CA2243632A1 - Cochlear electrode array employing dielectric partitions - Google Patents

Abstract

An electrode array (10) for stimulation of the cochlea includes an elongated tapered carrier (15) on which a multiplicity of separately controlled electrode contacts (20) are carried. A set of flexible fins (100, 110, 120) extend from the carrier in particular axes so as to cause the outside dimension of the array plus the fins to be greater than the typical cross section of the cavity in which the array is to be inserted. The fins are made from compliant, dielectric material so that they can be folded against the body of the carrier as it is inserted into the cochlea so that they slide past obstructions and accommodate variations in the cross-sectional dimensions of the cavity, e.g., the scala tympani (5). When in place; the fins unfurl so that they touch the walls of the cavity into which they are inserted, thereby forming a series of separate longitudinal compartments (35), at least most of which contain at least one separate stimulating electrode. The fins confine electrical currents injected through most of the contacts to flow preferentially through different portions of the wall of the cavity in which the electrode array is inserted, thereby selectively stimulating/activating cells encompassed by the compartments.

Description

COCHLE~R ELECTRODE ARRAY
El~PLOYING DIELECTRIC PARTITIONS

R~ekground of the Tnvention S The present invention relates to impl~nt~hle stimulation devices, e.g., cochlear prothesis used to electric~lly stimulate the auditory nerve, and more particularly to an electrode array employing rliele~tric partitions that may be used with such implantable stimulating devices.
A cochlear prosthesic provides sen~tions of sound for p~tiPnt~
10 suffering from sensorineural deafiless. It operates by direct electrical stirnulation of the auditory nerve cells, bypassinj, the defective cochlear hair cells that normally transduce acoustic energy into electrical activity in such nerve cells. In addition to stim~ tinp~ the nerve cells, the electronic ci~ y and the electrode array of thecochlear prosthecic must ~e,~."~ the function of the separating the acoustic signal 15 into a number of parallel ch~nnels of information, each r~l~se ~ting the intensity of a narrow band of frequencies within the acoustic spectrum. Ideally, each çh~nnel of information would be conveyed selectively to the subset of auditory nerve cells that normally tr~n~mitted information about that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from high frequencies at 20 the basal end of the cochlear spiral to progressively lower frequencies towards the apex. In practice, this goal tends to be dif~lcult to realize because of the anatomy of the cochlea.
After extensive research in many centers employing a variety of surgical sites and approaches for the implantation of cochlear electrode arrays, a 2~ con~en~lc has generally emerged on the use of the scala tympani, one of the three parallel ducts that, in parallel, make up the spiral-shaped cochlea. The electrode ar;ay to be implanted in this site typically consists of a thin, elongated, flexible carrier containing several longitutlin~lly disposed and separately connP,cte~l stimu~ating electrode contacts, perhaps 6-24 in number. Such electrode array is 30 pushed into the scala tympani duct to a depth of about 20-30 mm via a surgical CA 02243632 l998-07-22 W 097126943 PCTrUS97/00936 opening made in the round window at the basal end of the duct. During use, electrical current is passed into the fluids and tissues immt~Ai~tPly surrounding the individual electrical contacts in order to create transient potential gradients that, if sufficie-ntly strong, cause the nearby auditory nerve fibers to generate action 5 potentials. The auditory nerve fibers arise from cell bodies located in the spiral ganglion, which lies in the bone adjacent to the scala tympani on the inside wall of its spiral course. Rec~llse the density of electrical current flowing through volume condnctors such as tissues and fluids tends to be highest near the electrode contact that is the source of such current, stimulation at one contact site tends to activate 10 seliectively those spiral ganglion cells and their auditory nerve fibers that are closest to that contact site.
The selectivity of stimulation at each site provides a means for conveying different sound perceptions. Typically, cells (and their correspondingauditory nerve fibers) that are in one region or area convey sound pei~Lions 15 within a given frequency band or channel. This selectivity of stimulation at each site provides a lower limit on the useful spacing available between ~cent sites.Tlhat is, if adjacent sites closer than that lower-limit spacing are stimulated simultaneously, then the signals carried by the neurons can no longer distinguish res~;live frequency bands separately, but rather will convey signals that are 20 cont~nin~t~cl by cross-talk between channels and may be perceived as unclear andlor excessively loud. This lower-limit spacing also effectively limits the m~xim~l number of parallel ch~nnt~1~ of information that can be conveyed about acoustic signals such as speech because the length of the scala tympani over which ~he complete range of speech signal frequencies is l~lesellted is fixed at about25 15-20 mm in length. Further, the actual selectivity of stimulation at each site is limited by the spreading of the injected electrical current through the volume-collductive tissues and fluids of the cochlea.
Several stimulation strategies have been described in the prior art for m~ximi7ing the selectivity. These include:

