These devices use rigid probes which must be in contact with the skin. The most commonly used skin hydration measuring devices are Skicon200, Corneometer CM820, Nova DPM 9003. However the method is very sensitive to environmental changes and requires several minutes to retrieve stable readings. Transepidermal water loss (TEWL) expressed in grams per square meter and per hour is used for studying the water barrier function of the human skin. The most well-established commercially available moisture detectors measure electrical properties such as capacitance and alternating current conductivity on the skin surface. The reduction in the efficiency of the barrier and moisture-maintaining functions of the skin results in easily dried, roughened skin which can be potentially more vulnerable to risk of infection. Sufficient amount of skin hydration and sebum makes the skin appear smooth, soft and supple whereas the lack of moisture can cause the skin to look dull and cracked, appearing older. Excessive sebum production can cause clogged pores possibly resulting in blemishes. This is important information in the pathogenesis of sebaceous glands disorders and pimples and acne. The sebum excretion rate (SER) reflects the amount of sebum production and is closely related to the physiological activities of the sebaceous glands. Sebum keeps the skin smooth and flexible by sealing and preserving moisture in the corneal layer and preventing evaporation and bacterial infections. Sebum is a mixture of fatty acids, triglycerides, proteins, and other molecules produced by the sebaceous glands in the dermis. Thus, the water-sebum system determines the condition of the skin and can be used as an indicator of skin health. Skin superficial lipids have been found to serve as water modulator in the stratum corneum. Lipid phase behavior in the stratum corneum is considered to be crucial for the skin barrier function. Skin health is associated with the stability of the functioning of the skin barrier, which depends on the continuity of the skin’s superficial lipids structure. Studies show that superficial lipids play an important role in the barrier function, creating a filter for interaction with the external environment. Ichthyosis vulgaris shows decrease of hydration level (~63%) while the level of superficial skin lipids does not vary significantly (~ ± 15%). Eczema leads to minor water loss (few percent) combined with noticeable oiliness drop (~25%), whereas psoriasis shows dramatic decrease of hydration (~70%) and oiliness (~40-70%) levels. Nevertheless, these mentioned disorders show peculiar skin conditions with respect to the balance between hydration and oiliness. This similarity in symptoms leads to complications with diagnostics, which often requires a biopsy, an invasive approach. The same symptoms are seen in individuals suffering from psoriasis, eczema and ichthyosis vulgaris. Skin conditions such atopic dermatitis shows drop in skin hydration level reflecting in a drop of water holding capacity of the skin, increased transepidermal water loss (TEWL) and defect in barrier function. Stratum corneum receive hydration underneath skin layers and to a lesser extent from the atmosphere. Epithelium remains flexible when it contains 10-20% water, but becomes brittle, when it drops below 10%. The presence of natural hygroscopic agents collectively referred to as natural moisturizing factor (NMF) and the SC intercellular lipids arrange orderly to form a barrier to prevent transepidermal water loss. The water retaining property of the SC is dependent on these two major components. The SC plays the role of the barrier to water loss and is composed of the corneocytes and an intercellular lipid bilayer matrix. Hydration and sebum retaining ability of the skin is primarily related to the stratum corneum (SC). Optimal balance between hydration and sebum levels provides the skin with a radiant, smooth texture and a natural pigmentation appearance, which is important from a cosmetic perspective. Skin hydration (moisture) and sebum (skin surface lipids) are considered to be important factors in skin health a right balance between these components is an indication of healthy skin and plays a central role in protecting and preserving skin integrity.
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