WO2008058232A2 - Spatially-focused actuation in a neural prosthesis - Google Patents

Abstract

A method for stimulating a neuron within the cochlea by implanting an electrode array into the cochlea, each electrode in the array being separated from the modiolar wall of the cochlea; replacing a portion of perilymph within the cochlea with a replacement medium; and actuating an electrode on the implanted array.

Description

SPATIALLY-FOCUSED ACTUATION IN A NEURAL PROSTHESIS

TECHNICAL FIELD

This invention relates to prosthetic devices, and in particular, to neural prostheses.

STATEMENT OF FEDERAL FUNDING

This invention was made with Government support under NlH Grant ROl

DC007528-01 entitled "Speech Processor Optimization for Cochlear Implants." The Federal Government may have certain rights in this invention.

RELATED APPLICATIONS

This application is a PCT application claiming the benefit of the priority date of US Provisional Application Number 60/864,856, filed November 8, 2006, the contents of which are incorporated herein by reference.

BACKGROUND

A cochlear implant includes a linear array of electrodes that is inserted into the cochlea along a cochlear axis thereof. Each electrode on the array is associated with nerve fibers that code for a particular frequency in the normal ear. When correctly implanted, electrodes associated with nerve fibers of a particular frequency arc placed near those portions of the cochlea having neurons that are likewise associated with that frequency.

To enable a patient to perceive an acoustic signal, one decomposes the signal into its constituent frequencies. Then, for each constituent frequency, one applies an appropriate voltage to the appropriate electrode. This causes delivery of electric current to the vicinity of the correct neurons, which in turn initiates action potentials in those neurons.

A difficulty that arises in practice is that the electric field causes stray action potentials in neurons other than the correct neurons. Stimulation of a single electrode excites many neurons along the cochlear axis, with neighboring electrodes exciting overlapping redundant collections of neurons.

This stimulation of stray action potentials, which is referred to herein as "ectopic stimulation," is believed to occur because the cochlea is filled with a conductive fluid. As a result, an electric field imposed by an electrode tends to drive a current that spreads both along the cochlear axis and in a direction perpendicular to the cochlear axis. The electric field component perpendicular to the cochlear axis, hereafter referred to as the transverse component, will tend to stimulate an action potential on the correct neurons. However, the electric field component along the cochlear axis, hereafter referred to as the axial component, will be available to stimulate action potentials in neighboring neurons, hereafter referred to as satellite neurons.

Ideally, the ratio of the transverse component to the axial component, hereafter referred to as selectivity ratio, will be high, This will minimize ectopic stimulation while allowing neighboring electrodes to excite only the correct neurons.

SUMMARY

In one aspect, the invention features a method for stimulating a neuron by placing an electrode across a gap from a neuron to be stimulated; filling the gap with a medium having a selected resistivity; and actuating the electrode.

In some practices, filling the gap with a medium includes replacing pre-existing fluid in the gap with a replacement medium having a resistivity higher than the resistivity of the pre-existing fluid.

Other practices include filling the gap with adipose tissue, filling the gap with hydrogcl, and filling the gap with a medium having a resistivity in excess of 600 ohm- cm.

In some practices, placing an electrode across a gap includes inserting a cochlear implant into a cochlea, the cochlear implant having the electrode disposed thereon. In another aspect, the invention includes a method for stimulating a neuron. The method includes altering an electrical property of a medium adjacent to the neuron, and applying an electrical stimulus to the neuron.

In some practices, altering an electrical property includes replacing a pre-existing medium with a new medium. Examples of pre-existing media include cochlear fluids, such as perilymph and/or cndolymph. Examples of new media include adipose tissue.

In another aspect, the invention features a neural prosthesis that includes an implant having a plurality of electrodes and a replacement medium for placement in electrical communication with the electrodes.

In some embodiments, the implant includes a cochlear implant.

Additional embodiments include those in which the replacement medium includes adipose tissue.

