Kyunghwan Yoon

Functional nanofibers for environmental applications

Nanofibers, mainly fabricated by electrospinning, have exhibited great potential for many emerging environmental applications. They can be considered as one of the safest nanomaterials due to their extremely long length (can be up to hundreds of kilometers) and their ability to be embedded within other media. Their high surface-to-volume ratio, large porosity (up to over 80%) and adjustable functionality are also much more effective than conventional non-woven and polymeric membranes in particulate separation and liquid filtration. Technology advances such as multiple-jet electrospinning and electroblowing for mass production of nanofibers have made it practical to use nanofibrous scaffolds as a unique and breakthrough component in separation media for both gas and liquid filtration. In this review, the opportunities and challenges of using functional nanofibers for several representative environmental applications are reviewed.

A Highly Reactive and Sinter‐Resistant Catalytic System Based on Platinum Nanoparticles Embedded in the Inner Surfaces of CeO2 Hollow Fibers

The catalytic properties of a supported system are strongly dependent on the types of metal and supporting material involved. Platinum nanoparticles (NPs) supported on ceria (CeO2) have shown higher catalytic activities for a wide variety of reactions, including water-gas shift, 6] CO oxidation, and hydrogenation when compared to those supported on other oxides. Therefore, CeO2 has been widely employed as a support for manufacturing automotive catalysts because of its peerless oxygen storing/releasing capabilities as well as its superior ability to stabilize noble metals. In particular, Pt/CeO2 catalysts are known to exhibit strong metal–support interaction effects with a potential to enhance the catalytic activities for reactions involving rapid oxygen and/or electron transfer between the metal and the support. Although there are many benefits in utilizing the Pt/CeO2 system in real-world catalytic applications, great challenges, such as low thermal stability and loss of catalytic activity owing to sintering, still need to be addressed. In catalysts for automotive exhaust treatment, these problems are partially mitigated by combining CeO2 with ZrO2 in a solid solution, but precious metal sintering remains a primary deterioration mechanism. The use of nanostructured composite materials is one strategy to address the sintering issue. Yan and co-workers recently synthesized CeO2 nanoparticles covered with PtNPs and then SiO2 shells to protect the PtNPs from aggregation during calcination, but their approach required multiple steps to generate the SiO2 shells and then dissolve them. As a major drawback, the Pt/CeO2 nanocomposites could only withstand calcination up to 450 8C after the removal of the SiO2 shells. Tsang and co-workers demonstrated the synthesis of a Pt/CeO2 core–shell NPs by a modified microemulsion method. The resultant catalyst showed significantly improved catalytic activity and selectivity in water-gas shift over methanation by controlling the thickness of the CeO2 protective layer. However, the Pt/CeO2 core–shell NPs exhibited considerable aggregation in the reaction medium, which may negatively impact their catalytic activity in a practical application. Herein, we report a simple, template-based procedure for the fabrication of CeO2 hollow fibers with PtNPs embedded in the inner surfaces (Figure 1). The first step involved the

High-flux thin-film nanofibrous composite ultrafiltration membranes containing cellulose barrier layer

A novel class of thin-film nanofibrous composite (TFNC) membrane consisting of a cellulose barrier layer, a nanofibrous mid-layer scaffold, and a melt-blown non-woven substrate was successfully fabricated and tested as an ultrafiltration (UF) filter to separate an emulsified oil and water mixture, a model bilge water for on-board ship bilge water purification. Two ionic liquids: 1-butyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium acetate, were chosen as the solvent to dissolve cellulose under mild conditions. The regenerated cellulose barrier layer exhibited less crystallinity (determined by wide-angle X-ray diffraction, WAXD) than the original cotton linter pulps, but good thermal stability (determined by thermal gravimetric analysis, TGA). The morphology, water permeation, and mechanical stability of the chosen TFNC membranes were thoroughly investigated. The results indicated that the polyacrylonitrile (PAN) nanofibrous scaffold was partially imbedded in the cellulose barrier layer, which enhanced the mechanical strength of the top barrier layer. The permeation flux of the cellulose-based TFNC membrane was significantly higher (e.g. 10×) than comparable commercial UF membranes (PAN10 and PAN400, Sepro) with similar rejection ratios for separation of oil/water emulsions. The molecular weight cut-off (MWCO) of TFNC membranes with cellulose barrier layer was evaluated using dextran feed solutions. The rejection was found to be higher than 90% with a dextran molecular weight of 2000 KDa, implying that the nominal pore size of the membrane was less than ∼50 nm. High permeation flux was also observed in the filtration of an emulsified oil/water mixture as well as of a sodium alginate aqueous solution, while high rejection ratio (above 99.5%) was maintained after prolonged operation. A variation of the barrier layer thickness could dramatically affect the permeation flux and the rejection ratio of the TFNC membranes, while different sources of cellulose, ionic liquids, and non-woven supports did not. As ionic liquids can be recycled and reused without obvious decomposition, the chosen method also demonstrates a benign pathway to fabricate the cellulose barrier layer for other types of membranes.

