What are the benefits of hyaluronic acid for the scalp and hair?

The hyaluronic acid is a molecule naturally present in the body, known for its remarkable ability to attract and retain water. Depending on its molecular weight, it can even capture up to 1,000 times its weight in water. This property, highly beneficial for the skin, may also be relevant for hair. Indeed, each hair fibre is composed of three superimposed layers: the medulla, the innermost layer; the cortex, accounting for nearly 80% of the fibre; and the cuticle, the external protective layer. The latter consists of flat cells that overlap like tiles and whose cohesion is essential for limiting water loss.

To date, there are few studies on the hair properties of hyaluronic acid. One such study evaluated the effects of hyaluronic acids of different molecular weights on the mechanical properties of hair fibres. For this purpose, hyaluronic acids of 1,460 kDa (high molecular weight), 370 kDa (medium molecular weight) and 42 kDa (low molecular weight) were used. Hair strands were treated with a spray containing 0.25 % of one of these hyaluronic acids, then subjected to tensile testing to measure their strength. The results showed that only low molecular weight hyaluronic acid significantly increased the tensile strength of hair fibres (+16 %), without altering their elongation at break.

This study also assessed the penetration capacity into hair fibres of these different molecular weights of hyaluronic acid. Using fluorescence microscopy, it was observed that low molecular weight hyaluronic acid exhibited a markedly higher fluorescence intensity than the high molecular weight form, indicating greater penetration. Cross-sectional images revealed that the low molecular weight hyaluronic acid reached all regions of the fibre, including the cortex, whereas the high molecular weight form was largely confined to the outer layers. This distribution difference suggests that the ability of low molecular weight hyaluronic acid to restore the mechanical properties of the hair fibre is directly related to this deeper penetration into the hair’s internal structure.

Another study reached the same conclusions. Researchers assessed the effect of four different biopolymer-enriched conditioners (collagen, hyaluronic acid, a 50:50 collagen–hyaluronic acid blend and a 50:50 collagen–hyaluronic acid blend with 30% chitosan) compared with a conditioner without biopolymers on the mechanical properties of human hair. Strands of brown hair from a 20-year-old woman were washed, dried, treated with one of the conditioners for 30 minutes, then rinsed and left to dry. The hair was subsequently subjected to mechanical testing (Young’s modulus and elongation at break) using a tensile testing machine. The results, shown in the graph below, indicate that the presence of hyaluronic acid in the conditioners significantly increases Young’s modulus, corresponding to an improvement in the strength of the hair fibres.

Additionally, other research has studied the association of hyaluronic acid with a cationic polymer, polyquaternium-10, to improve its affinity for keratinous surfaces such as hair fibres. This complex, named BHA-10, exploits the electrostatic interaction between negatively charged hyaluronic acid and positively charged polyquaternium-10 to overcome the natural repulsion between hyaluronic acid and the hair fibre. Zeta potential measurements and radiolabelling tests have shown that this complexation increases the adsorption of hyaluronic acid on hair by a factor of ten after a simple water rinse, while prolonging its retention for at least one hour under continuous rinsing. This durable attachment not only enhances the softening and hydrating effects of hyaluronic acid, but also forms a flexible polymer film which, by coating the hair fibres, can protect the hair.

Depending on its molecular weight and formulation, hyaluronic acid exhibits different modes of action : at low molecular weight, it can penetrate as far as the cortex to reinforce the internal structure, whereas when combined with certain polymers or used at high molecular weight, it forms a protective surface film, thus shielding the hair from external aggressors and limiting dehydration.

Note : It is sometimes claimed that hyaluronic acid has the capacity to stimulate hair growth or slow hair loss. However, while injections of hyaluronic acid may indeed produce this effect, no study indicates that topically applied hyaluronic acid can do the same.

Hyaluronic acid is also beneficial for the scalp, an often overlooked area.

Just like facial skin, the scalp can be prone to dehydration, for example following overly frequent washes with harsh shampoos. Thanks to its hygroscopic properties, hyaluronic acid acts like a sponge to retain water within the tissues and restore the scalp’s water balance. Moreover, when of high molecular weight, hyaluronic acid can form a film on the scalp’s surface, helping to protect it from external aggressors and counteract the drying effect of certain surfactants present in shampoos.

Beyond its hydrating properties, hyaluronic acid also exhibits noteworthy anti-inflammatory effects for irritated scalps, redness or itching. Studies in vitro have shown that this molecule can modulate the immune response by acting on certain mediators of inflammation, such as interleukin-8 produced by skin cells under stress. By reducing the release of these cytokines, hyaluronic acid limits local inflammation and soothes discomfort. Furthermore, its high affinity for the CD44 receptor allows it to inhibit the production of prostaglandin E2 (PGE2) and matrix metalloproteinases, which also play a role in itching and irritation.

To hydrate and soothe the scalp, hyaluronic acid is preferably applied in the form of a serum or lightweight lotion, after shampooing, on slightly damp hair. Moisture facilitates its penetration and optimises its capacity to retain water within the tissues. It is recommended to gently massage with the fingertips, using circular motions, to promote even distribution of the product. Some shampoos also incorporate hyaluronic acid to compensate for the drying effects of certain surfactants.

Regarding the ends, hyaluronic acid may be applied before or after washing, depending on supplier recommendations. Most often, it is incorporated into moisturising masks applied to towel-dried hair. When combined with film-forming agents, hyaluronic acid also helps to smooth the cuticle, thereby reducing water loss and frizz. It is also found in serums formulated for dry hair, hyaluronic acid being a lightweight, non-greasy active ingredient.

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