US 7455687 B2
The present invention is directed to an expandable polymer link hybrid stent for implantation in a body lumen, such as a coronary artery along with a method of making the stent. The stent generally includes a series of metallic cylindrical rings longitudinally aligned on a common axis of the stent and interconnected by a series of polymeric links. The polymer links are formed by applying polymer layers between the rings and laser ablating the excess material. The polymeric material forming the polymeric links, provides longitudinal and flexural flexibility to the stent while maintaining sufficient column strength to space the cylindrical rings along the longitudinal axis. The metallic material forming the rings provides the necessary radial stiffness.
1. An intravascular stent, comprising:
a plurality of metallic cylindrical rings with a first delivery diameter and a second implanted diameter, the rings being expandable in a radial direction and aligned along a longitudinal axis of the stent;
a plurality of polymeric links positioned between the plurality of cylindrical rings, the polymer links being configured to impart column strength to the stent; and
at least some of the metallic rings being embedded in a polymer layer.
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This invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient's body lumen, such as blood vessel, to maintain the patency thereof. These devices are useful in the treatment of atherosclerotic stenosis in blood vessels.
Stents are generally tubular-shaped devices which function to hold open a segment of a blood vessel, coronary artery, or other anatomical lumen.
Various means have been described to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location includes mounting the expandable stent on an expandable member, such as a balloon, provided on the distal end of an intravascular catheter, advancing the catheter to the desired location within the patient's body lumen, inflating the balloon on the catheter to expand the stent into a permanent expanded condition and then deflating the balloon and removing the catheter. One of the difficulties encountered using prior stents involved maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery. Once the stent is mounted on the balloon portion of the catheter, it is often delivered through tortuous vessels, including tortuous coronary arteries. The stent must have numerous properties and characteristics, including a high degree of flexibility in order to appropriately navigate the tortuous coronary arteries. This flexibility must be balanced against other features including radial strength once the stent has been expanded and implanted in the artery. While other numerous prior art stents have had sufficient radial strength to hold open and maintain the patency of a coronary artery, they have lacked the flexibility required to easily navigate tortuous vessels without damaging the vessels during delivery.
Generally speaking, most prior art intravascular stents are formed from a metal such as stainless steel, which is balloon expandable and plastically deforms upon expansion to hold open a vessel. The component parts of these types of stents typically are all formed of the same type of metal, i.e., stainless steel. Other types of prior art stents may be formed from a polymer, again all of the component parts being formed from the same polymer material. These types of stents, the ones formed from a metal and the ones formed from a polymer, each have advantages and disadvantages. One of the advantages of the metallic stents is their high radial strength once expanded and implanted in the vessel. A disadvantage may be that the metallic stent lacks flexibility which is important during the delivery of the stent to the target site. With respect to polymer stents, they may have a tendency to be quite flexible and are advantageous for use during delivery through tortuous vessels, however, such polymer stents may lack the radial strength necessary to adequately support the lumen once implanted.
What has been needed and heretofore unavailable is a stent which has a high degree of flexibility so that it can be advanced through tortuous passageways and can be readily expanded and yet have the mechanical strength to hold open the body lumen into which it expanded. The present invention satisfied these needs.
The present invention is directed to an expandable hybrid stent for implantation in a body lumen, such as a coronary artery and a method of making the same. The stent generally consists of a series of metallic cylindrical rings used in connection with a series of polymeric links and in some cases a polymeric coating. The cylindrical rings are radially expandable and longitudinally aligned along a common axis of the stent. The rings are interconnected by one or more of the polymeric links and both the rings and links may be coated with the polymeric coating. The polymeric material forming the links provides longitudinal and flexural flexibility to the stent while maintaining sufficient column strength to space the cylindrical rings along the longitudinal axis, the metallic material forming the rings provides the necessary radial stiffness after the stent is implanted into a body lumen, and the polymeric coating may provide further structural integrity.
