SpringActive Odyssey Ruggedized Ankle Prosthesis

2018 OPORP Investigator Vignette

Investigator: Jeff Ward, PhD; SpringActive, Inc.

We were tasked with taking our powered ankle prosthesis technology, making it ruggedized, waterproof, building a powered ankle that is capable of supporting running gait, as well as walking gait, and navigating those transitions.

The idea behind the ankle is that you have a tuned spring in the system so that, when you’re taking that step and you’re rotating over your ankle joint, you’re going from here to here, you’re storing energy into that spring. Over the duration of that motion, there’s a motor in the system that is also putting additional energy into that spring, so that all of the energy and power is there when you step forward, when your toe comes off the ground.

The device is controlled by the motion of the tibia, so we’re looking at the dynamics of the tibia in space to know where we need to set the force in the spring, essentially. It’s all based on the motion of the tibia.

The nice thing about that is you’re not required to know anything about ground stiffness. So if you’re going to walk over sand or a soft surface, typically, the ground sort of moves away from you; your tibia falls forward a little faster; and the device pushes even more. So the surface stiffness doesn’t impact your gait as much as it would if you were relying on force sensing from the floor.

Also, you can transition activities in real time; 500 times a second, you’re looking at what is the tibia doing. So you can transition from a walking step to a running step within the step. And this is happening continuously, and we’re not looking for gait activities within the step; like we’re not trying to detect heel strike; we’re not trying to look for foot flat or toe off. We’re just looking at what the tibia is doing.

And we even found that it can walk backwards fairly well. So it sort of reaches back for the step—as you go.

So we’re experimenting with lots of different other types of control activities as well. But where we’re at right now is pretty smooth transitions as the speed increases and from walking to running.

The system is completely waterproof. We’ve done lots of testing with just having it suspended in a tank, actuating back and forth, as well as we have a test machine at the office that will load the device. We’re submerging it in water and then loading it for several thousands of cycles. And we haven't had any electrical issues.

It works on declines the same way. It’s looking at the motion of the tibia. So, right now, one of the things we are working on is to get the push-off reduced when you’re going down an incline because, when you’re going down an incline, the motion of the tibia is much greater. So it wants to push a little bit harder. We’ve been working on activity recognition algorithms, so we know, when we are going down an incline, we can slow that down. Right now it’s fine. I mean, like you kind of walk slower; it’s manageable. The same with stairs; it’s manageable, but we want to optimize it.

Sure, so the weight of this unit is 2.4 kilograms, so it’s about 0.2 kilograms heavier than the current Empower. And it’s actually on par with the last prototype. The last prototype was not submersible. So we basically added the ability to be dunked in water, support the running gait, and we didn’t reduce the weight with this iteration. I think there are opportunities now to get another couple hundred grams out of the system. We’ve been working on advanced FEA techniques, and now we’re able to look at the entire—instead of doing a component-by-component FEA analysis based on the anticipated loads, we’re able to look at the entire system and assembly and the software and really get some accurate predictions on those loads. So that is going to help us. That high-performance design is going to help us trim down our safety factors even more and get additional weight out of the system.

The battery life is 4,000 steps, so 4,000 continuous steps. He’s wearing the battery on his hip right now. The future iterations of this would integrate the battery onto the device, and the battery would get much smaller as well because they’ve just recently increased the energy density of some of these lithium ion cells. And so that’s going to take the battery—that’s going to shrink the battery size down a lot.

It’s a tension spring, so we have mechanical limits inside there that will—because—so you can calculate the life of a spring fairly accurately. And so now we’ve basically put mechanical limits to know—so we always know what the life of the spring is, even when you’re running on it.

This particular unit was designed to support a 100-kilogram user because the demands of running are tremendous on the system. When he’s running, there’s 800, 900 pounds of force in the spring in the foot, so it’s a tremendous amount of load on all of those internal components. For walking, for example, you’re looking at, on average, 3.2 watts per kilogram peak power at the ankle joint. For running, that goes up to 10. So it’s—the loads and the demands on the system are much higher, so we’ve designed this for a 100-kilogram user.

Commercialization-wise, SpringActive is, has formed a relationship with Ossur to commercialize a civilian version of this technology that is primarily designed to support walking gait. And a lot of the technology that has been developed in here, a lot of the things that we’re learning, are going into that civilian commercialization effort.