Patent Publication Number: US-8112874-B2

Title: Stent island removal system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 11/825,985 which was filed on Jul. 9, 2007, now U.S. Pat. No. 7,823,263 B2 which claims the benefit of U.S. Patent Application No. 60/830,208 which was filed on Jul. 11, 2006, both of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to implantable medical devices, such as stents. 
     2. Description of the State of the Art 
     A typical stent is a cylindrically shaped device, which holds open and sometimes expands a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been subjected to angioplasty or valvuloplasty. 
     Stents have been made of many materials including metals and polymers. Polymer materials include both nonbioerodable and bioerodable plastic materials. In some applications, a polymeric bioerodable stent may be more advantageous than a metal stent due to its biodegradeability and greater flexibility relative to the metal stent. 
     The cylindrical structure of a stent is typically composed of a scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or planar films of material rolled into a cylindrical shape. Furthermore, the pattern that makes up the stent allows the stent to be radially expandable and longitudinally flexible. Longitudinal flexibility facilitates delivery of the stent and rigidity allows a stent to hold open a lumen of a tubular organ. Generally, the pattern should be designed to maintain the longitudinal flexibility and rigidity required of the stent. The stent should also have adequate strength in the circumferential direction. 
     A number of techniques have been suggested for the fabrication of metallic and polymeric stents from tubes and planar films or sheets. Examples of such techniques include laser cutting or etching a pattern onto a material. Laser cutting may be performed on a planar film of a material which is then rolled into a tubular configuration. Alternatively, a desired pattern may be etched directly onto a tube. Other techniques involve cutting a desired pattern into a sheet or a tube via chemical etching or electrical discharge machining Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter, and U.S. Pat. No. 5,906,759 to Richter. 
     Additionally, a polymeric wire may be coiled to form a polymeric stent. In yet another method, a polymeric stent may be formed from a tube by laser cutting a pattern of cylindrical rings and connecting rings in the tube itself. See, e.g., U.S. Pat. No. 6,585,755 to Jackson et al. 
     “Islands” refer to pieces or portions of metal or polymeric tubing material that are not intended to be part of a stent pattern, and that remain attached to stent struts after laser cutting. The pieces or portions may have a relatively weak physical attachment to a strut or may be wedged between struts with no physical attachment to the struts. Stent islands are conventionally removed from the stent by inserting a metal mandrel through the stent and tapping the mandrel against a surface. This method of removing stent islands after laser cutting the stent causes vibrations that essentially shake the islands out of the stent. However, such vibrations may also damage the stent. In particular, regions of a stent that are exposed to high stress during stent use may be damaged, which may weaken the stent in these high stress regions. 
     Additionally, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active or bioactive agent or drug. Polymeric scaffolding may also serve as a carrier of an active agent or drug. 
     Furthermore, it may be desirable for a stent to be biodegradable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers should be configured to completely erode only after the clinical need for them has ended. 
     SUMMARY OF THE INVENTION 
     Various embodiments of the present invention include a system for removing stent islands from a laser cut tube comprising: a tube including at least one hole having an opening in an outer surface that is in fluid communication with an interior of the tube; a stent positioned over the tube, wherein the stent comprises a scaffolding including a pattern of interconnecting struts formed by laser cutting the pattern into a second tube, wherein the stent comprises a stent island, the island being pieces or portions of material of the second tube that are not intended to be part of the pattern that remain attached to the struts of the stent after the laser cutting; and a high pressure gas source connected to the tube to supply a compressed gas to the interior of the tube, wherein when the high pressure gas source supplies the compressed gas to the interior of the tube, the compressed gas exits the tube through the at least one hole and facilitates removal of the island. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a stent. 
         FIG. 2  depicts a stent island removal tool according to an embodiment of the invention. 
         FIGS. 3A-C  depict hole patterns according to various embodiments of the invention. 
         FIGS. 4A-B  depict a stent island removal tool according to an embodiment of the invention. 
         FIGS. 5A-C  depict hole opening patterns according to various embodiments of the invention. 
