Transparent Neural Electrodes Made of Graphene for Studying Dynamics of Brain Circuits
Graphene has recently emerged as an attractive material for neural sensing and stimulation, owing to its flexibility, transparency, excellent electrical conductivity and low noise characteristics. Transparency of graphene is particularly important for developing completely transparent neural electrode arrays for simultaneous neuroimaging and electrophysiology from the same population of neurons. Simultaneous functional optical imaging and electrophysiology can enable studying dynamic neural circuits with high spatio-temporal resolution by combining high spatial resolution of calcium imaging with high temporal resolution of electrical recordings. We have developed micro-fabrication techniques to build transparent graphene microelectrodes on flexible substrates. Electrochemical characterizations and in vivo neural recording experiments showed that graphene electrodes can achieve a significant improvement in signal-to-noise ratio and substantial reduction in electrical interference noise compared to gold electrodes. We demonstrated that brain slices from rats can be imaged through transparent graphene electrodes by confocal microscopy, while the neural activity was simultaneously recorded by the graphene electrode. Both excitation and emission light penetrated through the graphene electrode without causing any light induced artifacts in the electrical recordings. Recordings by the graphene electrode and calcium transients measured by the confocal microscopy were found to be consistent, showing short population bursts during induced epileptiform activity. The temporal resolution of the recordings with the graphene electrode enabled detection of high frequency population discharges, which could not be resolved by the calcium fluorescence responses. In contrast, calcium imaging responses were able to capture complex network contributions of individual neurons which were not evident in the electrical recordings. Experiments with the slices have shown that the graphene electrode was able to measure very fast population spikes with durations less than 5 ms, as well as slow field potentials, which were not detectable by calcium imaging. The capability to record brain activity from a large number of neurons and interacting neural circuits, while simultaneously resolving individual cells and their connections through optical imaging, may greatly illuminate our understanding of how brain circuits process information.
Duygu Kuzum1, Hajime Takano2, Euijae Shim1, Guanqing Hao1, Jason C Reed1, Douglas A. Coulter2, Ertugrul Cubukcu1, Brian Litt1.