Abstract:
A removable stent for implantation into a lumen in a human body. The stent is made from a soft, flexible fiber having an outer surface. An outer bioabsorbable/degradable coating is applied to the outer surface of the flament causing it to become rigid. The coating softens in vivo through absorption and/or degradation such that the stent is readily passed or removed from the lumen as a softened filament after a pre-determined period of time through normal flow of body fluids passing through the lumen or by manual removal.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of art to which this invention relates is medical devices, in particular, removable stent devices having bioabsorbable or biodegradable polymer coatings.  
         BACKGROUND OF THE INVENTION  
         [0002]    The use of stent medical devices, or other types of endoluminal mechanical support devices, to keep a duct, vessel or other body lumen open in the human body has developed into a primary therapy for lumen stenosis or obstruction. The use of stents in various surgical procedures has quickly become accepted as experience with stent devices accumulates, and the number of surgical procedures employing them increases as their advantages become more widely recognized. For example, it is known to use stents in body lumens in order to maintain open passageways such as the prostatic urethra, the esophagus, the biliary tract, intestines, and various coronary arteries and veins, as well as more remote cardiovascular vessels such as the femoral artery, etc. There are two types of stents that are presently utilized: permanent stents and temporary stents. A permanent stent is designed to be maintained in a body lumen for an indeterminate amount of time. Temporary stents are designed to be maintained in a body lumen for a limited period of time in order to maintain the patency of the body lumen, for example, after trauma to a lumen caused by a surgical procedure or an injury. Permanent stents are typically designed to provide long term support for damaged or traumatized wall tissues of the lumen. There are numerous conventional applications for permanent stents including cardiovascular, urological, gastrointestinal, and gynecological applications.  
           [0003]    It is known that permanent stents, over time, become encapsulated and covered with endothelium tissues, for example, in cardiovascular applications. Similarly, permanent stents are known to become covered by epithelium, for example, in urethral applications. Temporary stents, on the other hand are designed to maintain the passageway of a lumen open for a specific, limited period of time, and preferably do not become incorporated into the walls of the lumen by tissue ingrowth or encapsulation. Temporary stents may advantageously be eliminated from body lumens after a predetermined, clinically appropriate period of time, for example, after the traumatized tissues of the lumen have healed and a stent is no longer needed to maintain the patency of the lumen. For example, temporary stents can be used as substitutes for in-dwelling catheters for applications in the treatment of prostatic obstruction or other urethral stricture diseases. Another indication for temporary stents in a body lumen is after energy ablation, such as laser or thermal ablation, or irradiation of prostatic tissue, in order to control post-operative acute urinary retention or other body fluid retention.  
           [0004]    It is known in the art to make both permanent and temporary stents from various conventional, biocompatible metals. However, there are several disadvantages that may be associated with the use of metal stents. For example, it is known that the metal stents may become encrusted, encapsulated, epithelialized or ingrown with body tissue. The stents are known to migrate on occasion from their initial insertion location. Such stents are known to cause irritation to the surrounding tissues in a lumen. Also, since metals are typically much harder and stiffer than the surrounding tissues in a lumen, this may result in an anatomical or physiological mismatch, thereby damaging tissue or eliciting unwanted biologic responses. Although permanent metal stents are designed to be implanted for an indefinite period of time, it is sometimes necessary to remove permanent metal stents. For example, if there is a biological response requiring surgical intervention, often the stent must be removed through a secondary procedure. If the metal stent is a temporary stent, it will also have to be removed after a clinically appropriate period of time. Regardless of whether the metal stent is categorized as permanent or temporary, if the stent has been encapsulated, epithelialized, etc., the surgical removal of the stent will resultingly cause undesirable pain and discomfort to the patient and possibly additional trauma to the lumen tissue. In addition to the pain and discomfort, the patient must be subjected to an additional time consuming and complicated surgical procedure with the attendant risks of surgery, in order to remove the metal stent.  
