Yong Doo Park

In vitro response of primary human bone marrow stromal cells to recombinant human bone morphogenic protein-2 in the early and late stages of osteoblast differentiation.

A number of factors must be added to human bone marrow stromal cells (hBMSCs) in vitro to induce osteogenesis, including ascorbic acid (AA), β‐glycerophosphate (GP), and dexamethasone (Dex). Bone morphogenic protein (BMP)‐2 is an osteoinductive factor that can commit stromal cells to differentiate into osteoblasts. However, it is still not clear whether the addition of BMP‐2 alone in vitro can induce hBMSCs to complete osteoblast differentiation, resulting in matrix mineralization. This study compares the effects of BMP‐2 and Dex, alone and combined, on the early and late stages of hBMSC differentiation. We found that BMP‐2 causes a significant induction of alkaline phosphatase (ALP) activity in hBMSCs, with a transcriptional upregulation of known BMP‐2‐responsive genes, and the stable expression of cbfa1 in the nucleus and the regions surrounding the nucleus in the early phase of osteoblast differentiation. However, continuous treatment with BMP‐2 alone at doses ranging from 100 to 300 ng/mL results in a less efficient enhancement of in vitro matrix mineralization, despite a significant induction of ALP activity at a concentration of 100 ng/mL. Our results reflect how the effects of BMP‐2 on hBMSCs can vary depending on the stage of osteoblast differentiation, and highlight the need to understand the role of BMP‐2 in primary hBMSCs derived from diverse sources in order to increase the efficiency of using BMP‐2 in osteoinductive therapies.

Sodium Alginate Hydrogel‐Based Bioprinting Using a Novel Multinozzle Bioprinting System

Bioprinting is a technology for constructing bioartificial tissue or organs of complex three-dimensional (3-D) structure with high-precision spatial shape forming ability in larger scale than conventional tissue engineering methods and simultaneous multiple components composition ability. It utilizes computer-controlled 3-D printer mechanism or solid free-form fabrication technologies. In this study, sodium alginate hydrogel that can be utilized for large-dimension tissue fabrication with its fast gelation property was studied regarding material-specific printing technique and printing parameters using a multinozzle bioprinting system developed by the authors. A sodium alginate solution was prepared with a concentration of 1% (wt/vol), and 1% CaCl(2) solution was used as cross-linker for the gelation. The two materials were loaded in each of two nozzles in the multinozzle bioprinting system that has a total of four nozzles of which the injection speed can be independently controlled. A 3-D alginate structure was fabricated through layer-by-layer printing. Each layer was formed through two phases of printing, the first phase with the sodium alginate solution and the second phase with the calcium chloride solution, in identical printing pattern and speed condition. The target patterns were lattice shaped with 2-mm spacing and two different line widths. The nozzle moving speed was 6.67 mm/s, and the injection head speed was 10 µm/s. For the two different line widths, two injection needles with inner diameters of 260 and 410 µm were used. The number of layers accumulated was five in this experiment. By varying the nozzle moving speed and the injection speed, various pattern widths could be achieved. The feasibility of sodium alginate hydrogel free-form formation by alternate printing of alginate solution and sodium chloride solution was confirmed in the developed multinozzle bioprinting system.

A Three-Dimensional Bioprinting System for Use With a Hydrogel-Based Biomaterial and Printing Parameter Characterization

Bioprinting is an emerging technology for constructing tissue or bioartificial organs with complex three-dimensional (3D) structures. It provides high-precision spatial shape forming ability on a larger scale than conventional tissue engineering methods, and simultaneous multiple components composition ability. Bioprinting utilizes a computer-controlled 3D printer mechanism for 3D biological structure construction. To implement minimal pattern width in a hydrogel-based bioprinting system, a study on printing characteristics was performed by varying printer control parameters. The experimental results showed that printing pattern width depends on associated printer control parameters such as printing flow rate, nozzle diameter, and nozzle velocity. The system under development showed acceptable feasibility of potential use for accurate printing pattern implementation in tissue engineering applications and is another example of novel techniques for regenerative medicine based on computer-aided biofabrication system.

In vitro evaluation of the performance of Korean pulsatile ECLS (T-PLS) using precise quantification of pressure-flow waveforms

