A smart artificial foot that uses springs and electronics to recycle energy otherwise lost during walking could help people with prosthetic legs walk as efficiently as the able-bodied.
Without the ankle’s springiness and ability to actively push off from the ground, amputees can expend up to twice the energy of able-bodied people when walking.
Biomechanics researcher Steven Collins at Delft University of Technology in the Netherlands, and Arthur Kuo of the University of Michigan in Ann Arbor drew on their experience of making bipedal robots and studies of the way a human foot loses and uses energy to come up with their design.
When the foot hits the ground during walking, it deforms as it takes our weight. The heat created by the deformation is soaked up by the foot’s 200 or so muscles.
“You have to replace that lost energy when your ankle then pushes off,” says Collins. “And that costs you even more energy.” Artificial feet are even more wasteful of energy.
Today’s artificial feet comprise long, flat carbon-fibre leaf springs hidden in a foot-like “cosmetic cell”. These springs are smaller versions of the powerful running springs used by athletes like Oscar Pistorius.
But because the spring gives slightly when the user pushes down on the ground, it acts as a considerable energy leech. “You would think it would help, but it doesn’t quite restore function fully,” says Collins.
He and Kuo have addressed that by building a foot that’s not only able to soak up energy on touchdown, but also has the electronic smarts to release it at just the right moment to help with a springy, more powerful push-off (see video, above).
Their system has a rear-foot section hinged to a forefoot (see diagram, right). As the rear foot contacts the ground, energy is stored in a large compressible coiled spring held in place by an electronic clutch. As the forefoot bends when the user starts to push-off for the next step, a sensor sends a signal that releases the clutch, unleashing the spring’s energy to help out.
After initial safety tests on 11 able-bodied subjects wearing the prosthetic foot on a special boot (see video), the system is looking promising. “We’re getting a big increase in push off – double that of a conventional prosthetic foot,” says Collins.
It will soon move on to tests on amputees. The technology is already creating something of a buzz.
“This foot stores and uses some of the energy expended during gait that would otherwise be unrecoverable. There is no commercially-available foot like this,” says Glenn Klute, a prosthetics engineer at the University of Washington in Seattle.
Klute will soon be supervising tests of the foot on 20 amputee volunteers at the Seattle Veterans Hospital. “It has strong potential to help lower-limb amputees walk farther and with less effort than the foot they’re currently wearing.”
Journal reference: PLoS ONE (DOI: 10.1371/journal.pone.0009307)