Spotlight on Career Development Awardees
Building a better artificial limb
VARQU provides a look into Dr. Patrick Aubin's lab as he and his group seeks to build the next generation of lower-limb prostheses.
Dr. Patrick Aubin is a research investigator with the VA Puget Sound Health Care System in the Rehabilitation Research & Development (RR&D) Center for Limb Loss and Mobility (CLIMB). He is also an affiliate assistant professor in the department of mechanical engineering at the University of Washington.
The types of artificial limbs available to Veterans vary greatly. It all depends on the unique needs of the amputee. "Our veterans are prescribed legs that help them attain their goals and fit their lifestyles which vary from individual to individual. For example, most people just need a foot that helps them do their daily activities, like grocery shopping [or] walking around the community," says Aubin.
"The term articulation is just a fancy word for a joint, so the knee and ankle are articulations," explains Aubin. The new prosthesis he is developing replaces the function of the muscles in the missing limb via a spring and clutch mechanism, which the user activates by walking.
"In the biarticular prosthesis you have a thigh cuff that allows the spring to be attached above the knee," says Aubin. "The clutch engages at midstance, when you bend your knee so that in late stance, the spring gets stretched and stores energy. In late stance the energy in the spring is returned to the knee and ankle." Here, Aubin and research engineer Chris Richburg adjust the clutch mechanism.
Aubin and Richburg adjust the prosthesis while research participant Lori A. Waddell looks on.
Aubin: "A motorized prosthetic device would try to replicate how your ankle functions... The device that we are trying to develop is a quasi-passive prosthetic device which consumes only a tiny amount of power from the onboard computer. The key elements of our device are a clutch and a spring. And that spring gets loaded through the natural kinematics of walking."
Waddell puts the new artificial limb through its paces on a treadmill in Aubin's lab.
"One of the goals of this project is to use computer simulation during the design phase to find out how someone would walk on the new device," says Aubin. In this picture, Waddell is wearing the new prosthesis while research team members Aubin and Anthony Anderson from the department of mechanical engineering at the University of Washington look on.
Dr. Patrick Aubin is a research investigator with the VA Puget Sound Health Care System in the Rehabilitation Research & Development (RR&D) Center for Limb Loss and Mobility (CLIMB). He is also an affiliate assistant professor in the department of mechanical engineering at the University of Washington. Dr. Aubin received his Ph.D. in electrical engineering, with a focus on robotics and controls, at the University of Washington. After completing his Ph.D., he received Fulbright and Whittaker International grants to carry out a research project in Lithuania for two years. Returning to the U.S. in 2013, he completed a postdoctoral fellowship at the Wyss Institute for Biologically Inspired Engineering, Harvard University, working to develop "soft" exoskeletons, or exosuits, to help soldiers in the field walk with heavy loads.
Dr. Aubin, what type of work is being done at CLIMB?
Dr. Aubin: Our center's mission is to preserve and enhance mobility in Veterans and others with lower-limb musculoskeletal impairment or limb loss. Basically we focus on helping people walk better. There's a biomechanics thrust, which is addressing the problems that come with arthritis and diabetes, preventing it and having improved treatments for it. And then there is the prosthetic and rehabilitation thrust, which is what my Career Development Award (CDA) is under. We are developing state-of-the-art prosthetic legs to help people walk better. We have about eight principle investigators, and each of those people has a research team that works with them. I am one of those principle investigators.
What area of research is your Career Development Award in? What will you study?
We are trying to design a better prosthetic leg for people who have had lower-limb loss. It really just boils down to helping them walk better and giving them a solution that is better than current commercial devices.
There are many different types of prostheses, for instance those that a professional runner would wear. What would be the typical prosthesis that a Veteran who lost a lower limb would wear?
