JAPAN NANONET BULLETIN - 84th Issue - November 23, 2006

Making skin beautiful with nanotechnology
— Frontline of cosmetic science —

The more red, blue, green and other colors of light overlap, the brighter and more transparent the light becomes. However, the more pigments overlap, the darker they get. Now, nano-scale technology of powder fabrication makes it possible to control colors. One of the key trends in the field of makeup is the creation of foundation based on additive color-mixing technology.

Dr. Takata says, “Human skin is translucent. The skin surface reflects only about 5% of incident light, and most incident light penetrates the skin, and gets scattered and absorbed in it repeatedly.” The light is absorbed by melanin and hemoglobin in the skin, and 40%-50% of the incident light eventually comes out of the skin. Melanin absorbs ultraviolet light as well as visual light, while hemoglobin absorbs green and yellow light with wavelengths of 500-600 nm. Hardly red light is absorbed by these types of bio-pigments and most of the red light comes out of the skin. The red light significantly affects the skin's beauty. Measuring the optical characteristics of dull skin shows that it is less reddish than beautiful skin. Dr. Takata set out to make dull skin look more beautiful -- not with red pigment, but with red interference light.

His team chose plate-like mica powder with a size of 20 μm coated with nanometer-thick titanium dioxide as the base material for the foundation. Some part of the incident light that exposes the powder is reflected by the surface of titanium dioxide films while other parts of incident light penetrates the titanium dioxide film and is reflected at the interface between the titanium oxide film and mica. This layer structure generates a phase difference between these two kinds of reflected light. The phase difference can be controlled by changing the thickness of the titanium dioxide film. Light with a specific wavelength having no phase difference can be intensified. The spectral reflectance of the titanium dioxide coated mica (TiO₂/Mica) exhibits a curve with a peak at around 440 nm in wavelength when the thickness of the titanium dioxide film is 130 nm, which leads the titanium dioxide coated mica to generate blue interference light. At a thickness of 150 nm, the peak of the curve appears at nearly 520 nm in wavelength, and thus green interference light is obtained. At 100 nm, the curve is a downward one bottoming out at around 530 nm, and thus red and blue interference lights are obtained. Dr. Takata says, “Dull skin is not only less reddish but also slightly less bluish. Therefore, the interference light generated by titanium dioxide coated mica with a 100 nm-thick titanium dioxide film is appropriate for such skin.”

Dr. Takata says, “The three key elements of making human skin beautiful are color correction, gloss correction and face shape correction. Interference light is used for correcting colors. Gloss correction is adjustment of reflective property of skin, and is used to show skin quality indispensable for making skin beautiful.” Titanium dioxide coated mica excels at correcting colors, but it gives the skin too much gloss due to its excessively strong specular reflection caused by its plate-shaped particles. Dr. Takata's team found that when inorganic nanoparticles are put into dispersion liquid containing titanium dioxide coated mica as seeds for growing crystals and are reacted after the addition of barium chloride, sodium sulfate and a complex-forming reagent to the liquid, composite powders of titanium dioxide coated mica covered with very fine spherical sodium sulfate particles are obtained. Dr. Takata says, “Very fine particles were attached to the surface of titanium dioxide coated mica and the fine spherical particles reflects light irregularly. Applying this type of composite powders to the skin results in not only matte-like surface reflection with low gloss through the diffused reflection from the composite powder, but also interference light caused by titanium dioxide coated mica.”

Dr. Takata studied the optical reflection characteristics of the composite powder through electromagnetic simulations based on the Finite Difference Time Domain Method (FDTD Method) jointly with Prof. Kazumi Fujima at the University of Yamanashi. Their research found that when 20%-50% of mica-titanium’s surface is covered with fine particles with diameters of 1.0-1.5 μm, light is scattered strongly. Dr. Takata says, “The center of a face applied sparsely with titanium dioxide coated mica covered with spherical particles brightens due to specular reflection of light, and the outer part of the face become darker because of diffuse reflection of light caused by the particles. This contrast makes the face look more three-dimensional. This is a type of face shape correction.” The shape of the fine particles on the surface of titanium dioxide coated mica can be changed to reflective plate-like, spherical or needle-like. Their shape-flexibility indicates another type of face shape correction. He says, “Reflective plate-like particles can collect light. Shading caused by wrinkles or bags can be removed with specular reflection of light at very small areas of the wrinkles or bags.” Controls of optical texture are possible with technology that designs surface morphology of powder on the nano-scale order.

Dr. Takata is also conducting an optical study on human skin jointly with Prof. Jun Yamada at Shibaura Institute of Technology. Their "Monte-Carlo method," based on numerical analysis of light propagation inside human skin, brought unexpected results. They predicted that light coming out of skin covered with powder having a specific refractive index is stronger than that coming out of skin covered with nothing. When light that penetrates the skin comes out of it, some of the light is reflected back into the skin

through total reflection, which occurs at the interface between the skin and air, generating evanescent light. The stronger light that is reflected from the skin covered with powder is probably resulted from a change in the evanescent light caused by powder application. Experiments have showed that scattered light becomes strongest when variously shaped particles of 0.1-1 μm in size are applied to the skin. Dr. Takata says, “If a material from which red light emerges specifically is available, chromatic correction with additive color mixing may not be necessary.” When a titanium oxide-iron oxide composite powder is used, it specifically scatters the reflection light in the skin, and is highly effective in letting light with a specific wavelength come out of the skin. Dr. Takata expects his research to provide a new angle for approaching cosmetic science.

Dr. Takata says that makeup involves applying non-skin materials to the skin, and that there must be a type of artificial beauty created through the combination of the two. He says, “I don’t know yet what this beauty is. What I want to do is not to simply reproduce beautiful human skin but to use cosmetics to create make-up skin that surpasses natural skin. I think that once we establish firm criteria for beautiful skin made up with cosmetics, a new attitude toward applying cosmetics will emerge.” Dr. Takata adds that the role of cosmetic scientists is to build a bridge between science and sensitivity to beauty, which cannot be quantified easily.