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Engineers develop materials that replicate the feeling of softness

A team of engineers and psychologists at the University of California San Diego explored what factors affect how human touch perceives softness and designed materials that replicate different levels of this perceived feeling.

In doing so, the team proved that it’s possible to design tactile materials and haptic interfaces to apply to technologies such as electronic skin, prosthetics, and medical robots.

“We provide a formula to recreate a spectrum of softness,” said co-leader Charles Dhong, an assistant professor in biomedical engineering at the University of Delaware and former postdoctoral fellow at UC San Diego. “In doing so, we are helping close the gap in understanding what it takes to recreate some aspects of touch.”

To begin, Dhong and his colleagues first examined indentation depth (how deep a finger can press into a material) as well as the contact area between material and fingertip, as these are the first two parameters to change as a fingertip presses into an object. 

To test how these parameters work independently, the team engineered nine elastomeric slabs, each one of which had its own ratio of indentation depth to contact area, and differed in the amount of surface micropatterning (consisting of bunches of raised microscopic pillars that allow fingertips to press deeper without changing the contact area), thickness, and stiffness (measured by Young’s modulus). They then tested the slabs on human participants.

“By creating these micropatterned surface structures, we produce discontinuous regions of contact where the finger presses in that are much smaller than the shadow it would cast on the surface,” said co-corresponding author Darren Lipomi, a professor of nanoengineering at UC San Diego.

The 15 human participants were instructed to identify the softer slab when presented with a pair of slabs. They were then asked to rank the nine slabs from softest to hardest. The slabs that were thicker, had barely any micropatterning, and had the lowest Young’s modulus were perceived as being softer than others.

Dhong, Lipomi, and colleagues were also able to create equations based on the results of their experiments. These equations can calculate how soft or hard a material will feel based on its thickness, Young’s modulus, and micropatterned areas.

“What’s interesting about this is that we’ve found two new ways to tune the perceived softness of an object — micropatterning and changing the thickness,” Dhong said. “Young’s modulus is what scientists typically turn to in terms of what’s soft or hard. It is a factor, but now we show that it’s only one part of the equation.”

After experimenting, the researchers concluded that the perception of softness is a basic sensation, not a mashup of several sensations.

“This means softness is a primary ingredient of the human sense of touch. It’s like how we have RGB for color displays,” Lipomi said. “If we can find the other ‘pixels of touch,’ can we combine them to make any tactile image we want? These are the fundamental things we would like to know going forward.”

This study is published in Science Advances.

By Olivia Harvey, Staff Writer

Image Credit: Shutterstock/fizkes

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