Bionic arms have improved incredibly over the past 10 years. Just the other day it was announced that three men had chosen to amputate their arms as a result of nerve damage, and replace them with mind-controlled prosthetics. Researchers have even managed to create devices that allow amputees to feel what they're touching with their new limbs. Veritasium investigates an affordable 3D-printed arm that could help millions of people around the world. And you can print one yourself.