Youngmi Koo

Enhanced mechanical properties and increased corrosion resistance of a biodegradable magnesium alloy by plasma electrolytic oxidation (PEO)

We report the enhanced mechanical properties of AZ31 magnesium alloys by plasma electrolytic oxidation (PEO) coating in NaOH, Na2SiO3, KF and NaH2PO4·2H2O containing electrolytes. Mechanical properties including wear resistance, surface hardness and elastic modulus were increased for PEO-coated AZ31 Mg alloys (PEO-AZ31). DC polarization in Hanks solution indicating that the corrosion resistance significantly increased for PEO-coating in KF-contained electrolyte. Based on these results, the PEO coating method shows promising potential for use in biodegradable implant applications where tunable corrosion and mechanical properties are needed.

Flow-induced corrosion of absorbable magnesium alloy: In-situ and real-time electrochemical study

An in-situ and real-time electrochemical study in a vascular bioreactor was designed to analyze corrosion mechanism of magnesium alloy (MgZnCa) under mimetic hydrodynamic conditions. Effect of hydrodynamics on corrosion kinetics, types, rates and products was analyzed. Flow-induced shear stress (FISS) accelerated mass and electron transfer, leading to an increase in uniform and localized corrosions. FISS increased the thickness of uniform corrosion layer, but filiform corrosion decreased this layer resistance at high FISS conditions. FISS also increased the removal rate of localized corrosion products. Impedance-estimated and linear polarization-measured polarization resistances provided a consistent correlation to corrosion rate calculated by computed tomography.

Aligned carbon nanotube/copper sheets: a new electrocatalyst for CO2 reduction to hydrocarbons

We controlled the morphologies of copper (Cu) nanostructure on aligned carbon nanotube (CNT) sheets, influencing the efficiency of the electrocatalytic reduction of CO2. Functionalized CNT sheets affected the pulsed electrodeposition of copper in terms of 3D growth, bonding, and electrochemical activity. CNT/Cu sheet electrocatalyst shows high performance in electrochemical reduction of CO2 to hydrocarbons at room temperature and atmospheric pressure. Reduction products were carbon monoxide (CO), methane (CH4), and ethylene (C2H4) gases. Carbon monoxide yields (178 μmol cm2 mA−1 h−1) and methane yields (346 μmol cm2 mA−1 h−1) at oxygen-plasma-treated CNT/Cu sheet electrodes were remarkably higher than other CNT/Cu and CNT sheets. Experimental results also show 3D morphology of copper growth on CNT sheets may play a critical role in hydrocarbon products from CO2.

Biodegradable, elastomeric coatings with controlled anti-proliferative agent release for magnesium-based cardiovascular stents.

Vascular stent design continues to evolve to further improve the efficacy and minimize the risks associated with these devices. Drug-eluting coatings have been widely adopted and, more recently, biodegradable stents have been the focus of extensive evaluation. In this report, biodegradable elastomeric polyurethanes were synthesized and applied as drug-eluting coatings for a relatively new class of degradable vascular stents based on Mg. The dynamic degradation behavior, hemocompatibility and drug release were investigated for poly(carbonate urethane) urea (PCUU) and poly(ester urethane) urea (PEUU) coated magnesium alloy (AZ31) stents. Poly(lactic-co-glycolic acid) (PLGA) coated and bare stents were employed as control groups. The PCUU coating effectively slowed the Mg alloy corrosion in dynamic degradation testing compared to PEUU-coated, PLGA-coated and bare Mg alloy stents. This was confirmed by electron microscopy, energy-dispersive x-ray spectroscopy and magnesium ion release experiments. PCUU-coating of AZ31 was also associated with significantly reduced platelet adhesion in acute blood contact testing. Rat vascular smooth muscle cell (rSMC) proliferation was successfully inhibited when paclitaxel was released from pre-loaded PCUU coatings. The corrosion retardation, low thrombogenicity, drug loading capacity, and high elasticity make PCUU an attractive option for drug eluting coating on biodegradable metallic cardiovascular stents.

High performance magnesium anode in paper-based microfluidic battery, powering on-chip fluorescence assay.

A high power density and long-lasting stable/disposable magnesium battery anode was explored for a paper-based fluidic battery to power on-chip functions of various Point of Care (POC) devices. The single galvanic cell with magnesium foil anode and silver foil cathode in Origami cellulose chip provided open circuit potential, 2.2 V, and power density, 3.0 mW/cm(2). A paper-based fluidic galvanic cell was operated with one drop of water (80 μl) and continued to run until it was dry. To prove the concept about powering on-chip POC devices, two-serial galvanic cells are developed and incorporated with a UV-light emitting diode (λ = 365 nm) and fluorescence assay for alkaline phosphatase reaction. Further, detection using smart phones was performed for quantitative measurement of fluorescent density. To conclude, a magnesium-based fluidic battery paper chip was extremely low-cost, required minute sample volumes, was easy to dispose of, light weight, easy to stack, store and transport, easy to fabricate, scalable, and has faster analysis times.

