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Infinitely Big Expectations from Infinitely Small Particles
Posted: May 20, 2008
page 2 of 3
Obviously, nanotechnology is being used, but what are the exact benefits of these nanoparticles over conventionally sized TiO2 and ZnO? It sounds strange but the benefit is that you cannot see them. It need not be explained that if the particle size of an inorganic sun filter is on the order of magnitude of 25 nm, sunscreen formulations are transparent while the nanoparticles are big enough to scatter UV radiation. However, one obvious question has been: If the particles are that small, do they then penetrate the skin? The short answer to this question is a simple no, but undoubtedly ellaboration is needed.
Gerhard J. Nohynek, Jürgen Lademann, Christele Ribaud and Michael S. Roberts recently reviewed all available literature describing skin penetration of sunscreen agents in a review beautifully titled, “Grey Goo on the Skin? Nanotechnology, cosmetic and sunscreen safety.”1 They reviewed literally every paper that has been published in this field and reached the same conclusion—that nanoparticles will penetrate into the stratum corneum layers of the skin but will never reach the deeper layers, the viable epidermis or the dermis. The next issue of Skin Physiology and Pharmacology will contain the much-anticipated sequel to this article; and a sequel to such a long review within one year alone shows that the number of new publications emerging in the field of nanotechnology is enormous.
In sunscreen technology, the benefit of size reduction is the transparency; however, too small is also not good because the capability to scatter and reflect UV light is lost. But where is nanotechnology used in the rest of cosmetics? To be honest, I personally have not really seen it, although some argue that even things like liposomes are now nanotechnology. If size really matters and an antiaging nanoparticle product would penetrate to the same depth as a sunscreen nanoparticle, and that is not beyond the stratum corneum, then the use of such particles for such applications does not sound like a viable idea since they would never reach the viable epidermis and the dermis, known to be the regions where the majority of biologically active ingredients is supposed to work.
Of course, other tricks could be devised to ensure that this would happen but nanoparticles being advertised in antiaging products would envoke the need for two different types of evidence. First of all, evidence that it works by penetrating the skin, since all current skin penetration evidence indicates the opposite. Secondly, evidence that there is a benefit of the size reduction; i.e., does a non-nanosized particle not give exactly the same result? Cosmetic scientists will come up with a couple of smart ideas to get the particles to penetrate the deeper layers of the skin but the advantage of this would be to have them do so only where necessary.
If you are still concerned about the potential skin penetration of these particles, it would be suggested to read the work of Mike Roberts.2 He calculated theoretically that the skin penetration of 30 nm nanoparticles is in the order of magnitude of 10-18 nmol/mL. If the safety hazard is zero and the exposure is zero, then the safety risk has to be zero as well.