The Robo-Flex, as he calls the device, is attracting attention because it solves the classic problem for ball joints—they swivel easily, but holding one in a set position requires constant force, the way your muscles must stay flexed to extend your arm. Doing so for robotic arms usually requires lots of energy and several bulky joints to hold weight at different angles. Merlo's solution was to cover a small titanium ball with nubs. The ball rolls smoothly in a socket lined with stainless-steel pins. Extend the pins, and they interlock with the nubs to prevent the ball from moving. Built into a prosthetic wrist, the joint requires no energy to stay locked and can support a 500-pound load. Not bad for an afternoon in the backyard.