Micromachined Stimulating Microelectrode Arrays

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Principal Investigator	Affiliation	Contract Number	Link
Ken Wise, Ph.D.	University of Michigan	N01-NS9-2304

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Micromachined Stimulating Microelectrode Arrays, 1999

WORK STATEMENT

ARTICLE C.1

BACKGROUND.

The Neural Prosthesis Program is developing safe and effective methods of selective electrical stimulation of portions of the nervous system for restoration of function in neurologically impaired individuals. Many potential neural prostheses, including visual, auditory, and motor prostheses, will not be feasible until microelectrode arrays are developed that allow multiple, small clusters of neurons to be independently stimulated. This project will involve research and development on thin-film microelectrode arrays capable of independently stimulating as many as 512 such small clusters of cells. Specifically, these microelectrode arrays are being designed to provide microstimulation at multiple sites in the visual cortex, the cochlear nucleus and the lumbrosacral spinal cord.

Micromachining of silicon, combined with integration of electronic circuits on the micromachined structure, permits fabrication of active circuit microelectrodes with multiple stimulating sites on multiple shanks. Microelectrode arrays currently under development have 64 stimulation sites placed along 8 or 16 penetrating shanks. These 64-site, two-dimensional microelectrode arrays can be assembled into a 3-dimensional array with 512 stimulating sites. These thin-film stimulating microelectrodes have several advantages over more conventional wire bundle microelectrodes for multiple site, highly selective stimulation. Their stimulating site density is at least an order of magnitude greater than wire bundle electrodes and permits stimulation site spacing with dimensions comparable to the dimensions of neurons. The designs provide circuitry which permits extracorporeally generated stimulus instructions for many neural stimulating sites to be combined into a single signal and then decoded by integrated electronics on the implant. The integrated electronics also permit the arrays to be designed with integrated telemetry, eliminating the need for tethering cables.

This RFP represents a competitive renewal of a contract that will expire in November, 1998. The work to be done in the proposed contract consists of continued development of integrated circuit stimulating microelectrode arrays. An implantable platform that includes telemetry for receiving power and data for the array will be developed. The telemetry platform will provide a leadless, self-contained implant. The contractor will supply test devices to other investigators in the Neural Prosthesis Program who are investigating the histopathological effects of central nervous system placement of micromachined electrodes. Expertise in materials science, micromachining, integrated circuit design, and bioengineering will be needed to perform this research. No animal or human testing is required. A bibliography listing publications from current and prior NPP supported studies is available from the Neural Prosthesis Program, NINDS, NIH, Room 8A13, Federal Bldg., Bethesda, MD. 20892-9155.

ARTICLE C.2

WORK STATEMENT

A. Independently and not as an agent of the Government, the contractor shall exert its best efforts to design and fabricate microelectrode stimulating arrays capable of independently stimulating multiple, small populations of neurons within the central nervous system (CNS). These arrays of microelectrodes shall have at least 64 independent sites and some arrays shall be capable of free-standing operation, receiving both power and telemetry signals by means of an electromagnetic link, requiring no percutaneous connecting cable.

B. Specifically:

1. The contractor shall develop passive and active (containing integrated electronics) microelectrode stimulation arrays. These arrays shall have at least 64 stimulation sites distributed on up to 16 shanks that can penetrate into neural tissue. The array shall include an integrated flexible cable of at least 1.5 cm. in length that can be connected to the multiplexing integrated circuit (IC) described in item 2 below. The process used to produce the arrays shall permit fabrication of microelectrode arrays with activated iridium stimulation site areas from 200 to 1000 square microns, stimulation site placement within neural tissue ranging from 1000 to 6000 microns beyond the point of electrode penetration, and adjacent shank spacing of as little as 400 microns. Shanks shall be designed to penetrate the pia without harmful dimpling and shall be capable of penetrating pial vessels.

