Patent Publication Number: US-2009222036-A1

Title: Embolic filter loop fabricated from composite material

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 10/410,896 filed Apr. 11, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of embolic protection devices. More specifically, the present invention relates to embolic protection filters fabricated from composite materials having certain radiopaque and elasticity characteristics. 
     BACKGROUND OF THE INVENTION 
     Intravascular devices such as embolic protection filters are generally placed in a body lumen such as an artery or vein, downstream of a therapeutic site to filter emboli contained in the blood. In a typical procedure such as percutaneous transluminal coronary angioplasty (PTCA), an embolic protection filter is loaded into a delivery device such as a catheter or sheath, and advanced in a collapsed position to a location downstream of a lesion. Once positioned downstream the lesion, the embolic protection filter is ejected from the delivery device and deployed in the blood vessel. An angioplasty catheter containing an angioplasty balloon is then advanced along the guidewire and positioned across the site of the lesion to be dilated. The angioplasty balloon is then inflated, forcing the lesion to rupture and dislodge from the wall of the vessel. The dislodged debris is then carried downstream towards the embolic protection filter, where it can be collected and stored for later removal. 
     A number of embolic protection filters have been developed to filter embolic debris contained in the blood stream. Typically, these devices contain a support structure coupled to a filter membrane that filters contaminants in the blood stream. The support structure generally includes a number of wires, ribs, or struts formed of a relatively elastic material that can be used to support the filter membrane in an expanded position within the vessel. At the conclusion of the therapeutic procedure, the embolic protection filter is then collapsed within a retrieval catheter and removed from the vessel. 
     To monitor the placement and operation of the embolic protection filter, a radiopaque coil or marker band disposed on the filter can be used to produce a radiopaque image on a fluoroscopic monitor located outside of the patient&#39;s body. In one technique, for example, a radiopaque coil comprising a relatively dense metal such as platinum or tantalum is placed about the portion of the filter frame that forms the proximal mouth of the filter where embolic debris enters the filter membrane. In use, the radiopaque coil produces a bright image on the fluoroscopic monitor, allowing the operator to visualize the filter within the body. While many designs in the art have focused on the use of separate radiopaque elements to provide the necessary radiopacity, such designs often entail added manufacturing and processing steps, adding to the complexity and, in some cases, the size of the embolic protection filter. As such, it is desirable to have a filter support structure formed of composite materials exhibiting certain desirable radiopaque and elasticity characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention relates to embolic protection filters fabricated from composite materials. An embolic protection filter in accordance with an exemplary embodiment of the present invention includes a support structure and a filter membrane for filtering embolic debris contained in the blood. The support structure may comprise a wire loop formed of a composite member having one or more outer layer(s) surrounding an inner core member. The various layers may be formed from materials exhibiting certain desirable characteristics such as elasticity, radiopacity and biocompatibility. In certain embodiments, the support structure may include a bend region formed by grinding away a portion of the outer surface of the composite member. Methods of forming the composite member, including drawing and cladding, are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a perspective view of an embolic protection filter in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a perspective view of the wire loop of  FIG. 1 , showing the wire loop in an expanded position; 
         FIG. 3  is a cross-sectional view of the wire loop along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a wire loop in accordance with an exemplary embodiment of the present invention, wherein the wire loop is formed of a composite member having three layers; 
         FIG. 5  is a perspective view of a wire loop in accordance with an exemplary embodiment of the present invention, wherein the wire loop includes a bend region; and 
         FIG. 6  is a graph of stress as a function of strain for a composite wire loop placed under tension. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized. 
       FIG. 1  is a perspective view of an embolic protection filter  10  in accordance with an exemplary embodiment of the present invention. Embolic protection filter  10  comprises a filter membrane  12  coupled to a filter frame  14  or other support structure configured to support the filter membrane  12  in an expanded position within a body vessel. Filter frame  14  includes a wire loop  16  formed of a composite member having radiopaque properties that permits the wire loop  16  to be visualized via fluoroscopy. The wire loop  16  also has certain desirable elasticity characteristics that permit the embolic protection filter  10  to be radially collapsed within a delivery device such as a catheter or sheath. As shown in  FIG. 1 , the ends  18 , 20  of the wire loop  16  may be attached to a tubular member  22  configured to slide along a guidewire  24  or other guiding member within the body, allowing movement of the embolic protection filter  10  relative to the guidewire  24 . In an alternative embodiment (not shown), the ends  18 , 20  of the wire loop  16  can be fixedly secured directly to the guidewire  24 , in which case the guidewire  24  can be used to advance the embolic protection filter  10  within the body. 
