Abstract:
A vascular stent comprising a drug-eluting outer layer of a porous sputtered columnar metal having each column capped with a biocompatible carbon-containing material is described. This is done by placing the stent over a close-fitting mandrel and rotating the assembly in a sputter flux. The result is a coating that is evenly distributed over the outward-facing side of the stent&#39;s wire mesh while preventing the sputtered columnar coating from reaching the inward facing side where a smooth hemocompatible surface is required. The stent is then removed from the mandrel, exposing all surfaces, and finally coated with a layer of carbon such as amorphous carbon or diamond-like carbon. The carbonaceous coating enhances biocompatibility without preventing elutriation of a therapeutic drug provided in the porosity formed between the columnar structures. The result is a stent that is adapted to both the hemodynamic and the immune response requirements of its vascular environment.

Description:
FIELD OF THE INVENTION 
     This invention relates to stents provided with coatings for eluting medication to prevent or lessen the severity of restenosis. 
     PRIOR ART 
     In order to minimize the response of surrounding tissue to the trauma of stent insertion and expansion, stent coatings must be biocompatible. A further requirement is that a stent coating must adhere to a substrate undergoing plastic deformation. This occurs during insertion and expansion of the stent into the vasculature system. Plastic deformation involves grain rotation and elongation, and intersection of slip planes with the substrate surface. The result is that on a scale below the grain size of the substrate, deformation is highly non-uniform, with some areas undergoing little or no deformation and others extreme deformation with associated increase in surface roughness and irregularity. Therefore, coating adhesion must be preserved through the deformation process. 
     Conventional stent coatings can be classified as being either passive or active. Passive coatings rely on biocompatible materials to minimize the body&#39;s response to placement of the stent into the vasculature. Generally recognized “passive” coating materials include carbon, iridium oxide, titanium, and the like, as disclosed in U.S. Pat. No. 5,824,056 to Rosenberg. U.S. Pat. No. 5,649,951 to Davidson discloses coatings of zirconium oxide or zirconium nitride. 
     Drug eluting or “active” coatings have proven more effective for the prevention of restenosis. Such stents generally comprise a surface polymer containing a therapeutic drug for timed release. A second coating may be added to extend the period of effectiveness by limiting the rate of drug diffusion from the first, drug-containing coating. This second coating may be a polymer, or a sputtered coating as described in U.S. Pat. No. 6,716,444 to Castro et al. 
     However, polymeric drug eluting coatings suffer from a number of disadvantages. First, they can have poor adhesion to the stent, especially while undergoing plastic deformation during insertion and expansion of the stent into the vasculature. Secondly, due to biocompatiblity/hemocompatibility issues some polymers actually contribute to restenosis. Finally, that part of the coating facing the inside of the vasculature lumen loses its medication content to the bloodstream with little beneficial effect. 
     U.S. Pat. No. 6,805,898 to Wu et al. attempted to overcome adhesion problems by introducing roughness to the vasculature-facing portion of the stent while leaving the blood-facing side in a polished condition for better hemocompatibility. Surface roughness was increased by means of grit blasting, sputtering, and the like. Not only did augmenting surface roughness improve adhesion between the polymer and the stent, it also allowed for a thicker polymer coating to be applied. However, the final stent configuration still had eluting polymer in contact with body tissue, allowing biocompatibility issues to persist. 
     U.S. Pat. No. 5,607,463 to Schwartz et al. carried out experiments in which it was shown that tissue response to polymers could be reduced by means of a barrier layer of tantalum and niobium thin films on the exposed polymer surfaces. Specifically, in vivo tests showed an absence of thrombosis, inflammatory response, or neointimal proliferation when a thin tantalum or niobium barrier layer covered a polymer. However, in the case of a drug eluting polymer, these coatings detrimentally isolated the drug from the tissue as well. 
