Prosthetic limb technology with limbs controlled by microchips or limbs with sensors and artificial intelligence are costly and are not readily accessible to many amputees, especially in developing countries.
The MIT engineers created a low-cost, passive prosthetic foot that uses a person’s body weight and size to fine tune its shape and stiffness. Because of that, a user’s walk is similar to an able-bodied gait, according to the researchers. The researchers also suggested that the foot could cost significantly less than existing products if it is manufactured on a wide scale.
The prostheses are designed on a framework developed by the MIT researchers that gives a way to predict a user’s biomechanical performance and walking behavior based on the mechanical design of a prosthetic foot.
“[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,” Amos Winter, associate professor of mechanical engineering at MIT, said in a press release. “Now we can do that. And that’s super powerful.”
Winter was approached by Jaipur Foot in 2012 to see if we could design a better, lighter foot that could be produced on a large scale at a low cost. India-based Jaipur Foot makes a passive prosthetic foot that is designed for amputees in developing countries. It donates more than 28,000 models every year to users in India and other countries.
“At that point, we started asking ourselves, ‘how should we design this foot as engineers? How should we predict the performance, given the foot’s stiffness and mechanical design and geometry? How should we tune all that to get a person to walk the way we want them to walk?'” said Winter.
The MIT research team looked for a way to quantitatively relate a prosthesis’s mechanical characteristics to walking performance. A lot of traditional prosthetics replicate the movements of able-bodied feet and ankles, but Winter and his team approached it differently by realizing that amputees who have a below-knee amputation can’t feel what a prosthetic foot is supposed to do.
“One of the critical insights we had was that, to a user, the foot is just kind of like a black box – it’s not connected to their nervous system, and they’re not interacting with the foot intimately,” said Winter.
The team designed a prosthetic foot out of machined nylon that could produce lower-log motions that were similar to how an able-bodied person’s lower leg moves when walking.
“This really opened up the design space for us,” Winter said. “We can potentially drastically change the foot, so long as we make the lower leg do what we want it to do, in terms of kinematics and loading, because that’s what a user perceives.”
They also looked for a way to relate the mechanics of the foot to the mechanics of the lower leg while the foot is in contact with the ground. To figure this out, the researchers used an existing dataset that had measurements of steps taken by an able-bodied walker with a set size and weight. The data showed that with each step, there was a recorded ground reaction force and the center of pressure changed as the foot rocked from heel to toe. The position and trajectory of the lower leg also changed.
Winter and the researchers created a mathematical model of a simple, passive prosthetic foot that outlines the stiffness, possible motion and shape of the foot. They used the ground reaction forces from the data set and plugged that into the model to predict how a user’s lower leg would translate in a single step.
The model could then be tuned to adjust the stiffness and geometry of the simulated prosthetic foot to create a lower-leg trajectory that could be close to the able-bodied swing.
“Ideally, we would tune the stiffness and geometry of the foot perfectly so we exactly replicate the motion of the lower leg,” Winter said. “Overall, we saw that we can get pretty darn close to able-bodied motion and loading, with a passive structure.”
To determine an ideal foot shape, the researchers used a “genetic algorithm” that eliminates unfavorable options to find the most optimal designs.
“Just like a population of animals, we made a population of feet, all with different variable to make different curve shapes,” Winter said. “We loaded them into simulation and calculated their lower leg trajectory error. The ones that had a high error, we killed off.”
The researchers used the lower errors to mix and match with other shapes to grow the population into an ideal shape. They also used a wide Bezier curve to describe the shape of the foot using a few select variables that are easy to vary in the genetic algorithm. As a result, the foot shape resembled the side-view of a toboggan.
Winter suggested that fine tuning the stiffness and shape of the Bezier curve to match a person’s body weight and size could make a prosthetic foot that creates leg motions that are similar to able-bodied walking.
“What’s cool is, this behaves nothing like an able-bodied foot – there’s no ankle or metatarsal joint – it’s just one big structure, and all we care about is how the lower leg is moving through space,” Winter said. “Most of the testing was done indoors, but one guy ran outside, he liked it so much. It puts a spring in your step.”
The research team has partnered with Italian rubber outsole manufacturer Vibram to design life-like covering for the team’s prosthesis. They plan to test the prosthetics and coverings on volunteers in India later this year. Winter suggests that the simple design of the prosthetic foot could be a more affordable and durable option for populations like soldiers who want to return to active duty.
“A common passive foot in the U.S. market will cost $1,000 to $10,000, made out of carbon fiber. Imagine you go to your prosthetist, they take a few measurements, they send them back to us, and we send back to you a custom-designed nylon foot for a few hundred bucks,” said Winter. “This model is potentially game-changing for the industry, because we can fully quantify the foot and tune it for individuals, and use cheaper materials.”
The research was published in the Journal of Mechanical Design and was funded in part by the MIT Tata Center for Technology and Design.