Electrode array and method of fabrication

An electrode array, having application as a cochlear implant, includes a tube formed of Parylene defining a hollow channel. A substrate formed primarily of Parylene is supported by the tube. In turn, a plurality of metallic electrodes and feed lines are supported by the substrate. Numerous voids are defined by the tube which opens into the hollow channel. The size and spacing of the voids regulate stiffness and curl of the tube to provide excellent fit within the cochlea.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention relates to an array of electrodes. Specifically, the subject invention relates to an array of electrodes for use as part of a cochlear implant.

2. Description of the Related Art

Cochlear implants are the most widely used neural prostheses, using current stimulation to bypass the non-functional hair cells of the cochlea to directly stimulate receptor cells that drive the auditory nerve. Typical cochlear electrode arrays are fabricated with bundles of wires coated in silicone. Such wire bundles are hand assembled and are limited in the number of electrodes. A typical cochlear electrode array utilizes only 16 to 24 electrodes because of large size relative to the size of the scala tympani. The low number of electrodes results in often poor pitch specificity. The relatively large size of these electrode arrays may also cause insertion damage and limit the depth of insertion. The low insertion depth limits the pitch range provided by the implant.

The subject invention is directed toward providing an electrode array providing greater pitch specificity, greater pitch range, while resulting in minimal insertion damage.

SUMMARY OF THE INVENTION AND ADVANTAGES

The subject invention provides an electrode array. The electrode array includes a tube having at least one wall wherein the tube defines a channel. A substrate comprised of a non-conductive material is supported at least partially by the tube. The electrode includes a plurality of electrodes. Each electrode comprises a conductive material and is supported by the substrate. A plurality of feed lines comprised of a conductive material are disposed primarily within the substrate. Each feed line is electrically connected to at least one of the plurality of electrodes. At least one wall of the tube defines a plurality of voids into the hollow channel for regulating stiffness and curl of the tube.

The subject invention also provides a method of fabricating the electrode array. The method includes the step of depositing a first composition on a carrier wafer. The first composition defines a longitudinal slit and forms a first layer of the substrate. The carrier wafer is etched through the longitudinal slit to define a channel underneath the first layer. The method also includes depositing a second composition comprising a polymer through the longitudinal slit and onto the carrier wafer to form the tube around the channel and seal the longitudinal slit. The method further includes the step of disposing a plurality of feed lines comprised of conductive material on the substrate opposite the tube. The plurality of electrodes is disposed on the substrate with each electrode electrically connected to at least one of the feed lines. The method further includes the step of etching the carrier wafer opposite the substrate to define voids with each void exposing an area of the tube The areas of the tube exposed by the voids are removed to define slots within the tube. The tube and substrate are then released from the carrier wafer.

The electrode array provides a lower profile than those of the prior art, resulting in less damage when inserted into a cochlea. The electrode array also provides a greater number of electrodes and allows for deeper insertion into the cochlear, resulting in improved pitch specificity and greater pitch range. Furthermore, the slots of the tube of the electrode array assist in providing a modiolus-hugging curl to position the electrodes as close as possible to the receptor cells and reduce insertion trauma.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures, wherein like numerals indicate like parts throughout the several views, an electrode array20is shown herein. The electrode array20is well suited for use as part of a cochlear implant (not shown), but other uses of the electrode array20are described further below and/or will be realized by those skilled in the art.

The electrode array20includes a substrate22at least partially supported by a tube24. In the illustrated embodiments, the substrate22and tube24each comprise a non-conductive material. Specifically, the substrate22and tube24of the illustrated embodiments each comprise a polymer and more specifically, the substrate22and tube24each comprises poly(p-xylene), known commonly by the trade name Parylene. In the illustrated embodiments, Parylene C is utilized to form the substrate22and tube24. However, in other embodiments, other types of Parylene, other types of polymers, and other types of non-conductive materials may alternatively be utilized to form the substrate22and tube24. Furthermore, electrically conductive materials, such as metals, may alternatively be used to form the tube24. Moreover, the substrate22and tube24may be formed of a combination of different materials.

The electrode array20may include more than one tube24connected to the substrate22, i.e., a plurality of tubes24. However, for purposes of illustrative simplicity, the electrode array20is shown and described herein with only a single tube24.

The tube24includes at least one wall26,28of defining a hollow channel30referred to hereafter as the tube channel30. In the illustrated embodiment, the tube24has a generally semicircular cross section defined by a curved wall26and a generally straight wall28, as shown inFIG. 16. The generally straight wall28of the illustrated embodiment is connected to the substrate22as described in further detail below. Other shapes for the tube24may be realized by those skilled in the art. Moreover, the use of the term “tube” does not necessarily imply a circular or curved shape.

