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

Progress in touch technology has lagged, in part because it involves multiple types of sensations, such as temperature and pain. In addition, some efforts to recreate touch have included systems designed to simulate a sense of moving one’s body — a complex sensation.

Dhong’s research focuses on a specific type of touch: using the fingers to detect fine textures. Some methods for evoking this kind of fine touch are already available. Your smartphone attracts your attention without sound, using a tiny vibrating device within. A refreshable Braille display for people with low vision or blindness 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 skin encounters as it brushes against 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 the sensation.

Dhong and his colleagues suspected that by altering only chemistry-related features, they could change how a surface feels.

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

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

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

In San Diego, Dhong presented recent work focusing on polymers, the go-to molecules for synthetic materials. Polymers are distinguished not only by their chemical formulas, but also by a characteristic known as crystallinity, which describes how neatly the 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 across the polymer. This time, too, they found that people could differentiate between the polymers based only on variations in the friction resulting from subtle changes to the crystallinity of the molecules.

About the researcher

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

He earned his bachelor’s degree in chemical and biomolecular engineering at the University of California, Berkeley, his doctorate at 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.