Multichannel Transcutaneous Cortical Stimulation System

THIS PAGE IS ARCHIVED MATERIAL.

BACKGROUND

The Neural Prosthesis Program (NPP), National Institute of Neurological Disorders and Stroke, National Institutes of Health develops implanted devices that interface directly with the nervous system to replace or supplement function in neurologically disabled individuals.

Significant advances have been made in demonstrating the feasibility of bypassing peripheral sensory organs such as the ear and the eye to restore lost sensory functions. Recently, a blind individual had an array of 38 microelectrodes with percutaneous leads implanted into her visual cortex for a period of three months. During this time she was able to see and describe visual phosphenes produced by electrical stimulation through the microelectrodes. Critical engineering parameters, such as phosphene thresholds as low as 4 microamperes and a two point resolution of 500 microns, were determined which now permit the design of a permanent implant controlled by electromagnetic signals passing across the skin. (Ref. Hambrecht, F.T., 1995, Visual prostheses based on direct interfaces with the visual system. In Brindley, G.S. and Rushton, D.N. eds., Baillière's Clinical Neurology: Neuroprostheses, Baillière Tindall, London, pp 147-165.) Because such a permanent implant would not require any leads passing through the skin, there should be little risk of infection and the implant could be left in place indefinitely.

Specifically, a transcutaneous stimulation system consisting of a computer controlled transmitter and a group of implantable receiver-stimulator modules, each with 256 stimulus channel outputs, is needed. Research and development are required to assure that the implanted portion of this system will be small enough to fit safely and comfortably beneath the scalp and that the stimulus outputs are flexible enough to provide the range of stimulus parameters necessary for producing patterns of phosphenes by intracortical microstimulation. This transcutaneous transmission system will interface not only with discrete wire microelectrodes but also with silicon microstimulating microelectrodes presently being developed by other investigators in the NPP. The extracorporeal portion of the system will include a computer controlled transmitter for sending power and control signals across the skin to the implanted receiver-stimulator modules.

This contract research is a new project not previously supported by the NPP. A bibliography listing publications resulting from NPP studies related to this project is available, free of charge, from the Neural Prosthesis Program, NIH, Federal Building, Room 916, Bethesda, MD 20892-9170. (fax: 301-402-1501, e-mail: fh2@cu.nih.gov)

STATEMENT OF WORK

I. Independently, and not as an agent of the government, the contractor shall exert its best efforts to design and fabricate a transcutaneous transmission system suitable for use in a human visual prosthesis. The system shall consist of: 1.) a group of appropriately packaged, implantable, 256 channel receiver-stimulator modules each with high density connectors suitable for connection to 256 cortical microelectrodes; 2.) an external transmitter interfaced to an external computer which can be used to control the implanted receiver-stimulator modules; 3.) a reverse telemetry system for monitoring key voltages in the receiver-stimulator modules. The contractor will not be required to furnish the microelectrodes nor perform any animal or human testing.

Specifically the Contractor shall:

A. Design the overall system such that it can be expanded in modules of 256 channels up to 1024 channels.

B. Design the receiver-stimulator module to meet the following target specifications. (Note: Priorities and limits will have to be assigned to some of the stimulus parameters under certain operational conditions to avoid conflicts. These rules of operation will be established shortly after the beginning of the contract in a joint meeting between the contractor and the Project Officer.)

1. Capable of passing truly simultaneous biphasic current pulses through at least 16 of any of the 256 microelectrodes that will be connected to its output with the capability of passing interleaved biphasic current pulses through any or all of the non-simultaneously pulsed microelectrodes.

2. Capable of stimulating each channel at repetition rates of 10 to 250 Hz. The repetition period should have a resolution of 250 microseconds.

3. Phase durations of each phase of a biphasic pulse pair controllable over the range of 50 to 750 microseconds (50 microsecond resolution/phase)

4. Output compliance voltages of at least +/- 5 volts.

5. Amplitude of each phase controllable over the range of0 to +/- 64 microamperes (0.5 microampere resolution) unless limited by the compliance voltage.

