S. Trigwell

Plasma treatment of polyurethane coating for improving endothelial cell growth and adhesion

The advantage of helium plasma treatment in enhancing endothelial cell growth and adhesion on polyurethane film coated on glass substrate is demonstrated with experimental data. Human coronary artery endothelial cell (HCAE) growth and attachment was studied on (1) bare glass substrate, used as control, (2) coated glass, with and without helium plasma treatment and (3) collagen-treated polyurethane-coated glass substrates. The untreated polyurethane film surface was rough (RMS = 690 nm) and highly hydrophobic (contact angle = 90°). Cell growth on the untreated polyurethane surface was poor (cell concentration ≈ 3750/cm2) compared to glass surface (cell concentration ≈ 17 665/cm2). The atmospheric helium plasma treatment of the polyurethane film resulted in oxidation of the surface, a slight increase in roughness (RMS = 735 nm) and a significant drop in hydrophobicity (contact angle = 79°). The critical surface tension (γ C) of polyurethane film was also increased by 2 dynes/cm due to helium plasma treatment. These changes resulted in enhanced HCAE cell growth in polyurethane film (cell concentration ≈ 16 230/cm2) compared to the untreated polyurethane film. The cell growth was also comparable to cell growth on a glass surface (17 665/cm2) and the collagen-treated polyurethane film surfaces (cell concentration ≈ 21 645/cm2), respectively. Moreover, the strength of cell attachment on a plasma-treated surface (cell retention R = 89%) under laminar flow was significantly higher than that on a glass surface (R = 71%). While the collagen-treated polyurethane surface had the highest number of HCAE cells, the cell adhesion was found to be poor (R = 42%) compared to that of a plasma-treated surface. Thus, the overall performance of the plasma-treated polyurethane film surface on endothelial cell growth was better than other substrates studied here.

Corrosion resistance of polyurethane-coated nitinol cardiovascular stents.

Corrosion of metal stents implanted inside an artery can have two adverse effects: (1) tissue reaction and possible toxic effects from the metal ions leaching out of the stent, and (2) loss of mechanical strength of the stent caused by corrosion. The corrosion resistance of Nitinol (Nickel-Titanium) stents and its modulation with different film thickness of polymer coating was studied against an artificial physiological solution using a Potentiostat/Galvanostat and an electrochemical corrosion cell. The corrosion rate decreased rapidly from 275 μm/year for an uncoated surface down to less than 13 μm/year for a 30 μm thick polyurethane coating. Stainless steel (316L) and Nitinol both contain potentially toxic elements, and both are subject to stress corrosion. Minimization of corrosion can significantly reduce both tissue reaction and structural degradation.

Photovoltaic devices based on high density boron-doped single-walled carbon nanotube/n-Si heterojunctions

A simple and easily processible photovoltaic device has been developed based on boron-doped single-walled carbon nanotubes (B-SWNTs) and n-type silicon (n-Si) heterojunctions. The SWNTs were substitutionally doped with boron atoms by thermal annealing, in the presence of B2O3. The samples used for these studies were characterized by Raman spectroscopy, thermal gravimetric analysis, transmission electron microscopy, and x-ray photoelectron spectroscopy. The fully functional solar cell devices were fabricated by airbrush deposition that generated uniform B-SWNT films on top of the n-Si substrates. The carbon nanotube films acted as exciton-generation sites, charge collection, and transportation while the heterojunctions formed between B-SWNTs and n-Si acted as charge dissociation centers. The current-voltage characteristics in the absence of light and under illumination, as well as optical transmittance spectrum are reported here. It should be noted that the device fabrication process can be made amenable t...

