Abstract:
A method of manufacturing a medical device having interior and exterior surfaces, the method including the steps of: a) shielding the exterior surface; and, b) exposing the interior surface to a plasma, wherein the shielding of the exterior surface substantially prevents exposure of the exterior surface to the plasma.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation-in-Part of application Ser. No. 11/704,650, filed on Feb. 9, 2007, which application claims the benefit of Provisional Application Ser. No. 60/771,834, filed Feb. 9, 2006, which applications are each incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention broadly relates to means for modifying surfaces by deposition and etching, and more specifically, to means for creating structures and materials selectively on the inside surfaces of medical devices to render the devices biocompatible, to provide drug elution capability and/or to promote cell growth on and cell attachment to the modified surface. 
       BACKGROUND OF THE INVENTION 
       [0003]    Many medical devices, such as stents and stent grafts, are designed and manufactured to be inserted into the wall or lumen of a blood vessel. When this is done, complications may arise from the body&#39;s natural reaction to a foreign object. For example, inserting a stent into a blood vessel may cause the growth of an undesirable thick layer of smooth muscle tissue, and this new growth may cause restenosis, or re-narrowing of the vessel. The effects of restenosis are often minimized through the use of drug eluting stents, in which a medicated coating on the stent prevents tissue growth for a period of time. Thrombus formation is another serious condition that may occur after insertion of a stent, and recent studies have shown that current drug eluting stents can not prevent, and may even promote, thrombosis formation. See, for example, Windecker, S. et al.  Randomized Comparison of a Titanium - Nitride - Oxide - Coated Stent With a Stainless Steel Stent for Coronary Revascularization, Circulation,  111:2617-2622 (2005). 
         [0004]    The inner surface of a healthy blood vessel is lined with endothelial cells, which play an important role in controlling thrombosis, inflammation and other factors. It has generally been found that endothelial cells do not readily attach to the smooth inner surfaces of electropolished metal stents or to the polymers typically used for drug eluting stents. U.S. Pat. No. 6,140,127 discusses the desirability of having endothelial cells attach to the inner walls of stents, and overcomes the previously described attachment issue by using an adhesion specific peptide. Similarly, U.S. Pat. No. 6,478,815 discusses means for overcoming the attachment issue, however in this instance a stent is made primarily of niobium which can be coated with iridium oxide or other materials to promote the growth of endothelial cells. Additionally, a roughened surface on a stent has been proposed as a further means for promoting cell growth on a stent. For example, U.S. Pat. No. 6,820,676 B2 and United States Patent Application Publication No. 2005/0232968 discuss the role of surface inhomogeneities and surface structures in promoting endothelial cell growth. 
         [0005]    While the growth of endothelial cells on the inner surface of a stent is highly desirable, the growth of smooth muscle tissue at the inner wall of the blood vessel, i.e., the portion in contact with the outer surface of the stent, is undesirable. It has been found that stents coated entirely with a drug imbibed polymer layer designed to prevent growth of smooth muscle tissue have been highly successful in reducing in-stent restenosis. Unfortunately, the smooth polymer surface also inhibits endothelial cell growth on the inside of the stent. For example, the use of a drug eluting coating on the outer surface of stents is taught in United States Patent Application Publication No. 2006/0200231, however tailoring the properties of the inner surface for endothelial cell growth is not addressed. Stents having outer and inner surfaces which function differently would overcome the defects described supra. 
         [0006]    Many references that discuss surfaces to control cell growth, i.e., to enhance cell growth in the case of endothelial cells or suppress cell growth in the case of smooth muscle cells, are based on plasma processing and physical vapor deposition. As stents have a generally open structure, when they are coated or treated in a plasma environment both inner and outer surfaces typically receive the same or very similar coatings or treatments. United States Patent Application Publication No. 2006/0200231 describes a well-know means of coating only the outside surface of an object like a stent. The stent is placed on a mandrel which prevents the inner surfaces from receiving a coating while the outer surface is coated. Heretofore, nothing in the prior art suggests a means for plasma treating or coating only the inner surface of a medical device such as a stent, while leaving the outer surface largely unaltered, or allowing the outer surface to receive a different coating or treatment. 
         [0007]    As can be derived from the variety of devices and methods directed at coating and treating implantable medical devices, many means have been contemplated to accomplish the desired end, i.e., surface specific coatings wherein a first surface promotes cell growth thereon and a second surfaces prevents cell growth thereon. Heretofore, tradeoffs between preventing cell growth on one surface and promoting cell growth on another surface were required. Thus, there is a long-felt need for a method to treat or coat only the inner surfaces of medical devices such as shunts, stent-grafts and stents, as a means of preparing the inner and outer surfaces of such devices so that they function differently. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention broadly comprises a method of modifying a surface to produce surface structures, coatings and inhomogeneities in order to promote cell growth on and/or attachment to the surface for a variety of applications. Generally, the subject invention includes plasma deposition and removal processes to produce nanometer scale surface structures and coatings primarily on the inner surfaces of devices having both inner and outer wall surfaces, e.g., stents, stent-grafts and shunts. Specifically, the invention includes methods for producing plasma glow discharges on the inside of medical devices. 
