US20060260930A1 - Multi-layered radiopaque coating on intravascular devices - Google Patents

Abstract

Intravascular devices having a radiopaque layer thereon for visualization are provided. The devices further includes a capping layer on the radiopaque layer to prevent exposure of the radiopaque material to surrounding tissues. A method of coating the device is also provided. The method includes using an unbalanced magnetic field magnetron to generate, from a source, metal atoms for coating and bombarding ions for compressing deposited metal atoms to the surface of the device.

Description

RELATED U.S. APPLICATIONS(S)
• This application claims priority to U.S. Provisional Application Ser. Nos. 60/252,005, filed Nov. 20, 2000, and 60/253,107, filed Nov. 27, 2000, both which are hereby incorporated herein by reference.
FIELD OF THE INVENTION
• The present invention relates to intravascular devices, and in particular, to intravascular devices having radiopaque coatings thereon for visualization.
BACKGROUND ART
• Many medical intravascular devices are used either temporarily or permanently inside the human body. An example of such an intravascular device includes a stent for use in, for instance, coronary angioplasty. Stents are small metal scaffolds used to mechanically hold open and support constricted coronary arteries. For proper positioning, stents may need to be visualized during and after deployment using imaging techniques such as x-ray radiography and x-ray fluoroscopy. However, due to the nature of the materials used to construct these intravascular devices and their small size, visualization of these devices can often be poor or non-existent.
• Certain “radiopaque” materials are known to be more effective in stopping energetic x-ray photons, and as a result, are more readily visualized during, for instance, x-ray imaging. However, incorporation of these radiopaque materials, including ones that are biocompatible, into the device substrate material can have an undesirable effect on other device characteristics, such as mechanical performance.
• Traditional methods for adding opacity to a device include the use of metal bands, electrochemical deposition (i.e., electro-plating), or coatings. In the case of metal bands or disks having radiopaque material, the bands or disks may be crimped, swaged, pressed or glued on to the device at selected points. However, bands have the potential for becoming loose, shifting, or even falling off. Moreover, bands may also cause abrasion to the intima (i.e., the lining of a vessel wall) during insertion of the device, especially if the bands have sharp edges or outward projections. The physiological response can often be a reclosure of the lumen, thereby negating the effect of the device. Additionally, cellular debris can be trapped between the intravascular device and the band, and the edges of the band can serve as a site for thrombus formation.
• Alternatively, a metal coating can be used as a marker and can be applied on to an intravasuclar device using chemical vapor deposition (CVD), physical vapor deposition (PVD), or electroplating. However, for the range of thickness required to make the coating x-ray opaque, CVD and conventional PVD methods do not appear to provide a coating which can exhibit sufficient adhesion to the surface of the device, especially a stainless steel substrate surface, to be reliable in a medical device application.
• On the other hand, electroless and/or electroplated coatings are often porous, and can present a biocompatibility problem, since the porous coating can act to entrap the plating chemicals. For devices constructed from, for instance, titanium alloys, embrittlement caused by the electroplating process can occur to significantly alter the mechanical properties and thus the function of the device.
• Ion beam assisted deposition (IBAD) of radiopaque materials has been used and can improve the adhesion of coatings to the substrate surface. IBAD employs conventional PVD to create a vapor of atoms of, for instance, a noble metal that coats the surface of the substrate, while simultaneously bombarding the substrate surface with ions at energies, typically in the range of 0.8 to 1.5 keV, to impact and condense the metal atoms on the substrate surface. An independent ion source is used as the source of ions.
• Coatings produced by traditional IBAD, however, are costly. When evaporating, atoms of expensive noble metal are emitted over a large solid angle compared to that subtended by the device or devices being coated, thus requiring a costly reclaiming process. Moreover, because an evaporator uses a molten metal, it must be located upright on the floor of the deposition chamber to avoid spilling, thereby restricting the size and configuration of the chamber and the devices being coated. Additionally, evaporators cannot deposit mixtures of alloys effectively because of the differences in the alloy components' evaporation rates. As such, the composition of the resulting coating constantly changes.
• Furthermore, with IBAD, the flux (i.e. stream) of bombarding ions and evaporant (i.e., atoms of metal being deposited) approach the substrate from different directions. To this end, the energy from the bombarding ions transferred to the evaporant atoms varies depending on the extent to which the two streams overlap. As a result, the growth mechanism of the coating can be inconsistent, and uniform coating properties are difficult to achieve even over the same device.