Effect of degumming condition on the solution properties and electrospinnablity of regenerated silk solution.

The application of silk on tissue engineering scaffolds has been studied intensively because silk has an electrospinning technique using a good blood compatibility, excellent cytocompatibility and biodegradability. Silk consists of two polymers, fibroin and sericin. In spite of importance of sericin, most studies were focused on the fibroin only and the effect of residual sericin on the electrospinning performance of silk has not been considered. In this study, regenerated silk with different residual sericin contents was prepared by controlling the degumming conditions. The effects of the degumming conditions on the solution properties and electrospinning performance of silk were examined. The fast protein liquid chromatography (FPLC) measurements confirmed that the molecular weight of the regenerated silk decreased slightly with increasing residual sericin content. More molecular aggregation of silk occurred with increasing sericin content, resulting in an increase in the solution turbidity of formic acid. All silk formic acid solutions exhibited almost Newtonian fluid behavior and the viscosity increased with increasing sericin content. Interestingly, the dope solution viscosity of silk increased remarkably at sericin contents <1% (or degumming ratio >25%) leading to significant improvements in electrospinnability and an increase in the fiber diameter of the silk web.

Shear‐Induced Orientation and Structure Development in Isotactic Polypropylene Melt Containing Modified Carbon Nanofibers

The shear‐induced crystallization behavior of isotactic polypropylene (iPP) nanocomposite melt containing modified carbon nanofibers (MCNFs) was investigated by rheo‐SAXS (small‐angle X‐ray scattering) and rheo‐WAXD (wide‐angle X‐ray diffraction) techniques using synchrotron radiation. Under quiescent conditions, the nucleating effect of MCNFs on crystallization of iPP was pronounced and the system exhibited a remarkably low saturation point (ca. 0.05 wt% of MCNF). In‐situ SAXS and WAXD results showed the development of shear‐induced crystalline structures and lamellar morphology in nanocomposite melts. Under the same shear conditions, the filled system exhibited notably faster kinetics compared with the unfilled system. The oriented crystalline fraction was found to decrease with the MCNF loading, indicating the competition between oriented crystals (induced by shear) and unoriented crystals (due to the nucleating effect of MCNF). At the early stages of crystallization, the amount of the oriented crystals increased with the MCNF concentration, suggesting that the nanofiller hindered the motion of polymer chains after the cessation of flow resulting in the delayed relaxation of stretched polymer segments. Dedicated to Prof. Phillip H. Geils seventy‐fifth birthday.

Atherosclerotic Carotid Stenoses of Apical versus Body Lesions in High-Risk Carotid Stenting Patients

BACKGROUND AND PURPOSE: Different lesion locations in the atherosclerotic carotid bulb stenosis have not been clearly defined. We sought to evaluate 2 locations of carotid bulb stenosis in high-risk patients and to determine the relationship of each location to atherosclerotic risk factors and clinical features. MATERIALS AND METHODS: Atherosclerotic carotid plaques of apical versus body lesions, defined according to the area and extent of plaque involvement, were retrospectively analyzed in 200 consecutive high-risk patients who underwent carotid stent placement because of ≥50% symptomatic stenosis. We evaluated interobserver concordance and assessed each type of lesion relative to 13 atherosclerotic risk factors, mode of symptom presentation, infarct pattern, procedure-related factors, and clinical outcomes, by univariate and multivariable logistic regression analysis. RESULTS: Interobserver concordance showed good agreement for differentiating apical and body lesions (κ = 0.745). Univariate analysis revealed that apical lesions (n = 108, 54%) were associated with pseudo-occlusion (P = .027), older age (P = .073), and alcohol intake (P = .080), whereas body lesions (n = 92, 46%) were associated with hyperlipidemia (P = .001), a wedge-shaped cortical infarct pattern (P = .057), and hyperperfusion syndrome (P = .083). Multivariable logistic regression analysis adjusted by age revealed that hyperlipidemia (P = .002; OR, 3.462; 95% CI, 1.595–7.515) and hyperperfusion (P = .026; OR, 6.727; 95% CI, 1.261–35.894) were independent predictors of body-type lesions. CONCLUSIONS: Atherosclerotic carotid bulb stenosis was found to have 2 distinct locations, body and apical. Hyperlipidemia and cortical wedge-shaped infarcts were more frequently associated with body than with apical stenosis at the time of presentation.

Effects of degumming conditions on electro-spinning rate of regenerated silk.

