Patent Publication Number: US-2013230639-A1

Title: Rotatable support elements for stents

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application a continuation of application Ser. No. 13/184,696, filed on Jul. 18, 2011, which is a continuation of application Ser. No. 11/418,722, filed May 4, 2006, which issued as U.S. Pat. No. 8,003,156, the entirety of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods and devices for coating stents. 
     2. Description of the State of the Art 
     This invention relates to radially expandable endoprostheses, that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success. 
     Stents are typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts, formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffolding gets its name because it physically holds open and, if desired, expands the wall of the passageway. Typically, stents are capable of being compressed or crimped onto a catheter so that they can be delivered to and deployed at a treatment site. Delivery includes inserting the stent through small lumens using a catheter and transporting it to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention with stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis remains a significant problem. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited than for those lesions that were treated solely with a balloon. 
     Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy uses medicated stents to locally administer a therapeutic substance. Effective concentrations at the treated site require systemic drug administration which often produces adverse or even toxic side effects. Local delivery is a preferred treatment method because it administers smaller total medication levels than systemic methods, but concentrates the drug at a specific site. Local delivery thus produces fewer side effects and achieves better results. 
     A medicated stent may be fabricated by coating the surface of a stent with an active agent or an active agent and a polymeric carrier. Those of ordinary skill in the art fabricate coatings by applying a polymer, or a blend of polymers, to the stent using well-known techniques. Such a coating composition may include a polymer solution and an active agent dispersed in the solution. The composition may be applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent then evaporates, leaving on the stent surfaces a polymer coating impregnated with the drug or active agent. 
     Accurately loading drugs, minimizing coating defects, and coating with pure coating materials favor improved coating quality. In addition, adequate throughput of the overall manufacturing process is also of concern. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is directed to embodiments of a method of drying a stent that may include disposing a stent in a chamber. The stent may have a coating material including a polymer and a solvent applied to a surface of the stent. The method may further include directing a fluid stream at the stent to remove at least a portion of the coating material, measuring a temperature of the fluid at a location adjacent to the stent, and controlling a temperature of the fluid stream based on the measured temperature so as to maintain a desired temperature adjacent to the stent. 
     A stent drying apparatus, which may include a chamber configured to receive a stent having coating material including a polymer and a solvent, is also disclosed. The chamber may be adapted to receive a fluid stream that contacts the coated stent. In some embodiments, the fluid stream removes at least a portion of the coating material. A temperature sensor may be positioned adjacent to the stent for measuring the temperature of the fluid stream contacting the stent. The apparatus may further include a controller connected to the sensor, wherein the controller adjusts a temperature of the fluid stream based on the measured temperature so as to maintain a desired temperature of the fluid stream contacting the stent. 
     Other aspects of the present invention are directed to embodiments of a fixture for supporting a stent during weighing that may include a pan that can support a stent greater than about 28 mm in length along the stent&#39;s entire length. Pan geometry may be optimized to minimize fixture mass. 
     A method of coating a stent comprising the step of modifying the flow rate of a coating material sprayed onto the stent while axially moving the stent is also disclosed. In these or other embodiments, the stent repeatedly passes from one end of the stent to another relative to a major axis of the stent adjacent to a fixed or movable spray nozzle. Some of these methods further include adjusting the axial speed during spraying of the stent based on the modified flow rate of the coating material to deposit a selected amount of coating material per pass. 
     Additional embodiments of invention coating methods include spraying a coating material from a nozzle onto a stent substantially concurrently with axially moving the stent. This axial movement causes the stent to pass repeatedly from one end of the stent to another. The method may further include controlling the rate of the movement to deposit a selected amount of coating material per pass. 
     In these or other embodiments, the method may include increasing the flow rate of material sprayed onto the stent as the stent is moved axially. The method may further include adjusting the stent&#39;s speed based on the increased flow rate to deposit a selected amount of coating material on the stent. 
     A method of coating a stent that includes increasing the flow rate of material sprayed onto a stent in which the stent is axially translated relative to the sprayed coating material during spraying of the stent. The motion causes the stent to pass repeatedly from one end of the stent to another. The method may further include adjusting the axial translation speed of the stent based on the increased flow rate of the coating material to deposit a selected amount of material per pass. 
     A stent coating device that includes a first support element for rotating a first end of a stent and a second support element for rotating a second end of the stent, wherein the first and second rates of rotation are the same as or different from each other is disclosed. 
     A device for coating a stent that includes a support element for rotating a stent, wherein the support element is capable of providing at least one pulse in a rotation rate of the support element during stent coating is also disclosed. 
     Other device embodiments are disclosed. For instance, a device that may include a stent support element is disclosed. The element may be capable of providing at least one pulse in the rate of axial motion of the stent during coating of the stent, wherein the pulse allows the support element to move axially relative to the stent. 
     Additional devices for coating stents may include a support element for rotating the stent. In these or other embodiments, the element includes at least three elongate arms converging inwardly from a proximal end to a distal end to form a conical or frusto-conical shape. The support element may be capable of being positioned within an end of a stent during coating. 
     Further aspects of the present invention are directed to embodiments of a method of coating a stent that may include rotating a stent with a first rotatable element supporting a proximal end of the stent and a second rotatable element supporting a second end of the stent. The first rotatable element may rotate at a different rate than the second rotatable element at least a portion of a time during coating of the stent. 
     Another aspect of the present invention is directed to embodiments of a method of coating a stent that may include rotating a stent with a rotatable element supporting at least a portion of a stent. The method may further include providing at least one pulse in a rotation rate of the rotatable element during coating of the stent. 
     Other aspects of the present invention are directed to embodiments of a method of coating a stent that may include rotating a stent with a first rotatable element supporting a proximal end of the stent and rotating the stent with a second rotatable element supporting a distal end of the stent, the first rotatable element having the same rotation rate as the second rotatable element. The method may further include providing at least one pulse in a rotation rate of the first rotatable element during coating of the stent, the first rotatable element having a rotation rate different from the second rotatable element during the pulse, the pulse causing the first rotatable element to rotate relative to the stent. 
     Some aspects of the present invention are directed to embodiments of a method of coating a stent that may include positioning a support element within an end of a stent to support the stent. The method may further include providing at least one translational pulse to the support element along an axis of the stent during coating of the stent, whereby the translational pulse causes the support element to move axially relative to the stent. 
     Other aspects of the present invention are directed to embodiments of a method of coating a stent that may include rotating a stent with a support element having at least three elongate arms converging inwardly from a proximal end to a distal end of each arm to form a conical or frusto-conical shape, the support element positioned so that the elongate arms converge inwardly within an end of the stent to support the stent. 
     Certain aspects of the present invention are directed to embodiments of a method of coating a stent that may include contacting a first axial portion of a stent with a support element such that a second axial portion does not contact the support element or any other support element. The method may further include applying a coating material to the second axial portion and inhibiting or preventing application of the coating material on the first axial portion. 
     Certain aspects of the present invention are directed to embodiments of an apparatus for supporting a stent that may include a first support rod for supporting a proximal end of a stent, the first support rod coupled to a first collet opposite the proximal end of the stent. The apparatus may further include a second support rod for supporting a distal end of the stent, the second support rod coupled to a second collet opposite the distal end of the stent. The apparatus may also include a third support rod for supporting the first collet and the second collet, the first collet being coupled to a proximal end of the third support rod, the second collet being coupled to a distal end of the third support rod, wherein the third support rod extends between the first collet and the second collet outside of and free of any contact with the stent. 
     Certain aspects of the present invention are directed to embodiments of a device for supporting a stent that may include a filament having a spiral coiled portion, the coiled portion designed to support the stent at a plurality of contact points between the stent and the spiral coiled portion along a least a portion of an axis of the stent. 
     Certain aspects of the present invention are directed to embodiments of a collet for supporting a stent during coating that may include a generally tubular member having an end surface for contacting an end of a stent. The end surface may include a projecting portion and a flat or relatively flat portion. The projecting portion may include at least three segments radiating from a center of the end surface to an edge of the end surface, a thickness of the segments in a plane of the surface decreasing from the center to the edge of the end surface, and a height of the projecting portion perpendicular to the plane of the surface decreasing from the center of the end surface to the edge. 
     Certain aspects of the present invention are directed to embodiments of a stent support for supporting a stent during coating that may include a generally tubular support member supporting a stent, the support element having at least one magnetic element. The stent support may further include an electromagnetic device positioned adjacent to the mandrel for generating an electrical field that allows the magnetic element to support, rotate, and/or translate the mandrel. 
     Some aspects of the present invention are directed to embodiments of a method of manufacturing an implantable medical device that includes purifying a polymer by: contacting the polymer with a fluid capable of swelling the polymer and removing all or substantially all of the fluid from the polymer such that an impurity from the polymer is completely or at least partially removed by the fluid. The fluid may be selected from the group consisting of isopropyl acetate and propyl acetate. The method may further include coating an implantable medical device with the purified polymer, or fabricating the implantable medical device from purified polymer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a three-dimensional view of a cylindrically-shaped stent. 
         FIG. 2A  depicts a schematic embodiment of a drying apparatus. 
         FIG. 2B  depicts a three-dimensional view of an embodiment of a chamber of the apparatus of  FIG. 2A . 
         FIG. 2C  depicts a radial cross-section of the chamber of  FIG. 2B . 
         FIGS. 3A-C  depict a weighing pan that can accommodate stents under 28 mm in length. 
         FIG. 4A  depicts an embodiment of a weighing pan for a stent. 
         FIG. 4B  depicts an overhead view of the pan of  FIG. 4A . 
         FIG. 4C  depicts an embodiment of a weighing pan for a stent. 
         FIGS. 5A-C  depict embodiments of weighing pans with asymmetric shapes for stents. 
         FIG. 6A  depicts an embodiment of a cross-shaped weighing pan for a stent. 
         FIG. 6B  depicts an overhead view of the pan of  FIG. 6A . 
         FIG. 6C  depicts an embodiment of a cross-shaped weighing pan for a stent. 
         FIG. 7  depicts an embodiment of a weighing pan with a ridge around its edge. 
         FIG. 8  depicts an exemplary schematic embodiment of a spray coating apparatus for coating a stent. 
         FIG. 9  depicts a mounting assembly for supporting a stent. 
         FIGS. 10A-D  depict the rotation rate as a function of time for representative pulses in rotation rate. 
         FIGS. 11A-B  depict mounting assemblies for supporting a stent. 
         FIG. 12  depicts a representative linear pulse showing linear translation rate versus time. 
         FIGS. 13A-H  depict the axial position of a support element as a function of time. 
         FIG. 14A  depicts an embodiment of a stent support system. 
         FIG. 14B  depicts a close-up view of one side of the stent support system of  FIG. 14A . 
         FIG. 15A  depicts a side view of an embodiment of a stent support system for coating a portion of a stent that has no contact points. 
         FIG. 15B  depicts an axial cross-section of the stent support system of  FIG. 15A . 
         FIG. 16A  depicts an alternative view of the stent support system of  FIG. 15A . 
         FIG. 16B  depicts an axial cross-section of one side of the stent support system of  FIG. 16A . 
         FIG. 17A  depicts an alternative view of the stent support system of  FIG. 15A . 
         FIG. 17B  depicts an axial cross-section of one side of the stent support system of  FIG. 17A . 
         FIG. 18  depicts an embodiment for coating a central portion of a stent that has no contact points. 
         FIG. 19A  depicts an exemplary embodiment of a stent support system. 
         FIG. 19B  depicts a close-up view of one side of the stent support system of  FIG. 19A . 
         FIG. 20  depicts a side view of an exemplary embodiment of a stent support system. 
         FIG. 21  depicts the stent support system of  FIG. 19A  coupled to a rotatable spindle. 
         FIG. 22A  depicts a two-dimensional view of a spiral coil support. 
         FIG. 22B  depicts an illustration of a prototype of a spiral coil support. 
         FIG. 23A  depicts a holder for manipulating and positioning a spiral coil support. 
         FIG. 23B  depicts a radial cross-section of the front of the holder of  FIG. 23A . 
         FIG. 24A  depicts a picture of a spiral coil support supported by a holder. 
         FIG. 24B  depicts a picture illustrating a close-up view of a spiral coil support positioned within a stent. 
         FIG. 25  depicts an axial view of a system for coating a stent supported by a spiral coil support. 
         FIGS. 26A-D  depict scanning electron micrograph (SEM) images of 28 mm stents coated using a spiral coil support to support the stent during coating. 
         FIG. 27  depicts a stent mounted on a stent support system. 
         FIGS. 28A-D  depict an exemplary embodiment of a collet. 
         FIGS. 29A-B  depict photographs with side views of an embodiment of a collet according to the present invention. 
         FIGS. 29C-D  depict photographs with overhead views of an embodiment of a collet according to the present invention. 