Ex vivo blood vessel bioreactor for analysis of the biodegradation of magnesium stent models with and without vessel wall integration

Current in vitro models fail in predicting the degradation rate and mode of magnesium (Mg) stents in vivo. To overcome this, the microenvironment of the stent is simulated here in an ex vivo bioreactor with porcine aorta and circulating medium, and compared with standard static in vitro immersion and with in vivo rat aorta models. In ex vivo and in vivo conditions, pure Mg wires were exposed to the aortic lumen and inserted into the aortic wall to mimic early- and long-term implantation, respectively. Results showed that: 1) Degradation rates of Mg were similar for all the fluid diffusion conditions (in vitro static, aortic wall ex vivo and in vivo); however, Mg degradation under flow condition (i.e. in the lumen) in vivo was slower than ex vivo; 2) The corrosion mode in the samples can be mainly described as localized (in vitro), mixed localized and uniform (ex vivo), and uniform (in vivo); 3) Abundant degradation products (MgO/Mg(OH)2 and Ca/P) with gas bubbles accumulated around the localized degradation regions ex vivo, but a uniform and thin degradation product layer was found in vivo. It is concluded that the ex vivo vascular bioreactor provides an improved test setting for magnesium degradation between static immersion and animal experiments and highlights its promising role in bridging degradation behavior and biological response for vascular stent research. STATEMENT OF SIGNIFICANCE Magnesium and its alloys are candidates for a new generation of biodegradable stent materials. However, the in vitro degradation of magnesium stents does not match the clinical degradation rates, corrupting the validity of conventional degradation tests. Here we report an ex vivo vascular bioreactor, which allows simulation of the microenvironment with and without blood vessel integration to study the biodegradation of magnesium implants in comparison with standard in vitro test conditions and with in vivo implantations. The bioreactor did simulate the corrosion of an intramural implant very well, but showed too high degradation for non-covered implants. It is concluded that this system is in between static incubation and animal experiments concerning the predictivity of the degradation.

TiO 2 deposition on AZ31 magnesium alloy using plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) has been used in the past as a useful surface treatment technique to improve the anticorrosion properties of Mg alloys by forming protective layer. Coatings were prepared on AZ31 magnesium alloy in phosphate electrolyte with the addition of TiO2 nanoparticles using plasma electrolytic oxidation (PEO). This present work focuses on developing a TiO2 functional coating to create a novel electrophotocatalyst while observing the surface morphology, structure, composition, and corrosion resistance of the PEO coating. Microstructural characterization of the coating was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) followed by image analysis and energy dispersive spectroscopy (EDX). The corrosion resistance of the PEO treated samples was evaluated with electrochemical impedance spectroscopy (EIS) and DC polarization tests in 3.5wt.% NaCl. The XRD pattern shows that the components of the oxide film include Mg from the substrate as well as MgO and Mg2TiO4 due to the TiO2 nanoparticle addition. The results show that the PEO coating with TiO2 nanoparticles did improve the corrosion resistance when compared to the AZ31 substrate alloy.

Functionalized liposomes loaded with siRNAs targeting ion channels in effector memory T cells as a potential therapy for autoimmunity

Effector memory T cells (TM) play a key role in the pathology of certain autoimmune disorders. The activity of effector TM cells is under the control of Kv1.3 ion channels, which facilitate the Ca(2+) influx necessary for T cell activation and function, i.e. cytokine release and proliferation. Consequently, the knock-down of Kv1.3 expression in effector TMs may be utilized as a therapy for the treatment of autoimmune diseases. In this study we synthesized lipid unilamellar nanoparticles (NPs) that can selectively deliver Kv1.3 siRNAs into TM cells in vitro. NPs made from a mixture of phosphatidylcholine, pegylated/biotinylated phosphoethanolamine and cholesterol were functionalized with biotinylated-CD45RO (cell surface marker of TMs) antibodies via fluorophore-conjugated streptavidin (CD45RO-NPs). Incubation of T cells with CD45RO-NPs resulted into the selective attachment and endocytosis of the NPs into TMs. Furthermore, the siRNA against Kv1.3, encapsulated into the CD45RO-NPs, was released into the cytosol. Consequently, the expression of Kv1.3 channels decreased significantly in TMs, which led to a remarkable decrease in Ca(2+) influx. Our results can form the basis of an innovative therapeutic approach in autoimmunity.

Inverse-Ordered Fabrication of Free-Standing CNT Sheets for Supercapacitor

Free-standing thin carbon nanotube (CNT) sheets are challenging to handle and control for device fabrication. In this paper, we report on the inverse-ordered fabrication method from thick CNT sheets to thin free-standing CNT sheets. As proof of the concept, thin CNT sheets for a supercapacitor were fabricated from 200 thick layers. The results show that the thin CNT sheet was electrochemically stable and had enhanced capacitive performance. The smaller the number of layers is, the larger the specific capacitances we have (from 10.10 F g(-1) to 51.37 F g(-1)). Energy and power density were increased from 0.50 to 2.57 Wh kg(-1) and from 0.33 to 2.31 kW kg(-1), respectively. This simple and scalable inverse-ordered method is capable to fabricate CNT sheets in various forms, allowing fast trials on various applications.

Biodegradability and platelets adhesion assessment of magnesium-based alloys using a microfluidic system

Magnesium (Mg)-based stents are extensively explored to alleviate atherosclerosis due to their biodegradability and relative hemocompatibility. To ensure the quality, safety and cost-efficacy of bioresorbable scaffolds and full utilization of the material tunability afforded by alloying, it is critical to access degradability and thrombosis potential of Mg-based alloys using improved in vitro models that mimic as closely as possible the in vivo microenvironment. In this study, we investigated biodegradation and initial thrombogenic behavior of Mg-based alloys at the interface between Mg alloys’ surface and simulated physiological environment using a microfluidic system. The degradation properties of Mg-based alloys WE43, AZ31, ZWEK-L, and ZWEK-C were evaluated in complete culture medium and their thrombosis potentials in platelet rich plasma, respectively. The results show that 1) physiological shear stress increased the corrosion rate and decreased platelets adhesion rate as compared to static immersion; 2) secondary phases and impurities in material composition induced galvanic corrosion, resulting in higher corrosion resistance and platelet adhesion rate; 3) Mg-based alloys with higher corrosion rate showed higher platelets adhesion rate. We conclude that a microfluidic-based in vitro system allows evaluation of biodegradation behaviors and platelets responses of Mg-based alloys under specific shear stress, and degradability is related to platelets adhesion.