In the case of a balloon expandable catheter system the cylindrical rings and the polymeric links remain closely coupled from the time the stent is crimped onto the delivery system to the time the stent is expanded and implanted into a body lumen. Accordingly, both the cylindrical rings and the polymeric links have first delivery diameters in the crimped state of the stent and second larger implanted diameters in the expanded state of the stent.
The metallic cylindrical rings can have undulations including peaks and valleys generally formed as U- and W-shaped members. The peaks of each cylindrical ring can be axially aligned with the valleys of each adjacent cylindrical ring to provide the desired flexibility or vice versa to provide rigidity. The resulting stent structure is a series of radially expandable cylindrical rings which are spaced longitudinally close enough to adequately cover the wall of a body lumen, but not so close as to compromise the longitudinal flexibility of the stent.
The metallic cylindrical rings can be interconnected with a series of polymeric links through a variety of connection types including anchoring mechanisms, bonding agents and coatings. The polymeric links provide flexibility and allow the stent to easily bend or flex along its longitudinal axis as the stent navigates through tortuous vessels or coronary arteries. The polymer links also provide a good platform for a drug coating. The combination of the flexible metallic cylindrical rings when used in connection with the polymeric links cumulatively provide a stent which is flexible along its length and about its longitudinal axis, yet is still relatively stiff in the radial direction after it is expanded in order to maintain the patency of a vessel and to resist collapse.
The stent embodying features of the invention can be readily delivered to the desired body lumen, such as a coronary artery (peripheral vessels, bile ducts, etc.), by mounting the stent on an expandable member of a delivery catheter, for example a balloon, and advancing the catheter and stent assembly through the body lumen to the target site. Generally, a crimping tool is used to crimp the stent onto the balloon portion of the catheter so that the stent does not move longitudinally relative to the balloon portion of the catheter during delivery through the arteries, and during expansion of the stent at the target site. During the crimping process the metallic cylindrical rings undergo a plastic deformation and radially compress.
After insertion of the stent to the desired location of delivery, the balloon is inflated to implant the stent. The metallic cylindrical rings of the stent are plastically deformed when expanded so that the stent will remain in the expanded condition and therefore they must be sufficiently rigid when expanded to prevent the collapse thereof in use. During expansion of the stent, portions of the cylindrical rings may tip outwardly resulting in projecting members on the outer surface of the expanded stent. These projecting members may tip radially outwardly from the outer surface of the stent and may embed into the vessel wall and help secure the expanded stent once it is implanted.
In one method of manufacturing the stent, a solution is first prepared using a biocompatible polymer and solvent. A mandrel with laser registration features is then coated with the solution to build a thickness approximately equivalent to the thickness of stent rings. The mandrel is placed in a laser rotary fixture where the laser is then used to cut a female pattern of the stent rings into the polymer coatings. The mandrel is then removed from the laser rotary fixture with its position registered in control memory. The stent rings are thereafter embedded into the conforming ring-cavities in the polymeric coating. A second coating is applied to the assembly until the desired coating thickness is achieved. Once cured, the assembly is placed back into the laser rotary fixture, and the polymer coating lining the perimeter of the desired stent profile is ablated to form the finished stent.
It is to be recognized that the stent of the present invention can be self-expanding or balloon-expanded and made by other processes. Moreover, the stent of the present invention can be modified to be used in other body lumens including highly tortuous and distal vasculature as well as to create whole or portions of other medical devices or markers placed on such devices.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying exemplary drawings.
