Human 2.0: Exoskeletons and Orthoses with Hugh Herr

In this episode, Audrow Nash interviews Hugh Herr, Director of the Biomechatronics Group at MIT. Herr talks about the accident that led to the amputation of both of his legs below the knee and how this shaped his rock climbing and academic career. Herr also discusses orthoses and exoskeletons developed by his research group, as well as the future of bionic technology.

Transcript below.

Among topics discussed is the lower-body exoskeleton below. This exoskeleton is the first lower-body exoskeleton to decreases the user’s energy expenditure when walking (measured by oxygen consumption). It works by assisting the calf muscle and using the body’s joints, rather than heavy mechanical joints. Hugh Herr says, “Subjects noticed that their legs felt heavier and awkward when they took the exoskeleton off.”

Below is a TED talk Hugh Herr gave in 2014. In this talk, Herr describes disability as a failure of technology, he discusses the design of his own prostheses and his lab’s research, and there is a dance performance by Christian Lightner and Adrianne Haslet-Davis, who lost her lower left leg in the Boston Marathon bombings.

Hugh Herr

His research program seeks to advance technologies that promise to accelerate the merging of body and machine, including device architectures that resemble the body’s musculoskeletal design, actuator technologies that behave like muscle, and control methodologies that exploit principles of biological movement. His methods encompass a diverse set of scientific and technological disciplines, from the science of biomechanics and biological movement control to the design of biomedical devices for the treatment of human physical disability.

His research accomplishments in science and technology have already made a significant impact on physically challenged people. The Transfemoral Quasipassive Knee Prosthesis has been commercialized by Össur Inc., and is now benefiting amputees throughhttp://biomech.media.mit.edu/people-hugh-herr/out the world.  In 2006, he founded the company iWalk Inc. to commercialize the Powered Ankle-Foot Prosthesis and other bionic leg devices.  Professor Herr’s work impacts a number of academic communities. He has given numerous invited and plenary lectures at international conferences and colloquia, including the IVth World Congress of Biomechanics, the International Conference on Advanced Prosthetics, the National Assembly of Physical Medicine and Rehabilitation, World Economic Forum, Google Zeitgeist, Digital Life Design, and the TEDMED Conference. He is Associate Editor for the Journal of NeuroEngineering and Rehabilitation, and has served as a reviewer for the Journal of Experimental Biology, the International Journal of Robotics Research, IEEE Transactions on Biomedical Engineering, and the Proceedings of the Royal Society: Biological Sciences. He has been invited to participate in joint funding proposals from other universities and corporations, and has served on research review panels including the National Institute of Health, the National Institute on Disability and Rehabilitation, and the Department of Veterans Affairs. In 2007, He was presented with the 13th Annual Heinz Award for Technology, the Economy and Employment. His work has been featured by various national and international media, including Scientific American Frontiers, Technology Review, National Geographic, the History Channel, and CNN.

Links:

• Download mp3 (18.3 MB)
• Link to the Biomechatronics Group’s webpage
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The transcript is edited for clarity.

Hugh Herr:
My name is Hugh Herr and I’m a professor at MIT. I co-direct the Center for Extreme Bionics. I also am the founder of a company called BionX Medical Technologies.

Audrow Nash:
Can you tell me a bit about how you have lost your legs from the knee down?

Hugh Herr:
I was an avid mountain climber. I started mountain climbing when I was seven years old. When I was in my early teenage years I was considered a child prodigy in climbing. When I was seventeen I was out with my climbing partner at the time. Our goal was to climb Mt. Washington in the winter by ascending Huntington’s Ravine, which is about an eight hundred foot, very steep ice wall. We ascended very quickly the eight hundred foot wall, and then decided to continue towards the summit. The weather conditions got worse and worse. Even though we went just five minutes above the head-wall of Huntington Ravine, we were not able to retrace our tracks and we descended the mountain.

By the time we got to tree line it was clear that we had gone off course, but we were stuck at that point. We really didn’t have the rational choice of retracing our tracks and going towards the summit. At that point, wind speeds were so high that one could not even stand. We were forced to go down this ravine system, it’s called the Great Gulf Region. It’s the wilderness side of Mt. Washington. It was there that we spent several days in most extreme bushwhacking that one can imagine. The average depth of snow was to the waist. Sometimes it was to the chest. We were just simply trapped in a white maze.

