Patent Publication Number: US-8974513-B2

Title: Medical implant detachment systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of pending U.S. patent application Ser. No. 12/122,636, filed May 16, 2008, which claims the benefit under 35 U.S.C. §119 of U.S. Provisional Patent Application Ser. No. 60/939,032, filed May 18, 2007. The foregoing applications are each hereby incorporated by reference into the present application in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to implantable devices (e.g., embolic coils, stents, and filters) having flexible electrolytic detachment mechanisms. 
     BACKGROUND 
     Implants may be placed in the human body for a wide variety of reasons. For example, stents are placed in a number of different anatomical lumens within the body. They may be placed in blood vessels to cover vascular lesions or to provide patency to the vessels. Stents are also placed in biliary ducts to prevent them from kinking or collapsing. Grafts may be used with stents to promote growth of endothelial tissue within those vessels. As another example, vena cava filters can be implanted in the vena cava to catch thrombus sloughed off from other sites within the body and carried to the implantation site via the blood stream. 
     As still another example, vaso-occlusive devices are used for a wide variety of reasons, including for the treatment of intravascular aneurysms. An aneurysm is a dilation of a blood vessel that poses a risk to health from the potential for rupture, clotting, or dissecting. Rupture of an aneurysm in the brain causes stroke, and rupture of an aneurysm in the abdomen causes shock. Cerebral aneurysms are usually detected in patients as the result of a seizure or hemorrhage and can result in significant morbidity or mortality. Vaso-occlusive devices can be placed within the vasculature of the human body, typically via a catheter, either to block the flow of blood through a vessel making up that portion of the vasculature through the formation of an embolus or to form such an embolus within an aneurysm stemming from the vessel. The embolus seals and fills the aneurysm, thereby preventing the weakened wall of the aneurysm from being exposed to the pulsing blood pressure of the open vascular lumen. 
     One widely used vaso-occlusive device is a helical wire coil having windings, which may be dimensioned to engage the walls of the vessels. These coils typically take the form of soft and flexible coils having diameters in the range of 10-30 mils. Multiple coils will typically be deployed within a single aneurysm. There are a variety of ways of discharging vaso-occlusive coils into the human vasculature. In addition to a variety of manners of mechanically deploying vaso-occlusive coils into the vasculature of a patient, U.S. Pat. No. 5,122,136, issued to Guglielmi et al., describes an electrolytically detachable vaso-occlusive coil that can be introduced through a microcatheter and deployed at a selected location in the vasculature of a patient. 
     This vaso-occlusive coil is attached (e.g., via welding) to the distal end of an electrically conductive pusher wire. With the exception of a sacrificial joint just proximal to the attached embolic device, the outer surface of the pusher wire is coated with an ionically non-conductive material. Thus, the sacrificial joint will be exposed to bodily fluids when deployed within the patient. A power supply is used to provide power to the core wire, with a conductive patch or intravenous needle located on or in the patient providing a ground return path. Applying a positive voltage to the pusher wire via the power supply relative to the ground return causes an electrochemical reaction between the sacrificial joint and the surrounding bodily fluid (e.g., blood). As a result, the sacrificial joint will dissolve, thereby detaching the vaso-occlusive coil from the pusher wire at the selected site. 
     While the use of electrolytically detachable vaso-occlusive coils has generally been successful, the period of time needed to detach the vaso-occlusive coils from the pusher wire is relatively long (currently, averaging from 30 to 40 seconds) and variable, resulting in an increase in procedure time. This problem is compounded by the need to deploy multiple vaso-occlusive coils within the patient. The relatively long and varying detachment time is due, in large part, to the relatively large and widely varying tissue impedance between the sacrificial joint and the ground electrode amongst patients. In addition, the bodily fluid surrounding the sacrificial joint may not be the optimum electrolyte (compared with saline) for inducing an electrochemical reaction in the detachment zone, thereby increasing the detachment time. Blood environment may also introduce variability in detachment time due to the possibility of blood clotting and the variations in blood constituents amongst patients. 
     Theoretically, the voltage of the electrical energy supplied to the sacrificial joint can be increased in order to reduce the detachment time. However, an increased voltage may cause bubbling resulting from gas generation byproducts during the electrochemical reaction, which may insulate the detachment zone adjacent the sacrificial joint from the electrolyte, thereby slowing or stopping the electrochemical reaction, and at the least, causing variability in detachment time. In addition, because gas bubbles are more likely to be contained within the sheath of the microcatheter used to deliver the vaso-occlusive coil, delivery systems are often designed, such that the sacrificial joint extends a certain distance (e.g., 1 mm) from the distal tip of the microcatheter to accommodate dimensional tolerance stackup in the pusher wire and the microcatheter. 
     Exiting the microcatheter this far, however, degrades kickback performance (i.e., reaction of the microcatheter in response to detachment of the vaso-occlusive coil is to be minimized) due to the stiffness of the distal end of the pusher wire relative to the stiffness of the vaso-occlusive coil. In addition, locating the sacrificial joint this far from the distal tip of the microcatheter may cause it to come into contact with previously deployed vaso-occlusive coils, thereby shorting the sacrificial joint through the coils, resulting in an increase and/or variation in the detachment time. Notwithstanding the bubbling issue, it may sometimes be difficult to ascertain that the sacrificial joint is in contact with the blood, which must occur to initiate the electrochemical reaction and subsequent detachment of the vaso-occlusive coil. 
     SUMMARY OF THE INVENTION 
     In accordance with a one aspect of the present inventions, an implant assembly comprises an elongated pusher member, and an implantable device (e.g., a vaso-occlusive device) mounted to the distal end of the pusher member. The implant assembly further comprises an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed. The implant assembly further comprises a return electrode carried by the distal end of the pusher member in proximity to, but electrically isolated from, the severable joint. For example, the return electrode may take the form of a coil disposed about the pusher member. The return electrode may be carried by the pusher member in such a manner that it remains with the implantable device or remains the pusher member when the severable joint is severed. The implant assembly further comprises a terminal carried by the proximal end of the pusher member in electrical communication with the severable joint. The use of a return electrode on the pusher member decreases the effective distance between the anodic severable joint and cathodic return electrode, thereby decreasing the detachment time and increasing the reliability, repeatability, and uniformity of the detachment process. 
     In one embodiment, the implant assembly further comprises another terminal carried by the proximal end of the pusher member in electrical communication with the return electrode. In another embodiment, the terminal in electrical communication with the severable joint is the only terminal carried by the proximal end of the pusher member. In an optional embodiment, one or both of the severable joint and return electrode comprise silver chloride in order to facilitate an electrolytic reaction between the severable joint and return electrode. For example, the severable joint and/or return electrode may comprise a silver core and a silver chloride coating. In another embodiment, the pusher member comprises an electrically conductive stiffening member through which the terminal and the severable joint are in electrical communication. 
     In accordance with another aspect of the present inventions, a medical system comprises an implant assembly that includes an elongated pusher member, an implantable device mounted to the distal end of the pusher member, an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed, and a return electrode carried by the distal end of the pusher member in proximity to, but electrically isolated from, the severable joint. The detailed features of the implant assembly can be similar to those described above. The medical system further comprises an electrical power supply having a terminal electrically coupled to the severable joint; for example, via a terminal carried by the proximal end of the pusher member and/or an electrically conductive stiffening member of the pusher member. 
     In some embodiments, the power supply has another terminal electrically coupled to the return electrode (e.g., via another terminal carried by the proximal end of the pusher member) or electrically coupled to a ground electrode that is separate from the return electrode. In one embodiment, the power supply is configured for delivering direct current to the implant assembly. In another embodiment, the medical system further comprises a delivery catheter configured for slidably receiving the implant assembly. 
     In accordance with other aspect of the present inventions, a method of implanting a medical device (e.g., a vaso-occlusive device) within a patient is provided. The method comprises introducing the medical device within the patient via a pusher member (e.g., through a delivery catheter), conveying electrical energy (e.g., direct electrical current) to a joint disposed on the pusher member, and conveying electrical energy from a return electrode carried by the pusher member (e.g., a coil disposed about the pusher member) to induce an electrolytic reaction between the joint and the return electrode. As a result of the electrolytic reaction, the joint is severed to detach the medical device from the pusher member at a target site (e.g., an aneurismal sac) within the patient. The return electrode may, e.g., remain with the medical device when the joint is severed or remain with the pusher member when the joint is severed. 
     In one method, the electrolytic reaction comprises releasing chloride ions from the return electrode. The electrical energy may be conveyed to the joint via the pusher member, and the electrical energy may be conveyed from the return electrode via the pusher member or from the return electrode to a ground electrode via the tissue of the patient. In another method, the pusher member is removed from the patient. 
     In accordance with another aspect of the present inventions, an implant assembly comprises an elongated pusher member that has a stiffening member and an electrically conductive sheath (e.g., a coil, mesh, or braid) disposed over the stiffening member. The stiffening member may be composed of a suitable material, such as stainless steel, and the electrically conductive sheath may be composed of a suitable material, such as copper or silver. The implant assembly further comprises an implantable device (e.g., a vaso-occlusive device) mounted to the distal end of the pusher member. The implant assembly further comprises an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed. The implant assembly further comprises a return electrode carried (e.g., mounted) by the distal end of the pusher member in proximity to, but electrically isolated from, the severable joint. In one embodiment, the return electrode takes the form of a coil disposed about the pusher member. The return electrode may be carried by the pusher member in such a manner that it remains with the implantable device or remains the pusher member when the severable joint is severed. 
     The implant assembly further comprises a terminal carried by the proximal end of the pusher member in electrical communication with the severable joint. The implant assembly further comprises an electrically conductive path extending between the terminal and one of the severable joint and the return electrode, wherein the electrically conductive path includes the electrically conductive sheath. The use of the electrically conductive sheath increases the conductance of the electrically conductive path between the terminal and the severable joint or return electrode, as compared to the case where a standard electrical conductor or the stiffening member is used without the electrically conductive sheath. 
     In one embodiment, the electrically conductive path extends between the terminal and the return electrode, in which case, the electrically conductive sheath and stiffening member are electrically isolated from each other. For example, the stiffening member may comprise an electrically conductive core wire and an electrically insulative coating disposed over the core wire, wherein the electrically insulative sheath is disposed over the electrically insulative coating. In another embodiment, the stiffening member has a proximal section having a first diameter and a distal section having a second decreased diameter, in which case, the electrically conductive path only extends between the terminal and the severable joint along the distal section of the stiffening member. 
     In another embodiment, the implant assembly further comprises another terminal carried by the proximal end of the pusher member in electrical communication with the return electrode, and the pusher member includes another electrically conductive sheath disposed over the stiffening member. In this case, the implant assembly may further comprise another electrically conductive path extending between the other terminal and another of the severable joint and the return electrode, such that the other electrically conductive path includes the other electrically conductive sheath. 
     In accordance with a further aspect of the present inventions, another implant assembly is provided. The implant assembly comprises an elongated pusher member, an implantable device mounted to the distal end of the pusher member, an electrolytically severable joint disposed on the pusher member, a terminal carried by the proximal end of the pusher member, and the implant assembly further comprises an electrically conductive path extending between the terminal and the severable joint, wherein the electrically conductive path includes the electrically conductive sheath. The features of the implant assembly can be the same as those described above. 
     In accordance with still another aspect of the present inventions, a medical system comprises either of the implant assemblies described above, and an electrical power supply having a terminal electrically coupled to the terminal of the implant assembly. If a return electrode is provided on the implant assembly, the medical system may be configured in one of two manners. In one example, the medical system further comprises a ground electrode separate from the return electrode, in which case, the power supply has another terminal electrically coupled to the ground electrode. In another example, the implant assembly has another terminal in electrical communication with the return electrode, in which case, the power supply has another terminal electrically coupled to the other terminal of the implant assembly. In an optional embodiment, the medical system further comprises a delivery catheter configured for slidably receiving the implant assembly. 
