(45c) Understanding Artificial Touch: Designing “Softness” and Molecular Discriminability for Haptic Devices

Of all the senses, the sense of touch is the least understood from an engineering perspective. Televisions and virtual reality devices have fully tricked and engaged the sense of sight, but no devices exist that can fully simulate the experience of touch, especially with regards to fine texture. We have approached understanding touch by decomposing into several fundamental sensations, akin to the red, green and blue pixels for the sense of sight.

In one study, we found that human subjects can differentiate between two silicon wafers that differed in surface energy alone. One wafer was coated with a flourosilane coating, creating a “hydrophobic”, low-energy surface while the other was plasma treated, creating a “hydrophilic”, high-energy surface. Human subjects were successful in telling the two wafers apart, and we modeled this interaction by sliding a patterned, PDMS-elastomer block and measuring friction forces. A computation model of an elastic “rate-and-state” friction model confirms differences in the friction forces between the two surfaces, which would not be evident from a simple constant coefficient of friction. Friction also depends on the applied pressure and sliding velocity, and since differences between the two surfaces can grow or diminish at certain pressures and velocities, we generated novel “discriminability” matrices to quantify friction differences between subjects across conditions. We show that patterned interfaces – such as our fingerprints – enhance friction differences between surfaces, especially to certain mechanoreceptors in the finger.

In a second study, we examined how human subjects determined whether an object was soft. In engineering, the indenting of a finger on a soft object can be modeled as Hertzian contact, which generates changes in the contact area or indentation depth of the finger. We decoupled the effect of indentation depth and contact area through the layering effect of thin elastic films and micropatterning. This generated an array of samples that vary in indentation depth or contact area, or both. We determined that human subjects were sensitive to both indentation depth and contact area, which was not known before. We found that indentation depth was more correlated to softness, and we also quantified the relative strength of indentation depth to contact area as a tactile stimulus. That is, to make an object appear softer, a change in indentation depth generates larger changes in perceived softness than similar changes in contact area.

Between both studies, we developed techniques to quantify tactile sensations. We offer engineering guidelines to design haptic devices to generate changes in texture and softness.