At 5 o’clock on a blistering morning in June 2007, U.S. Marine Cpl. William Gadsby helped lead a team of infantrymen into the farmland surrounding Karma, an agricultural hub in Iraq’s volatile Anbar Province. Karma is pancake-flat, with sightlines for miles, and after a few hours on patrol, Gadsby grew worried. We’ve been out here too long, he thought. They’re probably tracking us.

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Around 10 a.m., he heard a deafening bang. A cloud of smoke enveloped him. He tried to run and he got nowhere: A remotely detonated bomb had turned his right leg into a mass of gore and gristle. All he felt was adrenaline. Ears ringing, he rolled and jerked away from the site of the explosion until he reached the side of the road. As he lay in the dirt, with a corpsman applying a tourniquet to his right leg, a sniper’s bullet pulverized his left knee.

More bullets zipped past. Gadsby hollered out orders, even as liters of blood poured out of his body. Once the insurgents had fled back into the farmland, his men flagged down a passing truck and loaded him into the back. His breathing was ragged and dry, and he flickered into and out of consciousness. At the field hospital, a priest read him his last rites. His eyes closed.

He awoke a day and a half later in the medical wing of a base in Germany. Miraculously, a trauma surgeon had preserved his left leg—but the right had been sawed off above the knee.

Months of pain followed: the endless physical therapy, the fitting of a prosthetic, the challenge of learning to walk again. Gadsby, 29 years old, faced it all head-on. After he was transferred to a base in Southern California, he took to spending his afternoons hobbling up and down the beach, because walking in sand took real effort, and he thought it would speed his recovery.

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It didn’t. Part of the problem was his prosthesis. It was a foot made from carbon fiber—top of the line, his doctors had assured him—and although it had some flex to it, the device still felt overly stiff. Every step sent a shock wave up his back. He was always sore.

“I thought, I live in an era where the technology is only expanding—every year, there’s a revolutionary breakthrough,” Gadsby, now a husband and father and social-worker-in-training, told me recently. “That gave me hope. Something to go on.”

In the spring of 2010, he read about a new type of prosthesis being developed by Hugh Herr, head of the biomechatronics group at MIT’s Media Lab. Herr himself was a double amputee: In 1982, when he was just 17, he’d lost both legs to frostbite sustained during a mountaineering expedition. While completing a master’s degree in mechanical engineering at MIT, a doctorate in biophysics at Harvard and postdoctoral work in biomechatronics at MIT, Herr had developed an increasingly sophisticated array of artificial knees, feet and ankles. His latest invention was a fully computerized ankle-foot system called the BiOM, which imitated a flesh-and-blood foot, propelling the user forward with each step. It bore no resemblance to any other prosthesis on the market.

“To me, this guy, Dr. Herr, was an inspiration,” Gadsby says. “Unlike the rest of us, he wasn’t sitting around, thinking, ‘Gee, I wish they could come up with a better gadget.’ He got those degrees so he could fix himself—and fix everyone else.”

***

For the past four years, the 30-odd members of the Media Lab’s biomechatronics group have worked out of a laboratory on the second floor of a gleaming glass complex on Amherst Street in Cambridge, not far from the Charles River. The space is high-ceilinged and bright, and dominated by a treadmill, which is used to test prostheses and exoskeletal devices. Amid the sleek fiberglass struts and polished machine parts, one object stands out: a flesh-colored rubber appendage known as a Jaipur Foot. Its presence in the lab is talismanic, commemorative. Until relatively recently, the Jaipur Foot, invented in 1971 by an Indian surgeon, represented the pinnacle of prosthetic science: an inanimate lump that aped the form of a foot without replicating its function.

“Wood, rubber, plastics,” Hugh Herr recited when I visited him in Cambridge earlier this year. “At the time of my accident, that was the reality. There were foot-ankle systems, but there was no computational intelligence. And a lot of key technological capabilities were not in place, like inexpensive, powerful, small microprocessors. A lot of sensing capability was not available. The same went for power supplies and motors.”

In person, Herr, 51, has a raffish air—more Parisian artist than hard-charging American scientist. He wears his thick hair swept back and favors dark blazers and colorful scarves. (In a shoot for an Italian edition of Wired magazine, he posed in a bespoke jumpsuit of fine linen; a blowup of the cover hangs prominently in the MIT lab.) But the impression is deceptive. Herr has confessed to being “stoic to a fault,” and when faced with questions he regards as trivial or uninteresting, he has a habit of going monosyllabic. “I just don’t express what’s inside,” Herr has been quoted as saying. “My students tend to be afraid of me, and I wish they weren’t.”