In one embodiment, the at least one wall26,28of the tube24defines a plurality of slots32into the tube channel30. Specifically, in the illustrated embodiments, the slots32are defined by the curved wall26. The slots32regulate the stiffness and curl of the tube24. More particularly, the size of the slots32and spacing of the slots32from one another define the ability of the tube24to bend and curl. The slots32in the illustrated embodiment are generally circular or ring shaped. Of course, other shapes for the slots32may also be suitable. Moreover, in other embodiments (not shown), the walls26,28of the tube24may be continuous, i.e., without any slots or other holes, to allow liquids to pass through the tube channel30.

In the illustrated embodiment, the substrate22includes an interconnect region34and an electrode region36. The interconnect region34allows for electrical connection of the electrode array20with at least one external device (not shown), as described further below.

The substrate22of the illustrated embodiment comprises a plurality of layers of Parylene. That is, several layers of Parylene are connected together to form the substrate22. Specifically, the substrate22comprises a first layer38of Parylene, also referred to as a base layer38. The first layer38is in contact with and affixed to the straight wall28of the tube24.

In the illustrated embodiment, a second layer40of Parylene is disposed above the first layer34. That is, the second layer40is disposed on the first layer38opposite the tube24. The second layer40and the tube24are integrally formed as described further below. That is, the second layer40and tube24are comprised of a single unit.

The substrate22also supports a plurality of feed lines42comprised of a conductive material. The feed lines42of the illustrated embodiment comprise a metal. More specifically, the feed lines42are formed of chromium-gold-chromium (Cr—Au—Cr). In the illustrated embodiment, the pitch, i.e., the distance between a point on one feed line42and a corresponding point on another feed line42, is about 10 μm. As such, a width of each feed line42is less than 9 μm, to maintain electrical isolation between the feed lines42.

In the illustrated embodiment, the feed lines42are disposed on the second layer40. The feed lines42run between the interconnect region34and the electrode region36. A third layer44of Parylene is disposed above the feed lines42and the second layer40. That is, the third layer44is connected to the second layer40opposite the tube24. As such, the feed lines42are disposed primarily within the substrate22, i.e., the feed lines42are encased within the substrate22. As such, the feed lines42are insulated by the non-conductive material of the substrate22. Specifically, the third layer44of Parylene electrically insulates the feed lines42.

The substrate22also supports a plurality of electrodes46for conducting electrical energy. The electrodes46each comprise an electrically conductive material, including, but not limited to, a metal. In the illustrated embodiment, the electrodes comprise titanium-iridium (Ti—Ir). However, in other embodiments, the electrodes46may be formed of different metals. Furthermore, other electrically conductive material, such as conductive polymers, could be used to form the electrodes46. Moreover, the various electrodes46need not be formed of the same type of material and could be formed by a combination of different materials.

In the illustrated embodiment, the electrodes46are supported in the electrode region26. Particularly, the electrodes46are disposed above the third layer44. At least one electrode46is electrically connected to at least one feed line42. As the feed lines extend to the interconnect region34, the feed lines42may electrically connect the electrodes46to the at least one external device. In the illustrated embodiment, each electrode46is electrically connected to one feed line42.

The electrode array20of the subject invention achieves a high density of electrodes46. More specifically, the pitch between the electrodes may be less than 300 μm. In the illustrated embodiment, as shown inFIG. 1, 32 electrodes46are supported in the electrode region26having a length of about 8 mm. That is, the pitch, or center-to-center spacing, between the electrodes46is the illustrated embodiment is about 250 μm. By increasing the density and number of electrodes46the electrode array20, when used as a cochlear implant, provides improved pitch specificity over the prior art.

Of course, other number of electrodes46may be achieved as will be realized by those skilled in the art. During experimentation, other electrode arrays20(not shown) were fabricated with 64 and 128 electrodes46.

The substrate22of the illustrated embodiment includes a fourth layer48of Parylene disposed above the third layer44. That is, the fourth layer is supported by the third layer44opposite the tube24.

The substrate22defines a peripheral edge50around its periphery. The electrode array20may include a curl strip52disposed adjacent at least a portion of the peripheral edge50. The curl strip52provides rigidity to the electrode array20and further regulates the amount of bend and/or curl of the array20. In the illustrated embodiment, the curl strip52is disposed on the fourth layer48. A fifth layer54of Parylene is disposed over the curl strip52. That is, the fifth layer54is disposed on the fourth layer48opposite the tube24. As such, the curl strip52is encased within the substrate22around the entire peripheral edge50.

The curl strip52of the illustrated embodiment is bimetallic. Specifically, the curl strip52is composed of Titanium-Iridium and Chromium-Gold (Ti—Ir/Cr—Au). Of course, other materials may be utilized to form the curl strip52. Furthermore, the curl strip52may be formed of various independent pieces or one continuous piece.