Our Veterans are prescribed legs that help them attain their goals and fit their lifestyles which vary from individual to individual. For example, most people just need a foot that helps them do their daily activities, like grocery shopping, walking around the community, etc. Those feet typically consist of a socket, which is a hard shell that goes over the residual limb. That's attached to what we call a pylon, which is typically an aluminum tube. And that connects to the actual prosthetic foot, which is typically carbon fiber—we call it a passive-elastic foot, meaning it can bend and flex, but it is not motorized in any way. Covering the foot is what we call the cosmesis, which is a flesh colored piece of plastic that looks like a foot. It is basically a shell. Those are the main components that make up a prosthetic lower limb.
Dr. Patrick Aubin (right) working on a new lower limb prosthesis with team member Chris Richburg, research engineer. (Photo by: Chris Pacheco, VA Puget Sound Health Care System)
What are the limitations of using a prosthesis like you describe?
If you think about an intact limb, there is a muscle that's called gastrocnemius (GAS), and it actually couples what the ankle is doing and the knee is doing. It connects from the calcaneus bone, which is your heel below your ankle to your femur (thigh bone), which is above the knee. People who have an amputation lose the function of their GAS. And typical commercial prostheses, like the ones I just described to you—basically a socket with a pylon and a passive elastic foot—also do not have GAS function. So current commercial prosthetic feet are missing the coupling between the ankle and knee joint.
If you pick your foot up and flex your foot or tap your foot, you are using the GAS muscle to flex your foot down. And you use it when you walk. The GAS is unique; it's what we call biarticular. That's a key word in the title of my project meaning to span across two articulations. The term articulation is just a fancy word for a joint, so the knee and ankle are articulations. The GAS muscle helps you walk by coupling the knee and ankle joint and assisting with the push off phase of gait. Biologically we have this built-in coupling between the ankle and the knee, and that helps us walk very efficiently. That unique coupling is one of the factors that help us do all the amazing things that we can do in a mobility sense.
So a Veteran who is using a standard prosthesis would not have this coupling. What are some of the difficulties he or she might have when walking?
The standard prostheses on the market that one might wear, they are pretty good at allowing people to walk. The passive-elastic foot is pretty good; it recovers most of the walking function that someone would need. But it doesn't do everything. One of the limitations that current amputees face, for example, is that they walk at a slower preferred walking speed. When they walk they expend more energy to walk. We call that the metabolic cost associated with walking. It is more difficult, more taxing for them to walk. It is more of a workout to do the same walking that you or I would do. They also have what we call a gait asymmetry, which just means that the limb that has been amputated is not mirroring in a variety of ways with what the intact limb is doing. One type of asymmetry would be ankle power.
Dr. Patrick Aubin and research participant Lori A. Waddell. (Photo by Chris Pacheco, VA Puget Sound Health Care System)
What would be the benefits of wearing a biarticulated prosthesis?
One of the first aims of the project will be to understand what happens if we can restore the function of the GAS muscle. How will an amputee walk with that restored function, versus without it? That's the first step, adding in that muscular function with prosthetic componentry and asking, "How does that change amputees' walking?" We are hoping to find that it helps them walk with less energy—so the metabolic cost will come down. It will be easier to walk. As well as help reduce some of those asymmetries [in gait] that I mentioned. Specifically, at the ankle, it might help with push off power. We divide walking into several stages; at the very beginning is initial contact—that is where your foot hits the ground. At the end there is swing phase where your leg is off the ground. This prosthetic foot might help initiate the leg into swing phase, which is propelling the leg forward when an amputee takes it off the ground.
What I've described to you is the passive-elastic prosthesis. The opposite of that is a motorized prosthetic device which has a motor and a battery and would try to replicate how your ankle functions and pushes off the ground to help you walk. The device that we are trying to develop is between those two. It is a quasi-passive prosthetic device which consumes only a tiny amount of power for the onboard computer. A powered prosthesis has a motor and a battery delivering power to the foot to help it walk forward. The key elements of our device are a clutch and a spring. And that spring gets loaded through the natural kinematics of walking. As you walk, just as your GAS muscle would normally get stretched in late stance, we have a spring that's replicating that function and is getting stretched. And then the energy that is stored in the spring gets returned to the user, to the ankle and to the knee. We are hoping to get some of the benefits of a powered device without powering it.
