Abstract:
A capacitor includes a container, a positive electrode, a negative electrode, and a fluid electrolyte. The positive electrode comprises a metal substrate and an active material provided in contact with the metal substrate, the active material comprising at least one of poly (ethylene 3,4-dioxythiophene) and a titanate.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application is a Continuation of U.S. patent application Ser. No. 10/449,645 filed May 30, 2003, the entire disclosure of which is incorporated by reference herein.  
         [0002]     This application is also related to U.S. patent application Ser. No. 10/448,556 filed May 30, 2003; U.S. patent application Ser. No. 11/003,183 filed Dec. 3, 2004 (now U.S. Pat. No. 7,079,377); U.S. patent application Ser. No. 11/440,922 filed May 25, 2006; and U.S. patent application Ser. No. 10/449,879 filed May 30, 2003 (now U.S. Pat. No. 6,842,328). 
     
    
     BACKGROUND  
       [0003]     The present invention relates generally to the field of capacitors. More specifically, the present invention relates to electrolytic capacitors for use in medical devices (e.g., implantable medical devices) or other types of devices.  
         [0004]     Since their earliest inception, there has been significant advancement in the field of body-implantable electronic medical devices. Today, such implantable devices include therapeutic and diagnostic devices, such as pacemakers, cardioverters, defibrillators, neural stimulators, drug administering devices, and the like for alleviating the adverse effects of various health ailments.  
         [0005]     Implantable medical devices may utilize a capacitor to perform various functions. For example, if the implantable medical device is a defibrillator, one or more capacitors may be used to provide a therapeutic high voltage treatment to the patient.  
         [0006]     One type of capacitor that may be used in such an application is an electrolytic or wet slug capacitor. Conventional wet slug capacitors may include a container formed from tantalum or a tantalum alloy that acts as the cathode for the electrolytic capacitor. An electrolyte (e.g., acid such as sulfuric acid) and an anode are provided within the container. In these types of capacitors, a native oxide may be formed on exposed surfaces.  
         [0007]     Since the electrolyte is electrically conductive, a conductor-insulator-conductor structure including metal, oxide coating, and electrolyte is present at both the anode and the cathode. Each of these conductor-insulator-conductor structures is itself a capacitor (e.g., an anode capacitor and a cathode capacitor).  
         [0008]     In the conventional wet slug capacitor, the anode capacitance is effectively electrically connected in series with the cathode capacitance. The amount of charge at the cathode and anode surfaces are substantially equal and of opposite sign. It should also be noted that the net capacitance of two capacitors connected in series is smaller than the smaller of the capacitances of the two capacitors. Because the oxide layer at the anode of a wet slug capacitor is usually much thicker than the thickness of the oxide layer at the cathode, the anode capacitance of a wet slug capacitor is generally smaller than the cathode capacitance.  
         [0009]     The capacitance of a wet slug capacitor can be described using the following equation:  
         C   Capacitor     =         C   Cathode     ·     C   Anode           C   Cathode     +     C   Anode             
 
 In general, it is desirable to increase the capacitance of the cathode to decrease the risk of forming hydrogen gas at the cathode and to make the capacitance of the anode more clearly observable. Although conventional wet slug capacitors having useful capacitances have been produced, there is a desire to increase the capacitance per unit area and capacitance per unit volume of the cathode coating material. Conventional cathode coating materials (e.g., tantalum), however, may provide a limited capacitance per unit area and limited capacitance per unit volume. For certain applications, it is desirable to provide a capacitor coating material that has a capacitance no less than approximately 10-20 milliFarads per square centimeter. 
 
         [0010]     Accordingly, it is desirable to provide a capacitor that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present description. The teachings disclosed extend to those embodiments that fall within the scope of the appended claims, regardless of whether they provide one or more of the aforementioned advantageous.  
       SUMMARY  
       [0011]     An exemplary embodiment relates to a capacitor that includes a container, a positive electrode, a negative electrode, and a fluid electrolyte. The positive electrode comprises a metal substrate and an active material provided in contact with the metal substrate, the active material comprising at least one of poly (ethylene 3,4-dioxythiophene) and a titanate.  
         [0012]     Another exemplary embodiment relates to a capacitor that includes a housing, a positive electrode comprising a substrate and a layer of poly (ethylene 3,4-dioxythiophene) provided on the substrate. The substrate includes a metal selected from the group consisting of titanium, tantalum, stainless steel, aluminum, niobium, zirconium, and alloys thereof. The capacitor also includes a negative electrode and a liquid electrolyte.  
