Abstract:
Implantable defibrillators are implanted into the chests of patients prone to suffering ventricular fibrillation, a potentially fatal heart condition. A critical component in these devices is an aluminum electrolytic capacitors, which stores and delivers one or more life-saving bursts of electric charge to a fibrillating heart. These capacitors make up about one third the total size of the defibrillators. Unfortunately, conventional manufacturers of these capacitors have paid little or no attention to reducing the size of these capacitors through improved capacitor packaging. Accordingly, the inventors contravened several conventional manufacturing principles and practices to devise unique space-saving packaging that allows dramatic size reduction. One embodiment of the invention uses thinner and narrower separators and top and bottom insulative inserts to achieve a 330-volt operating, 390-volt surge, 190-microfarad, 30-Joule aluminum electrolytic capacitor which is 33 percent smaller than conventional capacitors having similar electrical traits.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns electrolytic capacitors, particularly those for use in medical devices, such as implantable defibrillators. 
     Every year more than half a million people in the United States suffer from heart attacks, more precisely cardiac arrests. Many of these cardiac arrests stem from the heart chaotically twitching, or fibrillating, and thus failing to rhythmically expand and contract as necessary to pump blood. Fibrillation can cause complete loss of cardiac function and death within minutes. To restore normal heart contraction and expansion, paramedics and other medical workers use a device, called a defibrillator, to electrically shock a fibrillating heart. 
     Since the early 1980s, thousands of patients prone to fibrillation episodes have had miniature defibrillators implanted in their bodies, typically in the left breast region above the heart. These implantable defibrillators detect onset of fibrillation and automatically shock the heart, restoring normal heart function without human intervention. A typical implantable defibrillator includes a set of electrical leads, which extend from a sealed housing into the heart of a patient after implantation. Within the housing are a battery for supplying power, heart-monitoring circuitry for detecting fibrillation, and a capacitor for storing and delivering a burst of electric charge through the leads to the heart. 
     The capacitor is typically an aluminum electrolytic capacitor, which includes two long strips of aluminum foil with two long strips of paper, known as separators, in between them. One of the aluminum foils serves as a cathode (negative) foil, and the other serves as an anode (positive) foil. Each foil has an aluminum tab, extending from its top edge, to facilitate electrical connection to other parts of the capacitor. 
     The foil-and-paper assembly, known as the active element, is rolled around a removable spindle or mandrel to form a cylinder and placed in a round tubular case, with the two tabs extending toward the top of the case. The paper is soaked, or impregnated, with a liquid electrolyte—a very electrically conductive solution containing positive or negative ions. And, the tubular case is sealed shut with a lid called a header. Extending from the header are two terminals connected respectively to the anode foil and cathode foil via the aluminum tabs. 
     In recent years, manufacturers of aluminum electrolytic capacitors have focused almost single-mindedly on improving the active element by developing aluminum foils, electrolytes, and multiple-anode arrangements that improve capacitor performance, specifically energy density—the amount of energy or charge a capacitor stores per unit volume. For example, because energy density is directly proportional to the surface area of the aluminum foil making up the capacitive element, manufacturers have focused on methods of etching microscopic hills and valleys into foils to increase their effective surface area. 
     In comparison, capacitor manufacturers have made little, if any, effort to improve packaging of the active element. For example, three leading manufactures of electrolytic capacitors—Rubycon, United Chemicon, and Roederstein—presently provide 330-360 volt, dual-anode aluminum electrolytic capacitors which have total volumes greater than about 6.5 cubic-centimeters (which is roughly the same size as a AA battery.) Yet, when the present inventors studied how this space was used, they determined that the ratio of the volume of the active element to the overall volume of these capacitors was only about 40 percent. Thus, the inventors concluded that about 60 percent of the total capacitor volume was wasted in the sense of failing to directly contribute to the performance of these electrolytic capacitors. 
     Accordingly, the inventors identified an unmet need to reduce the size of electrolytic capacitors, especially those intended for implantable defibrillators, through better packaging. 
     SUMMARY OF THE INVENTION 
     To address this and other needs, the inventors devised several improvements intended to reduce the overall size of electrolytic capacitors, particularly those intended for implantable defibrillators. With these improvements, the inventors built an exemplary 360-volt operating, 390-volt surge, 190-microfarad, 15.9-Joule aluminum electrolytic capacitor about 33 percent smaller than conventional capacitors with comparable electrical traits. 