     In accordance with still another aspect of the present inventions, an implant assembly comprises an elongated pusher member, and an implantable device (e.g., a vaso-occlusive device) mounted to the distal end of the pusher member. The implant assembly further comprises an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed. The implant assembly further comprises a terminal carried by the proximal end of the pusher member in electrical communication with the severable joint. In one embodiment, the pusher member comprises an electrically conductive stiffening member through which the terminal and the severable joint are in electrical communication. 
     The implant assembly further comprises a return electrode carried by the distal end of the pusher member. In one embodiment, the return electrodes takes the form of a coil disposed about the pusher member. The return electrode is electrically isolated from the severable joint and is configured to remain with the pusher member when the severable joint is severed. Although the present inventions should not be so limited in their broadest aspects, the return electrode need not be composed of more expensive and electrically limiting biocompatible materials suitable for chronic implantation, since the return electrode remains with the pusher member. In one embodiment, the implant assembly further comprises another terminal carried by the proximal end of the pusher member in electrical communication with the return electrode. In another embodiment, the terminal in electrical communication with the severable joint is the only terminal carried by the proximal end of the pusher member. 
     In accordance with yet another aspect of the present inventions, a medical system comprises an implant assembly that includes an elongated pusher member, an implantable device mounted to the distal end of the pusher member, an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed, and a return electrode carried by the distal end of the pusher member. The detailed features of the implant assembly can be similar to those described above. The medical system further comprises an electrical power supply having a terminal electrically coupled to the severable joint; for example, via a terminal carried by the proximal end of the pusher member and/or an electrically conductive stiffening member of the pusher member. 
     In accordance with yet another aspect of the present inventions, a method of implanting a medical device (e.g., a vaso-occlusive device) within a patient is provided. The method comprises introducing the medical device within the patient via a pusher member (e.g., through a delivery catheter), conveying electrical energy (e.g., direct electrical current) to a joint disposed on the pusher member, and conveying electrical energy from a return electrode carried by the pusher member (e.g., a coil disposed about the pusher member) to induce an electrolytic reaction between the joint and the return electrode. As a result of the electrolytic reaction, the joint is severed to detach the medical device from the pusher member at a target site (e.g., an aneurismal sac) within the patient. The return electrode remains with the pusher member when the joint is severed. The electrical energy may be conveyed to the joint via the pusher member, and the electrical energy may be conveyed from the return electrode via the pusher member or from the return electrode to a ground electrode via the tissue of the patient. In one method, the pusher member is removed from the patient. 
     In accordance with yet another aspect of the present inventions, an implant assembly comprises an elongated pusher member, and an implantable device (e.g., a vaso-occlusive device) mounted to the distal end of the pusher member. The implant assembly further comprises an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed. The implant assembly further comprises a terminal carried by the proximal end of the pusher member in electrical communication with the severable joint. In one embodiment, the pusher member comprises an electrically conductive stiffening member through which the terminal and the severable joint are in electrical communication. 
     The implant assembly further comprises a return electrode carried by the distal end of the pusher member, and electrically isolated from the severable joint. The return electrode may be carried by the pusher member in such a manner that it remains with the implantable device or remains the pusher member when the severable joint is severed. 
     The implant assembly further comprises an electrically insulative sheath (e.g., one composed of a polymeric material) fixably coupled to the pusher member and circumferentially surrounding the severable joint and the return electrode. In one embodiment, the return electrode circumferentially extends around the severable joint, and the insulative sheath is disposed about the return electrode. In this case, the return electrode may be, e.g., a coil or a continuous cylinder. The implant assembly may comprise an electrically insulative spacer mounted to the distal end of the pusher member to prevent contact between the severable joint and the return electrode. 
     In one embodiment, the insulative sheath is configured to prevent diffusion of an electrolyte from a detachment region between the severable joint and the return electrode. In this manner, the electrically insulative sheath may maintain the ideal electrolytic environment within the detachment region between the severable joint and the return electrode in order to facilitate detachment of the implant assembly. In one embodiment, one or both of the severable joint and the return electrode has a hydrophilic coating, so as to, e.g., facilitate wicking of an electrolyte within the detachment region when desired. 
     In accordance with yet another aspect of the present inventions, a medical system comprises an implant assembly that includes an elongated pusher member, an implantable device mounted to the distal end of the pusher member, an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed, a return electrode carried by the distal end of the pusher member, and an electrically insulative sheath fixably coupled to the pusher member and circumferentially surrounding the severable joint and the return electrode. The detailed features of the implant assembly can be similar to those described above in other embodiments. The medical system further comprises an electrical power supply having terminal electrically coupled to the severable joint; for example, via a terminal carried by the proximal end of the pusher member and/or an electrically conductive stiffening member of the pusher member. 
     In accordance with still another aspect of the present inventions, a method of implanting a medical device (e.g., a vaso-occlusive device) within a patient using a pusher member is provided. A joint is disposed on the pusher member and a return electrode is carried by the pusher member. The method comprises introducing an electrolyte within a detachment region between the joint and the return electrode. For example, the electrolyte may be wicked into the detachment region. In one method, the electrolyte is introduced within the detachment region before the medical device is introduced into the patient. 
     The method further comprises introducing the medical device within the patient via a pusher member (e.g., through a delivery catheter), and substantially preventing the electrolyte from being diffused away from the detachment region using an electrically insulative sheath. In one embodiment, the insulative sheath is fixably coupled to the pusher member. The method further comprises conveying electrical energy (e.g., direct electrical current) to a joint disposed on the pusher member, and conveying electrical energy from a return electrode carried by the pusher member (e.g., a coil disposed about the pusher member) to induce an electrolytic reaction between the joint and the return electrode. As a result of the electrolytic reaction, the joint is severed to detach the medical device from the pusher member at a target site (e.g., an aneurismal sac) within the patient. In one method, the pusher member is removed from the patient. 
     In accordance with yet another aspect of the present inventions, a medical system comprises an implant assembly that includes an elongated pusher member, an implantable device (e.g., a vaso-occlusive device) mounted to the distal end of the pusher member, and an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed. The medical system further comprises an electrical power supply coupled to the implant assembly, the power supply configured for conveying pulsed electrical energy (e.g., direct electrical current) to the severable joint. By way of non-limiting example, the pulsed electrical energy may have a duty cycle within the range of 5 percent to 20 percent, and a frequency in the range of 5 KHz to 20 KHz. Pulsing the electrical energy delivered to the severable joint will tend to decrease the detachment time and increase the reliability, repeatability, and uniformity of the detachment process. 
     In one embodiment, the implant assembly further comprises a terminal carried by the proximal end of the pusher member in electrical communication with the severable joint, wherein the terminal of the power supply is electrically coupled to the terminal of the implant assembly. In another embodiment, the power supply has another terminal electrically coupled to a return electrode, which may be carried by the pusher member. The terminals of the power supply have different electrical potentials. 
     In one embodiment, the power supply includes a constant current source for conveying the electrical energy, e.g., at an amplitude within the range of 0.25 mA to 10 mA. In another embodiment, the power supply includes a constant voltage source for conveying the electrical energy, e.g., at an amplitude within the range of 0.5V to 11V. In an optional embodiment, the power supply includes a constant current source, a constant voltage source, and a controller configured for initially conveying the electrical energy from the constant current source, and subsequently conveying the electrical energy from the constant voltage source. In another embodiment, the medical system comprises a delivery catheter configured for slidably receiving the implant assembly. 
     In accordance with a further aspect of the present inventions, a method of implanting a medical device (e.g., a vaso-occlusive device) within a patient is provided. The method comprises introducing the medical device within the patient via a pusher member (e.g., through a delivery catheter), and conveying pulsed electrical energy (e.g., direct electrical current) to a joint disposed on the pusher member to induce an electrolytic reaction at the joint. By way of non-limiting example, the pulsed electrical energy may have a duty cycle within the range of 5 percent to 20 percent, and a frequency in the range of 5 KHz to 20 KHz. As a result of the electrolytic reaction, the joint is severed to detach the medical device from the pusher member at a target site (e.g., an aneurismal sac) within the patient. 
     In one method, the electrical energy is conveyed to the joint via the pusher member. An optional method comprises conveying pulsed electrical energy from a return electrode (e.g., one carried by the pusher member) to induce the electrolytic reaction between the joint and the return electrode. In one method, the electrical energy is conveyed to the joint from a constant current source, e.g., one having an amplitude within the range of 0.25 mA to 10 mA. In another method, the electrical energy is conveyed to the joint from a voltage source, e.g., one having a voltage within the range of 0.5V to 11V. In an optional method, the pulsed electrical energy is initially conveyed to the joint from a constant current source, and subsequently conveyed to the joint from a constant voltage source. In another method, the pusher member is removed from the patient. 
     In accordance with still further aspects of the present inventions, a medical system is provided, which comprises an implant assembly that includes an elongated pusher member, an implantable device (e.g., a vaso-occlusive device) mounted to the distal end of the pusher member, and an electrolytically severable joint disposed on the pusher member, wherein the implantable device detaches from the pusher member when the severable joint is severed. The medical system further comprises an electrical power supply coupled to the implant assembly. The power supply includes a constant current source (e.g., one having an amplitude in the range of 0.25 mA to 10 mA), a constant voltage source (e.g., one having an amplitude in the range of 0.5V to 11V), and a controller configured for conveying electrical energy from the constant current source to the severable joint (e.g., for a time period in the range of 0.5 seconds to 1 second), and subsequently conveying electrical energy from the constant voltage source to the severable joint. The electrical energy may be, e.g., direct electric current. 
     The initial electrical energy from the constant current source may quickly break through the oxide layer on the severable joint, whereas the electrical energy from the constant voltage source may minimize bubbling at the detachment region, thereby decreasing the detachment time and increasing the reliability, repeatability, and uniformity of the detachment process. 
     In one embodiment, the implant assembly further comprises a terminal carried by the proximal end of the pusher member in electrical communication with the severable joint, wherein the terminal of the power supply is electrically coupled to the terminal of the implant assembly. In another embodiment, the power supply has another terminal electrically coupled to a return electrode, which may be carried by the pusher member. The terminals of the power supply have different electrical potentials. In another embodiment, the medical system comprises a delivery catheter configured for slidably receiving the implant assembly. 
     In accordance with a yet another aspect of the present inventions, a method of implanting a medical device (e.g., a vaso-occlusive device) within a patient is provided. The method comprises introducing the medical device within the patient via a pusher member (e.g., through a delivery catheter), conveying electrical energy from a constant current source to a joint disposed on the pusher member to degrade an oxide layer on the joint, and subsequently conveying electrical energy from a constant voltage source to the joint to induce an electrolytic reaction at the joint. The electrical energy may be, e.g., direct electrical current. As a result of the electrolytic reaction, the joint is severed to detach the medical device from the pusher member at a target site (e.g., an aneurismal sac) within the patient. 
     In one method, the electrical energy is conveyed to the joint via the pusher member. An optional method comprises conveying electrical energy from a return electrode (e.g., one carried by the pusher member) to induce the electrolytic reaction between the joint and the return electrode. In another method, the pusher member is removed from the patient. 
