This bionic leg can be controlled through thinking about an action. Image: RIC Users are able to climb stairs, walk on sloped terrains and flex an ankle just by thinking about the action. Image: RIC This illustration demonstrates how neural impulses communicate with the leg's computer. The computer is able to learn a user's intended actions and provide an intuitive movement. Image: RIC

About a year ago, Zac Vawter climbed all 103 flights of stairs of the Willis Tower in Chicago. On its own, this accomplishment would be pretty unremarkable, but Vawter, who lost his leg four years ago during a motorcycle accident, happened climb more than 2,000 steps while wearing a prosthetic leg. Even crazier yet? Vawter could control this prosthetic leg with his mind, sending instructions from his brain, down through nerves that would communicate with his mechanical limb.

In the world of prosthetics, this was a breakthrough moment. Previously, if leg amputees needed to climb the stairs, they’d either have to rely heavily on their good leg to propel them upwards or use a remote control that required them to stop and start at the base of each stair, which creates a movement that’s more robotic than human.

>In the world of prosthetics, this was a breakthrough moment.

At the time Vawter climbed the stairs, this new leg, a collaboration between the Rehabilitation Institute of Chicago (RIC), a team of designers at Vanderbilt University and prosthetics company Freedom Innovations, was the most advanced lower limb prosthetic in development. It still is, but an improvement in technology, recently outlined in an issue of the New England Journal of Medicine, is extending the leg’s capabilities even further, now allowing amputees to control a range of movements—walking up stairs, rotating an ankle, navigating sloped terrains—just by thinking about doing them. “This is where prosthetic limbs will go,” says Steven Reinecke, EVP of Research and Development at Freedom Innovations. “Right now people have a sophisticated support system. The goal is try to mirror the natural body as much as possible."

Today most prosthetics are little more than glorified support systems, and it's extraordinarily difficult to get a protheses made of sensors and metal to mimic human movement. But the research team at RIC is getting close. Since around 2005, RIC has been working on a method called targeted muscle reinnervation, which reappropriates nerves from an amputated limbs to healthy muscles. In the case of Vawter, nerves from his lower leg were reattached to healthy hamstring muscles.

“This person has essentially been rewired,” explains Dr. Levi Hargrove, one of RIC's lead researchers on the project. So essentially, the neural impulses that are sent from the brain–the ones telling the body to stand, walk or change positions–communicate with the prosthetic leg through sensors, and a computer then translates those instructions into actions. Over time, an algorithm learns the patterns of a user’s intended actions and can begin to react to their thoughts, thus making a mechanical limb function intuitively or a least lot more like a normal human leg.

Though Vawter used an earlier version of the limb to climb the Willis Tower, the first time he used the leg to autonomously navigated sloped terrains and rotate an ankle was in April of this year. "When we said to Zac, “You’re in control, you can do whatever you want, that was kind of a surreal moment,” recalls Hargrove. “It was an overwhelming feeling for me—and I think for him as well that this has the potential to help a lot of people."

>'This person has essentially been rewired.'

A technology like this is obviously attractive to the military, and $8 million in funding proves that. But while the RIC team was working with the department of defense in administering this technology to wounded veterans, it was focused mostly on upper body prosthetics. “The Army said, ‘We love the technology, but we have there are 10 times more leg amputations than arm amputations–can you do anything to improve the control of prosthetic legs?”

People have been using neural information to control prosthetic limbs for decades, but all of them have been for the upper-body. Legs, on the other hand, are a little more difficult to control, and it was until recently that the technology was advanced enough—and light enough—to really even begin thinking about making a prosthetic limb like this. “All of the innovation will come to the electronics,” says Reinecke, adding that around 10 years ago the sensors and materials would have made a leg like this around 300 pounds.

Today, thanks to lightweight materials like graphite and micro-technologies, the leg is around 10.2 pounds and the research team is shooting to get it under 10. The research still has a ways to go. Error rates, which mostly refer to the number of times a user scuffs his or her feet on the ground while walking, need to be lowered in order to reduce falls. And Freedom Innovations still has a lot of work to do translate a research-based technology into a consumer product. For as sophisticated as the leg is, it still could be lighter, and the machine-like noise could stand to be more subtle.

Everyone involved says they hope to have something on the market for clinical testing in as little as two to three years. In consumer time that feels like an eternity away, but in science it's a remarkably quick pace. “Five years ago I thought there was no way to achieve all that we’ve achieved,” says Hargrove. “I thought it would take 10 years to get to where we are right now.”