Skin barrier 101
In order to talk about the skin barrier, we first need to reflect on the importance of our skin. The skin plays an essential role in protecting the body from temperature variations, microorganisms, the environment, radiation, physical impact and toxins. At the same time, the skin supports the organs of the human body, and prevents water and other nutrients from leaving it. This is only possible thanks to a complex and highly organized structure, which allows us to divide the skin into three layers: the epidermis, the more external layer, the dermis, in the middle, and the hypodermis, more deeply. As you would expect, we owe the epidermis that the main characteristics of the skin barrier, which we will delve into in this article.
The acid mantle 
In addition to the secretions produced by the cells of the “living epidermis”, the stratum corneum also contains substances from sebum, an oil produced by the sebaceous glands, and sweat, a mixture of water and salts produced by the sweat glands. Taken together, these secretions form what is called a hydrolipidic film or acid mantle: a mixture with slightly acidic pH that remains constant even when the skin is in contact with alkaline substances. The acid mantle also provides an unpleasant environment for harmful microorganisms, and promotes the growth of a healthy microbiota.
Skin desquamation 
In addition to the low permeability of the stratum corneum, dead cells desquamation provides a new skin barrier defense mechanism, as it allows to eliminate microorganisms and impurities deposited on its surface. It is the peeling process of the skin that gives provides the healthy epidermis a cell lifecycle of approximately 28 days, which that can be extended as we age and when the skin is dry. Moreover, desquamation of the stratum corneum depends on several enzymes, whose proper functioning is linked to the presence of water. Therefore, and for the stratum corneum to desquam normally, it is important that the epidermis is properly moisturized.
How to protect the skin barrier?
Knowing the epidermal physiology allows us to answer this question:
- Maintain skin hydration
Most of the water that reaches the epidermis comes through the dermis. However, if the skin barrier is weakened by excessive washing, weather changes, hormonal changes or even aging, this water is easily lost, leaving the skin dehydrated. Therefore, it is essential to use hydrating cosmetics, which are capable of providing water to the skin, as well as components of the natural moisturizing factor and lipids similar to those that exist in the statum corneum.
- Promote the “living epidermis” metabolism
In addition to the compounds mentioned in the previous point, there are several ingredients that stimulate the cells of the “living epidermis” to produce these same compounds. Niacinamide is one of the main ingredients with this ability, but coenzyme Q10, glyceryl glucoside and Inula helenium extract are also great ones.
- Use sun protection and antioxidants
Yes, the skin has its own antioxidants. But when sun exposure and pollution are too high, these antioxidants are quickly consumed. Therefore, it is important to use sun protection and antioxidants everyday, in order to protect our natural reserves.
Melanin & co. 
The sun is essential for vitamin D production, but it can also damage the epidermal cell’s DNA while generating free radicals. Hence, sun exposure is the main responsible for skin aging, and it is estimated that it contributes to 70% of this process. One of skin's defense mechanisms against sun exposure is the presence of trans-urocanic acid, which originates from filaggrin. But the skin's main line of defense is undoubtedly melanin, whose production increases with increasing data in cellular DNA after sun exposure.
An antioxidant reserve 
In addition to sun exposure, there are many other sources of free radicals that can be harmful to the skin, such as the pollution from nitrous gases and particulate matter. Therefore, the epidermis has a variety of natural antioxidants, which are capable of maintaining its balance, such as catalase and superoxide dismutase enzymes, but also vitamins A, C and E, or glutathione.
A wall of brick and mortar 
The epidermis can also be divided into layers (strata) according to the characteristics its cells acquire as it renews itself. In the stratum corneum, its outermost layer with older cells, those are already dead. But contrary to what it may seem, this is a great advantage! These are the cells that provide the epidermis with a structure that is resistant to the entry and exit of water and substances, thus maintaining skin's hydration, and making it difficult to penetrate by most substances and microorganisms from the environment. In addition to being composed of keratin rich dead cells, each cell resembles a brick, surrounded by a mixture of fats, which constitutes the intercorneocyte cement (the mortar).
The “live epidermis” 
The portion we call the “living epidermis” is below the stratum corneum. And these are highly active cells! They produce compounds which are capable of retaining water in the skin, such as ceramides, cholesterol and fatty acids, composing intercorneocyte cement, but also filaggrin, a protein that generates the natural moisturizing factor, consisting of urea, PCA of sodium, sodium lactate, calcium, amino acids, among other compounds. In addition to producing these essential compounds for proper epidermal functioning, the cells of the “living epidermis” are constantly evolving until they become a new stratum corneum, thus constituting the skin barrier.