Abstract:
A method of making a catheter balloon or other expandable medical device, and a balloon or other device formed thereby, in which at least a portion of a tubular, wrapped sheet of polymeric material is heated with laser radiation to form a fused seam extending along at least a section of the length of the tubular body. In one embodiment, the portion of the sheet heated by laser radiation is less than the entire area of the sheet, so that the fused seam is formed by heating portions of the sheet without heating sections of the sheet spaced apart from the fused seam. In one embodiment, the sheet of polymeric material comprises a polymer having a porous and preferably a node and fibril microstructure, which in one embodiment is selected from the group consisting of expanded polytetrafluoroethylene (ePTFE) and expanded ultra high molecular weight polyethylene.

Description:
BACKGROUND OF THE INVENTION 
   This invention generally relates to medical devices, and particularly to intracorporeal devices for therapeutic or diagnostic uses such as balloon catheters, and vascular grafts. 
   In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of a dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient&#39;s coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient&#39;s coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with fluid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not over expand the artery wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom. 
   In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant a stent inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion. Stent covers on an inner or an outer surface of the stent have been used in, for example, the treatment of pseudo-aneurysms and perforated arteries, and to prevent prolapse of plaque. Similarly, vascular grafts comprising cylindrical tubes made from tissue or synthetic materials such as polyester, expanded polytetrafluoroethylene, and DACRON may be implanted in vessels to strengthen or repair the vessel, or used in an anastomosis procedure to connect vessels segments together. 
   In the design of catheter balloons, characteristics such as strength, compliance, and profile of the balloon are carefully tailored depending on the desired use of the balloon catheter, and the balloon material and manufacturing procedure are chosen to provide the desired balloon characteristics. A variety of polymeric materials are conventionally used in catheter balloons. Use of polymeric materials such as PET that do not stretch appreciably consequently necessitates that the balloon is formed by blow molding, and the deflated balloon material is folded around the catheter shaft in the form of wings, prior to inflation in the patient&#39;s body lumen. However, it can be desirable to employ balloons, referred to as formed-in-place balloons, that are not folded prior to inflation, but which are instead expanded to the working diameter within the patient&#39;s body lumen from a generally cylindrical or tubular shape (i.e., essentially no wings) that conforms to the catheter shaft. 
   Catheter balloons formed of expanded polytetrafluoroethylene (ePTFE) expanded in place within the patient&#39;s body lumen without blow molding the ePTFE tubing have been disclosed. Prior methods of forming the ePTFE balloon involved wrapping a sheet of ePTFE on a mandrel and then heating the wrapped sheet in an oven to fuse the layers of wrapped material together. Heating the wrapped sheet in an oven will heat the entire sheet of ePTFE. One difficulty has been further processing of the tube by stretching the tube, after the layers of wrapped material are fused together. 
   It would be a significant advance to provide a catheter balloon with improved performance characteristics and ease of manufacture. 
   SUMMARY OF THE INVENTION 
   This invention is directed to a method of making a catheter balloon or other expandable medical device, and the balloon or other device formed thereby, in which a sheet of polymeric material is wrapped to form a tubular body, and at least a portion of the tubular body is heated with a localized heat source such as laser radiation to form a fused seam extending along at least a section of the length of the tubular body. In a presently preferred embodiment, the portion of the sheet heated by the localized heat is less than the entire area of the sheet, so that the fused seam is formed by heating portions of the sheet at the desired location of the fused seam without heating sections of the sheet spaced apart from the fused seam. 
   In a presently preferred embodiment, the expandable medical device is a balloon for a catheter. A balloon formed according to the method of the invention can be used on a variety of suitable balloon catheters including coronary and peripheral dilatation catheters, stent delivery catheters, drug delivery catheters, and the like. Although discussed below primarily in terms of the embodiment in which the medical device is a balloon for a catheter, it should be understood that other expandable medical devices are included within the scope of the invention including stent covers and vascular grafts. 
   In a presently preferred embodiment, the sheet of polymeric material comprises a polymer having a porous structure, which in one embodiment is selected from the group consisting of expanded polytetrafluoroethylene (ePTFE), ultra high molecular weight polyolefin, polyethylene, and polypropylene. In one embodiment, the porous material has a node and fibril microstructure. ePTFE and expanded ultra high molecular weight polyethylene typically have a node and fibril microstructure, and are not melt extrudable into tubular form. The node and fibril micro structure is produced in the material using conventional methods in which the material is heated, compacted, and stretched, as described in greater detail below. However, a variety of suitable polymeric materials can be used in the method of the invention including conventional catheter balloon materials which are melt extrudable. In one presently preferred embodiment, the polymeric material is not melt extrudable and is thus formed into a balloon by bonding wrapped layers of the polymeric material together. 
