Louisiana Assistive Technology Access Network

Bionic Engineer

Monday, 02 May 2011 13:21

Hugh Herr's PowerFoot could make life easier for countless amputees. Herr should know; he wears two of them.

Charles J. Murray, Senior Technical Editor, Electronics & Test -- Design News, November 29, 2010

Hugh Herr was 17 years old when he designed his first prosthetic leg, just months removed from a brush with death and fresh off a grueling period of rehab. At the time, Herr was an unlikely engineer. An academic underachiever in high school, he'd been more attracted to mountain climbing than to physics. But in June of 1982, wearing two crude prosthetic limbs, he got the itch to resume some semblance of his old life.

"I wanted to design limbs so I could return to mountain climbing," he recalls.

So Herr, with only a brief stint in vocational school and a rudimentary knowledge of tool and die-making to serve as a starting point, launched a career. He began building prosthetic limbs. He earned degrees from MIT and Harvard. He climbed, designed, and then climbed some more, in an interwoven pattern that continues today.

And now that pattern may be starting to pay off, not only for Herr, but for countless users of prosthetic limbs. His PowerFoot, being commercialized by his start-up firm, iWalk, offers an alternative for below-knee amputees who want to get back to an active lifestyle. The foot enables them to climb stairs, traverse ramps, walk fast and exert a level of force that's comparable to that of a biological ankle. And it does it all while enabling users to walk with a normal gait.

Moreover, Herr's twin passions for mountain climbing and engineering design have led him to conclude that technology can and will conquer all disabilities.

"I've designed limbs for vertical rock and ice climbing," says Herr, who now serves as an associate professor of media arts and sciences at MIT. "Through that process I've actually climbed at a more advanced level with artificial limbs than with biological limbs, and that has inspired me. I've realized that technology can fundamentally change human capacity."

Indeed, Herr's efforts to change human capacity are gaining some traction. He designed a prosthetic knee that's been commercialized by an Icelandic company, Ossur Inc. He has also created elastic shoes to boost aerobic endurance for walkers and runners, has built load-bearing exoskeletons for soldiers, and has helped patients disabled by strokes, cerebral palsy and multiple sclerosis. Plus, his PowerFoot is being used by about 30 individuals, including war veterans and Herr himself.

"We get comments from users who say, ‘I've got my leg back,'" Herr says. "Some say their biological leg can't keep up, especially when they are walking up steep hills and steps."

Inspiration from Adversity

Herr might not be dreaming up such breakthrough technologies, however, if not for his brush with death in 1982. At the time, the 17-year-old rock-climbing prodigy was already considered to be one of the best in the United States, having scaled the face of an 11,000-ft peak in the Canadian Rockies at age 8.

But on a January day in ‘82, Herr's life took a disastrous turn. While scaling Mount Washington in New Hampshire, Herr and a friend made the mistake of continuing toward the summit in white-out snow, without a compass or an adequate map. They found themselves trapped in zero-visibility conditions, barely able to see one another. Quickly, the temperatures on the mountain dropped to below zero, while they struggled to locate a shelter.

"It was the most extreme form of bushwhacking," Herr recalls. "The average depth of snow was to the waist, sometimes to the chest. Even though we were both at an Olympic level of fitness, we could barely walk a mile a day."

Nevertheless, in four days they trudged to within a few miles of a roadway, despite the fact that both were suffering from severe frostbite. When passersby found them, Herr's health had been irreversibly affected. In March, both of his legs were amputated below the knees.

By June of '82, however, Herr was again ready to do battle with the mountains. Because he'd taken vocational classes in high school, he was able to use his knowledge of tool and die-making to fashion specialized limbs for rock and ice climbing. He began climbing again while still in the rehab center and, through it all, developed a deeper understanding of the power of technology. "I realized that the artificial part of my body is a blank slate from which to create," he recalls. "The limitations were really limitations of technology. My biological body was not disabled. My artificial limbs were disabled."

Moreover, the "new" Hugh Herr viewed education differently. Up to that point, he says, he'd been a "C/D student." Now, he knew he would have to change. "Most of the men in my family worked construction of some kind," he says. "And I knew that it would be too uncomfortable for me to be on a construction site for very long. All the elements added up to my desire to go to college."

Herr earned a bachelor's degree in physics from Millersville University of Pennsylvania, followed it with an M.S. in mechanical engineering from MIT, and then got his Ph.D. from Harvard. "I started to study math and computer science and physics and developed an absolute passion for those disciplines," he recalls. "That passion more or less replaced my passion for mountain climbing."

