FUNCTIONAL PROPERTIES OF GLYCERIN 231 7. serves to accelerate wound healing 8. aids in the digestion of desmosomes 9. provides a skin protection function 10. enhances corneocyte desquamation 11. does not interfere with biochemical processes in the skin To provide the physiochemical and biochemical basis for the above functionalities of glycerin, reference to a series of seminal scientific papers will be made. EARLIER STUDIES A stereomicroscopic test for moisturizing efficacy (5) developed by Highley et al. is based upon the ability of moisturizers to prevent or alleviate soap-induced dry skin by using the back of the hand as a substrate. One of the advantages of this procedure is that it permits comparative efficacy measurements on moisturizing preparations regardless of their form or water content. Another advantage is that by means of this test using panels of small size, information is generated, which could otherwise be obtained only through clinical testing on large populations. A numerical grading system for relative skin dryness (70 to 80) was developed by means of which a numerical measure of moisturizing efficacy was obtained (differences of 30 to 40 between untreated and treated hands are rated as good). This technique clearly demonstrated that the use of 25% glycerol in water, over a period of four days, was distinctly superior to the use of similar solutions of both propylene glycol and sorbitol. The comparative effectiveness of these polyols is shown in Table III. In a significant paper (6) published in 1984 by Bissett et al., "Skin conditioning with glycerol," a human skin condition study conducted during winter (in Cincinnati, OH) using outside lower leg dry skin was reported. The panels consisted of male and female volunteers aged 25 to 55. One milliliter of test material was applied twice daily to an approximately 150-cm 2 skin area. All panelists used two test materials, one for each leg. Two trained graders using a 0-5 scale did visual grading of the skin. Panelists were allowed to continue their normal bathing/showering practices. Results after two weeks of testing are shown in Table IV. Similar type in vivo studies conducted on dry pig skin showed similar efficacy as the glycerol concentration was increased up to 20%, beyond which increased concentration produced only a small additional benefit. Several other polyols were tested (four weeks of treatment) as skin conditioners on dry Table III Effectiveness of Some Commonly Used Cosmetic Ingredients Stereomicroscope ratings by trained observers Treated Untreated Difference Ingredient tested hand hand (untreated-treated) Glycerin (25% in H20 ) 34 Propylene glycol (25% in H20 ) 71 Sorbitol (25% in H20) 73 68 +34 70 -1 87 +14

232 JOURNAL OF COSMETIC SCIENCE Table IV Improvement in Human Dry Skin Condition by Various Concentrations of Glycerol Grade reduction a Treatment (skin improvement) 5% Glycerol 0.86 10% Glycerol 1.44 20% Glycerol 1.74 40% Glycerol 1.80 • Combined results (after two weeks of treatment) of three studies, no one study involving all treatments. Water was the control in all studies. Starting grade averages were 2.02 to 2.71. pig skin to determine their effectiveness. Liquids such as propylene glycol and 1,3- propanediol (tested as 10% and 50% aqueous solutions) did not provide a skin- conditioning benefit. Aqueous solutions (10% and 50%) of crystalline polyols such as erythritol, xylitol, and sorbitol provided only a marginal benefit. In the discussion section of the Bissett article, the researchers made the following comments: "The exact nature of glycerol's mechanism of action is not clear. Glycerol is a nonvolatile, hygroscopic liquid. These properties presumably allow it to retain water in the skin. The water-glycerol mixture then hydrates and plasticizes the skin to prevent dehydration and the resultant physical damage in a stressful environment. Immediately after application, glycerol also provides a masking of the scales on the skin surface. However, the benefit diminishes with time and is lost with washing. The long-term benefits of glycerol reported here are in the absence of this 'cosmetic cover-up.' Whether glycerol in the viable epidermis can also affect the generation of new stratum corneum is not known. Alteration of the course of tissue synthesis might result in a stratum corneum more resistant to dehydration." This is a significant publication because the researchers thoroughly documented the skin conditioning properties of glycerin and presented insights concerning its potential mechanisms of action. Much of their insights are substantiated in later studies that are summarized in this review. LATER STUDIES With the use of sophisticated bioengineering equipment, Batt et al. (7) studied the changes in the physical properties of the stratum comeurn following treatment with glycerol. Preparations containing the humectant were applied topically to the skin of young adults, and the physical effects on the stratum comeurn were examined using instrumental techniques. The effects measured were reductions in transepidermal water loss and electrical impedance, smoothing of the skin surface profile, and an increase in the coefficient of friction. These effects were found to accompany an improvement in the expertly assessed condition of the skin. Found to last for periods in excess of eight hours, these effects were similar to those observed transiently after the topical application of water. These researchers summarized their views in their 1988 article as follows: "The data