W O 97126943 PCTrUS97/00936 ~ipol~rSfim~ tion - Bipolar stimulation provides two closely spaced electrode contacts within the scala tympani which are used to provide both a ? sourcc and s;nk for the stirnulating electrical current (see e.g., U.S. Patent No. 4,gl9,647), instead af the monopolar configuration in which the sink for all chstnne1s is a common electrode located outside of the cochlea. With bipolar stimulation, the rate at which the current density decreases with distance from the electrodes is much greater than with monopolar stimulation. There is a significant disadvantage, however, in that the ~mount of current requirecl to produce adequate stim~ tion at each site and lû the power required to pass that current through the tissues is much higherthan for monopolar stimulation. This is a significant disadvantage for the efficient design and operation of implanted mic~u.,.it~ circuitry in a portable battery-powered system.
nir~rtion~1 Contst~ In some electrode ~esign~, the individual contacts are shaped like cigar bands, causing the stim~ tin~ current to radiate in all directions equally. By using smaller contstcts that occupy only a por~ion of the transverse cross-section of the electrode carrier at a particular lon~itl1~lin~1 position, the current density can be made asymmetrical (see, e.g., I;'.S. Patent Nos. 4,686,765; 4,819,647). If the design of the electrode array and its p1~cem~nt by the surgeon permits the contacts to be reliably pos itioned so as to be facing the medial wall of the scala tyrnpani, in which the spiral ganglion cells reside, the selectivity will be somewhat improved. l'he improvement tends to be limit~, however, by the t~nden&y of stimulating current to disperse broadly through the relatively ~5 conductive fluids of the scala tympani. Furthermore, the small surface areaof the contacts will increase their electrical impedance and, hence, the voltage re~uired to deliver a particular stimulating current.
Sp;ral-Sh~ed ~ iers - Regardless of the design of the electrode contacts and the tissues in which they reside, the current density is always W 097126943 PCTrUS97/00936 highest nearest the contact surface. One strategy, therefore, that has been used with small contacts that face the medial wall is to embed them in an elastomeric carrier that is molded into the shape of the cochlear spiral (see U.S. Patent Nos. 4,686,765; 4,819,647). Upon insertion, the carrier regains its spiral shape, drawing the contacts close to the medial wall. The fabrication of such an electrode array is somewhat complicated, however.
Furthermore, special instruments and techniques must be used by the surgeon in order to hold the electrode straight in order to effect insertion into the round window opening.
1{~ Sp~k-Filling ~rriers - Yet another technique known in the art to position directional contacts near the medial wall is to make the electrode array relatively thick in cross-section. This can be done by using a mold whose ~imt~n~ions are sized closely to the cross-section~ m~n~ions of the scala tympani (see U.S. Patent Nos. 4,686,765; 4,819,647). Other techniques that achieve this same purpose could include adding flexible fins along the lateral edge to push the electrode towards the medial wall, or by making some or all of the carrier from a material that swells, inflates, or otherwise cll~nges its (limçn~ions after insertion. One problem with these techniques is that there is a fairly large range of variability in the ~1im~n~ions of the scala tympani from one patient to another and there are often irregularities in cross-sectional area along the length of an individual scala Ly~ i. As the electrode contacts get closer to the medial wall, even small fluctuations in the actual gap and the points of actual contact with the side walls can cause large changes in the distribution of the stimulating currents from each site, which may disrupt the orderly tonotopic r~,~sentation and the balance of loudness between channels. Furthermore, the su~eon who ~erforms the implant generally prefers an electrode that is as thin as possible to improve the ch?~nces of being able to insert it successfully in any conditions that may obtain.

