Abstract:
An implantable tissue stimulating electrode device comprising: a carrier member; one or more biocompatible electrodes positioned on the carrier member, said electrodes having a surface; and an ionically conductive layer disposed at least over a portion of the surface of one or more electrodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a National Stage application of International Patent Application No. PCT/AU2009/000851, filed Jul. 2, 2009, and claims priority from Australian Patent Application No. 2008903558, filed Jul. 10, 2008. The content of these applications is hereby incorporated by reference herein. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to devices that deliver neural stimulation to the body, such as cochlear implants. 
     2. Related Art 
     Cochlear implant hearing prostheses are a well-recognized therapeutic means of restoring a sense of hearing for profoundly deaf persons. Using one or more microphones to convert incoming sound into electrical signals, these prostheses process the amplitude and frequency information contained within these signals to create several discrete channels of electro-neural stimulation for delivery to the human organ of hearing called the cochlear. 
     Since the hearing cells targeted by these prostheses are spatially distributed along the spiral pathways of the human cochlea, electrical stimulation intended to evoke perceptions of both the amplitude and frequency of incoming sound must be distributed spatially in a likewise fashion. As a consequence, each stimulation channel is thus assigned to deliver electrical stimulation in response to a particular range of incoming sound frequencies. 
     While thousands to perhaps millions of frequency distinguishing neural cells are implicated in normal hearing, current art electro-stimulation hearing prostheses are constrained by size and other factors to deliver only twenty or so discrete channels of stimulation. This number is sufficient to allow for speech recognition and conversation in quiet environments, but makes it difficult for speech to be easily discernible when background noise levels increase. 
     Users of languages predominantly more tonal than English are especially challenged since the limited number discrete channels of stimulation cannot replicate the inflection of a word to distinguish its meaning. It is also an unfortunate fact that while many users of current art devices report some enjoyment of musical sounds, tonal appreciation of even the simplest of tunes is beyond them as there are simply not enough stimulation channels available to fully restore the sense of hearing. 
     This limitation arises from the need to deliver a time constrained, minimum level of electric charge in order to evoke a hearing-like sensation with a particular apparent loudness or intensity. Current art electrodes are already close to the minimum size necessary to prevent non-reversible galvanic reactions, electrode corrosion, and the creation of chemical species that can destroy the viability of the sensory cells targeted by the delivered stimulation. These reactions increasingly occur as the electrode to body tissue interface impedance, and hence the voltage developed during the application of stimulation current, rises as the surface area of the electrode is reduced. 
     Since the avoidance of such reactions is crucial to the long-term success of these prostheses, current art intra-cochlear electrodes are required to be of a particular size so as to have an outer area of at least approximately 1 square millimeter. As a consequence, in addition to limiting the total number of stimulating electrodes that can be accommodated within the cochlear, the size of the electrode also limits the depth to which the intra-cochlear electrode array can be inserted within the cochlear and the degree to which low frequency hearing can be restored, since this where the neural cells associated with low frequency perception are located. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
     SUMMARY 
     According to a first aspect of the present invention, an, implantable tissue stimulating electrode device is disclosed. The electrode device comprises: 
     a carrier member; 
     one or more biocompatible electrodes positioned on the carrier member, said electrodes having a surface; and 
     an ionically conductive layer disposed at least over a portion of said surface of said one or more electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example only, embodiments of the invention are now described with reference to the accompanying drawings, in which: 
         FIG. 1  depicts the principal parts of the implantable component of a cochlear implant system; 
         FIG. 2A  is an enlarged simplified view of a prior art intracochlear electrode array positioned in a cochlea; 
         FIG. 2B  is an enlarged simplified view of one embodiment of an intracochlear electrode array according to the present invention positioned in a cochlea; 
         FIGS. 3A and 3B  depicts two enlarged views of the intracochlear electrode array according to the present invention; and 
         FIG. 4  is a still further enlarged cross-sectional view of one electrode of the intracochlear electrode array according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The depicted embodiments of the present invention are shown as part of a cochlear implant system. It is to be understood that the present invention has application to other implantable prostheses including but not limited to auditory prostheses. 
     One example of an implantable component of a cochlear implant system that can use the electrode member of the present invention is depicted generally as  10  in  FIG. 1 . The component  10  has a hermetically sealed and biocompatible titanium housing  11  that contains a receiver/stimulator unit. This unit receives signals transmitted from an external component using a radio frequency (RF) transcutaneous magnetic induction link. Antenna coil  12  comprises the implanted part of such a link. 
     Extending from the housing  11  via a feedthrough is a cable  13  that extends to an implantable tissue stimulating intracochlear electrode array  20 . It will be noted that a series of wires  14  extend through the cable  13  to a plurality of biocompatible electrodes  21  making up the array  20 . Not all of the wires  14  are depicted for reasons of clarity. The component  10  is also shown with an optional reference electrode  15  that can be placed externally or internally of the cochlea. 
