Heung Jae Chun

A biodegradable, injectable, gel system based on MPEG-b-(PCL-ran-PLLA) diblock copolymers with an adjustable therapeutic window.

In situ-forming gel systems have drawn increasing attention for their potential use in a variety of biomedical applications. Here, we examined an in situ-forming gel system comprised of MPEG-b-PCL and MPEG-b-(PCL-ran-PLLA) diblock copolymers with different PLLA contents (0-10 mol%) in the PCL segment. The crystalline region of the PCL-ran-PLLA segment decreased with increasing PLLA content. The MPEG-b-(PCL-ran-PLLA) diblock copolymer solutions were liquid at room temperature and only MPEG-b-(PCL-ran-PLLA) diblock copolymer solutions with a PLLA content < or = 5 mol% in the PCL segment showed a sol-to-gel transition as the temperature was increased. The viscosity change associated with sol-to-gel phase transition depended on the PLLA content in the PCL segment. A MPEG-b-PCL diblock copolymer solution incubated in vitro showed increasing viscosity without degradation, whereas the viscosity of MPEG-b-(PCL-ran-PLLA) diblock copolymer solutions continuously and sharply decreased with increasing PLLA content in the PCL segment. As the amount of PLLA increased, the size of in vivo-formed MPEG-b-(PCL-ran-PLLA) gels after initial injection tended to gradually decrease because of hydrolytic degradation of the PLLA in the PCL-ran-PLLA segment. An immunohistochemical examination showed that in vivo MPEG-b-(PCL-ran-PLLA) diblock copolymer gels provoked only a modest inflammatory response. Collectively, our results show that the MPEG-b-(PCL-ran-PLLA) diblock copolymer gel described here could serve as a minimally invasive, therapeutic, in situ-forming gel system that offers an experimental window adjustable from a few weeks to a few months.

Fabrication of core-shell microcapsules using PLGA and alginate for dual growth factor delivery system.

To effectively harness the great potential of stem cells, we designed a dual growth factor delivery system for the application toward stem cell differentiation into specific lineages. This system carries a core-shell structure within microcapsules made of poly(L-lactide-co-glycolide) (PLGA) and alginate, which were fabricated using a coaxial electro-dropping method. Both PLGA and alginate were supplied from the inner and outer nozzles, respectively. The size and shape of microcapsules were greatly varying depending on the variables: nozzle size, applied voltage, volumetric feeding ratio (PLGA:alginate), feeding rate, and polymer concentrations. Once proper conditions were met, single or multi PLGA cores were found settled within the microcapsules. From the microscopic images, wrinkled surfaces of microcapsules were observed, along with the PLGA cores inside the alginate domain. When two different microcapsules were made, switching the position of bone morphogenetic protein (BMP)-2 and dexamethasone (Dex) for either core or shell domain, their release profiles were very unique on a temporal basis, based on their location in the microcapsules. An initial burst of biomolecules was highly suppressed when either biomolecule was loaded in the PLGA core. It was clear that the osteogenic biomolecules encapsulated in the microcapsule could be released together and their concentrations were disparate at each time point. Meanwhile as the hydrogel constructs including rat bone marrow stromal cells (BMSCs) and osteogenic factor-loaded microcapsules were cultured for up to 4 weeks, the gene expressions levels of osteopontin, type I collagen, and osteocalcin were significantly upregulated as compared to the control group. The present coaxial system was very effective in manufacturing PLGA core-alginate shell microcapsules and in encapsulating multiple biomolecules essential for stem cell differentiation.

Pervaporation separation of water-isopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes

The pervaporation separation of water–isopropanol mixtures was carried out using carboxymethylated poly(vinyl alcohol) (CMPVA) composite membranes. Carboxymethylated PVA (CMPVA) was synthesized by reacting PVA with various concentrations of monochloroacetic acid. Substitution efficiency of the CMPVA ranged from 12–32%. The cross-sectional structure of the composite membrane for pervaporation was confirmed by scanning electron microscopy (SEM) exhibiting a 20-μm active skin layer. Glass transition temperature of the CMPVA was in the range of 74–84°C, and decreased with increasing substitution efficiency. Degree of swelling and permeation flux for water–isopropanol in pervaporation increased with the substitution degree of carboxymethylation. CMPVA composite membrane, having 16% substitution efficiency, showed the following pervaporation performance; permeation flux of 831 g/m2 h and separation factor of 362 measured at 80°C and 85 wt % feed isopropanol concentration.

