j. Cosmet. Sci., 54, 321-333 (July/August 2003) Evaluation of post-application rheological changes in cosmetics using a novel measuring device: Relationship to sensory evaluation KEN KUSAKARI, MAR1 YOSHIDA, FUMIAKI MATSUZAKI, TOSHIO YANAKI, HIROSHI FUKUI, and MUNEHIRO DATE, Shiseido Research Center, Shiseido Co., Ltd., Tsuduki-ku, Yokohama 224-8558 (K.K., M.Y., F.M., T.Y., H.F.), and Kobayasi Institute of Physical Research, Kokubunji, Tokyo 185-0022 (M.D.), Japan. Accepted for publication October 18, 2002. Synopsis A novel measuring device was developed to study the changes in the rheological properties of cosmetics after application, and the relationship of the results to subjective sensory evaluation was examined. The device can measure the frictional force (0.001-0.1 N) between a probe and the surface of the sample stage, which moves reciprocally at a constant speed (1-8 cm/s). Two different probes (block-type and roller-type) were used under a constant load (0.02-0.3 N). The relationship between frictional force and the spreading resistance was examined using massage gels with the block-type probe. The changes in the measured force with the block-type probe correlated well with the changes in the spreading resistance experienced upon massaging the skin. This indicates that the spreading properties of cosmetics can be evaluated with this device. With respect to stickiness, no clear correlation was found between the measured frictional force and the skin sensation of moisturizing creams and essences using the block-type probe. With the roller-type probe, however, the stickiness of cosmetics due to thickening of polymers, oils, and other ingredients after application to the skin was reflected in the friction curves. The frictional force at 10 min correlated with the post-application stickiness. INTRODUCTION The sensation produced by application to the skin is one of the most important prop- erties of cosmetics, in addition to the long-term physical and physiological effects. With the advancement of formulation technologies, an understanding of the factors underly- ing skin sensation has become more important than ever. In the actual development of cosmetics, sensory analysis is used to find the best formulation for a certain product from a number of candidates. However, this procedure is subjective by nature, can be time- consuming, and sometimes does not afford clear-cut results. To overcome these prob- Address all correspondence to Ken Kusakari. 321

322 JOURNAL OF COSMETIC SCIENCE lems, Aust et•/. (1) developed a more objective and controlled descriptive method. This method involves a trained descriptive panel that is capable of identifying and defining attributes of test products through the use of reference materials and is able to measure the relative intensities of product attributes on a numerical scale. The descriptive method is able to yield consistent data compared with less objective consumer evalua- tion, but it still has difficulties in evaluating large numbers of test samples. As an alternative to sensory evaluation, objective evaluation methods by means of instrumental measurements have been studied using conventional rheometers (e.g., the cone/plate viscometer or reciprocation-type rheometer) (2-5) or measuring systems that simulate the application processes of cosmetics (6-8). The use of i, vivo friction mea- surements to evaluate skin condition has also been studied by many investigators (9- 13). These measurements basically depend on the detection of the force conducted to the probe through a thin film of the sample, although there are differences in probe geometry. Despite the above research, instrumental predictions of subjective sensation on the skin are still rarely used in the actual development of cosmetics. This is partly due to the intrinsic technical difficulties of simulating human sensation processes, which are highly complex and sensitive. The reported procedures may be inadequate to obtain the nec- essary information, since many sensory attributes are used in the subjective evaluation process, such as spreadability, stickiness, richness, dewiness, greasiness, and so on. Moreover, most studies focused on the initial properties of cosmetics and only a few have involved successive measurements (e.g., 0, 5, 10, and 30 rain after the application). Continuous monitoring of the properties of cosmetic samples after application may be the most effective approach to instrumental prediction of subjective sensation. In this paper, we describe a novel rheological measuring device developed to study the rapid changes in the physical properties of cosmetics after application, and we discuss the relationship of the results to the results of sensory evaluation. The characteristics of the system are described in detail, focusing on the detection of the changes in spreadability and stickiness after application. EXPERIMENTAL STRUCTURE OF THE MEASURING DEVICE Figure 1 shows the novel rheological measuring device used in this study. The device can measure frictional force (0.001-0.1 N) between a probe and the surface of the sample stage, which reciprocates horizontally at a constant speed (amplitude: 30 ram speed: 1-8 cm/s). Two different probes (block-type and roller-type Figure 1 C, D) were used with a constant load (0.02-0.3 N). The block-type probe has a curved contact surface (curvature radius: 10 ram) covered with polyimide film. The roller-type probe has an aluminum roller (diameter: 10 ram) equipped with ball bearings (MF63, NSK, Tokyo). The bearings were lubricated with low-viscosity oil (decamethylcyclopentasiloxane, Shin- Etsu Chemical, Tokyo) each time, just before the measurement, and the baseline friction (the friction measured without sample application) was less than 0.001 N at 0.05 N probe load. The surface of the sample stage was covered with polyimide film. The detected force was conducted through a leaf spring (10 x 25 x 0.2 ram, phosphor bronze) to a