How is your prototype made? What are the components?
In its current incarnation, which will likely change in the next five years, you have a thigh cuff that allows the spring to be attached above the knee. And then there is a clutch that's in series with that spring. The clutch engages at midstance, when you bend your knee so that in late stance, the spring gets stretched and stores energy. In late stance the energy in the spring is returned to the knee and ankle. During swing phase the biarticular element is passive to allow the user to get ready for the next step. The foot we are using is currently just a standard passive-elastic foot. It has a single pin joint at the ankle which allows for some dorsiflexion and plantar flexion of the foot. The key element is the spring and clutch which replicates how the GAS muscle functions.
How does the clutch work in this prosthesis?
A clutch is kind of like the brakes on your bike, so when the clutch is disengaged, it is like the brakes are turned off and the wheels can spin freely. In the prosthetic foot, when the clutch is disengaged the knee is not coupled to the ankle. The spring can move freely—it's in what we call a transparent mode—so basically no energy is stored when the user moves around.
When the clutch is engaged, it is like the brakes locking onto the wheel, it stops the wheel from moving. For the biarticular foot it is simulating when the GAS muscle is active, the muscle locks up so that knee extension and ankle dorsiflexion stretch the GAS—so in the biarticular foot, the clutch grabs on to the spring essentially. And then as you extend your knee it stretches that spring, much in the same way you would stretch your GAS muscle when you walk.
When you walk you are unconsciously doing this. Your muscles engage, they flex, they stretch, and they store energy like a spring. And then they recoil and that energy gets returned back to the joints. The clutch is basically replicating turning the muscle on and off—in the device there's a computer that controls the clutch. The computer, which is about the size of a postage stamp, replicates the timing of the GAS muscle by measuring the gait cycle with sensors.
The key thing for our foot is it doesn't have a heavy battery or a heavy motor. The spring is getting stretched through the natural walking pattern. That's beneficial, because motors and batteries are expensive and they are also heavy and complicated. So we can eliminate those components from the device.
How will you test the new prosthesis?
First we build it, which is the phase we are in now. An interesting point is how we use simulation to inform the design of the device. If you think about how an airplane or car is built right now, engineers will first build a computer simulation of the system to test how it works. Say you were going to build an airplane wing. You first design and build the wing in simulation and test it in the computer model to see how it will perform, and then adjust it in the computer model until you are happy with the performance. Then you would build an actual prototype once the design has been optimized.
When you think about how we build devices that interact with humans, that step is not available. It's very difficult to use simulation or computer modeling to help design a prosthetic device. We are very much stuck in a time (until now) where we actually build a prosthetic limb/foot and have someone walk on it and see how it works. And then build it again, until you get it right. You lose out on the benefits of computer simulated design—which means you can alter the design any number of times and see how it changes the performance or the function of the device, before you build an actual prototype.
One of the goals of this project is to use computer simulation during the design phase to find out how someone would walk on the new device. We have a computer model of someone walking, so we are hoping to build the device in the computer and then use these simulations to determine if it would be helpful to a lower limb amputee.
Where do you see your research going in the next two to five years?
The CDA project goes for five years, and we are just getting started. For that project, success would really be developing a prosthetic foot that is commercially viable, that shows that it is beneficial over the current standard of care. Within the broader scope of my research program, I'm very interested in how we can augment human performance (walking performance in particular or mobility performance) through the use of robotic and artificial intelligence aids—basically anywhere there is a machine and a human user and they are working together.
I'm working on a smart walking cane right now for people who have knee arthritis. The idea is to take simple existing devices, like a walking cane or knee brace, and make them smarter and work better by exploiting recent advances in robotics, artificial intelligence, and mechatronics. So how do we make walking canes more effective and help people walk better? How do we make the human and the machine work together in a way that really benefits the users and makes it easier for them to walk? The overarching goal here is to increase people's mobility so they can get out and do the things they like to do, simple things like playing with their grandchildren.