Abstract:
The present embodiments are directed to implantable electrode arrays having virtual electrodes. The virtual electrodes may improve the resolution of the implantable electrode array without the burden of corresponding complexity of electronic circuitry and wiring. In a particular embodiment, a virtual electrode may include one or more passive elements to help steer current to a specific location between the active electrodes. For example, a passive element may be a metalized layer on a substrate that is adjacent to, but not directly connected to an active electrode. In certain embodiments, an active electrode may be directly coupled to a power source via a conductive connection. Beneficially, the passive elements may help to increase the overall resolution of the implantable array by providing additional stimulation points without requiring additional wiring or driver circuitry for the passive elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/516,818, filed Apr. 8, 2011 and entitled, “VIRTUAL ELECTRODES,” the entire contents of which are specifically incorporated herein by reference without disclaimer. 
     
    
     GOVERNMENT INTEREST 
       [0002]    This invention was made with government support under Grant or Award no. 00065-5000-55800534 awarded by Department of Energy. The government has certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0003]    This invention relates to bio-stimulation devices and more particularly relates to virtual electrodes for implantable bio-stimulation devices. 
       BACKGROUND 
       [0004]    Electrical stimulator devices are used to stimulate various types of organic tissue. For example, electronics may be interfaced with the nervous system of a human body through use of neurostimulators. A neurostimulator is a device that may be implanted into human tissue, and provides stimulation to neurons through electrical pulses. For such reasons, neurostimulators may be referred to as implanted pulse generators (IPGs). 
         [0005]    A typical neurostimulator includes one or more electrodes. Some neurostimulators include arrays of electrodes configured in implantable devices. For example,  FIG. 1  illustrates a typical system  100  for neurostimulation. The system  100  includes an implantable electrode array  102  which may be implanted in organic tissue  106 . For example, the implantable electrode array  102  may be implanted in a human brain or near optical or auditory nerves. The system  100  also includes a controller  104  coupled to the implantable electrode array  102 . The electrode controller  104  is often located outside of the body. In certain systems, however, the electrode controller  104  may also be implantable. 
         [0006]      FIG. 2  illustrates one configuration of a electrode array  102 . As illustrated, the electrode array  102  includes a plurality of electrodes  202   a - d , and a current return  204 . A dielectric backing  107  mechanically supports the array and helps directing the charge injection into the tissue. The electrode array  102  may be implanted so the electrodes are in contact with organic tissue  106 . In most prior systems, the current return  204  is placed in a region relatively distal from the electrode array  102 . Each of the electrodes is coupled to a power source which injects charge on the electrodes  202   a - d . The charge is injected through the organic tissue  106  to the current return  204 , thus stimulating the organic tissue  106  between the electrodes  202   a - d  and the current return  204 . Typically, only the portions of organic tissue  106  directly proximate each of the electrodes  202   a - d  are stimulated. 
         [0007]    In order to increase the stimulation resolution of an electrode array  102 , additional electrodes  202  may be included in the electrode array  102 . For example, in the case of a retinal prosthesis, the electrode array  102  may originally have included nine electrodes  202 . In an effort to increase resolution, the electrode array  102  could eventually include sixty-four electrodes  202  and then two hundred electrodes  202  or more in later versions. 
         [0008]    In prior stimulator systems, as the number of electrodes increases, for the same size device, each electrode has to be smaller. This causes the current density at the vicinity of the stimulating electrodes to grow accordingly, to a point in which further miniaturization may lead to current density magnitudes that can damage the tissue. Additionally an additional wire and an additional driver circuit are typically required for each electrode, and the wire needs to have a large enough section to allow the appropriate amount of current to flow. These wires make the implant bulkier, mechanically stiffer, and in general harder to conform to delicate anatomical features. 
       SUMMARY 
       [0009]    The present embodiments are directed to implantable electrode arrays having virtual electrodes. The virtual electrodes may improve the resolution of the implantable electrode array without the burden of corresponding additional wiring and complexity of electronic circuitry. In a particular embodiment, a virtual electrode may include one or more passive elements. For example, a passive element may be a metalized layer on a substrate that is adjacent to, but not directly connected to an active electrode. In certain embodiments, an active electrode may be directly coupled to a power source via a conductive connection. Beneficially, the passive elements may help to increase the overall resolution of the implantable array by providing additional stimulation points without requiring additional wiring or driver circuitry for the passive elements. 
