Patent Document

RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Application Ser. No. 61/016,180, filed Dec. 21, 2007 entitled A System and Method for Detecting Implant Detachment, herein incorporated by reference in its entirety. This application also incorporates by reference U.S. Provisional Application Ser. No. 60/604,671, filed Aug. 25, 2004 entitled Thermal Detachment System For Implantable Devices; U.S. Provisional Application Ser. No. 60/685,342 filed May 27, 2005 entitled Thermal Detachment System For Implantable Devices; U.S. patent application Ser. No. 11/212,830 filed Aug. 25, 2005 entitled Thermal Detachment System For Implantable Devices; U.S. Provisional Application Ser. No. 60/952,520 filed Jul. 27, 2007 entitled Detachable Coil Incorporating Stretch Resistance; and U.S. Provisional Application Ser. No. 61/016,154 filed Dec. 21, 2007 entitled System and Method For Locating Detachment Zone Of A Detachable Implant. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to systems and methods for delivering implant devices to a target site or location within the body of a patient. The present invention also relates to a method of detecting implant detachment within the body of a patient. 
     BACKGROUND OF THE INVENTION 
     Delivery of implantable therapeutic devices by less invasive means has been demonstrated to be desirable in numerous clinical situations. For example, vascular embolization has been used to control vascular bleeding, to occlude the blood supply to tumors, to occlude fallopian tubes, and to occlude vascular aneurysms, particularly intracranial aneurysms. In recent years, vascular embolization for the treatment of aneurysms has received much attention. Implants used to treat aneurysms are often convoluted or coiled lengths of wound wire and are referred to as “microcoils.” Microcoils work by filling an aneurysm causing the blood flow through the aneurysm to slow or stop, thereby inducing thrombosis within the aneurysm. 
     Microcoils are extremely flexible and have very little structural integrity. In order to make them easier to retrieve and reposition, recent efforts have been directed to making them stretch-resistant. For example, a stretch-resistant embolic coil having a stretch-resistant member passing through the interior lumen of the coil is described in U.S. Pat. No. 5,582,619 to Ken, herein incorporated by reference in its entirety. U.S. Patent Publication No. 2004/0034363 to Wilson, herein incorporated by reference in its entirety, also discloses an embolic coil with a stretch resistant member having a distal end attached near the distal end of the coil and a proximal end of the member attached to a delivery catheter. 
     Several different treatment modalities have been employed in the prior art for deploying implant devices. For example, numerous repositionable detachment systems for implant devices have been described in the prior art including U.S. Pat. Nos. 5,895,385 to Guglielmi et al. and 5,108,407 to Geremia et al., the contents of which are hereby incorporated by reference. Several systems, such as those disclosed in U.S. Pat. No. 6,500,149 to Gandhi et al. and U.S. Pat. No. 4,346,712 to Handa et al., the contents of which are hereby incorporated by reference, describe the use of a heater to detach and deploy the implant device. 
     While implant delivery and detachment systems are known in the art, they do not provide the user feedback that the implant has indeed detached from the delivery device. This is especially important in cases where the detachment relies on the application of heat or an electrolytic process where an element of time is involved. These delivery devices leave the user in the position of wondering whether heat etc., has been applied long enough to cause detachment. Hence, there exists a need for a method of detecting whether an implant has properly and effectively detached within the body of a patient. 
     SUMMARY OF THE INVENTION 
     The present invention is an implant delivery and detachment system used to position and deploy implantable devices such as coils, stents, filters, and the like within a body cavity including, but not limited to, blood vessels, fallopian tubes, malformations such as fistula and aneurysms, heart defects (e.g. left atrial appendages and sepal openings), and other luminal organs. 
     The system comprises an implant, a delivery catheter (generically referred to as the pusher or delivery pusher), a detachable joint for coupling the implant to the pusher, a heat generating apparatus (generically referred to as the heater), and a power source to apply energy to the heater. 
     The present invention also includes a method for detecting detachment of an implant. In particular, detachment of an implant is detected by measuring the change in the electrical resistance of the delivery system. 
     The present invention may also be used in conjunction with the delivery mechanism disclosed in U.S. patent application Ser. No. 11/212,830 filed Aug. 25, 2005 entitled “Thermal detachment system for implanting devices,” which is incorporated by reference herein in its entirety. 
     In one aspect of the present invention, the implant is coupled to the pusher using a tether, string, thread, wire, filament, fiber, or the like. Generically this is referred to as the tether. The tether may be in the form of a monofilament, rod, ribbon, hollow tube, or the like. Many materials can be used to detachably join the implant to the pusher. One class of materials are polymers such as polyolefin, polyolefin elastomer such as those made by Dow marketed under the trade name Engage or Exxon marketed under the trade name Affinity, polyethylene, polyester (PET), polyamide (Nylon), polyurethane, polypropylene, block copolymer such as PEBAX or Hytrel, and ethylene vinyl alcohol (EVA); or rubbery materials such as silicone, latex, and Kraton. In some cases, the polymer may also be cross-linked with radiation to manipulate its tensile strength and melt temperature. Another class of materials is metals such as nickel titanium alloy (Nitinol), gold, and steel. The selection of the material depends on the capacity of the material to store potential energy, the melting or softening temperature, the power used for detachment, and the body treatment site. The tether may be joined to the implant and/or the pusher by welding, knot tying, soldering, adhesive bonding, or other means known in the art. In one embodiment where the implant is a coil, the tether may run through the inside lumen of the coil and be attached to the distal end of the coil. This design not only joins the implant to the pusher, but also imparts stretch resistance to the coil without the use of a secondary stretch resistant member. In other embodiments where the implant is a coil, stent, or filter; the tether is attached to the proximal end of the implant. 
     In another aspect of the present invention, the tether detachably coupling the implant to the pusher acts as a reservoir of stored (i.e. potential) energy that is released during detachment. This advantageously lowers the time and energy required to detach the implant because it allows the tether to be severed by application of heat without necessarily fully melting the material. The stored energy also may exert a force on the implant that pushes it away from the delivery catheter. This separation tends to make the system more reliable because it may prevent the tether from re-solidifying and holding the implant after detachment. Stored energy may be imparted in several ways. In one embodiment, a spring is disposed between the implant and pusher. The spring is compressed when the implant is attached to the pusher by joining one end of the tether to one of either the pusher or implant, pulling the free end of the tether until the spring is at least partially compressed, then affixing the free end of the tether to the other of the implant or the pusher. Since both ends of the tether are restrained, potential energy in the form of tension on the tether (or compression in the spring) is stored within the system. In another embodiment, one end of the tether is fixed as in the previous embodiment, and then the tether is placed in tension by pulling on the free end of the tether with a pre-determined force or displacement. When the free end of the tether is then affixed, the elongation (i.e. elastic deformation) of the tether material itself stores energy. 