         FIG. 29E  depicts a photograph with a view of an embodiment of a collet according to the present invention with a stent mounted on the collet. 
         FIG. 30  is an exemplary embodiment of a stent support system with a mandrel capable of magnetic levitation. 
         FIG. 31  depicts an overhead view of a coating system illustrating stations corresponding to various processing steps. 
         FIG. 32  shows the swell percentage of a polymer sample in various solvents obtained by measuring the size of the sample at selected times. 
         FIG. 33  shows the swell percentage of a polymer sample in various solvents obtained by measuring the weight of the sample at selected times. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention relate to coating implantable medical devices such as stents. More generally, embodiments of the present invention may also be used in coating devices including, but not limited to, self-expandable stents, balloon-expandable stents, stent-grafts, vascular grafts, cerebrospinal fluid shunts, pacemaker leads, closure devices for patent foramen ovale, and synthetic heart valves. 
     In particular, a stent can have virtually any structural pattern that is compatible with a bodily lumen in which it is implanted. Typically, a stent is composed of a pattern or network of circumferential and longitudinally extending interconnecting structural elements or struts. In general, the struts are arranged in patterns, which are designed to contact the lumen walls of a vessel and to maintain vascular patency. A myriad of strut patterns are known in the art for achieving particular design goals. A few of the more important design characteristics of stents are radial or hoop strength, expansion ratio or coverage area, and longitudinal flexibility. Embodiments of the present invention are applicable to virtually any stent design and are, therefore, not limited to any particular stent design or pattern. One embodiment of a stent pattern may include cylindrical rings composed of struts. The cylindrical rings may be connected by connecting struts. 
     In some embodiments, a stent may be formed from a tube by laser cutting the pattern of struts in the tube. The stent may also be formed by laser cutting a metallic or polymeric sheet, rolling the pattern into the shape of the cylindrical stent, and providing a longitudinal weld to form the stent. Other methods of forming stents are well known and include chemically etching a metallic or polymeric sheet and rolling and then welding it to form the stent. 
     In other embodiments, a metallic or polymeric filament or wire may also be coiled to form the stent. Filaments of polymer may be extruded or melt spun. These filaments can then be cut, formed into ring elements, welded closed, corrugated to form crowns, and then the crowns welded together by heat or solvent to form the stent. 
       FIG. 1  illustrates a conventional stent  10  formed from a plurality of struts  12 . The plurality of struts  12  are radially expandable and interconnected by connecting elements  14  that are disposed between adjacent struts  12 , leaving lateral openings or gaps  16  between adjacent struts  12 . Struts  12  and connecting elements  14  define a tubular stent body having an outer, tissue-contacting surface and an inner surface. 
     The cross-section of the struts in stent  10  may be rectangular- or circular-shaped. The cross-section of struts is not limited to these, and therefore, other cross-sectional shapes are applicable with embodiments of the present invention. Furthermore, the pattern should not be limited to what has been illustrated as other stent patterns are easily applicable with embodiments of the present invention. 
     Coating a Stent 
     As indicated above, a medicated coating on a stent may be fabricated by spraying a coating composition including polymer and drug on the stent. Spray coating a stent typically involves mounting or disposing a stent on a support, followed by spraying a coating material from a nozzle onto the mounted stent. 
     A spray apparatus, such as EFD 780S spray device with VALVEMATE  7040  control system (manufactured by EFD Inc., East Providence, R.I., can be used to apply a composition to a stent. A EFD 780S spray device is an air-assisted external mixing atomizer. The composition is atomized into small droplets by air and uniformly applied to the stent surfaces. Other types of spray applicators, including air-assisted internal mixing atomizers and ultrasonic applicators, can also be used for the application of the composition. 
     To facilitate uniform and complete coverage of the stent during the application of the composition, the stent can be rotated about the stent&#39;s central longitudinal axis. Rotation of the stent can be from about 0.1 rpm to about 300 rpm, more narrowly from about 30 rpm to about 200 rpm. By way of example, the stent can rotate at about 150 rpm. The stent can also be moved in a linear direction along the same axis. The stent can be moved at about 1 mm/second to about 12 mm/second, for example about 6 mm/second, or for a minimum of at least two passes (i.e., back and forth past the spray nozzle). 
     A nozzle can deposit coating material onto a stent in the form of fine droplets. An atomization pressure of a sprayer can be maintained at a range of about 5 psi to about 30 psi. The droplet size depends on factors such as viscosity of the solution, surface tension of the solvent, and atomization pressure. The flow rate of the composition from the spray nozzle can be from about 0.01 mg/second to about 1.0 mg/second, for example about 0.1 mg/second. Only a small percentage of the composition that is delivered from the spray nozzle is ultimately deposited on the stent. By way of example, when a composition is sprayed to deliver about 1 mg of solids, only about 100 micrograms or about 10% of the solids sprayed will likely be deposited on the stent. 
     Depositing a coating of a desired thickness in a single coating stage can result in an undesirably nonuniform surface structure and/or coating defects. Therefore, the coating process can involve multiple repetitions of spraying forming a plurality of layers. Each repetition can be, for example, about 0.5 second to about 20 seconds, for example about 10 seconds in duration. The amount of coating applied by each repetition can be about 1 microgram/cm 2  (of stent surface) to about 50 micrograms/cm 2 , for example less than about 20 micrograms/cm 2  per 1-second spray. 
     As indicated above, the coating composition can include a polymer dissolved in a solvent. Each repetition can be followed by removal of a significant amount of the solvent(s). In an embodiment, there may be less than 5%, 3%, or more narrowly, less than 1% of solvent remaining in the coating after drying between repetitions. When the coating process is completed, all or substantially all of the solvent may be removed from the coating material on the stent. Any suitable number of repetitions of applying the composition followed by removing the solvent(s) can be performed to form a coating of a desired thickness or weight. Excessive application of the polymer can, however, cause coating defects. 
     A stent coated with coating material can be dried by allowing the solvent to evaporate at room or ambient temperature. Depending on the volatility of the particular solvent employed, the solvent can evaporate essentially upon contact with the stent. Alternatively, the solvent can be removed by subjecting the coated stent to various drying processes. Drying time can be decreased to increase manufacturing throughput by heating the coated stent. For example, removal of the solvent can be induced by baking the stent in an oven at a mild temperature (e.g., 60° C.) for a suitable duration of time (e.g., 2-4 hours) or by the application of warm air. A stent is typically dried in an oven as the final drying step when the deposition stages are completed. 
     Evaporation of the solvent(s) can be induced by application of a warm gas between each repetition which can prevent coating defects and minimize interaction between the active agent and the solvent. The stent may be positioned below a nozzle blowing the warm gas. A warm gas may be particularly suitable for embodiments in which the solvent employed in the coating composition is a non-volatile solvent (e.g., dimethylsulfoxide (DMSO), dimethylformamide (DMF), and dimethylacetamide (DMAC)). The temperature of the warm gas can be from about 25° C. to about 200° C., more narrowly from about 40° C. to about 90° C. By way of example, warm gas applications can be performed at a temperature of about 60° C., at a flow speed of about 5,000 feet/minute, and for about 10 seconds. 
     The gas can be directed onto the stent following a waiting period of about 0.1 second to about 5 seconds after the application of the coating composition so as to allow the liquid sufficient time to flow and spread over the stent surface before the solvent(s) is removed to form a coating. The waiting period is particularly suitable if the coating composition contains a volatile solvent since such solvents are typically removed quickly. As used herein “volatile solvent” means a solvent that has a vapor pressure greater than 17.54 Torr at ambient temperature, and “non-volatile solvent” means a solvent that has a vapor pressure less than or equal to 17.54 Ton at ambient temperature. 
     Any suitable gas can be employed, examples of which include air, argon, or nitrogen. The flow rate of the warm gas can be from about 20 cubic feet/minute (CFM) (0.57 cubic meters/minute (CMM)) to about 80 CFM (2.27 CMM), more narrowly about 30 CFM (0.85 CMM) to about 40 CFM (1.13 CMM). The warm gas can be applied for about 3 seconds to about 60 seconds, more narrowly for about 10 seconds to about 20 seconds. By way of example, warm air applications can be performed at a temperature of about 50° C., at a flow rate of about 40 CFM, and for about 10 seconds. 
     Drying 
     Regardless of the method used for drying a stent, it is important for the drying process to be performed in a consistent manner for each layer and each stent. The same or similar processing conditions or parameters should exist for each layer of coating material applied for each stent. The reason for this is that drying process parameters can influence the molecular structure and morphology of a dried polymer and drug coating. Drug release parameters depend upon on molecular structure and morphology of a coating. Therefore, drug release parameters depend upon parameters of the drying process. For example, generally, the rate of a drying process is directly proportional to the resultant drug release rate of a resultant coating. 
     Since the temperature of a drying process is directly related to the rate of drying, it is important to control the drying temperature to obtain coating consistency. In general, the more consistent the temperature during the drying process from layer to layer and stent to stent, the more consistent the resultant coating in a given stent and from stent to stent. 
     Various embodiments of a method and device for drying a stent in a consistent manner are described herein.  FIG. 2A  depicts a schematic embodiment of an apparatus  100  for drying a coating material applied to an implantable medical device, such as a stent. Apparatus  100  includes a chamber  102  configured to receive a stent having a coating material applied by a coating apparatus.  FIG. 2B  depicts a three-dimensional view of an embodiment of chamber  102 . As shown, chamber  102  is cylindrically-shaped. However, chamber  102  is not limited to the shape depicted. In other embodiments, chamber  102  may be a conduit with a cross-section including, but not limited to, square, rectangular, oval etc. A coated stent for drying can be inserted into a side opening  104  in chamber  102 . Side opening  104  can be positioned, for example, at any location on the surface of chamber  102  adjacent to a desired drying position within chamber  102 . 
     Additionally, chamber  102  has a proximal opening  106  for receiving a stream of heated fluid or gas, as shown by an arrow  110 , and a distal opening  108  through which the fluid exits, as shown by an arrow  112 . The fluid may be an inert gas such as air, nitrogen, oxygen, argon, etc. 
     As shown in  FIG. 2A , apparatus  100  further includes a heated fluid source  114  that can include a heating element  116  for heating the fluid used for drying a coated stent. For example, fluid source  116  can be a blower with a heating coil. The temperature of the heated fluid may be adjusted by controlling the heat supplied by heating element  116 . 
       FIG. 2C  depicts a radial cross-section of chamber  104  along a line A-A in  FIG. 2A . A coated stent  118  is shown disposed over a support  120  within chamber  102 . Support  120  with stent  118  is disposed through opening  104  and is positioned so that the heated fluid contacts stent  118  to dry the coating. Stent  118  is secured by collets  122  on support  120 . Temperature sensors  124  and  126  are positioned adjacent to stent  118  to measure the drying temperature of the applied coating. Temperature sensors  124  and  126  are positioned as close as possible to stent  118  without significantly disrupting the flow of heated fluid past stent  118 . In one embodiment, there is no or substantially no offset or difference in temperature between the drying temperature of the coating on the stent and the temperature measured by sensors  124  and  126 . In other embodiments, sensors  124  and  126  are positioned far enough away so that there is an offset in the measured temperature and the drying temperature. Such an offset can be taken into account in the control system described below. Temperature sensors  124  and  126  can be thermistors, thermocouples, or any other temperature measuring devices. 
     Temperature sensor  124  measures the drying temperature to gather feedback (T F ) for controlling the drying temperature of stent  118 . Sensor  124  is coupled to a control system  132  by a sensor wire  128 . Any suitable control system, such as a closed loop system, can be used for maintaining the drying temperature of the coating at a desired temperature (T D ). A temperature, T F , measured by sensor  124  is transmitted via wire  128  to control system  132 . Control system  132  compares T F  to T D  and then transmits a signal  134  to fluid source  114 . Signal  134  carries instructions to fluid source  114  to adjust the temperature of the fluid stream supplied to chamber  102  if the difference in temperature is larger than a selected tolerance. In some embodiments, the desired temperature T D  can be a function of time or thickness. 
     Temperature sensor  126  monitors a drying temperature of the stent that is independent of a control system. A monitored temperature, T M , can be transmitted by a temperature sensor wire  130  to a display or an automated system (not shown). The control system, in some instances, may be unable to maintain a desired drying temperature. For example, the drying temperature may deviate substantially from a desired temperature. A user can be alerted to the deviation by a displayed T M  and take appropriate action, such as shutting down the drying process and discarding the coated stent. Alternatively, an automated system can be configured to take appropriate action such as shutting down the drying process. 
     Table 1 presents data for stents dried at two different temperatures using an embodiment of the drying apparatus discussed herein. Ten different sample runs were performed at each temperature. The stents were Xience-V stents obtained from Guidant Corporation, Santa Clara, Calif. The coating runs included application of a primer layer and a drug-matrix or reservoir layer over the primer layer. The coating material for the primer layer was poly(n-butyl methacrylate) (PBMA) in an acetone/cyclohexanone solvent. The coating material for the reservoir layer was poly(vinylidene fluoride-co-hexafluoropropene) copolymer (PVDF-HFP) (e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare, N.J.) in an acetone/cyclohexanone solvent. 
     Ten samples were dried at each temperature. Each sample was dried over a 24 hour period. Table 1 provides the percentage of solvent released during the drying period for each sample at each temperature. 
     As expected, a larger percentage of solvent was released at the higher temperature. At 35° C., approximately 72% to 77% of the solvent was released from the stent samples and at 65° C. all or substantially all of the solvent was released. At 65° C., for samples 1, 4, 5, 7, and 9, the measured percentage released was greater than 100%. This is likely due to errors in weighing the stent samples before and after drying. At 35° C., the standard deviation was less than 4° C. and the percent coefficient of variation (% CV) was less than 5° C. At 65° C., the standard deviation and % CV were both less than 2° C. The relatively low values of standard deviation and % CV suggest that the drying method tends to dry stents relatively consistently. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Percentage of solvent released from coated stent dried at two 
               