         FIGS. 6A-E  depict the profiles of holes according to various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a stent  10  that includes a number of interconnecting structural elements or struts  11 . In general, the pattern of the stent struts is designed so that stent  10  can be radially compressed and expanded. Stent  10  may include portions of struts that are straight or relatively straight, an example being a straight portion designated by reference numeral  12 . Stent  10  may also include portions of struts that are bent, such as the portions designated by reference numerals  13 ,  14 , and  15 . Bent portions  13 ,  14 , and  15  may bend further when stent  10  is crimped radially inwardly. Bent portions  13 ,  14 , and  15  may bend further outward when stent  10  is expanded radially outwardly. In some embodiments, a stent such as stent  10  may be fabricated by laser cutting a strut pattern on a tube. In other embodiments, chemical etching may be used to form a strut pattern on a tube. 
       FIG. 1  depicts exemplary stent islands  16  and  17  formed after laser cutting the stent into a pattern or chemically etching the pattern. Island  16  has a relatively weak physical attachment to a strut  18  and island  17  is wedged between struts  19  and  20 . Due to the relatively weak attachment, the invention provides a method and an apparatus for removing such islands by blowing a gas stream at or near the islands. The apparatus and method are designed to consistently remove stent islands through the use of a pressurized fluid such as a gas. The use of pressurized fluids minimizes any damage to the stent. 
     Embodiments of the present invention include a tube with at least one hole or opening that is adapted to receive a stent thereon. In some embodiments, a stent having islands is mounted over the tube and a fluid conveyed through or into the tube flows out of the hole. The stent with islands is mounted or positioned on the tube hole such that the conveyed fluid flowing through the hole facilitates removal of an island. 
     One embodiment of a stent island removal tool  100  is shown in  FIG. 2 . Tool  100  is composed of a tube  110  with an outer diameter that is the same or larger than the inner diameter of a stent  120  that is positioned over tube  110 . In certain embodiments, stent  120  has a slip fit, meaning stent  120  is slidably mounted to allow axial and rotational movement. One end  130  of the tube may be occluded and the other end  135  is connected to a high-pressure fluid source  140 . In another embodiment, both ends of tube  130 ,  135  may be occluded, and the pressurized fluid  140  would enter from the surface of the tube (not shown). One or more openings  150  are located on the surface of tube  110  to allow the compressed fluid to exit openings  150 . In one embodiment, openings  150  are located at other circumferential positions of tube  110 . The compressed fluid blows radially through openings  150 , blowing the islands off stent  120 . 
     In certain embodiments, stent  120  may be mounted on tube  110  using an interference fit or press fit. In such an embodiment, the outer diameter of tube  110  is the same or substantially the same as the inner diameter of stent  120 . The fit is tight enough that the stent does not slide axially or rotate during stent removal. 
     In several embodiments where stent  120  is slidably mounted, islands in various portions of the stent may be removed by translating the stent axially, as depicted by arrow  160 , rotationally as depicted by arrow  170 , or both, to remove islands in the various portions of the stent. In other embodiments, tube  110  may be moved axially or rotationally with respect to stent  120  such that the at least one hole may remove islands in various portions of the stent. In further embodiments, both stent  120  and tube  110  are moved with respect to each other. In one embodiment, the relative rotational movement of the stent and tube can be provided by attaching the tube to a fixture that rotates the tube. In another embodiment, relative axial movement of the stent and tube can be provided by attaching the tube to a fixture that vibrates longitudinally. Such a fixture can also rotate the stent to provide both relative rotational and translational motion. 
     In several embodiments, a tube  300  for removing stent islands may have more than one opening  350 , as illustrated in  FIGS. 3A-C . The openings may be arranged in different patterns such as circular patterns ( 3 A), longitudinal patterns ( 3 B), or helical ( 3 A), but the patterns are not limited thereto. 
