Dedicated nail biters, pay attention. You know what happens when you're nibbling a particularly annoying piece of nail? They never seem to rip downwards, only ever across. Well, Roland Ennos and a team of undergraduate researchers can now explain why your fingernails only splinter in one direction. They have discovered that the central layer of the nail is effectively reinforced by long thin cell structures, directing fractures across the nail no matter which way you try to rip(p. 735).

Ennos admits that this piece of research was inspired by his own nail nibbling habits, but with well-trimmed nails of his own, he needed a willing nail donor before he could begin investigating the tissue's biomechanical properties. Fortunately, his student Stephanie Shayler was happy donating her talons to science. After harvesting 3 mm-long fragments, Shayler slit the nail at the tip and tried tearing downwards. But the nail always resisted the downward rip; the nail seemed to be reinforced along its length. Puzzled by the nails' anisotropic characteristics, Shalyer used precision cutting equipment to cut the nail lengthways, and across, to test her nails'toughness. The nails were twice as tough along their length than across their width. More surprisingly, Shayler's nails were almost as tough as horses'hooves!

But how was the nail structured to resist rips in one direction, and not the other? Using a scanning electron microscope, Shayler and Ennos could clearly see three keratin-laden layers. The top and bottom layers were both made up from flat overlapping slate-like sheets' explains Ennos, and could rip in almost any direction. But when Ennos saw the core layer, sandwiched between the outer two, it was clear why nails only rip in one direction. The middle layer was built up from long thin keratin-packed cells, arranged with their long axes across the nail's width, resisting cracks that might penetrate to the nail's bed. But how tough was the central layer relative to the outer two?

This time Laura Farren set about trying to isolate each of her fingernails'layers to test their toughness. But peeling the layers apart proved tricky,until Farren realised that they were relatively easily separated when her fingernails were soft after a swim. Using the same precision cutting equipment, Farren tested each layer's toughness and discovered that the central layer was four times tougher along its length than its width, and six times tougher than either the upper or lower layers.

So why does the nail need the brittle outer layers when the fibrous core seems to provide most of the nail's inner strength? Ennos explains that,although the core layer strengthens the nail along its length, nails are still relatively fragile across their width. He suspects that it's the tile-like outer layers that give long talons their bending strength, but he adds this study has only started to scratch the surface of fingernail design'.

Farren, L., Shayler, S. and Ennos, A. R.(
2004
). The fracture properties and mechanical design of human fingernails.
J. Exp. Biol.
207
,
735
-741.