         [0009]    The present invention also broadly comprises a method of manufacturing a medical device having interior and exterior surfaces, the method includes the steps of: a) shielding the exterior surface; and, b) exposing the interior surface to a plasma, wherein the shielding of the exterior surface substantially prevents exposure of the exterior surface to the plasma. In some embodiments, the medical device further includes a first cross-sectional shape; while the step of shielding the exterior surface further includes the step of: contacting the exterior surface of the medical device with an inner surface of a hollow electrically conducting tube, the inner surface having a second cross-sectional shape substantially similar to the first cross-sectional shape; and, the step of exposing the interior surface to the plasma further includes the step of: igniting a hollow cathode discharge within the hollow electrically conducting tube. In other embodiments, the step of exposing the interior surface to the plasma further includes the step of: simultaneously sputtering the tube and the medical device. In some of these embodiments, the step of simultaneously sputtering the tube and the medical device modifies the interior surface of the medical device to include an inhomogeneous surface having at least two materials, while in some of these embodiments, the inhomogeneous surface includes a plurality of individual regions and each of the individual regions includes at least two materials and is separated from others of the individual regions by a material boundary. In still yet other embodiments, the step of exposing the interior surface to the plasma further includes the step of: cooling the hollow electrically conducting tube. 
         [0010]    In further embodiments of the present invention, the medical device further includes a first cross-sectional shape; while the step of shielding the exterior surface further includes the step of: contacting the exterior surface of the medical device with an inner surface of a hollow electrically insulating tube, the inner surface having a second cross-sectional shape substantially similar to the first cross-sectional shape; and, the step of exposing the interior surface to the plasma further includes the step of: igniting a discharge within the hollow electrically insulating tube using a radio frequency power. In some of these embodiments, the radio frequency power includes a capacitively coupled radio frequency field, while in others of these embodiments, the radio frequency power includes an inductively coupled radio frequency field. In some embodiments, the step of exposing the interior surface to the plasma further includes the step of: cooling the hollow electrically insulating tube. 
         [0011]    In yet further embodiments of the present invention, the medical device further includes a first cross-sectional shape; while the step of shielding the exterior surface further includes the step of: contacting the exterior surface of the medical device with an inner surface of a hollow electrically insulating tube, the inner surface having a second cross-sectional shape substantially similar to the first cross-sectional shape; and, the step of exposing the interior surface to the plasma further includes the step of: igniting a discharge within the hollow electrically insulating tube using a microwave power. In some embodiments, the step of exposing the interior surface to the plasma further includes the step of: cooling the hollow electrically insulating tube. 
         [0012]    In still yet further embodiments, the step of exposing the interior surface to the plasma is performed in an inert gas, while in other embodiments, the step of exposing the interior surface to the plasma is performed in a reactive gas selected from the group consisting of: oxygen, nitrogen, methane and mixtures thereof. In still other embodiments, the step of exposing the interior surface to the plasma is performed in a precursor gas, and the precursor gas is selected to deposit a coating on the interior surface, and in some of these embodiments, the precursor gas is selected from the group consisting of: a hydrocarbon, a metal containing compound, oxygen, nitrogen and mixtures thereof. In some embodiments, the coating includes a plurality of clusters and each of the clusters includes a lateral dimension from about ten nanometers to about one thousand nanometers. In other embodiments, each of the clusters have a size and a distance from others of the clusters, and in some of these embodiments, the size of each of the clusters and the distance from others of the clusters are chosen to preferentially bind at least one biological structure having a specific size. 
         [0013]    In yet further embodiments, the step of exposing the interior surface to the plasma removes material from the interior surface of the medical device, while in other embodiments, the present invention method further includes the step of: c) coating at least the interior surface of the medical device with a biodegradable polymer after the step of exposing the interior surface to the plasma. In some embodiments, a medical device is constructed according to the present invention method. 
         [0014]    The present invention further broadly comprises a medical device having an interior surface, an exterior surface and means for exposing the interior surface to at least one plasma. In some embodiments, the at least one plasma includes a first plasma and a second plasma, the first plasma deposits a plurality of clusters on the interior surface and the second plasma etches the interior surface. In other embodiments, the first and second plasmas produce a plurality of surface structures on the medical device. In some of these embodiments, each of the surface structures includes a lateral dimension from about ten nanometers to about one thousand nanometers, while in others of these embodiments, each of the surface structures includes a height from about one hundred nanometers to about ten thousand nanometers. In some embodiments, each of said clusters includes a size and a distance from others of the clusters, and in other embodiments, the size of each of the clusters and the distance from others of the clusters are chosen to preferentially bind at least one biological structure having a specific size. 
