Abstract:
Providing an electrode structure capable of realizing an electrode array which allows each of the electrodes to be individually controlled while allowing them to be densely arranged and placed in a living body. According to the present invention, an electrode control circuit electrically connected to an electrode body is fixed to a rear portion of the electrode body within a front-viewed contour of the electrode body. This electrode control circuit may be contained in a recess formed in the rear portion of the electrode body, or it may be fixed to the back face of the electrode body. Conversely, an electrically conductive material layer covering the electrode control circuit may be used as the electrode body. A plurality of such bioelectrodes may be arranged in a two-dimensional form on a substrate or connected by a connection line including an electrical wire. Such configurations allow the bioelectrodes to be densely arranged.

Description:
TECHNICAL FIELD 
     The present invention relates to a bioelectrode to be attached to a living body (such as an organ of various experimental animals or that of a man) so as to give electrical stimuli to that living body or measure an electric potential, electric current or similar quantity in that living body. 
     BACKGROUND ART 
     Electrode arrays having a number of electrodes arranged in various patterns are used as a nerve interface for electrically stimulating a nerve or measuring a nerve action potential. To fulfill this intended purpose, the electrode array should be implanted in a living body and the wires which are bonded to the metallic (e.g. platinum) electrodes should be connected to a stimulator or measurement apparatus. As one method for enabling the multielectrode configuration of the electrode array, the technique of equipping each individual electrode with a small semiconductor chip capable of performing electrode control as well as other functions has been proposed and demonstrated (high-function electrode). 
     For example, in the case where the sense of sight has been lost due to dysfunction of the visual cells in the retina for converting light into electric signals (examples of the dysfunction include age-related macular degeneration and retinitis pigmentosa), while there is no problem in the ganglion cells in the retina or the optic nerves connecting the retina and the brain, the vision can be virtually restored by taking a visual image of the scene in front of the eyes using a camera or similar device and giving the ganglion cells or other remaining retinal cells two-dimensional electrical stimuli corresponding to that image. Such a system for providing a vision substitution by giving electrical stimuli to the retina is called the “artificial vision device” (for example, see Patent Literature 1). 
     Non Patent Literature 1 discloses a visual stimulation experiment performed on a rabbit using an artificial vision device employing suprachoroidal transretinal stimulation (STS). The measurement was performed as follows: As shown in  FIGS. 1A and 1B , a flexible substrate with an array of 3×3 electrodes arranged as shown in  FIG. 3  was planted in an eyeball (sclera) of a rabbit, and electrical stimuli were given to the retina from the choroid side. Meanwhile, electrodes were attached to the visual cortex on the brain of the rabbit, and the electric potential at that point (electrically evoked visual potentials) was measured. 
     The amount of electric current supplied to the electrodes attached to the eyeball was set at various values and the change in the brain wave was measured, with the point in time of the supply of the electric current (or stimulus) defined as the zero point. Consequently, as shown in  FIG. 2 , it was confirmed that the peak height (response) of the brain wave increases with an increase in the current value (stimulus). The time delay from the stimulation (approximately 20 msec) was roughly equal to the transmission delay of the vision investigated by another experiment. These facts confirm that this electrode array substrate  80  (in  FIG. 3 ) was correctly acting as an eyeball-stimulating electrode. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2006-187409 A 
     Non Patent Literature 
     Non Patent Literature 1: Toshihiko Noda et al., “Totsugata Shigeki Denkyoku To CMOS Chippu Wo Tousai Shita Furekishiburu Jinkou Shikaku Debaisu No Sakusei To Kinou Jisshou (Creation and Functional Demonstration of Flexible Artificial Vision Device Equipped with Convex Stimulating Electrodes and CMOS Chips)”, Mar. 26, 2011, The 58 th  Spring Meeting of the Japan Society of Applied Physics and Related Societies 2011 (at Kanagawa Institute of Technology) 
     SUMMARY OF INVENTION 
     Technical Problem 
     As shown in  FIG. 3 , the electrode array substrate  80  described in Non Patent Literature 1 has an electrode control circuit chip  82  for each electrode  81  to control the amount and timing of the electric current or voltage supplied to the electrode  81  as well as other parameters. This configuration requires only four wires (for positive and negative power sources as well as first and second operation control lines) to be connected to the entire substrate regardless of the number of electrodes  81 , thus allowing a considerable number of electrodes  81  to be embedded in a living body (e.g. an eyeball). 
     The electrode control circuit chip  82  for controlling each electrode  81  is small in size. However, the spacing of the electrodes  81  cannot be small since the chip space must be provided for each individual electrode  81  as shown in  FIG. 4 . Therefore, in particular, it is difficult to densely arrange the electrodes  81  in a small part (e.g. eyeball) of the living body. 
