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CUE 2006

Mechanical Engineering: A Step Ahead—Recreating the Biomechanics of the Foot

Jerome Rifkin has always been an active outdoorsman, but when he broke his hip in a mountain biking accident several years ago, it was the simple act of walking that he learned to appreciate.

While recovering from his injury, Rifkin developed some insight into the mechanics of walking that he hadn't learned as a biomechanics undergraduate at the University of Florida. Wanting to turn those insights into something useful, Rifkin turned his focus toward creating a prosthetic foot that mimics the dynamic behavior and flexibility of the real thing.

"I had an intuition about how my feet worked, and I wanted to recreate that," Rifkin says. He spent the next five years doing independent research and working on prosthesis designs in his free time.

Now 33, Rifkin is a graduate student in mechanical engineering at CU-Boulder, where he continues his research and development of a prosthesis with funding from a National Institutes of Health grant through the Small Business Innovation Research Program.

One of his first prototypes won the Best New Industrial Product award at the 2005 Colorado Inventors' Showcase.

"I think I won the competition because I got people to try on my foot and see how it felt," explains Rifkin, who attaches his prosthesis to an air cast, allowing non-amputees to take it for a stroll. Those who have tried it say that Rifkin's prototype feels significantly more natural and stable than the best product on the market today, a carbon-fiber foot originally designed for running.

Called the "Liberator," Rifkin's design was developed specifically for walking and is flexible enough to hike up a mountain trail or use in an urban environment. The design incorporates two joints—the subtalar, or mid-foot joint, and the metatarsophaelangeal joint—providing flexibility at the toe. Each has six degrees of freedom, allowing the prosthesis to make firm contact on uneven ground, while steel cables control the range of motion and help the foot spring forward with each step.

"I was quite surprised how natural this foot felt when I walked on it," says Professor Larry Carlson, Rifkin's advisor in mechanical engineering, who has been reviewing Rifkin's designs, asking hard questions, and suggesting design alternatives.

Over the past year, Rifkin has been testing a series of functional prototypes made of machined aluminum and rapid prototyped plastic on a force treadmill in the Department of Integrated Physiology. The special treadmill developed by Associate Professor Rodger Kram allows the ground reaction forces to be measured and graphed on a computer to determine the prosthetic foot's performance over a series of steps. Video is also captured to compare its range of motion to that of a natural foot.

A "step-wise" evolution of Rifkin's prosthetic design.

Initial tests look positive, and Rifkin is now developing a manufacturable prototype made of magnesium, which will be both strong and lightweight. He hopes to test the device with amputees later this spring. Meanwhile, he has been developing a business plan for his new company, Tensegrity Prosthetics, and looking for investors.

"An estimated 175,000 people use a prosthetic foot today, and, unfortunately, the numbers are expected to increase," he says. "I want to do all I can to help these people."

For more information visit: http://tensegrityprosthetics.com
Department web site: http://me-www.colorado.edu