Bionic Limbs Remain Far Out of Reach
TV gave us the world's first bionic man in 1973. Science is way behind.
TV gave us the world’s first bionic man in 1973. Science is way behind.
Wired Science (“A True Bionic Limb Remains Far Out of Reach“):
A decade ago, researchers seemed on the cusp of creating a working interface between body and machine. Even back then, arms controlled by myoelectric signals were old news; more-advanced limbs would read commands directly from the brain. In 2003 scientists at Duke announced that monkeys could control a robotic arm via electrodes implanted in their brains. A year later, a similar device allowed a quadriplegic human patient at Brown University to play Pong with his thoughts. In 2008 researchers at the University of Pittsburgh showed off a monkey that could use a neurally controlled robotic arm to eat marshmallows. Surely if a monkey could use a robot arm to feed itself, it wouldn’t be long before amputees used them to tie their shoes and pilots flew jets with their minds.
Such advances are needed more than ever. There are approximately 185,000 limb amputations in the US every year—the causes range from diabetes to workplace injuries to battlefield trauma. In 2005, the most recent year for which data is available, 1.6 million Americans were living without a limb. As of January 2012 there were 1,421 amputees from the Afghanistan war and the second Iraq war. Of those, 254 lost upper limbs, like Lehman, and 420 lost more than one limb. Many of the rest, presumably, lost single legs or feet, but the statistics don’t break out those details. In fact, the numbers seem low—but they’re the best ones the military will share.
What the military will share, though, is money to study limb replacements. In 2006 the Defense Advanced Research Projects Agency began a program to build, in four years, an arm “directly controlled by neural signals” that would have abilities “almost identical to a natural limb in terms of motor control and dexterity, sensory feedback (including proprioception), weight, and environmental resilience.” With its deadline now three years past, those ambitions now look wildly unrealistic. After $153 million in funding and years of engineering, the best any amputee can get is a heavy, clunky arm that moves slowly, can’t feel anything, and often misreads its user’s intentions.
“The human arm is amazing,” says Rahul Sarpeshkar, a bioengineer at MIT who pioneered the design of ultralow-power circuitry for bionic interfaces. “It does a lot of very intelligent local computation that the brain doesn’t even do. We don’t understand the coding schemes that biology employs. We don’t understand how its feedback loops work together.” In other words, the science hasn’t yet caught up with the fiction. A true bionic limb—one that responded to mental commands with precision and fluidity, one that transmitted sensory information, one that its user could feel as it moved through space—would require a depth of understanding and technological complexity that is simply beyond today’s prosthetic experts. “It’s not that we’re not going to be able to do it,” Sarpeshkar says. “But it’s higher-hanging fruit than people think.” In other words, this is more than just an engineering problem. It’s a problem of basic science.
Animal brains keep track of body parts with a sort of sixth sense called proprioception. You know exactly where your right arm is, not because it feels hot or sore or is touching anything, but because you just know. Receptors in your limbs send position-and-motion data through your nervous system, and it all gets collated, somehow, into an unconscious awareness. My arm is up there; my arm is down here. “There are muscle receptors. There are tendon receptors. There are capsule receptors, even skin sensors, all contributing in a very complex way that we don’t understand,” Kuiken says. “I don’t think there’s going to be a single spot in the brain where you can put a dense array of electrodes and get a strong percept of proprioception.”
Ironically, old-fashioned mechanical arms first prototyped two centuries ago are better at giving feedback than anything invented since. A cable attached to a harness opens the hook or flexes the elbow when the user pulls it by reaching forward or shrugging. Pick something up and force on the prosthesis, translated to your stump, tells you how heavy it is. Many users actually prefer cable-driven arms to the myoelectric, motor-driven type.
So what would it take to build an artificial arm that could send proprioceptive feedback to the brain? In the 1930s the neurosurgeon Wilder Penfield found that electrically stimulating the surface of the brain caused patients to feel sensations and twitches in specific parts of the body. That’s where the monkeys come in again. Miller’s graduate students are working on using the same kind of electrode array in Thor’s skull to send electrical signals directly into a part of the brain that is thought to receive proprioceptive input. (Complicating matters further, this area may also handle tactile somatosensory feedback—touch.) The idea is to make a monkey believe that a lever is jerking in its hand. Eventually, the thinking goes, they’ll be able to embed sensors in the arm that transmit the same kind of data.
So far the test animals do seem to react as if the handle had moved. After training them to push the handle to the right when they feel it move, Miller’s graduate students send a signal to the electrodes, and the monkey moves its hand as if it could feel the handle moving against it. But no one knows what sensation the input is actually producing. There’s no way to ask the monkey. Researchers at Caltech and the University of Pittsburgh, currently working on a neural interface for a fancy motorized arm created at the Johns Hopkins University Applied Physics Laboratory, plan to integrate this kind of sensory data into human trials in April 2013.
Of course, spending all those resources to get comprehensible signals into and out of a brain means buying a central assumption: that the brain is the center of control for the limbs. But what if it isn’t? It takes 300 milliseconds, almost a third of a second, for the human arm to send a message to the conscious brain and for the brain to respond. If that was the only way to control the hand, we’d never manage to balance trays or hang up clothes. Those activities require a degree of fine-motion control that seems to outpace the speed of signals going to and from the brain.
That’s what makes some researchers think that the brain has learned to delegate fast-response tasks to the spinal cord. It’s closer to the arm and can respond up to 10 times faster—in just 30 milliseconds. “The moment-to-moment timing of the hand’s muscle contractions is dependent on sensory feedback that is never going to the brain,” says Loeb, the USC biomedical engineer. “It’s being handled locally.” In other words, the spinal cord isn’t just a dumb trunk line. It’s a coprocessor.
Unfortunately, tapping into the spinal cord is even tougher than tapping into the brain. “It’s really hard,” Donoghue says with a rueful laugh. “The spinal cord moves around a lot, so the mechanical problem is huge. Sticking an electrode in there that stays in place is a big challenge.”
Fascinating stuff. Much more at the link.