in the following description , the terms “ separator film ,” “ separator sheet ,” and “ sheet of separator material ” are used interchangeably , and are meant to indicate a relatively thin material that provides physical separation between a first electrode and an associated opposite polarity electrode , thereby preventing a short circuit between the electrodes . exemplary power sources comprising opposite polarity electrodes include a capacitor or a battery . referring now to the drawings , fig1 to 3 illustrate the present thermo - encapsulation apparatus 10 comprised of an electrode holding fixture 100 and a heater assembly 200 . the electrode holding fixture 100 may be joined to a mounting base 20 comprised of one or more rigid plates 22 , 24 and 26 . electrode holding fixture 100 is preferably separated from mounting base 20 by standoffs 28 , 30 , 32 and 34 in order to thermally isolate the fixture 100 and to provide clearance for an electrode ejection tool 190 , which will be subsequently described herein . electrode holding fixture 100 and mounting base 20 may also be mounted on a slide assembly 40 comprised of a plate 42 , outer guide rails 44 and 46 , and a center rail 48 . electrode holding fixture 100 is horizontally movable along plate 42 , as indicated by bidirectional arrow 99 . this is done by manipluating handle 50 . in that manner , the electrode assembly 100 is withdrawn from beneath the heater assembly 200 to enable easy loading of an electrode and separator film therein . a proximity sensor 21 is provided to detect the correct positioning of the electrode holding fixture 100 beneath the heater assembly 200 prior to heat sealing . the heater assembly 200 is preferably joined to a suspension plate 60 with standoffs 62 , 64 , 66 and 68 disposed there between . the standoffs thermally isolate heater assembly 200 from suspension plate 60 . the thermo - encapsulating apparatus 10 also comprises a main block 80 upon which the slide assembly 40 is mounted . the main block 80 further includes posts 82 and 84 . a linear actuator ( not shown ) such as a pneumatic or hydraulic cylinder , or a linear stepper motor may be mounted on posts 82 and 84 and operatively connected to the heater assembly 200 . the linear actuator is used to lower the heater assembly 200 and bring it into contact with the electrode holding fixture 100 during the heat sealing process . the electrode holding fixture 100 will now be described with particular reference to fig2 , 4 and 5 . electrode holding fixture 100 is comprised of a platen 102 , an electrode holding die 110 and an electrode support 140 . platen 102 has an upper surface 104 and a lower surface 106 . electrode holding die 110 is disposed on the upper surface 104 of the platen 102 and includes a deformable body 112 , preferably of an elastomeric material , having an upper surface 114 with a pocket 116 formed therein . the pocket 116 has an upper portion 118 , a lower portion 119 and a wall surface 120 shaped to correspond to the perimeter 4 of an electrode 2 . electrode holding fixture 100 is further comprised of the electrode support 140 disposed in the lower portion 119 of the pocket 116 of the elastic body 112 . the electrode support 140 includes an upper surface 142 and a lower surface 144 and is preferably made of a dielectric material that is both electrically and thermally insulative . the electrode support 140 is preferably shaped to correspond to the perimeter 4 of the electrode 2 which it supports . that is to provide uniform support and thermal contact with electrode 2 during heat sealing . the dielectric material may be a machinable glass ceramic such as mica or macor ®, which is manufactured and sold by corning inc . of corning n . y . other structurally strong , heat - resistant ceramics that can be cast and fired to near net shape may also be used . the electrode holding die 110 may be comprised of a base plate 122 that is joined to the platen 102 with suitable fasteners ( not shown ). the elastic body 112 is molded to the base plate 122 . a recess or step 304 provided in the upper surface 104 of the platen 102 has a depth sufficient to receive the electrode holding die 110 therein . that sub - assembly has the spaced apart outer upper surfaces 306 of the platen 102 being substantially coplanar with the upper surface 124 of the holding die base plate 122 . the elastic body 112 is made of a heat - resistant elastomer having a relatively low thermal conductivity that is capable of recovering relatively quickly to its former shape from a deformation force . one preferred elastomer is a polysiloxane elastomer , commonly known as silicone rubber , having a shore a durometer of between about 40 to about 90 . urethane is another suitable elastomer for this purpose . turning now to fig3 and 6 , the heater assembly 200 is