A replacement limb that moves, feels and responds just like flesh and blood. It's the holy grail of prosthetics research. The Pentagon's invested millions to make it happen. But it's been elusive — until, quite possibly, now.
The body's own nerves are arguably the biggest barrier towards turning the dream of lifelike replacements into a reality. Peripheral nerves, severed by amputation, can no longer transmit or receive any of the myriad sensory signals we rely on every day. Trying to fuse them with robot limbs, to create a direct neural-prosthetic interface, is no easy task.
But now a team of scientists believe they've overcome that massive barrier. Their research is still in the early stages. But if successful, it'd yield artificial arms and legs that can move with agility; discern hot from lukewarm from freezing; and restore even the subtlest sensations of touch.
"We think the interface problem is key to enabling the neuro-prosthetic concept," Dr Shawn Dirk, one of the researchers behind the finding, tells Danger Room. "And solving that is how we're going to give amputees their bodies back."
Dirk, alongside colleagues at Sandia National Laboratories, the University of New Mexico and the MD Anderson Cancer centre, set out to develop a synthetic substance that could act as a scaffold — that is, an artificial structure that can support tissue growth — successfully merging severed nerves with robotic limbs.
Of course, researchers have already made plenty of efforts to directly integrate nerves and prosthetics. But, according to Dirk, they typically "didn't use technology that was compatible with nerve fibres", which are tightly bundled and flexible. "Nerves need to grow and move around; they're not going to integrate well with a stiff interface."
Yes, the material comprising the scaffold had to be flexible and fluid, but it also needed to be extremely conductive. Nerve signals are highly localised, and also very, very subtle. An effective neural-prosthetic interface would need to transmit thousands of different signals per second to mimic the behaviour of a real limb and its relationship to the brain and body.
To create that ideal interface, Dirk and his colleagues developed their own biocompatible polymers, meant to mimic the properties of nerve tissue. The material is also porous, so that nerves can extend through it, and lined with electrodes, to vastly enhance conductivity.
When surgeons placed the scaffolds onto the severed leg nerves of rats, it didn't take long before the rats' own nerve fibres started to grow through the scaffold and fuse back together. Even better, the synthetic material wasn't rejected by the rats' immune systems.
"There was a very limited inflammatory response," Dirk says. "That's important, because we're looking for an interface that won't be rejected by the body. We want something that can last years, decades, and hopefully entire lifetimes."
The finding marks a huge, huge improvement over previous research efforts. Even Darpa, the Pentagon's far-out research arm and a leader in prosthetic science, couldn't seem to figure out a direct neural-prosthetic interface that was adequately sensitive and had a lifespan longer than a few months. In 2010, the agency asked for new research proposals that'd solve both those problems.
And while new prototype prosthetics have some incredible abilities, none of them include a direct interface. In fact, they've been designed to avoid one altogether. One Pentagon-funded project used "targeted muscle reinnervation surgery" to develop prosthetics that transmit signals from a bundle of nerves in the chest. Another, led by Johns Hopkins scientists, uses brain-implanted micro-arrays to transmit cues to an artificial limb.
A direct neural-prosthetic interface still remains years away. But if this polymer holds up in subsequent tests, it'll mean prosthetics far more lifelike than even the most impressive artificial limbs currently in development. Most importantly, in the words of Darpa, prosthetics hooked right into the nervous system "would incorporate the [artificial] limb into the sense-of-self."
Image: Sgt Ray Lewis/Bouhammer.com
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