           [0005]    Similar complications and problems, as in the case of metal stents, may well result when using permanent stents made from non-absorbable biocompatible polymer or polymer-composites although these materials may offer certain benefits such as reduction in stiffness.  
           [0006]    It is known to use bioabsorbable and biodegradable materials for manufacturing temporary stents. The conventional bioabsorbable or bioresorbable materials from which such stents are made are selected to absorb or degrade over time, thereby eliminating the need for subsequent surgical procedures to remove the stent from the body lumen. In addition to the advantages attendant with not having to surgically remove such stents, it is known that bioabsorbable and biodegradable materials tend to have excellent biocompatibility characteristics, especially in comparison to most conventionally used biocompatible metals in certain sensitive patients. Another advantage of stents made from bioabsorbable and biodegradable materials is that the mechanical properties can be designed to substantially eliminate or reduce the stiffness and hardness that is often associated with metal stents, which can contribute to the propensity of a stent to damage a vessel or lumen.  
           [0007]    However, there are disadvantages and limitations known to be associated with the use of bioabsorbable or biodegradable stents. The limitations arise from the characteristics of the materials from which such stents are made. One of the problems associated with the current stents is that the materials break down too quickly. This improper breakdown or degradation of a stent into large, rigid fragments in the interior of a lumen, such as the urethra, may cause obstruction to normal flow, such as voiding, thereby interfering with the primary purpose of the stent in providing lumen patency. Alternatively, they take a long time to breakdown and stay in the target lumen for a considerable period of time after their therapeutic use has been accomplished. There is thus a long-term risk associated with these materials to form stones when uretheral stents made from longer degrading biodegradable polymers.  
           [0008]    Accordingly, there is a need in this art for novel, temporary stents, wherein the stents remain functional in a body lumen for the duration of a prescribed, clinically appropriate period of time to accomplish the appropriate therapeutical purpose, and, then soften and are removable as an elongated string-like member without producing fragments, which may cause irritation, obstruction, pain or discomfort to the patient, and without the need for a surgical procedure.  
           [0009]    In a preferred embodiment of the present invention, the temporary stent readily passes out of the body, or is removed as, a limp, flexible string-like member, and irritation, obstruction, pain or discomfort to the patient is either eliminated, or if present, is minimal.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide a stent for insertion into a body lumen which is manufactured from a flexible filament member, such as a suture, and then coated with a biodegradable or bioabsorbale polymer such that the member is formed into a relatively rigid stent, and when in the body, softens back into a flexible filament member which is easily passed or removed from the body lumen after a specific therapeutic period of time.  
           [0011]    Therefore, an implantable stent is disclosed for use in body lumens, wherein such lumens exist as part of the natural anatomy or are made surgically. The stent is an elongate, hollow member having a helical or coiled structure, and in a preferred embodiment has a helical structure having a plurality of coils. The structure has a longitudinal axis and a longitudinal passage. The coils have a pitch. The structure is made from a flexible, limp filament or fiber, such as a surgical suture, having an exterior polymeric coating. The polymeric coating is a bioabsorbable or biodegradable polymer, or blend thereof At body temperature, the coating is solid, and of sufficient thickness to effectively cause the flexible, limp member to be maintained in a substantially rigid, fixed state as a structure. The rate of degradation or absorption of the coating in vivo is sufficient to effectively soften or be removed from the outer surface of the filament within the desired therapeutic period. This effectively provides that as the coating degrades, softens or is absorbed in vivo, it loses its mechanical integrity. This allows the filament to revert to its natural, flexible limp state, causing the stent structure to effectively collapse, and the filament may be removed or eliminated from the lumen.  
           [0012]    Upon in vivo exposure to body fluids, the progressively degrading and/or absorbing coating causes the stent to soften and collapse into a flexible filament that can readily pass out of the body lumen, either by manipulation or through natural expulsion with body fluids, thereby minimizing the possibility of causing obstruction, pain or discomfort.  
