subject: The Science Of Skin Moisturizers [print this page] Author: Author: . Greg Maguire & Dr. Ahmed Al-Qahtani" href="http://www.articlesbase.com/authors/dr.-greg-maguire-&-dr.-ahmed-al-qahtani/251550.htm">Dr. Greg Maguire & Dr. Ahmed Al-Qahtani
The term moisturizer is often used as a marketing term with little or no scientific meaning. Consumers think of moisturizers as actively increasing the water content of the skin. Dermatologists see moisturizers as bland oleaginous substances that are topically applied to the skin. The term moisturizer does not necessarily imply that moisture or water is being added to the skin. Moisturizers are a key component of basic skin care especially when there is alteration of the epidermal barrier function and reduced water content in the epidermis. Moisturizers are used to restore the barrier function of the epidermis, to cover tiny fissures in the skin, provide a soothing protective film, and increase the water-content of the epidermis. They may, thus, slow evaporation and add to the skins moisture, thereby maintaining hydration and improving the appearance and tactile properties of dry and aging skin.
For many years, epidermal water content has been known to be crucial for skin plasticity and the prevention of dry skin. Traditionally, moisturization was believed to inhibit transepidermal water loss (TEWL) by occlusion. Water originates in the deeper epidermal layers and moves upward to hydrate cells in the stratum corneum (SC), where the water is eventually lost to evaporation.
The barrier function of the stratum corneum (SC), an important component in hydrating the skin, depends on the presence of unique intercellular lipids in the SC. In contrast, the water-holding capacity of the SC that keeps our skin surface smooth and soft is dependent on the amounts of intracellular lipids and water-soluble small molecules such as amino acids of the SC as well as skin surface lipids. In various dermatoses that involve the epidermis, the SC barrier function becomes defective together with the development of scaling and fissuring on the skin surface. However, barrier function and water-holding capacity of the SC are not always interdependent of each other. For example, xerotic conditions that occur in normal individuals at certain ages, such as infantile xerosis, dry skin of in middle-aged females, and senile xerosis develop without any abnormality in the SC barrier function. Skin care of xerosis that employ moisturizers, which are utilized to hydrate the SC by improving defective water-binding capacity of the SC with various humectants, is effective to prevent the development of pruritus and secondary eczematization. Atopic dermatitis and nummular eczema (coin shaped patches) subsequent to senile xerosis are definitely exacerbated by hypersensitivity reactions to various environmental protein antigens (allergens) that penetrate through damaged SC forming fine fissures. Although such treatment with moisturizing agents can improve the water-holding capacity of the SC, such treatment cannot effectively restore the barrier function of the SC. Thus, the effects of moisturizers are distinct from those of skin-protective barrier creams.
Most moisturizers contain substances called humectants. These are substances that are capable of attracting water, and hence conserve the water in the skin. One of the oldest and best examples is glycerine, also called glycerol. This has been the standard humectant for many decades and is generally regarded as safe.
Moisturizers are usually made in the form of an emulsion. An emulsion is a "mixture" of oil and water. Because of their chemical composure, oil and water do not naturally mix. The atoms within water have slight negative and positive charges on them, and so they are drawn to other charged particles. Water molecules are not able to bind to the uncharged molecules in oil, so they cannot properly mix with the oil and instead form a separate phase when placed in the same container. Therefore, a chemical that attracts both oil and water must be added to the mix so that the oil and water phases will combine. This chemical is known as a surfactant and must have two parts, one that binds water and one that binds oil. Surfactants are found in many household products. One of the most common examples of a surfactant is dishwashing soap containing chemicals that bind both water and oil. When combined with water, the mixture is able to bind the greasy residues on dishes, pulling them off as the mixture runs over the dish. Water alone would simply slide over the oil, thus not able to remove the oil from the dish.
When water is combined with an oil-based component and a surfactant, an emulsion is made when the components undergo a process that thoroughly mixes the three together. To ensure that the final product is water-based, the amount of water added to the mixture should be somewhat greater than the amount of oil-soluble substance added, so that mixing will produce an aqueous phase with small amounts of oil dispersed throughout. If too much of the oil-based product is added, the water and surfactant phase will not be able to "dissolve" and a separate oil phase will form. Alternatively, the emulsion will instead become oil-based, with small amounts of water mixed into it. The choice of surfactant affects the ratio of water to oil that can be mixed and still remain in one phase. Some surfactants are able to bond with more oil molecules than others, meaning that more oil can be dissolved in a given amount of water.
Emulsions provide a convenient way to deliver oil-based products. After the product is applied, excess water will evaporate from the skin, leaving a layer of the oil-soluble product behind with some water. In addition to delivering the oil-based product, the surfactant may bind the remaining water to the skin to increase its water content. The surfactant is able to do this because it is also still bound to the lipid or oil-based product that forms the protective barrier for the skin.
Considering the skins structure, the SC architecture is the most important factor in water flux and retention in the skin, and in overall level of moisturization. The four key processes for the formation and functioning of the SC are the corneocyte process, SC lipid process, natural moisturizing factor (NMF) process, and desquamation process (exfoliation). Corneocytes are the physical barrier of the SC and, when hydrated, contribute to elasticity. The lipid bilayer of the SC function as a moisture barrier and although the layer prevent entry of many chemicals, this layer is also the means of entry for most topically applied substances. The NMF is found within corneocytes and is a mix of hygroscopic molecules that, by helping maintain hydration in the corneocyte, keep the SC hydrated. Half of the NMF is amino acids derived from the protein filaggrin in keratinocytes, and the other half is composed of salts, including lactates, urea, and electrolytes. Production of NMF is directly related to external humidity. In desquamation, corneodesmosomes are degraded by water-dependent hydrolytic agents. When there is low moisture in the SC, these enzymes do not work efficiently. Corneocytes accumulate on the skin surface producing the signs of dry skin, e.g., when the moisture content is less than 10%, and when there is loss of continuity of the SC. The moisturizing treatment involves repairing the skin barrier, retaining/increasing water content, reducing TEWL, restoring the lipid barriers ability to attract, hold and redistribute water, and maintaining skin integrity and appearance. Moisturizers perform these functions by acting as humectants, emollients, and occlusives. Moisturizers containing collagen and other proteins, i.e., keratin and elastin, claim to rejuvenate the skin by replenishing essential proteins. Moisturizers also act to reduce skin friction and increase skin hydration by providing water directly to the skin from their water phase and by increasing occlusion, as measured as a decrease in TEWL
About the Author:
Drs. Al-Qahtani and Maguire are Co-Founders of A & G Skin Solutions, Inc. of Irvine, California, USA www.agskinsolutions.com Both Al-Qahtani and Maguire are NIH-supported research scientists, professors at medical schools, with numerous peer-reviewed publications. Maguire was awarded the NIH's prestigious Fogarty Fellowship for his work in studying the nervous system, and Al-Qahtani has received numerous international awards for his work in immunology and medicine. Both professors have been working on stem cells dating back to 1997. Dr. Maguire is currently President of the San Diego Neuroscience Group at the Scripps Research Institute (http://www.scripps.edu/services/sdneuro/ )