     Other and further aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The drawings illustrate the design and utility of preferred embodiment(s) of the invention, in which similar elements are referred to by common reference numerals. In order to better appreciate the advantages and objects of the invention, reference should be made to the accompanying drawings that illustrate the preferred embodiment(s). The drawings, however, depict the embodiment(s) of the invention, and should not be taken as limiting its scope. With this caveat, the embodiment(s) of the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a plan view of a medical system arranged in accordance with one embodiment of the present invention, wherein the medical system particularly delivers a vaso-occlusive device into a patient using a bipolar electrolytic delivery means; 
         FIG. 2  is a plan view of a medical system arranged in accordance with another embodiment of the present inventions, wherein the medical system particularly delivers a vaso-occlusive device into a patient using a monopolar electrolytic delivery means; 
         FIG. 3  is a block diagram of an optional power supply that can be used in either of the medical systems of  FIGS. 1 and 2 ; 
         FIG. 4  is a perspective view of one embodiment of a vaso-occlusive device that can be delivered in either of the medical systems of  FIGS. 1 and 2 ; 
         FIG. 5  is a cross-sectional view of one embodiment a bipolar implant assembly that can be used in the medical system of  FIG. 1 ; 
         FIG. 6  is a cross-sectional view of one embodiment a monopolar implant assembly that can be used in the medical system of  FIG. 2 ; 
         FIG. 7  is a cross-sectional view of another embodiment a bipolar implant assembly that can be used in the medical system of  FIG. 1 ; 
         FIG. 8  is a cross-sectional view of another embodiment a monopolar implant assembly that can be used in the medical system of  FIG. 2 ; 
         FIG. 9  is a diagram illustrating relative voltage differences in a monopolar arrangements that utilizes an intermediate return electrode; 
         FIG. 10  is a cross-sectional view of another embodiment a bipolar implant assembly that can be used in the medical system of  FIG. 1 ; 
         FIG. 11  is a cross-sectional view of still another embodiment a bipolar implant assembly that can be used in the medical system of  FIG. 1 ; 
         FIG. 12  is a cross-sectional view of yet another embodiment a bipolar implant assembly that can be used in the medical system of  FIG. 1 ; and 
         FIGS. 13A-13C  are cross-sectional views illustrating a method of delivering a vaso-occlusive device within an aneurysm of the patient utilizing the medical systems of  FIG. 1  or  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Referring generally to  FIGS. 1 and 2 , a medical system  10  constructed in accordance with one embodiment of the present inventions will be described. The medical system  10  is used in vascular and neurovascular indications, and particularly in the treatment of aneurysms, such as cerebral aneurysms. The medical system  10  utilizes an electrolytic detachment means to deploy vaso-occlusive devices, such as helical coils, within an aneurysm. Alternatively, the medical system  10  can be utilized to deploy implantable devices other than vaso-occlusive devices. For example, the medical system  10  can alternatively be used to deploy stents and vena cava filters, which are described in further detail in U.S. Pat. No. 6,468,266, which is expressly incorporated herein by reference. 
     To this end, the medical system  10  generally comprises a delivery catheter  12  that can be intravenously introduced within a patient to access a target site within the vasculature, an implant assembly  14  that can be slidably disposed within the delivery catheter  12 , and an electrical power supply  16  that can supply electrical energy to the implant assembly  14  to effect the electrolytic detachment process. 
     Various types of implant assemblies  14  will be described herein, all of which include a pusher member  18 , an electrolytically severable joint  20 , and a detachable vaso-occlusive implant  22  mounted to the distal end of the pusher member  18 . As will be described in further detail below, the vaso-occlusive implant  22  detaches from the pusher member  18  when the joint  20  is electrolytically severed. 
     Some of the implant assemblies  14  described herein use bipolar electrolytic means to detach the vaso-occlusive implant  22  from the pusher member  18  at the severable joint  20 , and others use monopolar electrolytic means to detach the vaso-occlusive implant  22  from the pusher member  18  at the severable joint  20 . In the bipolar cases (shown specifically in  FIG. 1 ), the implant assembly  14  includes positive and negative terminals  28 ,  30  disposed on the proximal end  24  of the pusher member  18 , and a return (ground) electrode (not shown in  FIG. 1 ) carried by the distal end  26  of the pusher member  18 . The positive terminal  28  is electrically coupled to the severable joint  20 , whereas the negative terminal  30  is electrically coupled to the return electrode. In the monopolar case (shown specifically in  FIG. 2 ), the implant assembly  14  includes a single terminal  28  disposed on the proximal end  24  of the pusher member  18 . In this case, the system  10  includes a return electrode  32  in the form of a ground patch electrode or ground needle electrode, and an optional intermediate return electrode (not shown in  FIG. 2 ) carried by the distal end  26  of the pusher member  18 . In either of the monopolar or bipolar arrangements, the severable joint  20  serves as an anode, and the return electrode or ground electrode serves as a cathode. 
     Notably, because of the close proximity of the severable joint  20  and return electrode in the bipolar case, there is a greater chance that the return electrode will induce gas bubbling that will adversely effect the detachment process. That is, greater volumes of bubbling at the return electrode that can displace electrolyte may be created, thus insulating the return path, and thereby causing a variable return electrode impedance (changing voltage drop at the severable joint  20 ). As will be described in further detail below, the pusher member  18  can be provided with various features that prevent or minimize such bubbling, so that the full advantages of a bipolar configuration can be achieved. These features can likewise be used in the monopolar configuration as well to reduce the chance of gas bubbling. 
     The power supply  16  conveys electrical energy to the implant assembly  14  (and in particular, the severable joint  20 ) and returns electrical energy either from the implant assembly  14  (and in particular, the return electrode) or the ground electrode, to effect the electrolytic detachment of the vaso-occlusive implant  22 . To this end, the power supply  16  has a positive terminal  34  configured to mate with the positive terminal  28  of the implant assembly  14  via a cable  38 , and a negative terminal  36  configured to mate with the negative terminal  30  of the implant assembly  14  ( FIG. 1 ) or the ground electrode  32  ( FIG. 2 ) via a cable  40 . Alternatively, in the case of a monopolar arrangement, the positive terminal  24  of the implant assembly  14  is mated directly to the positive terminal  34  of the power supply  16 , and in the case of a bipolar arrangement, the positive and negative terminals  28 ,  30  of the implant assembly  14  are mated directly to the positive and negative terminals  34 ,  36  (which may be configured in a front-to-back relationship instead of the side-by-side relationship illustrated in  FIG. 2 ) of the power supply  16 . For the purposes of this specification, the terms “positive” and “negative” with respect to a terminal is relative and merely means that the positive terminal has a greater voltage potential than that of the negative terminal. 
     In a monopolar arrangement, the power supply  16  preferably includes a constant current source (not shown in  FIGS. 1 and 2 ) from which the electrical energy is conveyed. In this manner, the detachment times are not affected by the widely varying tissue impedances between the remotely positioned severable joint  20  and ground electrode  32  amongst different patients. A suitable amplitude range for the constant current source is between 0.25 mA and 10 mA. The bipolar arrangement is particularly advantageous, because the varying tissue impedance will not adversely affect the detachment time due to the close proximity between the severable joint  20  and return electrode. As such, the power supply  16  preferably includes a constant voltage source (not shown in  FIGS. 1 and 2 ), which results in a predictable return path voltage drop that avoids over-driving the voltage at the severable joint  20  (anode), which may otherwise cause gas generation (i.e., bubbling). A suitable amplitude range for the constant voltage source is between 0.5V and 11V. 
     In either of the bipolar configuration or monopolar configuration, the electrical energy takes the form of continuous direct electrical energy; that is, electrical energy that continually flows in one direction only. In an optional embodiment, the power supply  16  is configured to pulse the direct electrical energy supplied by the constant current source or constant voltage source. It has been discovered that pulsing the electrical energy eliminates or minimizes bubbling at the detachment zone. A suitable frequency range and duty cycle for pulsing the electrical energy is 5 KHz to 20 KHz and 5% to 20%, respectively. 
     In an optional embodiment illustrated in  FIG. 3 , the power supply  16  includes both of a constant current source  42  and a constant voltage source  44  coupled to a radio frequency (RF) oscillator  46 , and a controller  48  for initially conveying the electrical energy from the constant current source  42  and subsequently conveying the electrical energy from the voltage current source  44 ; that is, by selectively coupling the constant current source  42  and constant voltage source  44  to the positive terminal  34  via switch  50 . This option works best in a bipolar arrangement, wherein the electrical energy can be delivered from the constant current source  42  to quickly break through the oxide layer on the severable joint  20  for a certain time period (e.g., 0.5 s to 1.0 s), and then the electrical energy can be delivered from the constant voltage source  44  to minimize bubbling at the detachment zone. 
     Referring back to  FIGS. 1 and 2 , the delivery catheter  12  includes an elongate, flexible, tubular member  52  composed of a suitable polymeric material and optionally reinforced with a coil or braid to provide strength or obviate kinking propensities. The delivery catheter  12  further includes a lumen (not shown) through which the implant assembly  14  can be selectively located. The delivery catheter  12  further includes a pair of radiopaque markers  58  disposed on the distal end  54  of the tubular member  52  to allow visualization of the delivery catheter  12  relative to the vaso-occlusive implant  22 . The delivery catheter  12  further includes a proximal fitting  60  disposed on the proximal end  56  of the tubular member  52  for introduction of the implant assembly  14 , as well as for the optional introduction of dyes or treatment materials. 
     Referring to  FIG. 4 , the vaso-occlusive implant  22  is standard and comprises a helically wound primary coil  62  having a proximal end  64 , a distal end  66 , and a lumen  68  extending therethrough. The materials used in constructing the primary coil  62  may be any of a wide variety of materials, and preferably, a radio-opaque material such as a metal or a polymer. Suitable metals and alloys for the wire making up the coil include super-elastic alloy, such as titanium/nickel alloy, known as “nitinol”, or include Platinum Group metals, especially platinum, rhodium, palladium, rhenium, as well as tungsten, gold, silver, tantalum, and alloys of these metals. In addition to being largely biologically inert, these metals have significant radio-opacity and their alloys may be tailored to accomplish an appropriate blend of flexibility and stiffness. Highly preferred is a platinum/tungsten alloy, e.g., 8% tungsten and the remainder platinum. 
     The primary coil  62  may also be made of radiolucent fibers or polymers (or metallic threads coated with radiolucent or radio-opaque fibers) such as Dacron (polyester), polyglycolic acid, polylactic acid, fluoropolymers (polytetrafluoroethylene), Nylon (polyamide), or even cotton or silk. If a polymer is used as the major component of the primary coil  62 , it is desirably filled with some amount of radio-opaque material, such as powdered tantalum, powdered tungsten, bismuth oxide, barium sulfate, and the like. 
     The primary coil  62  may generally be composed of a wire having a diameter in the range of 0.0025 inches to 0.006 inches, which is then wound into a primary form having a diameter between 0.003 inches and 0.025 inches. But for most neurovascular applications, a diameter between 0.008 to 0.018 inches provides sufficient hoop strength to hold the primary coil  62  in place within the chosen body site, lumen, or cavity, without substantially distending the wall of the site and without moving from the site as a result of the repetitive fluid pulsing found in the vascular system. The axial length of the primary coil  62  will usually fall in the range of 0.5 cm to 100 cm, more usually 2 cm to 40 cm. Depending on the usage, the primary coil  62  may have 10-75 turns per centimeter, preferably 10-40 turns per centimeter. All of the dimensions here are provided only as guidelines, and the invention, when applied to vaso-occlusive devices, should not be limited thereto. Only dimensions that are suitable for use in occluding sites within the human body, however, are included in the scope of this invention as applied to vaso-occlusive devices. 
     Depending on the desired therapeutic effect and the shape of the site to be treated, the primary coil  62  may later be treated or accessorized in numerous ways in order to enhance its therapeutic effect. The primary coil  62  may be made to form various secondary shapes, often through the use of heat treatment, that may be better suited to fill a particular treatment site, as disclosed in U.S. Pat. Nos. 5,853,418 and 6,280,457, the entireties of which are hereby expressly incorporated herein by reference. Alternatively, the primary coil  62  may have little or no shape after introduction into the vascular space, as disclosed in U.S. Pat. No. 5,690,666, the entirety of which is hereby expressly incorporated herein by reference. In addition, external materials may be added to the outside of the primary coil  62  in an effort to increase its thrombolytic properties. These alternative embodiments are disclosed in U.S. Pat. Nos. 5,226,911; 5,304,194; 5,549,624; and 5,382,259; the entireties of which are hereby expressly incorporated herein by reference, and 6,280,457, the entirety of which has previously been incorporated by reference. 
