IMPLANTABLE ELECTRODE ARRAY

FIELD OF THE INVENTION

The present invention relates to electrode arrays and implantable devices for use in prosthetic devices implanted to electrically stimulate a nerve in a recipient, for example a cochlear prosthesis implanted to stimulate the auditory nerve fibers of a patient. It may also be applied to electroacoustic devices, which provide mechanical stimulation as well as electrical stimulation. BACKGROUND OF THE INVENTION Intra-cochlear electrode arrays are typically implanted in the scala tympani of the cochlea in close proximity to the ganglion cells to thereby stimulate the auditory nerve fibers of a patient suffering sensorineural hearing loss. Sensorineural hearing loss is due to abnormalities in the function of transducing sound energy into auditory nerve impulses. Typically the abnormality is caused by defective hair cells within the cochlea. The intra-cochlear electrode array, in operation, electrically stimulates the spiral ganglion cells located in the modiolus of the cochlea so as to stimulate the auditory nerve fibers and create a perception of sound for the user.

An objective of cochlear implant electrode carrier design is to position and retain the electrodes as close as possible to the modiolus. Minimizing the distance between the electrodes and the modiolus is intended to improve the effectiveness and localization of the stimulation current and minimize the threshold current amplitude required for stimulation. The further the electrode contacts are positioned from the ganglion cells, the greater the magnitude of the current stimulation required to reach the ganglion cell threshold stimulation and the greater the overspread of the current resulting in resolution loss. Cochlear implants typically consist of an array of electrodes that are embedded in a carrier. Typically, the carrier is either of straight configuration or, to better achieve the objective of positioning the electrodes close to the modiolus, preshaped in a spiral or curved configuration. Upon insertion, the straight configuration follows the outer wall of the cochlea. The preshaped configuration must generally be held straight or approximately straight for insertion and upon insertion will be released back to its preshaped configuration such that it is closer to the modiolus. For

example, as described in commonly assigned United States Patent No. 6,421 ,569, a preshaped curved cochlear implant electrode array is disclosed having a surgical stylet resident in a lumen of the carrier to straighten the carrier during insertion into the cochlea. After insertion, the stylet is removed and the curved configuration of the preshaped carrier allows it to conform to the inner wall of the scala tympani.

United States Patent No. 6,195,586 discloses a space-filling intracochlear electrode array consisting of the electrode array embedded in a carrier and a separate positioner to fill the space behind the electrode array to position and retain the electrode array against the modiolus of the cochlea. The positioner also includes a lumen for receiving a stylet during insertion of the electrode array and positioner in the cochlea.

In commonly assigned United States Patent No. 5,545,219, another cochlear electrode implant assembly is disclosed having an auxiliary positioning member. The positioning member assumes an arched configuration against the constraint of the radially outer wall or lateral wall and exerts a force on the electrode carrier to position the electrode elements in close proximity to the spiral ganglion cells of the cochlea.

These two-part design space-filling assembly or stylet receiving cochlear electrodes have disadvantages. The space-filling designs apply a permanent static pressure against the both the medial inside wall and the lateral outside wall of the cochlea which may have adverse affects on the surrounding structures. They additionally tend to displace a larger volume of perilymph. The stylet design cochlear electrodes, once positioned, are fixed in place and are difficult to reposition or remove during explantation. Additionally, including a lumen in the electrode increases the manufacturing complexity of the array. In United States Patent No. 6,151,526 another design of electrode array having a ribbed electrode for cochlear stimulation is disclosed. The ribbed electrode consists of a flexible carrier having embedded electrode contacts along a front side and a flexible rib extending from a rear side directly opposite each electrode contact. Once inserted in the desired position, the ribbed electrode is fixed by slight pulling back of the surgeon to engage the ribs with the outside or lateral wall of the cochlea. As for the stylet design cochlear electrodes, once positioned and fixed, the ribbed

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design electrode array is difficult to reposition and remove during explantation if necessary from the cochlea without damaging the delicate structures of the cochlea.

