Abstracts

Rationale: Transparent neural electrodes have recently emerged as a novel technology enabling simultaneous functional optical imaging and electrophysiology from the same population of neurons. Simultaneous imaging and electrophysiology can combine high spatial resolution of cellular optical imaging with high temporal resolution of electrophysiology. Among several alternatives, graphene is the most promising candidate to build completely transparent neural electrode arrays owing to its other superior characteristics such as flexibility, high electrical conductivity and potential for functionalization of its surface.Methods: Previously, we demonstrated that neural activity could be simultaneously imaged using calcium imaging by confocal or two-photon microscopy through a single transparent graphene electrode during electrophysiological recordings in hippocampal slices. Here, we demonstrate completely transparent graphene electrode arrays of various sizes and densities using advanced nanofabrication techniques. We engineered graphene electrodes to optimize their impedance for electrophysiological recordings and their charge injection capacity for electrical stimulation. Impedance and charge capacity of the electrodes were characterized by electrochemical impedance spectroscopy and cyclic voltammetry measurements. The results were fitted to equivalent circuit models to analyze capacitive and resistive nature of graphene interface.Results: We recorded epileptiform activity from cortex in acute experiments with anesthetized rats. High signal to noise ratio allowed us to record low amplitude physiological cortical potentials and evoked potentials by whisker stimulation. We demonstrated that neural activity could be simultaneously imaged using calcium imaging through optimized transparent graphene electrodes during electrophysiological recordings (Fig. 1). Both excitation and emission light penetrates through the graphene electrodes during imaging without causing any light induced artifacts in the electrical recordings. Recordings (Fig. 1 c) by the graphene electrode and calcium transients (Fig. 1 b) 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. Recordings by graphene electrodes were able to capture very fast population spikes with durations less than 5 ms, as well as slow field potentials.Conclusions: Transparent graphene electrode technology can be a key enabler for studying dynamics of epileptic circuits with high spatio-temporal resolution by combining high spatial resolution of calcium imaging with high temporal resolution of electrical recordings.