By Gopinathan K. Menon
May 15, 2017
Pretty much everyone in the business of personal care—including the scientists (Formulation, Claims Substantiation, and Measurement Science departments), managers, directors, and vice-presidents, as well as marketing and sales professionals—know or at least believe they understand the barrier properties of skin. It is evaluated by measuring cutaneous water loss, or more precisely trans-epidermal water loss (TEWL). Usually, this is accomplished by a non-invasive instrument (e.g., evaporimeter, Tewameter, Finn chamber, or AquaFlux) under controlled environmental conditions in the laboratory or clinic. The values are usually expressed or converted (in the case of an older instrument such as the Meeco electrolytic moisture analyzer) to Grams/square meter/hour. However, it is worth considering the quote from Kottner and coworkers: TEWL is regarded as one of the most important parameters for characterizing skin barrier function but an agreed upon definition of what a "normal" TEWL is does not exist.1
Over the last several decades, a lot has been published about the skin barrier: how the infant skin barrier is different from adults,2 how ethnic background influences barrier function,3 the effect of age on skin barrier,4 and even how the barrier differs in the same individual depending on the season or even time of the day.5 Dry skin is usually associated with a defective barrier, which opens up a myriad of possibilities when designing moisturizers, cleansers, and soaps with claims of improving the barrier and/or not disrupting the barrier. Claim substantiation centered around the skin barrier is a common topic in skin care product development. I doubt if there is any personal care company or ingredient supplier who does not embrace the skin barrier theme today.
So, what is the skin barrier? There is a very successful and well-attended Gordon Research Conference held on the topic every two years (including 2017). The conference was instituted in 1989 and attracts scientists globally from fields as diverse as molecular biology, engineering, biophysics, dermatology, and pharmaceutical sciences. Such a grand effort with so many stakeholders demonstrates that the "skin barrier" has more to it than TEWL can reveal. Although "barrier" conjures the image of a rigid brick wall, the skin with its multifaceted defense against environmental aggressors is actually an interactive boundary layer—literally and figuratively the touch screen of our body.6 Keeping this perspective while discussing the more popular and understandable term "barrier" is worthwhile when we consider the multiple defensive functions of skin ( see Table 1).
Table 1. Types of skin barriers.
| Type of Skin Barrier | Description and Function |
|---|---|
| Permeability | Keep water in/out (primarily carried out by stratum corneum lipids, but also by tight junctions in stratum granulosum. |
| UV | Mediated by melanin, especially when it forms a microparsol above the nuclei in keratinocytes of the basal layer. Other physiological factors that aid include DNA repair; up-regulation p53 in vitiligo, etc. |
| Physical | Impact resistance (a cushioning provided by the matrix components of the dermis). |
| Oxidant | Mediated by antioxidants such as vitamin E, coenzyme Q10, and enzymes (e.g., superoxidase dismutase, catalase, and peroxidase). |
| Thermal | Antifreeze proteins/heat shock proteins. |
| Antimicrobial | Antimicrobial peptides such as beta defensins, cathelicidins, psoriain, etc. as well as specific types of lipids (elaborated by keratinocytes). |
| Immune | The innate (epidermal) and adaptive (via mediation of Langerhans cells) immune system. |
| Human relations | Xenophobia based on skin color. |
| Physiological | Enzymes that detoxify xenobiotics, such as cytochome 450 dependent enzymes. |
If we consider the skin barrier in other animals, such as birds and marine mammals, the list would also include a 'toxin mantle' to discourage predators and a "barrier to biofowling".7,8 All these defensive functions are inter-related and regulated in a coordinated fashion. So, when one of the functions is altered, several other functions are also affected, primarily due to the intricate cross-talk and signaling between the different cell types that maintain functional homeostasis of skin under constantly changing environmental stimuli.9 Thus, essentially every product can be envisaged as having an effect on skin's barrier functions, be it anti-aging, skin lightening, tanning, or cleansing products. Very often these effects are underestimated. On the other hand, the effects of cleansers and their ability to remove barrier lipids is often overestimated. As most investigators in the barrier field know, it takes a harsh solvent/surfactant and prolonged treatment to remove the barrier lipids that are within the extracellular space between the corneocytes in human subjects. In fact, during normal usage, or even an exaggerated wash procedure, cleansers mostly remove the surface lipids derived from sebum, and possibly some of the barrier lipids from the outermost stratum disjunctum ( the cells that are in various stages of being exfoliated) that does not contribute to the permeability barrier.