         [0010]    In the proposed scheme, the excitation waveform used in the electrodes has higher frequency components than what the body can react to. The neural cells being stimulated respond then to an averaged stimulus over time. 
         [0011]    Embodiments of an apparatus for stimulating biological tissue are described. In one embodiment, the apparatus includes a first active electrode configured to receive a current from a current source and injecting it into organic tissue. The apparatus may also include a second active electrode configured to return current emitted by the first active electrode to ground. Additionally, the apparatus may include a region defining a virtual electrode disposed between the first active electrode and the second active electrode. 
         [0012]    In further embodiments, a portion of the current emitted by the first active electrode is collected in the region defining the virtual electrode. Additionally, the time-average current density present in the region defining the virtual electrode is sufficient to stimulate biological tissue in proximate to the region defining the virtual electrode. The region defining the virtual electrode may include one or more passive elements. The passive elements may include a conductive layer disposed on a substrate in the region defining the virtual electrode. Additionally, an electrical insulation barrier may be disposed between the first and second active elements and the one or more passive elements. In one embodiment, the one or more passive elements are shaped in a pattern of a cross, a center point of the cross being disposed at a center point of the region defining the virtual electrode. In another embodiment, a majority portion of the region defining the virtual electrode comprises the one or more passive elements. 
         [0013]    In such embodiments, the apparatus may include a plurality of active electrodes arranged in an array. Additionally, the apparatus may include a plurality of regions defining virtual electrodes, the virtual electrodes positioned between the plurality of active electrodes in the array. This is accomplished by means of tiling the same electrode pattern over a larger region. 
         [0014]    Embodiments of systems for stimulating biological tissue are also presented. In one embodiment, the system includes an implantable bio-stimulator device. The implantable bio-stimulator device may have a first active electrode configured to inject current into tissue from a current source, a second active electrode configured to sink the current injected by the first active electrode, and a region defining a virtual electrode disposed between the first active electrode and the second active electrode. Additionally, the system may include a current source coupled to the implantable bio-stimulator device and configured to supply current to the first and second active electrodes in the implantable bio-stimulator array. In a further embodiment, the system includes an electrode controller coupled to the implantable bio-stimulator device, and configured to control operation of the implantable bio-stimulator device. 
         [0015]    The system may also include one or more conductors coupling the first and second active electrodes to the electrode controller. The electrode controller may also include one or more driver circuits coupled to the first and second active electrodes, the driver circuit configured to supply current from the current source to the first and second active electrodes according to a timing sequence. 
         [0016]    Methods of stimulating biological tissue are also presented. In one embodiment a method includes providing a stimulating current to a first active electrode in an implantable bio-stimulator device, and collecting return current from a second active electrode in the implantable bio-stimulator device. The first active electrode and the second active electrode may be arranged in an array configuration with one or more regions defining virtual electrodes disposed adjacent to the first active electrode and the second active electrode. 
         [0017]    Additionally, the method may include providing the stimulating electrical charge in a pulse current pulse having finite pulse duration, the pulse duration sufficient to allow a portion of the electrical charge injected by the first electrode to accumulate in a region defining a virtual electrode. The region defining a virtual electrode may have one or more passive elements configured to steer the injected electrical charges to a predetermined position within the region defining the virtual electrode. 
         [0018]    Additionally, the methods may include providing a plurality current waveforms to a plurality of active electrodes in an implantable array of bio-stimulator electrodes comprising both active electrodes and virtual electrodes, wherein the duration, timing, waveform, and firing sequences of the injected current is sufficient to generate a stimulation current in the virtual electrodes. A sequence of the waveforms may be applied to preselected active electrodes in the array of electrodes according to a predetermined pattern. 