         [0015]    It is a general object of the present invention to provide a medical device including an interior surface having different characteristics than the device&#39;s exterior surface. 
         [0016]    It is another general object of the present invention to provide a medical device having an interior surface which includes surface structures, coatings and/or inhomogeneities. 
         [0017]    It is yet another object of the present invention to provide a method of producing a plasma glow discharge on the inside of a medical device while substantially shielding the outside of the device from such discharge. 
         [0018]    These and other objects and advantages of the present invention will be readily appreciable from the following description of preferred embodiments of the invention and from the accompanying drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which: 
           [0020]      FIG. 1  is a cross sectional view of a portion of a typical stent taken generally along a plane parallel to the longitudinal axis of the stent; 
           [0021]      FIG. 2  is a cross sectional view of a representation of a hollow cathode discharge system; 
           [0022]      FIG. 3  is a cross sectional view of an embodiment of a present invention apparatus for coating and/or treating an inner surface of a stent; 
           [0023]      FIG. 4   a  is a cross sectional view of an arrangement for capacitively coupling RF power into a tube to produce a plasma; 
           [0024]      FIG. 4   b  is a cross sectional view of an arrangement for inductively coupling RF power into a tube to produce a plasma; 
           [0025]      FIG. 5  is a cross sectional view of an arrangement having a tube inserted within a microwave cavity so that microwave radiation may reach an interior of the tube; 
           [0026]      FIG. 6  is a cross sectional view of an array of short tubes used to coat or treat a number of devices, e.g., stents, together; 
           [0027]      FIG. 7  is a cross sectional view of a substrate having a discontinuous coating of atoms; 
           [0028]      FIG. 8  is a cross sectional view of the substrate of  FIG. 1  after etching; and, 
           [0029]      FIG. 9  is a cross sectional view of a medical device manufactured according to an embodiment of present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects. 
         [0031]    Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims. 
         [0032]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described. 
         [0033]    Adverting now to the figures,  FIG. 1  shows a cross sectional view of a portion of a typical stent  10  taken generally along a plane parallel to longitudinal axis  12  of stent  10 . Stent  10  is constructed from a plurality of struts  14 , however for clarity, only two struts  14  are shown in  FIG. 1 . Struts  14  form a cage or scaffold, which holds open the lumen of a blood vessel and define a generally cylindrical envelope having longitudinal axis  12 . Struts  14  have inner surfaces  16  and outer surfaces  18 , while portions  20  represent the cut ends of struts  14 . As discussed infra, the present invention method alters inner surfaces  16  through a coating or treatment without substantially altering outer surfaces  18  during the same processing. It should be appreciated that inner surface  16  of stent  10 , i.e., the interior surfaces of the medical device, refers to the portion of the medical device which may be viewed from longitudinal axis  12 . Therefore, outer surface  18  or exterior surfaces refer to the portion of the medical device which may not be viewed from longitudinal axis  12 . 
         [0034]    It is well known in the art of plasmas and plasma deposition that it is possible to produce a glow discharge inside of a tube, even a tube with a diameter of 1 millimeter (mm) or less, for example, using hollow cathode discharges. As one of ordinary skill in the art appreciates, hollow cathode discharges are primarily used as sources of electrons for a variety of applications such as ion beam neutralization, plasma enhancement and electron beam evaporation.  FIG. 2  shows a representation of hollow cathode discharge system  22 . Tube  24  has a source of gas  26  flowing through it and is held at a negative voltage with respect to a second electrode  28  by power supply  30 . It should be appreciated that gas  26  may be an inert gas, e.g., argon, a reactive gas, e.g., oxygen, nitrogen, methane or mixtures thereof, or a precursor gas, e.g., hydrocarbon, metal containing gases, oxygen, nitrogen or mixtures thereof. In the embodiment shown in  FIG. 2 , tube  24  is a small tube. It should be appreciated that second electrode  28  could be a grounded surface which is part of a vacuum chamber, and need not be a discrete electrode as shown in  FIG. 2 . Alternatively, tube  24  could be the grounded surface and electrode  28  could be raised to a positive potential with respect to tube  24 . 
         [0035]    The general principal of operation of hollow cathode discharge system  22  is that electrons  32  emitted from inner surface  34  of tube  24  are confined by reflections at the opposite wall and effectively produce ions  36  in the gas flowing in tube  24  until electrons  32  exit end  38  of tube  24  and are collected by anode  28 . Systems similar to hollow cathode discharge system  22  have been used to deposit material and plasma treat surfaces. See, e.g., U.S. Pat. No. 5,716,500 which describes the use of a hollow cathode discharge system as a source of coating material. Systems similar to hollow cathode discharge system  22  are usually operated at sub-atmospheric pressures, but it is also possible to operate some hollow cathode discharge systems at atmospheric pressures. See, e.g., “Characterization of Hybrid Atmospheric Plasma in Air and Nitrogen,” 49 th  Annual Technical Conference Proceedings of the Society of Vacuum Coaters, 2006. Known methods of using hollow cathode discharge systems include placing a substrate to be coated or modified outside of the hollow cathode tube, e.g., tube  24 . Contrarily, in the present invention, a substrate to be treated or coated lines the inside wall of the hollow cathode discharge system, i.e., inner surface  34  of tube  24 , making the substrate an electrode in the plasma discharge system. Although the extremely small discharge volume in typical hollow cathode discharge systems limits their usefulness for etching or depositing on most substrates, their very size and shape make them ideal for etching or depositing on the inner surface of small objects having generally cylindrical shapes, such as stents, grafts and shunts. 
