MIT scientists have developed a low-cost prosthetic foot which they say can be customised according to the size and weight of an individual, and is affordable for rural populations in India and other developing countries.
Prosthetic limb technology has advanced by leaps and bounds, giving amputees a range of bionic options, including artificial knees controlled by microchips, sensor-laden feet driven by artificial intelligence, and robotic hands that a user can manipulate with her mind.
However, such high-tech designs can cost tens of thousands of dollars, making them unattainable for many amputees, particularly in developing countries.
Researchers from Massachusetts Institute of Technology (MIT) in the US developed a passive prosthetic foot that they can tailor to an individual.
Given a user’s body weight and size, the researchers can tune the shape and stiffness of the prosthetic foot, such that the user’s walk is similar to an able-bodied gait.
They estimate that the foot, if manufactured on a wide scale, could cost an order of magnitude less than existing products.
“Walking is something so core to us as humans, and for this segment of the population who have a lower-limb amputation, there’s just no theory for us to say, ‘here’s exactly how we should design the stiffness and geometry of a foot for you, in order for you to walk as you desire’,” said Amos Winter, an associate professor at MIT.
In 2012, soon after Winter joined the MIT faculty, he was approached by Jaipur Foot, a manufacturer of artificial limbs based in Jaipur, India.
The organisation manufactures a passive prosthetic foot, geared toward amputees in developing countries, and donates more than 28,000 models each year to users in India and elsewhere.
“They’ve been making this foot for over 40 years, and it’s rugged, so farmers can use it barefoot outdoors, and it’s relatively life-like, so if people go in a mosque and want to pray barefoot, they’re likely to not be stigmatised,” Winter said.
“But it’s quite heavy, and the internal structure is made all by hand, which creates a big variation in product quality,” Winter said.
The organisation asked Winter whether he could design a better, lighter foot that could be mass-produced at low cost.
The team first looked for a way to quantitatively relate a prosthesis’ mechanical characteristics to a user’s walking performance – a fundamental relationship that had never before been fully codified.
Instead of designing a prosthetic foot to replicate the motions of an able-bodied foot, researchers looked to design a prosthetic foot that would produce lower-leg motions similar to those of an able-bodied person’s lower leg as they walk.
“We can potentially drastically change the foot, so long as we make the the lower leg do what we want it to do, in terms of kinematics and loading, because that’s what a user perceives,” said Winter.
Researchers developed a mathematical model of a simple, passive prosthetic foot, which describes the stiffness, possible motion, and shape of the foot.
They plugged into the model the ground reaction forces from the dataset, which they could sum up to predict how a user’s lower leg would translate through a single step.
With their model, they then tuned the stiffness and geometry of the simulated prosthetic foot to produce a lower-leg trajectory that was close to the able-bodied swing – a measure they consider to be a minimal “lower leg trajectory error.”
To pinpoint an ideal foot shape, the group ran a “genetic algorithm” – a common technique used to weed out unfavorable options, in search of the most optimal designs.