PNAS: Scientists have scrutinised how different liquids affect our nails to find out which is best for carrying treatments - such as for fungal infections and psoriasis - deep into the nail. They found that, surprisingly, water was ten times faster at working its way down through the nail surface than the other two solvents examined in the study. Researchers report that solvents diffuse through the human nail in a size- and concentration-dependent manner. Treating nail diseases, such as fungal infections, is difficult because the tightly woven keratin network of the nail acts as a barrier to efficient drug delivery.Richard Guy and colleagues traced the diffusion of three deuterated solvents—water, propylene glycol, and dimethyl sulfoxide—across human nails using stimulated Raman scattering microscopy, which detects intramolecular vibrations at specified frequencies. The authors measured the intensity of these vibrations as functions of time and depth into the nail. Because the bonds in deuterated compounds vibrate at distinct frequencies from compounds containing normal hydrogen, the deuterated solvents could be distinguished from the non-deuterated keratin in the nails. The authors found that water penetrated about 100 µm into the nail after about 30 minutes, whereas the other two solvents penetrated only 40-50 µm after approximately 1 day. The results differed sharply from the behavior predicted by a simple model that assumes constant diffusivity, suggesting that the diffusivity increases as the nail takes up solvent because the solvent opens up the keratin structure. Consistent with this hypothesis, all three solvents appeared to compromise the integrity of the outer nail. The results may help guide the development of improved drug delivery platforms for treating nail diseases, according to the authors.