Asymmetric Membrane Structure Research Articles

A study of the structure of micro and ultra filtration membranes: The Voronoi tessellation as a stochastic model to simulate the structure

A stochastic model is presented to describe the structure and pore geometric properties of certain cellular micro and ultra filtration membranes. The model is based on a Voronoi tessellation, this is a contiguous space-exhaustive division of three-dimensional (3D) space. It is developed from a Poisson distribution of points each of which represents the nucleus of a cell which when developed coincides with an individual pore within the membrane space. The model is first applied to statistically homogeneous isotropic media and the statistics of the tessellation are computed and compared with the limited published data available. By applying first a constraint function and then a linear affine transformation to the tessellation a model is produced to simulate the structure of an anisotropic microfiltration membrane, cellulose acetate. The results computed from the model are compared with data obtained from image analysis of scanning election micrographs, SEM, of an actual membrane. Nonlinear transformations of the basic tessellation are applied to produce simulations of asymmetric cellular structures. The computed results are again compared with SEM image analysis data of polyether sulphone membranes. By applying a range of transformations the basic model can be used to simulate the structure of cellular homogeneous isotropic, anisotropic and asymmetric membranes. Furthermore, by using a combination of these the structure of composite membranes can also be modelled. Being based on statistical geometry the pore structure, shape, volume, cross-section area and perimeters are described by distribution functions and are not simply based on a mean length dimension implicit in all deterministic models which are used extensively in membrane research and application work. The implications to problems such as predicting the flux — transmembrane pressure characteristics, flux decline during filtration and backflashing are discussed.

Evaluation of the structure of asymmetric hollow-fiber dialysis membranes by dye adsorption

To develop high performance dialysis membranes, the asymmetric structure should be positively accepted and the relationship between asymmetric structure and diffusive permeability should be further clarified. A little information on the asymmetric structure can be obtained from SEM observation. The objective of the present study is to propose a new method for evaluating asymmetric hollow-fiber dialysis membranes by dye adsorption. Adsorption experiments were carried out using mini-modules composed of test membranes (AM-SD-M, PS-UW, PEPA, PAN-DX, PAN-CX2) of which the inside was packed with paraffin to make solutes diffuse only from the outside inward, or of which the outside was packed with paraffin to make solutes diffuse only from the inside outward. The amount of solutes (cytochrome C, Evans blue, Congo red, ethidium bromide) transferred into the membranes was plotted as a function of time. In membranes of asymmetric structure the shape of the transfer rate curves differed with the direction of transfer, and a difference in the rate of solute transfer was produced which is due to the asymmetric structure of the membrane. Further, depending on the solute size and on the membrane, the directional difference in the transfer rate curves appeared either in the middle portion or the initial portion of the curves. It is concluded that the adsorption technique can be used to evaluate the asymmetric structure and diffusive permeability.

Defects in actin-cap formation in Vav-deficient mice implicate an actin requirement for lymphocyte signal transduction

Background: Antigen-receptor interactions on lymphocytes result in local clustering of actin, receptors and signaling molecules into an asymmetric membrane structure termed a cap. Although actin polymerization is known to be required, the mechanisms underlying cap formation are unclear. We have studied the events underlying cap formation using mice bearing a null mutation in vav (vav−/−), a gene that encodes a guanine-nucleotide exchange factor for the GTPase Rac.Results: Lymphocytes from vav−/− mice failed to form T-cell receptor caps following activation and had a defective actin cytoskeleton. The vav−/− T cells were deficient in interleukin-2 (IL-2) production and proliferation, and the peak of Ca2+ mobilization was reduced although of normal duration. Activation of Jun N-terminal kinase or stress-activated kinase (JNK or SAPK) and mitogen-activated protein kinase (MAPK) and the induction of the transcription factor NF-ATc1 and egr-1 genes was normal. Despite the reduced Ca2+ mobilization, translocation of cytoplasmic NF-ATc to the nucleus was normal, reflecting that the lower levels of Ca2+ in vav−/− cells were still sufficient to activate calcineurin. Treatment of lymphocytes with cytochalasin D, which blocks actin polymerization, inhibited cap formation and produced defects in signaling and IL-2 transcriptional induction in response to antigen-receptor signaling that were nearly identical to those seen in vav−/− cells. In transfection studies, either constitutively active Vav or Rac could complement constitutively active calcineurin to activate NF-AT-dependent transcription.Conclusions: These results indicate that Vav is required for cap formation in lymphocytes. Furthermore, the correlation between cap formation, IL-2 production and proliferation supports the hypothesis that an actin-dependent pathway is a source of specialized growth regulatory signals.

