CHICAGO – Zac Vawter considers himself a test pilot. After losing his right leg in a motorcycle accident, the 31-year-old software engineer signed up to become a research subject, helping to test a trailblazing prosthetic leg that’s controlled by his thoughts.

He will put this groundbreaking bionic leg to the ultimate test Sunday when he attempts to climb 103 flights of stairs to the top of Chicago’s Willis Tower, one of the world’s tallest skyscrapers.

If all goes well, he’ll make history with the bionic leg’s public debut. His whirring, robotic leg will respond to electrical impulses from muscles in his hamstring. Vawter will think, “Climb stairs,” and the motors, belts and chains in his leg will synchronize the movements of its ankle and knee.