fig1 and 2 are schematic representations of implantable devices 100 that may have occasion to be explanted . each implantable device 100 includes a device body 105 and a tail 110 . each device body 105 includes a biocompatible housing that houses elements configured to perform activities when the corresponding implantable device 100 is implanted in a body . for example , each device body 105 can include mechanical , electrical , chemical , biological , and / or optical components that perform , e . g ., diagnostic , therapeutic , monitoring , and / or other activities after implantation . in some implementations , implantable devices 100 are implantable pulse generators . an implantable pulse generator ( ipg ) is an implantable device that delivers electrical pulses to cells or tissue . the electrical pulses can be delivered for therapeutic , functional , diagnostic , or other purposes . when implantable device 100 is an implantable pulse generator , the elements housed in device body 105 can include , e . g ., one or more power storage devices such as a battery , one or more electrodes for delivery of the electrical pulses , and / or control circuitry for managing the delivery of the electrical pulses . tails 110 are generally flexible , elongate members that are each attached to a corresponding device body 105 at an attachment site 115 . each attachment site 115 is found on a surface 120 of device body 105 of a corresponding implantable device 100 . surface 120 extends laterally outward from attachment site 115 . tails 110 are thus thinner than device bodies 105 in that at least a portion of device bodies 105 have a larger cross - section than tails 110 . in some implementations , the cross - sections of tails 110 are generally uniform along their entire length . in other implementations , the cross - sections of tails 110 are not uniform along their entire length but may include elements that can be detached or otherwise removed to achieve a uniform cross - section . each tail 110 extends longitudinally from attachment site 115 to an end 135 . tails 110 can be formed from one or more flexible polymeric , ceramic , or metallic biocompatible materials , such as nylon , polytetrafluoroethylene , silicone , polyurethane , polyester , polypropylene , titanium , carbon fiber , stainless steel , noble metals , glass fibers , and the like . in some implementations , tails 110 can be formed from bioresorbable materials if the expected duration of implantation is shorter than the time for resorbtion . the material forming tails 110 can be shaped into wires or other elongate strands . in some implementations , the wires or strands can be woven or threaded to form a stranded tails 110 . in other implementations , tails 110 can be solid . tails 110 can be completely passive or can include one or more active components . active tails 110 can include one or more mechanical , electrical , chemical , biological , and / or optical components that contribute to the performance of activities by implantable device 100 . for example , active tails 110 can include active components such as optical fibers , fluid conduits , and / or electrical leads that are covered by one or more electrical insulators . in operation , device bodies 105 of implantable devices 100 can be implanted at relatively “ deep ” sites , while tails 110 can extend to relatively superficial sites closer to the skin or other surface . this arrangement can have a number of different advantages . for example , a tail 110 can guide medical personnel from a superficial site to a device body 105 of an implanted device 100 that it implanted at a deeper site with minimal collateral tissue damage , as discussed further below . as another example , if an active tail 110 includes a charging element that contributes to the conversion of energy from outside a body into potential energy , the charging element can be positioned close to the skin surface so that a relatively small amount of the energy from outside the body is dissipated by transmission through the body . fig3 is a schematic representation of a tail 110 that includes active components , namely , one or more leads 305 and one or more electrodes 310 . leads 305 are wires that place electrodes 310 in electrical contact with components in device body 105 of an implantable device 100 . the conducting path formed by leads 305 and electrodes 310 traverses an outer surface 315 of tail 110 so that electrodes 310 can deliver electrical pulses originating in device body 105 to sites that are relatively remote from device body 105 . fig4 is a schematic representation of a tail 110 that includes active components , namely , a pair of leads 405 and a charging element 410 . fig5 is a sectional representation of charging element 410 along the line 5 - 5 of fig4 . charging element 410 includes a coiled conductor 415 that is housed in a biocompatible casing 420 . leads 405 are wires that place coiled conductor 415 in electrical contact with components in device body 105 of implantable device 100 . coiled conductor 415 can respond to a magnetic or electric field generated outside the body in which implantable device 100 is implanted . the motion of electrons in coiled conductor 415 under the influence of such a magnetic or electric field can be converted into potential energy and stored , e . g ., at a rechargeable battery or other energy storage device in device body 105 of implantable device 100 . the conducting path formed by leads 405 to coiled conductor 415 allows charging element 410 to be positioned close to the skin surface while device body 105 is implanted at a deeper site . such a positioning can reduce the dissipation of the magnetic or electric field from