Tiny Implantable "Microcoils" in the Brain Activate Neurons Via Magnetic Fields

Precise stimulation could be useful for visual prosthetics or brain-computer interfaces

3 min read

Eliza Strickland is a Senior Editor at IEEE Spectrum covering AI and biomedical engineering.

The tiny microcoil that stimulates brain cells via magnetic fields is shown on top of a penny for size comparison.
Image: Seung Woo Lee

The neuroscientists and doctors who are pioneers in the new medical field of brain hacking (no, that’s not an official term) would like to have precise control over the 86 billion neurons in the human brain. All those neurons turn on and off in complex patterns to manage the body and behavior. The brain hackers would like to treat the brain like a control panel, flipping certain patches of neurons on or off to help with disorders like epilepsy and depression. 

But activating specific neurons isn’t exactly easy. The typical technique for neural stimulation uses implanted metal electrodes that send current through surrounding brain cells, but these activate relatively broad swaths of neurons, and scar tissue that gradually forms around the metal can block the flow of current. A new technique called optogenetics enables researchers to control individual cells with flashes of light, but that requires somehow getting a light into the brain

A tiny “microcoil” that could be inserted into the brain offers a new way. Running a current through the insulated coil generates magnetic fields that activate neurons through electromagnetic induction, and the shape of these fields enables researchers to target specific patches of cells. What’s more, the magnetic fields would pass easily through any scar tissue that might build up.

Diagram shows the microcoil inserted into the brain, with magnetic fields passing through scar tissue.

Image: Seung Woo Lee

In some ways, this technique is similar to a magnetic field therapy already in use for depression. In transcranial magnetic stimulation (TMS), a big coil is placed on the patient’s scalp to activate neurons in parts of the brain that regulate mood. “One of the big reasons people like TMS is that it’s non-invasive,” says Shelley Fried, who led the new research at his neural prosthetics lab at Harvard Medical School. “But it’s hard to activate the neurons accurately from so far away.”

Neuroscientists previously thought that a coil tiny enough for implantation in the brain wouldn’t generate a strong enough field to activate neurons, Fried tells IEEE Spectrum. His team proved the conventional wisdom wrong. 

The researchers used microfabrication techniques to make a microcoil of silicon and copper that measures just 100 micrometers wide—about twice the diameter of a human hair. (That’s comparable in size to existing electrode implants.) They then showed that this microcoil, with its sharply bent copper microwire, generated a sufficiently strong field around the bend in its tip to do the job. And by inserting the probe alongside certain vertically oriented neurons in the cortex, they could position the tip to activate just those neurons, and not activate the horizontal branches of neurons extending from other regions. 

Diagram shows microcoil implanted in brain generating an electric field that selectively activates vertically oriented neurons.

Image: Seung Woo Lee

Fried and his colleagues describe the microcoil in a paper in Science Advances. They tested the hardware in anesthetized mice, implanting the microcoils in the part of the motor cortex that controls the whiskers. Sending a current through the microcoil reliably caused the whiskers to twitch.

One expert in the field praised the microcoil’s miniaturized design and its ability to produce highly focused fields, but questioned its advantages in producing real clinical results. “Comparisons made so far show similar responses to electrical stimulation, which is the gold standard,” says Harbaljit Sohal, an investigator at the Feinstein Institute for Medical Research, in Manhasset, Long Island, N.Y., who has microfabricated flexible electrodes for neural implants. “There needs to be a clear benefit of the technology over electrical stimulation before groups will start to adopt the technology.” 

Fried says the microcoil could be particularly useful when doctors need very accurate stimulation, and cites visual prosthetics as one example. The visual prosthetic is a cool idea whose time may have come: In October, the company Second Sight implanted a stimulator in a blind person’s visual cortex in a first-of-its-kind experiment. To send information about very specific parts of the visual field, such a device must activate very specific parts of the visual cortex, Fried says. “If you activate them all, your brain sees a blur rather than fine detail.”

He also mentions sensory feedback in brain-computer interface (BCI) systems that allow mind-control of robotic limbs or other external machines. If a paralyzed person is using a mind-controlled robot arm to pick up a glass of water, they could have both implanted electrodes to record movement commands from the motor cortex and implanted microcoils that provide tactile feedback from the metal fingers. “The user needs to know that the thumb has a good grip, but the fingers need to press harder,” Fried says, noting that a microcoil could send different signals to the patches of cortex responsible for those different digits. 

For their next steps, the Harvard researchers will implant microcoils in monkeys’ brains and see if they can reliably stimulate specific patches of neurons while the animals are awake. They’ll also test the microcoils’ impact on slices of human brain tissue in the lab. 

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