     In another aspect of the present invention, a heater is disposed on or within the pusher, typically, but not necessarily, near the distal end of the pusher. The heater may be attached to the pusher by, for example, soldering, welding, adhesive bonding, mechanical boding, or other techniques known in the art. The heater may be in the form of a wound coil, heat pipe, hollow tube, band, hypotube, solid bar, toroid, or similar shape. The heater may be made from a variety of materials such as steel, chromium cobalt alloy, platinum, silver, gold, tantalum, tungsten, mangalin, chromium nickel alloy available from California Fine Wire Company under the trade name Stable Ohm, conductive polymer, or the like. The tether is disposed in proximity to the heater. The tether may pass through the lumen of a hollow or coil-type heater or may be wrapped around the heater. Although the tether may be disposed in direct contact with the heater, this is not necessary. For ease of assembly, the tether may be disposed be in proximity to, but not actually touching, the heater. 
     The delivery catheter or pusher is an elongate member with distal and proximal ends adapted to allow the implant to be maneuvered to the treatment site. The pusher comprises a core mandrel and one or more electrical leads to supply power to the heater. The pusher may taper in dimension and/or stiffness along the length, with the distal end usually being more flexible than the proximal end. In one embodiment, the pusher is adapted to be telescopically disposed within a delivery conduit such as a guide catheter or microcatheter. In another embodiment, the pusher contains an inner lumen allowing it to be maneuvered over a guide wire. In still another embodiment, the pusher can be maneuvered directly to the treatment site without a secondary device. The pusher may have a radiopaque marking system visible with fluoroscopy that allows it to be used in conjunction with radiopaque markings on the microcatheter or other adjunctive devices. 
     In another aspect of the present invention, the core mandrel is in the form of a solid or hollow shaft, wire, tube, hypotube, coil, ribbon, or combination thereof. The core mandrel may be made from plastic materials such as PEEK, acrylic, polyamide, polyimide, Teflon, acrylic, polyester, block copolymer such as PEBAX, or the like. The plastic member(s) may be selectively stiffened along the length with reinforcing fibers or wires made from metal, glass, carbon fiber, braid, coils, or the like. Alternatively, or in combination with plastic components, metallic materials such as stainless steel, tungsten, chromium cobalt alloy, silver, copper, gold, platinum, titanium, nickel titanium alloy (Nitinol), and the like may be used to form the core mandrel. Alternatively, or in combination with plastic and/or metallic components, ceramic components such as glass, optical fiber, zirconium, or the like may be used to form the core mandrel. The core mandrel may also be a composite of materials. In one embodiment, the core mandrel comprises an inner core of radiopaque material such as platinum or tantalum and an outer covering of kink-resistant material such as steel or chromium cobalt. By selectively varying the thickness of the inner core, radiopaque identifiers can be provided on the pusher without using secondary markers. In another embodiment, a core material, for example stainless steel, with desirable material properties such as kink resistance and/or compressive strength is selectively covered (by, for example, plating, drawing, or similar methods known in the art) with a low electrical resistance material such as copper, aluminum, gold, or silver to enhance its electrical conductivity, thus allowing the core mandrel to be used as an electrical conductor. In another embodiment, a core material, for example, glass or optical fiber, with desirable properties such as compatibility with Magnetic Resonance Imaging (MRI), is covered with a plastic material such as PEBAX or polyimide to prevent the glass from fracturing or kinking. 
     In another aspect of the present invention, the heater is attached to the pusher, and then one or more electrical conductors are attached to the heater. In one embodiment a pair of conductive wires run substantially the length of the pusher and are coupled to the heater near the distal end of the pusher and to electrical connectors near the proximal end of the pusher. In another embodiment, one conductive wire runs the substantially the length of the pusher and the core mandrel itself is made from a conductive material or coated with a conductive material to act as a second electrical lead. The wire and the mandrel are coupled to the heater near the distal end and to one or more connectors near the proximal end of the pusher. In another embodiment, a bipolar conductor is coupled to the heater and is used in conjunction with radiofrequency (RF) energy to power the heater. In any of the embodiments, the conductor(s) may run in parallel to the core mandrel or may pass through the inner lumen of a substantially hollow core mandrel (for example, a hypotube). 
     In another aspect of the present invention, an electrical and/or thermally insulating cover or sleeve may be placed over the heater. The sleeve may be made from insulating materials such as polyester (PET), Teflon, block copolymer, silicone, polyimide, polyamide, and the like. 
     In another aspect of the present invention, electrical connector(s) are disposed near the proximal end of the pusher so that the heater can be electrically connected to a power source through the conductors. In one embodiment, the connectors are in the form of a plug with one or more male or female pins. In another embodiment, the connector(s) are tubes, pins, or foil that can be connected with clip-type connectors. In another embodiment, the connector(s) are tubes, pins, or foil that are adapted to mate with an external power supply. 
     In another aspect of the present invention, the pusher connects to an external power source so that the heater is electrically coupled to the power source. The power source may be from battery(s) or connected to the electrical grid by a wall outlet. The power source supplies current in the form of direct current (DC), alternating current (AC), modulated direct current, or radiofrequency (RF) at either high or low frequency. The power source may be a control box that operates outside of the sterile field or may be a hand-held device adapted to operate within a sterile field. The power source may be disposable, rechargeable, or may be reusable with disposable or rechargeable battery(s). 
     In another aspect of the present invention, the power source may comprise an electronic circuit that assists the user with detachment. In one embodiment, the circuit detects detachment of the implant and provides a signal to the user when detachment has occurred. In another embodiment, the circuit comprises a timer that provides a signal to the user when a pre-set length of time has elapsed. In another embodiment, the circuit monitors the number of detachments and provides a signal or performs an operation such as locking the system off when a pre-set number of detachments have been performed. In another embodiment, the circuit comprises a feedback loop that monitors the number of attachment attempts and increases the current, voltage, and/or detachment time in order to increase the likelihood of a successful detachment. 
