The two men told researchers at Case Western Reserve University that wiring some of their remaining nerves to a robotic arm — albeit only during visits to a lab — felt more like grasping objects with their own hand than with a tool.
"This feels like normal sensation," one of the men, Igor Spetic of Madison, Ohio, said in an interview.
When researchers touched different spots on his artificial hand, "sometimes it felt like a cotton ball," he said. "Sometimes like sandpaper."
An unexpected benefit: The phantom pain both men have felt since losing their limbs in industrial accidents has nearly disappeared since they began the experiment, the researchers reported Wednesday in the journal Science Translational Medicine.
It will take years of additional research before robotic hands really let people feel what they touch. But the new research is an important step, said Dr. Michael Boninger, who directs the University of Pittsburgh Medical Center's rehabilitation institute and wasn't involved with the experiments.
Beyond better function, getting feedback from the limb "would be a spectacular thing to be able to have, that you feel like the arm is your own," he said.
People with natural limbs take for granted the intuitive control that a sense of touch allows. Reach for something and your hand naturally grasps with just enough force to hang on. But users of prosthetic hands have to watch carefully every motion, judging by eye how tightly to squeeze so they don't either drop something or crush it. Consequently, many amputees abandon prosthetic hands, or don't use them as much as they'd like.
Here's how it works: The team at Case Western and the Cleveland Veterans Affairs Medical Center implanted electrodes around three nerves in the stump of the men's arms.
Wires snake from under the skin. During monthly visits to a lab, scientists hook the men up to a machine that sends electrical signals between the stump and a prosthetic hand. Sensors on the hand can convey a sense of touch from 16 to 19 spots.
At first, it just felt like electrical tingles, Spetic said.
But as researchers adjusted the pattern and intensity of those signals, he started to feel pressure in his missing fingers, and even different textures.
The real test was when the men tried plucking grapes and cherries from their stems. Blindfolded, they crushed a lot of fruit until the sensory feedback was switched on and they could gentle their grasp.
"We can change what they're feeling and how they're feeling it," said Case Western biomedical engineer Dustin Tyler, who led the research.
It's working by reactivating dormant areas of the brain that produced the sense of touch for that hand, he said.
What explains the disappearance of that phantom pain that Spetic described as like a vice gripping his missing fist? Tyler said researchers don't know but it may be that making the brain sense a hand is there again, rather than missing, could affect how it interprets pain signals.
While the men can't use touch-enabled hands outside of the lab, the under-the-skin electrodes still are working up to two years after they were implanted, important evidence that long-term use of this approach is possible. Tyler hopes to begin real-world study soon.
In related research Wednesday, scientists at Chalmers University of Technology in Sweden reported that a man there has been wearing a more lifelike prosthetic arm since January 2013, one that attaches to a metal rod implanted in his bone.
Bone-anchored prosthetics aren't new, but this one allowed for some direct connections to nerves of the remaining arm, allowing a greater range of motion than standard prostheses and some more fine motor control. The study found it has held up to heavy daily use. As for sensation, researchers reported that the man perceived tingling sensations from things he touched at a few locations, and they plan more work to enhance that sensory feedback.