These and other features of the invention will be apparent from the following detailed description and the drawings, in which:

DESCRIPTION OF DRAWINGS

I⁷IG I shows a cochlear implant inserted into a cochlea; FlG. 2 shows a retinal prosthesis; and FlG. 3 shows a vestibular prosthesis; and

FIG 4 shows a cochlear implant with electrodes in contact with aqueous bubbles in a hydrophobic medium.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRI PTION

FlG. 1 shows a cochlear implant 10 inserted along a cochlear axis 12 of a cochlea 14. An electrode array 16 extends along the cochlear implant 10. Spaced apart along the length of the array 16 are electrodes 18, each of which is individually actuable by a signal processor (not shown). Each electrode 18 is assigned to a range of frequencies according to the neurons on the modiolar wall that electrode is intended to excite.

In operation, the signal processor decomposes an acoustic signal into constituent frequencies and, for each band of frequencies, excites an electrode 18 corresponding to that band. When the cochlear implant 10 is correctly inserted, an electrode 18 assigned to a particular range of frequencies will be placed proximate to that portion of the cochlea 14 that is sensitive to that range of frequencies. As a result, actuation of that electrode 18 will result in the patient perceiving the corresponding frequencies. When this procedure is repeated across the range of frequencies, the net result is that the patient perceives a facsimile of the original acoustic signal.

The extent to which the facsimile perceived by the patient matches the original acoustic signal depends in part on whether each electrode 18 stimulates only the targeted neurons 20, and not the satellite neurons 22 that are adjacent to the targeted neurons 20. This, in turn, depends in part on the flow of current between the electrode 18 and the satellite neurons 22, which in turn is influenced by the electrical properties of whatever medium 24 lies between the electrode 18 and the neurons 20, 22.

In a natural cochlea, the medium 24 is the highly-conductive perilymphatic fluid. This medium 24 is much more conductive than the bone surrounding the cochlea 14. As a result, actuating an electrode 18 causes current to flow both radially outward from the implant 10, referred to herein as "transverse current," and in a direction along the cochlear axis 12, referred to herein as the "axial current."

The transverse current at the target neurons is desirable since it is this current that travels toward, and excites, the targeted neuron 20. The axial current, however, tends to cause current to flow toward adjacent satellite neurons 22. Some of this axial current may eventually find itself redirected transversely from the targeted neuron 20, in which case it would tend to stimulate one or more satellite neurons 22. Thus, to suppress the resulting ectopic stimulation, it is desirable to suppress the axial current. One method for suppressing axial current is to replace cochlear fluid within at least a portion of the cochlea with a medium 24 having a high resistivity, and in particular, with resistivity that is between that of cochlear iluid and that of the surrounding bone.

Preferably, the resistivity of the medium 24 is close to, or equal to that of the bone. However, any increase in resistivity over that provided by the perilymph will tend to suppress axial current.

As used herein, "cochlear fluid" refers to cither endolymph, or perilymph or both. The act of "replacing" cochlear fluid includes both the complete or partial replacement of cochlear fluid. The medium 24 can be any material, tissue, or combination thereof.

Replacement, whether in full or in part, can be earned out by replacing cochlear fluid with an insulating fluid, such as a fluid containing silicone.

Alternatively, replacement can be carried out by transplanting tissue or by stimulating growth of fibrous tissue in the cochlear duct.

For example, one can introduce, into the cochlea, a suspension of cells having a tendency to establish a tissue of higher resistivity than that of the cochlear fluid. For example, one can introduce autologous adipose cells into the cochlea. This is an example of transplanting tissue into the cochlear duct.

Replacement can also be carried out by causing the formation of resistive fibrous tissue in the cochlea. This can be achieved by attracting fibroblasts to the cochlear duct. One can attract fibroblasts by, for example, introducing, into the cochlea, a suspension of permanent or absorbable particles, such as micro-spheres that arc selected to promote ilbrous tissue formation. In this case, fibroblasts will tend to migrate to the cochlear duct to encapsulate the particles, eventually replacing cochlear fluid with fibrous tissue.

