The human sense of touch can pick up on differences as minute as a single layer of molecules, according to new research.

Image credits Pedro Figueras.

Our sense of touch comes in very handy. Through it, we can easily discriminate between a wide range of surfaces, from wood, paper and metal to glass and plastics, chiefly due to differences in texture and because every material sucks up the heat from our fingers at different rates. So we understand the ‘how’, but until now we didn’t know how finely tuned our tactile sense is, i.e. what the smallest difference they can pick up on is. Such knowledge is crucial if we are to create life-like prosthetics that can accurately recreate our sense of touch, in the development of virtual and augmented realities, and many other advanced technologies.

New research from the University of California San Diego says that our sense of touch is, in fact, so refined it can pick up on differences of a single layer of molecules.

A touching subject

Modern technology such as PCs, game consoles, smartphones, or TVs, let us experience the world more freely and fully than ever before. We can hear and watch events unfolding on the other side of the planet — even on other side of other planets — but these devices don’t allow us to feel what’s happening. Mixing in that ingredient “is a driving force behind this work,” says San Diego nanoengineering Ph.D. student and paper co-author Cody Carpenter.

“Reproducing realistic tactile sensations is difficult because we don’t yet fully understand the basic ways in which materials interact with the sense of touch,” adds Darren Lipomi, a professor of nanoengineering at UC San Diego who led the research efforts.

For their research, the team asked participants to try and distinguish between various unassuming silicon wafers only by dragging or tapping a finger across their surface. The wafers were almost identical, differing only in their single, topmost layer of molecules.

So how could they pick up on such minute differences? The team was placing their hopes on stick-slip friction, the jerking motion that occurs as two objects at rest start sliding against one another. The phenomenon draws on the fact that, generally speaking, the static friction coefficient between two bodies is greater than their dynamic friction coefficient — it takes more force to start sliding two objects against each other than to keep them sliding. This difference means that immediately after movement starts and the dynamic coefficient takes over, there’s a short but powerful burst in the bodies’ relative velocity, hence the jerking motion.

The sound you make when running a wet finger along the rim of a wine glass is generated by stick-slip-induced vibrations in the glass. That’s also why an ungreased door hinge will squeak, and why stopping trains make that infernal racket when stopping. It’s all stick-slip-induced vibrations dissipating as sound.

Hands-on approach

Image credits Simon Wijers.

The team ran several trials to see if their theory would hold. During the first, participants were presented with two wafers. One was covered in a single oxidized layer rich in oxygen atoms, the other with a Teflon-like layer of fluorine and carbon atoms. Both wafers looked identical so participants couldn’t tell them apart by appearance.

In another test, 15 subjects were presented with three surfaces and had to identify the one surface that differed from the other two. Subjects correctly identified the differences 71% of the time during this trial.

Finally, subjects were given three strips of silicon wafer, each patterned with a different sequence of 8 patches of oxidized and Teflon-like surfaces. Each strip thus encoded a binary language (1s and 0s), with the patterns corresponding to a letter in the ASCII alphabet. Participants were asked to read these sequences, using their fingers to tell which patches were oxidized and which were covered in the Teflon-like material. During this trial, 10 out of 11 subjects correctly decoded the word (which was “Lab”, spelled with upper and lowercase letters) more than 50% of the time. Subjects spent an average of 4.5 minutes to decode each letter.

“This is the greatest tactile sensitivity that has ever been shown in humans,” Lipomi explains.

From the data recorded during the trials, the team determined that materials should be distinguishable by how fast a finger drags and how much force it applies to the surface. They constructed a mock finger out of organic polymer, attached it to a force sensor, and ran it across the surfaces used during their study at different combinations of force and speed. Further processing of the data revealed that certain combinations of these two factors lend themselves well to distinguishing materials, while others create too much noise (chaotic data) to be used in such applications.

“Our results reveal a remarkable human ability to quickly home in on the right combinations of forces and swiping velocities required to feel the difference between these surfaces. They don’t need to reconstruct an entire matrix of data points one by one as we did in our experiments,” Lipomi said.

“It’s also interesting that the mock finger device, which doesn’t have anything resembling the hundreds of nerves in our skin, has just one force sensor and is still able to get the information needed to feel the difference in these surfaces,” he adds. “This tells us it’s not just the mechanoreceptors in the skin, but receptors in the ligaments, knuckles, wrist, elbow and shoulder that could be enabling humans to sense minute differences using touch.”

Starting from these results, researchers and engineers could develop technologies that would allow artificial-skin systems to feel the world around us very much like our own, biological skins. Alternatively, the paper could form the foundation for systems that can recreate the feel of any material, which would level-up virtual reality systems dramatically.

The paper “Human ability to discriminate surface chemistry by touch” has been published in the journal Materials Horizons.