     U.S. Patent Application Pub. No. 2004/0172124 to Vallana et al. optimized the coating configuration by limiting the drug-eluting material to only that portion of the stent surface in contact with the vasculature. This was done by confining the drug eluting polymer to outward facing channels which were micro-machined into the stent mesh elements. All other stent surfaces were coated with hemocompatible carbon. Thus, the use of a biocompatible-problematic carrier polymer was minimized, but not eliminated. 
     In addition, U.S. Pat. No. 6,820,676 to Palmaz shows that, independent of the stent&#39;s surface composition, the surface texture of the stent or coating has an effect on the ability of proteins to adsorb into the stent surface, ultimately allowing thrombosis formation. It was shown that the surface texture can be controlled by grain size and other means to prevent protein adsorption and subsequent thrombosis. 
     Thus, even though much work has been done to develop stent systems comprising drug eluting polymers while minimizing, and even eliminating, thrombosis, inflammatory response and neointimal proliferation, further improvements are required to fully realize these goals. The present stent coating is believed to accomplish just that. 
     SUMMARY OF THE INVENTION 
     In the present invention, the drug-eluting outer layer of a stent consists of a porous sputtered metal or ceramic coating rather than a conventionally deposited polymer. This is done by placing the stent over a close-fitting mandrel and rotating the assembly in a sputter flux. The result is a coating that is evenly distributed over the outward-facing side of the stent&#39;s wire mesh while preventing the sputtered coating from reaching the inward facing side where a smooth hemocompatible surface is required. The stent is then removed from its mandrel, exposing all surfaces, and finally coated with a layer of carbon such as amorphous carbon or diamond-like carbon. The carbonaceous coating enhances biocompatibility without preventing elution of the therapeutic drug. The result is a stent that is adapted to both the hemodynamic and the immune response requirements of its vascular environment. 
     These and other objects and advantages of the present invention will become increasingly more apparent by a reading of the following description in conjunction with the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a stent  12  supporting a blood vessel  10  according to the present invention. 
         FIG. 2  is a cross-sectional view of a wire  14  comprising the stent  12  shown in  FIG. 1 . 
         FIG. 2A  is an enlarged cross-sectional view of the indicated portion of the stent wire  14  shown in  FIG. 2  comprising a biocompatible porous columnar coating  16  supported on the stent wire  14  with a thin carbonaceous material  18  providing a cap on each of the columns as well as covering the inside-facing surface  14 B of the wire. 
         FIG. 3  is a cross-sectional view of the stent portion shown in  FIG. 2A , but with the capillary spaces between the columnar coating  16  infused with a medication compound  20 . 
         FIG. 3A  is an enlarged cross-sectional view of the indicated portion of  FIG. 3 . 
         FIG. 4  is a cross-sectional view showing the interface between the stent and blood vessel  10  after deployment of the stent  12 . 
         FIG. 5  is an SEM photograph of a fracture cross-section of a porous columnar titanium nitride coating with porous carbon caps. 
         FIG. 6  is a SEM photograph showing sputtered columnar aluminum nitride adhering to a substrate that has been subjected to plastic deformation. 
         FIG. 7A  is a schematic view of an unstrained stent wire  14  in a zero stress state. 
         FIG. 7B  is a schematic view of the stent wire  14  shown in  FIG. 7A  having been strained within its elastic limit and depicting the resulting tension and compression stress forces therein. 
         FIG. 7C  is a schematic view of the stent wire  14  shown in  FIG. 7B  being elastically strained and provided with a columnar coating  16  that is an unstrained state. 
         FIG. 7D  is a schematic view of the stent wire  14  shown in  FIG. 7C  in a relaxed, unstrained state and depicting the resulting tension and compression stress forces in the columnar coating  16 . 
         FIG. 7E  is a schematic view of the stent wire  14  shown in  FIG. 7C  having been expanded past its elastic limit and depicting the resulting tension and compression stress forces in both the wire and the columnar coating  16 . 
