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TERRY GROSS, host:

This is FRESH AIR. I'm Terry Gross.

My guest, Hugh Herr, sometimes jokes that he feels sorry for people who have both legs because those legs can't be upgraded like his can. Herr had both legs amputated below the knee after getting frostbite during an unexpected snowstorm while mountain-climbing. That was back in 1982, when he was 17.

Now he walks on robotic, prosthetic legs. As we'll hear, they do things that his intact legs never could have done. He expects that their capabilities will keep improving, and he can say that with authority since he helped design them.

Herr has worked at MIT's Leg Lab and Media Lab and now directs MIT's Biomechatronics Research Group. He's the holder or co-holder of 14 patents related to assistive devices, including a computer-controlled artificial knee.

One aspect of prosthetics he's focusing on is how amputees can control the movements of their robotic limbs by signals from their nerves or brains. Herr is one of the people profiled in the new book about innovative technologies developed in MIT's Media Lab. The book, "The Sorcerers and Their Apprentices," is by former Media Lab director Frank Moss.

Hugh Herr, welcome to FRESH AIR. Would you describe the artificial legs that you designed that you're wearing today?

Mr. HUGH HERR (Director, Biomechatronics Group, Massachusetts Institute of Technology): The legs that I'm wearing today, they're called PowerFoot BiOMs. And they're an ankle-foot prosthesis. So my legs are amputated below the knee, about six inches. So from there down to the floor, I'm artificial. I'm titanium, carbon, silicon, a bunch of nuts and bolts.

My limbs that I wear have five computers, 12 sensors and muscle-like actuator systems that enable me to move throughout my day.

GROSS: How do you activate the computers? Like if I want to move my foot, I just think move the foot, and it moves. How do you get your foot to move?

Mr. HERR: Well, in the future, I'll have neural implants within my biological body. But for now, the artificial limb itself has its own intelligence. So when I push on it, and when I change its positions, it responds in a natural way.

So when I want to walk faster, I simply walk faster, and it - the artificial limb outputs the right torques and energies.

GROSS: Did you design the computer part, or did you have computer specialists come in for that?

Mr. HERR: My laboratory at MIT, what we first do is we model how the human body works in standing, walking and running, what the muscles are doing, what the tendons are doing, how the muscles are controlled with spinal reflexes.

And those - that biological science motivates what we build. So we end up with putting forth novel architectures of how motors interact with springs, interact with sensors, interact with onboard microprocessors. So we do most of the design work of the artificial limb.

GROSS: So you described the limbs that you're wearing today. Do you have other artificial lower legs at home that you wear for other occasions?

Mr. HERR: Yeah, absolutely. I have - let's see. I have a running pair. I have a bionic walking pair, limbs that are waterproof. I have various limbs for climbing mountains. Some are - just sense steep ice walls, spiked feet that penetrate ice walls, other feet that wedge into small rock fissures, others that stand on small rock edges the width of a coin. So, probably about eight pair or so.

GROSS: Wow, I think it's amazing that you can still do all those things that you mentioned - climbing mountains, climbing ice - with your artificial legs. Are there things your legs, your artificial legs can do that your real legs couldn't have?

Mr. HERR: Oh absolutely. My legs were amputated in 1982. And I quickly realized that from the knees down, I was a blank palette for which to create. I could create any type of limb that would afford me various physical capabilities.

So I quickly realized that, well, if I want to climb a mountain, I could make my artificial limbs very, very lightweight, lighter than human flesh. So I made - I used carbon composites with a density much lower than flesh and bone. But I had the same strength in my upper body. So I could do more pull-ups before fatiguing.

When I climb a steep ice wall, I no longer have calf fatigue. When I'm on my front points, it feels like I'm standing on a sidewalk. It's fantastic.

GROSS: You know, I like this idea that your prosthetic legs can keep getting better and better and designed for all kind of specialty needs while our real limbs get more worn out with age.