     The depicted embodiment of the electrode array  20  comprises a carrier member  22  and a plurality of the electrodes  21 . It will be appreciated that a single electrode or less than or more than the number of electrodes depicted in the drawings could be mounted on the carrier member  22 . As described below, an ionically conductive layer  40  is also disposed at least over the electrodes  21 . As depicted in  FIG. 2B , the electrode array  20  is designed to be inserted into the scala tympani  31  of the cochlea  30  of an implantee. Due to its construction, the electrode array  20  can be inserted relatively further into the scala tympani  31  than that is safely possible using current typical electrode arrays (depicted as  20   a  in  FIG. 2A ). It will be noted that the leading end  23   a  of the array  20   a  does not reach as close to the end  32  of the scala tympani as is achieved by the leading end  23  of the array  20  depicted in  FIG. 2B . 
     As shown in  FIG. 4  the ionically conductive layer  40  can comprise any material that has a structure that allows conduction of ion species  43  through the layer to a surface of the electrode  21 . At the same time, the layer  40  preferably has a structure that partially, substantially or wholly prevents tissue growth over the electrode  21 , following implantation. 
     The ionically conductive layer  40  should be biocompatible, dimensionally stable, compliant, and/or capable of remaining in contact with at least a surface of the electrode  21  and, if required, the material of the carrier member  22 . It can comprise a polymeric material. In one embodiment, the layer  40  can comprise an ionically conductive elastomer or a hydrogel. The hydrogel can undergo hydration prior to implantation and/or be hydrated by exposure to bodily fluids on implantation. 
     The hydrogel making up the ionically conductive layer  40  can be formed from polyacrylic acids, poly(meth)acrylic acids, polyalkylene oxides, polyvinyl alcohols, poly(N-vinyl lactams), polyacrylamides, poly(meth) acrylamides, or pressure sensitive adhesives such as a N-vinyl-pyrrolidone/acrylic acid copolymer. 
     The ionically conductive layer  40  can be further disposed over some, the majority or all of the carrier member  22 . The layer  40  can further act as a lubricant and serve to assist in placement of the electrode array  20  in a desired location within an implantee. 
     The ionically conductive layer  40  can serve to host and release, when appropriate, beneficial chemical and/or bioactive agents  42  at the site of implantation of the electrode array  20 . For example, anti-inflammatory, anti-bacterial, and/or anti-viral agents could be released from the layer  40 . In another embodiment, cellular growth factors could be released from the layer  40 . 
     The electrodes  21  are formed from a metal or alloy. In the depicted embodiment, the electrodes  21  are platinum rings. 
     As depicted in  FIG. 3B , the electrodes  21  are chemically etched or otherwise have undergone surface modification so as to increase the surface area of the electrode  21  than would otherwise be the case. As depicted in both  FIG. 3B  and  FIG. 4 , one or more channels  41  can be formed in the electrode  21 . 
     As shown in  FIG. 4 , the presence of the channels  41  results in the ionically conductive layer  40  when applied to the electrode  21  moving relatively down and into the channels  41 . The channels  41  can be substantially shaped in the form of grooves extending substantially perpendicularly with respect to the carrier member  22 . As depicted in  FIG. 4 , the channels  41  can include recesses  44  extending substantially perpendicularly with respect to the channels  41  (i.e., substantially parallel to the carrier member  22 ). If a hydrogel, hydration of the layer  40  following disposition in the channels  41  serves to mechanically entrap the hydrogel layer  40  in the channels  41  and on the surface of the electrode  21 . This serves to create a relatively more intimate contact between the electrode  21  and the hydrogel  40  so ensuring a relatively low impedance ionic interface between the electrode  21  and the hydrogel  40 . 
     The carrier member  22  can be formed of a silicone elastomeric material, such as a Silastic material. The carrier member  22  preferentially adopts a spirally curved configuration but can be straightened or be straight prior to implantation. 
     The carrier member can have at least  22  electrodes. However, as depicted in  FIG. 2B , it is envisaged that significantly more electrodes  21  can be disposed on the carrier member  22  in the present invention. Each electrode  21  can have an outer area of less than 1 mm 2 . 