Partly, the stoicism may be a response to life in the spotlight. Even before he lost his legs, Herr was a sensation in the rock-climbing world—a handsome kid from a Mennonite farm in Pennsylvania putting up wild and hairy routes that even hardened veterans had trouble replicating. His accident, the result of a botched winter ascent of New Hampshire’s Mount Washington, slowed him down for a few months, but soon he was climbing again, using prosthetics he designed in his own workshop. And something strange was happening: His climbing was improving. He had flexible rubber feet that helped him scuttle up tricky cracks, and specialized crampons for scaling ice walls. Again, the media came calling—magazines, newspapers, TV.

At the same time, he continually ran into evidence of a prejudice against people like him. “My father told me this story about how, shortly after my limbs were amputated, a person came up to him in the hospital and said, ‘Oh, I’m so sorry. He wasn’t married, was he?’ I had become instantly subhuman!” Herr marveled. “It was fascinating. We’re all so programmed to think that an unusual body is a weak one.”

He was determined to change that. A middling high-school student, he now consumed mathematics textbooks by the crateload. In his early 20s, he enrolled at Millersville University, a small school a few miles from the family farm in Lancaster, Pennsylvania. While an undergraduate, he obtained his first patent, for a prosthetic sock that leveraged a system of inflatable bladders and microprocessors to help the wearer walk better and more comfortably. The device—along with a sterling grade-point average—caught the attention of MIT’s admissions staff, and in the early 1990s Herr moved to Cambridge to work on his master’s degree. He invented ceaselessly, always tinkering, building, improving. The patents piled up: for artificial joints, computer-powered ankles, biomimetic joint actuators.

The prosthetics industry had seemed trapped in another century, and Herr wanted to haul it into the digital age. “There was a long stretch of time where there was a lot of technological advancement in other sectors, but not in our field,” Elliot Weintrob, a Virginia prosthetist who sells BiOM devices, told me. “Yes, you had the emergence of carbon fiber, but the improvements were incremental: Lighter carbon fiber, stronger carbon fiber. OK, what’s the next level? The next level was power. Because no matter how much spring you’ve got in that carbon fiber, until you start trying to replace the action of the muscle, you’re inherently limited. That was Hugh Herr’s genius—he understood that.”

In 2007, Herr founded a bionics company called iWalk (the name was later changed to BiOM), and set about bringing to life the advanced technology that had always fascinated him. Research and development in prosthetics had not been particularly well funded or attractive to engineers and scientists, but things were rapidly changing. “With the war on terror, and the conflicts in Iraq and Afghanistan, and all these returning injured, Congress had unleashed millions in research money,” Herr recalled. “Another driver was that the key disciplines relevant to bionics had matured, from robotics to tissue engineering. And they were maturing to a level where we could actually build bionics as envisioned by Hollywood and science-fiction writers.”

Herr trained his focus on the ankle, a dauntingly complex part of human anatomy, and one traditionally underserved by prosthetics technology. By late 2009, testing was underway on the PowerFoot BiOM, the first lower-leg system to use robotics to replace muscle and tendon function. Using onboard microprocessors and a three-cell ion lithium battery, the device actually propelled the user forward with each step, in the manner of organic muscle. For propulsion, the BiOM relied on a custom-built carbon-fiber spring—each time the user stepped down on the device, the spring was loaded with potential energy. On the up-step, that energy was supplemented with a small battery-powered motor.

But Herr and his team knew that all steps are not created equal: Scrambling up a steep slope requires a very different gait—and very different parts of the body—from walking across a tennis court. So they developed a proprietary algorithm that measured the angle and speed of the initial heel strike of the BiOM, and controlled, via the microprocessors, the speed and angle of descent on the next step.

The BiOM weighed about five pounds—more or less the weight of a human ankle and foot—and was fitted to the user’s residual limb with a simple carbon fiber socket. Tests indicated that the device returned about 200 percent of the body’s downward energy. A top-flight carbon-fiber prosthetic returned only 90 percent.

Tens of millions of dollars in venture capital poured in. Ditto for emails and letters from amputees desperately eager to serve as BiOM guinea pigs. That barrage has not stopped. “It’s overwhelming,” Herr told me, shaking his head. “It’s emotionally taxing and heartbreaking.”

