370 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 17- 16- 15- 14- 13- 12- 11- 10- 9- 8- 7- 6- .. 5 4 3- 2- 1- O- I I I I I I I I I 5 30 60 90 120 150 180 210 240 Time (min) Figure 2. Effects on transepidermal water loss of topically applied water (O) and aqueous glycerol solu- tions at concentrations of 5% (•') and 15% (&), (p 0.05 for all glycerol solutions by comparison with the water control at ten minutes and all subsequent time points, Student's paired t-test). The broken line shows the pretreatment value. Mean values ñ S.E. are given for each data point (n = 10). It was found that a simple approach to skin friction was sufficient to monitor the effects of water and humectants such as glycerol. The apparatus used was a weighted cylinder located on the end of a viscometer spindle in such a way that it was free to slide vertically under the effects of gravity. The cylinder was supported by the skin, whilst it was rotated at a predetermined speed by the viscometer motor (12,13). The torque applied to maintain constant speed of rotation divided by the weight of the cylinder produced the value of dynamic friction (ix). MEASUREMENT OF ELECTRICAL IMPEDANCE To a first approximation (14) the stratum corneum behaves electrically in a manner

GLYCEROL TREATMENT OF SKIN 371 54 53 52 51 5O 49 • 47 • 45 o '• 8- •: 7- 6- 5- 4- • 1 I I i I I I I I 0-02 I 2 3 4 5 6 7 Time (h) Figure 3. Effects of oil-in-water cream containing 10% glycerol (I) on transepidermal water loss (p 0.02 by comparison with the non-glycerol control cream (O), at all time points between one and seven hours, Student's paired t-test). The broken line shows the pretreatment value. Mean values -+ S.E. are given for each data point (n = 10). similar to a simple electric circuit (Figure 1). At a given frequency of applied alter- nating voltage it resembles a resistor (R•) and capacitor (C) connected in parallel. Water increases the capacitance of the outer layers of dry stratum corneum (15), and the increased plasticity of hydrated skin improves the electrical contact that an electrode makes when gently held by its own weight on the skin thus any instrument which measures the contact resistance, internal resistance, capacitance, or a combination of these parameters (impedance at a stated frequency) should detect an increased water content after hydration. In order to examine the electrical characteristics of the stratum corneum, a phase delay