Capillary membrane shapes

Figure 2.1 Polymeric membrane shapes and cross-sectional structures. Tubular membranes are similar to flat sheet membranes because they are cast on a macroporous tube as support. Capillary membranes are hollow fibers with larger diameter, that is, >0.5 mm.

Four types of diffusion mechanisms can be utilized to effect separation in porous membranes. In some cases, molecules can move through the membrane by more than one mechanism. These mechanisms are described below. Knudsen diffusion gives relatively low separation selectivities compared to surface diffusion and capillary condensation. Shape selective separation can yield high selectivities. The separation factor for these mechanisms depends strongly on pore-size distribution, temperature, pressure, and interactions between the solute being separated and the membrane surfaces. 

The reactors are cylindrical in shape and can carry up to 30 mg of resin. Polymer sieves at the top and bottom of the cylinders serve for liquid feed and withdrawal. The array of reactors is attached to a capillary system allowing feed to either columns or rows. This distribution system is said to provide uniform charges to the various reactors. A specific detail of the reaction system is that mixing is achieved by pneumatic actuation using a fluoropolymer membrane (Figure 4.36). 

Fig. 6.10 Methods of preparation of bilayer lipid membranes. (A) A Teflon septum with a window of approximately 1mm2 area divides the solution into two compartments (a). A drop of a lipid-hexane solution is placed on the window (b). By capillary forces the lipid layer is thinned and a bilayer (black in appearance) is formed (c) (P. Mueller, D. O. Rudin, H. Ti Tien and W. D. Wescot). (B) The septum with a window is being immersed into the solution with a lipid monolayer on its surface (a). After immersion of the whole window a bilayer lipid membrane is formed (b) (M. Montal and P. Mueller). (C) A drop of lipid-hexane solution is placed at the orifice of a glass capillary (a). By slight sucking a bubble-formed BLM is shaped (b) (U. Wilmsen, C. Methfessel, W. Hanke and G. Boheim)...

Polymerization allows deoxygenated hemoglobin to exist as a semisolid gel that protrudes into the cell membrane, distorting RBCs into sickle shapes. Sickle-shaped RBCs increase blood viscosity and encourage sludging in the capillaries and small vessels. Such obstructive events lead to local tissue hypoxia and accentuate the pathologic process. 

An interesting feature of many cells is the permanent presence on the plasma membrane of flask-shaped regions termed caveolae. They are abundant in certain capillary endothelial cells, and appear to have a role in cholesterol binding, although many other functions have been suggested [62]. 

The major issue found in testing is the corrosion of the foam material and resultant contamination of the membrane. The high manufacturing cost of the metal or carbon foam with the required pore shape, size, and distribution also is a challenge. Further study and testing of the corrosion mechanism, selection of appropriate coating, a capillary process involved in the tiny pores, and related water retention are necessary to identify whether the new material and concept can be finally applied in the plate. 

The Vel data as a function of flow rate, Q, are shown for a 10 g/mol molecular weight polystyrene in Figure A. Both the Ubbelohde viscometric data and the membrane viscometer data are platted on the same graph for a 0.6 urn pore membrane at a low concentration of 100 ppm. The flow is Newtonian. The actual agreement of the capillary and membrane viscosities at low flow rates is always excellent when << Dj., and the concentration is extremely low. At small pore size, high concentrations, and high shear rates the flow can become non-Newtonian. The latter effects are only briefly discussed in this paper, but it is this effect that offers an oportunity to characterize the shape rather than the overall size. Even for a relatively large pore (0.6, Hi , membrane the shear rates vary from 100 s at E mi/Hr to 10 s at 200... 

There are differences in the ease of extravasation of macromolecules from the bloodstream into different tissues [14, 104, 105]. Capillaries in the liver, spleen, and bone marrow have incomplete basal membranes and are lined with endothelial cells which are not continuously arranged. Capillaries in the muscle have a somewhat tighter arrangement, and there is an almost impermeable barrier which isolates the central nervous system from circulating blood. The rate of glomerular filtration of macromolecules depends on their hydrodynamic radius, the threshold being approx. 45 A [106]. Structure of the macromolecule is of utmost importance, since shape, flexibility, and charge influence the penetration and possible readsorption in the tubular epithelia [100]. 

The membrane skeleton acts as an elastic semisolid, allowing brief periods of deformation followed by reestablishment of the original cell shape (reviewed by Bennett and Gilligan, 1993). Erythrocytes in the human bloodstream have to squeeze repeatedly through narrow capillaries of diameters smaller than their own dimensions while resisting rupture. A functional erythrocyte membrane is pivotal to maintaining the functional properties of the erythrocyte. This importance is apparent when examination is made of many hemolytic anemias, where mutation of proteins involved in the structure of the submembranous cytoskeleton, and its attachment to the lipid bilayer, result in a malformed or altered cytoskeletal architecture and a disease phenotype. 

Heavy proteinuria is caused by increased permeability of the glomerular capillary wall for macromolecules. Mainly the size, the charge, and the shape influence the passage of macromolecules through the glomerular capillary wall. The glomerular basement membrane and the slit diaphragm represent the main barriers for the filtration of macromolecules. 

Molecularly imprinted polymers with a variety of shapes have also been prepared by polymerizing monoliths in molds. This in situ preparation of MIPs was utilized for filling of capillaries [20], columns [21], and membranes [22, 23]. Each specific particle geometry however needs optimization of the respective polymerization conditions while maintaining the correct conditions for successful imprinting. It would be advantageous to separate these two processes, e.g., to prepare a molecularly imprinted material in one step, which then can be processed in a mold process in a separate step to result the desired shape. 

Thermoporometry. Thermoporometry is the calorimetric study of the liquid-solid transformation of a capillary condensate that saturates a porous material such as a membrane. The basic principle involved is the freezing (or melting) point depression as a result of the strong curvature of the liquid-solid interface present in small pores. The thermodynamic basis of this phenomenon has been described by Brun et al. [1973] who introduced thermoporometry as a new pore structure analysis technique. It is capable of characterizing the pore size and shape. Unlike many other methods, this technique gives the actual size of the cavities instead of the size of the openings [Eyraud. 1984]. 

Although Knudsen diffusion, shape selectivity, and molecular sieving play an important role in the separation of mixtures, the mechanisms which control the majority of the multicomponent separations in zeolite membranes are surface diffusion, and sometimes, capillary condensation. In addition, molecular simulations and modeling of M-S diffusion in zeolites [69,70] show that the slower moving molecules are also sped up in some mixtures [71,72] in the presence of fast-diffusing molecules and other times, slower molecules inhibit diffusion of faster molecules because molecules have difficulty passing one another in zeolite pores [73]. 

Osmometry is a technique for measuring the concentration of solute particles that contribute to the osmotic pressure of a solution. Osmotic pressure governs the movement of solvent (water in biological systems) across membranes that separate two solutions. Different membranes vary in pore size and thus in their ability to select molecules of different size and shape. Examples of biologically important selective membranes are those enclosing the glomerular and capillary vessels that are permeable to water and to essentially aU small molecules and ions, but not to large protein molecules. Differences in the concentrations of osmoticaUy active molecules that carmot cross a membrane cause those molecules that can cross the membrane to move to establish an osmotic equilibrium. This movement of solute and permeable ions exerts what is known as osmotic pressure. 

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