(Soundbite of laughter)

GROSS: So yours can improve, when real ones tend not to after a certain age.

Mr. HERR: Yeah, it's interesting because my biological body will degrade in time due to normal, age-related degeneration. But the artificial part of my body improves in time because I can upgrade. I can get the best computer, the best motor system, the best artificial intelligence.

So I predict that when I'm 80 years old, I'll be able to walk with less energy than is required of a person that has biological legs. I'll be more stable. I'll probably be able to run faster than a person with biological limbs

So it's quite interesting that in fact my artificial part of my body is in some sense immortal.

GROSS: Yeah, yeah, that's kind of amazing. Now are you wearing shoes over your artificial feet?

Mr. HERR: I'm now wearing my bionic walking limbs, and yes, I do have shoes on. But when I climb mountains, I do not use shoes. Sticky climbing rubber is attached right to the prosthetic foot. I need not use a mountain boot.

GROSS: And why are you wearing shoes now? Is that for - so that people don't stare at you, or...?

Mr. HERR: Just to try to attempt to fit into society.

(Soundbite of laughter)

GROSS: Right. Do you ever, like, wear shorts and just, like, let the prosthetics show?

Mr. HERR: Oh absolutely. It's a lot of fun. So in terms of industrial design, the approach that my team takes is as follows. We want the bionic limb to have a human-like shape, but we don't want the bionic limb to look human. We want it to look like a beautiful machine, to express machine beauty as opposed to human beauty.

And the reason is we want the user one evening to, you know, pull a black sock over their bionic limb and have their limb appear to be fully biological and then the very next evening, go to a fancy party where they pull that sock off, and they expose the fact that part of their body is bionic.

So that interplay between human and machine beauty is something we accelerate and allow.

GROSS: So in talking about machine beauty, how would you aesthetically describe the beauty of your artificial legs?

Mr. HERR: Well, we all know of machine beauty. We have beautiful cars. Some people find the Mercedes a beautiful machine. It's certainly not a human beauty; it's a machine beauty.

Many of us find the iPod, Apple products, that are so well-designed beautiful, again not a human beauty but a machine beauty. So what we want is we don't want the user of these prosthetic limbs to be ashamed of their body, ashamed of the fact that part of their body is artificial. We want them to celebrate it.

So to do that, we need to put forth synthetic structures that are elegant.

GROSS: If you're just joining us, my guest is Hugh Herr. He's the director of the Biomechatronics Research Group at MIT, and this is a group that's designing bionic limbs. And he uses his own designs. He was amputated from a few inches below the knee down, after a mountain-climbing accident in 1982, when he was still in his teens.

He's also one of the people profiled in the new book "The Sorcerers and their Apprentices," about the MIT Media Lab by former director of the lab Frank Moss. Let's take a short break here. Then we'll talk some more. This is FRESH AIR.

(Soundbite of music)

GROSS: If you're just joining us, my guest is Hugh Herr. He's the director of the Biomechatronics Research Group at the MIT Media Lab. And he's helped design the artificial limbs that he's wearing as a result of a mountain-climbing accident. He lost his legs. They were amputated from the knees down.

You started needing artificial limbs when - was it when you were 17 that you had your accident?

Mr. HERR: Yes, the tender age of 17, that's correct.

GROSS: So tell us what happened.

Mr. HERR: My climbing partner at the time, Jeff Batsor(ph), we traveled to New Hampshire to climb Mount Washington. It was January. We headed up towards what's called the Huntington's Ravine, and we climbed about a 700-foot ice face called Odell's Gully.

When we got to the top of the face, we decided to continue towards the summit of the mountain, and the weather conditions became more and more severe. The snow was blinding. The wind speeds were very, very high. We could barely stand.

So even though we ventured just a few minutes above the top of the face, we were not able to make it back. We were not able to retrace our tracks and return to the same gully system.