         [0036]      FIG. 3  shows a cross sectional view of an embodiment of a present invention apparatus for coating and/or treating inner surface  40  of stent  42 . Stent  42  is inserted into tube  44  so that stent struts  46  (shown in cross-section as in  FIG. 1 ) are in contact with inner surface  48  of tube  44 . When hollow cathode discharge plasma  50  is created within tube  44 , as described above, primarily inner surface  40  of struts  46  will be exposed to plasma  50  while outer surface  52  of struts  46 , which are in contact with inner surface  48  of tube  44 , will not receive as much exposure to plasma  50 . In this way, inner surface  40  of stent  42  can be altered through a coating, a plasma etch treatment or a combination of both, while outer surface  52  of stent  42  is left almost unchanged, i.e., outer surface  52  is substantially shielded from exposure to plasma  50 . 
         [0037]    Various methods exist for using the present invention to treat or coat inner surface  40  of stent  42  or other medical devices having inner and outer surfaces. For example, a precursor gas such as methane or acetylene could be used alone or in combination with other gases such as argon to produce a carbon containing coating on inner surface  40 . The formation of a coating by a plasma discharge in a precursor gas, or plasma enhanced chemical vapor deposition (PECVD) is well know in the art and many precursor gases, such as hexamethyldisiloxane, tetrafluoroethylene, and those containing metals such as titanium isopropoxide can be used. 
         [0038]    Alternatively, the hollow discharge tube, e.g., tube  44  shown in  FIG. 3  could be made of a material that is meant to be deposited on inner surface  40  of strut  46 . For example, if tube  44  were made of titanium, because a significant portion of inner wall  48  of tube  44  is exposed through openings  54   a  and  54   b  in stent  42 , i.e., the areas within and between struts  46 , the bombardment of inner surface  48  of tube  44  by energetic ions, e.g., ions  36  shown in  FIG. 2 , will sputter titanium onto inner surface  40  of strut  46 . Because plasma  50  will also bombard inner surface  40  of strut  46 , not all of the titanium that is deposited will remain, however some will remain and mix with inner surface  40 . Alternatively, by choosing a tube material that has a significantly different sputter yield than the stent material, it has been found that two or more materials may be effectively co-deposited to create an inhomogeneous surface on the inside surface of a stent without the use of lithography. It is believed that such a surface is conducive to endothelial cell growth. See, e.g., U.S. Pat. No. 6,820,676. It should be appreciated that, as used herein, sputter and sputtering is intended to mean removal of material by ion bombardment, and in some embodiments, includes the subsequent deposit of the removed material onto another surface, e.g., ion bombardment of an inner surface of a hollow electrically conducting tube removes material therefrom which is subsequently deposited on a medical device held within the hollow tube. 
         [0039]    If it is desired to simply expose the inner surface of a device such as a stent to the energetic ion bombardment, for example to roughen the device or plasma activate the device for further processing, the hollow cathode discharge system tube can be made of a biocompatible, low sputter yield material, e.g., carbon. Because the device is biased at a negative voltage with respect to the anode, it will be impacted by ions that have been accelerated to high energy. Therefore, the surface of the device can be aggressively plasma etched, a coating can be put down with PECVD, or both can be done simultaneously. 
         [0040]    In addition to a hollow cathode discharge, it is possible to create a plasma on the inside surface of a medical device by other means. For example, an inductively or capacitively coupled radio frequency (RF) field can produce a glow discharge on the inside surface of an electrically insulating tube. The tube must have a low enough conductivity that the RF fields are not shielded from the interior portion. A gas, which can be inert or can contain a precursor for depositing a coating, can flow through the tube. In this case, because the stent or device may itself shield the interior of the tube from the RF fields, the treatment or deposition can take place remotely from where the power is coupled.  FIG. 4   a  shows a cross sectional view of an arrangement for capacitively coupling RF power into a tube to produce a plasma and  FIG. 4   b  shows a cross sectional view of an arrangement for inductively coupling RF power into a tube to produce a plasma. In  FIG. 4   a , plasma discharge device  58  comprises electrically insulating tube  60  and has separate electrodes  62  placed on opposite sides of tube  60  in a manner well known in the art. Radio frequency power supply  64  is connected to electrodes  62 . Gas  66  is admitted into tube  60  and excited by power supply  64 . Gas  66  may include any of the gases discussed supra, e.g., inert, reactive or precursor. The medical device, e.g., stent  67 , is located remotely from the electrodes, as explained above, and is treated or coated in the flow of ionized and excited gas  68  downstream from the plasma generation portion, i.e., the area within tube  60  between electrodes  62 , of plasma discharge device  58 .  FIG. 4   b  shows an alternative form a plasma discharge device, i.e., device  70 , wherein electrodes  62  of device  58  are replaced by coil of wire  72 . Coil  72  inductively couples power from power supply  64  into ionized and excited gas  68  in a manner well-known to those skilled in the art. 
