A brain implant with mobile electrodes that can seek out and connect to individual neurons in the brain has enabled monkeys to regain control of a paralysed wrist.
The inventors of the implant say it could help people paralysed by spinal injuries to regain control of their limbs, or control robotic limbs.
The implant exploits the fact that even when the neural connection between a brain region and the muscles it controls is severed or damaged by, say, a stroke or spinal injury, the controlling neurons remain active.
For example, people living with quadriplegia who try to move their arm still generate arm-movement signals in the motor cortex of their brain, even after several years of paralysis.
Implants that use those signals have allowed monkeys to control mechanical devices, and a paralysed man to control a robotic arm and check his email.
Now a brain implant with moving electrodes has shown the potential of that approach to give people back control of their own limbs. It was created by physiologist Chet Moritz and colleagues at the Washington National Primate Research Center in Seattle.
Previous implants collect signals from large collections of neurons, and need complex software to process them into a clean output signal.
Moritz's system, though, uses only 12 moving electrodes – just 50 micrometres wide – to seek out and connect to just a single neuron. This produces a much simpler and tidier output signal.
After being inserted into the brain's motor cortex, the device can sense where the strongest signal is coming from, and move the electrodes towards it.
Piezoelectric motors can move the 12 electrodes in small 1-micrometre increments and will back off when necessary to avoid damaging nerve cells.
Two macaque monkeys were fitted with the implants, and the roving electrodes were used to connect to single neurons that control wrist muscles. The signals picked up were carried to the muscle groups they would usually control using wires and electrodes.
The macaques were then anaesthetised to block the nerves that normally control the same wrist muscles.
The team found that the monkeys could learn to exploit the alternative connection provided by the implant to bypass the nerve block and contract their muscles as they tried to grab a tempting food reward.
Moritz hopes this approach could restore movement to humans who have lost control of muscles due to nerve damage. Implants like these could also control prosthetic limbs more precisely because they relay signals from carefully chosen neurons, rather than having software calculate a signal from recordings of many different cells.
However, a way must be found to keep the implant's fine electrodes in place in a moving patient, he says.
"Maintaining stable recordings for long periods of time is one of the remaining challenges in neural engineering," says Moritz. "But given that the users of this device would be patients confined to a wheelchair, their physical activity will necessarily be limited."
So far, even when used on highly active monkeys, it has been possible to maintain connections to stable neurons for up to four weeks, he says.