               
                 temperatures over a 24 hour period. 
               
            
           
           
               
               
               
            
               
                 Sample 
                 Stents Dried at 35° C. 
                 Stents Dried at 65° C. 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 72.23 
                 101.66 
               
               
                 2 
                 74.56 
                 97.76 
               
               
                 3 
                 74.31 
                 97.52 
               
               
                 4 
                 72.54 
                 101.40 
               
               
                 5 
                 72.29 
                 102.40 
               
               
                 6 
                 73.57 
                 98.38 
               
               
                 7 
                 76.60 
                 101.44 
               
               
                 8 
                 63.85 
                 97.28 
               
               
                 9 
                 75.59 
                 100.53 
               
               
                 10  
                 74.02 
                 98.47 
               
               
                 Average 
                 73.46 
                 99.68 
               
               
                 Standard Deviation 
                 3.62 
                 1.98 
               
               
                 % CV 
                 4.93 
                 1.99 
               
               
                   
               
            
           
         
       
     
     Weighing of Stents 
     The amount of coating material applied to a stent is typically determined by comparing the weight of an uncoated and coated stent. The weight of a coated stent can be taken to be the dry coating weight. Stents are typically weighed using a microbalance, for example, the UMX5 Microbalance from Mettler-Toledo, Inc. of Columbus, Ohio. A sample to be weighed, such as a stent, is disposed on a weighing pan of coupled to a balance for weighing. The maximum capacity of the UMX5 Microbalance including a weighing pan (including its support) is 2.1 g. The weight of a coated stent is approximately 0.4 g. Therefore, the maximum weight of a weighing pan and its support is about 1.7 g for the above-mentioned balance. 
     Weighing pans that are currently available for microbalances have a maximum surface diameter of under about 28 mm. Therefore, such pans cannot accommodate stents longer than about 28 mm without portions of the stent hanging over an edge of the pan. Hanging portions during weighing are undesirable due to the risk of the stent rolling off the pan. Such pans also have a flat surface with no raised features that facilitate loading of the stent or reduce or prevent rolling off of the stent from the pan. 
       FIGS. 3A-C  depict a prior art weighing pan  160  that can accommodate stents under 28 mm in length. Weighing pan  160  is circular with a diameter, D P , of 27.57 mm and has a top surface  162  for supporting a stent. Weighing pan  160  is coupled to a support rod  164  on a bottom surface (not shown). Support rod  164  is configured to be coupled with a balance. Surface  162  of weighing pan  160  is substantially flat and has no features such as indentations or ridges. The weight of weighing pan  160 , Mettler Toledo, part number ME-211185, and support rod  164  is approximately 1.2 g. 
     Stents over 28 mm in length are not accommodated by current weighing pans. However, increasing the diameter of the current weighing pans to accommodate such stents may result in a weighing pan that exceeds the weighing capacity of a typical microbalance. What is needed is a weighing pan that can accommodate stents over 28 mm in length without exceeding the capacity of a typical microbalance. 
     Embodiments of the present invention relate to a fixture that includes a weighing pan for weighing stents that are greater than about 28 mm in length. In one embodiment, a fixture for supporting a stent during weighing may include a plate or pan. At least a portion of an area of a surface of the pan can support a stent greater than about 28 mm in length along an entire length of the stent without end portions of the stent hanging over an edge of the pan. For example, a portion of an area of the surface can support a 28 mm, 33 mm, or 38 mm stent along its entire length. In certain embodiments, a geometry of the pan is optimized to minimize a mass of the pan, and thus the fixture. In particular, the mass of the fixture may be optimized so that the weight of the fixture and a stent does not exceed the capacity of a selected microbalance. 
     In one embodiment, the fixture may include a pan having a plurality of holes in a surface of the pan to reduce the mass of the fixture. The pan may have a shape that includes, but is not limited to, circular, oval, rectangular, etc. The holes tend to reduce the mass of the pan while still allowing the surface of the pan to support a stent along its entire length. 
       FIGS. 4A-C  depict exemplary embodiments of a fixture for supporting a stent. In  FIGS. 4A-B , a fixture  170  has a circular pan  172  and a support rod  174 .  FIG. 4A  depicts a three-dimensional view and  FIG. 4B  depicts an overhead view of pan  172 . Pan  172  includes a plurality of holes  176  which reduce the mass of pan  172  so that the mass of fixture  174  does not exceed the maximum capacity of a microbalance. Pan  172  has a diameter D P  that is large enough to support a stent over 28 mm in length along its entire length. It is desirable for the holes in a pan to be relatively evenly distributed to enhance the stability of the fixture. 
     The mass of a pan may be optimized to a desired mass by varying the size and number of holes. As the size of the holes decreases, the number of holes required to reduce the mass of the pan a selected amount increases. This is illustrated in  FIG. 4C  by an exemplary pan  178  with holes  180  which are smaller than holes  176  in pan  172 . Pan  178  has a larger number of holes  180  than pan  172  has of holes  176 . 
     In some embodiments, the pan of a fixture may have an asymmetric shape that is longer along one axis, for example, an oval, rectangular, or dumbbell shape. The surface along the longer axis may be configured to accommodate a stent greater than 28 mm in length along its entire length.  FIGS. 5A-C  depict exemplary pans with asymmetric shapes which include an oval pan  184 , a rectangular pan  186 , and a dumbbell-shaped pan  188 . An axis A-A corresponds to a longer axis of the shapes in  FIGS. 5A-C . A length L A  corresponds to the length along axis A-A. L A  is long enough to support a stent greater than 28 mm. A length along a shorter axis need not be long enough to support the stent. Pans  184 ,  186 , and  188  can include holes  185 ,  187 , and  189 , respectively, to further optimize the mass of the pan. 
     As the asymmetry of a pan increases, a fixture may tend to become less stable. Additionally, a stent has an increased risk of rolling off a pan that is highly asymmetric. A more stable geometry of the pan may include at least two intersecting elongated portions. At least one of the intersecting portions may be adapted to support a stent greater 28 mm in length along its entire length.  FIGS. 6A-B  depict a fixture  190  with a cross-shaped pan  194  supported by a support rod  192 . Cross-shaped pan  194  includes intersecting rectangular portions  196  and  198 . A length L c  of rectangular portion  196  is long enough to support a stent longer than about 28 mm along its entire length. The mass of the cross-shaped pan can further be optimized with holes.  FIG. 6C  depicts a fixture  200  with a cross-shaped pan  202  with holes  204 . 
     In some embodiments, a weighing pan may include surface features at or adjacent to at least a portion of an edge of the pan to facilitate placement or removal of a stent on the pan. The features may also inhibit a stent from falling or rolling off the pan. For example, the features may include, but are not limited to, ridges, bumps, or protrusions along at least a portion of the pan.  FIG. 7  depicts a fixture  210  including a pan  212  with holes  220  supported by a support member  214 . Pan  212  includes a ridge  216  extending all the way around the edge of pan  212 . Ridge  216  can also be discontinuous, i.e., ridge  212  can include a series of discrete portions separated by gaps. 
     Improving Throughput of Coating Process 
     A further aspect of the present invention relates to manipulation of spray coating parameters to obtain desired processing goals and coating characteristics. Spray coating parameters that may be manipulated can include, but are not limited to, flow rate of coating material, axial translation speed of stent, rotation speed, nozzle height, and atomization pressure. 
       FIG. 8  depicts an exemplary schematic embodiment of a spray coating apparatus  250  for coating a stent  252 . A syringe pump  256  pumps coating material from a reservoir  254  that is in fluid communication with a spray nozzle  258 . Nozzle  258  can be in fluid communication with pump  256  through a hose  260 . Nozzle  258  provides a plume  260  of coating material for depositing on stent  252 . A flow rate of coating material provided by nozzle  258  can be varied by changing the pump rate of pump  256 . 
     Stent  252  is supported by a stent support  262 , such as a mandrel, other support devices known in the art, or support devices as described herein. Support  262  is configured to rotate stent  252  about its cylindrical axis, as shown by an arrow  264 . Support  262  is also configured to axially or linearly translate stent  252  with respect to plume  260 , as shown by an arrow  266 . In other embodiments, nozzle  258  can be translated along the cylindrical axis of stent  252  rather than or in addition to axially translating stent  252 . 
     Coating material is deposited on stent  252  as it is translated through plume  260  from a proximal end  268  to a distal end  270  of stent  252 . After a selected number of passes through plume  260 , the deposited coating material is allowed to dry or subjected to a drying process known in the art or described herein prior to further deposition of coating material. The deposition and drying steps are repeated until a desired amount of coating material is deposited on stent  252 . 
     One aspect of the present invention relates to increasing the rate of the coating process or increasing throughput of the coated stents while maintaining coating quality. In one embodiment, the rate of the coating process can be increased by increasing the flow rate of coating material through nozzle. This can be accomplished by increasing the pump rate of coating material, as described above. An increased flow rate of coating material increases the amount or mass of coating material deposited per unit time on the stent or per pass of the nozzle over the stent. 
     However, increasing the flow rate, and mass per pass, can have deleterious effects on coating quality. First, an increased mass per pass can lead to defects in the coating, as well as inconsistencies in the coating from layer to layer and stent to stent. Second, increasing the mass per mass increases the drying time of the deposited coating material. Due to the increased drying time, there is an increased likelihood of the nozzle clogging between passes. Residual coating material in the nozzle can dry and reduce or prevent flow of coating material through the nozzle. Embodiments of the method described herein allow an increase in throughput and flow rate of coating material while maintaining coating consistency resulting in an acceptable level of defects. In addition, the increased flow rate does not lead to an increase in nozzle clogging. 
     Certain embodiments of a method of coating a stent may include modifying a flow rate of coating material sprayed onto a stent by a coating apparatus. The method may further include adjusting an axial translation speed of the stent based on the modified flow rate during spraying of the coating material to obtain a selected amount of deposition of coating material per pass of the stent relative to the nozzle. In one embodiment, an axial translation speed of the stent may be controlled to obtain a selected amount of deposition or mass per pass of coating material. 
     In an embodiment, the flow rate of the coating material directed at the stent from a nozzle of the coating apparatus may be increased. The axial translation speed may then be increased to compensate for the increased flow rate. The increased axial translation speed tends to decrease the mass per pass at the increased flow rate. The axial translation speed may be increased so that the mass per pass does not result in a drying time that results in clogging of the nozzle during drying of deposited coating material on the stent. In addition, the axial translation speed may be increased so that the mass per pass results in a relatively consistent coating from pass to pass and that has an acceptable quantity and degree of defects. 
     Table 2 illustrates the effect of pump rate and linear or axial speed on the mass per pass and coating quality. The coating quality was based on a visual inspection of the coated stent under a microscope. Inspections are performed of a coated stent at a magnification between 40× and 100× under a microscope. The coating quality is based on size and number of defects including the presence of cobwebs and rough spots. In general, in order to pass, a stent must be free of defects over a certain size and have less than a specified number of cobwebs and rough spots. 
     Coating runs were performed on a 28 mm stent for different pump rates of coating material and linear or axial speed of the stent under a nozzle. The mass per mass in each run was determined by the difference in weight of the stent before and after coating. 
     The coating runs included application of a primer layer and a drug-matrix or reservoir layer over the primer layer. The coating material for the primer layer was poly(n-butyl methacrylate) (PBMA) in an acetone/cyclohexanone solvent. The coating material for the reservoir layer was poly(vinylidene fluoride-co-hexafluoropropene)copolymer (PVDF-HFP) (e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare, N.J.) in an acetone/cyclohexanone solvent. 
     The coating equipment included a Havard syringe pump obtained from Instech Laboratories, Inc., Plymouth Meeting, Pa. The nozzle and mandrel used were developed in-house. 
     As shown in Table 2, Runs 1 and 2 were performed at a pump rate of 12 mg/sec and linear or axial speed of 6 mm/sec. The mass per pass differed slightly between the two runs and the coating quality was a “pass.” 
     The linear or axial speed for Runs 3-6 was increased from 6 mm/sec to 12 mm/sec from Runs 1 and 2. The pump rate for Runs 3-6 was the same as Runs 1 and 2. Table 2 shows, as expected, that the increase in the linear or axial speed caused a decrease in the mass per mass. As indicated above, an increase in linear or axial speed tends to decrease the mass per pass. The mass per pass of Runs 3-6 is slightly more than a third of the mass per pass of Runs 1 and 2. In addition, the coating quality was a “pass” for each of Runs 3-6. 
     In Runs 7-10, both the pump rate and linear or axial speed were increased over that used in Runs 3-6 from 12 ml/hr to 18 ml/hr and 12 mm/sec to 18 mm/sec, respectively. In general, the increase in the pump rate tends to increase the mass per pass. However, the increase in the linear or axial speed compensates for the increase in the increased pump rate since the mass per pass of Runs 7-10 is less than Runs 3-6. The coating quality is a “pass” for Runs 8-10 and a “fail” for Run 7. 
     In Runs 11 and 12, the pump rate and linear or axial speed were both further increased over that used in Runs 7-10 from 18 ml/hr to 24 ml/hr and from 18 mm/sec to 24 mm/sec, respectively. In this case, the increase in the pump rate resulted in an increase in the mass per pass. The increase in the linear or axial speed only partially compensated for the increase in flow rate. In each of Runs 11 and 12, the coating quality was a pass. Nominal refers to the center or standard parameter run. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Test coating runs for different pump rates and linear or axial speed. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Mass Per Pass 
                 Pump Rate 
                 Axial Speed 
                   
               
               
                 Test 
                 (μg/pass) 
                 (ml/hr) 
                 (mm/sec) 
                 Visual Inspection 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Test Conditions for 28 mm Stent 
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 92 
                 12 
                 6 
                 Pass 
               
               
                 2 
                 95 
                 12 
                 6 
                 Pass 
               
               
                 3 
                 35 
                 12 
                 12 
                 Pass 
               
               
                 4 
                 34 
                 12 
                 12 
                 Pass 
               
               
                 5 
                 36 
                 12 
                 12 
                 Pass 
               
               
                 6 
                 36 
                 12 
                 12 
                 Pass 
               
               
                 7 
                 24 
                 18 
                 18 
                 Fail 
               
               
                 8 
                 23 
                 18 
                 18 
                 Pass 
               
               
                 9 
                 23 
                 18 
                 18 
                 Pass 
               
               
                 10 
                 19 
                 18 
                 18 
                 Pass 
               
               
                 11 
                 38 
                 24 
                 24 
                 Pass 
               
               
                 12 
                 32 
                 24 
                 24 
                 Pass 
               
            
           
           
               
               
            
               
                   
                 Nominal Conditions for 28 mm Stent 
               
            
           
           
               
               
               
               
               
            
               
                 13 
                 30 
                 5 
                 6 
               
               
                   
               
            
           
         
       
     
