Hyaluronic acid, a source of hydration.

Water is one of the major constituents of our body and accounts for about 60% of body weight. In the skin, it is primarily found in the dermis. The hydration of the epidermis, which is essential for its suppleness, softness, and appearance, is mainly governed by mechanisms of diffusion from the dermis and evaporation at the skin's surface. The dermis contains connective tissue characterized by an abundant extracellular matrix, primarily composed of protein fibers (collagen and elastin) and glycosaminoglycans, including hyaluronic acid. Together, they form a compressible gel that retains water like a sponge, also allowing for its circulation and that of dissolved molecules. While hyaluronic acid is mainly present in the dermis, recent histological studies have shown that it also exists in the epidermis, where it binds water molecules that are then retained by the hydrolipidic film.

Hyaluronic acid is the key molecule for skin hydration and has the ability to attract and retain up to 1000 times its weight in water.

The highly hygroscopic nature of the hyaluronic acid, combined with its healing and anti-inflammatory properties, makes it an interesting active ingredient to use in cosmetics. There are several forms of hyaluronic acid that can penetrate the skin to varying depths. When it is of high molecular weight (1000 - 1400 kDa), hyaluronic acid remains on the surface of the epidermis where it creates a film limiting the evaporation of water molecules. The low molecular weight active ingredient (20 - 300 kDa), on the other hand, is capable of crossing the horny layer to lodge itself in the deeper layers of the epidermis and retain the water molecules diffused towards the surface.

By ensuring skin hydration, hyaluronic acid strengthens the skin barrier, which slows down the appearance of aging signs, reduces the risk of infections, and promotes the healing process.

Many clinical studies have focused on the moisturizing power of hyaluronic acid when applied topically. A few of these are compiled in the table below, to more concretely highlight the moisturizing properties of this active ingredient.

The moisturizing virtues of hyaluronic acid can also benefit the hair. Indeed, this active ingredient, which does not leave a greasy effect, can deeply hydrate hair fibers and strengthen the hydrolipidic film present on their surface. This film aims to protect the hair from external aggressions (vigorous brushing, wind, temperature variations...) and to keep them hydrated. By providing protection and hydration to the hair fibers, hyaluronic acid also prevents their breakage and the appearance of split ends, resulting in softer and stronger hair.

Hyaluronic acid treatments are particularly beneficial for dry or damaged hair and this active ingredient is often found in conditioners, treatments that protect the hair and make styling easier. A scientific study has indeed focused on the effects of a conditioner containing 1% hyaluronic acid as an active substance. This treatment increased the Young's modulus, which quantifies the elasticity of the fibers, by 25%. The thickness of the hair also significantly increased by 6% and the researchers noted an overall improvement in hydration and the appearance of the hair fibers.