Scientists at the University of California, Berkeley, (UC Berkeley) have demonstrated yet another use for the ever-versatile wonder material graphene, using it as the basis for an advanced sensor that can image electrical signals from living cells and tissue in real time. The team's "graphene camera" was used to record electrical activity of a beating heart in action, and could also open up new sensing capabilities when it comes to the brain.
Graphene is a two-dimensional sheet of carbon measuring just a single atom in thickness, and its incredible list of properties have captured the imagination of scientists from a wide range of research areas. These characteristics include a remarkable thinness, a high thermal and electrical conductivity and a status as the strongest man-made material.
The UC Berkeley scientists teamed up with chemists from Stanford University to explore how the material could usher in a new breed of advanced medical sensors. The work builds on previous research demonstrating that an electrical field can influence the way a sheet of graphene reflects or absorbs light, something the team explored by placing a roughly 1 cm2 of the material on a beating heart from a chicken embryo.
“When cells contract, they fire action potentials that generate a small electric field outside of the cell,” explains study author Halleh Balch. “The absorption of graphene right under that cell is modified, so we will see a change in the amount of light that comes back from that position on the large area of graphene.”
The technique took some tweaking. Initially, the electric field generated by the beating heart muscle cells was too small to make a noticeable difference to the reflectance of the graphene. The team was able to amplify it adding a thin waveguide underneath, which works with an input laser light that is beamed through a prism and bounces the light off of the graphene about 100 times, before exiting the device.
“One way of thinking about it is that the more times that light bounces off of graphene as it propagates through this little cavity, the more effects that light feels from graphene’s response, and that allows us to obtain very, very high sensitivity to electric fields and voltages down to microvolts,” Balch says.
The team was able to use this "graphene camera" to study cardiac cells measuring just ten microns across in real time and produce an optical image of the faint electric fields they generated through their beating. While electrodes and chemical dyes can be used to measure this electrical activity in cells, they can only do so at one particular location, while the sheet measures the voltage across an entire area of tissue. The team imagines combining these sensing techniques by recording the electrical signals of cells while simultaneously imaging dyed tissues.
“The ease with which you can image an entire region of a sample could be especially useful in the study of neural networks that have all sorts of cell types involved,” says first author of the study, Allister McGuire, from Stanford University. “If you have a fluorescently labeled cell system, you might only be targeting a certain type of neuron. Our system would allow you to capture electrical activity in all neurons and their support cells with very high integrity, which could really impact the way that people do these network level studies.”
The "graphene camera," or coupled waveguide-amplified graphene electric field (CAGE) sensor, as it is called, could be used to test out candidate drugs on heart muscle ahead of clinical trials, to see whether they trigger abnormal changes. This was demonstrated by administering the chicken embryo with a drug to inhibit muscle proteins, which caused the heart to stop beating, while allowing the team to observe it had no effects on the electrical field.
The device could also open up new possibilities in direct sensing of the brain. Electrode arrays are used today to study the electrical activity of brain cells, though can only do so at a few hundred locations. The strong graphene sheets could be placed on the surface to gain a broader picture of continuous electrical activity.
“One of the things that is amazing to me about this project is that electric fields mediate chemical interactions, mediate biophysical interactions – they mediate all sorts of processes in the natural world – but we never measure them. We measure current, and we measure voltage,” Balch says. “The ability to actually image electric fields gives you a look at a modality that you previously had little insight into.”
The research was published in the journal Nano Letters, while the video below shows a sequence of images generated by the device, depicting a single beat of the chicken embryo heart.