     Stent Support Assemblies 
     Another aspect of the present invention relates to reducing or eliminating coating defects that can result from stent contact with supports, such as mandrels, during coating. A shortcoming of the above-described method of medicating a stent through application of a coating is the potential for coating defects. While some coating defects can be minimized by adjusting the coating parameters, other defects occur due to the nature of the interface between the stent and the apparatus on which the stent is supported during the coating process. A high degree of surface contact between the stent and the supporting apparatus can provide regions in which the liquid composition can flow, wick, and collect as the composition is applied. As the solvent evaporates, the excess composition hardens to form excess coating at and around the contact points between the stent and the supporting apparatus. Upon the removal of the coated stent from the supporting apparatus, the excess coating may stick to the apparatus, thereby removing some of the needed coating from the stent and leaving bare areas. Alternatively, the excess coating may stick to the stent, thereby leaving excess coating as clumps or pools on the struts or webbing between the struts. 
     Thus, it is desirable to minimize the influence of the interface between the stent and the supporting apparatus during the coating process to reduce or coating defects. Accordingly, the present invention provides for embodiments of a method and device for supporting a stent during the coating application process that minimizes the influence of the interface. 
     As described above, spray coating a stent typically involves mounting or disposing a stent on a support, followed by spraying a coating material from a nozzle onto the mounted stent. A stent can be supported, for example, on a mandrel or rod that supports the stent along its length by positioning the stent over the mandrel. A stent can also be supported at its ends with supports having a variety of geometries, such as tapered or untapered cylinders. 
     As indicated above, the interface or contact points between a stent support and the stent can lead to defects. This is particularly the case where the support and the stent do not move relative to one another during the coating process. The lack of relative movement leads to stationary contact points between the stent and the support. 
     The contact area between a support and stent can be minimized by sizing a support such as a mandrel so that its diameter is less than the inside diameter of the stent. Similarly, the ends of a stent can be supported loosely over tapered or untapered cylinders. Thus, as the mandrel rotates, the contact points continuously change. A disadvantage of this approach is that the stent can stick to the support members, resulting in stationary contact points. 
     Embodiments of the present invention relate to a method of and a device for shifting or changing the contact points of a stent with a support during a coating process. In certain embodiments, the shift in contact points may be accomplished by a pulse in the rotation rate of a support member that rotates and causes a stent to rotate. A “pulse” generally refers to a rise and/or fall in a quantity during a period of time. Shifting or changing a contact point, area, or interface refers to a change of a point, area, or interface of contact of a stent with a support from one location of the support to another location of the support. 
       FIG. 9  depicts a mounting assembly  18  for supporting a stent  15  including a rod or mandrel  20  and support elements  22 . Mandrel  20  can connect to a motor  24 , which provides rotational motion to mandrel  20 , as depicted by an arrow  26 , during the coating process. Motor  24  is also capable of providing rotational pulses for shifting contact points, as described below. Another motor  28  can also be provided for moving mandrel  20  and thus stent  15  in a linear or axial direction, back and forth, along a rail  30 . Motor  28  is also capable of providing translational pulses to mandrel  20  along its axis. Pulsing motors include an electronic gearing system that can cause a pulse or change in the rotation rate or cause a pulse in the axial translation of a support element. 
     Mandrel  20  is illustrated as having two regions with a larger diameter. The two regions can be support elements  22  for applying a torque to stent  15 . In commercially useful embodiments, any number of support elements  22  can be used to adequately support stent  15 . As shown, support elements  22  are sized larger than the outer diameter of mandrel  20  so as to prevent mandrel  20  from being in contact with the inner surface of stent  15 . Alternatively, in other embodiments, mandrel  20  can be free of support elements  22  so that the stent is supported by and in contact with mandrel  20 . 
     Additionally, support elements  22  are sized smaller than the inner diameter of stent  15  so as to provide for minimum contact between support elements  22  and the inner surface of stent  15 . In the case of a mandrel  20  free of support elements  22 , mandrel  22  is sized smaller than the inner diameter of stent  15 . 
     Support elements  22  of small diameter, as compared to the inner diameter of stent  10 , results in an axis x M  about which support elements  22  rotate, that is offset from an axis x S , about which stent  15  rotates. Axis x S  is positioned longitudinally through the center of stent  15 . Since support elements  22  and stent  15  rotate about different axes, x M  and x S , support elements  22  and stent  15  do not have a 1:1 rotation. Thus, the contact points or area between support  22  and stent  15  continuously change. However, coating material can cause stent  15  to stick to support elements  22 , resulting in a stationary contact point. 
     Sticking of stent  15  to support elements  22  is reduced or prevented by pulsing the rotation rate of mandrel  20  and elements  22 . The pulses in rotation rate cause a stent that is stuck to one or both of support elements  22  to break free and resume rotation about axis x S  with continuously changing contact points. 
     Numerous variations of the form of a pulse that could break a stent free are possible.  FIGS. 10A-D  each depict the rotation rate as a function of time for representative pulses. The pulse duration can be, for example, between 0.0001 sec and 1 sec. A pulse duration outside this range can be contemplated by one of skill in the art. R 1  is the rate prior to the pulse, R 2  is the maximum rate of the pulse, and R 3  is a final rate of the pulse (in  FIGS. 10A-C ). R 1  can also correspond to the rotation rate of the opposing non-pulsing element.  FIGS. 10A-D  depict pulses that have an increase in rotation rate. However, pulses with a decrease in rotation rate can also be used to reduce or prevent sticking, as well as shifting contact points, as described below. 
     In  FIGS. 10A-C , the rotation rate increases from R 1  to a peak at R 2  and then decreases to R 3 . R 3  is greater than R 1  in  FIG. 10A , less than R 1  in  FIG. 10B , and the same as R 1  in  FIG. 10C . The maximum rate, R 2  of the pulse can be greater than 10%, 30%, 60%, 80%, or more narrowly greater than 100% of the rate prior to the pulse, R 1 . 
     The pulses can be performed at a specified frequency during the coating process to reduce or eliminate sticking of the stent that can occur. The frequency can be greater than 0.1 Hz, 0.5 Hz, 1 Hz, 3 Hz, or more narrowly 5 Hz. Alternatively, the pulses can be performed at irregular or unequal intervals. 
     A pulse causes a rotation of elements  22  relative to stent  15 . The relative rotation caused by an increase in rate from R 1  to R 2  can be greater than 10°, 30°, 45°, 90°, or 270°. The amount of rotation can be controlled by the maximum rate of the pulse, the degree of acceleration, and the duration of the pulse. It is believed that the higher rate R 2  or degree of acceleration, the greater is the relative rotation of elements  22  to stent  15 . 
       FIG. 11A  depicts a mounting assembly  40  which supports stent  45  via support elements  42  and  44 . Support elements  42  and  44  have a cylindrical cross-section and taper inwardly toward stent  45 . As shown, the ends of stent  45  can rest on the tapered portions of support elements  42  and  44 . A variety of shapes can be contemplated by one of ordinary skill in the art for support elements  42  and  44 . 
     Support elements  42  and  44  can connect to motors  46  and  48 , respectively, which provide rotational motion to support elements  42  and  44 , as depicted by arrows  50  and  52 , during the coating process. Motors  46  and  48  are also capable of providing rotational pulses for shifting contact points, as described below. Motors  54  and  56  can also be provided for providing pulses to support elements  42  and  44  in a linear direction along rails  58  and  60 , respectively. Motors  54  and  56  are also used to position support elements  42  and  44  relative to one another. Another motor (not shown) can also be provided to move assembly  40  and thus stent  45  in a linear direction, back and forth, during coating. 
     Support elements  42  and  44  can be positioned by motors  54  and  56 , respectively, so that support elements  42  and  44  and stent  45  have the same or substantially the same axes of rotation. Opposing forces exerted from support elements  42  and  44 , for securely pinching stent  45  against support elements  42  and  44 , should be sufficiently strong so as to prevent any significant movement of stent  45  on mounting assembly  40 . The forces can be sufficiently strong so that there is a 1:1 rotation of stent  45  with support elements  42  and  44 . 
     However, the exerted force should not compress stent  45  so as to distort the body of stent  45 . Over or under application of support force can lead to problems such as stent  45  resting too loosely on mounting assembly  20 , stent  45  bending, opposing ends of stent  45  flaring on support elements  42  and  44 , and increased contact between stent  10  and support elements  42  and  44 , all of which can lead to coating defects. 
     Additionally, stent  45  can be disposed loose enough on support elements  42  or  44  so that there is not 1:1 rotation between stent  45  and support elements  42  or  44 . Support elements  42  and  44  can be positioned relative to one another so that stent  45  has an axis of rotation different from an axis of rotation of the support elements. In this case, the contact points or area between support elements  42  and  44  and stent  10  continuously change. 
     When there is 1:1 rotation between a support element and the stent, contact points between the support and the stent tend not to change if the rotation rate of elements  42  and  44  are the same. As discussed above, contact points that do not change for all or a substantial part of the coating process can lead to coating defects. A shift in contact points during coating can reduce or eliminate defects due to contact points during coating. Providing a rotational pulse to a support element can shift the contact points between the support element and the stent. Contact points that are uncovered due to a shift may then be covered by coating material. A repeated shifting of contact points during coating tends to allow coating material to cover defects or uncoated regions caused by contact points during the coating process. 
     Motor  46  can rotate support element  42  at the same or different rate as motor  48  rotates support element  44 . Thus, motor  46  and motor  48  can provide rotational pulses independent of one another to support elements  42  and  44 , respectively, to shift contact points between stent  45  and support elements  42  and  44 . The pulses can take the form of those shown in  FIGS. 10A-D . As discussed above, a rotational pulse causes a rotation of support elements  42  or  44  relative to stent  45 . An increase in rotation rate of element  42  from R 1  to R 2  causes element  42  to rotate ahead of stent  302  resulting in a shift of contact points between stent  45  and element  42 . The relative rotation can continue as long the rotation rate of element  42  differs from element  44 . However, if both ends of the stent are stuck, coupled, or attached to the respective collets resulting in 1:1 rotation at each end, a difference in velocity between the ends could damage or destroy the stent. The stent could be twisted and damaged or destroyed by a torsional force. 
     The pulses can be performed at a specified frequency during the coating to reduce or eliminate the defects caused by contact points. A pulse rotates elements  42  or  44  relative to stent  45  resulting in different contact points after the pulse. As above, the relative rotation can be greater than 10°, 30°, 45°, 90°, or 270°. The degree of rotation can be controlled by the maximum rate of the pulse, the degree of acceleration, and the duration of the pulse. 
     When stent  45  is loosely supported on support elements  42  and  44  so that there is not a 1:1 rotation, rotational pulses can reduce or prevent sticking of stent  45  to support elements  42 . Sticking can also be reduced or prevented by rotating support elements  42  and  44  at different rates. Rotating support elements  42  and  44  at different rates can reduce or prevent sticking of the ends of stent  10  to elements  42  or  44 . The difference in torque at the different ends of stent  10  causes the slower end to pull back on the faster end and the faster end to pull forward on the slower end to reduce or prevent sticking or to break free an end that is sticking. The difference in torque between elements  42  and  44  should be small enough so as not to cause excessive flexure to the stent. 
     Contact points between stent  45  and support elements  42  and  44  can be shifted by pulsing support elements  42  and  44  in a linear direction. Sticking of stent  45  to support elements  42  or  44  can also be reduced or prevented by linear pulses. Motor  54  or motor  56  can provide linear pulses independent of one another to support elements  42  and  44 , respectively. A linear pulse involves an axial translation of support elements  42  or  44  by motors  54  or  56 , respectively. 
     Motors  54  or  56  can pulse support elements  42  or  44 , respectively, either inward toward stent  45  or outward away from stent  45 . A representative linear pulse showing linear translation rate versus time is depicted in  FIG. 12 . The rate corresponds to the linear translation rate of support element  42  or  44  relative to the assembly  40  which can be translated back a forth during coating. In  FIG. 12 , the translation rate increases from zero to R max  and then returns to zero. For example, support element  42  can be pulsed inward toward stent  45  with a pulse having the functional form in  FIG. 12 . Support element  42  can be pulsed outward in a similar manner. 
     Pulses or a suitable combination of pulses can cause axial translation of supports  42  and  44  with respect to stent  45  in a number of ways.  FIGS. 13A-H  show the axial position of an end of support element  42  or  44  as a function of time. “x 1 ” is the initial position of a point on a support element, “x 2 ” is the maximum linear deviation from the initial position. “x 3 ” is the final position of the support element in  FIGS. 13C-H .  FIGS. 13A-B  depict movement of a support element from a position x 1  to a position x 2 . For example, for support element  42 ,  FIG. 13A  depicts a movement inward and  13 B is a movement outward from the stent.  FIGS. 13C-D  depict a movement of a support element from a position x 1  to a position x 2  followed by a movement back to x 1 .  FIGS. 13E-F  depict a movement of a support element from a position x 1  to a position x 2  followed by a movement to a position between x 1  and x 2 .  FIGS. 13G-H  depict a movement from of a support element from a position x 1  to a position x 2  followed by a movement to a position that is further from x 2  than x 1 . 
     The linear pulses tend to reduce or prevent sticking of the stent to support element  42  or  44 . When support element  42  or  44  is translated inward toward the stent, the stent tends to slide or ride upward along the upper tapered portion of support element  42  or  43 . Similarly, when support element  42  or  44  is translated outward away from the stent, the stent tends to slide or ride downward along the upper tapered portion of support element  42  or  44 . 
