RALEIGH, N.C.—According to a study at North Carolina State University, UVB-damaged skin slightly enhanced titanium-dioxide or zinc-oxide nanoparticles penetration in sunscreen formulations, but no transdermal absorption was detected (Toxicol Sci. June 3, 2011). Sunscreens containing titanium-dioxide (TiO(2)) and zinc-oxide (ZnO) nanoparticles (NP) are effective barriers against UVB damage to skin, although little is known about their disposition in UVB-damaged skin. Therefore, researchers exposed pigs to UVB, giving them a moderate sunburn. For in vitro studies, skin in flow-through-diffusion-cells were treated for 24 hours with four sunscreen formulations: 10 percent-coated-TiO(2)-in-oil/water (o/w); 10 percent-coated-TiO(2)-in-water/oil (w/o); 5 percent-coated-ZnO-in-o/w; and 5 percent-uncoated-ZnO-in-o/w. TiO(2) primary particle size was 10 x 50 nm with mean agglomerates of 200 nm; ZnO mean was 140 nm. Skin was processed for light (LM), 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 TiO(2) NP 17-layers into stratum corneum (SC), while ZnO remained on the surface. TOF-SIMS showed TiO(2) and ZnO epidermal penetration in both treatments. Perfusate analyzed by TEM/EDX or inductively coupled-plasma-mass-spectrometry (ICP-MS) detected no titanium or zinc, indicating minimal transdermal absorption. In vivo, skin was dosed 24 hours occluded with formulations and at 48 hours. TiO(2)-NP in o/w formulation penetrated 13-layers into UVB-damaged SC, while only seven-layers in normal; TiO(2) in w/o penetrated deeper in UVB-damaged SC. Coated and uncoated Zn-NP in o/w were localized to the upper 1-2 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 titanium within epidermis and superficial dermis, while zinc was limited to SC and upper epidermis in both treatments.