the following detailed description , which references and incorporates fig1 - 7 , describes and illustrates one or more exemplary embodiments of the invention , specifically a new foil structure and method of manufacture , several new foil assemblies , new capacitors incorporating the foil structure and foil assemblies , and an implantable defibrillator incorporating one or more of the new capacitors . 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 . fig1 shows an enlarged perspective view of a foil structure 8 which the inventors call a “ perforated - core - etched ” foil . foil structure 8 can be made of aluminum , tantalum , hafniium , niobium , titanium , zirconium , and combinations of these metals . however , the invention is not limited to any particular foil composition or class of foil compositions . foil structure 8 includes opposing surfaces 8 a and 8 b which define an average foil thickness 8 t and a set of perforations 8 p which extend through foil structure 8 from surface 8 a to surface 8 b . surfaces 8 a and 8 b include respective sets of surface cavities ( or depressions ) 9 a and 9 b , which have generally cylindrical , conical , or hemispherical shapes . however , the invention is not limited to any particular cavity form , class of cavity forms , or combination of cavity forms . surface cavities 9 a have an average maximum depth da which is less than thickness 8 t , and surface cavities 9 b having an average maximum depth db which is also less than thickness 8 t . as fig1 shows , depths da and db are measured along dimensions generally perpendicular to respective surfaces 8 a and 8 b . cavities 9 a and 9 b also have respective average maximum cross - sectional areas sa and sb ( which are not shown in the figure .) cross - sectional area is measured in a plane substantially parallel to one of surfaces 8 a and 8 b . in the exemplary embodiment , average maximum depths da and depths db are approximately equal to one third or one quarter of thickness 8 t , and cross - sectional areas sa and sb are substantially equal and range inclusively between about 0 . 16 and 0 . 36 square - microns . however , other embodiments use different equal and unequal depths da and db and different and unequal cross - sectional areas sa and sb . more generally , the exemplary embodiment adheres to the constraint that the sum of average maximum depths da and db is less than thickness 8 t . adherence to this constraint ensures that a significant number of cavities 9 a are separated from a significant number of cavities 9 b by a solid region of foil material . these regions of solid material not only provide foil structure 8 with greater structural integrity but also greater surface area than conventional tunnel - etched foils . however , in some embodiments of the invention one or more of cavities 9 a intersect one or more of cavities 9 b , thereby forming openings through the foil . the number of these intersections and resultant openings can be regulated through selection of appropriate cavity formation techniques and cavity depths . in addition to surface cavities 9 a and 9 b , fig1 shows that foil structure 8 includes a set of one or more perforations ( or holes ) 8 p . perforations 8 p have an average maximum cross - sectional area sp measured in a plane substantially parallel to one of surfaces 8 a and 8 b . although perforations 8 p have a generally circular cross - section in the exemplary embodiment , other embodiments use perforations with elliptical , triangular , square , or rectangular cross - sections . thus , the invention is not limited to any particular shape or class of shapes . the layout or arrangement of perforations 8 p takes any number of forms , including for example , a random distribution and a specific pattern with each perforation having a predetermined position relative to other perforations . the number of perforations per unit area is chosen to optimize relevant criteria , such as capacitor electrical performance or foil structural properties . in the exemplary embodiment , average maximum cross - sectional area sp of perforations 8 p is larger than average maximum cross - sectional areas sa and sb of cavities 9 a and 9 b . more precisely , area sp in the exemplary embodiment ranges between about 500 square - microns and 32 square - millimeters . in other embodiments , area sp ranges between 2 - 50 , 10 - 75 , 25 - 100 , or 2 - 100 times larger than surface areas sa and sb . additionally , the exemplary embodiment provides a total perforation area ( number of perforations times average maximum cross - sectional area sp ) which is no more than about 20 percent of the foil surface area . the inventors have devised a number of ways of making foil structure 8 . for example , one method initially core - etches a foil using conventional etching techniques to form cavities 9 a and 9 b and then perforates the core - etched foil . another method entails initially perforating a foil to form perforations 8 p and then etching the perforated foil to form cavities 9 a and 9 b . ( for more details on a conventional etching technology , see , for example , u . s . pat . no . 