     The vaso-occlusive implant  22  further includes a stretch-resisting filament  70 , which extends through the coil lumen  68  and is secured to the primary coil  62  at two locations to prevent axial stretching of the primary coil  62  in the event that the pusher member  18  must be withdrawn or repositioned to change the position of the vaso-occlusive implant  22 . Specifically, the proximal and distal ends of the stretch-resisting filament  70  are respectively affixed to the proximal and distal ends  64 ,  66  of the primary coil  62 . Alternatively, the stretch-resisting filament  70  only extends through a portion of the lumen  68  and is attached to the primary coil  62  at a location between the proximal and distal ends  64 ,  66  of the primary coil  62 . 
     The distal end of the stretch-resisting filament  70  may be secured to the primary coil  62  by melting, gluing, or otherwise fixedly attaching the stretch-resisting filament  70  to the primary coil  62 , either at the distal end  66  or some location between the proximal and distal ends  64 ,  66  of the primary coil  62 . In the illustrated embodiment, the distal end of the stretch-resisting filament  70  is glued or melted and reformed into a distal cap  72 , the diameter of which is larger than the inner diameter of the primary coil  62 . Alternatively, the stretch-resisting filament  70  may be tied in a knot (not shown), which may or may not be attached to the primary coil  62 . These methods of attachment are disclosed in more detail in U.S. Pat. No. 5,582,619, the entirety of which is hereby expressly incorporated herein by reference. 
     In a preferred embodiment, the stretch-resisting filament  70  is fibrous and desirably polymeric. Suitable polymeric materials can be either thermosetting or thermoplastic and can comprise a bundle of threads or a single filament. Thermoplastics are preferred because they allow simplification of the procedure for constructing the assembly, since they may be melted and formed into the distal cap  72 . Simple tools, such as soldering irons, may be used to form the distal cap  72 . Thermosetting plastics would typically be held in place by an adhesive. Suitable polymers include most biocompatible materials that may be made into fibers, including thermoplastics, e.g., polyesters such as polyethyleneterephthalate (PET), especially Dacron; polyamides, including the Nylons; polyolefins, such as polyethylene, polypropylene, polybutylene, their mixtures, alloys, block, and random copolymers; polyglycolic acid; polylactic acid; fluoropolymers (polytetrafluoroethylene) or even silk or collagen. The stretch-resisting polymer may be made from materials used as dissolvable sutures, for instance, polylactic acid or polyglycolic acid, to encourage cell growth in the aneurysm after their introduction. Highly preferred is polypropylene, for instance, in the form of 10-0 and 9-0 polypropylene suture material. The diameter of the polymer is typically between about 0.0001 inches and about 0.01 inches. 
     The vaso-occlusive implant  22  further includes an anchor coil  74  coaxially situated in the coil lumen  68 . The anchor coil  74  is preferably soldered or welded to the inner surface of the primary coil  62 . In the illustrated embodiment, the anchor coil  74  is preferably less than 2.6 mm long, preferably about 1.0 mm long. The anchor coil  74  has a distal hook  76  to which the stretch-resisting filament  70  is attached. The anchor coil  74  may be composed of the same material as the primary coil  62 . The vaso-occlusive implant  22  further includes a polymeric plug  78  that is slipped over the distal end of the pusher member  18  and into the proximal end  64  of the primary coil  62 . The assembled joint is then heated, so as to allow the thermoplastic of the polymeric plug  78  to flow and secure the primary coil  62  to the pusher member  18 . 
     Referring now to  FIG. 5 , one embodiment of a bipolar implant assembly  14 ( 1 ) will now be described. The bipolar implant assembly  14 ( 1 ) comprises the previously described vaso-occlusive implant  22  and a pusher member  18 ( 1 ). The pusher member  18 ( 1 ) comprises an elongated stiffening member  80 , which includes an electrically conductive coil wire and an electrically insulative coating disposed over the core wire. The core wire of the stiffening member  80  can be composed of any suitable electrically conductive and rigid material, such as stainless steel, and the coating can be composed of any suitable electrically insulative material, such as polyimide, polytetrafluoroethylene (PTFE), tetrafluoroethylene (TFE), polyparaxylxylene (e.g., Parylene), polyethyleneterephthalate (PET), polybutyleneterephthalate (PBT), cyanoacrylate adhesives, or other suitable insulating layer. 
     In the illustrated embodiment, the stiffening member  80  tapers from a large diameter section  81  to a small diameter section  83 . The core wire of the stiffening member  80  can be ground to effect this taper. In the illustrated embodiment, the diameter of the core wire in the large diameter section  81  of the stiffening member  80  is 0.004 inches, and the diameter of the core wire in the small diameter section  83  is 0.0025 inches. The insulative coating may have a suitable thickness (e.g., 0.00035 inches). Notably, the large diameter section  81  of the stiffening member  80  provides the pusher member  18 ( 1 ) with lateral rigidity, as well as tensile strength, whereas the small diameter section  83  of the stiffening member  80  provides the pusher member  18 ( 1 ) with the desired lateral flexibility adjacent the vaso-occlusive implant  22  to minimize kickback during detachment of the vaso-occlusive implant  22 . 
     A distal region of the core wire at the small diameter section  83  is either not coated with the insulative coating or a portion of the insulative coating is removed (e.g., using laser ablation) to expose a portion of the core wire, thereby forming the electrolytic severable joint  20 , which serves as the anode of the bipolar implant assembly  18 ( 1 ). Preferably, the length of the severable joint  20  is relatively short (e.g., 0.002 inches). As a result, the severable joint  20  has a narrow range of circumferential contact with the electrolyte, so that the dissolution of the core wire will be limited to a narrow circumferential band, rather than a broad one, thereby resulting in a quicker erosion through the thickness of the core wire. 
     The pusher member  18 ( 1 ) further comprises an electrically conductive coil that serves as a return electrode  86  (i.e., the cathode of the bipolar implant assembly  18 ( 1 )). The return electrode coil  86  may be formed by winding a wire having a suitable diameter (such as, e.g., 0.00175 inches) around a mandrel. The return electrode coil  86  has suitable dimensions; for example, an inner diameter of 0.006 inches (and thus, an outer diameter of 0.0095 inches) and a length of 0.75 mm. In the illustrated embodiment, a length of the wire forming the return electrode coil  86  is not wound, so as to make a straight tail  88  for coupling to an electrical conductor, as will be described in further detail below. The return electrode coil  86  circumferentially extends around the severable joint  20  and is spatially isolated from the severable joint  20  via a spacer element  90  mounted to the stiffening member  80  at a location proximal to the severable joint  20  using a suitable adhesive. The return electrode coil  86  may be composed of a suitable electrically conductive material, such as silver or copper. In the illustrated embodiment, the spacer element  90  takes the form of a coil coated with an electrically insulative material, such as, e.g., such as polyimide, PTFE, TFE, Parylene, PET, PBT, cyanoacrylate adhesives, or other suitable insulating layer. Alternatively, the spacer element  90  may take the form of a tube composed of an electrically insulative material, such as, e.g., polyetheretherketone (PEEK). 
     Significantly, while the stiffening member  80  serves as a tensioning element during deployment and retraction of the vaso-occlusive implant  22 , the return electrode coil  86  serves as a compression element, while allowing the distal end of the pusher member  18 ( 1 ) to remain laterally flexible. Thus, when loading the vaso-occlusive implant  22  in axial compression (such as during deployment), the return electrode coil  86  can compress against a structure distal to the severable joint  20 , thereby avoiding compression loading of the detachment zone, and thus reducing any possibility of kinking, fatiguing, or otherwise damaging the severable joint  20  prior to detachment. 
     In the illustrated embodiment, the return electrode coil  86  is composed of silver with a thick layer of silver chloride, which results in a high total charge capacity per unit length of wire. This feature provides a facile solid to liquid phase electrochemical reaction that does not evolve gaseous bubbles. The electrochemical reaction occurs at a very low bias voltage and is relatively insensitive to magnitude of electrical current. Thus, the return electrode coil  86  can be placed closer to the severable joint  20  without introducing gaseous bubbles, which as discussed above, can insulate the detachment zone from electrolytes needed for the electrochemical reaction, thereby prolonging detachment of the vaso-occlusive implant  22 . 
     The electrochemical reaction at the return electrode coil  86  with the electrolyte, such as sodium chloride, releases chlorine ions into the electrolyte in accordance with the equation: AgCl(s)+1e − →Ag(s)+Cl − (aq), E 0 =0.22 V HSE. This electrochemical reaction requires low voltage, has rapid charge transfer, and results in fast ion diffusion. Silver chloride has the unusual property of being minimally soluble in water, with the chloride released from the return electrode coil  86  being drawn to the severable joint  20 . Notably, in the illustrated embodiment, the severable joint  20  is composed of stainless steel (i.e., iron, chrome, and nickel). The resulting iron chloride, nickel chloride, and chrome chloride hexahydrate is highly soluable in water. The electrochemical reaction at the severable joint  20  releases iron into the electrolyte, thereby dissolving the severable joint  20  in accordance with the equations: Fe(s)−2e − , Fe(s)−3e − →Fe 2+ (s), Fe 2+ (s); Fe 2+ (s)+2Cl − (aq)→FeCl 2 (aq). 
     The return electrode coil  86  can be chloridized in any suitable manner. In one embodiment, the return electrode coil  86  is composed of pure silver, which is chloridized by placing it in a saline solution while the windings are stretched to 50-100% open pitch. The return electrode coil  86  is connected to a power supply, and a suitable electrode current (e.g., 0.1 mA) is conveyed between the coil  86  as an anode and a return electrode as a cathode for a suitable period of time (e.g., 10 minutes). The open pitch of the return electrode coil  86  will naturally close when an outer sheath (described below) is heat shrunk over the coil  86 . 
     The pusher member  18 ( 1 ) further comprises a radiopaque marker, and in particular a platinum marker coil  92 , circumferentially extending around the stiffening member  80  just proximal to the return electrode coil  86 . The marker coil  92  may be formed by winding a wire having a suitable diameter (e.g., 0.002 inches) around a mandrel. The marker coil  92  has suitable dimensions; for example, an inner diameter of 0.005 inches (and thus, an outer diameter of 0.009 inches) and a length of 3.0 mm. The marker coil  92  may have an open pitch (e.g., 10%) to increase its lateral flexibility. The marker coil  92  is bonded to the stiffening member  80  using a suitable adhesive. 
     Prior to such bonding, the tail  88  of the return electrode coil  86  is proximally threaded through the lumen of the marker coil  92  and connected to an electrical conductor  94  via suitable means, such as soldering or welding, or bonding using an electrically conductive adhesive, such as a silver-filled epoxy. The electrical conductor  94  may be a copper or silver wire that is coated with an electrically insulative material, such as, e.g., polyimide, to ensure electrical isolation of the electrical conductor  94  from the stiffening member  80 , and thus, electrical isolation between the return electrode coil  86  and the severable joint  20 . The electrical conductor  94  has suitable dimensions, such as, e.g., a wire diameter of 0.0015 inches and a total diameter (including insulation) of 0.002 inches. 
     The pusher member  18 ( 1 ) further comprises an electrically insulative sheath  96  disposed over the assembly, including the return electrode coil  86 , marker coil  92 , and the stiffening member  80 . The sheath  96  may be composed of a suitable polymeric material, such as PTFE or TFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). In the illustrated embodiment, the sheath  96  is heat shrunk over the assembly. 
     Significantly, the sheath  96  circumferentially surrounds both the severable joint  20  and the return electrode coil  86 . In addition to providing the distal end of the pusher member  18 ( 1 ) with an increased compressive strength (along with the return electrode coil  86 ), the existence of the sheath  96  reduces the possibility of previously-deployed vaso-occlusive devices from short-circuiting the severable joint  20 , which can prolong detachment time. In addition, the sheath  96  tends to exclude bodily fluids (e.g., blood) from the interior of the pusher member  18 ( 1 ), thereby reducing diffusion and convection of an ideal electrolytic environment away from the detachment region when the implant assembly  14  is exposed to the bodily fluids. The ideal electrolytic environment can be created by introducing an ideal electrolyte, such as a sodium chloride solution (saline) into the detachment region, for example, by soaking the distal end of the implant assembly  14  within the saline prior to introduction of the implant assembly  14  into the delivery catheter  12 . 
