KP Ananthapadmanabhan PhD,a James J. Leyden MD,b Stacy S. Hawkins PhDc
aJL Winkle College of Pharmacy, University of Cincinnati, Cincinnati, OH bDepartment of Dermatology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA cUnilever Research & Development, Trumbull, CT
Rebuilding the Skin Barrier With Advanced Skincare Benefit Technologies Beyond the immediate visual dryness reduction and improved skin hydration benefits, moisturization technologies are progressing towards improving damaged skin barrier from wash-off and leave-on formats. As discussed earlier, ensuring minimal changes to skin pH is the first step towards skin barrier repair.Another approach to barrier repair is the use of complex lipid mixtures consisting of ceramides, fatty acids, and cholesterol that can form SC bilayer-like film on the skin surface.43,44 Such films can be expected to reduce the water loss and help repair the barrier. This can be considered as an “outside-in” approach as the barrier reinforcement is essentially coming from the outside. This is similar to the use of an occlusive such as petrolatum. However, because of the differences in the composition of the film, their performance can be different from that of petrolatum.Pro-lipid technologies aim to supply ingredients that the skin can utilize to rebuild the barrier from within. The early work by Rawlings et al involving the use of triglyceride oils as a source of essential fatty acids that skin can utilize to increase the synthesis of ceramides is noteworthy.45 Subsequent work by Harding et al in living skin equivalent (LSE) models showed that fatty acids, especially palmitic acid, can be utilized by skin to increase synthesis of ceramides in skin.46 In these studies, radiolabeled palmitic acid and stearic acid were applied to LSE topically as well as in the culture medium. Analysis of SC lipids after 24 hours of incubation showed the presence of radiolabeled ceramides in them indicating the incorporation of the fatty acids into the building of ceramides (see Figure 6). In this regard, recent work by Bouwstra and team is also noteworthy.47 The latter group showed that addition of deuterated palmitic acid to their cultured skin model resulted in elongation of the fatty acid to longer chain fatty acids such as C22 and C24 fatty acids. This shows another route by which palmitic acids can be taken up by skin to build SC bilayer lipids. Note that in vivo leave-on and wash-off study results with products containing fatty acids such as palmitic and stearic acids have clearly shown that they do penetrate into deeper layers of skin.9,28 In vivo demonstration of conversion of fatty acids, ideally using deuterated fatty acids, may be a logical next step in this research.Similar to the case of fatty acids, externally applied ceramides from leave-on formats in LSE-type models have also been suggested to be taken up by skin in the creation of ceramides.48 Noting that the skin barrier in the case of LSE models is relatively weak compared to a healthy corneum, penetration of ceramides into deeper layers for incorporation is yet to be validated. In fact, some of the recent studies using Raman imaging and microscopy suggests that externally applied ceramides tend to get trapped in skin’s furrows and valleys rather than penetrating deeper into skin.49 Given the two-tailed structure of ceramides, its high molecular weight and its low solubility in typical cleansing and leave-on systems, its lack of penetration into skin is not surprising.Cleansing Technologies Based on the above discussion, a strategy for developing cleanser technologies to meet the various market needs is outlined in Figure 7. Current market trends for various cleansing applications is shown in Figure 8. For a specific type of cleanser
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