Mayurachat Ning Gulari

Microelectrodes, Microelectronics, and Implantable Neural Microsystems

Lithographically defined microelectrode arrays now permit high-density recording and stimulation in the brain and are facilitating new insights into the organization and function of the central nervous system. They will soon allow more detailed mapping of neural structures than has ever before been possible, and capabilities for highly localized drug-delivery are being added for treating disorders such as severe epilepsy. For chronic neuroscience and neuroprosthesis applications, the arrays are being used in implantable microsystems that provide embedded signal processing and wireless data transmission to the outside world. A 64-channel microsystem amplifies the neural signals by 60 dB with a user-programmable bandwidth and an input-referred noise level of 8 muVrms before processing the signals digitally. The channels can be scanned at a rate of 62.5 kS/s, and signals above a user-specified biphasic threshold are transmitted wirelessly to the external world at 2 Mbps. Individual channels can also be digitized and viewed externally at high resolution to examine spike waveforms. The microsystem dissipates 14.14 mW from 1.8 V and measures 1.4 1.55 cm2.

A Microassembled Low-Profile Three-Dimensional Microelectrode Array for Neural Prosthesis Applications

This paper describes the design and micro- assembly process of a low-profile 3-D microelectrode array for mapping the functional organization of targeted areas of the central nervous system and for possible application in neural prostheses. The array consists of multiple planar complimentary metal-oxide-semiconductor stimulating probes and 3-D assembly components. Parylene-encapsulated gold beams supported by etch-stopped silicon braces allow the backends of the probes to be folded over to reduce the height of the array above the cortical surface. A process permitting parylene to be used at wafer level with bulk-silicon wet release has been reported. Spacers are used to fix the microassembled probes in position and are equipped with interlocking structures to facilitate the assembly process and increase yield. Four-probe 256-site 3-D arrays operate from plusmn5 V with an average per-channel power dissipation of 97 muW at full range stimulation with pulse widths of 100 mus at 500-Hz frequency. Thirty-two sites can be stimulated simultaneously with maximum currents of plusmn127 muA and a current resolution of plusmn1 muA. The microassembly techniques allow a variety of 3-D microstructures to be created from planar components.

Silicon neural recording arrays with on-chip electronics for in-vivo data acquisition

This paper describes a 64 site, 8 channel silicon microelectrode for single-unit neural recording. The probe features integrated CMOS circuitry for electronic positioning of the active recording sites with respect to active neurons. On-chip capacitively coupled pre-amplifiers eliminate the DC baseline polarization of the electrode while providing a per channel gain of 1000. Time-division multiplexing circuitry is provided for sampling the 8 active channels onto one data lead. The on-chip circuitry consumes 834 /spl mu/W of power from /spl plusmn/1.5 V supplies and occupies 4.34 mm/sup 2/ of die area. The probe is fabricated using an 18 mask, single-sided, micromachined CMOS process with a 3 /spl mu/m minimum feature size.

A shuttered probe with in-line flowmeters for chronic in-vivo drug delivery

This paper describes a micromachined microprobe for in-vivo drug delivery at the cellular level. The probe was developed to support research in neuropharmacology as well as for eventual use in neural prostheses. Extending a standard process for the realization of electrical recording and stimulating probes, the drug delivery probe adds microchannels, fluidic cables, dielectric shutters over the injecting orifices, and in-line flowmeters to verify the intended dose. These fluidic components require three masks in addition to the normal probe process. The fluidic cables are 1.5X more flexible than the polyimide tubing formerly used and can be integrated with the probes, while the shutters reduce the unintended delivery of drugs into tissue by a factor of 25 compared with an open orifice. The flowmeter operates in pulsed mode as a hot wire anemometer, with a power dissipation of 1.5 mW and a flow resolution of about 150 pL/sec. The temperature rise in tissue near the flowmeter is less than 1/spl deg/C. Process compatibility with on-chip microvalve and micropump structures has also been explored.

