Multilayer CVD graphene electrodes using a transfer-free process for the next generation of optically transparent and MRI-compatible neural interfaces

Abstract

Multimodal platforms combining electrical neural recording and stimulation, optogenetics, optical imaging, and magnetic resonance (MRI) imaging are emerging as a promising platform to enhance the depth of characterization in neuroscientific research. Electrically conductive, optically transparent, and MRI-compatible electrodes can optimally combine all modalities. Graphene as a suitable electrode candidate material can be grown via chemical vapor deposition (CVD) processes and sandwiched between transparent biocompatible polymers. However, due to the high graphene growth temperature (≥ 900 °C) and the presence of polymers, fabrication is commonly based on a manual transfer process of pre-grown graphene sheets, which causes reliability issues. In this paper, we present CVD-based multilayer graphene electrodes fabricated using a wafer-scale transfer-free process for use in optically transparent and MRI-compatible neural interfaces. Our fabricated electrodes feature very low impedances which are comparable to those of noble metal electrodes of the same size and geometry. They also exhibit the highest charge storage capacity (CSC) reported to date among all previously fabricated CVD graphene electrodes. Our graphene electrodes did not reveal any photo-induced artifact during 10-Hz light pulse illumination. Additionally, we show here, for the first time, that CVD graphene electrodes do not cause any image artifact in a 3T MRI scanner. These results demonstrate that multilayer graphene electrodes are excellent candidates for the next generation of neural interfaces and can substitute the standard conventional metal electrodes. Our fabricated graphene electrodes enable multimodal neural recording, electrical and optogenetic stimulation, while allowing for optical imaging, as well as, artifact-free MRI studies.

Surface modification of multilayer graphene electrodes by local printing of platinum nanoparticles using spark ablation for neural interfacing

“…It should be noted that the surface roughness of multilayer graphene electrodes with the same growth condition was reported to be 6.75 nm (ref. 11) which is smaller than the surface roughness of printed Pt NPs.…”

“…In a practical application scenario of a neural interface, the oxide layer underneath the graphene electrodes is removed and substituted with parylene, as shown in ref. 11. Therefore, to ensure a more conclusive result the stability tests should be repeated and extended for the final device.…”

“…The electrode diameter is 340 μm which leads to 68 320 μm 2 surface area after subtracting the holes’ surface area (these holes are considered in the mask design as explained in ref. 11).…”

“…To ensure a safe measurement for both materials (Pt and graphene) and facilitate the comparison between pre-NP and post-NP measurements, the overlap (−0.6 V to 0.6 V) between the previously used water window ranges for graphene (−0.8 V to 0.6 V) and Pt (−0.6 V to 0.8 V) is chosen. 11 The measurements are performed at various scan rates (0.1 V s −1 , 0.2 V s −1 , 0.6 V s −1 , and 1 V s −1 ) 3 times to ensure that the third stabilized cycle is used for the calculation of the CSC. Both the total and cathodic CSC are calculated and expressed in μC cm −2 after dividing the calculated charge (based on the third scan) by the electrode surface area.…”

“…11,12 However, increasing the number of layers has only impact on the quantum capacitance up to a threshold of 6 layers, 10 while each added layer reduces the graphene's optical transparency. 11,13…”