We dug in when we were not able to move any more out of exhaustion by creating kind of these snow caves. During the day we would walk. We would probably make two miles of progress in a complete kind of marathon effort in the deep snow. We made it within a few miles of the roadway and then couldn’t walk anymore because of severe frostbite. We were later discovered, or found by a person out snowshoeing for the day, and we were plucked from the mountain via helicopter and then treated for severe frostbite and hypothermia.

Audrow Nash:
Then, so from there because of the severe frostbite you had both of your legs amputated from the knee down?

Hugh Herr:
That’s right. After a few months of effort by my medical team to “save” my biological limbs, they were amputated. We were on the mountain late January. My legs were amputated mid-March.

Audrow Nash:
Afterwards, you wanted to return to rock climbing. You were told otherwise, though.

Hugh Herr:
Yeah. I didn’t know what I would be able to do with my new body, and my father said, “If you want to climb, you should climb.” I really had no clear examples of what life would be using prostheses. My limbs are amputated. I went through about a month or two of healing, and then I was fitted with my first pair of artificial limbs. I was shocked at their lack of technological sophistication. They were actually made of plaster of Paris and it was recommended that I not walk without canes or crutches because the limbs might actually break. They were kind of these crazy trainer limbs. The first weekend I went home from the rehabilitation center they didn’t allow me to take my legs because they knew what I was capable of. The next weekend they were stupid enough to allow me to take my legs and I went climbing with my brother, Tony.

Audrow Nash:
How did you begin to adjust your prosthetic limbs for climbing and various purposes?

Hugh Herr:
I quickly abandoned the notion that the limbs needed to look human, and I quickly focused on function. I thought to myself what would be optimal designs for the vertical world of rock and ice climbing. What emerged was a series of different feet for rock and ice surfaces, so feet that could stand on small rock edges the width of a coin, feet that could wedge into rock fissures even where the human foot could not penetrate, feet that could penetrate ice walls. I made my legs exceedingly lightweight to increase my strength to weight ratio. Through design, I was able to etch out a few advantages. There were disadvantages, but I gained enough advantages that it was only twelve months after my limbs were amputated that I was climbing better with artificial limbs than I’d ever achieved with normal biological limbs before the accident.

Audrow Nash:
How did you transition into academia from this, and how did you know that that is the way you should pursue, or continue?

Hugh Herr:
To be honest, the men in my family, my father and brothers and grandfathers, were in the business of construction. Given how uncomfortable artificial limbs were at the time, I couldn’t imagine being on a construction site for the rest of my life. I also couldn’t imagine being on the construction site for the rest of my life. I decided to go to college, which I’d never plan to do. My intent was to be the best climber in the world before the accident. I went to college and I signed up for very basic math courses and computer science courses. I developed an extraordinary passion for the topics and couldn’t stop studying. I just absolutely loved the material. It’s interesting because science and a passion for science and math actually replaced somewhat my passion for climbing mountains.

Audrow Nash:
Then you began to apply what you were learning to your own prostheses with what you were learning? When did you start doing that?

Hugh Herr:
I did have the goal of continuing to design prostheses. The experience of designing new climbing legs and succeeding so wildly beyond everyone’s expectation, including my own, was very inspirational for me. I realized the power of technology to heal, to rehabilitate, even to extend human capability beyond innate capabilities. That was also a clear motivation in my acquisition of knowledge and going back to school. My first patent was obtained, issued very close to the time when I was graduating from undergraduate school in physics. It was on the mechanical interface between the residuum and the artificial limb. The idea was to use fluid bladders in certain configurations. I later proposed to my future MIT advisor that I would build this interface and have actuation pumps and pressure sensing and continually modulate the pressure field around the residuum.

Audrow Nash:
On the spot where the prosthetic limb interfaces with the residual limb is what you’re talking about?

Hugh Herr:
Yes.

Audrow Nash:
So you would use some sort of fluid to make a vacuum so that it would stay very well attached?

Hugh Herr:
Yes, a vacuum offering suspension, but also offering loading support.

Audrow Nash:
Loading support.