Before describing in detail an exemplary embodiment of a hybrid stent with polymeric and metallic components in accordance with the present invention, it is instructive to briefly describe a typical stent implantation procedure and the vascular conditions which are typically treated with stents. Referring now to
With most lesion treatment procedures, the treated artery suffers a degree of trauma, and in a certain percentage of cases may abruptly collapse or may slowly narrow over a period of time due to neointimal hyperplasia which is referred to as restenosis. To prevent either of these conditions, the treated artery is often fitted with a prosthetic device, such as the stent 10 of the present invention. The stent provides radial support for the treated vessel and thereby prevents collapse of the vessel 16, and further provides scaffolding to prevent plaque prolapse within the lumen. The stent may also be used to repair an arterial dissection, or an intimal flap, both of which are sometimes found in the coronary arteries, peripheral arteries and other vessels. In order to perform these functions, the stent must be accurately placed across the lesion site. Therefore, it is critical that the stent be sufficiently radiopaque so that the physician can visually locate the stent under fluoroscopy during the implantation procedure. However, it is equally important that the stent not be too radiopaque. If the stent is overly radiopaque, i.e., too bright, the physician's view of the lumen is compromised. This makes assessment of subsequent restenosis difficult. In cases where the balloon markers are very close to the stent, the stent can blend in with the markers. Without precise visualization of the stent ends, accurate placement of the stent in a lesion, particularly in the case of an ostial lesion, can be compromised.
With continued reference to
Referring now to
The metallic cylindrical rings 12 of this embodiment are formed from tubular members and may be relatively flat in transverse cross-section. Thus, after implantation into the artery 16 as shown in
The stent patterns shown in
The polymeric links 18 which interconnect adjacent cylindrical rings 12 may have cross-sections similar to the cross-sections of the undulating components of the expandable cylindrical rings. The polymeric composition of the links may enable the links to be more flexible than comparatively sized metallic links. The number and location of the links connecting the rings together can be varied in order to vary the desired longitudinal and flexural flexibility in the stent assembly structure in the unexpanded as well as expanded condition of the stent. These properties are important to minimize alteration of the natural physiology of the body lumen into which the stent assembly is implanted and to maintain the compliance of the body lumen which is internally supported by the stent assembly. Generally, the greater the longitudinal and flexural flexibility of the stent assembly, the easier and the more safely it can be delivered to the target site.
With reference to
The cylindrical rings 12 can be nested such that adjacent rings slightly overlap in the longitudinal direction so that one ring is slightly nested within the next ring and so on. The degree of nesting can be dictated primarily by the length of each cylindrical ring, the number of undulations in the rings, the thickness of the rings, and the radius of curvature, all in conjunction with the crimped or delivery diameter of the stent. If the rings are substantially nested one within the other, it may be difficult to crimp the stent to an appropriate delivery diameter without the various struts overlapping. It is also contemplated that the rings may be slightly nested even after the stent is expanded, which enhances vessel wall coverage. In some circumstances, it may not be desirable to nest one ring within the other, which is also contemplated by the invention.
For the purpose of illustration only, the stent 10 is shown as a flat pattern in
In keeping with the invention, the polymeric links 18 are formed from a flexible polymeric material, that is bendable and flexible to enhance longitudinal and flexural flexibility of the stent 10. The polymeric material forming the links can be taken from the group of polymers consisting of polyurethanes, polyolefins, polyesters, polyamides, flouropolymers and their co-polymers, polyetherurethanes, polyesterurethanes, silicone, thermoplastic elastomer (C-flex), polyether-amide thermoplastic elastomer (Pebax), fluoroelastomers, fluorosilicone elastomer, polydimethyl siloxones (PDMS), aromatic PDMS, silicon thermoplastic urethanes, poly (glycerol sebacate) (PGS) (developed by Yadong Wang, MIT) and commonly referred to as biorubber) styrene-butadiene rubber, butadiene-styrene rubber, polyisoprene, neoprene (polychloroprene), ethylene-propylene elastomer, chlorosulfonated polyethylene elastomer, butyl rubber, polysulfide elastomer, polyacrylate elastomer, nitrile, rubber, a family of elastomers composed of styrene, ethylene, propylene, aliphatic polycarbonate polyurethane, polymers augmented with antioxidants, polymers augmented with image enhancing materials, polymers having a proton (H+) core, polymers augmented with protons (H+), butadiene and isoprene (Kraton), polyester thermoplastic elastomer (Hytrel), methacrylates, ethylene, acetate, alcohol, and polyvinyl alcohol. The polymeric links can also be translucent or radiopaque. In the case of a radiopaque link, the polymer may be modified with a radiopaque material.
