Functional synthetic membranes

With either type of dialysis, studies suggest that recovery of renal function is decreased in ARF patients who undergo dialysis compared with those not requiring dialysis. Decreased recovery of renal function may be due to hemodialysis-induced hypotension causing additional ischemic injury to the kidney. Also, exposure of a patient s blood to bioincompatible dialysis membranes (cuprophane or cellulose acetate) results in complement and leukocyte activation which can lead to neutrophil infiltration into the kidney and release of vasoconstrictive substances that can prolong renal dysfunction.26 Synthetic membranes composed of substances such as polysulfone, polyacrylonitrile, and polymethylmethacrylate are considered to be more biocompatible and would be less likely to activate complement. Synthetic membranes are generally more expensive than cellulose-based membranes. Several recent meta-analyses found no difference in mortality between biocompatible and bioincompatible membranes. Whether biocompatible membranes lead to better patient outcomes continues to be debated. 

Western blotting has become an important, modern technique for analysis and characterization of proteins. The procedure consists of, first, the electrophoretic transfer (blotting) of proteins from polyacrylamide gels to synthetic membranes. The transferred blots are then probed using immunological detection methods to identify proteins of specific structure and/or function. In this experiment, bovine serum will be fractionated by SDS-PAGE and the proteins blotted onto a nitrocellulose membrane. Serum glycoproteins will be identified by their specific interaction with the lectin concanavalin A. 

Figure 6.20 Purchase price in 2003 US dollars for ultrafiltration plants as a function of plant capacity. Data of Rogers corrected for inflation [20]. Reprinted from Synthetic Membrane Processes, A.N. Rogers, Economics of the Application of Membrane Processes, p. 454, G. Belfort (ed.), Copyright 1984, with permission from Elsevier...

The discussion of synthetic membranes can be structured in terms of the function or the structure of the membrane used in a particular application. For instance, one can consider whether a membrane is used to separate mixtures of gas molecules vs particles from liquids (function) vs whether the membrane structure is primarily microporous or dense (structure). In fact, function and stmcture are linked, but to facilitate the consideration of physical science issues related to membranes appropriate for this reference, emphasis on functional aspects are probably most appropriate. This approach reflects the fact that the use of a membrane generally involves one or more physical sci-... 

Another well-defined synthetic membrane is a planar bilayer membrane. This structure can be formed across a 1-mm hole in a partition between two aqueous compartments by dipping a fine paintbmsh into a membrane-forming solution, such as phosphatidyl choline in decane. The tip of the brush is then stroked across a hole (1 mm in diameter) in a partition between two aqueous media. The lipid film across the hole thins spontaneously into a lipid bilayer. The electrical conduction properties of this macroscopic bilayer membrane are readily studied by inserting electrodes into each aqueous compartment (Figure 12.14). For example, its permeability to ions is determined by measuring the current across the membrane as a function of the applied voltage. 

We now describe synthetic membranes in which the molecular-recognition chemistry used to accomplish selective-permeation is DNA hybridization. These membranes contain template-synthesized gold nanotubes with inside diameter of 12 nm, and a transporter DNA-hairpin molecule is attached to the inside walls of these nanotubes. These DNA-functionalized nanotube membranes selectively recognize and transport the DNA strand that is complementary to the transporter strand relative to DNA strands that are not complementary to the transporter. Under optimal conditions, single-base mismatch transport selectivity is obtained. 

It was much later, and after the aforementioned work of Rosaro and Kunin, in which it was proven that various synthetic membranes doped with different species could function as ion exchangers, and that these systems were used for quantitative work. Indeed, the properties of these membranes were of... 

Another issue relating to dialysis membranes is their ability to stimulate the immune system. If the dialysis filter membrane does not activate the complement system (C3a and C5a) when it comes in contact with the patient s blood, it is considered biocompatible. In the acute setting, the incidence of hypotension, fever, bronchoconstric-tion, and thrombocytopenia are lower in patients dialyzed with biocompatible filters. The most biocompatible dialyzers use a synthetic membrane of PS, PAN, or PMMA. Although not definitive, biocompatible membranes may be associated with fewer adverse events during dialysis, lower rates of hospitalization, reduced death rates, and slower declines in residual renal function. ... 

One of the main functions of the plasma membranes of living cells is to control the transport processes into and out of the cells of many substances and thus to regulate the composition of the intracellular fluid. The fluid usually contains solutes at concentrations which are quite different from their corresponding values in the bathing medium. This is achieved by the ability of the membrane to discriminate among various solutes so that some are allowed through, others are kept inside or outside the cell, and still others are carried actively. In addition, important processes such as oxidative metabolism, protein synthesis and several other synthetic processes are intimately connected with and dependent on membrane processes. In fact, continued existence of the cell is critically dependent on its having a functional plasma membrane. 

Thermal polymers of amino acids are able to make spherical and planar membranes. This gives us both theoretical and practical tools for studying the composition and functions of natural membranes. Having such models, in which we can manipulate amino acid composition, we may produce synthetically membranes resembling natural ones, and in some aspects surpass them in desired properties. 

Synthetic membrane processes perform versatile functions with the membrane acting as a barrier interface between feed and product. In liquid separations, for example, they are used to separate particles that span four orders of magnitude from dissolved ions to bacteria (Figure 1.1). Virtually all membrane processes are pressure driven, do not involve a phase change, and consume much less energy than alternate separation processes. 

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