4 , 395 , 305 to whitman , which is entitled chemical etching of aluminum capacitor foil and incorporated herein by reference .) perforations 8 p can be formed using lasers , chemical etchants , or mechanical dies , for example . conceptually , cavities 9 a and 9 b could also be formed using lasers . thus , the invention is not limited to any particular technique or combination of techniques for forming perforations 8 p and cavities 9 a and 9 b . in one embodiment of the invention , further processing of the foils , particularly those intended for electrolytic capacitors , entails applying an insulative , or dielectric , coating to one or both sides of the foils . examples of suitable coatings include metallic oxides such as aluminum or tantalum oxide . foil structure 8 can be combined with other foils structures to form various electrically and / or mechanically advantageous foil assemblies . many of these assemblies are particularly useful as multiple anodes structures in flat , semi - cylindrical , and cylindrical capacitors . in particular , the inventors devised several foil assemblies that combine foil structure 8 with core - etched and tunnel - etched foils . for example , one foil assembly stacks two or three foils incorporating foil structure 8 to form a dual - or triple - foil assembly which can serve as a dual or triple anode . another foil assembly stacks a core - etched foil between two foils incorporating foil structure 8 . table 1 describes these and several other foil assemblies . in the table , p denotes a perforated foil similar to foil structure 8 ; c denotes a core - etched foil ; and t denotes a tunnel - etched foil . thus , for example , foil assembly 7 comprises a foil similar to foil structure 8 between two tunnel - etched foils . other novel assemblies result from combining two or more of these assemblies . for instance , combining two assembly 1s yields a pppp structure , and combining assemblies 2 and 3 yields a ppppcp structure . additionally , still other novel assemblies result from inserting insulators and electrolyte - impregnated substrates , such as paper , between adjacent foils of an assembly . fig2 shows a perspective view of an exemplary electrolytic capacitor 10 which incorporates one or more foils incorporating foil structure 8 or one or more of the foil assemblies described above . in addition to incorporating these novel foils and foil assemblies , capacitor 10 embodies many novel space - saving features . these features and their advantages are addressed in a co - pending u . s . pat . application ( attorney docket number 279 . 086us1 ) which is entitled smaller electrolytic capacitors for implantable defibrillator , and which was filed on the same day as the present application . this application is incorporated herein by reference . more specifically , fig2 shows that capacitor 10 has a diameter 10 d of about 14 . 5 millimeters and a total height 10 h of about 30 millimeters , thereby occupying a total volume of about five cubic - centimeters . capacitor 10 also 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 . 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 . fig3 a cross - sectional view taken along line 3 — 3 in fig2 shows that case 12 has a thickness 12 t and groove 12 a is spaced a distance 12 d from lip 12 b . 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 . ( some embodiments compress or flatten groove 12 a to reduce capacitor height and volume .) header 14 , which comprises a rubber layer 14 a and a phenolic - resin layer 14 b , has a total thickness 14 t of about two millimeters . fig3 also shows that capacitor 10 includes an active element 20 wound around mandrel region 28 and two pairs of insulative inserts 30 a - 30 b and 32 a - 32 b respectively positioned adjacent the top and bottom of active element 20 . mandrel region 28 has an exemplary width or diameter 28 w of about 2 . 5 millimeters . and , insulative inserts 30 a - 30 b and 32 a - 32 b comprise respective pairs of paper disks , with each disk having a thickness of one one - thousandth of an inch and a diameter of about 14 millimeters the insulative inserts ensure electrical isolation of conductive portions of active element 20 from anode tab 25 and rivets 15 and 17 and from the bottom interior surface of case 12 . ( as an alternative to insulative inserts , other embodiments enclose substantially all of active element 20 within an insulative bag .) for clarity , fig3 omits a 1 . 