Viable tissue, penetrant diffusion

Normal stratum comeum, taken in its entirety, is a dense molecular continuum penetrable only by molecular diffusion. It is virtually an absolute barrier to microbes, preventing them from reaching the viable tissues and an environment suitable for their growth. The outermost stratum comeum is continuously being shed in the form of microscopic scales (natural desquamation) and, to a limited depth, is laced with tiny crevices. Many... 

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,... 

The development of a blophyslcally based model of chemical absorption via human skin Is described. The simulation has been used to analyze the In vivo penetration kinetics of a broad range of molecular species. Four first-order rate constants are Identified with the percutaneous absorption process k -penetrant diffusion through the stratum corneum k2 transport across the viable epidermal tissue to the cutaneous microcirculation k - a retardation parameter which delays the passage of penetrant from stratum corneum to viable tissue k - the elimination rate constant of chemical from blood to urine. 

Most studies on percutaneous absorption emphasize the penetration of drugs, toxins, and other solutes into and through the skin as described in this book. Percutaneous absorption is also dependent on the clearance of solutes from the skin and transport into deeper layers of the skin (Figure 13.1). Clearance mechanisms in the avascular viable epidermis, diffusion in the dermis, and export from/within the dermis include solute diffusion and physiological transport by the dermal blood and lymphatic systems. This chapter focuses on the nature of these clearance mechanisms and how they affect the rates of percutaneous absorption and the levels of solute in skin and tissue. We consider first the role of blood flow, followed by the role of binding, transport to deeper tissues, and then the role of lymphatics. 

Transport of nutrients, viable cell culturing and penetration In a tissue-regeneration purpose, on absorbable scaffold as well as on permanent support, cell viability requires diffusion of nutrients, oxygen, and metabolic waste through the substitute. In native tissue, this delivery takes place across the thin wall of the capillaries network [21]. 

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