     In another aspect of the present invention, the construction of the system allows for extremely short detachment time. In one embodiment the detachment time is less than 1 second. 
     In another aspect of the present invention, the construction of the system minimizes the surface temperature of the device during detachment. In one embodiment, the surface temperature at the heater during detachment is under 50° C. In another embodiment, the surface temperature at the heater during detachment is under 42° C. 
     In another aspect of the present invention, detachment of the implant is detected by measuring a change in the electrical resistance of the delivery system, specifically the heater zone, to detect implant detachment. 
     These and other aspects and features of the present invention will be appreciated upon consideration of the following drawings and detailed descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional side view of a first embodiment of a detachment system according to the present invention; 
         FIG. 2  illustrates a cross-sectional side view of a second embodiment of a detachment system according to the present invention; 
         FIG. 3A  illustrates example direct signaling current according to the present invention; 
         FIG. 3B  illustrates example alternating signaling current according to the present invention; 
         FIG. 4  illustrates a cross-sectional side view of a third embodiment of a detachment system according to the present invention; 
         FIG. 5  illustrates example temperature data of the surface of a detachment system as a function of time according to the present invention; 
         FIG. 6  illustrates a cross-sectional side view of an electrical connector of a detachment system according to the present invention; 
         FIG. 7  illustrates a cross-sectional side view of radiopaque layers of a detachment system according to the present invention; and 
         FIG. 8  illustrates a cross-sectional side view of a detachment system including a stent according to the present invention; 
         FIG. 9  illustrates a side view of a implant device according to the present invention; 
         FIG. 10  illustrates a perspective view of a coil and expander of the delivery system of  FIG. 9 ; 
         FIG. 11  illustrates a partial cut-away view of a coil and expander in an expanded state over a pusher of a delivery system according to the present invention; and 
         FIG. 12  illustrates a partial cut-away view of a system according to  FIG. 11  in a contracted state. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to  FIG. 1 , a detachment system  100  of the present invention, and specifically the distal portion of the detachment system  100 , is illustrated. The detachment system  100  includes a pusher  102  that is preferably flexible. The pusher  102  is configured for use in advancing an implant device  112  into and within the body of a patient and, specifically, into a target cavity site for implantation and delivery of the implant device  112 . Potential target cavity sites include but are not limited to blood vessels and vascular sites (e.g., aneurysms and fistula), heart openings and defects (e.g., the left atrial appendage), and other luminal organs (e.g., fallopian tubes). 
     A stretch-resistant tether  104  detachably couples the implant  112  to the pusher  102 . In this example, the tether  104  is a plastic tube that is bonded to the pusher  102 . A substantially solid cylinder could also be a design choice for the tether  104 . The stretch resistant tether  104  extends at least partially through the interior lumen of an implant device  112 . 
     Near the distal end of the pusher  102 , a heater  106  is disposed in proximity to the stretch resistant tether  104 . The heater  106  may be wrapped around the stretch resistant tether  104  such that the heater  106  is exposed to or otherwise in direct contact with the blood or the environment, or alternatively may be insulated by a sleeve, jacket, epoxy, adhesive, or the like. The pusher  102  comprises a pair of electrical wires, positive electrical wire  108  and negative electrical wire  110 . The wires  108  and  110  are coupled to the heater  106  by any suitable means, such as, e.g., by welding or soldering. 
     The electrical wires  108 ,  110  are capable of being coupled to a source of electrical power (not shown). As illustrated the negative electrical wire  110  is coupled to the distal end of the heater  106  and the positive electrical wire  108  is coupled to the proximal end of the heater  106 . In another embodiment, this configuration may be reversed, i.e., the negative electrical wire  110  is coupled to the proximal end of the heater  106  while the positive electrical wire  108  is coupled to the distal end of the heater  106 . 
     Energy is applied to the heater  106  from the electrical wires  108 ,  110  in order to sever the portion of the tether  104  in the proximity of the heater  106 . It is not necessary for the heater  106  to be in direct contact with the tether  104 . The heater  106  merely should be in sufficient proximity to the tether  104  so that heat generated by the heater  106  causes the tether  104  to sever. As a result of activating the heater  106 , the section of the stretch resistant tether  104  that is approximately distal from the heater  106  and within the lumen of an implant device  112  is released from the pusher  102  along with the implant device  112 . 
     As illustrated, the implant device  112  is an embolic coil. An embolic coil suitable for use as the implant device  112  may comprise a suitable length of wire formed into a helical microcoil. The coil may be formed from a biocompatible material including platinum, rhodium, palladium, rhenium, tungsten, gold, silver, tantalum, and various alloys of these metals, as well as various surgical grade stainless steels. Specific materials include the platinum/tungsten alloy known as Platinum 479 (92% Pt, 8% W, available from Sigmund Cohn, of Mount Vernon, N.Y.) and nickel/titanium alloys (such as the nickel/titanium alloy known as Nitinol). 
     Another material that may be advantageous for forming the coil is a bimetallic wire comprising a highly elastic metal with a highly radiopaque metal. Such a bimetallic wire would also be resistant to permanent deformation. An example of such a bimetallic wire is a product comprising a Nitinol outer layer and an inner core of pure reference grade platinum, available from Sigmund Cohn, of Mount Vernon, N.Y., and Anomet Products, of Shrewsbury, Mass. 
     Commonly-assigned U.S. Pat. No. 6,605,101 provides a further description of embolic coils suitable for use as the implant device  112 , including coils with primary and secondary configurations wherein the secondary configuration minimizes the degree of undesired compaction of the coil after deployment. The disclosure of U.S. Pat. No. 6,605,101 is fully incorporated herein by reference. Furthermore, the implant device  112  may optionally be coated or covered with a hydrogel or a bioactive coating known in the art. 
     The coil-type implant device  112  resists unwinding because the stretch resistant tether  104  that extends through the lumen of the implant device  112  requires substantially more force to plastically deform than the implant device  112  itself. The stretch resistant tether  104  therefore assists in preventing the implant device  112  from unwinding in situations in which the implant device  112  would otherwise unwind. 
     During assembly, potential energy may be stored within the device to facilitate detachment. In one embodiment, an optional spring  116  is placed between the heater  106  and the implant device  112 . The spring is compressed during assembly and the distal end of the tether  104  may be tied or coupled to the distal end of the implant device  112 , or may be melted or otherwise formed into an atraumatic distal end  114 . 
