Skin diffusion processes

The in vitro diffusion studies for each sample were carried out by using the Franz diffusion cells with a diffusional area of about 1.76cm2. The acceptor compartment of the apparatus was filled with the buffer solution pH 6, USP [21], and maintained at 37 0.5°C via a circulating water system. The diffusion membrane (the cellulose membrane with a molecular weight cut-off point of 1000 or the hairless mouse skin) previously prepared was placed between die donor and the acceptor compartments of the assembly. An accurately weighed 4g of sample was then placed in the donor cell and the diffusion process was started. The solution in the acceptor compartment was continuously stirred with a small magnetic stirrer to maintain the sink conditions. Aliquots from the receptor cells were removed at 0.5,2,4, 8 and 24 h time intervals and replaced with equal... 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

In certain situations, the diffusion process occurs concurrently with a particular reaction in the membrane. For instance, when a prodrug (i.e., estradiol acetate) diffuses through the skin, an enzyme in the viable epidermis converts the prodrug to estradiol and acetate. A diester prodrug (i.e., PNU-82, 899) diffuses across a Caco-2 cell monolayer, which extensively metabolizes the diester to a monoester. 

Reverse osmosis membranes are characterized by an MWCO of -100 Da, and the process involves transmembrane pressures (TMP) of 10-50 bar (1000-5000 kPa), which are 5-10 times higher than those used in UF [11,36]. Unlike UF, the separation by RO is achieved not by the size of the solute but due to a pressure-driven solution-diffusion process [36]. Like UF membranes, RO membranes are uniquely stmctured films from synthetic organic polymers and consist of an ultrathin skin layer superimposed on a coarsely porous matrix [3]. The skin layer of the RO membrane is nonporous, which may be treated as a water-swollen gel, and water is transported across membrane by dissolving in this gel and diffusing to the low-pressure side... 

The corrosion resistance of enamel is generally attributed to a surface layer of silica. Acid first dissolves the surface alkali oxide leaving a hydrated porous layer which is mainly silica, that acts as a barrier through which corrosive agents must difiltse for further attack to occur. As the affected layer deepens, the diffusion process slows down so that the enamel becomes protected by a skin of acid resisting silica. 

HowevCT, be aware that, in spite of these surface effects, DEET enhanced permethrin depot formation in fat as well as the skin surface. Eurther stratnm comenm partitioning studies demonstrated that permethrin was least likely to partition into the stratum eomeum when DEET was present. These observations are unique in that DEET readily diffuses into the skin, and in spite of it small molecular wright of 191.3 and log octanol-water partition coefficient of 1.69, it has the capacity to retain pesticide levels in the skin layers. Although DEET may enhance this retention of pesticides in the skin layers and probably preclude acute toxicoses, this skin depot is available for dermal absorption albeit by a slow diffusion process across the skin. 

The diffusant molecule from a topically applied formulation has three potential routes of entry to the subepidermal tissue (1) the transappenda-geal route, (2) the transcellular route and (3) the intercellular route (Fig. 2) [ ] Percutaneous absorption refers to the overall process of mass transport of substances applied topically and includes their transport across each layer of the skin and finally their uptake by the microcirculation of the skin. The process of percutaneous absorption can be described by a series of individual transport events occurring in sequence. First, deposition of a penetrant molecule onto the stratum corneum, then the diffusion through it and through the viable epidermis, the passage through the upper part of the papillary dermis, and finally uptake into the microcirculation for subsequent systemic distribution [1,3,4]. The viable tissue layers and the capillaries are relatively permeable, and the peripheral circulation is sufficiently rapid,... 

Common observations show that the chemical lesion does not appear within the very first second of contact with a corrosive. They occur progressively but quickly, from the surface of the skin toward the deeper layers. After the contact, a diffusion process through the different skin layers starts. Then, the chemical reacts with biochemical components, leading to cellular and tissular injury (Fig. 3.2). Four main parameters condition the seriousness of a chemical skin lesion ... 

The rate of cure for a one-part sealant material in which the cure is initiated upon contact with moisture is important in determining the depth of cure. One-part sealants cure from the outer surface that is exposed to surface or environmental moisture, and the cure proceeds inward, toward the center of the sealant material. Curing beyond the outer surface depends on moisture diffusing into the sealant material. This diffusion process is slowed significantly or stopped completely when the sealant cures sufficient to create the desired tight moisture barrier. Because the sealant underneath the cured surface remains uncured due to the exclusion of water from the bulk of the sealant, the phenomenon is often called skinning. Once the... 

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