GLYCEROL EFFECT ON STRATUM CORNEUM LIPID 63 mectants, have been found to plasticize the stratum corneum even under conditions of low humdiity (30% RH) (21,22). It has been suggested that such materials act to break hydrogen bonds between keratin chains, rendering the stratum corneum more pliable (20). Although this mechanism may be correct, we propose that the ot-hydroxyacids may also act directly on the skin lipids to prevent the transition of liquid crystal to solid crystal state. Because glycerol is a small polar molecule, we suspect that it maintains fluidity of the lipid membrane through interaction with the polar headgroups of the bilayer rather than by penetrating the alkyl chains. On the other hand, the structure of the ot-hydroxyacids, particularly the longer-chain (C•o, C•2) species, may enable these compounds to penetrate the fatty acid chains of the bilayer, interrupting the close packing of these chains and thereby enhancing the fluidity of the membrane. Studies are planned to test this hypothesis using our model lipid system. Glycerol is also frequently added to biological samples prior to freezing, to prevent the disruption of membranes and denaturation of proteins. Probably glycerol interferes with the formation of large crystals (23). APPLICABILITY OF MODEL TO IN VIVO SKIN RESPONSES An important objective in this work is to establish the predictive nature of the behavior of the model skin lipid in response to cosmetic ingredients. Insofar as the present results show that glycerol helps to maintain the normal state of the intercellular lipid under extremes of relative humidity, our model is consistent with in viva studies of the effect of glycerol, where cracking, flaking, and roughness were reduced by the application of glycerol to the skin. Bissett and McBride (16) used an animal model (pig epidermis) as well as human skin to demonstrate that glycerol visibly improves dry skin in a dose-de- pendent manner and that this improvement is accompanied by an increase in skin levels of glycerol. The related liquid polyols, ethylene glycol, propylene glycol, and 1,3-pro- panediol evaporated rapidly from the skin surface and supplied no conditioning effects. Solutions of the crystalline polyols erythritol, xylitol, and sorbitol crystallized on the skin upon evaporation and provided negligible conditioning benefits. Glycerol, being a nonvolatile liquid, is retained in the skin for long periods. The conditioning action of glycerol depends on the ability of this compound to penetrate the skin. These authors (16) supposed that the mechanism of action of glycerol, once in the skin, was to retain water, preventing the physical changes that accompany dehydration. Our data suggest an alternative mechanism. Batt and Fairhurst (17) also measured in viva skin conditioning by glycerol in humans. As in the study cited above, glycerol was found to penetrate and accumulate in the stratum corneum, where its presence showed resistance to soap-and-water washing. Glycerol gave long-term benefits, such as an improved visible appearance and reduced surface roughness. Highley et al. (9) have shown that serial treatment of the hands with 25% glycerol solution following handwashing with soap prevents induction of skin dryness. This effect is observed even after a final handwash not followed by glycerol application i.e., glycerol does not simply mask skin dryness. We confirmed these results in our labora- tory and demonstrated that glycerol specifically prevents soap-induced cracking, rather than erythema (unpublished observations).

64 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Regarding true hydration of the skin, our model lipid predicts that glycerol, present at 5- 15% of the intercellular lipid, will reduce water loss and that 15% adds water to the skin, from the atmosphere, when the skin is exposed to high humidity. The model predicts that glycerol will have no effect on the water content of skin exposed to low humidity but that glycerol will act as a skin conditioner, maintaining the pliability of the tissue. It is difficult to make the comparison between these in vitro results and in vivo studies of skin hydration by glycerol because the latter studies do not measure water content per se. Rather, one infers this from transepidermal water loss (TEWL) measurements. There have been conflicting opinions as to the significance of TEWL values. It was originally believed that moisturizing agents occlude the skin and reduce TEWL. How- ever, more recent evidence indicates that increased skin hydration parallels increased water flux (24,25). Thus, a true "moisturizer" may cause an increase in TEWL. In the case of glycerol, these trends are dependent on the environment. It has been found that the addition of glycerol reduces TEWL of skin under high humidity (65% RH) (16). On the other hand, when glycerol is added to skin in vivo at low humidity, TEWL values increase (12,14), persumably due to the attraction of water from the lower epidermal layers into the stratum corneum, where it then evaporates. Glycerol thus acts as a conduit for water loss under dry atmospheric conditions. This effect probably accounts for the visible improvement in skin by glycerol under dry conditions. Unfortunately, our low hu- midity data cannot be compared with these in vivo findings because our model does not include a hydrated reservoir beneath the stratum corneum lipid. This is a shortcoming of the model and must be borne in mind when comparing our experimental results with those of in vivo studies. In total, the model lipid appears promising as a predictive tool for studies of skin moisturization/conditioning. REFERENCES (1) S. E. Friberg and D. W. Osborne, Samll angle x-ray diffraction patterns of stratum corneum and a model structure for its lipids, J. Disp. Sd. and Technol. 6, 485-495 (1985). (2) A.M. Kligman, in The Epidermis, W. Montagna, Ed. (Academic Press, New York, London, 1964). (3) G. Imokawa and M. Hattori, A possible function of structural lipids in the water-holding properties of the stratum corneum, J. Invest. Dermatol., 84, 282-284 (1985). (4) C. Prottey, P.J. Hartop, J. G. Black, and J. I. McCormack, The repair of impaired epidermal barrier function in rats by the cutaneous application of linoleic acid, Br. J. Dermatol., 94, 13-21 (1976). (5) P.M. Elias, B. E. Brown, P. Fritsch, J. Goerke, G. M. Gray, and R. J. White, Localization and composition of lipids in neonatal mouse stratum granulosum and stratum corneum, J. Invest. Der- matol., 73, 339-348 (1979). (6) P.M. Elias, Lipids and the epidermal permeability barrier, Arch. Dermatol. Res., 270, 95-117 (1981). (7) S. E. Friberg, I. Kayali, and L. Rhein, Direct role of linoleic acid in barrier function: Difference between linoleic and oleic acid, J. Disp. Sci. and Technol. (in press). (8) K. Laden, The role of glycerol in skin hydration, J. $oc. Cosmet. Chem., 13, 455-458 (1962). (9) D. R. Highley, V. O. Savoyka, J. J. O'Neill, and J. B. Ward, A stereomicroscopic method for the determination of moisturizing efficacy in humans,J. Soc. Cosmet. Chem., 27, 351-363 (1976). (10) M. A. Lampe, A. L. Burlingame, J. A. Whitney, M. L. Williams, B. E. Brown, E. Roitman, and P. M. Elias, Human stratum corneum lipids: Characterization and regional variations, J. Lipid Res., 24, 120-130 (1983).