Environmental barrier, skin

The skin is the largest organ of the human body. One of its most important functions is to assist the immune system by serving as a barrier that protects underlying structures from trauma, infection, and exposure to harmful environmental elements. The skin also holds in place essential organs and fluids necessary for life. Any significant injury to this outer protective layer may potentially compromise an individual s overall health. 

General functions of the skin are outlined in Table 1. These functions include containment of tissues and organs, multifaceted protections, environmental sensing, and body temperature regulation. Some skin functions are inextricably entwined. For instance, containment and the barrier functions are to some extent inseparable. Active sweating is accompanied by increased peripheral blood flow, which in turn is tied in with greater nourishment of the cells of the skin as needed to promote their proliferation, differentiation, and specialization. 

From the environmental point of view, the three principal routes of entry of xenobiot-ics into the human body are percutaneous, respiratory, and oral. In multicellular animals, the extracellular space is filled with interstitial fluid. Thus, regardless of how a compound enters the body (with the exception of intravenous administration), it enters interstitial fluid after penetrating the initial cellular barrier (such as skin, intestinal mucosa, or the lining of the respiratory tract). From the interstitial fluid, the compound penetrates the capillaries and enters the bloodstream, which distributes it throughout the body. 

Noven/DOT Matrix Alza/D-Trans delivery/ absorption through skin drug/formulation from environmental aggrevates Clear barrier offers aesthetics Pouching as primary/ secondary package Barrier property on carrier web critical Chemical inertness... 

The role of the liquid applied barrier dressing is to provide a biocompatible protective coating over the tissue for the purpose of protecting it from bacteria and environmental contamination. From a physical and dynamic point of view, the barrier coating must include proper stress-strain physical properties for reasons shown in Fig. 2.2. The skin (epidermis and dermis) and subcutaneous soft tissue are not smooth and stretch and retract (stress-strain) as the body moves to lift an arm or leg, for example. The barrier must experience the same stress-strain and flexing phenomena and remain adhered to the tissue, otherwise the barrier would disbond... 

Dry, scaly skin is characterized by a decrease in the water retention capacity of the stratum corneum (SC),1 with water content diminished to less than 10%. Barrier function of the SC is usually declined, and transepidermal water loss (TEWL) is increased because of an abnormality on barrier homeostasis.2 People feel tightness of their skin, and the skin surface becomes rough, scaly, and sensitive. Hyperkeratosis, abnormal scaling, and epidermal hyperplasia are usually observed in the dry skin.2 Keratinization also shows abnormal features.2 These phenomena are commonly observed in atopic dermatitis and psoriasis.3 Dermatitis induced by environmental factors such as exposure to chemicals, low humidity, and UV radiation also shows these features. Thus, many researchers have been investigating the cause and treatment of dry skin, and there is currently great interest in adequate model systems for dry skin studies. In this chapter, I will describe several model systems of dry skin for clinical research of dermatitis associated with skin surface dryness and also mention recent studies to improve the dry skin. 

Cure time is very important for sealants as well as adhesives. Often the sealant will be required to function as a barrier or resist the movement of substrates very soon after it is applied. With construction sealants, for example, it may not be possible to delay the environmental conditions until after the adhesive cures. Thus, curing time becomes a critical parameter in selection of the sealant. ASTM C 679 covers a method for determining the time that a mechanic can work the sealant into the joint before the sealant starts to skin or solidify. 

It is generally accepted that the stratum comeum represents the primary electrical barrier in skin. Though impedance results vary from subject to subject and from site to site on the same individual, the electrical response of skin can be modeled as a simple RC network. Nonideal behavior is associated with environmental conditions, the hydration of the skin, and the integrity of the stratum comeum. 

Stratum corneum, the nonliving layer of skin, is refractory as a substrate for chemical reactions, hut it has a strong physical affinity for water. The chemical stability of stratum corneum is evident in its mechanical barriers which include insoluble cell membranes, matrix-embedded fibers, specialized junctions between cells, and intercellular cement. The hygroscopic properties of stratum corneum appear to reside in an 80 A-thick mixture of surface-active proteins and lipids that forms concentric hydrophilic interfaces about each fiber. This combination of structural features and surface-active properties can explain how stratum corneum retains body fluids and prevents disruption of living cells by environmental water or chemicals. 

Stratum corneum, the outermost layer of mammalian epidermis, functions physiologically as the principal diflFusion barrier to molecules penetrating the skin and as a protective physical barrier to mechanical insults at the skin surface. Data suggest that these functions are critically dependent on the specific morphological and macromolecular organization of the membrane mosaic (16, 17, 18, 19, 20). Thus, alterations of biophysical properties arise from environmental factors acting directly on the membrane or upon the keratinization process, and they affect... 

Figure 2 The organization of the skin as a biologic barrier. (Reproduced from Smith RP (1992) The organization of the skin as a biological barrier. A Primer of Environmental Toxicology, p. 73. Philadelphia Lea Febiger, with permission from Lea Febiger.)...

Staphylococci exist in air, dust, sewage, water, milk, and food or on food equipment, environmental surfaces, humans, and animals. Normally, this species lives in the human oropharynx, nose, large intestine, vagina, and on the skin without causing harm. However, if a breach in the skin or mucosal barrier occurs, S. aureus gains access to nearby tissues or the bloodstream where it can colonize and cause disease. The relationship between S. aureus and its human host, then, is dynamic in nature, capable of quickly shifting from mutualistic or commensualistic to parasitic. 

Epithelial barrier models for the skin [48,49], respiratory tract [50], BBB, and intestine [39] are constructed to study and predict the absorption, penetration, and metabolism of drugs or environmental toxins through these barriers. All the models are physically tight structures, and generally involve cells cultured at the air-liquid interface on porous membrane support, such as a porous polycarbonate filter. The use of a permeable support allows cells to be grown in a polarized state under more natural conditions promote Cell differentiation and enhance cell functions. 

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