SUMMARYFingernails 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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The indentation properties of human fingernails at varying humidity are reported. The samples were indented using both microindentation, to obtain their Vickers hardness and also nanoindented using a Berkovich indenter tip. The relative humidity (RH) of the samples was controlled by using salt solutions with a sealed and enclosed environment surrounding the testing equipment. It was shown that the Vickers hardness of the samples is sensitive to RH, with recovery of the nail material more readily occurring for nails tested at >55% RH. This recovery mechanism is discussed in terms of the structure of the nails, and this approach is also suggested as a technique for following recovery mechanisms in natural materials under varying humidity. The hardness obtained by nanoindentation is similar to previously published data, but does not change with humidity. The modulus of the nails is also insensitive to relative humidity, but in the same range as the value derived from the microindentation tests.

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