In Vivo Imaging of ZnO Nanoparticles from Sunscreen on Human Skin with a Mobile Multiphoton Tomograph

Reports on the toxicity of inorganic nanoparticles used in sunscreen, in particular of zinc oxide and titanium dioxide, have raised concerns about a possible particle penetration through the skin barrier. We used two-photon imaging to visualize the distribution of zinc-oxide nanoparticles after topical application of a commercially available sunscreen on human skin in vivo and to investigate a possible penetration of nanoparticles beyond the stratum corneum. Two-photon imaging is in particular suitable for these investigations since the excitation and the nonlinear signal light from zinc-oxide nanoparticles as well as the endogenous skin autofluorescence are all spectrally well-separated which allows a clear identification of the signal origin by detection in two-spectral channels. Furthermore, microscopic modifications in the cutaneous structure like skin wrinkles which exhibit different thicknesses of the dermal layers and at the same time are regions where nanoparticle accumulation can be specifically investigated. The results indicate no penetration of nanoparticle through the barrier of the stratum corneum of healthy skin even in microscopic wrinkles.