     In one embodiment, the stretch resistant tether  104  is made from a material such as a polyolefin elastomer, polyethylene, or polypropylene. One end of the tether  104  is attached to the pusher  102  and the free end of the tether  104  is pulled through the implant  112  with the proximal end of the implant  112  flush to either the heater  106  (if no spring  116  is present) or to the compressed spring  116 . A pre-set force or displacement is used to pre-tension the tether  104 , thus storing energy in an axial orientation (i.e. co-linear or parallel to the long axis of the pusher  102 ) within the tether  104 . The force or displacement depends on the tether material properties, the length of the tether  104  (which itself depends on the tether&#39;s attachment point on the pusher and the length of the implant). Generally, the force is below the elastic limit of the tether material, but sufficient to cause the tether to sever quickly when heat is applied. In one preferred embodiment wherein the implant to be deployed is a cerebral coil, the tether has a diameter within the range of approximately 0.001 to 0.007 inches. Of course the size of the tether can be changed to accommodate different types and sizes of other implants as necessary. 
     Turning to  FIG. 2 , another embodiment of a detachment system of the present invention, detachment system  200 , is illustrated. Detachment system  200  shares several common elements with detachment system  100 . For example, the same devices usable as the implant device  112  with detachment system  100  are also usable as the implant device  112  with detachment system  200 . These include, e.g., various embolic microcoils and coils. The implant device  112  has been previously described with respect to detachment system  100 . As with the implant device  112 , the same identification numbers are used to identify other elements/components of detachment system  100  that may correspond to elements/components of detachment system  200 . Reference is made to the description of these elements in the description of detachment system  100  as that description also applies to these common elements in detachment system  200 . 
     With detachment system  200 , an interior heating element  206  is used to separate a section of a stretch resistant tube  104  and an associated implant device  112  from the detachment system  200 . Detachment system  200  includes a delivery pusher  202  that incorporates a core mandrel  218 . The detachment system  200  further includes a positive electrical wire  208  and a negative electrical wire  210  that extend through the lumen of the delivery pusher  202 . 
     To form the internal heating element  206 , the positive electrical wire  208  and the negative electrical wire  210  may be coupled to the core mandrel  218  of the delivery pusher  202 . Preferably, the electrical wires  208 ,  210  are coupled to a distal portion of the core mandrel  218 . 
     In one embodiment, the positive electrical wire  208  is coupled to a first distal location on the core wire  218 , and the negative electrical wire  210  is coupled to a second distal location on the core wire  218 , with the second distal location being proximal to the first distal location. In another embodiment, the configuration is reversed, i.e., the positive electrical wire  208  is coupled to the second distal location and the negative electrical wire  210  is coupled to the first distal location on the core wire  218 . When the positive electrical wire  208  and the negative electrical wire  210  are coupled to the distal portion of the core mandrel  218 , the distal portion of the core mandrel  218  along with the electrical wires  208 ,  210  forms a circuit that is the interior heating element  206 . 
     The heater  206  increases in temperature when a current is applied from a power source (not shown) that is coupled to the positive electrical wire  208  and the negative electrical wire  210 . If a greater increase in temperature/higher degree of heat is required or desired, a relatively high resistance material such as platinum or tungsten may be coupled to the distal end of the core mandrel  218  to increase the resistance of the core mandrel  218 . As a result, higher temperature increases are produced when a current is applied to the heater  206  than would be produced with a lower resistance material. The additional relatively high resistance material coupled to the distal end of the core mandrel  218  may take any suitable form, such as, e.g., a solid wire, a coil, or any other shape or material as described above. 
     Because the heater  206  is located within the lumen of the tube-shaped tether  104 , the heater  206  is insulated from the body of the patient. As a result, the possibility of inadvertent damage to the surrounding body tissue due to the heating of the heater  206  may be reduced. 
     When a current is applied to the heater  206  formed by the core mandrel  218 , the positive electrical wire  208 , and the negative electrical wire  210 , the heater  206  increases in temperature. As a result, the portion of the stretch resistant tether  104  in proximity to the heater  206  severs and is detached, along with the implant device  112  that is coupled to the tether  104 , from the detachment system  200 . 
     In one embodiment of the detachment system  200 , the proximal end of the stretch resistant tether  104  (or the distal end of a larger tube (not shown) coupled to the proximal end of the stretch resistant tether  104 ) may be flared in order to address size constraints and facilitate the assembly of the detachment system  200 . 
     In a similar manner as with detachment system  100 , energy may be stored within the system with, for example, an optional compressive spring  116  or by pre-tensioning the tether  104  during assembly as previously described. When present, the release of potential energy stored in the system operates to apply additional pressure to separate the implant device  112 , and the portion of the stretch resistant tether  104  to which the implant device  112  is coupled, away from the heater  206  when the implant device  112  is deployed. This advantageously lowers the required detachment time and temperature by causing the tether  104  to sever and break. 
     As with detachment system  100 , the distal end of the stretch resistant tether  104  of detachment system  200  may be tied or coupled to the distal end of the implant device  112 , or may be melted or otherwise formed into an atraumatic distal end  114 . 
       FIG. 4  illustrates another preferred embodiment of a detachment system  300 . In many respects, the detachment system  300  is similar to the detachment system  200  shown in  FIG. 2  and detachment system  100  shown in  FIG. 1 . For example, the detachment system  300  includes a delivery pusher  301  containing a heater  306  that detaches an implant device  302 . Detachment system  300  also utilizes a tether  310  to couple the implant device  302  to the delivery pusher  301 . 
     In the cross-sectional view of  FIG. 4 , a distal end of the delivery pusher  301  is seen to have a coil-shaped heater  306  that is electrically coupled to electrical wires  308  and  309 . These wires  308 ,  309  are disposed within the delivery pusher  301 , exiting at a proximal end of the delivery pusher  301  and coupling to a power supply (not shown). The tether  310  is disposed in proximity to the heater  306 , having a proximal end fixed within the delivery pusher  301  and a distal end coupled to the implant device  302 . As current is applied through wires  308  and  309 , the heater  306  increases in temperature until the tether  310  breaks, releasing the implant device  302 . 