A New Achilles

That passion has also led Herr to an understanding of the human ankle that didn't exist previously.

Herr's knowledge of the ankle started growing in 2003, shortly after he began analyzing the biomechanics of it. His study led to an important conclusion: springs - in the form of ligaments and tendons - are critical to the function of the human leg.

"The Achilles tendon is the dominant energy storage element in the ankle," he says. "It allows the calf muscle to exert very high power on the ankle joint." Herr says that while you're walking, the ankle produces by far the highest power of any joint in the body, typically reaching about 700W in a grown man.

Knowing that, Herr reasoned that amputees needed a power boost. He supplied his prosthetic ankle with a so-called "series elastic actuator" - that is, a small dc electric motor, a transmission and a series spring. Together, the three elements work with a carbon composite spring foot to provide the power that would otherwise be missing from an amputee's step.

The PowerFoot, as it's known, accomplishes that by controlling the compression of the series spring. During operation, its dc motor produces torque, then transmits it through the gearhead of the transmission, where it is converted to a linear force (in a recent embodiment, this is also accomplished with a ball screw). The linear force, in turn, compresses the series spring, storing energy in the ankle to provide stepping power. To flex the foot, the direction of motor rotation is reversed.

Herr says he modeled the prosthetic ankle on the concept of the catapult. To rely solely on a motor to provide power would have been a mistake, he says. Any such motor would have been enormous. But by using the spring to store energy, he minimized the motor size and more effectively mimicked the operation of the biological Achilles tendon.

"When the foot is in contact with the ground, the ankle behaves like a spring," Herr says. "When the foot is off the ground, it optimizes its position for the next foot strike."

From an engineering standpoint, the trick was to enable the artificial foot to do the right things at the right times. Herr and his team at MIT's Media Lab did that by employing a six degree-of-freedom inertial measurement system. Using 12 three-axis accelerometers from Freescale Semiconductor, the ankle estimates the angle of the foot and of the ground's surface.

"So if you're walking on an incline, the ankle will orient itself to the incline," Herr explains.

The accelerometers accomplish all that by measuring the acceleration of gravity at multiple points and then sending analog signals to one of five microcontrollers. Similarly, they also detect compression and de-compression of the series spring, essentially by determining if two accelerometers are getting closer together or farther apart. All of the power for the system is supplied by a lithium-ion phosphate battery from A123 Systems, an MIT spin-off that makes batteries for electric cars.

"The inertial measurement system acts as the eyes," Herr says. "It knows where you are in absolute space, and it uses pattern recognition to get some idea of what the terrain looks like."

Herr says he deliberately chose off-the-shelf components for his ankle. "I didn't want this to be a science project," he says. "I wanted it to benefit people as quickly as possible."

Elimination of Disability

Widespread adoption of Herr's PowerFoot will depend in part on how the device is viewed by the insurance industry. With conventional prosthetics already costing in the thousands of dollars, many prospective PowerFoot users will need an insurance boost to help with the additional costs of Herr's device.

Early indications are that the PowerFoot will offer a significant advantage, however, especially for amputees who want to remain especially active. "A conventional prosthesis is a carbon-fiber leaf spring - as you step, it compresses and returns about half the energy you need," says Matt Williams, a biomedical engineer who is a Post-Doctoral Fellow at Providence VA Medical Center. "The PowerFoot has the carbon-fiber foot for shock absorption, but it actually goes through a step-cycle. When you go to ‘toe-off,' the motor engages and actually gives you a little boost."

Herr claims the best conventional prosthetic legs deliver one-eighth as much power and 100 percent less energy than the PowerFoot. "This is a whole new day," he contends. "It's like comparing a broken down Chevy to a Maserati."

Whatever success is achieved by the PowerFoot, however, Herr isn't finished innovating. As director of the Biomechatronics Group at MIT's Media Lab, he has worked with the Defense Advanced Research Projects Agency (DARPA) on exoskeletons for the military. He foresees a day when individuals will wear exoskeletons, using them to run to work while barely breaking a sweat. He predicts amputees will don robots and control them with neural interfaces. He even foresees a day when robotic carpets and furniture will incorporate the ability to soften the blow when the elderly fall. Ultimately, he believes new technologies will eliminate disability by the end of this century.

"Incredible strides are being made in depression, social and emotional conditions, epilepsy, hearing and vision impairment," Herr says. "We're already seeing the elimination of disabilities. It's hard, with a straight face, to label someone like me as disabled."

References: IEEE, MIT Media Lab - http://biomech.media.mit.edu/publications/AuICRA2006.pdf