     The present invention has been developed in view of the previously described problem. Its primary objective is to provide an electrode structure capable of realizing an electrode array which allows each of the electrodes to be individually controlled while allowing them to be densely arranged and placed in a living body. 
     Solution to Problem 
     A bioelectrode according to the present invention developed for achieving the aforementioned objective is characterized in that an electrode control circuit electrically connected to an electrode body is fixed to a rear portion of the electrode body within a front-viewed contour of the electrode body. 
     The phrase “within a front-viewed contour of the electrode body” means that the portion concerned lies within the contour of the electrode body when the electrode body is viewed from the side that is to be in contact with, pushed onto or stuck into a living body (the front side of the electrode body). 
     Specifically, the electrode control circuit may be contained in a recess formed in the rear portion of the electrode body. It is also possible to fix the circuit to the back face (rear face) of the electrode body, without forming any recess in the rear portion of the electrode body. Furthermore, as opposed to forming a recess in a solid electrode body and containing the circuit in that recess, the electrode control circuit may be covered with an electrically conductive layer and use this layer as the electrode body. 
     These bioelectrodes may be singly used. Alternatively, they can be arranged in a two-dimensional form (array) on a substrate and used as an electrode array, or be connected by a connection line including an electrical wire and used as a connection-type bioelectrode. 
     Advantageous Effects of the Invention 
     In the bioelectrode according to the present invention, since the control circuit for controlling the electrode body is fixed to the rear portion of the electrode body within the front-viewed contour of the electrode body, the electrode body can be implanted in a compact area of a living body without additionally providing a lateral space for the electrode control circuit. This is particularly advantageous when many electrode bodies need to be implanted in a living body, since the electrode bodies can be densely arranged so as to give stimuli to the living body or measure biopotential or other quantities at a higher level of planer (or linear) density. 
     By forming a recess in the rear portion of the electrode body and containing the electrode control circuit in that recess, the entire length of the bioelectrode inclusive of the electrode body and the electrode control circuit can be reduced. In the case of implanting this bioelectrode in a living body, a liquid-tight separation between the electrode control circuit and the living body can be achieved by simply sealing the open side of the recess. Therefore, it is easy to prevent both the invasion on the living body by the electrode control circuit and the invasion of biological solutions into the electrode control circuit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are illustrations showing the setup of a visual stimulation experiment performed on a rabbit using an artificial vision device. 
         FIG. 2  is a graph showing the magnitude of the stimulus and the change in the brain wave, which is a result of the aforementioned visual stimulation experiment performed on the rabbit. 
         FIG. 3  is a plan view of the electrode array substrate used in the aforementioned visual stimulation experiment performed on the rabbit. 
         FIG. 4  is a sectional view showing the structure of the electrode array substrate used in the aforementioned visual stimulation experiment performed on the rabbit. 
         FIGS. 5A-5C  are a side view, sectional view and bottom view, respectively, of a bullet-shaped electrode body used in an artificial vision device as one embodiment of the bioelectrode according to the present invention. 
         FIG. 6  is a plan view of an electrode control circuit chip used in the artificial vision device of the aforementioned embodiment. 
         FIG. 7  is a sectional view showing the structure of an electrode array substrate used in the artificial vision device of the aforementioned embodiment. 
         FIG. 8  is a perspective view of the electrode array substrate used in the artificial vision device of the aforementioned embodiment. 
         FIG. 9  is a sectional view showing the structure of an electrode array substrate using an electrode body as another embodiment of the present invention. 
         FIG. 10  is a sectional view showing the structure of an electrode array substrate using a plate-shaped electrode body as another embodiment of the present invention. 
         FIG. 11  is a sectional view showing the structure of an electrode array substrate using a coating electrode body as another embodiment of the present invention. 
         FIGS. 12A-12C  are sectional views of other embodiments of the present inventions, where  FIG. 12A  is a bioelectrode with the control circuit fixed to the rear face,  FIG. 12B  is a bioelectrode with the control circuit encapsulated, and  FIG. 12C  is a bioelectrode coated with an electrically conductive resin or ceramic encapsulation material. 
         FIG. 13  is a perspective view of a connection-type bioelectrode consisting of the bioelectrodes connected by a connection line including a conductor wire. 