     To reduce the transfer of heat from the heater  306  to the surrounding tissue of the patient and to provide electrical insulation, an insulating cover  304  is included around at least the distal end of the outer surface of the delivery pusher  301 . As the thickness of the cover  304  increases, the thermal insulating properties also increase. However, increased thickness also brings increased stiffness and a greater diameter to the delivery pusher  301  that could increase the difficulty of performing a delivery procedure. Thus, the cover  304  is designed with a thickness that provides sufficient thermal insulating properties without overly increasing its stiffness. 
     To enhance attachment of the tether  310  to the implant device  302 , the implant device  302  may include a collar member  322  welded to the implant device  302  at weld  318  and sized to fit within the outer reinforced circumference  312  of the delivery pusher  301 . The tether  310  ties around the proximal end of the implant device  302  to form knot  316 . Further reinforcement is provided by an adhesive  314  that is disposed around the knot  316  to prevent untying or otherwise unwanted decoupling. 
     In a similar manner as with detachment systems  100  and  200 , energy may be stored within the system with, for example, an optional compressive spring (similar to compressive spring  116  in  FIG. 1  but not shown in  FIG. 4 ) or by axially pre-tensioning the tether  104  during assembly. In this embodiment, one end of the tether  310  is attached near the proximal end of the implant device  302  as previously described. The free end of the tether  310  is threaded through a distal portion of the delivery pusher  301  until it reaches an exit point (not shown) of the delivery pusher  301 . Tension is applied to the tether  310  in order to store energy in the form of elastic deformation within the tether material by, for example, placing a pre-determined force on the free end of the tether  310  or moving the taut tether  310  a pre-determined displacement. The free end of the tether  310  is then joined to the delivery pusher  301  by, for example, tying a knot, applying adhesive or similar methods known in the art. 
     When present, the release of potential energy stored in the system operates to apply additional pressure to separate the implant device  302 , and the portion of the tether  310  to which the implant device  302  is coupled, away from the heater  306  when the implant device  302  is deployed. This advantageously lowers the required detachment time and temperature by causing the tether  310  to sever and break. 
     The present invention also provides for methods of using detachment systems such as detachment systems  100 ,  200 , or  300 . The following example relates to the use of detachment system  100 ,  200 , or  300  for occluding cerebral aneurysms. It will, however, be appreciated that modifying the dimensions of the detachment system  100 ,  200 , or  300  and the component parts thereof and/or modifying the implant device  112 ,  302  configuration will allow the detachment system  100 ,  200 , or  300  to be used to treat a variety of other malformations within a body. 
     With this particular example, the delivery pusher  102 ,  202 , or  301  of the detachment system  100 ,  200 , or  300  may be approximately 0.010 inches to 0.030 inches in diameter. The tether  104 ,  310  that is coupled near the distal end of the delivery pusher  102 ,  202 , or  301  and is coupled to the implant device  112 ,  302  may be 0.0002 inches to 0.020 inches in diameter. The implant device  112 ,  302 ; which may be a coil, may be approximately 0.005 inches to 0.020 inches in diameter and may be wound from 0.0005 inch to 0.005 inch wire. 
     If potential energy is stored within the detachment system  100 ,  200 , or  300 , the force used to separate the implant  112 ,  302  from the pusher typically ranges up to 250 grams force. 
     The delivery pusher  102 ,  202 , or  301  may comprise a core mandrel  218  and at least one electrically conductive wire  108 ,  110 ,  208 ,  210 ,  308 , or  309 . The core mandrel  218  may be used as an electrical conductor, or a pair of conductive wires may be used, or a bipolar wire may be used as previously described. 
     Although the detachment systems  100 ,  200 , and  300  have been illustrated as delivering a coil, other implant devices are contemplated in the present invention. For example,  FIG. 8  illustrates the detachment system  300  as previously described in  FIG. 4  having an implant that is a stent  390 . This stent  390  could similarly be detached by a similar method as previously described in regards to the detachment systems  100 ,  200 , and  300 . In a further example, the detachment systems  100 ,  200 , or  300  may be used to deliver a filter, mesh, scaffolding or other medical implant suitable for delivery within a patient. 
       FIG. 7  presents an embodiment of a delivery pusher  350 , which could be used in any of the embodiments as delivery pusher  102 ,  202 , or  301 , which includes radiopaque materials to communicate the position of the delivery pusher  350  to the user. Specifically, the radiopaque marker material is integrated into the delivery pusher  350  and varied in thickness at a desired location, facilitating easier and more precise manufacturing of the final delivery pusher  350 . 
     Prior delivery pusher designs, such as those seen in U.S. Pat. No. 5,895,385 to Guglielmi, rely on high-density material such as gold, tantalum, tungsten, or platinum in the form of an annular band or coil. The radiopaque marker is then bonded to other, less dense materials, such as stainless steel, to differentiate the radiopaque section. Since the radiopaque marker is a separate element placed at a specified distance (often about 3 cm) from the tip of the delivery pusher, the placement must be exact or the distal tip of the delivery pusher  350  can result in damage to the aneurysm or other complications. For example, the delivery pusher  350  may be overextended from the microcatheter to puncture an aneurysm. Additionally, the manufacturing process to make a prior delivery pusher can be difficult and expensive, especially when bonding dissimilar materials. 
     The radiopaque system of the present invention overcomes these disadvantages by integrating a first radiopaque material into most of the delivery pusher  350  while varying the thickness of a second radiopaque material, thus eliminating the need to bond multiple sections together. As seen in  FIG. 7 , the delivery pusher  350  comprises a core mandrel  354  (i.e. the first radiopaque material), preferably made from radiopaque material such as tungsten, tantalum, platinum, or gold (as opposed to the mostly radiolucent materials of the prior art designs such as steel, Nitinol, and Elgiloy). 
     The delivery pusher  350  also includes a second, outer layer  352 , having a different radiopaque level. Preferably, outer layer  352  is composed of a material having a lower radiopaque value than the core mandrel  354 , such as Elgiloy, Nitinol, or stainless steel (commercially available from Fort Wayne Metals under the trade name DFT). In this respect, both the core mandrel  354  and the outer layer  352  are visible and distinguishable from each other under fluoroscopy. The outer layer  352  varies in thickness along the length of the delivery pusher  350  to provide increased flexibility and differentiation in radio-density. Thus the thicker regions of the outer layer  352  are more apparent to the user than the thinner regions under fluoroscopy. 
