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CHICAGO — Zac Vawter, a software engineer who lives in Yelm, Thurston County, knew about advances in bionic technology when a motorcycle wreck led to the amputation of his right leg just above the knee in 2009.

As doctors at Harborview Medical Center in Seattle battled for three days to try to save his leg, Vawter asked about the method that uses the mind to move a prosthetic limb. The technology previously had been used only in arms.

Four years and an $8 million grant from the U.S. Army’s Telemedicine and Advanced Technology Research Center later, Vawter, 32, is considered the “test pilot” of the bionic leg that can tackle slopes, stairs and in-chair movement markedly better than existing devices.

Rather than tissue and bone, his 10-pound right-leg prosthetic is made of aluminum, two motors, sensors and a powerful computer system. The bionic leg, which could transform the lives of wounded veterans, accident victims and the elderly, can basically read Vawter’s mind.

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A team of researchers led by Levi Hargrove at the Rehabilitation Institute of Chicago’s Center for Bionic Medicine reported its results with the novel prosthetic in the current issue of The New England Journal of Medicine. The institute unveiled the latest prototype leg this week, in conjunction with the journal report.

“In my mind, it’s still the same thing in terms of moving my ankle down or up, or extending my leg forward or back,” said Vawter, who spends most of his days using a typical prosthetic at home in Yelm but travels to Chicago several times a year. “It’s just walk like I would normally walk. It’s not special training or buttons or tricks. That’s a big piece of what I think is groundbreaking and phenomenal about this work.”

Additional refinements are needed to make the thought-controlled bionic leg commercially viable, Hargrove said in a telephone interview. Vawter is allowed to use the machine only a week at a time during his Chicago visits.

The new leg allows Vawter to seamlessly transition between walking and standing, with the biggest difference showing up when he is climbing stairs. With a standard prosthetic leg, Vawter always steps up first with his healthy left leg, then pulls the right leg along. With the thought-controlled leg, he is able to walk foot over foot, he said. Someone watching him climb wouldn’t know he had a prosthesis based on his gait, Hargrove said, though they may hear the motor whirring.

“It’s still a prosthetic, but it’s in-between the leg I wear every day and prior to amputation,” said Vawter, who can’t yet jump to the rim of a basketball net with the robotic leg, as he could before the amputation. “It’s a dramatic improvement over my current prosthetic, but there is still a long way to go.”

The rate of errors, including the risk of falls, was shaved to just 1.8 percent with the new device, down from 12.9 percent with the standard robotic leg prosthesis.

The new device may be available within three to five years for the 1 million Americans with leg amputations, Hargrove said. The approach may benefit the more than 1,600 amputees injured while serving in the U.S. military, many of whom are young and want to continue active lives. It may also help older people who want to remain at home, particularly those who have trouble standing and caring for themselves because of amputations, he said.

Current prosthetic legs cost a few thousand dollars, with robotic devices as much as $100,000, Hargrove said. There is no price target yet for the thought-controlled bionic leg, he added.

Col. John Scherer sees the technology’s potential. From Fort Detrick, Md., he directs Army research intended to restore as much function as possible to veterans suffering from traumatic injuries involving limited mobility, or vision and hearing loss.

Of the 1,600 amputees from the Iraq and Afghanistan wars, Scherer said, about 80 percent of that population has lower-extremity amputations. “We’re obviously funding projects … to make … advancements so our injured service members can resume the life that they want to live.”

Vawter’s transformation began with an accident. In 2009, when he sped a Suzuki motorcycle too quickly through a curve on a rural Washington road. The resulting crash forced the amputation.

He became the test pilot of the device through his surgeon, Dr. Douglas Smith of the University Washington Medical Center, a contributor on the paper and an expert on the use of targeted muscle reinnervation. He performed Vawter’s amputation, and rewired his severed nerves into surviving leg muscles, laying the groundwork for advanced prosthetics. Within months, Vawter could flex his upper leg when prompted to try to plant or rotate his missing foot. His brain was sending clear, electrical signals to a body part that didn’t exist. Vawter began working with the rehabilitation center in December 2010.

Before he could strap on the bionic lower limb, engineers in Chicago had to “teach” the prosthetic how to read his motor intentions from tiny muscle contractions in his right thigh. At the institute’s Center for Bionic Medicine, Vawter spent countless hours with his thigh wired up with electrodes, imagining certain movements on command with his missing knee, ankle and foot.

Using pattern-recognition software, engineers discerned, distilled and digitized those recorded electrical signals to catalog a repertoire of movements. The prosthetic could thus be programmed to recognize the subtlest contraction of a muscle in Vawter’s thigh as a specific motor command.

“It’s a fascinating world,” Vawter, who works at Engineered Software in Lacey, said while testing the prosthesis inside a 14th-floor laboratory at the rehabilitation institute. “It’s neat to see the way that they’re using software, algorithms and machine learning to make this work.”

The prosthetic’s knee and ankle joints are powered by two small motors. Thirteen mechanical sensors are embedded along the prosthetic, including an accelerometer, gyroscope and sensors to determine how much weight Vawter puts on the leg or how fast he’s moving.

Small metal electrodes attach to Vawter’s remaining leg to catch his neural signals.

Vawter used an earlier prototype of the leg to climb 103 stories of the Willis Tower in Chicago last year. The latest version has substantial software improvements that allow him complete independence and a seamless range of motion directed by his thoughts.

The prosthetic used at Willis Tower last year only had modes for walking and stair climbing and required a team of engineers to monitor the device, Hargrove said.

“This is a huge breakthrough,” he said. “It’s no longer, OK, we’re giving you the choice to walk or climb stairs. We are giving you the choice to do whatever you want to do. That wasn’t possible (before).

“He has freedom. … He could walk out of the building with it and go until the battery died. There isn’t anything stopping him from doing that other than maybe security at the front desk.”

Vawter said he had “fallen down a whole bunch of times” while wearing his everyday prosthetic, but not once while moving around on his bionic leg.

He said he could move a lot faster, too, which would be helpful for keeping up with his 5-year-old son and 3-year-old daughter. But first, Vawter added, he needs to persuade Hargrove’s team to let him wear it home.

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