Young-Sam Cho

Moving least-square method for the band-structure calculation of 2D photonic crystals

The moving least-square (MLS) basis is implemented for the real-space band-structure calculation of 2D photonic crystals. A value-periodic MLS shape function is thus proposed in order to represent the periodicity of crystal lattice. Through numerical examples, this MLS method is proved to be a promising scheme for predicting band gaps of photonic crystals.

Fabrication of dual-pore scaffolds using SLUP (salt leaching using powder) and WNM (wire-network molding) techniques.

In this study, a novel technique was proposed to fabricate dual-pore scaffolds combining both SLUP (salt leaching using powder) and WNM (wire-network molding) techniques. This technique has several advantages: solvent-free, no limit on the use of thermoplastic polymers as a raw material, and easiness of fabricating scaffolds with dual-scale pores that are interconnected randomized small pores. To fabricate dual-pore scaffolds, PCL and NaCl powders were mixed at a certain ratio. Subsequently, needles were inserted into a designed mold, and the mixture was filled into the mold thereafter. Subsequently, after the mold was pressurized, the mold was heated to melt the PCL powders. The PCL/NaCl structure and needles were separated from the mold. The structure was sonicated to leach-out the NaCl particles and was dried. Consequently, the remaining PCL structure became the dual-pore scaffold. To compare the characteristics of dual-pore scaffolds, control scaffolds, which are 3D plotter and SLUP scaffolds were fabricated.

Fabrication of 3D alginate scaffold with interconnected pores using wire-network molding technique

In this study, we fabricated 3D porous scaffold by ‘Wire-Network Molding’ technique with alginate gel which has been used for cartilage regeneration because of the chemical similarity. Firstly, prepared ETPCS-S wires with size of rectangular cross section 600 μm by 600 μm, 400 μm by 400 μm, respectively, and the wires are inserted in designed mold. Secondly, sterilized 2 wt% alginate gel within hMSC (human Mesenchymal stem cell) was injected into the assembled mold. The concentration of hMSC in the used alginate gel is about 5000 cells per scaffold. For the gelation of alginate gel, the mold was soaked in 5 wt% CaCl2 solution for 5 min. Subsequently, wires are separated from the mold and the mold is removed from alginate gel. Consequently, the remained alginate scaffold has interconnected pores with a configuration of wire-network. Additionally, to analyze the cell-culturing characteristics, 1-day, 3-day, and 7-day cultured scaffolds which encapsulate hMSC are assessed using MTS assay. Consequently, the optical density of 400 μm-WNM scaffolds and 600 μm-WNM scaffolds are clearly more increased than control scaffolds without pores.

Deformation-induced bandgap tuning of 2D silicon-based photonic crystals

We address the issue of tuning the absolute bandgap in 2D silicon-based photonic crystals by mechanical deformation. The moving least-square (MLS) method, recently proposed by the authors for photonic bandgap materials, is employed for the real-space computation of band structures. The uniaxial tension mode is shown to be more effective for bandgap tuning than both pure and simple shear deformations. We verify that bandgap modifications are strongly influenced by the deformation-induced distortion of interfaces between inclusions and matrix. This result ensures the usefulness of real-space technique for the accurate calculation of strained photonic bandgap materials.

Finite element analysis of quasistatic crack propagation in brittle media with voids or inclusions

A three-level finite element scheme is proposed for simulation of crack propagation in heterogeneous media including randomly distributed voids or inclusions. To reduce total degrees of freedom in the view of mesh gradation, the entire domain is categorized into three regions of different-level meshes: a region of coarse-level mesh, a region of intermediate-level mesh, and a region of fine-level mesh. The region of coarse-level mesh is chosen to be far from the crack to treat the material inhomogeneities in the sense of coarse-graining through homogenization, while the region near the crack is composed of the intermediate-level mesh to model the presence of inhomogeneities in detail. Furthermore, the region very near the crack tip is refined into the fine-level mesh to capture a steep gradient of elastic field due to the crack tip singularity. Variable-node finite elements are employed to satisfy the nodal connectivity and compatibility between the neighboring different-level meshes. Local remeshing is needed for readjustment of mesh near the crack tip in accordance with crack growth, and this is automatically made according to preset values of parameters determining the propagation step size of crack, and so the entire process is fully automatic. The effectiveness of the proposed scheme is demonstrated through several numerical examples. Meanwhile, the effect of voids and inclusions on the crack propagation is discussed in terms of T-stresses, with the aid of three-level adaptive scheme.

The fabrication of well-interconnected polycaprolactone/hydroxyapatite composite scaffolds, enhancing the exposure of hydroxyapatite using the wire-network molding technique.

In this study, the fabrication method was proposed for the well-interconnected polycaprolactone/hydroxyapatite composite scaffold with exposed hydroxyapatite using modified WNM technique. To characterize well-interconnected scaffolds in terms of hydroxyapatite exposure, several assessments were performed as follows: morphology, mechanical property, wettability, calcium ion release, and cell response assessments. The results of these assessments were compared with those of control scaffolds which were fabricated by precision extruding deposition (PED) apparatus. The control PED scaffolds have interconnected pores with nonexposed hydroxyapatite. Consequently, cell attachment of proposed WNM scaffold was improved by increased hydrophilicity and surface roughness of scaffold surface resulting from the exposure of hydroxyapatite particles and fabrication process using powders. Moreover, cell proliferation and differentiation of WNM scaffold were increased, because the exposure of hydroxyapatite particles may enhance cell adhesion and calcium ion release.

