Monkeys are being trained to control what might be the world's most sophisticated and human-like robot arm. But they never touch the prosthetic limb or fiddle with a remote control; they guide it with their thoughts alone. If trials are successful, people with spinal cord injuries could soon learn to do the same.

In 2008, Andrew Schwartz, of the University of Pittsburgh in Pennsylvania, published a landmark paper describing how two rhesus macaques learned to feed themselves marshmallows and fruit using a crude robotic limb controlled by electrodes implanted in their brains (Nature).

No brain-controlled prosthetic limb had ever carried out a more complex real-world task. Still, Schwartz envisioned a more elegant and nimble device that paralyzed people could use -- something much closer to a human hand.

Enter the Modular Prosthetic Limb (MPL), a bionic limb that closely approximates the form and agility of a human arm and hand. Born from the U.S. Defense Advanced Research Projects Agency's Revolutionizing Prosthetics program, and designed by Michael McLoughlin's team at the Johns Hopkins University Applied Physics Laboratory in Maryland, the MPL is made from a combination of lightweight carbon fiber and high-strength alloys.

The device has 22 degrees of freedom, compared with the human arm's 30, and can grasp precisely and firmly without crushing fragile objects. The wrist and elbow rotate with ease and, like an average human limb, it weighs just under 4.5 kilograms.

"I would say it's very close to human dexterity," says McLoughlin. "It can't do absolutely everything -- it can't cup the palm, for example -- but it can control all fingers individually. I don't think there is another limb that approaches it."

A prototype of the MPL has been tested by people who've had one or both arms amputated. Researchers surgically redirect nerves that would normally control the arm into unused chest muscle, where nerve signals are interpreted by electrodes that guide the robotic limb.

"One of our patients, Jesse Sullivan, was able to use the arm almost from time zero. It was a very natural thing to do," says McLoughlin. "The brain still thinks the arm is there and if you can tap into those signals, you can really achieve something amazing."

But people paralyzed from the neck down cannot benefit from this technique, as brain signals cannot reach the chest. So in his work with rhesus macaques, Schwartz developed an array of 100 electrodes that eavesdrop on 100 neurons in the motor cortex. Once he'd learned the electrical language the cortex uses to guide arm movement, he converted those signals into instructions for a crude robotic limb with a two-finger clamp.

Now, Schwartz is training his monkeys again, except this time he wants to teach them to use the five-fingered MPL and perform the kind of everyday but complex tasks we take for granted.

If the monkeys demonstrate that it is possible to steer the arm with brainpower alone, Schwartz and colleagues will give people with spinal cord injuries a chance to try the MPL.

"For someone with spinal cord injury, it's a huge deal for them to be able to feed themselves," says McLoughlin. "Nobody has achieved this level of a control in humans with a brain-controlled prosthetic. We want to take it to a higher level than in the past."

 

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