Hugh Herr:
A pressure field that would be very comfortable around the end of the limb, enabling a person to walk without discomfort.

Audrow Nash:
Can you tell me a bit about the prostheses that you’re wearing now?

Hugh Herr:
I’m wearing two bionic legs. Both legs have three microprocessors and twelve sensors. The microprocessors are brains, if you will. The devices control a muscle-like actuator that moves and stiffens and powers my synthetic ankles.

Audrow Nash:
What sensors are on board?

Hugh Herr:
The sensors measure position, speed, acceleration, force and temperature, as well as joint position and speed.

Audrow Nash:
What sensing are you doing, contact with the ground?

Hugh Herr:
No. The device has tendon-like series springs, and also a parallel elasticity. The torque sensing is of the torque that the series elastic actuator sees and the parallel spring sees.

Audrow Nash:
I see.

Hugh Herr:
Either one can produce net torque on a joint.

Audrow Nash:
Can you describe how they look a little bit?

Hugh Herr:
They look interesting. There’s two black batteries that sit on top, and there’s black anodized metal shields coming around the ankle. Then inside those shields are all the electronics and then the motor system. Then attached to that is a synthetic foot made of again, black carbon composite.

Audrow Nash:
What kind of grip on the bottom?

Hugh Herr:
I wear the carbon foot, and glued to that carbon foot is a typical rubber that you’d find on the bottom of a shoe.

Audrow Nash:
What kind of battery life?

Hugh Herr:
We get a few thousand fast walking steps. For me and my lifestyle, that’s fine to get through the day. If one wants to walk say, ten thousand steps, one would just take a spare battery or two. The batteries are modular. You just snap them in like a power tool.

Audrow Nash:
I see. How does this compare to other prostheses?

Hugh Herr:
All other foot/ankle prostheses in the world are human powered, meaning the energy of the human attached to them drives the movement. The legs that I’m wearing are bionic. They inject mechanical energy into a person’s stride in a way that’s similar to what the muscles used to do that were lost on amputation.

Audrow Nash:
Can you talk a bit about how they are fit to your residual limbs?

Hugh Herr:
At MIT I have a team of people thinking about fit. What is the nature of comfort? What are the mathematics of comfort? What is the science of comfort? We build mathematical descriptions of the tissues of the ends of the residuum and theories of how the shape of the structure that comes around the residuum and the stiffness of that shape to optimize comfort. Basically, the end of my limb goes into a cup-like device that supports my weight, and so the shape of that cup is critically important to comfort. We mathematically derive that shape, and then we 3D print the structure.

Audrow Nash:
You’re identifying where there are hard and soft spots in the residual limb so that you can match them or match them with their complement, so soft to hard, hard to soft so that it’s more comfortable?

Hugh Herr:
Yeah. Then what’s very complex is the shape, the equilibrium shape of the synthetic skin, if you will. That’s based on tissue, how compliant the tissues are, that shape. Mathematically we understand the compliance of the tissues in all the regions.

Audrow Nash:
What do you mean, compliance of the tissue?

Hugh Herr:
How soft or how stiff they are.

Audrow Nash:
Okay. What about stretch of say, the skin? Does that apply as well?

Hugh Herr:
In a different way. I wear a second skin silicone liner, so the design of that second skin relates to the skin’s strain field, or the amount of stretch in the skin.

Audrow Nash:
I see. Are they attached by this vacuum principle or how would you attach them to the residual limb?

Hugh Herr:
There’s a number of ways to attach. You can have a pin and a lock at the base of the limb. You can have suction. You can have a sleeve that spans from the biological leg across the socket. There’s a number of ways to hold it on.

Audrow Nash:
What have been some challenges in designing this?

Hugh Herr:
The interface. If you ask a thousand persons with limb amputation what’s the number one problem they want solved, probably all thousand would say the mechanical interface, the mechanical attachment. Why? Because it’s uncomfortable. It’s a very, very important problem. It’s a very complex problem because no one understands what comfort is for any device, or there’d be a shoe or a prosthesis. We’re developing that science, and it’s very exciting. We want to be able to produce sockets that are comfortable. We want to produce them fast at low cost.

Audrow Nash:
What are a means of doing this? You 3D print the part. How do you model the residual limb?

