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PubMed2006

Carbide-derived carbons: a comparative study of porosity based on small-angle scattering and adsorption isotherms.

Porous carbons have received much attention recently for potential applications in energy generation and storage, molecular sieving, and environmental remediation. Property optimization for specific applications rests largely on controlling the volume, size, and shape of the pores at the synthetic level. Direct atom-scale experiments which might accurately and reliably measure these quantities are problematic, so indirect methods such as gas sorption are generally employed. Here we apply a second indirect method, small-angle X-ray scattering (SAXS), to study porosity in carbide-derived carbons (CDC). The results qualitatively confirm and reinforce model-dependent conclusions drawn from gas sorption isotherms. In particular, both techniques indicate the onset of broad polydispersity under the same processing conditions for particular porous carbon materials.

Journal Article↗
PubMed2005

Characterization of porosity in organic and metal-organic macrocycles by hyperpolarized 129Xe NMR spectroscopy.

Hyperpolarized (129)Xe NMR spectroscopy is used to establish the solid-state porosity of shape-persistent macrocycles with either an organic or metal-organic framework. These studies show that even upon removal of cocrystallized solvent molecules, the macrocycles maintain a porous or channeled structure. The technique can provide valuable information about systems for which X-ray crystallographic analysis is not feasible. [structure: see text]

Journal Article↗
PubMed1997

Fabrication of hydroxyapatite ceramic with controlled porosity.

Porous hydroxyapatite (HAp) ceramics were fabricated using poly vinyl butyral (PVB) as a porosifier. The effects of preparation conditions involving PVB particle size, sintering time, and forming pressure (die-pressing technique) on the resultant pore size/structure as well as the pore size distribution were investigated. The experimental results showed that the HAp ceramics with controlled pore characteristics such as pore volume fraction, pore size and pore structure are achievable. It provides the possibility to design HAp ceramics with diverse porosities simulating that of natural bone.

Journal Article↗
PubMed2005

NMR measurements of hyperpolarized 3He gas diffusion in high porosity silica aerogels.

Hyperpolarized 3He is used to non-destructively probe by NMR the structure of custom-made and commercial silica aerogels (97% and 98.5% porous). Large spin-echo signals are obtained at room temperature and very low magnetic field (2 mT) even with small amounts of gas. Attenuation induced by applied field gradients results from the combined effects of gas diffusion and confinement by the porous medium on atomic motion. Nitrogen is used as a buffer gas to reach equivalent 3He pressures ranging from 5 mbars to 3.5 bars. The observed pressure dependence suggests a nonuniform structure of the aerogels on length scales up to tens of micrometers. A description by broad phenomenological distributions of mean free paths is proposed, and quantitatively discussed by comparison to numerical calculations. The investigated aerogel samples exhibit different effective diffusion characteristics despite comparable nominal porosities.

Journal Article↗
PubMedDate not supplied

Effect of well defined dodecahedral porosity on inflammation and angiogenesis.

Porosity is an important factor in the healing of prosthetic devices. To better understand this phenomenon, porous polyurethane scaffolds were produced by a variation of the phase inversion/porogen extraction technique in which a prepacked column of spherical porogen particles was infiltrated with a polymer solution before polymer precipitation and porogen extraction. Scaffolds contained pores of well defined shape (approaching open faced pentagonal dodecahedra), narrow size distributions (66.1 +/- 1.3 microm, 84.2 +/- 1.7 microm, and 156.9 +/- 1.2 microm) and high interconnectivity (interconnecting windows of 30.1 +/- 0.8 microm, 41.9 +/- 1.5 microm, and 76.4 +/- 2.0 microm, respectively). A high degree of accessible macroporosity (>80%) could be achieved while limiting the mostly inaccessible microporosity to below 2%. The neovascularization and inflammatory responses to the scaffolds were evaluated in the subcutaneous rat model for 4 weeks. The inflammatory response index and foreign body giant cell index could be reduced by 56% (p < 0.05) and 21% (p < 0.02), respectively, when the pore size was increased from 66 microm to 157 microm, whereas the vascularization index and arteriolar index remained unchanged. Thus, a significant decrease in inflammatory response could be achieved without adversely affecting the degree of neovascularization by increasing the size of the pores.

Animals↗
PubMed1996

An in-situ method to measure a soil's undisturbed pore gas radon concentration, diffusion length for radon and air filled porosity.