We'd mistakenly descended down what's called the Great Gulf Region of Mount Washington, and it's the wilderness side of Mount Washington. And it was there that we spent several days trying to stay alive, digging snow caves, hugging each other to stay warm.

Through that process, we made to within just a few miles of a roadway of civilization, but at that point, we were no longer able to walk. We'd suffered some severe frostbite and hypothermia, and because of the frostbite, we could only walk a few paces before losing our balance and toppling over.

Fortunately, a person was out snow-shoeing for the day, discovered human footprints, our footprints, followed them to our snow cave, and we were rescued from the mountain via helicopter.

GROSS: Did you think you would survive before you were rescued?

Mr. HERR: No, I didn't believe that we would make it even one day, and we made it nearly four days. The conditions were severe. It was minus-20-degrees Fahrenheit, snowing continuously. The average depth of snow was to the waist. Sometimes the snow came up to our chest. We could barely move.

So I thought we would freeze to death the first night, but it turns out by hugging one another, by sharing body heat, we were able to survive for quite a long time. But near the end, when we were both not able to walk even three paces before toppling over, that's when we both gave up hope, and we actually stopped hugging each other. And our strategy was the sooner we die, the better. We were in excruciating pain.

GROSS: So when you were rescued, and you found out that you would live, but then you found out that your legs were going to be amputated, what went through your mind in terms of that mix of the positive side, knowing that you'd live, and the negative side of knowing that you would never physically be the same again?

Mr. HERR: Well, our physical condition, to me, was the least of my concern. We were plucked from the mountain, and we were told immediately that a rescuer had died, a volunteer rescuer, Albert Dow(ph).

GROSS: Rescuing you?

Mr. HERR: Yeah, he was - he and a partner were in Huntington's Ravine, and while descending, they were hit by an avalanche. And Albert perished. So the news of that was horrible, horrible. You know, I really, at that time I really didn't care what was happening with my physical body. I was just devastated that a fellow climber had perished.

GROSS: I can have some sense of how devastating that must have been. When you did find out - when did you find out that you would be losing your legs? Did you know that before the surgery, or did you wake up and find that they were gone?

Mr. HERR: My medical team spent several months trying to save my biological limbs. There were periods where I would have two surgeries a week, where they would go in, and they'd try to clean the tissues, to restore circulation to my lower legs.

But those efforts did not work, and on mid-March of 1982, both of my legs were amputated. I was aware going into surgery that I would come out of the surgery without feet. When I awoke from the surgery, and I looked at my legs, I was shocked at how high the amputation was, how short my legs had become as a result of the amputation. So that was very traumatic for me.

GROSS: The amputation eventually changed your life around because you found your calling, which is designing artificial limbs. But before finding your calling, did you go through a period of abject depression?

Mr. HERR: Yeah, after the - immediately after the accident, I was very, very angry with myself of putting myself and my partner in that situation, as well as other climbers, and someone ended up getting hurt. Someone ended up dying.

So I wasn't really depressed. It was a severe, intense anger, and out of that anger came this extraordinary swelling of energy and a desire to not wallow in self-pity but to do something worthwhile with my life.

I viewed at the time that to give up, to wallow in self-pity, would have been a disgrace to the memory of Albert Dow, the rescuer who had died.

GROSS: Would you describe the first prosthetic legs that you were given, what they were capable of doing and what it physically felt like to wear them?

Mr. HERR: After my legs were amputated, I was home in a wheelchair for a month, and then I was sent to a rehabilitation center. And at that center, I was fitted with my first prostheses. The prostheses were made of plaster of Paris, and they actually told me to not walk without canes because the added force of walking without canes may actually fracture the plaster.

(Soundbite of laughter)

Mr. HERR: So the devices were very, very crude, and I remember distinctly, I was in a wheelchair, they put these artificial limbs on me made of plaster, and I pressed up on parallel bars with arms fully extended, and then I slowly put weight on my legs, and I never felt such excruciating pain in my life. I think I stood there for a second and immediately sat down again.