           [0013]    Yet another aspect of the present invention is the above-described stent made from a fiber which is radio-opaque.  
           [0014]    Yet another aspect of the present invention is a method of using the stents of the present invention in a surgical procedure to maintain the patency of a body lumen. A stent of the present invention is provided. The stent is an elongate, hollow member and in a preferred embodiment has a helical structure having a plurality of coils. The member has a longitudinal axis. The coils have a pitch. The structure is made from a flexible, limp filament or a fiber, having an outer surface and an exterior polymeric coating. The stent is inserted into a body lumen. The exposure to in vivo body fluids causes the exterior coating to absorb and/or degrade and soften, thereby causing the stent structure to collapse and return to a limp, flexible filament that can then be either eliminated by the passage of body fluids or manually removed.  
           [0015]    These and other aspects of the present invention will become more apparent from the following description and examples, and accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a perspective view of a preferred embodiment of a stent device of the present invention mounted to the distal end of an applicator instrument.  
         [0017]    [0017]FIG. 2 is a perspective view of the stent and applicator of FIG. 1, prior to loading the stent onto the applicator instrument.  
         [0018]    [0018]FIG. 3 is a side view of a stent device of the present invention, having a helical configuration.  
         [0019]    [0019]FIG. 4 is a cross-sectional view of the fiber used to make the stent of FIG. 3 taken along View Line  4 - 4  illustrating a circular cross-section.  
         [0020]    [0020]FIG. 5 is a side view of the stent and applicator device of FIG. 1, where the device is shown in the ready position, prior to application.  
         [0021]    [0021]FIG. 6 is a side view of the stent and applicator device of FIG. 5, illustrating the position of the stent relative to the applicator when the stent is partially deployed by engaging the applicator trigger.  
         [0022]    [0022]FIG. 7 illustrates the relative positions of the stent to the applicator of FIG. 6 when the stent is fully deployed by fully engaging the applicator trigger.  
         [0023]    [0023]FIG. 8 illustrates the stent of the present invention fully deployed in the urethra and prostate of a patient, providing for a patent lumen.  
         [0024]    [0024]FIG. 9 illustrates a stent of the present invention emplaced in the urethra of a patient after the coating has degraded, been absorbed or otherwise broken down or softened; showing the stent being removed from the body as an elongated, soft, flexible filament.  
         [0025]    [0025]FIG. 10 is a schematic of a mandrel used to manufacture stents in Example 3. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Referring to FIGS.  1 - 9 , a preferred embodiment of a stent of the present invention is illustrated. As seen in FIG. 3, the stent  10  is seen to be a helical structure having a series of connected coils  20 . The coils are made from filament  100 . The term filament as used herein is defined to include not only filaments but fibers as well, and is used interchangeably with the term fiber. It is preferred that filament  100  be a continuous filament, however, it is possible to make stent  10  from two or more sections of filament which are subsequently connected or hinged together. As seen in FIG. 4, the filament  100  is seen to have inner flexible member  110  and outer coating  130 . The inner flexible member  110  is seen to have outer surface  115 . Covering the outer surface  115  of flexible member  110  is the outer coating  130 . Outer coating  130  is seen to have inner surface  135  and exterior surface  140 . Preferably, inner surface  135  is in contact with, and affixed to, the outer surface  115 . The stent is seen to have a longitudinal axis  70 , and internal passageway  11 . The stent  10  is seen to have a first distal section  30  of coils  20  connected to a second section  50  of coils  20 , wherein the sections  30  and  50  are connected by hinged connecting fiber  60 . The distal section  30  of coils adjacent to hinged connecting fiber  60  forms an anchoring section which is inserted distal to the external sphincter. The proximal section  50  of the stent  10  is maintained within the prostatic urethra. Proximal section  50  is seen to have coils  20  having diameter  24 , and also has passageway  51 . The distal section  30  of stent  10  has coils  20  having a diameter  22 . Distal section  30  also has a passageway  31 . Passage ways  31  and  51  are in communication to form passageway  11  of stent  10 . As seen in FIG. 4, one preferred embodiment of the stent  10  of the present invention has a filament  100  having a circular cross-sectional configuration. The filament  100  may have various configurations depending upon the application including round, square, polygonal, curved, oval, and combinations thereof and equivalents thereof. Those skilled in the art will appreciate that certain cross-sectional configurations will provide different advantages in the stent. For example, the advantages of fiber of the present invention having a round cross-section include ease of the stent manufacturing process due to a possible on-line, one-step transition from the fiber to the stent in future manufacturing processes, flexibility during the stent deployment by being able to tailor the length of the stent during a surgical procedure to fit a particular patient&#39;s anatomy, and the use of commercially available filaments such as sutures.  