Previous work has shown that work has shown that for soils of insignificant permeability (soils with permeability less than about 10(-12) m2) the important soil parameters for characterizing radon mobility in the soil are the soil's steady-state pore gas random concentration at depth (Cs), the soil's bulk diffusion length for radon (L) and the soil's air filled porosity (pa). Existing methods to measure these parameters have been based wholly or in part on measurements of soil samples taken to a laboratory for analysis. The drawbacks of this approach are twofold: (1) since soil structure can be quite heterogeneous, the sample may not have characteristics indicative of the site as a whole, and (2) since the parameters are dependent on soil structure and the soil structure of the sample may be changed in th e process of acquiring the sample, one may be changing the parameters that one is trying to measure. These problems can be avoided by using a totally in-situ method to measure Cs, L, and pa. This paper describes a totally in-situ method for simultaneously measuring the important soil parameters, based on measurements of the radon concentration as a function of time for the gas in a cavity in the soil.

Diffusion↗
PubMed2006

Use of a void network model to correlate porosity, mercury porosimetry, thin section, absolute permeability, and NMR relaxation time data for sandstone rocks.

The Pore-Cor void network model is used to construct stochastic realizations of the void structures of five sandstone samples of varying lithography. A close match was achieved to experimental porosity and mercury intrusion curves. The samples were resin impregnated and the fragments of voids revealed in thin sections photographed by backscatter electron microscopy at two magnifications. The sizes of these pore fragments matched those derived from a simulated microtoming of the network model much more closely than the sizes derived from the traditional capillary bundle approximation. Absolute permeabilities of the network were calculated by finding the flow capacity of the entire flow network, based on parametrized Navier Stokes equations with Klinkenberg correction, applied to each pore-throat-pore arc. A match to the experimental trend was obtained, although the network model considerably underestimated the experimental values. The results were also compared with the semiempirical equations of Thomson et al. and Kozeny and Carmen modified to accept thin section image analysis. Finally, the simulated pore and throat size distributions were compared to proton NMR transverse (T2) spin-echo relaxation times. Although the shapes of the distributions differed markedly, the mean values trended together. The capillary bundle approximation, however, gave a poor match to the NMR data.

Journal Article↗
PubMed2004

Phase diagram of superfluid 3He in 99.3% porosity aerogel.

We report on continuous-wave NMR measurements of the energy gaps of the A-like and B-like superfluid phases of 3He at 28.4 mT confined to a 99.3% porosity silica aerogel. The gaps are suppressed by the presence of the aerogel in a temperature-independent manner, but the suppression is considerably stronger than expected from the suppression of T(c). We then use our measurements to calculate the free energy ratio between the A-like and B-like phases. The equilibrium AB transition temperature, derived from where this ratio reaches unity, is consistent with previous measurements of the initial displacement of the pinned AB interface on warming. On this basis, we present for the first time the equilibrium phase diagram of the A-like and B-like phases of superfluid 3He in aerogel.

Journal Article↗
PubMed2004

Quantification of micrometre-sized porosity in quasicrystals using coherent synchrotron radiation imaging.

Phase contrast radiography has been applied to investigate porosity in the bulk of quasicrystals. The pores act on X-rays as lenses, leading to drastic changes in the phase contrast images of pores. Depending on the recording conditions and pore features, different imaging regimes are accessed. The variation of the contrast makes the direct characterization of pores from image data, in terms of a precise determination of their size and volume, unreliable. The contrast in Fresnel diffraction images of pores was thus investigated with the support of numerical simulations. A phase retrieval procedure was carried out in order to obtain the correct information from the pore images.

Journal Article↗
PubMed1985

Extraction of glycolytic enzymes: myo-inositol as a marker of membrane porosity.

