Visualisation of H 2 O 2 penetration through skin indicates importance to develop pathway-specific epidermal sensing.

Elevated amounts of reactive oxygen species (ROS) including hydrogen peroxide (H ₂ O ₂ ) are observed in the epidermis in different skin disorders. Thus, epidermal sensing of H ₂ O ₂ should be useful to monitor the progression of skin pathologies. We have evaluated epidermal sensing of H ₂ O ₂ in vitro, by visualising H ₂ O ₂ permeation through the skin. Skin membranes were mounted in Franz cells, and a suspension of Prussian white microparticles was deposited on the stratum corneum face of the skin. Upon H ₂ O ₂ permeation, Prussian white was oxidised to Prussian blue, resulting in a pattern of blue dots. Comparison of skin surface images with the dot patterns revealed that about 74% of the blue dots were associated with hair shafts. The degree of the Prussian white to Prussian blue conversion strongly correlated with the reciprocal resistance of the skin membranes. Together, the results demonstrate that hair follicles are the major pathways of H ₂ O ₂ transdermal penetration. The study recommends that the development of H ₂ O ₂ monitoring on skin should aim for pathway-specific epidermal sensing, allowing micrometre resolution to detect and quantify this ROS biomarker at hair follicles.Graphical abstract.