The skin is the outermost structure of our body and therefore it has the function to preserve an essential element for our survival: water.
Water is the primary source of hydration of the body and in particular is the most important parameter for the functional balance and skin health, that contain a total of approximately 67% water, which is distributed very differently in the various tissues. The water from the deeper tissues of the skin, follows a well-defined pathway within the different layers of the skin: from the bloodstream it reaches the dermis, then it spreads regularly and consistently up to the upper layers of the epidermis reaching the outermost surface consists of the stratum corneum, where it is responsible for preserving skin hydration.
The natural moisture of the skin is the result of several biological mechanisms with specific functions, which take place in the dermis, epidermis and stratum corneum. This movement of the water depends mainly on the secretion of the sweat glands, an active process under the control of the nervous system, and of sebaceous glands, which together with the aggregated polypeptide from the dacay of the corneocytes, substances of purine and carbohydrate nature, derived from the loss of nucleus of keratinocytes, and from lipids of epidermis, are a mixture of lipophilic and water-soluble substances, whose task is to protect the skin and, in particular, in maintaining the hydration of the stratum corneum. Then the free water in the stratum corneum, evaporates through a process called “perspiratio insensibilis” or trans epidermal water loss (TEWL – Trans Epidermal Water Loss), which consists in the evaporation constant and passive of water in vapor form in the outermost layer of epidermis. Its size is very variable and depends on the degree of environmental humidity, temperature and ventilation. The movement of water between cells in different layers of the epidermis also depends on proteins called aquaporins, which are present in the epidermis where they form appropriate channels to convey water and water-soluble ingredients, transporting them to the surface. A right hydration of the epidermis and stratum corneum, is only possible if the water supply from the dermis is sufficient to balance the small losses and if the water retention capacity in the upper compartments of the skin is efficient and consistent.
In the dermis, the water is retained thanks to the presence of glycosaminoglycans (GAGs), hydrophilic polymers able to retain large amounts of water of the extracellular matrix. The dermis is particularly rich in water: it contains about 70% of the water supply of the entire skin, and this concentration is hardly to be changed by external events but mainly depends on the hydration state of the whole organism, and on the efficiency of protein synthesis by fibroblasts. In case of systemic dehydration, the dermal water supply becomes the primary source from which to draw to compensate for the lack of water. Another cause of depletion of dermal water supply is the qualitative and quantitative alteration of the molecules able to bind water, as in the case of photodamage in which the dermal structures are altered by UV radiation and lose their ability to retain water.
The structure of the skin acts as a barrier essential to maintain hydration of the skin within physiological values. The main factors of this barrier responsible for maintaining hydration are:
These components are subjected to a continuous renewal process determined by the ability of the epidermal keratinocytes to reproduce and differentiate continuously, thus giving rise to the various layers of the epidermis.
Different factors, both exogenous and endogenous, may affect these fragile defense mechanisms, causing lower or higher skin dehydration. The extent of skin hydration depends on the humidity, the hygroscopic properties of the stratum corneum and the presence of naturally moisturizing factors, without which you have dry skin. In addition to age and genetic predisposition to skin aging, many external factors can lead to lower or higher skin dehydration. The main factors of dehydration are chemical (eg, the solvent action and loss of skin lipids caused by the continuous use of surfactants) or due to unfavorable climatic or environmental conditions: wind, cold, and humidity, separately or in combination, cause dehydration of the stratum corneum leading to dry, rough, scaly and cracked skin, redness, itching and irritation. Even prolonged contact with water, despite the protection of the sebaceous-sweating film, causes a depletion in NMF. Indeed, a topical application of water alone causes a stress on all the stratum corneum that causes an alteration of its barrier function. The more exposed areas, such as face and hands, are the most affected; however also other areas of the skin are particularly prepared to excessive dryness often associated with a more or less evident flaking skin.
A dehydrated skin favors the entry of foreign substances, the onset of inflammation and causes loss of elasticity and tone. The application of topical cosmetic formulations on the dehydrated skin is able to restore smoothness, softness and distensibility. Therefore it is necessary to intervene with moisturizing treatments that prevent skin dehydration, and at the same time restoring the optimal content of water and other substances, which may be impaired as a result of multiple factors. Topical treatments are able to intervene on the hydration of the stratum corneum and also to convey compounds able to bring water, and to restructure the skin barrier which is essential to effectively replenish and restore the water and lipid content of the skin (collagen, hyaluronic acid, urea, ceramides, plants lipids and fatty acids treatments). The moisturizing efficacy of a formulation is due not only to the presence of specifically moisturizing ingredients, but also to the choice of vehicle and cosmetic form that contributes to the transfer of such property.