Papers by Author: Jai Hyuk Choi

Abstract: Various composition of calcium-silicate thinfilms were deposited to the thickness of 1μm by electron-beam deposition. The composition was varied by changing the composition of source. The Si concentration of thinfilm was ranged from 25 at% to 85 at%, evaluated by EDS. The specimens were immersed into PBS solution at 37 °C. Changes of the morphology and calcium phosphate formation on the specimens were examined by SEM. If the concentration of calcium is high, the calcium phosphate was quickly formed on the specimen. However, the calcium phosphate formed on the high calcium oxide concentration exhibited very poor adhesion onto the substrate. There is a range of appropriate calcium oxide concentration to use as a surface modification method for dental or orthopedic implants.

Abstract: Exposed dentine with patent tubules allows the movement of tubule fluid leading to dentine sensitivity. Occlusion of patent dentinal tubules effectively reduces the state of dentine hypersensitivity. Strontium chloride (SrCl2) has been well known as a component of desensitizing dentifrice. Recently, new dentifrice containing sodium metasilicate was on the market for desensitizing dentifrice. The aim of this in vitro study was to compare the occlusion effect of dentinal tubule between the sodium metasilicate and the existing desensitizing dentifrices. Sixtyfour human dentine specimens were embedded into a Teflon mold and were grinded with the use of silicone carbide papers to expose the surface of dentin. The dentin surface was etched with 37% phosphoric acid for 60 seconds and was treated with ultrasonic washing for 1 hour. The dentifrices used in the study are classified into four groups: G1-dentifrice containing Sodium metasilicate, G2- dentifrice containing Strontium chloride, G3-dentifrice containing Hydroxyapatite and G4-BSI reference dentifrice (control group). Toothpaste slurries were prepared as 20 gram of toothpaste in 80 ml of distilled water. Thirty-two specimens were brushed with the toothpaste slurries using V8 Cross Brushing Machine. Tooth-brushing was performed 5,000 times using a back-and–forth stroke. The pictures of the sizes of dentinal tubules were taken by scanning electron microscopy (×3000) and changes of tubule sizes were analyzed by digital analysis. The results showed that the difference of tubule size in descending order: G1-3.30±1.80, G2-2.82±1.73, G3-1.49±1.14, G4- 1.04±0.94. The dentifrice containing Sodium metasilicate (22.5%) showed the highest dentinal tubule occlusion. Moreover, the dentifrice containing Sodium metasilicate statistically significantly increased occlusion of dentinal tubule as compared to BSI reference dentifrice (p<0.05). Thus, new desensitizing dentifrice containing sodium metasilicate was more effective than existing desensitizing dentifrice on occlusion of dentin tubules.

Abstract: The surface morphology of commercially pure titanium was investigated with varying electrical waveform. Controlling of electrical pulse width is effective to change pore sizes. Longer electrical pulse width was induced, larger size of pores were generated.

Abstract: The surface properties of scaffolds are important since cell affinity is the most crucial factor to be concerned when the biodegradable polymeric material is utilized as a scaffold in tissue engineering. The surface of biodegradable non-porous poly (lactic-co-glycolic acid) (PLGA) scaffolds were treated by atmospheric pressure dielectric barrier discharge (APDBD). The wetting angle of APDBD treated PLGA were decreased from the untreated PLGA of 73° to 42°. FTIR-ATR analyses showed hydroxyl groups were not detected regardless of treated condition, but the intensities of both ether groups and carbonyl groups were increased with treatment time and oxygen flow rate. Treatment time and oxygen flux are equally effective to make the PLGA surface more hydrophilic.

Abstract: Feed drive system of machine tools may experience serious vibrations or unstable motions during high-speed operation. In this paper, a 6 D.O.F (Degree-Of-Freedom) lumped parameter model was proposed in order to analyze vibrations of a ball screw feed-drive system, which takes planar motion. The equations of motion were derived for the proposed model. Natural frequencies of the system and transient responses due to driving motor speed control input were also analyzed. Experimental modal analysis and operational vibration measurements of a laboratory ball screw feed drive system were made. Both theoretical analysis and experimental measurement results showed good agreement with each other. Consequently we concluded that the proposed 6 D.O.F model could be well applicable to the vibration analysis of a ball screw feed-drive system.