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.