Originally Published MPMN September 2009

NEED TO KNOW

Neurosurgery Researchers Have Microelectrodes on the Brain

Stephanie Steward

Doctors at the University of Utah (Salt Lake City; www.utah.edu) have put on their thinking caps to find a better way of performing electrocorticography (ECoG)—the science of sticking electrodes onto the brain to record electrical activity from the cerebral cortex. Invasive and risky, ECoG has been used to develop devices that help treat epilepsy or allow people to control prosthetic limbs. But the Utah scientists have been working on enhancing microsized versions of ECoG electrodes to create grids that can be set on top of the brain’s surface instead of inserted into it. Though it still requires going inside the skull, the less-invasive method of using micro-ECoGs could potentially be used for cognitive and mood disorder treatment, drug screening and development, and neuroscience research.

“I see micro-ECoG as a disruptive technology that will eventually displace the currently used ECoG grids,” says Bradley Greger, assistant professor of bioengineering at the University of Utah and coauthor of the Journal of Neurosurgery study on the technology. “By using large numbers of microelectrodes, we are trying to match our sensor, the micro-ECoG grid, to the fundamental unit of neural signal processing, the cortical column, so that we can maximize the amount of information extracted from the brain while minimizing the level of invasiveness,” he explains.
Greger says that he and his coauthor, neurosurgeon and University of Utah assistant professor of neurosurgery Paul House, have learned a lot about the design considerations for the microelectrodes based on their initial experiments. The grids used in the published study were made of fine platinum wires that were then embedded in a layer of silicone. “These are the same materials used in the clinical ECoG grids currently used for localizing seizure foci,” says Greger. “Since these grids were very similar to FDA-approved grids—except for the electrode size—we didn’t have any strong biocompatibility concerns.”
Going from a large, low-impedance electrode in the standard ECoG grids to the very small high-impedance electrodes in the micro-ECoG grid did present electromagnetic interference issues, however. “Basically, you pick up a lot of dreaded 60-Hz electromagnetic resonance,” explains Greger. “We addressed this by adding local low-impedance references and having their wires follow the same pathway as all of the signal wires.” They also are working to develop shielded and impedance-converting cables for use in their next set of experiments.
“The key to all of this is to be able to use lithography fabrication techniques to reduce costs and increase quality and control over design parameters,” Greger says. “Another important issue is being able to handle the larger amount of data generated by the micro-ECoG grids.” To address this issue, Greger and House are currently working with Blackrock Microsystems (Salt Lake City; www.blackrock.dreamhosters.com) to develop the software and analysis tools necessary to take advantage of data gathered by the micro-ECoG electrodes and reduce them to outputs that are useful to clinicians. With such tools, the scientists hope to see lithographic parylene micro-ECoG grids become available in the next few years.
In the meantime, the investigators are aiming their efforts at examining real-time movement decodes and developing real-time algorithms for detecting and classifying arm and finger movements for prosthetic applications. “That is, can we determine at any given moment if a patient wants to move their arm and, if so, in what direction it is going to be moving?” asks Greger.
There is some debate among scientists as to whether prosthetics should be controlled from nerve signals collected by electrodes in or on the brain, or by electrodes planted in a residual limb. For Greger, it’s all about getting the maximum amount of information out of the nervous system with the minimum amount of risk. If a patient can control a prosthetic limb using an electrode in a peripheral nerve, he says, then that is probably the right approach. However, some patients may not have any functional peripheral nerves, or it may not be possible to extract enough information from them. These patients can only benefit from a neural prosthetic device that can interface directly with the cerebral cortex, he explains. “Patient and clinical need should always drive device design.”
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