All cochlear electrode arrays displace cochlear fluid within the cochlea and this may adversely affect the patient. Therefore it is desirable to limit the amount of cochlear fluid displaced by minimizing the size of the cochlear implant. Another concern regarding the size of the electrode array implant is the size of- the cochleostomy required to insert the cochlear electrode array. Cochlear electrode arrays are usually inserted into the scala tympani through the round window or through a cochleostomy drilled into the basal part of the cochlea Any cochleostomy and displacement of the cochlear fluid may cause traumatic damage to auditory structures and degrade residual hearing. It is accordingly preferable to minimise the size of the electrode array, and hence reduce the size of the required cochleostomy or opening. It is an object of the present invention to provide an alternative intra- cochlear electrode array which will allow electrodes to be positioned close to the modiolus. SUMMARY OF THE INVENTION

According to one aspect, the present invention provides an electrode array device for insertion into the scala tympani, including a flexible carrier having a basal end and a tip, a plurality of electrodes disposed along the carrier, and electrical connections for each electrode so that stimuli can be delivered to selected electrodes, the carrier being preformed into a wave shape such that parts of the carrier operatively engage the radially outer wall of the scala tympani and thereby urge other parts of the carrier into proximity with the radially inner wall.

The present invention further provides an intra-cochlear implant incorporating such a device.

The wave shape may be generally sinusoidal, with changes in dimensions of wavelength and amplitude ( width) to accommodate the shape of the scala tympani. It is not necessary that the shape be regular, although this is preferred. It may be an essentially two dimensional shape, with limited side to side flexibility, or a more three dimensional shape, such as a coil or three dimensional spiral. It is

preferred that there be multiple engagements with the inner and outer wall, so as to correctly position the device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanying drawings, in which:

FIG. 1A is a simplified side elevational view of the electrode array device of an embodiment of the invention;

FIG. 1B is a plan view of the electrode array device of FIG. 1A;

FIG. 2A is a partial view of the electrode array device of FIG. 1 A; FIG. 2B is a cross-sectional view of the electrode array device of FIG. 2A taken along line A-A in accordance with an embodiment of the invention;

FIG. 2C is a cross-sectional view of the electrode array device of FIG. 2A taken along line A-A in accordance with an embodiment of the invention;

FIG. 3 is a schematic illustration in longitudinal section of the inserted cochlear electrode array in accordance with the embodiment of FIG. 1 ;

FIG. 4 is a simplified side elevational view of the electrode array device of another embodiment of the invention;

FIG. 5 is a simplified side elevational view of the electrode array device of another embodiment of the invention; and FIG 6 is a simplified view of the device.

DESCRIPTION OF PREFERRED EMBODIMENT

The present invention will be described with reference to various implementations and embodiments. It will be appreciated that the present invention may be implemented using many alternative structures, and that the implementations described are not intended to be limitative of the scope of the invention.

It will further be well understood that the present invention is intended to be used in conjunction with the rest of an intracochlear implant system. Such systems are available commercially, for example from the present applicant/assignee, or from other entities such as Advanced Bionics Corporation. Whilst it will be understood that stimulation strategies and speech coding, for example, will be required to implement a practical system, the present invention is

not limited to any particular arrangement, and as such the sound processing and coding will not be described in any detail.

The present invention may be used in conjunction with a conventional cochlear implant system, wherein an external speech processing device receives sound signals, processes them, and transmits data to an implanted device including a electrode array. Alternatively, the sound processor could be fully or partly implanted, or positioned within the ear. The arrangement may be a totally implanted device. Acoustic stimuli may be provided in addition to electrical stimuli, whether via the ear canal, bone conduction, through stimulation of the middle or inner ear structures, or in any other suitable way. The present invention may be applied in conjunction with any type of cochlear stimulation system.

FIG. 1A shows a side elevational view of the perimodilar cochlear electrode array device 10 of an embodiment of the invention. In general terms, the perimodilar electrode array device has a "snake" or "wave" shape carrier 8 with electrode contacts or pads 12 partially embedded and located on the wave peaks 14 of the carrier. The peaks 14 and troughs 16 are joined by substantially smooth straight median sections 18. The basal end 34 of the carrier is provided with a mark 70 to indicate insertion depth. The other end of the carrier forms the tip 20. A plurality of wires or leads 40 discussed in more detail with reference to FIG. 2B extend through the carrier from a receiver/stimulator 60 and each wire is electrically connected to a respective electrode 12.