Hugh Herr:
We take an MRI image of the limb, and that tells us where the shape of the bones and where the skin is and where the muscles are. We then use robotic palpation tools to compress the tissues and measure displacement speeds and forces, which tell us the fundamental constants of the tissues. We then build a continuum mechanical model. If you have a mathematical description of the biological segment, you can then apply pressure fields. The model will tell you how the tissues deform. Then we define particular tissue deformations that are healthy and comfortable.

Audrow Nash:
Is the goal is to make this cheap and replicable for the general person?

Hugh Herr:
Yeah. My vision of the future is that each human individual will have a digital representation of their body. When they need any type of a bionic device, that digital body will be used to compute an optimal personalized bionic device for the human.

Audrow Nash:
This can refer to clothing as well.

Hugh Herr:
Correct. Clothing, shoes, bras, bike seats, bionic limbs, exoskeletons, neural implants that go inside the body. Everything will be personalized.

Audrow Nash:
Everything is optimized, and this relies heavily on 3D printing or additive manufacturing?

Hugh Herr:
3D printing is a tool. It’s not the dominant science. That is to say, one could build these interfaces with molding processes and not using digital fabrication. Digital fabrication is sometimes nice because it reduces the fabrication frequency, but it’s not necessary. It’s not the secret sauce.

Audrow Nash:
What’s the role of industry and academia in producing these prostheses?

Hugh Herr:
As stated, I’m a professor. The typical model of a professor in the US is they receive some type of grant monies and they conduct research with a team comprising both staff and students. There are inventions. Patents are filed. Patents are owned by the university. The university then licenses the intellectual property to founders of a company. Those founders can be the professors and students that invented, and often that’s the model, the most successful model of translation. In my case that’s exactly what happened. There was patents generated. I’m the inventor. My students are the inventors. A company’s established and then there’s a contractual relationship between the company and the university, licensing the IP within a certain domain of use. That’s what was done with my company, BionX Medical Technologies.

Audrow Nash:
What role does the company play in developing the technology?

Hugh Herr:
At university we do science. We do research that’s publishing. We test hypotheses. We do things that have never been done before, never been tested before. At the company, basically crude prototypes that we build at the university are commercialized. What that means is getting them light enough, most important strong enough, durable enough, manufacturable, at the right price point. Then all the sales and clinical services necessary to distribute that product globally.

Audrow Nash:
What does it look like when someone puts on the BiOM ankle for the first time?

Hugh Herr:
The design in the ankle is biomimetic, inspired by nature, by the human body. The algorithms that are running on these small computers on the ankles control the motor as if the motor were made of muscles and tendon and the whole structure was bone and whatnot. The whole thing moves as if it was made of flesh and bone, even though it’s made of synthetics. Because of that, when it’s fit to a human, the human is used to those dynamics, so there’s little to no training required. Often in minutes the person is saying things like they have their life back, they have their leg back, and they’re either crying out of happiness or laughing out of happiness. It’s fun to go to these fittings, actually.

Audrow Nash:
I’d like to move into talking about exoskeletons. First, would you define what an exoskeleton is and then tell us how it relates to your work with prostheses.

Hugh Herr:
My definition of an exoskeleton is a device that attaches to the body, intimately to the body, that augments physicality. What I mean by augments, it enables a human being to do something that’s beyond natural capability for an innate, healthy body. The word orthoses is a medical term for a robot that attaches to the body that enables a person with a disability or some condition to move more naturally. Exoskeletons are wearable robots that augment.

Audrow Nash:
Can you tell me a bit about metabolic cost as a parameter for designing exoskeletons?

Hugh Herr:
One goal in exoskeletal design is to augment a human by pedal locomotion. An excellent metric of evaluation is the amount of food energy a person requires when using the exoskeleton versus not using the exoskeleton. Why? My view is that if the exoskeleton increases energy levels of the human, the human won’t want to use it. It won’t have sufficient value. It’s a very important metric of evaluation.

Audrow Nash:
Basically they’ll discard it and not use it if the metabolic cost is higher than the task, or higher than it would be to accomplish otherwise?

Hugh Herr:
You know, unless they want to get exercise. If they want to actually be augmented and enhance physicality, no they won’t use it.