     One improvement contributing to this size reduction is the use of one or more separators having a thickness less than the standard one-thousandth of an inch used in conventional electrolytic capacitors. The exemplary embodiment uses combinations of paper separators with thicknesses of 0.000787, 0.0005, and 0.00025 inches. For conventional cylindrically wound active elements, reducing separator thickness reduces the space necessary to contain the separators. In turn, this allows one to reduce the diameter and volume of the active element and thus the total volume of the capacitor, or alternatively to increase the size of other components of the active element to increase energy density for a given total volume. 
     In devising this improvement, the inventors recognized that the conventional practice of using thick paper separators stems from at least three design objectives that are of lesser relevance to implantable defibrillators. The first is that thicker paper reduces electrolyte depletion, or evaporation, and thus generally increases capacitor life. However, the inventors determined that electrolyte depletion has much less effect on capacitor life in medical device applications than it does in the typical applications that govern how conventional electrolytic capacitors are built. In particular, implanted defibrillators are generally not subject to the same long-term temperature variations and extremes that conventional capacitors are designed to withstand. 
     Secondly, conventional manufacturers used the standard thick paper because it is less likely to tear or break during fabrication, particularly during the conventional high-speed process of winding the foil-and-paper assembly around a spindle. Thus, using the thick paper allows conventional manufacturers to make capacitors faster. However, manufacturing speed is not very important to defibrillator makers who need to make many fewer capacitors than conventional manufacturers and thus can generally afford more time making them. 
     Thirdly, conventional manufacturers use the thick papers to reduce the chance of anode and cathode foils contacting each other and therefore causing capacitor failure during functional testing. Since failed capacitors are generally discarded or recycled, using thick papers ultimately reduces manufacturing waste. However, waste is of less concern when making a small number of capacitors for implantable defibrillators than it is when making millions of capacitors as do most conventional manufacturers. 
     Another improvement contributing to the 33-percent size reduction is the use of separators with end margins less than two millimeters. The end margins are the portions of the separators which extend beyond the width of the cathode and anode foils. Conventional paper separators are about four-to-six millimeters wider than the aluminum foils of the active element, with the excess width typically divided to form equal top and bottom margins of two-to-three millimeters. Thus, when wound into a roll and stood up on one end, the top and bottom margins increase the overall height of the active element and the overall height of the case needed to contain the active element. 
     Conventional manufacturers use the large end margins for at least two reasons: to protect the foils from damage during high-speed manufacturing processes, and to insulate the foils of the active element from an aluminum case after insertion into the case. In particular, during high-speed winding, the foil and paper can easily become misaligned or skewed so that the edges of the foil extend beyond the edges of the papers, making them prone to bending, creasing, or tearing. The large, conventional end margins allow room for misalignment while also protecting the foil edges during high-speed winding. After insertion into a tubular case, the end margins separate the edges of the rolled foil from the top and bottom of the case, preventing the electrically conductive case from shorting the anode and cathode foils. 
     In devising this improvement, the inventors determined that the end margins could be greatly reduced, even eliminated completely in some embodiments, by more carefully winding the foils and separators during manufacture. Additionally, the inventors devised other ways of insulating foils from cases, while reducing capacitor size. 
     Specifically, the exemplary embodiment of the invention, which has little or no end margins, includes insulative inserts, for example, flat paper disks, between the bottom of the active element and the bottom of the case and between the top of the active element and the underside of a lid on the case. Other embodiments enclose substantially all of the active element within an insulative bag. 
     Other improvements include reducing the thickness of the capacitor lid, or header, by about 50 percent, reducing the space between the underside of the lid and the top of the active element, reducing the diameter of the normally empty mandrel region of the active element, and reducing thickness of the aluminum tube. Like the use of thinner separators, smaller end margins, and insulative inserts, these ultimately allow reductions in the size of electrolytic capacitors and implantable defibrillators which incorporate them. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an exemplary aluminum electrolytic capacitor  10  incorporating various space-saving features to achieve a 33-percent size reduction over conventional electrolytic capacitors; 
     FIG. 2 is a cross-sectional view of electrolytic capacitor  10  taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a cross-sectional view of a layered capacitive assembly  21  used to form active element  20  of FIG. 2; 
     FIG. 4 is a perspective view of a unique foil structure  33  included within some alternative embodiments of capacitive assembly  21 ; 
     FIG. 5 is a partial cross-sectional view of an insulative bag  40  enclosing active element  20  in an alternative embodiment of capacitor  10 ; and 
     FIG. 6 is a block diagram of generic implantable defibrillator  50  including a capacitor that has one or more of the novel features of capacitor  10 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description, which references and incorporates FIGS. 1-6, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only 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. 