         [0027]    The stent  10  is preferably manufactured from a flexible, polymeric filament  100  having a desired cross-sectional configuration. The length and overall diameter of the stent  10  will depend upon a number of factors including the anatomy of the patient, the size of the anatomy and the type of surgical procedure which has effected the urethral lumen. For example, the overall length of a stent  10  useful in the practice of the present invention will be sufficient to effectively maintain the lumen passage open. Typically the length for urethral applications in and adult male, the length will be about 10 mm to about 200 mm, more typically about 20 mm to about 100 mm, and preferably about 40 mm to about 80 mm. The diameter of a stent  10  of the present invention will be sufficient to effectively maintain patency of the lumen. For prostatic urethral applications, where the stent has two sections having different diameters, typically the diameter in the prostatic urethra will typically be about 2 mm to about 25 mm, more typically about 4 mm to about 15 mm, and preferably about 6 mm to about 10 mm. The diameter of the section used to anchor distal to the external sphincter will be about 2 mm to about 25 mm, more typically about 4 mm to about 15 mm, and preferably about 6 mm to about 10 mm. The major cross-sectional dimension of a fiber used to manufacture a stent of the present invention will be sufficient to provide effective support and flexibility. Typically, when utilizing a circular cross-section, the diameter for urethral applications will be about 0.1 mm to about 4 mm, more typically about 0.5 mm to about 3 mm, and preferably about 1 mm to about 2 mm. The pitch, length, diameter and fiber diameter of the stents of the present invention will be sufficient to effectively provide sufficient support in response to radial stress of the urethral vessel walls, while providing for ease of insertion and stability while inserted in the urethral lumen, as well as desired flexibility and lumen patency. The pitch of the stent is defined to be the number of coils per unit length. In this patent application specification, for this example, pitch is defined as the number of coils per centimeter of stent length. Typically, for urethral applications, the pitch will be about 2.5 to about 100, more typically about 3 to about 20, and preferably about 5 to about 10. Although it is preferred for urethral applications that there be no space between adjacent coils, the stents of the present invention may have spaces between adjacent coils.  
         [0028]    The flexible members  110  coated with coatings  130  to form filaments  100  of the present invention will preferably be selected to have sufficient flexibility and softness and limpness to effectively provide for a stent that will collapse and be easily removed from a body lumen. The materials useful for the flexible member include flexible, limp monofilament and braided string-like members. It is particularly preferred to use conventional nonabsorbable sutures, such as monofilament or braided polypropylene, silk, polyester, nylon and the like and equivalents thereof The flexible members may also be conventional absorbable sutures, monofilament or braided, including 95/5 lactide/glycolide, and polydioxanone, and the like. The flexible member  110  may also be made from yarn type materials made from biocompatible fibers that are “spun” together to form the yarn.  
