By Cleber Barros, Technical Researcher, Vinia

If you are a cosmetic formulator and you develop products for skin care, then you’ve probably had to formulate at least one product with a moisturizing claim.

There are several ways to promote skin hydration, and knowing each one of them is extremely important so you can develop products that are truly effective in promoting skin moisturization. For example, did you know that preventing water from leaving the skin is not the only way to contribute to its hydration? Yup, there are many other mechanisms that your products can work on to promote the moisturization of the skin.

That’s why I chose this subject to write about, so you can learn how to develop innovative and effective moisturizing products based on the natural process of skin hydration.

Cellular structures of the stratum corneum

The skin is the largest organ of the human body, and it has many functions that are vital for the maintenance of the body’s homeostasis, such as thermal and water flux control. These functions are only possible because the skin has a barrier that is composed by a majority of dead cells filled with keratin – the corneocytes – and a lipidic extracellular matrix.

In order for this barrier to function well, as well as maintain our health and prevent microbial contamination on the inside of the body, is vital that the skin is always hydrated.

The stratum corneum is the outer layer of the skin, and it is responsible for the barrier function. The stratum corneum is composed of a layer of anucleated dead cells (the corneocytes) that are surrounded by a protein shell called ‘cornified envelope’. These corneocytes contain keratin and other molecules called the natural moisturizing factor; besides an extracellular matrix, in which the corneocytes are immersed.

This structure can be described as the brick and mortar complex (in this metaphor, the corneocytes represent the brick, and the extracellular matrix, the mortar).

When the relative humidity is high, the keratin fibers absorb large amounts of water, even doubling the weight of the corneocytes when they are completely filled.

On the other hand, in conditions of low relative humidity, the keratin filaments are solidified, which influences negatively on the viscoelastic and water absorption properties of the stratum corneum.

However, the keratin is not the only component of the corneocytes capable of retaining water. There are also small hygroscopic compounds inside these cells which are called the natural moisturizing factor (known as NMF). The NMF is composed mostly by amino acids and their derivatives, which are formed by the breakdown of a protein called filaggrin.

The filaggrin is a large histidine rich protein that is derived from a precursor known as profilaggrin, which is a high molecular weight protein localized on the granular layer. As the granular cells differentiate into cornified cells, the profilaggrin is degraded into a lower molecular weight protein: the filaggrin. At this stage, filaggrin starts to aggregate filaments, catalyzing the processes that will form the keratin molecules. Still, the formation of the keratin, as said before, is not the only function of the filaggrin, because it continues to be degraded almost instantaneously after the formation of the keratin, this way forming the amino acids and their compounds which will form the NMF.

The conversion of filaggrin to NMF occurs as the corneocytes are moving to the more superficial layers of the stratum corneum.

There are some factors that can reduce the formation of the NMF. Low air humidity is one of them. In conditions where the air humidity is <10%, the enzymes that proteolyse the filaggrin are less effective. Other factors that negatively affect the formation of the NMF are chronological aging and sunlight exposure.

The NMF compounds can be classified as very effective humectants, since they have the property to bind to the water from the atmosphere, attaching it to the skin. Unfortunately, some studies has shown that these compounds are easily lost, even a simple cleansing routine is enough to wash the NMF compounds away.

Besides the amino acids and their derivatives, there are also other molecules that are considered as part of the NMF, but are external to the corneocytes. Some of them are: urea, lactates and electrolytes.

Another compound of the skin that is long used in the cosmetic industry is hyaluronic acid (HA). HA is one of the major compounds of the dermis, but some researches have shown that it is also present in the epidermis, where it plays an important role in the barrier function and also in the skin hydration.

HA, as the NMF, has an humectant function, but it is also essential for the cellular mechanism once it influences the cell-cell and cell-matrix connections. Recent studies have also shown that the HA also plays a role on the differentiation of the keratinocytes and the extracellular lipids formation.

