EFFECTS OF AGE ON SKIN PROPERTIES 463 (2) H. G. Vogel, Influence of age, treatment with corticosteroids and strain rate on mechanical properties of rat skin, Blochim. Biophys. Acta, 286, 79-83 (1972). (3) H. G. Vogel, Correlation between tensile strength and collagen content: Effect of age and cortisol treatment, Conn. Tiss. Res., 2, 177-182 (1974). (4) H. G. Vogel, Measurement of some viscoelastic properties of rat skin following repeated load, Conn. Tiss. Res., 4, 163-168 (1976). (5) H. G. Vogel, Tensile strength, relaxation and mechanical recovery in rat skin as influenced by maturation and age,J. Med., 7, 2, 177-188 (1976). (6) H. G. Vogel, Strain of rat skin at constant load (creep experiments), Gerontology, 23, 77-86 (1977). (7) H. G. Vogel, Age dependence of mechanical parameters in rat skin following repeated strain, Akt. Geronto/., 8, 601-618 (1978). (8) H. G. Vogel and W. Hilgner, Influence of age and desmotropic drugs on the step-phenomenon observed in rat skin, Arch. Dermatol., 264, 225-241 (1979). (9) If. G. Vogel, Directional variations of mechanical parameters in rat skin depending on maturation and age,J. Invest. Dermatol., 76, 493-497 (1981). (10) H. Holzmann, G. W. Korting, D. KobeIt and H. G. Vogel, Priifung der mechanischen Eigenschaften von menschlicher Haut in Abhiingigkeit von Alter und Geschlecht, Arch. klin. exp. Derre., 239, 355-367 (1971). (11) H. G. Vogel, in press (1983). (12) H. G. Vogel, Attempts to compare in vivo and in vitro measurement of mechanical properties in rat skin, Bioengineering and the Skin, 3, 1, 39-46 (1981). (13) H. G. Vogel, Mechanical properties of rat skin as compared by in vivo and in vitro measurement, Bioengineering and the Skin, 3, 3, 198-209 (1982). (14) H. G. Vogel and K. Denkel, Methodological studies on biomechanics of rat skin comparing in vivo and in vitro results, Bioengineering and the Skin, 4, 1, 71-79 (1982).

j. Soc. Cosmet. Chem., 34, 465-476 (December 1983) Friction of skin LESZEKJ. WOLFRAM, Clairol Incorporated, 2 Blachley Road, Stamford, CT 06902. Received September 1983. Presented at the IFSCC/SCCJoint Conference on Skin, San Francisco, September 1983. INTRODUCTION The process of consumer evaluation of skin product attributes involves two key subjective perceptions: visual and tactile. While the former addresses primarily the aesthetics of treated skin (shiny, dull, oily, etc.), the latter deals with the much more complex problems of skin texture, its suppleness and smoothness, roughness, greasiness and dryness, etc. Tactile perception is a dynamic process. In its simplest form, it involves a sequence of two events--a contact and a movement of a probe (finger) across the tested surface. The relative movement of two surfaces, gliding over each other, is resisted by friction. The latter can be objectively evaluated and a number of techniques have been evolved to carry out such measurements on skin. This review updates the available information in the field of skin friction and discusses its utility in terms of generating sound and readily interpretable data related to consumer- perceptible attributes. METHODOLOGY OF MEASUREMENT AND FRICTIONAL PROPERTIES OF SKIN A variety of techniques have been employed to evaluate skin friction. The first Amontons' law which gives us a definition of the coefficient of friction g, (g = F/W where F is the frictional resistance to movement and W is the weight of or load on the probe), states the basic requirements for its measurement. Thus, any technique that can simultaneously measure the normal load between the contacting surfaces and the force necessary to initiate motion can be used to determine g. While all of the techniques developed for the measurement of skin friction conform to this requirement, three general approaches in basic instrument design have been adopted: 1. Gravity (tension) loading/pully combination devices (1), 2. Measurement of the deceleration of the rotating probe (2-6), and 3. Deflection of a spring (7-9). Figure 1 illustrates the basic design of the first approach. The instrumentation is simple 465