Implantable, Non-contacting Nerve Stimulating Transducer

Abstract

An implantable transducer for stimulating a nerve without contacting it includes a subcutaneous receiver coil having a low number of turns, of the order of six, which is implantable in an accessible location. A magnetic core including first and second separable sections is placed to form a complete magnetic path around the nerve desired to be stimulated, and an excitation coil having about three turns is wound around one section of the core. A pair of conducting wires connect the excitation coil with the receiver coil. The nerve is stimulated by placing an external coil of like dimensions adjacent the receiver coil and pulsing it electrically.

Description

BACKGROUND AND SUMMARY

The present invention relates to the selective and controlled stimulating of nerves in animals, including stimulation of the nerve trunks of human beings.

It is known that an electrical signal passed through a nerve trunk through direct contact can cause stimulation. A contactless inductive transducer for nerve stimulation has also been reported, "Contactless Nerve Stimulation and Signal Detection by Inductive Transducer", J. A. Maass and M. M. Asa, IEEE Transactions on Magnetics, Vol. MAG-6, No. 2, June, 1970. Heretofore, however, these prior systems were not able to operate subcutaneously in a continuous manner or for an extended period of time without causing a rise in temperature that is inconsistent with the biological tissue surrounding the implanted device. Devices have been reported in which a ferrite core, split into semi-torroidal halves, surround the nerve trunk being stimulated. To my knowledge, the prior systems employ wires passing through the skin for electrically energizing the stimulating ferrite core. This, of course, is disadvantageous because of the inability to absolutely prevent sepsis of infection through wires passing through the skin and because of the unsightliness and adverse psychological reaction to the exposed wires.

In my invention, I include a semi-torroidal magnetic member which partially surrounds the nerve desired to be stimulated and has wound on it three turns of wire to provide an excitation coil. The magnetic path is closed by means of a laminated keeper member which is used to complete a low reluctance path about the nerve. A pair of wires connect the excitation coil with a receiver coil, located subcutaneously in a known, convenient location so that is may be accessed externally. The receiver coil has only six turns. A transmitting coil dimensioned similarly to the subcutaneous receiving coil includes 20 turns.

When the external transmitting coil is placed in proximity to the subcutaneous receiving coil and pulsed electrically, the resulting magnetic field excites the receiving coil and a pulse is transmitted through the connecting wires to the excitation coil to excite the nerve.

I have found that the present system matches the impedance of the magnetic core to that of the nerve trunk and reduces the energy required to stimulate the nerve. This results in operating temperatures which are compatible with living tissue, so that operation can be effected for an extended period of time without undue temperature rise. Further, the present system provides an implanted stimulation system which can be energized and controlled without passing wires through the skin.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment.

THE DRAWING

FIG. 1 is a diagrammatic showing of the elements of the present system which are located subcutaneously to excite a nerve trunk; and

FIG. 2 is an electrical schematic diagram showing the primary elements of the stimulation system.

DETAILED DESCRIPTION

Referring first to FIG. 1, reference numeral 10 generally designates means for providing a low reluctance path surrounding the nerve trunk 11. The means 10 includes a semi-torroidal, C-shaped core 12 defining a central lumen 13 in which the nerve 11 is placed. The C-shaped core may be formed by wrapping silicon steel tape around a suitable form. Any suitable material having a high saturation flux density may be used alternatively; and the word "magnetic" as used herein refers to such material.

The means 10 also includes a generally rectilinear keeper member 14 which completes the low reluctance path and comprises a set of laminated plates engaging the open surfaces of the C-shaped core.

Both the C-shaped core and liminated keeper member 14 are surgically implanted, so they are preferably enclosed in any known suitable non-antigenic material, such as are well-known in the implantation of various devices and organs within the body, as indicated by the dashed lines 15 and 16.

An excitation or stimulation coil generally designated by reference numeral 17 and preferably comprising three turns of enameled copper wire is wound around the C-shaped core 12, and these wires are twisted as at 18 to prevent stray pick-up and routed to a suitable, possibly more easily accessed location directly beneath the skin where they are wound into a receiver coil generally designated by reference numeral 20. The receiver coil 20 as well as the connecting wires 18 are also ensheathed in a suitable non-antigenic material. The receiver coil 20 preferably comprises six turns of wire formed to a diameter of approximately 2.54 .times. 10.sup.-.sup.2 meters in diameter with an inductance of 6 .times. 10.sup.-.sup.6 henrys.

Turning now to FIG. 2, the excitation coil 17 wound on the C-shaped core 12 is schematically shown at 17a, the wires 18a, and the receiver coil 20 at 20a.

