Cosmetics are not miracle products!

The ingredients found in a cosmetic product typically penetrate the stratum corneum, the most superficial layer of the skin, to disperse into the deeper layers. However, there must be boundaries: the active ingredients in cosmetic products should never enter the bloodstream. Their action should remain superficial and localized.

When a cosmetic product is applied to the skin, it must first penetrate the stratum corneum, which is composed solely of dead cells known as corneocytes. If it contains small-sized actives such as caffeine (molar mass = 194.2 g/mol) and/or hydrophilic compounds, it takes the so-called trans-cellular pathway (movement of actives from cell to cell). If it is a lipophilic care product (composed of fatty substances), the inter-cellular route is favored: the actives then circulate in the intercellular lipid cement that ensures the cellular cohesion of the stratum corneum, and then in the interstitial fluid that fills the spaces between the cells of the deeper layers of the skin.

Typically, in cosmetics, molecules act at the level of the epidermis but do not reach, or reach in very small quantities, the dermis. For an active ingredient to diffuse to the dermis, several intrinsic and extrinsic conditions must be met:

• The molecule must be of small size, like caffeine or low molecular weight hyaluronic acid;

• Skin hydration should be optimal, and the skin should be neither too oily nor too dry;

• Small amphiphilic molecules, which have an affinity for both lipids and water, are the ones that penetrate the deepest;

• The thickness of the product layer applied to the skin and the frequency of application have an influence on the penetration of active ingredients.