Scientists can perform these measurements with rows of microelectrodes – networks of tiny tubes – inserted into cell membranes. But this approach is limited. Researchers can only determine the voltage in certain cells into which the electrode is inserted.
“Recording the voltage of a single point – say in the brain – is a bit like trying to watch a movie by watching one pixel on a computer screen. You can somehow tell when things are happening, but you can’t really see the action, you can’t see the connection of information at different points in space, ”Cohen says. The new graphene device gives a more complete picture because it records stresses at each individual point where tissue and carbon atoms touch.
“What we can do with our graphene device is simultaneously paint the entire surface,” 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 due in part to 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 hard, which has made it a long-standing experimental favorite among quantum physicists and materials scientists.
But in the field of biological feeling it is more of a novice. “The method itself is quite interesting. This is new in the sense that graphene is used, ”says 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 doubts that graphene-based devices could become real competition for them in the future. Producing large arrays of microelectrodes can be very complex and expensive, Zeck says, but making large sheets of graphene could be more practical. The new device is approximately 1 centimeter square, but graphene sheets thousands of times larger are already commercially available. Using them to make “cameras”, scientists could monitor electrical impulses through larger organs.
For over ten years, physicists have known that graphene is sensitive to electrical voltages and fields. But combining that insight with the messy reality of biological systems posed design challenges. For example, because the team did not inject graphene into the cells, they had to amplify the effect of the electric fields of the cells on the graphene before they recorded it.
The team relied on their knowledge of nanophotonics – technologies that use light on a nano scale – to translate even small changes in graphene reflectivity into a detailed picture of the heart’s electrical activity. They layered graphene on the waveguide, a glass prism coated with silicon and tantalum oxides, creating a zigzag path for light. Once the light hit the graphene, it entered the waveguide, which bounced it back to the graphene, and so on. “This has increased the sensitivity we have because you go through the surface of graphene more than once,” says Jason Horng, co-author of the study and Balch’s laboratory assistant during his doctorate. “If graphene has some changes in reflectivity, then that change will intensify.” This increase meant that small changes in graphene reflexivity could be detected.
The team was also able to record the mechanical movement of the whole heart – the depletion of all cells at the beginning of the heartbeat and their subsequent relaxation. As the heart cells pulsed, they dragged toward the graphene sheet. This caused the light leaving the graphene surface to refract slightly, in addition to the changes that the electric fields of the cells already had on their reflection. This led to an interesting observation: when the researchers used a muscle inhibitor drug called blebbistatin to prevent the cells from moving, their light-based images showed that the heart had stopped, but the tension was still spreading through its cells.