A Three-Dimensional 64-Site Folded Electrode Array Using Planar Fabrication

Neuroscience and neuroprosthetic devices are increasingly in need of more compact less invasive 3-D electrode arrays for interfacing with neural tissue. To meet these needs, a folding 64-site 3-D array architecture has been developed. The microstructure, in which four probes and two platforms are fabricated as a single planar unit, results in a low-profile (<; 350-μm) narrow-platform (0.604-mm2 silicon footprint) implant for cortical use. Signals are routed from 177-μm2 iridium sites through polysilicon lines to the probe back end and then across 4-μm-thick parylene-encased electroplated-gold folding lead transfers to the associated platform. Three levels of interconnect with a 10-μm minimum pitch are utilized for the 32 leads that traverse the platforms. After rapid microassembly, micromachined latches are used to fasten the folded device. Two flexible parylene cables with gold leads at a 20- μm pitch are monolithically integrated with the probes to minimize tethering and avoid any need for lead bonding within the array, and these cables carry the neural signals to a remote circuit module or percutaneous connector. With thin (~15-μm) boron-doped shanks at a ~ 200-μm pitch, the array displaces only 1.7% of the 0.64-mm2 instrumented tissue area, assuming a 100-μm recording range. Neural signals were recorded in vivo from the guinea pig auditory cortex.

Silicon Microelectrodes with Flexible Integrated Cables for Neural Implant Applications

This paper describes two different cable structures that can be integrated with silicon microprobes and fabricated using etch-stops and a wet release at wafer level. One is a serpentine silicon ribbon cable and the other is a parylene cable. They provide highly flexible biocompatible interconnects between the implanted microelectrodes and implanted or external microsystems. Silicon microelectrodes integrated with these cable structures demonstrate consistent and reliable neural recording both acutely and chronically.

An integrated position-sensing system for a MEMS-based cochlear implant

An interface providing multi-point microstimulation and position sensing has been developed for a cochlear prosthesis, integrating a MEMS-based electrode array with signal-processing electronics. The array incorporates piezoresistive polysilicon sensors for position and tip contact to minimize insertion damage and optimize implant placement. The signal-processing chip (2.4mm times 2.4mm) operates from 3V and performs command validation, stimulus generation, sensor selection, 5b offset compensation, and signal conditioning. The calibrated sensors have typical gauge factors of 15 and provide tip contact signals of more than 100mV

A Low-Profile Three-Dimensional Silicon/Parylene Stimulating Electrode Array for Neural Prosthesis Applications

This paper describes a low-profile three-dimensional silicon/parylene microelectrode array as basis for practical neural prostheses for use in the central nervous system. The circuit areas of the silicon probes, containing mixed-signal CMOS circuitry for neural stimulation/recording, can be folded over to reduce the overall height of the microassembled array above the cortical surface. The low-profile structure is implemented using multiple gold beams spaced by orthogonal silicon braces. An integrated silicon/parylene batch process is introduced to encapsulate these interconnects and achieve high yield

A cochlear electrode array with built-in position sensing

A modiolus-hugging thin-film electrode array is being developed for a cochlear prosthesis. The array contains embedded sensors for position and wall contact in order to minimize tissue damage during array insertion. The piezo-resistive polysilicon position sensors have gauge factors of typically 15, permitting array position to be determined to less than 50/spl mu/m and providing wall contact output signals of more than 50mV. The thin-film array substrate was designed to realize devices that hug the modiolus wall in order to minimize stimulation thresholds. The array substrate is pneumatically actuated to maximize insertion depth and perceived frequency range.

A low-profile three-dimensional neural stimulating array with on-chip current generation

This work describes a low-profile silicon microelectrode array for selectively stimulating in the central nervous system. The array consists of a number of 64-site 8-channel planar CMOS probes, a platform to support the probes on the cortical surface, spacers to hold the probes orthogonal to the platform, and a hybrid chip for platform address decoding. It features integrated circuitry with on-chip current generation to deliver biphasic currents from -127/spl mu/A to +127/spl mu/A to selected sites with 1/spl mu/A resolution and fold-down structures to reduce the vertical rise above the cortex for chronic implants. The output stimulating current has a differential nonlinearity of less than 0.8LSBs and a biphasic mismatch of less than 0.23LSBs. The average power dissipation to generate full-range biphasic pulses with pulse widths of 100/spl mu/s at a repetition rate of 500 Hz is about 97/spl mu/W per channel. The probes and the hybrid chip are fabricated in a 3/spl mu/m, p-sub/n-epi/p-well 2P/1M micromachined CMOS technology; the other structures use a simplified passive process.