         FIG. 8A  is a cross-sectional view of a polymer  26  used as a reinforcing material between individual columns  16 . 
         FIG. 8B  is a cross-sectional view of the stent shown in  FIG. 8A  provided with a diffusion limiting polymeric coating  28 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It has been found that coatings having a columnar structure can be made to adhere strongly to a substrate even while the substrate undergoes extensive plastic deformation. This is possible because the porous film consists of many strongly adhering individual columns rather than a single thin film coating. Furthermore, it has been shown that when thin columnar coatings are themselves coated with a biocompatible material such as carbon, the carbon continues the original columnar structure rather than disposing itself as a continuous non-porous barrier layer. This is described in U.S. Patent Application Pub. No. 2004/0176828 to O&#39;Brien, which publication is assigned to the assignee of the present invention and incorporated herein by reference. These characteristics are put to use in the present invention as a medication-carrying structure on a stent for the purpose of eluting the medication into surrounding tissue to lessen or prevent restenosis. 
     Referring now to the drawings,  FIG. 1  shows a cross-section of a blood vessel  10  with a stent  12  inserted and expanded therein. In the current invention, the medication eluting coating is limited to that portion of the stent in contact with tissue, which is exemplified by the blood vessel  10 . 
     The stent  12  is comprised of a plurality of wires  14  forming an elongated hollow tube and disposed so as to be capable of circumferential expansion. Commonly used stent materials include platinum, Nitinol, and even medical grade 316L stainless steel containing about 16% nickel. The wires  14  provide for an elongated, expandable hollow tube that can, in a preferred embodiment, increase in diameter when the ends of the hollow tube are moved closer relative to each other and decrease in diameter when the ends are moved apart. A design objective is to have as little length change as possible when the stent is expanded. Physicians have a hard enough time lining up a stent with a lesion without it acting like an accordion. 
     The stent  12  is positioned in the vasculature of a patient during or after a procedure, such as an angioplasty, atherectomy, or other interventional therapy, and then expanded to an appropriate size (i.e., approximately the same diameter as the vessel  10  in the region where placed), thus supporting that vascular region. When in its expanded configuration, the stent  12  provides support to the vascular walls thereby preventing constriction of the vascular region in which it is located and maintaining the vascular lumen open. This is often referred to as maintaining vascular patency. 
       FIG. 2  represents a cross-section of a wire  14  comprising the vascular stent  12 . The stent wire  14  has a roughly circular cross-section comprising an outside-facing surface  14 A and an inside-facing surface  14 B. The outside-facing surface  14 A of the stent wire faces the blood vessel wall and serves as a substrate provided with a coating  16  of columnar material to a thickness of about 0.1 μm to about 20 μm. Sputtering causes the columnar material to first be physically absorbed with some implantation into the wire material. This is due to the kinetic energy generated by the sputtering process prior to the column growing to its desired length. 
     While sputtering is a preferred method for depositing the columnar coating  16 , other suitable thin film deposition method can be used. These include chemical vapor deposition, pulsed laser deposition, evaporation including reactive evaporation, and thermal spray methods. Also, while the wire  14  is shown having a circular cross-section, that is not necessary. Other embodiments of the stent  12  comprise wires  14  having triangular, square, rectangular, hexagonal, and the like cross-sections. 
     As shown in  FIGS. 3 and 3A , each column of the coating  16  comprises an intermediate portion  16 A extending to a base  16 B adhered to the inside-facing surface  14 B of the wire  14  and a tip  16 C. Each column is of a relatively consistent cross-section along its length extending to the base  16 B and tip  16 C. That way, the columns are discrete members that only adhere to the wire substrate at their base  16 B, but do not join to an immediately adjacent column. Titanium nitride is a preferred material for the columnar coating  16 , although other useful materials include, but are not limited to, boron, aluminum, calcium, gold, hafnium, iridium, molybdenum, niobium, platinum, rhenium, ruthenium, silicon, silver, tantalum, titanium, tungsten, yttrium, and zirconium, and carbides, oxides, nitrides, oxynitrides, carbonitrides thereof. 