A suitable material for replacing the perilymph would be any biocompatible material that would avoid causing severe inflammatory or immunological reactions. Other properties of suitable materials include being noninfectious, nonallergenic, nonmigratory, noncarcinogenic, stable after implantation, and resistant to both adsorbtion and phagocytosis. In addition, for ease of insertion, the material or tissue should be liquid or liquefiable so that it can readily be injected into the cochlea 14.

Replacement of the perilymph by a replacement medium 24 would generally be accomplished by perfusion. This would proceed by injecting the replacement medium 24 through one cochlear window 42 and extracting the perilymph through the other cochlear window 43. In some cases, only the scala vcstibuli would be infused with the replacement medium 24. This would tend to electrically isolate adjacent turns of the cochlea 14 from each other. In other cases, both the scala vestibuli and the scala tympani can be infused with the replacement medium 24.

One material suitable for replacing the perilymph is an autologous graft of adipose tissue. Such tissue can be easily harvested from subcutaneous fat deposits, partially digested, and liquefied for case in injecting into the cochlea 14. Moreover, adipose tissue is relatively inert, and when retrieved from another portion of the same patient, assures biocompatibility. Moreover, once injected into the cochlea 14, adipose tissue is unlikely to be reabsorbed, and will eventually re-establish the extracellular matrix found in native adipose tissue.

The resistivity of adipose tissue is on the order of 2500 ohm-cm, which is significantly higher than that of the perilymph, which is in the order of 50 ohm-cm, but lower than that of the surrounding bone, which is on the order of 5000 ohm-cm.

Other potentially suitable materials for use in replacing the perilymph include nonnabsorbablc materials (e.g., hydrogels, paraffins, polymer micro spheres suspended in resorbable fluids, and silastic materials), as well as absorbable materials (e.g., collagen, hyaluronic acids, and polylactic acid). In addition, certain commercially available preparations may be used. One such preparation is sold under the trade name

"ΛRTEFILL" by Artcs Medical, Inc. of San Diego, California. Alternatively, a mixture of any of the foregoing materials may be used. Referring to FIG. 4, another method of suppressing axial current is to replace cochlear fluid with a hydrophobic material 42, such as an oil, and to then form a bubble 44 of aqueous conducting fluid extending between the modiolar wall 46 and an electrode 18. In such a case, the aqueous conducting fluid, which could be saline, would enhance conductivity across the gap between the modiolar wall 46 and the electrode 18. At the same time, the high resistivity hydrophobic medium 42 between the bubbles 44, would suppress any currents in the axial direction.

An aqueous bubble 44 can be introduced by providing a main tube 48 that extends along the length of the electrode array 16 and providing subsidiary tubes 50 extending from the main tube 48 to each of the electrodes 18. As part of the implantation process, a surgeon would inject saline into the main tube 48. The saline would then pass into each of the subsidiary tubes 50 and emerge through orifices 52 near each electrode 18. This would result in the formation of aqueous bubbles 44 extending across the gap between each electrode 18 and the modiolar wall 46.

In an alternative environment, it is the bubble 44 that is hydrophobic and the surrounding medium 42 that is hydrophilic. In that case, it is the hydrophobic medium that is conductive.

In general, for a cochlea 14, resistivities in excess of 600 ohm-cm can be expected to significantly reduce axial current, with resistivities in excess of 7500 ohm-cm likewise being useful for reducing axial current.

The principles described herein arc applicable to many other prosthetic devices designed to stimulate target nerves without stimulating adjacent nerves. For example, in the case of an ocular implant 26, as shown in FIG. 2, one may wish Io stimulate different portions of the retina 28 using electrodes 18 in a two-dimensional array of electrodes separated from the surface of the retina 28 by a conductive medium 30. In such a case, replacement of the conductive medium 30 with a replacement medium 24 having a higher resistivity may result in less spreading of current, thereby reducing the inadvertent stimulation of those portions of the retina 28 that arc adjacent to the targeted portion of the retina 28. In the case of a vestibular prosthesis, as shown in FIG. 3, one may insert an electrode 33A,B in each of two semi-circular canals 34, 36 so that one electrode may be used to signal a change in pitch angle and another electrode may be used to signal a change in roll angle. The low resistivity of the fluid within the canals may result in cross- talk, so that actuation of an electrode 33A in one canal 34 may stimulate neurons in the other canal 36. The likelihood of such cross-talk would be reduced if the perilymphatic fluid connecting the canals were replaced with a more resistive replacement medium 38.

