GLYCEROL TREATMENT OF SKIN 375 30- 20- OJ 0 10 20 30 40 50 60 70 80 90 100 Relative humidity (ø•o) at 24øC +-0-5 ø Figure 7. Increase in phase delay readings from untreated skin with increasing ambient relative humidity. Figures show mean _+ S.E. of ten readings per data point on a typical subject. the glycerol solutions at 10 minutes and all subsequent time points (p 0.05 by comparison with the water control, Student's paired t-test). TEWL values on glycerol- treated skin remained lowered for at least four hours. TEWL was also markedly decreased by the application of glycerol in emulsion systems (Figure 3) (p 0.02 at all time points between one and seven hours by comparison with the non-glycerol control, Student's paired t-test). SKIN SURFACE TOPOGRAPHY A mean Rtm value of 70 + 3.3 (S.E.) Ixm was obtained on the forearms at ambient temperature and relative humidity. When excess water was applied for 1 minute and then patted dry, there was a rapid mean decrease of 15 Ixm in the Rtm value (p 0.05, Student's paired t-test) which disappeared within 30 minutes (Figure 4). The skin smoothing from the addition of water was just detectable to the naked eye and very reproducible. Application of an aqueous solution of glycerol to the skin surface (Figure 4) enhanced and prolonged the smoothing effects of water (p 0.02 at 30 minutes, one hour, and two hours, Student's paired t-test). Similar effects were found when glycerol was incor- porated into an oil-in-water emulsion (p 0.02 at two and five hours, Student's paired t-test). The effect was still discernible after 24 hours (Figure 5).

376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 80• 70- 60- (n 50-• --•40- 30- 20- 10- • 0 10 20 30 40 50 60 Time(min) Figure 8. Effects on phase-delay readings from skin treated with water (O) and 15% glycerol solution (A). The broken line shows the pretreatment value. Mean values + S.E. are given for each data point (n = 5). COEFFICIENT OF FRICTION Untreated forearm skin which had equilibrated with the environment was found to have a coefficient of friction of kt = 0.20 _ 0.01 (S. E.). Topically applied water showed a marked effect. A mean peak value of kt = 0.75 - 0.09 (S. E.) was observed between two and five minutes after treatment (Figure 6), after which friction fell within ten minutes to the pretreatment value. Application of 15 % aqueous glycerol (Figure 6) gave a similar mean peak friction value, kt = 0.68 - 0.04 (S.E.), after a short lubrication period (15 minutes). Elevated readings were recorded for periods in excess of two hours.