         [0041]    Alternatively, microwave power can be used to produce a discharge. In this instance, the tube that holds the medical device can be inserted into a microwave cavity, also known as a waveguide, in a manner well known to those of ordinary skill in the art.  FIG. 5  shows a cross sectional view of an arrangement of discharge device  73  having tube  74  inserted within microwave cavity  76  so that microwave radiation  78  may reach interior  80  of tube  74 . Gas  82 , which may include any of the gases described supra, can flow through tube  74  and the medical device to be treated or coated, e.g., stent  84 , can be placed in a portion of tube  74  outside of cavity  76 , e.g., portion  86 , where ionized gas  88  can reach interior surfaces  90  of medical device  84 . It should be appreciated that medical device  84  is placed outside of cavity  76  so that its conductivity does not interfere with the propagation of microwaves  78 . As discussed above, gas  82  can be an inert gas intended to modify the surface of medical device  84  through physical bombardment with ions, can be a reactive gas or can contain a precursor gas used to deposit a coating onto interior surface  90  of device  84 . 
         [0042]    It should be appreciated that the present invention method may be used to produce large numbers of devices simultaneously. For example, a number of stents can line the inside of a long tube and be coated or treated at one time. Alternatively, an array of shorter tubes, as shown in the cross sectional view in  FIG. 6 , can be used to simultaneously coat or treat a number of devices. In the embodiment shown in  FIG. 6 , tubes  92 , each of which holds one or more medical devices, e.g., stents  94 , for treatment or coating, are arrayed in holder  96 . Holder  96  includes hollow gas manifold  98  which is connected to tubes  92 . Gas manifold  98  is fed by gas line  100  such that gas  102  flowing in line  100  is distributed substantially evenly to tubes  92 . Assembly  104  is electrically insulated by means such as insulators  106  and is connected electrically to power supply  108 . When power supply  108  applies a sufficient negative voltage to assembly  104 , simultaneous hollow cathode discharges exist in tubes  92 , which treat and/or coat inside surfaces  110  of medical devices  94  therein. 
         [0043]    The inventive method of the present invention can be used in a variety of ways to alter the interior surfaces of medical devices. For example, it is possible to create an inhomogeneous surface by depositing a discontinuous coating of atoms of a first substance on a substrate comprising a second substance. In some embodiments, the substrate can then be etched via physical sputtering, while in other embodiments, the steps of depositing and etching are performed simultaneously. This deposition and etching sequence is described in U.S. Patent Application Nos. 60/771,834 and 11/704,650, which applications have been incorporated herein by reference and form the basis of priority for this application. In further embodiments, the discontinuous coating of atoms forms a plurality of clusters, each of the plurality of clusters having lateral dimensions from about ten nanometers to about one thousand nanometers. In yet further embodiments, the inhomogeneous surface includes a plurality of structures, each of the structures having heights from about ten nanometers to about ten thousand nanometers. The above described embodiments of the present invention are shown in  FIGS. 7 and 8 .  FIG. 7  is a cross sectional view of a substrate having a discontinuous coating of atoms, more specifically, a coating of aluminum oxide (Al 2 O 3 ) clusters  112  randomly spaced about titanium substrate  114  thereby forming coated substrate  116 , while  FIG. 8  is a cross sectional view of coated substrate  116  after etching. The following discussion is perhaps best understood in view of both  FIGS. 7 and 8 . 
         [0044]    Ultra thin coatings deposited using physical vapor deposition, or in other words those layers having average thicknesses from less than a monolayer, i.e., a single atomic layer, to tens of monolayers, do not ordinarily condense as a uniform coating. Rather, the atoms nucleate as clusters whose size and spacing are determined by such factors as substrate temperature, chemical binding energy between the coating and substrate, energy of the arriving atoms, etc. Therefore, the average height of these clusters may be significantly greater than the average thickness of the overall coating, while the regions between the clusters are merely bare substrate material. The instant invention makes use of differences in etch rates that can exist between such clusters and the underlying substrate material, in order to produce structures that have dimensions of tens to hundreds of nanometers in breadth and height in and on the substrate. 
