Patent Publication Number: US-2023157863-A1

Title: Intrauterine device with controlled copper ion elution

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 15/441,539, entitled, “Intrauterine Device with Controlled Copper Ion Elution,” filed Feb. 24, 2017, which is a continuation-in-part of U.S. patent application Ser. No. 14/289,530, entitled “Intrauterine Device with Controlled Copper Ion Elution,” filed May 28, 2014, now U.S. Pat. No. 10,188,546, which claims priority to U.S. Provisional Patent Application No. 61/890,714, entitled “Intrauterine Device with Controlled Copper Ion Elution,” filed on Oct. 14, 2013. The full disclosures of the above-listed patent applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to medical devices. More specifically, the invention relates to intrauterine devices for contraception. 
     BACKGROUND 
     An intrauterine device (IUD) is a small, often T-shaped device, containing copper or hormone (e.g., levonorgestrel), which is inserted into the uterus to provide long-acting, reversible contraception. As of 2007, IUDs are the most widely used form of reversible contraception, with more than 180 million users worldwide. IUDs typically are one of two types—copper or hormonal. 
     Copper IUDs primarily work by disrupting sperm motility and damaging sperm so that the sperm are prevented from joining with an egg. Copper acts as a spermicide within the uterus, increasing levels of copper ions, prostaglandins, and white blood cells within the uterine and tubal fluids. The increased copper ions in the cervical mucus inhibit the sperm&#39;s motility and viability, preventing sperm from traveling through a thickened cervical mucus and/or destroying it as it passes through. Copper IUDs have a typical first-year contraceptive success rate greater than 98 percent. 
     Most copper IUDs have a plastic T-shaped frame that is wound with pure electrolytic copper wire and/or has copper collars (sleeves). The arms of the frame hold the IUD in place near the top of the uterus. The Paragard® intrauterine copper contraceptive device (www.paragard.com) and the generic TCU 380A are currently the most common copper IUDs. Copper IUDs can typically be implanted for up to 10 years. 
     The mechanism of action of a copper IUD is essentially a predictable electrochemical process of corrosion, driven by the IUD&#39;s immersion in the uterine fluid. Over time, the copper element (wires, beads, tubes, etc.) corrodes and releases copper ions. Typically, the release rate is governed by the surface area and volume of copper, the chemistry (especially pH) of the uterine fluid, and the specific composition and surface properties of the copper elements. The frame of the IUD on which the copper is attached is typically made of a flexible, non-corrosive polymer. Since typical polymers are not electrochemically active, they do not influence the ion release rate of the copper elements. Therefore, the copper ion release characteristics of a typical plastic-frame IUD are strictly governed by the amount, preparation and configuration of the copper itself. One challenge with copper IUDs has been that increased effectiveness typically means increased amounts of copper on the IUD, and increased amounts of copper typically lead to increased side effects, primarily increased menstrual and intermenstrual bleeding, and increased pelvic pain. Balancing effectiveness with minimization of side effects has been a continuing challenge for making copper IUDs. 
     One advantage of the copper IUD is its ability to provide emergency contraception up to five days after unprotected sex. It is generally known as the most effective form of emergency contraception available. Another advantage is that it contains no hormones, so it can be used while breast feeding, and fertility returns quickly after removal. Lastly, copper IUDs have been shown to be clinically effective for up to 10 years of continuous use. Disadvantages include the possibility of heavier menstrual periods and more painful menstrual cramping in some women. 
     A hormonal IUD, such as the Mirena® Levonorgestrel-releasing intrauterine system (www.mirena-us.com), uses the controlled release of a hormonal contraceptive such as Levonorgestrel (a progestin). The hormonal contraceptive released from a hormonal IUD prevents ovulation from occurring. The hormone also thickens the cervical mucus so that sperm cannot reach the fallopian tubes. Hormonal IUDs can typically be implanted for up to 5 years. 
     Hormonal IUDs do not increase bleeding as copper-containing IUDs do. Instead, they can often reduce menstrual bleeding or prevent menstruation altogether, and thus they are sometimes used as a treatment for menorrhagia (heavy menstrual bleeding). Hormonal IUDs have common side effects, however. The most common side effects with levonorgestrel IUDs are hormone-related effects, such as headaches, nausea, breast tenderness, depression and cyst formation. 