comprised of an upper platform 202 to which a base plate 204 of a dielectric material is secured by fasteners ( not shown ). inner and outer hold - down plates 206 and 208 are , in turn , secured to the base plate 204 to hold a heating element 230 in place . the dielectric base plate 204 and the inner and outer hold - down plates 206 , 208 are preferably formed from a machinable ceramic material , such as macor ® or mica . details of the heating element 230 are shown in fig6 , 7 , 7 a and 7 b . it is noted that in these drawings , the heating element 230 has been inverted from its installed position on the base plate 204 of the heater assembly 200 . this was done to more clearly show key features thereby . as shown , the t - shaped heating element 230 is comprised of a cutting and sealing portion 232 that extends from terminal 234 to terminal 236 . the cutting and sealing portion 232 includes a proximal head 238 , a cutting edge 240 and a sealing shoulder 242 . the cutting edge 240 and sealing shoulder 242 provide for cutting and sealing the separator film around the electrode , as will be explained subsequently . the head portion 238 includes relatively small flanges 238 a , 238 b that provide the heating element its t - shaped structure . the head portion 238 supported on the base plate 204 is held in position thereon by the edges 206 a , 208 a of the outer and inner hold - down plates 206 , 208 capturing the respective flanges 238 a , 238 b . heating element 230 is formed approximately in an omega ( ω ) shape . its cutting and sealing portion 232 is shaped to match a corresponding portion of the wall surface 120 of the pocket 116 of the elastic body ( fig5 ) which , in turn , corresponds to the shape of the electrode perimeter 4 being sealed in a separator film . in this manner , heating element 230 cuts and seals the separator film around the perimeter of the electrode except for the portion that is wrapped around the straight edge of the electrode , i . e ., the “ open ” portion of the omega shape between the terminals 233 and 235 . heating element 230 may also include an additional central terminal 244 for a more secure attachment to the base plate 204 . fasteners , which are partially shown in fig3 , engage with through holes in the terminals 234 , 236 and 244 to secure the heating element 230 to the dielectric base 204 . the relatively large terminals 234 , 236 and 244 also provide for easy connection to power supply wires ( not shown ). additionally , the terminals 234 , 236 and 244 prevent hot spots in the heating element at the terminals , thereby providing better control of the heat sealing process . the base plate 204 and the inner and outer hold - down plates 206 , 208 are joined to the upper platform 202 by fasteners 222 ( only shown for plates 204 and 208 in fig3 ) that are preferably countersunk therein . then , when there is a need in manufacturing to process electrodes with a different shape , only heating element 230 and the hold - down plates 206 , 208 need to be changed . the platform 202 and the base plate 204 are provided with a sufficient number of threaded openings to accommodate such changeovers . in a like manner and with regard to the electrode holding fixture , only the electrode holding die 110 and the electrode support 140 need to be changed . that way , the manufacturing changeover to process a different electrode batch is made faster , simpler , and lower in cost compared to prior heat sealing systems . the upper platform 202 of the heater assembly 200 is further provided with notches 214 and 216 ( fig6 ) and a large through hole 218 for receiving polymeric plugs 223 , 225 and 227 , respectively . the polymeric plugs 223 , 225 and 227 provide stress relief to reduce the likelihood of cracking the dielectric base plate 204 at the fasteners . certain additional preferred features of the thermo - encapsulating apparatus 10 will now be described along with the method for using the apparatus , and the advantages thereof . fig8 is a perspective view of the apparatus shown with an electrode and separator sheet loaded in a pocket of the electrode holding die . to begin the process , the electrode holding fixture 100 is withdrawn from beneath the heater assembly 200 using slide assembly 40 ( fig1 ). a piece of separator sheet 3 is placed over the pocket 116 in the elastic body 112 of the electrode holding die 110 . an electrode 2 having a perimeter 4 corresponding to the shape of the pocket 116 is placed upon the sheet 3 in alignment with the pocket . the electrode 2 is then forced downwardly into the pocket 116 . this draws the separator sheet 3 down into pocket . the separator sheet 3 is now in contact with the electrode support 140 and is contiguous with the wall surface 120 of the pocket 116 in the elastic body 112 . referring also to fig9 a , a distal portion 5 of the separator sheet 3 is folded over the top surface 6 of the electrode 2 , thereby fully enveloping electrode 2 in the separator sheet . apparatus 10 preferably also includes a tool 160 ( fig8 ) for immobilizing the ends of the respective distal and proximal portions 5 , 7 of the sheet 3 prior to pressing the heater assembly 200 and the electrode holding fixture 100 together . holding tool 160 is comprised of an elongated rod 162 that is operatively connected to a handle 163 . handle 163 is joined to the fulcrum 164 by a pin 165 . the fulcrum 164 is joined to the base 161 , which in turn is joined to the platen 102 . tool 160 further comprises a spring loaded plunger 166 embedded in the handle 163 . to immobilize and hold the ends of the distal and proximal separator sheet portions 5 , 7 , handle 163 is pressed downwardly , thereby raising the elongated rod 162 above the upper surface 124 of the base plate 122 . the ends of the distal and proximal portions 5 , 7 of the sheet 3 are placed beneath rod 162 , and the handle 163 is released . the plunger 166 then forces handle 163 upwardly , as indicated by arrow 98 . by the action of fulcrum 164 , the rod 162 is forced downwardly , as indicated by arrow 97 , thereby pinching and immobilizing the ends of sheet 3 against the base plate 122 . it is noted that electrodes for electrochemical capacitors and batteries used to power implantable medical devices typically have a perimeter with at least one relatively straight portion . referring also to fig4 and 5 , pocket 116 in the elastic body 112 is provided with a corresponding straight portion 117 of wall 120 , which is distally positioned with respect to the rod 162 of the holding tool 160 . thus , the fold of the separator sheet to form the wrap around electrode 2 is preferably made along the straight portion 117 and around the corresponding straight edge of electrode 2 . in that manner , puckers and wrinkles in the separator sheet prior to sealing are minimized . in the preferred embodiment , the electrode support 140 is also movable within the pocket 116 of the elastic body 112 and is continually forced upwardly . that way , when the electrode 2 is wrapped in the separator sheet 3 and disposed in the pocket 116 of the elastic body 112 , the electrode support 140 is forced against the separator material . in that manner , electrodes having a substantial variation in their thickness can be processed in the apparatus 10 and tightly sealed in the separator film . referring in particular to fig4 and 10 , springs 141 and 143 are provided for applying an upward force on the electrode support 140 . the respective outer ends 145 and 146 of the springs 141 and 143 abut against the bottoms of respective upper counterbores 101 and 103 in the platen 102 . the respective inner ends 147 and 148 of the springs 141 and 143 are in contact with the lower surface 144 of the electrode support 140 to thereby apply an upward bias thereto , as indicated by arrows 96 . shoulder bolts 151 and 152 pass through holes that continue in platen 102 from the upper counter bores 101 and 103 to the lower counterbores 105 and 107 . the shoulder bolts 151 and 152 are engaged with threaded holes in the electrode support 140 and serve as locators for maintaining the springs 141 and 143 in position . as indicated by bidirectional arrows 95 , electrode support 140 thus “ floats ” within the lower portion 119 of the pocket 116 . its upward travel is stopped when the heads of bolts 151 and 152 bottom out in the lower counterbores 105 and 107 , and its downward travel is stopped when surface 144 contacts surface 104 of the platen 102 . when electrode 2 and separator sheet 3 are first loaded into pocket 116 , the upper surface 6 of the electrode 2 is above the upper surface 114 of the elastic body 112 , and the bolt heads 151 and 152 bottom out in the lower counterbores 105 and 107 . however , during the heat sealing step , when the heater assembly 200 is pressed against the electrode holding fixture 100 , the lower surface 206 of the housing plate 220 pushes down on the separator sheet 3 and electrode 2 until the upper electrode surface 6 is substantially coplanar with the upper surface 114 of the elastic body 112 . this occurs regardless whether there is a substantial thickness variation between individual electrodes being sealed , with springs 141 and 143 compressing as needed to adjust the position of electrode support 140 . ( however , the maximum electrode thickness is ultimately limited to the depth of the pocket minus the sum of the electrode support and twice the separator sheet thickness .) it is also noted that the fit of the electrode 2 and separator film 3 in pocket 116 is snug , thereby providing a very tight fit of the separator film around the electrode as compared to prior art