Effect of Water on the Mechanical and Frictional Behaviors of Human Fingernails

The fingernails are generally used for a variety of functions including protecting, foraging, and gripping. The achievement of these functions depends on the mechanical properties of fingernails. In this article, the deformation recovery of human fingernails in water was investigated. The effect of water on the mechanical and frictional behaviors of fingernails was studied using a nano indentation and scratch tester. Owing to the swelling of fingernail matrix, the indents under a peak indentation load of 20 N and scratches under a constant load of 70 mN can be fully recovered after the fingernail samples were dipped in water for 10 min. The recovery of these deformations in water reveals obvious anisotropy on the different sections of fingernails. The recovery speed of the indents and scratches on the surface of fingernails is the fastest, and the slowest recovery happens for the deformations on the longitudinal section. With the water in hydrated fingernails evaporating in air, the microhardness of fingernails shows a quick increase in the first 100 min and levels off after 200 min. Finally, the microhardness of fingernails in the fully dehydrated state is 2–3 times larger than that in the hydrated state. The water in fingernails reduces the fluctuation of friction force on surface and protects the surface of fingernails from scratch damage. Owing to the special orientation of filament in the intermediate layer of fingernails, the friction on the cross section of fingernail shows an anisotropic behavior. Either on the dehydrated or hydrated fingernails, the friction coefficient along the parallel direction of filament is lower than that along the vertical direction of filament.