         FIGS. 14A and 14B  are model views showing application examples of the connection-type bioelectrode, where  FIG. 14A  shows an application to an eyeball and  FIG. 14B  shows an application to a brain. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An artificial vision device as one embodiment of the bioelectrode according to the present invention is hereinafter described. As shown in  FIGS. 5A-5C , the bioelectrode  10  used in the artificial vision device of the present embodiment uses a bullet-shaped electrode body  11 . The electrode body  11  may be made of any material compatible with living bodies. Examples of the available materials include: metallic materials, such as platinum (Pt), gold (Au), titanium (Ti) or an alloy of these kinds of metal; electrically conductive compounds, such as iridium oxide (IrOx) or titanium nitride (TiN); and electrically conductive polymers, such as poly(3,4-ethylenedioxythiophene) or PEDOT. Naturally, it is also possible to use such electrically conductive materials for the surface coating while creating the inner part of the electrode body from resin, ceramic or other non-conductive materials. In the case of using an inner part made of a resin, ceramic or similar non-conductive material, it is necessary to form an electrical channel between the electrode control circuit chip and the electrically conductive coating on the surface (as will be described later). Compared to resin, using a metallic or ceramic material allows the electrode control circuit chip to be encapsulated for a longer period of time within a hermetic space surrounded by the metallic or ceramic material. Accordingly, it is preferable to use a metallic material for the electrode body  11  or create the inner part of the electrode body  11  from ceramic. In this case, the electrode body  11  can double as the encapsulation material (which will be described later). Although the following descriptions deal with the case of a bullet-shaped electrode body  11  with a size of 500 μm, the present invention is not limited to this size but may have any size that allows the electrode body to be implanted in a living body, e.g. from 10 μm to 500 μm. The “size” of the electrode body  11  in the present context means the depth, width or height of the electrode body  11 , whichever is the largest. 
     As shown in  FIGS. 5B and 5C , the electrode body  11  of the present embodiment has, in its rear portion, a recess  12  for containing an electrode control circuit chip  20 . 
     The electrode control circuit chip  20  is a single chip on which a circuit for controlling electric current and/or voltage supplied to the electrode body  11  is created. As shown in  FIG. 6 , it includes five electrode pads  21   a - 21   e  and three control circuits (or similar elements)  22   a - 22   c  arranged among those pads. The electrode pads  21   c  and  21   d  are the terminals for receiving positive and negative DC power from an external source, while the electrode pads  21   a  and  21   b  are the terminals for receiving externally-supplied operation control signals. As will be described later, these four electrode pads  21   a - 21   d  are individually and respectively connected to the four electrical wires provided in a substrate  31 . Based on the operation control signals thus supplied from an external control circuit, the electrode control circuit chip  20  performs various mathematical operations and appropriately controls the electric current and/or voltage supplied from the positive and negative power sources so that a controlled amount of current and/or voltage will be fed to the central electrode pad  21   e . The electric current and/or voltage fed to the electrode pad  21   e  is supplied to the electrode body  11  through a wire provided in the substrate  31 . 
     This electrode control circuit chip  20  is contained in the recess  12  in the rear portion of the electrode body  11  and fixed by filling the surrounding space with a resin or similar material. In other words, the electrode control circuit chip  20  is encapsulated in the recess  12  in the rear portion of the electrode body  11  with a resin or similar encapsulation material. A resin, metallic or ceramic material can be used as the encapsulation material. Compared to resin, using a metallic or ceramic material allows the electrode control circuit chip to be encapsulated for a longer period of time within an hermetic space surrounded by the metallic or ceramic material. Therefore, they can be suitably used as the encapsulation material for implantation in a living body. When the electrode control circuit chip  20  is inserted into the electrode body  11 , these two components should be oriented in a specified way, and furthermore, a mark should be put on the outside of the electrode body  11  so that one can locate the position of the contained electrode control circuit chip  20  around the axis. 
     As shown in  FIG. 8 , a plurality of electrode bodies  11  (bioelectrodes  10 ) each of which has the electrode control circuit chip  20  contained in the recess  12  in its rear portion are two-dimensionally arranged on and fixed to a flexible substrate  31 . In this process, each electrode body  11  should be placed at a specific position and in a specific direction with respect to the substrate  31  so that the electrode pads  21   a - 21   e  of the electrode control circuit chip  20  will be correctly brought into electrical connection with the counterpart electrode pads on the substrate  31 . As a result, as shown in  FIG. 7 , the wires  32  provided in the substrate  31  are connected to the electrode pads  21   a - 21   e  of the electrode control circuit chip  20 , allowing the operation control signals and power from external sources to be correctly supplied to each electrode control circuit chip  20 . The region surrounding the bottom portion of the electrode body  11  placed on the substrate  31  is sealed with a sealant  33 . 
