Abstract:
Implantable heart monitors, such as defibrillators and cardioverters, detect abnormal heart rhythms and automatically apply corrective electrical shocks to the hearts of patients. A critical component in these devices are the capacitors that produce the electrical shocks. One problem with some of these capacitors is that during operation they generate internal gases, which over time accumulate and exert pressure on their cases, often forcing the cases to swell or bulge and potentially compromising capacitor and monitor performance. Accordingly, the inventors devised novel capacitors that include titanium and/or other hydrogen-getting materials and structures, for preventing the development of excessive pressures within capacitor cases.

Description:
TECHNICAL FIELD  
         [0001]    The present invention concerns implantable heart monitors, such as defibrillators and cardioverters, particularly structures and methods for capacitors in such devices.  
         BACKGROUND  
         [0002]    Since the early 1980s, thousands of patients prone to irregular and sometimes life-threatening heart rhythms have had miniature heart monitors, particularly defibrillators and cardioverters, implanted in their bodies, specifically in the upper chest area above their hearts. These devices detect onset of abnormal heart rhythms and automatically apply corrective electrical therapy, specifically one or more bursts of electric current, to hearts. When the bursts of electric current are properly sized and timed, they restore normal heart function without human intervention, sparing patients considerable discomfort and often saving their lives.  
           [0003]    The typical defibrillator or cardioverter includes a set of electrical leads, which extend from a sealed housing into the walls of a heart after implantation. Within the housing are a battery for supplying power, monitoring circuitry for detecting abnormal heart rhythms, and at least one capacitor for delivering bursts of electric current through the leads to the heart.  
           [0004]    The capacitor is often times an aluminum electrolytic capacitor, which takes a flat or cylindrical form. The flat form of this type capacitor generally includes a stack of flat capacitor elements or modules, each comprising two or more aluminum foils and an electrolyte-soaked separator between them. The stack of flat modules, often D-shaped, are housed in a sealed aluminum case of similar shape. The cylindrical form includes one long capacitor module that is rolled up and housed in a round tubular, or cylindrical, aluminum case.  
           [0005]    One problem with both the flat and cylindrical forms of these capacitors is that during normal operation their capacitor modules electro-chemically generate gases, such as hydrogen, that are trapped inside the sealed cases. Over the life of some of these capacitors, the trapped gases accumulate and exert considerable pressure on the cases, often forcing them to swell and permanently distort. This swelling is problematic not only because of the cramped spacing within implantable heart monitors, but also because it causes portions of some foils to separate from adjacent separators and to be starved of electrolyte. This starvation increases equivalent series resistances (ESR) and reduces capacitance, or energy-storage capacity, of the capacitors.  
           [0006]    To address this problem, some capacitor manufacturers have sought to make their sealed cases with thicker walls to resist swelling. However, the inventors have recognized that this solution is of limited value because it often increases the size and weight of capacitors and/or reduces the space available for components, such as aluminum foil, which contribute to the total capacitance, or energy-storage density, of the capacitors. Additionally, some capacitor manufacturers have introduced organic nitro-compounds to the electrolyte of the capacitor to reduce production of hydrogen gas. However, these compounds have not proven to successfully reduce hydrogen gas build-up in all cases.  
           [0007]    Accordingly, the inventors identified an unmet need for better ways of avoiding or reducing capacitor swelling, particularly for capacitors in implantable heart monitors.  
         SUMMARY  
         [0008]    To address this and other needs, the inventors devised novel structures and related capacitors and devices that include hydrogen- or other gas-getting materials and thus prevent the development of excessive pressures within their cases. One exemplary capacitor includes at least aluminum and titanium. Another exemplary capacitor includes the titanium in the form of a titanium and titanium-oxide coating on an aluminum cathode. In this embodiment, the titanium absorbs or adsorbs hydrogen gas, and the titanium oxide, which has a much higher dielectric constant than the aluminum oxide present in conventional aluminum electrolytic capacitors, increases capacitance.  
           [0009]    Other aspects of the invention include an implantable heart monitor, such as pacemaker, defibrillator, cardioverter, or defibrillator-cardioverter, which comprises one or more of the novel capacitors or other related structures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a cross-sectional view of an exemplary structure embodying the present invention.  