The polymeric links can be formed with a curved pattern 24 to enable the stent to have higher flexibility and deliverability than a strut with substantially straight links. The links can also be formed in a number of different patterns according to design requirements. For example, the links can be formed with more or less surface area, larger or smaller cross-sections, variable radial thickness, a greater or lower number of curves or oscillations, and a variety of other shapes according to design requirements.
The polymeric links 18 when used in connection with the metallic cylindrical rings 12 enable the stent 10 to have higher flexibility and deliverability than all metal stents. The cylindrical rings are formed out of a metal, such as stainless steel and can be interconnected with the links through many types of connections including bonding agents, coatings and anchoring mechanisms. By comparison, an all metal stent generally consists of a series of metallic cylindrical rings interconnected by metallic links or struts. In the case of a metallic stent where the rings and links are laser cut from a unitary tube, the design of the stent may be a compromise between flexibility and rigidity, where the stent must be flexible enough to conform to the curvature of the body lumen it is inserted into and the stent must be rigid enough to remain in its expanded state once implanted.
In the stent of the present invention, the polymeric links 18 enable the stent 10 to exceed the longitudinal flexibility of conventional metallic stents made to similar dimensions. Such flexibility is achieved by factors such as choice of polymeric material, type of pattern, and desired final size. To account for the required radial strength, the cylindrical rings 12 are formed from a metallic material as in conventional metallic stents. Accordingly, it is possible with the present invention to produce a stent having radial strength equivalent to a conventional all-metallic stent while offering longitudinal flexibility exceeding the metallic stent. With the addition of more rings to the stent, radial stiffness can also be increased over a conventional stent while maintaining a high degree of flexibility.
The stent patterns shown in
As shown in
An alternative link 57 is shown in
In another embodiment, as shown in
In another embodiment shown in
Additional configurations are contemplated to increase the strength and reduce the profile of the polymer to metal attachment. Additional anchoring mechanisms that include geometric features designed to generate a lock and key type of fit may include hooks incorporated into either or both the links or rings whereby the links and rings are securely fastened together. In such cases the hooks can also be arranged in multiple fusion where the anchoring design specifies such an arrangement. The ring-to-link attachment can be accomplished in additional ways including using a bonding agent. For example, when using a bonding agent, adhesive can be applied at each end of the polymeric link and at corresponding rings. The adhesive can be any biocompatible adhesive that is well known, such as a cyanoacrylite-based adhesive. Several adhesives can be used including epoxy based adhesives, Locitite 401, 1-06FL, and M-11FL, the latter two of which are urethane-based adhesives. Other adhesives such as epoxy based adhesives can be used without departing from the spirit and scope of the invention. In addition, a coating applied to the entire stent can also be used to interconnect the links and rings.
The polymeric links may have cross-sections similar to the cross-sections of the undulating components of the expandable cylindrical rings. In one embodiment, the cross-section of the links may fall between approximately 0.0012 inch (0.03 mm) and 0.01 inch (0.25 mm). Compared to a conventional metallic link of similar size and shape, the polymeric link connecting adjacent rings provides increased flexibility to the stent by allowing adjacent rings to more freely conform to a patient's vasculature.
One method of making the rings is to laser cut the rings from a tubular member, such as stainless steel tubing to remove portions of the tubing in the desired pattern for the rings. Other possible ring materials include titanium, tantalum, nickel titanium, cobalt-chromium, gold, palladium, platinum and platinum-iridium. In accordance with the invention, it is preferred to cut the tubing in the desired pattern by means of a machine-controlled laser as is well known in the art.
Direct laser cutting produces edges which are essentially perpendicular to the axis of the laser cutting beam, in contrast with chemical etching and the like which produce pattern edges which are angled. Hence, the laser cutting process essentially provides stent cross-sections, from cut-to-cut, which are square or rectangular, rather than trapezoidal. The cross-sections have generally perpendicular edges formed by the laser cut.