       FIG. 11B  depicts a part of a mounting assembly with support elements  62  and  64  supporting stent  55 . Support elements  62  and  64  are untapered cylindrical elements. Support elements  62  and  64  can be sized so that there is a 1:1 rotation between stent  55  and the support elements. Rotational and linear pulses can be used to shift contact points between support elements  62  and  64  and stent  55 . Support elements  62  and  64  can be sized smaller than the inner diameter of stent  55  so that the support elements and stent  55  have a different axis of rotation. Sticking of stent  55  to support elements  62  and  64  can be reduced or prevented by rotational and linear pulses. 
     In further embodiments, assembly  40  can include only one rotatable support element, e.g., support element  42 , with the opposing support element being rotationally fixed. An end of stent  45  can fit loosely over an opposing support element so that there is not a 1:1 rotation of stent  45  with the support element. Such a support element can have a variety of shapes including those pictured in  FIGS. 11A-B . 
       FIGS. 14A-B  depict an additional exemplary embodiment of a stent support assembly  300  that provides for changing or shifting of contact points of a stent  302  with a stent support. Stent  302  has a geometry composed of a plurality of undulating and intersecting elements or struts  302 A. Assembly  300  includes a rotary spindle  304  and a rotary spindle  306  that can rotate independently of one another. 
     A support element  308  and a support element  310  are coupled to rotary spindle  304  and rotary spindle  306 , respectively. Rotary spindles  304  and  306  can rotate support elements  308  and  310 , and thus, stent  302  supported by elements  308  and  310 . Support element  308  supports a proximal end  309  of stent  302  and support  310  supports a distal end  311  of stent  302 . Rotary spindles  304  and  306  are each connected to motors (not shown) which provide rotational motion to support elements  308  and  310  and to stent  302 . The rotational motors are also capable of providing rotational pulses, as described above. Additionally, rotary spindles  304  and  306  are also connected to linear motors (not shown) for positioning support elements  308  and  310  relative to one another and for providing translational pulses to support elements  308  and  310  in a linear or axial direction. Another motor (not shown) can also be provided to move an assembly including spindles  304  and  306 , and thus, stent  302  in a linear direction, back and forth, during coating. 
     Support element  308  includes three elongate arms  312  coupled to spindle  304  and support element  310  also includes three elongate arms  314  coupled to spindle  306 . Elongate arms  312  and  314  converge inwardly to form a conical or frusto-conical shape for supporting stent  302 . As depicted in  FIG. 14B , elements  312  and  314  have cylindrical cross-sections and have pointed ends  318  that taper to a thin tip. Elements  312  and  314  can have other cross-sectional shapes such as triangular, square, rectangular, etc. Also, ends  318  can be flat or rounded. Elongate arms  312  and  314  may be, for example, wires or rods. 
     The ends of spindles  304  and  306  each have projecting portions  305  and  307 . Portion  307  has an inner edge  307 A, an edge  307 B that tapers inward toward stent  302  to meet edge  307 A, an edge  307 C that tapers outward toward stent  302  to meet edge  307 B. An edge  307 D tapers inward toward stent  302  to meet edge  307 C. Spindle  304  has corresponding elements including edge  305 B, edge  305 C, and edge  305 D. Edge  307 D has a longer a more gradual taper than edge  305 D. 
     As shown in  FIG. 14B , elongate arms  312  and  314  are coupled to rotation spindles  304  and  306 . Edges  307 A and  307 C have holes sized to receive elements  314 . Elements  314  are disposed through the holes to couple elements  314  to portion  307 . Elements  312  are similarly coupled to or disposed in portion  305 . Elongate arms  312  and  314  can be, for example, screwed, glued, riveted, or friction-fitted into the holes. 
     Elongate arms  312  and  314  extend into and support a proximal end  309  and a distal end  311 , respectively of stent  302 . Each of elongate arms  312  and  314  have at least one point or area of contact with struts  302 A at proximal end  309  and distal end  311 , respectively. Elongate arms  312  and  314  are sized to be capable of supporting the stent, for example, so that an elongate arm can support stent  302  without passing through a narrow portion  303  of the stent pattern. Support elements  308  and  310  are positioned within ends  309  and  311  by translating the support elements with respect to each other using the linear motors mentioned above. 
     In one embodiment, elongate elements  312  and  314  can be rigid such that the elements exhibit no or relatively no bending while being positioned within a stent or during coating. Alternatively, elongate elements  312  or  314  can be flexible. As flexible elements  312  or  314  are positioned into an end of stent  302 , the flexible elements bend inward and can exhibit an outward radial force that facilitates securing the elongate elements to stent  302 . 
     Stent  302  can be rotated during coating by rotating rotary spindles  304  and  306  which rotate support elements  308  and  310 , and thus, stent  302 . Elements  308  and  310  can be positioned within ends  309  and  311  so that there is 1:1 rotation between elements  308  and  310  and stent  302 . As described for elements  42  and  44  in  FIG. 11A , the relative distance between elements  308  and  310  can be decreased so that elongate elements  312  and  314  securely pinch ends  309  and  311  of stent  302 . The opposing forces exerted from support elements  308  and  310  should be sufficiently strong so as to prevent any significant movement of stent  302 . However, the exerted force should not compress stent  302  so as to distort the body of stent  302 . When support elements  308  and  310  rotate at the same rate or rotate in phase, the contact points between elements  308  and  310  with stent  302  tend to remain the same, as described for elements  42  or  44  in  FIG. 11A . 
     Providing a rotational pulse to, for example, element  308  causes a shift or change in the contact points between each of elements  312  and stent  302 . Referring to  FIG. 10A-D , an increase in rotation rate of element  308  from R 1  to R 2  causes element  308  to rotate ahead of stent  302  resulting in a shift of contact points between stent  302  and element  308 . The relative rotation can continue as long the rotation rate of element  308  differs from element  310 . The degree of relative rotation would depend on the length of the stent and the torsional stiffness of the stent in order to cause separation of the support elements. 
     For example, for a pulse represented by  FIG. 10A , the final rotation rate R 3  is greater than R 1  which causes element  308  to continue to rotate ahead of or faster than stent  302 . The final rotation rate must be the same for both spindles if the stent is keyed into element  308  for 1:1 rotation, otherwise the stent can be damaged through torsion. Similarly, for the pulse of  FIG. 10D , element  308  continues to rotate ahead of or faster than stent  302  as long as the rotation rate is at R 2 . For a pulse shown by  FIG. 10B , the final rotation rate is less than R 1  which causes element  308  to rotate behind or slower than stent  302 . The final rotation rate, R 3 , is the same as the pre-pulse rotation rate, R 1 , in  FIG. 10C . Thus, element  308  stops rotating relative to stent  302  when the rotation rate of element  308  returns to R 3 . After the pulse, element  308  has different contact points with stent  302 . 
     As described above, pulses can be performed at a specified frequency resulting in a change in contact points at the specified frequency. Alternatively, pulses can be performed at irregular intervals. 
     Similar to the embodiment in  FIG. 11A , contact points between stent  302  and elements  308  and  310  can be shifted by pulsing element  308  or  310  in a linear direction. The linear motors can provide linear pulses to elements  308  or  310 . As above, a linear pulse is an axial translation of element  308  or  310 . 
     The linear pulses tend to reduce or prevent sticking of the stent to elongate elements  312  or  314 . Translation of support element  308 , for example, inward toward stent  302  causes struts  302 A of stent  302  to slide or ride upward along elements  312 . Similarly, translation of support element  308  outward from the stent, causes struts  302 A to slide or ride downward along elements  312 . A representative linear pulse is shown in  FIG. 12 . The rate corresponds to the linear translation rate of support element  308  or  310  relative to an assembly that includes system  300  which translates back a forth during coating. For example, support member  308  can be pulsed inward toward stent  302  with a pulse having the functional form in  FIG. 12 . Support element  308  can be pulsed outward in a similar manner. 
     Pulses or a suitable combination of pulses can cause axial translation of supports  308  and  310  with respect to stent  302  in a number of ways. As described above,  FIGS. 13A-H  show the axial position of an end of support element  308  or  310  as a function of time. 
     Furthermore, translational pulses of elements  308  or  310  can be performed at a specified frequency resulting in a change in contact points at the specified frequency. Alternatively, pulses can be performed at irregular intervals. 
     In other embodiments, assembly  300  can include an element disposed axially within stent  302  at least between support element  308  and support element  310 . Alternatively, the member may extend between spindle  304  and spindle  306 . The member may include, but is not limited to, a rod or wire. The member may be coupled to the support members at each end in a manner than allows independent rotation of the rotary spindles. 
     In additional embodiments, assembly  300  can include only one rotatable support element, e.g., support element  308 , with an opposing support element being rotationally fixed. Distal end  311  of stent  302  can fit loosely over an opposing support element so that there is not a 1:1 rotation of stent  302  with the support element. Such a support element can have a variety of shapes including those pictured in  FIGS. 11A-B  and  FIGS. 14A-B . 
     A further aspect of the present invention relates to eliminating coating defects that can result from stent contact with supports, such as mandrels, during coating. Additional embodiments of the present invention involve selectively coating portions of a stent that have no contact points. 
       FIGS. 15A-B  depict views of an exemplary embodiment of a stent support assembly  400  for coating a stent  402  having a proximal end  402 A and a distal end  402 B.  FIG. 15A  depicts a side view of assembly  400  and  FIG. 15B  depicts an axial cross-section of assembly  400 . As described below, assembly  400  is configured to selectively coat portions of stent  402  that have no contact points. An exemplary stent  402  has a geometry composed of a plurality of undulating and intersecting elements or struts  403 . 
     As shown in  FIG. 15B , assembly  400  includes a rotation spindle  404  and a rotation spindle  406  that can rotate independently of one another. A support mandrel  408  is coupled to rotation spindle  404  at a proximal end  409 A of mandrel  408  as shown. A distal end  409 B of mandrel  408  is free floating, i.e., not coupled or connected to another support element. A support mandrel  410  is coupled to rotation spindle  406  at a proximal end  411 A of mandrel  410 . A distal end  411 B is free floating. Thus, distal end  409 B of support mandrel  408  and distal end  411 B of support mandrel  410  are separated by a gap  413 . As described above, gap  413  is adjustable. 
     A masking sheath  420  is disposed over spindle  404  and can be translated axially over rotation spindle  404 . A shuttle sheath  430  is positioned over support mandrel  408  which can axially translate over support mandrel  408 . In a similar manner, shuttle sheath  421  is disposed over spindle  406  and shuttle sheath  431  is disposed over support mandrel  410 . 
     Either or both rotation spindle  404  and rotation spindle  406  can be axially translated as shown by arrows  414  and  415 , respectively. Prior to coating, stent  402  is loaded onto support mandrel  408  by sliding stent  402  onto support mandrel  408 . Alternatively, stent  402  can also be loaded onto support mandrel  410 . Prior to loading stent  402  on support mandrel  408 , one or both of the rotation spindles can be axially translated to their maximum separation position or at least to a position that allows loading of stent  402 . 
     As shown in  FIGS. 15A-B , stent  402  is loaded so that a portion  416  of stent  402  is over support mandrel  408  and a portion  418  extends beyond distal end  409 B of support mandrel  408 . Portion  418  is free of contact points with support mandrel  408  and any other support element. Portion  416  can be long enough so that there is adequate support for portion  418 , for example, so that there is no or substantially no sagging of portion  418 . It is expected that the longer the stent, a larger percentage of the length of stent  402  should be over mandrel support  408 . For example, portion  416  can be less than 30%, 40%, 50%, 60%, or 70% of the length of stent  402 . It may be desirable for portion  418  to be as large as possible since, as described below, a coating material is applied to a majority of portion  418 . 
       FIGS. 16A-B  depict views of assembly  400  showing masking sheath  420  translated axially from rotation spindle  420 , as shown by an arrow  422 , to mask or cover at least portion  416  of stent  402  in  FIG. 15A .  FIG. 16A  depicts a side view of assembly  400  and  FIG. 16B  depicts an axial cross-section of one side of assembly  400 . As shown, a portion  424  is unmasked. Preferably sheath  420  masks portion  416  and a small axial section of portion  418  to inhibit or prevent exposure of portion  416  to coating material. For example, the small axial section may be less than 1%, 3%, 5%, 8%, 10%, or less than 15% of a length of stent  402 . 
     After positioning masking sheath  420 , a coating material  425  is applied to unmasked portion  424  from a spray nozzle  427  positioned above stent  402 . Stent  402  is rotated by rotation spindle  404 , as shown by an arrow  426  during coating. Additionally, stent  402  is axially translated by axially translating rotation spindle  404  along with masking sheath  420 , as shown by an arrow  428 , during coating. Alternatively or additionally, spray nozzle  427  can be translated along unmasked portion  424 . 
     At least one pass of spray nozzle  427  from one end of portion  424  to the other can be made over stent  402 . After a desired amount of coating material is applied to unmasked portion  424 , the coating applied on stent  402  is dried according to methods known to a person of skill in the art or by methods disclosed herein. Stent  402  can be dried at the same location as it is sprayed or moved to a drying station (not shown). Alternatively, the coating can be dried at the same time it is being sprayed. The spraying-drying cycle can be repeated a number of times until a desired amount of coating material has been applied to the stent. 
     To coat the masked portion of stent  402 , stent  402  is loaded onto support mandrel  410  by axially translating support mandrel  408  toward support mandrel  410 , decreasing adjustable gap  413  shown in  FIGS. 15A-B . Mandrel  408  is axially translated so that distal end  402 B of stent  402  engages distal end  411 B of mandrel  410 . Distal end  411 B is tapered to facilitate engagement of stent  402  on distal end  411 B. 
       FIG. 17A  depicts a side view of assembly  400  showing shuttle sheath  430  positioned over support mandrel  408  axially translated towards support mandrel  410  as shown by an arrow  432 .  FIG. 17B  depicts an axial cross-section of one side of assembly  400  showing shuttle sheath  430  over support mandrel  408 . As shuttle sheath  430  translates, it pushes against stent  402  at proximal end  402 A so that stent  402  is pushed off support mandrel  408  and onto support mandrel  410 . 
