Characterisation of a Novel Flexing Diffusion Cell (CutaFlex TM) for Assessing Dermal Exposure to Nanoparticles

Abstract

Nanoparticles are thought to present a unique hazard to human health. Furthermore, the increasing use of nanomaterials in consumer products has not been accompanied by relevant risk assessments. It is conceivable that skin flexion may assist the translocation of nanoparticles across the stratum corneum. However, current in vitro methodology to study dermal absorption involves the exclusive use of immobile skin within diffusion cells. Therefore, a novel skin-flexing diffusion cell system (“CutaFlex™”) was developed to incorporate reproducible skin flexing (2 flexes min-1; 6 mm maximum amplitude). The initial aims of this Thesis were to characterise the CutaFlex™ system to eliminate the possibility of flexion-induced (experimental) skin damage, demonstrate equivalence with historical permeability data to model compounds and assess the effect of skin flexing on barrier disruption. Subsequent work aimed to investigate the hypothesis that nanoparticles require dermal flexion to penetrate intact skin. In supporting these aims, this Thesis also performed work to assess the correlation between direct measurements of skin barrier function (using tritiated water) and transepidermal water loss (TEWL), the effect of flexing on the performance of topical skin protectants (barrier creams) and to further validate in vitro diffusion cell measurements against in vivo data acquired under identical conditions. The results demonstrated that skin flexing did not alter skin barrier function and that the CutaFlex™ system was in general agreement with historical measurements of skin permeability. Furthermore, controlled chemical or physical damage to the stratum corneum was not exacerbated by skin flexing. Skin flexion did not facilitate the dermal absorption of a range of nanoparticles (quantum dots). However, differences in the partitioning of nanoparticles into the stratum corneum were observed (independent of the degree of flexing), with greater amounts of negatively charged nanoparticles found in the superficial layers of the stratum corneum in comparison with positive or neutral nanoparticles. Flexing had a modest effect on the performance of a skin barrier cream which was limited to low dose applications; an effect tentatively ascribed to flexion-induced movement of cream to previously untreated areas. A poor correlation was found between 3H2O water permeability and TEWL flux. Most importantly, there was excellent agreement between in vitro skin permeability studies and in vivo studies (which used a surrogate measure of skin permeability). To summarise, the data in this Thesis has led to the development and characterisation of a novel diffusion cell (CutaFlex™), capable of simultaneously flexing skin whilst performing dermal absorption measurements comparable with the OECD-compliant models.