Lasers Power Pentagon’s Next-Gen Artificial Limbs

By Katie Drummond

The Pentagon’s already got brain-controlled prosthetics, and they are a major improvement over old-school artificial limbs. The devices are far from perfect, however. They rely on metal implants, which aren’t compatible with the body’s tissues, and they can only transmit a few signals at a time — turning what should be a simple movement into a Herculean task.

Now, Darpa-funded researchers are convinced they’ve found a way to make prosthetics truly life-like: laser beams.

A team led by experts at Southern Methodist University is making swift progress towards prosthetic devices that rely on fiber-optics, and would offer a wearer the kind of seamless movement and sensation experienced with a flesh-and-blood limb.

“Already, we’re tantalizingly close,” Dr. Marc Christensen, the program’s leader, tells Danger Room. “We haven’t seen anything that’s been a deal-breaker yet.”

It all started in 2005, when researchers at Vanderbilt realized they could trigger a nerve using infrared light. The finding catalyzed a handful of research projects investigating the prospect of laser-powered prostheses, and Darpa last year doled out $5.6 million for the creation of the Neurophotonics Research Center, led by SMU, for the development of prosthetic devices powered by infrared lasers.

A fiber-optic prosthetic for a human patient would likely be a cuff — loaded with optical cables — affixed at one end to a prosthetic, and attached at the other to the body’s severed nerves. Those are a decade off, but already, researchers say they’ve nearly climbed the project’s biggest hurdle: Developing sensors with enough sensitivity to detect — and trigger — the infinitesimally small perturbations of a single activated nerve.

That’s thanks to Professor Volkan Otugen, director of SMU’s Micro-Sensor Laboratory. He developed entirely new micro-sensors for the project. The soft spheres are a few hundred microns in diameter — small enough to fit hundreds onto a single optical fiber — and the consistency of Jell-O. That unique composition would make the sensors compatible with the body’s tissues, unlike metal implants that can cut into delicate tissue, wear down within years, and risk being rejected by the body. And one optical fiber can transmit a ton of signals at a single time and even stimulate a single neuron, making a bundle of them able to transmit exponentially more signals, much faster, with way more specificity, than systems relying on electrodes.

Let’s say you were trying to grab a coffee cup. Even a bleeding-edge, brain-based prosthetic would only offer a few degrees of movement, and because electrical signals are relatively slow, you couldn’t move as quickly as someone with a real arm. “It would be akin to bench-pressing 250 pounds to lift a mug,” Christensen says.

With a fiber-optics prosthetic, touching the cup would catalyze optical fibers to pulse a specific message out of infrared light through the hundreds of micro-sensors, which would stimulate sensory nerves that could then — as they do with a flesh-and-blood arm — transmit the specific, nuanced sensory message to the brain. The brain would then send feedback to the arm’s motor nerves, which would trigger specific movements in those trusty micro-sensors. Those movements change the pattern of infrared light circulating in and out of the sensors, which triggers highly specific muscle movement.

“It’s the same way the internet put thousands of phone calls on one wire,” Christensen says of the method, which he expects to test in mammals next year. “Right now a prosthetic can pick up or transmit maybe two signals. We think we can turn that number into thousands.”

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