Researchers have discovered that by regulating moisture levels, our fingerprints play a key role in our ability to grip objects.
Fingerprints are unique to primates – and, incidentally, koalas – and scientists have long wondered why they evolved, and in particular what advantages they have over the smooth pads found on animals like cats or bears.
These regions, found at the tips of our fingers and toes, are made up of tiny “waves” of skin called epidermal ridges. Compared to flat skin they have a much greater density of sweat glands, which curiously tend to respond to anxiety and emotional states rather than temperature changes.
Now, a new study led by Seoul National University (SNU) in South Korea has found that these ridges interact with our sweat glands to regulate friction between our skin and an object, giving us the ability to grip a variety of different surfaces.
“This work demonstrates that profound influence of friction in the way that we perceive the tactile attributes of an object,” the researchers write in a paper in the journal PNAS.
“During contact with solid objects, the ridges are important for grip and precision manipulation by regulating moisture levels – from either external sources or the sweat pores – so that friction is maximised, and catastrophic slip is inhibited.”
The team, led by SNU’s Seoung-Mok Yum, used spectroscopic and tomographic imaging techniques to study the way moisture behaved on a fingertip in contact with a glass surface.
When the fingertip was initially dry, it secreted sweat and thus increased friction until close contact between the epidermal ridges and the glass blocked the sweat glands. When the fingertip was initially wet, the “valleys” between the ridges acted like fluid channels to help evaporate excess moisture.
Over a series of experiments, the team showed that the fingertip adjusted moisture levels up or down to maximise friction between the finger and a surface.
“Abundant low-flow sweat glands and epidermal furrows have provided primates with the evolutionary advantage in dry and wet conditions of manipulative and locomotive abilities not available to other animals,” the researchers conclude.
But understanding the underlying mechanisms of our grip is not just an interesting evolutionary problem.
In a wider context, the authors note that this research “will contribute to the development of more realistic tactile sensors, for example, in robotics and prosthetics, and also haptic feedback systems, for example for touch screens and virtual reality environments”.
Lauren Fuge is a science journalist at The Royal Institution of Australia.
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