Other methods of forming the rings of the present invention can be used, such as chemical etching; electric discharge machining; laser cutting a flat sheet and rolling it into a cylinder; and the like, all of which are well known in the art at this time.
Several processes can be used for forming and incorporating the polymeric links within the stent. One method is to first coat a core mandrel with a polymer coating.
In keeping with the invention, a mandrel 82 is shown in
After curing the solution on the mandrel, a CO2 laser (or Eximer lasers, solid state lasers and their harmonic wavelengths, picosecond lasers, or femtosecond lasers) is used to cut a ring cavity 86 into the polymer identical to the metallic rings of the stent. The cavity 86 is shown in
After the stent rings 12 are cut (preferably by laser) they are pressed and crimped into the ring cavity 86 in the polymer layer 84.
The link to ring attachment strength of the stent can be further enhanced by increasing the surface area at the attachment location on a microscopic scale. The degree of surface roughness of the ring can vary from a mild surface etch to a more aggressive etch which creates pits which allows the polymer to flow into. Surface roughening/texturing is common for most materials including austenitic stainless steel and cobalt-chromium. These materials are particularly susceptible to corrosion of the grain boundaries and can be attacked with a variety of etchents such as FeCl3(ferric chloride), CoCl3 (cobalt chloride), NaS2O8 (sodium persulfate), Hcl (hydrochloric acid), etc. The grain boundaries are attacked causing larger grains to fall out and dissolve thereby forming a pit.
Reverse current process can also be used to remove material and make a surface texture on the rings. The surface texture can be formed at the specific attachment location for the purposes of enhancing the link to ring attachment strength. This can be done by applying a mask or resist on the rings and then selectively removing this mask thereby exposing the attachment locations. During the surface etching of the rings, the resist protects these areas of the rings where a polished surface finish is desired.
The stent of the present invention also can include a superelastic ring material. The term “superelastic” refers to an isothermal transformation, more specifically stress inducing a martensitic phase from an austenitic phase (commonly referred to as stress induced martensite). Alloys having superelastic properties generally have two phases: a martensitic phase, which has a relatively low tensile strength and which is stable at relatively low temperatures, and an austenitic phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensitic phase. The austenitic phase also typically has better corrosion properties. Superelastic characteristics generally allow the metal stent to be deformed by collapsing and deforming the stent and creating stress which causes the metal to change to the martensitic phase. The stent is restrained in the deformed condition to facilitate the insertion into a patient's body, with such deformation causing the phase transformation. Once within the body lumen, the restraint on the stent is removed, thereby reducing the stress therein so that the superelastic stent can return towards original undeformed shape by the transformation back to the austenitic phase. A basic discussion of this phenomenon can be found in Wayman and Deuring, “An Introduction to Martensite and Shape Memory,” which appears in Engineering Aspects Of Shape Memory Alloys, Deuring et al. editors (Butterworth-Heinemann Ltd. 1990, Great Britain.)
The rings of the stent can be formed from a superelastic material such as NiTi (nickel titanium) and undergo an isothermal transformation when stressed. The stent is first compressed to a delivery diameter, thereby creating stress in the NiTi alloy so that the NiTi is in a martensitic state having relatively low tensile strength. While still in the martensitic phase, the stent is mounted onto or within a catheter by known methods.