     Based on effectiveness and the types of side effects, copper IUDs are preferable for many women. Although currently available copper IUDs are very effective, it would still be advantageous to have improved copper IUDs. For example, it would be advantageous to have copper IUDs that were very effective but had minimal side effects. 
     BRIEF SUMMARY 
     Disclosed herein are various embodiments of an improved copper IUD. Generally, the described embodiments provide an IUD with a desirable (or “controlled”) release rate of copper ions. Different embodiments may be configured with different copper ion release rates. For example, a particular copper ion release rate may be identified to provide a desired balance between IUD effectiveness and reduced side effects, compared to currently available devices. 
     In one aspect of the present application, an intrauterine contraceptive device may include a frame comprising a first metal having a first galvanic potential and at least one metallic member coupled with the frame, the at least one metallic member comprising a second metal having a second galvanic potential different from the first galvanic potential. In some embodiments, the first galvanic potential may be more anodic than the second galvanic potential. In alternative embodiments, the first galvanic potential may be more cathodic than the second galvanic potential. 
     In some embodiments, the frame may be made entirely of metal. Alternatively, the frame may be made partially of metal and partially of a non-metallic substance. For example, the first metal may in some embodiments be Nitinol. The second metal may be copper. In some embodiments, for example, the metallic members are multiple copper sleeves disposed around the frame at various locations. Some embodiments include the combination of the first metal being Nitinol and the second metal being copper. In embodiments where the second metal is copper, the first galvanic potential and the second galvanic potential may be configured to achieve approximately a desired copper elution rate of the at least one metallic member. In all of these embodiments, the non-metallic substance may be a polymer, a silicone rubber or other engineered polymer plastic, for example. Optionally, the device may further include a retrieval string attached to one end of the metal frame. 
     In another aspect of this disclosure, a method of preventing conception may involve advancing an intrauterine contraceptive device into a uterus, where the intrauterine device includes a frame comprising a first metal having a first galvanic potential, and at least one metallic member coupled with the frame, comprising a second metal having a second galvanic potential different from the first galvanic potential. In some embodiments, the second metal may be copper, and the first and second galvanic potentials may be configured to achieve approximately a desired copper ion elution rate. In some embodiments, the first metal may be Nitinol, and advancing the contraceptive device into the uterus may involve advancing a distal end of an introducer device through a cervix, into the uterus, and advancing the contraceptive device out of the distal end, thus allowing the Nitinol of the frame to expand to contact a wall of the uterus. 
     In yet another aspect of the disclosure, a method of manufacturing an IUD for contraception that has a desired copper elution rate may involve: selecting an elongate shape memory member comprising a first metal having a first galvanic potential; selecting at least one attachment member comprising a second metal having a second galvanic potential that is different from the first galvanic potential; attaching the attachment member(s) to the shape memory member; and shaping the shape memory member to form a frame of the intrauterine contraceptive device. The first metal and the second metal may be specifically selected as materials for the shape memory member and the at least one attachment member, in order to achieve the desired copper elution rate. 
     In some embodiments, the shape memory member may be a wire. Merely by way of example, the first material may be Nitinol, stainless steel, titanium, cobalt-based alloys, tantalum, platinum, gold, silver, or MP35N. In one embodiment, for example, the first metal is Nitinol, and the second metal is copper. Some embodiments may include multiple copper sleeves, which may be attached to the frame, for example, by passing the multiple copper sleeves over the shape memory member. In such an embodiment, the method may also include positioning the multiple sleeves along the shape memory member at different positions before or after shaping the shape memory member. The attachment member(s) may have any of a number of different forms, such as but not limited to sleeves, wires, ribbons, coils, tubes and beads. 
     In some embodiments, the first galvanic potential is more anodic than the second galvanic potential. Alternatively, the first galvanic potential may be more cathodic than the second galvanic potential. In some embodiments, the frame is made entirely of the first metal. In other embodiments, the frame is made partially of the first metal and partially of a non-metallic substance. For example, the frame may include a non-rigid material, such as but not limited to a flexible polymer thread, a polyester thread, a nylon thread or other biocompatible fabric. Optionally, the method may also include attaching a retrieval string to a bottom portion of the frame. 
     In one embodiments, shaping the shape memory member may involve forming the frame to include: a bottom loop; a middle portion comprising two parts of the elongate shape memory member extending upward from the bottom loop; multiple twists in the middle portion, forming a spring portion; two bends in the elongate shape memory member at a location above the multiple twists, where the elongate member crosses over itself; a first arm extending from a first bend of the two bends; a second arm extending from a second bend of the two bends; a first end loop at an end of the first arm; and a second end loop at an end of the second arm, where the first end loop and the second end loop in the elongate shape memory member comprise tissue contact surfaces. 