Preparation of silica-based microporous inorganic gas separation membranes

Fully inorganic silica-based membranes have been prepared using sol-gel procedures starting from sodium silicate solutions. An asymmetric membrane structure is obtained by multiple deposition of colloidal silica sols onto commercial ceramic supports. The microporous structure of the silica which forms the active top layer remains stable up to 873 K and is characterised by a mean pore diameter of 1.6 nm, a narrow pore size distribution and a porosity of 35%. The continuous silica top layer of the membrane was analysed by SEM and checked by gas permeation measurements. The permeabilities of CH 4, N 2 and CO 2 were measured for membranes deposited onto zirconia supports as a function of temperature. The preliminary experimental results indicate reproducible and relatively high fluxes as well as a transport mechanism which basically follows Knudsen flow with contributions from surface and microporous diffusion.

Preparation and structure of cross‐linked asymmetric membranes based on polystyrene and divinylbenzene

AbstractAsymmetric polystryene‐based membranes were prepared by photocross‐linking with divinylbenzene (DVB) combined with phase‐inversion steps. The primary variables examined were original polystrene concentration in the membrance casting solution and degree of cross‐linking. The effects of these upon membrane morphology and reverse osmosis performance were determined. At higher DVB concentration, skin was formed at the top due to phase separation and at the bottom due to preferential polymer absorption at the support interface. An attempt was made to increase membrane ductility by incorporating low levels of liquid polybutadiene or styrene‐butadiene block copolymer, but no significant change was measured. © 1994 John Wiley & Sons, Inc.

Evaluation of membranes for use in on-line cell separation during mammalian cell perfusion processes.

In this study two microporous hollow fibre membranes were evaluated for their use as cell retention device in continuous perfusion systems. A chemically modified permanent hydrophillic PTFE membrane and a hydrophilized PP membrane were tested. To investigate the filtration characteristic under process conditions each membrane was tested during a long term perfusion cultivation of a hybridoma cell line. In both cultivations the conditions influencing membrane filtration (e.g. transmembrane flux) were kept constant. Filtration behaviour was investigated by monitoring transmembrane pressure and protein permeability. Transmembrane pressure was measured on-line with an autoclavable piezo-resistive pressure sensor. Protein permeability was determined by quantitative evaluation of unreduced, Coomassie stained SDS-PAGE. The membrane fouling process influences the filtration characteristic of both membranes in a different way. After fermentation the PP membrane was blocked by a thick gel layer located in the big outer pores of the asymmetric membrane structure. The hydraulic resistance was higher but the protein permeability was slightly better than of the PTFE membrane. For this reason the PP membrane should be preferred. On the other hand, transmembrane pressure decreases slower when the PTFE membrane is used, which favours this membrane for long term cultivations, especially when low molecular weight proteins (< 30 KD) are produced.

