Hi Guys! I came across this article when researching for my cancer project and since spring is slowly starting to arrive and it
seems that more and more people are out and about in the sun, I thought this
was a good article to post about.
In this article titled Characterization of Optical Properties of ZnO Nanoparticles for Quantitative Imaging of Transdermal Transport, a group of Swiss and Australian scientists used laser
imaging to assess the toxicity of zinc nanoparticles in common sunscreens. The
use of zinc nanoparticles, in this study zinc oxide (ZnO), in modern sunscreen
formulas was adopted for many practical and also aesthetic reasons. ZnO has a
high optical absorption in both the UVA and UVB light and is ideal for
protecting the skin from light radiation. Aesthetically, the zinc makes
sunscreen more cosmetically pleasing as smaller zinc particles make the
sunscreen more transparent. ZnO is regarded as safe for topical applications.
So what’s the big issue?
While ZnO is quite safe if applied to the skin and if it
stays on the skin surface, it can also be very harmful to other cells in the
body. Adding ZnO to sunscreen is only completely safe if the zinc particles do
not go past the stratum corneum, the
uppermost layer of skin. The negative cytotoxic effects of ZnO nanoparticles in
epithelial, neural stem cells, and several types of immune cells have been
confirmed as cited in the article.
What did they do?
Non linear optical microscopy was used to assay the ZnO in
vivo in order to provide a clear image of the subsurface skin layers. Comercial
formulation Zinclear_60CCT was used for the ZnO sample. The original
formulation contained 60% ZnO nanoparticles in organic solvent. The
nanoparticles were then coated with stearic acid to make the surface
hydrophobic in order to allow for its dispersion in silicone based non-polar
liquids, such as sunscreen formulas. In the figure below, the relative size and shape of the ZnO nanoparticles can be observed. The physical properties of the ZnO
nanoparticles were determined using light scattering, fluorescence, UV/Vis
Spectrometry, transmission electron microscopy, and absorption. Skin samples
were taken from mostly female donors between the ages of 30 and 40 years who
underwent abdominoplasty surgery. 10µL of the 60% ZnO formulation was applied
to 1.33 cm2 human skin patches which were fixed in a Franz cell and
incubated at 37 Celsius.
Conclusions?
Basically, this study found that in organic solvent, the ZnO
nanoparticles remained at about 20nm in diameter. In polar solvents, the
diameter increased to about 200nm. It was found that the 20nm primary size ZnO
nanoparticles show very small absorption in the stratum corneum and no detectable absorption in the viable
epidermis or following layers. The figure below shows the optical image generated from the study which shows that the ZnO particles did not penetrate the skin as well as the distribution of ZnO based on skin depth. The nanoparticles were primarily situated in the
uppermost skin layer with some protrusion to skin folds. Basically, it was
concluded that sunscreen is in fact not toxic to our bodies through skin
absorption. So rest assured that your sunscreen will not be your one way ticket to the emergency room.
References:
Zhen Song, Timothy A. Kelf, Washington H. Sanchez, Michael S. Roberts,
Jaro Rička, Martin Frenz, and Andrei V. Zvyagin, "Characterization of
optical properties of ZnO nanoparticles for quantitative imaging of
transdermal transport," Biomed. Opt. Express 2, 3321-3333 (2011)
