Smart Bionic Limbs are Reengineering the Human

There aren't enough bionic men on the planet to produce a proper stereotype. Even so, David the Farmer seems atypical. Ruddy, red-haired, and impossibly cheerful, he meets us on the ?gravel path outside his workshop. What I was expecting?a grizzled retiree limping stiffly through his daily chores?bears no resemblance to this 30-something mechanic climbing down from a massive tractor without hesitation, weaving between ATVs and scattered engine parts, moving from task to task with no evidence that he's part machine. After a few minutes, there are clues, though: He always turns on his right leg, and his pants gather around his left ankle, hinting at a limb that's slightly skeletal and decidedly nonbiological.

Today that left leg will be replaced. That's why engineers from ??ssur, one of the world's largest prosthesis-makers, drove an hour west from the company's headquarters in Reykjav?k, Iceland, to the farm where David Ingvason lives and works. David the Farmer??the nickname they've given their star prosthesis tester, though he is actually employed as a full-time, on-site ?mechanic?is one of a limited pool of amputees fitted with the Symbionic Leg: an artificial knee, ankle, and foot that are integrated into a single bionic limb.

On the farmland and surrounding terrain, in tall grass, and on moss-sprayed plains of volcanic rock, Ingvason regularly destroys his leg. He fouls the motors in muck and sludge, burns them out through unremitting use, and generally grinds one of the most sophisticated auto-adaptive devices on the planet, each one worth more than some sedans, into an inert, cybernetic paperweight. According to ?ssur's new technology search manager, Magn?s ?Oddsson, all Ingvason has to do is call and they'll hand-deliver a new limb. More often, he swings by Reykjav?k himself wearing a backup leg and asking for a repair or replacement. Whatever David the Farmer wants, he gets?the punishment he metes out to his leg, and the data that result, are simply too useful.

?ssur began selling the Symbionic model as the world's first ?commercially available bionic leg last fall. It represents a significant shift in prostheses. The traditional half-measures, the stand-ins for lost limbs and senses, are now being imbued with machine intelligence. Ingvason's leg is, in fact, a robot, with sensors that detect its environment and gauge his intentions, and processors that determine the angle of his carbon-fiber foot as it swings forward. The same approach is being applied to prosthetic arms, in which complex algorithms determine how hard to grasp a water bottle or when to absorb the impact of a fall. Vision- and hearing-based prostheses bypass faulty organs and receptors entirely, processing and translating raw sensor data into signals that the brain can interpret. All of these bionic systems actively adapt to their users, restoring the body by serving it.

Take, for example, one of the most common prosthesis failures. A mechanical knee typically goes rigid as the heel lands, supporting the user's weight, then unlocks when pressure is applied to the toe. If that toe contact comes too early the leg collapses under its owner. The Symbionic Leg isn't so easily fooled. Force sensors and accelerometers keep track of the leg's position relative to the environment and the user. Onboard processors analyze this input at a rate of 1000 times per second, deciding how best to respond?when to release tension and when to maintain it.

Since the leg knows where it is throughout each stride, achieving a rudimentary form of proprio?ception, it takes more than a stubbed toe to trigger a loose knee. If the prosthesis still somehow misreads the situation, the initial lurch of the user falling should activate its stumble-?recovery mode. Like antilock brakes for the leg, the actuators will slow to a halt, and magnetically controlled fluid in the knee will become more viscous, creating resistance, as the entire system strains to keep the person from crumpling or toppling.

The result, Ingvason says, is that he rarely falls, or no more often than someone with two biological legs. He can drive ATVs, hike across glaciers, even ride a horse while herding sheep. "I don't have to think about it," he says. Before he went bionic, Ingvason fell constantly. "With the old knee, it was every day, often more than once in a day," he says. "If I was walking and the toe hit something while swinging forward and I stepped on it, then I just went down. Now I'm walking on uneven ground and high grass and sand and mud and everything."

Ingvason's newly delivered limb is another Symbionic Leg, loaded with upgraded software that will allow the knee and the ankle to communicate with each other. ?ssur plans to develop this feature over the coming years, ?establishing what Oddsson calls networked intelligence. After putting it on, Ingvason limps, awkwardly at first, across dirt and gravel, past the rusting hulks of trucks and cars. Within a few minutes, the robot has calibrated itself.

With Ingvason's pant leg hitched up, it's impossible not to watch the limb in action. It's harsh and alien. The gray polymer shell, which partially conceals aircraft-grade aluminum, seems too skinny to support his weight; the ankle, too delicate for the 10,000 newtons of force it was built to withstand. But the leg is nimble and so quick to react, it's as though he were born with it.

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