     The transitions in thickness of the outer layer  352  can be precisely created at desired locations with automated processes such as grinding, drawing, or forging. Such automated processes eliminate the need for hand measuring and placement of markers and further eliminates the need to bond a separate marker element to other radiolucent sections, thus reducing the manufacturing cost and complexity of the system. 
     In the present embodiment, the delivery pusher  350  includes three main indicator regions of the outer layer  352 . A proximal region  356  is the longest of the three at 137 cm, while a middle region  358  is 10 cm and a distal region  360  is 3 cm. The length of each region can be determined based on the use of the delivery pusher  350 . For example, the 3 cm distal region  360  may be used during a coil implant procedure, as known in the art, allowing the user to align the proximal edge of the distal region  360  with a radiopaque marker on the microcatheter within which the delivery pusher  350  is positioned. The diameter of each of the regions depends on the application and size of the implant. For a typical cerebral aneurysm application for example, the proximal region  356  may typically measure 0.005-0.015 inches, the middle region  358  may typically measure 0.001-0.008 inches, while the distal region  360  may typically measure 0.0005-0.010 inches. The core mandrel  354  will typically comprise between about 10-80% of the total diameter of the delivery pusher  350  at any point. 
     Alternately, the delivery pusher  350  may include any number of different regions greater than or less than the three shown in  FIG. 7 . Additionally, the radiopaque material of the core mandrel  354  may only extend partially through the delivery pusher  350 . For example, the radiopaque material could extend from the proximal end of the core mandrel  354  to three centimeters from the distal end of the delivery pusher  350 , providing yet another predetermined position marker visible under fluoroscopy. 
     In this respect, the regions  356 ,  358 , and  360  of delivery pusher  350  provide a more precise radiopaque marking system that is easily manufactured, yet is readily apparent under fluoroscopy. Further, the increased precision of the markers may decrease complications relating to improper positioning of the delivery pusher during a procedure. 
     In operation, the microcatheter is positioned within a patient so that a distal end of the microcatheter is near a target area or lumen. The delivery pusher  350  is inserted into the proximal end of the microcatheter and the core mandrel  354  and outer layer  352  are viewed under fluoroscopy. The user aligns a radiopaque marker on the microcatheter with the beginning of the distal region  360 , which communicates the location of the implant  112 ,  302  relative to the tip of the microcatheter. 
     In some situations, for example, small aneurysms where there may be an elevated risk of vessel damage from the stiffness of the delivery pusher  350 , the user may position the proximal end of the implant slightly within the distal end of the microcatheter during detachment. The user then may push the proximal end of the implant  112 ,  302  out of the microcatheter with the next coil, an adjunctive device such as guidewire, or the delivery pusher  102 ,  202 ,  301 , or  350 . In another embodiment, the user may use the radiopaque marking system to locate the distal end of the delivery pusher outside the distal end of the microcatheter. 
     Once the implant device  112 ,  302  of the detachment system  100 ,  200 , or  300  is placed in or around the target site, the operator may repeatedly reposition the implant device  112 ,  302  as necessary or desired. 
     When detachment of the implant device  112 ,  302  at the target site is desired, the operator applies energy to the heater  106 ,  206 , or  306  by way of the electrical wires  108 ,  110 ,  208 ,  210 ,  308 , or  309 . The electrical power source for the energy may be any suitable source, such as, e.g., a wall outlet, a capacitor, a battery, and the like. For one aspect of this method, electricity with a potential of approximately 1 volt to 100 volts is used to generate a current of 1 milliamp to 5000 milliamps, depending on the resistance of the detachment system  100 ,  200 , or  300 . 
     One embodiment of a connector system  400  that can be used to electrically couple the detachment system  100 ,  200 , or  300  to the power source is shown in  FIG. 6 . The connector system  400  includes an electrically conductive core mandrel  412  having a proximal end surrounded by an insulating layer  404 . Preferably the insulating layer  404  is an insulating sleeve such as a plastic shrink tube of polyolefin, PET, Nylon, PEEK, Teflon, or polyimide. The insulating layer  404  may also be a coating such as polyurethane, silicone, Teflon, paralyene. An electrically conductive band  406  is disposed on top of the insulating layer  404  and secured in place by molding bands  414 , adhesive, or epoxy. Thus, the core mandrel  412  and the conductive band  406  are electrically insulated from each other. The conductive band  406  is preferably composed of any electrically conductive material, such as silver, gold, platinum, steel, copper, conductive polymer, conductive adhesive or similar materials, and can be a band, coil, or foil. Gold is especially preferred as the conductive material of the conductive band  406  because of the ability of gold to be drawn into a thin wall and its ready availability. The core mandrel  412  has been previously described and may be plated with, for example, gold, silver, copper, or aluminum to enhance its electrical conductivity. 
     The connector system  400  also includes two electrical wires  408  and  410  which connect to the conductive band  406  and core member  412 , respectively, and to a heating element at the distal end of a delivery system such as those described in  FIGS. 1 ,  2 , and  4  (not shown in  FIG. 6 ). These wires  408  and  410  are preferably connected by soldering, brazing, welding, laser bonding, or conductive adhesive, or similar techniques. 
     Once the user is ready to release the implant  112 ,  302  within the patient, a first electrical clip or connector from a power source is connected to a non-insulated section  402  of the core mandrel  412  and a second electrical clip or connector from the power source is connected to the conductive band  406 . Electrical power is applied to the first and second electrical clips, forming an electrical circuit within the detachment system  100 ,  200 , or  300 , causing the heater  106 ,  206 , or  306  to increase in temperature and sever the tether  104 ,  310 . 
     Once the detachment system  100 ,  200 , or  300  is connected to the power source the user may apply a voltage or current as previously described. This causes the heater  106 ,  206 , or  306  to increase in temperature. When heated, the pre-tensioned tether  104 ,  310  will tend to recover to its unstressed (shorter) length due to heat-induced creep. In this respect, when the tether  104 ,  310  is heated by the heater  106 ,  206 , or  306 ; its overall size shrinks. However, since each end of the tether  104 ,  310  is fixed in place as previously described, the tether  104 ,  310  is unable to shorten in length, ultimately breaking to release the implant device  112 ,  302 . 
     Because there is tension already within the system in the form of a spring  116  or deformation of the tether material  104 ,  310 ; the amount of shrinkage required to break the tether  104 ,  310  is less than that of a system without a pre-tensioned tether. Thus, the temperature and time required to free the implant device  112 ,  302  is lower. 
