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

LINEAR SKIN RHEOMETER 331 120 400 80 60 20 I I ! I I I "•B 0 I 2 3 4 5 6 Time (hour,) Figure 8. Relative hydration performance of products A and B (as measured by impedance). occlusive lipidic films. Indeed, in the case of products A and B, product A might be expected to leverage a greater increase in stratum corneum hydration due to its higher glycerol content (4% [w/w] glycerol in A, in contrast to 3% [w/w] in B) and formulation (gel network, in contrast to a simple oil-in-water emulsion in B). For products A and B, therefore, one would expect to be able to measure (a) absolute significant increases in softness for both treatments and (b) differing relative changes in skin softness for both treatments in accordance with their apparent hydration performance. Results of the study using the LSR to measure the effects of products A and B on stratum corneum mechanics are presented in Figure 9. Both products induced significant in- creases (p 0.05 paired t-test vs pretreatment baseline) in skin softness at all time points up to and including six hours after application. Moreover, product A induced greater increases in skin softness than product B throughout the time course, signifi- cantly so (p 0.05 paired t-test) at six hours after application. These results compare favorably with the relative hydration profiles of the two products (Figure 8). The LSR is, thus, able to measure subtle changes in stratum corneum mechanics in response to hydration and to distinguish between the effect of topical application of moisturizing products and differing relative hydration performance. CONCLUSIONS The control system used for the LSR provides the instrument with an inherently more accurate and reliable measurement capability because it employs closed-loop (feedback) control. The GBE, in contrast, uses an open-loop method of control whereby a prede- termined current is applied to the solenoid and assumed to be transformed into the desired force. As no determination of the actual force generated is made at the time of measurement, it is difficult to know with certainty the true force applied to the skin.