For illustrative purpose and ease of discussion, the electrode array device 10 is shown with six electrodes 12 in the array. It will be appreciated that any number of electrodes may be used. Further, as will be discussed further below, if desired electrodes could be positioned on the troughs, or in other positions along the array, although the latter is not presently preferred.

It will be appreciated that the implementation of a practical implanted device may further include extra-cochlear electrodes or other electrodes placed to act, for example, as a reference electrode, or to capture telemetry or neural response information.

The electronic circuitry provided in the receiver/stimulator 60 together with the electrode array performs the selective stimulation of the auditory nerve cells to represent the detected sound signals. The electrodes are preferably located in

close proximity to the ganglion cells. As is understood in the art, the ganglion cells are arranged in an orderly tonotopic sequence, from lower frequencies towards the apex of the spiral cochlea to higher frequencies towards the basal end of the cochlea. Any suitable speech processing and electrode mapping strategy may be used in conjunction with the present invention. The external lead from the carrier to the receiver/stimulator may include known leads.

The electrode pads 12 will be positioned at each wave peak 14. Of course, the electrode array may be configured such that one or several wave peaks may be provided without an electrode. There may be reasons for providing a wave peak without an electrode, for example, it may be possible to treat a patient with hearing loss only at certain frequencies in which case electrodes are only required at specific locations in the cochlea The overall height of the wave including peak height 22 and trough depth 24 is slightly larger (for example 10%) than the maximum width of the scala tympani at the final location of the electrode. In the apical direction, the scala tympani becomes narrower and as the electrode extends deeper, the wave height decreases. The wave height of the carrier is smallest at the tip 20 of the carrier and progressively larger toward the basal end 34 electrode pads. In one embodiment, the carrier and electrode array is inserted 19mm and the wave height will be for example 1.34mm at the tip and 1.85mm at the base (based on mean cochlea size measurements of 1.7 at the base, and 1.2 at the tip). Additionally, the distance between peaks 30, i.e. wave length, and distance between peaks and troughs may vary along the carrier. The wave carrier shown in FIG 1A is symmetric about a common point 26 such that the distances between peaks 30 and troughs 28 are kept constant, however, the wave carrier may take asymmetrical shape having varying peak and trough heights and/or peak and trough distances. For example, the wave shape may be asymmetric, and the wave shape could be changed to vary the insertion properties in different parts of the electrode array device. The wave length may vary along the length of the electrode array device, which may allow positioning of electrodes relative to certain cochlea locations, for example, denser electrodes at high frequencies. The asymmetric properties may also be employed to vary forces and hence optimise insertion. The dimension of the height of the wave may vary from less than the internal dimension of the cochlea to significantly

greater than 10% (up to approximately 50%). Hence the force of the electrode pad against the modilar wall can be varied. The variation could be such that all waves are the same or different for each wave so the force is different at each pad. Additionally, the sections 18 between the peaks and troughs may be other than straight.

It will be appreciated that the present invention may be applied to an array of any desired length, including shorter electrode arrays intended for insertion to a reduced depth, for example for use with electro-acoustic devices, or where it is desired to stimulate only the higher frequency regions of the cochlea. Figure 2A illustrates a single electrode 12, and figures 2B and 2C illustrate possible cross-sectional shapes. In figure 2B, the carrier can be seen, with electrode 12 and connecting wires 40. Preferably, a separate wire connects each electrode to the implanted stimulator unit ( not shown). In this way, the specific stimuli intended for a particular electrode can be delivered with accuracy. This aspect of the implementation is largely in accordance with conventional electrode array carrier construction. In figure 2C, a cavity or passage 52 is provided, to allow for the delivery of pharmacologically active substances, if this is desired. Arrangements for delivery of such substances are known from the prior art, and may be applied with the present electrode array carrier. The tip 20 of the carrier is preferably a softip arrangement to prevent tip fold over as published in United States Patent Publication No. US 2004/0243212A1 (United States Application No. 10/825,358 filed 16 April 2004, which is incorporated herein by reference).