In Vivo Osteogenic Differentiation of Human Adipose-Derived Stem Cells in an Injectable In Situ–Forming Gel Scaffold

The sol-to-gel transition occurring at around body temperature makes the MPEG-PCL diblock copolymer an ideal candidate material for use as an injectable in situ-forming gel containing human adipose tissue-derived stem cells (hADSCs). The sol can be prepared at room temperature, and the gel forms at body temperature. Solutions of the copolymer containing hADSCs and osteogenic factors injected into rats formed gel scaffolds at the injection sites. The gels thus formed showed the interconnective pore structure required to support growth, proliferation, and differentiation of hADSCs. Bromodeoxyuridine-labeled hADSCs were confirmed to be present in gels formed in vivo. Bone formation was observed only in gel implants containing both hADSCs and osteogenic factors. Subcutaneous implantation of the in situ-forming gel scaffold demonstrated that hADSCs embedded in the gel stimulated much lower host tissue responses than did the gel alone, probably because of the unique immunomodulatory properties of hADSCs. In conclusion, our data on hADSCs embedded in an in situ gel scaffold suggest that this formulation may provide numerous benefits as a noninvasive alternative for tissue-engineered bone formation.

Polymeric Scaffolds for Regenerative Medicine

Regenerative medicine, one of the most exciting and dynamic life science fields, is an emerging biomedical technology for assisting and accelerating the regeneration and repair of lost or damaged organs or body parts. Modern regenerative medicine is increasingly using three-dimensional structured scaffolds because they represent a wide range of morphological and geometric in vivo possibilities that can be tailored for each specific regenerative medicine application. This review focuses on polymeric scaffolds, a highly promising regenerative medicine strategy, summarizing some important issues related to various natural and synthetic scaffolding biomaterials, techniques on the design and fabrication of three-dimensional polymeric scaffolds to mimic the properties of the extracellular matrix, and clinical applications of polymeric scaffolds for tissue regeneration.

Graft copolymerization of mixtures of acrylic acid and acrylamide onto polypropylene film

Liquid phase ultraviolet irradiation in the presence of benzophenone as a photosensitizer and barium hydroxide as a pH controller were used to graft the mixtures of acrylic acid and acrylamide to a polypropylene surface. The surface of the grafted polypropylene samples were characterized by Fourier transform infrared spectroscopy-attenuated total reflectance, electron spectroscopy for chemical analysis, scanning electron microscopy, and a contact angle meter. The pH value of the reaction medium that produced the graft with equal molar ratio was found to be ∼ 3.77. The optimal reaction condition was found at a monomer feed of 25%, a reaction time of 30 min, and a benzophenone concentration of 1%. Surface tension of the samples increased to a value of 40 dyn cm−1 due to the graft of the hydrophilic monomers.

Evaluations of Poly(vinyl alcohol)/Alginate Hydrogels Cross-linked by γ-ray Irradiation Technique

In this work, we prepared hydrogels for wound dressing from a mixture of poly(vinyl alcohol) (PVA) and alginate using the60Co γ-ray irradiation technique. We examined the physical properties of these hydrogels, including gelation, water absorptivity, and gel strength, to evaluate the applicability of these hydrogels for wound dressings. The biocompatibility of these hydrogels was also evaluated in vitro, in cultures of mouse fibroblasts, and in vivo, by subcutaneous implantation studies in rats. The gel content and strength increased upon increasing the radiation dose and upon decreasing the concentration of alginate. The degree of swelling was inversely proportional to the gel content and strength. The degree of cytotoxicity of the γ-ray-treated hydrogels was ca. 60% compared to the (−) control (serum) after 1 day of incubation. When the incubations were prolonged up to 2 days, the toxicity of all the samples decreased remarkably and reached that of the control. Subcutaneous implantation studies in rats indicated that foreign body reactions occurring around the implanted hydrogels were moderate and became minimal upon increasing the implantation time.