SUMMARY OF THE INVENTION
• The present invention provides, in accordance with one embodiment, an intravascular device having substrate, a layer of a radiopaque material disposed on the substrate surface and having a thickness sufficient for visualization, and a capping layer to prevent exposure of the layer of radiopaque material to surrounding tissue. In an embodiment, the device may include a transition layer between the layer of radiopaque material and the capping layer to enhance bonding of the capping layer to the layer of radiopaque material. The device may also include an adhesion layer to promote bonding of the layer of radiopaque material to the substrate.
• The present invention also provides, in another embodiment, a method for coating an intravascular device for visualization. The method includes providing an intravascular device having a substrate surface. Next, a flux of atoms of a radiopaque material and a flux of bombarding ions may be generated in a manner which permits the flux of atoms of a radiopaque material and bombarding ions to travel towards the substrate surface of the device in a co-linear fashion. Thereafter, the atoms of radiopaque material may be deposited onto the substrate surface, and the bombarding ions permitted to impact the atoms of the radiopaque material deposited on the substrate surface to provide a substantially uniform layer of radiopaque material. If desired, co-linear fluxes of metal atoms and of bombarding ions and may be directed in a similar manner on to the layer of radiopaque material to provide a capping layer.
BRIEF DESCRIPTION OF DRAWINGS
• FIG. 1 illustrates an instravascular device in accordance with one embodiment of the present invention.
• FIG. 2 illustrates a cross sectional view of the intravascular device shown inFIG. 1.
• FIG. 3 illustrates a detailed cross sectional view of an intravascular device in accordance with one embodiment of the present invention.
• FIG. 4 illustrates a detailed cross sectional view of an intravascular device in accordance with another embodiment of the present invention.
• FIG. 5 illustrates a method for depositing various layers onto an intravascular device in accordance with an embodiment of the present invention.
• FIG. 6 illustrates a standard prior art magnetron.
• FIG. 7 illustrates a plasma cloud generated on a magnetron of the present invention.
• FIGS.8A-B illustrate various configurations of magnetrons for use in accordance with embodiments of the present invention.
• FIG. 9 illustrates a coating apparatus for use in the manufacturing of a device of the present invention.
DESCRIPTION OF VARIOUS EMBODIMENTS
• InFIG. 1, there is shown anintravascular device10 in accordance with one embodiment of the present invention. Thedevice10, as illustrated by the cross sectional view inFIG. 2, includes a body11 havingsubstrate surface12, alayer14 of a radiopaque material on thesubstrate surface12, and acapping layer16 on thelayer14 of the radiopaque material.
• Thedevice10, illustrated inFIG. 1 as a stent, may be made so that the body11 can be provided with the ability to expand while, at the same time, being flexible, so that thedevice10 can be deployed to a site ofstenosis15 within, for instance, a coronary artery13. In addition, the body11 ofdevice10 may be made from a strong material sufficient to provide adequate radial strength at the site of stenosis to prevent closure. In one embodiment, the body11 ofdevice10 may be made from a metallic material, which includes, but is not limited to, stainless steel, nickel-based steel, cobalt-chrome, titanium alloys, and nitinol.
• With reference now toFIGS. 2 and 3, thelayer14 of radiopaque material provides thedevice10 with opacity, so that thedevice10 may be visualized by, for instance, x-ray imaging. In addition to providing opacity, thelayer14 may be pliable and malleable to permit thelayer14 to expand and flex along with the body11 ofdevice10. To that end, thelayer14 may be made from a ductile metal, for instance a noble metal, an example of which may be gold. Thelayer14 should also be substantially pure and free of contaminating material, so as to minimize interference with its bond to thesubstrate surface12. Furthermore, to permit sufficient opacity, while maintaining flexibility and malleability, thelayer14, in accordance with an embodiment of the invention, may be provided with a thickness having a range of from about 1 micron to about 15 micron, with a preferred thickness having a range of from about 3 microns to about 9 microns. In the instance where gold may be used forlayer14; a thickness ranging from about 5 microns to about 6 microns is preferred. The thickness of thelayer14 of radiopaque material may vary depending on the type of application or use for which thedevice10 is employed.
• In certain situation, it may not be desirable to expose thelayer14 of radiopaque material to surrounding tissues. To that end, cappinglayer16 may be provided to prevent exposure of surrounding tissues to the radiopaque material ondevice10. It should be noted that the integrity of thecapping layer16 must be maintained while thedevice10 is being maneuvered and deployed. Accordingly, thecapping layer16 may be made from a strong, substantially scratch resistant, yet malleable material with a thickness ranging from about 0.5 micron to about 1 micron. In addition, as thecapping layer16 may be positioned adjacent living tissues, it may be desirable to manufacture thecapping layer16 from a biocompatible material. In an embodiment of the invention, thecapping layer16 may be made from a metallic material, including platinum alloys, platinum-iridium, palladium, and tantalum. Thecapping layer10, like thelayer14 should be substantially pure and free of contaminating material.
