Page last updated at 04:29 GMT, Friday, 12 June 2009 05:29 UK

Fingerprint grip theory rejected

Fingerprint
The exact function of fingerprints is unclear

Scientists say they have disproved the theory that fingerprints improve grip by increasing friction between people's hands and the surface they are holding.

Nobody is sure of the exact function of fingerprints, but improved grip has long been a favoured theory.

However, in a series of tests to measure friction between a finger and a piece of acrylic glass, it was debunked by a University of Manchester team.

The study appears in the Journal of Experimental Biology.

It's always nice to knock down an urban myth with good data
Dr Jon Barnes
University of Glasgow

Dr Roland Ennos designed a machine which enabled him to measure the amount of friction generated by a fingerprint when it was in contact with the acrylic glass.

The machine was then strapped to the index finger of one of his students.

Dr Ennos expected the amount of friction to increase in proportion to the strength at which the acrylic glass was pushed against the finger.

This would have supported the theory that the fingerprint was helping to improve grip by ramping up friction levels.

However, the results showed that friction levels increased by a much smaller amount than had been anticipated.

Like rubber

Dr Ennos realised that the skin was not behaving like a normal solid, where friction is proportional to the strength of the contact.

Instead, it was behaving like rubber, where the friction is proportional to the contact area between the two surfaces.

To corroborate his suspicion, Dr Ennos varied the area of each fingerpad that came into contact with the surface by using narrow and wide strips of acrylic glass.

This showed that friction did increase as more of the fingerprint came into contact with the surface - so the skin was behaving just like rubber.

But crucially, further measurements revealed that the area of skin in contact with the acrylic glass was always 33% less than if the fingerpads were completely smooth.

This confirmed that fingerprints do not improve our grip, because they actually reduce our skin's contact with the objects that we hold.

In fact, in some circumstances they even seem to loosen our grip.

Instead, Dr Ennos believes that fingerprints may have evolved to help us grip on to rough surfaces, or they may allow our skin to stretch and deform more easily, protecting it from damage.

Alternatively, they may allow water trapped between our finger pads and the surface to drain away and improve surface contact in wet conditions.

Other researchers have suggested that the ridges could increase our fingerpads' touch sensitivity.

Dr Jon Barnes, a biomechanics expert at the University of Glasgow, has carried out research into the adhesive properties of tree frogs' feet.

He said other work, using artificial surfaces, had produced similar results to those in Dr Ennos' work.

"It's always nice to knock down an urban myth with good data," he said.



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