What Exactly Does 'Soft' Mean?

In a joint effort from a team of engineers and psychologists, the University of California - San Diego has revealed fundamental insights into what factors affect the perception of softness. Using the findings, the team has developed innovative design tricks that allow engineers to create materials that replicate different levels of softness. Eventually, the researchers hope the technology can be used to mimic sensations of softness within tactile materials and haptic interfaces such as electronic skin, prosthetic limbs and robots used for touch-sensitive medical tasks.

Simulating softness

The findings were published in the Aug. 30 issue of Science Advances and explain how the team explored the intricate factors that affect how humans experience the sensation of touch, with a specific focus on the perception of softness. Using the insights, the team was able to develop equations that calculate different levels of softness based on the thickness and stiffness of the material, as well as the nature of micropatterned areas. The equation also works in reverse and can calculate how thick or micropatterned a material should be to mimic a specific level of softness.

"We provide a formula to recreate a spectrum of softness. In doing so, we are helping close the gap in understanding what it takes to recreate some aspects of touch," explains Charles Dhong, co-leader of the study and postdoctoral fellow at UC San Diego. "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."

Pinpointing softness parameters

To develop the equation, Dhong and his colleagues investigated two parameters used to measure the softness of a material. The first was indentation depth, i.e. how far a fingertip can press into a material. The second was contact area between the fingertip and the material, which generally increases with softness. To explore the impact indentation depth and contact area have on the perception of softness, the team engineered materials that separate the two parameters.

Next, they engineered nine elastomeric slabs, each with different indentation depth to contact area ratios. Each slab also featured unique levels of thickness, stiffness and micropatterning. The latter refers to microscopic columns that dot the surface of the slabs and allow a fingertip to create a deeper press indentation without increasing the contact area.

"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," says Darren Lipomi, co-author of the study and professor of nanoengineering at UC San Diego.

Softness identified as an independent sensation

After testing the slabs on 15 participants and asking them to rate levels of softness, the team concluded that the perception of softness is an independent sensation, not a product of other combined sensations.

"This means softness is a primary ingredient of the human sense of touch. It's like how we have RGB for color displays," says Lipomi. "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."

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