         [0011]    [0011]FIG. 2 is a perspective view of an exemplary flat aluminum electrolytic capacitor  100  including a generic pressure-relief mechanism  120 , embodying the present invention.  
         [0012]    [0012]FIG. 3 is a perspective view of an exemplary cylindrical electrolytic capacitor  200  including a generic pressure-relief mechanism  220  embodying the present invention.  
         [0013]    [0013]FIG. 4 is a block diagram of an exemplary implantable heart monitor  400  embodying the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]    The following detailed description, which incorporates FIGS.  1 - 4  and the appended claims, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit, but to exemplify and teach, are shown and described in sufficient detail to enable those skilled in the art to implement or practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art.  
         [0015]    [0015]FIG. 1 shows an exemplary structure  100  incorporating teachings of the present invention. Structure  100  includes an aluminum substrate  110  and coat structures  120  and  130 .  
         [0016]    Aluminum substrate  110  has opposing major surfaces  112  and  114 , which define a nominal thickness  116 . In the exemplary embodiment, aluminum substrate  110  consists essentially of a commercially available high-purity aluminum, and nominal thickness  116  lies in the range of 5-150 micrometers (im.) (Other embodiments use other thicknesses, aluminum concentrations, and possibly even other base metals.) Also in the exemplary embodiment, surfaces  112  and  114  are roughened by chemical etching or other suitable procedure. In some embodiments, the roughened surfaces have an effective surface area 2-5 times that of the “unroughened” surface, and still other embodiments have an effective surface area 200-300 times that of the unroughened surface. Affixed respectively to surfaces  112  and  114  are coat structures  120  and  130 .  
         [0017]    Coat structure  120  includes a non-aluminum hydrogen-absorbent (or gas-getting) layer  122  and a non-aluminum-based dielectric layer  124 . Coat structure  130 , which contacts major surface  114  of substrate  110 , similarly includes a non-aluminum hydrogen-absorbent (or gas-getting) layer  132  and a non-aluminum dielectric layer  134 . As used herein, the term “absorb” and its derivatives includes adsorb.  
         [0018]    In the exemplary embodiment, non-aluminum hydrogen-absorbent layers  122  and  132  consist essentially of titanium and have a substantially uniform thickness in the range of 10-1000 nanometers, for example, 500 nanometers. Dielectric (or insulative) layers  124  and  134  consist essentially of titanium oxide and have a substantially uniform thickness in the range of 0.5-5.0 nanometers. (As used herein the term titanium oxide includes any form of oxidized titanium and thus encompasses, for example, one or more of the following: TiO, TiO 2 , Ti 2 O 3  and Ti 3 O 5 .) Notably, the combination of aluminum and titanium exhibits an increase hydrogen solubility compared to pure titanium, exhibiting for example a hydrogen solubility of 180-310 parts per million (ppm) at room temperature. Titanium oxide has a dielectric constant that ranges from 28 to 60, exceeding the 7-10 range associated with aluminum oxide.  
         [0019]    Other embodiments use titanium-based alloys, titanium-containing compositions, or other gas-absorbent materials, such as palladium, zirconium, niobium, vanadium, and combinations of these materials, that also absorb hydrogen. Some embodiments use palladium-, zirconium-, niobium-, and vanadium-based alloys. Other embodiments also use other dielectrics, such as palladium oxide, zirconium oxide, niobium oxide, or vanadium oxide which may also have a higher dielectric constant than aluminum oxide.  
         [0020]    An exemplary method of forming structure  100  entails providing an aluminum substrate, such as an aluminum foil of desired thickness and surface texture, and completely sputter coating one or both sides of the substrate with titanium to the desired uniform thickness. An exemplary titanium source has a purity of 99.5 percent. Some embodiments may mask off sections of the foil to prevent adherence of the titanium coat and thus define coated and non-coated regions. Still other embodiments may apply titanium to achieve a thickness gradient. Other embodiments may use other physical- or chemical-vapor deposition techniques to deposit the titanium.  
