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.

332 JOURNAL OF COSMETIC SCIENCE • 90 • 8o ß 70 •. 6o .,o 5o = 40 ß o 30 E 2O E =. 10 • o I I I I I I 0 1 2 3 4 $ 6 -O-A -•-B Time (hours) Figure 9. Relative performance of products A and B as measured by LSR. While open-loop techniques can be used successfully in perfectly stable environments, no account can be taken of instantaneous fluctuations in such a system (notably, in this case, subject movement). With the LSR closed-loop system, the true force applied to the skin is measured at a rate of 1 KHz and corrective action taken within 1 ms to restore that measured force to the required value. This system helps ensure that the test sequence is reliable, repeatable, and can dynamically adjust for the inevitable variations that occur during in vivo testing. Put another way, because this system allows instanteous compensation of error resulting from the conversion of an electrical signal to a mechani- cal force, the need for the friction-free gas-bearing arrangement of the GBE is elimi- nated. This allows the deployment of a compact, efficient, and flexible new instrument. ACKNOWLEDGMENTS The authors wish to thank Dr Paul Stevens (Paul Stevens Mechanical Design, Tunbridge Wells, Kent, U.K.) for his significant skill and expertise in the design of the LSR, and also Dr Chris Gummer of Procter & Gamble HABC Ltd for his guidance throughout the development project. REFERENCES (1) M. S. Christensen, C. W. Hargens III, S. Nacht, and E. H. Gans, Viscoelastic properties of intact human skin: Instrumentation, hydration effects and the contribution of the stratum corneum. J. Invest. Dermato/. 69, 282-286 (1977). (2) C. W. Hargens Ill, "The Gas Bearing Electrodynamometer (GBE) Applied to Measuring Mechanical Changes in Skin and Other Tissues", in Bioengineering and the Skin, R. Marks and P. A. Payne, Eds. (MTP Press, Hingham, MA, 1981), pp. 113-122.