sealing apparatus . however , the silicone rubber material of the elastic body 112 is sufficiently soft so that in spite of the tight fit , damage to the separator sheet 3 by the relatively hard electrode 2 is minimized . it will be apparent that other spring arrangements can be used to achieve the same result of a floating electrode support . for example , the electrode support 140 and the platen 102 can be configured to have a single spring disposed in a counterbore around the shaft 184 of the ejection tool 180 . alternatively , other springs such as leaf springs could be used . with the electrode 2 and separator sheet 3 loaded into the pocket 116 of the elastic body 112 , and with the ends of the separator sheet held down , the heat sealing step is now performed . the electrode holding fixture 100 is replaced beneath the heater assembly 200 using slide assembly 40 . the heating element 230 is energized by an electrical power supply ( not shown ) and heated to the desired temperature . referring now to fig8 , heater assembly 200 is lowered , as indicated , by a suitable linear actuator ( not shown ), until the heating element 230 contacts the separator material on the upper surface 114 of the elastic body 112 . heater assembly 200 is preferably provided with a pair of guide pins 224 and 226 ( fig3 ). guide pin has a “ key ” shape while guide pin 226 has a cylindrical shape and they engage with respectively shaped holes 109 and 309 in the platen 102 . in this manner , the heating element 230 is precisely brought into contact with the upper surface 114 of the elastic body 112 in the required position with respect to the perimeter 4 of electrode 2 . referring also to fig8 a , the thermo - encapsulating apparatus 10 may also include a temperature sensing device , such as thermocouple probe 270 , that is provided for measuring and controlling the temperature of the heating element 230 . this is important because the present thermo - encapsulating apparatus 10 is useful for enveloping an electrode structure in a variety of separator materials having a wide range of melting temperatures . typical separators are of a polymeric material requiring temperatures of from about 100 ° c . to about 500 ° c . for being thermally sealed . one preferred material is polytetrafluoroethylene , which is sealed at a relatively high temperature range of about 200 ° c . to about 500 ° c ., preferably about 400 ° c . polyethylene is another preferred separator material that is sealed at a temperature range of about 130 ° c . to about 250 ° c . however , those skilled in the art will be able to readily determine at what temperature a particular polymeric material melts . that way , the present apparatus 10 makes it possible to regulate the temperature at which the separator sheet portions 5 , 7 are fused to each other during the sealing step of the process . thermocouple probe 270 is comprised of a female connector 272 , a male connector 274 , and a protective sheath 276 extending from the distal end 278 of connector 274 . the male connector 244 may be secured to a mounting block 275 , which in turn is secured to upper dielectric plate 203 by suitable fasteners ( not shown ). a pair of thermocouple wires ( not shown ) is connected to spades 280 , 281 that extend from the proximal end 282 of the male connector 274 . these thermocouple wires are insulated , and extend through the protective sheath 276 , which is typically a thin - walled metallic tube . within the distal tip 284 of sheath 276 , the thermocouple wires are joined to form the thermocouple junction , which is the point at which the temperature measurement is made by probe 270 . the thermocouple wires may be made of chromel and alumel alloys , thereby providing a k - type thermocouple . spades 280 and 281 are insertable into corresponding receptacles that are connected to a pair of terminals ( not shown ) in female connector 276 . additional thermocouple wires ( not shown ) are connected to these respective terminals , and extend out through a strain relief bushing 286 . these thermocouple wires in turn are connected to a thermocouple circuit board ( not shown ) that detects the voltage potential produced at the thermocouple junction and converts the potential into temperature data . this type of thermocouple instrument is well known and is manufactured and sold commercially by various companies such as omega engineering of stamford , conn . small bores ( not shown ) are provided through the upper platform 202 and the base plate 204 for receiving the distal portion 288 of sheath 276 . the distal portion 288 extends downwardly through the bores so that its distal tip 284 is in direct contact with the proximal head 238 of the heating element 230 . in that manner , an accurate and responsive measurement of the temperature of the heating