The fourth and fifth layers48define a plurality of openings56. Each opening56encircles at least one electrode46to allow electrical contact with the at least one electrode46. Preferably, each opening56encircles just one electrode46. In the illustrated embodiment, each opening56has a generally circular shape and a diameter less than 200 μm. However, other shapes and sizes for the openings56may be suitable as realized by those skilled in the art.

The electrode array20of the illustrated embodiment provides significantly greater flexibility than silicon-substrate devices of the prior art and are robust enough to withstand repeated flexing. Testing of the electrode array20results in only a 20% impedance drop after 6000 cycles of twisting the array20to a helical radius of about 2 mm. This testing also revealed that no shorting between the feed lines42and saline after the 6000 cycles of twisting.

The subject invention also provides an exemplary method of fabricating the electrode array20. However, other methods of fabricating the electrode array20described above may be realized by those skilled in the art.

Referring toFIG. 2, the method utilizes a carrier wafer60. The carrier wafer60of the illustrated embodiment comprises silicon (Si). Of course other materials may also be suitable for forming the carrier wafer60, as realized by those skilled in the art.

The method includes the step of depositing a first composition (not separately numbered) on the carrier wafer60as shown inFIG. 2. The first composition forms the first layer38described above. The first composition is preferably a non-conductive material and more preferably, the first composition is a polymer. Most preferably, the first composition is Parylene C and is applied by vapor deposition.

Referring toFIG. 3, a longitudinal slit62is defined in the first layer38. In the illustrated embodiment, lithography is utilized to demarcate the area of the slit62on the first layer38. Then, the area of the first layer38is removed using a directional oxygen (O2) plasma reactive ion etching (RIE) to define the slit62. Of course, other techniques for defining the slit62may be realized by those skilled in the art. In the illustrated embodiment, the longitudinal slit62has a length of about 16 mm and a width of about 5-20 μm.

The method also includes etching the carrier wafer60through the longitudinal slit62to define a channel64underneath the first layer38, as shown inFIG. 4. The channel64in the carrier wafer60is referred to hereafter as the carrier channel64. In the illustrated embodiment, the etching of the carrier wafer60to form the carrier channel64is accomplished using xenon difluoride. However, those skilled in the art realize may realize other suitable techniques for generating the carrier channel64.

In the illustrated embodiment, the maximum depth of the carrier channel64is about 150-200 μm below a top level of the substrate60. Said another way, the depth of the carrier channel64is about 150-200 μm below the first layer38. Of course, other depths may alternatively be generated. The width of the carrier channel64in the illustrated embodiment is about 100-300 μm. The carrier channel64may have a generally semicircular cross section, as is shown inFIG. 4. However, those skilled in the art realize that the carrier channel64may any of numerous shapes and sizes.

The method further includes the step of depositing a second composition (not separately numbered). The second composition is preferably a non-conductive material and more preferably, the second composition is a polymer. Most preferably, the second composition is Parylene C and is applied by vapor deposition.

In the illustrated embodiment, the second composition is deposited through the longitudinal slit and around the carrier channel64to form the tube24described above having the tube channel36. The second composition is also deposited above the first layer38to form the second layer40described above. Accordingly, the second composition seals the longitudinal slit62. As such, the tube24and second layer40are formed of a unitary material in the illustrated embodiment.

The feed lines42are disposed on the second layer40opposite the tube24according to the method. The feed lines42are applied to the second layer40using evaporated metal and lift off.

The method further includes depositing a third composition (not separately numbered) atop the second layer40and the feed lines42, as shown inFIG. 77. The third composition forms the third layer44described above. The third composition is preferably a non-conductive material and more preferably, the third composition is a polymer. Most preferably, the third composition is Parylene C and is applied by vapor deposition. The third layer44(as well as the fourth and fifth layers44,54) acts as insulation to the feed lines42. A plurality of apertures45are defined in the third layer44using directional O2RIE or other suitable techniques. Each aperture45exposes at least one of the feed lines42.

Referring toFIG. 8, the method continues with the step of disposing a plurality of electrodes46on the substrate22. In the illustrated embodiment, the electrodes are disposed in the apertures45such that each electrode46electrically connected to at least one of the feed lines42. Each electrode46in the illustrated embodiment is generally circular shaped with a diameter of less than 200 μm. Preferably, each electrode46has a diameter of about 180 μm. Of course, other suitable shapes and sizes for the electrodes46will be contemplated by those skilled in the art.

In the illustrated embodiment, the electrodes46are formed using a lift-off technique. More specifically, photoresist (not shown) is placed on the exposed substrate22, the photoresist is removed from areas where the electrodes46are to be disposed, the Ti—Ir metal (not shown) is disposed on the exposed substrate22such that the Ti—Ir metal bonds in the areas where the photoresist has been removed, and acetone (not shown) is applied to the substrate22. The acetone eats away at the remaining photoresist and removes the Ti—Ir metal, except for the electrodes46.

