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Our 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.
 

Skin Structure

First, let’s talk a little about the structure of your 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.
 

How cosmetic moisturizers work

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 include ingredients like glycerin, urea, and pyrrolidone carboxylic acid (PCA). Humectants work by attracting water from the dermis below and helping to keep that water bound in the stratum corneum. Glycerin is used frequently because of its low cost and high efficacy, but a tacky feel on the skin is one of the drawbacks of formulating with high levels of humectants.

 

When optimizing skin formulations, cosmetic chemists try to reduce these negative properties of humectants.
 

Occlusive Agents

Occlusive agents increase moisture levels by providing a physical barrier to epidermal water loss. Ingredients with occlusive properties include petrolatum, waxes, oils, and silicones. Some occlusive agents like petrolatum can leave a heavy feel so they are often combined with other ingredients, like emollients, to improve consumer appeal.
 

Emollients

Emollients provide some occlusivity and improve the appearance of the skin by smoothing flaky skin cells. There are many different types of emollient esters and oils available to a formulation chemist. 3,4 Emollients are generally grouped by their ability to spread on the skin. By combining emollients with the different spread rates you can tailor the skin feel of a moisturizer. You can test for these differences by using different emollients in a standard base lotion. Additionally, emollient lipids similar to those naturally found in the skin may also increase the rate of barrier repair. 5
 

Putting it together

Each of these ingredient types has a different mechanism of action and most cosmetic moisturizers will use a combination of these ingredients to create a synergistic effect and mitigate certain aesthetic or financial drawbacks. Product claims and skin feel are also considerations to be aware of, so don’t be afraid to experiment with your options when creating a moisturizer. For tips on creating successful skin formulations, see our previous article on HLB formulating.
 

References

1. Harding CR; The Stratum Corneum: Structure and Function in Health and Disease, Dermatologic Therapy; 2004. Vol. 17, pp 6 -15.

2. Wickett RR, Visscher MO; Structure and Function of the Epidermal Barrier,
American Journal of Infection Control, 2006. Vol. 34, Issue 10, pp S98 – S110.

3. Flynn TC, Petros J, Clark RE, Viehman GE; Dry Skin and Moisturizers, Clinics in Dermatology, 2001. Vol. 19, pp 387 – 392.

4. Rawlings AV, Canestrari DA, Dobkowski B; Moisturizer Technology versus Clinical Performance. Dermatologic Therapy; 2004. Vol. 17, pp 49 – 56.

5. Mao-Qiang M, Brown BE, Wu-Pong S, Feinglod KR, Elias PM; Exogenous Non-physiologic vs. Physiological Lipids. Divergent Mechanisms for Correction of Permeability Barrier Dysfunction. Archives of Dermatology, 1995. Vol. 131, pp 809 – 816