Jeong Koo Kim

An Improvement in Shock Absorbing Behavior of Polyurethane Foam with a Negative Poisson Effect

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.

Polyurethane Foam with a Negative Poisson's Ratio for Diabetic Shoes

This study fabricated polyurethane foam after transforming the cell structure from a convex polyhedral shape to a concave shape. Polyurethane was synthesized and fabricated after changing the cellular structure of the foam using two methods. Scanning electron microscopy showed that the cellular structure was a more concave structure than in control foam. The Poisson’s ratio of the experimental foam was negative. The average range of the Poisson’s ratio was –3.4~0, versus 0.3~1.3 for the control foam.

Tissue-engineered semi-microporous segmented polyetherurethane vascular prostheses.

We investigated the biocompatibility of semi-microporous segmented polyetherurethane (SPEU) that was coated with human microvascular endothelial cells. Three types of prosthesis were examined: (1) non-porous (control), (2) SPEU without a collagen coating (PNC) and (3) collagen-coated SPEU (PC). The attachment and proliferation of endothelial cells on SPEU (PNC and PC prostheses) was twice as great as in the control prostheses and endothelial cell adhesion in the PC prostheses was 1.4-times greater than in the PNC prostheses. The degree of endothelial cell coverage in the control prostheses was significantly lower than in the PNC and PC prostheses (P < 0.05), and there was no significant difference between the PNC and PC prostheses in the degree of endothelial cell coverage. As regards the retention of endothelial cells under simulated blood-flow conditions, the PNC prostheses exhibited a greater decrease in endothelial cell coverage, as compared to the PC prostheses (P < 0.05). Similarly, the PNC prostheses exhibited less platelet adhesion than the PC prostheses (P < 0.05). In summary, the semi-microporous SPEU offered better surfaces to anchor and culture for endothelial cells, and had superior anti-coagulant properties, as compared to nonporous SPEU. The coating of SPEU with collagen influenced both the attachment of endothelial cells and blood coagulation on the SPEU.

Enhanced neurite outgrowth of rat neural cortical cells on surface-modified films of poly(lactic-co-glycolic acid).

Neural cortical cells, isolated from prenatal rat cerebra, were grown on surface-modified poly(lactic-co-glycolic acid, 65:35) (PLGA) films coated with poly-D-lysine (PDL) with either laminin (LN), fibronectin (FN) or collagen (CN). Immunocytochemistry showed that the isolated cells were highly immunopositive for both neurofilament and MAP-2 with well-organized neurites and somatodendritic localization. The presence of PDL with LN or FN on the PLGA films was essential for increased neural cell growth. Also, PLGA films coated with either PDL/LN or PDL/FN mixtures had higher neurite outgrowth and regular differentiation.

Hydrophilicity of Surfactant-Added Poly(dimethylsiloxane) and Its Applications

This paper discusses the hydrophilicity of surfactant-added poly(dimethylsiloxane) (PDMS) and its applications. The contact angle of water droplets was measured in order to study the short- and long-term stability of the hydrophilicity of PDMS combined with various concentrations of the surfactant Silwet L-77 as a wetting agent. In the short-term, the contact angle was measured every 20 s for 10 min and decreased dramatically with time as the concentration of surfactant was increased from 0 to 1%. In water droplets observed for 800 h, the hydrophobic recovery of the surfactant-added PDMS was very weak and minor. As applications, a micropump and pneumatic actuators were fabricated using only surfactant-added PDMS. The basic properties of surfactant-added PDMS, such as the contact angle of the water head in the microchannel and Youngs modulus, were measured and discussed.