       FIG. 2  is a perspective view of the wire loop  16  of  FIG. 1 , showing the wire loop  16  in an expanded position. As illustrated in  FIG. 2 , wire loop  16  is formed from a single, composite wire member having a circular, hoop-like shape that forms an opening  26  that receives embolic debris contained in the bloodstream. The size of the opening  26  can be selected to permit the embolic protection filter  10  to be deployed in various locations within the body, such as the coronary or peripheral arteries. 
       FIG. 3  is a transverse cross-sectional view of the wire loop  16  along line  3 - 3  in  FIG. 2 . As can be seen in  FIG. 3 , wire loop  16  may comprise a composite wire member having an outer layer  28  concentrically disposed about an inner core member  30 . The outer layer  28  and inner core member  30  may be formed of differing materials selected to exhibit various desirable mechanical properties, including stiffness (i.e. modulus of elasticity), biocompatibility, durability, tensile strength, compressive strength, and radiopacity. In certain embodiments, for example, the outer layer  28  may be formed of a superelastic or pseudoclastic metal such as nickel-titanium alloy (Nitinol), which exhibits characteristics such as high elasticity and biocompatibility. Superelastic metals such as Nitinol are capable of providing excellent resistance to bio-corrosion while providing a greater level of elasticity than other metals frequently used in the art, such as stainless steel or titanium. Other superelastic materials that can be used to form the outer member  28  include silver-cadmium (Ag—Cd), gold-cadmium (Au—Cd), gold-copper-zinc (Cu—Au—Zn), copper-aluminum-nickel (Cu—Zn—Al), copper-gold-zinc (Cu—Au—Zn), copper-zinc (Cu—Zn), copper-zinc-aluminum (Cu—Zn—Al), copper-zinc-tin (Cu—Zn—Sn), copper-zinc-silicon (Cu—Zn—Si), iron-beryllium (Fe 3 Be), iron-nickel-titanium-cobalt (Fe—Ni—Ti—Co), iron-platinum (Fe 3 Pt), indium-thallium (In—Tl), iron-manganese (Fe—Mn), nickel-titanium-cobalt (Ni—Ti—Co), and copper-tin (Cu—Sn). 
     While superelastic alloys have certain desirable biocompatibility and elasticity characteristics, it should be understood that the outer layer  28  may be formed of any number of suitable materials, including stainless steel (e.g. type 304 or 316), titanium, Beta III Titanium, or precursors of superelastic alloys. Precursor alloys are similar in chemical composition to superelastic alloys, but have not been processed to impart superelasticity during use, thereby exhibiting linear stress-strain characteristics in the body. Such alloys are further described in co-owned U.S. Pat. No. 5,238,004 to Sahatjian et al., the entire contents of which are incorporated herein by reference. 
     The outer layer  28  may also be configured to exhibit certain shape-memory properties within the body. Using a superelastic material such as Nitinol, wire loop  16  can be configured to assume a particular shape, such as that illustrated in  FIG. 2 , when deployed from the delivery catheter and exposed to temperature within the body. The shape-memory material may be binary or ternary in form, and can be configured such that the wire loop  16  reverts back to a collapsed shape when loaded into the delivery device and cooled using, for example, a cooling fluid. 
     The outer surface  32  of the wire loop  16  may also include a lubricious (e.g. hydrophilic) coating such as polytetraflouroethylene (PTFE) to permit the wire loop  16  to be easily collapsed within the delivery device, and to reduce trauma to the vessel wall during deployment. Moreover, the wire loop  16  may include an anti-thrombogenic agent such as heparin (and its derivatives), urokinase, or PPack (dextrophenylalanine proline arginine chloromethylketone) to reduce thrombosis from occurring when the wire loop  16  is placed into contact with the vessel wall. 