     To further lessen the response of contacted tissue to the presence of the stent  12 , the inside-facing surface  14 B of the wire  14  as well as each columnar tip  16 C is coated with a carbonaceous material  18 , such as amorphous carbon or diamond-like carbon. During this operation, the carbon  18  assumes the morphology of a “cap” adhered to each tip  16 C of the porous columnar coating  16  supported on the outside-facing surface  14 A of the stent wire  14 . The carbon caps  18 , which are also preferably provided by a sputtering process, are at a thickness of about 0.05 μm to about 2.0 μm. That is, the porosity of the drug-eluting columnar coating  16  is maintained. This is because while the thickness of the carbon cap is sufficient to impart biocompatibility to the columnar tip  16 C, it is insufficient to form a continuous coating that could detrimentally isolate the drug eluting porosity inherent in the columnar structure. The carbon  18  that coats the bare metal inside-facing surface  14 B of the stent wire  14  forms a smooth continuous pore-free layer suitable for contact with blood. 
     Finally, as shown in  FIGS. 3 and 3A , the capillary spaces between the columns of the coating  16  and the carbon cap  18  are infused with medication  20  to inhibit restenosis. This can be done by various methods well known to those skilled in the art including spraying the stent with a medication solution, dipping the stent into a medication solution, immersing the stent in a medication solution under vacuum conditions and centrifuging the medication solution into the porosity. 
       FIG. 4  shows the interface between the treated stent wire  14  and the blood vessel  10  after deployment of the stent  12  therein. Medication  20  residing in the capillary spaces of the columnar coating  16  is directed into the vessel  10  supported by the stent with the vessel tissue only contacting the biocompatible carbonaceous caps  18 . 
     It is to be appreciated that the schematics of  FIGS. 1 to 4  do not illustrate the extremely high surface area present in the inter-columnar capillaries.  FIG. 5  is a SEM photograph of a fracture cross-section of a porous columnar coating illustrating the volume of empty space therein and the internal surface roughness of the capillaries. In this case, the porous columnar coating consists of titanium nitride, which is widely used as a permanent implantable coating for bioelectrodes. Also visible in the photograph is the carbon cap on each individual titanium nitride column, comprising the outer 200 nm to 300 nm of the coating. Deposition of the carbon layer was done with the mandrel removed from the stent mesh. The mesh was fixtured to expose all surfaces of the stent to sputter flux. The stent was rotated in the sputter flux during deposition, which was done with DC sputtering of a carbon target in argon process gas. Typical conditions are 7 mTorr, 250 Watts, no bias. The result is a stent that presents a relatively thick, porous eluting layer containing therapeutic medication to the blood vessel wall, while presenting a smooth, hemocompatible face to the flowing blood. 
       FIG. 6  illustrates adhesion of a porous columnar coating of aluminum nitride even after extensive plastic deformation of the substrate. Reactive DC sputtering was used. The process gas was pure nitrogen at a pressure of about 5.3 mTorr. Power was set at 250 W on a 3 inch diameter planar target with no bias. Deposition time was 4 hours. 
     In that respect, a further aspect of the invention relates to controlling the stress state of each column comprising the coating  16  supported on the stent wires  14 . Fixturing the stent  12  on a mandrel (not shown) subjected to a sputter flux provides for coating the outside-facing surface  14 A thereof with the columnar coating  16  while protecting the inside-facing surface  14 B of the stent wire  14 . Increasing the degree of expansion over the mandrel to higher levels, within the elastic limit of the stent wire  14 , and sputtering in that expanded state, lessens the overall stress on the columnar coating  16  when the stent  12  is finally inserted and expanded in the blood vessel  10 . Then, when the stent is plastically deformed upon deployment into the vasculature, the individual columns are less likely to delaminate from the wire substrate as their connection to the substrate is in a relatively less stressed state. The associated carbon caps  16  experience the same compression and tension stress forces because they essentially “ride” on the tips  16 B of each column. This is illustrated in  FIGS. 7A to 7E . 