   In the method of the invention, the laser radiation is only applied at the desired location of the fused seam, so that the entire sheet of polymeric material does not have to be heated to fuse the layers of material together. Additionally, the laser radiation is applied at a specific power level to the selected area of the tubular body, to precisely heat the polymeric material to a desired temperature. By controlling the location and the power of the laser radiation, the method of the invention limits and controls the sintering of the polymeric material which would otherwise occur in materials such as ePTFE due to heating. Sintering ePTFE results in a change in crystal structure when the ePTFE is heated at or above its melting point of about 344° C. and allowed to cool, as described in McCluken, M., et al., Physical Properties and Test Methods for Expanded Polytetrafluoroethylene (PTFE) Grafts, Special Technical Publication 898, American Society for Testing and Materials (ASTM), pp. 82-84, 1987, incorporated by reference herein in its entirety. Specifically, after melting, the ePTFE recrystalizes to a different crystal structure having a lower melting point and lower strength. The sintered ePTFE has a higher stiffness. During formation of the fused seam in the method of the invention, only the sections of the ePTFE which form the fused seam are heated, and the other sections of the ePTFE tube are not heated and are therefore not sintered. As a result, subsequent processing such as for example stretching of the ePTFE tube is facilitated, because the stiffer sintered form of the ePTFE is only at the fused seam of the ePTFE tube, and a strong highly sintered fused seam is formed which holds together during such subsequent stretching of the ePTFE tube. In one embodiment, the tube of ePTFE is stretched to a greater degree after formation of the fused seam than would be possible if the entire tube was 100% sintered. As a result, the balloon has a decreased wall thickness, for example, of less than about 0.001 inch single wall thickness, and therefore an improved lower profile. 
   In the embodiment in which the polymeric material of the balloon has a node and fibril microstructure, after the fused seam is formed, the tube of polymeric material is typically stretched, sintered, compacted, and sintered a final time, to form the balloon. As discussed above, heating the desired area of the fused seam with laser radiation, sinters the polymeric material at the site of the fused seam. Thus, an intermediate product in the formation of the catheter balloon of the invention is a tube of polymeric material (e.g., ePTFE) having a fused seam, in which the polymeric material forming the fused seam is more highly sintered than the polymeric material forming sections of the sheet adjacent to the fused seam. During subsequent processing of the intermediate polymeric tube to form the balloon by heating and thereby sintering the polymeric material, the entire area of the tube is typically heated. In one embodiment, about 100% of the polymeric material of the finished balloon is sintered, as defined in the McClurken, M., et al. ASTM publication, incorporated by reference herein above. However, in alternative embodiments, the percent sintering is less than 100%, and preferably about 80% to less than 100%, and more specifically about 90% to less than 100%. As a result, in one embodiment, the polymeric material forming the fused seam is fully sintered and the polymeric material forming sections of the sheet adjacent to the fused seam is not fully sintered, i.e., partially unsintered or semi-sintered, so that the balloon comprises a wrapped sheet of polymeric material (e.g., ePTFE) having a fused seam, in which the polymeric material forming the fused seam is more highly sintered than the polymeric material forming sections of the sheet adjacent to the fused seam. The method of the invention provides for selective heating of the wrapped polymeric material due to the use of a laser or other localized heat source to heat the material. As a result, during formation of the fused seam, the sintering of the polymeric material which would otherwise occur in materials such as ePTFE is limited to the location of the fused seam. Moreover, the method provides for precise control over the temperature of the polymeric material during heat fusing to form the fused seam, and consequently over the sintering of the polymeric material at the fused seam. Additionally, heating the polymeric material with a laser to form the fused seam provides an improved reduced manufacturing time, and ease of manufacturing. These and other advantages of the invention will become more apparent from the following detailed description and accompanying exemplary FIGURES. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevational view, partially in section, of a stent delivery balloon catheter embodying features of the invention. 
       FIG. 2  is a transverse cross sectional view of the balloon catheter shown in  FIG. 1 , taken along line  2 — 2 . 
       FIG. 3  illustrates the formation of a layer of the balloon of  FIG. 1 , in which the sheet of polymeric material is spirally wrapped around a mandrel and fused during wrapping. 
       FIG. 4  is a partially in section view of the balloon shown in  FIG. 3 , taken along line  4 — 4 , in which the sections of polymeric material about one another. 