         [0019]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0021]      FIG. 1  is a schematic block diagram illustrating a system for biostimulation according to the prior art; 
           [0022]      FIG. 2  is a perspective view diagram illustrating one embodiment of an implantable electrode array according to the prior art; 
           [0023]      FIG. 3  is a logical diagram illustrating an embodiment of a stimulation current path; 
           [0024]      FIG. 4  is a logical diagram illustrating another embodiment of a stimulation current path; 
           [0025]      FIG. 5  is a schematic diagram illustrating one embodiment of an implantable electrode array having a virtual electrode; 
           [0026]      FIG. 6  is a graphical diagram illustrating a comparison of time-accumulated normalized current density magnitude across an active electrode and a virtual electrode in an embodiment that does not include a passive element; 
           [0027]      FIG. 7  is a schematic diagram illustrating one embodiment of an electrode array having passive elements; 
           [0028]      FIG. 8A  is a graphical diagram illustrating a pattern of time-accumulated normalized current density at 10 μm from the embodiment of an implantable sensor array shown in  FIG. 7 ; 
           [0029]      FIG. 8B  is a graphical diagram illustrating a pattern of time-accumulated normalized current density at 30 μm from the embodiment of an implantable sensor array shown in  FIG. 7 ; 
           [0030]      FIG. 8C  is a graphical diagram illustrating a pattern of time-accumulated normalized current density at 50 μm from the embodiment of an implantable sensor array shown in  FIG. 7 ; 
           [0031]      FIG. 9  is a graphical diagram illustrating a comparison of time-accumulated normalized current density magnitude across an active electrode and a virtual electrode in the embodiment of an implantable sensor array shown in  FIG. 7 ; 
           [0032]      FIG. 10  is a schematic diagram illustrating another embodiment of an electrode array having passive elements; 
           [0033]      FIG. 11A  is a graphical diagram illustrating a pattern of time-accumulated normalized current density at 10 μm from the embodiment of an implantable sensor array shown in  FIG. 10 ; 
           [0034]      FIG. 11B  is a graphical diagram illustrating a pattern of time-accumulated normalized current density at 30 μm from the embodiment of an implantable sensor array shown in  FIG. 10 ; 
           [0035]      FIG. 11C  is a graphical diagram illustrating a pattern of time-accumulated normalized current density at 50 μm from the embodiment of an implantable sensor array shown in  FIG. 10 ; 
           [0036]      FIG. 12  is a graphical diagram illustrating a comparison of time-accumulated normalized current density magnitude across an active electrode and a virtual electrode in the embodiment of an implantable sensor array shown in  FIG. 10 ; 
           [0037]      FIG. 13  is a schematic diagram illustrating another embodiment of an electrode array having passive elements; 
           [0038]      FIG. 14  is a graphical diagram illustrating a comparison of time-accumulated normalized current density magnitude across an active electrode and a virtual electrode in the embodiment of an implantable sensor array shown in  FIG. 13 ; 
           [0039]      FIG. 15  is a schematic flowchart diagram illustrating one embodiment of a method of using an implantable electrode array having a virtual element. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    Various features and advantageous details are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well known starting materials, processing techniques, components, and equipment are omitted so as not to unnecessarily obscure the invention in detail. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the invention, are given by way of illustration only, and not by way of limitation. Various substitutions, modifications, additions, and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those skilled in the art from this disclosure. 
         [0041]      FIGS. 3A-3B  are logical diagrams illustrating embodiments of a stimulation current path. In  FIG. 3 , the external current return  204  is eliminated. For example, in the implantable electrode array  102  of  FIG. 3 , the electrodes  202   a - d  may be configured such that electrode  202   a  is coupled to a current source and electrode  202   b  is configured as a current return for current emitted by electrode  202   a . Thus, a flow of current between electrode  202   a  and electrode  202   b  may be established. As described in the paragraphs to follow, a configuration in which a first electrode (e.g.,  202   a ) is a current source and a second electrode (e.g.,  202   b ) is a current return may be beneficial for controlling positioning of current density in tissue surrounding the electrodes (e.g.,  202   a - b ) in the electrode array.  FIG. 4  illustrates a second embodiment, wherein a diagonal current path is established between a first electrode (e.g.,  202   a ) and a second electrode (e.g.,  202   c ). 