Hydrogen Storage in Carbon-Based Nanostructured Materials

Carbon nanostructures represent a revolution in science and hold the potential for a large range of applications because of their interesting electrical, mechanical, and optical properties. Multiwall carbon nanotubes and carbon nanofibers of herringbone formation were grown by chemical vapor deposition on different catalysts from a number of hydrocarbon sources. After the total or particle removal of the catalyst system, the carbon nanostructures were analyzed for hydrogen uptake. Six samples of nanofibers grown on a Pd-based catalyst system (with a surface area of 425–455 m2/g) were controlled oxidized in air, such that they had different ratios of Pd/C varying from 0.05 to 0.9 mole ratio. The hydrogen uptake experiments were performed volumetrically in a Sievert-type installation and showed that the quantity of desorbed hydrogen (for pressure intervals ranging from 1 to 100 bars) by the carbon nanostructures free of any metal catalyst particles was between 0.04 and 0.33% by weight. For the samples of nanofibers that contained Pd in various Pd/C ratios, palladium revealed catalytic properties and supplied atomic hydrogen at the Pd/C interface by dissociating the H2 molecules. The results show a direct correlation between the Pd/C ratio and the quantity of hydrogen absorbed by these samples. A saturation value of about 1.5 wt.% was reached for a high ratio of about 1:1 of Pd/C. The multiwall carbon nanotubes grown on a Fe:Co:CaCO3 catalytic system and purified by acid cleaning and air oxidation showed a hydrogen uptake value of 0.1 to 0.2 wt.%.

Use of non-thermal atmospheric plasmas to reduce the viability of Bacillus subtilis on spacecraft surfaces

Atmospheric pressure glow-discharge (APGD) plasmas have been proposed for sterilizing spacecraft surfaces prior to launch. The advantages of APGD plasmas for the sterilization of spacecraft surfaces include low temperatures at treatment sites, rapid inactivation kinetics of exposed microbial cells, physical degradation and removal of microbial cells, physical removal of organic biosignature molecules, and short exposure times for the materials. However, few studies have tested APGD plasmas on spacecraft materials for their effectiveness in both sterilizing surfaces and removal of microbial cells or spores. A helium (He)+oxygen (O2) APGD plasma was used to expose six spacecraft materials (aluminum 6061, polytetrafluoroethylene (PTFE), polycarbonate, Saf-T-Vu, Rastex, and Herculite 20) doped with spores of the common spacecraft contaminant, Bacillus subtilis, for periods of time up to 6 min. Results indicated that greater than six orders of magnitude reductions in viability were observed for B. subtilis spores in as short of time as 40 s exposure to the APGD plasmas. Spacecraft materials were not affected by exposures to the APGD plasmas. However, Saf-T-Vu was the only material in which spores of B. subtilis adhered more aggressively to plasma-treated coupons when compared to non-plasma treated coupons ; all other materials exhibited no significant differences between plasma and non-plasma treated coupons. In addition, spores of B. subtilis were physically degraded by exposures to the plasmas beginning at the terminal ends of spores, which appeared to be ruptured after only 30 s. After 300 s, most bacteria were removed from aluminium coupons, and only subtle residues of bacterial secretions or biofilms remained. Results support the conclusion that APGD plasmas can be used as a prelaunch cleaning and sterilization treatment on spacecraft materials provided that the biocidal and cleaning times are shorter than those required to alter surface properties of materials.

Portable Free-Fall Electrostatic Separator for Beneficiation of Charged Particulate Materials

A portable free-fall electrostatic separator capable of analyzing gram quantities of charged powders is presented. Unlike a Faraday pail, in which only the net average charge-to-mass (Q/M) ratio of the particles sampled by the instrument is measured, an electrostatic separator is capable of separately measuring the charge-to-mass ratios of positively and negatively charged sampled powders. Thus, with an electrostatic separator it is possible to measure the mass fractions of powders that are charged with different polarities and the respective charge-to-mass ratios, along with the mass fraction of particles that are uncharged or charged below a threshold level. We describe a method of measuring the total charge of the collected particles in real time by incorporating an electrometer to integrate the current flowing through the collecting electrode to the high voltage power supply. In this manner, both the total charge and total mass of powder deposited on the two electrodes are measured in near real time, providing information on charge-to-mass ratio of the aerosol cloud sampled. Such real time measurements are often needed to analyze the electrostatic charging properties of small quantities of dispersed powder, particularly in such applications where the charge characteristics are of high importance.

