Human fingerprints have a self-regulating moisture mechanism that could lead to improvements in prosthetic limbs, according to a new study. Epidermal ridges on the hands and feet of primates have a higher density of sweat glands than elsewhere on their bodies, which allows precise regulation of skin moisture to give greater levels of grip when manipulating objects, according to the study’s authors. The moisture-regulating fingerprints help to increase friction when in contact with smooth surfaces, boost grip on rough surfaces, and enhance tactile sensitivity.
“Primates have evolved epidermal ridges on their hands and feet. During contact with solid objects, fingerprint ridges are important for grip and precision manipulation,” said study co-author Mike Adams, PhD, a professor in product engineering and manufacturing at the University of Birmingham. “They regulate moisture levels from external sources or the sweat pores so that friction is maximized, and we avoid ‘catastrophic’ slip and keep hold of that smartphone.
“Understanding the influence of finger pad friction will help us to develop more realistic tactile sensors—for example, applications in robotics and prosthetics and haptic feedback systems for touch screens and virtual reality environments,” Adams said.
Ultrasonic lubrication is commonly used in touch screen displays that provide sensory haptic feedback, but its effectiveness is reduced when a user has dry compared with moist finger pads. Moreover, being able to distinguish between fine-textured surfaces, such as textiles, by touch relies on the induced lateral vibrations, but the absence of sliding friction inhibits our ability to identify what we are touching.
Fingerprints are unique to primates and koalas, and appear to have the dual function of enhancing evaporation of excess moisture whist providing a reservoir of moisture at their bases that enables grip to be maximized.
The researchers discovered that when finger pads are in contact with impermeable surfaces, the sweat from pores in the ridges makes the skin softer and thus dramatically increases friction. However, the resulting increase in the compliance of the ridges causes the sweat pores eventually to become blocked and prevents excessive moisture that would reduce our ability to grip objects.
Using high-tech laser-based imaging technology, the researchers found that moisture regulation could be explained by the combination of this sweat pore blocking and the accelerated evaporation of excessive moisture from external wetting as a result of the specific cross-sectional shape of the epidermal furrows when in contact with an object.
These two functions result in maintaining the optimum amount of moisture in the fingerprint ridges that maximizes friction whether the finger pad is initially wet or dry.
The study, “Fingerprint ridges allow primates to regulate grip,” was published in Proceedings of the National Academy of Sciences.
Editor’s note: This story was adapted from materials provided by the University of Birmingham.