         [0042]    There are several distinctions that can be drawn from a comparison of the present embodiments with the prior art system of  FIG. 2 . For example, in the present embodiments, the active electrodes  202   a - d  are also configurable as current returns, where as in the prior art the current return  204  is not included as one of the elements of the electrode array  102 . Indeed, it is the configurations in which the current is drawn between the electrodes in the array that helps facilitate distribution of charge to the virtual electrode discussed below with respect to  FIG. 5 . As illustrated in various embodiments herein, an current injection electrode (e.g.,  102   a ) may situated in a coplanar orientation with the current return electrode (e.g.,  102   c ). Indeed, because the current injection electrode and the current return electrode are part of a common electrode array  102 , they may be formed on a common substrate. 
         [0043]      FIG. 5  is a schematic diagram illustrating one embodiment of an implantable electrode array  102  having a virtual electrode  502 . In one embodiment, the implantable electrode array  102  may include a plurality of electrodes  202   a - d  that are actively powered. The virtual electrode  502  may be passively powered by charge injection for the actively powered electrode  202   a - d . For example, a current may be injected on a first electrode  202   a  for 0.1 μs and a second electrode  202   c  that is situated diagonally across from the first electrode  202   a  may be configured as a current collector for the current injected on the first electrode  202   a . In such an embodiment, charge may flow through the virtual electrode region  502 . In such an embodiment, charge may be rapidly injected from neighboring electrodes  202   a - d  using predetermined spatial and temporal patterns which are configured to increase the time-average amount of charge present in the area of the virtual electrode  502 . 
         [0044]      FIG. 6  is a graphical diagram illustrating a comparison of accumulated normalized current density magnitude across an active electrode  202   a - d  and a virtual electrode  502  in an embodiment that does not include a passive element. As illustrated, there is some increase in the current density in the region corresponding to the virtual electrode, but the current density is still far lower than the current density at the center of the active electrodes  202   a - d.    
         [0045]      FIG. 7  is a schematic diagram illustrating another embodiment of an electrode array  102  having one or more passive elements  702  comprising the virtual electrode  502 . In this embodiment, the passive elements  702  may include a plurality of conductive arms positioned adjacent to the electrodes  202   a - d . The passive elements  702  do not actually touch one another. The passive elements  702  may be formed of a bio-compatible conductive material, such as gold, platinum, or other suitable conductive materials. In one embodiment, the passive elements  702  at least partially insulated from the electrodes  202   a - d . Thus, in one embodiment, current density may accumulated on the passive electrodes  702 , thereby creating a region of relatively higher charge density at the center point of the virtual electrode  502 . 
         [0046]    One benefit of including the passive elements  702  is that the passive elements  702  provide a low-resistance path between the current injection electrode (e.g.,  102   a ) and the current return electrode (e.g.,  102   c ). Thus, the charge will be more likely to follow a path along the passive elements  702  from the current injection electrode to the current return electrode. Accordingly, if the passive elements  702  are positioned in a region defining a virtual electrode  502 , then the virtual electrode  502  will exhibit a higher current density than would be likely without the passive elements  702 .  FIGS. 9A-C  illustrate this as well. Various geometries may be used to generate a variety of current density characteristics in the virtual electrode  502 . 
         [0047]      FIG. 8A  is a graphical diagram illustrating a pattern of accumulated normalized current density at 10 μm from the embodiment of an implantable sensor array  102  shown in  FIG. 7 . In this embodiment, a greater level of current density can be seen as the relatively light portions of  FIG. 7 . Although it appears that the greatest current density corresponds to the position of the active elements  202   a - d , there does appear to be a higher current density concentration at the center of the virtual electrode  502  which is indicated by the lighter shaded portions in the middle of  FIG. 8A .  FIG. 8B  is a graphical diagram illustrating a pattern of accumulated normalized current density at 30 μm from the embodiment of an implantable sensor array  102  and  FIG. 8C  is a graphical diagram illustrating a pattern of accumulated normalized current density at 50 μm from the implantable sensor array  102 . 
         [0048]      FIG. 9  is a graphical diagram illustrating a comparison of accumulated normalized current density magnitude across an active electrode  202   a - d  and a virtual electrode  502  in the embodiment of an implantable sensor array  102  shown in  FIG. 7 . In this embodiment, a significant increase in the current density over the embodiment of  FIG. 5  which did not include a passive element. Thus, it appears that in at least this “cross” configuration, the passive element facilitates collection of greater charge density at the center of the virtual electrode  502 . 
