Human skin, nature layers

Human skin is the outer layer of the body, that covers and protects it from any exposure to the surroundings (wind or rain, etc.). The natural substances that compose the skin are very elaborate and complex. Further, the composition of the skin changes with age, and is different for different people. This also true for the animal world. 

The inclusion of ethanol in commercially available transdermal systems has naturally provoked curiosity concerning its role as an enhancer in human skin, particularly in vivo. The mechanism by which ethanol compromises the human stratum comeum in vivo was investigated by Bommannan et al. [125] using ATR-IR in studies analogous to those described previously in this chapter [61 ]. Those in vivo studies on the untreated ventral forearm of healthy adults had revealed a depth-dependent ordering and reduction of the intercellular lipids relative to the superficial layers. Consequently, in the ethanol experiments, the measurement site (about 20 cm ) was tape stripped four times prior to ethanol treatment in order to isolate the effect of ethanol from the inherent lipid changes in untreated SC. The examination site was treated for 30 minutes with absolute ethanol (10 ml) and then spectrally examined periodically over the... 

Marrow-Tech, Inc. (Elmsford, NY) has also developed a human skin model. Marrow-Tech s skin equivalent consists of (1) a dermal layer of fibroblasts and naturally secreted collagen and (2) an epidermal layer of keratinocytes separated by a dermal-epidermal junction. Whereas Testskin uses bovine collagen, Marrow-Tech s skin model consists solely of human tissue. 

Human skin has a multifunctional role, primary among which is its role as a barrier against both the egress of endogenous substances such as water and the ingress of xenobiotic material (chemicals and drugs). This barrier function of the skin is reflected by its multilayered structure (Fig. 5.1). The top or uppermost layer of the skin known as the stratum comeum (SC) represents the end product of the differentiation process initially started in the basal layer of the epidermis with the formation of keratinocytes by mitotic division. The SC, therefore, is composed of dead cells (comeocytes) interdispersed within a lipid rich matrix. It is the brick and mortar architecture and lipophilic nature of the SC, which primarily accounts for the barrier properties of the skin [1,2]. The SC is also known to exhibit selective permeability and allows only relatively lipophilic compounds to diffuse into the lower layers. As a result of the dead nature of the SC, solute transport across this layer is primarily by passive diffusion [3] in accordance with Pick s Law [4] and no active transport processes have been identified. 

A natural concentration gradient across human skin exists the inner dermal layer is essentially fully hydrated whereas the outer stratum corneum water content depends on external factors such as relative humidity and temperature typically, stratum corneum water content is around 20-30% of the tissue dry weight. Of this, approximately 25-35% is associated with some structural component of the tissue (i.e. bound ) with the remainder free (Walkley, 1972). Increasing the water content of the barrier stratum corneum is well known to... 

Healthy, normal human skin is kept moist by natural body oils contained in sebum, a secretion of the skin s sebaceous glands. The oily sebum helps the epidermis, or outer layer of the skin, retain the 10-30% of water it normally contains. However, some people suffer from skin that is nam-rally dry, or dry because of aging or contact with materials like paint thinner that dissolve and remove the sebum. Such individuals may find some relief by using OTC products called moisturizers. 

Human skin is coated by a protective liquid layer of approximately 50 % sweat and 50 % sebum. Of the two surface liquids, sebum tends to be the more cosmetically problematic because it causes skin to appear shiny or greasy in light and it distributes unevenly over the skin in layers that can be tens of microns thick in areas such as the face. Not only is sebum cosmetically undesirable, but it can also be medically undesirable in that excessive amounts of sebum collecting on the face can aggravate existing acne conditions. There remains a need to improve cosmetic facial powders that will reduce the amount of sebum (and therefore shiny appearance) collecting on the face, but that will not disrupt the natural look and feel of the skin. 

In terms of the mechanisms of action, the pyrrolidones partition well into human horny layer. They may act by altering the solvent nature of the membrane and pyrrolidones have been used to generate reservoirs within skin membranes. Such a reservoir effect offers potential for sustained release of a permeant from the stratum corneum over extended time periods. However, as with many other potential enhancers, clinical use of pyrrolidones is problematic due to adverse reactions. An in vivo vasoconstrictor bioavailability study demonstrated that pyrrolidones caused erythema in some volunteers, although this effect was relatively short-lived. Also, a toxic hygroscopic contact reaction to NMP has been reported recently [21]. 

Yannas s invention mimics the structure and function of the upper two layers of natural skin. It consists of an upper layer about 0.023 mm thick made of an elastic silicone material that, like the human epidermis, acts as a harrier to the loss of moisture from the body. Silicones are siloxane polymers to which are bonded various organic radicals (groups of atoms that contain carbon). The chemical structures of siloxane and a typical silicone polymer are shown on page 50. 

More recently, Bommannan et al. [61] used ATR-IR to examine the SC as a function of serial tape stripping. The nonuniform and inhomogeneous nature of the membrane has inevitably led to questions concerning the exact location of the barrier within the SC. With ATR-IR in conjunction with tape stripping, it has been possible to probe, at discrete intervals across the forearm SC in human subjects, spectral features associated with both the intercellular lipids and the hydration level of the membrane, key determinants of skin barrier function. On average, each tape-stripping procedure removed one SC layer per strip and enabled the progressive examination of 50-75% of the SC tissue. 

---