         [0013]     Another exemplary embodiment relates to a capacitor that includes a housing and a positive electrode comprising a substrate and a titanate material provided on the substrate. The substrate includes a metal selected from the group consisting of titanium, tantalum, stainless steel, aluminum, niobium, zirconium, and alloys thereof. The capacitor also includes a negative electrode and a liquid electrolyte.  
         [0014]     Another exemplary embodiment relates to a medical device that includes an electrolytic capacitor including a positive electrode, a negative electrode, and a fluid electrolyte. The positive electrode includes a metal substrate and an active provided in contact with the metal substrate, the active material including poly (ethylene 3,4-dioxythiophene. The medical device is configured for implantation into a human to provide a therapeutic high voltage treatment.  
         [0015]     Another exemplary embodiment relates to a medical device that includes an electrolytic capacitor including a positive electrode, a negative electrode, and a fluid electrolyte. The cathode includes a titanate selected from the group consisting of beryllium titanate, magnesium titanate, calcium titanate, strontium titanate, barium titanate, radium titanate, lead titanate, cadmium titanate, niobium titanate, strontium titanate, organic titanates, and combinations thereof. The medical device is configured for implantation into a human to provide a therapeutic high voltage treatment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The invention will be explained in more detail in the following text using the attached drawings, in which:  
         [0017]      FIG. 1  is a schematic drawing showing an implantable medical device shown in the form of a defibrillator implanted within a human body;  
         [0018]      FIG. 2  is a schematic drawing of a capacitor bank that is provided within the implantable medical device shown in  FIG. 1 ;  
         [0019]      FIG. 3  is a schematic drawing showing the capacitor bank coupled to a battery;  
         [0020]      FIG. 4  is a schematic cross-sectional view of one of the capacitors provided within the capacitor bank shown in  FIG. 2  according to an exemplary embodiment;  
         [0021]      FIG. 5  is a schematic cross-sectional view of one of the capacitors provided within the capacitor bank shown in  FIG. 2  according to another exemplary embodiment;  
     
    
     DETAILED DESCRIPTION  
       [0022]     With reference to the accompanying Figures, the present disclosure relates to capacitors (e.g., electrolytic capacitors, etc.) for use in medical devices (e.g., implantable medical devices, etc.), methods of producing such capacitors, and medical devices which utilize such capacitors. While the subject matter herein is presented in the context of the use of such capacitors in the field of medical devices, such capacitors may be utilized in alternative applications, as will be appreciated by those of skill in the art.  
         [0023]     Referring to  FIG. 1 , a system  11  including an implantable medical device (IMD) is shown as being implanted within a body or torso of a patient  31 . The system  11  includes a device  12  in the form of an implantable medical device that for purposes of illustration is shown as a defibrillator. The defibrillator is configured to provide a therapeutic high voltage (e.g., between approximately 500 Volts and approximately 850 Volts, or, desirably, between approximately 600 Volts and approximately 800 Volts) treatment for the patient  31 . While the implantable medical device is shown and described as a defibrillator, it should be appreciated that other types of implantable medical devices may be utilized according to alternative embodiments, including but not limited to a pacemaker, cardioverter, neural stimulator, drug administering device, or other implantable medical device. According to still other alternative embodiments, non-implantable medical devices or other types of devices that are not medical devices may utilize capacitors as are shown and described in this disclosure.  
         [0024]     The device  12  includes a container or housing that is hermetically sealed and biologically inert according to an exemplary embodiment. The container may be made of a conductive material. One or more leads  16  are electrically coupled between the device  12  and the patient&#39;s heart  20  via a vein  22 . Cardiac electrodes  18  are provided to sense cardiac activity and/or provide a voltage to the heart  20 . At least a portion of the leads  16  (e.g., an end portion of the leads) may be provided adjacent or in contact with one or more of a ventricle and an atrium of the heart  20 .  
         [0025]     A capacitor bank  40  including a plurality of capacitors is provided within the device  12 . A schematic view of the capacitor bank  40  is shown in  FIG. 2 , and shows a group of five capacitors  42  connected in series and provided within the capacitor bank  40 . The size and capacity of the capacitors  42  may be chosen based on a number of factors, including the amount of charge required for a given patient&#39;s physical or medical characteristics. According to other exemplary embodiments, the capacitor bank  40  may include a different number of capacitors  42  (e.g., less than or greater than five capacitors). According to still other exemplary embodiments, a different number of capacitor banks  40  may be provided within the implantable medical device having any suitable number of capacitors  42  provided therein.  
