Hyaluronic Acid Benefits and Actions in Topical Application.

Hyaluronic acid (HA) is an extremely hydrophilic complex sugar, naturally present in the human body in the joints, muscles, eyes and especially in the skin. Nearly 50% of its total quantity is found in the dermis, the deep layer of the skin. It constitutes the extracellular matrix and serves as a support for collagen and elastin fibers. It also ensures skin hydration because it acts like a molecular sponge: it captures water molecules and retains them.

Epidermal and dermal cells are capable of synthesizing HA. Because the dermis is much thicker than the epidermis, it contains four to nine times more HA. Nevertheless, it has been shown that for the same amount of tissue, the epidermis synthesizes four times more hyaluronic acid than the dermis.

Skin cells lose their ability to produce optimal amounts of HA during the aging process. This is why babies' skin is so soft and smooth: it is rich in hyaluronic acid. However, as we age, dehydration occurs; the skin becomes thinner and wrinkles appear. The decrease in HA production is also accompanied by a decrease in the suppleness, elasticity, and tone of the skin. In mature skin, HA is still present in the dermis, while in the epidermis it has completely disappeared.

Today, the reasons for the precipitous drop in HA with aging are unknown. In order to fight against this phenomenon, it is recommended to continue to “recharge” the skin with HA from adolescence. It is already known that hyaluronic acid ingredient taken orally has no benefits for the skin, as the skin cells are not able to extract it from the bloodstream. The “recharging” of the skin with this compound is more reliable through the topical application of hyaluronic acid skin care products.

But how does HA work on the skin when applied topically?

The process of hydration of the skin proceeds according to a very complex mechanism. Depending on the molecular weight of the hyaluronic acid, it will act differently on skin hydration. The higher the molecular weight of the hyaluronic acid, the more the physicochemical properties are predominant, while the biological properties will prevail in the case of fragments of low molecular weight.

• High molecular weight hyaluronic acid (>1,800kDa) increases the viscosity of a formula and forms a viscoelastic film when applied to the skin. Thus, high molecular weight hyaluronic acid benefits the hydration of the upper layers of the epidermis, which is manifested by a decrease in trans-epidermal water loss due to its occlusive action. Moreover, this film-forming effect increases the penetration of the other active ingredients in the formula.

• Low molecular weight hyaluronic acid (> 100-1000 kDa) allows a deeper hydration of the skin thanks to its deep penetration. It interacts with skin cells and extracellular matrix components. It is effective in improving skin texture and reducing wrinkles.

• Tiny hyaluronic acid fragments (50 kDa and less) are very useful in the fight against aging. After their penetration into the deep layers of the skin, they serve as signaling molecules for the epidermal cells to synthesize new endogenous high molecular weight hyaluronic acid molecules, which provides positive benefits on overall hydration. In addition, hyaluronic acid fragments reduce the creation of pro-inflammatory interleukins, which are responsible for the generation of free radicals capable of damaging the components of the extracellular matrix and the skin cells themselves.

Regardless of its weight, when applied topically, hyaluronic acid is a molecule that is generally well tolerated by all skin types and has no sensitizing potential. It does not present any undesirable side effects in cosmetics.

It is in aesthetic medicine in the range of anti-wrinkle injections that hyaluronic acid can be irritating. Nevertheless, it should be noted that according to the ANSM (National Agency for the Safety of Medicines and Health Products), adverse effects related to hyaluronic acid injections affect only 0.1 to 1% of people.

• MICHELOTTI A. & al.Anti-aging and filling efficacy of six types hyaluronic acid based dermo-cosmetic treatment: double blind, randomized clinical trial of efficacy and safety.Journal of Cosmetic Dermatology (2014).

• JUNCAN A. M. & al. Advantages of hyaluronic acid and its combination with other bioactive ingredients in cosmeceuticals. Molecules (2021).