     The inner core member  30  may comprise a metal or metal/polymer blend having a density generally greater than that of the outer layer  28  to enhance the radiopacity of the wire loop  16 . The inner core  30  may be formed of a continuous solid mass extending through the entire length of the wire loop  16 , or may be in a powder-form. Examples of some materials that can be used as the inner core member  30  include bismuth, gold, iridium, palladium, platinum, silver, stainless steel, tantalum, titanium, tungsten, or alloys of these materials. The inner core member  30  and/or the outer layer  28  may also be loaded with a radiopaque filler material such as bismuth (Bi), barium (e.g. BaSO 4 ), or tungsten (W) to enhance the radiopacity of the wire loop  16 , if desired. 
     In certain embodiments, the inner core member  30  may have a higher modulus of elasticity, and hence greater stiffness, than the material used to form the outer layer  28 . By way of example, the inner core member  30  may comprise platinum having a modulus of elasticity of about 171 GPa whereas the outer member  28  may comprise a nickel-titanium alloy having a modulus of elasticity of about 40-75 GPa, depending on whether the material is in its martensite or austenite phase. Generally, the smaller the difference between the modulus of elasticity of the outer layer  28  and the inner core member  30 , the smaller the variation of modulus of the overall wire support  16  in comparison to support structures containing a single (i.e. non-composite) material. For larger variations, the size of the inner core member  30  can be reduced to produce a wire loop  16  in which the elastic properties of the wire loop  16  are dominated by the outer layer  28 . 
     The location of the various layers  28 , 30  used to form the composite wire loop  16  may also be changed to alter the particular mechanical characteristics of the device. For example, in certain embodiments the outer layer  28  may comprise a radiographically dense material to enhance the radiopacity of the composite wire loop  16  whereas the inner core member  30  may comprise a relatively elastic material such as superelastic nickel-titanium alloy to impart elasticity to the wire loop  16 . The location of each layer forming the composite may be selected to change one or more properties of the wire loop  16 , such as tensile strength, compressive strength, bendability, durability or torquability. 
       FIG. 4  is a cross-sectional view of a wire loop  116  in accordance with another exemplary embodiment of the present invention, wherein wire loop  116  is formed of a composite wire member having three layers  128 , 130 , 134 . Similar to the embodiment of  FIGS. 2-3 , wire loop  116  may include an outer layer  128  formed of a material having certain desirable elasticity and biocompatibility characteristics, and an inner core member  130  formed of a relatively dense material (e.g. platinum) to impart the desired radiopacity and resolution to the wire loop  116 . Alternatively, the location of the layers may be interchanged such that the outer layer  128  is be formed of a relatively dense material whereas the inner core member  130  is formed of a material having certain desirable elasticity characteristics. In either embodiment, the outer surface  132  of the wire loop  116  may also optionally include an anti-thrombogenic agent or lubricious coating to reduce trauma to the vessel wall. 
     Wire loop  116  may further include a third, intermediate layer  134  disposed between the outer layer  128  and inner core member  130  selected to exhibit various properties described herein. In certain embodiments, for example, the intermediate layer  134  may be formed from a material having a modulus of elasticity between that of the outer layer  128  and inner core member  130  to blend the stiffness transversely across the wire loop  116 . By way of another example, the intermediate layer  134  may be formed of a material that acts as a tie-layer to more effectively bond the outer layer  128  to the inner core member  130 . 
     While the exemplary embodiments illustrated herein show composite wire loops having a transverse cross-sectional dimension that is substantially circular in shape, other configurations such as square, triangular, hexagonal, octagonal, trapezoidal, etc. may be employed. For example, the outer layer(s) of the composite wire loop may be rectangular in cross-sectional area with a rectangular-shaped inner core member. In some embodiments, the wire loop may be formed from composite tubing, wherein the inner layer(s) of the tubing can be formed of materials having the desired radiopacity characteristics whereas the outer layer(s) exhibit the desired elasticity properties. The wire loop may comprise a single strand of composite wire, or may comprise multiple composite members that are wound or braided together, forming a multifilament wire loop. 