W O 97~26943 PCT~US97~0936 Sep~rate (~ont~t~t Pl~cPment~ - Another technique for ma~imi7ing the selectivity of stimulation sites is to drill into the scala ly~ ani through its Iateral wall at multiple locations and place a separate stimulating electrode ineach site, as described by IChouard and MacLeod (1976). Before sealing the holes, small plugs of a nonconductive material such as silicone elastomer are inserted into the holes so as to flank each electrode in an attempt to prevent its currents from spreading lon~ lin~lly in the conductive fluid of the scala tympani. Several problems developed with this technique that caused it to be abandoned. Only one side of the cochlear spiral is surgically ~cçssible in this manner ~nd even then, it is difficult to ~lrOllll the multiple fenestrations without r~m~in~ the extremely delic~te "~emb~ es that separate the three pa~lel canals. Further, even if ~ccur~tely-sized plugs could be in.st~ll~, they tend to block only lf~ngihldin~l conduction and not lateral conduction out of the scala tympani and into ~dj~ -ont, overlying 1~ turns of the spiral; in fact, the scar tissue that eventually seals over the fenestr~ti- ns tends to be more conductive than the bone that it replaces, actually ch~nnelin~ stimulation currents laterally rather than in the desired medial direction.

20 Summ~ry of the Inv~ntion The cochlear electrode array that is the subject of the present invention includes a single elonga,tedt tapered carrier on which a multiplicity of electrode contacts are carried. The electrode array is clç~ignecl to be inserted into the scala tympani via an opening at or near the round window, in the conventional 2~ manner. A set of thin fins project from the body of the carrier in particular axes.
These fins are made of a highly flexible but resilient material that can be folded ag~inst the body of the carrier so as to slide past obstructions and to accommodate ~anations in the cross-sectional riimen~ions of the scala tympani.

W O 97/26943 PCTrUS97/00936 In a ~le~ll~d embodiment, the flexible fins are extensions of the silicone elastomer that forms the body of the carrier and are molded as one with the body of the carrier in a single injection molding process. In other embodiments, the fins may be formed or added to the carrier employing a variety of other materials S and processes known in the art.
In accoldance with one aspect of the invention, the fins should be made of a dielectric material, i.e. a material with a much higher resistivity toelectrical current than any of the surrounding tissues of the cochlea. Upon insertion, the elastic fins are d~sig11ed to unfurl so that they touch the walls of the 10 scala tympani, effectively separating it into a series of separate longitu~lin~l col"~ nlents, each of which contains a sep~te stimulating electrode. Re~ se the fins have a much higher resistivity than the surrounding tissue, current can flow to or from the eleçtrode in each col,~pa~ lent only through the portion of the wall of the scala tympani that lies within that co"ll)~ llllent, thereby çnh~nçing the ~5 selectivity of the spiral ganglion cells ~jaçent to that colllp~u~lllent.
It is a feature of the invention to provide a cochlear electrode array that produces a predictable and highly selective activation of spiral ganglion cells at each of a large number of closely spaced sites along the longit~ldin~l axis of the cochl~
It is a further feature of the invention to provide a cochlear electrode alTay that is readily insertable in scalae tympani that have a wide range of ~limP~ ions and even partial obstructions.
It is another feature of the invention to provide a cochlear electrode array that has a minim~l likelihood of producing damage to the cochlea upon its 2~ initial insertion, after prolonged function, and even in the event that surgical removal ~ndlor replacement is required.
It is yet an additional feature of the invention to provide a cochlear ~ectrode array that minimi7es the electrical power required to achieve adequate stimulation and perceived loudness.