The Twin-Pulse Life Support System (T-PLS) is a novel pulsatile extracorporeal life support system developed in Korea. It has been reported that the T-PLS achieves higher levels of tissue perfusion of the kidney during short-term extracorporeal circulation and provides more blood flow to coronary artery than nonpulsatile blood pumps. However, these results lack pulsatility quantifications and thus make it hard to analyze the effects of pulsatility upon hemodynamic performance. We have adopted the concepts of hemodynamic energy, energy equivalent pressure (EEP), and surplus hemodynamic energy (SHE) to evaluate pulsatility performance in the different circuit configurations of the T-PLS and a membrane oxygenator (MO) in vitro. In a mock system, three different circuits were constructed depending on the location of an MO: pump-MO-pump (serial), MO-pumps (parallel A), and pumps-MO (parallel B). In parallel A, a low-resistance MO was used to preserve the pulsatility from the pump. All circuits showed good pulsatility in terms of EEP (serial: 13.2% ± 3.2%, parallel A: 10.0% ± 1.6%, parallel B: 7.00% ± 1.1%; change from aortic pressure to EEP; p < 0.003). The SHE levels were 17,404 ± 3,750 ergs/cm3, 13,170 ± 1,486 ergs/cm3, and 9,192 ± 1,122 ergs/cm3 in each circuit setup (p < 0.001). Although EEP levels were somewhat lower, both parallel types provided higher pump output compared with the serial type (serial: 1.87 ± 0.29 l/min, parallel A: 3.09 ± 0.74 l/min, parallel B: 3.06 ± 0.56 l/min; p < 0.003 except parallel A vs. parallel B, p = 0.90). Conclusively, the precise quantifications of pressure flow waveforms, EEP, and SHE are valuable tools for evaluating pulsatility of the mechanical circulatory devices, and are expected to be used as additional performance indexes of a blood pump. The pulsatility performances are different according to circuit setups. However, the parallel A circuit could achieve higher pump output and generate adequate pulsatility level. Thus, the parallel A circuit is suggested as the optimal configuration in T-PLS applications.

Heparinized Micropatterned Surfaces for the Spatial Control of Human Mesenchymal Stem Cells

In this study, a heparinized micropattern surface was prepared for the spatial control of human mesenchymal stem cells (hMSCs) that can differentiate into the desired tissues. Poly(styrene-co-vinylbenzyl N,N-diethyl-dithiocarbamate) (poly(ST-co-VBDC)) was synthesized as a photoreactive polymer; poly(ethylene glycol) methacrylate (PEGMA) was polymerized on the poly(ST-co-VBDC) coated surface by UV irradiation. XPS spectra revealed the residual DC moieties on the PEGMA-grafted surface and the linear chain growth of PEGMA was monitored according to irradiation time. After chemical immobilization of heparin onto this PEGMA surface, surface micropatterning was carried out by additional photopolymerization of PEGMA using a photomask. After incubation for 4 hour, the hMSCs adhered to the heparinized surface, while the hydrophilic PEGMA surface demonstrated no cell adhesion even after basic fibroblast growth factor (bFGF) treatment. Good alignment of hMSCs on the pattern-surface was distinctly observed along micron-sized grooves due to the presence of both heparin and bFGF. This heparinized micropattern surface can be used to study in vitro hMSCs responses with various heparin-binding growth factors in tissue engineering fields as well as cellular array for the spatial control of hMSCs.

A Durability Study of a Paracorporeal Pulsatile Electro‐Mechanical Pneumatic Biventricular Assist Device

In 2002, the paracorporeal pulsatile electro-mechanical pneumatic ventricular assist device (VAD) began to be developed by the Korea Artificial Organ Center at Korea University under a Health & Medical Technology Research and Development program which finished in 2008. In vitro durability testing was conducted on the paracorporeal pulsatile pneumatic VAD to determine device durability and to evaluate device failures. The 1- and 2-year reliability of the paracorporeal pulsatile pneumatic VAD was shown to be 91.2% and 54.9%, respectively, with an 80% confidence level. Failure modes were analyzed using fault tree analysis, with customized software continuously acquiring data during the test period. After this period, 21 in vivo animal tests were done, with 14 cases of left atrium to left ventricle (LV) inflow cannulation (36Fr)/outflow grafting to descending aorta, and seven cases of apex cannulation of LV to descending aorta (12 mm). The longest postoperative day (182 days) in Korea was recently recorded in in vivo animal testing (bovine, 90 kg, male, 3.5-4.0 L/min flow rate, and 55 bpm).

Evaluation of acid-treated hyaluronic acid-based hydrogelation

Though hyaluronic acid (HA)-based hydrogel has drawn great attention in biomedical society, it’s long molecular weights sometimes have been problematic due to its difficulty in handling. After reduction of its high molecular weight into smaller sizes with various concentrations of hydrogen chloride solutions, its chemical and biological properties have been examined by changes in viscosity, FTIR spectroscopy and gel permeation chromatography as well as cellular interactions. While FTIR analysis indicated maintenance of its original chemical structures, its viscosity has been remarkably reduced and its extent was dependent upon the employment of acid concentrations. After controlling its molecular weight to approximately 100 kDa and coupling of aminopropymethacrylate to the treated HA, we evaluated in vitro cellular interactions and cell proliferations of the HA-poly(ethylene oxide) (PEO) hydrogel.

