The effect of humidity on the fracture properties of human fingernails
2008
SUMMARY Fingernails are a characteristic anatomical feature of primates and their
function is dictated by the environment in which they are utilised. The
present study examined the mechanical properties of human fingernails as a
function of relative humidity (RH) and the subsequent moisture content of the
nail material. Nail clippings were stored at a range of RH values and then
weighed in order to determine their moisture content. There was a non-linear
relationship between the moisture content of nails and the RH of their local
environment. The in vivo moisture content of nails, measured from 55%
to 80% RH, was between 14% and 30%, similar to other keratinous materials such
as claws, hooves and feathers. Cutting tests on the nail samples showed that
the work of fracture was between 11 and 22 kJ m –2 , rising to
a peak at 55% RH and falling at higher and lower humidities. At all RH values
there was anisotropy within the nail between the proximal and lateral
directions, the work of fracture being greater proximally. This anisotropy was
greatest at 55% RH, at which point the proximal work of fracture was double
the lateral value. These results suggest that the mechanical behaviour of
human fingernails is optimised at in vivo conditions; they resist
tearing most strongly under these conditions and particularly resist tearing
into the nail bed. At more extreme humidity levels the fracture properties of
the nail deteriorate; they are brittle when fully dry and fracture and split
when wet.
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