In vivo biofunctionality comparison of different topographic PLLA scaffolds.

In this work, the in vivo biodegradation of, biocompatibility of, and host response to various topographic scaffolds were investigated. Randomly oriented fibrous poly(L-lactide) (PLLA) nanofibers were fabricated using the electrospinning technique. A PLLA scaffold was obtained by salt leaching. Both the electrospun PLLA nanofibers and the salt-leaching PLLA scaffolds formed three-dimensional pore structures. Cytotoxicity studies, in which rat muscle-derived stem cells (rMDSCs) were grown on electrospun PLLA nanofibers or the salt-leaching PLLA scaffolds, revealed that the rMDSCs cell count on the PLLA nanofibers was slightly higher than that on the salt-leaching PLLA scaffolds. An in vivo study was carried out by implanting the scaffolds subcutaneously into rats to test the biodegradation, biocompatibility, and host response at regular intervals over 0-4 weeks. The degradation of the PLLA nanofibers 1, 2, and 4 weeks after initial implantation was more extensive than that observed for the salt-leaching PLLA scaffolds. PLLA nanofibers seeded the growth of larger fibrous tissue masses due to in vivo cellular infiltration into the randomly oriented fibrillar structures of the PLLA nanofibers. In addition, the inflammatory cell accumulation in PLLA nanofibers was lower than that in the salt-leaching PLLA scaffolds. These results indicate that the electrospun PLLA nanofibers may serve as a good scaffold to elicit fibrous cellular infiltration, to minimize host response, and to enhance tissue-scaffold integration.

Time-course Transcriptional Profiling of Human Amniotic Fluid-derived Stem Cells Using Microarray

PURPOSE To maintain the homeostasis of stem cells and prevent their ability to initiate tumorigenesis, it is important to identify and modify factors that prevent or accelerate stem cell senescence. We used microarrays to attempt to identify such factors in human amniotic fluid (HAF)-derived stem cells. MATERIALS AND METHODS To identify gene expression changes over a time course, we compared gene expression profiles of HAF-derived stem cells in different passages (1(st), 2(nd), 4(th), 6(th), 8(th), and 10(th)) using a Sentrix Human illumina microarray. RESULTS Of the 25,804 genes in the microarray chip, 1,970 showed an over 2-fold change relative to the control (the 1(st) passage)-either upregulated or downregulated. Quantitative real-time PCR validated the microarray data for selected genes: markedly increased genes were CXCL12, cadherin 6 (CDH6), and folate receptor 3 (FOLR3). Downregulated genes included cyclin D2, keratin 8, insulin-like growth factor 2 (IGF2), natriuretic peptide precursor B (NPPB) and cellular retinoic acid binding protein 2 (CRABP2). The expression pattern of the selected genes was consistent with the microarray data except for CXCL12 and IGF2. Interestingly, the expression of NPPB was dramatically downregulated along the time course; it was almost completely shut-down by the 10(th) passage. In contrast, FOLR3 mRNA expression was dramatically increased. CONCLUSION Taken together, although a function for NPPB and FOLR3 in stem cell senescence has not been reported, our results strongly suggest that NPPB and/or FOLR3 play a significant role in the regulation of stem cell senescence.

Effect of γ-Ray irradiation on surface oxidation of ultra high molecular weight polyethylene/zirconia composite prepared byin situ ziegler-natta polymerization

Novel ultra-high molecular weight polyethylene (UHMWPE)/zirconia composites were previously prepared by thein situ polymerization of ethylene using a Ti-based Ziegler-Natta catalyst supported on to the surface of zirconia, as a bearing material for artificial joints. Tribological tests revealed that a uniform dispersion of zirconia in UHMWPE markedly increased the wear resistance. The effects of zirconia content on the oxidation behavior of the γ-ray-treated UHMWPE/zirconia composite surfaces were examined. The oxidation index that estimates the oxidation degree as the content of total carbonyl compounds was monitored using Fourier transform infrared spectroscopy-attenuated total reflectance. The changes in the surface composition due to the oxidation were confirmed by electron spectroscopy for chemical analysis. The extent of oxidation decreased with increasing zirconia content, which was attributed to the increased crystallinity as well as the decreased polymer portion of the UHMWPE/zirconia composites.