In this Oct. 25, 2012 photo, Zac Vawter practices walking with an 

In this Oct. 25, 2012 photo, Zac Vawter practices walking with an experimental "bionic" leg at the Rehabilitation Institute of Chicago. After losing his right leg in a motorcycle accident, the 31-year-old software engineer signed up to become a research subject, helping test a trailblazing prosthetic leg that's controlled by his thoughts. He will put this leg to the ultimate test Sunday, Nov. 4 when he attempts to climb 103 flights of stairs to the top of Chicago's Willis Tower, one of the world’s tallest skyscrapers. (AP Photo/Brian Kersey)
Associated Press
In this Oct. 25, 2012 photo, Zac Vawter practices walking with an experimental "bionic" leg at the Rehabilitation Institute of Chicago. After losing his right leg in a motorcycle accident, the 31-year-old software engineer signed up to become a research subject, helping test a trailblazing prosthetic leg that's controlled by his thoughts. He will put this leg to the ultimate test Sunday, Nov. 4 when he attempts to climb 103 flights of stairs to the top of Chicago's Willis Tower, one of the world’s tallest skyscrapers. (AP Photo/Brian Kersey)
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