Accurate mapping of neural circuits and interfacing with neurons for control of brain-machine interfaces require simultaneous large-scale and high spatiotemporal resolution recordings and stimulation of neurons in multiple layers and areas of the brain. Conventional penetrating micro-electrode arrays (MEAs) are limited to a few thousand electrodes at best, with limited volumetric 3D spatial resolution. This is mainly due to the types of fabrication technologies and available designs and materials for making such probes. Based on the strengths and shortcomings of the available MEAs, we present a new fabrication technology for a new class of 3D neural electrode array that provides the characteristics of a near-ideal neural interface. This research addresses some of the limitations of previous works in terms of electrode scale, density and spatial coverage (depth and span). In order to realize a scalable 3D out-of-plane array of extremely dense, slender, and sharp needles with recording sites at each of their tips, a number of techniques are developed.These includes: 1- A custom-developed silicon DRIE process to make deep (500 µm) high aspect-ratio (20-30) thru-wafer holes with controlled sidewall slope, 2- A method of extending the thru-wafer holes depth by aligning and then fusion bonding multiple silicon substrates already having holes etched in them, 3- A process for conformal refilling of ultra-deep (~2 mm) ultra-high aspect-ratio (80-100) holes with dielectric and conductive films using LPCVD process, 4- Methods of forming recording sites using self-aligned mask-less metallization processes, and 5- A method based on wet silicon etching to dissolve away the support substrate containing the refilled holes to release the electrodes.Using these technologies, we have fabricated millimeter-long (1.2mm), narrow (10-20µm diameter), sharp (submicron tip size), high-density (400 electrodes/mm2) high-count (5000+) silicon electrode arrays. Electrodes robustness, insertion and recording functionality have been demonstrated by acute in vivo recordings in rats under anesthesia using 2×2 and 3×3 arrays, where local field potentials (LFP) have been recorded.Innovative features of this technology could be utilized to produce arrays with arbitrary 3D design to target specific brain structures to achieve 3D spatial coverage over the convoluted topography of the brain. These include: 1- Length of side-by-side electrodes can be varied from tens of microns to several millimeters independently. 2- Electrodes spacing can be modified by the designer to obtain a desirable density and distribution of the array needles. 3- Electrode cross-sectional size can be controlled to obtain extremely fine, sharp and slender needles, crucial for minimizing tissue damage and improving chronic stability of implanted probes. 4- Any desired distribution of electrodes with customizable length, diameter and pitch across the array can be obtained to realize near-ideal application-specific neural probes.Potential capabilities of this work are investigated. These include integration of optical waveguides and chemical sensors and drug delivery channels to create sophisticated multi-modal multi-channel probes for electrophysiological studies of brain at the cellular levels. Limitations of developed DRIE, bonding, and LPCVD processes and tissue volume displacement by high-density arrays are discussed and a number of solutions are proposed. These results suggest maximum electrode length of ~2.5 mm and 20 µm thick electrodes. A maximum density of ~225 electrodes/mm2 for 10 µm thick electrodes is suggested for chronic applications.
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Sea of Electrodes Array (SEA): Customizable 3D High-Density High-Count Neural Probe Array Technology
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