       FIG. 5  is a graph showing the temperatures at the surface of the PET cover  304  of the detachment system  300 . As can be seen, the surface temperature of the detachment system  300  during detachment does not vary linearly with time. Specifically, it only takes just under 1 second for the heat generated by the heating coil  306  to penetrate the insulating cover  304 . After 1 second, the surface temperature of the insulating cover  304  dramatically increases. Although different outer insulating material may slightly increase or decrease this 1-second surface temperature window, the necessarily small diameter of the detachment system  100 ,  200 , or  300  prevents providing a thick insulating layer that may more significantly delay a surface temperature increase. 
     It should be understood that the embodiments of the detachment system  100 ,  200 , or  300  include a variety of possible constructions. For example, the insulating cover  304  may be composed of Teflon, PET, polyamide, polyimide, silicone, polyurethane, PEEK, or materials with similar characteristics. In the embodiments  100 ,  200 , or  300  the typical thickness of the insulating cover is 0.0001-0.040 inches. This thickness will tend to increase when the device is adapted for use in, for example, proximal malformations, and decrease when the device is adapted for use in more distal, tortuous locations such as, for example, cerebral aneurysms. 
     In order to minimize the damage and possible complications caused by such a surface temperature increase, the present invention detaches the implant device  112 ,  302  before the surface temperature begins to significantly increase. Preferably, the implant device  112 ,  302  is detached in less than a second, and more preferably, in less than 0.75 seconds. This prevents the surface temperature from exceeding 50° C. (1220 F), and more preferably, from exceeding 42° C. (107° F.). 
     Once the user attempts to detach the implant device  112 ,  302 , it is often necessary to confirm that the detachment has been successful. The circuitry integrated into the power source may be used to determine whether or not the detachment has been successful. In one embodiment of the present invention an initial signaling current is provided prior to applying a detachment current (i.e. current to activate the heater  106 ,  206 , or  306  to detach an implant  112 ,  302 ). The signaling current is used to determine the inductance in the system before the user attempts to detach the implant and therefore has a lower value than the detachment current, so as not to cause premature detachment. After an attempted detachment, a similar signaling current is used to determine a second inductance value that is compared to the initial inductance value. A substantial difference between the initial inductance and the second inductance value indicates that the implant  112 ,  302  has successfully been detached, while the absence of such a difference indicates unsuccessful detachment. In this respect, the user can easily determine if the implant  112 ,  302  has been detached, even for delivery systems that utilize nonconductive temperature sensitive polymers to attach an implant, such as those seen in  FIGS. 1 ,  2 , and  4 . 
     In the following description and examples, the terms “current” and “electrical current” are used in the most general sense and are understood to encompass alternating current (AC), direct current (DC), and radiofrequency current (RF) unless otherwise noted. The term “changing” is defined as any change in current with a frequency above zero, including both high frequency and low frequency. When a value is measured, calculated and/or saved, it is understood that this may be done either manually or by any known electronic method including, but not limited to, an electronic circuit, semiconductor, EPROM, computer chip, computer memory such as RAM, ROM, or flash; and the like. Finally, wire windings and toroid shapes carry a broad meaning and include a variety of geometries such as circular, elliptical, spherical, quadrilateral, triangular, and trapezoidal shapes. 
     When a changing current passes through such objects as wire windings or a toroid, it sets up a magnetic field. As the current increases or decreases, the magnetic field strength increase or decreases in the same way. This fluctuation of the magnetic field causes an effect known as inductance, which tends to oppose any further change in current. Inductance (L) in a coil wound around a core is dependant on the number of turns (N), the cross-sectional area of the core (A), the magnetic permeability of the core (μ), and length of the coil (l) according to equation 1 below: 
     
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       
                         0.4 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           N 
                           2 
                         
                         ⁢ 
                         A 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         μ 
                       
                       l 
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     The heater  106  or  306  is formed from a wound coil with proximal and distal electrically conductive wires  108 ,  110 ,  308 , or  309  attached to a power source. The tether  104 ,  310  has a magnetic permeability μ 1  and is positioned through the center of the resistive heater, having a length l, cross sectional area A, and N winds, forming a core as described in the previous equation. Prior to detachment, a changing signaling current i 1 , such as the waveforms shown in  FIGS. 3A and 3B , with frequency f 1 , is sent through the coil windings. This signaling current is generally insufficient to detach the implant. Based on the signaling current, the inductive resistance XL (i.e. the electrical resistance due to the inductance within the system) is measured by an electronic circuit such as an ohmmeter. The initial inductance of the system L 1  is then calculated according to the formula: 
     
       
         
           
             
               
                 
                   
                     L 
                     1 
                   
                   = 
                   
                     
                       
                         X 
                         L 
                       
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           f 
                           1 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     This initial value of the inductance L 1  depends on the magnetic permeability μ 1  of the core of the tether  104 ,  310  according to Equation 1, and is saved for reference. When detachment is desired, a higher current and/or a current with a different frequency than the signaling current is applied through the resistive heater coil, causing the tether  104 ,  310  to release the implant  112 ,  302  as previously described. If detachment is successful, the tether  104 ,  310  will no longer be present within the heater  106 ,  306  and the inside of the heater  106 ,  306  will fill with another material such as the patient&#39;s blood, contrast media, saline solution, or air. This material now within the heater core will have a magnetic permeability μ 2  that is different than the tether core magnetic permeability μ 1 . 
     A second signaling current and frequency f 2  is sent through the heater  106 ,  306  and is preferably the same as the first signaling current and frequency, although one or both may be different without affecting the operation of the system. Based on the second signaling current, a second inductance L 2  is calculated. If the detachment was successful, the second inductance L 2  will be different (higher or lower) than the first inductance L 1  due to the difference in the core magnetic permeabilities μ 1  and μ 2 . If the detachment was unsuccessful, the inductance values should remain relatively similar (with some tolerance for measurement error). Once detachment has been confirmed by comparing the difference between the two inductances, an alarm or signal can be activated to communicate successful detachment to the user. For example, the alarm might include a beep or an indicator light. 
     Preferably, the delivery system  100 ,  300  used according to this invention connects to a device that automatically measures inductance at desired times, performs required calculations, and signals to the user when the implant device has detached from the delivery catheter. However, it should be understood that part or all of these steps can be manually performed to achieve the same result. 
