j. Soc. Cosmet. Chem., 35, 171-181 (May/June 1984) A new method to evaluate the softening effect of cosmetic ingredients on the skin MOTOJI TAKAHASHI, MASAMI YAMADA, YASUHIKO MACHIDA, Shiseido Laboratories, 1050 Nippa-cho, Kohoku-ku, Yokohama, Japan 223, and YUKIO TSUDA, Department of Legal Medicine, Yokohama City University School of Medicine, Yokohama, Japan 232. Received August 11, 1983. Synopsis An apparatus was developed for measuring the changes of rheological properties of the stratum corneum with time when cosmetic ingredients were applied. By using this instrument, skin softening effects were evaluated from the ratio of dynamic elastic moduli before and after treatment. The extent of plasticization of the stratum corneum by aqueous humectant solutions varied depending on their water-holding capacities. It was determined that the higher the water-holding capacity, the more the plasticizing effect. Furthermore, the effects of water, liquid oil, surfactants, and protein denaturing agents on stratum corneum softening were examined. Although the stratum corneum was immediately softened by the application of water, the elastic modulus gradually recovered to the non-treated level as water evaporated. On the other hand, the mechanical properties were not affected at all by the application of polar or non-polar oils which are widely used in cosmetic products. It was also found that dimethyl sulfoxide (DMSO) and lactic acid markedly plasticized the stratum corneum and that the effect of surfactant solution was characterized by an increase of elasticity even after water evaporated. INTRODUCTION The mechanical properties of the stratum corneum and their relationship to water con- tent have been extensively investigated, and it is well known that the flexibility of the stratum corneum depends on its content of water and hygroscopic substances (1-6). However, most of the previous studies have been concerned with an extension method which has some shortcomings in evaluating softening effects of topical moisturizer on stratum corneum. First, it must be considered that in these experiments the mechanical properties could not be measured on the same stratum corneum before and after treatment. The results from the one sample of stratum corneum may not be compared directly with the results from another since no two stratum corneum samples have the same cross-sectional area. Secondly, this method has the disadvantage that the change of mechanical properties with time could not be examined sequentially after the application of cosmetic ingre- dients to the stratum corneum. Lastly, these experiments have the disadvantage that the test samples of stratum corneum must be soaked in a solution to examine the effects 171

172 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of humectants. These treatment conditions are considered to be far different from the usual application of cosmetic products to the skin. This paper describes a method which overcomes the shortcomings described above. It can precisely estimate the change of skin-softening effects of humectants with time. Experimental results obtained with the method are presented. EXPERIMENTAL MATERIALS The ingredients used were cosmetic grade with no further purification. Stratum cor- neum was removed from human abdominal skin (35-year-old male) by the heat-tryp- sinization method described by Kligman and Christophers (7). MECHANICAL MEASUREMENT The rheological properties of the strips of stratum corneum (20 X 3 mm) were measured by a specially constructed dynamic measuring instrument at 25øC, 50% relative hu- midity (RH). Figure 1 shows a schematic drawing of its principal parts. The measure- MOVING COIL TYPE SPECIMEN I CROSSHEMOTORSERVO 1 - \ RECORDER INITIAL STRAIN SET CIRCUIT OPERATION CIRCUIT MEASUREMTIMECIRCUITSET PRINTER Figure 1. Schematic drawing of principal parts of the apparatus.