         [0021]    Formation of the titanium oxide in the exemplary embodiment entails exposing the titanium-coated aluminum substrate to ambient air; however, other embodiments use other procedures for forming the titanium oxide. For instance, some may form the oxide under more specific oxygenated, pressurized, and temperature-controlled conditions.  
       Exemplary Flat Capacitor  
       [0022]    [0022]FIG. 2 shows a pictorial cross-section of an exemplary flat aluminum electrolytic capacitor  200 , incorporating exemplary structure  100 . Capacitor  200  includes a flat-form or pan-type case  210 , a capacitor module  220 , and capacitor terminals  230  and  232 .  
         [0023]    Case  210 , which has a D-shape (not visible in this cross-sectional view), includes at least one wall portion  211 . Wall portion  211 , as shown in inset  2 A, includes an aluminum substrate  212  which is affixed to a coat structure  214 . In the exemplary embodiment, the interface between substrate  212  and coat structure  214  is etched; however, in other embodiments, the interface is smooth or unreached. Coat structure  216 , which is similar in form and function to structure  100 , includes a non-aluminum hydrogen- or gas-ion-getting layer  216  and a non-aluminum-based dielectric  218 . In the exemplary embodiment, substrate  212  comprises titanium, and non-aluminum-based dielectric layer  218  comprises titanium oxide. Coat structure  216  is subject to similar material and form variations as structure  100 .  
         [0024]    Capacitor module  220 , generally representative of one or more stacked capacitor modules, includes a cathodic electrode structure  100 ′, a separator structure  222  and an anodic electrode structure  224 . Specifically, cathodic electrode structure (or cathode)  100 ′ has the same structural format and material composition as structure  100 . Separator structure  222 , which is impregnated with an electrolyte, such as an ethylene-glycol base combined with polyphosphates or ammonium pentaborate, separates cathodic electrode structure  100 ′ from anodic electrode structure  224 . Anodic electrode structure (anode)  224  includes one or more conductive layers, although only one layer is depicted in the simplified figure. For example, some embodiments provide an anodic structure having three or more stacked conductive layers. Additionally, anodic electrode structure  224  may itself include a coat structure based on that of structure  100 , as indicated by broken-line layers  225  and  226 .  
         [0025]    In the exemplary embodiment, cathodic electrode structure  100 ′ has a capacitance greater than that of anodic electrode structure  224 . For example, the cathode capacitance is 100-1000 micro-Farads per square centimeter, and the anode capacitance is 0.8-1.4 micro-Farads per square centimeter. And, separator structure  222  comprises one or more layers of kraft paper impregnated with an electrolyte. Other embodiments, however, use other types of separators. Also, some embodiments include additional separator structures to separate capacitor module  220  from conductive elements in other capacitor modules and/or from portions of capacitor case  210 . Still other embodiments include a heterogeneous set of capacitor modules, with one or more of the modules incorporating teachings of structure  100 .  
         [0026]    Coupled to electrode structures  100 ′ and  224  are capacitor terminals  230  and  232  Capacitor terminal  230  is coupled to cathodic electrode structure  100 ′, and capacitor terminal  232  is coupled to anodic electrode structure  224 . In some embodiments, cathodic electrode structure  100 ′ is electrically coupled to case  210  at a connection point  219 . FIG. 2 shows this electrical connection as a broken line  233 .  
         [0027]    In operation, capacitor  200  generally functions in a conventional manner, with the exception that the cathodic electrode structure and/or case-wall structure provide one or more performance advantages. For example, during charging and discharging of the capacitor, interaction of the electrolyte with the cathodic electrode frees hydrogen ions from the electrolyte, and some of these hydrogen ions pair up or unite to form H 2  molecules, or hydrogen gas. In contrast to conventional aluminum electrolytic capacitors that allow this hydrogen gas to accumulate and exert a mounting pressure on the capacitor case and internal capacitor components, the titanium material in the capacitor, particularly the titanium in the cathodic electrode structure, absorbs hydrogen ions and/or hydrogen gas and thus reduces or eliminates the mounting pressure. More precisely, it is presently believed that some portion of the adsorbed hydrogens atoms diffuse into the titanium coat structure as absorbed hydrogen and that some portion combine with the titanium to produce TiH 2  film, according to  
         2H ads +Ti→TiH 2 ,  
         [0028]    where the “ads” subscript denotes adsorbed atoms. (See A. M. Shams El. Din et. al, Aluminum Desalination 107, 265-276 (1996.)) Other embodiments may use other materials to absorb hydrogen or to absorb other gases and ions. Titanium itself may absorb gases other than hydrogen.  
