Diffusion barrier, skin

The section discusses diffusion across a number of diffusion barriers in parallel. Diffusion across the skin represents one of the best examples to illustrate steady diffusion involving two or more independent diffusional pathways in parallel [6],... 

Permeability coefficients for phenol in isolated skin patches from nude mice have been determined (Behl et al. 1983). The permeability coefficient increased as the concentration of the applied aqueous phenol solution increased doubling the concentration from 20 to 40 g/L resulted in a 12-fold increase in mean permeability coefficient (0.007-0.085 cm/hour). The value obtained for the permeability coefficient when 60 g/L was applied to the skin patch (0.169 cm/hour) was similar to that obtained for skin patches in which the stratum comeum had been removed. It was concluded that phenol concentrations exceeding 20 g/L may destroy a diffusion barrier normally provided by the intact stratum comeum, permitting increased percutaneous absorption. 

In the light of these observations and the newer trends in product formulation, it was decided to study the in vitro release and permeation of propranolol hydrochloride from various hydrophilic polymeric matrices using the cellulose membrane and the hairless mouse skin as the diffusion barriers and to evaluate the effects of some of the additive ingredients known to enhance drug release from dermatological bases. 

The Methocel matrix, formulation A, was further investigated using the hairless mouse skin as the diffusion barrier. Here the drug release was observed to be reduced significantly to 1.3mg/ (24h) compared with 11.75 mg/(24h) through the cellulose membrane. The formulation was further modified with the inclusion of... 

PORE. I A minute cavity in epidermal tissue as in skin, leaves, or leather, having a capillary channel to the surface that permits transport of water vapor from within outward but not the reverse. 2. A void of interstice between particles of a solid such as sand minerals or powdered metals, that permits passage of liquids or gases through the material in either direction. I11 some structures, such as gaseous diffusion barriers and molecular sieves, the pores ate of molecular dimensions, i.e 4-10 A units. Such microporous structures are useful for filtration and molecular separation purposes in various industrial operations. 3. A cell in a spongy structure made by gas formation (foamed plastic) that absorbs water on immersion but releases it when stressed. 

The outermost layer of the skin, the cornified layer or stratum corneum, has been identified as the principal diffusion barrier for substances, including water [2,3]. It is approximately 10 to 20 pm thick when dry but swells to several times this thickness when fully hydrated [17], It contains 10 to 25 layers lying parallel to the skin surface of nonviable cells, the corneocytes, which are surrounded by a cell envelope and imbedded in a lipid matrix. This architecture is often modeled as a wall-like structure, with the corneocytes as protein bricks embedded in a lipid mortar [18]. Similarly to the viable epidermis, desmosomes (corneodesmosomes) contribute to the cell cohesion. 

The substrate also has an important influence on diffusion of the dye. The diffusion rate increases with caustic soda treatment or mercerization of cotton. However, with regenerated cellulose fibers, which have a marked skin-core structure, the outer parts of the fiber can act as a diffusion barrier. 

When oedema fluid collects in the tissues of the skin, it gives a puffy look to the skin of the face. In the lung, the capillaries run close to the alveoli, and reduction in plasma oncotic pressure can result in fluid accumulation in the alveolar wall and in the alveoli. This fluid increases the diffusion distance for oxygen between blood and alveolar air and acts as a diffusion barrier, reducing gas exchange. If severe, lung (pulmonary) oedema can result in development of abnormal blood gas concentrations. Treatment of pulmonary oedema is critical as it can develop into a life-threatening situation. 

Before any dmg applied topically can act either locally or systemically it must penetrate the barrier layer of the skin, the stratum comeum (Fig. 9.21). This behaves like a passive diffusion barrier with no evidence of metabolic transport processes, dmgs being absorbed by transcellular or intercellular pathways. 

The principle of the method consists of rapidly extracting the anthocyanins from the skins, gently at first and then under more extreme conditions, where the diffusion barriers are broken down. Acidity is used as a vector to facilitate extraction. In addition, it is recommended that the grapes should be roughly crushed and the resulting flesh diluted by half. Crushing the seeds also results in partial extraction of their tannins, which is necessary to assess the characteristics of the grapes. The solutions are aqueous, pH 1 (HCl n/10) and pH 3.2 (solution with 5 g/1 tartaric acid neutralized by 1/3). 

A major diffusion barrier of the skin is considered to be the stratum corneum. Absorption of chemicals... 

Skin. The skin s unique molecular transport and barrier properties pose a challenge for transdermal dmg dehvery. Diffusion of dmgs through the stratum corneum, the outer layer primarily responsible for the skin s limited permeabUity, varies by dmg, by skin site, and among individuals. Until recently, virtuaUy aU dmgs appHed to skin were topical treatments. 

The chemical incorporated in a vehicle should reach the surface of the skin at a suitable rate and concentration. If the site of action lies in the deeper layers of the epidermis or below, the substance must cross the stratum comeum, if the skin is intact. Both processes, diffusion from the dosage form and diffusion through the skin barriers, are inextricably linked. They should be considered simultaneously and can be influenced by the choice of formulation. 

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