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.

Keywords: Engineering; Other nanotechnology.

Fig. 1. Wafer-scale transfer-free fabrication process steps of graphene-based neural electrodes.

Fabrication process steps a Oxide deposited on both sides of a DSP Si wafer, patterned, and etched on the backside, b Mo deposition and pattern, c Graphene growth, d Al (1%Si)/Ti deposition and pattern on the electrodes and contact pads, e Parylene-C deposition, f Al/Ti hard mask deposition and pattern for parylene etching followed by a DRIE process, g Frontside oxide removal followed by Mo wet etching, second parylene deposition on the backside, and parylene etching on the frontside, h Hard mask wet etching, i Cutting the sample.

Fig. 2. Suspended multilayer graphene electrodes with different polymers (Parylene C and PDMS).

a Suspended graphene electrodes with parylene-C substrate, b Suspended graphene electrode with PDMS substrate, c Optical image of the electrode before (yellow) and after (blue) Mo removal, d Raman Spectroscopy on graphene electrodes.

Fig. 3. Sheet resistance and optical transmittance characterization of graphene with different thicknesses.

a Sheet resistance of three different graphene recipes (growth time: 20, 40, and 60 min) showing the maximum, upper quartile, median (red line), average (plus sign), lower quartile, and minimum values, b Optical transmittance measurements for different graphene growth times (the effect of the glass slide is removed).

Fig. 4. Electrochemical impedance spectroscopy (EIS) for graphene, Pt, and Au electrodes with the same size and geometry.

a Average impedance magnitude and b Phase angle plots (±standard deviation shaded in gray) for fifteen graphene electrodes, c Proposed equivalent circuit model for the multilayer graphene electrode and the average values of the parameters used in the equivalent circuit model, d Impedance magnitude and e Phase angle plots for fifteen graphene electrodes in black (average values), Au electrodes in orange, and Pt electrodes in blue. All electrodes are of the same size and geometry.

Fig. 5. Cyclic voltammetry (CV) and voltage transient measurement for graphene, Pt, and Au electrodes.

a CV curves for graphene, Pt, and Au electrodes with scan rates 1, 0.6, 0.2, and 0.1 V/s from left to right, respectively, b Voltage-transient measurements for graphene, c Au, and d Pt electrodes.

Fig. 6. Photo-induced artifact test for graphene and Au electrodes in a PBS solution.

Normalized power spectrum of the recorded signal from Au and graphene electrodes (zoomed-in) after shining light with 10 Hz frequency.

Fig. 7. MRI compatibility test for graphene and Pt electrodes immeresed in a phantom.

a Immersed Pt and graphene electrodes in a phantom, b T2^*-weighted image with no artifact from the electrodes, c T2^*-weighted image acquired with EPI readout resulting in an artifact-free imaging, d T2^* maps of the electrodes without any artifact, e Baseline magnitude image, B₀ maps, and the high-pass filtered image of the B₀ maps of the electrodes.