GROSS: And was that because the amputation was recent, or is that because of the nature of the prosthetics?

Mr. HERR: More the former than the latter. When you - when a limb is first amputated, as you can imagine, it's very swollen, it's very, very sore, the residual limb. And then to actually stand requires that very high forces and pressures are exerted on the residual limb, and that's often done very early, before the residual limb has fully healed. So you can imagine how painful that is.

But after a while, after six months, after 12 months, the residual limb takes on a uniform volume, a uniform shape. It doesn't atrophy in time. Then if the prosthesis is fit well, it can actually be reasonably comfortable.

GROSS: Did you get to the reasonably comfortable point?

Mr. HERR: Yeah, it took about a year. It's very interesting, as an amputee, the more experienced one is as an amputee the better it gets, the more comfortable the prosthesis becomes because that residual limb is that constant shape, and the skin becomes conditioned.

GROSS: At what point did you think about designing a better version of the prosthetic leg?

Mr. HERR: The prostheses that were provided to me were designed for the horizontal world and for walking very, very slowly on a horizontal surface. They were not designed for fast walking, for running. They were not designed for the vertical world of rock and ice climbing.

After the accident, I dreamed of returning to my chosen sport of mountain climbing. So it immediately became apparent to me that what I needed to do was design my own artificial limbs. So I had training in machining and carving metals and woods and making artifacts. So I went into the shop, and I started grinding and cutting and designing various limbs, designing various foot structures that were conducive to the vertical world of rock and ice climbing.

GROSS: What was the first prosthetic like that you designed?

Mr. HERR: Well, very early, initially I put a rock-climbing boot over the prosthetic foot. And then I said that's silly, and I threw out the shoe.

(Soundbite of laughter)

Mr. HERR: And then I realized, well, the foot need not look like a human foot. So, you know, to climb a vertical rock face, I really don't need a heel. so I cut off the heel, prosthetic heel. It was just extra weight that I didn't need. And I started then optimizing how - the angle of the foot relative to the calf of the prosthesis, the size of the foot. My rock climbing feet are the size of baby feet. They're very, very small, very, very short so I can get the center of my body over my feet on a vertical wall.

So through all this tinkering, I was actually able to climb at a more advanced level after my accident, with artificial limbs, than I had ever achieved before my accident with normal biological limbs.

GROSS: Hugh Herr will be back in the second half of the show. He heads the Biomechatronics Research Group at MIT's Media Lab. He wears and designs prosthetic legs. I'm Terry Gross, and this is FRESH AIR.

(Soundbite of music)

GROSS: This is FRESH AIR. I'm Terry Gross.

We're talking about wearing and designing state-of-the-art prosthetic legs. My guest, Hugh Herr, does both. His legs below the knees were amputated after getting frostbite in 1982 when he barely survived a snowstorm while mountain climbing. He heads the Biomechatronics Research Group at the MIT's Media Lab. He is the holder or co-holder of 14 patents related to assistive devices, including a computer controlled artificial knee. He's one of the people profiled in the new book about the MIT Media Lab, "The Sorcerers and Their Apprentices."

When we left off, we were talking about his early attempts to design prostheses that were better than the ones he was given. Those were designed for walking slowly. He wanted legs designed for running and for climbing in what he describes as the vertical world.

This might be a silly question but if you're in the vertical world and you're climbing a mountain that's putting a lot of pressure, isn't it, on the join between the prosthetic limb and what's left of your leg. I mean what prevents you from falling off of your own prosthetics?

Mr. HERR: Falling out of my own legs?

GROSS: Yeah. Mm-hmm.

(Soundbite of laughter)

Mr. HERR: Not...

GROSS: Is that a stupid question?