     To facilitate wicking of the saline into the detachment zone, a hydrophilic coating can be applied to one or both of the severable joint  20  and return electrode coil  86  as a rehydratable gel or water soluble polymer, such as polyvinyl alcohol. Preferably, the hydrophilic coating is weakly anchored to the severable joint  20  so as not to hinder detachment of the vaso-occlusive device  22 . Thus, in spite of the sheath  96  substantially isolating the detachment region from the exterior environment, the hydrophilic quality of the detachment region allows liquid to wick into the detachment zone upon soaking of the implant assembly  14 ( 1 ) in the liquid. In an optional embodiment, the hydrophilic material may comprise of, or contain, readily soluble salt or salts, such as sodium chloride, other metal chloride, metal chlorate, or metal sulfate. In the presence of water, these salt(s) dissolve, providing an ion-rich electrolyte that accelerates electrochemical reaction and dissolution at the severable joint  20 . In this optional case, the implant assembly  14  need not be soaked in the electrolytic solution, but rather water, since the electrolytic solution is created within the detachment region as the water makes contact with the severable joint  20  and/or return electrode coil  86 . 
     The pusher member  18 ( 1 ) further comprises an electrically conductive hypotube  98  composed of a suitable electrically conductive material, such as stainless steel. The core wire in the proximal end of the stiffening member  80  is exposed and is bonded to the interior of the hypotube  98  using a suitable electrically conductive bonding material, such as, e.g., silver-filled epoxy. The distal end of the hypotube  98  abuts the proximal end of the sheath  96 . The hypotube  98  may have suitable dimensions, e.g., an outer diameter of 0.012 inches, and an inner diameter of 0.006 inches. Thus, any portion of the hypotube  98  forms the positive terminal  28  (shown in  FIG. 1 ) that electrically communicates with the severable joint  20  via a forward electrical path that includes the core wire of the stiffening member  80 . 
     The pusher member  18 ( 1 ) further comprises another electrically insulative sheath  100  disposed over a portion of the hypotube  98 , and an electrically conductive terminal coil  102 , which serves as the negative terminal  30  (shown in  FIG. 1 ), mounted around the insulative sheath  100 . The insulative sheath  100  may be composed of a suitable polymeric material, such as PTFE or TFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). In the illustrated embodiment, the insulative sheath  100  is heat shrunk over the hypotube  98 . The terminal coil  102  may be composed of a material, such as platinum, and is electrically coupled to the return electrode coil  86  via a return electrical path that includes the electrical conductor  94  and the tail  88  of the return electrode coil  86 . 
     To this end, the electrical conductor  94 , which is connected to the return electrode coil  86  via the tail  88 , is proximally threaded through the hypotube  98 , and distally bent about the proximal end of the hypotube  98 , so that the proximal end of the electrical conductor  94  can be placed between the insulative sheath  100  and the terminal coil  102 . Preferably, the electrical conductor  94  is disposed on the insulative sheath  100 , and then the terminal coil  102  is bonded over the electrical conductor  94  and insulative sheath  100  using soldering or welding or an electrically conductive adhesive, such as, e.g., silver-filled epoxy. 
     Referring now to  FIG. 6 , one embodiment of a monopolar implant assembly  14 ( 2 ) will now be described. The monopolar implant assembly  14 ( 2 ) comprises the previously described vaso-occlusive implant  22  and a pusher member  18 ( 2 ). The pusher member  18 ( 2 ) comprises an elongated stiffening member  180  that includes an uninsulated electrically conductive core wire. The core wire of the stiffening member  180  can be composed of any suitable electrically conductive and rigid material, such as stainless steel. In the illustrated embodiment, the stiffening member  180  comprises a proximal section  185  and a distal section  187  that are coupled together via a crimped bushing  189 . Alternatively, the proximal section  185  and distal section  187  of the stiffening member  180  can be soldered or welded together. The core wire of the distal section  187  may have a uniform diameter equal to the smallest diameter of the proximal section  185 . 
     In the illustrated embodiment, the stiffening member  180  tapers from a large diameter section  181  to a small diameter section  183 . The core wire of the stiffening member  180  can be ground to effect this taper. In the illustrated embodiment, the diameter of the core wire in the large diameter section  181  of the stiffening member  180  is 0.010 inches, and the diameter of the core wire in the small diameter section  183  is 0.0025 inches. 
     Like the previously described stiffening member  80 , the large diameter section  181  of the stiffening member  180  provides the pusher member  18 ( 2 ) with lateral rigidity, as well as tensile strength, whereas the small diameter section  183  of the stiffening member  180  provides the pusher member  18 ( 2 ) with the desired lateral flexibility adjacent the vaso-occlusive implant  22  to minimize kickback during detachment of the vaso-occlusive implant  22 . 
     The formation of the electrolytic severable joint  20 , which serves as the anode of the monopolar implant assembly  14 ( 2 ), can be the same as that described above with respect to the pusher member  18 ( 1 ). Any portion of the larger diameter section  181  of the stiffening member  180  can serve as the positive terminal  28  (illustrated in  FIG. 2 ) that electrically communicates with the severable joint  20  via a forward electrical path that includes the core wire of the stiffening member  180 . 
     The pusher member  18 ( 2 ) further comprises an electrically conductive coil  186  that serves as an intermediate return electrode; that is, a return electrode between the severable joint  20  and the ground electrode  32  (shown in  FIG. 2 ). The return electrode coil  186  may be formed by winding a wire having a suitable diameter (such as, e.g., 0.00175 inches) around a mandrel. The return electrode coil  186  has suitable dimensions; for example, an inner diameter of 0.006 inches and a length of 0.75 mm. The return electrode coil  186  circumferentially extends around the severable joint  20  and is spatially isolated from the severable joint  20  via a spacer element  190  mounted to the stiffening member  180  at a location proximal to the severable joint  20  using a suitable adhesive. The return electrode coil  186  may be composed of a suitable electrically conductive material, such as silver or copper. The spacer element  190  may take the same form and be constructed of the same materials as the spacer element  90  described above. 
     Like the previously described return electrode  86 , the return electrode coil  186  serves as a compression element and may be coated with a layer of silver chloride to further prevent or reduce the generation of gaseous bubbles. Unlike the previously described return electrode  86 , the return electrode coil  186  is not electrically coupled to a terminal. Instead, as illustrated in  FIG. 9 , two electrochemical circuits are created: one between the severable joint  20  and the return electrode coil  186 , and one between the return electrode coil  186  and the ground return electrode  32 . The large surface area of the return electrode coil  186  provides an electrochemical circuit with a lower impedance return path to ground than the electrolyte itself. When a voltage is applied between the severable joint  20  and the ground return electrode  32 , the return electrode coil  186  will be at a voltage in between the severable joint  20  and the ground return electrode  32 , as illustrated in  FIG. 9 . Thus, the return electrode coil  186  shortens the diffusion distance for metal ions and provides a reducing surface that can plate these ions out of the electrolyte, thus lowering metal ion concentration at the detachment region. This increases the rate of metal ion dissolution and reduces the magnitude of over-voltage required. This, in turn, reduces bubbling at the detachment region, thereby shortening the detachment time and making the detachment process more reliable. 
     The pusher member  18 ( 2 ) further comprises a radiopaque marker, and in particular a platinum marker coil  192 , circumferentially extending around the stiffening member  80 . The marker coil  192  may be formed by winding a wire having a suitable diameter (e.g., 0.002 inches) around a mandrel. In the illustrated embodiment, the distal end of the marker coil  192  is disposed within the proximal end of the return electrode coil  186 . To this end, the inner diameter of the marker coil  192  can be 0.002 inches (and thus, an outer diameter of 0.006 inches) and a length of 3.0 mm. Like the previously described marker coil  92 , the marker coil  192  may have an open pitch (e.g., 10%) to increase its lateral flexibility, and may be bonded to the stiffening member  80  using a suitable adhesive, such as epoxy. The inner surface of the return electrode coil  186  may be bonded to the outer surface of the marker coil  192  using a suitable adhesive, such as epoxy. 
     The pusher member  18 ( 2 ) further comprises an electrically insulative sheath  196  disposed over the assembly, including the return electrode coil  186 , marker coil  192 , and the stiffening member  180 . The sheath  196  may be composed of a suitable polymeric material, such as PTFE or TFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). In the illustrated embodiment, the sheath  196  is heat shrunk over the assembly. 
     Significantly, the sheath  196  circumferentially surrounds both the severable joint  20  and the return electrode coil  186 . Thus, like the previously described sheath  96  of the bipolar pusher member  18 ( 1 ), the sheath  196  increases the compressive strength of the pusher member  18 ( 2 ), and reduces diffusion and convection of an ideal electrolytic environment away from the detachment region when the implant assembly  14 ( 2 ) is exposed to the bodily fluids. As previously described, such ideal electrolytic environment can be created by introducing the ideal electrolyte into the detachment region or introducing water into the detachment region previously coated with salt. To facilitate wicking of the saline or water into the detachment zone, a hydrophilic coating can be applied to one or both of the severable joint  20  and return electrode coil  186  in the same manner previously described above. 
     Referring now to  FIG. 7 , another embodiment of a bipolar implant assembly  14 ( 3 ) will now be described. The bipolar implant assembly  14 ( 3 ) differs from the previously described bipolar implant assembly  14 ( 1 ) in that it includes an exposed detachment region. To this end, the bipolar implant assembly  14 ( 3 ) comprises the previously described vaso-occlusive implant  22  and a pusher member  18 ( 3 ). The pusher member  18 ( 3 ) comprises an elongated stiffening member  280  that includes an electrically conductive core wire and an electrically insulative coating disposed over the core wire. The core wire of the stiffening member  280  can be composed of any suitable electrically conductive and rigid material, such as stainless steel, and the coating can be composed of any suitable electrically insulative material, such as polyimide, PTFE, TFE, Parylene, PET, PBT, cyanoacrylate adhesives, or other suitable insulating layer. 
     In the illustrated embodiment, the stiffening member  280  tapers from a large diameter section  281  to a small diameter section  283 . The core wire of the stiffening member  180  can be ground to effect this taper. In the illustrated embodiment, the diameter of the core wire in the large diameter section  281  of the stiffening member  280  is 0.004 inches, and the diameter of the core wire in the small diameter section  283  is 0.0025 inches. 
     Like the previously described stiffening member  80 , the large diameter section  281  of the stiffening member  280  provides the pusher member  18 ( 3 ) with lateral rigidity, as well as tensile strength, whereas the small diameter section  283  of the stiffening member  280  provides the pusher member  18 ( 3 ) with the desired lateral flexibility adjacent the vaso-occlusive implant  22  to minimize kickback during detachment of the vaso-occlusive implant  22 . The construction of the core wire, coating, and formation of the electrolytic severable joint  20 , which serves as the anode of the bipolar implant assembly  14 ( 3 ), can be the same as that described above with respect to the pusher member  18 ( 1 ). 
     The pusher member  18 ( 3 ) further comprises an electrically conductive coil  286  that serves as a return electrode (i.e., the cathode of the bipolar implant assembly  14 ( 3 ). The return electrode coil  286  may be formed by winding a wire having a suitable diameter (such as, e.g., 0.002 inches) around a mandrel. The return electrode coil  286  has suitable dimensions; for example, an inner diameter of 0.003 inches (and thus, an outer diameter of 0.007 inches) and a length of 0.75 mm. In the illustrated embodiment, the return electrode coil  286  has an open pitch (e.g., 20%) to increase its lateral flexibility. The return electrode coil  286  may be composed of a suitable electrically conductive material, such as silver or copper. The return electrode coil  286  circumferentially extends around the stiffening member  280 , and in particular, is bonded to the stiffening member  280  at a location proximal to the severable joint  20  using a suitable adhesive, such as epoxy. Like the previous return electrode coil  86 , the return electrode coil  286  serves as a compression element and may be coated with a layer of silver chloride to further prevent or reduce the generation of gaseous bubbles. 
