It sounds like something straight out of science fiction: artificial limbs that not only move, flex, and feel like their flesh counterparts, but also respond directly to one's thoughts and even translate sensory feedback -- the feeling of grass beneath one's feet or the sensation of a limb floating in space -- straight back to the brain.
Thanks to an aggressive push in funding from the US military in an effort to the improve the lives of injured veterans, those advancements are no longer such farfetched dreams. While the idea of "Blade Runner"-level prosthetics is still a far-off fantasy, impressively capable, thought-controlled bionic limbs are now a modern-day reality thanks to pioneering research between the Rehabilitation Institute of Chicago (RIC), DARPA, and a growing sector of companies developing the next generation of artificial limbs.
Just last month, the RIC announced that its research in bionics has yielded the first thought-controlled robotic leg. The research had already seen its fair share of headlines -- including a climb of 103 flights of stairs on the single prosthetic unit -- but the team led by Dr. Levi Hargrove waited to conclusively publish its findings in the The New England Journal of Medicine. The bionic hardware, which was more than eight years in the making, was coupled with a groundbreaking approach -- targeted muscle reinnervation surgery -- that empowers the brain to move parts of the bionic limb with nerves that are rerouted to healthy muscles.
But thought-controlled limbs are only the beginning. With a goal to one day provide lower-cost, sensory-enabled limbs that may use implants to generate even more precise movement, bionics research is on course to fundamentally change the world of prosthetics over the course of the next decade.
TMR: Controlling your ankle with your hamstring
In 2009, Zac Vawter was in a motorcycle accident that resulted in the amputation of his right leg below the knee. It turns out that, at roughly the same time, the RIC and Northwestern University were developing a procedure that would allow researchers to rewire nerves from damaged muscles to healthy ones, using the still-intact neural impulses to reroute movement.
Called targeted muscle reinnervation, or TMR, the surgery -- first developed in 2009 by doctors Gregory Dumanian and Todd Kuiken -- has worked for bionic hands and elbows, which can use rewired nerves placed onto larger muscles like biceps and pectorals to translate contractions in those healthy muscles to wrist and arm movements.
So the now-32-year-old Seattle native volunteered to undergo TMR and become a part of a multiyear research that's backed by $8 million in funding from the US Army and additional financial support from DARPA, another big player in prosthetics that has helped foster cutting-edge bionic arms with its Revolutionizing Prosthetics program and advance the science that enables their movement with its Reliable Neural-Interface Technology program.
So how exactly does TMR work with respect to Vawter's condition?
"We take nerves that would have gone down to his ankle and rewire them to his hamstring," said Dr. Annie Simon, a biomedical engineer on Hargrove's team at the RIC.
That means that when Vawter, post surgery, thinks of moving his ankle, the rewired nerves force his hamstring muscle to contract. Over-the-skin electrodes that are placed within the molded plastic that connects the bionic leg to the residual limb pick up on that contraction and translate it, through RIC's algorithms, into precise movement below the knee.
"It learns and performs activities unprecedented for any leg amputee, including seamless transitions between sitting, walking, ascending and descending stairs and ramps, and repositioning the leg while seated," Hargrove said.
Because Vawter's amputation was below the knee, he was left with healthy nerves that made him a perfect candidate for TMR. "If the nerve is healthy, it still carries that information that would have went to the missing joint, even 10 to 20 years post-amputation," explained Simon.
However, because the use of electrodes over the skin is noninvasive, even those with more complicated amputations that have not undergone TMR surgery can use the RIC's bionic leg. "If you have the surgery you can control the ankle a little bit better," Simon said, stressing that someone who has undergone TMR will have a few more sensors and a more precise nerve network to work with.
Hargrove and his team at the RIC think their bionic leg will available for home trials in three to five years. Currently, the RIC has only one device -- developed over an eight-year period by Vanderbilt University -- capable of being controlled with one's thoughts, and Vawter has yet to push its boundaries such as using it in the home.
"If there is a difference between what he intended to do and what the prosthesis does, we want it to just be the equivalent of, say, stubbing your toe," said Simon. Vanderbilt and prosthetic manufacturer Freedom Innovations are currently working on a second iteration of the leg that will near consumer-level quality and robustness.
Still, Vawter -- who uses a regular mechanical prosthetic for everyday use -- is optimistic. "For the first time since my injury, the bionic leg allows me to seamlessly walk up and down stairs and even reposition the prosthetic by thinking about the movement I want to perform. This is a huge milestone for me and for all leg amputees," he said in the RIC press release.
Sensory feedback: Technically possible, but still a long way off
"I think you'll see bionic legs become very popular within 10 years," Hargrove said. But, he added, "the way that they're controlled will be variable."
Hargrove points out that there are already a number of high-tech prosthetics on the market, ones that forgo nerve rewiring and rely on simple electrodes and internal motors. For instance, the Bebionic3 is a prosthetic arm that comes with a multitude of grip patterns and the ability to move independent fingers with such precision that it's been called the "Terminator arm."
Still, thought-driven nerve control with the help of TMR is the ideal future for artificial limbs if science is to realistically replicate the functionality of a human hand or leg. To go even further, Hargrove suggests, one must imagine the use of implanted electrodes that could enable sensory feedback and employ substantially more natural neural interfacing.
"What we're not doing as well at yet is providing feedback to users about where the arm or leg is in space or the type of ground that they're on," he said, adding that sensory feedback is a growing area of research that still has a long path ahead to clinical use. "In order to get these advancements, we would need cutting-edge sensors that could perhaps be implanted in the body that could directly interface with the nerve."
Despite the breadth of the remaining ground to cover, Hargrove -- very much like the test subject that wears his team's bionic leg -- is ready for the future. "They'll continue to work better and better and better," he said.
"We hope we're laying the foundation here."