(Soundbite of laughter)

Mr. HERR: Not very much. So what was absolutely terrifying was the threat of my prosthetic foot being wedged so tightly in a rock fissure that I would not be able to get it out. And then, about a minute later, my arms would fatigue and then I'd, you know, fall and eventually my stuck prosthetic foot would catch me. And since the connection between the artificial limb and my body isn't that secure, I would simply fall out of my own legs.

(Soundbite of laughter)

Mr. HERR: So then I'd be at the bottom of the mountain after falling, and I'd look up and there'd be an artificial stuck in the crack hundreds of feet up

(Soundbite of laughter)

Mr. HERR: There's a funny story. One time I fell and the ropes eventually caught me. But right after the rope caught me I slammed into the rock face and it was with such a force that one of my prosthetic feet broke in half.

GROSS: Oh, god.

Mr. HERR: And it tumbled down the mountain. And at that early point, I was still wearing rock climbing boots. And we yelled down to a tourist at the base of this rock wall as they walked by in their hike to please find boot and put it next to our backpacks at the base of the wall. So this tourist went over and they grabbed the boot and they said, there's something in it.

(Soundbite of laughter)

GROSS: So if that accident happened and it was to your real leg what would've happened

Mr. HERR: Probably my tibia would have just been shattered into many, many pieces.

GROSS: So in a way you survived...

Mr. HERR: Yeah. There's so many benefits.

(Soundbite of laughter)

GROSS: ...better with the artificial limb than you would have with the real one?

Mr. HERR: Absolutely. Oh gosh. Gosh, absolutely.

GROSS: Mm-hmm. Mm-hmm.

Mr. HERR: I can walk through high grass and not have any fear of snakes. I would love for his snake to try to bite me. Yeah.

(Soundbite of laughter)

Mr. HERR: A mighty surprise.

GROSS: So one of the things you could do with your artificial legs is change their height. So tell us some of the ways that comes in handy for you and why you would choose one height over another.

Mr. HERR: My brothers are about they're five foot about 10 inches. And with my artificial limbs I'm about six-two, six-three.

(Soundbite of laughter)

Mr. HERR: So just, you know, in day-to-day life I've augmented my height.

GROSS: Why did you want to be taller?

Mr. HERR: It's fun to be tall. There's...

GROSS: I wouldn't know.

(Soundbite of laughter)

Mr. HERR: There's social advantage to being higher in stature.

(Soundbite of laughter)

Mr. HERR: In climbing, sometimes the hand and footholds are very, very far apart and it's advantageous to be tall. Sometimes the handholds are very close together and it's actually better to be shorter. So when I climb I look up into rock face and I kind of decide, depending on how the face looks, what height I should be and what foot I should use and whatnot.

GROSS: So you can adjust the height as you're wearing the prosthetics.

Mr. HERR: Yeah. It's just a simple turn of an Allen key and I can be as short as five feet and as tall as I'd like.

GROSS: Wow. And have you used this to trick friends?

Mr. HERR: Yeah. My first several weeks in undergraduate school I decided to conduct the following experiment; every day I went to school I increased my height by one inch. So I wanted to see how long would it take for someone to notice that I was increasing in stature. And I think I got up to nearly eight feet tall and I had to touch the ceiling practically to remain balanced. And someone finally said you seem to be getting taller. And I said, of course, college is a growing experience.

(Soundbite of laughter)

Mr. HERR: One can have so much fun with artificial limbs.

GROSS: What have you done to try to make the limbs as comfortable as possible? We've talked a lot about function. But just in terms of like comfort in the join between the flesh and the metal.

Mr. HERR: Well, my first patent was issued while I was in undergraduate school, and the patent described a way of connecting the artificial limb to the human body, mechanically. It described using fluid bladders that surround the residual limb, and that are sandwiched between the outer part of the artificial limb and the human body. And the user vary the pressures within these bladders and whatnot. So that was kind of my first design project. Today at MIT, I have an entire team working on novel ways of connecting artificial limbs to the human body, where there's - where the material using an onboard power supply, the material will stiffen or it will become soft depending on whether the person's sitting or standing or running.

