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A Budget Exoskeleton Allows Paraplegics To Walk—For The Price Of A Car

The Phoenix lets paraplegic people sit, stand, and walk. It costs just $40,000. Here’s how the designers pulled it off.

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In 2005, Steven Sanchez was trying to do a flip off a BMX dirt ramp when he was paralyzed from the belly button down. 11 years later, with no miracle surgery to speak of, he stands like any other tourist in line at the Vatican.

"I had this awesome robotic suit on, and nobody cared," he says. "They just waited for me to move up like everyone else moved up." It was a moment of incredible, touristy normalcy, provided by a bit of practice—and the Phoenix exoskeleton.

Created by the Berkeley-based company SuitX, the Phoenix is a 27-pound, $40,000 robot frame that can allow paraplegic people to walk. Even still, its specs might not sound so impressive on paper. It can only cruise at 1.1 mph on level ground. Wearers still need to use crutches or a walker to balance. But costing two to four times less than competitors like ReWalk, it’s amongst the lightest exoskeletons in the world. And maybe most importantly, "it feels like you’re actually walking," Sanchez says. A long time collaborator on the Phoenix project, he says competitors made him feel like a robot had hijacked his lower half.

How is this possible, that an exoskeleton came around that’s lighter, cheaper, and better feeling?

"It’s really not much about the power, it’s about cleverness," says SuitX CEO Dr. Homayoon Kazerooni. "We've designed this machine with minimal hardware that just allows for walking and nothing else."

Building The Bare Minimum Product

Kazerooni is a UC Berkeley professor and director of the Berkeley Robotics and Human Engineering Laboratory. Before founding SuitX with a group of his PhD students in 2013, he founded exoskeleton company Ekso Bionics, and worked on high-profile projects like the military’s HULC suit, which allows soldiers to carry 200 pounds without feeling a burden.

What makes makes the Phoenix so different from its predecessors is its spartan design, born from biomechanics rather than the practices of industrial robot construction and the benchmarks of military contracts.

As Kazerooni explains it, if you’ve ever seen a video of something like MIT's Cheetah robot, you’ll notice that it runs almost like a linebacker, with a strong, stocky presence. That’s because it’s loaded with virtual muscles, motors at every hinge point, in a design overkill approach he likens to a 500 horsepower car that you can’t steer—while a 200 horsepower car with traction control could get you around a track faster.

In an industrial context, this approach might make sense. A single model of Kuka robot arm may be used to paint car parts at one factory and lift a concrete staircase in another, so its joints have been loaded with powerful motors to allow for any style of movement. Similarly, robots built for DARPA’s robot challenge are asked to twist valves, walk through rubble, and climb stairs. The tasks asked of these machines necessitates strength and complication.

The problem is that with every motor you add to the system, the more infrastructure it needs: wires, batteries, sensors—all of which add weight, expense, and more potential fail points.

"My idea and philosophy is less is more," Kazerooni says. "We want to give a minimal amount to the user to be independent. We’re not saying this is a complete [mobility] machine, but if we come up with something that’s robust and simple—walks, stops, sits, and stands—that’s hugely enabling!"

Indeed, when I ask Sanchez why he’s most excited about the possibility of owning an exoskeleton, he says that getting around on his wheelchair is mostly fine. The world is built to accommodate the wheels. However, he’s plagued by dangerous pressure sores from sitting all day, forcing him to lift himself from his chair or lay down. "If I could stand up every half hour for 5-10 minutes, be able to walk around in a circle, get some sort of blood flow, that’s huge for my body," he says. "That’s hours more I could theoretically sit."

With that philosophy in mind, Phoenix has been built not to defeat obstacle courses, but to allow someone who is paraplegic and otherwise bound to a wheelchair to stand, sit, and walk on a level grade at 1.1mph.

By limiting the scope of what this new exoskeleton could do, the SuitX team was able to reassess its bionic knee. To do so, they took a closer look at how the human knee worked. Kazerooni is quick to caution that biomimicry isn’t an outright solution to building an exoskeleton, because our man-made materials simply can’t challenge 6 billion years of evolution. But while they couldn’t reconstruct a human knee’s twisting and leaping prowess without heavy machinery, they could mimic a simpler function: walking. The human knee doesn’t need to exert great force for walking on level ground—in fact, it can operate with almost no energy at all—so they constructed their knee to work similarly, like a relatively simple, locking hinge.

In turn, the only driving motors in Phoenix are at the hip joints. When the user hits a forward button on their crutches, their left hip swings forward. At this moment, the onboard computer signals the knee to become loose, flex, and clear the ground. As the foot hits, the knee joint stiffens again to support the leg. This computer-choreographed process repeats for the right leg.

As it happens, this hinged knee joint has another benefit. If the wearer hits something midstep, like a rock or a curb, a powered knee would blindly drive the leg forward anyway, likely leading to a fall. The hinge naturally absorbs such resistance and allows the wearer a chance to compensate.

Sanchez, who meets with the SuitX team each month to test the product, says that as a result, he’s only fallen two or three times over four years. It was never hard, and as he’s the first to admit, the BMX rider in his blood is constantly pushing the system to its limits.

Sci-Fi Mobility Need Not Be A Luxury

The Phoenix goes on sale next month for $40,000. Despite winning two National Science Foundation awards, and despite lowering the price of an exoskeleton from luxury car territory to the midrange sedan, Kazerooni is still unsatisfied with what he’s done.

"I still think $40,000 is way too expensive," he says. "You can buy a motorcycle for $10,000! All that technology in there? That’s amazing! You can buy a Hyundai car for $15,000!" Scale will inevitably lower the price, Kazerooni hopes, to the range of a motorcycle or low end car in the next two or three years. In the meantime, he’s working to adapt his system for children.

As for Sanchez, even if he acquires a Phoenix of his own, he still imagines that most of his day will be spent in a wheelchair, because he finds it so practical. But the exoskeleton has another big benefit besides mobility alone. It breaks what he calls the "bubblish" experience of his chair, a literal and figurative perimeter around him that makes people afraid to approach him and constantly apologize for being in his way, even when they aren’t.

"Having that separation is pretty damaging on all sorts of spiritual levels," Sanchez says. "Having the ability to come up and hug someone, without bending over or feeling awkward because of the wheelchair, it’s a huge difference in life."

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