A Flex-Rigid, Multi-Channel ECoG Microelectrode Array: Reliable Electrical Contact&Long-Term Stability in Saline

Abstract

An Electrocorticography Micro-Electrode Array (ECoG MEA) is a promising signal-acquisition solution for weakly-invasive brain-computer interfaces. This PhD Thesis proposes a microfabrication scheme for a high-density ECoG MEA and investigates its long-term performance in saline. The ECoG device contains 124 circular electrodes of 100, 300 and 500 um diameters, situated on concentric hexagons (150 mm2 total recording area). The reference electrode, situated beside them, is to be bent to the array backside to become a skull-facing electrode (2.5 mm2 surface area). Metallization paths connect the electrodes to the assembly pads of 4 x 32 SMD Omnetics connectors. The ECoG device was realized as a polyimide-metal-polyimide stack on a silicon wafer. A DRIE process shaped a silicon interposer out of the carrier wafer, serving as mechanical platform for the assembly of fine-pitch electrical connectors. Electrical characterization was performed by means of electrochemical impedance spectroscopy. The electrode impedance scaled with electrode area. The strength of the solder joints was tested by means of pull tests. Electrical and mechanical tests revealed that removing the bottom polyimide from the solder-joint area enables a more reliable electrical contact. The array was implanted on the primary visual cortex (V1) of a macaque and recorded natural electrophysiological signals: the larger the electrode, the larger the signal. The skull-facing reference electrode provided signals of greater average Power Spectral Density (PSD) than common average referencing. However, the onset of parasitic short-circuits formation was encountered ca. 3-4 months after implantation. Accelerated soak tests were performed on planar-capacitor IDE structures to simulate the formation of parasitic short-circuits. The influence of curing, adhesion and sterilization on the water-barrier properties of polyimide and parylene coatings was monitored. Based on the results, a new ECoG MEA was fabricated and stored under accelerated soak conditions. The proposed ECoG MEA can be beneficial for the design, microfabrication and long-term stability of future flexible microdevices.