         [0045]    In the embodiment shown in  FIGS. 7 and 8 , Ti substrate  114  is used as a base layer upon which Al 2 O 3  clusters  112  are deposited. Al 2 O 3  clusters  112  are attached to Ti substrate  114  and approximately several nanometers in height and approximately several nanometers in diameter. Under ion bombardment, the sputter yield of Al 2 O 3  clusters  112 , i.e., the number of Al 2 O 3  atoms ejected from coated substrate  116  per incident ion, is approximately a few percent of that of the atoms ejected from Ti substrate  114 . Thus, after depositing clusters  112  on Ti substrate  114 , coated substrate  116  is subjected to ion bombardment to cause sputtering. Initially, coated substrate  116  will be etched only in those areas not covered by Al 2 O 3  clusters  112 . By continuing to etch coated substrate  116  until Al 2 O 3  clusters  112  are removed, the resulting etched substrate  118  will have high aspect ratio structures  120  with spacings that reflect the original spacing of the Al 2 O 3  clusters  112 . Thus,  FIG. 8  shows the results of coating Al 2 O 3  clusters  112  on Ti substrate  114  to form coated substrate  116 , and the subsequent removal of Al 2 O 3  clusters  112  by ion bombardment. It has been found that even if the substrate material, e.g., Ti substrate  114 , has a low sputter yield surface, such as a native oxide, removing that surface will require the same length of time in all locations. Therefore, the difference in sputter rates for the deposited clusters  112  and substrate  114  will still dictate the vertical size of the resulting structures  120 . It should be noted that as used herein lateral dimension or diameter is used to refer to diameters  122 , while vertical size, height and depth are used to refer to height  124 . 
         [0046]    Although coating a substrate with Al 2 O 3  is described in the foregoing embodiment, one of ordinary skill in the art will recognize that a wide variety of coating materials may be used, e.g., metals, oxides, nitrides and alloys, and such variations are within the spirit and scope of the claimed invention. However, it has been found that metal oxides such as Al 2 O 3  as well as oxides of Titanium (Ti), Molybdenum (Mo), Niobium (Nb), Chromium (Cr) and others have very low sputter yields and are, therefore, particularly advantageous when used for coating a substrate. Such materials are good candidates for producing randomly spaced clusters of atoms on a nanometer scale, such as Al 2 O 3  clusters  112 . Hereinafter, such nanometer scale coatings are referred to as a “nanomask.” 
         [0047]    As those skilled in the art will appreciate, the nanomask, e.g., Al 2 O 3  clusters  112  may be deposited using a source of the mask material or may be deposited reactively by, for example, sputtering a metal in a chamber containing oxygen (O 2 ), nitrogen (N 2 ), or some other compound forming gas. Any number of well-known means, such as sputtering, cathodic arc evaporation, thermal evaporation and chemical vapor deposition can deposit discontinuous clusters  112 . As mentioned previously, the deposition conditions strongly affect clusters  112  size and spacing, and conditions are chosen which produce the desired results. 
         [0048]    For the purposes of bone growth, nucleation characteristics resulting in a discontinuous coating of clusters  112  having diameters from about several nanometers to about several hundreds of nanometers, and heights from about several nanometers to about several hundreds of nanometers, have been found to be particularly advantageous. The dimensions of resulting structures  120  of course still depend on the ratio of the etch rate of substrate  114  to the etch rate of clusters  112 . Although the aforementioned embodiment is described in terms of preferentially bonding to bone, one of ordinary skill in the art will recognize that a substrate have clusters of different dimensions than previously set forth will preferentially bond to other types of cells, and such variations are within the spirit and scope of the claimed invention. In a preferred embodiment, resulting structures  120  have lateral dimensions, i.e., diameters  122 , from approximately ten (10) to several hundreds of nanometers across and heights  124  from approximately ten (10) to ten thousand (10,000) nanometers. 
         [0049]    The height H of a given resulting structure  120  will be: 
         [0000]    
       
      
       H=R×h,  
      
     
         [0000]    Where h is the height of the initial cluster  112  that produced structure  120  and R is the ratio of the etch rate of substrate  114  to the etch rate of cluster  112 . Of course, a given cluster  112  will not have a single height, but will be domed or otherwise irregular, and therefore, the resulting structure  120  may also be irregularly shaped. For example, as is well known from published sputter yields for Al 2 O 3  and Ti, an Al 2 O 3  nanomask deposited on a Ti substrate and sputtered using 500 electron volts (eV) under Argon (Ar) will result in a ratio R of approximately 17. Therefore, if a nanomask cluster of atoms had a height h of 10 nanometers, the height H of the resulting structure would be approximately 170 nanometers. 