Effect of Hydroxyapatite-Tricalcium Phosphate Micro-Particles on the Cellular Interaction of In Situ Hyaluronic Acid-Poly(Ethylene Oxide) Hydrogel

In vitro cell behaviors of calvarial osteoblasts (MC3T3) were evaluated by seeding them on both the surface and inside of in situ hyaluronic acid-poly(ethylene oxide) (HA-PEO) hydrogel, either after or before incorporation of mixture micro-particles of hydroxyapatite-β-tricalcium phosphate on/inside the hydrogel, respectively. Cellular behaviors such as adhesion and proliferation on the surface and inside the gel were evaluated with light microscopy and a microplate reader by focusing on the interactions of cell-HA-PEO as well as cell-hydroxyapatitetricalcium phosphate micro-particle surface in the gel. Cell adhesion and spreading seemed to be enhanced by supplying the micro-particles to the inside the HA-PEO hydrogel, compared to the results of the HA-PEO hydrogel itself.

Characterization of Hyaluronic Acid-Poly(Ethylene Oxide) Hydrogel Dependent upon Poly(Ethylene Oxide)

Characterization of an hyaluronic acid (HA)-poly(ethylene oxide) (PEO) hydrogel was performed by changing the number of side arms in a PEO molecule. Verification of grafting chemistry and mechanical strength, as well as swelling behaviors and surface morphologies of the HA-PEO hydrogels were analyzed by employing different x-linking molecular weights (MW) of PEO ranging from 1.7 to 5.0 kDa at a fixed low MW HA in the hydrogel network. Methacrylation to the HA successfully obtained via Michael type reaction between the methacrylate arm groups in HA and the thiol end groups in PEO was observed by XPS. Hydrogel formation was markedly dependent upon the numbers of thiol groups in the PEO molecule. Furthermore the lower MW PEO-based HA hydrogel demonstrated stronger mechanical properties but lower water absorption and the smaller pore sizes on its surface and cross section.

DEVELOPMENT OF WEARABLE ELECTROPNEUMATIC BIVENTRICULAR ASSIST DEVICE

LAR ASSIST DEVICE Changmo Hwang, Gi Seog Jeong, Min Woo Jung, Jung Soo Kang, Jung Joo Lee, Yong Doo Park, Kyu Baek Lee, Kyung Sun. Korea Artificial Organ Center, Korea University, Seoul, Korea; Brain Korea 21 Program for Biomedical Science, Korea University College of Medicine, Seoul, Korea; Department of Biomedical Engineering, Korea University College of Medicine, Seoul, Korea; Department of Thoracic and Cardiovascular Surgery, Korea University College of Medicine, Seoul, Korea. The conventional pneumatic drivers for ventricular assist device(VAD) have been relatively large in size and heavy in weight. Recently, portable size drivers are developed and applied in clinical area to give the patients extended scope of activity and convenience of daily life. Additionally, they contributed out of hospital home discharge of pneumatic VAD patients in the clinical aspect. There is still possibility of reducing the size and weight of the pneumatic driver for wearable size. In this paper, simple driving mechanism is suggested for electropneumatic biventricular assist device actuator with smaller size and reduced weight. Our electropneumatic actuator is composed of brushless DC motor, ball screw, two push plates and bellows as major part. The push plates modulate the volume of the bellows. This bellows volume change generates the pressure of the pneumatic circuit for blood pumping. The bellows is manufactured by dipping process with durable polyurethane. The bellows mechanism enabled smaller driver size of 20x30x8cm and reduced weight of 4.0kg including the actuator, controller and battery. To restore the gas leakage, gas compensating system is included, and the short range wireless monitoring is possible with bluetooth module. This wireless monitor/controlling function will improve the mobility of the patients. From the in vitro experiment result, this driver can support 6.5L/min at 100 bpm. And the in vivo animal study showed good performance as biventricular assist device. NUMERICAL STUDY OF THE UNSTEADY BLOOD FLOW IN THE BLOOD SAC OF THE PNEUMATIC VENTRICULAR ASSIST DEVICE GS Jeong, CM Hwang, K Sun. Brain Korea 21 Program for Biomedical Science, Korea University College of Medicine, Seoul, Republic of Korea; Korea Artificial Organ Center, Korea University, Seoul, Republic of Korea; Department of Biomedical Engineering, Korea University College of Medicine, Seoul, Republic of Korea; Department of Thoracic and Cardiovascular Surgery, College of Medicine, Korea University, Seoul, Republic of Korea. Many researchers have studied the hemocompatibility of blood contacting surface in the pneumatic VAD(Ventricular Assist Device), which is widely used in cardiac support. The blood sac in the pneumatic VAD is important in the point of hemocompatibility, because it contacts with blood directly. Among many factors that affect the hemocompatibility, the characteristic of blood flow is important also in the point of thrombosis formation and hemolysis. The recirculation flow and the excessive shear stress imposed to the blood cells lead to thrombosis formation and hemolysis. In this study, the three dimensional numerical analysis about the hemodynamics in the blood sac was performed. Commercial code ADINA based on finite element method is employed to compute blood flow in the blood sac. To analyze the blood flow, FSI(Fluid-Structure Interaction) method is included. The results showed the higher shear stress region at the inlet and outlet region. The recirculation flow was observed in the center of blood sac during systolic period. This numerical study suggests important information in designing the blood sac of pneumatic VAD in the point of hemodynamics.