Graphene Circuits Enhanced to Monitor, Image Brain Signals

Model of graphene atomic structure (CORE-Materials, Flickr)

15 June 2018. Engineers and neuroscientists discovered a technique for enhancing the conductivity of graphene electrodes to make them better able to record optical images of brain activity in lab mice. A team from University of California in San Diego describes its process in the 5 June issue of the journal Advanced Functional Materials (paid subscription required).

Researchers from the neuroelectronics lab of engineering professor Duygu Kuzum are seeking ways of using graphene electrodes to monitor brain functions. Current methods use electronic techniques for tracking physiological signals in the brain, which are limited compared to optical technologies. Optical imaging, say the researchers, is able to monitor calcium loads on nerve cells in the brain, an indicator of brain signals, down to the level of individual cells.

Implanted graphene electrodes, which can be placed beneath the skull on the surface of brain tissue, offer an opportunity to capture these calcium signals. Graphene is a material closely related to graphite like that used in pencils, one atom in thickness and arrayed in an hexagonal atomic pattern. The material is very light, strong, chemically stable, and can conduct both heat and electricity, with applications in electronics, energy, manufacturing, and health care. The ultra-thin nature of graphene makes possible flexible and transparent electrodes, better able to monitor brain functions than stiffer and opaque metals.

The conductivity of graphene, however, presents a problem. While graphene can conduct an electrical current, it has high impedance, which slows the flow of electrons in the current, and reduces its usefulness for capturing images. In their report, Kuzum and colleagues describe a technique for enhancing the conductivity of graphene, with nanoscale particles of platinum. Graphene, the team discovered, offers few pathways through which electrons can flow, contributing to its high impedance. The platinum nanoparticles provide alternative pathways for electrons, and the researchers found a dusting of platinum added to the surface of graphene reduces impedance to 1 percent of graphene alone, while retaining 70 percent of the material’s transparency.

“This technique is the first to overcome graphene’s electrochemical impedance problem without sacrificing its transparency,” says Kuzum in a university statement. “By lowering impedance, we can shrink electrode dimensions down to single cell size and record neural activity with single cell resolution.”

Kuzum’s group collaborated with UC-San Diego neuroscientist Takaki Komiyama to test the graphene-platinum electrodes with lab mice. The electrodes were implanted at 50 and 250 micrometers inside the mice brains’ surface, where they recorded brain signals. The team also sent laser beams through the electrodes while recording nerve cell signals. As a result, the researchers could simultaneously record nerve cell signals while visualizing spikes in calcium on individual nerve cells, thus identifying the individual nerve cells responsible for those signals.

“This work opens up new opportunities to use optical imaging to detect which neurons are the source of the activity that we are measuring,” notes Kuzum. “This has not been possible with previous electrodes. Now we have a new technology that enables us to record and image the brain in ways we could not before.” The team plans to reduce the size of the electrodes and build them into high-density arrays.

The following brief (20 second) video shows spikes of calcium imaged 250 micrometers inside the brain tissue of a test mouse.

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