• With particular reference now toFIG. 3, to enhance a bond between the cappinglayer16 and thelayer14 of radiopaque material, thedevice10 may be provided with atransition layer17. Thetransition layer17, in one embodiment, comprises a mixture of the radiopaque material and the material comprising thecapping layer16, for example, a mixture of gold or platinum-iridium. Such a mixture can provide a graded interface between the cappinglayer16 and thelayer14 of radiopaque material which, it has been found, can result in superior adhesion between the twolayers14 and16. It is noted that providing thetransition layer17 with a thickness of about 1 micron or less, and in particular, a thickness ranging from about 0.25 micron to about 0.5 micron, can add to the superior adhesion between the cappinglayer16 and thelayer14 of radiopaque material, while maintaining the diameter of thedevice10 within an optimal range.
• Now referring toFIG. 4, in certain embodiments, anadhesion layer18 may be provided between thesubstrate surface12 and thelayer14 of radiopaque material to enhance the bonding of thelayer14 to thesubstrate surface12. Theadhesion layer18, in an embodiment, includes a mixture of chromium, palladium, and the radiopaque material. Such a mixture, similar to the mixture in thetransition layer17, provides a graded interface to which thelayer14 of radiopaque material can better adhere. In order to maintain the diameter ofdevice10 within an optimal range, theadhesion layer18 may be provided with a thickness that is about 1 micron or less, and more particularly, from about 0.25 microns to about 0.5 microns.
• Referring now toFIG. 5, the various layers described above may be deposited on thedevice10 using, in one embodiment, an unbalancedmagnetic field magnetron20. Themagnetron20, as shown in cross sectional view inFIG. 5, includes asource21 of metal atoms for use in coating thedevice10. Thesource21, typically a plate comprising the metal atoms for coating, may be positioned in front of an array ofmagnets22 placed within acooling block23. Thecooling block23, in an embodiment, may be made from, for instance, copper. If desired, coolingchannels24 may be provided within thecooling block23 to permit fluid, like water, to circulate therethrough to cool down themagnetron20.
• The array ofmagnets22 within thecooling block23 preferably permits themagnetron20 to generate an unbalancedmagnetic field26 when thecooling block23 is biased with an electrical charge ranging from about −200VDC to about −1000VDC. It should be noted that in a standard (i.e., balanced magnetic field) magnetron, as shown inFIG. 6, the field lines61 in themagnetic field60 emanate from theouter magnets62 and terminate tightly in thecentral magnet63. However, by providing the magnetron20 (FIG. 5) of the present invention withmagnets22 of uneven strength, for example, outer magnets that are stronger than central magnet, not allfield lines27 emanating from the outer magnets terminate in the central magnet. These field lines27, of course, eventually returns to the array of magnets, but not in a tight pathway as that observed with field lines in the standard magnetron. It should be noted that although the array ofmagnets22 inFIG. 5 is arranged in a circular format, the array can be arranged in any geometric shape, so long as an appropriate unbalanced magnetic field can be generated.
• Themagnetic field26, which has electrons trapped therein, in the presence of noble atoms, for instance, argon or xenon atoms, can cause an ionization event to create aplasma cloud28 within themagnetic field26. In particular, when noble atoms traveling through themagnetic field26 come into contact with the electrons therein, the noble atoms become ionized (i.e., positively charged ions). This ionization process generates aplasma cloud28 within themagnetic field26. As illustrated inFIG. 7, theplasma cloud28 above thesource plate21 and within themagnetic field26 can conform to the shape of the array ofmagnets22. In this instance, theplasma cloud28 is toroidal in shape.
• It should be noted that since thecooling block23 is biased with a negative electrical charge, the positively charged noble ions within theplasma cloud28 are drawn towards thesource21 of metal atoms positioned in front of the array ofmagnets22. In addition, since themagnetic field26 is unbalanced, theplasma cloud28 may not be as well contained therein, making it easier to extract or draw the noble ions from theplasma cloud28 towardssource21. Upon hitting thesource21 of metal atoms, the noble ions can causemetal atoms29 to come off the surface of the source21 (i.e., sputtering) and travel towards thedevice10. Because of the manner in which the noble ions impinge the surface of thesource21, themetal atoms29 coming of the source tend to travel in a substantially straight path towards thesubstrate surface12 ofdevice10. The movement of metal atoms towards thedevice10 creates what is know as a flux ofmetal atoms29. The flux ofmetal atoms29, upon hitting thesubstrate surface12 ofdevice10, coats thesubstrate surface12 with a layer of themetal atoms29.