The peak and trough of the wave carrier 8 is preferably smooth and highly flexible to facilitate insertion into the cochlea. In an embodiment the flexibility is maximised at the peak 14 and trough 16 so the electrode straightens slightly as it is inserted. This can be achieved by keeping the cross section of the electrode to a minimum. To increase flexibly at the peak and trough relative to the straight median sections 18, the cross section at the peak and trough may be slightly smaller. For example, the cross section 32 will be 0.3mm diameter at the tip (cs _t ) and 0.4mm at the base (cs _b ). The cross section is preferably slightly thicker out of plane than in plane as shown in FIG. 1B to ensure bending only happens in the desired plane. A typical cross sections, which can be seen for example in figure

2B, is oval. This is illustrated more clearly with respect to figure 6. The width may be, for example, 0.6 to 1.0 mm.

Fabrication of the carrier and electrode array may be achieved using a two piece mould. The carrier 8 may be of any flexible biocompatible material such as silicone to support the electrode pads and wires. The electrodes 14 may be of metal or metal alloy such as platinum electrode pads, and the wires 40 may be of a metal or metal alloy such as platinum iridium (PtIr) alloy to connect to the stimulator 60. More generally, the pads and wires may be any biocompatible conductor. Typically electrode pads 12 punched from platinum sheet are positioned at each peak of the lower half of the mould. An insulated PtIr wire is welded to each pad and positioned to follow the mould shape. The repeatable position of each pad facilitates automation of this process. The top half of the mould is then positioned and the mould filled with silicone rubber material which is then cured to form the complete electrode. The softip shape 20 described above may be included as an integral part of the mould. The perimodilar electrode device 10 design facilitates fabrication.

It will be appreciated that while this is a preferred fabrication process, any suitable process may be used to produce the inventive structures.

The perimodilar electrode array device 10 may be inserted through a cochleostomy as known for other prior electrodes arrays. However, the perimodilar electrode array device has a mean electrode cross section that requires cochleostomy in the range of 0.5 to 1mm positioned as for existing cochlear electrode arrays anterior posterior to the oval window. The electrode array device 10 may alternatively be inserted through the round window of the cochlea. Small cochleostomies and round window insertions are preferred for minimally traumatic cochlea surgery and hence help preserve residual hearing. The usual procedures and precautions appropriate to insertion of any intracochlear electrode array are applicable to the present invention. Any insertion mode deemed appropriate by the surgeon may be used. Once inserted through the cochleostomy or round window, the electrode is gently pushed until fully inserted as shown in FIG. 3. A marker 70, such as a feature on the electrode carrier, may indicate full insertion. Upon a full, correct insertion, the electrodes 12 are positioned in close proximity to the modiolar or

inside wall 50 of the scala tympani of the cochlea, and the troughs 16 are positioned in close proximity to the lateral or outside wall 52 of the scala tympani of the cochlea. The action of the troughs contact with the lateral wall urges the peaks and hence the electrodes towards the modilar wall. Typically the force on the walls will be the minimum that will reliably maintain electrode position. The force is easily optimised through adjustment of the following electrode array carrier parameters: wave height, cross section and carrier durometer. It is expected that the dimensions indicated above the wave height and cross section with a carrier durometer of 30-60 (shore A) will give an appropriate force on the cochlea walls.

In other embodiments, the electrode array device 10 may have features out of the plane as shown in FIG. 1 A. This may allow features to further optimise forces on the cochlea for example, the electrode array device could be made hollow in whole or in part, as shown in FIG. 2C by cavity 42. This may allow variation of stiffness and hence optimisation of forces to facilitate insertion. Cavity 42 or hollow sections 42 of this embodiment may be used for drug delivery. In another embodiment, the carrier 8 of the perimodilar electrode array device 100 may be formed in a coil rather than a wave as shown in FIG. 4. Potential of the electrode pads in the coil carrier may then be moved closer to or more distant from the modilar wall by rotating the coil electrode. It will be appreciated that other shapes, both regular and irregular, could be used for the electrode carrier in order to create a structure which will locate the electrodes as desired.

In another embodiment, the electrode pads or contacts 12 may also be placed on the troughs 16 as well as the peaks 14 as shown in FIG. 5. Once inserted, the result in half the electrode pads being close to the modiolar wall and half being against the lateral wall - lateral wall electrodes 36 and perimodilar electrode pads 12 alternating. This arrangement may allow for additional control of the location of stimuli. It will be understood that the foregoing description of a number of embodiments of the present invention is for purposes of illustration only, and that the various structural and operational features and relationships disclosed herein

are susceptible to a number of modifications and changes none of which entails any departure from the scope of the invention as defined in the appended claims.