     The pusher member  18 ( 3 ) further comprises a radiopaque marker, and in particular a platinum marker coil  292 , circumferentially extending around the stiffening member  280 . The marker coil  292  may be formed by winding a wire having a suitable diameter (e.g., 0.002 inches) around a mandrel. In the illustrated embodiment, the inner and outer diameter of the marker coil  292  is preferably the same as the inner and outer diameter of the return electrode coil  286 ; that is, an inner diameter of 0.003 inches and an outer diameter of 0.007 inches. The length of the marker coil  292  may be 3.0 mm. Like the previously described marker coil  92 , the marker coil  292  may have an open pitch (e.g., 10%) to increase its lateral flexibility, and may be bonded to the stiffening member  80  using a suitable adhesive, such as epoxy. 
     The pusher member  18 ( 3 ) further comprises an interconnecting flex coil  295 , the proximal end of which is circumferentially disposed around the distal end of the marker coil  292 , and the distal end of which is circumferentially disposed around the proximal end of the return electrode coil  286 . The flex coil  295  is composed of an electrically conductive material, such as stainless steel, and is suitably bonded to the marker coil  292  and return electrode coil  286  using an electrically conductive adhesive, such as silver-filled epoxy. As such, the marker coil  292  and return electrode coil  286  are electrically coupled together. The flex coil  295  may be formed by winding a wire having a suitable diameter (e.g., 0.00175 inches) around a mandrel. The flex coil  295  has suitable dimensions; for example, an inner diameter of 0.007 inches (and thus, an outer diameter of 0.0105) and a length of 30 mm. In the illustrated embodiment, the flex coil  295  has a closed pitch. 
     The pusher member  18 ( 3 ) further comprises an electrical conductor  294  connected to the external surface of the marker coil  292  via suitable means, such as soldering or welding, or bonding using an electrically conductive adhesive, such as silver-filled epoxy. The electrical conductor  292  may be a copper or silver wire that is coated with an electrically insulative material, such as, e.g., polyimide, to ensure electrically isolation of the electrical conductor  292  from the stiffening member  280 , and thus, electrical isolation between the return electrode coil  286  and the severable joint  20 . The electrical conductor  292  has suitable dimensions, such as, e.g., a wire diameter of 0.0015 inches and a total diameter (including insulation) of 0.002 inches. 
     The pusher member  18 ( 3 ) further comprises an electrically insulative sheath  296  disposed over the assembly, including the proximal end of the marker coil  292 , the electrical conductor  294 , and the portion of the stiffening member  280  extending proximally from the marker coil  292 , with the distal end of the sheath  296  abutting the proximal end of the flex coil  295 . The sheath  296  may be composed of a suitable polymeric material, such as PTFE or TFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). In the illustrated embodiment, the sheath  296  is heat shrunk over the assembly. 
     The pusher member  18 ( 3 ) further comprises a first electrically conductive hypotube  298  composed of a suitable electrically conductive material, such as stainless steel. The core wire in the proximal end of the stiffening member  280  is exposed and is bonded to the interior of the hypotube  298  using a suitable electrically conductive bonding material, such as, e.g., silver-filled epoxy. The distal end of the hypotube  298  abuts the proximal end of the sheath  296 . The hypotube  298  may have suitable dimensions, e.g., an outer diameter of 0.012 inches, an inner diameter of 0.006 inches, and a length of 150 cm. Thus, any portion of the hypotube  298  forms the positive terminal  28  (shown in  FIG. 1 ) that electrically communicates with the severable joint  20  via the forward electrical path that includes the core wire of the stiffening member  80 . 
     The pusher member  18 ( 3 ) further comprises a second electrically conductive hypotube  299  composed of a suitable electrically conductive material, such as stainless steel. The proximal end of the electrical conductor  294  is exposed and is bonded to the interior of the hypotube  299  using a suitable electrically conductive bonding material, such as, e.g., silver-filled epoxy. The hypotube  299  may have suitable dimensions, e.g., an outer diameter of 0.012 inches, an inner diameter of 0.006 inches, and a length of 10 mm. Thus, any portion of the hypotube  299  forms the negative terminal  30  (shown in  FIG. 1 ) that electrically communicates with the return electrode coil  286  via return electrical path that includes the electrical conductor  294 , marker coil  292 , and flex coil  295 . 
     The pusher member  18 ( 3 ) further comprises a reinforcing mandrel  300  around which the proximal end of the first hypotube  298  and the distal end of the second hypotube  299  is bonded using a suitable adhesive, such as epoxy. The reinforcing mandrel  300  may be a stainless steel wire that is coated with an electrically insulative material, such as, e.g., polyimide, to ensure electrically isolation between the first and second hypotubes  299 ,  300 , and thus, electrical isolation between the severable joint  20  and the return electrode coil  286 . The reinforcing mandrel  300  has suitable dimensions, such as, e.g., a wire diameter of 0.004 inches and a length of 10 mm. 
     Referring now to  FIG. 8 , another embodiment of a monopolar implant assembly  14 ( 4 ) will now be described. The monopolar implant assembly  14 ( 4 ) differs from the previously described monopolar implant assembly  14 ( 2 ) in that the intermediate return electrode is configured to remain with a vaso-occlusive implant  23  when detached from a pusher member  18 ( 4 ). As discussed above with respect to the monopolar pusher member  18 ( 2 ), the large surface area of the intermediate return electrode reduces bubbling at the detachment region. 
     The vaso-occlusive implant  23  is similar to the previously described vaso-occlusive implant  22  in that it comprises the primary coil  62 , stretch resisting filament  70 , and distal cap (not shown). The vaso-occlusive implant  23  differs in that it comprises an intermediate return electrode  386  in the form of an electrically conductive hypotube disposed within the proximal end of the primary coil  62 . In the illustrated embodiment, the proximal end of the primary coil  62  has an open pitch (e.g., 4 of the proximal turns are open pitched) that is wound around, and mounted to, the intermediate return electrode  386  via suitable means, such as soldering or welding, or bonding using an electrically conductive adhesive, such as silver-filled epoxy. In an alternative embodiment, the intermediate return electrode  386  may take the form of an electrically conductive coil. The intermediate return electrode  386  is preferably composed of a biocompatible material suitable for chronic implantation. Notably, because the primary coil  62  is electrically coupled to the intermediate return electrode  386 , the effective surface area of the intermediate return electrode  386  is substantially increased to the extent that the primary coil  62 , itself, is electrically conductive, thereby further minimizing the chance of gaseous bubbling. 
     The pusher member  18 ( 4 ) comprises an elongated stiffening member  380  that includes an uninsulated electrically conductive core wire. The core wire of the stiffening member  380  can be composed of any suitable electrically conductive and rigid material, such as stainless steel. In the illustrated embodiment, the stiffening member  380  tapers from a large diameter section  381  to a small diameter section  383 . The core wire of the stiffening member  380  can be ground to effect this taper. In the illustrated embodiment, the diameter of the core wire in the large diameter section  381  of the stiffening member  380  is 0.010 inches, and the diameter of the core wire in the small diameter section  383  is 0.0025 inches. 
     Like the previously described stiffening member  80 , the large diameter section  381  of the stiffening member  380  provides the pusher member  18 ( 4 ) with lateral rigidity, as well as tensile strength, whereas the small diameter section  383  of the stiffening member  380  provides the pusher member  18 ( 4 ) with the desired lateral flexibility adjacent the vaso-occlusive implant  22  to minimize kickback during detachment of the vaso-occlusive implant  22 . 
     The pusher member  18 ( 4 ) further comprises a radiopaque marker, and in particular a platinum marker coil  392 , circumferentially extending around the stiffening member  380 . The marker coil  392  may be formed by winding a wire having a suitable diameter (e.g., 0.002 inches) around a mandrel. The marker coil  392  has suitable dimensions; for example, an inner diameter of 0.003 inch and a length of 3.0 mm. The marker coil  392  may have an open pitch (e.g., 10%) to increase the lateral flexibility of the marker coil  392 . 
     The pusher member  18 ( 4 ) further comprises an electrically insulated coil  395  having an electrically conductive wire and an electrically insulative coating disposed thereon. The insulated coil  395  may be formed by winding a wire having a suitable diameter (e.g., 0.00175 inches) around a mandrel. In the illustrated embodiment, the insulated coil  395  has an open pitch (e.g., 50%) to increase the lateral flexibility of the coil  395 . The proximal windings of the insulative coil  395 , which are stripped of the insulative coating, are circumferentially mounted around the distal end of the stiffening member  380  via suitable means, such as soldering or welding, or bonding using an electrically conductive adhesive, such as silver-filled epoxy. 
     The distal end  397  of the insulated coil  395  is straightened and extended through the lumen of the return electrode  386  of the vaso-occlusive device  23 . A region of the straight section  397  is either not coated with the insulative material or a portion of the insulative material is removed (e.g., using laser ablation) to expose a portion of the wire, thereby forming the electrolytic severable joint  20 . The formation of the electrolytic severable joint  20 , which serves as the anode of the monopolar implant assembly  14 ( 4 ), can be the same as that described above with respect to the pusher member  18 ( 2 ). 
     Like the previous intermediate return electrode  186 , the return electrode  386  may be coated with a layer of silver chloride to further reduce or prevent the generation of gaseous bubbles. To facilitate wicking of the saline or water into the detachment zone, a hydrophilic coating can be applied to one or both of the severable joint  20  and return electrode  386  in the same manner previously described above. In an alternative embodiment, only an edge of the intermediate return electrode  386  is exposed to the severable joint  20 , thereby improving electrolyte perfusion and reducing the overall diameter of the implant assembly. 
     As illustrated, the return electrode  386  circumferentially extends around the severable joint  20 . The insulative coating proximal and distal to the severable joint  20  provides a mechanical spacer that prevents contact between the severable joint  20  and the return electrode  386 . Any portion of the proximal end of the stiffening member  380  can form the positive terminal  28  (shown in  FIG. 2 ) that electrically communicates with the severable joint  20  via a forward electrical path that includes the coil wire of the stiffening member  380  and the coil  395 . 
     The straight section  397  of the insulated coil  395  is threaded through the stretch resisting filament  70  and bent 180 degrees to form a link with the stretch resisting filament  70 . The straight section  397  is then wound around, and mounted to, the return electrode  386  via suitable means, such as soldering or welding, or bonding using an electrically conductive adhesive, such as silver-filled epoxy. The newly formed windings of the insulated coil  395  fit between the open pitch windings of the primary coil  62  to minimize any increase in the outer diameter of the return electrode  386 . 
     The pusher member  18 ( 4 ) further comprises an electrically insulative sheath  396  disposed over the assembly, including the insulated coil  395 , marker coil  392 , and the stiffening member  380 . The sheath  396  may be composed of a suitable polymeric material, such as PTFE or TFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). In the illustrated embodiment, the sheath  396  is heat shrunk over the assembly. 
     Although the implant assembly  14 ( 4 ) has been described as a monopolar assembly, a bipolar implant assembly can be constructed by connecting the return electrode directly to ground return through a wire attached to the delivery catheter  12  (shown in  FIGS. 1 and 2 ) or a wire extending through the pusher member  18 ( 4 ). In this case, the primary coil  62  of the vaso-occlusive implant  22  can be electrically insulated from the return electrode  386 . 
     Referring now to  FIG. 10 , another embodiment of a bipolar implant assembly  14 ( 5 ) will now be described. The bipolar implant assembly  14 ( 5 ) differs from the previously described bipolar implant assembly  14 ( 1 ) in that it utilizes electrically conductive sheaths in the forward electrical path between the severable joint  20  and the positive electrode  28 , and in the return electrical path between the return electrode and the negative terminal  30  (shown in  FIG. 1 ). 
