Organic Electrochemical Transistors for Reading Brain Waves

Strange supersensitive transistors could be ideal for brain-computer interfaces

2 min read
Organic Electrochemical Transistors for Reading Brain Waves
Photo: École Nationale Supérieure des Mines de Saint-Étienne

A new type of implantable electrode can get much more sensitive readings of brain waves and cost substantially less, according to a scientist studying the device. Organic electrochemical transistors (OECTs) consist of conductive polymers and liquid electrolytes, which could make for an easy interface between, say, the surface of the brain and conventional silicon electronics.

Device physicists have been studying OECTs since the early part of this century, says George Malliaras, head of the bioelectronics department at École Nationale Supérieure des Mines de Saint-Étienne, France, but they don’t yet fully understand how they work. In a research published in Science Advances last week, Malliaras and his colleagues determined that the performance of the device is directly related to the thickness of the polymer channel, a piece of information that will help in designing these electrodes.

OECT is about two orders of magnitude more sensitive than a silicon-based device

In a standard silicon-based electrode, the transconductance—the measure of how the device amplifies signals from the brain—is determined by the surface area of the electrode. In an array of electrodes to be fitted onto a human skull or the surface of a rat’s brain, it’s hard to increase the area of the electrode by very much. Malliaras and his team found that transconductance in the OECT is determined not by area but by thickness. OECTs work by the exchange of ions from the electrolyte into the polymer; that means devices with thicker polymer channels work better than thinner ones, because the whole bulk of the device, and not just the surface, comes into play.

They placed two OECT electrodes—one 230 nanometers thick, the other 870 nm—on the skull of a volunteer and took a standard electroencephalogram reading. The transconductance on the thicker electrode was about twice what it was on the thinner one. Testing other sizes, they found thicker channels worked better, even when the channel was slightly more than 1 micrometer thick.

Because it takes advantage of the transistor volume instead of just the surface, “a very small change in electrical potential can be amplified by the device into a very large change in current,” says Malliaras. He says the OECT is about two orders of magnitude more sensitive than a silicon-based device.

Because the devices are polymer based, they can be fabricated using printing processes, which should make them cost just a few cents each, as opposed to several dollars for conventional electrodes, Malliaras says. He expects electrodes for use on the outside of the body could be available in the near future. Implantable devices would need to go through government regulatory approval, and thus would take longer.

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This Implant Turns Brain Waves Into Words

A brain-computer interface deciphers commands intended for the vocal tract

10 min read
A man using an interface, looking at a screen with words on it.

A paralyzed man who hasn’t spoken in 15 years uses a brain-computer interface that decodes his intended speech, one word at a time.

University of California, San Francisco

A computer screen shows the question “Would you like some water?” Underneath, three dots blink, followed by words that appear, one at a time: “No I am not thirsty.”

It was brain activity that made those words materialize—the brain of a man who has not spoken for more than 15 years, ever since a stroke damaged the connection between his brain and the rest of his body, leaving him mostly paralyzed. He has used many other technologies to communicate; most recently, he used a pointer attached to his baseball cap to tap out words on a touchscreen, a method that was effective but slow. He volunteered for my research group’s clinical trial at the University of California, San Francisco in hopes of pioneering a faster method. So far, he has used the brain-to-text system only during research sessions, but he wants to help develop the technology into something that people like himself could use in their everyday lives.

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