         [0029]    Moreover, the titanium oxide in the cathodic electrode structure has a higher dielectric constant than that of aluminum oxide and thus increases the capacitance of the cathodic electrode structure (assuming all other factors equal.). This increase in cathodic capacitance in turn reduces the voltage on the cathode because according to the relationship  
         
       C 
       anode 
       ×V 
       anode 
       =C 
       cathode 
       ×V 
       cathode  
     
         [0030]    where C anode  and C cathode  denote the respective capacitance of the anodic and cathodic structures and V anode  and V cathode  denote the respective voltages across the anodic and cathodic structures, V cathode  is inversely proportional to C cathode . Since hydrogen ions are liberated from the electrolytes at a specific voltage, the reduced cathodic voltage can ultimately inhibit or prevent hydrogen-ion liberation in the first place, further reducing the accumulation of hydrogen gas and its distortion potential.  
       Exemplary Cylindrical Capacitor  
       [0031]    [0031]FIG. 3 shows an exemplary cylindrical aluminum electrolytic capacitor  300  which incorporates teachings of the present invention and functions in a manner similar to capacitor  200 . Capacitor  300  includes terminals  310  (only one visible in this view), a case  320 , and a rolled capacitor module  330 .  
         [0032]    Specifically, terminals  310  are fastened to a top or header  322  of case  320  via rivets  324  (only one visible in this view). Case  320 , which consists essentially of aluminum in this exemplary embodiment, includes one or more portions that incorporate a coat structure  326  as shown in inset  3 A. (Other embodiments may form the case from other metals and materials alone or in combination with each other or aluminum.) In the exemplary embodiment, coat structure  326  has a similar structural format, material composition, and functionality as that shown and/or described for coat structure  214  in FIG. 2. Rolled capacitor module  430  includes at least one elongated capacitor module, which, as inset  3 B shows, has a cross-sectional structure resembling that shown and/or described for capacitor module  220  in FIG. 2. Rolled capacitor module  330  is rolled around a mandrel region  332 .  
       Exemplary Implantable Cardiac Rhythm Manager  
       [0033]    [0033]FIG. 4 shows an exemplary implantable cardiac rhythm manager  400  that includes one or more capacitors that incorporate teachings of the exemplary embodiments. Specifically, manager  400  includes a lead system  410 , which after implantation electrically contact strategic portions of a patient&#39;s heart, a monitoring circuit  420  for monitoring heart activity through one or more of the leads of lead system  410 , and a therapy (or pulse-generation) circuit  430  which includes one or more capacitors  432  that incorporate one or more of the teachings related to capacitor  200  or  300 . Capacitors  432  are rated for an operating voltage of 390 volts and energy storage of about 14 Joules. Manager  400  operates according to well known and understood principles to generate electrical pulses and perform defibrillation, cardioversion, pacing, and/or other therapeutic or non-therapeutic functions.  
       Other Exemplary Applications  
       [0034]    In addition to aluminum electrolytic capacitors and implantable cardiac rhythm management systems or devices, the teachings of the present invention are applicable to other systems, devices, and components. For example, other types of capacitors that liberate hydrogen or other gases during operation may include the cases, anodes, and/or cathodes based on the present teachings. Also, other systems and devices that use capacitors, such as those related to photographic flash equipment, may incorporate one or more of the present teachings.  
       Conclusion  
       [0035]    In furtherance of the art, the inventors have devised not only unique structures that enhance operation of capacitors by preventing development of excessive internal pressures, but also related devices, systems, and methodologies. One exemplary capacitor includes aluminum structures coated with titanium or titanium oxide or more generally with non-aluminum-based, gas- or gas-ion-getting materials or high-dielectric-constant materials.  
         [0036]    The embodiments described herein are intended only to illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The actual scope of the invention, which embraces all ways of practicing or implementing the teachings of the invention, is presently defined by the following claims and their equivalence.