Scientists are making astonishing breakthroughs in sense of touch for amputees with bionic arms — and they are moving ever closer to natural feeling robotic limbs.

For the research, scientists used two male amputees who each lost an arm after traumatic injuries. Both men were implanted with neural devices embedded with electrodes that were attached to the nerves of the arm at the point where those nerves would normally carry signals from the hand.

They then tested the men's ability to distinguish sensations when their nerves were stimulated through the device. The results were astounding: Not only were the men able to sense touch, they were able to determine the strength of the sensation.

The research is a joint effort by University of Chicago neuroscientist Sliman Bensmaia, Ph.D., and Dustin Tyler, Ph.D., the Kent H. Smith professor of biomedical engineering at Case Western Reserve University, who works with a team trying to make bionic hands mimic natural hands in movement and sensation.

"If you want to create a dexterous hand for use in an amputee or a quadriplegic patient, you need to not only be able to move it, but have sensory feedback from it," said Bensmaia. "To do this, we first need to look at how the intact hand and the intact nervous system encodes this information, and then, to the extent that we can, try to mimic that in [an artificial hand]."

Earlier research from Bensmaia's lab showed how the nervous system responds to intensity of touch — people determine how hard an object is pressing it against the skin. That work suggested that the number of times nerve fibers fire in response to a given stimulus, known as the population spike rate, determines the intensity of touch.

The new study enhances the development of "prosthetics that feel" or "neuroprosthetics." In a separate paper published earlier this year, Bensmaia and his team tested the sensory abilities of a robotic fingertip equipped with touch sensors. That paper published by Bensmaia and a team led by Robert Gaunt, Ph.D., from the University of Pittsburgh, announced that for the first time, a paralyzed human patient was able to experience sensations through a robotic arm that he controlled with his brain.

In that study, researchers tapped directly into the patient's brain, through an electrode implanted in the areas of the brain responsible for hand movements and for touch, which allowed the man to both move the robotic arm and feel objects through it.

Using the same behavioral techniques that are used to test human sensory abilities, Bensmaia's team, led by Benoit Delhaye and Erik Schluter, tested the finger's ability to distinguish different touch locations, different pressure levels, the direction and speed of surfaces moving across it, and the identity of textures scanned across it.

The robotic finger (with the help of machine learning algorithms) proved to be almost as good as a human finger at most of these sensory tasks. By combining the results from the  study, Bensmaia and team can begin building neuroprosthetics that closely mimic natural sensations of touch.

With these results, Bensmaia and team have crossed the chasm of natural feelings in robotic limbs. Without natural sensations, prosthetics could never achieve the dexterity of natural hands. In the study, Bensmaia gave the analogy of playing a piano, which requires fluid touch, hard and soft, relying on sensory signals from fingers.

"The idea is that if we can reproduce those signals exactly, the amputee won't have to think about it; he can just interact with objects naturally and automatically. Results from this study constitute a first step towards conveying finely graded information about contact pressure," Bensmaia said.

The results of both studies were published in Science Translational Medicine (STM) and reported by Science Daily.