     FIG. 1 shows a perspective view of an exemplary 360-volt operating, 390-volt surge, 190-microfarad, 15.9-Joule (stored) electrolytic capacitor  10  which incorporates various space-saving features of the present invention. Capacitor  10  has a diameter  10   d  of about 14.5 millimeters and a total height  10   h  of about 30 millimeters, and a total volume of about five cubic-centimeters. Thus, capacitor  10  has an energy density of about 3.2 Joules per cubic-centimeter. 
     In contrast, conventional electrolytic capacitors with comparable electrical characteristics and of about the same diameter have heights greater than or equal to about 40 millimeters and total volumes greater than or equal to about 6.6 cubic-centimeters, with energy densities around 2.4 Joules per cubic-centimeter. Thus, the exemplary capacitor is about 33 percent smaller than conventional capacitors with similar electrical traits. 
     More specifically, capacitor  10  includes a cylindrical aluminum case  12 , a header (or lid)  14 , and two aluminum terminals  16  and  18 . Two rivets  15  and  17  fasten terminals  16  and  18  to header  14 . Aluminum case  12 , which houses an active element  20  (not visible in this view), includes a circumferential seating groove  12   a  and a rolled lip  12   b , both of which secure header  14  to case  12 . 
     FIG. 2, a cross-section taken along line  2 — 2  in FIG. 1, shows that case  12  has a thickness  12   t  and that groove  12   a  is spaced a distance  12   d  from lip  12   b . In the exemplary embodiment, thickness  12   t  is about 0.010 inches, and distance  12   d  is about 0.145 inches. Additionally, groove  12   a  has a radius of about 0.035 inches, and lip  12   b , which is formed by rolling over the top edge of case  12 , has a radius of about 0.015 inches. Groove  12   a  and lip  12   b  each have a smaller radius than the corresponding features of conventional capacitors. In another embodiment, case  12  is vertically compressed to completely flatten or reduce the height of groove  12   a  and thus to further reduce the height and volume of capacitor  10 . 
     FIG. 2 also shows that header  14  comprises two bonded layers  14   a  and  14   b  and has a total thickness  14   t . Layer  14   a  consists of rubber, and layer  14   b  consists of a phenolic resin. Although thickness  14   t  is about two millimeter in the exemplary embodiment, it ranges inclusively between 0.5 to 2 millimeters in other embodiments. In contrast, conventional aluminum electrolytic capacitors use headers that are about three to four millimeters thick. 
     FIG. 2 also shows that capacitor  10  includes an active element  20  comprising about  19  turns of a layered capacitive assembly  21  around mandrel region  28  and two pairs of insulative inserts  30   a - 30   b  and  32   a - 32   b  separating the top and bottom of active element  20  from interior surfaces of case  12 . For clarity, FIG. 2 omits a plastic insulative sheath that surrounds the vertical surfaces of active element  20 . In the exemplary embodiment, this sheath is a piece of transparent tape having a width of 1.125 inches (or 28.6 millimeters). 
     FIG. 3, a cross sectional view of layered capacitive assembly  21 , shows that it includes a cathode  22 , a three-foil anode  24 , and four electrolyte-impregnated separators  26   a ,  26   b ,  26   c , and  26   d . Cathode  22  and anode  24  each have a width (or height)  22   w , which partly determines a minimum height of case  12 . Though not shown in FIG. 3 for clarity, cathode  22  and anode  24  also include insulative or dielectric coatings, for example aluminum or tantalum oxide, on at least their facing surfaces. In this exemplary embodiment, cathode  22  and three constituent foils  24   a ,  24   b , and  24   c  of anode  24  are about 24 millimeters wide and 100 micrometers thick. Cathode  22  is about 422 millimeters long and anode  24  is about 410 millimeters long. 
     Although not shown in FIG. 3, anode foils  24   a ,  24   b , and  24   c  are connected to a single aluminum anode tab  25  (which is shown in FIG.  2 ). Alternatively, individual anode tabs can be connected to each anode members, and to each other to form a joint or composite anode tab. For more details on these or other types of tabs incorporated in other embodiments of the invention, see co-pending U.S. Pat. applications 09/063692 , filed on Apr. 21, 1998, and 09/076023 , filed on May 11, 1998, now issued as U.S. Pat. No. 6,110,233, which are respectively entitled Electrolytic Capacitor and Multi-Anodic Attachment and Wound Multi-Anode Electrolytic Capacitor with Offset Anodes and which are incorporated herein by reference. 