     In another aspect of this application, a method of manufacturing an IUD for contraception that has a desired copper elution rate may involve: selecting a wire comprising a first metal having a first galvanic potential; selecting at least one attachment member comprising a second metal having a second galvanic potential that is different from the first galvanic potential; attaching the at least one attachment member to the wire; and shaping the wire to form a frame. The shaping step may result in a frame that has: a bottom loop; a middle portion; multiple twists in the middle portion; two bends in the elongate shape memory member at a location above the multiple twists in the middle portion, where the elongate member crosses over itself; two arms extending from the two bends; a first end loop at an end of one of the two arms; and a second end loop at an end of the other of the two arms. 
     These and other aspects and embodiments will be described in further detail below, in reference to the attached drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a front view of a prior art, T-shaped, copper IUD with a plastic support structure; 
         FIG.  2    is a bar graph illustrating a galvanic series for a number of common metals and alloys; 
         FIG.  3    is a front view of a copper IUD with a metal frame, according to one embodiment; 
         FIG.  4    illustrates potentiodynamic polarization curves for copper sleeves tested in simulated uterine fluid at 37° C., pH=7.1; 
         FIG.  5    illustrates potentiodynamic polarization curves for bare Nitinol frames tested in simulated uterine fluid at 37° C., pH=7.1; and 
         FIG.  6    illustrates potentiodynamic polarization curves for finished device assemblies (copper sleeves crimped onto Nitinol frames) tested in simulated uterine fluid at 37° C., pH=7.1. 
     
    
    
     DETAILED DESCRIPTION 
     The mechanism of action of a copper IUD is essentially a predictable electrochemical process of corrosion, driven by the IUD&#39;s immersion in the uterine fluid. Over time, the copper elements of the IUD (copper wires, beads, tubes, or the like) corrode and release copper ions. For a given IUD, the copper ion release rate can be measured using standard laboratory tests in simulated uterine fluid (SUF). Typically, the release rate is governed by the surface area and volume of copper, the chemistry (especially pH) of the SUF, and the specific composition and surface properties of the copper elements. In currently available IUDs, the primary driver influencing the copper release rate is the amount of exposed copper surface area. In other words, to achieve a higher copper ion release rate, an IUD would typically have an increased copper surface area, which generally means the IUD will contain more copper overall. 
     Referring now to  FIG.  1   , a typical, prior art, T-shaped, copper IUD  10  is illustrated. The IUD  10  includes a frame  12 , copper sleeves  14  disposed around the frame  12  in three areas, and a retrieval string  16 . The frame  12  is typically made of a flexible polymer and is often T-shaped for positioning in the uterus. Since typical polymers are not electrochemically active, they do not influence the ion release rate of the copper elements  14 . Therefore, the copper ion release characteristics are strictly governed by the amount of surface area of the copper sleeves  14  themselves. 
     Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte. The strength of this electrical potential between two metals or alloys is called the galvanic potential. The galvanic reaction can be exploited for positive benefit. For example, a galvanic potential between dissimilar metals can be used in primary batteries to generate an electrical voltage. The measured voltage is representative of the transfer of charged metal ions from one of the metals (the anode) to the other (the cathode). The stronger the galvanic effect between the two metals, the higher the transfer rate of ions from the anode to the cathode. 
     To determine the galvanic potential between two metals, we use a galvanic series, an example of which is illustrated in  FIG.  2   . In a galvanic series, metals are ranked by the electrical potential they develop in a given electrolyte against a standard reference electrode— i.e., their galvanic potential versus the reference electrode. The relative position of two metals on such a series determines which metal will be the anode and which will be the cathode (if they are put in contact in an appropriate electrolyte). The magnitude of the distance between the two metals on the galvanic series will determine the strength of the galvanic potential between them. A specific galvanic series can be determined for any metals or alloys of interest, tested in a specific electrolyte (e.g., SUF) at an appropriate pH. 
