Abstract:
A fluid bearing assembly provides support to stent tubing while the stent tubing is undergoing laser cutting to form a stent pattern. The fluid bearing assembly supports the stent tubing and provides a fluid barrier between the bearing and the stent tubing thereby providing nearly frictionless movement between the support bearing and the stent tubing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates generally to improvements in the manufacture of expandable stents and, more particularly, to new and improved methods and apparatus for direct laser cutting of stents in providing stents of enhanced structural quality. 
         [0002]    Stents are expandable endoprosthesis devices which are adapted to be implanted into a patient&#39;s body lumen, such as a blood vessel or coronary artery, to maintain the patency of the artery. These devices are typically used in the treatment of atherosclerotic stenosis in blood vessels, coronary arteries, and the like. 
         [0003]    In the medical arts, stents are generally tubular-shaped devices which function to hold open a segment of a blood vessel or other anatomical lumen. Stents are very high precision, relatively fragile devices and, ideally, the most desirable stents incorporate a very fine precision structure cut from a very small diameter, thin-walled cylindrical tube. In this regard, it is extremely important to make precisely dimensioned, smooth, narrow cuts in the thin-walled tubing in extremely fine geometries without damaging the narrow struts that make up the stent structure. Prior art stents typically are cut by a laser and held by collet in a computer controlled machine that translates and rotates the stent as the laser cuts through the outer surface of the metal tubing. In order to stabilize the stent tubing, typically a bushing surrounds the stent tubing and is positioned between the laser and the collet holding the stent. Prior art bearings or bushings create a small amount of friction between the stent tubing and the bearing which can cause slight imperfections in the laser cutting process as the stent tubing is moved relative to the bearing. 
         [0004]    Accordingly, the manufacturers of stents have long recognized the need for improved manufacturing processes and to reduce the amount of friction between the bearing and the stent tubing during the laser cutting process. The present invention fulfills these needs. 
       SUMMARY OF THE INVENTION 
       [0005]    In general terms, the present invention provides a new and improved method and apparatus for direct laser cutting stents by enabling greater precision, reliability, structural integrity and overall quality. 
         [0006]    The present invention provides an improved system for producing stents with a fine precision structure cut from a small diameter, thin-walled, cylindrical tube. The tubes are typically made of stainless steel, other biocompatible materials, or biodegradable materials, and are fixtured under a laser and positioned utilizing a CNC machine to generate a very intricate and precise pattern. Due to the thin wall and the small geometry of the stent pattern, it is necessary to have very precise control of the laser, its power level, the focus spot size, and, importantly, the precise positioning of the laser cutting path. 
         [0007]    In keeping with the invention, a stent tubing is held in a collet in a CNC machine so that the stent tubing is able to rotate and translate relative to a fixed laser beam. In order to support the stent tubing, a bearing or bushing supports the stent tubing just proximal to the laser beam (between the collet and the laser beam). In this manner, the stent tubing is prevented from sagging or deflecting away from the laser beam, which would otherwise create inaccuracies in the cut stent pattern. In the present invention, a fluid bearing includes a housing having a gas inlet port and a fluid inlet port on its outer surface. A bearing is positioned within the housing and the bearing has multiple blades that are aligned with the gas inlet port on the housing. The bearing is free to rotate within the housing without touching the housing. The gas inlet port is positioned to inject a high pressure gas on the blades in order to impart a high speed rotation of the bearing within the housing. The fluid inlet port on the housing is positioned to inject fluid onto an inner surface of the bearing so that as the bearing rotates at high speed, a thin film of fluid adheres to the inner surface of the bearing. The stent tubing is inserted through an inner diameter of the bearing so that the bearing supports the stent tubing just proximal of the laser beam. As the bearing rotates at high speed, on the order of about 1,000 to 10,000 rpm (or higher), the film of fluid adheres to the inner surface of the bearing so that the film of fluid is between the inner surface of the bearing and the outer surface of the stent tubing. Since the bearing is rotating at high speed and a film of fluid is formed between the inner surface of the bearing and the outer surface of the stent tubing, the stent tubing is centered within the bearing thereby creating a near frictionless environment as the stent tubing translates and rotates relative to the laser beam. The fluid film has very low friction and will not place a significant resistive load on the stent tubing as the collet or CNC system rotates and translates the stent tubing relative to the laser beam. 
         [0008]    The advantages of the present invention will be apparent from the following more detailed description when taken in conjunction with the accompanying drawings of exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a plan view depicting a prior art laser cutting assembly for cutting a pattern in stent tubing. 
           [0010]      FIG. 2  is a perspective view of a prior art bushing used to support stent tubing during a laser cutting operation. 
           [0011]      FIG. 3  is a partial elevational view of a prior art laser cutting assembly in which a bushing receives the stent tubing for support during a laser cutting process. 
           [0012]      FIG. 4  is a plan view of a laser cutting assembly in which a fluid bearing is used to support stent tubing during a laser cutting process. 
           [0013]      FIG. 5  is a perspective view depicting a fluid bearing assembly for use in supporting stent tubing during a laser cutting process. 
           [0014]      FIG. 6  is a partial cross-sectional view depicting a fluid bearing for supporting stent tubing during a laser cutting process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Referring to  FIGS. 1-3 , a typical prior art laser assembly is shown in which a laser beam is used to cut a pattern in stent tubing. The stent tubing is mounted in the collet of a CNC controller which will move the stent tubing in a translational and rotational direction while the laser beam cuts through one wall of the stent tubing to form a pattern. As shown, a bushing is used to support the stent tubing between the collet and the laser beam (or proximal to the laser beam). The prior art bushings typically support the stent tubing, however, because the inner diameter of the support bushing is closely matched to the outer diameter of the stent tubing, there is some amount of drag or friction between the bushing and the stent tubing. The control system must supply sufficient force to overcome the inertia of the tubing and the drag caused by the interface between the bushing and the stent tubing, and at the same time accurately position the stent tubing for laser cutting. It is therefore a goal to reduce cutting errors due to sticking and choppiness in the movement of the stent tubing and to improve yields. 