         [0050]    In order to control the nucleation characteristics of the nanomask coating, it is possible to change the chemical binding energy between substrate  114  and the coating material, e.g., Al 2 O 3 . For example, a very thin layer of a material having weak chemical bonding with the nanomask material, such as a hydrocarbon, may be deposited onto the substrate prior to the deposition of the coating material. Such a low energy coating, as it is known, will result in fewer, larger nuclei of the nanomask material, clusters  112 . Alternatively, it is possible to use plasma cleaning as an integral part of the coating process to change the nucleation characteristics. In that case, an initial high voltage can be applied to substrate  114  in order to clean substrate  114  and remove any residual contamination. This cleaning may be done with the deposition source off or it may be carried out during the initial stages of deposition. Times for such cleaning may range from less than a minute to several minutes. 
         [0051]    For purposes of cell attachment, coated substrate  116  may not require etching in order to form preferred sites for cell growth. In certain cases, it is possible that material boundaries formed between substrate  114  and clusters  112  will produce enough of discontinuity in surface characteristics to stimulate the attachment of cells at the locations of clusters  112  and/or therebetween clusters  112 . It has been found, for example, that material boundaries on such scales may result in relatively large local electric fields, which may enhance the attachment of biological materials at those locations. For example, a discontinuous coating of Gold (Au) on Ti may result in large chemical potentials at the boundaries of the two materials that stimulate biological materials, such as proteins, to locate preferentially at those boundaries. As one of ordinary skill in the art will appreciate, other types of dissimilar materials are also candidates for such nanoscale coating clusters, and such variations are within the scope of the claimed invention. 
         [0052]    Clusters  112  may be deposited on otherwise smooth portions of substrate  114  or it is also possible to form clusters  112  on the surfaces of a sintered powder, thereby creating a surface with two roughness scales. In addition, if clusters  112  are porous they may be infused with bioactive materials, such as superoxide dismutuse to inhibit inflammation or proteins to promote bone growth. 
         [0053]    As described supra, once clusters  112  are deposited on substrate  114 , thereby forming coated substrate  116 , structures  118  can be produced by etching coated substrate  116 . Any etching known in the art may be used, such as reactive or non-reactive ion etching. For example, introducing an inert gas such as Argon at a pressure from approximately one (1) mTorr to one hundred (100) Torr, and applying a voltage to coated substrate  116  that is high enough to cause physical sputtering, typically between one hundred (100) and one thousand (1000) volts (V), will result in the desired etching. The sputtering voltage may be direct current (DC), pulsed DC, radio frequencies (RF) in the megahertz range, or an intermediate frequency, i.e., alternating current (AC), and such voltage should be applied under conditions that produce a glow discharge. The gas used may be inert, such as Ar, or can be chosen to accentuate the difference in sputtering rates between clusters  112  and substrate  114 . For example, if clusters  112  are a metal oxide and substrate  114  is a polymer, it is known in the art that a plasma containing O 2  will etch the polymer very quickly while etching the metal oxide slowly. Such a process is known as reactive ion etching and relies on chemical processes as well as physical bombardment to remove material. 
         [0054]    The above described etching processes are common in the electronics industry, where etch masks are routinely used to produce specific desired patterns in integrated circuits, for example. However, in those cases the patterns that define the final structure are made using lithography, which is an expensive process. In the method of the instant invention, the patterns are formed on the surfaces of implantable devices by choosing deposition conditions that form a random pattern of clusters of atoms, and therefore is far more cost effective and simple to perform than lithography processes. 
         [0055]    The deposition of clusters  112  and subsequent etching of coated substrate  116  may be done in one continuous operation, or may be performed sequentially. An example of a continuous operation is depositing Al 2 O 3  clusters  112  onto Ti substrate  114  using RF sputtering. During deposition of clusters  112 , a voltage may also be applied to substrate  114 . The voltage should be kept low enough that it will not cause clusters  112  to be removed faster than they are deposited. However, once clusters  112  are properly deposited on substrate  114 , the voltage may be increased to cause sputtering of both clusters  112  and substrate  114  in such a way that there is a net removal of material, and the formation of nanostructures  120  as described above. It has been found that using RF sputtering to deposit clusters  112  is a relatively inefficient deposition process. That is, a relatively intense RF plasma is needed to produce even a small deposition rate of a nanomask material such as Al 2 O 3 . However, because the nanomask material is so thin on average, a low deposition rate is often acceptable. The advantage of using RF sputtering arises once the nanomask is deposited. By leaving the RF power on and applying a DC voltage to coated substrate  116 , the intense RF plasma provides a dense source of ions which are available to etch coated substrate  116 . In other words, applying a DC voltage to coated substrate  116  in the presence of RF plasma will produce a far greater etch rate than applying the same voltage in the absence of RF plasma. Even though there are still sputtered atoms arriving at coated substrate  116 , they are removed as quickly as they arrived by the combined effect of the dense plasma and high substrate voltage. 
         [0056]    Alternatively, the deposition and etching steps may be sequential. If both steps are accomplished using sputtering, this may be accomplished by simply turning off the power to the deposition source of clusters  112  and turning on the power to substrate  114 . Or alternatively, the deposition and etching steps may take place in separate chambers. 
