A Fully Transparent, Flexible μECoG Array Based on Highly Conductive and Anti-reflective PEDOT:PSS-ITO-Ag-ITO Thin Films

Abstract

Integrative neural interfaces combine neurophysiology and optogenetics with neural imaging, providing numerous opportunities for neuroscientists to study the structure, function, and diseases in neural circuits. Such a comprehensive interface demands miniature electrode arrays that are highly transparent, mechanically flexible, and biocompatible. Compared to implanted electrodes, microscale electrocorticogram (μECoG) arrays are less invasive, therefore lowering the risk of infection, seizure, and stroke. Common transparent μECoG arrays are made of a single material and at most two materials, including indium tin oxide (ITO), ultrathin metals, graphene, poly-(3, 4-ethylenedioxythiophene)/ poly(styrenesulfonate) (PEDOT:PSS), etc. , making it hard to possess the excellent combination of properties, like high transmittance, low electrical resistance, mechanical flexibility, stability, and biocompatibility. In this paper, we structured an ultra-flexible, fully transparent, highly conductive, and peak-transmittance-tunable μECoG array with a PEDOT:PSS-ITO-Ag-ITO multilayer assembly on a thin Parylene C substrate. Each array consisted of 32 transparent microelectrodes distributed uniformly and divided equally into two 5 mm× 11 mm panels. The transmittance of the PEDOT:PSS-ITO-Ag-ITO assembly under the 550 nm targeted wavelength on the Parylene C substrate was ~7% higher than that of a single ITO layer of the equivalent thickness. The average impedance of the microelectrodes at 1 kHz was ~45 kΩ and increased to ~59 kΩ after four weeks of soaking test, suggesting the stability of the electrodes for long-term electrophysiology recording. Cyclic voltammetry was performed to confirm the increased charge storage capacity. These microelectrodes based on PEDOT:PSS-ITO-Ag-ITO also showed a neglectable signal-to-noise ratio (SNR) changes under blue, green, yellow and red light-emitting diodes (LEDs) compared with no light. In vivo recording from the primary visual cortex (V1) of an anesthetized rat validated the efficacy of the transparent electrodes for recording ECoG activity in living brain tissues.