Papers by Author: Jae Bong Choi

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Authors: K.O. Park, Jae Bong Choi, Jong Chul Park, D.J. Park, Jeong Koo Kim
Abstract: Polyurethane foam was fabricated by ‘two-component method’ for changing cell structures. Compression force applied immediately to the polyurethane foam just after complete foam formation at the top of the mold for generation cell structure of negative Poisson effect. That is what we called pressure-controlled method. The polyurethane foam, produced by pressurecontrol method (CT), has significant higher resilience (52.3%) and similar level of shock absorption (47.5%) compared with control polyurethane foam (resilience is 21.5%, shock absorption is 54%). The PU foam with negative Poisson’s ratio showed excellent resilience with shock absorbance. The pressure-control method divided into two parts (CT0, CT1). The CT1 method is to apply compression force to the foam with time-delayed after foam formation. The PU foam produced by CT1 showed lower stress relaxation time, stress relaxation ratio, and maximum stress than CT0. Hence, CT1 foam is superior to other polyurethane foam as shock absorbing materials, such as shoes for diabetic patients.
Authors: Moon Kyu Lee, Jae Bong Choi, Kui Won Choi, H.N. Lim
Abstract: In the area of biomaterials, the structures with negative Poisson’s ratio are able to be applied to the polymer component of prosthesis, artificial blood-vessel and catheter. To induce its characteristic, previous studies postulated many structural shapes such as non-convex shape with reentrant corners and re-entrant honeycomb. In this study, we proposed the rotational particle structures and investigated the Poisson’s ratio and the ratio (Ee/E) of the elastic modulus of these structures based on structural design variables using finite element method. The auto-meshing preprocessor was coded using MATLAB in order to construct numerical models as design variables and perform finite element analysis (FEA) effectively. Three selected design variables were the ratio of fibril’s length to particle’s diameter (0.2~2.0), the ratio of fibril’s length to its width (0.02~0.2) and the angle of fibril about horizontal axis (0 degree ~ tangential angle). Finite element model has 2D plain stress quadratic element and composed of 515 particles and 6-linked fibrils per each particle. For all of 213 cases, one side of each model is applied a tension, 0.1% strain and analyze Poisson’s ratio and the ratio (Ee/E) of the elastic modulus. As the ratio of fibril’s length to particle’s diameter increased and the ratio of fibril’s diameter to fibril’s length decreased fixing the fibril’s angle with 45 degree, the negative Poisson effect of rotational particle structures increased. The ratio of elastic modulus of these structures decreased with Poisson’s ratio. The results show the reasonable values as compared with the previous analytical results.
Authors: Jung Bok Lee, Seong Mi Yu, Sang Gil Lee, Jae Bong Choi, Jeong Koo Kim
Abstract: PLGA (75:25)/hydroxyapatite (HA) composite films were fabricated by solvent-casting method to investigate the effect of various hydroxyapatite content ratio to the PLGA film for cellular attachment and proliferation. Mechanical property of the composite film was characterized by tensile test. The ultimate tensile strength of 10% HA content film was two folds higher than control group. The surface of the film was characterized by contact angle measurement. The PLGA/HA composite film was more hydrophilic than control film. In vitro chondrocyte responses to the composite films were measured by cellular attachment and proliferation test. The attached and proliferated cells were significantly higher on PLGA/HA (10%) composite film than control group (1.44 times higher in attachment test and 1.31 times higher for 6th-day at culture in proliferation assaying, p<0.05). Base on these finding, the PLGA/HA (10%) composite was effective for the cell attachment for the initial stage of cultivation and cell proliferation.
Authors: Jae Bong Choi
Abstract: The objective of this study was to quantify the zonal difference of the in situ chondron’s Poisson effect under different magnitudes of compression. Fluorescence immunolabeling for type VI collagen was used to identify the pericellular matrix (PCM) and chondron, and a series of fluorescent confocal images were recorded and reconstructed to form quantitative three-dimensional models. The zonal variations in the mechanical response of the chondron do not appear to be due to zonal differences in PCM properties, but rather seem to result from significant inhomogeneities in relative stiffnesses of the extracellular matrix (ECM) and PCM with depth.
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