Thin-Film Intracortical Recording Electrodes

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Principal Investigator	Affiliation	Contract Number	Link
Ken Wise, Ph.D.	University of Michigan	N01-NS7-2364

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ARTICLE C.1 INTRODUCTION

Chronic recording of single unit activity from large numbers of neurons in the central nervous system (CNS) is desired by neural prosthesis researchers who need long-term multielectrode connections with neurons for prosthetic control signals. Neurophysiologists also need multiple-unit recording electrodes for studying cortical interactions in chronic preparations. Micromachining of penetrating microelectrode structures and thin-film definition of recording sites and insulated conductor paths on these structures makes possible multiple, precisely-shaped microelectrode sites on shanks as small as 60 microns wide and 15 microns thick. On the same microstructure, integration of electronic circuitry designed to switch between different recording sites and to buffer the weak neural signals detected at the sites permits the activity from many recording sites to be recorded in an unshielded environment. The buffered signals from neurally active sites can be multiplexed onto a small number of output leads.

The Neural Prosthesis Program (NPP) is supporting the development of a microelectrode array capable of recording from up to 64 sites located on a probe with multiple penetrating shanks. The broad objectives of the project are to develop multi-channel penetrating electrode arrays for both CNS and peripheral nervous system (PNS) recording that will operate reliably in the hostile ionic environment within the body over decades of use in chronic implants. This will require the integration of micromachined electrode structures with active electronics and with integral flexible ribbon cables or with integral telemetry systems. This project will support the development and testing of the chronic recording electrode arrays. Electrode systems that include active electronics and ribbon cables will be tested in-vitro and in chronic animals. Probes will be furnished to the NPP for testing by other investigators.

A current bibliography listing publications resulting from Neural Prosthesis Program studies in this and related areas is available free of charge from the Neural Prosthesis Program, NIH, Federal Building, Room 9l6, Bethesda, MD 20892, e-mail: w2h@cu.nih.gov.

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ARTICLE C.2 STATEMENT OF WORK

I. Independently and not as an agent of the Government, the contractor shall exert its best effort to design, fabricate, and test multiple-site intracortical recording microelectrode probes. The contractor shall also acquire or develop associated cables, connectors, and head mounts to produce a reliable signal path between the recording sites on a microelectrode probe and an extracoporal mating connector.

Specifically the contractor shall:

A. During the first year of the contract, design and fabricate a microelectrode recording probe.

1. A single probe shall have at least eight penetrating shanks, each with sufficient strength to penetrate the pia-arachnoid and cortex in primates without probe breakage. Shanks shall not exceed 60 microns in thickness or width and shall be at least 2.5 mm in length.

2. Each probe shall have at least forty-eight recording sites distributed along the shanks.

3. Each probe shall have at least 4 signal output lines.

4. Each probe shall include electronic switching circuitry to enable each of the recording sites to be directly connected to at least one of the signal output lines of the probe for neural recording. Switching an output line between different recording sites shall require no more than 5 milliseconds.

5. At least one recording site on each shank shall connect to the switching circuitry by two independent conductor paths. These two paths shall be capable of being switched to two different output lines to permit verification of electrical continuity of the conductor path to the recording site and shall permit direct monitoring of the voltage developed between the recording site and a large return electrode when small-signal impedance test currents are passed through the site.

B. For the recording probe of A., develop or obtain a flexible microcable, connector, and head mount system to permit the probe, when implanted in cortical tissue, to communicate with an extracorporal connector.

1. At the probe, the output lines and any required control and power lines shall connect to a flexible microcable that is at least 2 cm. in length. The 4 mm. of this cable closest to the probe shall be at least as flexible as a 100 micron diameter gold wire.

2. The non-probe end of the microcable shall terminate on a percutaneous connector/mount assembly suitable for mounting on the head of a mammal.

3. The portions of the probe, cable, percutaneous connector and mount system that contact tissue shall be made of biocompatible materials.

4. The probe, cable, and percutaneous connector shall be capable of sustaining sterilization with steam or ethylene oxide (either is adequate for this requirement) without changes in operating characteristics.

5. The probe, cable, and percutaneous connector shall be designed to provide chronic neural recordings from the mammalian cortex. In this application, chronic refers to periods greater than 6 months.