Glycerol is another hygroscopic compound present on the skin. It is produced by the sebaceous gland and it also plays an important role in skin hydration. The mechanism of the function of glycerol is still unknown, but some studies revealed that some alterations on the intrinsic hydration of the skin are more connected with the amount of glycerol produced by the sebaceous gland than with the amount of sebum produced by them (contrary to what was thought). This discovery, according to the researchers, is really important to the development of moisturizing cosmetics. Now we know that the addition of glycerol can contribute to the potential of moisturization in these products.

Lipidic structures of the stratum corneum

Besides the cellular structures, skin also has an lipidic content which helps to maintain its hydration. These structures are the compounds that form the extracellular matrix, or the ‘mortar’ where the corneocytes are imerse.

This extracellular matrix is composed of 50% ceramides, 10% fatty acids, 25% cholesterol, and 15% of cholesterol derivatives and glucosylceramides.

Most of the stratum corneum lipids are synthetized by the keratinocytes on the upper spinous and granular layers. When they reach the interface between the spinous and granular layers, these lipids are transported to the stratum corneum, where they will compose the extracellular matrix, forming an apolar, water-impermeable layer.

The extracellular lipids, due to their hydrophobic characteristics, form a physical barrier against the transepidermal water and electrolytes loss. These lipids are organized in a series of parallel lamellar membranes, and this organization is vital for the formation of a narrow and extremely efficient barrier. Several studies report the disorganization of this structure, as well as the deficiency of its production, as factors that determine several skin hydration issues, such as atopic skin, dermatitis and senile xerosis.

New members of the ceramides family are constantly being found, but ceramide-1 (the most polar between the ceramides) plays a major role on the extracellular lipids organization.

Some research demonstrated that the signs of skin aging are intrinsically related to the deficiency on the ceramides content, even on skin with a healthy appearance. It happens due to the disorganization of the lipids, that is caused by the absence of ceramides.

Cholesterol increases the fluidity of extracellular lipids, besides contributing to the elastic properties of the skin.

Lamellar bodies are also vital for the stratum corneum maintenance, once it contains enzymes such as hydrolases and proteases which are responsible for the synthesis of lipids of the corneum layer and for the physiological process of desquamation.

Sebum, which is produced by sebaceous glands, is responsible for the formation of a hydrolipidic mantle on the surface of the skin, thus participating in the formation of the epidermal barrier. Human sebum is composed of 47% fatty acids, 17% ester waxes, 11% scalene, 7% cholesterol, 3% triglycerides and 2% cholesterol esters.

The absence of the sebum production is associated with diverse skin disorders, such as senile xerosis and atopic skin.

Fatty acids form a substrate that composes the acidity of the skin surface, an important defense mechanism against pathological microorganisms. Besides, it helps to maintain the homeostasis of the skin barrier.

When topically applied, lipids that are not naturally present on the skin (such as petrolatum, for instance) can help on the treatment of skin disorders related to the deficiency of skin hydration by forming an occlusive barrier that prevents transepidermal water loss. However, these lipids can’t be absorbed by the skin, so applying these products is only a palliative measure.

On the other hand, the lipids similar to the ones which are naturally present on our skin are absorbed when topically applied, thus increasing the amount of extracellular lipids of the stratum corneum, helping the effective treatment of skin dehydration.

Studies have shown that the use of a single group of lipids in cosmetics is not the most effective measure to improve skin hydration. In some cases, the use of ceramides alone can even worsen the skin condition. Still, the association of the three main classes of skin lipids (ceramides, cholesterol, and fatty acids) is a reliable way to restore the skin barrier function, thus improving skin hydration to its ideal condition.

So skin hydration is a complex process that involves several cellular and extracellular pathways. For teaching purposes, I divided this article into two chapters (one for the cellular and one for the lipidic structures).

However, it is important to remember that the skin is an active organ. Therefore, all these processes happens at the same time, all the time, on our skin.

The cellular pathways of skin hydration occur by the binding of water to hydrophilic compounds, such as the keratin filaments and the NMF compounds.

However, the lipid processes that contribute to skin hydration occur due to the formation of an occlusive barrier that prevents the transepidermal water loss.

So when you’re formulating a moisturizing cosmetic, it’s important to remember that you must use ingredients that associate both of the skin hydration mechanisms. This way you make sure that the product will have an effective and permanent result.