Audrow Nash:
How do you measure this?

Hugh Herr:
Typically for aerobic exercise it’s measured by monitoring how much oxygen a person uses when they breathe and how much carbon dioxide is expelled. With those two rates one can compute the calories burned per unit of time.

Audrow Nash:
Would you describe the exoskeleton used to decrease metabolic costs for walking?

Hugh Herr:
The first exoskeleton that was envisioned and published in the world that I’m aware of was in the nineteenth century, over a hundred years ago. It was a Russian inventor called Jagen. His dream was to augment the Russian Army, Russian soldiers. It wasn’t until 2014 that someone succeeded in building an exoskeleton that augments walking and running. My group was the first to accomplish that goal. It’s an exoskeleton that spans the foot and comes up about mid-calf, just below the knee. Fundamentally it’s an artificial calf muscle. The calf muscle is the most important muscle in bi-pedal walking. It supplies about eighty percent of the power to walk. It’s where humans are most inefficient, so the exoskeleton adds an artificial calf muscle and injects energy into the gait like a calf muscle to reduce the metabolic cost of the biological calf.

Audrow Nash:
What does it look like and what kind of actuators are you using?

Hugh Herr:
The actuators are electric based. It looks very interesting. It’s like, this crazy structure, this crazy shoe that goes higher than is normal and has these springy struts that go up on the side of the leg, and then a motor that sits on kind of a shin guard that pulls on that strut and powers movement.

Audrow Nash:
Yes, and so it’s using some sort of tether and you’re pulling that, and that’s how you flex the calf to augment walking?

Hugh Herr:
Flex the ankle, yes.

Audrow Nash:
You’re using the body as its own joint for this. Correct?

Hugh Herr:
It’s very important that exoskeletons are very, very, very light weight. Whenever you add weight to the legs, metabolic cost increases. In an effort to reduce the weight or the mass of the device, we actually use the body’s joints as the bearing instead of putting a synthetic bearing in the device.

Audrow Nash:
What kind of sensing is done with the device?

Hugh Herr:
All the electronics are up high by design so you can walk through a deep, deep puddle of water and not affect the system. We measure positions and speeds and accelerations, essentially. Then from those data we can compute torques, forces, given a model of the structure.

Audrow Nash:
You can infer where the person is in their gait and then determine if you should pull to assist the gap?

Hugh Herr:
Essentially.

Audrow Nash:
Why did you choose to revolve around the ankle?

Hugh Herr:
Again because the calf muscle is the most important muscle in walking.

Audrow Nash:
Are there other advantages to it as well, being that there’s not so much squishy …

Hugh Herr:
Yeah. The shin bone is where the leg is most stiff, most rigid, and it’s a tremendous plate of bone to apply forces on from an exoskeleton.

Audrow Nash:
Can you only use this for walking? Could someone use it for running or other activities?

Hugh Herr:
The device is very versatile. It’s powered, it has smart computation, so yes you can walk, run up and down hills, whatever you want to do. It’s as versatile as the human leg itself.

Audrow Nash:
What kind of efficiency gains are we getting?

Hugh Herr:
There’s a number of peer-reviewed manuscripts. Again, we were the first in 2014 to augment the human in a peer review publication. That first publication, the human subjects were wearing a twenty-three kilogram backpack and we augmented by eight percent. We later conducted a separate study on unloaded humans, just regular walking, and augmented by an average ten percent. The variation is huge, so you have N subjects. Some subjects are as high as almost thirty percent, some are low, and everything in between.

Audrow Nash:
What do you believe creates that variation?

Hugh Herr:
It’s a really interesting question. No one really knows, but everyone in the field has observed this phenomenon, that when you put a single exoskeletal design on N people, you get huge variation. My belief is you take ten people and you could distribute them in terms of athletic capability. Some people have a very smart spinal cord and they can exploit any tool or device very, very quickly. Others are clueless. That’s a possibility where training may be effective to mitigate that variance.

Audrow Nash:
Now, people that use it, what do they say it feels like?

Hugh Herr:
The human body is extraordinary, so a person begins to use it, they very quickly … Initially they feel the energy that it provides, but very quickly the human body gets used to it. Then you take it off and you kind of stumble, and your own biological normal legs feel heavy, awkward and slow. This tells us that wearing bionic structures will just be commonplace in the future, and innate, normal bodies will just be boring.

