Multiphoton tomography of micro- and nanocosmetics in human skin: in vivo studies involunteers and patients with ex vivo validation

BACKGROUND: A holy grail in human skin permeation is to be able to quantify how compounds and nanoparticles are absorbed, distributed and exert their effects. Traditionally, the processes used have either been destructive or relying upon interpreting what has exited the skin after absorption, be it in in vivo plasma or urine or through ex vivo skin. Multiphoton imaging and other modalities have revolutionized this landscape. SUMMARY: In this review article, we summarise our experiences in using multiphoton microscopy and fluorescence lifetime imaging in characterizing the skin absorption, distribution and redox effects of nanoparticulate and related delivery systems. Much of our work has been focused on the absorption and safety of nanozinc oxide which started off being an apparently unsafe product to now being a FDA GRAS substance. We have found that the skin is a remarkable physical barrier to zinc oxide nanoparticles, as well as to other nanoparticles, under a wide range of formulation and method of use conditions. However, this work has only been made possible for us by the development of confocal, multiphoton and fluorescence lifetime imaging technologies that can be safely used on human skin.