       FIG. 5  is a partially in section view of an alternative embodiment of the balloon shown in  FIG. 3 , in which the sections of wrapped polymeric material overlap one another. 
       FIG. 6  illustrates an alternative embodiment of the formation of a layer of the balloon of  FIG. 1 , in which the sheet of polymeric material is wrapped around the mandrel by folding the sheet radially around the mandrel. 
       FIG. 7  is a transverse cross sectional view of the sheet of polymeric material wrapped around the mandrel shown in  FIG. 6 , taken along line  7 — 7 , in which the sections of polymeric material abut one another. 
       FIG. 8  is a transverse cross sectional view of an alternative embodiment of the sheet of polymeric material wrapped around the mandrel shown in  FIG. 6 , in which the sections of wrapped polymeric material overlap one another. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates an over-the-wire type stent delivery balloon catheter  10  embodying features of the invention. Catheter  10  generally comprises an elongated catheter shaft  12  having an outer tubular member  14  and an inner tubular member  16 . Inner tubular member  16  defines a guidewire lumen  18  configured to slidingly receive a guidewire  20 , as best illustrated in  FIG. 2  illustrating a transverse cross section view of the distal end of the catheter shown in  FIG. 1 , taken along line  2 — 2 . The coaxial relationship between outer tubular member  14  and inner tubular member  16  defines annular inflation lumen  22 . An inflatable balloon  24  disposed on a distal section of catheter shaft  12  has a proximal skirt section  25  sealingly secured to the distal end of outer tubular member  14  and a distal skirt section  26  sealingly secured to the distal end of inner tubular member  16 , so that its interior is in fluid communication with inflation lumen  22 . An adapter  30  at the proximal end of catheter shaft  12  is configured to provide access to guidewire lumen  18 , and to direct inflation fluid through arm  31  into inflation lumen  22 . In the embodiment illustrated in  FIG. 1 , an expandable stent  32  is mounted on uninflated balloon  24 , with an expandable stent cover  35  on the stent  32 . In the embodiment illustrated in  FIG. 1 , the uninflated balloon  24  has a wingless, low profile configuration prior to inflation. The distal end of catheter may be advanced to a desired region of a patient&#39;s body lumen  27  in a conventional manner and balloon  24  may be inflated to expand stent  32 , seating the stent  32  in the body lumen  27 . 
   In the embodiment illustrated in  FIG. 1 , balloon  24  has a first layer  33  and a second layer  34 . In a presently preferred embodiment, the balloon  24  has at least one layer comprising a microporous polymeric material, and preferably a microporous polymeric material having a node and fibril microstructure, such as ePTFE. In the embodiment illustrated in  FIG. 1 , first layer  33  is formed of ePTFE, and the second layer  34  is formed of a polymeric material preferably different from the polymeric material of the first layer  33 . Although discussed below in terms of one embodiment in which the first layer  33  is formed of ePTFE, it should be understood that the first layer may comprise other materials including ultra high molecular weight polyethylene. The second layer  34  is preferably formed of an elastomeric material, including polyurethane elastomers, silicone rubbers, styrene-butadiene-styrene block copolymers, polyamide block copolymers, and the like. In a preferred embodiment, layer  34  is on the interior of balloon  24 , although in other embodiments it may be on the exterior of the balloon  24 . Layer  34  formed of an elastomeric material limits or prevents leakage of inflation fluid through the microporous ePTFE to allow for inflation of the balloon  24 , and expands elastically to facilitate deflation of the balloon  24  to a low profile deflated configuration. The elastomeric material forming layer  34  may consist of a separate layer which neither fills the pores nor disturbs the node and fibril structure of the ePTFE layer  33 , or it may at least partially fill the pores of the ePTFE layer. 