, It is still another feature of the invention to provide a cochlear electrode array that is readily manufacturable.

~..
Rrief nes~ on of the nr~w~
S The above and other aspects, features and advantages of the present invention will be more a~a.~ nl from the following more particular description thereof, presented in conjunction with the following drawings, wherein:
FIG. 1 shows a perspective view of an electrode array made in accordance with one embodiment of the present invention;
FIG. 2A is a top schematic ~ sentation of the electrode array of FI~i. l;
FIG. 2B is an e~cr~n-le~l top view of one of the co-,-pa~ ents of the elect~ode array;
FIG. 2C is an e7cp~n~led front view of one of the co",pa,l",ents of the electrode array;
FIG. 3A is a cross-sectional view taken along the line A-A in FIG.
2A;
FIG. 3B is a cross-sectional view taken along the line B-B in FIG.
2A;
FIG. 3C is a cross-sectional view taken along the line C-C in FIG.
2A;
FIG. 3D shows the desired fit of the apical cross-section A-A in the ~ala tympani; and FIG. 4 shows a crclss-sectional view of an ~ltern~tive embodiment of e 2~ the invention that includes an additional elongated electrode contact that may be used as a reference or return eleclrode.
Corresponding reference ch~ t~rs indicate c<: lles~)onding components throughout the several views of the drawings.

