Still in its prototype phase, the foot was built by inventor and mechanical engineer Jerome Rifkin. It bends like a normal foot and ankle, and conforms to the terrain underneath it. Rifkin built something that combined the natural step of a bionic foot with the simplicity and low cost of a mechanical prosthetic. His jointed foot has a heel, a forefoot, a big toe-and no joint at the ankle. Instead, a novel midfoot joint, which connects the heel and forefoot, does the job of both the ankle and the arch. Like an ankle joint, it flexes up and down to give the wearer a more natural step.

As with a natural midfoot joint, the prosthetic creates a flexible arch in the middle of the foot. A spring and cable connect it to a second joint at the toe, to create extra push-off at the end of each step. Other tensioned steel cables serve as the tendons and ligaments that govern its range of motion-the user doesn't control it, it simply responds to the pressure of walking.

With input from 11 amputee test users, Rifkin is refining his fifth (and, he hopes, final) prototype, made primarily of magnesium for its strength and low weight. Early results indicate that the one-pound foot reduces the amount of energy required for each step because it uses the force absorbed by the spring and joints to help propel the foot forward. "It's the equivalent of taking a 50-pound pack off your back," he explains. That's on par with the best bionic feet, without all the expensive motors and artificial intelligence."