     Referring now to  FIG.  3   , in one embodiment, an IUD  20  may include a metal frame  22  (or “shape memory member”), multiple copper sleeves  24  (or “copper elements”), and a retrieval string  26 . This embodiment of the IUD  20 , as well as other, alternative embodiments, is described in further detail in pending U.S. patent application Ser. No. 13/795,940, entitled “Intrauterine Contraceptive Device,” filed Mar. 12, 2013, which is hereby incorporated by reference in its entirety. In this embodiment, the shape memory member  22  is generally made of a wire or wire-like material, and the copper sleeves  24  are generally tubular or partially tubular structures. In alternative embodiments, however, the shape memory member  22  and/or the sleeves  24  may take any of a number of different forms. For example, in one embodiment, the shape memory member  22  may be configured more like a flat ribbon that a cylindrical wire. In other embodiments, the sleeves  24  may be replaced by any other suitable attachment members, such as balls, coatings, clamps, tape or the like. Thus, the sleeves  24  are merely one example of a type of attachment member that might be used in forming the IUD  20 . 
     In various embodiments, the elongate shape memory member  22  may have a two-dimensional (i.e., predominantly flat) configuration, as shown, or may have a more three dimensional (i.e., slightly bent out of plane) configuration. The elongate shape memory member  22  may include a loop portion  36  at the bottom (or “vertex”) of the IUD  20 , two generally parallel portions  35  extending vertically upward from the loop portion  36 , multiple twists  34 , a middle portion, two bends  32  that form the shape memory member  22  into two, approximately oppositely directed arms  31 , and two tissue contact surfaces (or “end points”)  28  at the ends of the two arms  31 . The copper sleeves  24  are disposed around the shape memory member  22  at multiple locations, such as near each of the tissue contact surfaces  28 , along the arms  31  between the bends  32  and the tissue contact surfaces  28 , and at or near the loop portion  36 . Different numbers and positions of the copper sleeves  24  may be provided in different alternative embodiments. 
     In this embodiment, the bottom loop portion  36  does not form any turns of a spring structure. Instead, to generate resilience in the arms  31  of the IUS  20 , this embodiment includes twists the  34  between the parallel portions  35  and the middle portion  30 . Twists  34  may be formed when the two portions  35  of the elongate member  22  cross each other more than once in a longitudinally extending helix containing multiple turns that run in series from closer to the loop portion  36  upward, until the arms diverge again from the twisted portion  34 .  FIG.  3    shows three twists  34 . However, other numbers of twists (e.g., 1-5, 6-10, greater than 10) are also possible. In this embodiment, the twists  34  act as the “spring portion” to confer laterally directed pressure to tissue contact surfaces  28 . 
     It may be desirable to be able to easily modify the properties of the elongate member  22 . For example, the anatomical dimensions or characteristics of the uterus or medical conditions of a patient may call for adjustments to the outward force produced by the IUD  20 . In such applications, the location of the twists  34  along the shape memory member  22 , the number of the twists  34 , and/or the tightness of the twists  34  may be used to provide variability in the properties of the device  20 . For example, increasing the number of twists  34  may increase the stiffness of the elongate member  22 , and decreasing the number of twists  34  may decrease the stiffness of the elongate member  22 . For another example, increasing the tightness of the twists  34  may increase the stiffness of the elongate member  22 . Decreasing the tightness of the twists  34  may decrease the stiffness of the elongate member  22 . Moving the twists  34  upward along the shape memory member  22  may increase the stiffness of the elongate member  22 , and moving the twists downward along the shape memory member  22  may decrease the stiffness of the elongate member  22 . With the device of  FIG.  3   , a variety of differently configured devices can be made available to physicians to apply in the appropriate circumstances. The variability provided by the twists  34  also allows much easier and more continuous customization of properties by merely modifying the nature of the twists  34 , rather than the thickness or material of the wire used to for the shape memory member  22  itself to produce similar variations in properties. Wires come in standard sizes and materials, and if some intermediate is desired, the expense can make producing such a modified device impractical. Controlling properties of the device with the properties of the twists  34  resolves this issue. 
     According to various embodiments, the galvanic effect can be exploited in the copper IUD  20  by coupling the copper elements  24  to the metal frame  22  or other metal structure. The material of the metal frame  22  or other elongate shape memory structure may be chosen to achieve a desired galvanic effect between itself and the copper elements  24 . In one embodiment, for example, the frame  22  may be made of Nitinol (see  FIG.  3   , on which Nitinol and copper are depicted). In other embodiments, however, any of a number of other suitable biocompatible metals (or combinations of metals) may be selected for making the frame  22 . In various embodiments, the type of material selected for the frame  22 , the characteristics of the material (thickness, for example), the overall size of the IUD  20  (and thus the amount of the material used to make the frame  22 ), and/or other characteristics of the frame  22  may be specifically selected to provide a particular copper elution rate. Similarly, the type of size, number, positioning, and/or other factors pertaining to the copper sleeves  24  may be specifically selected to provide a particular embodiment with a particular copper elution rate. In other words, any of a number of characteristics of the frame  22  and/or the sleeves  24 , such as but not limited to their materials, may be selected to provide a desired copper elution rate. 