• The presence of an unbalancemagnetic field26 on themagnetron20 also makes it less difficult for some of electrons within theplasma cloud28 to escape therefrom. These escaping electrons tend to followfield lines27 away fromplasma cloud28, leaving theplasma cloud28 net positive. Since thecloud28 generally would want to remain net neutral,noble ions30 are permitted to escape towards thesubstrate surface12 of thedevice10. This flux ofnoble ions30, upon bombarding thesubstrate surface12, act to impact and condense themetal atoms29 onto thesubstrate surface12. In this manner, a layer ofmetal atoms29 having uniform thickness throughout may be generated. It should be appreciated that the escaping electrons may collide with noble atoms present in an area between themagnetron20 and thedevice10. When such an event occurs, additionalnoble ions30 can be generated to bombard thesubstrate surface12 to impact and condense themetal atoms29 on thesubstrate surface12.
• To further enhance the movement of positively chargednoble ions30 towards thedevice10, thedevice10 may be biased with an electrical charge ranging from about −20VDC to about −100VDC. The biasing of thedevice10 with a negative voltage can further generate a desired effect. In particular, a negative chargeddevice10 can reduce the number of electrons that may impinge on thedevice10. In this manner, the amount of heat typically generated from electrons hitting thedevice10 can be minimized.
• It should be noted that unlike existing technology which employs different independent sources for generating coating metal atoms and for generating bombarding noble ions (e.g., IBAD), the unbalancedmagnetic field magnetron20 of the present invention permits bombardingnoble ions30 to be generated from the same source, that is theplasma cloud28, used to generatemetal atoms29 for coating thedevice10. In addition, while themagnetron20 of the present invention permits the flux ofmetal atoms29 and flux of bombardingions30 to both travel in a co-linear, or same, direction from theplasma cloud28 towards thedevice10, it can also control the ratio of bombarding ions flux to metal atoms flux, so that such ratio can be maintained at a substantial constant. The ability to impart a co-linear flow and the ability to maintain a constant ratio of metal atoms to bombarding noble ions further enhances the generation of a substantially uniform coating on thedevice10. Furthermore, energy imparted by the bombarding ions, typically in the range of from about 50 eV to about 250 eV, is substantially less than that observed with the IBAD method (e.g., 0.8 keV to 1.5 keV). By imparting relatively less energy per bombarding ion but to relatively more bombarding ions, the unbalancedmagnetic field magnetron20 can increase compression events of the metal atoms deposited on thesubstrate surface12, while decreasing the likelihood of back-sputtering of metal atoms from thesubstrate surface12.
• Although the description above references onemagnetron20 in the coating ofdevice10, it should be appreciated thatmultiple magnetrons20, such as that shown in FIGS.8A-B may be used, especially if multiple metal atoms are used to provide multiple layers on thedevice10. In an embodiment wherein at least twomagnetrons20 are used, the magnetrons may be set up in mirrored fashion or in coupled fashion.
• In the mirrored fashion, as illustrated inFIG. 8A, eachmagnet80 in onemagnetron20 may be situated so that it faces amagnet80 in the opposingmagnetron20 having a same magnetic pole. By positioning themagnets80 in this mirrored fashion,magnetic fields81, such as that shown inFIG. 8A, can be generated to permit relatively few ionizing events, so as to reduce the amount of noble ion bombardment of the substrate surface.
• In the coupled fashion, as illustrated inFIG. 8B, eachmagnet80 in onemagnetron20 may be situated so that it faces amagnet80 in the opposingmagnetron20 having an opposite magnetic pole. By positioning themagnets80 in the coupled fashion,magnetic fields82, such as that shown inFIG. 8B, can be generated to permit relatively more ionizing events, so as to increase the amount of noble ion bombardment of the substrate surface.
• An example for coating thedevice10 is hereinafter provided. In one embodiment of the invention,device10, which in this example may be a stent approximately 2.5 cm in length and approximately 4 mm in diameter, may first be cleaned using any number of appropriate solvent cleaning protocols designed to remove contaminants on all surfaces of thedevice10. Thedevice10 may subsequently be dried. Next, an ion pre-clean process may be employed to further clean thesurface12 ofdevice10 prior to coating.
• Looking now atFIG. 9, thedevice10 may be taken to acoating apparatus90 and placed on afixture91 positioned within avacuum chamber92. Next, a vacuum of about 1E-3 to 1E-9 torr may be drawn fromchamber92 usingvacuum pump98. Vacuum pumping may thereafter be throttled byvalve93 and a noble gas, for instance, argon or xenon, may be introduced from asource94 throughport95 intochamber92. Thechamber92 may continue to be filled with the noble gas to a pressure ranging from about 0.1 mtorr to about 100 mtorr. Next, an electrical charge of about −200VDC to about −1000VDC may be applied todevice10, while thedevice10 is moved through a magnetic field between themagnetrons96. In the presence of the electrical charge, the noble atoms become ionized and are pulled toward the surface of thedevice10 to rid the surface of oxides and other unwanted contaminants. This pre-cleaning process of thedevice10 may last from about 5 to about 60 minutes, depending on the initial cleanliness of thedevice10.