outside the body while allowing the motion induced by such a field in coiled conductor 415 to be efficiently conveyed to device body 105 . fig6 is a schematic representation of one example of an implantable device 100 , namely , an implantable microstimulator 600 . implantable pulse generator 600 includes a generally cylindrical device body 105 and tail 110 that is attached to a surface 120 which caps device body 105 . device body 105 can be dimensioned to be implantable through the cannula of a closed surgical device . device body 105 houses electrical circuitry , power storage devices , and other components for the delivery of electrical stimuli to cells or tissue . implantable pulse generator 600 also includes a stimulating electrode 605 and a counter electrode 610 that are separated by an insulator 615 . stimulating electrode 605 is positioned on a cap 620 of device body 105 . counter electrode 610 and insulator 615 are generally cylindrical members with the same circumferential outer surface geometry . counter electrode 610 and insulator 615 are joined at a seam 625 with their outer surfaces aligned . fig7 is a schematic representation of an explant tool 700 . explant tool 700 includes a handle 705 that is fixed to an elongate shank 710 at a site 712 . shank 710 extends longitudinally away from handle 705 to terminate in a tip 715 . tip 715 can have a truncated conical shape ( as shown ), a blunted , bullet shape , a truncated pyramidal shape , or other shape that includes one or more slanted surfaces that slope to spread tissue during forward penetration of tip 715 into the body . fig8 is a schematic representation of the truncated conical implementation of tip 715 in additional detail . in the vicinity of tip 715 , shank 710 includes an outer circumference 720 . the truncated cone of tip 715 includes a slanted surface 725 that slopes inward from outer circumference 720 to an end 730 of tip 715 . outer circumference 720 can be dimensioned to be the same size or larger than a lateral size an implantable device that is to be explanted , as discussed further below . tip 715 and at least a portion of shank 710 define a passage 735 . passage 735 extends longitudinally into the truncated cone of tip 715 and through the forward portion of shank 710 . shank 710 includes a cutout 740 that communicates with end 730 via passage 735 . cutout 740 ends the generally circular outer diameter 720 of shank 710 in a rounded edge 745 . passage 735 can be dimensioned to receive the tail of an implantable device that is to be explanted , as discussed further below . in some implementations , end 730 of tip 715 can include one or more elements to aid in the penetration of tip 715 into a body . for example , end 730 can include a blade or other sharpened edge for cutting through tissue . as another example , end 730 can be serrated or include abrasive elements for sawing through tissue upon rotation . fig9 is a schematic representation of another implementation of tip 715 . in the illustrated implementation , slanted surface 725 defines one or more lateral depressions 905 . a blade 910 or other cutting element is recessed in each depression 905 . blade 910 is oriented laterally relative to shank 710 and can cut tissue that moves into depression 905 during the forward penetration of tip 715 into a body . fig1 is a schematic representation of another implementation of tip 715 . in the illustrated implementation , slanted surface 725 defines one or more longitudinal depressions 1005 . a blade 1010 or other cutting element is recessed in each depression 1005 . blade 1010 is oriented longitudinally along shank 710 and can cut tissue that moves into depression 905 upon rotation of tip 715 during the penetration of a body . fig1 - 17 schematically represent an illustrative example of the use of explant tool 700 to explant an implantable device 100 from a body 1100 . with reference to fig1 , body 1100 includes a body surface 1105 by which medical personnel can access implantable device 100 . body surface 1105 is generally the skin . implantable device 100 is implanted in body 1100 with device body 105 positioned at a relatively deep site 1110 and with tail 110 extending to a superficial site 1115 closer to surface 1105 . thus , superficial site 1115 is at a depth d 1 beneath surface 1105 and surface 120 of device body 105 is at a depth d 2 beneath surface 1105 . depth d 1 is smaller than depth d 2 . with reference to fig1 - 13 , after a sufficiently long implantation term , implantable device 100 will often become encased in a tissue capsule 1205 . medical personnel can make an incision 1305 in the vicinity of end 135 of tail 110 using a scalpel 1210 or other device . the exposed portion of tissue capsule 1205 can also be cut and the terminal portion 1310 of tail 110 can be teased out from body 1100 . since superficial site 1115 is at a depth d 1 beneath body surface 1105 , incision 1305 is relatively shallow despite the implantation of device body 105 of device 100 at a deeper location . in implementations where tail 110 includes one or more active components , such as charging element 410 , that are relatively large , the active components can be detached or otherwise removed from the remainder of tail 110 . for example , tail 110 can be cut to remove charging element 410 . with reference to fig1 and 15 , the terminal portion 1310 of tail 110 can be inserted into end 710 of explant tool 700 by threading tail 110 through passage 735 and out into cutout 740 . the terminal portion 1310 of tail 110 can then be grasped between the jaws 1505 of hemostat or other pliers 1510 . explant tool 700 can be advanced into incision 1305 by medical personnel manipulating handle 705 ( not shown ). pliers 1510 can maintain tension on tail 110 while explant tool 700 is advanced . with reference to fig1 and 16 , while maintaining grip on terminal portion 1310 of tail 110 , further manipulation can be used to advance tip 715 along tail 110 and deeper into body 1100 toward device body 105 of implantable device 100 . if necessary , explant tool 700 can be pushed or rotated using handle 705 to aid in the penetration of tip 715 through tissue capsule 1205 that surrounds tail 110 . such manipulation can bring blades , serrated edges , and / or abrasive elements to bear on tissue capsule 1205 . as tip 715 penetrates into body 1100 along tail 110 , slanted surface 725 spreads tissue capsule 1205 to create a channel 1605 . channel 1605 can thus pass solid objects with diameters that are the same size or smaller than outer circumference 720 of shank 710 . with reference to fig1 , following tail 110 , tip 715 of explant tool 700 can reach site 1110 where device body 105 of implantable device 100 is implanted . if necessary , tip 715 of explant tool 700 can be used to penetrate any tissue capsule 1205 around surface 120 of device body 105 to clear the way for withdrawal of device body 105 through channel 1605 . implantable device 100 can be explanted through channel 1605 by pulling explant tool 700 , along with the gripped terminal portion 1310 of tail 110 , away from body 1100 . fig1 is a schematic representation of another implementation of explant tool 700 . in the illustrated implementation , shank 710 of explant tool 700 includes a relatively flexible portion 1810 and a relatively inflexible portion 1805 that is joined to tip 715 . relatively inflexible portion 1805 is less flexible than relatively flexible portion 1810 and hence better suited for penetrating into a body . in particular , a relatively less flexible and harder dissection tip can facilitate penetration of the tissue and decrease deformation of the dissection tip . the different flexibilities of relatively inflexible portion 1805 and relatively flexible portion 1810 can be achieved in a number of different ways . for example , portion 1805 can be made from a first , relatively inflexible , polymer and portion 1810 can be made from a second , relatively flexible , polymer . as another example , portion 1805 and portion 1810 can have identical compositions but different structures . for example , passage 735 can be larger in portion 1810 than in portion 1805 . as yet another example , the materials used to form portions 1805 , 1810 can be handled in different ways and / or include different compositions . in some implementations , portions 1805 , 1810 are joined at a seam 1815 . in other implementations , shank 710 can be formed as a unitary structure and portions 1805 , 1810 can be separated by a gradient or other transition in flexibility . fig1 is a diagrammatic view of a positioning member 1900 . fig2 is a view of positioning member 1900 from above . positioning member 1900 is a generally planar member that includes an interior wall 1920 . interior wall 1920 extends from a first face 1910 to a second face 1915 of positioning member 1900 to define a passage 1905 therethrough . interior wall 1920 also defines a lateral tooth 1925 that extends toward the interior of passage 1905 . positioning member 1900 is dimensioned to slidably receive the portion of shank 710 that includes cutout 740 . fig2 is a schematic representation of the reception of this portion of shank 710 in positioning member 1900 . as shown , cutout 1905 is dimensioned to pass this portion of shank 710 with lateral tooth 1925 extending into cutout 740 . in extending into cutout 740 , lateral tooth 1925 can prevent the entry of a tail 110 into cutout 740 . lateral tooth 1925 does not however , extend into passage 735 . rather , passage 735 can pass a tail 110 of an implantable device 100 . the portion of shank 710 that includes cutout 740 can be slid forward and backward ( i . e ., in and out of the page ) through positioning member 1900 . fig2 schematically represents the use of positioning member 1900 and explant tool 700 to explant an implantable device 100 from a body 1100 . as shown , the portion of shank 710 that includes cutout 740 is received in positioning member 1900 . positioning member 1900 is maintained outside of body 1100 and can rest against body surface 1105 . tail 110 is threaded into tip 715 and follows passage 735 past positioning member 1900 and out of body 1100 . positioning member 1900 thus prevents the entry of tail 110 into cutout 740 before tail 110 exits body 1100 . a number of implementations have been described . nevertheless , it will be understood that various modifications may be made . for example , other charging elements , including optical ( e . g ., photovoltaic cells ) and mechanical ( e . g ., piezoelectric devices ) elements , can be used . as . another example , a tail 110 need not be teased out from the body before insertion into passage 735 of an explant tool 700 . as yet another example , a positioning member 1900 need not include a tooth 1925 . accordingly , other implementations are within the scope of the following claims .
systems and techniques for explanting implantable devices are described . in one aspect , a device includes an explant tool configured to explant an implantable device from a body . the explant tool includes an elongate shank comprising a forward portion defining a longitudinal passage and a front tip attached to the forward portion and defining an opening to the longitudinal passage defined by the forward portion . the front tip comprises a slanted surface that slopes in a direction so as to spread tissue during forward penetration of the front tip into the body .