Robotic Ankles Step Up

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May 31, 2007 — Two robotic devices that mimic the human ankle could give amputees more natural, energy-efficient gaits. These spring-enabled, motor-driven devices could one day evolve into prostheses that attach directly to bone and draw on neural implants and sensors for automatic feedback and control.

"Human beings aren't disabled. It's the technology that is provided to human beings that is disabled," said said Hugh Herr, assistant professor and director of the Biomechatronics Group at the Massachusetts Institute of Technology's Media Lab in Boston.

Herr speaks from first-hand experience. He lost both legs below the knee to frostbite in a rock-climbing accident more than 25 years ago. When he was fitted with his first pair of artificial legs, he wasn't happy with them.

"My doctors told me that this was the best, and I should live with and accept what I was given," he said.

But Herr didn't. Instead he devoted his career to developing something better.

Herr and his group recently demonstrated the latest design of their Active Ankle-Foot Prosthesis, which is part of a five-year research project funded by the Veteran's Administration. The goal is to create an entire leg prosthetic comprised of sophisticated electronics and sensors that receive signals from a person's brain and use those signals to control limb motion.

The current design has sensors, springs and a motor that work together to mimic how the human ankle stores and releases energy as a person walks.

When an able-bodied person walks, the heel contacts the ground in front. Next, the ankle extends until the foot becomes flat. The ligaments and tendons in the ankle and foot absorb energy during this phase and, as the foot rolls on its ball, release the energy to allow the ankle to extend and propel the body upward and forward.

An amputee wearing a conventional prosthetic receives a push at the ankle, but from passive springs that do little to propel the body. As a result, these people expend 20 to 30 percent more metabolic energy as compared to able-bodied folks.

Herr and his group recently showed that by using the Active Ankle-Foot Prosthesis, an amputee could walk with 20 percent less metabolic energy than conventional prostheses.

At Arizona State University's Polytechnic campus, associate professor Thomas Sugar and his team are working on another robotic ankle model. They have developed a device named Sparky, for "spring ankle with regenerative kinetics." Sugar's device lacks sensors but has springs specifically adjusted to a person's weight.

A motor adjusts the position of the springs so that they store and release the energy to propel the ankle forward. The device weighs just two pounds—compared to a human ankle and shinbone, which weigh four or five pounds.

Sugar's prosthetic is part of a three-year project administered by the U.S. Army Medical Research and Materiel Command and could be commercialized by 2009.

"The biggest problem is batteries," said Sugar. "How can you have enough battery power for day-to-day walking?" Small batteries need to be charged more frequently, while large batteries are too heavy.

For now Herr and his team think they have solved the battery problem. They have started up a Cambridge, Mass.-based company called iWalk to begin commercializing their foot-ankle system next year.