     The use of the ionically conductive layer  40  of the present invention provides for an electrode array  20  having relatively smaller electrodes  21  than hitherto usually used without suffering the disadvantage of undesirably increased interface impedance. The layer  40  also serves to prevent undesirable in-growth tissue over the electrodes  21  following the implantation. The relative decrease in the size of the electrodes  21  allows for manufacture of a relatively narrower carrier member  22  and potentially an increase in the number of electrodes  21  on the carrier member  22 . An increase in the number of electrodes  21  will potentially allow for improvement in the quality of the sound perception delivered to an implantee, particularly when listening to music. A relatively narrow carrier member  22  also provides the opportunity for potentially deeper insertion of the electrode array  20  into, for example, the cochlea  30 . In the case of the cochlea, relatively deeper insertion has the advantage of allowing stimulation of neural cells responsible for perception of relatively low frequencies. The decrease in width of the carrier member  22  also potentially reduces the likelihood of damage to the sensitive structures within the cochlea  30  during and following implantation. 
     In one embodiment, the electrode member is for use in conjunction with a cochlear implant system. The electrode member can comprise an intracochlear electrode array. Still further, the intracochlear electrode member can be suitable for insertion into the scala tympani of the cochlea of an implantee. 
     In one embodiment, the ionically conductive layer comprises any material that has a structure that allows conduction of ion species through the layer to a surface of the electrode. At the same time, the layer preferably has a structure that partially, substantially or wholly prevents tissue growth over the electrode. 
     In a further embodiment, the ionically conductive layer can be biocompatible, dimensionally stable, compliant, and/or capable of remaining in contact with at least the portion or all of the surface of the electrode and, if required, the material of the carrier member. It can comprise a polymeric material. In one embodiment, the layer can comprise an ionically conductive elastomer or a hydrogel. The hydrogel can undergo hydration prior to implantation and/or be hydrated by exposure to bodily fluids on implantation. 
     In a further embodiment, the hydrogel can be formed from polyacrylic acids, poly(meth)acrylic acids, polyalkylene oxides, polyvinyl alcohols, poly(N-vinyl lactams), polyacrylamides, poly(meth) acrylamides, or pressure sensitive adhesives such as a N-vinyl-pyrrolidone/acrylic acid copolymer. 
     In a further embodiment, the ionically conductive layer can be further disposed over some, the majority or all of the carrier member. The layer can further act as a lubricant and serve to assist in placement of the electrode member in a desired location within an implantee. 
     In a still further embodiment, the ionically conductive layer can serve to host and release, when appropriate, beneficial chemical and/or bioactive agents at the site of implantation of the electrode member. For example, anti-inflammatory, anti-bacterial, and/or anti-viral agents could be released from the layer. In another embodiment, cellular growth factors could be released from the layer. 
     In a still further embodiment, the electrodes are formed from a metal or alloy. In one embodiment, the electrodes are platinum. The electrodes can be chemically etched or otherwise have undergone surface modification. The etching or surface modification can be performed to increase the surface area of the electrode than would otherwise be the case. In one embodiment, one or more channels can be formed in the electrode. 
     Where the channels are present, the ionically conductive layer when applied to the electrode can move down and fill the channels. If a hydrogel, hydration of the layer following disposition in the channels can serve to mechanically entrap the hydrogel in the channels and on the surface of the electrode. This can serve to create a relatively more intimate contact between the electrode and the hydrogel so ensuring a relatively low impedance ionic interface between the electrode and the hydrogel. 
     The carrier member can have a leading end and a trailing end. In the case of an intracochlear electrode array, the leading end can be firstly insertable into the cochlea. A plurality of electrodes can be disposed on the carrier member between the leading end and the trailing end. The electrodes can be mounted in a longitudinal array with each having at least one wire, preferably at least two, extending back from each electrode and at least towards the trailing end. A cable can extend from the trailing end of the array back to an implantable housing of a receiver/stimulator unit. 
     The carrier member can be formed of a silicone elastomeric material, such as a Silastic material. The carrier member preferentially adopts a spirally curved configuration but can be straightened or be straight prior to implantation. 
     The carrier member can have at least  22  electrodes, however, it is envisaged that significantly more electrodes can be disposed on the carrier member in the present invention. Each electrode can have an outer area of less than 1 mm 2 . 
     The use of the ionically conductive layer of the present invention provides for an electrode member having relatively smaller electrodes than hitherto usually used without suffering the disadvantage of undesirably increased interface impedance. The layer also serves to prevent undesirable in-growth tissue over the electrodes following implantation. The relative decrease in the size of the electrodes allows for manufacture of a relatively narrower carrier member and potentially an increase in the number of electrodes on the carrier member. An increase in the number of electrodes will potentially allow for improvement in the quality of the sound perception delivered to an implantee, particularly when listening to music. A relatively narrow carrier member also provides the opportunity for potentially deeper insertion of the electrode member, such as into the cochlea. In the case of the cochlea, relatively deeper insertion has the advantage of allowing stimulation of neural cells responsible for perception of relatively low frequencies. The decrease in width of the carrier member also potentially reduces the likelihood of damage to the sensitive structures within the cochlea during and following implantation. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.