About this Article
Written by: Marianne Case
Written on: December 4th, 2003
Tags: chemical engineering, lifestyle
Thumbnail by: Chino/Illumin
About the Author
In the Fall 2003 semester, Marianne Case was a junior majoring in Chemical (Biochemical) Engineering with a minor in Neuroscience. She is from Tacoma, WA. She is active in the USC branch of AIChE and enjoys playing the violin.
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Volume V Issue V > The Chemistry Behind Moisturizers
The skin is a complex organ with many layers. Its structure is designed to minimize moisture loss from the body while preventing foreign materials from entering. To accomplish these functions, the skin must have a protective covering of lipids, or oil-soluble molecules. Exposure to everyday conditions can strip the skin of its protective lipid covering. Therefore moisturizers containing some oil-soluble components are often used to restore the skin to its natural condition. Most moisturizers use a mixture of oil and water soluble components called an emulsion. More recently, researchers have begun incorporating liposomes, or small spheres of lipids, into moisturizers. Not only are these bubble-like structures useful for restoring the skin's protective lipid layer, but they can also carry active ingredients inside of them, increasing the potency of some cosmetic products.
Nothing hinders a smile like dry skin. Skin must maintain a certain amount of water inside to remain stretchy and feel comfortable. However, splashing water directly onto your face makes it feel drier than before. So how do moisturizers add water to your skin and keep it there? They require a combination of water and oil to work properly. A few different techniques for delivering moisturizers to your skin exist. Moisturizers often incorporate emulsion systems, which are mixtures of oil and water. They may also add a more recently developed technology that uses vesicles, or microscopic "bubbles" made out of biological components. In this article we will first describe emulsions, and then go into detail about a kind of vesicle called a liposome, which is commonly used in moisturizers. We will also explain why utilizing a vesicular system is generally superior to using emulsions only. First, however, a brief review of the skin's components is helpful.

The Skin

The skin is made up of three main layers (see Fig. 1): the epidermis, the dermis, and the hypodermis [1].
As the deepest layer of skin, the hypodermis contains a layer of fat. It also houses blood vessels and nerves that are generally larger than those found in the dermis layer [2].
Within the dermis lie the blood vessels, nerves, and hair shafts, as well as sweat and sebaceous glands. The sebaceous glands produce an oily substance called sebum and secrete it into the hair shaft. Sebum migrates up to the top of the skin through the hair shaft, where it covers the outer layer of the epidermis, providing a protective barrier for the skin from the outside world. It consists of fatty acids, cholesterol, waxes, triglycerides, and a variety of other substances [1]. A small coating of sebum will protect the skin effectively. An excess of sebum leads to overly oily skin and can cause acne.
Figure 1: The various layers and components of human skin.
Within the epidermis are multiple sublayers. On the very surface of the epidermis lies a layer of fat [1]. Just beneath the fat is a layer of cornified cells, called the stratum corneum [1]. Beneath the cornified cells lies the horny layer, which consists of three sublayers [1]. At the bottom of the epidermis lies the basal layer, where cells are created. These cells then migrate up through the layers of the skin all the way to the stratum corneum, where they fall off the body [1]. This upward cell movement helps defend the skin from harmful invaders by acting like an ocean current, moving foreign debris up and out of the skin.

Why the Skin Needs Moisturizers

The epidermis is most relevant to this discussion, since cosmetic products target the cells in that section and are generally not able to migrate past it into the dermis [1]. Most cosmetic products that claim to improve the skin's appearance and texture act on the stratum corneum layer of the epidermis [3] because it is the outermost layer of the skin -- when it is scaly, the skin appears rougher. These products often fill in gaps between the cornified cells of the stratum corneum, creating the appearance of smoother skin. They may also serve as occlusives, meaning that they create a barrier on the skin through which water can not permeate. It may seem disadvantageous to apply such a barrier to the skin because it would prevent the skin from absorbing water in the air. The water content inside the skin, however, is generally much higher than that of the air. The moisture content in the stratum corneum should be about 15% [2]. The water content increases deeper in the skin, reaching about 80% in the dermis [4]. Therefore, in the absence of a barrier, water is much more likely to leave the skin than to enter it. Applying a barrier helps the skin retain more water.