     Anode tab  25 , shown in FIG. 2, is ultrasonically welded to rivet  15  and thus electrically connected to terminal  16 . In this embodiment, anode tab  25  is folded over itself; however, other embodiments omit this fold to reduce the space between header  14  and the top of active element  20 . Though not visible in FIG. 2 or FIG. 3, cathode  22  includes a cathode tab which is similarly connected via rivet  17  to terminal  18 . 
     Cathode  22  and anode foils  24   a ,  24   b , and  24   c  are made of an electrically conductive material, such as aluminum or tantalum foil, with the anode etched to enhance its effective surface area. Examples of suitable etched foil structures include conventional core-etched and tunnel-etched foils, and a novel perforated-core-etched foil as well as various combinations of these foils. For instance, one embodiment forms anode  24  by stacking a core-etched or tunnel-etched foil with two perforated-core-etched foils. FIG. 4 shows an example of a perforated-core-etched foil  33 . 
     Foil  33  includes two opposing surfaces  33   a  and  33   b  that define an average foil thickness  33   t  and a number of perforations, or holes,  33   p  that extend all the way through the foil. Surfaces  33   a  and  33   b  include respective sets of surface cavities  34   a  and  34   b , which have respective average maximum depths D a  and D b  and respective average cross-sectional areas S a  and S b  (measured in a plane generally parallel to the foil). In the exemplary embodiment, the perforations, which are formed using laser, etch, or mechanical means, have an average cross-sectional area that is 2-100 times larger than the average cross-sectional areas of the cavities. Depths D a  and depths D b  are approximately equal to one third or one quarter of thickness  33   t , and cross-sectional areas S a  and S b  are equal and range inclusively between about 0.16 and 0.36 square-microns. The layout or arrangement of perforations can take any number of forms, including, for example, a random distribution or a specific pattern with each perforation having a predetermined position relative to other perforations. Perforations  33   p , which can be any shape, for example, circular, have a cross-sectional area ranging between approximately 500 square-microns and 32 square-millimeters in the exemplary embodiment. Additionally, the exemplary embodiment limits the total surface area of perforations  10   p  to about 20 percent of the total area of foil  33 . 
     The perforated-coil-etched foil can be made either by perforating a conventional core-etched foil or core-etching a perforated foil. Further details of the perforated core-etched foil are disclosed in co-pending U.S. patent application 09/165,779, entitled High-Energy Density Capacitors for Implantable Defibrillators. This application was filed on the same day as the present application and is incorporated herein by reference. 
     In addition to cathode  22  and three-part anode  24 , FIG. 3 shows that capacitive assembly  21  includes thin electrolyte-impregnated separators  26 , specifically  26   a ,  26   b ,  26   c , and  26   d . Separators  26   a ,  26   b ,  26   c , and  26   d , each of which consists of kraft paper impregnated with an electrolyte, such as an ethylene-glycol base combined with polyphosphates or ammonium pentaborate, distinguish in at least two ways from separators used in conventional electrolytic capacitors. 
     First, in contrast to conventional separators which are one-thousandth of an inch or more in thickness to improve fabrication yield and reduce electrolyte depletion, separators  26   a - 26   d  are each less than one-thousandth of an inch in thickness. In the exemplary embodiment, each of the separators has one of the following thicknesses: 0.000787, 0.0005 inches, and 0.00025 inches, with thicker papers preferably placed nearer the center of the active element to withstand the greater tensile stress that interior separators experience during winding. 
     However, various other embodiments of the invention use combinations of these thicknesses, combinations of these thickness with other thicknesses, and combinations of other thicknesses. Additionally, other embodiments of invention combine one or more thin separators with one or more conventional separators. Ultimately, the use of one or more thinner separators reduces the diameter of the active element for a given length of separator (assuming all other factors are equal). 
     Second, in contrast to conventional separators which are about four to six millimeters wider than the anode and cathode foils to provide large two to three millimeter end margins, separators  26  have a width  26   w  which is less than four millimeters wider than cathode  22  and anode  24  to provide smaller end margins  27   a  and  27   b . For example, in the exemplary embodiment, width  26   w  is about 27 millimeters, or three millimeters wider than cathode  22  and anode  24 , to provide end margins  27   a  and  27   b  of about 1.5 millimeters. Other embodiments of the invention provide at least one end margins of about 1.75, 1.25, 1, 0.75, 0.5, 0.25, and even 0.0 millimeters. 
