Development of Biomimetic Tactile Sensing Systems for Cosmetics and Cosmetic Ingredients

The comfortable application of creams and powders and the texture of human skin and hair are essential factors in the design of cosmetics and cosmetic raw materials. However, the mechanisms underlying these diverse and delicate tactile sensations are poorly understood. We developed a "biomimetic tactile sensing system" to reproduce the interfacial phenomena that occur on the skin surfaces and evaluated the "moist" and "dry" sensations of surface-treated cosmetic powders and the texture of organogel and dispersion formulations. This tactile sensing system consists of a finger model contact probe that mimics the fingerprint and mechanical properties of a human finger and a sinusoidal motion friction evaluation device that can reproduce natural and smooth motions. The finger model contact probe, which mimics the fingerprint and mechanical properties of a human finger, was designed such that the elastic modulus, which reflects hardness, and the surface energy, which affects adhesion, were comparable to those of human skin. In addition, grooves of hundreds of micrometers were engraved to imitate fingerprints. A scotch yoke mechanism that converts elliptical motion into sinusoidal motion was introduced into a sinusoidal motion friction evaluation device to reproduce natural and smooth motions. We analyzed the relationship between sensory evaluation and friction data for cosmetics and cosmetic ingredients and constructed a physical model of tactile sensation evocation. For example, the "moistness" of cosmetic powder was strongly felt when the friction coefficient in the sliding process was low, and a gap existed where the frictional force reached its maximum value. Commercially available makeup cosmetics and sunscreens were characterized based on their friction dynamics and classified accordingly. The wax derived from rice bran and rice paraffin was shown to have high oil-gelling ability, and the resulting gel was smooth to the touch, indicating that it is suitable as a raw material for lipstick and cleansing products.