       FIG. 7A  shows an unstrained stent wire  14 . The wire has a generally elongate U-shape comprising spaced apart struts  14 C and  14 D joined together by a union portion  14 E. Datum points  22  and  24  are indicated adjacent to the terminus of the respective struts  14 C,  14 D. In actuality the struts comprise a continuous structure such as a mesh and have no “terminus”. When the stent is placed over a supporting mandrel (not shown), the distance between the datum points  22 ,  24  is increased, as indicated by the opposing directions of the respective vector arrows  22 A and  24 A in  FIG. 7B . The stresses set up in the union portion  14 E include both tension forces (+σ e ) and compression forces (−σ e ) within the elastic limits of the wire. The goal is to stress the union portion  14 E of the wire  14  within its elastic limits so that the tension and compression strains create an opposite elastic pre-strain in the coating when the stent is removed from the mandrel. The struts  14 C,  14 D remain relatively unstressed. 
       FIG. 7C  shows the stent wire  14  in the same stressed state illustrated in  FIG. 7B , but after the sputtered columnar coating  16  is applied. The columnar coating  16  is in a zero stress state. Then, as shown in  FIG. 7D , when the stent is removed from the mandrel, it elastically springs back with the distance between the datum points  22 ,  24  being at or near to their original spacing shown in  FIG. 7A . The columnar coating  16  is now in a stressed state opposite to that shown for the substrate in  FIG. 7C . In that respect, the columnar coating  16  on the outside-facing surface  14 A is in a tension state within the elastic limits of the wire coating material (+σ e ) while the columnar coating on the inside-facing surface  14 B is in a compression state (−σ e ). 
     In  FIG. 7E , the wire  14  undergoes plastic deformation during the stent&#39;s expansion and placement in the vasculature. This is depicted by the opposing directions of the respective vector arrows  22 A,  24 A. In this final state, the stress in the coating  16  is the stress due to deformation of the wire surface at the union portion  14 E minus the coating pre-stress, as shown in  FIG. 7E . Therefore, the final tension (−σ f ) and compression (+σ f ) forces in the coating  16  are somewhat less than they would have been had the columnar coating been provided on the stent wire in a completely relaxed state in comparison to the actual stressed state the union portion  14 E was in during the deposition process. The difference is the amount of elastic deformation in the union portion  14 E of the stent wire  14  while the coating was being deposited ( FIG. 7C ). 
     The elastic limit of the stent wire  14  can be determined by placing the stent over increasingly larger diameter mandrels, until the spring back upon removal does not return the stent to its original dimension. Alternately, the film pre-stress can be achieved by using a nickel titanium shape memory alloy which can be made to assume the partially expanded configuration by heating in the sputter chamber. 
     Another aspect of the invention is shown in  FIG. 8A . This embodiment relates to the use of a polymer  26  that is provided with a medication and infused into the porous columnar coating  16  to improve biocompatibility while increasing coating strength and adhesion. Suitable polymers include (but are not limited to) polyurethane, silicone, polyesters, polycarbonate, polyethylene, polyvinyl chloride, polypropylene methylacrylate, para-xylylene. 
     As shown in  FIG. 8B , a polymer  28  can also be used to moderate and control the diffusion of the medication from the capillaries of the porous coating  16  into the surrounding tissue. In that case the polymeric coating  28  is added to the porous layer after it is infused with the therapeutic medication. 
     Because the process coats all surfaces of the stent, it allows selection from a wider range of substrate materials, including those which improved radiopacity characteristics. This is an important consideration for locating the stent correctly during placement in the vasculature. 
     It is appreciated that various modifications to the invention concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.