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Scientists can take these measurements with sets of microelectrodes embedded in cell membranes — networks of tiny tubes. But that view is limited. Researchers can only determine the voltage of specific cells that have been inserted into an electrode.
“Recording a point voltage (e.g. in the brain) is like trying to watch a movie by looking at a pixel on your computer screen. You can know when things happen, but you can’t see the plot, you can’t see the correlation of information at different points in space,” Cohen says. The new graphene device creates a more complete image because it records stresses at all points where it touches tissues and carbon atoms.
“What we can do using our graphene device is to represent the entire surface at once,” says Halleh Balch, lead author of the study, who was a doctoral student at Berkeley in the experiment. (He is currently a postdoctoral researcher at Stanford.) It is due in part to the unique nature of graphene. “Graphene is atomically thin, which makes it sensitive to the local environment, basically because every part of its surface is an interface,” he says. Graphene also conducts electricity well and is relatively hard, which has made it a long-standing favorite among quantum physics and scientific materials.
But in the area of biological detection, it’s newer. “The method itself is quite interesting. It’s new in the sense that graphene is used, “said Gunther Zeck, a physicist at Vienna University of Technology who was not involved in the study. He has worked with microelectrodes in the past and suspects graphene-based devices could become real competition in the future. Manufacturing large sets of microelectrodes can be very complex and expensive. Zeck says it can be done, but making large sheets of graphene might be more practical.The new device is about a square centimeter square, but graphene sheets that are thousands of times larger are already on sale, using these to make “cameras,” allowing scientists to track electrical impulses through larger organs.
Physicists have for more than a decade been concerned that graphene is sensitive to voltage and electric fields. But combining this approach with the confusing realities of biological systems presented design challenges. For example, because the group did not insert graphene into the cells, they had to increase the effect of the electric fields in the cells on the graphene before recording.
The group used their knowledge of nanophotonics — technologies that use light at the nanoscale — to accurately visualize the electrical activity of the heart to convert weak changes in graphene reflection. They layered graphene on a waveguide, a glass prism coated with silicon and tantalum oxides, which created a zig-zag path for light. When light hit the graphene, it entered the waveguide, which bounced off the graphene and so on. “This has increased our sensitivity to the fact that the surface of graphene passes through it several times,” says Jason Horng, author of the study and a member of Balch’s lab during his PhD. “If graphene has some change in reflection, that change will be exacerbated.” This increase allows small changes in the reflection of graphene to be detected.
The group also managed to capture the mechanical movement of the whole heart: wrinkle all the cells at the beginning of the heartbeat and later relax. As the heart cells made a pulse, they crawled against the graphene sheet. This caused the light coming out of the surface of the graphene to refract slightly, in addition to the changes that the electric fields of the cells already underwent in their reflection. This led to an interesting observation: when the researchers used a muscle called blebystatin to prevent the cells from moving, light-based recordings showed that the heart stopped, but that the tension was still spreading through the cells.
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