Electro-spun silk webs are potentially good candidates as tissue engineering scaffolds owing to their good bio- and cyto-compatibility. However, the low fabrication rate of electro-spun silk mats has been one of the obstacles in the mass production of such nanofibrous silk mats in applications to the biomedical field. In this study, the effects of degumming ratio and silk concentration on the electro-spinning process were investigated by using regenerated silk with different residual sericin contents and different silk concentrations in terms of the morphology and structure of the electro-spun silk web. The rate of production of electro-spun silk mats could be increased by approximately 5 fold at a degumming ratio of 19.5%. The electro-spinning rate of silk was affected by two main factors: (1) dope solution viscosity and (2) degumming ratio of silk. The conductivity of the silk dope solution, however, had little effects on the electro-spinning of regenerated silk. A constant spun fiber morphology was observed within the electro-spinning rate range (0.3-1.4 ml/h). Fourier transform infrared spectroscopy showed that partial β-sheet crystallization occurred during electro-spinning. The molecular conformation was relatively unaffected by the electro-spinning rate of silk.

A Bioreducible Polymer for Efficient Delivery of Fas‐Silencing siRNA into Stem Cell Spheroids and Enhanced Therapeutic Angiogenesis

This paper describes a bioreducible polymer for efficient delivery of Fas-silencing small interfering RNA (Fas siRNA) into human mesenchymal stem cells (hMSCs). The genetically modified hMSCs could be formulated as spheroids with enlarged sizes (≥ 800 μm in diameter) to enhance their angiogenic efficacy. The Fas siRNA delivered into the cytoplasm of hMSCs via the bioreducible polymer could efficiently inhibit Fas expression, leading to effective inhibition of hypoxia-induced apoptosis in the core of an enlarged spheroid of hMSCs. The enlarged spheroid not only increased the number of viable hMSCs but also significantly enhanced the secretion of bioactive angiogenic growth factors per hMSC relative to small spheroids that were not treated with Fas siRNA. Twenty-eight days after transplantation into mouse with ischemic hindlimbs, the enlarged hMSC spheroids significantly enhanced cell survival, limb salvage, blood perfusion, and vessel formation while substantially reducing muscle degeneration and tissue fibrosis. This study demonstrates a promising strategy that combines siRNA and spheroid-based stem cell therapies using a stimuli-responsive nonviral siRNA carrier for the efficient treatment of ischemic disease.

Effects of adding injection–compression to rapid heat cycle molding on the structure of a light guide plate

This study investigates the effects of adding injection–compression to rapid heat cycle molding (RHCM) (rapid heat cycle injection–compression molding (RICM)) on the physical quality and optical anisotropy of a molded light guide plate (LGP). Transcription ratio of microstructure, uniformity of part thickness and birefringence were experimentally evaluated on a 7 inch LGP of nominal thickness of 1.12 mm (including a microstructure array of 30 µm diameter and 14 µm height). The designed mold was equipped with rapid heating and compressing facilities and a microstructured nickel stamper was fabricated by UV LIGA process. In addition, to investigate the efficacy of RICM, experiments involving conventional injection molding (CIM), ICM, and RHCM were conducted in parallel with RICM using the same mold. RHCM and RICM yielded excellent transcription ratios for the microstructure, while CIM and RICM provided high thickness uniformity and low birefringence. Thus, RICM obtains high transcription ratio of microstructure, uniform thickness and low birefringence.

Computational Flow Dynamics of the Severe M1 Stenosis Before and After Stenting

Purpose Computational flow dynamic (CFD) study has not been widely applied in intracranial artery stenosis due to requirement of high resolution in identifying the small intracranial artery. We described a process in CFD study applied to symptomatic severe intracranial (M1) stenosis before and after stenting. Materials and Methods Reconstructed 3D angiography in STL format was transferred to Magics (Materialise NV, Leuven, Belgium) for smoothing of vessel surface and trimming of branch vessels and to HyperMesh (Altair Engineering Inc., Auckland, New Zealand) for generating tetra volume mesh from triangular surface-meshed 3D angiogram. Computational analysis of blood flow in the blood vessels was performed using the commercial finite element software ADINA Ver 8.5 (ADINA R & D, Inc., Lebanon, MA). The distribution of wall shear stress (WSS), peak velocity and pressure in a patient was analyzed before and after intracranial stenting. Results Computer simulation of wall shear stress, flow velocity and wall pressure before and after stenting could be demonstrated three dimensionally by video mode according to flow vs. time dimension. Such flow model was well correlated with angiographic finding related to maximum degree of stenosis. Change of WSS, peak velocity and pressure at the severe stenosis was demonstrated before and after stenting. There was no WSS after stenting in case without residual stenosis. Conclusion Our study revealed that CFD analysis before and after intracranial stenting was feasible despite of limited vessel wall dimension and could reveal change of WSS as well as flow velocity and wall pressure.