     As shown in  FIG. 8 , the electrode array substrate  30  thus created as one example of the bioelectrode substrate has the bioelectrodes  10  densely arranged without the electrode control circuit chips  20  placed in between. Therefore, it is possible to give precise electrical stimuli to target sites of the living body. Since the electrode control circuit chip  20  is contained in the recess  12  of each electrode body  11  and sealed on the flexible substrate  31  with the sealant  33 , the various component substances in the electrode control circuit chip  20  will not penetrate into the living body. Thus, the living body is safely protected and can be correctly tested without being influenced by those substances. Conversely, biological solutions are prevented from invading into the electrode control circuit chip  20  and corroding the electric circuits or obstructing electrical conductions. Such characteristics of the bioelectrode  10  are important since, in some cases, the implanted bioelectrode  10  needs to be left in the living body for a number of years. 
     Another example of the bioelectrode according to the present invention is shown in  FIG. 9 . This bioelectrode  50  is similar to the previous embodiment in that the electrode control circuit chip  20  is contained in the recess in the rear portion of the electrode body  11 . The characteristic point is that the supply of the electric current and/or voltage from the electrode control circuit chip  20  to the electrode body  11  is directly performed from an electrode pad  51  provided in the upper portion (on the side closer to the tip of the electrode body  11 ) of the electrode control circuit chip  20  to the electrode body  11 . This configuration simplifies the pattern of the conductor wires in the substrate  31 . 
     In any of the previously described examples, the electrode body  11  is bullet shaped. However, as shown in  FIG. 10 , the electrode body  15  may be shaped like a plate having a circular or rectangular form (or any other form). 
     Furthermore, as opposed to those examples in which a solid body of an electrically conductive material is used as the electrode body and the electrode control circuit chip is contained in the recess formed in that body, a bioelectrode  60  as shown in  FIG. 11  may be constructed by covering the electrode control circuit chip  20  with an electrically conductive material layer  16  formed by application, plating or similar processes. 
     Furthermore, as shown in  FIG. 12A , it is possible to fix the electrode control circuit chip  20  to the rear portion of the electrode body  70   a  instead of containing it in the electrode body. In this case, the electrode control circuit chip  20  is fixed to the electrode body  70   a  by attaching it to the electrode body  70   a  with its electrode pads electrically in contact with the rear portion of the electrode body  70   a , and subsequently covering it with a resin material  70   b  or the like and curing the material. In other words, the electrode control circuit chip  20  is encapsulated on the rear portion of the electrode body  70   a  with the resin material  70   b  as the encapsulation material. This bioelectrode  70  is not fixed to the substrate  31  as in the previous embodiments, and therefore, can be singly used. For example, as shown in  FIG. 13 , it is possible to create, as one example of the bioelectrode connection line, a connection-type bioelectrode  90  having a plurality of bioelectrodes  70  connected by a connection line  75 . Connecting the bioelectrodes  70  by the connection line  75  including a conductor wire allows the bioelectrodes  70  to be arbitrarily arranged at desired sites of a living body.  FIG. 14A  shows an example of attaching the electrodes to the fundus of the eyeball, while  FIG. 14B  shows an example of attaching them to the brain. For such an application using the connection line, a bioelectrode  71  as shown in  FIG. 12B  can also be used, in which the electrode control circuit  20  is encapsulated in the bioelectrode  71  using an electrically conductive material  71   a  as the encapsulation material (including the case where this is a metallic material). A similar bioelectrode  72  is shown in  FIG. 12C , which can be obtained by encapsulating the electrode control circuit  20  with an electrically non-conductive resin, ceramic or similar material  72   a  as the encapsulation material forming the inner part of the bioelectrode  72 , and subsequently coating this part with an electrically conductive material  72   b.    
     REFERENCE SIGNS LIST 
       10 ,  50 ,  60 ,  70 ,  71 ,  72  . . . Bioelectrode 
       11 ,  15 ,  17  . . . Electrode Body 
       12  . . . Recess 
       16  . . . Electrically Conductive Material Layer 
       18  . . . Resin 
       20  . . . Electrode Control Circuit Chip 
       21   a - 21   e  . . . Electrode Pad 
       22   a - 22   c  . . . Electrode Control Circuit 
       30  . . . Electrode Array Substrate 
       31  . . . Substrate 
       32  . . . Wire in the Substrate 
       33  . . . Sealant 
       51  . . . Electrode Pad 
       75  . . . Connection Line 
       80  . . . Electrode Array Substrate 
       81  . . . Electrode 
       82  . . . Electrode Control Circuit Chip 
       90  . . . Connection-Type Bioelectrode