Zinc oxide nanoparticles (ZnO-NP) are commonly used in sunscreens and their use is currently debated due to potential toxicity. The removal of topically applied nanoparticles is an area that has been unexplored, but is relevant due to safety concerns, and the lack of suitable data to support ZnO-NP safety on damaged skin. We aimed to investigate the penetration and removal of ZnO-NPs from intact and wounded in vivo and ex vivo human skin.
Ex / in vivo human skin was damaged by tape-stripping and/or microneedling followed by application of 2mg/cm2 ZnO-NP (approximately 60 nm in size). After 2-hr incubation the skin was washed three times using soap and water. Multiphoton tomography was used to assess ZnO-NP signal before and after each wash.
Washing once removed more than 85 % of the detectable ZnO-NP signal (p < 0.05) from ex vivo intact skin and 83 % from ex vivo tape-stripped skin (p < 0.05). However, only 28 % of ZnO-NP signal (p = 0.5) was removed from puncture sites. A similar trend was found in vivo with removal of 85 % of the detectable ZnO-NP signal (p < 0.05) from intact skin and 93 % from tape-stripped skin (p < 0.05).
Soap and water washing is effective for the removal of ZnO-NPs from the superficial layers of intact and tape-stripped (x40) skin, but not from puncture wounds.