Dr Roland Ennos

Mechanics of plant and animal structures

Using a range of techniques we are investigating the engineering of structures in both plants and animals. We have studied the anchorage systems of a wide range of plants and shown that the buttress roots of tropical trees have a mechanical function, preventing failure at the junction between the roots and trunk. These highly stressed regions are packed with mechanical fibres at the expense of reduced investment into vessels, with concomitant reduction in hydraulic permeability. We are also investigating (in collaboration with Prof. Sue Hartley, University of Sussex) the role of silica bodies in grasses in providing mechanical defence against herbivory [Link to pictures]; their abrasiveness helps to wear down the teeth of herbivores. Finally we have determined the mechanical design of human fingernails.

In collaboration with Dr Stephen Eichhorn, of the Materials Science Centre, we are now studying how nail mechanics varies with humidity and between people, and are investigating the comparative morphology of claws and nails. We are also starting to investigate how the mechanical design of our finger-pads allows us to optimize our grip.

Mechanics of human fingernails

Instrumented scissors used to cut fingernails

Instrumented scissors, used to cut fingernails in different directions, show that fingernails are twice as difficult to cut towards the base of the nail as across it. Consequently cracks are deflected around the end of the nail, protecting the nail bed from damage.

Scanning electron micrograph of the torn surface of a human fingernail

Scanning electron micrograph of the torn surface of a human fingernail, showing the three-layer structure.

Diagrammatic representation of fingernail structure

Diagrammatic representation of fingernail structure. The thick middle layer, with its transversely oriented fibres, deflects the cracks The thinner outer layers, meanwhile, have fibres running in all directions. They give the nail its bending strength and, as they wrap around the edges of the nail, prevent cracks from forming.

Silica bodies and abrasive defences of grasses

Backscattered EM of a grass leaf

A backscattered EM of a grass leaf, showing rows of silica bodies.

A modified Martindale wear machine

A modified Martindale wear machine is used to test the abrasiveness of grasses. Samples are held beneath a moving plate which abrades an underlying Perspex plate.