net~ A l~escrir~tion of tlle Invention The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the 5 invention. The scope of the invention should be determined with reference to the claims.
An example of a prior electrode array, and the manner of making such an array, is taught in United States Patent Nos. 4,686,765 and 4,819,647, both of which are incorporated herein by reference.. Many of the te~*ingc of these 10 patents, e.g., r~gal~ling physiology of the cochlea, m~Pn~l~ for the electrodes and carrier/body, manufacturing techniques, sizes, (1imensions of the scala tympani,etc., apply equally well to the present invention.
A prer~ d embodiment of an electrode array 10 made in accordal~ce with the invention is shown in FIGS 1, 2A, 2B, and 2C. FIG. 1 shows a 15 perspective view of the electrode array 10; FIG. 2A is a top schematic representation of the electrode array 10; FIG. 2B is an e~cp~nded top view of one small section or co-l.palllllent 3~ of the electrode array 10; and FIG. 2C is ane~cr~n~ l front view of one of the co,llpal l,llents 35 of the electrode array.
As seen in FIGS. 1, 2A, 2B and 2C, the electrode array 10 includes a 20 body 15, a multiplicity of individual contacts 20 and their ~csoci~t~cl wire leads 30 coursing through the body 15, plus fins 100, 1 lû, and 120. The outside dimensions of the electrode array plus fins at various points along the length of the array is carefully sized so as to be at least slightly larger than available cross-sectional ciimen~ions of the scala tympani in most human beings. Typical cross-sectional 25 profiles at three points along the array, ciecign~ed A-A, B-B and C-C in FIG 2A, are shown in FIGS 3A, 3B and 3C, respectively.
After insertion of such an electrode 10 to the intended depth in the scala tympani, the various fins will touch and be somewhat bent or co~ c;ss~d by their contact with the walls of the scala tympani. The desired fit of the apical cross-section A-A in the scala tympani is shown in FIG. 3B.
In the ~l~Ç~ d embodiment that is illustrated in the figures, the fins lie in only two axes. One pair of ims 100 and 110 projects perpendicularly from 5 the body so as to create a lon~ihl-lin~l barrier in the vertical axis of the spiral. It is a feature of this arrangement that any stiffn~os~ contributed by fins 100 and 110 contributes to the desired ~?l~elly of the electrode array that it flex readily in only this vertical axis, particularly in the more apical regions of the scala tympani where the electrode array must curl tightly along this axis to conform to the spiral shape of 10 the cochlea. This flexion p~ ly is further enh~nced in the pr~Ç~ d emborlim~nt by the gathering of leads 30 into a vertically aligned "rib" 35 as illustrated in the cross-sectional view in FIG 3B.
A multiplicity of fins 120 project perpendicularly from body 15 in the medial direction of the transverse plane, lying orthogonal to and joining with 15 fims 100 and 110. Transverse fins 120 effectively divide the scala tympani 5 (seen in FIG. 3D) into a set of longitudinally separate col-lpa-~lllents 35 (FIGS. 2A, 2B) within each of which there is one individual electrode contact 20. When all of the various fins have unfurled so as to touch the walls of the scala tympani as shown in FIG 3D, the electrical current inJected from each contact must pass through the 20 bone that forms the medial wall 3 (FIG. 3D) of each separate co.llpal~ ent, and thence into the subjacent portion of the spiral ganglion 6. Thus, all or most of the stimulating current delivered to a given contact tends to be directed to, and hence flow through, the spiral ganglion, where it can effectively stimulate the auditory neurons 7, rather than being ~ ip~t~d in other paths that do not intersect these25 neurons.
Also shown in FIG 3D is an elongated electrode contact 50 that is inserted into the scala vestibuli 8 so as to provide a return pathway for stimulation current injected into the cochlea fiom one or more individual contacts 20.
Elongated electrode contact 50 lies parallel to all or much of the length of electrode W O 97/26943 PCT~US97/00936 array 10. This arrangement further enhances the tendency of stimulation ~;ullt;nls to flow parallel to spiral ganglion neuron 7 stimulating them efficiently and selectively.
It should be a~pl~ciated that transverse fins 12() add little or no 5 stiffnloss to the electrode array 10 in the axis along which the array must flex to accommodate the cochlear spiral. Furthermore, the transverse fins can be bent orfurled in either longit~l-lin~l direction so that the electrode array can slide out of the scala tympani even if connective tissue grows into some or all of the various colllpal ~Illents 35 created between the fins. A tab 40 (FIG. 2A) projecting from the 10 array at its basal end can be used as a handle whereby the surgeon pushes or pulls the electrode array to effect insertion or removal of the electrode array. The tab 40 also provides a marker in~lic~tin~ that the electrode array has been inserted to the intend~l depth when the tab 40 is aligned with the round window opening.
In another embodiment of the invention shown in FIC~. 4, an 15 elongated electrode contact 50 is added to the lateral surface of body 15 along all or most of the length of the array (e.g., along the back side of the array in the region between sectional lines A and C of FIG. 2A). This elongated electrode contact 50may be used as the return electrode for some or all of the stimulating pulses applied to individual contacts 20. As described in a separate patent application of the 20 inventor, Serial No. 08/516,758, filed 08/18/95, which application is inco~ led herein by reference, this arrangement causes each site of stimulation to behave in a quasi-bipolar mode, further reducing any tendency for stimulation current to spread longit~l-lin~lly. This arrangement also increases the tendency for the current ~de~ign~t~d "i" in FIG. 4) flowing through the spiral ganglion to follow a course 25 that lies parallel to the elongated processes of neurons 7, which is more efficient for activating those neurons.
The electrode contacts 20 and 50, if present, may be made of any biocompatible electrode material such as platinum and its alloys, iridium or anodized tantalum. The associated electrode leads 30 may be made of any similarly W O 97/26943 PCT~US97/00936 biocompatible conductive material. The mech~nical properties, shape and ,limçn~ions of leads 30 and their disposition within body 15 can be used to modify the flexibility and other h~n(lling propert;es of the electrode array 10 so as to improve its insertability into the scala tympani. For example, it may be advantageous to use one or more dirr~l~.,t calibers of individual round or fl~ttP-nP~
wires with various of the apical or basal contacts, or to use a ribbon cable in which a multiplicity of wires are held together by bonds or envelopments of dielectricmaterial.
Alternatively or additionally, some or all of body 15 may be fabricated from a stiffer material fflan the material used for fins 100, 110 and 120.
This can be accomplished. for example, by molding body 15 as a ~r~ from a silicone elastomer having a relatively high duramater value and then inserting this ~llC~llll into the mold used to add fins made from a lower duramater elastomer.
(Note: the "d--r~m~ter" is the tough, fibrous membrane forming the outermost of the three coverings of the brain. l'hus, as used here, a relatively high dù~ aler va1ue meams a to~l~hnçss that is relatively high colll~a,ed to the to~ghnPc~ of the P.r.) It may be advantageous for the l"eroll,~ to contain various wells, pockets or other shape r~atul~s to f~~~ilit~te the p~ pment of contacts 20 and leads 30 during fabrication of electrode array 10.
It is desirable for the electrode array 10 to be relatively stiff in all directions at the basal end of the electrode array 10, a cross-sectional view ofwhich basal end is shown in F~G. 3C. In order to achieve this, the relatively large number of electrode leads 30 present at this point are dispersed throughout the relatively thick body 15 rather than gathering into a rib 35 as illust~tP~ in FIG 3B.
While the invention herein disclosed has been described by means of spPcific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.

