Zac Vawter, a 31-year-old amputee, aims to make medical history Sunday when he attempts to climb 103 stories to the top of the Willis Tower with his state-of-the-art bionic leg.



Researchers at the Rehabilitation Institute of Chicago equipped Vawter with the prosthetic limb after he lost his right leg in a motorcycle accident in 2009. Their innovative design allows him to control the leg with his thoughts, a groundbreaking medical achievement that's been years in the making.



"The first time that we went up and down stairs was a little clunky and not particularly smooth," said Vawter, a software engineer from Yelm, Wash. "Now I'm comfortable taking a hand off of the railing."



The climb, called "SkyRise Chicago," will be the bionic limb's first public appearance and its most grueling test, said lead researcher Levi Hargrove. Vawter said his goal was to make it the top in one hour.



With about 2,700 people registered to climb alongside him, the event is serving as a fundraiser for the institute.



Hargrove said the leg worked by responding to electrical impulses from the muscles in Vawter's upper leg, including his rewired hamstring. That's where the surgeon who amputated his leg reattached the dangling nerves that previously carried signals past his knee.



The procedure, known as targeted muscle reinnervation, allowed Hargrove and his team at the institute's Center for Bionic Medicine to tap into the preserved neural signals to control the prosthetic limb.



"He just thinks about moving his ankle," Hargrove said as an example. "He thinks about doing those movements and the signals travel down the nerves and are redirected onto hamstring muscle. The body doesn't know that the ankle is not contracting. It is very intuitive for him."



That the U.S. Department of Defense is funding the five-year, $8-million research project hints at the bionic leg's potential. Hargrove said injured veterans who lost legs in combat had much to gain from the new technology.



But while bionic arms have been available for several years, their lower-limb equivalents won't be on the market for at least a few more. After the climb, Vawter has to return the leg to researchers who will work to fine tune its steering.



Hargrove said safety was his top concern and the largest hurdle to overcome before the leg could be sold commercially. A malfunctioning leg could cause its wearer to stumble, or worse — fall down a flight of stairs.



"We have to make sure our system is really safe and robust to prevent those sorts of injuries," Hargrove said. "It'll be a few years."



mholtz@tribune.com

