Engineers working to add muscle sense, or proprioception, to prosthetic limbs found that tactile feedback on the skin allowed blindfolded test subjects to more than double their ability to discern the size of objects grasped with a prosthetic hand. The results will be presented June 7 at the World Haptics 2017 conference in Fürstenfeldbruck, Germany, by researchers from Rice University (Rice) and the Research Center E. Piaggio of the University of Pisa, Italy, and the Italian Institute of Technology (IIT), Genova.
“Humans have an innate sense of how the parts of their bodies are positioned, even if they can’t see them,” said Marcia O’Malley, PhD, professor of mechanical engineering at Rice. “This muscle sense is what allows people to type on a keyboard, hold a cup, throw a ball, use a brake pedal, and do countless other daily tasks.”
Researchers in O’Malley’s Mechatronics and Haptic Interfaces (MAHI) Lab have worked for years to develop technology that would allow people with amputations to receive proprioceptive feedback from prosthetic limbs. While some proprioceptive technologies require surgically implanted electrodes, the Rice Haptic Rocker has a noninvasive user interface—a rotating arm that brushes a soft rubber pad over the skin of the user’s arm. At rest, when the prosthetic hand is fully open, the rocker arm does not stretch the skin. As the hand closes, the rocker arm rotates, and the more the hand closes, the greater the skin is stretched.
In the paper being presented in Germany, O’Malley and colleagues demonstrated that 18 able-bodied test subjects performed significantly better on size-discrimination tests with a prosthetic hand when they received haptic feedback from a simple skin-stretch device on the upper arm. The study is the first to test a prosthesis in combination with a skin-stretch rocking device for proprioception, and is a finalist for the best paper award at the conference.
In experiments at Rice beginning late last year, University of Pisa graduate student Edoardo Battaglia and Rice graduate student Janelle Clark tested MAHI’s Rice Haptic Rocker in conjunction with the Pisa/IIT SoftHand. They measured how well blindfolded subjects could distinguish the size of grasped objects with and without proprioceptive feedback.
“We’re using the tactile sensation on the skin as a replacement for information the brain would normally get from the muscles about hand position,” Clark said. “We’re essentially mapping from feedback from one source onto an aspect of the prosthetic hand. In this case, it’s how much the hand is open or closed.”
In tests, subjects used the SoftHand to grasp objects of varying shapes and sizes, ranging from grapefruit-size balls to coins. To close the hand, subjects flexed a muscle in their forearm. Electrodes taped to each subject’s arm picked up electric signals from the flexing muscle and transmitted those to the motor in the SoftHand. For the size-discrimination test, subjects were blindfolded and asked to grasp two different objects. They were then asked which of the two was larger. Without haptic feedback, the blindfolded subjects had to base their guesses on intuition. They chose correctly only about 33 percent of the time, which is what one would expect from a random choice. When they performed the same tests with feedback from the Rice Haptic Rocker, the subjects correctly distinguished the larger from smaller objects more than 70 percent of the time.
The researchers are following up to see if people with upper-limb amputations get a similar benefit from using the haptic rocker in conjunction with the SoftHand.
Editor’s note: This story was adapted from materials provided by Rice University.