The subject holding a cherry tomato / Russell Lee
Janet Fang 09 Oct 2014, 04:28

This is the closest we’ve come to robotic arms and hands that function and feel like the real thing. Two new neuroprosthetic developments, published as a pair of studies in Science Translational Medicine this week, use implanted electrodes to help mimic the control, freedom of movement, and sense of touch that many of us might take for granted. One development restored the ability to discern familiar sensations, and the other recreated reliable motor control—both have been subjected to long-term, real-world use by amputees. 

Case Western Reserve University’s Dustin Tyler and colleagues developed a prosthetic hand that connects to under-the-skin electrodes. Two to three electrodes were implanted around major nerve bundles in two adult males who lost their hands in accidents. After implantation surgery, the men were able to perform everyday tasks for up to two and a half years with no problems. 
    
The team also improved sensory perception by sending electrical pulses through the men’s prosthetic hands. Varying the intensity of the stimulation excited different neurons with different patterns—resembling what happens in our hands naturally when we touch and grip. Afterwards, the patients reported feeling like they were grasping objects with a natural extension of their body, rather than an external tool. "The work reactivates areas of the brain that produce the sense of touch,” Tyler says in a news release. "When the hand is lost, the inputs that switched on these areas were lost."

Familiar sensations were restored over multiple points across the hand. When blindfolded, one man reported “feeling” a cotton ball brushing against the back of his prosthesis, while the other “felt” water running across his artificial hand. That’s because electric signals were sent by a computer into nerves in their arms and to their brains. They were also able to control their hands with more dexterity. In the picture above, one recipient is even holding a cherry tomato. Other delicate tasks that they were able to do include grabbing (and not crushing) grapes, pulling stems off cherries, and squeezing toothpaste onto a toothbrush. 

In another study, a team led by Max Ortiz-Catalan from Chalmers University of Technology focused on recreating the freedom of movement of a natural arm. They developed an “osseointegrated” arm that connected directly to the bone, nerves, and muscles of an adult male whose arm was amputated above the elbow over 10 years ago. The artificial arm is anchored to the bone in the stump by a titanium rod serving as an extension of the skeleton. 

This osseointegrated approach uses implanted electrodes woven under the skin to provide constant sensory feedback, helping to stimulate nerves for more precise control. The recipient used his prosthesis throughout his daily activities—from clamping his trailer load to tying his children's skates, even occasionally sleeping with it attached—without any problems. He couldn't do these things with his previous prosthetic, which used surface electrodes.

"We have used osseointegration to create a long-term stable fusion between man and machine, where we have integrated them at different levels. The artificial arm is directly attached to the skeleton, thus providing mechanical stability,” Ortiz-Catalan explains in a university statement. “The human's biological control system, that is nerves and muscles, is also interfaced to the machine's control system via neuromuscular electrodes. This creates an intimate union between the body and the machine.” 

Images: Russell Lee (top), Ortiz Catalan et al., Science Translational Medicine, 2014 (middle)

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