Cationic Nanocylinders Promote Angiogenic Activities of Endothelial Cells

Polymers have been used extensively taking forms as scaffolds, patterned surface and nanoparticle for regenerative medicine applications. Angiogenesis is an essential process for successful tissue regeneration, and endothelial cell–cell interaction plays a pivotal role in regulating their tight junction formation, a hallmark of angiogenesis. Though continuous progress has been made, strategies to promote angiogenesis still rely on small molecule delivery or nuanced scaffold fabrication. As such, the recent paradigm shift from top-down to bottom-up approaches in tissue engineering necessitates development of polymer-based modular engineering tools to control angiogenesis. Here, we developed cationic nanocylinders (NCs) as inducers of cell–cell interaction and investigated their effect on angiogenic activities of human umbilical vein endothelial cells (HUVECs) in vitro. Electrospun poly (l-lactic acid) (PLLA) fibers were aminolyzed to generate positively charged NCs. The aninolyzation time was changed to produce two different aspect ratios of NCs. When HUVECs were treated with NCs, the electrostatic interaction of cationic NCs with negatively charged plasma membranes promoted migration, permeability and tubulogenesis of HUVECs compared to no treatment. This effect was more profound when the higher aspect ratio NC was used. The results indicate these NCs can be used as a new tool for the bottom-up approach to promote angiogenesis.

Beneficial effects of freezing rate determined by indirect thermophysical calculation on cell viability in cryopreserved tissues.

Many types of mammalian cells, such as sperm, blood, embryos, etc., have been successfully cryopreserved for the last few decades, while no optimal method for the cryopreservation of mammalian tissues or organs has been established, showing a poor survival after thawing with a low recovery of function. In this study, the freezing rate was determined by indirect thermodynamic calculation, and its potential effect on the cryoprotection of human saphenous veins and tissue-engineered bones was investigated. The vein segments were frozen according to the calculated freezing rate, using rate-controlled freezing devices, with a freezing solution composed of 10% dimethylsulphoxide and 20% fetal bovine serum in RPMI 1640 media. The efficacy of indirect calculation was assessed by the cell viability measured using fluorescence double-staining methods. The results indicated that the freezing rate determined by indirect calculation significantly (P < 0.05) maintained the post-thaw cellular viability of the blood vessel, particularly in terms of the endothelial cells. However, it exerted relatively less protective effect on the osteoblastic cell-cultured scaffolds. These results suggest that freezing-induced injuries may occur in tissues, and the freezing rate determined by indirect thermophysical calculation can be used for the optimization of tissue cryopreservation by minimizing the injuries.

Effect of PLGA Nano-Fiber/Film Composite on HUVECs for Vascular Graft Scaffold

Potential application of the PLGA nanofiber/ film composite for vascular tissue regeneration was examined. The composite was prepared by solvent-casting and electro-spinning method. The PLGA nano-fiber was coated on the surface of the prepared PLGA film. The surface of the composite film was characterized by contact angle measurement and scanning electron microscope (SEM). The film made by only nano-fiber showed more hydrophobic than control (PLGA film made by solvent-casting); however, optimal amount of nano fibers coating could assist cell adhesion and spreading on the film. For human umbilical vein endothelial cells (HUVECs) proliferation on 6-day culture, the NF-30 (30min. nano-fiber spread specimen) composite was 1.5 times more cells proliferated than that of the NFO-1h (nano-fiber only specimen). The SEM images showed that the HUVECs on the NF-30 specimen spread widely than the other groups. The result indicates that the nano-fibers on films have an effect on cell adhesion and spreading because the adequate amount of nano-fibers on the surface of the films provide similar structure to natural ECM morphology. Conclusively, nanofiber/ film composite provided an adequate environment for cell proliferation and showed the potential application for vascular tissue regeneration.

Application of Hydrogen Peroxide Gas Plasma Method for Porous Polyurethane Sterilization

Tissue-engineering must be either manufactured aseptically or sterilized after processing. To extend protection of medical devices against microbial contamination, various sterilization methods have been suggested. Hydrogen peroxide gas plasma sterilization has been applied in hospitals worldwide for almost a decade. In this study, we investigated the sterilization efficacy of hydrogen peroxide gas plasma sterilizer with porous polyurethane sample. The result is suggested that hydrogen peroxide gas plasma can be applicable to the sterilization of polymer scaffold for tissue engineering materials.

MG-63 Cell Proliferation with Static or Dynamic Compressive Stimulation on an Auxetic PLGA Scaffold

The effect of dynamic compressive stimulation on MG-63 cell proliferation on an auxetic PLGA scaffold was investigated. The estimated Poisson ratio of the prepared auxetic scaffold specimens was approximately (−)0.07, while the Poisson ratio estimated for conventional scaffold specimens was (