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.