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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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Fine-particle TiO2 is manufactured as nanoparticles that are typically 10--20 nm in size, and some suppliers promote this as the particle size. However, this is somewhat misleading because these nanoparticles form tightly bound aggregates (see above definition), which obviously have a somewhat larger size. These aggregates are the smallest particles actually present in the system and the forces required to break apart the aggregates are far greater than those encountered during manufacturing or application of these products onto skin.
The story does not end there. These aggregates join together to form loosely bound agglomerates. In the dry, powdered form of inorganic sunscreen in which these materials are supplied, the agglomerates have particle sizes greater than 1 micron, placing them well outside the nano-scale range. Yet the Mie Theory states that these agglomerates must be broken down to some degree in the final formulations because such large particles would not be effective as sunscreens and would look white and opaque on skin. The as-yet unanswered question is to what degree the agglomerates break down, and what the particle size is in the final formulation. This is difficult to determine in emulsions because the results from many particle sizing techniques are confounded by the size of the emulsion droplets, making it difficult to distinguish the size of other particles, such as inorganic sunscreens.
The situation is not helped by the plethora of particle-sizing techniques available and the different results given by these methods. Some techniques measure the component nanoparticles while others measure the size of aggregates and/or agglomerates. Also, any measurement is dependent upon how the samples are prepared. This is why the particle sizes quoted by manufacturers can vary so widely; in order to compare different grades, it is necessary to measure them by the same technique and with the samples prepared in the same way.
Safety of Inorganic Sunscreens
This nanomaterials debate seems to be getting sidetracked by arguments over the "true" particle size of inorganic sunscreens and whether or not they really are nanomaterials. The key question remains: Are they safe? Based on the large body of evidence the exists, the answer has to be an overwhelming, "Yes!"
Much of the concern around nanotechnology has been based on the premise that nanomaterials are so new that they have not been properly tested to ensure safety. This may be true of some nanomaterials but it is important to recognize the distinction between inorganic sunscreens and other types of nanoparticles.