Another advantage of replacing the perilymph with a more resistive replacement medium is that there will be significant gradient in the electric field between the electrode 18 and the targeted neuron 20. This will tend to result in a pieccwisc linear electric field distribution along the neuron, with a steeper slope along one extent of the neuron and a shallower slope along the remaining extent of the neuron, and discontinuity where the steeper slope transitions into the shallower slope. Since the initiation of an action potential on a neuron to some extent depends on the second derivative of the potential field, this results in enhancing the likelihood of an action potential at that neuron 20.

Because the foregoing gradient effect decreases rapidly with distance from the actuating electrode 18, the likelihood of an action potential at only the correct neurons is enhanced.

Having described the invention, and a preferred embodiment thereof, what is claimed as new, and secured by letters patent is:

Claims

1. A method for stimulating a neuron within a cochlea, the method comprising

implanting an electrode array into the cochlea, each electrode on the electrode array being separated from the modiolar wall of the cochlea;

replacing a portion of the perilymph in the cochlea with a replacement medium; and

actuating an electrode on the implanted electrode array.

2. The method of claim 1, wherein replacing a portion of the perilymph with a replacement medium comprises replacing perilymph with a replacement medium having a resistivity higher than the resistivity of the perilymph.

3. The method of claim 1, further comprising selecting the replacement medium to be a hydrogel.

4. The method of claim 1 , wherein replacing a portion of the perilymph comprises replacing the perilymph with a replacement medium having a resistivity in excess of 600 ohm-cm.

5. The method of claim 1, wherein implanting an electrode array comprises implanting a cochlear implant into the cochlea, the cochlear implant having the electrode array disposed thereon.

6. The method of claim 1 , wherein replacing a portion of the perilymph comprises introducing a hydrophobic medium, and introducing an aqueous bubble extending between an electrode and the modiolar wall of the cochlea.

7. The method of claim 1 , wherein replacing a portion of the perilymph comprises introducing a hydrophilic medium, and introducing a hydrophilic bubble extending between an electrode and the modiolar wall of the cochlea.

8. The method of claim 1 , further comprising selecting the replacement medium to include adipose tissue.

9. The method of claim 1, further comprising selecting the replacement medium to be a hydrophobic medium.

10. A method for stimulating a neuron, the method comprising:

altering an electrical property of a medium adjacent to the neuron, and

applying an electrical stimulus to the neuron.

1 1. The method of claim 9, wherein altering an electrical property comprises replacing a pre-existing medium with a new medium.

12. The method of claim 10, wherein the pre-existing medium comprises cochlear perilymph.

13. A neural prosthesis comprising:

an implant having a plurality of electrodes; and

a replacement medium for placement in electrical communication with the electrodes.

14. The neural prosthesis of claim 12, wherein the implant comprises a cochlear implant.

15. The neural prosthesis of claim 12, wherein the replacement medium comprises adipose tissue.

16. The neural prosthesis of claim 11, further comprising a fluid delivery network for delivering fluid through orifices on the electrode array for extrusion into the replacement medium.

17. The neural prosthesis of claim 11. wherein the replacement medium has a resistivity less than the perilymph resistivity.

18. A method for stimulating a neuron, the method comprising

placing an electrode across a gap from a neuron to be stimulated;

filling the gap with a medium having a selected resistivity; and

actuating the electrode.

19. An implantable neural prosthesis comprising

a plurality of electrodes, and

a replacement medium for placement in electrical communication with the electrodes.

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