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Posted: Oct 30, 2007
Does coating nanoparticles make them safe(r) for cosmetics?
(Nanowerk Spotlight) Our title today poses a loaded question. The cosmetics industry of course would argue that their products are already safe, whether they use nanoparticulate ingredients or not. On the other hand, there are research reports that show that nanoparticles could cause DNA damage and could accumulate in organs (with unknown consequences). It has not conclusively been proven, or disproven, that nanoparticles in cosmetics applied to the (healthy) skin are able to penetrate the skin and get into the body.
Sunscreens are a good example for the pro & contra discussion about nanoparticles in cosmetics. Most people use sunscreen for two reasons: to avoid getting sunburn and to avoid getting skin cancer. If applied frequently and thoroughly, sunscreens do prevent sunburn. However, no one has ever determined that sunscreens actually prevent skin cancer. Another, mostly aesthetic, limitation with sunscreens is that they don't rub into the skin very easily. You rub, and rub, and rub, but still your skin has that pasty, white appearance. That's due to the two most common active ingredients in sunscreens - zinc oxide (ZnO) and titanium dioxide (TiO2). These inorganic materials are used in sunscreen in order to reflect UV radiation and reduce the amount of organic materials necessary to achieve a specific SPF (sun protection factor) value, but the drawback is they leave that unsightly white film.
To resolve this problem, manufacturers have started using nanoparticles in place of the bulk forms of zinc oxide and titanium dioxide because the smaller particle size reduces the visibility of the cream. This could potentially mean solving one problem by creating another because TiO2 nanoparticles - a major component of photovoltaic cells - emit photoelectrons when exposed to UV light. These electrons, in turn, induce the formation of peroxides, free radicals and other reactive oxygen species (ROS) which interact with lipids and DNA, causing damage which may lead to a host of medical problems. Researchers have now found clear evidence that titanium dioxide nanoparticles catalyze DNA damage. Fortunately, they also came up with a solution - by coating them - that would allow these nanoparticles to be used with less risk in cosmetics.
The controversy over the use of nanoparticles in everyday cosmetics has been going on for a while now. At best, the evidence is inconclusive – it's too early to say whether there is a risk or not. Regulators have no specific research findings to act on. The Therapeutic Goods Administration (TGA), Australia's version of the U.S. Food and Drug Administration) put it this way (in February 2006; there has been no update since):
"There is evidence from isolated cell experiments that zinc oxide and titanium dioxide can induce free radical formation in the presence of light and that this may damage these cells (photo-mutagenicity with zinc oxide). However, this would only be of concern in people using sunscreens if the zinc oxide and titanium dioxide penetrated into viable skin cells. The weight of current evidence is that they remain on the surface of the skin and in the outer dead layer (stratum corneum) of the skin."
Cosmetic firms of course claim that their products are safe and comply with all the relevant laws and regulations. On the other hand, they fight tooth and nail to have to label their products containing synthetic nanoparticles. This was made very clear by a statement from Dr. Gerhard Nohynek of L'Oréal France who said at the NanoRegulation Conference in St. Gallen/Switzerland in early September:
"Labeling warnings should inform the consumer about potential risks, and not hypothetical risks – and there is no evidence that nanoparticles in sunscreens pose a health risk. If such a labeling warning would be obligatory and given that the cosmetic industry is very sensitive to consumer concerns, some producers could remove nanoparticle-based UV filters from sunscreens in order to avoid labeling. Thus such a regulation may create a genuine public health risk – given 1 million new cases per annum of skin cancers in the EU & US – by labeling a hypothetical risk. This would be detrimental to public health."
