Touch is perhaps the least understood of the five senses. Many factors can influence how an object feels to people, and for that reason, manipulating touch has until now largely eluded the grasp of scientists. A recent study published in the August 30 issue of Science Advances, however, is looking to bridge that comprehension gap. A joint effort between engineers and psychologists from the University of California, San Diego (UC San Diego) has unlocked a few keys that could help in designing materials that more realistically simulate the sensation of touch.
Experimental Process
The team used a high-volume dataset of materials with widely differing feel characteristics. By measuring various materials for factors such as Young’s modulus, micropatterned areas, and material thickness, the researchers were able to develop an equation for calculating the perceived softness of a given item. Importantly, the equation can be used in reverse to determine what characteristics a design-phase material needs in order for it to be felt in a certain way.
One of the study’s most interesting findings was the relative lack of importance of Young’s modulus, which traditionally was considered among the most important identifiers of touch. “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,” said lead author Charles Dhong.
Instead of focusing on Young’s modulus, the researchers sharpened their focus to two key traits: indentation depth and contact area. These two variables are typically dependent on one another—as a surface gives in (indentation depth), contact area must necessarily increase. To test the variables independently, the scientists needed to create materials that would allow one to move without triggering a proportional change in the other.
To meet this challenge, they designed nine individual elastomeric slabs, each with a unique indentation depth/contact area profile. Thanks to micropatterning, each slab could be pressed with a similar amount of force and yield different contact areas. This allowed the researchers to test how the different variables influenced test subjects’ perceptions of feel and isolate which factors were most meaningful.
Implications for Haptic Design
The most interesting finding to be taken from the study was that softness, while influenced by many factors, is actually a sensation unto itself. Softness is experienced as a gradient, much in the way that color is perceived on a spectrum. The researchers argue that softness, therefore, is “one of the pillars of touch.” The finding represents a crucial piece of a puzzle, which, once completed, could have far-reaching impacts. Everything from prostheses to smart clothing to soft robotics depends on better controlling how humans perceive touch. Being able to turn the softness of a material used in such applications up or down could unlock new design possibilities in each of these areas and more.