Nanomaterials in Personal Care: Opportunities and Safety Considerations

By: Julian Hewitt and Mindy Goldstein, PhD

Posted: November 26, 2008, from the December 2008 issue of Cosmetics & Toiletries.

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Inorganic sunscreens: Perhaps the most common types of nanoparticles in personal care are the UV-attenuating grades of titanium dioxide and zinc oxide. A small particle size is required with these materials to provide effective UV protection in addition to transparency on the skin.

Other inorganic particles: Nano-scale forms of, for example, iron oxide, silica and alumina, also have found application in cosmetic products as colors, fillers or rheological additives.

Of these nano-sized materials, nanoemulsions and vesicles are designed to break down into their constituent ingredients when applied to skin or hair. Thus, in terms of assessing risk, these materials act the same as conventional cosmetic emulsions and delivery systems. Assessing the safety of such systems is then simply a matter of considering the safety of the bulk constituents; the fact that they are in a nano-form in the formulation is irrelevant. For this reason, the primary focus of safety and regulatory agencies has been toward nanomaterials that remain as nano-objects when applied to skin or hair. Summarizing this, the International Cooperation on Cosmetic Regulation (ICCR) has proposed the definition of a cosmetic nanomaterial to be: a stable, insoluble material, purposely manufactured, where the primary particle has at least two dimensions on the order of 1 to 100 nm.

The most commonly used type of cosmetic nanomaterials are inorganic sunscreens and hence these have become the focus of much current debate in regard to nanotechnology in personal care. One aspect of that debate is whether these materials are, in fact, nanomaterials, so this must first be examined.

Efficacy and Particle Size of Inorganic Sunscreens
The optical properties of inorganic oxides of various particle sizes can be calculated by the use of Mie theory,3 a generally well-known theory related to inorganic oxides used as pigments or sunscreens that explains how the optical properties of scattering and absorption vary with particle size and refractive index. Applying this theory to titanium dioxide4 will result with the conclusion that a particle size of less than 100 nm is necessary to achieve effective UV protection while maintaining cosmetic elegance; i. e., the product is transparent in a thin film. The optimum size is calculated to be around 50 nm. Many grades of fine particle TiO2 are available today from various sources and most show good efficacy as sunscreens; however, particle sizes are claimed to range anywhere from 10 nm to well above 100 nm—contradicting the theoretical calculations. The answer to this paradox lies in the confusion arising from different definitions for particle size and the variation in results given by different particle techniques.