     In some embodiments, stent island removal can be performed with little or no relative movement of the stent with respect to the stent island removal tube.  FIGS. 4A-B  are an illustration of a stationary stent island removal procedure. Holes in tube  410  are not shown for the sake of clarity. Alternatively or additionally, stent  420  has an interference fit with tube  410  such that the stent remains stationary on the tube while the blowing occurs. In some embodiments, rotational and axial movement of stent  420  may be reduced or prevented on tube  410  with collets  450 , at the ends of stent  420 . Alternatively, only one collet  450  may be used around one end of stent  410  if the other end of the stent is pressed against a stationary portion of the tool. In one embodiment, tube  410  has a sufficient number of holes such that all or most of the islands on the stent are removed.  FIG. 4B  depicts tube  410  showing a high density of holes  460 . In other embodiments, the holes are in a pattern coincident with the likely location of stent islands for a given stent pattern. Different stent patterns may require different tube hole patterns. 
     The force of the fluid through the holes should be large enough to remove a stent island. Force depends on the flow rate of the fluid conveyed into the tube, and the pressure of the fluid in the tube. If a compressible fluid, such as a gas is used, then the temperature of the gas is an additional factor in determining the force out of the tube. The force of the gas should not be high enough such that it causes damage to the stent. Because the islands may vary in size, different sized holes may be necessary. The size of the holes should be determined relative to the islands and width of the struts. In several embodiments, the size of the holes is determined such that the force per unit area applied on the islands is high enough to remove the island, but not so high as to damage the surrounding struts. If the hole is smaller, then a higher force may be necessary to remove the islands. 
     The method of removing stent islands can include multiple passes of the stent over the gas flowing from the opening(s). The gas flow the flow can be continuous, variable, or pulsed by controlling the gas supply. Continuous flow is a constant flow over time. A variable flow is a changing flow over time. A pulsed flow is an on-off type of flow, and may be continuous or variable. The pulses may occur at regular intervals, or irregular intervals. 
     The gas velocity, the gas flow direction, the gas temperature, the gas pulse frequency, etc. can be varied to provide a force needed to remove the stent islands. Various types of gases may be used in the present invention, including but not limited to air, argon, oxygen, nitrogen, carbon dioxide. 
     In one embodiment, the tube is cooled to make the stent islands more brittle so they may be removed more easily. In another, the gas may be cooled, further cooling the islands. In a further embodiment, both the tube and the gas may be cooled. 
     It is also contemplated that the cross-section of the openings can have a variety of shapes. In some embodiments, the shape of the opening can be circular, oval, square, or rectangular. In particular embodiments, the openings may be slits.  FIGS. 5A-C  depict various arrangements of slits in a tube.  FIG. 5A  depicts a tube  550  with a pattern of slits  510  arranged axially, with the major axis of the slits  510  aligned with the longitudinal axis.  FIG. 5B  depicts a tube  551  with a pattern of slits  511  arranged circumferentially, with the major axis of the slits  511  aligned around the circumference of the tube.  FIG. 5C  depicts a tube  552  with a pattern of slits  512  arranged helically, with the major axis of the slits  512  aligned at an angle to the longitudinal axis. Using a longer rectangular slit may allow the fluid to exit as a thin sheet rather than a point flow of fluid through a circular opening. 
     The size of the cross-section of the holes between the inside and outside of the tube can be constant or can vary.  FIG. 6A  depicts a tube  600  with a hole  650  with a constant cross-section.  FIG. 6B-C  depict tubes  610  and  620  with holes  651  and  652  having cross-sections that increase and decrease, respectively. Hole  651  acts to disperse fluid flow, while hole  652  acts to concentrate fluid flow. 
     In further embodiments, the holes can be adapted to direct airflow in a selected direction.  FIGS. 6D-E  depict tubes  630  and  640  with holes  653  and  654  that direct fluid flow at an acute angle to the surface of the tubes. 
     In several embodiments, the tube is made of metal. In other embodiments, the tube may be made of a polymeric material. The selection of material may be based on operating temperature, pressure, duty cycle, compatibility of the tube material with the stent material or gas, cost, but is not limited thereto. In some embodiments, the tube is stainless steel. 
