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Edited by J. Edward Colgate, Northwestern University, Evanston, IL, and accepted by Editorial Board Member John A. Rogers October 8, 2020 (received for review June 4, 2020)
Significance
Why have primates evolved epidermal ridges on the volar regions of the hands and feet and with a much greater density of sweat glands than flat skin, which respond to anxiety rather than act as a thermoregulation mechanism? 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 the friction is maximized and catastrophic slip is inhibited. An understanding of the underlying mechanisms involved has become particularly important with the almost ubiquitous contact of the finger pads with flat screens and recent developments in haptic feedback using ultrasonic vibrations for which the performance is critically related to the friction.
Abstract
Fingerprints are unique to primates and koalas but what advantages do these features of our hands and feet provide us compared with the smooth pads of carnivorans, e.g., feline or ursine species? It has been argued that the epidermal ridges on finger pads decrease friction when in contact with smooth surfaces, promote interlocking with rough surfaces, channel excess water, prevent blistering, and enhance tactile sensitivity. Here, we found that they were at the origin of a moisture-regulating mechanism, which ensures an optimal hydration of the keratin layer of the skin for maximizing the friction and reducing the probability of catastrophic slip due to the hydrodynamic formation of a fluid layer. When in contact with impermeable surfaces, the occlusion of the sweat from the pores in the ridges promotes plasticization of the skin, dramatically increasing friction. Occlusion and external moisture could cause an excess of water that would defeat the natural hydration balance. However, we have demonstrated using femtosecond laser-based polarization-tunable terahertz wave spectroscopic imaging and infrared optical coherence tomography that the moisture regulation may be explained by a combination of a microfluidic capillary evaporation mechanism and a sweat pore blocking mechanism. This results in maintaining an optimal amount of moisture in the furrows that maximizes the friction irrespective of whether a finger pad is initially wet or dry. Thus, 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.
Footnotes
↵1S.M.Y. and I.K.B. contributed equally to this work.
↵2Present address: Mechatronics R&D Center, Samsung Electronics Co., Ltd., Hwasung 18448, Republic of Korea.
- ↵3To whom correspondence may be addressed. Email: gunsik{at}snu.ac.kr or m.j.adams{at}bham.ac.uk.
Author contributions: I.-K.B., D.H., M.J.A., and G.-S.P. designed research; S.-M.Y., I.-K.B., J.K., K.J., Seontae Kim, K.E., J.J., Seonmyeong Kim, M.S., M.J.A., and G.-S.P. performed research; S.-M.Y., I.-K.B., and D.H. contributed new reagents/analytic tools; S.-M.Y., I.-K.B., J.K., K.J., M.-G.L., Sungwan Kim, and G.-S.P. analyzed data; and S.-M.Y., I.-K.B., M.J.A., and G.-S.P. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission. J.E.C. is a guest editor invited by the Editorial Board.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2001055117/-/DCSupplemental.
Data Availability.
All data discussed in this paper are available at Figshare, https://doi.org/10.6084/m9.figshare.13139489.v1 (45).
Published under the PNAS license.
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Fingerprint ridges allow primates to regulate grip
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