The method also includes the step of depositing a fourth composition (not separately numbered) atop the third layer40, as shown inFIGS. 9 and 10. The fourth composition forms the fourth layer48described above. The fourth composition is preferably a non-conductive material and more preferably, the fourth composition is a polymer. Most preferably, the fourth composition is Parylene C and is applied by vapor deposition.

The method further includes the step of disposing the curl strip52on the substrate22. Preferably, the curl strip52is disposed adjacent the peripheral edge50, as shown inFIGS. 11 and 12. After disposition of the curl strip52, a fifth composition (not separately numbered) is deposited atop the fourth layer48and the curl strip52. The fifth composition forms the fifth layer54described above. The fifth composition is preferably a non-conductive material and more preferably, the fifth composition is a polymer. Most preferably, the fifth composition is Parylene C and is applied by vapor deposition.

The method also includes the step of forming the plurality of openings56in the fourth and fifth layers48,52to expose the electrodes46. The openings56are created using directional O2RIE or other suitable techniques. The openings56allow electrical conduction with the electrodes46. Preferably, one opening56is formed for each electrode46.

The use of five compositions forming five layers38,40,44,48,54is not absolutely necessary in forming the substrate22of the electrode array20. Several of these layers38,40,44,48,54could be combined and/or the compositions applied in combination.

Referring toFIGS. 13 and 14, the method continues with the step of etching the carrier wafer60opposite the substrate22to define voids68with each void68exposing an area (not separately numbered) of the tube24. The creation of the voids68is performed using directional RIE or other suitable techniques.

After the voids68have been defined in the carrier wafer60, the method proceeds with removing the areas of the tube24exposed by the voids68to define the slots32within the tube24. The slots32are generated using oxygen plasma.

The method further includes the step of releasing the tube24and substrate22from the carrier wafer60. In the illustrated embodiment, the tube24and substrate22are released from the carrier wafer60by dissolving the carrier wafer60using a solution including potassium hydroxide (KOH). Preferably, the solution is about 2% KOH. However, other techniques may be implemented as realized by those skilled in the art.

FIGS. 16 and 17show cross sectional views of the electrode array20after release from the carrier wafer60. Furthermore,FIG. 18shows a perspective view of a portion of the electrode array16andFIG. 19shows a close up top view of two electrodes46and a plurality of feed lines42.

Curl may be induced into the electrode array20by utilizing a wire74. The wire74is disposed through the tube channel30to hold the array20in a curled position.

The electrode array20provides high electrode46density, built in curl, integrated positioning, and tailored stiffness for use as part of the cochlear implant. The substrate22is flexible and robust enough to withstand the tight helical pitch of the cochlea. This substrate22can hug the modiolus of the cochlear for close proximity to neural receptors. The size and spacing of the openings56can be altered to make the array stiff enough for insertion in the cochlea yet pliable enough to curl. The openings56also provide a lumen where a stylet wire (not shown) can be advanced to straighten the array for insertion into the cochlea and then fed off the array to allow it to curl into the cochlea. Alternatively, a microelectromechanical systems (MEMS) solution may be implemented to perform insertion of the electrode array20into the cochlea.

The electrode array20may also find application as part of a neural probe (not shown). Polymer neural probes are of increasing interest because they more closely match the tissue compliance then harder silicon or glass probes. This match in compliance improves the synchronicity of probe motion with that of the tissue, so that the probes do not tear through the tissue in response to micro motions. However, the typical drawback of polymer probes is that they are highly flexible and need to be strengthened with stiffeners in order to penetrate the brain tissue.

The electrode array20may also find application with for dispensing cell growth promoters or pharmaceuticals to the surrounding tissue through the tube channel30, either with or without the slots32. The tube channel(s)30beneath the electrodes20may be used to deliver other fluid locally to surrounding cells. Electrochemical reactions can be studied locally, by stimulating cells chemically and recording the electrical response via the electrodes46. Alternatively or additionally, the electrodes46may be used to electrically stimulate cells.

Prior art polymer electrode arrays have been designed with drug delivery channels. However, the channels were fabricated using sacrificial photoresist. The photoresist is sandwiched between two polymer layers such that when it is dissolved away it leaves an empty space (channel) in the polymer. The sacrificial process of the prior art has the drawback of limiting the cross-sectional dimensions of the channel to the maximum thickness the resist can be spun on, typically less than 100 μm. In comparison, the cross-sectional dimensions of the tube channel30of the present invention are only limited by the thickness of the carrier wafer64, thus providing cross-sectional dimensions up to about 500 μm. Furthermore, these cross-sectional dimensions can even be extended beyond that through bonding of carrier wafers64.