Gayatri E. Perlin

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.

An Implantable 64-Channel Wireless Microsystem for Single-Unit Neural Recording

This paper reports an implantable microsystem capable of recording neural activity simultaneously on 64 channels, wirelessly transmitting spike occurrences to an external interface. The microsystem also allows the user to examine the spike waveforms on any channel with 8 bit resolution. Signals are amplified by 60 dB with a programmable bandwidth from < 100 Hz to 10 kHz. The input-referred noise is 8 ¿Vrms. The channel scan rate for spike detection is 62.5 kS/sec using a 2 MHz clock. The system dissipates 14.4 mW at 1.8 V, weighs 275 mg, and measures 1.4 cm 1.55 cm.

An Ultra Compact Integrated Front End for Wireless Neural Recording Microsystems

Abstract-The design and performance of an integrated front end for high-channel-count neural recording microsystems is presented. This front end consists of a 3-D micromachined microelectrode array, realized using a new architecture that allows simple and rapid microassembly. A 64-site 3-D multiprobe, realized using the new architecture, interfaces with tissue volumes of less than 0.01 mm3 and has a footprint of 1 mm2. For amplification, filtering, and buffering of the recorded neural signals, a custom signal-conditioning circuit provides high gain (60 dB), low noise (4.8 μVrms), and low power (50 μW) in an area of 0.098 mm2. In addition, this circuitry implements bandwidth tuning, offset compensation, and wireless gain programmability. This new approach to system integration uses a microfabricated parylene overlay cable to electrically interconnect the 3-D array and signal-conditioning circuitry. In vivo results obtained using this integrated microsystem front end in its most compact form are presented.

Neural Recording Front-End Designs for Fully Implantable Neuroscience Applications and Neural Prosthetic Microsystems

An implantable neural recording front-end has been designed in two versions. The first is a multi-channel signal-conditioning ASIC for use with any neural recording probe technology. This ASIC was implemented in a commercial 0.5 mum CMOS process, includes 16 parallel amplifier channels, and measures 2.3 mm2 The amplifiers have a gain of 59.5 dB, a high cutoff frequency at 9.1 kHz and consume 75 muW per channel. The low cutoff frequency is independently tunable on each channel to accept or reject field potentials. This chip is small enough to be chronically packaged for experiments in awake behaving animals or it can be integrated into a fully implantable neural recording microsystem. The second version of the front-end is a neural recording probe with integrated signal conditioning circuitry on the back-end implemented in a 3 mum CMOS process. This version dissipates 142 muW and includes 64 to 8 site selection, 8 per-channel amplifiers each having a gain of 50.2 dB, a tunable low cutoff frequency, and a 7 kHz upper cutoff frequency. Real-time site impedance and circuit testing has been integrated in this design

An Implantable Microsystem for Wireless Multi-Channel Cortical Recording

This paper reports a 64-channel microsystem for chronic neural recording. The system scans all the channels simultaneously, detects any neural spikes, and reports their occurrence wirelessly to the external world. Any one of the selected channels can also be digitized and read out wirelessly to allow inspection of the full analog waveform. The neural signals are amplified with 60 dB of gain, a programmable bandwidth, and an 8 muVrms input-referred noise level before being processed digitally. For a 2 MHz clock, the channel scan rate for spike detection is 62.5 kS/Sec and the total system power dissipation at 1.8 V is 14.4 mW. The implantable version of the microsystem weighs 275 mg and measures 1.4 cm times 1.55 cm.

A compact architecture for three-dimensional neural microelectrode arrays

A new architecture is presented for achieving three-dimensional electronic interfaces to the nervous system using planar microfabricated two-dimensional arrays. This architecture overcomes many of the limitations of existing approaches and enables flexible electrode configurations with minimal overhead in size. A 64-channel (4×4×4) 3-D array using this architecture is demonstrated with sites on 100μm centers, interfacing with a volume of tissue less than 0.1mm3. The 1mm2 footprint of the device is the smallest ever reported.

A neural amplifier with high programmable gain and tunable bandwidth

A neural recording amplifier having programmable gain and bandwidth is presented. The gain can be digitally programmed using 6 bits from 100× to 1100× in steps of 100×. The low-frequency cutoff can be varied from less than 10Hz to above 100Hz to accept or reject field potentials while the high-frequency cutoff is fixed at 9kHz. The input referred noise of this amplifier is 4.8μV<inf>rms</inf> and it consumes 50μW operating from ±1.5V. Implemented in a 0.5μm technology, the amplifier occupies an area of 0.098mm². This amplifier has been successfully demonstrated in-vivo and compared to a commercial amplifier.

The effect of the substrate on the extracellular neural activity recorded micromachined silicon microprobes

The influence of a highly-doped silicon substrate on the neural activity recorded by thin-film microelectrode arrays has been explored using top, back, and double-sided recording sites. Probes having shank widths from 25-50 /spl mu/m and site spacings (in depth) from 20-40 /spl mu/m were used. The realization of back-looking and double-sided sites requires one mask in addition to the normal 8-mask passive probe process. Back-looking sites record as well as top sites even though separated by only a few microns from the surrounding silicon substrate, indicating that the substrate acts as an insulator and does not shunt local current from the extracellular space. On wide substrates, back-looking sites can thus be used to ensure a spherical recording field. As the substrate width is scaled to dimensions of a few microns, it should leave the extracellular field relatively undisturbed while providing mechanical support and suppressing crosstalk.

Chronic Neural Recording with a 64-Channel Cortical Microsystem

This paper presents initial neural recording results with a 64-channel wireless cortical microsystem. The system can be programmed and powered using an inductive telemetry link, switching an RF carrier between 4 and 8MHz to enter command information. Recorded neural activity is returned to the external world wirelessly over an on-off-keyed 80MHz carrier. The system can be programmed to detect positive, negative, and biphasic spikes on all the channels simultaneously or can monitor one channel with 8 bits of resolution. It has recorded single units from guinea pig auditory cortex over periods of 30 days.

MBE back-illuminated silicon Geiger-mode avalanche photodiodes for enhanced ultraviolet response

We have demonstrated a wafer-scale back-illumination process for silicon Geiger-mode avalanche photodiode arrays using Molecular Beam Epitaxy (MBE) for backside passivation. Critical to this fabrication process is support of the thin (< 10 μm) detector during the MBE growth by oxide-bonding to a full-thickness silicon wafer. This back-illumination process makes it possible to build low-dark-count-rate single-photon detectors with high quantum efficiency extending to deep ultraviolet wavelengths. This paper reviews our process for fabricating MBE back-illuminated silicon Geigermode avalanche photodiode arrays and presents characterization of initial test devices.