Semi-permeable barriers

Human skin is the largest organ in the human body. It is fundamentally important to health as the semi-permeable barrier - the first line of defence - between the body and the external world. However, it remains relatively inaccessible to conventional magnetic resonance imaging, firstly because it is thin and therefore requires high spatial resolution, and secondly because it is characterized by relatively short T2 relaxation times, particularly in the outermost stratum comeum. Conventional studies have not usually achieved a resolution better than 70-150 pm, with an echo time of the order of a millisecond or so. As a planar sample, skin has proved amenable to GARField study where it has been possible to use both a shorter echo time and achieve a better spatial resolution, albeit in one direction only. Such studies have attracted the interest of the pharmaceutical and cosmetic industries that are interested in skin hydration and the transport of creams and lotions across the skin. 

Cell Membrane 10-20 nm Doubled-layered membrane main semi-permeable barrier of cell 5-10% of cell dry wt 50% protein, 30% lipid and 20% carbohydrate... 

The third main class of separation methods, the use of micro-porous and non-porous membranes as semi-permeable barriers (see Figure 2c) is rapidly gaining popularity in industrial separation processes for application to difficult and highly selective separations. Membranes are usually fabricated from natural fibres, synthetic polymers, ceramics or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibres or spiral-wound sheets. For the micro-porous membranes, separation is effected by differing rates of diffusion through the pores, while for non-porous membranes, separation occurs because of differences in both the solubility in the membrane and the rate of diffusion through the membrane. Table 2 is a compilation of the more common industrial separation operations based on the use of a barrier. A more comprehensive table is given by Seader and Henley.1... 

Cevc, G., Schatzlein, A., and Richardsen, H. (2002), Ultradeformable lipid vesicles can penetrate the skin and other semi-permeable barriers unfragmented. Evidence from double label CLSM experiments and direct size measurements, Biochim. Biophys. Acta, 564,21-30. 

By definition, enzyme immobilization is the conversion of an enzyme to a form with artificially restricted mobility and retention of catalytic function W. This restricted mobility allows for containment and recovery of the enzyme and is often achieved by either conversion to an insoluble form (for example by linking to insoluble particles) or by containment within a semi-permeable barrier (for example entrapment within an ultrafiltration membrane). In the course of this immobilization, enzymes can acquire four advantageous properties ... 

Complex relaxation is not however confined to biological tissue. Following a freeze-thaw cycle, non-exponential decay is observed for the water protons in an agarose gel (12) and is also readily observed in meat models made from completely synthetic man-made structures (13) In view of the absence of membranes or any semi-permeable barriers in these wholly fabricated materials, the general relevance of compartmentalisation to the observation of complex relaxation needs to be re-examined. 

Membrane separation. Here the separation takes place by a selective diffusion of one or more gaseous components across a semi-permeable barrier. In most applications, the membrane is a micro-porous solid, but there are also liquid membranes. The feasibility may be evaluated by using an index of perselectivity aq defined by means of solubility (S/) and diffusivity (D,) ratios as follows ... 

A membrane can be viewed as a semi-permeable barrier between two phases. This barrier can restrict the movement of molecules across It In a very specific manner. The membrane must act as a barrier between phases to prevent Intimate contact. The semi-permeable nature la essential to Insuring that a separation takes place. 

Membrane classification can be done according to several viewpoints. A major division can be made between biological and synthetic membranes. Biological membranes are semi-permeable barriers that separate either the inside from the outside of the cell, or enclose internal cell structures, but these will not be addressed in this work. Commonly used membranes in separation or bioconversion processes are made of synthetic polymers or ceramics (Table 4). 

Membranes are semi-permeable barriers that are used to isolate and separate constituents from a fluid stream. The separatirai process can be accomplished through a number of physical and chemical properties of the membrane as well as the material being separated. Separation can occur through processes such as size, ionic char, solulnlity, and combinations of several processes. Membranes can remove materials ranging from large visible particles to molecular and ionic chemical species. Membrane materials are diverse and can consist of synthetic polymers, natural fabrics, porous metals, porous ceramics, or liquids. The surface of the membrane can be chemically or biologically altered to perform separations on specific chemical... 

The permeable CPC containing a permeable or semi-permeable barrier OS and certain amount of activated carbon-based air-permeable interliners (such as Mark IV NBC Suit (UK), Paul Boye s CBRN Protective Suit (France),and JSLIST overgarment (USA)i ) were therefore developed to provide protection over a longer period of wearing (up to 24 h).i °... 

A membrane is a selective barrier between two homogeneous phases (gas or liquid). The selectivity is based on the fact that the different components in the feed will diffuse through the barrier at different rates. This is illustrated in figure 5.7 in which some of the terms are defined. It is worth remembering that, in general, a membrane is not a complete semi-permeable barrier and some of the solute, which should ideally be retained in the concentrate, passes into the permeate. 

As a semi-permeable barrier between two fluid phases, a membrane can be used to separate chemical species if the membrane is designed to be selectively permeable to some species and not others. The selectivity of the membrane... 

The lipid composition of membranes is a sensitive indicator of changes in environmental temperature. The fluidity of a membrane is critical to its functioning as a semi-permeable barrier, and is directly related to the fatty acid composition of the membrane. In artificial lipid membrane the liquid to crystalline transition occurs at lower temperatures for phospholipids containing higher proportions of shorter chain fatty acids or increased degree of unsaturation. Bacterial membranes with a greater proportion of unsaturated fatty acids are better able to function at low temperatures. 

An alternative approach is the use of a membrane inlet system. Polymer membranes have been used, for several decades, for the selective introduction of molecules into the ion source region of a mass spectrometer. The membrane acts as a semi-permeable barrier and by selecting a hydrophobic material, typically a silicone polymer, an inlet can be constructed which allows organic compounds to diffuse through the membrane and into the inlet of a mass spectrometer while acting as a barrier to water. This idea has been combined with PTR-MS to detect various compounds in water [7]. A water pump is used... 

It has been stated that a filter medium is a porous (or at the very least semi-permeable) barrier placed across the flow of a suspension to hold back some or all of the suspended material. If this barrier were to be very thin compared with the diameter of the smallest particle to be filtered (and perforated with even sized holes), then all the filtration would take place on the upstream surface of the medium. Any particle smaller than the pore diameter would be swept through the pores, and any particle larger than that (assuming the particles to be rigid) would remain on the upstream surface. Some of the larger particles, however, would be of a size to settle into the individual pores and block them. The medium surface would gradually fill with pores blocked in this way, until the fluid flow reduced to below an acceptable level. At this point filtration would be stopped and the medium surface would be brushed or scraped clean (although many automatic filters have their surface continuously brushed or scraped). 

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