     The bipolar implant assembly  14 ( 5 ) comprises the previously described vaso-occlusive implant  22  and a pusher member  18 ( 5 ). The pusher member  18 ( 5 ) comprises an elongated stiffening member  480  that includes a proximal stiffening member element  485  and a distal stiffening member element  487 . The proximal stiffening member element  485  comprises an uninsulated core wire, and the distal stiffening member element  487  comprises an insulated core wire. The core wires of the stiffening member elements  485 ,  487  can be composed of any suitable electrically conductive and rigid material, such as stainless steel, and the coating can be composed of any suitable electrically insulative material, such as polyimide, PTFE, TFE, Parylene, PET, PBT, cyanoacrylate adhesives, or other suitable insulating layer. 
     The distal end of the proximal stiffening member element  485  includes a forked member  489  in which the proximal end of the distal stiffening member element  487  is mounted. The proximal end of the core wire in the distal stiffening member element  487  is left exposed, so that the proximal stiffening member element  485  is in electrical communication with the distal stiffening member element  487 . An uninsulated wire is wrapped around the distal end of the proximal stiffening member element  485  to form a coil  491  that firmly secures the distal stiffening member element  487  within the forked member  489  of the proximal stiffening member element  485 . 
     In the illustrated embodiment, the proximal stiffening member element  585  tapers from a large diameter section  481  to a small diameter section  483 . The core wire of the proximal stiffening member element  485  can be ground to effect this taper. In the illustrated embodiment, the diameter of the core wire in the large diameter section  481  is 0.010 inches, and the diameter of the core wire in the small diameter section  483  is 0.0025 inches. The diameter of the core wire in the distal stiffening member element  487  is smaller than the core wire in the small diameter section  483  of the proximal stiffening member element  485 ; for example 0.0015 inches. The insulative coating on the core wire of the distal stiffening member element  487  may have a suitable thickness (e.g., 0.00035 inches). 
     Like the previously described stiffening member  80 , the large diameter section  481  of the proximal stiffening member element  485  provides the pusher member  18 ( 4 ) with lateral rigidity, as well as tensile strength, whereas the small diameter section  483  of the proximal stiffening member element  485  and even smaller diameter distal stiffening member element  487  provide the pusher member  18 ( 5 ) with the desired lateral flexibility adjacent the vaso-occlusive implant  22  to minimize kickback during detachment of the vaso-occlusive implant  22 . The electrolytic severable joint  20 , which serves as the anode of the bipolar implant assembly  14 ( 5 ), is formed on the distal stiffening member element  487  in the same manner as the severable joint  20  is formed on the core wire of the stiffening member  80  of the pusher member  18 ( 1 ). 
     The pusher member  18 ( 5 ) further comprises an electrically conductive coil  486  that serves as a return electrode (i.e., the cathode of the bipolar implant assembly  14 ( 5 ). The return electrode coil  486  may be formed by winding a wire having a suitable diameter (such as, e.g., 0.00175 inches) around a mandrel. The return electrode coil  486  has suitable dimensions; for example, an inner diameter of 0.006 inches (and thus an outer diameter of 0.0095 inches) and a length of 0.75 mm. The return electrode coil  486  may be composed of a suitable electrically conductive material, such as silver or copper. In the illustrated embodiment, the return electrode coil  486  has an open pitch (e.g., 20%) to increase its lateral flexibility. The return electrode coil  486  circumferentially extends around the severable joint  20  and is spatially isolated from the severable joint  20  via a spacer element  490  mounted to the distal stiffening member element  487  at a location proximal to the severable joint  20  using a suitable adhesive. The spacer element  490  may take the same form and be constructed of the same materials as the spacer element  90  described above. Like the previous return electrode coil  86 , the return electrode coil  486  serves as a compression element and may be coated with a layer of silver chloride to further prevent or reduce the generation of gaseous bubbles. 
     The pusher member  18 ( 5 ) comprises an electrically conductive sheath  493  bonded around the smaller diameter section  483  of the proximal stiffening member element  485  using suitable means, such as silver-filled epoxy or shrink tubing. In the illustrated embodiment, the electrically conductive sheath  493  extends from the proximal end of the small diameter section  483  of the proximal stiffening member element  485  to the distal end of the small diameter section  483  of the proximal stiffening member element  485  just proximal to the securing coil  491 . The pusher member  18 ( 5 ) further comprises an electrically insulative sheath  497  disposed over the proximal stiffening member element  485 , electrically conductive sheath  493 , and securing coil  491 . The pusher member  18 ( 5 ) further comprises another electrically conductive sheath  498  suitably bonded around the electrically insulative sheath  497  coincident with the large diameter section  481  of the proximal stiffening member element  485 , using suitable means, such epoxy. The pusher member  18 ( 5 ) further comprises an electrically insulative sheath  496  disposed over the assembly, including the return electrode coil  486 , marker coil  492 , and flex coil  495 . 
     The electrically conductive sheaths  493 ,  498  may take the form of, e.g., a mesh, braid, or coil. In the embodiment illustrated in  FIG. 10 , the electrically conductive sheaths  493 ,  498  take the form of mesh. While the core wire of the stiffening member  480  is preferably composed of a material that has a greater durometer than the material from which the electrically conductive sheaths  493 ,  498  are composed; for example, stainless steel, the electrically conductive sheaths  493 ,  498  are preferably composed of a material that is more electrically conductive than the material from which core wire of the stiffening member  480  is composed; for example, silver or copper. The electrically insulative sheaths  496 ,  497  may be composed of a suitable polymeric material, such as PTFE or TFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). 
     Significantly, the sheath  496  circumferentially surrounds both the severable joint  20  and the return electrode coil  486 . Thus, like the previously described sheath  96  of the bipolar pusher member  18 ( 1 ), the sheath  496  increases the compressive strength of the pusher member  18 ( 5 ), and reduces diffusion and convection of an ideal electrolytic environment away from the detachment region when the implant assembly  14 ( 5 ) is exposed to the bodily fluids. As previously described, such ideal electrolytic environment can be created by introducing the ideal electrolyte into the detachment region or introducing water into the detachment region previously coated with salt. To facilitate wicking of the saline or water into the detachment zone, a hydrophilic coating can be applied to one or both of the severable joint  20  and return electrode coil  486  in the same manner previously described above. 
     The pusher member  18 ( 5 ) further comprises a radiopaque marker, and in particular a platinum marker coil  492 , circumferentially extending around the small diameter section  483  of the proximal stiffening member element  485 . The marker coil  492  may be formed and constructed of the same material as the marker coil  92  described above. The pusher member  18 ( 5 ) further comprises a flex coil  495  circumferentially extending around the small diameter section  483  of the proximal stiffening member element  485  just proximal to the marker coil  492 . The flex coil  495  is composed of an electrically conductive material, such as stainless steel. In the illustrated embodiment, the flex coil  495  has a closed pitch. The marker coil  492  and flex coil  495  preferably have the same diameter as the return coil  486 . 
     The pusher member  18 ( 5 ) further comprises an electrical conductor  494  connected between the return electrode coil  486  and the other electrically conductive sheath  498  via suitable means, such as soldering or welding, or bonding using an electrically conductive adhesive, such as silver-filled epoxy. The electrical conductor  492  may be a copper or silver wire. In the illustrated embodiment, the electrical conductor  494  is disposed on the outside of the electrically insulative sheath  497  to ensure electrically isolation of the electrical conductor  492  from the stiffening member  480 , and thus, electrical isolation between the return electrode coil  486  and the severable joint  20 . The electrical conductor  492  has suitable dimensions, such as, e.g., a wire diameter of 0.0015 inches and a total diameter (including insulation) of 0.002 inches. 
     The pusher member  18 ( 5 ) further comprises an electrically insulative sheath  496  disposed over the assembly, including the return electrode coil  486 , marker coil  492 , and flex coil  495 . The sheath  496  may be composed of a suitable polymeric material, such as PTFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). In the illustrated embodiment, the sheath  496  is heat shrunk over the assembly. 
     A portion of the core wire in the proximal stiffening member element  485 , and in the illustrated embodiment the proximal tip of the proximal stiffening member element  485 , is left exposed to form the positive terminal  28  (shown in  FIG. 1 ) that electrically communicates with the severable joint  20  via a forward electrical path that includes the stiffening member  480  and the electrically conductive sheath  493 . Advantageously, the stiffening member  480  provides the necessary pushability for the implant assembly  14 ( 5 ), while the high electrically conductive sheath  493  significantly decreases the electrical conductance along the portion of the forward electrical path that is coincident with the small diameter section  483  of the proximal stiffening member element  485  where the electrical conductance would otherwise decrease relative to the large diameter section  481  of the proximal stiffening member element  485 . 
     The pusher member  18 ( 5 ) further comprises an electrically conductive terminal ribbon  499 , which serves as the negative terminal  30  (shown in  FIG. 1 ), mounted around the other electrically conductive sheath  498  at a location at the proximal end of the proximal stiffening member element  485 . The terminal ribbon  499  may be composed of a material, such as silver or copper, and is electrically coupled to the return electrode coil  486 . Thus, the terminal ribbon  499  electrically communicates with the return electrode coil  486  via a return electrical path that includes the other electrically conductive sheath  498  and electrical conductor  494 . Advantageously, the high electrically conductive sheath  498  significantly decreases the electrical conductance along the return electrical path compared to a case where the electrical conductor  494  extends the full length between the return electrode coil  486  and the terminal ribbon  499 . 
     Referring now to  FIG. 11 , still another embodiment of a bipolar implant assembly  14 ( 6 ) will now be described. The bipolar implant assembly  14 ( 6 ) differs from the previously described bipolar implant assembly  14 ( 5 ) in that it utilizes only one electrically conductive sheath in the return electrical path between the return electrode and the negative terminal  30  (shown in  FIG. 1 ). The bipolar implant assembly  14 ( 5 ) comprises the previously described vaso-occlusive implant  22  and a pusher member  18 ( 6 ). 
     The pusher member  18 ( 6 ) comprises an elongated stiffening member  580  that is similar to the previously described stiffening member  480 , with the exception that substantially the entire length of the stiffening member  580  is insulated. Thus, the stiffening member  580  includes a tapered proximal stiffening member element  585  having a large diameter section  581  and a small diameter section  583 , and a distal stiffening member element  587 . The distal end of the proximal stiffening member element  585  has forked member  589  in which the proximal end of the distal core wire element  587  is mounted via a securing coil  591 . The construction and dimensions of the proximal stiffening member element  585 , distal stiffening member element  587 , and securing coil  591  may be the same as the proximal stiffening member element  485 , distal stiffening member element  487 , and securing coil  491  described above, with the exception that both stiffening member elements  585 ,  587  comprise core wires coated with an electrically insulative material. 
     Like the previously described stiffening member  80 , the large diameter section  581  of the proximal stiffening member element  585  provides the pusher member  18 ( 6 ) with lateral rigidity, as well as tensile strength, whereas the small diameter section  583  of the proximal stiffening member element  585  and even smaller diameter distal stiffening member element  587  provide the pusher member  18 ( 6 ) with the desired lateral flexibility adjacent the vaso-occlusive implant  22  to minimize kickback during detachment of the vaso-occlusive implant  22 . The electrolytic severable joint  20 , which serves as the anode of the bipolar implant assembly  14 ( 6 ), is formed on the distal stiffening member element  587  in the same manner as the severable joint  20  is formed on the stiffening member  80  of the pusher member  18 ( 1 ). 
     The pusher member  18 ( 6 ) comprises an electrically conductive sheath  598  suitably bonded around the proximal stiffening member element  585  using suitable means, such epoxy. The electrically conductive sheath  598  may take the form of, e.g., a mesh, braid, or coil. In the illustrated embodiment, the electrically conductive sheath  598  is a coil. The electrically conductive sheath  598  is composed of a material that is more electrically conductive than the material from which the stiffening member  580  is composed; for example, silver or copper. 