125inch - wide plastic insulative sheath that surrounds the vertical surfaces of active element 20 . active element 20 comprises about 19 turns of a layered capacitive assembly 21 . as the cross - section in fig4 shows , capacitive assembly 21 includes a cathode 22 , an anode structure 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 . in this exemplary embodiment , cathode 22 and the one or more constituents of anode structure 24 are about 24 millimeters wide and 100 microns thick . cathode 22 is about 422 millimeters long , and anode structure 24 is about 410 millimeters long . anode structure 24 can assume a variety of novel forms , the simplest being a single foil member incorporating foil structure 8 of fig1 . some embodiments provide anode structure 24 with one or more of the novel foil assemblies described using table 1 . although not shown in fig4 the exemplary embodiment connects anode structure 24 to one anode tab regardless of the number of foils constituting the anode structure . ( fig3 shows an exemplary aluminum anode tab 25 .) other embodiments , however , provide individual anode tabs for each anode members , with the tabs connected together 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 . patent applications ser . no . 09 / 063692 and 09 / 076023 ( attorney docket numbers 279 . 094us1 and 279 . 101us1 ) 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 fig3 is ultrasonically welded to rivet 15 and thus electrically connected to terminal 16 . the exemplary embodiment folds anode tab 25 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 fig3 or 4 , cathode 22 includes a cathode tab which is similarly connected via rivet 17 to terminal 18 . in addition to cathode 22 and anode 24 , fig4 shows that capacitive assembly 21 includes thin electrolyte - impregnated separators 26 , specifically 26 a , 26 b , 26 c , and 26 d . in the exemplary embodiment , separators 26 a - 26 d each consists of kraft paper impregnated with an electrolyte , such as an ethylene - glycol base combined with polyphosphates or ammonium pentaborate , and each has a thickness less than 0 . 001 inches . more specifically , the exemplary embodiment uses one or more papers 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 . additionally , each of separators 26 a - 26 d has a width 26 w which is less than four millimeters wider than cathode 22 and anode 24 to provide end margins 27 a and 27 b . 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 . although the exemplary capacitor 10 has a wound or cylindrical configuration , the invention is not limited to any particular type or category of configurations . for example , fig5 a , 5 b , and 5 c show other capacitor configurations encompassed by the invention . more specifically , fig5 a shows a semi - cylindrical ( or “ d ”) capacitor ; fig . sb shows an asymmetric semi - cylindrical capacitor ; and fig5 c shows a flat ( or more precisely a rectangular parallelepiped ) capacitor . fig6 show a cross - sectional view of a capacitive assembly 60 particularly useful for flat capacitors such as the one shown in fig5 c . in particular , capacitive assembly 60 includes an anode structure 62 between two cathode foils 64 a and 64 b . electrolyte - impregnated separators 63 a and 63 b lie respectively between anode structure 62 and cathode foils 64 a and 64 b . in the exemplary embodiment separators 63 a and 63 b each comprise two or more layers of kraft paper of thicknesses similar to separators 26 of fig4 . anode structure 62 comprises one or more of the foil assemblies identified in table 1 . fig7 shows one of the many applications for exemplary capacitor 10 : a generic implantable defibrillator 70 . more specifically , defibrillator 70 includes a lead system 72 , which after implantation electrically contacts strategic portions of a patient &# 39 ; s heart , a monitoring circuit 74 for monitoring heart activity through one or more of the leads of lead system 72 , and a therapy circuit 76 which delivers electrical energy through lead system 72 to the patient &# 39 ; s heart . therapy circuit 76 includes an energy storage component 76 a which incorporates at least one capacitor having one or more of the novel features of capacitor 10 . defibrillator 70 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 . in furtherance of the art , the inventors devised a new foil structure which combines the durability of core - etched foils with the electrolyte flow advantages of tunnel - etched foils . in addition to devising methods for making the new foil structure , the inventors applied the new foil structure to build new capacitors and implantable defibrillators . 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 .
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 . to reduce the size of these devices , capacitor manufacturers have developed special aluminum foils , for example core - etched and tunnel - etched aluminum foils . unfortunately , core - etched foils don &# 39 ; t work well in multiple - anode capacitors , and tunnel - etched foils are quite brittle and tend to break when making some common types of capacitors . accordingly , the inventors devised a new foil structure having one or more perforations and one or more cavities with a depth less than the foil thickness . in an exemplary embodiment , each perforation and cavity has a cross - sectional area , with the perforations having a larger , for example , 2 to 100 times larger , average cross - sectional area than the cavities . other embodiments of the invention include foil assemblies , capacitors , and implantable defibrillators that benefit from properties of the new foil structure .