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A Robot That Walks the Walk

From Aliens to Avatar, exoskeletons have been storming around for so long and with increasingly compelling CGI effects that a person could be forgiven for forgetting they’re still mostly the stuff of science fiction. Mostly, anyway. An interdisciplinary team out of the Harvard School of Engineering and Applied Sciences—including roboticists, experts in biomechanics and apparel design, and mechanical and electrical engineers, Ph.D. candidate and Olin alum Brendan Quinlivan among them—have made significant steps toward bringing a wearable robot to reality, and perhaps to market. The January 2017 issue of Science Robotics published their latest study, in which they isolated the overall energy savings that result from the assistance their so-called “exosuit” provides.

The system is made of four simple parts: An actuator unit, essentially a motor and a pulley, is either attached to a person’s waist or worn as a backpack. The actuator attaches to a bowden cable, or an ordinary bicycle brake cable that transmits the force to the lower joints on the limbs. And a soft textile interface provides attachment points to the cable and works in parallel to the body’s muscles. The use of textile also distinguishes this system from the rigid linkages of a traditional exoskeleton (picture Iron Man here), which apply torque to the joints. The mass of those systems combined with the “kinematic constraints of movement from the rigid linkages end up increasing the metabolic costs,” Quinlivan says. For an average healthy person, in other words, the balance of energy required to wear an exoskeleton exceeds the gains it can provide.

For Quinlivan and his team, finding that sweet spot—the balance between the amount of assistance the suits offers and the metabolic cost of wearing it—is the goal. The basic system underneath it all is quite simple, Quinlivan says: “You retract the cable in the motor by turning the pulley, and that shortens the cable near the ankle joint. The theory is that if we retract the cable down there, we can generate a force—the shortening between those points. That force is in parallel to the underlying ankle muscles, and therefore we reduce the torques and the powers that those ankle muscles have to produce.” Ultimately, the suit reduces the wearer’s energy consumption, as they need to produce less of that power themselves.

Research labs have been trying to reduce the mass of these robotic systems for years, to reduce the associated metabolic costs. A team in Belgium accomplished the first reduction in 2013, and Quinlivan says a handful of others are achieving similar results. In their January study of seven healthy individuals, the Harvard team demonstrated that increasing the assistance applied to the ankle joint reduced the subject’s energy use up to 23 percent, compared to walking with the exosuit on but powered off. “To our knowledge, the 23 percent reduction found in this study is the highest reduction that’s been shown with a tethered exoskeleton or exosuit,” Quinlivan says.

The Harvard team is working toward two applications for their suit: One is for healthy individuals walking long distances with a heavy load on their back, such as soldiers or firefighters, for example. DARPA’s Warrior Web program funds part of that initiative. In the second application, similar technology could improve the gait of stroke patients or those who suffer from multiple sclerosis, for example. “In that case, instead of trying to reduce metabolic cost, we’re trying to help the individual walk with a more natural gait,” Quinlivan says. For a stroke patient who can’t lift their toes, “We provide a small amount force to pull the toes up during the swing phase of the gait cycle, which is when the foot is off the ground.” That project has been in the works for about two years; in partnership with ReWalk Robotics, they hope to make this technology available to rehab clinics. In time, they may be able to tailor it to assist patients with Parkinson’s disease, or cerebral palsy, or the elderly.

The exosuit’s inherent flexibility and mobility could lead to applications for a variety of patient populations in the future. Whereas some of the most promising exoskeletons rely on pneumatics and are therefore tethered to a wall, “One of the most promising things about our suits is that they can be made mobile,” Quinlivan says. That distinction may be what edges them one step closer to commercialization.