▻ Complex and fascinating biological micro-events unfold daily in our skin, some of which we still have to fully comprehend. Among these processes, skin ageing is one of the most perceivable and it is closely associated with loss of skin moisture. Hyaluronic acid is the key molecule involved in dermis moisturisation and a better understanding of its metabolism can help us to increase our awareness towards its acknowledgedly useful applications.
What we generally define as skin aging is the result of both intrinsic and extrinsic processes. While the former is innate and ultimately unpreventable in the skin just like in any other organ, the latter is caused by exposure to external agents to which the skin is highly susceptible, namely ultraviolet irradiation, which causes photoaging. The combined effects of these two processes provoke skin dryness, loss of elasticity, collagen degradation, epidermal atrophy and wrinkles. The key molecule responsible for keeping the skin hydrated is hyaluronic acid.
Hyaluronic acid (HA) or hyaluronan is a glycosaminoglycan naturally present in the extracellular matrix, a highly organised network of extracellular macromolecules, such as collagen and enzymes, which not only provide the cell with a structural framework, but also influences its functions via biochemical interactions.
Hyaluronic acid is profusely and widely distributed in young human body, being more abundant in the skin, rather well present in the synovial fluid, in the vitreous of the eye and in the umbilical cord, and present in smaller amounts in a surprisingly vast array of body organs.
In our skin, HA is produced within the dermis by specialised cells known as dermal fibroblasts through devoted enzymes called hyaluronan synthases. Ultimately HA is degraded into fragments of varying size by the hyaluronidases enzyme and ultimately dissolved in our body.
Both endogenously-produced and exogenously-administered hyaluronic acid come in various molecular weights, which in turn depend upon the length of the its chain. Thus, concentration and limiting viscosity, which are measures for molecular weight, are positively correlated with the propensity of hyaluronan to create viscoelastic polymeric structures.
Even though our body naturally synthesises hyaluronans, its spontaneous production progressively diminishes as we age. One of the most evident histochemical changes in senescent skin is the gradual disappearance of epidermal hyaluronan. This way, the skin loses the main agent responsible for binding and retaining water, ending up in loss of skin moisture.
As a matter of fact, for being hydrophilic, HA molecules display a unique capacity to bind water with ease, creating hydrated molecules which can retain up to 6,000 times their weight in water: a greater capacity to hold water than any other natural or synthetic polymer. Due to this exceptional water retention property, hyaluronan provides the skin with volume, plumpness and a smoother, youthful appearance.
Nonetheless, hyaluronans for personal care with different molecular weights generate different effects onto our skin. Generally speaking, longer chains tend to remain on the surface and shape a protective layer, reinforcing its barrier function against external agents. It acts as a humectant too, pulling water from the atmosphere in humid environments and reducing transepidermal water loss. Moreover, as the HA molecule mechanically holds water, this, in turn, allows small molecules to penetrate and excludes or slows down the passing of larger ones. Otherwise, when the molecular weight is small, shorter chains get rapidly absorbed by the skin delivering water to its inner layers. Seeing as how small-molecular weight HA penetrates the skin so effectively, it is often used to increase the absorption rate – and, thus, the efficacy – of other cosmeceutical ingredients, such as peptides.
Since hyaluronans are a completely natural product of the human body, their intake is virtually free from major risks. As a matter of fact, multiple clinical studies and toxicity assessments have confirmed not only the effectiveness of hyaluronic acid, but also its safety for a widespread use in cosmetics and the absence of any significant adverse reaction.
For the reasons we just went through and its paramount role in skin hydration, hyaluronic acid is widely adopted in cosmeceutics as skin conditioning and viscosity-increasing agent, keeping away all the undesired signs of skin-ageing and, at the same time, helping our body to stay healthier.