     The large end margins of conventional separators are necessary to prevent damage to foil areas during high-speed fabrication and to insulate the cathode and anode foils from case  12 . However, the inventors recognized that they are not necessary in all applications, particularly defibrillator applications, where high-speed fabrication is of little concern or where the inventors have devised other ways of insulating the foils from the top and bottom of aluminum case  12 . 
     In particular, FIG. 2 shows that the exemplary embodiment provides two pairs of insulative inserts  30   a - 30   b  and  32   a - 32   b , which prevent other conductive portions of capacitor  10 , specifically anode tab  25  and rivets  15  and  17  and the interior surface of case  12 , from shorting cathode  22  and anode  24 . In the exemplary embodiment, these inserts are two pairs of paper disks, with each disk having a thickness of one one-thousandth of an inch and a diameter of about 14 millimeters. However, other embodiments of the invention use not only thinner or thicker inserts, but also different insert materials and numbers of inserts. For example, in one alternative embodiment, one or both pairs of inserts  30   a - 30   b  and  32   a - 32   b  consist of a polymeric insulator, and in another embodiment, inserts  30   a  and  30   b  consist of different material combinations, such as paper and a polymeric insulator. 
     As an alternative to insulative inserts, other embodiments enclose substantially all of active element  20  within an insulative bag. FIG. 5 shows an exemplary embodiment of an insulative bag  40  enclosing substantially all of active element  20 , with the exception of the anode and cathode tabs. In this embodiment, bag  40  comprise materials similar to the insulative inserts. 
     FIG. 2 also shows that capacitive assembly  21  of active element  20  is wound around a mandrel (not shown), which has been removed after winding to leave an empty mandrel region or cavity  28 . In this exemplary embodiment, mandrel region  28  has a width or diameter of about 2.5 millimeters, or more generally less than about 3.5 millimeters. In contrast to conventional electrolytic capacitors which have mandrels or mandrel regions with 3.5-millimeter diameters, the smaller mandrels of the present invention allow use of about 2-5 percent more aluminum foil without increasing the total volume of the capacitor. Another embodiment of the invention uses about the same amount of foil as conventional capacitors with the smaller mandrel region, thereby reducing the diameter of the active element without reducing energy density. 
     Mandrels with diameters less than 3.5 millimeters are not used in manufacturing conventional electrolytic capacitors primarily because they increase the difficulty in rolling the cathodes, anodes, and separators around them. Indeed, a smaller-diameter mandrel increases the tensile stress on the cathode, anode, and separators, leading them to break or tear during high-speed winding and thus to increase manufacturing waste. In addition, the smaller diameter mandrels tend to break and require replacement more often than larger mandrels. Thus, conventional capacitor manufactures avoid smaller mandrels to increase manufacturing yield and to accelerate manufacturing. However, these conventional objectives are of lesser importance when making small numbers of capacitors for implantable medical devices, specifically defibrillators. 
     EXEMPLARY EMBODIMENT OF IMPLANTABLE DEFIBRILLATOR 
     FIG. 6 shows one of the many applications for space-saving electrolytic capacitor  10 : a generic implantable defibrillator  50 . More specifically, defibrillator  50  includes a lead system  52 , which after implantation electrically contacts strategic portions of a patient&#39;s heart, a monitoring circuit  54  for monitoring heart activity through one or more of the leads of lead system  52 , and a therapy circuit  56  which delivers electrical energy through lead system  52  to the patient&#39;s heart. Therapy circuit  56  includes an energy storage component  56   a  which incorporates at least one capacitor having one or more of the novel features of capacitor  10 . Defibrillator  50  operates according to well known and understood principles. 
     In addition to implantable defibrillators, the innovations of capacitor  10  can be incorporated into other cardiac rhythm management systems, such as heart pacers, combination pacer-defibrillators, and drug-delivery devices for diagnosing or treating cardiac arrhythmias. They can be incorporated also into non-medical applications, for example, photographic flash equipment. Indeed, the innovations of capacitor  10  are pertinent to any application where small, high energy, low equivalent-series-resistance (ERS) capacitors are desirable. 
     CONCLUSION 
     In furtherance of the art, the inventors have devised unique space-efficient packaging for aluminum electrolytic capacitors which allows either reduction of the actual size (total volume) of capacitors with specific electrical traits or improvement in the electrical traits of capacitors of a specific total volume. For example, in their exemplary embodiment, the inventors use thinner and narrower separators and top and bottom insulative inserts to achieve a capacitor which is about 33 percent smaller than conventional capacitors having similar electrical traits. 
     The embodiments described above 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 concepts and principles of the invention, is defined only by the following claims and their equivalents.