Papers by Author: Sung Jae Lee

Abstract: Porous metallic biomaterials produced using additive manufacturing technologies have gained popularity in orthopaedic implant applications. Porous metals provide enhanced biological anchorage for the bone tissue surrounding implants, and to promote rapid bone ingrowth. However, fragility associated with highly-porous metals is still a major concern. Standard mechanical testing yields merely structural parameters (i.e. stiffness and strength), but infers nothing about local strains in the micro-architecture such as struts. This study aims to develop a technique applicable for direct measurement of strain in porous titanium (Ti) structures. A low rigidity lattice Ti sample was specifically designed and fabricated using the Selective Laser Melting (SLM) technology. A novel compression test was performed, in which the surface lattice pattern of the Ti cube during the entire compression process was captured. Customized Matlab code was then used to compare lattice images in the unloaded and loaded states to compute strains. The results of full-field strain calculations were presented to demonstrate the capacity of the method. The characterization of localized strains from experiments can aid in the understanding of the mechanics of porous Ti structures, the relationship between microstructures and overall mechanical property, and the interpretation of failure patterns observed in complicated microstructures such as strut.

Abstract: Polymeric calcium phosphate cements (PCPC) derived from biodegradable poly-g-glutamic acid (g-PGA) were prepared in an attempt to improve the mechanical strength of calcium phosphate cement (CPC). The characteristics of the PCPCs were compared to those of cement incorporated with citric acid. The diametral tensile and compressive strengths of the CPC incorporated with g-PGA were significantly higher than that of cement incorporated with citric acid at equivalent concentrations (p<0.05). The maximal diametral tensile and compressive strengths of the CPC incubated for 1 week in physiological saline solution were approximately 18.0 and 50.0 MPa, respectively. However, the initial setting time of the PCPC was much slower than that of CPC incorporated with citric acid. The formation of ionic complexes between calcium ions and g-PGA was observed using FT-IR spectroscopy. Hydroxyapatite (HA) formation was retarded by g-PGA incorporation according to scanning electronic microscopy (SEM) and powder X-ray diffraction (XRD) observations.

Abstract: Dynamic stabilization of the lumbar spine has gained increasingly popularity. These nonfusion systems are alleged to maintain or restore the intradiscal pressure to magnitudes of the intact spine and have little negative effect on the adjacent segments to the stabilized one. Compliant Nitinol alloy (Ni-Ti) has been used in the manufacture of the rods of the dynamic stabilization systems. In this study, we investigated a dynamic stabilization system with its coiled rods made of Nitinol alloy. Four porcine lumbar spines (T12-L6) were prepared: intact, fixed by a conventional rigid fixation system, fixed by a dynamic stabilization system with 2-coiled rods, fixed by a dynamic stabilization system with 3-coiled rods. Intradiscal pressures were measured at all levels before and after insertion of the implant. Our results show that the rigid stabilization system resulted in great changes of disc pressure in flexion and extension regardless of spinal levels. However, Intradiscal pressures (IDPs) remained relatively unchanged in models fixed by the dynamic stabilization systems. Changes in intradiscal pressure can lead to altered metabolism within the disc, with potential long-term disc degeneration. These results suggest that the dynamic stabilization systems are superior over traditional fusion implants in maintaining the intradiscal pressure to the intact level at surgical level and adjacent level and can therefore be considered as an alternative method to fusion surgery in these indications while the intradical pressure is preserved.

Abstract: 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.

Abstract: The goal of this study was to investigate effects of fibrin reinforcement of collagen sponges on fibroblasts-mediated contraction and in vivo tissue regeneration, especially angiogenesis. Human dermal fibroblasts (HDFs)-populated collagen sponges reinforced with or without fibrin were cultivated via the free-floating method in vitro. They were then evaluated using xenogeneic implantation into nude mice. The HDFs-populated collagen sponges reinforced with fibrin exhibited significantly decreased HDFs-mediated contraction in vitro (p<0.05). Microvascular and cellular densities of the collagen sponges were significantly higher with fibrin than without (p<0.01). Cell ingrowths, neovascularization, and deposition of ECM matrix were more evenly distributed in the fibrin-reinforced collagen matrices. The results demonstrated that fibrin reinforcement of porous collagen sponges can reduce cell-mediated contraction in vitro while enhancing functional integration with surrounding tissue in vivo.

Abstract: Fibrin is a natural polymer with excellent biocompatibility and widely used as a cell delivery vehicle in tissue engineering. However, fibrin of low concentration is not able to promote cell growth and differentiation within a desired time because of contraction and biodegradation of cell-seeded matrices. In this study we investigated effects of combining fibrin with collagen on growth and osteogenic differentiation of bone marrow stromal cells (BMSCs). Rabbit BMSCs-populated fibrin hydrogels with or without collagen were fabricated and cultured by the free floating method for 4 weeks. The DNA content of fibrin/collagen matrix significantly increased the growth of BMSCs compared to the fibrin-only matrix at 2week. Alkaline phosphatase activity was significantly higher in the fibrin/collagen matrix (71.0 nmol of p-nitrophenol /min/disc) than the fibrin-only matrix (45.1 nmol of p-nitrophenol /min/disc). Deposition of calcium was not significantly different between two groups. Histological examination also revealed more matured organization and deposition of collagen fibers and more concentric calcium deposition in the fibrin/collagen matrix compared to the fibrin-only matrix. These results indicated that fibrin/collagen matrix could be more effective than fibrin alone in supporting growth and osteogenic differentiation of BMSCs.

Abstract: A culture system that is capable of providing even and uniform distribution and deposition of cells and extracellular matrix (ECM) is desired to enhance biological functions of the tissue-engineered artificial dermis (TEADs). For this purpose, we have developed a perfusion culture system that offers uniform exchange of nutrients and gases along the scaffold. Viability and effectiveness of the system were investigated by comparing biological and mechanical properties of TEADs. Results showed that the TEADs constructed by the perfusion culture system revealed significantly increased cell growth, ECM synthesis, and elastic modulus compared to those by the conventional static culture system. In addition, histological findings indicated that cells were more evenly distributed and ECM deposition increased in TEADs with the perfusion culture system. Therefore, it can be suggested that the perfusion culture system can constitute a more promising approach for constructing the TEADs.