Smaller, softer, lower-impedance electrodes for human neuroprosthesis: a pragmatic approach

Abstract

Finding the most appropriate technology for building electrodes to be used for long term implants in humans is a challenging issue. What are the most appropriate technologies? How could one achieve robustness, stability, compatibility, efficacy, and versatility, for both recording and stimulation? There are no easy answers to these questions as even the most fundamental and apparently obvious factors to be taken into account, such as the necessary mechanical, electrical and biological properties, and their interplay, are under debate. We present here our approach along three fundamental parallel pathways: we reduced electrode invasiveness and size without impairing signal-to-noise ratio, we increased electrode active surface area by depositing nanostructured materials, and we protected the brain from direct contact with the electrode without compromising performance. Altogether, these results converge toward high-resolution ECoG arrays that are soft and adaptable to cortical folds, and have been proven to provide high spatial and temporal resolution. This method provides a piece of work which, in our view, makes several steps ahead in bringing such novel devices into clinical settings, opening new avenues in diagnostics of brain diseases, and neuroprosthetic applications.

Keywords: Intracortical microelectrodes; brain-conformability; carbon nanotubes; conductive polymers; hydrogel; micro-electrocorticography (ECoG); neural recording; neural stimulation.

Figure 1

(A) Impedance spectra of a 0.031 cm² Pt electrode, uncoated (black) and with electrodeposited Au-MWCNT (pink), Au-agar (green), PPy-CNT (dark green), PEDOT-agar (blue), or PEDOT-CNT (red). (B) Cyclic voltammograms between −0.5 and 0.5 V of a 0.031 cm² Pt electrode, uncoated (black), or with electrodeposited Au-agar (green), PPy-CNT (dark green), PEDOT-agar (blue), or PEDOT-CNT (red). (C), Cyclic voltammograms between −1.0 and 0.6 V of a 0.031 cm² Pt electrode coated with PPy-CNT (dark green) and PEDOT-CNT (red).

Figure 2

(A) Optical image of a single sharp microelectrode. (B) Scanning electron micrograph of a single sharp microelectrode. (C,D) Schematic datasheet of a 4-core quartz-platinum/tungsten tetrode microelectrode (Thomas Recording, Giessen, Germany). (E) Tetrode scanning electron micrograph (s.e.m.).

Figure 3

(A) Impedance spectra of a single intracortical electrode, uncoated (black), or coated with Au-CNT (green) PPy-CNT (red), or PEDOT-CNT (blue), electrodeposited and CNT coated by direct growth with CVD. (B) Cyclic voltammograms of a single intracortical electrode, uncoated (black) or coated with Au-CNT (green), PPy-CNT (red) or PEDOT-CNT (blue), electrodeposited, and CNT coated by direct growth with CVD.

Figure 4

Scanning electron micrograph of (A) as grown CNT-CVD coatings, (B) Au-MWCNT coating, (C) PPy-CNT coating, (D) PEDOT-CNT coating.

Figure 5

Scanning electron micrograph of (A) a single tip Au-agar coated microelectrode, (B) Au-agar coating, (C) Au-SWCNT coating.

Figure 6

Impedance spectra of a single intracortical electrode uncoated (black), and PEDOT-CNT-coated (red) and after stimulation experiments.

Figure 7

Photographs of (A) a 64-electrode array with 4.2 × 4.2 mm total recording area and 140 μm diameter recording sites. (B) A 64-electrode array with 3.6 × 18.0 mm total recording area and 100 μm diameter recording sites; (C) A 64-electrode matrix with 5 × 5 mm total recording area, designed for the marmoset; (D) A 128-electrode array with 4.2 × 8.4 mm total recording area and 100 μm diameter recording sites; (E) A 64-electrode array with 12.5 × 27.5 mm total recording area and 200 μm diameter recording sites. (F) A 16-electrode array with 1.2 × 1.2 mm total recording area and 100 μm diameter recording sites.

Figure 8

s.e.m. images of: (A) a 100 μm Au-agar-coated recording site; (B) Au-CNT coating; (C) PPy-CNT coating; (D) PEDOT-CNT coating.

Figure 9

Impedance spectra of 100-μm-diameter ECoG array recording sites (mean and standard deviation from 64 recording sites each) before (black) and after Au-agar (green), PPy-CNT (red) and PEDOT-CNT (blue) electrodeposition.

Figure 10

SEP response of a 4 × 4 micro-ECoG array (inset) with a 1 × 1 mm total recording area, 100 μm diameter recording sites, and 300 μm inter-electrode pitch, coated with PEDOT-CNT to the first 12-ms truncated Gaussian of a 9-Hz train of rat multiwhisker deflections of 0.8° mm. The data are averaged on 20 repetitions of the stimulation pattern.

Figure 11

(A) Optical images of the microspheres after 3, 6, 9, 12, and 13 h of electrodeposition; (B) impedance magnitude at 1 kHz and diameters of the microspheres after 3, 6, 9, 12, and 13 h of electrodeposition; (C) Impedance after 13 h of electrodeposition; (D) an example of spike activity recorded using a 100 μm microsphere; (E) from left to right different top view optical magnifications of a typical microsphere.

Figure 12

(A) One possible ultraflexible micro-ECoG array layout. (B) Details of the two groups of five recording sites 200° × 200° μm and 100° × 100° μm in size, and a scanning electron micrograph of a 100 × 100 μm Au recording site (inset). (C) Other possible ultraflexible micro-ECoG array layouts and detail of a group of eight 200 × 200° μm recording sites (inset).

Figure 13

(A) Impedance spectra of the 100 × 100 μm and 200 × 200 μm ultraflexible ECoG array recording sites (mean and standard deviation from 10 recording sites each) before and after PEDOT-CNT electrodeposition. (B) Averaged SEP elicited using a 9-Hz train of truncated Gaussian 0.8-mm rat multiwhisker deflection (first 12 ms response) using PEDOT-coated 100 × 100 μ m electrodes (average on 10 electrodes).

Figure 14

Impedance spectra of (A) 140-μm-diameter ECoG array recording sites (mean and standard deviation from 64 recording sites each) coated with PEDOT-CNT composite, before and after fibrin encapsulation, and (B) intracortical PEDOT-CNT microelectrode before and after fibrin encapsulation and in vivo recording. s.e.m. images of a single recording/stimulation site encapsulated with fibrin are shown in the respective insets.