• Once the ion pre-cleaning process is completed, the coating process may begin. Vacuum pumping inchamber92 may be throttled further and a noble gas, such as argon, may be introduced throughport95 at a rate of about 33 sccm (standard cubic centimeter) and to a pressure of from about 1 mtorr to about 20 mtorr within thechamber92. Themagnetrons96 may next be electrically charged to a voltage in the range of from about −500VDC to about −1000VDC. In the presence of the electrical charge, the noble gas becomes ionized to generate a plasma cloud within themagnetic field97 on each of themagnetrons96. In one embodiment, themagnetrons96 may be placed approximately 8 in. apart with thedevice10 positioned in therebetween. If desired, onemagnetron96 may be oriented at an angle of plus 20°, while theother magnetron96 may be oriented at an angle of minus 20° from the normal of anouter surface device10 to enhance deposition on to surfaces of the three-dimensional device10. Subsequently, a charge of about −40VDC may be applied to thedevice10, so that a current draw on thedevice10 cab be maintained at about between 7 to 9 mA. Thereafter, metal atoms, such as gold, on onemagnetron96, may be deposited on to thedevice10 at a rate, for example, of between 165-185 Å/min. The rate of deposition should be such that the ratio between the metal atoms flux and the bombarding ions flux can be maintained at a substantial constant. The constant, as it will be appreciated, can vary depending on the device being coated. After between about 7.5 and about 8.0 microns have been deposited, the gold deposition may be stopped to provide a radiopaque layer. Next, metal atoms, such as platinum or platinum-irridium, on the opposingmagnetron96, may be deposited at a similar rate to a thickness of between about 0.5 and about 1.0 micron to provide a capping layer. After an appropriate cooling period, thevacuum chamber92 may be vented to atmosphere using nitrogen gas, and thedevice10 removed.
• It should be appreciated that atransition layer17, such as that shown inFIG. 3, may be provided between the cappinglayer16 and theradiopaque layer14 to enhance bonding between the twolayers14 and16. To deposit thetransition layer17 onto thedevice10, the magnetron depositing, for instance, gold atoms onto thedevice10, may be controlled to decrease the amount at which gold atoms are being deposited, while the opposing magnetron used for depositing platinum atoms or a mixture of platinum-iridium atoms may be controlled to simultaneously deposit the platinum or platinum-iridium atoms at an increasing amount. The amount of platinum or mixture of platinum-iridium atoms being deposited can continue to increase, while the amount of gold atoms being deposited can continue to decrease until platinum or mixture of platinum-iridium atoms comprise 100 percent of the deposition. Such a process can provide a gradedtransition layer17, preferably with a thickness ranging from about 0.25 microns to about 0.5 microns, which can result in superior adhesion of thecapping layer16 to theradiopaque layer14.
• Anadhesion layer18, such as that shown inFIG. 4, may also be deposited between thesubstrate surface12 and thelayer14 of radiopaque material. To deposit theadhesion layer18 onto thesubstrate surface12, one magnetron may be used to generate a flux of chromium atoms onto thesubstrate surface12. Thereafter another magnetron may be used to generate a flux of palladium atoms, so that the palladium atoms may be simultaneously deposited onto thesubstrate surface12 with the chromium atoms. The magnetron generating the chromium flux may next be controlled to decrease the amount at which chromium atoms are being deposited, while the opposing magnetron used for generating the palladium flux may be controlled to increase the amount at which palladium atoms are being deposited. This process can continue until palladium atoms comprise100 percent of the deposition. At a point at which only palladium atoms are being deposited, a flux of gold atoms may be generated for deposition onto thesubstrate surface12. The amount of palladium atoms being deposited thereafter may be decreased, while the amount of gold atoms being deposited may be increased, until gold atoms comprise 100 percent of the deposition. Such a process can provide a gradedadhesion layer18, preferably with a thickness ranging from about 0.1 micron to about 0.2 microns, which can result in superior adhesion of thelayer14 of radiopaque material to the substrate surface.
• Alternatively, instead of generating a flux of chromium atoms and a flux of palladium atoms from different magnetrons, a mixture of chromium-palladium atoms may be positioned on one magnetron to permit a flux of chromium-palladium atoms to be generated. The amount of chromium-palladium atoms being deposited may thereafter be decreased, while the amount of gold atoms being deposited may be increased, until gold atoms comprise 100 percent of the deposition.
• While the invention has been described in connection with the specific embodiments thereof, it will be understood that it is capable of further modification. Furthermore, this application is intended to cover any variations, uses, or adaptations of the invention, including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as fall within the scope of the appended claims.