Fabrication of cylindrical PCL scaffolds using a knitting technique and assessment of cell proliferation in the scaffolds

In this study, a three-dimensional (3D) cylindrical polycaprolactone (PCL) scaffold was fabricated from PCL monofilaments using a knitting machine. First, two types of multi-filaments were fabricated from PCL monofilaments with diameters of 120 μm using a knitting machine. The approximate diameters of the fabricated multi-filaments were 400 μm and 660 μm, respectively. These two types of multi-filaments were used to fabricate 2D PCL sheets. Next, the fabricated 2D PCL sheet was rolled up to form a cylindrical structure with a diameter of 5 mm. Subsequently, the end of the sheet was sewed onto the intimate surface of the rolled up sheet. Finally, the cylindrical knitted PCL structure was cut into several pieces, resulting in cylindrical scaffolds with lengths and diameters of 5 mm. The porosities of the 400 μm multi-filament and 660 μm multi-filament scaffolds were measured as 76.6% and 77.2%, respectively. Moreover, the compressive moduli of 400 μm multi-filament and 660 μm multi-filament scaffolds were measured as 13.3 MPa and 11.2 MPa, respectively. Additionally, to analyze cell proliferation, scaffolds were seeded with 1×106/20 μl in media osteosarcoma cells and cultured for 1, 3, 7, and 14 days and were analyzed using a CCK-8 assay. The results were compared with bio-plotter scaffolds, which have a porosity of 77% and size of 4.43 by 4.43 by 5 mm3, which is approximately the same as the scaffolds fabricated here. The scaffolds fabricated in this study showed better cell proliferation than the bio-plotter scaffold.

Fabrication of Dual-pore Scaffolds Using a Combination of Wire-Networked Molding (WNM) and Non-solvent Induced Phase Separation (NIPS) Techniques

In this study, to fabricate dual-pore scaffolds with interconnected pores, Non-solvent Induced Phase Separation (NIPS) and Wire-Network Molding (WNM) techniques were combined. First, a mold with uniform slits was prepared, and needles were inserted into the mold. Subsequently, polycaprolactone (PCL) pellets were dissolved in tetrahydrofuran (THF) at a specified ratio. The slurry was mixed using hot plate stirrer at 1200 rpm for 24 hours at 40 °C. The PCL slurry was subsequently injected into the mold. Thereafter, to exchange the THF (solvent) with the ethanol (non-solvent), the mold was soaked in an ethanol bath. After removing the mold from the ethanol bath, the needles were removed from the mold. Consequently, dual-pore scaffold with interconnected pores was obtained. The surface morphology of the fabricated scaffolds were observed using Scanning Electron Microscope (SEM). Moreover, cell culture experiments were performed using the CCK-8 assay, and the characteristics of cells grown on the dual-pore scaffolds were assessed and were compared with the NIPS-based 3D plotting scaffold.

Analysis of degradation rate for dimensionless surface area of well-interconnected PCL scaffold via in-vitro accelerated degradation experiment

Until now, many researchers have explained the degradation rate of biodegradable scaffold with respect to several parameters such as porosity, pore size, and strand diameter. In this study, to analyze the degradation rate of Polycaprolactone (PCL) scaffold, accelerated degradation experiment using sodium hydroxide (NaOH) was used. For the experiment, PCL scaffolds were fabricated by bioplotter with respect to porosity, pore size, and strand diameter, respectively. Each fabricated scaffold was put into a vial filled with 5 mol-NaOH 5 mL and trapped-air was removed using vacuum desiccator. After that, all vials were placed in the waterbath which was maintained with 37°C For 24 days, seven vials were taken out from the waterbath for every 2 days and each scaffold was dried after rinsing with D.I. water. Afterwards, the degradation rate was analyzed for each type of PCL scaffolds using the measured mass. Among them, one type of scaffolds, which has the strand diameter of 300 μm and the pitch between strands of 800 μm, was used for the measurement of molecular weight change via gel permission chromatography (GPC). To show the each conventional parameter could not explain alone the degradation rate, the calculated degradation rates were analyzed with respect to porosity, pore size, and strand diameter, respectively. Afterwards, every degradation rate of all types of scaffolds was recorded with respect to the dimensionless surface area which is surface area/ S0. S0 is the surface area of sphere which has same volume of respective scaffold. Consequently, the dimensionless surface area was found to be a single parameter irrelevant to the type of PCL scaffold to explain the in-vitro degradation rate of accelerated NaOH experiment.

Numerical Analysis of the Adhesive Forces in Nano-Scale Structure

Nanohairs, which can be found on the epidermis of Tokay gecko’s toes, contribute to the adhesion by means of van der Waals force, capillary force, etc. This structure has inspired many researchers to fabricate the attachable nano-scale structures. However, the efficiency of artificial nano-scale structures is not reliable sufficiently. Moreover, the mechanical parameters related to the nano-hair attachment are not yet revealed qualitatively. The mechanical parameters which have influence on the ability of adhesive nano-hairs were investigated through numerical simulation in which only van der Waals force was considered. For the numerical analysis, finite element method was utilized and van der Waals force, assumed as 12-6 Lennard-Jones potential, was implemented as the body force term in the finite element formulation.