But the trick to sending alternating current through the skin that is powerful enough to reach the spine, yet painless, is disguising it in the right overlapping frequency, or carrier wave. At low frequencies, like the kind that runs through your 60-hertz wall outlet, a 10-milliamp current zaps nerves in the skin that communicate pain—it hurts. But at 10 kilohertz, electricity slips by these nerves unnoticed. Inanici used a tablet to control each stimulator’s current, and found she could crank it up to 120 milliamps and keep it noninvasive. “Most people tolerate it easily,” Inanici says. “It’s like a buzzing or tingling sensation.” (For the trial, they kept the level between 40 and 90 milliamps.)
Then, once the researchers had wired up the volunteers, the participants resumed their hand activities. For one person, the effect of that spinal cord tingling was immediate. He could squeeze a ping pong ball between his index finger and thumb and drop it into a bucket—moving his digits for the first time since his injury. “The immediate response in the very first session was really unexpected,” says Inanici. “It was thrilling.”
Others, including Owen, improved slowly but noticeably. “It was not a light switch for me, but by the second week, I could stack more blocks,” says Owen. “And I wasn’t fast, I wasn’t amazing. But it was a lot better.”
After four months of training—including two with stimulation—every person more than doubled their pinching strength. Several doubled their grasping strength. Inanici says one person regained enough dexterity to drive without an assistive device. Another could handle their catheter well enough to insert it on their own. Owen decided to try painting. At the beginning of the experiment, she recalls, “I was like, ‘I can kind of hold a brush and some paints, and why don’t I give this a go?’” So she ordered a paint-by-numbers kit of a portrait of a dog. “It was kind of hard, and I don’t think it turned out very well, but I’m still really impressed by it,” she says.
Still, when she brought it to the lab, “that was really an emotional moment for me,” Inanici says. “Even a small contribution to people’s well being and quality of life is really so rewarding.”
Why did it work? Inanici thinks it’s because the device made more movement possible, which made rehab easier, which made more movement possible—and so on in a cycle. Renewed activity enticed the neurons to build stronger interconnections, demonstrating neuroplasticity. “By practicing the movement again and again, these activated neural structures become stronger,” says Inanici. “They connect with each other better. So, after a while, they do not need any external stimulation anymore.”