Ultrathin, flexible brain implant offers better monitoring of seizures

November 15, 2011

Photograph of a 360-channel high-density active electrode array. Inset: a closer view showing a few unit cells. (Credit: Jonathan Viventi et al./Nature Neuroscience)

Researchers have developed a high-density, flexible brain implant that could one day be used to treat epileptic seizures more effectively.

They used an advanced electrocorticographic (ECoG) device — an electrode array that conforms to the brain’s surface — to take an unprecedented look at the brain activity underlying seizures.

The device also promises to enable advanced neuroprosthetic devices, which have been limited until now by the irregular topography of the brain.

“Someday, these flexible arrays could be used to pinpoint where seizures start in the brain and perhaps to shut them down,” said Brian Litt, M.D., the principal investigator and an associate professor of neurology at the University of Pennsylvania School of Medicine in Philadelphia.

“This group’s work reflects a confluence of skills and advances in electrical engineering, materials science and neurosurgery,” said Story Landis, Ph.D., director of NIH’s National Institute of Neurological Disorders and Stroke (NINDS), which helped fund the work. “These flexible electrode arrays could significantly expand surgical options for patients with drug-resistant epilepsy.”

In an animal model, the researchers saw spiral waves of brain activity not previously observed during a seizure. Similar waves are known to ripple through cardiac muscle during a type of life-threatening heart rhythm called ventricular fibrillation.

“If our findings are borne out in human studies, they open up the possibility of treating seizures with therapies like those used for cardiac arrhythmias,” said Dr. Litt. A stimulating electrode array might one day be designed to suppress seizure activity, working like a pacemaker for the brain, Dr. Litt said.

The arrays currently used to record seizure activity in patients being considered for surgery consist of electrodes attached to a rubbery base about the thickness of a credit card. These arrays are placed on the surface of the brain, but they are not flexible enough to mold to the brain’s many folds. The electrodes are widely spaced and allow for only limited brain coverage. It is also necessary to individually wire each passive sensor at the electrode-tissue interface.

High-density, flexible electrode array

To overcome this constraint, the researchers developed new devices that integrate 750 ultrathin, flexible silicon nanomembrane transistors into the electrode array, enabling new dense arrays of thousands of amplified and multiplexed sensors that are connected using fewer wires. The new array is made of a pliable material 260 nm wide (about one quarter the thickness of a human hair). “This technology allows us to see patterns of activity before and during a seizure at a very fine scale, with broad coverage of the brain,” said Jonathan Viventi, Ph.D., the study’s lead author and an assistant professor at the Polytechnic Institute of New York University and New York University.

The flexibility of the array allows it to conform to the brain’s complex shape, even reaching into grooves that are inaccessible to conventional arrays. With further engineering, the array could be rolled into a tube and delivered into the brain through a small hole rather than by opening the skull, the researchers said.

The researchers tested the flexible array on cats. The team evaluated the array in multiple contexts and brain areas. They found that it could record brain responses as the cats viewed simple objects, and sleep rhythms while the cats were under anesthesia. In one set of experiments, the researchers recorded brain activity during seizures that were induced with a drug. “We were able to watch as spiral waves began and became self-sustaining,” said Dr. Litt.

Using their flexible array technology, the researchers hope to identify these spiral brainwaves in people with epilepsy, to monitor seizures, and perhaps to control them. “We should be able to model the spirals and determine what kind of waveform can stop them. Or we can watch the spirals terminate spontaneously and try to reproduce what we see by stimulating the brain electrically,” Dr. Litt said.

The team will discuss their findings at the 2011 Society for Neuroscience meeting, Nov. 12–16 in Washington, D.C.

Ref.: Jonathan Viventi, et al., Flexible, foldable, actively multiplexed, high-density electrode array for mapping brain activity in vivo, Nature Neuroscience, published online November 13, 2011; [DOI:10.1038/nn.2973]