The fracture properties and mechanical design of human fingernails

Abstract

Fingernails are a characteristic feature of primates, and are composed of three layers of the fibrous composite keratin. This study examined the structure and fracture properties of human fingernails to determine how they resist bending forces while preventing fractures running longitudinally into the nail bed. Nail clippings were first torn manually to examine the preferred crack direction. Next, scissor cutting tests were carried out to compare the fracture toughness of central and outer areas in both the transverse and longitudinal direction. The fracture toughness of each of the three isolated layers was also measured in this way to determine their relative contributions to the toughness. Finally, the structure was examined by carrying out scanning electron microscopy of free fracture surfaces and polarized light microscopy of nail sections.When nails were torn, cracks were always diverted transversely, parallel to the free edge of the nail. Cutting tests showed that this occurred because the energy to cut nails transversely, at approximately 3·kJ·m -2 , was about half that needed (approx. 6·kJ·m -2 ) to cut them longitudinally. This anisotropy was imparted by the thick intermediate layer, which comprises long, narrow cells that are oriented transversely; the energy needed to cut this layer transversely was only a quarter of that needed to cut it longitudinally. In contrast the tile-like cells in the thinner dorsal and ventral layers showed isotropic behaviour. They probably act to increase the nail's bending strength, and as they wrap around the edge of the nail, they also help prevent cracks from forming. These results cast light on the mechanical behaviour and care of fingernails.

Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration

“…On the other hand, the enhanced understanding of keratins has fueled the research area of biological keratinous materials with the aim to create bioinspired materials. Some keratinized materials with interesting properties, such as skin [137], quills [138,139], fingernails [140], horns [141,142], whelk egg capsules [116], and bird feathers [143,144], have been studied, with the hopes to obtain mechanisms and principles to design new functional materials, such as light-weight composites, and energy-absorbent materials [145]. This is a new and fascinating area, awaiting more and in-depth explorations.…”

“…cat) show an a-type X-ray diffraction pattern [29]; both are grouped as Nails in this section. Nails serve as a stiff backing to the soft terminal pads, preventing the skin from rolling back- wards over the distal phalanx [140]. Fingernails are one characteristic feature of primates [200]; they are used to lever up objects, open cracks, scratch and fight, during which the loadings are usually from below and cause upward bending forces.…”

“…Scissor cutting tests on fingernails at different layers and different orientations are reported to investigate how the structure design resists bending forces and prevent crack propagation [140]. This is not a perfect fracture toughness test, but is all that is available in the literature.…”

Fingerprints are unlikely to increase the friction of primate fingerpads

“…The skin is attached firmly to the bone around the edges of the nail bed, and the free edge of the nail helps prevent the skin from being pushed around the end of the finger (Farren et al, 2004). The skin is also attached directly to the bone via subcutaneous fibrous tissue (Cauna, 1954), which travels through fatty tissue that cushions the pad.…”

The effect of humidity on the fracture properties of human fingernails

“…An earlier study (Farren et al, 2004), performed by carrying out cutting tests with instrumented scissors, showed that this design is admirably suited to limit and control fracture. The lateral orientation of the fibres in the thick intermediate layer ensures that the work of fracture is greater proximally than laterally, deflecting cracks laterally away from the nail bed and allowing self-trimming.…”