     The inductance between the attached and detached states can also preferably be determined without directly calculating the inductance. For example, the inductive resistance XL can be measured and compared before and after detachment. In another example, the detachment can be determined by measuring and comparing the time constant of the system, which is the time required for the current to reach a predetermined percentage of its nominal value. Since the time constant depends on the inductance, a change in the time constant would similarly indicate a change in inductance. 
     The present invention may also include a feedback algorithm that is used in conjunction with the detachment detection described above. For example, the algorithm automatically increases the detachment voltage or current automatically after the prior attempt fails to detach the implant device. This cycle of measurement, attempted detachment, measurement, and increased detachment voltage/current continues until detachment is detected or a predetermined current or voltage limit is attained. In this respect, a low power detachment could be first attempted, followed automatically by increased power or time until detachment has occurred. Thus, battery life for a mechanism providing the detachment power is increased while the average coil detachment time is greatly reduced. 
     Referring now to  FIGS. 9 through 12 , there is shown an embodiment of a delivery system  500  for use with the present invention that includes a detachment detection capability. The delivery system  500  operates under the principle that electrical current passing through a coil held in an expanded, open gap configuration will encounter more resistance than electrical current passing through a coil in a contracted, closed gap configuration. In the expanded configuration, the electrical current must follow the entire length of the coiled wire. In the contracted configuration, the electrical current can bridge the coils and travel in a longitudinal direction. 
     The delivery system  500 , illustrated in  FIG. 9 , is generally similar to the previously described detachment system  300  of the present invention seen in  FIG. 4 , including a delivery pusher  301 , containing a heater coil  306  that detaches an implant device  302 . The detachment system  500  similarly utilizes a tether  310  to couple the implant device  302  to the delivery pusher  301 . 
     The heater coil  306  is preferably a resistance-type heater having a plurality of loops  306 A, as seen in  FIG. 10 , that connect to a voltage source through a connector system at the proximal end of the delivery pusher  301 , such as the connector system  400  described in  FIG. 6 . 
     The delivery system  500  also includes a heater coil expander  502  that serves two functions. First, it expands the heater coil  306  such that the heater coil  306  maintains a friction-fit attachment to the inside of the insulating cover  309 , thereby connecting the two. Second, the heater coil expander  502  expands the heater coil  306  in such a manner that electricity is forced to flow around each individual loop  306 A of the coil  306  in order to maximize the resistance of the coil  306 . 
     Maximizing the coil resistance not only serves to heat the coil  306  when voltage is passed through it, it also sets an initial value (or “normal” value) for the resistance provided by the coil  306 , which can be used to compare a changed resistance state, indicating detachment of the implant  302 . Hence, the heater coil expander  502  must also be capable of undergoing change when subjected to heat. In this regard, the heater coil expander  502  may be made of any suitable sturdy material capable of holding the heater coil  306  in an expanded, biased state while also being capable of melting or being otherwise reduced by the heat of the heater coil  306  in order to yield to the bias of the heater coil  306  to return to an unbiased state. Examples of acceptable materials include, but are not limited to, polymers and monofilament. 
     It will be appreciated that the energy necessary to break the tether  310  will be predetermined and will constitute a lower energy than the predetermined energy necessary to transform or to otherwise melt or reduce the heater expander  502 . Accordingly, it will be possible determine that the tether  310  will have been broken and the implant  302  detached when the resistance of the heater coil  306  changes due to the transformation of the heater expander  502 . 
     The heater coil expander  502  shown in  FIGS. 9 and 10  operates by longitudinally, or radially and longitudinally, expanding a heater coil  306  which is normally a closed gap coil in a relaxed state. In other words, the individual loops  306 A contact each other when the heater coil  306  is not stretched or radially expanded. Preferably, the heater coil expander  502  may have a coiled shape as seen in  FIG. 10 . Alternately, the heater coil expander may have a continuous, tubular shape with helical ridges. It should be understood that a variety of different expander shapes that expand the loops  306 A of the heater coil  306  from each other are suitable for use in the present invention. 
     Preferably the power source (previously described in this embodiment and connected to the connector system  400 ) also includes a measuring instrument for measuring the resistance of the heater coil  306 . In this respect, the power source (preferably located in a hand-sized unit) includes an indicator that communicates when a change in resistance has occurred and therefore when detachment of the implant has occurred. 
     An alternative embodiment of the heater coil expander  512  is shown in  FIGS. 11 and 12 . The heater coil expander  512  operates in conjunction with the heater coil  306  so that the heater loops are in an open gap state, as shown in  FIG. 11 , and a pusher  350 , as previously described with reference to  FIG. 7 , that conducts electricity. The heater coil  306  is sized to snugly fit around the pusher  350  in a contracted state. The heater coil expander  512  operates to separate the heater coil  306  from the pusher  350 , electrically isolating the heater coil  306  therefrom, as illustrated in  FIG. 11 . As the heat from the heater coil  306  melts or otherwise reduces or degrades the heater coil expander  512 , the heater coil  306  resumes a contracted state (i.e., reduced diameter configuration), making electrical, if not physical, contact with the pusher  350 , as shown in  FIG. 12 . In this respect the individual loops are shortened, significantly reducing the resistance of the circuit and thereby indicating detachment has occurred. 
     Another alternative embodiment of the present invention, the heater coil expander  502  may be sized to expand the heater coil  306  against a conductive reinforcement circumference  312 , shown in  FIG. 9 . Hence, when the heater coil  306  is in its initial expanded position and in contact with the electrically conductive reinforcement circumference  312 , the loops  306 A of heater coil  306  are shorted and maintain a low initial resistance that is registered by the controller for the circuit (i.e., the measurement device of the power source). 
     When the heater coil  306  is energized, the initial resistance is noted and the heater coil expander  502  melts, degrades or otherwise reduces. The heater coil  306  then contracts, releasing the conductive reinforcement circumference  312 , and the implant  302 , The loop  306 A of heater coil  306  are no longer shorted out by the reinforcement circumference  312 , and the circuit experiences a change in resistance as the electrical current must travel through each of the individual loops  306 A. This increase in resistance signifies the implant  302  is detached. 
     Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. For example, the heater coil or heater coil expander could be constructed to activate a switch that provides a user indication of detachment in some manner. Additionally, a visual indicator may be associated with the change in resistance to provide easy indication of detachment. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.

Technology Category: g