A three-dimensional neural recording array.
Arnold Christiaan Hoogerwerf · Deep Blue (University of Michigan) · 1992
Simultaneous multipoint extracellular recording of the potentials generated by individual neurons is the most important technique for studying the central nervous system (CNS) at the cellular level. This thesis demonstrates the development of the first reproducible microstructure capable of recording these potentials in three dimensions. The structure is micro-assembled from planar silicon components, micromachined using deep-boron etch-stops. It consists of a number of two-dimensional probes penetrating a perpendicular platform and fixed in parallel by silicon spacers. An automated electrical contact technology with end-point detection based on nickel electroplating was developed to form lead transfers between the probes and the platform, producing contact resistances below 15 $\Omega$. Three-dimensional recording arrays have been constructed having interprobe separations of 150$\mu$m and employing four four-shank probes. Such arrays are potentially capable of recording from all of the active neurons in a 800$\mu$m x 650$\mu$m x 200$\mu$m volume of cortical tissue. They have demonstrated easy penetration of the pia archnoid and minimal dimpling of the cortex in guinea pigs. Histology results indicate very little tissue reaction; with normal tissue appearing between the shanks. The first chronic recordings of neural activity have been obtained. Electronics can be integrated in the array by replacing the current passive probes with active ones and by mounting additional circuitry on the platform. A first-generation platform circuit has been designed that can interact with four active probes and can function as an interface for a future platform DSP. The chip scans the signals from 32 selected probe recording sites for the presence of neural spikes using an 800 kHz sampling frequency. The user-programmable 5-bit detection threshold corresponds to an input dynamic range from 20 $\mu$V to 640 $\mu$V. The circuit can also perform a 6-bit A/D conversion on the signal from a single recording site at a conversion rate of 80 kHz and provides electroplating access to the 16 probe-platform lead transfers during the assembly of the array. The circuit has been fabricated in a commercial foundry using a 2$\mu$m double-metal double-poly CMOS process in a die area of 4.4 mm x 4.75 mm. While design errors prevented the chip from being fully functional on first-pass, the circuitry that could be tested showed performance close to the design targets.