Steroid eluting cuff electrode for peripheral nerve stimulation

An improved electrode for establishing electrical contact with a nerve of a patient. This electrical contact may be used to sense neural activity on the nerve or to artificially stimulate it to perform various medical treatments. The electrode has an outer cuff of an insulating material which is sutured around the nerve to be contacted. A drug impregnated layer of material is positioned inside of the outer cuff. This material is a polymeric matrix which permits the drug to leach out at a predetermined rate. The drug is a steroid such as dexamethasone sodium phosphate. The actual electrical contact is produced by metallic foil which is positioned on the surface of the drug impregnated layer of material which is located inside of the cuff. An insulated lead electrically couples the metallic foil to an electronic circuit located remote from the nerve. The leaching out of the drug serves to control irritation, swelling, and impedance of the nerve/electrode interface.

CROSS-REFERENCE TO COPENDING APPLICATIONS 
This application is related to Ser. No. 07/446,594, filed Dec. 6, 1989, now 
U.S. Pat. No. 3,009,228, entitled "Steroid Eluting Intramuscular Lead" and 
Ser. No. 07/446,865, filed Dec. 6, 1989, now U.S. Pat. No. 3,031, entitled 
"Nerve Electrode with Biological Substrate", both of which assigned to the 
assignee of this application. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to implantable medical devices and 
more particularly relates to electrodes for electrically coupling to nerve 
tissue. 
2. Description of the Prior Art 
The use of electrodes to monitor electrical activity and stimulate body 
tissue is quite old. U.S. Pat. No. 1,662,446 issued to R. H. Wappler 
teaches an early electrode system. The Wappler electrode is used for acute 
stimulation only, and is not implantable. 
An early stimulation electrode which is chronically implantable is taught 
by S. I. Schwartz, et al in U.S. Pat. No. 3,421,511, herein incorporated 
by reference. U.S. Pat. No. 3,654,933 issued to Hagfors, herein 
incorporated by reference, teaches an improved stimulation electrode for 
chronic implantation. Clinical experience with the electrodes taught by 
Schwartz, et al and Hagfors may produce excess irritation in certain 
applications. This irritation results in swelling of the nerve tissue and 
ultimately an unacceptable increase in impedance of the nerve 
tissue/electrode interface. 
U.S. Pat. No. 4,341,221 issued to Testerman teaches an improved nerve 
electrode for chronic implantation which uses gel electrodes. While 
offering some improvement, excess irritation is yet experienced in some 
patients. 
Cardiac pacing leads which sense and stimulate activity in the myocardial 
muscle tissue have been in use for some time. U.S. Pat. No. 4,711,251 
issued to Stokes teaches an endocardial pacing lead having steroid drug 
embedded in the distal tip. U.S. Pat. Nos. 4,506,680; 4,577,642; and 
4,606,118 teach similar endocardial pacing leads, all of which have an 
embedded steroid drug. U.S. Statutory Invention Registration No. H356 
discloses a pacing lead suitable for epicardial application which elutes a 
steroid drug. Because cardiac pacing leads establish electrical contact 
with muscle tissue rather than nerve tissue, the total contact area is 
extremely small to increase current density. Therefore, the area of muscle 
tissue to be treated by the embedded drug is so small that only minute 
quantities of drug need be eluted. 
SUMMARY OF THE INVENTION 
The present invention overcomes the disadvantages of the prior art by 
directly treating the irritation of the nerve tissue caused by contact 
with the electrode. 
This area is treated with an anti-inflammatory drug such as a steroid which 
is topically applied at a predetermined chronic rate. 
The steroid drug is embedded in a polymeric matrix which permits the drug 
to leach out at the desired rate. The polymeric matrix is a layer of 
material which is applied to the outer substrate of the electrode system. 
The metallic electrode elements are positioned on the polymeric matrix 
layer opposite the outer substrate. 
The outer substrate is wrapped about the nerve at the desired location, and 
the edges of the outer substrate are sutured to hold the electrode in 
place. The metallic electrode elements contact the nerve tissue directly 
and are electrically coupled to an insulated lead which couples to remote 
electronic circuitry. The polymeric matrix thus treats the entire surface 
area of the nerve tissue which is wrapped with the cuff formed by the 
sutured outer substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a plan view of a neurological lead suitable for chronic 
implantation incorporating the present invention. The proximal end of the 
lead has connector pins 6 and 7 which establish electrical contact with 
electronic circuitry remote from the site of the nerve. This electronic 
circuitry (not shown) may include sensing and monitoring functions and/or 
pulse generation functions for stimulation. Connector pins 6 and 7 are 
electrically coupled to insulated wires 4 and 5 which join at point 8 and 
are covered distally by outer insulating sheath 9. The dashed lines show 
the location 10 and 11 of the extension of wires 4 and 5. 
By way of illustration, outer substrate 15 is an insulator of a flexible 
polymeric material having a length of about 3/8 inch and a width of about 
1/4 inch. It is fixedly attached to outer insulating sheath 9. Attached to 
outer insulating sheath 9 is polymeric matrix layer 16 which is not shown 
in this illustration. The position of metallic foils 12 and 13 are shown 
by the dashed lines. 
FIG. 2 is a closeup view of the electrode structure as seen from the side 
which directly contacts the nerve tissue. Extension 10 is a conductor coil 
which is electrically coupled to metallic foil 13. Similarly, extension 1; 
is a conductor coil which is electrically coupled to metallic foil 12. 
Polymeric matrix layer 16 is shown as attached to the entire surface of 
outer substrate 15. Molded portion 14 of outer substrate 15, because of 
its increased thickness, tends to ensure ease of implantation by causing 
bending of outer substrate 15 at the desired points. 
Polymeric matrix layer 16 may be fabricated in a variety of ways. By way of 
example and not to be deemed as limiting of the present invention, a 
preferred mode is a mixture of 0.2 milligrams of dexamethesone sodium 
phospate with 0.5 cubic centimeters of silastic medical adhesive. The 
mixture is molded to the desired shape and allowed to cure. After curing 
polymeric matrix layer 16 is fixedly attached to outer substrate 15 with 
silastic medical adhesive. 
FIG. 3 is a closeup of a cross-section of the electrode structure. 
Polymeric matrix layer 16 is attached to outer substrate 15. Metallic foil 
12 is attached to polymeric matrix layer 16 using silastic medical 
adhesive. Metallic foil 13 (not shown) is similarly attached. 
FIG. 4 is a view of the electrode assembly after it has been sutured into 
place for chronic implantation. Outer substrate 15 is wrapped about nerve 
20 as shown. It is fixedly attached using sutures 21 at the edges of outer 
substrate 15. Molded portion 14 is shown as ensuring that outer substrate 
15 is wrapped in the proper way. Outer insulating sheath 9 is shown as 
proceeding proximally from the electrode assembly parallel to nerve 20. 
Having thus described the preferred embodiments, those of skill in the art 
will be readily able to apply the present invention without departing from 
the scope of the claims which are hereto attached.