The receiver coil 20a forms the secondary coil of a transformer generally designated by reference numeral 25 and including a primary coil 26. Everything to the left of the dashed line 32 in FIG. 2 is located external to the body, and everything to the right of that line is surgically implanted in the body, as already mentioned.

The coil 26 is a transmitter coil, and it preferably is a 20- turn coil 2.54 .times. 10.sup.-.sup.2 meters in diameter with an inductance equal to 28.6 .times. 10.sup.-.sup.6 henrys. It has the same diameter as the receiver coil 20a so that when the two are superimposed, the mutual magnetic coupling will be enhanced.

A switch 27 is connected in series with a capacitor 28 and the transmitting coil 26. A dc power source 29 is connected by means of a switch 31 to charge the capacitor 28. The switches 27 and 31 may be silicon controlled rectifiers and they are controlled, in turn, by circuitry (called a "controller" and represented by block 30) which coordinates the charging and discharging of the capacitor 28 so that the coil 26 is periodically pulsed by the charged capacitor 28 when the switch 27 closes.

The circuitry for charging the capacitor 28 and for controlling the closing of the switch 27 while preventing shorting of the power source is disclosed in my co-pending application Ser. No. 300,571 entitled "External Inductive Neural Stimulator System", filed concurrently; and the subject matter of that application is incorporated by reference herein for the purpose of enabling the practice of the present invention briefly, the charging and discharging of capacitor 28 occur at mutually exclusive times to avoid shorting the source 29 through the coil 26.

Using the present invention, the sciatic nerve of a dog has been successfully stimulated with a non-contacting nerve stimulator operating at temperatures compatible with living tissue. The capacitor 28 had a capacitance of 500 .times. 10.sup.-.sup.6 farads, and the transmitting coil 26 and receiving coil 20a are as has already been described.

In operation, when the switch 27 closes, the capacitor 28 discharges through the transmitting coil 26. The transmitting coil is located next to the receiving coil 20a, which is located subcutaneously; and because of the mutual inductance existing between them, a voltage is generated at the terminals of the receiving coil 20a. The voltage is transmitted by means of the connecting wires 18a to the stimulation coil 17a on the C-shaped core member 12. A low reluctance path about the nerve 11 is provided by the keeper member 14 which may be physically bonded to the C-shaped core 12 with any suitable adhesive.

The resulting voltage at the terminals of the stimulation coil 17 (or 17a in FIG. 2) generated an electric field which produces a current sufficient to stimulate the nerve trunk passing through the lumen 13 of the low magnetic reluctance means 10. Sixty volts on the capacitor 28 has been adequate to produce visual contractions of the muscles of the rear leg of a dog.

Thus, by means of the present invention, I have been able to stimulate nervous tissue with an induced electric field produced by a non-contacting transducer. A receiving coil, stimulating coil and low reluctance magnetic path means surrounding the nerve desired to be stimulated are all implanted wholly beneath the skin, and the receiving coil is energized externally. Thus, there is no need for wires to pass through the skin; and with my design, I have found that operation of the unit does not produce excess temperatures, considering the environment in which the unit is placed.

Having thus described in detail a preferred embodiment of the invention, persons skilled in the art will be able to modify certain of the structure which has been illustrated and to substitute equivalent elements for those disclosed while continuing to practice the principle of the invention; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims.

Claims

I claim:

1. Apparatus for external stimulation of a nerve in a body without contacting the body comprising:

a transducer adapted to be implanted in said body including

low reluctance magnetic means comprising first and second magnetic members for surrounding a nerve selected to be stimulated and providing a central lumen through which said nerve passes when said first and second members are in assembled relation;

an excitation coil including approximately three turns of conducting wire wound around one of said members;

a receiver coil adapted to be implanted in said body and including approximately six turns of wire;

conductor means connecting said excitation coil with said receiving coil and adapted to be located subcutaneously; and

controller means adapted to energize said receiver coil externally of said body for transmitting electrical pulses to said receiver coil.

2. The system of claim 1 wherein said external excitation means includes a transmitter coil having a diameter approximately equal to the diameter of said receiving coil and is placed next to said receiving coil; and means for periodically exciting said transmitter coil with pulsed electrical current.

3. The system of claim 2 wherein said receiver coil has six turns of conducting wire with an approximate diameter of 2.5 .times. 10.sup.-.sup.2 meters and an inductance equal to approximately 6 .times. 10.sup.-.sup.6 henrys.

4. The system of claim 3 wherein excitation means comprises a capacitor; means for charging said capacitor; and a switch connected in series with said transmitter coil whereby when said switch closes, said capacitor is discharged to provide said pulsed electrical current.