     The wire loop can be drawn-formed, for example, by removing the center portion of a rod comprised of the outer layer material to form a bore through the rod, and then inserting a smaller rod of inner core material through the bore. With the ends of the wire loop sealed (e.g. by crimping), the wire loop is then drawn through a series of dies of decreasing diameter until the desired dimensions of the wire loop are achieved. The drawn-formed wire loop can then be subjected to various heat-treatment steps, if desired, to anneal, harden, or impart superelastic and/or shape-memory properties to the layers. Such processes are described in co-owned U.S. Pat. No. 5,628,787 to Mayer, the entire contents of which are incorporated herein by reference. 
     Other manufacturing methods such as cladding or plating may also be possible, depending on the particular type of materials used and mechanical properties desired. In a cladding process, for example, one or more metallic outer layers and a metallic inner core member having certain desirable radiopacity and elasticity characteristics can be bonded together at a suitable temperature and pressure, causing the various layers to diffuse together at each interface to form an alloy. 
     The type of materials used to form the outer layer(s) and the inner core member of the wire loop can be selected based on the particular manufacturing process implemented. For example, a drawing process may be particularly desirable for forming composite wire loops formed of superelastic materials, such as Nitinol, imparted with shape-memory and/or superelasticity characteristics that may be affected at certain temperatures. 
     In one exemplary method, a wire loop may be first formed, for example, by a drawing or cladding process, and then subsequently ground at a middle region to form a bend region on the wire loop. As shown in  FIG. 5 , for example, a wire loop  216  similar to that described above with respect to  FIGS. 2-3  may include a bend region  236  formed by grinding away a portion of the outer surface of the wire loop  216 . Because of the reduction in diameter of the wire loop  216  at the bend region  236 , the wire loop  216  can be subjected to further bending, and in some cases, loaded into a delivery device having a smaller profile. 
     Example 
     An exemplary composite wire loop suitable for use in an embolic protection filter may be formed as follows. A 0.0010-inch to 0.0100-inch diameter drawn-formed composite wire may include a nickel-titanium outer layer that forms about 80% of the total transverse cross-sectional area of the wire loop, and an inner core member comprising a 90% platinum (Pt) 10% nickel (Ni) metal composition that forms the remaining 20% of the total transverse cross-sectional area of the wire loop. 
     Once drawn, the composite wire member is then inserted into a shaping mandrel having a shape similar to the final, formed wire loop illustrated, for example, in  FIG. 2 . The shaping mandrel and accompanying composite wire is then subjected to a heating process wherein the composite wire is heated to a temperature of 500° C. for a period of approximately 10 minutes. During this time, the nickel-titanium outer layer is heated to a temperature above the austenitic finish temperature A f  (i.e. the temperature at which the material completely transforms to austenite), thereby setting the hoop-like shape of the composite wire. Once the desired shape-memory and superelasticity characteristics have been imparted to the nickel-titanium outer layer, the composite wire member is then cut to a specific length and attached to the other filter components to form the embolic protection filter. 
     Referring now to  FIG. 6 , a tensile stress-strain curve is shown for the composite wire loop. As shown in  FIG. 6 , the composite wire loop reaches a relatively constant loading plateau at about 95 ksi of applied stress, wherein the stress level of the composite wire remains relatively constant. At point  38 , after about 7% strain, the composite wire loop reaches its plastic yield point, thereafter forming a permanent set in the material. At point  40 , at about 195 ksi of applied stress, the composite wire loop reaches its ultimate tensile strength, causing the wire loop to break. As illustrated in  FIG. 6 , the desired superelasticity characteristics of the composite wire loop are maintained, as evidenced by the constant stress plateau at about 95 ksi, despite the presence of the platinum inner core member. 
     Having thus described the several embodiments of the present invention, those of skill in the art will readily appreciate that other embodiments may be made and used which fall within the scope of the claims attached hereto. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size and arrangement of parts without exceeding the scope of the invention.