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The following excerpt is adapted with permission from an article by Kelly Dobos of Kao Corp., titled, "How Do Skin Moisturizers Work," which was featured on the Chemist's Corner. The article discusses how moisturizers work on skin with respect to the three main ingredient categories: humectants, emollients and occlusive agents.
Human skin has many important functions, including the prevention of water loss. Dry skin, or xerosis, is a common problem that many consumers seek to treat with cosmetic moisturizers. Whether for the face, hands, feet, or entire body, moisturizer formulations are an important part of any cosmetic chemist’s tool kit.
First, let’s talk a little about the structure of human skin. The upper layer of the skin, the epidermis, is further subdivided into four distinct layers. These layers from bottom to top are the Stratum basale, the Stratum spinosum, Stratum granulosum, and the Stratum corneum. There presence of a fifth layer, the Stratum lucidum, can be seen in thicker areas of the epidermis like the soles of your feet.
Keratinocytes, the main type of cells in the epidermis, migrate up from the dermis and undergo many changes to become a flat, keratin rich corneocyte before being shed. During this progression through the layers of the epidermis, lipids are released into the spaces between cells, and the skin’s own natural moisturizing factor (NMF) is generated. These lipids form a barrier to water loss and help retain the skin’s NMF. Disruption of this lipid matrix and subsequent loss of hydration can lead to dry, flaky skin.1, 2 Cosmetic moisturizers are used to help repair the lipid barrier and restore hydration.
Although there are some more specialized approaches, like the use of alpha-hydroxy acids or quaternary conditioning agents, most cosmetic moisturizers improve the condition of dry skin by utilizing one of three major ingredient types including: humectants, emollients and occlusive agents.