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Moisturization is an essential part of skin care. Almost every chemical measured in my cosmetic life has demonstrated moisturizing benefits to human skin. Maybe I was lucky—perhaps I chose good moisturizing molecules to measure; however, I have often said that if I were to grind up a computer disc, even that would moisturize my skin. From this, one could conclude that moisturizing human skin would and should be easy.
This became a problem to select benchmark chemicals as positive and negative controls for skin moisturization. The problem arose when selecting a negative control, as it seems that everything moisturizes the human skin. The only thing needing moisturization was human skin itself; therefore, I chose untreated skin as a negative control and glycerin as a positive control. I applied a product and expressed the result of that as a relative performance moisturization (RPM), where untreated skin had value of 0% and glycerin-treated skin a value of 100%. After six hours, the worst moisturizer of all chemicals tested turned out to be water.
Water and glycerin are interesting molecules when discussing skin moisturization. There is a large content of water within the human body that stays inside the body thanks to an efficient barrier called the stratum corneum. But the barrier itself dries out towards the surface since the external environment is significantly less moist than the internal human body. The barrier is moisturized via the production of the natural moisturizing factor (NMF), which is a mixture of amino acids, urea, lactate and other polar hygroscopic molecules.
One of the most hygroscopic molecules is glycerin but it is not a part of the NMF. It is a raw material applied in large quantities via cosmetic products to give skin back its youthful appearance and to allow it to maintain flexibility. It must be applied from the exterior since there is no natural glycerin present within the skin—at least, that is what was first thought. Then it was realized that the phospholipids present in the cell membranes of actively proliferating keratinocytes are broken down during the transformation of these cells, resulting in terminally differentiated keratinocytes and glycerin. This glycerin will penetrate into cells and act as a skin moisturizer. But what regulates the uptake of glycerin into cells?
Aquaporins is a word that is underlined with red squiggles in my Word documents. Software engineer Bill Gates probably does not know what it means and he is not alone, since most consumers are in the dark about aquaporins. It might even be new for you readers but you are excused. In the second edition of Dry Skin and Moisturizers–Chemistry and Function by Marie Lodén and Howard Maibach, published in 2006, there are only two references in the index to aquaporin-3, the epidermal water/glycerol transporter. In a very recent article in the Journal of Investigative Dermatology by Mariko Hara-Chikuma and Alan S. Verkman, titled "Roles of Aquaporin-3 in the Epidermis," aquaporin-3 (AQP-3) is reviewed and readers learn that it is involved in skin hydration, wound healing and skin tumorigenesis.