6. At least 20 probe-cable-connector assemblies shall be supplied to the Project Officer over the 3 year period of the contract for evaluation by other members of the Neural Prosthesis Program.

C. Evaluate the probe-cable-connector(PCC) assemblies developed in B. in-vitro and in a mammalian cortex (excluding chimpanzees).

1. Test the PCC assembly in-vitro in a simulation of the conditions to be expected in-vivo. In-vitro testing shall include soak testing (0.9% NaCl solution) of the probe and microcable, testing of the connect-disconnect function of the percutaneous connector and testing of the mechanical shock resistance of the complete system.

2. Investigate the electrical stability of the PCC assembly over a period of at least 6 months when recording in mammalian cortex by observing the integrity of the electrical insulation at all points of the PCC assembly and studying the stability of the electrode recording surface.

3. Demonstrate chronic neural recording over periods of at least 6 months using the PCC. Modify the PCC design as needed in response to any problems revealed by the chronic recording.

D. Design, fabricate and test an active circuit multiplexed recording probe with the following characteristics:

1. The probe shall have at least 64 recording sites distributed along 8 or 16 pitchfork shanks. The sites shall have characteristics suitable for recording single unit neural activity.

2. The probe shanks shall have a maximum width of 80 microns, a maximum thickness of 60 microns and a minimum length of 2.5 mm.

3. The probe electronics shall include at least eight preamplifier/filters and a multiplexer with characteristics suitable for amplifying the neural spike potentials at selected recording sites and multiplexing these amplified signals onto a single output line.

a. The preamplifiers shall have an input impedance greater than 20 megohms at 1000 Hz, a bandwidth of at least 500 Hz to 6 KHz, a noise level referred to the input over the specified bandwidth of less than 20 microvolts r.m.s. for a source impedance of 2 megohms, and shall tolerate up to 200 millivolts of DC offset at the input.

b. The multiplexer shall combine the outputs of the preamplifiers onto a single, multiplexed line. The output impedance of this line shall be less than 500 ohms and it shall be short-circuit proof.

c. The electronic circuitry shall operate at a voltage of five volts or less.

4. The probe shall include an electronic switch matrix between the recording sites and the preamplifiers with the following characteristics:

a. The switch matrix shall be externally programmable. Switching the matrix to a new electrode configuration shall require less than 1000 ms.

b. The switch matrix shall allow selection of any set of adjacent recording sites, along a single shank or across shanks, as inputs to the preamplifiers. Additional combinations of selected electrodes are desired but completely random selection of recording sites is not required.

c. On probe cross talk signals from non selected recording sites shall be at least 40 db below signals from the selected recording sites.

5. The probe shanks shall have sufficient strength to penetrate the pia-arachnoid and cortex in primates without probe breakage.

6. The height of the non-penetrating portion of the probe that contains the switch matrix, the preamplifiers and the multiplexer shall be no more than 1 mm.

7. The probe circuitry shall be designed to require no more than 7 conductors in the cable from the probe to the connector. The probe shall be compatible with the microcable and connector assembly developed in B.

8. The probe electronics shall be covered with a biocompatible coating that will permit stable operation of the probe in a 0.9% NaCl saltwater bath for at least 1 year.

9. The contractor shall deliver 5 active recording microelectrodes by the end of the contract period.

E. Develop and fabricate a subdural platform for mounting single or multiple electrode probes giving special consideration to the following.

1. The platform shall be of a size and configuration to permit it to rest on the pia-arachnoid surface of the cortex with minimal damage to underlying structures, and it shall be designed to minimize motion between the probe shank and the neural substrate. The height of the platform shall be less than 3 mm.

2. Communication between extracorporal electronics and the implanted platform shall be through a telemetry system that utilizes an extracorporal inductive coil placed over the platform.

3. The inductive telemetry system on the platform shall provide power to operate at least 1 probe and shall have telemetering capacity to control and receive signals from at least one probe with the characteristics of the probe developed in C.

4. The platform shall incorporate the necessary hardware to substitute a flexible ribbon cable for the inductive telemetry system.

5. The contractor shall deliver ten platforms with mounted microelectrodes by the end of the contract period.