Electroosmotic transport of mannitol across human nail during constant current iontophoresis

Objectives: This work aimed to elucidate the role of electroosmosis during trans-nail iontophoresis.

Methods: Passive and iontophoretic experiments were performed after short hydration (10-15 min) of human nail tips. The electroosmotic fluxes of mannitol were determined during anodal and cathodal iontophoresis and at different pH values. Passive controls were also carried out. Four sets of experiments were performed: (a) three anodal delivery experiments using different nails, at pH 4.0, 5.0 and 7.4, (b) one anodal delivery experiment that kept the same nails across two pH stages, (c) one experiment, comprising an anodal delivery stage (pH 4 and 7.4) followed by a cathodal delivery stage (pH 7.4 and 4), which kept the same nails across the different polarities and pH stages, and (d) a passive experiment keeping the same nails across different pH values (4 and 7.4).

Results: The fluxes of mannitol measured were very variable and little difference between passive and electroosmotic transport was observed. Cathodal and anodal fluxes were not always significantly different. Experiments which minimised internail variability suggested that the nails were negatively charged at physiological pH, and that this negative charge was lost at pH 4.

Conclusions: These results suggest a modest and highly variable contribution of electroosmosis to the iontophoretic transungual flux.