Water is a vital element for our body to function properly. About 20% of the body’s total water is found in the skin (dermal and epidermal layer). It is necessary to keep the skin in its optimal state of hydration in order to maintain the skin’s elasticity and for it to establish a correct barrier function. However, there are some situations where water is easily lost through transcutaneous evaporation, resulting in dehydrated, rough, tight skin that lacks suppleness and is prone to wrinkles.
Cosmetics, and in particular moisturisers, are designed to combat the dehydration that skin often suffers from. One of the main components of creams are humectants, substances with a high hygroscopic capacity (they capture water), such as hyaluronic acid. Applying it in cosmetics improves the skin’s state of hydration because it acts as a moisturising sponge: it captures water from the ambient humidity and deposits it on the skin.
What is hyaluronic acid?
Hyaluronic acid (HA) is a structural polysaccharide of the glycosaminoglycans (GAGs) group. These are long, unbranched chains of repeating complex carbohydrate units: N-acetylglucosamine and glucuronic acid.
It plays a key role in skin hydration, and as a support molecule maintaining the integrity of the dermis. Its role as a hydrating molecule is due to the fact that it is capable of retaining hundreds of times its weight in water, and this characteristic is a consequence of the large number of hydroxyl groups and negative charges that form hyaluronic acid. It is also attributed an important role in the integrity of the extracellular matrix as it triggers the neoformation and regeneration of collagen and stimulates the growth of connective tissue cells, thus helping to rebuild the skin’s support tissues.
These two characteristics mean that hyaluronic acid is used in cosmetics mainly as a hydrating ingredient, but also as a dermal filler for expression lines and wrinkles. Hyaluronic acid is also the main component in eye drops, collyrium, contact lens fluid, etc.
How important is it for the skin?
Hyaluronic acid occurs naturally in numerous tissues and organs of the body: epidermis, connective tissue, cartilage, synovial fluid, etc.
Hyaluronic acid is part of the structural glycosaminoglycans found in the extracellular matrix of connective tissue, such as dermal connective tissue. It is associated with collagen and/or proteoglycan molecules, conferring elasticity, strength and lubrication to skin tissues.
The elimination of glycosaminoglycans from the interior of collagen and elastin structures, the main components of connective tissue, causes these fibres to lose functionality, become less flexible and less elastic. This occurs naturally with ageing, but there are also some factors that can accelerate this process, such as stress, sun exposure, poor diet, genetics, etc.
Hyaluronic acid is naturally lost with ageing, so mature skin loses hydration, and becomes less supple and less elastic.
What is its role in Cosmetics?
In cosmetic formulations, hyaluronic acid is included in order to take advantage of its surface hydrating action largely associated with its film-forming capacity. Hyaluronic acid remains on the surface of the skin, creating a protective layer that retains moisture in order to maintain skin hydration. However, its hydrating function on the skin’s surface depends on climatic conditions. In places where the air humidity is high, hyaluronic acid will capture ambient water to hydrate the skin. In contrast, in conditions where the air humidity is low, hyaluronic acid will capture water from the skin itself that will be lost by transepidermal evaporation. The best hydrating formula, therefore, is one combining hyaluronic acid with hydrophobic emollients with an occlusive function. These will create a film on the skin that will not allow water to pass through, so the occlusive agents will seal in the hydration provided by the hyaluronic acid.
Another function attributed to it in cosmetics is as a filler for expression lines and wrinkles, this occurs with hyaluronic acid with a lower molecular weight, sufficient for it to penetrate to the dermis layer, where the support fibres that provide elasticity and turgidity to the skin are synthesised.
Types of hyaluronic acid
One of the ways of classifying hyaluronic acid, therefore, is according to molecular weight:
- HA with high molecular weight (1,000,000 – 2,000,000,000 Da) creates a film on the surface of the skin, thus preventing the loss of lipids and other molecules.
- HA with lower molecular weight (< 1000,000 Da) is able to penetrate through the dermis and exerts a structural function in the extracellular matrix of the dermis.
Hyaluronic acid can also be classified according to its origin:
- Animal origin
- Bacterial origin: obtained by bacterial fermentation biotechnology.
Another way to classify hyaluronic acid is according to the type of modification the salt undergoes:
- Polymeric: hyaluronic acid conjugated with polymeric chains (sodium hyaluronate crosspolymer), also called cross-linked. It is conjugated in the polymeric network and can be released in a sustained manner into the skin.
- Cationically charged: this type of cationic-charged hyaluronate (Hydroxypropyltrimonium hyaluronate) adheres strongly to negatively charged surfaces such as skin and hair, giving the molecule not only hydrating but also conditioning properties.
- Hydrolyzed: hyaluronic acid (Hydrolized Hyaluronic Acid) with a very low molecular weight, which gives it greater percutaneous penetration. It helps repair and regenerate skin cells, as well as having an antioxidant function by protecting against free radicals. It also increases the skin’s deep hydration. Unlike the smaller size, the hydrolysate does not keep the molecule whole but instead it is divided into parts. Most studies measuring the effectiveness of hyaluronic and the literature on the subject are based on the whole molecule.
- Acetylated: (Sodium Acetylated Hyaluronate) very low molecular weight hyaluronic acid with greater affinity for the skin, so that its penetration is greater. It provides a highly hydrating action to repair the skin’s barrier function. In addition, this type of hyaluronic acid is highly resistant to the hyaluronidase enzymes that naturally degrade it.
How to choose the right hyaluronic acid?
The characteristics to bear in mind when selecting the most suitable hyaluronic acid for my formulation:
- It must be of the appropriate molecular weight according to the functionality intended for the cosmetic product.
- The grade of purity vs. price. There are many manufacturers in the market, what will make one better than another is the grade of purity of the hyaluronic acid production. This is an important index that, via HPLC, measures the presence of the molecule, therefore, the purity of the ingredient itself.
- It must comply with the regulation according to the application of the product. If it is intended for Cosmetics, it must comply with European Regulation 1223/2009. If the product is intended for eye drops, collyrium or contact lenses, it must be an ocular grade hyaluronic acid, i.e. it must comply with the European Pharmacopoeia regulations. Finally, if it is intended for dermal injections in beauty salons or beauty centres, it must be of injectable grade and have CEP certification, which requires a higher level of quality and purity.
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