     As shown in  FIG. 17A , stent  402  is positioned on support mandrel  410  in a manner that is similar to support mandrel  408 . In particular, stent  402  is loaded on support mandrel  410  so that a portion  436  of the stent is over support mandrel  410  and a portion  438  extends beyond distal end  411 B of support mandrel  410 . Portion  438  is free of contact points with support mandrel  410  and any other support element. Portion  436  can be long enough so that there is adequate support for portion  438 , for example, so that there is no or substantially no sagging of portion  438 . Portion  438  includes at least the portion of stent  402  that was not coated while stent  402  was loaded on mandrel support  408 . 
     In a manner similar to that described above, a masking sheath translates axially from rotation spindle  406  to mask or cover portion  436 . After positioning the masking sheath, coating material  425  is applied to the unmasked portion from spray nozzle  427  positioned above stent  402 . Stent  402  is rotated by rotation spindle  406  during coating. Additionally, stent  402  is axially translated by axially translating rotation spindle  406  during coating. Alternatively or additionally, spray nozzle  427  can be translated along an axis of the unmasked portion. At least one pass of spray nozzle  427  can be made over stent  402 . After a desired amount of coating material is applied to the unmasked portion, the coating applied on stent  402  is dried. The spraying-drying cycle can be repeated a number of times until a desired amount of coating material has been applied to the stent. 
     After coating stent  402  on support mandrel  410 , stent  402  is loaded back onto support mandrel  408  if it is desired to apply additional coating to portion  418 . Stent  402  may be loaded back onto support mandrel  410  in a similar manner as the transfer of stent  402  from support mandrel  408  to support mandrel  410 . For example, either or both of the support mandrels can be axially translated toward one another so that proximal end  402 A of stent  402  engages distal end  409 B of mandrel  408 . Distal end  409 B is tapered to facilitate engagement of stent  402  on distal end  409 B. A shuttle sheath positioned over support mandrel  408  can push against stent  402  at proximal end  402 B so that stent  402  is pushed off support mandrel  410  and onto support mandrel  408 . The sequence of steps described above involving coating on support mandrel  408 , drying the coated portion, transferring stent  402  to support mandrel  410 , coating stent  402  on support mandrel  410 , can be repeated a selected number of times until a specified loading of coating is applied to stent  402 . 
     In some embodiments, support mandrels  408  and  410  could be covered, coated, or jacketed with a lubricious material, such as Teflon, to reduce or eliminate defects that could be caused by interaction of the inside diameter (ID) of stent  402  with the outside surface of the mandrels. Additionally, in one embodiment, the outside diameter of the mandrels can be sized to allow a slip or friction-fit so that there is a 1:1 rotation of stent  402  with the mandrels. 
     In alternative embodiments, stent  402  can be coated by sequential spray coating of any number of contactless portions of stent  402 . For example, stent  402  can be disposed so that a proximal portion is over support mandrel  408  and a distal portion is over support mandrel  410  with a center portion having no contact with either mandrel.  FIG. 18  depicts an embodiment for coating a central portion  440  of stent  402 . Proximal and distal portions (not shown) of stent  402  are masked by masking sheaths  442  and  444 . Masking sheaths  442  and  444  are disposed over proximal and distal portions of stent  402  after disposing the proximal and distal portions over support mandrel  408  and support mandrel  410 , respectively. 
     Additional aspects of the present invention relate to devices and methods for supporting a stent during coating, processing, or handling that reduce or eliminate defects in the stent coating. Various embodiments include a system for supporting a stent having support members that contact proximal and distal portions of a stent that are connected with a connecting member that extends between the support members outside of and free of any contact with the stent. 
       FIG. 19A  depicts a three-dimensional view of an exemplary embodiment of a stent support assembly  500 . Support  500  includes pins or rods  502  and  504  which are coupled to collets  506  and  508 , respectively. Collets  506  and  508  are coupled to a proximal and a distal end of a support bar  512 , respectively. A stent can be supported on pins  502  and  504  between collets  506  and  508 . 
       FIG. 19B  depicts a close-up side view of a proximal end  505  of support  500 . As depicted in  FIG. 19B , pin  502  can be embedded in one side of collet  506  and rod  510  can be embedded in an opposite side of collet  506 . Pin  504  and rod  511  can be coupled to collet  508  in a similar manner. Rod  511  extends through a distal end of support bar  512  and rod  510  extends through a proximal end of support bar  512 . As shown in  FIGS. 19A and 19B , rod  510  and rod  511  can have a larger diameter on the side opposite of collets  506  and  508 , respectively. In other embodiments, rod  510  and rod  511  have the same diameter on both sides of collets  506  and  508 , respectively. 
       FIGS. 19A and 20  show that support bar  512  is shaped in the form of a “C-clamp.” Support bar  512  is designed so that it may be held, grasped, or manipulated during and/or before processing of a stent. Thus, other shapes of support bar  512  may be contemplated that allow support assembly  500  to be grasped, handled, or manipulated by a human hand or mechanically without contacting or interfering with a mounted stent. For example, support bar  512  can also be U-shaped. 
     In the exemplary embodiment depicted in  FIG. 19A , pin  504 , along with collet  508  and rod  511 , are rotationally and axially fixed about and along axis  516 . Pin  502 , along with collet  506  and rod  510  are rotationally and axially movable about and along axis  518 . As shown, rod  510  has a spring  520  that allows pin  502  to be retracted axially, as shown by an arrow  522 , a specified distance. The retracted pin is then returned to and held at its original position by the force of spring  520 . Retracting pin  502  allows a stent to be mounted or a mounted stent to be removed from support assembly  500 . Pin  502  can be retracted by pulling rod  510  as shown by an arrow  523 . Releasing rod  510  then allows pin  502  to return to its original position. 
     When pin  502  is retracted, a stent can be mounted by inserting a proximal end of a stent over pin  502  and a distal end of a stent over pin  504 . Pin  502  is then pushed forward by the force of spring  520  to secure the stent on pins  502  and  504 . Similarly, a mounted stent can be removed when pin  502  is retracted. The ability to load and unload a stent on a support with contact points limited pins  502  and  504 , reduces or eliminates the potential for defects due to handling or manipulation of the stent. 
       FIG. 20  depicts a side view of an exemplary embodiment of support assembly  500 . Representative dimensions are provided for support assembly  500 . All dimensions are in centimeters. The representative dimensions are provided by way of example only and in no way are intended to limit support assembly  500 . 
       FIG. 21  depicts a stent  532  mounted on support assembly  500 . Stent  532  can be rotated by support  500 , for example, during application of a coating to the stent. Rod  510  is coupled or engaged in a chuck in a rotatable spindle  525  that is coupled to a stationary support  526 . Rotatable spindle  525  rotates rod  510  as shown by an arrow  528  which rotates rod  510  and collet  506  as shown by arrows  530 . 
     A stent  532  can be mounted such that a length of stent  532  is less than a distance between collets  506  and  508  such that the stent can move from side to side during rotation. In another embodiment, a proximal end  534  and distal end  536  of stent  532  are at least partially in contact with a surface of collets  506  and  508 , respectively. Collet  506  and pin  502  are positioned axially to contact stent  532  to allow a 1:1 rotation of collet  506  and stent  532 . Collet  506  can have a protrusion or pin  538 , as shown in  FIG. 19A , positioned on its surface to reduce or prevent sticking of proximal end  534  to collet  506  during rotation. Protrusion  538  can make contact with a proximal end  534  that is sticking to collet  506 , causing it break free. 
     In addition to use as a support during coating, support assembly  500  can be used as a support during various portions of stent processing. Support assembly  500  can be used generally for handling or manipulating a stent between and during processing steps. Support  500  can be used in operations such as weighing, inspection, and transport. When weighing a stent, the support bar can be held by hand and the spring-loaded end-pin can be grasped by the other hand. 
     For example, a coated stent  532  that is to be weighed can be loaded on a weigh pan by holding stent  532  over the weigh pan, retracting spring-loaded pin  502  and allowing coated stent  532  to be placed onto the weigh pan. Retracted pin  502  is allowed to return to a relaxed position. Thus, stent  532  may be loaded on the weigh pan without any additional contact of stent  532  with support  500 . When the stent weighing process (or any other process) is complete, spring-loaded pin  502  can be retracted again to allow enough space for fixed pin  504  to be placed into distal end  536  of stent  532 . Spring-loaded pin  502  can then be inserted into proximal end  534  of stent  532  to secure stent  532  onto support  500  for further transport, handling, or processing. 
     Further aspects of the present invention relate to devices and methods for supporting a stent during coating, processing, or handling that reduce or eliminate defects in the stent coating. Various embodiments include a system for supporting a stent with reduced contact points of a support with a stent. The stent support includes a spiral coil or spiral mandrel that can be disposed within a stent to support the stent during coating. 
       FIGS. 22A and 22B  depict exemplary embodiments of spiral coil supports for supporting a stent.  FIG. 22A  depicts a two-dimensional view of a spiral coil support  550  having a coiled portion  552 , two straight portions  556  and  557  on either side of coiled portion  552 . Peaks  558  of coiled portion  552  provide contact points with an inner surface of a stent to support the stent. An outside diameter  564  of coiled portion  552  from a peak  558  to a valley  560  is sized to be smaller than an inside diameter of a stent. Thus, as support  550  rotates the contact points alternate along the coil. Alternatively, diameter  564  is sized to obtain a friction fit or press fit between an inner surface of the stent and coiled portion  552 . A “pitch” of a spiral coil refers to a distance from any point on the coil to a corresponding point on the coil measured parallel to the axis of the coil. For example, spiral coil support  550  has a pitch  562 . 
       FIG. 22B  depicts an illustration of a prototype of a spiral coil support having a pitch  568 . The spiral coil is made from wire with a 0.017 inch outside diameter. 
     An advantage of a support as depicted in  FIGS. 22A and 22B  is that the number of contact points with a stent may be controlled by controlling pitch  562  of coiled portion  552 . Increasing pitch  562  reduces the number of contact points of coiled portion  552  with a supported stent. However, the support provided to the stent is reduced by increasing the pitch. In one embodiment, a coil with a pitch of less than three, less than four, less than five, less than six, or more narrowly, less than seven pitch per inch may be used for coating a 28 mm stent. In an embodiment, the pitch may be controlled so that the total contact points may be less than three, less than four, less than five, or more narrowly less than seven. 
     An example of a spiral coil support  550  depicted in  FIG. 22A  has dimensions as follows: Length of straight portion  556 =0.15 inch, Length of straight portion  557 =1.05 inch, Length of coiled portion  552 =2 inch, Diameter  564 =0.05±0.005 inch, and Pitch  562 =0.2 inch. 
       FIG. 23A  depicts a holder  570  for a spiral coil support for manipulating and positioning a spiral coil support. Holder  570  has a cylindrical cross-section. In an embodiment, holder  570  may be used to manipulate and position a spiral coil support before and after coating. Holder  570  can also support a spiral coil support during coating. Holder  570  includes a proximal tapered section  572  having a cavity or hole  574 . Cavity  574  is capable of receiving a straight portion  556  or  557  of a spiral coil support. Straight portion  556  or  557  of a spiral coil support can be coupled within hole  574  by a press fit and/or by gluing. Straight portion  556  or  557  and hole  574  can also be threaded for a screw fit. 
       FIG. 23B  depicts a radial cross-section of the front of tapered section  572 . A middle section  576  has a larger diameter than a distal section  578 . Distal section  578  can be sized and adapted to engage into a rotatable spindle for rotating holder  570  during coating. For example, distal section  578  includes a tapered portion  578 A for adapting to a support structure or rotatable spindle. An example of holder  570  depicted in  FIGS. 22A-B  has dimensions as follows: L 1 =1.65 inch, L 2 =1.2, L 3 =0.45, L 4 =0.125 inch, L 5 =0.051, D 1 =0.0995 to 0.1 inch, D 2 =0.017 to 0.0175 inch, D 3 =0.125 inch, θ 1 =45°, and θ 2 =45°. 
       FIG. 24A  depicts a picture of a spiral coil support  580  supported by a holder  582 . Holder  582  is used to position support  580 , as shown by an arrow  586 , within a stent  584  to be coated.  FIG. 24B  depicts a close-up view of support  580  disposed within stent  584 . When holder  582  is coupled with a rotating spindle, it can rotate support  580  and stent  584  as shown by an arrow  588 . 
       FIG. 25  depicts an axial view of an assembly  590  for coating a stent  592  supported by spiral coil support  594 . The coiled portion of spiral coil support  594  supports stent  592  at contact points  596 . Straight portion  598  of spiral coil support  594  is coupled to a holder  600  like that shown in  FIG. 23A . Holder  600  can rotate as shown by an arrow  602  which rotates support  594  and stent  592 . A straight portion  604  can be coupled to a fixed or rotatable spindle or member  606 . Assembly  590  can also axially translate stent  592  relative to a nozzle spraying coating material onto stent  592 . 
       FIGS. 26A-D  depict scanning electron micrograph (SEM) images of 28 mm stents coated using an embodiment of a spiral coil support or spiral mandrel. The stent used in the examples are Xience V medium stent obtained from Guidant Corporation in Santa Clara, Calif. The pump rate of coating material during spraying of the stent was 3 ml/hr and the rotation rate of the stent was 100 rpm. The drying nozzle was set to 55° C. at an air pressure of 20 psi.  FIG. 26A  depicts an outside surface of a coated stent.  FIG. 26B  depicts an end ring of a coated stent.  FIG. 26C  depicts a close-up view of an outside surface of a coated stent.  FIG. 26D  depicts a close-up view of an inside surface of a coated stent. 
     Another aspect of the present invention relates to a device that allows reduced contact area between a support for a stent and the ends of a stent during coating of the stent.  FIG. 27  depicts a side view of a stent support assembly  700  including a mandrel  702  and support collets  704  for supporting a stent. Assembly  700  also includes collets  706  for securing a stent mounted over a mandrel in an axial direction. One or both of members  708  can be rotatable spindles for rotating mandrel  702  which rotates a stent mounted on mandrel  702 . At least one of members  708  can be rotationally and axially fixed. 
     As indicated above, it is generally desirable to minimize the contact area between an end of a stent and collets  706  to reduce or prevent coating defects. Additionally, minimizing such contact area tends to reduce undesirable wicking or flow of coating material from the stent to the collets. Contact points can create defects such as uncoated or insufficiently coated areas on a stent surface. Wicking can also result in uncoated or insufficiently coated areas. 