     The pusher member  18 ( 6 ) further comprises an electrically conductive coil  586  that serves as a return electrode (i.e., the cathode of the bipolar implant assembly  14 ( 6 )), a radiopaque marker, and in particular a platinum marker coil  592 , and a flex coil  595  that circumferentially extend around the stiffening member  590  in the same manner as the respective return electrode coil  486 , marker coil  492 , and flex coil  495  described above. That is, the proximal end of the return electrode coil  586  is bonded around the securing coil  591 , and the marker coil  592  and flex coil  595  are bonded around the electrically conductive sheath  598  along the small diameter section  583  of the proximal stiffening member element  585 . The return electrode coil  586  is spatially isolated from the severable joint  20  via a spacer element  590  mounted to the distal stiffening member element  587  at a location proximal to the severable joint  20  using a suitable adhesive. 
     The return electrode coil  586 , marker coil  592 , flex coil  595 , and spacer element  590  may take the same form and be constructed of the same materials as the respective return electrode coil  486 , marker coil  492 , flex coil  495 , and spacer element  490  described above. Like the previous return electrode coil  86 , the return electrode coil  586  serves as a compression element and may be coated with a layer of silver chloride to further prevent or reduce the generation of gaseous bubbles. 
     The pusher member  18 ( 6 ) further comprises an electrically insulative sheath  596  disposed over the assembly, including the return electrode coil  586 , marker coil  592 , and flex coil  595 . The sheath  596  may be composed of the same material and have the same dimensions as the sheath  496  described above. Significantly, the sheath  596  circumferentially surrounds both the severable joint  20  and the return electrode coil  586 . Thus, like the previously described sheath  96  of the bipolar pusher member  18 ( 1 ), the sheath  596  increases the compressive strength of the pusher member  18 ( 6 ), and reduces diffusion and convection of an ideal electrolytic environment away from the detachment region when the implant assembly  14 ( 6 ) is exposed to the bodily fluids. As previously described, such ideal electrolytic environment can be created by introducing the ideal electrolyte into the detachment region or introducing water into the detachment region previously coated with salt. To facilitate wicking of the saline or water into the detachment zone, a hydrophilic coating can be applied to one or both of the severable joint  20  and return electrode coil  586  in the same manner previously described above. 
     The core wire in a portion of the proximal stiffening member element  585 , and in the illustrated embodiment the proximal tip of the core wire, is left exposed to form the positive terminal  28  (shown in  FIG. 1 ) that electrically communicates with the severable joint  20  via a forward electrical path formed only by the stiffening member  580 . Any portion of the electrically conductive sheath  598  may serve as the negative terminal  30  (shown in  FIG. 1 ). Thus, the entire electrically conductive sheath  598  forms the forward electrical path to the return electrode coil  586 . Advantageously, the high electrically conductive sheath  586  significantly decreases the electrical conductance along the electrical path to the return electrode coil  586  compared to a standard wire that may otherwise extend between the return electrode coil  586  and the negative terminal  30 . 
     Referring now to  FIG. 12 , yet another embodiment of a bipolar implant assembly  14 ( 7 ) will now be described. The bipolar implant assembly  14 ( 7 ) differs from the previously described bipolar implant assembly  14 ( 5 ) in that it utilizes coils, instead of mesh, for the electrically conductive sheaths. To this end, the bipolar implant assembly  14 ( 7 ) comprises the previously described vaso-occlusive implant  22  and a pusher member  18 ( 7 ). 
     The pusher member  18 ( 7 ) comprises an elongated stiffening member  680  that is similar to the previously described stiffening member  480 . In particular, the stiffening member  680  includes a tapered proximal stiffening member element  685  having a large diameter section  681  and a small diameter section  683 , and a distal stiffening member element  687 . The distal end of the proximal stiffening member element  685  has forked member  689  in which the proximal end of the distal core wire element  687  is mounted via a securing coil  691 . The construction and dimensions of the proximal stiffening member element  685 , distal stiffening member element  687 , and securing coil  691  may be the same as the proximal stiffening member element  485 , distal stiffening member element  487 , and securing coil  491  described above. 
     Like the previously described stiffening member  80 , the large diameter section  681  of the proximal stiffening member element  585  provides the pusher member  18 ( 7 ) with lateral rigidity, as well as tensile strength, whereas the small diameter section  683  of the proximal stiffening member element  685  and even smaller diameter distal stiffening member element  687  provide the pusher member  18 ( 7 ) with the desired lateral flexibility adjacent the vaso-occlusive implant  22  to minimize kickback during detachment of the vaso-occlusive implant  22 . The electrolytic severable joint  20 , which serves as the anode of the bipolar implant assembly  14 ( 7 ), is formed on the distal stiffening member element  687  in the same manner as the severable joint  20  is formed on the stiffening member  80  of the pusher member  18 ( 1 ). 
     The pusher member  18 ( 7 ) comprises an electrically conductive sheath  693  bonded around the smaller diameter section  683  of the proximal stiffening member element  685  using suitable means, such as silver-filled epoxy or shrink tubing. In the illustrated embodiment, the electrically conductive sheath  693  extends from the proximal end of the small diameter section  683  of the proximal stiffening member element  685  to the distal end of the small diameter section  683  of the proximal stiffening member element  685  just proximal to the securing coil  691 . The pusher member  18 ( 7 ) further comprises an electrically insulative sheath  697  disposed over the proximal stiffening member element  685 , electrically conductive sheath  693 , and securing coil  691 . The pusher member  18 ( 7 ) further comprises another electrically conductive sheath  698  suitably bonded around the electrically insulative sheath  697  coincident with the large diameter section  681  of the proximal stiffening member element  685 , as well as a large portion of the small diameter section  683  of the proximal stiffening member element  685 , using suitable means, such epoxy. The pusher member  18 ( 7 ) further comprises an electrically insulative sheath  696  disposed over the assembly, including the return electrode coil  686 , marker coil  692 , and portion of the electrically conductive sheath  698 . 
     The electrically conductive sheaths  693 ,  698  may take the form of, e.g., a mesh, braid, or coil. In the embodiment illustrated in  FIG. 12 , the electrically conductive sheaths  693 ,  698  take the form of coils. The electrically conductive sheaths  693 ,  698  are preferably composed of a material that is more electrically conductive than the material from which core wire of the stiffening member  680  is composed; for example, silver or copper. The electrically insulative sheaths  696 ,  697  may be composed of a suitable polymeric material, such as PTFE or TFE, and have suitable dimensions (e.g., a wall thickness of 0.002 inches and an inner diameter of 0.006 inches). 
     The pusher member  18 ( 6 ) further comprises an electrically conductive coil  686  that serves as a return electrode (i.e., the cathode of the bipolar implant assembly  14 ( 6 )), and a radiopaque marker, and in particular a platinum marker coil  692  that circumferentially extend around the stiffening member  690  in the same manner as the respective return electrode coil  486  and marker coil  492  described above. That is, the proximal end of the return electrode coil  686  is bonded around the securing coil  691 , and the marker coil  692  is bonded around the electrically conductive sheath  698  along the small diameter section  683  of the proximal stiffening member element  685 . Notably, the electrically conductive sheath  698  serves as a flex coil, and thus, a separate flex coil is not needed in this embodiment. 
     The return electrode coil  686  is spatially isolated from the severable joint  20  via a spacer element  690  mounted to the distal stiffening member element  587  at a location proximal to the severable joint  20  using a suitable adhesive. The return electrode coil  686 , marker coil  692 , and spacer element  690  may take the same form and be constructed of the same materials as the respective return electrode coil  486 , marker coil  492 , and spacer element  490  described above. Like the previous return electrode coil  86 , the return electrode coil  686  serves as a compression element and may be coated with a layer of silver chloride to further prevent or reduce the generation of gaseous bubbles. 
     Significantly, the sheath  696  circumferentially surrounds both the severable joint  20  and the return electrode coil  686 . Thus, like the previously described sheath  96  of the bipolar pusher member  18 ( 1 ), the sheath  696  increases the compressive strength of the pusher member  18 ( 7 ), and reduces diffusion and convection of an ideal electrolytic environment away from the detachment region when the implant assembly  14 ( 7 ) is exposed to the bodily fluids. As previously described, such ideal electrolytic environment can be created by introducing the ideal electrolyte into the detachment region or introducing water into the detachment region previously coated with salt. To facilitate wicking of the saline or water into the detachment zone, a hydrophilic coating can be applied to one or both of the severable joint  20  and return electrode coil  686  in the same manner previously described above. 
     A core wire at a portion of the proximal stiffening member  685 , and in the illustrated embodiment the proximal tip of the core wire, is left exposed to form the positive terminal  28  (shown in  FIG. 1 ) that electrically communicates with the severable joint  20  via a forward electrical path formed only by the stiffening member  680 . Any portion of the electrically conductive sheath  698  may serve as the negative terminal  30  (shown in  FIG. 1 ). Thus, like the previously described electrically conductive sheaths  593 ,  598 , the electrically conductive sheaths  693 ,  698  increase the conductance of the forward electrical path to the severable joint  20  and the return electrode path from the return electrode coil  686 . 
     Having described the arrangement and function of the medical system  10 , a method of its use in occluding an aneurysm  702  within a blood vessel  700  will now be described with reference to  FIGS. 13A-13C . The vaso-occlusive device  23  (shown in  FIG. 8 ) can similarly be delivered to the aneurysm  702  in the following manner, but for the purposes of brevity, only delivery of the vaso-occlusive device  23  will be described in detail. 
     Turning specifically to  FIG. 13A , the delivery catheter  12  is steered just within a neck  704  of the aneurysm  702 . At this point, the vaso-occlusive device  22  is in its undeployed shape, and is coupled to the pusher member  18  via the electrolytically severable joint  20 . The implant assembly  14  is situated within the lumen of the delivery catheter  12 , such that the vaso-occlusive device  22  resides within the distal end  54  of the delivery catheter  12 . 
     Turning to  FIG. 13B , the pusher member  18  is then pushed in the distal direction relative to the delivery catheter  12 , causing the vaso-occlusive device  22  to extend out of the distal end  54  of the delivery catheter  12 , through the neck  704 , and into the aneurysm  702 . As the vaso-occlusive device  22  is pushed out of the delivery catheter  12 , the portion of the vaso-occlusive device  22  that is free from the constraints of the delivery catheter  12  can assume its deployed shape. 
     Turning to  FIG. 13C , the pusher member  18  continues to be pushed in the distal direction relative to the delivery catheter  12  until the entire vaso-occlusive device  22  is deployed within the aneurysm  702 . The vaso-occlusive device  22  is then detached from the pusher member  18  by conveying an electrical current through the pusher member  18  to electrolytically dissolve the severable joint  20 . 
     In a bipolar arrangement (shown in  FIG. 1 ), detachment of the vaso-occlusive device  22  can be accomplished by conveying electrical current from the positive terminal  34  of the power supply  16  to the positive terminal  28  of the pusher member  18 , and along the forward electrical path within the pusher member  18  to the severable joint  20 , and conveying electrical current from the return electrode on the pusher member  18 , back along the return electrical path within the pusher member  18 , and then from the negative terminal  30  of the pusher member  18  to the negative terminal  36  of the power supply  16 . Additional vaso-occlusive devices  22  can be deployed within the aneurysm  702  as needed by removing the pusher member  18  from the delivery catheter  12 , inserting another implant assembly  14  within the lumen of the delivery catheter  12 , and repeating the steps illustrated in  FIGS. 13B and 13C . 
     In a monopolar arrangement (shown in  FIG. 2 ), detachment of the vaso-occlusive device  22  can be accomplished by conveying electrical current from the positive terminal  34  of the power supply  16  to the positive terminal  28  of the pusher member  18 , and along the forward electrical path within the pusher member  18  to the severable joint  20 , and conveying electrical current from the return electrode (if available) on the pusher member  18 , back along the return electrical path within the patient&#39;s body, and then from the ground electrode  32  (shown in  FIG. 2 ) to the negative terminal  36  of the power supply  16 . 
     Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.