In another embodiment, the stent may also be coated with a therapeutic agent to perform a variety of functions, from preventing blood clots to promoting healing. As an example, an active agent coated on the stent can inhibit the activity of vascular smooth muscle cells. More specifically, the active agent is aimed at inhibiting abnormal or inappropriate migration and proliferation of smooth muscle cells. The active agent can also include any substance capable of exerting a therapeutic or prophylactic effect in the practice of the present invention. The agent can also be for enhancing wound healing in a vascular site or improving the structural and elastic properties of the vascular site. The dosage or concentration of the active agent required to produce a favorable therapeutic effect should be less than the level at which the active agent produces toxic effects and greater than the level at which non-therapeutic results are obtained. The dosage or concentration of the active agent required to inhibit the desired cellular activity of the vascular region can depend upon factors such as the particular circumstances of the patient; the nature of the trauma; the nature of the therapy desired; the time over which the ingredient administered resides at the vascular site; and if other therapeutic agents are employed, the nature and type of the substance or combination of substances. Therapeutic effective dosages can be determined empirically, for example by infusing vessels from suitable animal model systems and using immunohistochemical, fluorescent or electron microscopy methods to detect the agent and its effects, or by conducting suitable in vitro studies. Standard pharmacological test procedures to determine dosages are understood by one of ordinary skill in the art.
Further, it is well known that the stent (when made from a metal) may require a primer material coating such as a polymer to provide a substrate on which a drug or therapeutic agent is coated since some drugs and therapeutic agents do not readily adhere to a metallic surface. The drug or therapeutic agent can be combined with a coating or other medium used for controlled release rates of the drug or therapeutic agent. Examples of therapeutic agents or drugs that are suitable for use with the polymeric materials include sirolimus, everolimus, actinomycin D (ActD), taxol, paclitaxel, or derivatives and analogs thereof. Examples of agents include other antiproliferative substances as well as antineoplastic, antiinflammatory, antiplatelet, anticoagulant, antifibrin, antithrombin, antimitotic, antibiotic, and antioxidant substances. Examples of antineoplastics include taxol (paclitaxel and docetaxel). Further examples of therapeutic drugs or agents that can be combined with the polymeric materials include antiplatelets, anticoagulants, antifibrins, antithrombins, and antiproliferatives. Examples of antiplatelets, anticoagulants, antifibrins, and antithrombins include, but are not limited to, sodium heparin, low molecular weight heparin, hirudin, argatroban, forskolin, vapiprost, prostacyclin and prostacyclin analogs, dextran, D-phe-pro-arg-chloromethylketone (synthetic antithrombin), dipyridamole, glycoprotein IIb/IIIa platelet membrane receptor antagonist, recombinant hirudin, thrombin inhibitor (available from Biogen located in Cambridge, Mass.), and 7E-3B® (an antiplatelet drug from Centocor located in Malvern, Pa.). Examples of antimitotic agents include methotrexate, azathioprine, vincristine, vinblastine, fluorouracil, adriamycin, and mutamycin. Examples of cytostatic or antiproliferative agents include angiopeptin (a somatostatin analog from Ibsen located in the United Kingdom), angiotensin converting enzyme inhibitors such as Captopril® (available from Squibb located in New York, N.Y.), Cilazapril® (available from Hoffman-LaRoche located in Basel, Switzerland), or Lisinopril® (available from Merck located in Whitehouse Station, N.J.); calcium channel blockers (such as Nifedipine), colchicine, fibroblast growth factor (FGF) antagonists, fish oil (omega 3-fatty acid), histamine antagonists, Lovastatin® (an inhibitor of HMG-CoA reductase, a cholesterol lowering drug from Merck), methotrexate, monoclonal antibodies (such as PDGF receptors), nitroprusside, phosphodiesterase inhibitors, prostaglandin inhibitor (available from GlaxoSmithKline located in United Kingdom), Seramin (a PDGF antagonist), serotonin blockers, steroids, thioprotease inhibitors, triazolopyrimidine (a PDGF antagonist), and nitric oxide. Other therapeutic drugs or agents which may be appropriate include alpha-interferon, genetically engineered epithelial cells, and dexamethasone.
While the invention has been illustrated and described herein in terms of its use as an intravascular stent, it will be apparent to those skilled in the art that the stent can be used in other body lumens. Further, particular sizes and dimensions, number of undulations per ring, materials used, and the like have been described herein and are provided as examples only. Other modifications and improvements may be made without departing from the scope of the invention.