         [0026]     As shown in  FIG. 3 , the capacitor bank  40  is coupled to a battery  50 . According to an exemplary embodiment, the battery  50  is included within the device  12 . According to alternative embodiments, the battery may be provided external to the device  12 . The capacitors  42  provided within the capacitor bank are configured to store energy provided by the battery  50 . For example, the system  11  may be configured such that when the device  12  determines that a therapeutic high-voltage treatment is required to establish a normal sinus rhythm for the heart  20 , the capacitors  42  in the capacitor bank  40  are charged to a predetermined charge level by the battery  50 . Charge stored in the capacitors  42  may then be discharged via the leads  16  to the heart  20 . According to another exemplary embodiment, the capacitors  42  may be charged prior to determination that a stimulating charge is required by the heart such that the capacitors  42  may be discharged as needed.  
         [0027]     In an exemplary embodiment, device  12  is configured to deliver an electric pulse energy to the heart  20  on the order of 30 J for a single defibrillation pulse. However, the energy stored in the capacitors  42  is generally somewhat larger due to losses along the delivery path during the release of the energy. It should be understood that the therapeutic high voltage treatment delivered to the patient&#39;s heart  20  may vary somewhat in intensity depending on the patients&#39; physiology and the details of the particular configuration of device  12 .  
         [0028]     Also, capacitors  42  may be configured to store energy from battery  50  and discharge that energy in a timely manner. For example, capacitors  42  may be configured so that capacitor charge times may be of the order of 10 seconds when using electrical currents of the order of 10 mA. Also, capacitors  42  may be configured so that the typical discharge times are of the order of 10 milliseconds. Thus, in this exemplary embodiment, the capacitors  42  are configured to deliver about 30 J of electrical energy in a total time window of about 10 seconds, using a charge current on the order of 10 mA.  
         [0029]     In order to provide these relatively low charge and discharge times, capacitors  42  generally have low internal resistance, or more generally speaking, impedance. The impedance behavior of capacitors  42  is typically characterized by its equivalent series resistance (ESR) value measured at a specified frequency. For capacitor bank  40 , the ESR measured at 120 Hz is typically of the order of 5 Ohms or less. Thus, capacitor bank  40  is able to provide timely delivery of the therapeutic high voltage treatment with minimal waste of energy lost in heating the device. It should be understood that other embodiments may have different charging and/or discharging characteristics depending on the needs of the device in which it is used.  
         [0030]     Various types of capacitors may be provided within the capacitor bank  40  according to various exemplary embodiments.  FIG. 4  shows a schematic cross-sectional view of a portion of a capacitor  60  according to a first exemplary embodiment. The capacitor  60  includes a container or housing  62  (e.g., a hermetically sealed container). According to an exemplary embodiment, the container comprises titanium. According to other exemplary embodiments, other materials may be used in place of or in addition to titanium (e.g., stainless steel, silver, valve metals such as aluminum, tantalum, niobium, zirconium, alloys of any of the previous materials, etc.). For example, an alloy of titanium/6% aluminum/4% vanadium may be used as the material for container  62 . In general, the material or materials used to form the container  62  are chosen based on the particular electrolyte used in the capacitor. Thus, the container  62  comprises a conductive material that resists corrosion from the electrolyte.  
         [0031]     Capacitor  60  generally includes a plurality of electrodes (e.g., cathode and anode). As shown in  FIG. 4 , capacitor  60  includes a cathode  68  that is provided within the container  62 . According to an exemplary embodiment, the cathode  68  is electrically isolated from an inner surface  64  of the container  62  and comprises an active or coating material  67  and a substrate  69 . According to an exemplary embodiment, substrate  69  comprises titanium. In other exemplary embodiments, substrate  69  may include stainless steel, silver, tantalum, niobium, zirconium, aluminum, alloys of these materials (e.g., Ti/6% Al/4% Va, etc.), etc. A cathode lead  70  is electrically coupled to the cathode  68  and extends through a wall  66  of the container  62 . The cathode lead  70  is electrically isolated from the container  62  by a feed-through  72 . According to an exemplary embodiment, the feed-through  72  comprises an insulating material (e.g., glass) that seals the cathode lead  70  from the container  62 . The feed-through  72  may also act to prevent material (e.g., electrolyte) from escaping the container  62  and to prevent foreign matter from entering the container  62  in the location of the cathode lead  70 .  
         [0032]     In an exemplary embodiment, the cathode  68  has a specific capacitance that is not less than about 10 milliFarads per square centimeter. In another exemplary embodiment, the cathode  68  has a specific capacitance that is not less than about 20 milliFarads per square centimeter.  
