Humans can feel differences in the chemical c

image: Research by the University of Delaware has shown that humans can sense tiny differences on a surface, down to the substitution of a single atom.
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Credit: Courtesy of Charles Dhong

Bumps and lines make up tactile technology such as Braille. But the human sense of touch is sharp enough to detect much smaller differences. Search by Charles Dong and his group at the University of Delaware discovered that humans can sense differences in the chemical composition of a surface, down to the substitution of a single atom.

This ability is one of the goals of Dhong’s work as an assistant professor in the Department of Materials Science and Engineering and the Department of Biomedical Engineering at UD. He explores new possibilities for tactile technologies and the mechanical forces that affect the perception of touch.

Dhong presented research on this subject to the American chemical society‘s national meeting in San Diego on Wednesday, March 23. And he and collaborator Jared Medina, an associate professor in UD’s Department of Psychological and Brain Sciences, is receiving new support for the development of better tactile aids for people with visual impairments. The $1.9 million grant, which began in February and runs for five years, comes from the national eye institute in collaboration with the National Federation of the Blind.

Current technology recreates the sense of touch using tiny motors and electricity. But the bumps and buzzes they generate aren’t so good at mimicking reality.

A new approach to controlling texture perception could have many applications, Dhong said. It could make it possible to design new types of surfaces or improve the integration of the sense of touch in virtual reality environments. It could also improve existing devices, such as braille displays, or provide information to surgeons performing remote operations.

“When you touch an object, you feel its surface, and you can change its feel by changing the friction between that surface and your finger. This is where the chemistry comes in,” Dhong said. “We think materials chemistry could open the door to recreating more nuanced sensations, whether you’re designing a product to feel a certain way or creating feedback devices for virtual reality.”

Other human senses have gotten much more attention in today’s technology. Visual characteristics are highly refined in computer monitors, smart phone screens, and virtual reality headsets. Audio devices also recreate voices, music and other sounds in high fidelity.

Advances in touch technology have lagged, in part because they involve multiple types of sensations, such as temperature and pain. Additionally, some efforts to recreate touch have included systems designed to simulate a sense of body movement – a complex sensation.

Dhong’s research focuses on a specific type of touch: using fingers to detect fine textures. Some methods for evoking this kind of fine touch are already available. Your smartphone grabs your attention without sound, using a tiny vibrating device inside. An updatable braille display for the visually impaired or blind uses an actuator to move pins up to create bumps.

This type of touch depends on a physical force – friction – which is the resistance that the skin encounters when it touches an object. While attributes such as the contours of a surface influence friction, so does chemistry. The structure of the molecules within a substance and the properties of its surface also influence sensation.

Dhong and his colleagues suspected that by changing only the chemistry-related characteristics, they could alter the feel of a surface.

In previous work, Dhong’s team had people touch single-molecule-thick layers of silane, a silicon-containing compound. None of the silane surfaces had detectable differences in smoothness.

But those who touched the surfaces could tell them apart based on chemical differences, including the substitution of one atom in each silane molecule for another, due to subtle changes in friction.

“Recent research has shown that people can detect physical differences between surfaces at a resolution as low as 13 nanometers,” Dhong says. “Now we say that the sense of touch can also identify chemical changes as small as the exchange of a nitrogen atom for a carbon atom.”

In San Diego, Dhong presented recent work focused on polymers, the benchmark molecules for synthetic materials. Polymers are distinguished not only by their chemical formulas, but also by a characteristic known as crystallinity, which describes the way in which chain-like molecules are organized. The polymers in these experiments had identical formulas and molecular weights. Only the degree of crystallinity differed.

In their experiments, the researchers focused on the perceived texture of thin layers of polymers. As with the silanes, they asked the subjects to slide their fingers over the polymer. This time, too, they discovered that people could differentiate between polymers based solely on variations in friction resulting from subtle changes in the crystallinity of molecules.

About the researcher

Charles Dhong, assistant professor of materials science and biomedical engineering in the College of Engineering, joined the faculty at the University of Delaware in 2019.

He received his BS in Chemical and Biomolecular Engineering from the University of California, Berkeley, his Ph.D. from Johns Hopkins University, and did postdoctoral research in nanoengineering at the University of California, San Diego.

His research focuses on understanding the mechanical forces that shape the human sense of touch.

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