     The present invention removes stent islands while maintaining stent integrity. The method and apparatus of the invention can be used to remove metal or polymeric stent islands from stents. Examples of stents for use in the invention include, without limitation, self-expandable stents, balloon-expandable stents, stent-grafts, and vascular grafts. 
     Polymers for use in fabricating a substrate of a stent or a coating for a stent can be biostable, bioabsorbable, biodegradable or bioerodable. Biostable refers to polymers that are not biodegradable. The terms biodegradable, bioabsorbable, and bioerodable are used interchangeably and refer to polymers that are capable of being completely degraded and/or eroded when exposed to bodily fluids such as blood and can be gradually resorbed, absorbed, and/or eliminated by the body. The processes of breaking down and eventual absorption and elimination of the polymer can be caused by, for example, hydrolysis, metabolic processes, bulk or surface erosion, and the like. 
     It is understood that after the process of degradation, erosion, absorption, and/or resorption has been completed, no part of the stent will remain or in the case of coating applications on a biostable scaffolding, no polymer will remain on the device. In some embodiments, very negligible traces or residue may be left behind. For stents made from a biodegradable polymer, the stent is intended to remain in the body for a duration of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. 
     The underlying structure or substrate of a stent can be completely or at least in part made from a biodegradable polymer or combination of biodegradable polymers, a biostable polymer or combination of biostable polymers, or a combination of biodegradable and biostable polymers. Additionally, a polymer-based coating for a surface of a device can be a biodegradable polymer or combination of biodegradable polymers, a biostable polymer or combination of biostable polymers, or a combination of biodegradable and biostable polymers. 
     Representative examples of polymers that may be used to fabricate or coat an stent include, but are not limited to, poly(N-acetylglucosamine) (Chitin), Chitosan, poly(hydroxyvalerate), poly(lactide-co-glycolide), poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate), polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolide), poly(L-lactic acid), poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide), poly(caprolactone), poly(trimethylene carbonate), polyester amide, poly(glycolic acid-co-trimethylene carbonate), co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules (such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronic acid), polyurethanes, silicones, polyesters, polyolefins, polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymers and copolymers other than polyacrylates, vinyl halide polymers and copolymers (such as polyvinyl chloride), polyvinyl ethers (such as polyvinyl methyl ether), polyvinylidene halides (such as polyvinylidene chloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics (such as polystyrene), polyvinyl esters (such as polyvinyl acetate), acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon 66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides, polyethers, polyurethanes, rayon, rayon-triacetate, cellulose, cellulose acetate, cellulose butyrate, cellulose acetate butyrate, cellophane, cellulose nitrate, cellulose propionate, cellulose ethers, and carboxymethyl cellulose. Another type of polymer based on poly(lactic acid) that can be used includes graft copolymers, and block copolymers, such as AB block-copolymers (“diblock-copolymers”) or ABA block-copolymers (“triblock-copolymers”), or mixtures thereof. 
     Additional representative examples of polymers that may be especially well suited for use in fabricating or coating an implantable medical device include ethylene vinyl alcohol copolymer (commonly known by the generic name EVOH or by the trade name EVAL), poly(butyl methacrylate), poly(vinylidene fluoride-co-hexafluororpropene) (e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare, N.J.), polyvinylidene fluoride (otherwise known as KYNAR, available from ATOFINA Chemicals, Philadelphia, Pa.), ethylene-vinyl acetate copolymers, and polyethylene glycol. 
     A non-polymer substrate of the stent may be made of a metallic material or an alloy such as, but not limited to, cobalt chromium alloy (ELGILOY), stainless steel (316L), high nitrogen stainless steel, e.g., BIODUR 108, cobalt chrome alloy L-605, “MP35N,” “MP20N,” ELASTINITE (Nitinol), tantalum, nickel-titanium alloy, platinum-iridium alloy, gold, magnesium, or combinations thereof “MP35N” and “MP20N” are trade names for alloys of cobalt, nickel, chromium and molybdenum available from Standard Press Steel Co., Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20% chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20% nickel, 20% chromium, and 10% molybdenum. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.