An artificial foot that recycles energy otherwise wasted between steps could make it easier for amputees to walk, its developers say.
Compared with conventional prosthetic feet, the new prototype device significantly cuts the energy spent per step, according to a news release.
A paper about the device is published in the Feb. 17 edition of in the journal PLoS ONE. The foot was created by Art Kuo, professor in the University of Michigan departments of biomedical engineering and mechanical engineering, and Steve Collins, who was then a University of Michigan graduate student. Now Collins is an associate research fellow at Delft University of Technology in the Netherlands.
The human walking gait naturally wastes energy as each foot collides with the ground in between steps. A typical prosthesis doesn’t reproduce the force a living ankle exerts to push off of the ground. As a result, test subjects spent 23% more energy walking with a conventional prosthetic foot, compared with walking naturally.
“We realized that humans normally must waste some energy during walking, and that is where some energy might be captured for another use, to benefit the patient,” Kuo told O&P Business News. “For amputees, what they experience when they’re trying to walk normally is what I would experience if I were carrying an extra 30 pounds,” Kuo said. “We have spent several years re-examining how humans walk, and realized that much of the scientific attention was on the ways that the body puts energy into gait, and not why gait uses energy. We found that the way humans walk, where the stance leg is kept relatively straight, saves energy for much of the stride but also causes losses when the next foot hits the ground.”
To test how stepping with their device compared with normal walking, the engineers conducted their experiments with non-amputees wearing a rigid boot and prosthetic simulator.
In their energy-recycling foot, the engineers put the wasted walking energy to work enhancing the power of ankle push-off. The foot naturally captures the dissipated energy. A microcontroller tells the foot to return the energy to the system at precisely the right time.
Kuo explained how the foot controls the energy.
“We use a spring with an automatic latching mechanism, so that energy is captured without need for intervention. The ‘intelligence’ is in the latch, so there are no decisions to be made, by either the user or the computer controlling the device.” Kuo said. “This is partially due to the computer, and partially due to the user. The latching mechanism is not able to release the energy until the full weight of the body transfers onto the front of the foot. Then the computer determines when the latch can be moved, and then energy is released as the user starts to transfer weight off the foot.”
Based on metabolic rate measurements, the test subjects spent 14% more energy walking in energy-recycling artificial foot than they did walking naturally. That represents a decrease from the 23% more energy they used in the conventional prosthetic foot, Kuo explained.
They are now testing the foot on amputees at the Seattle Veterans Affairs Medical Center.
Kuo predicts his foot to be in the commercial market after several testing phases.
“We need to make many refinements and test them on patients. There are many hurdles with safety and reliability, and practical issues such as cost and appearance. It will take several years of development to reach market,” he said.