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Glassy Carbon Electrocorticography Electrodes on Ultra-Thin and Finger-Like Polyimide Substrate: Performance Evaluation Based on Different Electrode Diameters

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Glassy Carbon Electrocorticography Electrodes on Ultra-Thin and Finger-Like Polyimide Substrate: Performance Evaluation Based on Different Electrode Diameters

Maria Vomero et al. Materials (Basel).

Abstract

Glassy carbon (GC) has high potential to serve as a biomaterial in neural applications because it is biocompatible, electrochemically inert and can be incorporated in polyimide-based implantable devices. Miniaturization and applicability of GC is, however, thought to be partially limited by its electrical conductivity. For this study, ultra-conformable polyimide-based electrocorticography (ECoG) devices with different-diameter GC electrodes were fabricated and tested in vitro and in rat models. For achieving conformability to the rat brain, polyimide was patterned in a finger-like shape and its thickness was set to 8 µm. To investigate different electrode sizes, each ECoG device was assigned electrodes with diameters of 50, 100, 200 and 300 µm. They were electrochemically characterized and subjected to 10 million biphasic pulses-for achieving a steady-state-and to X-ray photoelectron spectroscopy, for examining their elemental composition. The electrodes were then implanted epidurally to evaluate the ability of each diameter to detect neural activity. Results show that their performance at low frequencies (up to 300 Hz) depends on the distance from the signal source rather than on the electrode diameter, while at high frequencies (above 200 Hz) small electrodes have higher background noises than large ones, unless they are coated with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS).

Keywords: ECoG; conformability; glassy carbon; neural implants; poly(3,4-ethylenedioxythiophene) (PEDOT); polyimide; pyrolysis.

Conflict of interest statement

The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure 1
Figure 1
Sketch with examples of different types of neural implants in different sections of the human nervous system with examples of main clinical applications.
Figure 2
Figure 2
Microfabrication process used to fabricate the ultra-conformable polyimide-based ECoG devices with GC electrodes: (1) Lithographically patterned SU-8 disks with diameters of 50, 100, 200 and 300 µm and height of 13 µm. (2) Pyrolysis of SU-8 structures (1000 °C) to obtain GC electrodes. (3) PECVD (plasma-enhanced chemical vapor deposition) of 50 nm DLC (diamond-like carbon) onto GC electrodes. (4) First layer of polyimide (PI, 4 µm), patterned to open the vias to the GC electrodes. (5) PECVD of 50 nm SiC (silicon carbide) on the tracks of the devices, followed by evaporated platinum (Pt, 300 nm) and 50 nm Si/DLC/SiC. (6) Second layer of PI (4 µm), patterned to access the metal bump pads. (7) Buffered oxide etch (BOE) for releasing the devices from the silicon wafer.
Figure 3
Figure 3
(A) Picture of an ECoG array after fabrication wrapped around a pipet with a diameter of about 8 mm. The inset shows one of the four PI ‘breathable’ or holed fingers with the different-diameter GC electrodes. (B) Profilometer data showing shape, height of GC electrodes (primary Y-axis) and vertical shrinkage in percentage (secondary Y-axis) of 40, 100, 220 and 340 µm diameter SU-8 structures after pyrolysis (1000 °C, in N2). The height of all the structures before pyrolysis was 13 µm. N = 5 samples per diameter.
Figure 4
Figure 4
Lateral/surface area ratio calculated for not-pyrolyzed SU-8 cylindrical test structures with different diameters (ranging from 40 to 340 µm) and with a height of 13 µm before pyrolysis.
Figure 5
Figure 5
EIS (N = 4) and CV (representative cases) plots before (A,C) and after (B,D) electrical stimulation (10 million biphasic pulses maintaining a current density of about 0.15 mC/cm2). Shade regions represents standard deviation.
Figure 6
Figure 6
Voltage transient response of the four GC microelectrodes with diameters of 300 µm (A), 200 µm (B), 100 µm (C) and 50 µm (D), before and after stimulation (10 million pulses at 0.15 mC/cm2). (EH) show the protocols used for the biphasic pulse stimulation of the different diameter GC electrodes.
Figure 7
Figure 7
X-ray photoelectron spectroscopy (XPS) survey spectra of the four-diameter GC electrodes before (A,C,E,G) and after (B,D,F,H) stimulation. Insets show pictures of the electrodes with the correspondent diameter.
Figure 8
Figure 8
ECoG device implanted over the left barrel cortex of a rat during an acute experiment. The array perfectly adapts to the curvature of the cortex enabling strong biotic/abiotic interface contact.
Figure 9
Figure 9
Representative interpolated maps of the averaged SEPs, first row recorded in different positions (A-B) or in the same position but stimulating all and single whiskers (B-C). The colormap reflects the SEP sizes normalized in the range [0, 100] independently for each experimental condition. The second row reports the corresponding SNR histograms, which show how the same electrode can take high or low values depending on its position or intensity of the signal.
Figure 10
Figure 10
(A) Picture of the array over the barrel cortex where the position of the barrels of interest are overlaid. (B,C) Interpolated maps of the averaged SEPs obtained after stimulating the C2 and the D4 whiskers showing the expected somatotopic shift. The colormap reflects the SEPs sizes normalized in the range [0, 100] independently for each experimental condition.
Figure 11
Figure 11
Representative continuous traces of one trial (A) recorded by all the electrode diameters after high-pass filtering the data above 200 Hz. The grey box (B) represents the magnification of a small time window of spontaneous activity in order to highlight the background noise for every electrode diameter before and after PEDOT:PSS deposition. Box (C) shows ECoG raw data recorded from pristine GC electrodes of various diameter.

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