         [0033]     An anode  78  is provided within the container  62 . According to an exemplary embodiment, the anode  78  comprises tantalum (e.g., a porous sintered tantalum slug). According to other exemplary embodiments, the anode  78  may comprise other materials in addition to or in place of tantalum (e.g. valve metals such as, aluminum, titanium, niobium, zirconium, etc.). The anode  78  is provided in the container  62  such that it is not in direct contact with (e.g., is spaced apart from) the cathode  68 . Typically, a separator is used to prevent anode  78  and cathode  68  from touching. The separator can be any of a number of suitable materials (e.g., cellulose, etc.) that separate the anode  78  and cathode  68  as well as allow a sufficient amount of electrolyte to pass through for the capacitor to function properly.  
         [0034]     The anode  78  is electrically coupled to an anode lead  74  that passes through a wall  66  of the container  62  via a feed-through  76 . The feed-through  76  may be constructed in a similar manner as described with respect to feed-through  72  and may act to electrically isolate the anode lead  74  from the container  62  in substantially the same manner as described with respect to cathode lead  70  and feed-through  72 .  
         [0035]     A fluid or liquid electrolyte  79  is provided in the container  62 . At least a portion of the electrolyte  79  is provided intermediate the cathode  68  and the anode  78 . The electrolyte  79  electrically associates cathode  68  and the anode  78 . According to an exemplary embodiment, the electrolyte may comprise ammonium salts (e.g., ammonium acetate) dissolved in a water and an organic solvent (e.g., glycol, etc.), phosphoric acid, etc. The particular electrolyte chosen may depend on a number of factors, such as the desired reactivity of the electrolyte with the cathode and anode, compatibility with the material or materials that make up the container  62 , desired breakdown voltage, etc.  
         [0036]      FIG. 5  shows a cross-sectional schematic view of a portion of a capacitor  80  according to a second exemplary embodiment. The capacitor  80  includes a container or housing  82  which may be constructed in a manner similar to that described with respect to the container  62 .  
         [0037]     A cathode  84  is integrally formed with the container  82  and comprises wall  88  and active or coating material  86 . In this embodiment, wall  88  functions as the substrate for active material  86 . The cathode  84  is electrically coupled to a cathode lead  90  that extends from the wall  88  of the container  82 .  
         [0038]     An anode  96  is provided within the container  82  such that the anode  96  is not in contact with (e.g., is spaced apart from) the cathode  84 . According to an exemplary embodiment, the anode  78  comprises tantalum. According to other exemplary embodiments, the anode  78  may comprise other materials in addition to or in place of tantalum (e.g., aluminum, titanium, niobium, zirconium, etc.).  
         [0039]     The anode  96  is electrically coupled to an anode lead  92  through a feed-through  94 . The feed-through  94  may be constructed in a similar manner to that described with respect to the feed-through  72  and the feed-through  76 .  
         [0040]     The anode  96  and the cathode  84  may be configured in a variety of ways. According to an exemplary embodiment, the anode  96  and the cathode  84  are configured to be similar to the anode  78  and the cathode  68 .  
         [0041]     A fluid or liquid electrolyte  98  is provided in the container  82 . At least a portion of the electrolyte  98  is provided intermediate the cathode  84  and the anode  96  and electrically associates the cathode  84  and the anode  96 . The electrolyte  98  utilized in the capacitor  80  may be the same as or may differ from that utilized in the capacitor  60 . In general, the same factors considered in choosing the electrolyte  79  also apply in choosing the electrolyte  98 .  
         [0042]     Referring to  FIGS. 4 and 5 , the active materials  67  and  86  may comprise a number of materials. In an exemplary embodiment, materials are chosen that have a relatively high capacitance. Generally, materials that have a high capacitance include materials that have a high surface area, have the ability to absorb protons, and/or have a high dielectric constant.  
         [0043]     In one exemplary embodiment, the active materials  67  and  86  comprise a conducting polymer such as poly (ethylene 3,4-dioxythiophene) (hereinafter “PEDT”). PEDT is generally a high surface area conductive polymer that is not lamellar. PEDT is a suitable material to use for the active materials  67  and  86  because of its high surface area and ability to absorb protons. PEDT also provides a number of other advantages over other conducting polymers. For example, PEDT is relatively thermally stable up to a temperature of approximately 125° C. Also, PEDT is generally more conductive than other thiophene polymers. While not wishing to be bound by theory, it is thought that PEDT&#39;s higher conductivity relative to other thiophene polymers is due to the directing effect of the bonds at the 3,4 positions (i.e., the oxygen bonds at the 3,4 positions prevent PEDT from conducting at those positions so that the 2,5 positions, the only positions readily available for bonding during polymerization, are the positions associated with maximum conductivity).  
