Is there penetration of titania nanoparticles in sunscreens through skin? A comparative electron and ion microscopy study

We report on a comparative study by Transmission Electron Microscopy (HRTEM) and Scanning Transmission Ion Microscopy (STIM) combined with Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-Ray Emission (PIXE) on ultra-thin and thin cross-sections, respectively, of various skin samples (porcine skin, healthy human skin, human skin grafted on a severe combined immuno-deficient mouse model) to which we applied topically various formulations containing titanium dioxide (TiO2) nanoparticles with primary particle sizes in the range from 20-100 nm. Whereas the HRTEM and STIM/PIXE images reveal clear differences - mainly related to the different thickness of the cross-sections - they unambiguously show that penetration of TiO2 nanoparticles is restricted to the topmost 3-5 corneocyte layers of the stratum corneum (SC).