Quirks & Quarks

You won't believe just how sensitive our sense of touch is

A study on the sensitivity of human touch could pave the way for advanced technologies such as better prosthetic devices and surgical robots.
Current sensor technologies can't begin to replicate the variety of textures that we can, from petting a soft cat to recoiling at rough sandpaper. (Pixabay)
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Touchy subject

Our sense of touch is so sensitive that we can feel the difference of just a single layer of molecules, researchers have found.

We can easily tell the difference between a range of surfaces, from the roughest of sand paper to a soothing caress. We read eye charts to test our visual acuity before getting behind the wheel of a car. Newborns take simple audio tests to check that they can hear before they go home with their parents. But in comparison, our sense of touch has been a bit neglected.

Recently, researchers in California put our tactile sense to the test.

Darren Lipomi is a professor of nanoengineering at the University of California San Diego's School of Engineering. He worked with a team of physicists, chemists, engineers and psychologists on stretchable electronics, such as wearable biomedical devices to monitor heartbeats and blood pressure. The goal is to create sensors that could interact with the user, like a glove or another article or clothing. 

Hands-on tests

So far, the field of "haptics" in virtual and augmented reality has been limited to how a smartphone or gaming console vibrates, Lipomi suggested. Those can't begin to replicate the variety of textures that we can, from petting a soft cat to wiping away sticking slime.

[T]his might be the greatest sensitivity demonstrated.- Darren Lipomi

By better understanding just how sensitive our sense of touch is naturally, Lipomi said it opens the door to inventive applications, including: 

  • Surgical robots.
  • Tactile therapies for premature infants.
  • Wearables for people with neurological disorders.

Lipomi said he was inspired not only by the biomedical applications but also virtual reality games where players are immersed in haptic suits, as well as his mindfulness meditation practice that involves feeling your hands on the ground for instance. The materials scientist started to pay more attention to the tactile sense.

To find out how sensitive the sense of touch really is, the researchers designed an experiment using silicon wafers — the building blocks of microprocessors found in computers and smartphones. 

We use dynamic feedback to interact with an object and unconsciously vary pressure and speed to home in cues to differentiate objects. (Tara Copeland/CBC)

One on type of wafer, they oxidized the surface to remove what it gets from the atmosphere. Another was given a Teflon-like surface. 

No one could tell the one-molecule difference in thickness just by looking, or by temperature or electrical conductivity, tests showed. 

Over many trials, the researchers found humans are capable of telling the difference just by dragging a finger across the surface.

"I might go so far as to say this might be the greatest sensitivity demonstrated," Lipomi said.

It turned out that humans excel at detecting minute changes in vibration and pressure.

Slip-stick friction is responsible for the beautiful sounds of a violin.

The researchers tested the hypothesis using a mechanical or mock finger to measure forces as the finger slid across the surface.

Slip and slide

Lipomi called it remarkable how we don't turn to a book to tell an apple from a cactus. Instead, we use dynamic feedback to interact with an object and unconsciously vary pressure and speed to produce different vibrational frequencies and texture to quickly and automatically home in cues to differentiate objects. 

In one test, the researchers tested for "stick-slip friction," the jerking motion that happens when two objects at rest start to slide against each other. It's what makes a musical note when you rub a wet finger on the rim of a wine glass, the creaking of a rusty door hinge, or the beautiful sounds of a violin. We're not detecting audible sound, but vibrations tranduced in the skin, he said.  

In another test, subjects were given silicon wafers containing different sequences of a 8-digit string of 0s and 1s corresponding to different letters in the ASCII alphabet used in computer programming. The researchers found 10 of 11 subjects could decode the bits needed to spell the word "Lab" with correct upper and lowercase letters, more than 50 per cent of the time.