     For the sake of comparison, a “baseline” copper ion elution rate may be defined as one in which there is no galvanic effect present (e.g., copper on a polymer frame, as in prior art IUDs). If the metal frame  22  (or other metallic structure) is selected to have a galvanic potential more anodic than the galvanic potential of the copper sleeves  24  (lower on the galvanic series), then the electrochemical driving force would be toward the copper sleeves  24 , thereby reducing the copper ion release rate relative to the baseline. Alternatively, if the metal frame  22  or structure that has a galvanic potential more cathodic than the copper sleeves  24  (higher on the galvanic series), then the electrochemical driving force would be away from the copper sleeve  24 , thereby increasing the copper ion release rate relative to the baseline. The greater the difference between the materials on the galvanic series, the more significant the decrease or increase in copper ion release rate. The galvanic potential is also related to the relative surface areas of the anode and the cathode, so the surface area ratio between the frame  22  and the copper sleeves  24  may also be selected to achieve a desired ion release rate. 
     In such a way, the material of the frame  22  and/or other elements in contact with the copper elements  24  may be selected to control the galvanic potential between the frame  22  and the copper  24 , thereby allowing specificity, control and optimization of the ion release rate. This may have a number of advantages. In some embodiments, the IUD  20  may achieve a copper release rate comparable or identical to that of a currently available (i.e., prior art) IUD with a lower amount (i.e., less surface area) of copper, by coupling the copper  24  with the metal frame  22  that is more cathodic than the copper  24 . The smaller surface area of the copper  24  may help reduce copper-related side effects, compared to prior art copper IUDs. Alternatively, in other embodiments, the IUD  20  may be configured to release copper ions at a rate similar or identical to that of a currently available copper IUD but for a longer period of time. This may be achieved by giving the copper sleeves  24  a greater exposed surface area than the copper element(s) of a currently available device, while coupling the sleeves  24  with the frame  22  made of a material that is more anodic than the copper elements  24 . In this case, the greater amount of available copper is balanced by a slower release rate to achieve, overall, a similar ion release profile. However, in this case, the slower release rate with a larger copper reservoir allows the elution to happen over a potentially longer period of time, thereby extending the effectiveness of the IUD  20  beyond the typical 10 years of device life. 
     In still other embodiments, the IUD  20  may achieve a copper ion release rate whose release curve is more curvilinear or more linear in comparison to the standard commercial copper IUD release rate curve. In this manner, a metallic IUD with a controlled release rate may be chosen to avoid the burst release of copper ions typically associated with commercial copper IUDs which implement a plastic core. The burst release of copper ions has been shown to be directly associated with the onset of clinical side effects of the standard commercial copper IUDs, including in particular, side effects experienced in the first 1 to 6 months of IUD use, including increased menstrual bleeding, intermenstrual bleeding, cramping and general pelvic pain. Such a reduction in clinical side effects without with the need for ancillary medications would be a significant advantage. 
     In various embodiments, a method for making the IUD  20  may first involve selecting the wire material for making the metal frame  22  and selecting (or perhaps manufacturing) the copper sleeves  24 . The metal frame  22  and/or any other metal structure(s) on IUD  20  may be made from any material with desired galvanic properties, including but not limited to Nitinol, stainless steel, titanium, cobalt-based alloys, tantalum, platinum, gold, silver, and MP35N. The frame  22  may be made from one material or multiple materials. In alternative embodiments, the frame  22  may be made entirely of metal or may have discrete active metallic portions or elements, some or all of which may be in contact with the copper elements  24 , with non-metallic parts between the metallic portions or with no parts at all between the metallic portions. The copper elements  24  may take any suitable shape, size, form and number, according to various embodiments. In the embodiment illustrated in  FIG.  3   , there are ten copper sleeves  24  disposed on the frame  22 . Alternative embodiments may include more or fewer sleeves  24  and/or smaller or larger sleeves  24 . In other alternative embodiments, the sleeves  24  may be replaced by, or used in combination with, other copper elements, including but not limited to wire, ribbon, coils, tubes, beads, and the like. Additionally, surface processing of the frame  22  and/or other metallic structure may be tailored to achieve a desired galvanic potential. For example, this may be accomplished through heat treatment, oxidation, electropolishing, passivation, and/or other chemical processing. 
