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Mind-Powered Prosthetic Limb

By Brennen Jensen
Drawing of prosthetic arms connected to a brain
Michael Mcloughlin is an engineer with the Johns Hopkins Applied Physics Lab and a principal investigator in APL’s Revolutionizing Prosthetics Program.

In June 2014, a Colorado man named Les Baugh visited the Johns Hopkins Applied Physics Laboratory and made history. He moved a cup and a rubber ball from one shelf to another.

This may sound trivial until you realize Baugh had both of his arms amputated at the shoulder following an accident 40 years earlier. He performed these actions while fitted with a pair of APL’s prototype Modular Prosthetic Limbs. What’s more, he was able to do so simply with his thoughts. To raise the cup, he just thought about doing it and the limb responded.

While APL prostheses have been successfully thought- operated by amputees since 2006, this was the first time a bilateral amputee had successfully operated a pair of them at once. It was but the latest accomplishment of the Revolutionizing Prosthetics program, a partnership between APL and the Defense Advanced Research Projects Agency. The program was launched in 2005 as a way to develop upper-limb prosthetic technology. “When I was a kid, I used to watch The Six Million Dollar Man,” says Michael McLoughlin, an engineer and the Revolutionizing Prosthetics principal investigator ”I never actually thought I would be doing these kinds of things.”

The brain prosthesis interface that APL developed works via electrodes on the skin detect ing the tiny electric signals that occur whenever a muscle is activated. If you want to move your index finger, for instance, your thoughts trigger muscles in your forearm, which in turn activate the correct ten-don to move the finger. With the Modular Prosthetic Limb, electrodes pass along these muscle-movement signals to a computer within the prosthesis, which interprets them to activate the correct motors to move the hand and fingers.

Now, that’s how it works for partial amputees who still have forearm muscles present. Those like Baugh who are missing an entire arm require a surgical process called targeted muscle reinnervation wherein the nerves that once activated the now-missing arm and hand muscles are rerouted to muscles in the chest where the electronic signals can be measured and analyzed by the computer in the prosthesis. This is a departure from how conventional prostheses have been designed in the past. “The whole idea is that the patient is not learning how to use the prosthesis, the prosthesis is learning to understand what the patient wants to do,” says McLoughlin. “That’s a fundamental shift in the way these kinds of devices work.”

While the thought-control aspect of APL’s prosthetic limb might have the greater gee-whiz factor, McLoughlin also stresses the significant electrical and mechanical engineering required to create the arm itself. Indeed, this was one of the reasons prosthetic arms lagged behind advances made in prosthetic legs. “What legs do is comparatively simple,” McLoughlin says. “We walk, we run, we stand. But the hand and arm have a tremendous variety of activities and movements. The prosthesis had to have similar size and weight to a human arm, requiring really small, high-powered motors to be in a very confined space.”

"The whole idea is that the patient is not learning how to use the prosthesis, the prosthesis is learning to understand what the patient wants to do. That's a fundamental shift in the way these kinds of devices work."

To provide the full range of arm and hand movement, APL’s prosthesis employs 26 joints and 17 motors. The design is also modular, composed of individual units that can fit together as needed to accommodate a variety of amputees, whether they are missing the entire arm, just the hand, or something in between. This added to the engineering challenges (requiring, for example, that the on-board computer be located in the palm of the hand), but it ultimately makes for a more efficient and flexible system.

The engineering hurdles have largely been overcome in these late-stage prototype prostheses. The next challenge is getting the limbs into wide-spread use by amputees—and it is largely an economic one. These complex devices would be extremely expensive to produce at present. But McLoughlin is confident that the key to reducing the per-unit costs lies in commercializing the technology beyond the field of prosthetics, especially in the field of robotics.

“There is interest in advanced, human-like arms for a lot of different robotic applications in manufacturing, home assistance, and especially for dangerous situations,” he says, such as bomb defusing. “The key to moving this forward is building these robotic capabilities that get the technology used for a lot of different things. That will ultimately drive the price down.”

futuristic illustration of the brain
top image by Mario Wagner; prosthetic arms drawing by Tom Jay

Through the Revolutionizing Prosthetics program, the Johns Hopkins Applied Physics Lab has developed a prosthetic limb that can be controlled by the user's mind.

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