Claims (10)

1. An electrode array (10) for insertion into a body cavity for nerve stimulation comprising:
a flexible body (15);
a multiplicity of separately-controlled, spaced-apart, electrode contacts (20) carried by said body; and fins (100, 110, 120) made from a compliant, dielectric material and projecting from said body so as to cause the outside dimension of said array plus said fins to be equal to or greater than the typical cross-section of the cavity in which said array is to be inserted.

2. The electrode array of Claim 1 wherein the body cavity in which the electrode array is inserted comprises the scala tympani (5) of the cochlea, and wherein the electrode array is adapted to selectively stimulate neurons of the auditory nerve.

3. The electrode array of Claims 1 or 2 wherein the number of electrode contacts comprises at least six, and wherein at least one fin resides on each side of each electrode contact except for an end electrode contact.

4. The electrode array of Claims 1, 2 or 3 wherein said fins comprise orthogonal fins lying in two axes; a first set of fins projecting substantially perpendicularly from the longitudinal axis of the flexible body so as to create a longitudinal barrier in a vertical axis; and a second set of fins, generally orthogonal to and joining the first set of fins, that project substantially perpendicularly from the body in a medial direction of a transverse plane; said fins effectively dividing the cavity into which the electrode array is inserted into a set of longitudinally separate compartments, within at least most of which resides one of said electrode contacts.

5. The electrode array of Claim 1 wherein the flexible body is made from a material that is stiffer than the material from which said fins are made.

6. The electrode array of Claim 2 wherein each of the multiplicity of electrode contacts includes a separate wire (30) through which electrical contact may be made with the respective electrode contact, each of the wires of the multiplicity of electrode contacts being embedded within the flexible body.

7. The electrode array of Claim 6 wherein the wires within the flexible body are gathered to form a rib in at least those regions of the electrode arraybetween its tip and midpoint.

8. The electrode array of Claim 6 wherein the wires within the flexible body are dispersed throughout the flexible body in that region of the electrode array near its basal end.

9. The electrode array of Claims 2, 3 or 4 further including an elongated electrode contact (50) insertable into the scala vestibuli so as to provide a return pathway for stimulation current injected into the cochlear from one or more of said electrode contacts (20).

10. An electrode array (10) for insertion into and stimulation of the cochlea comprising:
a multiplicity of separately controlled electrode contacts (20), and fins (100, 110, 120) made from a compliant, dielectric material that cause electrical currents injected through most of said contacts to flow preferentially through different portions of the wall of the cavity in which said array is to be inserted.