A ‘Camera’ graphene the image of the activity of living heart cells
Scientists can take these measurements with arrays of microelectrodes – networks of small tubes – inserted into cell membranes. But this approach is limited. Researchers can only determine the voltage on specific cells that have had an electrode inserted into them.
“Recording a point voltage – say, in the brain – is more or less like trying to watch a movie by watching a pixel on your computer screen. “You can somehow tell when things are happening, but you can not really see the plot, you can not see the correlations of information at different points in space,” says Cohen. The new graphene device produces a more complete picture because it records tensions at every single point where tissues and carbon atoms are affected.
“What we are able to do using our graphene device is image the entire surface at once,” says Halleh Balch, lead author of the study, who was a doctoral student at Berkeley during the experiment. (She is currently a postdoctoral researcher at Stanford.) This is partly a consequence of the unique nature of graphene. “Graphene is atomically thin, which makes it extremely sensitive to the local environment, because basically every part of its surface is an interface,” she says. Graphene also conducts electricity well and is quite strong, which has made it an experimental lover for a long time among quantum physicists and materials scientists.
But in the area of biological sensitivity, it is more of a newcomer. “The method itself is quite interesting. “Novel is a novel, in the sense that graphene is used,” said Gunther Zeck, a physicist at the Technical University of Vienna who was not involved in the study. He has worked with microelectrodes in the past and he suspects graphene-based devices could become real competition for them in the future. Producing large arrays of microelectrodes can be very complex and costly, Zeck says, but making large sheets of graphene can be more practical. The new device is approximately 1 square centimeter, but thousands of times larger graphene sheets are already available in the market. Using them to make “cameras”, scientists can track electrical impulses across larger organs.
For more than a decade, physicists have known that graphene is sensitive to voltages and electric fields. But combining that insight with the erratic realities of biological systems posed design challenges. For example, because the team did not introduce graphene into cells, they had to amplify the effect of cell electric fields on graphene before recording it.
The team withdrew from their knowledge of nanophotonics – technologies that use light at the nanoscale – to translate even weak changes in graphene reflection into a detailed picture of the heart’s electrical activity. They layered the graphene over a waveguide, a glass prism coated with silicon oxide and tantalum, which created a zigzag path for light. As soon as the light hit the graphene, it entered the guide, which turned back to the graphene, and so on. “This has increased the sensitivity we have because you go through the surface of graphene many times over,” says Jason Horng, a co-author of the study and Balch’s lab friend during his PhD. “If graphene has any change in reflection, then that change will be amplified.” This magnification meant that small changes in graphene reflection could be detected.
The team also managed to capture the mechanical movement of the whole heart – pulling out all the cells at the beginning of a heartbeat and their subsequent relaxation. As the heart cells pulsated, they crawled against the graphene sheet. This caused the light that was leaving the surface of the graphene to be easily refracted, in addition to the changes that the electric fields of the cells already had in its reflection. This led to an interesting observation: When researchers used a muscle inhibitor called blebbistatin to prevent cell movement, their light-based recordings showed that the heart had stopped, but tension was still spreading through its cells.