the reproliferation until 1000 h. The difference in durability depending on synthesis route was analysed with respect to the amount of silver released at each interval. In case of ion exchanged Ag-HAp, more than 60% of overall silver was released during initial 10 minitues. On the contrary, coprecipitated Ag-HAp released less than 40% of overall silver during the same period, meaning the comparatively uniform release behavior.