     In some embodiments, the sleeves  24  may be complete tubes. In these embodiments, the sleeves  24  may be passed onto the frame  22  before the frame  22  is bent and twisted to assume its final shape. Alternatively, each sleeve  24  may be a partial tube, and each may include a slit, for example, so that the sleeves  24  can be placed over the frame  22  after the frame  22  has been formed. In some embodiments, some sleeves  24  may be placed over the frame  22  before it is formed into its final shape, and other sleeves  24  may be placed over the frame  22  after it is shaped. 
     Description of Corrosion Testing 
     To determine the baseline corrosion behavior of a Nitinol frame and copper sleeves, such as those described above in reference to  FIG.  3    (together referred to below as the “ContraMed IUD”), and to evaluate the effects of different surface processing on the Nitinol frame, preliminary potentiodynamic polarization testing was conducted. Processing of the Nitinol surface was targeted to achieve a rest potential (“Erest”) similar to or higher than that of the copper sleeves, in order to both optimize the intentional corrosion (and ion release) of the copper and minimize any deleterious galvanic corrosion effects on the Nitinol frame. A review of the literature of electropolished, etched, chemically-polished, and heat-treated (with oxide) Nitinol revealed that rest potentials can be of a wide range, from less than −420 mV to as high as +20 mV vs. SCE. 
     Potentiodynamic polarization testing was conducted according to ASTM F2129 on copper sleeves and Nitinol frames made from different starting wires and processed in different ways. This testing was conducted in phosphate buffered saline (PBS) at 37° C. at a pH of 7.1. PBS was chosen for this testing for simplicity as well as to allow comparison of the Nitinol corrosion behavior with data from the literature. The testing was conducted using a Gamry Cyclic Polarization Corrosion Apparatus. 
     Testing was also conducted in simulated uterine fluid (SUF) per the following composition: NaHCO3 0.25; NaH2PO4 2H20 0/072; Glucose 0.50; CaCl2 0.167; KCl 0.224; NaCl 4.97. The Simulated Uterine Fluid solution was purged with nitrogen gas, and pH was adjusted to 7.1 before testing. The Rest Potential (“Er”) was monitored for one hour before the corrosion test began. Specimens were subjected to a scan from below the zero current potential to 1.0 V at a rate of 1 mV/s. No reverse scan was conducted, and the test was stopped after a scan of −1 mV/s to 0.8 V (vs. Er). 
     Results 
     As described previously, the ideal Nitinol surface processing scheme would achieve a rest potential (Erest) that is similar to or higher than that of the copper sleeves (−286.1 mV), in order to both optimize the intentional corrosion (and ion release) of the copper and minimize any deleterious galvanic corrosion effects on the Nitinol frame. At the same time, the Nitinol surface should demonstrate excellent resistance to breakdown. Based on the work of Rosenbloom and Corbett, a Nitinol surface with an average breakdown potential in excess of 600 mV (as tested in PBS at 37° C.) is generally considered to be in an optimum state for use as a medical implant. 
       FIGS.  4 - 6    illustrate plots of all the curves for a given sample group on the same axes.  FIG.  4    illustrates potentiodynamic polarization curves for copper sleeves tested in simulated uterine fluid at 37° C., pH=7.1.  FIG.  5    illustrates potentiodynamic polarization curves for bare Nitinol frames tested in simulated uterine fluid at 37° C., pH=7.1.  FIG.  6    illustrates potentiodynamic polarization curves for finished device assemblies (copper sleeves crimped onto Nitinol frames) tested in simulated uterine fluid at 37° C., pH=7.1. As illustrated in  FIGS.  4 - 6   , the copper curves and the Nitinol curves are very different. Copper corrodes via generalized corrosion, while Nitinol, due to its formation of a protective surface oxide layer, corrodes via breakdown (i.e., pitting). As expected, the corrosion of the assembled devices is dominated by corrosion of the copper sleeves, and no breakdown behavior is observed in the potentiodynamic polarization curve. 
     Although various embodiments and features are described herein, the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Thus, the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above.