         [0049]      FIG. 10  is a schematic diagram illustrating another embodiment of an electrode array  102  having passive elements  702 . In this embodiment, the passive elements are configured to take up the majority of the area of the virtual electrode  502 , leaving only a cross-shaped gap between the passive elements. Further, in this embodiment, the passive elements  702  are configured to match the contour of the active electrode  202   a - d , thereby gaining greater electromagnetic coupling with the active electrodes  202   a - d.    
         [0050]      FIG. 11A  is a graphical diagram illustrating a pattern of accumulated normalized current density at 10 μm from the embodiment of an implantable sensor array  102  shown in  FIG. 10 . In this embodiment, it can be seen by the lighter cross-shaped portion in the center of the virtual electrode  502  that a relatively high level of current density is achievable than with the previously discussed embodiments.  FIG. 11B  is a graphical diagram illustrating a pattern of accumulated normalized current density at 30 μm and  FIG. 11C  is a graphical diagram illustrating a pattern of accumulated normalized current density at 50 μm. These show the highest current density at 50 μm. 
         [0051]      FIG. 12  is a graphical diagram illustrating a comparison of accumulated normalized current density magnitude across an active electrode  202   a - d  and a virtual electrode  502  in the embodiment of an implantable sensor array  102  shown in  FIG. 10 . As can be seen from this graph, the current density achievable in the virtual electrode  502  is at or above the same level that is achievable in the active electrodes  202   a - d . Thus, by tuning the size and geometry of the passive element, a variety of different current density levels and patters are achievable in the virtual electrode  502 . 
         [0052]    One of ordinary skill in the art will recognize other geometries for the passive elements  702  which may be suitable for various applications. For example, triangle shapes, star shapes, and other similar geometries may be used. In each case, one of ordinary skill in the art will appreciate that the current density in the region defining the virtual electrode  502  may be tuned by adjustment of the geometry including shape and size of the one or more passive elements  702 . 
         [0053]      FIG. 13  is a schematic diagram illustrating another embodiment of an electrode array  102  having passive elements  702 . In the embodiment of  FIG. 13 , a region of the passive elements  702  adjacent to the active elements  202   a - d  is shaped to match a contour of the active elements  202   a - d . In such an embodiment, a greater degree of electromagnetic coupling between the active elements  202   a - d  and the passive elements  702  may be achieved. For example,  FIG. 14  is a graphical diagram illustrating a comparison of accumulated normalized current density magnitude across an active electrode and a virtual electrode in the embodiment of an implantable sensor array shown in  FIG. 13 . 
         [0054]      FIG. 15  is a schematic flowchart diagram illustrating one embodiment of a method  1500  of using an implantable electrode array  102  having a virtual element  502 . In one embodiment, a method  1500  includes providing  1504  a stimulating current to a first active electrode (e.g.,  202   a ) in an implantable bio-stimulator device  102 , and collecting  1506  return current from a second active electrode (e.g.,  202   c ) in the implantable bio-stimulator device. The first active electrode  202   a  and the second active electrode  202   c  may be arranged in an array configuration  102  with one or more regions defining virtual electrodes  502  disposed adjacent to the first active electrode  202   a  and the second active electrode  202   c.    
         [0055]    Additionally, the method  1500  may include providing the stimulating current in a pulse current pulse having finite pulse duration, the pulse duration sufficient to allow a significant portion of the current emitted by the first electrode  202   a  to reach into a region defining a virtual electrode  502 . The region defining a virtual electrode  502  may have one or more passive elements  702  configured to direct the current to a predetermined position within the region defining the virtual electrode  502 . 
         [0056]    Additionally, the methods  1500  may include providing a plurality of current pulses to a plurality of active electrodes  202   a - d  in an implantable array  102  of bio-stimulator electrodes comprising both active electrodes  202   a - d  and virtual electrodes  502 , wherein a duration of the pulses is sufficient to generate a stimulation current in the virtual electrodes  502  as illustrated in  FIGS. 6 ,  9 ,  12  and  14 . A timing and/or sequence of the pulses may be applied to preselected  1502  active electrodes in the array  102  of electrodes according to a predetermined pattern. 
         [0057]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.