Hi-Fi fingerprints

April 1, 2009
Of all the species of animals, only primates have fingerprints and only homo sapiens have fingerprint patterns made up of intricate loops and whorls.

by Bill Landers
[email protected]

Of all the species of animals, only primates have fingerprints and only homo sapiens have fingerprint patterns made up of intricate loops and whorls. Other primates just have straight parallel ridges. So what are fingerprints for and why are ours so different? It's a pretty safe bet they're not just a convenient plot device for mystery writers and fingerprints evolved long before the FBI found a use for them.

Why are fingerprints exclusive to primates? Probably because primates are the only species with opposable thumbs. Walking on two feet instead of four opened up vast new possibilities for our forepaws. We use our hands and fingers to sense and manipulate our environment, not just for locomotion. Dental hygiene wouldn't be possible without them! Fingerprints, as you'll soon discover, make our hands even more useful.

Until recently, no one really knew for what fingerprints were for . The two leading theories were:

  • Fingerprint ridges improve our grip on slippery objects
  • They have something to do with our sense of touch.

Of course, they could do both, but a paper just reported in the journal, Science, (January 2009, Vol. 323) by two French physicists (Alexis Prevost & Georges Debregeas: “A Novel Biomimetic Haptic Sensor to Study the Physics of Touch”) seems to confirm the importance of fingerprints to tactile sensitivity. Without them, we probably couldn't tell the difference between silk and sand.

Our sense of touch comes from structures located a few millimeters below the skin surface called Pacinian corpuscles. These cells are sensitive to pressure and vibration. But how can cells located below the skin detect fine differences in texture on the skin? Were it not for fingerprints, we probably couldn't.

The function of fingerprints

To test the function of fingerprints, the scientists built a patch of artificial skin using a flesh-like elastomer embedded with tiny haptic (touch) sensors that mimicked the sensitivity of our own Pacinian corpuscles. The sensors were placed several millimeters below the skin surface just like the corpuscles in human skin. Then they gently rubbed the fleshy sensor array against glass cover slides. Some glasses were smooth while others were etched with tiny ridges just like fingerprints.

The result is that the haptic sensors were 100 times more sensitive to the vibrations caused by ridged cover glasses than smooth ones. Moreover, Pacinian corpuscles are especially sensitive to certain frequencies of vibration and the vibrations from the slides that had been etched with ridges that matched the height and spacing of human fingerprints produced vibrations in exactly the right frequency range to be detected by Pacinian corpuscles.

In effect, fingerprint ridges act just like the needle in a phonograph. A phonograph needle amplifies the vibrations of the needle against surface variations in the groove of a record to produce sound.

Fingerprints amplify the tiny vibrations from textures moving against the fingerprint ridges. We hear the vibrations with our Pacinian corpuscles and the brain interprets the information as texture. The fidelity of our fingerprints is so fine; we can detect objects as small as 1 micrometer (A human hair is about 50 micrometers.).

The loops and whorls serve a function too. If they were parallel, we'd only be able to detect vibrations coming from certain angles. The loops give our fingers 360 degrees of fine tactile sensitivity.

No matter which way something moves across our fingers, some of our fingerprint ridges will detect it.

So the next time you feel the caress of satin against your fingertips, thank your hi-fi fingerprints!