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.

LINEAR SKIN RHEOMETER 333 (3) D. Maes, J. Short, B. A. Turek, and J. A. Reinstein, In vivo measuring of skin softness using the gas bearing electrodynamometer, Int. J. Cosmet. Sci., 5, 189-200 (1983). (4) E. R. Cooper, P.J. Missel, D. P. Hannon, and G. B. Albright. Mechanical properties of dry, normal, and glycerol-treated skin as measured by the gas-bearing electrodynamometer, J. Soc. Cosmet. Chem., 36, 335-348 (1985). (5) C. W. Hargens III, "The Gas Bearing Electrodynamometer", in Handbook of Non-Invasive Methods and the Skin, J. Serup and G. B. E. Jemec, Eds. (CRC Press, Boca Raton, FL, 1995), pp. 353-357. (6) V. Rogiers, M.P. Derde, V. Verleye, and D. Roseeuw, Standardised conditions needed for skin surface hydration measurements, Cosmet. Toiletr. 105, 73-82 (1990). (7) J. L. Leveque, M. Escoubez, and L. Rassneur, Water-keratin interaction in human stratum corneum, Bioeng. Skin, 3, 227-230 (1987). (8) J. L. Leveque, "Water-Keratin Interactions," in Bioengineering of the Skin: Water and the Stratum Corneum, P. Elsher, E. Berardesca, and H. I. Maibach, Eds. (CRC Press, Boca Raton, FL, 1994), pp. 13-22.