     It is desirable to minimize the interface between the end of a stent and the apparatus supporting the stent end during the coating process to minimize coating defects. Accordingly, the present invention provides for a device for supporting a stent during the coating application process. The invention also provides for a method of coating the stent supported by the device. 
     Various embodiments of the present invention include a collet that is shaped so as to minimize contact area of a stent end with a surface of the collet. Certain embodiments of the collet have an end surface for contacting a stent end that includes a raised or projecting portion and a flat or relatively flat portion. In one embodiment, the projecting portion may have at least three segments radiating from a center to the edge of the surface. The thickness of the segments in a plane of the surface may decrease from the center to the edge of the surface. Additionally, a height of the projecting portion perpendicular to the surface may decrease from the center of the surface to the edge. 
       FIGS. 28A-D  depict an exemplary embodiment of a collet  710  having a head section  712  and a body section  714 .  FIG. 28A  depicts a three-dimensional view of collet  710 . An end surface of head section  712  has flat portions  716  divided into sections by a projecting portion  718 . Projection  718  has a top surface  720  and a sidewall surface  722  with a hole  723 . A mandrel for supporting a stent may be inserted and disposed in hole  723 . 
     Body section  714  has slots  715  running parallel to an axis of body section  714 . Slots  715  are designed to allow collet  710  to engage and couple to a rotatable spindle or other support structure. In one embodiment, body section  714  has six slots  715  spaced 60° apart. In other embodiments, body section  714  has seven, eight, nine, or ten slots. 
       FIG. 28B  depicts a two-dimensional radial projection of the end surface of collet  710  which shows three segments  724  of projection  718  radiating from the center of the end surface of collet  710  to the edge of the end surface. A thickness  726  of segments  724  of projection  718  decrease from the center to the edge of the surface. Adjacent segments  724  may radiate in directions that are 120° apart. Other embodiments may include four, five, or six segments radiating from the center to the edge of the end surface spaced apart, for example, by 90°, 72°, or 60°, respectively. 
       FIGS. 28C-D  depict side views of collet  710 .  FIG. 28C  depicts a view that is parallel to a length of one of segments  724 . As shown in  FIG. 28C , segments  724  have a pitch, θ P , which facilitates self-centering of the stent on the surface of the collet. In one embodiment, θ P  can be 30°. In other embodiments, θ P  can be greater than 30°, greater than 45°, or greater than 60°.  FIG. 28D  depicts a view facing along a direction that is between two adjacent segments  724 . As shown in  FIGS. 28A ,  28 C, and  28 D, a height  728  of segments  724  decrease from the center to the edge of the surface. 
     In some embodiments, a radius of collet  710  may be sized so that stent ends are in contact with segments  724  near the edge of the surface of the end of collet  710 . Segments  724  are relatively thin near the edge of the surface resulting in a relatively small contact area of the stent end with the collet. Thus, wicking of coating material from the stent to collet  710  is relatively low or nonexistent. Also, defects due to contact of the stent with collet  710  tend to be reduced or eliminated. Additionally, the decrease in height outward from the center tends to allow the stent to center itself on the surface which results in a consistent contact area between the stent and collet  710 . 
     An example of collet  710  depicted in  FIGS. 28A-D  has the following dimensions: W S =0.10 inch, D H =0.022 inch, D B =0.099 inch, L B =0.22 inch, L C =0.29 inch, and L CP =0.32 inch. 
       FIGS. 29A-E  depict photographs of an embodiment of the collet according to the present invention.  FIGS. 29A-B  depict photographs with side views of the collet and  FIGS. 29C-D  depict photographs with overhead views of the collet.  FIG. 29E  depicts a photograph of a stent mounted on the collet. 
     Another aspect of the present invention relates to a method and device that reduces operator or machinery contact with a mandrel that supports a stent during processing. Stent handling or manipulation, whether manual or automated, risks exposing a stent to damage and/or contamination with undesired impurities. Embodiments of the present invention tend to reduce or eliminate damage to a stent and/or stent coating that may be caused by handling or manipulation of a stent during processing. Damage can result from contaminants or undesired contact with surfaces. Such embodiments also reduce or eliminate damage resulting from exposure to a stent to undesired contaminants. 
     As described above, coating a stent requires a number of processing steps which involve handling and manipulation of a stent. The process involving application of a coating, a drug-polymer coating, for example, may include loading or mounting a stent on a mandrel. A conventional mandrel for supporting a stent during processing is coupled mechanically through direct or indirect physical contact to fixtures and/or rotatable spindles. For example, mandrel  702  in  FIG. 27  is coupled mechanically through direct physical contact to members  708 . Such mechanical rotation devices typically require lubricants or other substances for their normal operation. During the course of processing a stent, such lubricants or substances can come into contact with a coated stent. 
     Furthermore, as discussed above, a coating is typically applied in stages with a drying step in between stages. The application step and drying step are repeated until a desired weight or loading of coating on the stent is achieved. After some or all of the drying steps, the stent may be weighed. When the application-drying stages are completed, the stent is typically dried in an oven to remove all or substantially all of the solvent remaining in the coating. 
     In conventional coating systems, the above-described procedure can require handling and manipulation of a mandrel holding the stent. For instance, after application of a coating layer, the mandrel may be moved to a drying station. The drying station, for example, may include a nozzle blowing a stream of heated gas on the coated stent. A stent may be transferred to an oven for a drying, which can be the final drying step. In addition, after one or more of the drying steps, a stent may be transferred to a weighing station, unloaded from a mandrel, and transferred to a scale for weighing. During each of these handling or manipulation procedures, the stent and/or stent coating can be damaged through contact with surfaces or exposure to undesired substances. 
     Embodiments of a system including a mandrel that can support a stent without physical or mechanical contact with other machines or devices during processing are described herein. In some embodiments, the mandrel can contact, hold, translate and/or rotate a stent. Limited operator or machinery contact with the mandrel reduces or eliminates damage or defects to a stent and/or stent coating that occur during processing. The mandrel of the system is supported, translated, and/or rotated through magnetic levitation. 
       FIG. 30  is an exemplary embodiment of a system  750  having a mandrel  752  that is supported by magnetic levitation. Mandrel  752  includes removable collets  754  and support collets  756 . A stent can be placed over mandrel  752  between removable collets  754  and supported directly on support collets  756 . A diameter of support collets  756  can be sized to be slightly smaller than an inside diameter of a stent to be coated. 
     Mandrel  752  includes permanent magnets  758  embedded within a proximal and a distal end of mandrel  752 . “Permanent magnets” refer to materials that possess a magnetic field which is not generated by outside influences such as electricity. In some embodiments, mandrel  752  can include magnets  758  in only one end of mandrel  752 . In other embodiments, magnets  758  can be embedded or disposed at other positions along mandrel  752 . 
     As shown in  FIG. 30 , mandrel  752  includes three magnets at each end. Generally, a mandrel  752  can include at least one magnet. In some embodiments, mandrel  752  can include more than four or more than five magnets. The size, number, and location of the magnets may be modified to obtain the desired movement of mandrel  752  as described below. 
     System  750  further includes electromagnetic coils  760 ,  761 , and  762  positioned adjacent to mandrel  752 . Coils  760 ,  761 , and  762  are electrically connected to an electromagnetic power supply  764  which provides power to coils  760 ,  761 , and  762  to generate an electrical field. Electromagnetic coils  760  and  761 , for example, generate an electrical field that allows the magnets  758  to support or levitate mandrel  752  without the physical contact with fixtures or members such as members  706  and  708  in  FIG. 27 . The absence of a mechanical coupling between mandrel  752  and a support member eliminates the need for lubricants or other substances that may be required and that can contaminate a stent or stent coating. 
     Power supply  764  further includes polarity switching equipment. The polarity switching equipment induces polarity changes in the electromagnetic field generated by coils  760 ,  761 , and  762  in a way that induces movement of magnets  758  to move mandrel  752  in a selected manner. For example, coils  760  and  762  can induce rotation of mandrel  752  as shown by an arrow  764  to rotate a stent during application of coating material to the stent. Also, coils  762  can induce translation of mandrel  752  as shown by an arrow  766 . 
     In additional embodiments, a coating system can use magnetic levitation to rotate or translate a stent during or between processing steps.  FIG. 31  depicts an overhead view of a coating system  800  illustrating stations corresponding to various processing steps. A coating station  802  is for application of a coating material, such as a polymer-solvent mixture, to a stent. A stent mounted on a mandrel, including magnets, may be rotated and translated under a spray nozzle using magnetic levitation, as described above. Electromagnetic coils for use in the magnetic levitation can be powered by a power supply (not shown). 
     After application of a coating layer at station  802 , the stent can be transferred to a drying station  804  that includes a nozzle that blows a stream of heated gas on the coated stent. The stent can be translated to and from station  802  and station  804  as shown by arrows  812  and  813  using magnetic levitation induced by coils  810  than run between the stations. 
     Additionally, the stent can be transferred to a drying station  808  for a final drying step. Drying station  808  may include an oven for drying the stent. The stent can be translated from station  802  to station  808  as shown by an arrow  814  using magnetic levitation induced by coils  816  than run between the stations. 
     A stent mounted on the mandrel can also be translated, as shown by an arrow  818 , using magnetic levitation induced by coils  820  from station  804  to a weighing station  806  after the coating on the stent is dried at station  804 . After weighing the stent at weigh station  806 , the stent can then be translated back to coating station  802 , as shown by an arrow  822 , using magnetic levitation induced by coils  824 . In addition, the stent can also be translated, as shown by an arrow  826 , using magnetic levitation induced by coils  828  from station  808  to weighing station  806  after the coating on the stent is dried at station  808 . 
     In other embodiments, a stent loaded on a magnetic mandrel can be translated to other processing stations or steps than those depicted in  FIG. 31 . 
     Purification of Coating Polymers 
     It is important to control the raw material purity of coating polymers for medical devices such as stents. A potential problem with polymers used in coating applications of stents is that such polymers can contain impurities that trigger adverse biological responses to the stent when implanted into a biological lumen. The polymers can contain impurities such as catalysts, initiators, processing aids, suspension aids, unreacted monomers, and oligomers, or other low molecular weight species, even though the polymer is sold as a “medical grade” polymer by the manufacturer. Thus, there tends to be a need to purify polymers used in coating applications. Various embodiments of the present invention relate to purifying such polymers. 
     It is desirable that after a polymer has been purified for the polymer to be substantially biologically inert. “Purified” refers to a polymer that has had impurities removed or significantly reduced. “Impurities” refer to traces of catalysts, initiators, processing aids, suspension aids, unreacted monomers, and oligomers, or other low molecular weight species, or any other chemical remaining in the polymer, that can cause or effectuate an adverse biological response greater than which would occur if the impurity is removed or significantly reduced. For example, “medical grade” poly(n-butyl methacrylate) (PBMA) can contain impurities such as suspension aids (e.g., starch) and unreacted monomers. 
     “Biologically inert” refers to a material that does not elicit a significantly greater adverse biological response than a biocompatible material, such as stainless steel, when implanted into a body vessel. Examples of biocompatible materials include metals such as stainless steel, titanium, and Nitinol, and organic materials such as collagen, fibronectin, polyethylene glycol, polysaccharides, TEFLON, silicone and polyurethane. 
     The coating for a stent including the purified polymer can have a drug-polymer layer, an optional topcoat layer, and an optional primer layer. The drug-polymer layer can be applied directly onto the stent surface to serve as a reservoir for a therapeutically active agent or drug which is incorporated into the drug-polymer layer. The topcoat layer, which can be essentially free from any therapeutic substances or drugs, serves as a rate limiting membrane for controlling the rate of release of the drug. The optional primer layer can be applied between the stent and the drug-polymer layer to improve the adhesion of the drug-polymer layer to the stent. 
     In one embodiment, poly(vinylidene fluoride-co-hexafluoropropene) copolymer (PVDF-HFP) (e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare, N.J.), can be used as a polymer for a drug-polymer layer or matrix. Polybutyl methacrylate (PBMA) can be used as the primer to improve adhesion between the metallic stent and the drug-polymer layer or matrix. 
     By using the methods described herein, the polymer for the drug-polymer layer can be purified to remove a significant amount of residual catalysts, initiators, processing aids, suspension aids, unreacted monomers, and oligomers or other low molecular weight species. Certain embodiments of a method of purifying a polymer mass may include washing the polymer mass with a solvent that dissolves an impurity, but not the polymer. The impurities, such as low molecular species including unreacted monomers and oligomers, should be miscible or substantially miscible in the solvent, while the polymer should be immiscible or substantially immiscible in the solvent. 
     In some embodiments, it may be advantageous for a solvent for use in purifying PVDF-HFP to have the following properties: (1) capable of swelling, but not dissolving PVDF-HFP; (2) capable of dissolving contaminants such as mineral oil; (3) the boiling temperature is low enough so that a solvent-washed polymer can be dried without using a convection oven; and (4) safety—Class III or Class II on the International Conference on Harmonization (ICH) Guidance list. 
     Representative examples of some solvents that may be used to purify PVDF-HFP include, but are not limited to, acetonitrile (ACN), isopropyl alcohol (IPA), methyl acetate (MA), ethyl acetate (EA), isopropyl acetate (ISPA), propyl acetate (PA), and mixtures of ethyl acetate and ethanol. The suitability of the above solvents for purifying PVDF-HFP can be evaluated by measuring the swelling of PVDF-HFP and the removal of contaminants from PVDF-HFP. The above-listed solvents were pre-screened by examining the physical parameters listed in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Physical parameters of the solvents evaluated for purification of PVDF-HFP. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 EA/EtOH* 
                 EA/EtOH 
                 EA/EtOH 
                   