Claims (7)

1-31. (canceled)

32. A method for coating an intravascular device, the method comprising:

providing an intravascular device having a substrate surface;

generating a flux of atoms of a radiopaque material and a flux of bombarding ions;

directing the flux of atoms of the radiopaque material and the flux of bombarding ions in a co-linear fashion toward the substrate surface;

depositing the atoms on to the substrate surface; and

permitting the bombarding ions to impact the atoms to the substrate surface, so as to provide a substantially uniform layer of radiopaque material.

33. A method as set forth inclaim 32, further comprising:

generating a flux of first metal atoms;

directing the flux of first metal atoms and a flux of bombarding ions in a co-linear fashion toward the layer of radiopaque material;

depositing the first metal atoms on to the layer of radiopaque material; and

permitting the bombarding ions to impact the first metal atoms to the radiopaque material, so as to provide a substantially uniformly capping layer.

34. A method as set forth inclaim 33, further comprising:

depositing a mixture of the atoms of radiopaque material and the first metal atoms between the layer of radiopaque material and the capping layer; and

generating a transition layer therebetween.

35. A method as set forth inclaim 34, wherein the step of depositing includes:

reducing an amount at which the atoms of the radiopaque material are being deposited; and increasing an amount at which the metal atoms are being deposited.

36. A method as set forth inclaim 33, further comprising:

depositing a mixture of second metal atoms and atoms of the radiopaque material between the substrate surface and the layer of radiopaque material; and

generating an adhesion layer therebetween.

37. A method as set forth inclaim 36, wherein the step of depositing includes:

reducing an amount at which the second metal atoms are being deposited; and

increasing an amount at which the atoms of the radiopaque material are being deposited.

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