   The ePTFE layer  33  is formed according to a method which embodies features of the invention, in which a sheet of polymeric material is wrapped to form a tubular body and then heated to fuse the wrapped material together. In accordance with a method of the invention, the wrapped material is fused by heating at least a portion of the polymeric material with laser radiation to form a fused seam extending along at least a section of the length of the tubular body.  FIG. 3  illustrates the formation of the ePTFE layer  33  of the balloon  24  of FIG.  1 . In the embodiment of  FIG. 3 , a sheet  40  of polymeric material is spirally wrapped around a mandrel  41  to form a tubular body  42 . A laser  43  emitting laser radiation  44  and the polymeric tubular body  42  are moved relative to one another, so that the laser radiation is applied to the spiral junction between sections of the wrapped sheet  40  to form fused seam  45 . Although laser radiation  44  is illustrated at a perpendicular angle to the sheet  40 , in one embodiment it may be tangential to the sheet  40 , and particularly for the embodiment in which multiple layers of polymeric material are wrapped on the mandrel  41  as discussed below, to minimize the penetration of the laser heat into layers of material beneath the layer of material being heat fused. The laser radiation  44  is applied to the spiral junction along the length of the polymeric tubular body and around the circumference thereof to fuse the sections of the wrapped sheet  40  together. In the embodiment of  FIG. 3 , the laser follows the winding pattern of the sheet  40  as it is wrapped onto the mandrel, so that the laser radiation is applied during the wrapping of the sheet  40 . For example, with the mandrel  41  rotating to wrap the sheet  40  onto the mandrel  41 , the laser  43  is moved along the length of the wrapped polymeric material. In an alternative embodiment (not shown), laser radiation is applied to the wrapped polymeric material as a separate processing step after the wrapping of the sheet  40  onto the mandrel is completed. 
   In the embodiment of  FIG. 1 , the sheet  40  is a long strip of polymeric material having longitudinal edges along the length of the strip which are longer than the width of the sheet  40 . The sheet  40  is wrapped on the mandrel  41  so that the longitudinal edges of the sheet  40  are brought together in an abutting or overlapping relation. In the embodiment of  FIG. 3 , the fused seam  45  is formed by spirally extending edges of the wrapped sheet  40  which abut one another, as best illustrated in  FIG. 4 , showing a partial sectional view of the assembly of  FIG. 3 , taken along line  4 — 4 . The laser radiation heats the abutting edges to form the fused seam  45 , so that the fused seam  45  joins the abutting edges together. The abutting edges are easily held together in position during application of the laser radiation, so that the method provides improved ease of manufacture of an accurate fused seam.  FIG. 5  illustrates an alternative embodiment in which the extending edge section of the wrapped sheet  40  overlaps the adjacent section of the wrapped sheet  40  so that the longitudinally adjacent section of the wrapped sheet has overlapping portions. The laser radiation heats the overlapping  20  portions to form the fused seam  45 , so that the fused seam  45  joins the overlapping portions. 
     FIG. 6  illustrates an alternative embodiment in which the sheet  40  is wrapped around mandrel  41  by folding the sheet around the circumference of the mandrel so that the longitudinal edges of the sheet  40  extend in a substantially straight line along the length of the mandrel  41 .  FIG. 7  illustrates a transverse cross section of the assembly of  FIG. 6 , taken along line  7 — 7 , showing the abutting longitudinal edges of the sheet  40 .  FIG. 8  illustrates an alternative embodiment in which the extending edge section of the wrapped sheet  40  overlaps the adjacent edge section of the wrapped sheet  40 . 
   The sheet  40  of polymeric material is preferably wrapped along a length of the mandrel to form a single layer of wrapped material. Alternatively, multiple layers of polymeric material are wrapped on the mandrel, by for example, wrapping the sheet  40  down the length of the mandrel  41  to form a first layer and then back again over the first layer one or more times to form additional layers. In the embodiment having multiple layers of material, the laser radiation  44  is preferably applied as the sheet  40  is being wrapped on the mandrel. The multiple layers of material may be different materials with different heat fusing temperatures, in which case the laser radiation is preferably applied to each layer in turn at a different setting to produce the different heat fusion temperature for that specific material. 
   The sheet  40  is preferably a polymeric material having a microporous structure, which in one embodiment has a node and fibril structure, such as ePTFE. Thus, the sheet  40  has preferably been stretched to form the desired microstructure (e.g., porous and/or node and fibril) before being wrapped on the mandrel  41 . In a presently preferred embodiment, the sheet  40  of ePTFE is semi-sintered before wrapping. The sheet  40  typically has a percent sintering of about 0% to about 80%, preferably about 20% to about 50%, of the polymeric material of the sheet  40 , as defined in the McClurken, M., et al. ASTM publication, incorporated by reference above, before wrapping. 
   The laser radiation is applied to the wrapped material at a specific power and for a specific duration to control temperature of the heated portion of the polymeric material. The power level of the laser depends on variables such as the type and angle of the laser. The ePTFE polymeric material is heated by the laser radiation to a temperature of about 330° C. to about 380° C., which is above the crystalline melting temperature of the ePTFE. The heat spread during the heating of the ePTFE material is limited, so that the portion of the sheet  40  which is heated to thereby form the fused seam has a width of about 0.1 mm to about 1.0 mm, and preferably about 0.1 mm to about 0.5 mm. The heated ePTFE forming the fused seam has a different crystal structure than the adjacent sections of the ePTFE which were not heated during formation of the fused seam and which consequently are not completely sintered. 