                   
               
               
                   
                 ACN 
                 IPA 
                 MA 
                 EA 
                 (75/25) 
                 (50/50) 
                 (25/75) 
                 ISPA 
                 PA 
               
               
                   
               
               
                 ICH 
                 II 
                 III 
                 III 
                 III 
                 III 
                 III 
                 III 
                 III 
                 III 
               
               
                 b.p. 
                 82 
                 82 
                 57 
                 76 
                 NA 
                 NA 
                 NA 
                 87 
                 102 
               
               
                 (° C.) 
               
               
                 Effect 
                 Swell 
                 Swell 
                 Dissolve 
                 Dissolve 
                 Swell 
                 Swell 
                 swell 
                 swell 
                 swell 
               
               
                 PVDF 
                   
                   
                   
                 40% 
               
               
                 Mineral 
                 &lt;0.02% 
                 Not 
                 soluble 
                 soluble 
                 soluble 
                 Not 
                 Not 
                 soluble 
                 soluble 
               
               
                 oil 
                 soluble 
                 soluble 
                   
                   
                   
                 soluble 
                 soluble 
               
               
                   
               
               
                 *the boiling point for ethanol is 78° C. 
               
            
           
         
       
     
     Based on the pre-screening data listed in the above table, the following solvents were selected for further study of the swell parameters of PVDF-HFP. These include 1) ACN; 2) IPA; 3) EA; 4) ISPA 5) EA/EtOH (75/25). 
     The swelling of PVDF-HFP was determined as a function of time. Two methods were used to measure the PVDF-HFP swell ratio in the solvents: size measurement and weight measurement. Prior to contacting a sample of PVDF-HFP with a solvent, a sample pellet of PVDF-HFP was weighed and pictures were taken by a light microscope. The diameter of the pellet was measured from the digital image. This measured diameter is the data for time zero. 
     The sample pellet was then placed into a vial containing about 5 grams of a selected solvent and shaken on a shaker table at 480 rpm. At selected time intervals, the pellet was taken out for weight and size measurement. By comparing the weight and size of the same pellet at the selected time intervals, the swell parameters of PVDF-HFP in different solvents were calculated. 
       FIG. 32  shows the swell percentage of a polymer sample in each solvent tested obtained by measuring the size of the sample at selected times. The swell percentage was calculated by the following formula: (volume(t)−initial volume)/initial volume×100%. The polymer sample in ACN had the highest swell percentage or ratio. There was no swelling of the polymer sample in the IPA. The polymer sample in the other three solvents had swell ratios below that of ACN. The swell ratios of the sample for each of solvents appear to flatten out after about 24 hours. Other than acetonitrile, isopropyl acetate and propyl acetate resulted in larger polymer swell for samples than the mixture of ethanol and ethyl acetate. 
       FIG. 33  shows the swell percentage of polymer samples in each solvent obtained by measuring the weight at selected times. As above, the polymer samples had the largest swell ratio in acetonitrile and isopropyl alcohol did not cause any swelling in the polymer sample. As before isopropyl acetate and propyl acetate had a greater effect on polymer swelling than the mixture of ethanol and ethyl acetate. 
       FIGS. 32-33  show that the swell percentage obtained from the different methods of determining the swell percentage yielded different values of the swell percentage. The difference may be due to precision of polymer pellet diameter measurement and the precision of the polymer pellet weight measurement. 
     The removal of contaminants was studied by analyzing an extraction phase. The extraction phase refers to the mixture of the solvent and any impurities or contaminants removed or extracted from the swelled sample polymer. The extraction phase of acetonitrile, isopropyl acetate, and propyl acetate was analyzed. The extraction phase was blow-dried with a stream of heated gas. The residual remaining after the extraction phase was blow-dried included contaminants extracted from the polymer sample. The residual was analyzed by Gel Permeation Chromotography (GPC) analysis. Acetone was used to dissolve the residual impurities or contaminants for the GPC injection. A person of ordinary skill in the art is familiar with the principles of GPC analysis. 
     The acetonitrile extracted more residual than either isopropyl acetate or propyl acetate. Table 4 lists the results of the GPC analysis of the residual for each solvent. In Table 4, the “retention time” is the characteristic time of passage of a component through a GPC system. For the extracts from acetonitrile, a large peak retention time of 19 minutes overwrote other small insignificant peaks. For the extracted residual from isopropyl acetate and propyl acetate, the results showed a mixture of small molecular weight and large molecular weight material. The large molecular weight material corresponds to the molecular weight of PVDF-HFP, which is about 280K. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Results of GPC analysis of extraction phase. 
               
            
           
           
               
               
               
               
            
               
                   
                 Extraction Solvents 
                 Retention Time 
                 M w  of the residual 
               
               
                   
                   
               
               
                   
                 Acetonitrile 
                 19 min 
                 63K 
               
               
                   
                 Isopropyl Acetate 
                 21 min 
                 24K 
               
               
                   
                   
                 15 min 
                 288K  
               
               
                   
                 Propyl Acetate 
                 21 min 
                 19K 
               
               
                   
                   
                 15 min 
                 278K  
               
               
                   
                   
               
            
           
         
       
     
     Generally, it is desirable to use a solvent to purify PVDF-HVP that (1) swells and extracts a substantial amount of low molecular weight materials from the polymer and that (2) dissolves as little as possible of the polymer. The data indicates that isopropyl acetate has a favorable balance of swelling and extraction with dissolution of the polymer. 
     The polymer sample purified by the isopropyl acetate was analyzed after extraction to determine drying time. The polymer sample was dried in a convection oven at three different temperatures. The change in weight was determined using Thermogravimetric Analysis (TGA). Table 5 summarizes the results from TGA. The data in Table 3 indicate that drying at 85° C. or 90° C. for 24 hours will remove the solvent to an acceptable level. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Results of TGA analysis of sample purified by isopropyl acetate. 
               
            
           
           
               
               
               
               
            
               
                 Oven 
                   
                   
                   
               
               
                 Temperature 
                 Drying 
                 Drop of Weight 
               
               
                 (° C.) 
                 Time 
                 in TGA Test 
                 Comments 
               
               
                   
               
               
                 80 
                 24 hr 
                 1.17% 
                 Polymer looked normal after dry 
               
               
                 85 
                 24 hr 
                 0.83% 
                 Polymer looked normal after dry 
               
               
                 90 
                 24 hr 
                 0.56% 
                 Polymer looked normal after dry 
               
               
                   
               
            
           
         
       
     
     Stent and Coating Materials 
     A non-polymer substrate for a coating of an implantable medical device may be made of a metallic material or an alloy such as, but not limited to, cobalt chromium alloy (ELGILOY), stainless steel (316L), high nitrogen stainless steel, e.g., BIODUR 108, cobalt chrome alloy L-605, “MP35N,” “MP20N,” ELASTINITE (Nitinol), tantalum, nickel-titanium alloy, platinum-iridium alloy, gold, magnesium, or combinations thereof. “MP35N” and “MP20N” are trade names for alloys of cobalt, nickel, chromium and molybdenum available from Standard Press Steel Co., Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20% chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20% nickel, 20% chromium, and 10% molybdenum. 
     In accordance with one embodiment, the composition can include a solvent and a polymer dissolved in the solvent and optionally a wetting fluid. The composition can also include active agents, radiopaque elements, or radioactive isotopes. Representative examples of polymers that may be used as a substrate of a stent or coating for a stent, or more generally, implantable medical devices include, but are not limited to, poly(N-acetylglucosamine) (Chitin), Chitosan, poly(3-hydroxyvalerate), poly(lactide-co-glycolide), poly(3-hydroxybutyrate), poly(4-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolide), poly(L-lactic acid), poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide), poly(L-lactide-co-D,L-lactide), poly(caprolactone), poly(L-lactide-co-caprolactone), poly(D,L-lactide-co-caprolactone), poly(glycolide-co-caprolactone), poly(trimethylene carbonate), polyester amide, poly(glycolic acid-co-trimethylene carbonate), co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules (such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronic acid), polyurethanes, silicones, polyesters, polyolefins, polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymers and copolymers, vinyl halide polymers and copolymers (such as polyvinyl chloride), polyvinyl ethers (such as polyvinyl methyl ether), polyvinylidene halides (such as polyvinylidene chloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics (such as polystyrene), polyvinyl esters (such as polyvinyl acetate), acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon 66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides, polyethers, polyurethanes, rayon, rayon-triacetate, cellulose acetate, cellulose butyrate, cellulose acetate butyrate, cellophane, cellulose nitrate, cellulose propionate, cellulose ethers, and carboxymethyl cellulose. Additional representative examples of polymers that may be especially well suited for use in fabricating embodiments of implantable medical devices disclosed herein include ethylene vinyl alcohol copolymer (commonly known by the generic name EVOH or by the trade name EVAL), poly(butyl methacrylate), poly(vinylidene fluoride-co-hexafluoropropene) (e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare, N.J.), polyvinylidene fluoride (otherwise known as KYNAR, available from ATOFINA Chemicals, Philadelphia, Pa.), ethylene-vinyl acetate copolymers, poly(vinyl acetate), styrene-isobutylene-styrene triblock copolymers, and polyethylene glycol. 
     “Solvent” is defined as a liquid substance or composition that is compatible with the polymer and is capable of dissolving the polymer at the concentration desired in the composition. Examples of solvents include, but are not limited to, dimethylsulfoxide (DMSO), chloroform, acetone, water (buffered saline), xylene, methanol, ethanol, 1-propanol, tetrahydrofuran, 1-butanone, dimethylformamide, dimethylacetamide, cyclohexanone, ethyl acetate, methylethylketone, propylene glycol monomethylether, isopropanol, isopropanol admixed with water, N-methylpyrrolidinone, toluene, and combinations thereof. 
     A “wetting” of a fluid is measured by the fluid&#39;s capillary permeation. Capillary permeation is the movement of a fluid on a solid substrate driven by interfacial energetics. Capillary permeation is quantified by a contact angle, defined as an angle at the tangent of a droplet in a fluid phase that has taken an equilibrium shape on a solid surface. A low contact angle means a higher wetting liquid. A suitably high capillary permeation corresponds to a contact angle less than about 90°. Representative examples of the wetting fluid include, but are not limited to, tetrahydrofuran (THF), dimethylformamide (DMF), 1-butanol, n-butyl acetate, dimethylacetamide (DMAC), and mixtures and combinations thereof. 
     Examples of radiopaque elements include, but are not limited to, gold, tantalum, and platinum. An example of a radioactive isotope is p 32 . Sufficient amounts of such substances may be dispersed in the composition such that the substances are not present in the composition as agglomerates or flocs. 
     Active Agents 
     Examples of active agents include antiproliferative substances such as actinomycin D, or derivatives and analogs thereof (manufactured by Sigma-Aldrich 1001 West Saint Paul Avenue, Milwaukee, Wis. 53233; or COSMEGEN available from Merck). Synonyms of actinomycin D include dactinomycin, actinomycin IV, actinomycin I 1 , actinomycin X 1 , and actinomycin C 1 . The bioactive agent can also fall under the genus of antineoplastic, anti-inflammatory, antiplatelet, anticoagulant, antifibrin, antithrombin, antimitotic, antibiotic, antiallergic and antioxidant substances. Examples of such antineoplastics and/or antimitotics include paclitaxel, (e.g., TAXOL® by Bristol-Myers Squibb Co., Stamford, Conn.), docetaxel (e.g., Taxotere®, from Aventis S.A., Frankfurt, Germany), methotrexate, azathioprine, vincristine, vinblastine, fluorouracil, doxorubicin hydrochloride (e.g., Adriamycin® from Pharmacia &amp; Upjohn, Peapack N.J.), and mitomycin (e.g., Mutamycin® from Bristol-Myers Squibb Co., Stamford, Conn.). Examples of such antiplatelets, anticoagulants, antifibrin, and antithrombins include aspirin, sodium heparin, low molecular weight heparins, heparinoids, hirudin, argatroban, forskolin, vapiprost, prostacyclin and prostacyclin analogues, dextran, D-phe-pro-arg-chloromethylketone (synthetic antithrombin), dipyridamole, glycoprotein IIb/IIIa platelet membrane receptor antagonist antibody, recombinant hirudin, and thrombin inhibitors such as Angiomax ä (Biogen, Inc., Cambridge, Mass.). Examples of such cytostatic or antiproliferative agents include angiopeptin, angiotensin converting enzyme inhibitors such as captopril (e.g., Capoten® and Capozide® from Bristol-Myers Squibb Co., Stamford, Conn.), cilazapril or lisinopril (e.g., Prinivil® and Prinzide® from Merck &amp; Co., Inc., Whitehouse Station, N.J.), calcium channel blockers (such as nifedipine), colchicine, proteins, peptides, fibroblast growth factor (FGF) antagonists, fish oil (omega 3-fatty acid), histamine antagonists, lovastatin (an inhibitor of HMG-CoA reductase, a cholesterol lowering drug, brand name Mevacor® from Merck &amp; Co., Inc., Whitehouse Station, N.J.), monoclonal antibodies (such as those specific for Platelet-Derived Growth Factor (PDGF) receptors), nitroprusside, phosphodiesterase inhibitors, prostaglandin inhibitors, suramin, serotonin blockers, steroids, thioprotease inhibitors, triazolopyrimidine (a PDGF antagonist), and nitric oxide. An example of an antiallergic agent is permirolast potassium. Other therapeutic substances or agents which may be appropriate agents include cisplatin, insulin sensitizers, receptor tyrosine kinase inhibitors, carboplatin, alpha-interferon, genetically engineered epithelial cells, steroidal anti-inflammatory agents, non-steroidal anti-inflammatory agents, antivirals, anticancer drugs, anticoagulant agents, free radical scavengers, estradiol, antibiotics, nitric oxide donors, super oxide dismutases, super oxide dismutases mimics, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO), tacrolimus, dexamethasone, ABT-578, clobetasol, cytostatic agents, prodrugs thereof, co-drugs thereof, and a combination thereof. 
     Other therapeutic substances or agents may include rapamycin and structural derivatives or functional analogs thereof, such as 40-O-(2-hydroxy)ethyl-rapamycin (known by the trade name of EVEROLIMUS), 40-O-(3-hydroxy)propyl-rapamycin, 40-O-[2-(2-hydroxy)ethoxy]ethyl-rapamycin, methyl rapamycin, and 40-O-tetrazole-rapamycin. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.