Detergent extraction of brain slices and mouse fibroblast 3T3 cells was performed to determine rates and relative amounts of extraction of inositol versus the glycolytic enzymes. The two detergents, Triton X-100 and Brij 58, led to similar results for extraction of myo-inositol. The extraction of enzymes from brain slices or cells varied with the detergent. In brain slices, a buffered solution containing 0.2% of the detergent Brij 58 led to the extraction of 85% of the inositol before 3% of the aldolase or before 37% of either lactate dehydrogenase or triose phosphate isomerase was extracted. In contrast, with 0.1% Triton X-100 in isotonic phosphate-buffered saline, when 70% of the inositol was extracted, 33% of the aldolase and 48% of the triose phosphate isomerase were extracted. Lesser amounts of aldolase and glyceraldehyde phosphate dehydrogenase were extracted than most of the other glycolytic enzymes under all conditions, implying that these enzymes may be interacting with non-extractable subcellular components. In 3T3 cells, both detergents were of similar effectiveness for inositol extraction. Triton X-100 caused 89% of the inositol to be released and Brij 58 caused 84% to be released. With the enzymes, Brij 58 caused between 15 and 38% extraction and Triton X-100 caused between 61 and 85% extraction of the different glycolytic enzymes. Thus Brij 58 was as effective as Triton X-100 in inositol extraction but not nearly as effective in glycolytic enzyme extraction. The results demonstrate that inositol leakage from tissues or cells is a better indicator of detergent-mediated alterations in membrane porosity than glycolytic enzyme leakage.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Hand-made valved conduit with high-porosity knitted graft and glutaraldehyde-treated autologous pericardial trileaflet valve.

In 10 patients with various congenital heart defects requiring reconstruction of right ventricle to pulmonary artery continuity, a new hand-made valved conduit was introduced. To avoid late stenosis with peel formation of the graft, high-porosity Golaski knitted graft was chosen. The graft was sealed by fibrin glue, and autologous pericardium was treated with glutaraldehyde and used to construct a trileaflet valve inside the graft. This hand-made graft was pliable with good conformability. The valve function was found good in the intermediate follow-up in 8 patients. This composite valved conduit may be a good alternative to the xenograft valved conduit when allograft conduit is not available.

Bioprosthesis↗
PubMedDate not supplied

Effects of plate luting on cortical vascularity and development of cortical porosity in canine femurs.

A 4.5 mm, six hole dynamic compression plate was applied to the lateral aspect of one femur in 16 dogs. The plate was luted with polymethylmethacrylate (PMMA) in eight dogs. Four dogs each from the luted and nonluted groups were euthanatized at weeks 5 and 10. Plated femurs were sectioned and processed for microradiography, microangiography, and correlated histology. At week 5, there was decreased vascularity in the outer one third of the cortex beneath luted plates and normal to increased vascularity in the inner two thirds of the cortex. Increased cortical porosity and a decreased percentage of osteocyte-filled lacunae were found in the cortex beneath luted plates at week 5. At week 10, vascularity was increased throughout the cortex under luted plates. Under nonluted plates, there was normal to increased vascularity at both time periods. Luting a bone plate with PMMA decreased vascularity to the outer one third of the cortex beneath the plate at week 5, and resulted in accelerated resorption of bone.

Animals↗
PubMed1977

The influence of the refractory investment on the development of porosity in cast structures.

Four series of standard tensile specimens were produced by casting into moulds composed of silica bonded, gypsum bonded, and phosphate bonded refractories. Two nickel base alloys were employed, one a standard dental casting alloy and the other a specially formulated ductile material. Subsequently, all specimens were subjected to mechanical testing, and radiographic and metallographic evaluation, in conjunction with porosity determinations, in order to ascertain how the nature of the refractory investment influences the structural quality of the castings produced.

Dental Alloys↗
PubMed1974

Porosity of the yeast cell wall and membrane.

The limiting sizes of molecules that can permeate the intact cell wall and protoplast membrane of Saccharomyces cerevisiae were determined from the inflection points in a triphasic pattern of passive equilibrium uptake values obtained with a series of inert probing molecules varying in molecular size. In the phase identified with the yeast protoplast, the uptake-exclusion threshold corresponded to a monodisperse ethylene glycol of molecular weight = 110 and Einstein-Stokes hydrodynamic radius (r(ES)) = 0.42 nm. In the cell wall phase, the threshold corresponded to a polydisperse polyethylene glycol of number-average molecular weight ( M(n)) = 620 and average radius (r(ES)) = 0.81 nm. The third phase corresponded to complete exclusion of larger molecules. The assessment of cell wall porosity was confirmed by use of a second method involving analytical gel chromatographic analyses of the molecular weight distribution for a single polydisperse polyglycol before and after uptake by the cells, which indicated a quasi-monodisperse threshold for the cell wall of M(n) = 760 and r(ES) = 0.89 nm. The results were reconciled with two situations in which much larger protein molecules previously have been reported able to penetrate the yeast cell wall.

Biological Transport↗