Cell surface changes during aging of human bladder epithelium examined by lectin probes and SEM

Ultrastructural changes have been shown to occur in the urinary bladder epithelium (urothelium) during the life span of humans. With increasing age, the luminal surface becomes more flexible and develops simple microvilli-like processes. Furthermore, the specialised asymmetric structure of the luminal plasma membrane is relatively more prominent in the young than in the elderly. The nature of the changes at the luminal surface is now explored by lectin-mediated adsorption visualised by scanning electron microscopy (SEM).Samples of young adult (21-31 y old) and elderly (58-82 y old) urothelia were fixed in buffered 2% glutaraldehyde for 10 m and washed with phosphate buffered saline (PBS) containing Ca++ and Mg++ at room temperature. They were incubated overnight at 4°C in 0.1 M ammonium chloride in PBS to block any remaining aldehyde groups. The samples were then allowed to stand in PBS at 37°C for 2 h before incubation at 37°C for 30 m with lectins. The lectins used were concanavalin A (Con A), wheat germ agglutinin (WGA), phytohaemagglutinin (PHA) and pokeweed mitogen (PWM) at a concentration of 500 mg/ml in PBS at pH 7.A.

Surface tension considerations for membrane casting systems

AbstractSurface energy considerations in the context of membrane preparation are examined. The viability of a membrane‐casting system can be successfully assessed with pertinent surface tension data. It is illustrated how surface tension data for multicomponent casting solutions can be used to resolve the surface structure of asymmetric reverse osmosis membranes.

Formation of membranes by means of immersion precipitation : Part I. A model to describe mass transfer during immersion precipitation

Equations and boundary conditions are derived for the isothermal diffusion processes in the coagulation bath and in the polymer solution after immersion of a cast (ternary) polymer solution into a (binary) coagulation bath. The mass transfer is expressed in terms of thermodynamic driving forces and frictional coefficients. The frictional coefficients in the ternary system are assumed to be interrelated through the Onsager reciprocal relations and to be related to the measurable frictional coefficients defined in the three limiting binary composition ranges. In combination with knowledge about the demixing processes which can take place in the polymer solution (liquid-liquid phase separation or solid-like aggregate formation), this model makes it possible to calculate the polymer concentration profile in the immersed film at the moment of demixing of the polymer solution as a function of several process parameters. The calculated concentration profile and its relation to the asymmetric structure of the ultimate membrane are presented in Part II 1 1 J. Membrane Sci., 34 (1987) 67. .

Ternary gas mixture separation in two‐membrane permeators

AbstractSingle‐stage separation of ternary gas mixtures of He, CO2, and N2 in a hollow‐fiber permeator that simultaneously houses two different types of membranes, cellulose acetate and silicone rubber, was investigated. The two membranes have reverse selectivities with respect to He and CO2. Such a two‐membrane permeator separates the feed into three streams, two permeates and one reject, each stream being enriched in a different component. The species permeabilities through each membrane were measured independently, and these values were utilized in a simulation that incorporates the fiber lumen pressure drops, the elastic deformation of the silicone capillaries, and the asymmetric structure of the CA membrane. Simulation results are compared with experimental data. Effects of flow pattern, relative permeation areas of the two membranes, permeate pressure ratios, feed composition, and membrane selectivities are studied. Advantages of the two‐membrane scheme over the conventional permeators with only one kind of membrane are discussed.

Ultrafiltration membranes — characterization methods

The experimental data obtained by application of two different techniques for measuring the porosity of asymmetric ultrafiltration membranes are reported. The measurements were carried out keeping the membranes in conditions similar to those utilized in most ultrafiltration processes. The techniques used were: (a) cut-off evaluation by means of proteins of known molecular weight; (b) pore distribution measurement by means of the so-called “bubble pressure” method. It has been verified that use of these methods supplies information on the structure of asymmetric membranes and also on the influence of the operative parameters. p]Limitations for the reliability of above techniques were taken into account, namely in connection with chemical properties of the membrane and experimental conditions. From the above considerations it is inferred that all techniques must be uniform in order to compare experimental data from different laboratories.

Lectin-resistant mutants of polarized epithelial cells.