Audrow Nash:
Now, what kind of upper limit do you think we can get on efficiency with this device?

Hugh Herr:
With the foot/ankle device it’s not known. I would say at least twenty-five percent, but it may be even higher, which is very, very exciting.

Audrow Nash:
Then will you benefit, you think, from augmenting other muscles or other movements?

Hugh Herr:
We don’t know. What’s intriguing is our recent paper on the device. What’s intriguing is when you add exoskeletal torque and power at the ankle, not only do you reduce the power requirements of the biological ankle, but also you reduce the power requirements of the knee and hip. Even though the exoskeleton doesn’t span the knee and hip we nonetheless, reduce muscular effort at those joints tremendously. It begs the question do you really need an exoskeleton that spans the whole leg, or is the foot/ankle sufficient?

Audrow Nash:
Why would you think that it decreases the expenditure at those muscles?

Hugh Herr:
It’s just the dynamics of the system. The body, when you give it power, it uses that power distributively across the entire leg in an optimal way. It’s a new gait. It’s a different gait … than normal.

Audrow Nash:
How does it look compared to normal?

Hugh Herr:
It looks normal to an untrained eye. It doesn’t look like a bizarre walk. It is a distinctly new gait. You give the human energy from a bionic structure and the body quickly figures out how to ultimately use that energy to minimize its own metabolic food energy. That’s what humans are really good at. We’re inherently lazy and we minimize energy really, really well. We’re terrible at detecting high stress levels and probabilities of injury. We injure our knee and we’re surprised. We injure our hip and we’re surprised. An incredible value of exoskeletons is to give the human something that they’re bad at. One class of exoskeletons in the future will be exoskeletons that enable us to do athletic performances without a threat of injury.

Audrow Nash:
How would those work?

Hugh Herr:
I don’t know, but they would have to detect high levels of stress and fatigue in biological structures and then tell the human to stop or to move in a different way. Imagine a world where our top athletes never injure. Imagine the impact to human athletic performance. Performance would just go through the roof, because often what mitigates top performance is injury and recovering from injury and not training harder because you might injure, yada, yada, yada.

Audrow Nash:
What other future applications of exoskeletons do you see?

Hugh Herr:
Another form of augmentation is maintaining one’s inherent physicality independent of age. Imagine a world when you run your best marathon time when you’re sixty, and you’ve been running marathons since the age of fifteen. Why? Because you’re a more skilled athlete. As you age, technology eliminates the impact of age related degeneration by neural implants, maybe by regenerative medicine, maybe by exoskeletons or various forms of augmentation. That’s a very palpable form of augmentation. Most people ethically would be fine with that and most people would be excited about maintaining quality of life as they age.

Audrow Nash:
What are some bottlenecks in exoskeletons? What technological bottlenecks are there?

Hugh Herr:
I’m not sure there are technological bottlenecks. I think that largely the components are sufficient. It’s really a problem with design and architecture. The human machine interaction is something that’s unknown, so putting a bionic structure on a human and adapting its control in real time in a kind of human machine optimization is critically important to the future of this area of design. Imagine putting on a device and you just starting to walk and run and it adjusts its behavior to optimize its performance with you in a collaborative effort.

Audrow Nash:
Is this one of your research interests moving forward?

Hugh Herr:
Absolutely.

Audrow Nash:
What are some of your near-term research goals?

Hugh Herr:
I want to do three things first and foremost. One is I want to advance better muscle-like actuators. Perhaps actuators that are better than biological ones, kind of the engine of BionX. I want to understand electrical interface between the peripheral human nervous system and devices, so how to talk to nerve endings, essentially. I also want to understand how to attach machines to the body mechanically in a comfortable way. Those three innovations will really define bionics in this century. If you solve all three, you’re more or less done. You’ve solved bionics, limb bionics that is.

Audrow Nash:
Thanks for being on Robots Podcast.

Hugh Herr:
Thank you.

Audrow Nash is a Software Engineer at Open Robotics and the host of the Sense Think Act Podcast

Audrow Nash is a Software Engineer at Open Robotics and the host of the Sense Think Act Podcast