An integrated telemetric multichannel sieve electrode for nerve regeneration applications.

Abstract

Chronic recording from individual axons and fibers in the peripheral nervous system (PNS) is a long pursued goal to further our understanding of the nervous system and for potential control of prosthetic devices and stimulation of paralyzed muscles. This thesis demonstrates the development of a new, totally implantable system to record neural signals from axons and fibers in the PNS using the nerve regeneration of the PNS. The system is composed of a nerve regeneration electrode (sieve electrode), a radio-frequency (RF) telemetry link, and on-chip signal processing and RF interface circuitry. The electrode structure contains a large number of small holes through which axons of a severed nerve will regenerate and reinnervate its target organs. The small holes are surrounded by 100-200 $\mu{\rm m}\sp2$ anodized iridium oxide sites, which can be used for both recording and stimulation and have impedances of less than 100$\Omega$ 1kHz and charge delivery capacities in the 4-6 mC/cm$\sp2$ range. The sieve electrode is fabricated using silicon micromachining. The fabrication process is single-sided, has high yield, requires only five masks, and is compatible with integrated silicon ribbon cables. These electrodes have been implanted between the cut ends of peripheral taste fibers of rats (glossopharyngeal nerve), and axons have functionally regenerated through holes, responding to chemical, mechanical, and thermal stimuli. In addition, spontaneous action potentials were recorded using passive electrodes with a percutaneous connector. An inductively-coupled RF telemetry link has been developed using two small circular coils to transmit power and bi-directional data to a small chronic system, eliminating the need for cables and wires. The link is driven by a high efficiency class E amplifier, which is amplitude-modulated for transmitting control data to implanted on-chip circuitry. The on-chip circuitry for a small implantable neural recording unit has been developed to interface with the RF telemetry link and for signal processing. The on-chip circuitry generates a 5 V supply, the clock signal, and control data from the RF signal, and allows simultaneous recording from any user-selectable 2-of-32 channels. These signals are amplified, multiplexed, and digitized with an on-chip low-power (2mW) current-mode analog-to-digital converter before transmission to outside world using passive telemetry. The full functionality of the on-chip CMOS circuitry has been verified by fabricating it at the University of Michigan. The on-chip circuitry contains more than 5000 transistors, dissipates about 90 mW of power, and results in an implantable unit which measures 5mm x 8mm x 2mm. This is the first chip of its kind that combines signal amplification and filtering, low-power analog-to-digital conversion, and bi-directional RF telemetry for power and data transfer all integrated monolithically on a single chip.