So if you're - when a person sits, the interface will become soft and loose with very little pressures. But if the person suddenly gets up and starts running it will stiffen.

GROSS: One has to be so smart to be able to do that.

Mr. HERR: Yeah. Many, many sensors and small microprocessor across the interface. So yeah, there's a lot of intelligence on the interface.

GROSS: Do these legs cost a fortune?

Mr. HERR: Yeah. They are currently too expensive to be made widely available across the entire world. Artificial limbs are generally affordable in places such as the United States. But a country, for example, like Afghanistan or Cambodia, there is no way that a person there would be able to afford the limbs. So that's something we, long-term, that we working in technology and business need to do a better job. It's - just because I was born in the United States, into a wealthy country, affords me the best medical devices. And that shouldn't be. We should have a system set up we could have broad application and distribution of these types of mechanisms.

GROSS: If you're just joining us, my guest is Hugh Herr. He's the director of the Biomechatronics Research Group at the MIT Media Lab. And he's helped design the artificial limbs that he's wearing as a result of a mountain climbing accident. In other words he was caught in a blizzard and got frostbite. He lost his legs. They were amputated from the knees down. And he and his research group have created these amazing computer-sensored artificial limbs that he says are actually much more functional that his legs used to be.

Let's take a short break here then we'll talk some more.

This is FRESH AIR.

(Soundbite of music)

GROSS: If you're just joining us, my guest is Hugh Herr. He's the director of the Biomechatronics Research Group at the MIT Media Lab, and what he does is design artificial limbs. He wears artificial limbs from the knees down. Well, from below the knees, down, as a result of a mountain climbing accident in which he suffered frostbite. In 1982 and his legs were amputated from below the knees down. He is one of the people profiled in the new book "The Sorcerers and Their Apprentices," which is about the MIT Media Lab and it was written by the former director of the lab Frank Moss.

You said earlier that one of the things you're hoping to accomplish with your artificial limbs is to find a way where they can be neurally(ph) connected to you so that you can't think move and then we'll move. How in the world are you thinking about going about doing that?

Mr. HERR: Yeah. I predict that as we march into this 21st century the changes we'll see in prosthetic designs, that the artificial prosthesis will become more intimate with the biological human body. There will be emergence, if you will. So there's several forms to that emergence. One is that the prosthesis will be attached to the body, mechanically, by a titanium shaft that goes right into the residual bone, where you can't take the artificial limb off. Another intimate connection will be electrical. The nervous system of the human will be able to communicate directly with the synthetic nervous system on the artificial limb, if you will.

GROSS: How? I mean I just can't imagine how that could possibly be done.

Mr. HERR: Well, there's a number of research groups worldwide developing what are called neural(ph) implants. For most persons with limb amputations, they would have what is called a peripheral neural(ph) interface, which would comprise of small electronic packages that are implanted into muscle in the residual limb to measure the degree to which muscle has been activated electrically by the spinal cord. And there's also these small electronic packages attached to nerves.

When a limb is amputated obviously key nerves are transected or amputated. So in the future, there will be electronic packages that will communicate, in a bidirectional way, with the amputated nerve. They'll measure motor information, how the person wishes to move, dissenting commands from the spinal cord, and they'll also be stimulation ports to stimulate the nerve endings to reflect the sensory information from the artificial limb onto the human nervous system, to close the loop between the human and the machine.

One day, amputees will not only be able to walk across sand but they'll be able to field the sand against their synthetic prosthesis.

GROSS: What you're describing seems so, just totally, far out. But I gave the fact that you've accomplished what you have so far makes me think well, maybe it's really possible.

Mr. HERR: Yeah. When you look at prosthetic innovation over history, what you'll see is spikes in innovation during and after every major war.

