Engineers from the University of Michigan, the developers of an artificial foot that recycles energy otherwise wasted in between steps, say that it could make it easier for amputees to walk.
“For amputees, what they experience when they are trying to walk normally is what I would experience if I were carrying an extra 30 lb,” says Art Kuo, professor in the University of Michigan departments of Biomedical Engineering and Mechanical Engineering.
Compared with conventional prosthetic feet, the new prototype device – created by Kuo and Steve Collins, who was then a University of Michigan graduate student and is now an associate research fellow at Delft University of Technology in the Netherlands – significantly cuts the energy spent per step.
The human walking gait naturally wastes energy as each foot collides with the ground in between steps. A typical prosthesis does not 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.
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 micro-controller tells the foot to return the energy to the system at precisely the right time.
According to Kuo, based on metabolic rate measurements, the test subjects spent 14% more energy walking in energy-recycling artificial foot than they did walking naturally – a significant decrease from the 23% more energy they used in the conventional prosthetic foot. “We know there is an energy penalty in using an artificial foot. We are almost cutting that penalty in half,” Kuo explains. “All prosthetic feet store and return energy, but they do not give you a choice about when and how. They just return it whenever they want. This is the first device to release the energy in the right way to supplement push-off, and to do so without an external power source.”
Other devices that boost push-off power use motors and require large batteries, however, because the energy-recycling foot takes advantage of power that would otherwise be lost, it uses less than 1W of electricity through a small, portable battery.
“Individuals with lower limb amputations, such as veterans of the conflicts in Iraq and Afghanistan or patients suffering from diabetes, often find walking a difficult task. Our new design may restore function and reduce effort for these users,” Collins says. “With further progress, robotic limbs may yet beat their biological forerunners.”
The foot is currently being tested on amputees at the Seattle Veterans Affairs Medical Center, with commercial devices based on the technology currently under development by an Ann Arbor, MI, company. Research was funded by the National Institutes of Health and the Department of Veterans Affairs.
A paper detailing the foot, Recycling Energy to Restore Impaired Ankle Function during Human Walking, was published in the February 17, 2010, edition of in the journal PLoS ONE. To read the full paper, click here.
The University of Michigan
College of Engineering
Ann Arbor, MI
engin.umich.edu