Stanford Engineers Develop More Stable and Efficient Prostheses for Various Terrains

Taking on a cobblestone street or a hiking trail with a prosthetic leg is a precarious plan – although possible, however, even in comparatively easy terrain, people who use prostheses to walk are more probable to fall than others. Currently, mechanical engineers at Stanford University have built a more stable prosthetic leg – and a better technique of designing them – that could make difficult terrain more manageable for persons who have lost a lower leg.

Mechanical engineers Steven Collins and Vincent Chiu discuss a new approach to designing prosthetic limbs. (Credit: Stanford University)

The basis of the new design is a type of tripod foot that responds to coarse terrain by actively transferring pressure between three various contact points. As essential as the foot is a tool, the team developed for rapidly emulating and refining their prototypes.

“Prosthetic emulators allow us to try lots of different designs without the overhead of new hardware,” said Steven Collins, an associate professor of mechanical engineering and a member of Stanford Bio-X. “Basically, we can try any kind of crazy design ideas we might have and see how people respond to them,” he said, without having to construct each idea individually, an effort that would require months or years for each different design.

Graduate student Vincent Chiu, postdoctoral researcher Alexandra Voloshina and Collins describe the creation and primary tests of their prosthetic emulator in a paper published in IEEE Transactions on Biomedical Engineering.

Adjusting to the terrain

About half a million people in the United States have had their lower limb amputated, with effects that go further than just making it tougher to move around. People with a leg amputation are five times more probable to fall during a year, which may be a reason as to why they are also less socially engaged. A better prosthetic limb could increase not only mobility but also the general quality of life.

One area of specific interest is designing prosthetic limbs that can better handle rough terrain. Chiu, Voloshina, and Collins thought that the solution could be a tripod with a rear-facing heel and two forward-facing toes. Equipped with position sensors and motors, the foot could regulate its orientation to respond to variable terrain, similar to someone with an intact foot could move their toes and flex their ankles to balance while walking over rough terrain.

But the engineers were aware that improving the design would be tough – even with simple designs, a conventional method can take several years. “First you have to come up with an idea and then you prototype it and then you make a nice machined version,” Chiu said. “It could take several years, and most of the time you find out that it doesn’t actually work.”

Accelerating design

Chiu and his team thought they could quicken the process by building an emulator, which flips the design process on its head. Instead of developing a prosthetic limb someone could test in the real world, the team instead assembled a basic tripod foot, and then connected it to strong off-board motors and computer systems that regulate how the foot responds as a user walks over all sorts of terrain.

In doing so, the team can place their design emphasis on how the prosthesis should work – how hard one toe must push off while moving, how bouncy the heel should be and so on – without having to be concerned about how to make the device lightweight and economical simultaneously.

Thus far, the team has exposed results from work with one participant, a 60-year-old man whose leg was amputated below the knee because of diabetes, and the early results are encouraging – making the team optimistic they can utilize those results and convert them into more capable prosthetics.

One of the things we’re excited to do is translate what we find in the lab into lightweight and low power and therefore inexpensive devices that can be tested outside the lab. And if that goes well, we’d like to help make this a product that people can use in everyday life.

Steven Collins, Associate Professor of Mechanical Engineering and Member of Stanford Bio-X, Stanford University

The National Science Foundation funded the research.