         [0044]     PEDT may be applied to the substrate  69  or the wall  88  of the container  82  using chemical and/or electrochemical oxidation of the monomer. In an exemplary embodiment, a solution of the monomer and a solvent alcohol (e.g., methanol, ethanol, etc.) is applied to the substrate  69  or the wall  88 . The solvent alcohol is evaporated leaving the monomer. A solution of water, an oxidizer (e.g., ammonium persulfates, etc.), and a doping agent (e.g., para-toluene sulfonic acid, etc.) is then applied to the monomer. The monomer reacts with the oxidizer and the doping agent to form poly (ethylene 3,4-dioxythiophene). In an alternative embodiment, the solution of water, oxidizer, and doping agent may be applied to substrate  69  or wall  88  first and then, after evaporating the water, the monomer is applied. Also, the doping agent may be provided in solution with the monomer rather than with the oxidizer. Many methods known by those of ordinary skill in the art can be used to apply the PEDT to the substrate  69  or the wall  88 . Thus, PEDT may be applied to the substrate  69  or the wall  88  in any manner that is suitable to provide the desired structure.  
         [0045]     In another exemplary embodiment, the active materials  67  and  86  comprise titanates such as titanates that are used in ceramic capacitors. For example, acceptable titanates include, but should not be limited to, alkaline earth titanates (i.e., beryllium titanate, magnesium titanates, calcium titanate, strontium titanate, barium titanate, radium titanate), organic titanates, lead titanate, cadmium titanate, niobium titanate, strontium titanate, etc. Titanates are suitable materials to use for active materials  67  and  86  because typically they have a high dielectric constant and an ability to absorb protons. In an exemplary embodiment, the titanates used have a dielectric constant that is not less than about 50, or, desirably, not less than about 100. For example, barium titantate exhibits a dielectric constant of approximately 1600. Also, titanates are relatively thermally stable up to a temperature of approximately 125° C.  
         [0046]     The titanates may be applied to the substrate  69  or the wall  88  of the container  82  in a variety of ways. In an exemplary embodiment, a suspension of water and the titanate or titanates are contacted with the substrate  69  or the wall  88  of the container  82 . The suspension is then heated to a temperature sufficient to thermally bond the titanate to the substrate  69  or the wall  88 . In another embodiment, the titanate or a precursor, such as barium acetate, is contacted with the substrate  69  or the wall  88  of the container  82 . The substrate  69  or the wall  88  and the titanates or titanate precursors are then heated to a sufficient temperature to bond the titanate to the substrate  69  or the wall  88 . In an alternative embodiment, the titanates are sputtered, brushed, etc. onto the substrate  69  or the wall  88 . The substrate  69  or wall  88  is then heated to bond the titanate(s) to the substrate  69  or wall  88 . If the substrate  69  or wall  88  is titanium then the bonding temperature is desirably between approximately 800° C. and approximately 1000° C., and the bonding temperature may be at or near the beta transition temperature of titanium (i.e., the temperature at which the structure of the titanium changes from hexagonal to cubic, which is approximately 865° C.).  
         [0047]     In an exemplary embodiment, the thickness of the active materials  67  and  86  is between approximately 0.025 inches and approximately 0.0001 inches or, desirably, between approximately 0.003 inches and approximately 0.0005 inches. In another exemplary embodiment, the thickness of the active materials  67  and  86  is no more than approximately 0.003 inches.  
         [0048]     The anodes  78  and  96  may also include PEDT or titanates in a similar manner and as disclosed with regard to the cathodes  68  and  84 . Thus, it should be understood that the use of PEDT and titanates is not limited to the cathodes  68  and  84 . Rather, these materials may be used in a variety of desirable configurations in an appropriate capacitor.  
         [0049]     As utilized herein, the terms “approximately,” “about,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.  
         [0050]     The construction and arrangement of the elements of the capacitor as shown in the preferred and other exemplary embodiments is illustrative only. Although only a few embodiments of the present inventions have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages of the subject matter recited in the claims. Accordingly, all such modifications are intended to be included within the scope of the present invention as defined in the appended claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the preferred and other exemplary embodiments without departing from the scope of the present invention as expressed in the appended claims.