         [0057]    It should be appreciated the above described sputtering of the hollow tube and medical device contained therein may occur simultaneously, and an example of such is shown in  FIG. 9 .  FIG. 9  shows a cross sectional view of medical device  122  manufactured according to an embodiment of present invention. Simultaneously sputtering both the hollow tube and medical device  122  modifies interior surface  124  of medical device  122  to comprise inhomogeneous surface  126 , wherein inhomogeneous surface  126  comprises at least two materials, e.g., first and second materials  128  and  130 , respectively. Inhomogeneous surface  126  includes a plurality of individual regions  132 , and each of these regions  132  comprises at least two materials, e.g., first and second materials  128  and  130 , respectively. Individual regions  132  are separated from other individual regions by material boundary  134 . 
         [0058]    Furthermore, the present invention method allows for the creation of different surfaces on the inside and outside of medical devices, e.g., stents, which serve different purposes. For example, it may be possible to first deposit a material only on the outside of the medical device that enhances the biocompatibility of that surface with respect to a lumen wall. This could be done using conventional deposition techniques such as sputtering, evaporation, spray coating, plasma polymerization or others while using a mandrel to prevent coating on the interior surface of the device. In a separate operation, the present invention method could be used to create another surface on the inside of the medical device that serves an alternative purpose, for example, biocompatibility with blood rather than tissue or promotion of endothelial cell growth via a rough surface or inhomogeneous surface. 
         [0059]    In some instances, it may be useful to use a drug that prevents cell growth for a period of time in combination with a medical device whose inner surface has been altered so that it promotes endothelial cell growth. In these instances, the textured inner surface may cause platelet attachment, which is undesirable, during the period of time when the drug is preventing cell growth. It has been found that this issue can be addressed by coating at least the inner surface of the medical device with a biodegradable polymer. The smooth surface of the polymer suppresses platelet attachment while the drug acts to prevent cell growth. When the polymer is gone, i.e., has degraded, and the drug no longer acts to prevent cell growth, the surface of the medical device that promotes endothelial cell growth is then exposed and becomes effective. 
         [0060]    A further advantage of the present invention relates to controlling the temperature of medical devices during their coating or treatment. For example, if the inside diameter of the hollow cathode or discharge tube is slightly smaller than the outside diameter of the device, the device will remain in intimate contact with the tube during processing. Therefore, if the tube is cooled, for example by a circulating liquid, the medical device can also be cooled during processing. This is particularly important for medical devices made of a nickel/titanium alloy known as Nitinol. Nitinol has the unusual properties of superelasticity and shape memory which result from the fact that Nitinol exists in a martensitic phase below a first transition temperature, known as M f , and an austenitic phase above a second transition temperature, known as A f . Both M f  and A f  can be manipulated by altering the ratio of nickel to titanium in the alloy as well as changing the thermal processing of the material. In the martensitic phase, Nitinol is very ductile and easily deformed, while in the austenitic phase Nitinol has a high elastic modulus. Applying stresses to materials at temperatures above A f  produces some martensitic materials, however when the stresses are removed, the material returns to its original shape. This results in a very springy behavior for Nitinol, referred to as superelasticity or pseudoelasticity. Furthermore, if the temperature is lowered below M f  and the Nitinol is deformed, raising the temperature above A f  will cause the Nitinol to recover its original shape. This property is described as shape memory. 
         [0061]    It is well known that if Nitinol is raised to too high a temperature for too long of a period of time, the A f  value will rise. Additionally, sustained temperatures above 300-400 degrees Centigrade will adversely affect typical A f  values used in medical devices. Likewise, if stainless steel is raised to too high a temperature, it can lose its temper, while other materials would also be adversely affected by exposure to such conditions. Therefore, the time-temperature history of a medical device during a coating operation is critical. In view of the foregoing, the present invention allows the temperature of a device to be controlled directly while uniformly treating or coating its interior surface. 
         [0062]    It should also be appreciated that the present invention method can also be used to selectively remove material from the interior surfaces of medical devices. For example, many polymer deposition processes used to coat devices are conformal, i.e., a process of spraying a dielectric material onto a device to protect it from moisture, fungus, dust, corrosion, abrasion, and other environmental stresses. Parylene, which is widely used as a coating material, is deposited by polymerizing a monomer vapor, and thereby coating parylene on all exposed surfaces. As has been discussed above, it may be desirable to remove such a polymer coating from the interior surface while leaving it on the exterior surface. Thus, the present method can be used to plasma etch a polymer using an oxygen containing plasma, thereby removing it from the interior surface while leaving it on the exterior surface as desired. 
         [0063]    Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, which modifications are intended to be within the spirit and scope of the invention as claimed. It also is understood that the foregoing description is illustrative of the present invention and should not be considered as limiting. Therefore, other embodiments of the present invention are possible without departing from the spirit and scope of the present invention.