The long-held notion that fingerprints marks help us grip more firmly appears to be wrong. Instead, a new study finds that the marks actually reduce the friction between skin and surfaces.
"Because there are all the gaps between the fingerprints, what they do is reduce the contact area with the surface," says Roland Ennos, a biomechanicist at the University of Manchester, UK, who led the study with colleague Peter Warman.
Rather than singe the prints off an unlucky student to compare hands with and without prints, Ennos rigged Warman's fingers to a special device that slides a weighted sheet of Perspex across a finger and measures the resulting frictional force.
Ennos and Warman determined that the amount of friction generated went up as more of the fingerprint was touching the sheet, but not by as much as expected. This indicated that the skin was behaving like rubber, where friction is proportional to the contact area between the two surfaces.
With most solids, friction depends on the force of contact between of the surfaces.
Further tests with varying widths of plastic sheet confirmed this behaviour.
Measurements taken from ink marks then revealed that fingerprints actually reduce the area in contact with a surface by about one third, compared with smooth skin. Therefore fingerprints actually reduce friction, Ennis says.
But if they're no good for grip why, then, do we have fingerprints?
A French team of researchers working with a mechanical hand loaded with tactile sensors found that fingerprint-like ridges improved the hand's tactile sensitivity.
Another possibility is that fingerprints help wick water off of our hands, improving grip on wet surfaces, Ennos says. Or alternatively, they might work in coordination with soft finger and foot pads to help skin fit more snugly to abrasive surfaces, reducing shear stress. "We very rarely get blisters on the soles of our feet or our fingertips," he says.
"It's an interesting paper," says Lynette Jones, a mechanical engineer at Massachusetts Institute of Technology in Cambridge. Ennos and Warman make a good case that fingerprints don't increase friction when our fingers apply the kinds of forces used to feel objects, but they didn't test much stronger gripping forces, she says.
Understanding exactly how fingerprints work could help in the design of prosthetic and robotic hands that grasp and feel every bit as well as natural hands.