As to the permeability barrier, assessed by TEWL, there are significant variations with anatomical location; often highest on the face and lowest on the forearm making it nearly impossible to define a "normal TEWL" for a subject.10 Unfortunately, most of the panelist testing done in early stages of product development are on the volar forearm. If a "barrier product" intended for use on the face is tested on the volar forearm, it is at best an inappropriate test due to the already mentioned differences of these anatomical regions. Again, the penetration of the active ingredients/formula may also be vastly different, thus impacting the proper appreciation of its efficacy. It may also be inappropriate to "strengthen" the permeability barrier of the face, when adaptive evolution has selected this area to be more permeable to facilitate evaporative cooling for thermoregulation.
Finally, there is a close relation between the barrier function and skin moisturization. Skin dysfunctions (winter xerosis, atopic dermatitis, psoriasis, or any of the ichchyotic conditions) that negatively impact its permeability barrier function also lead to dry skin conditions. In these cases, a barrier product is valuable. It is more difficult to effectively treat skin that already has a healthy barrier. To quote Dr. Peter Elias, possibly the most prominent leader in skin barrier research, all barrier products are good moisturizers, but not all moisturizers are good barrier products. As cosmetic scientists designing skin care care products, we could benefit from paying heed to when the barrier is good enough to allow for optimal skin functions.
References
1. J. Kottner, A. Lichterfeld, and U. Blume-Peytavi, Transepidermal water loss in young and aged healthy humans: a systematic review and meta-analysis, Arch. Dermatol. Res., 305, 315-323 (2013).
2. G.N. Stamatas, J. Nikolovski, M.A. Luedtke, N. Kollias, and B.C. Wiegand, Infant skin microstructure assessed in vivo differs from adult skin in organization and at the cellular level, Pediatr. Dermatol., 27, 125-131 (2010).
3. K. Sugino, G. Imokawa, and H.I. Maibach, Ethnic difference of stratum corneum lipid in relation to stratum corneum function, J. Invest. Dermatol., 100, 587 (1993).
4. R. Ghadially, B.E. Brown, S.M. Sequeira-Martin, K.R. Feingold, and P.M. Elias, The aged epidermal permeability barrier. Structural, functional, and lipid biochemical abnormalities in humans and a senescent murine model, J. Clin. Invest., 95, 2281-2290 (1995).
5. G. Yosipovitch, G.L. Xiong, E. Haus, L. Sackett-Lundeen, I. Ashkenazi, and H.I. Maibach, Time-dependent variations of the skin barrier function in humans: transepidermal water loss, stratum corneum hydration, skin surface pH, and skin temperature, J. Invest. Dermatol., 110, 20-23 (1998).
6. G.K. Menon, Caveolins in epidermal lamellar bodies: skin is an interactive interface, not an inflexible barrier, J. Invest. Dermatol., 120(4), xv-xvi (2003).
7. G.K. Menon and J.P. Dumbacher, A 'toxin mantle' as defensive barrier in a tropical bird: evolutionary exploitation of the basic permeability barrier forming organelles, Exp. Dermatol., 23, 288-290 (2014).
8. C. Baum, W. Meyer, D. Roessner, D. Siebers, L.G. Fleischer, A zymogel enhances the self-cleaning abilities of the skin of the pilot whale (Globicephala melas), Comp. Biochem. Physiol. A Mol. Integr. Physiol., 130, 835-847 (2001).
9. P.M. Elias and E.H. Choi, Interactions among stratum corneum defensive functions, Exp. Dermatol., 14, 776-776 (2005).
10. S. Marrakchi and H.I. Maibach, Biophysical parameters of skin: map of human face, regional, and age‐related differences, Contact Dermatitis, 57, 28-34 (2007).