What is dermal penetration?
When we apply a cream to our skin, we expect that the active ingredients in the product will be absorbed and will eventually reach all layers of the epidermis and dermis. This penetration is called dermal penetration or skin penetration. However, it should be noted that not all products can penetrate through the epidermis into deeper layers. This depends on the active ingredients in the product, on the vehicle, and on many other factors.
What can be absorbed through the skin?
There are many substances that can be absorbed into the skin. These include water, as well as numerous organic and inorganic molecules. The vehicle used to develop a cosmetic product is an important factor regarding dermal penetration, as some ingredients have a high affinity for certain types of formulation solutions. As an example, this would apply to oil-in-water emulsions.
What is the difference between skin penetration and permeation?
Skin penetration refers to the transport of a chemical species from outside the body, through the surface of the skin, and into deeper layers thereof. Penetration can be due to diffusion or to transportation by blood flow. Skin permeation, on the other hand, refers to transport through the skin that is mediated by a carrier-mediated process. There are many molecules that will not permeate the skin on their own but can do so when they are coupled with such carriers.
Skin absorption of cosmetics
Cosmetics have been traditionally used topically, that is, they are placed on the skin surface in order to protect it, moisturize it, give color to it, etc. Over time, the concept of using “cosmeceuticals” has evolved, and many cosmetic companies have come up with products that claim to have a therapeutic effect when applied to the skin. The use of cosmetics has been associated with systemic effects, which can be harmful if certain guidelines are not followed (e.g., excessive exposure to ultraviolet radiation).
There is a growing demand for cosmetic products that may exhibit dermal penetration and therefore provide an agent that will affect deeper layers of the skin. However, this need must be balanced against the potential risks associated with such penetration. There are many examples of cosmetic ingredients that have been reported to penetrate through the skin and reach deeper layers. In contrast, there are numerous reports that describe penetration as negligible or absent even after repeated application of a particular product for a prolonged period of time.
Why is dermal penetration important in the efficacy of cosmetics?
The efficacy of many cosmetic products depends on their ability to deliver an active ingredient into the skin. The vehicle used for this purpose is designed to assist penetration, but it is usually not possible to achieve significant delivery without proper formulation. When developing a new cosmetic product, it is important to determine whether active ingredients can be absorbed through the skin and whether this will lead to the desired results, either locally or systemically.
The efficacy of cosmetics is often evaluated by clinical tests, in which the product is simply applied and its ability to penetrate through the skin is not quantified. Even though this is a useful measure for determining whether it might be possible to achieve dermal penetration, such experiments do not provide information on how much of the active ingredient penetrates and reaches deeper layers. Measuring dermal absorption of a cosmetic product is a complex task and requires the application of the correct analytical methods. Most dermatologists who have been asked to analyze dermal penetration have applied incorrect methodologies and arrived at erroneous conclusions.