Microcoils May Improve Upon Electrodes in BCIs

The silicon and copper microcoil is 100 micrometers wide, about twice the diameter of a human hair. Photograph courtesy of Seung Woo Lee.

Researchers have improved upon the technology behind brain implants used to activate neural circuits responsible for vision, hearing, or movement. Such neural stimulation systems have been based on electrodes and are used to restore vision and hearing, to treat neurological disorders, and for brain-computer interfaces (BCIs) that can give people with paralysis the ability to communicate or move objects. The advance involves running a current through an insulated microcoil-similar in size to existing electrode implants-to generate magnetic fields that activate neurons through electromagnetic induction, which allow researchers to target specific patches of cells and overcome some limitations of electrode-based neural stimulation.

The device, developed by the study's lead author Seung Woo Lee, PhD, and senior author Shelley Fried, PhD, of the Massachusetts General Hospital department of neurosurgery, takes advantage of the fact that the passage of electric current through a bent wire will induce a magnetic field. The coil they designed, while similar in size to electrodes, generated magnetic fields in excess of the thresholds required to activate neurons. Neuroscientists previously thought that a coil small enough for implantation in the brain wouldn't generate a strong enough field to activate neurons, Fried told IEEE Spectrum. The study is published in the December 9 issue of Science Advances.

Testing in brain tissue samples from mice revealed that the microcoils could activate neurons, and did so more selectively than is possible with metal electrodes. "Electrode-based neural stimulation devices, especially those that target the cortex, have several significant limitations. The environment within the brain can erode a metal electrode over time, and the brain's natural foreign-body response can lead to scarring, which can impede passage of electrical fields," said Lee.

Electric fields most effectively activate neurons when they are oriented along the length of nerve cells, but most implantable electrodes generate fields that spread uniformly in all directions. In contrast, magnetic fields extend in specific directions, allowing selective targeting of neurons with the same orientation while simultaneously avoiding the activation of other neurons.

"Our next steps will be to continue improving coil design to reduce power and enhance selectivity, to confirm that the enhanced effectiveness of these coils will persist over time," said Fried, who is also an associate professor of neurosurgery at Harvard Medical School. "More stable long-term performance of these microcoils and the high-resolution signals produced by ever greater selectivity in neuron activation would significantly improve currently available neural prostheses and open up many new applications," including sensory feedback in BCIs that allow mind-control of robotic limbs. A user of a BCI to move a robotic arm could have implanted electrodes to record movement commands from the motor cortex and implanted microcoils that provide tactile feedback from the metal fingers. Using the example of picking up glass of water, Fried said, "The user needs to know that the thumb has a good grip, but the fingers need to press harder."

Editor's note: This story was adapted from materials provided by Massachusetts General Hospital.