element 230 can be made by the thermocouple probe 270 . in one preferred embodiment , the proximal head 238 of heating element 230 and at least the distal portion 288 of the sheath 276 are coated with a thin film of electrically insulative and thermally conductive material that is unaffected by high temperatures . one suitable thin film coating is diamond - like carbon ( dlc ). it will be apparent to those skilled in the art that other known temperature measurement probes may be used instead of probe 270 to measure the temperature of heating element 230 . for example , a platinum resistance thermometer ( prt ) may be used in a similar configuration . fig1 a is a detailed cross - sectional view of the pressing of the sealing portion of the heating element against the separator sheet and the elastic body 112 near the perimeter of the electrode . as the cutting edge 240 of the heating element 230 cuts the overlapping layers 5 and 7 of separator sheet , the sealing shoulder 242 faces the electrode 2 and firmly presses a narrow band of the separator layers together . then , the layers 5 and 7 are fused together under high pressure and heat at the shoulder 242 , thereby enveloping the electrode 2 within the separator sheet 3 . it can be seen that the upper surface 114 of the elastic body 112 near heating element 230 is elastically deformed during cutting and sealing . this deformation is reversed when the heating element 230 is removed . the apparatus preferably further comprises a film tensioning block 170 mounted on the outer hold - down plate 208 . the tensioning block is operatively associated with the elastic body 112 . when the heater assembly and the electrode holding fixture are pressed together , the elastic body 112 and the film tensioning block 170 pinch the separator portions 5 , 7 together and apply tension to the separator material wrapped around the electrode 2 . referring to fig3 , 9 and 9 a , film tensioning block 170 is joined to the outer hold - down plate 208 . the inner sidewall 172 of the tensioning block 170 is positioned with respect to the outer sidewall 121 of the elastic body 112 so that when the heater assembly 200 and electrode holding fixture 100 are pressed together , a gap is formed between them . the gap width is slightly less than two times the thickness of the separator film , so that the distal and proximal separator sheet portions 5 , 7 are pinched between the sidewalls 121 and 172 during downward motion of the tensioning block 170 , as shown in fig9 a . this downward motion results in a downward pulling of the distal and the proximal portions 5 , 7 of the separator sheet 3 , as indicated by arrow 94 . this , in turn , results in tensioning of the distal and proximal separator portions 5 , 7 around the electrode 2 , as indicated by arrow 93 . it is to be understood that the tensioning of the separator portions 5 , 7 occurs immediately prior to contact of the heating element 230 with them , while they are still free to be displaced horizontally . then , the cutting and sealing portion 232 of heating element 230 “ bites ” into the separator portions 5 , 7 , cuts and seals them together as described previously . in this manner , a superior fit of the sealed separator sheet 3 to the electrode 2 is achieved . as shown in fig1 a , in order for the tension to be more strongly applied to the separator material , a friction - reducing film 126 is provided on the upper surface 114 of the elastic body 112 . the friction reducing film has a lower coefficient of friction than the relatively “ tacky ” silicone rubber material of the elastic body 112 . this enables the proximal portion 7 of the separator material 3 to more easily slip along upper surface 114 during tensioning . in one embodiment , the friction reducing film is a polyimide . in a preferred embodiment , the elastic body of the electrode holding die includes a clearance notch positioned to receive a wire contact extending from the electrode . the notch , in combination with an associated tab on the heating element , enables sealing of the separator sheet around the wire . referring to fig5 , 5 a and 5 b , clearance notch 130 is located in elastic body 112 in a position corresponding with the contact wire 9 extending laterally from the electrode 2 . this is typically in the “ corner ” of the electrode , i . e . where the straight edge of the electrode intersects the curved portion . clearance notch 130 extends from pocket wall surface 123 to the outer sidewall 125 of the elastic body 112 and includes a recessed portion 128 , a wire sealing portion 132 and a barrier portion 134 . the bottom surfaces of wire sealing portion 132 and barrier portion 134 form a contiguous surface 133 . the wire 9 rests upon the separator sheet when the electrode 2 and the separator sheet 3 are first placed in the