Two lectin-resistant mutants derived from Madin Darby canine kidney cells, with constitutive alterations in the asparagine-linked carbohydrate moieties, retained the characteristic structural and functional epithelial polarity of the parental cells. A ricin-resistant cell line was unable to incorporate galactose-sialic acid into glycoproteins and, from the pattern of cross-resistance to other lectins, appears to be different from previously described lines resistant to this lectin: the mutation in a concanavalin A-resistant line results, probably, in the production of defective carbohydrate cores of glycoproteins. In spite of glycosylation defects which result in an increased electrophoretic mobility of many cellular glycoproteins, both mutants retained the typical asymmetric structure of the plasma membrane (microvilli on the apical surface, junctional elements on the basolateral surface), functional tight junctions, and unidirectional active transport of electrolytes and water. These results suggest that glycoproteins with terminal galactose-sialic acid moieties are not critically involved in the development and maintenance of polarity in epithelial cells. The mutant cells, particularly the ricin-resistant line, exhibited, however, morphological and electrophysiological changes which suggest a quantitative effect of the mutations on intracellular traffic of membranes and tight junction formation. The cell lines described in this paper, the first lectin-resistant mutants of epithelial lineage, should prove useful tools for studying the peculiarities of glycosylating pathways in polarized cells.

Lectin-resistant mutants of polarized epithelial cells.

Two lectin-resistant mutants derived from Madin Darby canine kidney cells, with constitutive alterations in the asparagine-linked carbohydrate moieties, retained the characteristic structural and functional epithelial polarity of the parental cells. A ricin-resistant cell line was unable to incorporate galactose-sialic acid into glycoproteins and, from the pattern of cross-resistance to other lectins, appears to be different from previously described lines resistant to this lectin: the mutation in a concanavalin A-resistant line results, probably, in the production of defective carbohydrate cores of glycoproteins. In spite of glycosylation defects which result in an increased electrophoretic mobility of many cellular glycoproteins, both mutants retained the typical asymmetric structure of the plasma membrane (microvilli on the apical surface, junctional elements on the basolateral surface), functional tight junctions, and unidirectional active transport of electrolytes and water. These results suggest that glycoproteins with terminal galactose-sialic acid moieties are not critically involved in the development and maintenance of polarity in epithelial cells. The mutant cells, particularly the ricin-resistant line, exhibited, however, morphological and electrophysiological changes which suggest a quantitative effect of the mutations on intracellular traffic of membranes and tight junction formation. The cell lines described in this paper, the first lectin-resistant mutants of epithelial lineage, should prove useful tools for studying the peculiarities of glycosylating pathways in polarized cells.

Studies on syntheses and permeabilities of special polymer membranes: 38. Formation mechanism of finger-like cavities in membranes from cellulose nitrate and single solvent

The relationship between the permeation characteristics of an aqueous polymer solution and the asymmetric structure of cellulose nitrate membranes was investigated under various conditions. The conditions and mechanism of the formation of finger-like cavities in cellulose nitrate membranes are discussed in some detail.

The structure and performance of aromatic polyamide type asymmetric RO membranes.

The structure and performance of aromatic polyamide type asymmetric RO membranes.

Studies on syntheses and permeabilities of special polymer membranes. 40. Formation conditions of finger-like cavities of cellulose nitrate membranes

The permeation characteristics for polymer aqueous solution and asymmetric structure of cellulose nitrate membranes, in particular the finger-like cavity in the back sponge layer of these membranes, were investigated by changing the preparation conditions of the membranes— the casting solvent, evaporation period, and gelation medium. The formation of the finger-like cavity was dependent on absorption of a water molecule into the casting mixture from the atmosphere during the solvent evaporation process and the exchange velocity between the casting solvent and the gelation medium. The mechanism and conditions for the formation of a finger-like cavity in cellulose nitrate membranes were discussed.

Assessment of Rhodopseudomonas sphaeroides chromatophore membrane asymmetry through bilateral antiserum adsorption studies.

