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.”