These days, Herr is something of a professional juggler: In addition to his posts at BiOM and the biomechatronics lab, he teaches classes at MIT and Harvard. He travels to lecture and to consult on other bionics projects. He still climbs when he can, although in recent years, the highest-profile mountaineer in the family has been his wife, Patricia Ellis Herr, whose 2012 book, Up: A Mother and Daughter’s Peakbagging Adventure, details a family quest to summit the 48 highest mountains in New Hampshire. The Herrs’ daughters, Alex, 11, and Sage, 9, are both avid climbers. Hugh joins them on hikes when he can but spends a large part of his waking life in the lab.

Before I left MIT, I asked Herr if he was comfortable with the roles he had assumed as an outspoken advocate for bionics and a very visible bionic man himself. He paused. “We’re constantly surrounded by messages about how technology is not doing us well: pollution and nuclear weapons and so on,” he said, finally, studying his legs. “I’m an example of the opposite trend. So, yes, I’m comfortable with it. God, yes.”

***

This past March, Herr flew to Vancouver to deliver an address at the TED Conference, the annual summit of science and tech cognoscenti. His presentation was heavily autobiographical: He discussed his accident, his first inventions and a pair of early prosthetics that allowed him to adjust his height from 5 feet to 6 1⁄2 feet plus. (“When I was feeling badly about myself, insecure, I would jack my height up,” he joked, “but when I was feeling confident and suave, I would knock my height down a notch, just to give the competition a chance.”)

Then the lights dimmed and went up again, and Herr introduced a professional ballroom dancer named Adrianne Haslet-Davis. In 2013, Haslet-Davis had lost part of her left leg when terrorists detonated a pair of bombs at the Boston Marathon; now, as the crowd sat rapt, she and her dancing partner, Christian Lightner, performed a delicate rumba. If you hadn’t spotted the glint of the prosthesis Herr had fitted her with, you would have been hard-pressed to know Haslet-Davis had ever been injured—her footwork was dazzlingly precise, meticulous, elegant.

The performance—a video of which has been viewed more than 2.5 million times online—was a testament to the healing power of high technology. It was also a high-profile showcase for the BiOM T2, the successor to the iWalk BiOM. The T2 uses the same basic architecture and algorithms as the original device, but the battery is lighter and longer-lasting and the motor more reliable. This fall, BiOM will release an Android application that will allow users to monitor steps and battery life and maintain some control over the propulsion levels. “If you’re just sitting in the office, you might dial it down a bit,” Charles S. Carignan, BiOM’s CEO, told me. “But let’s say you want to go out and climb a few steep hills. Well, then you’d probably want some extra power.”

BiOM says it has distributed more than 900 BiOM ankle systems, with nearly half going to veterans such as William Gadsby. Paul Pasquina, a colonel in the Army Medical Corps and chief of the Integrated Department of Orthopaedics and Rehabilitation at Walter Reed Army Medical Center, calls the technology “revolutionary.” Non-powered prostheses, he said, cannot mimic the natural gait, and users try to compensate with other muscle groups. That can lead to pain, degeneration, osteoarthritis and severe musculoskeletal and cognitive stress. Bionics, Pasquina said, can, when combined with aggressive rehabilitation, better compensate for a lost limb and improve balance and function. “The more you’re able to simulate natural human motion, the better for the individual,” Pasquina said. “In that sense, I believe, the technology speaks for itself.”

But a BiOM T2 lists for about $40,000, and Herr has had trouble stirring up the same enthusiasm among civilian insurers. Last year, he and several of his patients testified in front of Congressional panels to persuade Medicare administrators to provide bionic limbs for amputees. In part, their argument centered on the preventive benefits of a BiOM. Sure, the device is expensive. But isn’t the cost justified if it saves insurers money on painkillers, osteoarthritis treatments and other measures needed to treat the side effects of traditional prostheses? Ultimately, a Medicare code was issued; a handful of workers’ compensation providers have also agreed to pay for the BiOM. Still, wider acceptance by the insurance industry remains elusive.

David Conrod, a communications professional who lost his leg decades ago in an industrial accident in Canada, was one of the patients to testify with Herr. His BiOM system is paid for by a workers’ compensation plan, but he said he expects that more health insurers will come around to the idea of bionic prosthetics. “People default to what they know, and they don’t know bionics yet,” he said. “There aren’t millions of people on these products. But I think this is such a value-add for amputees...that it will become common. Many, many people will wear legs like mine.”

