EVALUATION OF SKIN FRICTION 15 The computerized friction instrument consists of a probe with an oscillating steel plate that permits objective measurements of the surface slide friction (1,2). The operating principles of the instrument have been described in detail previously (1). The probe is applied perpendicular to the surface and the friction is measured at an axial load of approximately 0.1 N. At this low load no twisting or wrinkling of the skin occurs. The topographical features of the steel probe were determined using profilometry, according to a previously described method (3). The surface of the probe was much smoother than normal skin (data not shown). SENSORY ASSESSMENT OF FRICTION Eleven females constituted the panel for sensory evaluation of the creams. They were all familiar with tactile measurements. Their mean age was 46 years, range 41-50. The perceived degree of friction was marked on a 15-cm visual analogue scale, where the endpoints of the line reflect a continuum from very low to very high friction. Before application of the creams, the panelists were "calibrated" by estimating the basal level of the skin friction. They were instructed that the basal level was anchored 6 cm from the left end of the line, and that the scale values emanated in two directions from this normalized point. The key benefit to the use of the line comes from its ability to diminish the variation in the panelists' rating and that it gives the panelists the oppor- tunity to use the scale in a way they found comfortable. The products were submitted to the panelists in random order. They were dispensed from coded volumetric syringes to the inner surface of the panelists' forearms. The application rate was 5 mg/cm 2. The panel members were asked to spread the product over and into the skin with fingertips. The ease of moving the fingertip over the surface during 10 s was evaluated as friction. The sensory magnitude of friction was estimated at product application and every fifth minute thereafter during 15 minutes. The fin- gertips were cleansed between each evaluation. CONSUMER TEST Fifteen users of skin care products were selected among subjects with no apparent connection to any of the tested creams. Their mean age was 4 ! year, range 29-57. Three products (A, B, and E) were dispensed to the arms of the volunteers as described above. The subjects were asked to spread the products on the skin surface and assess the degree of liking during spreading. Fifteen minutes later they were again asked to assess the degree of liking of the skin resistance. The results were marked on an analogue scale, where the left end reflected very unpleasant friction and the right very pleasant friction. The subjects were also asked to mark on a five-point hedonic scale the term that best represented their attitude about the feeling: Score Sensation 1 Much too slippery 2 Too slippery 3 Pleasant 4 Too stiff 5 Much too stiff and tacky

16 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS CALCULATIONS AND STATISTICS The results from creams A and E were compared statistically with the results from cream B. In the subjective evaluation, the distance from the left end to the marker on the analogue scale yielded numerical values used for statistical analysis. These results and the results from the five-point hedonic scale were evaluated statistically using the Wilcoxon signed rank test. Student's t-test was used to compare the values from the instrumental mea- surements. Probability values less than 0.05 were regarded as significant. RESULTS AND DISCUSSION The friction of the skin following application of five moisturizing creams was studied. The friction was evaluated with a new friction instrument as well as by panelists trained in sensory evaluation. During application of the products the friction of the skin was lower than that of untreated skin. This result was obtained with instrumental as well as with sensory O O eA ß B [] C 20 /•D &E 15 10 $ 5 $ 5 10 15 20 25 30 Time, rain Figure l. Instrumental evaluation of the frictional resistance of the skin following application of five moisturizing creams. The results are the mean from seven subjects - S.D. s = Significantly different from cream B.