GROSS: Mm-hmm.

Mr. HERR: So World War II, for example, there's a tremendous progress in prosthetic design. The current conflict that we now find ourselves in - in Iraq and Afghanistan, we are now seeing tremendous innovation spike. There's several reasons for that. One is there is a lot of funding, now, from the U.S. government to fund research in this area of science and technology.

Another reason why it's an interesting time is that over the last several decades key disciplines have matured that are relevant to prosthetic design -robotics, tissue engineering, artificial intelligence. And we now find ourselves in a position that if we cleverly integrate these disciplines we can put forth truly bionic limbs that fundamentally improve the quality of life of persons with limb amputations.

GROSS: Well, it's nice to have such hope for the future for people who have amputations. And I'm also - I'm sure this would also apply to people who don't. I'm sure eventually this technology will help people who haven't lost limbs...

Mr. HERR: Absolutely.

GROSS: ...but maybe are just like weak or in pain or, you know, need an assist of some sort. Or elderly people. Like what are you working on now that would apply in those areas?

Mr. HERR: Yeah. So our conversation has been about prosthetic limbs. But we're also working here at MIT on exoskeletal robotic structures. And that's a robot that wraps around a biological limb. And that biological limb could be impaired or it could be completely normal and healthy. So in the former case, when the biological limb is impaired due to age-related degeneration or due to stroke or spinal cord lesion, given a few examples, the robot pushes on the impaired biological limb in just the right way to allow a person to stand, walk and even run.

We're also working on exoskeletal structures that argument human capability beyond what nature intended. So Fancy Pants, these robotic robots that you wear on your legs that allow you to run with less stress to your biological leg, allow you to walk and run with less energy, food energy - we predict in the future that when a person, for example, goes jogging, they'll routinely wear robots. Why? To protect their joints, their knees' joints and hip joints and whatnot, from the risk of injury.

And if you - in the future, if you have a knee injury you'll wear robots to protect your joints from further degradation.

GROSS: You know how a lot of older people use walkers because they're either too weak to walk without an assist or they have a hip problem or a leg problem? Can you envision robotic technology that would make that, you know, by comparison primitive walker obsolete?

Mr. HERR: Oh, absolutely. Absolutely. And it could be in a form of, again, an exoskeletal structure that wraps around the legs of the elderly person. Or it could be implants. It could be implants into key muscles within the leg that help the nervous system of the elderly person fire the muscles in the appropriate way for balance.

GROSS: Why do you still climb? You were in, you know, a horrible blizzard. You came so close to dying. You lost the bottom of your legs. You lost your feet. Somebody died trying to rescue you. You felt really guilty and angry at yourself. And you designed these artificial limbs which in part have enabled you to keep climbing. Why would you go back to it after such catastrophe?

Mr. HERR: Well, I've been climbing mountains since the age of seven. To me it's as natural as walking. Climbing is a vertical dance. It requires both physical power and also grace, graceful movements. So it's always been a part of my life. It's absolutely exhilarating and wonderful and it's something that I didn't want to give up. And it was really the first expression of how I could exploit technology attached to my body in a way that was positive and that would help others.

GROSS: Did you ever climb that mountain again, the one where you got lost and got hypothermia and frostbite?

Mr. HERR: I have. Yes. And I have two daughters. They are six and eight years old, and my daughters have climbed the mountain as well. So it's wonderful to now see them ascending these glorious mountains that I ascended when I was young as well.

GROSS: Well, Hugh Herr, I want to thank you so much for talking with us. It's really been a pleasure. I wish you well and thank you so much.

Mr. HERR: Oh, thank you so much. It's been my pleasure.

GROSS: Hugh Herr heads the Biomechatronics Research Group at MIT's Media Lab and is one of the people profiled in the new book "The Sorcerers and Their Apprentices."

You can see pictures of him wearing his prosthetic legs on our website freshair.npr.org.

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