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Skin has a very complicated structure, as schematically presented in Fig. 28. It possesses a protective function preventing penetration of pollutions and microorganisms inside the body. The outermost cellular layer of skin is epidermis, which consists of stratum corneum (SC) (mostly dead cells) and four layers of living cells. Stratum corneum is a lipid-protein biphasic structure, having a thickness of only 10-20 mm on most surfaces of the body. Due to cell membrane keratinization, tightly packing of cells, and lipid bridges between them, SC is a dense medium with a poor penetration for foreign molecules. The excellent diffusional resistance of the SC makes the transdermal delivery of immersion agents and water lost by skin difficult. To understand the transport and barrier functions of the skin, it is important to have knowledge of the distribution of water and ions within the different layers. Water content is known to influence: various physical characteristics, such as brittleness, elasticity, tensile strength, and viscoelasticity; barrier characteristics; electrical resistance; thermal conductivity; and appearance. The SC receives water from within the body, but water also may be taken up from the environment. From within the body, water reaches this tissue from the sweat glands and by diffusion from underlying tissues. In vivo, the diffusion of water across the SC is a passive process that can be modified with the application of osmotic agents. The water content of the innermost layer of the SC is in equilibrium with the adjacent moist granular layer. The outside cell layer, however, is in equilibrium with the environment and it is certainly drier than the innermost cornified layer. Thus, there exists a concentration gradient causing transepidermal water lost (TEWL).
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