LINEAR SKIN RHEOMETER 329 In practice, these calculations are performed automatically by a simple software program, allowing rapid and simple calibration of absolute force and displacement. SKIN MEASUREMENT USING THEIR LSR For direct comparison with the GBE reproducibility data obtained by Maes eta/. (3), the reproducibility of the LSR was estimated by the same method. Forty consecutive iden- tical measurements were performed on the back of the hand of a female volunteer. Results were analyzed to determine the coefficient of variation of the measurement. To determine the ability of the LSR to measure sensitive changes in stratum corneum mechanics in response to simple hydration, the following study was performed: Two moisturizing formulae of differing hydration performance (products A and B hydration performance was determined by impedance measurements using a Nova TM Dermal Phase Meter 9003, see below) were applied to the back of the hands of 13 female subjects (aged 18-35). The dorsal surface of the hand was chosen for mechanical measurements (a) to conform to previous measurement sites using the GBE (3) and (b) because it is relatively simple to immobilize the hand effectively. The study was performed in a controlled- environment chamber (temperature 20 + IøC relative humidity 45 + 5%). The plastic stub on the end of the LSR wire probe was attached to skin on the back of the hand via a circular piece of double-sided tape (5 mm diameter). LSR measurements were then performed in triplicate. Baseline measurements were performed before product applica- tion. Test products were then applied at a rate of 2 l•l/cm 2 to the entire back of the hand according to a predetermined randomization schedule. LSR measurements were per- formed at one, three, and six hours after product application. As the whole dorsal surface of each hand was used for product treatment, inclusion of an untreated control was not possible. Results were, therefore, expressed as mean difference to initial pretreatment baseline. Hydration performance of products A and B was assessed by randomized application at the same rate as above (2 t•l/cm 2) to 5 x 5-cm sites on the volar forearms of 12 female subjects (aged 18-35 the volar forearm was chosen as the site for hydration measure- ments because of its smooth, hairless morphology and its utility as a standard in this type of testing (6). Each forearm also contained an untreated 5 x 5-cm control site. The study was performed within a controlled-environment chamber (temperature 20 + iøC rela- tive humidity 45 + 5%). Impedance measurements were performed using a Nova TM Dermal Phase Meter 9003 with the standard measuring probe DPM 9103 (Nova In- struments, USA) at one, two, four, and six hours after application, and results were expressed as mean difference to untreated control. RESULTS Forty consecutive measurements on the same subject and same site indicated that the coefficient of variation of the measurement was only 2.9% (Figure 7). This demonstrates very good reproducibility of the measurement technique and compares very favorably with the value of 3% obtained by Maes et al. (3) for the GBE. The variation measured is almost certainly due to movement of the subject during the probe cycle. This has always been the main source of error in these types of sensitive measurements, and

330 JOURNAL OF COSMETIC SCIENCE 12 10 o 15o 200 250 300 350 400 1/DSR Figure 7. Reproducibility of LSR measurement (n = 40 coefficient of variation = 2.9% mean = 281.8 Fm/g SEM = 1.3 Fm/g). various means have been employed to minimize subject movement during readings [e.g., use of a precast plaster mold by Maes et al. (3)]. However, like Cooper et al. (4), we have found that the use of no restraint is preferable and we employ a simple sloping table on which subjects rest their hands. The results of the study using the Nova DPM 9003 to measure the hydration efficacy of products A and B can be seen in Figure 8. Both products induced significant increases (p 0.05 paired t-test vs untreated control) in apparent stratum corneum hydration (as measured by impedance changes) up to, and including, six hours after application. Moreover, product A increased stratum corneum hydration significantly more (p 0.05 paired t-test) than product B at all time points up to and including six hours after application. Water exerts considerable influence on the mechanical properties of the human stratum corneum due to its complex interactions with keratin (7,8). This plas- ticization of the stratum corneum, an essentially viscoelastic material, has been described as skin "softening" (2,3). In the case of topical application of a moisturizing formula, the extent of this softening effect is directly related to the ability of the product to deliver and maintain increased water concentrations within the stratum corneum. This is usu- ally achieved by the delivery of humectant compounds such as glycerol and/or use of