Imagine, for a moment, losing both your arms. If you’re very lucky, you’ll lose them below the elbow. That means you’ll be able to wear a prosthesis and use your shoulder muscles to open and close your hand.
If you’re unlucky, you’ll lose your arm at your shoulder. At that point, you may not end up using a prosthesis, because they’re awkward, difficult and very hard to control. There’s no way to wire the muscle into the arm, so you use chin switches to trigger one motor at a time to move each joint.
Jesse Sullivan was very unlucky. An electrical lineman, he received a 7400 volt burn which caused him to lose both his arms at the shoulder. And then he got very lucky, meeting Dr. Todd Kuiken, who was interested in new strategies for giving amputees control over their prosthetic limbs.
As Kuiken explains, “It’s very tempting to think that the way to control a prosthetic limb is to use the nerves” – i.e., listen to the signals the nerves are delivering and use them to trigger motors in the limb. Unfortunately, this is a lot harder than it looks. The nerve signals are very weak, the transmitters are very fragile, and it requires embedding electrodes deep in the shoulder.
So Kuiken tried something different, something he’d tried in rats, but never in a human. He rewired Jesse’s arm nerves into his pectoral muscle. When the nerves healed, Jesse could think “open hand” and his arm nerves would move his pectoral muscle. With a heavily modified prosthetic arm, this pectoral movement could trigger a motor in the arm and close the hand.
This, by itself, is pretty amazing. By rewiring four nerves, Kuiken enabled Jesse to bend his elbow, close or open his hand, and extend his wrist, all by using his existing nerves, not moving his chin. But it got more exciting – as the nerves regrew in the pectoral muscles, sensory nerves regrew as well. So Jesse can now “feel” his arm by sensing pressure or temperature on his chest. This meant that Kuiken could design a prosthesis – the first ever – that allowed Jesse to feel how much pressure he was applying in a grip.
Sullivan and Kuiken show off both the new technoogy they’ve developed… and the old tech, which Sullivan says he still uses more often and prefers, based mostly on reliability. Sullivan tells us he’s able to do laundry, paint rooms in his house, mow his lawn and trim his hedges… and Kuiken tells us that Sullivan once came back into the lab with 16 titanium bolts in his prosthetic arm broken from trying to pull-start his mower. In other words, Sullivan is the perfect tester to see whether this technology can really work in the real world.
It sounds like there’s a tremendous room to improve the technology. The arm Sullivan is wearing has a 400Mhz processor and less than 64KB of RAM – with improvements in computational technology, it’s easy to see how the prosthetics could process more information, provide more feedback and become more reliable. It would be beautiful to see the techniques doctor and patient are pioneering used by people around the world to regain their independence despite disabling injuries.