The asymmetric structure of the Rhodopseudomonas sphaeroides chromatophore membrane was examined in detail by crossed immunoelectrophoresis techniques. Because these methods are quantitative and allow increased resolution and sensitivity, it was possible to analyze simultaneously the relative transmembrane distribution of a number of previously identified antigenic components. This was demonstrated by analysis of immunoglobulin samples that were adsorbed by preincubation with either isolated chromatophores or osmotically protected spheroplasts. The photochemical reaction center, the light-harvesting bacteriochlorophyll a-protein complex, the L-lactate dehydrogenase, and reduced nicotinamide adenine dinucleotide dehydrogenase (EC 1.6.99.3) were found to be exposed on the chromatophore surface (cytoplasmic aspect of the membrane within the cell). Other antigenic components were found to be exposed on the surface of spheroplasts (periplasmic aspect of the in vivo chromatophore membrane). Antigens with determinants expressed on both sides of the chromatophore membrane were also identified. Charge shift crossed immunoelectrophoresis confirmed the suggested amphiphilic character of the pigment-protein complexes and identified several additional amphiphilic membrane components.

The asymmetric unit membrane (AUM) structure in the lumenal plasma membrane of urothelium: The effect of trypsin on the integrity of the plaques

The lumenal plasma membrane of most mammalian urothelia is distinguished morphologically both in situ and in vitro by regions (plaque) of thickened membrane structure. Within plaque regions are regular arrays of hexagonally shaped (11.4 nm) particles which project from the free surface into the lumen, imparting an asymmetry to the unit membrane structure. When the urothelial lumenal surface is exposed to the action of trypsin, a symmetric unit membrane structure is observed along the entire length of the lumenal plasma membrane. Thickened plaque regions can no longer be distinguished from interplaque regions. Negatively stained preparations reveal that after trypsin treatment many pieces of lumenal plasma membrane are stripped of particulate structures. Hexagonally shaped particles, freed from the membrane continuum by trypsin, can be recovered and visualized in the supernatant fraction. Freeze-fractured membranes after trypsin treatment suggest that only the portion of the plaque particle protruding from the surface is removed, and a hydrophobic core of the plaque particle remains within the membrane interior. In correlation with the action of trypsin on the asymmetric unit membrane (AUM), there is a loss of the scalloped profile normally characterizing the lumenal plasma membrane so that membrane pieces after trypsinization take on a more linear ( in situ ) or more rounded ( in vitro ) shape. In essence, the lumenal plasma membrane as a whole loses its distinctive morphology and reverts after exposure to trypsin to a membrane system indistinguishable in form from many cell types.

Asymmetric structure of the purple membrane.

There is both functional and structural evidence that bacteriorhodopsin is oriented asymmetrically across the purple membrane of Halobacterium halobium. To assess the degree of asymmetry, the x-ray diffraction data from the membrane have been analyzed for possible electron-density profiles. A recent theory predicts that only a limited number of profiles are consistent with the continuous diffraction data, and two possible profiles have been found. Both profiles indicate that the protein molecules span a lipid bilayer in the membrane. Both profiles are asymmetric; there are more lipid molecules in one half of the membrane than in the other, and the bacteriorhodopsin molecule shows a slight complementary asymmetry.

A study of the structure of reverse osmosis membranes by means of the scanning electron microscope

SUMMARYReverse osmosis, or hyperfiltration, is a desalination process that uses membranes through which brackish or sea water is passed under pressure. The desalination properties of the cellulose acetate reverse‐osmosis membranes are due to their particular structure.A study was made of the structure of the Loeb‐Sourirajan type cellulose acetate membranes by means of the scanning electron microscope (SEM). The specimens were prepared as follows: the samples were freeze‐dried and then fractured by applying a bending stress at liquid nitrogen temperature.Different results were obtained depending on which of the two surfaces of the membrane had been subject to bending stress. The scanning micrographs of membrane cross‐sections confirmed (Gittens et al., 1973), a three‐layer structure of the membranes: a first dense layer which is responsible for desalination, a second layer, and a porous sub‐layer which is a very good natural support for the dense one. The asymmetric morphology of the membrane was correlated with its particular transport properties. A study was also made of the influence that the conditions of preparation exerted on the structure of cellulose acetate asymmetric membranes; the micrographs confirmed that the evaporation step has a marked effect on the membrane morphology. The technique has been extended to observe the recently perfected, anisotropic polyamide membranes.