Measurement of Surface Topography, Packing Density, and Surface Charge Distribution of an Electrostatically Deposited Powder Layer by Image Analysis of Fluorescent Latex Spheres

A novel method of nonintrusive measurement of surface profile, packing density, and surface charge distributions of a powder layer deposited on a substrate is reported. The method employs the deposition of electrostatically charged monodispersed fluorescent latex spheres (FLS), approximately 2 m in diameter, on the surface of: (1) the substrate before deposition, (2) the powder layer after deposition, and (3) the film formed by curing the powder layer. The surface topography in all cases was mapped using an epi-fluorescent microscope with a vertical resolution of - 2 m in the z axis and - 10 m in the x and y axes. An area of 1 cm 2 1 cm is scanned in 1 mm segments, providing approximately 100 data points per cm 2 for the surface topography. For each measurement of surface topography, the substrate was positioned on the microscope stage in a manner such that the reference points (x, y, and z) remained the same for all measurements of the substrate. The surface profiles, with respect to the same reference points, were plotted using Origin 6.0 software for 3D presentation of the topography. The method was also applied to map the surface charge density distribution of electrostatically charged surfaces. The FLS imaging method provides a new tool for examination of surface profiles, packing density, and charge distribution of powder layers on a microscopic scale not provided by optical or atomic force/electrostatic force microscopy (AFM/EFM). While AFM and EFM are very effective in providing similar information with nanometer resolution, they cannot be directly applied on a larger macroscopic scale to study powder layers and for a larger surface area (up to 1 cm 2 or greater) involving deposited particles in the range of 1-50 m in diameter. For AFM, the range in the z-axis is limited to - 3 m and the x-y scan area is limited to 100 m 2 100 m. The FLS method has a much wider range but it is operated manually; an automated scanning process is required for rapid measurement. A comparison of the FLS and EFM techniques as they apply to analyzing charge distribution on coal surfaces is presented.

Lunar and Martian Dust Dynamics

In order to support human and robotic explorations of Mars and beyond,the National Aeronautics and Space Administration (NASA) has undertaken lunar missions. One technological problem is the mitigation of dust hazards associated with both lunar and Mars missions. There are many factors that transformed lunar regolith into an ocean dust like volcanic eruption, meteorite impact, solar radiation and thermal cycling, solar plasma and bombardment by interplanetary dusts. A detailed understanding of lunar and Martian dust dynamics is needed for designing and implementing dust hazard mitigation methods to facilitate successful missions to the moon and Mars.

Electrostatic and Gravitational Transport of Lunar Dust in the Airless Atmosphere of the Moon

Since its formation about 4.6 billion years ago, the surface of the moon has become covered with a thick layer of fine dust which has been preserved in a pristine, ultrahigh vacuum condition without any atmosphere-related changes of its physical properties and chemical compositions. The lunar surface has no atmosphere. Yet, Apollo and surveyor missions observed there is an atmospheric glow in the horizon and streamers of dust arising from the surface and scattering solar radiation during the sunrise. Apollo astronauts saw fountains of lunar dust rising hundreds of kilometers above the surface. The Apollo landing sites showed that the moon is virtually an ocean of electrostatically charged dust containing fractured glass-like particles with a high adhesive property. We present here an analysis of electrostatic charging by the photoemission and solar plasma electrons based on the current understanding of surface dust and the environmental conditions of the moon. The photon- particle interactions causing the particles to be positively charged during the day negatively charged during the night by solar wind including the effect of the earths magnetosphere. The cyclical charging and charge neutralization processes are briefly discussed in view of the observed electrostatic levitation of the dust. As the moon rotates on its axis, the dividing boundary line between the illuminated (sunlit) and the shaded (dark) regions, called the terminator, across which the charge polarity of dust layer and the intensity and the electric field change, also rotates. The reversal of direction of the electric field along the dividing line and the rotational motion cause dust trajectories to cross the terminator. Possible inhalation hazards of the toxic surface reactive dust with sharp edges in the nano- and micrometer size ranges are considered to be high in view of the increased lung deposition efficiency caused by the electrostatic image and space charge forces. A clear understanding of the lunar dust dynamics is needed for developing effective dust hazard mitigation for future robotic and human missions to the moon.

Research needs in electrostatics for Lunar and Mars space missions

The paper gives an overview over the potential dust-related hazards for crew and equipment on the surfaces of Moon and Mars, the currently available technologies for dust characterization and dust mitigation and directions for research needed to make future manned space exploration safe and efficient for crew and equipment. Primary research needs for future Mars and Moon missions related to electrostatics include: (1) in-situ characterization of dust properties including particle size, particle charge and magnetic properties of particles, (2) dust mitigation strategies to protect mission equipment including solar cells, camera ports, optical mirrors, mechanical components and EVA suits, and (3) research to understand the specific health hazards coming from exposure (inhalation, ingest ion and skin contact) to Lunar and Martian dust and development of appropriate monitoring and protective devices for flight crews.