Moisturization research was spearheaded in the 1950s when Blank demonstrated that low moisture content of the skin is a prime factor in dry skin conditions.1 In the last 50 years, many scientists have devoted their lives to researching moisturization and have begun to unravel the mysteries of skin hydration (see Chapter 11). It is now known that the symptoms of dry skin can be treated by increasing the hydration state of the stratum corneum (SC) with occlusive or humectant ingredients and by smoothing the rough surface with an emollient. Moisturizers represent a multibillion dollar market in the US. Commonly used moisturizers are oil-in-water emulsions, such as creams and lotions, and water-in-oil emulsions such as hand creams. There are two main types of ingredients: occlusives and humectants. A good moisturizer usually contains both components. This chapter will identify and discuss the mechanisms of action of the main components found in popular moisturizers.
MECHANISM OF ACTION OF MOISTURIZERS
There are many moisturizers on the market but they all have the same goal: to increase water content in the SC. This can be accomplished by preventing water evaporation from the skin by using occlusive ingredients or by increasing the integrity of the skin barrier (see Chapter 11). The mainstay of increasing the integrity of the skin’s barrier involves providing fatty acids (such as linoleic acid, Fig. 32-1), ceramides, cholesterol, and controlling the calcium gradient. Increasing the skin’s ability to hold onto water is another strategy for moisturizing skin. Increasing levels of natural moisturizing factor (NMF), glycerin (glycerol), and other humectants such as hyaluronic acid will help skin hold onto water. Lastly, increasing the ability of the epidermis to absorb important components for the circulation, such as glycerol and water through aquaporin channels, will also aid in increasing skin hydration.
Occlusives coat the SC to retard transepidermal water loss (TEWL). They are usually oily substances that have the ability to dissolve fats and are therefore widely used as a component in skin care cosmetics. An occlusive is one of the best choices to treat dry skin because it provides an emollient effect as well as decreases TEWL. Two of the best occlusive ingredients currently available are petrolatum and mineral oil. Petrolatum, for example, exhibits a water vapor loss resistance 170 times that of olive oil.2 However, petrolatum has a greasy feeling that may render agents containing it cosmetically unacceptable. Other commonly used occlusive ingredients include paraffin, squalene, dimethicone, soybean oil, grapeseed oil, propylene glycol, lanolin, and beeswax.3 In addition, “natural” oils such as sunflower oil have been increasing in popularity. Occlusive agents are only effective while present on the skin; once removed, TEWL returns to the previous level. Interestingly, it is not desirable to lower TEWL ...