The skin as a barrier is a popular belief that hardly matches the experimental factuality. The skin is the outer part of the body and harbors several physiological functions. It participates in the immune response, contributes to the maintenance of the thermal balance and exerts tactile functions. Its enzymatic panoply accompanies or opposes the settlement of those microorganisms that constitute the surface microbiome, and it is kept elastic, flexible and resilient by the lipids and the moisturizing factors it produces.

The skin is the largest organ of the human body. It has a surface of about two square meters and weights a couple of kilograms or so. Some scientists say that the skin is the envelope that contains the body and others say that the skin is a barrier keeping the inside in and the outside out. Of these definitions, the first one is the most simplistic one and the second one is the furthest one from the truth. It is experimentally observed that the skin regulates the exchange of gases between the body and the external world. It has been shown that the epidermis and the upper dermis take up oxygen from the surroundings, and not only from the blood vessels.¹ It is also well known that water vapor crosses the skin. This phenomenon is called transepidermal water loss (TEWL) and several devices enable one to determine how many grams of water vapor cross the skin per square meter per hour. The skin regulates the thermal exchanges with the external world, as well as the exchange of liquids such as sweat and sebum. The skin also modulates the penetration of topically applied foreign molecules (called xeno-biotics) such as vitamins, irritants, vaso-dilating agents, anti-oxidants and the likes. These are just a few examples indicating that the skin is permeable to many substances, in both directions, and is therefore far from behaving as a barrier.

The zealots of the idea that skin is a barrier might well propose topical treatments to “improve” the barrier, and indeed it is sometimes given to read that a certain product “improves” the barrier function. This statement rests on clinical measurements of the TEWL before and after the application of the product on the skin and is often an indication that the product that “improves” the barrier is indeed occlusive.

Omissions of Occlusion
Occlusive products have two main drawbacks. On the one hand, they hinder the natural process of gaseous exchange between the body and the surrounding. On the other hand, to be dispersed in an emulsion they require large amounts of surfactants. Surfactants are known to indiscriminately kill microorganisms, denature proteins and disorganize lipids. We do know that many surface proteins are enzymes committed to exfoliation and moisture production, we also know that many bacteria are friendly saprophytes and we know for sure that smooth surfaces are associated with well-structured lipid lamellae. In other words, products claimed to “improve” the barrier are at risk of disturbing many of the natural processes on the skin surface. This is to say that to “improve” a barrier that does not exist, certain products jeopardize existing mechanisms necessary to achieve defense and wellbeing. A contrario, it might be worthwhile recalling that the two best-selling moisturizers in the US market do indeed increase the transepidermal water loss, as if better gaseous exchanges with the environment provide more agreeable sensations to the skin or keep it in a more comfortable state.

A better approach to the description of the skin could be to consider the skin as an interface between the inside and the outside. The understanding of the role of the skin as an interface would allow one to conceive and design products for topical application with very diversified functions such as hydration, cooling, warming, soothing, and even protection and repair. The possibility of such a set of functions can be easily considered when one thinks of the skin as an interface, but can’t even be imagined, when one visualizes the skin as a simple, dull, passive barrier. Liposomes carry water into the stratum corneum, zanthalene exerts a soothing effect by acting on the thermal receptors of the skin, vitamin E protects human epidermis in vivo against UV-induced oxidation, niacinamide boosts DNA repair.

Of course, to exert their functions, those xeno-biotics have to penetrate the stratum corneum, and even the simple idea of penetration is conceptually impossible for the zealots of the visualization of the skin as a barrier. Other xeno-biotics can be added to the panoply of the skin care chemist, as long as they can be made to penetrate. The penetration of topically applied molecules into the stratum corneum and the epidermis depends on several parameters. Unfortunately, the penetration of a molecule into the epidermis is not an easy-to-describe phenomenon that depends on the size or charge of the molecule. Perhaps the best telling parameter for penetration of a molecule is the so-called partition coefficient, expressed as:

Partition Coefficient = Log K(OW)


that is experimentally measured by adding the molecule to an octanol/water mixture (OW) and by taking the logarithm of the ratio [K(OW)] of the amount that dissolves in the octanol phase to the amount that dissolves in the water phase. The penetration will be “easier” for xeno-biotics with partition coefficients similar to the partition coefficient of the stratum corneum. The penetration of active molecules can be enhanced by excipients having partition coefficients very different from the partition coefficient of the active: by doing so the net result would be to push the active toward the stratum corneum and achieve the crossing thereof.  To facilitate the penetration into the epidermis one should avoid excipients having partition coefficients similar to the one of the active: if so, the active could well be trapped inside the formula layered onto the surface and not be released into the stratum corneum.

What about the “actives” that should not penetrate, such as the UV filters? One can follow the same line of reasoning; select those filters with partition coefficient different from the one of the stratum corneum and formulate them with excipients having partition coefficients similar to the one of the UV filter itself: by doing so one increases the chances of keeping the UV filters on the surface of the skin. Understanding the skin as an interface allows the blossoming of creative thinking and innovative production! 

Reference:
1. Stücker M et al (2002). The cutaneous uptake of atmospheric oxygen contributes significantly to the oxygen supply of human dermis and epidermis.
J. Physiol. 538:985-994

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Paolo Giacomoni, PhD
Insight Analysis Consulting
[email protected]
516-769-6904

Paolo Giacomoni acts as an independent consultant to the skin care industry. He served as executive director of research at Estée Lauder and was head of the department of biology with L’Oréal. He has built a record of achievements through research on DNA damage and metabolic impairment induced by UV radiation as well as on the positive effects of vitamins and antioxidants. He has authored more than 100 peer-reviewed publications and has more than 20 patents.