A Graphene ‘Camera’ Images the Activity of Living Heart Cells

WHEN ALLISTER MCGUIRE was working on his doctorate at Stanford University, he bought a lot of fertilized chicken eggs from Trader Joe’s. McGuire doesn’t study chickens; he’s a chemist, and he was buying eggs because he was developing a device for imaging electrical activity in beating hearts. Chicken embryo hearts just happened to be well suited for testing it.

Well, maybe not the ones incubated from these particular eggs. “Those didn’t go very well,” he recalls.

In a proof-of-principle experiment described in Nano Letters in June, McGuire and a group of physicists from UC Berkeley detailed how they created and ultimately successfully used a “camera” for recording electrical activity in living cells—which can be hard to monitor across large tissues in real time using other methods.

It’s not an optical camera; this one is made from carbon atoms and lasers. To build it, the team started with an extremely thin sheet of carbon, made up of only a single layer of atoms arranged in a honeycomb pattern. This is called graphene. Graphene’s reflectivity changes when it is exposed to electric fields: It becomes either more like a mirror that reflects light very well, or more like a dark object that does not reflect light at all.

To test how well it could record the electrical activity of living tissue, the team used cardiac muscle cultured from chicken embryos. (Eventually, McGuire realized that eggs from a biomedical distributor worked better.) The researchers placed the beating heart tissue on top of the graphene sheet and watched to see how the electrical signal—a voltage and an electric field—that controls the heartbeat might make the sheet’s reflectivity change. Whenever voltage developed inside a cell, they believed, the accompanying electric field would change the amount of light returning from the graphene underneath it. Then they set a laser to constantly throw light onto the sheet and measured how much of it bounced back. Indeed, after adding a very sensitive charge-coupled device that converts properties of light into digital signals, they finally produced images of the heart’s electrical activity.

Biologists have long been interested in measuring electrical activity not only in living heart muscle, but also in brain cells. In these tissues, the cells must use electrical signals to communicate or to synchronize their behavior. “Every cell has a membrane around it, and the membrane is made out of a greasy insulating substance—out of lipids. The water, the aqueous solutions on both sides of the membrane, are basically conductors,” says Adam Cohen, a professor of chemistry, chemical biology, and physics at Harvard University who was not part of the experiment. “Many cells use the voltage across the membrane as a way of sending signals very fast and coordinating activity.”

 

graphene set up
COURTESY OF HALLEH BALCH

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