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, 15 (1), 832-54

A Multi-Channel, Flex-Rigid ECoG Microelectrode Array for Visual Cortical Interfacing

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A Multi-Channel, Flex-Rigid ECoG Microelectrode Array for Visual Cortical Interfacing

Elena Tolstosheeva et al. Sensors (Basel).

Abstract

High-density electrocortical (ECoG) microelectrode arrays are promising signal-acquisition platforms for brain-computer interfaces envisioned, e.g., as high-performance communication solutions for paralyzed persons. We propose a multi-channel microelectrode array capable of recording ECoG field potentials with high spatial resolution. The proposed array is of a 150 mm2 total recording area; it has 124 circular electrodes (100, 300 and 500 µm in diameter) situated on the edges of concentric hexagons (min. 0.8 mm interdistance) and a skull-facing reference electrode (2.5 mm2 surface area). The array is processed as a free-standing device to enable monolithic integration of a rigid interposer, designed for soldering of fine-pitch SMD-connectors on a minimal assembly area. Electrochemical characterization revealed distinct impedance spectral bands for the 100, 300 and 500 µm-type electrodes, and for the array's own reference. Epidural recordings from the primary visual cortex (V1) of an awake Rhesus macaque showed natural electrophysiological signals and clear responses to standard visual stimulation. The ECoG electrodes of larger surface area recorded signals with greater spectral power in the gamma band, while the skull-facing reference electrode provided higher average gamma power spectral density (γPSD) than the common average referencing technique.

Figures

Figure 1.
Figure 1.
(A) ECoG electrode array; (B,D) Signal-acquisition area; (C) Reference electrode (top-view); (E) Reference electrode fixed to the array's backside (cross-section).
Figure 2.
Figure 2.
(Left) Connector-assembly region; (Right) A free-standing flex-rigid ECoG array.
Figure 3.
Figure 3.
Microfabrication flow.
Figure 4.
Figure 4.
EIS test set-up with LabView-controlled multiplexer (MUX); CE stands for counter electrode, Ref for the external reference electrode and WE for the ECoG array.
Figure 5.
Figure 5.
Receptive field mapping task. Movement of the bar stimulus over the receptive field of an electrode is reflected as a temporary increase in ECoG signal power.
Figure 6.
Figure 6.
A 4″ wafer which contains three flex-rigid ECoG devices.
Figure 7.
Figure 7.
ECoG signal-acquisition area.
Figure 8.
Figure 8.
(A,B) Soldering pad; (C) Reflowed photoresist profile transferred to PI.
Figure 9.
Figure 9.
(A) Solder paste dispensed on pads; (B) Soldered connector leg; (C) Four soldered connectors after flux-clean.
Figure 10.
Figure 10.
(A) ECoG array after wafer release; (B) The ECoG array with its skull-facing reference electrode.
Figure 11.
Figure 11.
Specimen types used in adhesion (AC) and electrical (B,C) tests.
Figure 12.
Figure 12.
Impedance spectral bands according to electrode type; circuit diagrams.
Figure 13.
Figure 13.
Impedance magnitude values at 100 Hz derived from the data of Figure 12, plotted against the inverse values of the electrode area (mean ± SD).
Figure 14.
Figure 14.
Values derived from Table 1 for the double layer capacitance C and charge-transfer resistance Rp plotted against the electrode area.
Figure 15.
Figure 15.
Electrophysiological signals (left image) recorded from electrode sites distributed over the array diameter (right image, #1–14).
Figure 16.
Figure 16.
Time-frequency plots of normalized PSD for three neighbouring electrodes of different sizes situated in the middle of the array layout (red circles in the right image of Figure 15).
Figure 17.
Figure 17.
Time course of normalized gamma-PSD ± s.d. derived from time-frequency plots (shown in Figure 16), using a skull-facing reference electrode (Left) and common average referencing (Right).

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