Waiting For an Arm and a Leg

Melding Body and Bionics

Of the many hurdles on the road to creating a Halfaker-friendly prosthetic, one of the most critical is the socket, the place where flesh and prosthetic meet. There have been plenty of socket innovations, including vacuum sockets that suspend the limb and suction sockets that add or remove fluid to maintain a consistent fit. But the best option would be to get rid of the socket altogether.

In 1952 Swedish orthopedist Per-Ingvar Branemark discovered that a titanium rod inserted in a rabbit´s bone fused well. He called it osseointegration, and the technique has worked wonderfully for dental implants, false teeth built on rods anchored in the patient´s underlying bone. In 1990 Branemark´s son, Rickard, an orthopedic surgeon at Sahlgren University Hospital in Gothenburg, Sweden, surgically implanted rods into human patients´ bones to act as a stable base for a prosthetic arm or leg. But several patients suffered complications. The skin never fused around the rod, acting as if it were a wound, and infections sprouted.

At Brown, molecular biologist Jeffrey Morgan and dean of engineering Clyde Briant are seeking ways to stop such infections. Briant is experimenting with titanium and alloys in search of a combination that is strong yet compatible with human tissue. Morgan is growing skin cells that will cling to the metal, forming a natural seal. It shouldn't be impossible: "Brown students," he observes, "have pierced noses."

Once science figures out better ways to attach artificial limbs, prosthetics themselves need to become smarter, able to act on signals sent directly from the brain. Consider the case of Jesse Sullivan, a power lineman from Dayton, Tennessee, who lost both arms at the shoulder after being electrocuted on the job in 2001. A year later, doctors transferred four nerves (which were no longer infusing muscle) that had controlled his left arm out of his shoulder area and into his pectoral muscles. Six months after that, Todd Kuiken, director of the Rehabilitation Institute of Chicago´s Neural Engineering Center for Artificial Limbs, detected signals in the nerves. Kuiken´s team studded the surface of Sullivan´s chest with electrodes and joined them with wires to a multi-jointed prosthetic. The goal was to connect brain to artificial arm by redirecting signals from Sullivan's severed nerves. It worked. When doctors asked Sullivan to think about opening his hand, the device, almost instinctively, sprung open. "It was the greatest feeling I'd had since I'd been hurt," Sullivan says. He can now eat, mow the lawn, and do his laundry, but his arm fulfills only a small fraction of the nerves´ potential power. The nerve for hand closing controls at least 20 muscles, Kuiken says, "and I'm using it for just two different signals. If we tease it out, we might get better and better control." Kuiken is now developing sensors that will allow Sullivan to feel what he is touching.

Another way to power artificial limbs is to bypass the nerves and tap directly into the brain. That´s what John Donoghue, director of Brown´s Brain Science program and the chief scientific officer at Cyberkinetics Neurotechnology Systems in Foxborough, Massachusetts, is working toward with the invention of BrainGate, a chip that was implanted in 2004 into paralyzed 25-year-old stabbing victim Matt Nagle. With the four-millimeter-square chip in his primary motor cortex, Nagle thinks about moving a cursor on the computer screen to the right. His neurons fire in a certain pattern, and those data are transmitted through a plug affixed to his skull to the computer, which moves the cursor. Soon, BrainGate´s developers got really ambitious. They lay a prosthetic arm, tethered to the computer, on Nagle´s lap and told him to open the hand. He did, just by thinking, and swore in amazement as the hand unfurled. Donoghue promises that future versions will operate wirelessly; Cyberkinetics is developing a control system that uses wireless transmitters and fully implantable power sources.