Fingertips Can Detect Single Atom Differences

The touch sensitivity of finger skin could not be
improved. There’s a purpose for fingerprints, too.

Note: Reporting at CEH will be sparse till between Christmas and New Year’s weekends. These findings from science news and journal papers published earlier in 2021 are too good to pass up. —Editor

Robot designers are improving their devices’ haptic capabilities—that is, their touch sensitivity. They are getting closer to distinguishing between hard and soft surfaces, so as to get a firm grip on solid objects but handle delicate objects, like an egg, with appropriate care (see example at New Scientist, 15 Dec 2021). None of them, though, can approach the sensitivity of the human finger. Look what scientists found in 2021.

Fingerprints Enhance Our Sense of Touch (Neuroscience News, March 15, 2021). Fingerprints usually come up in the context of forensics: identifying criminals by the unique patterns of ridges on their fingertips. Some of us have to supply fingerprints for our jobs. But what do they do for us?

The hand contains tens of thousands of sensory neurons. Each neuron tunes in to a small surface area on the skin — a receptive field — and detects touch, vibration, pressure, and other tactile stimuli. The human hand possesses a refined sense of touch, but the exact sensitivity of a single sensory neuron has not been studied before.

And just how sensitive is a single sensory neuron? Dr. Eva Jarocka et al. “measured the electrical activity of the sensory neurons in human fingertips when they stimulated with raised dots swept over the skin.” They refined the data and mapped it onto images of a fingerprint. The result? “Human Touch Receptors Are Sensitive to Spatial Details on the Scale of Single Fingerprint Ridges.” This was the title of their paper in the Journal of Neuroscience, published 21 April 21, 41 (16) 3622-3634; DOI: https://doi.org/10.1523/JNEUROSCI.1716-20.2021 (open access).

See also “How the Brain Makes Sense of Touch” in Neuroscience News, 10 Feb 2021. Experiments with rat whiskers showed those mammals have exquisite sensitivity to multiple aspects of a surface, including shape, texture and temperature. “When scientists have looked in the human brain, the same cell types and the same mechanisms have come up,” says Dr Carl Petersen at EFPL, “one of Europe’s most vibrant and cosmopolitan science and technology institutions.”

Sensitivity to details of touch allows humans to engage in many highly-refined activities. Credit: Illustra Media

Continuing on about fingerprints, “We submit that the receptor organs underlying subfields essentially measure mechanical events at individual ridges,” the authors say in their Significance Statement. Look at those ridges right now on a finger, using a magnifier if you have one. That’s amazingly precise. But why do we have fingerprint ridges?

Taken together, these results suggest that the internal sensitivity topography of a neuron’s receptive field was largely conserved across scanning directions but could be influenced by direction-dependent shear deformations of the skin surface. In addition, most neurons retain the distinctiveness of the features of their receptive fields with reference to other neurons’ fields.

This appears to mean that a fingertip, with its ridges and valleys, enhances the responsiveness of its underlying neurons. Try rubbing a fingertip across a rough surface in different directions. Do you feel slight differences? Those come from the collective response of tens of thousands of ridges slightly bending this way or that, each with their own “direction-dependent shear deformations.” Each neuron remembers its own response. The brain puts together a map of the surface based on thousands of inputs—a very advanced method of maximizing data input from a field, similar to radar and other scanning algorithms.

For more amazing details of fingerprint ridges, including how the hand uses both fast-acting and slow-acting neurons to build the sensory field, read the open-access paper. But that’s not all: there’s more….

Your finger can feel the change of a single atom in a material (New Scientist, 4 May 2021). Leah Crane reported on experiments at the University of Delaware that showed individuals able to distinguish atomic differences in material they touched.

Generally, what we feel with our fingers are physical bumps in a material’s surface structure. Charles Dhong at the University of Delaware and his colleagues set out to find whether it would be possible to feel a chemical difference in which the internal molecular structures of two materials slightly vary but their surfaces are equally smooth.

Dr Dhong devised experiments to have subjects try to detect chemical differences between surfaces that were virtually identical except for single atom substitutions. Most of the time, the differences were detectable.

“When we make our samples, physically they’re almost identical, the differences are on a sub-nanometre scale,” says Dhong. “But when test subjects felt them, some people said that some felt a little gritty and other ones were more pleasant and velvety.”

The chemical difference between the two compounds that the testers were best able to tell apart caused a slight change in how much friction they felt when running a finger over them. This alteration wasn’t due to bumps in each material, but rather the way their molecules fitted together.

The findings were published in Soft Matter 24 March 2021 as Nolin et al., “Predicting human touch sensitivity to single atom substitutions in surface monolayers for molecular control in tactile interfaces,” DOI https://doi.org/10.1039/D1SM00451D (paywall).

These findings are reminiscent of how eyes can detect single photons. One could not design detectors for both these senses that could perform any better than the ones with which our bodies are endowed. The same could be said for all the senses: smell, taste, and hearing. We have the ultimate versions. Your marvelous body is all you need for a thankful Christmas.

Many times the Bible uses the phrase “the hand of God” as a metaphor for his creative power and sovereign will. “I made the earth and created man on it; it was my hands that stretched out the heavens, and I commanded all their host” (Isaiah 45:12). At Christmas time, we celebrate how the hand of God brought salvation to humankind through the birth of the Savior. “Oh sing to the Lord a new song, for he has done marvelous things! His right hand and his holy arm have worked salvation for him” (Psalm 98:1)

Credit: Decision Magazine cover, Dec 1966.

“When I look at your heavens, the work of your fingers,
the moon and the stars, which you have set in place,
what is man that you are mindful of him,
and the son of man that you care for him?” (Psalm 8:3-4)