a. During each year of the contract, the contractor shall design, in cooperation with other investigators in the NPP, a stimulating microelectrode array suitable for stimulation of selected neural populations within the CNS. The contractor shall fabricate and supply at least 30 arrays to other investigators for in-vivo evaluation. The contractor shall cooperate with other investigators in the evaluation of these microelectrode arrays.

b. During the first year of the contract, the contractor shall design and fabricate a microelectrode stimulation array that incorporates active electronic multiplexing for the purpose of directing current controlled stimulus pulses from an external stimulator to one of several stimulation sites.

i. Multiplexor electronics on the array shall direct each of four stimulator input signals to one of 16 stimulation sites. The multiplexor switches shall support currents up to +/-100 microamps and shall permit monitoring of the voltage at a selected site when not stimulating.

ii. The number of lead wires needed to connect to the active microelectrode stimulation array shall not exceed 12 wires.

iii. Stable operation in 0.9 % saline at 37 degrees C. for a period of one year shall be demonstrated.

c. By the end of the second year of the contract, the contractor shall design and fabricate a biocompatible, microelectrode stimulation array that includes stimulus waveform generation circuitry and electronic multiplexing.

i. The microelectrode stimulation array shall have at least 4 stimulator circuits capable of generating stimulus pulses of -100 to 100 microamperes in 1 microampere steps. The stimulator circuitry shall be capable of producing cathodic first, charge balanced stimulus pulses of 30 to 500 microseconds per phase with a variable delay of up to 500 microseconds between the phases.

ii. Probes shall have multiplexor circuitry between the stimulators and electrodes that permits stimulation through any electrode. Switching from one electrode site to another shall take less than 200 microseconds.

iii. The probe operation shall include a mode that connects the stimulation sites through a low impedance to ground when not passing stimulating current.

iv. The probe design shall permit anodic bias of the stimulation electrodes at 0.25 or .5 volts.

v. The probe shall include an operating mode for electrode impedance monitoring and for electrode recording that permit recording or impedance checking of the microelectrodes.

vi. The number of lead wires needed to connect to the active microelectrode stimulation array shall not exceed 12 wires.

2. The contractor shall develop implantable multiplexing IC (s) capable of interfacing with each of the microelectrode stimulation arrays of item 1.

a. The multiplexor shall require no more than 2 leads to connect to an extracorporal stimulator module.

b. It shall be possible to anchor the multiplexing IC (or its carrier) to the skull using titanium screws.

c. Passivation of the IC and associated cable shall permit operation in 0.9% saline at 37 degrees C. for a period of at least one year.

d. The IC shall permit simultaneous stimulation pulses at 4 or more electrode sites.

e. A cable suitable for connecting between the multiplexing IC and a percutaneous connector that can be threaded subcutaneously for distances of up to 10 cm shall be devised and fabricated.

3. The contractor shall develop an integrated telemetry system and associated support circuitry that integrates with one of the 64 site microelectrode stimulation arrays of item 1 to provide a leadless stimulator system with power and telemetry supplied by electromagnetic coupling from an extracorporal coil. The implanted portion of the telemetry system can be integrated with the microelectrode array to provide a free floating unit or can attach to the microelectrode array via a flexible cable.

4. The contractor shall design and fabricate a 3 dimensional stimulating array that provides at least 512 separate stimulation sites in a 3-dimensional grid with spacing between shanks of 400 microns. The microelectrode stimulation arrays designed in item 1 can be utilized as building components of this 3-dimensional array if appropriate. Specifications for electrode stimulating material and site spacing shall be as in item 1.

a. The breaking of some of the array shanks shall not impair the function of the remaining shanks in an array.

b. No more than 12 wires shall be required to connect to the 3 dimensional array.

5. The contractor shall design and construct software and external hardware to support the in-vitro testing and evaluation of the microelectrode stimulation arrays.

6. In the performance of this contract, the Contractor shall coordinate its experimental program through the Project Officer with results of experimental findings developed by other collaborators in the Neural Prosthesis Program. Adjustments and changes so indicated shall be approved in advance by the Project Officer and if necessary, by the Contracting Officer.