   After the fused seam  45  is formed, the tubular body is typically further processed prior to being bonded to the layer  34  to form the balloon  24 . Preferably, the tubular body is further processed by being stretched, sintered, compacted, and then sintered again, to provide the desired properties such as the desired dimension, and dimensional stability (i.e., to minimize axial shortening occurring during inflation of the balloon). For example, in one embodiment, the tubular body is longitudinally stretched to thereby increase the length of the tubular body by about 50% to about 200%. The controlled, localized delivery of heat to form the fused seam  45  facilitates the subsequent stretching of the tubular body. Although the heating of the ePTFE to form the fused seam results in a recrystallization of the ePTFE at the fused seam, the adjacent sections of the ePTFE tubular body are not sintered/recrystallized, and consequently are easier to stretch than the portions of the tubular body forming the fused seam. In one embodiment, the tensile strength of the tubular body after formation of the fused seam is about 2,000 psi to about 20,000 psi. Changes to other characteristics of the polymeric material, such as the porosity, melting point, strength and flexibility of the material are localized at the fused seam  45  during the fusing of the wrapped material together in the method of the invention. After the longitudinal stretching, the tubular body is preferably compacted and heated to further sinter the material, to provide the desired performance characteristics for balloon  24 . In one embodiment, after the longitudinal stretching, the tubular body is heated to completely sinter the material, so that the percent of the polymeric material of the ePTFE layer  33  which is sintered is about 100%. The tubular body is typically heated in an oven at about 360° C. to about 380° C., or to at least the melting point of the ePTFE. In another embodiment, after the longitudinal stretching, the tubular body is incompletely sintered, so that the percent of the polymeric material of the ePTFE layer  33  which is sintered is about 80% or greater, or more specifically about 90% or greater, but less than 100%. 
   The completed ePTFE layer  33  is then combined with or bonded to the elastomeric liner  34  to complete the balloon  24 , and the balloon  24  is secured to the catheter shaft  12 . 
   The dimensions of catheter  10  are determined largely by the size of the balloon and guidewires to be employed, catheter type, and the size of the artery or other body lumen through which the catheter must pass or the size of the stent being delivered. Typically, the outer tubular member  14  has an outer diameter of about 0.025 to about 0.04 inch (0.064 to 0.10 cm), usually about 0.037 inch (0.094 cm), the wall thickness of the outer tubular member  14  can vary from about 0.002 to about 0.008 inch (0.0051 to 0.02 cm), typically about 0.003 to 0.005 inch (0.0076 to 0.013 cm). The inner tubular member  16  typically has an inner diameter of about 0.01 to about 0.018 inch (0.025 to 0.046 cm), usually about 0.016 inch (0.04 cm), and wall thickness of 0.004 to 0.008 inch (0.01 to 0.02 cm). The overall length of the catheter  10  may range from about 100 to about 150 cm, and is typically about 143 cm. Preferably, balloon  24  may have a length about 0.5 cm to about 6 cm, and an inflated working diameter of about 2 to about 10 mm. 
   Inner tubular member  16  and outer tubular member  14  can be formed by conventional techniques, for example by extruding and necking materials already found useful in intravascular catheters such a polyethylene, polyvinyl chloride, polyesters, polyamides, polyimides, polyurethanes, and composite materials. The various components may be joined using conventional bonding methods such as by fusion bonding or use of adhesives. Although the shaft is illustrated as having an inner and outer tubular member, a variety of suitable shaft configurations may be used including a dual lumen extruded shaft having a side-by-side lumens extruded therein. Similarly, although the embodiment illustrated in  FIG. 1  is an over-the-wire stent delivery catheter, balloons of this invention may also be used with other types of intravascular catheters, such as rapid exchange dilatation catheters. Rapid exchange catheters generally comprise a distal guidewire port in a distal end of the catheter, a proximal guidewire port in a distal shaft section distal of the proximal end of the shaft and typically spaced a substantial distance from the proximal end of the catheter, and a short guidewire lumen extending between the proximal and distal guidewire ports in the distal section of the catheter. 
   While the present invention is described herein in terms of certain preferred embodiments, those skilled in the art will recognize that various modifications and improvements may be made to the invention without departing from the scope thereof. Moreover, although individual features of one embodiment of the invention may be discussed herein or shown in the drawings of the one embodiment and not in other embodiments, it should be apparent that individual features of one embodiment may be combined with one or more features of another embodiment or features from a plurality of embodiments.