Permeable to water-soluble

A major problem with natural biocompatible materials (collegan, chitosa, etc.) used for tissue barriers is that they absorb water and are permeable to water soluble/... 

Lopour P et al. (1990) Silicone rubber-hydrogel composites as polymeric biomaterials. II Hydrophilicity, permeability to water-soluble low-molecular-weight compounds. Biomaterials 11 (6) 397—402... 

Relative to the gastrointestinal mucosa the pulmonary epithelium possesses a high permeability to water soluble molecules, which is an advantage with dmgs such as sodium cromoglicate (IX), a bischromone with two carboxylic acid groups and a pK of approximately 1.9. The dmg is well absorbed from the... 

Certain proteins present in biomembranes make them selectively permeable to water-soluble molecules and ions. 

Atmospheric exposure trials, carried out in Cambridge, established the fact that when rusty specimens were painted in the summer, their condition, after some years exposure, was very much better than that of similar specimens painted in the winter It was found that steel weathered in Cambridge carried spots of ferrous sulphate, deeply imbedded in the rust, and that the quantity of ferrous sulphate/unit area was very much greater in the winter than in the summer this seasonal variation was attributed to the increased sulphur dioxide pollution of the atmosphere in the winter, caused by the combustion of coal in open grates. It was concluded that there was a causal relationship between the quantity of ferrous sulphate and the effective life of the paint. It was suggested that these soluble deposits of ferrous sulphate short-circuit the resistance of the paint film and, since paint films are very permeable to water and oxygen, the ferrous sulphate will become oxidised and hydrolysed with the production of voluminous rust, which will rupture the film at numerous points, thus giving rise to the characteristic type of failure seen on painted rusty surfaces. 

The blood-brain barrier (BBB) forms a physiological barrier between the central nervous system and the blood circulation. It consists of glial cells and a special species of endothelial cells, which form tight junctions between each other thereby inhibiting paracellular transport. In addition, the endothelial cells of the BBB express a variety of ABC-transporters to protect the brain tissue against toxic metabolites and xenobiotics. The BBB is permeable to water, glucose, sodium chloride and non-ionised lipid-soluble molecules but large molecules such as peptides as well as many polar substances do not readily permeate the battier. 

Capillaries are the site of exchange between blood and the interstitial fluid surrounding tissue cells. Tissues with a higher metabolic rate have a more extensive capillary network, that is, a greater number of capillaries per unit area. Because of extensive branching of these vessels, the cells of the body are typically within 20 pm of the nearest capillary. Consequently, the distance that substances must travel between blood and the cells is minimized. Capillaries are permeable to water and small water-soluble substances, such as glucose, amino acids, lactic acid, and urea, and impermeable to proteins. 

One of the major drawbacks of SOD as putative pharmaceutical agent is its small plasma half-life. It has been suggested that this shortcoming might be corrected by the chemical modification of SOD, for example, by binding to water-soluble polymers [17] or the entrapment in liposomes [18]. However, it seems that the membrane-permeable, low molecular weight compounds to be more promising for the use as SOD mimics. Two major types of... 

Bibette has used this method to study the effect of osmotic pressure on the stability of thin films in concentrated o/w emulsions [96], by means of an osmotic stress technique. The emulsion is contained in a dialysis bag, which is immersed in an aqueous solution of surfactant and dextran, a water-soluble polymer. The bag is permeable to water and surfactant, but impermeable to oil and polymer. The presence of the polymer causes water to be drawn out of the emulsion, increasing the phase volume ratio and the deformation of the dispersed droplets (Fig. 10). 

From the data in Table III. 1 it may be seen that cellophane and cellulose acetate have very large permeabilities to water also, the solubility of water in these materials is great, so they are clearly unsuited as moisture barriers. Of the elastomers listed, silicone rubber has the highest permeability to air. 

Generally, the barrier or rate-controlling films are more permeable to water than the carrier films. The materials used for this purpose consisted of a base, film forming water-soluble polymer in combination with at least one hydrophilic component such as hydroxypropyl methylcellulose or polyvinylpyrrolidone. The polymers used were the same as those for the carrier films. Some recent developments are discussed herein. 

As discussed above for simple adsorption, polymer sorption can be treated in both thermodynamic and kinetic contexts. The quantity of an analyte that is sorbed by a polymer at equilibrium is referred to as the solubility of the analyte, while the rate at which the analyte is transported through the polymer is referred to as permeability. Although high solubility is generally a prerequisite for high permeability (on any reasonable time scale), there are some notable exceptions. Poly-siloxanes and polytetrafluoroethylene (Teflon ), for example, are quite permeable to water, but the solubility of water is not particularly large in either material. 

Chromates have been widely successful because most paint films are permeable to water. If the rate of moisture permeability is matched to the solubility of the chromate pigment used then enough chromate ions may migrate to the metal surface to initiate and sustain the formation of passive layers [5.51]. The effectiveness of chromates in both the cathodic and anodic areas is simplified and illustrated in Figure 5.11 [5.58]. 

Formulations containing an absorption promoting substance, such as propylene glycol or sodium lauryl sulphate, may increase the permeability of the stratum comeum to water-soluble drugs. Propylene glycol is a commonly used vehicle in topical corticosteroid preparations for veterinary use. Various aprotic solvents, which include dimethylacetamide, dimethylformamide, dimethylsulphoxide, tetrahydrofurfuryl alcohol, and 2-pyrrolidone, serve as penetration enhancers of polar drugs (Barry, 1983). Dimethylsulphoxide... 

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