Safety Evaluation of Sunscreen Formulations Containing Titanium Dioxide and Zinc Oxide Nanoparticles in UVB Sunburned Skin: An In Vitro and In Vivo Study

Sunscreens containing titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles (NP) are effective barriers against ultraviolet B (UVB) damage to skin, although little is known about their disposition in UVB-damaged skin. Pigs were exposed to UVB that resulted in moderate sunburn. For in vitro studies, skin in flow-through diffusion cells were treated 24 h with four sunscreen formulations as follows: 10% coated TiO2 in oil/water (o/w), 10% coated TiO2 in water/oil (w/o), 5% coated ZnO in o/w, and 5% uncoated ZnO in o/w. TiO2 (rutile, crystallite) primary particle size was 10 x 50 nm with mean agglomerates of 200 nm (range ca. 90nm--460nm); mean for ZnO was 140 nm (range ca. 60--200nm). Skin was processed for light microscopy, scanning (SEM) and transmission electron microscopy (TEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). UVB-exposed skin had typical sunburn histology. TEM showed TiO2 NP 17 layers into stratum corneum (SC), whereas ZnO remained on the surface. TOF-SIMS showed TiO2 and ZnO epidermal penetration in both treatments. Perfusate analyzed by TEM/energy dispersive x-ray spectroscopy or inductively coupled plasma mass spectrometry detected no Ti or Zn, indicating minimal transdermal absorption. In vivo, skin was dosed at 24 h occluded with formulations and at 48 h. TiO2 NP in o/w formulation penetrated 13 layers into UVB-damaged SC, whereas only 7 layers in normal skin; TiO2 in w/o penetrated deeper in UVB-damaged SC. Coated and uncoated Zn NP in o/w were localized to the upper one to two SC layers in all skin. By SEM, NP were localized as agglomerates in formulation on the skin surface and base of hair. TOF-SIMS showed Ti within epidermis and superficial dermis, whereas Zn was limited to SC and upper epidermis in both treatments. In summary, UVB-damaged skin slightly enhanced TiO2 NP or ZnO NP penetration in sunscreen formulations but no transdermal absorption was detected.