6. Have an anodic bias supply adjustable over the range of 0 to 0.75 volts referenced to a standard calomel electrode (SCE). In a functional system, the actual voltage of this bias must be translated to correspond to the use of a large surface area platinum reference electrode that will be used in place of a SCE. Each channel must be connected to the anodic bias supply through large bias resistors (e.g. 10 megohm)

7. Train length on each channel of 1 to 255 biphasic pulse pairs.

8. Train delay times for each channel referenced to a reference timing pulse (e.g., a reference pulse for channel #1) adjustable over the range of 0 to 12700 microseconds (100 microsecond resolution).

9. Maximum total output current of each module at least 1024 microamperes.

10. Have built-in safety features that include the ability to sense the failure of any of the output drivers or other modes of operation that could result in charge imbalance and tissue damage.

a. Since it is assumed that space will not permit output coupling capacitors on each channel, a desired feature would be the ability to disconnect a shorted output driver from its power supply.

11. The modules should be capable of independent operation, i.e. should any of the modules fail, such failure should not affect the operation of the remaining modules.

12. Have a reverse telemetry system for monitoring the following voltages within each module on demand

a. The voltage waveform developed across any remotely selected microelectrode during stimulation.

b. The anodic bias supply.

c. The positive and negative power supply voltages

13. Contained in a hermetic package

a. Suitable for implantation between the scalp and the skull with dimensions no greater than 5 mm thick, 30 mm wide, and 30 mm long, not including any receiving antennae which can be external to the package.

b. With 4 separate 66 contact, high-density, low disconnect force, connectors in the package walls that interface with appropriate connectors terminating the leads from the microelectrodes or on dummy loads during in-vitro testing.

c. With smooth, rounded edges and attachment sites for immobilizing the package to the skull in an appropriate manner.

C. Design the external transmitter portion of the system to supply power and full control of up to four receiver-stimulator modules.

1. The transcutaneous transmission signal should be capable of reliably operating through human scalp tissue with thicknesses ranging from 2 mm to 10 mm.

2. The transmitter shall be under the control of a computer whose output can be altered by keyboard entries, by software or by a remote interface that will eventually be controlled by an image sensing device such as a television camera. (The contractor does not need to supply the image sensor, nor any interface electronics between the image sensor and the computer, only the remote interface that will allow full control of the receiver-stimulator modules.)

3. Although the external portions of the systems to be supplied under this contract do not need to be "patient portable", the design should not ignore the fact that future generations of the system will be portable.

D. Fabricate and test, in-vitro, a complete system with 1024 channels.

1. Document the ability of the system to deliver the specified design stimuli at the specified rates without significant cross talk between channels.

2. Test functional receiver-stimulator packages with integral connectors for hermeticity and proper electrical operation in an accelerated aging environment of heated saline solution for a period of at least 6 months.

E. Before the end of the first year of the contract, furnish the Project Officer with 5 empty but hermetically sealed receiver-module packages with at least one 66 contact connector for in-vivo testing by other investigators in the Neural Prosthesis Program (NPP).

F. Before the end of the second year of the contract, furnish the Project Officer with at least 5 fully functional, 256 channel systems, complete with mating connectors so that other investigators in the NPP can attach microelectrode leads and test the complete system in animals.

G. Before the end of the third year of the contract, furnish the Project Officer with at least 2 fully functional, 1024 channel systems, complete with mating connectors for attachment to microelectrode leads for human testing by other NPP investigators.

H. Should failures occur during the above mentioned in-vitro or in-vivo testing, the remaining required deliverables will be reduced, upon mutual agreement between the contractor, the Project Officer and the Contracting Officer, to allow resources to be applied to the redesign and fabrication of relevant parts of the system.

I. The contractor shall coordinate his work, through the Project Officer, with other investigators in the NPP.