pocket 116 of the elastic body 112 . the separator sheet , in turn , rests upon notch surface 133 . ( for the sake of simplicity of illustration , and in order to depict the interaction of wire 9 with notch 130 , the separator sheet is not shown in fig5 b .) referring also to fig3 and 7 , heating element 230 is provided with a wire sealing tab 231 that is shaped to match the surfaces of the wire sealing portion 132 of clearance notch 130 . when heater assembly 200 is lowered and heating element 230 contacts the separator portions 5 , 7 and the upper surface 114 of the elastic body 112 , sealing tab 231 contacts electrode wire 9 and pushes downwardly on it . the surface 133 of notch 130 deforms elastically as shown in fig5 b , and the separator portions 5 , 7 ( not shown ) disposed above and below wire 9 correspondingly deform as well . the heat from sealing tab 231 fuses the portions 5 , 7 together proximate to the wire 9 . this provides a superior seal of the separator material around the wire 9 . the barrier portion 134 of notch 130 is made sufficiently narrow to prevent unwanted heat transfer inwardly from sealing tab 231 to the electrode 2 . the electrode 2 may include a j - bend 11 , a glass - to - metal - seal 12 and contact wire 14 joined to electrode wire 9 prior to the separator heat sealing process . these contacts are disposed in recessed portion 128 of the clearance notch 130 during sealing . if the glass - to - metal seal 12 is larger than the depth of the recessed portion 128 , the elastic body 112 will temporarily deform as needed where the seal contacts the recessed portion 128 . with the separator sheet 3 having been cut and sealed around the electrode 2 as described , the heater assembly 200 is now withdrawn upwardly from the electrode holding fixture 100 . the electrode holding fixture 100 with the electrode 2 sealed in the separator sheet 3 is withdrawn from beneath the heater assembly 200 using slide assembly 40 ( fig1 ). the sealed electrode 2 is then removed from the pocket 116 of the elastic body 112 . because of the tight fit of the electrode 2 and enveloping separator sheet 3 in the pocket 116 of the elastic body 112 , the thermo - encapsulating apparatus 10 preferably includes an ejection tool 180 for ejecting the sealed electrode from the pocket . referring to fig4 , 9 and 11 , the ejection tool 180 is comprised of a piston 182 disposed in a cavity 154 in the electrode support , a shaft 184 having an upper portion 183 connected to the piston 182 and passing through a hole 156 in the electrode support , and a lower portion 185 passing through a hole 307 in the platen 102 of the electrode holding fixture 102 . the upper surface 187 of piston 182 is made large with respect to the size of electrode support 140 , and is provided with a radiused edge . these features prevent damage to the separator sheet 3 during ejection of the sealed electrode 2 . although the ejection tool 180 may be operated by contact with an operator &# 39 ; s finger , it is preferably connected to an actuator for easier operation . referring to fig9 and 11 , actuator 190 is comprised of a lever 191 that is connected to the ejection tool 180 by a pin 192 . the lever 191 is also connected to the platen 102 by a fulcrum block 193 and a pin 194 . when an operator pushes down on the handle 195 of lever 191 as indicated by arrow 92 , the ejection tool 180 moves upwardly as indicated by arrow 91 . this results in ejection of the sealed electrode 2 , as indicated by arrow 90 . a spring loaded plunger 196 , which is joined to the lever 191 and in contact with the bottom surface 106 of the platen 102 , provides a force on the lever 191 to return it to its home position . it is to be understood that while the present invention has been described in terms of “ upper ” and “ lower ” surfaces , and with the heater assembly located “ above ” the electrode holding fixture , there is no requirement that the apparatus be oriented and operated as shown with respect to gravity . these terms are simply used to indicate locations of certain elements with respect to each other and the appended drawings , and are not intended to be limiting with regard to the overall construction of the apparatus and its use . it is , therefore , apparent that there has been provided , in accordance with the present invention , an apparatus and method for thermal encapsulation of a battery or capacitor electrode within a protective polymer film . while this invention has been described in conjunction with preferred embodiments thereof , it is evident that many alternatives , modifications , and variations will be apparent to those skilled in the art . accordingly , the present invention is intended to embrace all such alternatives , modifications and variations that fall within the broad scope of the appended claims .