Titanium Dioxide (TiO2) is the engineered nanomaterial (ENP) that appears most commonly in commercial products, including cosmetics and sunscreens, which are applied directly to the skin. Because nanoscale TiO2 has been shown to be toxic to some cells under some conditions, many research studies have been conducted to investigate whether nano TiO2 can penetrate the skin barrier and pose a potential threat to cells beneath it.
The majority of those studies have shown that topically applied TiO2 does not penetrate the outermost layer of skin. That's because when ENPs are contained in liquids, small clusters of ENPs tend to aggregate, accumulating into larger clusters that are too large to pass through the skin. But a new paper published by members of Bren professor Arturo Keller's lab suggests that exposure to sunlight can cause ENPs that are aggregated in liquid to break up into smaller clusters that can pass through the skin.
The paper, titled "Photoinduced Disaggregation of TiO2 Nanoparticles Enables Transdermal Penetration," was lead-authored by Bren School PhD student Samuel Bennett; his co-authors were Professor Keller, former Keller student Dongxu Zhou (PhD 2012), and PhD student Randall Mielke. The paper was published in the Nov. 14 issue of the journal PloSO ONE and is available online here.
"Our research has found that when some liquid mixtures of nanoparticles are exposed to natural sunlight, small nanoparticles can separate from the larger cluster of nanoparticles," the researchers write. "Given this phenomenon, we investigated whether photo-induced disaggregation could assist nanoparticles [to move] across a skin barrier.We found that titanium dioxide nanoparticles applied topically to pig skin, an analog for human skin, in the presence of sunlight — they could penetrate the skin."
The authors explain in the paper's abstract that the phenomenon of "photo-induced disaggregation" has not been previously reported or considered, either in coagulation theory or in "transdermal toxicological paradigms." They found that after just a few minutes of exposure to sunlight, the diameter of TiO2 aggregates in liquid was reduced from ~280 nm to ~230 nm, meaning that disaggegration was taking place. Further, pig skin that was exposed to the liquid nanoparticle suspension in natural sunlight had nearly three times the amount of TiO2 inside the skin as did skin that was exposed to the same suspension in the dark.
These findings, say the authors, "are in stark contrast to much of the literature and may have important health consequences, Although the TiO2 particles used in this study were bare and not contained in an organic matrix, as is the case with some sunscreens, our results suggest that topically applied TiO2, found in cosmetics and other skin-care products, may be much more mobile than previously thought."