***

And yet to spend any time with Hugh Herr is to understand that he is already thinking beyond a world where bionics are used only to enable wounded people and toward a future where bionics are an integral part of everyday life. In less than 20 years, he told me, “it will be common to step outside and see someone wearing a robot, meaning a bionic of some kind.”

One afternoon at the biomechatronics lab, I watched a group of Herr’s doctoral students test an exoskeletal leg brace on the treadmill. The device, constructed from fiberglass struts, is intended to supplement the wearer’s capability—a construction worker might don one to lift a heavy load, or a Marine might wear one to walk an extra 50 miles with a pack on her back. Lately, the lab has become a veritable factory of similarly high-end bionics, from robotic limbs that can “read” the ground ahead and adjust power input and angle accordingly, to the pieces of a fully autonomous exoskeleton—an invention Herr and his team unveiled earlier this year to much fanfare in the Journal of NeuroEngineering and Rehabilitation.

“When you view the human being in terms of its locomotory function, some aspects are quite impressive,” Herr said. “Our limbs are very versatile: We can go over very rough terrain, we can dance, we can stand still. But...our muscles, when they do positive work, 75 percent is thrown out as heat and only a quarter is mechanical work. So we’re pretty inefficient, we’re pretty slow and we’re not terribly strong. These are weaknesses we can fix.”

The next frontier for bionics, Herr believes, is neurally controlled devices. For now, the BiOM works independently from the brain, with an algorithm and a processor governing the prosthetic’s movement. But Herr is working on sensors that can tap into the body’s nervous system—eventually we could see a prosthetic controlled by the brain, muscles and nerves.

Of course, as Herr is quick to acknowledge, it is impossible to think of the mating of flesh and robotics without thinking of the dystopian fiction of Philip K. Dick or movies like the Transformers series, where machines have eclipsed humanity. “The fear is that the mating will be such that the human, however that’s defined, is no longer in control,” he allowed. Herr recently presided over the founding of the Center for Extreme Bionics at MIT, which will explore more experimental forms of robotic engineering. As part of the center’s activity, he hopes to convene a group of lawyers, scientists and philosophers to help guide “policy around augmentation.”

“We’re going to advance technologies in this century that just fundamentally change human capability,” he told me. “And there’s real beauty in that—there’s real advantage to humanity in that you can eradicate disability. There’s also real risk, so we need to develop policy commensurate with these new technologies. And in my view the drivers of policy around augmentation technology should focus on enhancing human diversity.”

Eventually, he suggested, prosthetics could become a lifestyle choice, like a nose piercing or a tattoo—“where our bodies are an art form and we can just create any type of body. Then we see a death of normalcy, a death of standard views of human beauty. Then you walk down the street 50 years from now and it’s like the cantina scene in Star Wars. That’s what I want.”

***

On a humid day this summer, I met William Gadsby at a restaurant in Northern Virginia, where he now lives with his wife, Tatiana, who is a computer programmer, and their 5-year-old son. Four years ago, after much lobbying, Gadsby received approval from the Veterans Administration to join an early BiOM testing program for above-the-knee, or transfemoral, amputees. (The device had been used for below-the-knee amputees because the diminished gait of transfemoral amputees is significantly more difficult to compensate for.) Running a hand through his close-cropped blond hair, Gadsby recalled reporting to his prosthetist’s office for the fitting—a lengthy process where the BiOM’s firmware is synced to the user’s gait.

“I don’t think most ‘organic’ people, as I refer to them, understand the energy return they get from their feet,” Gadsby said. “But when you’re on that carbon-fiber foot...you’re using upwards of 100 percent more energy just to get around, and man, it hurts. It does. With the BiOM, it felt like I was going from using a bicycle to a Ferrari. I was getting energy return. I was getting propulsion. It felt real.”

I followed him out to the parking lot. Gadsby fished his carbon-fiber foot out of his backpack for me to hold. It was light, but when he told me to smack the sole against my palm, I saw what he meant—there wasn’t much give. “Now watch this,” he said, and took off across the pavement at an impressive clip, the BiOM pistoning away underneath him. He returned grinning.

“Now I can hike,” he said. “I can drive all the way to Florida. I can cart a bunch of heavy suitcases when we go on vacation. I can throw my son on my shoulders and walk around with him. I can be a dad. The bottom line is that I’ve always tried to make sure my wounds aren’t my family’s wounds. The BiOM allows me to do that.”