         [0016]    In keeping with the present invention, as shown in  FIGS. 4-6 , a laser cutting assembly  10  includes a CNC controller  12  and a laser beam assembly  14 . The laser beam assembly is well known in the art and includes numerous components such as a focusing lens, coaxial gas jet, and the laser beam itself. The laser cutting assembly  10  also includes a collet  16 , which is well known in the art, and is used for the purpose of holding a stent tubing  18  and moving the stent tubing in a translational and rotational direction. The stent tubing  18  is mounted in the collet  16  and the stent tubing extends away from the collet so that it is positioned directly under the laser beam assembly  14 . Typically, the laser beam assembly, and the laser beam itself, remain stationary during the stent cutting process, while the stent tubing translates and rotates while the laser beam removes material from the tubing. 
         [0017]    In further keeping with the invention, a fluid bearing  20  is provided to support the stent tubing  18 . More specifically, the fluid bearing includes a housing  22  that will be anchored at one end so that the housing is stationary and firmly supports the stent tubing. The housing has a gas inlet port  24  and a fluid inlet port  26  on its outer surface  28 . Depending upon the specific requirements, more than one gas inlet port  24  and fluid inlet port  26  can be provided and spaced along the outer surface  28  of the housing  22 . Typically, the multiple gas inlet ports or fluid inlet ports would extend in alignment circumferentially around the outer surface  28  of the housing  22 . A bearing  30  is contained within the housing  22  so that the bearing can rotate at high speed within the housing without hitting or touching the walls of the housing. The bearing  30  has one or more blades  32  that are positioned in grooves  33  in the housing  22  which align with the gas inlet port  24 . When high pressure gas is injected into gas inlet port  24 , the gas will impinge on the blades  32  thereby causing the bearing  30  to rotate at high speeds. For example, it is contemplated that the bearing  30  will rotate at speeds between 1,000 rpm up to 10,000 rpm. In another embodiment, it may be appropriate for the bearing  30  to rotate at speeds between 10,000 rpm up to 100,000 rpm. 
         [0018]    With reference in particular to  FIG. 6 , the bearing  30  has one or more fluid channels  34  that will allow fluid to pass from the fluid inlet port  26  into a cavity  36  and through the fluid channels  34  onto the inner surface  38  of the bearing  30 . 
         [0019]    In use, the present invention provides a low friction fluid film between the bearing and the stent tubing so that the amount of friction between the bearing and the stent tubing is substantially reduced from the prior art devices. Again, referring to  FIGS. 4-6 , a fluid is injected through fluid inlet port  26  on the housing  22  and a high pressure gas is injected through gas inlet port  24 , also on housing  22 . The high pressure gas impinges on the blades  32  which cause the bearing  30  to rotate at a high speed as previously disclosed. As the bearing  30  rotates at high speed, the fluid is forced from the fluid inlet port into cavity  36  where it can flow through fluid channels  34  in the bearing  30  and onto the inner surface of the bearing  38 . As the bearing  30  rotates at high speeds, the shear between the bearing and the fluid will cause a film of fluid to adhere to the bearing inner surface  38  and this film will separate the outer surface  40  of the stent tubing  18  from the bearing  30 . The fluid film has a very low coefficient of friction, and accordingly will not place a significant resistive load on the stent tubing as the collet  16  attempts to rotate and translate the stent tubing relative to the laser beam. Further, the high rotational speeds of the bearing  30 , in conjunction with the film fluid that adheres to the inner surface  38  of the bearing, act to center the stent tubing  18  relative to the bearing  30 . This further allows the laser beam to precisely cut the stent pattern so that a more accurate stent pattern can be reproducably manufactured. 
         [0020]    In one embodiment, the space between the blades  32  and the grooves  33  in the housing  22  may be sufficient to allow the high pressure gas to be directed toward the stent tubing. This serves several purposes including allowing the gas to exhaust from the bearing  30 , thereby allowing more gas to be injected to drive the bearing rotation. Further, as the gas exhausts, it may exert a pressure in the direction opposite to the flow of fluid along the stent tubing thereby forcing fluid out of the space between the bearing  30  and the stent tubing  18  in only one direction. This will prevent contamination of the region opposite to the exit location, namely where the laser is cutting the pattern in the stent tubing. 
         [0021]    In an alternative embodiment, the blades  32  may be either flat fins or have a tilted configuration such as the blades found in a turbine (not shown). In either configuration, the grooves  33  that receive the blades  32  will be configured to accommodate the blades as the bearing  30  rotates. Further, the blades also can have a curved configuration and still provide the rotational forces on the bearing as described. In one embodiment, the blades have a rectangular shape and are substantially flat fins. 
         [0022]    The fluid used with the present invention can be water, saline or any thin oil such as a mineral oil. Further, the high pressure gas typically will be air. 
         [0023]    The housing  22  can be formed from any rigid material such as stainless steel, while the bearing  30  is formed from a low friction material such as a polymer, including such polymers such as PTFE. 
         [0024]    It will be apparent from the foregoing that the present invention provides a new and improved method and apparatus for laser cutting stents thereby enabling greater precision, reliability and overall quality in forming precise stent patterns in stent tubing. Other modifications and improvements may be made without departing from the scope of the invention. Accordingly, it is not intended that the invention be limited, except by the appended claims.