Distribution of ZnO nanoparticle containing formulations in skin measured by in vivo multiphoton fluorescence/SHG/HRS tomography

Objective: Among other scattering centers TiO₂ and ZnO nanoparticles are commonly used as UV filters in commercial sunscreen products. Their penetration into the skin is controversially discussed in the literature. ZnO in biological tissue is harder to detect than TiO₂ because zinc is also present in the human organism in dissolved form. Multi-photon tomography providing nonlinear effects can be used to investigate the penetration of ZnO nanoparticles into the human skin in vivo.

Material and methods: The distribution of ZnO nanoparticles (30nm in size) in the horny layers of the epidermis, as well as in the furrows, wrinkles and orifices of the hair follicles was analyzed using effects of second harmonic generation (SHG) and hyper-Rayleigh scattering (HRS) induced by ZnO nanoparticles. These methods permitted to distinguish between the particular ZnO and dissolved forms of zinc.

Results: A detection limit of 0.16µg/cm² was estimated for ZnO. The investigations could show that ZnO nanoparticles penetrate only into the outermost layers of the stratum corneum, the furrows and the orifices of the hair follicles and do not reach the viable epidermis.

Conclusion: Multi-photon tomography is a well suited method for measureing the penetration of different formulations into the skin, including ZnO nanoparticles.

Acknowledgement: We would like to thank Beiersdorf Company, Hamburg, providing the sunscreen, Prof Michael Roberts of the School of Pharmacy & Medical Sciences & Sansom Institute for Health Research, University of South Australia, for stimulating and helpful discussion and the Foundation “Skin Physiology” of the Donor Association for German Science and Humanities for financial support.