This is an English translation of a news release in Japanese on July 17.
The Skin Care Research Laboratory and Material Science Research Laboratory of Kao Corporation (President: Michitaka Sawada) have discovered that adhesion of airborne particulates such as dust and pollen to skin can be inhibited by use of a surface covering film with an irregular convexo-concave microstructure.
Fine particles, such as dust, pollen, and pollution particulates, are known to floating invisibly in the air and can easily attach to skin, causing problems such as irritation (itchiness, rough skin, etc.) and dullness. Furthermore, long-term exposure to such particulates has been reported to accelerate skin aging factors like flecks and wrinkles. A survey conducted by Kao showed that some respondents in Japan were concerned about adhesion of these particles during the pollen season, while more than 80% of respondents in China, Thailand, and Vietnam, where air pollution levels are high, answered that skin condition deterioration was caused by fine pollutant particulates.*1
There are three types of forces of attraction between solids like skin and airborne substances, namely, Van der Waals, electrostatic, and liquid bridge forces. Each can vary depending on the size of the objects in contact with one another. For fine particles ranging from 2.5-30 μm, Van der Waals force is the most important. Thus, Kao has focused attention on understanding how to reduce Van der Waals force between fine particulates and skin in order to develop technology to control fine particle adhesion.
The strength of Van der Waals force is characterized as, "the closer the distance between interacting substances, the stronger the force becomes". Thus, Kao examined the effect of extending the distance between the spherical object (particle) and adhesion surface (skin) (Fig. 1). In order to simulate this force, convexo-concave irregularities with varied roughness on the surface were prepared. This led to identification of areas where fine particles sized 2.5-30 μm showed lower levels of adhesion (Fig. 2).
Results of the simulation revealed that formation of fine convexo-concave irregularities on a flat surface clearly inhibited adhesion of fine particles. Based on these findings, Kao investigated how to form a film with a fine irregular convexo-concave microstructure on the surface of skin. Utilizing UV scattering agents, such as titanium oxide and zinc oxide, it was possible to form film on the skin surface with a fine irregular convexo-concave microstructure, which demonstrated significantly reduced adhesion of fine particulates, like pollen and dust.
However, when UV scattering agents are blended under practical conditions, the ability to reduce adhesion is generally lost because the surface of the film is submerged under compounding oil (Fig. 3, left). Kao overcame this problem by making the UV scattering agents difficult to wet with an oil agent. This led to successful formation of a fine convexo-concave film with UV scattering agents on the surface (Fig. 3, right).
Experiments conducted with an artificial leather surface to which a sunscreen with the new technology had been applied clearly confirmed the ability to inhibit adhesion of model pollen, as the level of adhesion was significantly reduced as compared to untreated leather (Fig. 4).
Additional experiments were conducted with the novel sunscreen applied to one side of the face of subjects and sunscreen without this technology applied to the other side. After 5 hours of exposure, dirt containing fine particulates was wiped from the skin with a nonwoven cloth and quantified. The results confirmed that the quantity of particulate dirt was significantly less on skin applied with sunscreen that included a fine convexo-concave film coated with UV scattering agents as compared to the conventional sunscreen film (Fig. 5).
Kao focused on ease of adhesion of airborne fine particles to skin depending on the amount of skin surface roughness. It was shown that adhesion of irritant particulates to skin could be inhibited by formation of a thin film comprised of a fine irregular convexo-concave microstructure. Furthermore, that result was practically achieved by controlling the wetting of UV scattering agents with oil.
Development of technology based on this discovery paves the way for unique products such as sun care applications that effectively protect skin from particulate irritants and pollutants.
Kao creates high-value-added products that enrich the lives of consumers around the world. Through its portfolio of over 20 leading brands such as Attack, Bioré, Goldwell, Jergens, John Frieda, Kanebo, Laurier, Merries and Molton Brown, Kao is part of the everyday lives of people in Asia, Oceania, North America and Europe. Combined with its chemical division, which contributes to a wide range of industries, Kao generates about 1,500 billion yen in annual sales. Kao employs about 33,000 people worldwide and has 130 years of history in innovation. Please visit the Kao Group website for updated information.