Nohynek also mentioned during his speech at NanoRegulation that "if labeling would be obligatory, some companies might no longer use nanoparticles in their sunscreens, resulting in poorer protection against UV radiation and, thereby, an increased skin cancer risk." (quoted from The Innovation Society newsletter)
We had to read this quote several times, because it doesn't seem to make sense, but we believe it more or less says that, rather than labeling a 'hypothetical risk' (by the way, labeling laws are about informing the consumer what's in a product, not necessarily making statements about their riskiness), some cosmetics firms would be willing to sell products that could be less effective (assuming nanoparticle-based sunscreens are more effective) in protecting against damaging UV radiation, thereby increasing the risk of skin cancer. And the culprit would not be the cosmetics industry but rather the regulators trying to help consumers get information what's in their products and help them when making their purchase decisions. How's that for being "very sensitive to consumer concerns"?
The cosmetic industry's stance is even more perplexing given the common sense approach that regulators are taking. The UK Government, for instance, suggested this (in their response to the report 'Nanoscience and nanotechnologies: opportunities and uncertainties’):
"The Government agrees that ingredients in the form of manufactured free nanoparticles should undergo a thorough safety assessment by the relevant scientific advisory body before they are used in consumer products. The DTI, and other relevant departments, will discuss with our European partners the most effective mechanisms for referral to the relevant scientific advisory committees and for responding to their advice to ensure the safety of manufactured unbound nanoparticles in cosmetics and other consumer products. We believe that disclosure of testing methodologies used by industry will help set the right climate of co-operation and advancement between industry, regulators, and the science community in developing best practice. Such an open approach will also help build public confidence... Government believes in consumers being able to make informed choices. Existing labelling requirements on consumer products would need to be revised to accommodate this."
The real question, though, is whether nanoparticles in cosmetics can actually penetrate healthy skin cells and travel through the nuclear membrane to reach the DNA. Furthermore, even if they did get that deep ('deep' being a relative term here – we are talking nanometers and micrometers) it is not known if the UV light could still reach the nanoparticles and sensitize reactive oxygen species in the skin.
But why incur this risk at all? Since the photocatalytic activity is the culprit in all the damage scenarios, Rafailovich, director of the Garcia Center for Polymers at Engineered Interfaces at Stony Brook, and her colleagues propose to eliminate the photocatalytic activity simply by effectively blocking the emission of the surface electrons. Thereby the ROS generated by the sunscreen itself could be reduced and prevent any subsequent damage that the sunscreen could potentially have caused.
In their paper, they demonstrate that this could be accomplished by chemical grafting of anti-oxidant molecules directly onto the TiO2 particle surface, using sonochemistry (the effect of sonic waves and wave properties on chemical systems). This would minimize free radical formation, while still providing protection against UV irradiation.
The scientists used an antioxidant formed from grape seed extracts and anionic polymer to create the polymer coating. Coated and uncoated particles with a mean diameter of 30 nm were suspended in a lotion-type emulsion, spread on a slide, and left to dry for 15 minutes. The SPF of both samples was then measured, using a UV spectrophotometer, to ensure that no degradation of screening occurred during the coating process. Both samples showed an equal SPF of 22.
To determine whether the polymer coating was effective in reducing the electron emission from the particles when exposed to light, the scientists measured florescence using Red Dye 28. After simulated sun exposure for 27 hours, most of the dye was removed from the uncoated particles, while the coated sample was identical in color to the control sample that was not exposed to UV light at all.
"Since the degradation of the dye florescence is known to result from the electron emission from the TiO2 particles' surface, these results indicate that even though the coating does not effect the SPF value, or the UV absorption, it is very efficient at preventing the emission of the electrons in the solution surrounding the particles and producing free radicals," say the authors.
They also suggest that the polymer coating could prevent discoloration (the dreaded white film) commonly found in TiO2 based-sunscreens.
After determining the effectiveness of the polymer coating in blocking the emission of the electrons, the scientists performed further tests to measure DNA damage caused by the free radicals. They exposed DNA, DNA with uncoated TiO2 nanoparticles, and DNA with coated TiO2 nanoparticles to UVA, UVB and UVC light for four hours. While results varied for the DNA and DNA mixed with TiO2 nanoparticles, the results showed nearly no damage to the DNA mixed with the polymer-coated nanoparticles.