         [0029]    The outer coatings useful for the stents and filaments of the present invention will be conventional biodegradable or bioabsorbable polymers, and blends thereof, including polymers made from monomers selected from the group consisting of lactide, glycolide, para-dioxanone, caprolactone, and trimethylene carbonate, blends thereof and copolymers thereof The effect of the degradation or absorption of the polymeric coating is to convert the filament back into a soft, flexible member after a predetermined time period, such that the stent effectively collapses, and the flexible member can then be easily removed or passed from the lumen. In a flow environment, the progressively degrading stent can readily pass through the body or be removed from the lumen without causing obstruction. The types of polymeric coatings that can advantageously provide stiffness to form a filament  100  include polymers with glass transition temperatures above room temperature and preferably above 55° C., and most preferably above about 120° C. These materials may be amorphous, that is, not display crystallinity. Polymers that have glass transition temperatures that are low, especially below room temperature, will generally require some crystallinity to provide the dimensional stability and stiffness to function in the present application. These can be described as semicrystalline. Regarding water soluble polymers for the coating, there are two general classes of water soluble polymers: ionic and non-ionic. In general of use are polyacrylamides, polyacrylic acid polymers, polyethers (especially the polyethylene glycols or polyethylene oxides), vinyl polymers such as some polyvinyl alcohols and some poly(N-vinyl pyrrolidone)s. Certain polysaccharide gums may also be useful; certain hydroxy celluloses, such as hydroxy methyl cellulose or certain hydroxy isopropyl cellulose are also useful.  
         [0030]    One can control the dissolution process by material selection. Altering molecular weight of the water soluble resin also provides a means of control.  
         [0031]    Utilization of polymer blending is particularly advantageous to achieve the necessary rates of dissolution. Polyamide (nylon) may be used as a component to advantage because it can provide mechanical strength, absorbs some water, etc.  
         [0032]    A possible preferred blend component is polyethylene glycol (PEG or polyethylene oxide, PEO), especially those higher molecular weight resins that are semicrystalline. The melting point of PEG is about 60° C., which is high enough to meet requirements of a coating useful in the present invention. Optionally, the PEO may be blended with nylon. In addition, biodegradable polymers made from poly glycolide/lactide copolymers, polycaprolactone, and the like may be used for the outer coating of the filament  100 . In addition, polyoxaesters can be utilized which are water soluble and degrade by hydrolysis. Other suitable polymers are found in U.S. Pat. No. 5,980,551, which is incorporated by reference.  
         [0033]    A stent must be designed to withstand radial stresses in order to perform its function of maintaining a passage through a lumen open. The mechanical capability of the stents of the present invention to withstand radial stresses when the stent is emplaced in the body lumen is provided primarily by the biodegradable/bioabsorbable material in the outer coating. The strength and stiffness and thickness of this material in the outer coating is sufficient to be effectively withstand the loads necessary to keep the stent functional. As the coating degrades and breaks down, it will have a sufficient thickness of properly selected biodegradable material to effectively be able to withstand the load necessary for the time period required to keep the lumen patent. In essence then, the coating can be designed to fulfill the mechanical requirements of keeping the body lumen patent or open for the specific therapeutic time period.  
         [0034]    After the coating has degraded/absorbed and effectively been removed from the stent structure by body fluids, the remaining filament returns to its soft, pliable, fibrillar state as a flexible member. The remaining soft filament is readily excreted or removed from the lumen.  
         [0035]    The coated filaments of the present invention may be made by conventional processes including co-extrusion, melt coating, solution coating or powder coating followed by spreading the coating by melting, etc., and the like. For example, when using a coating process, the inner flexible member can be a mono-filament extruded material or can be made from a multi-filament braid. The outer coating can be added on top of the flexible member either by melt coating or solution coating by passing the inner core through a bath, through coating rollers, brushes, spraying and/or a die.  
         [0036]    In another embodiment of the present invention, the polymers and blends that are used to form the coating can be used as a drug delivery matrix. To form this matrix, the coating material would be mixed with a therapeutic agent. The variety of different therapeutic agents that can be used in conjunction with the polymers of the present invention is vast. In general, therapeutic agents which may be administered via the pharmaceutical compositions of the invention include, without limitation: anti-infectives such as antibiotics and anti-viral agents; analgesics and analgesic combinations; anti-inflammatory agents; hormones such as steroids; bone regenerating growth factors; and naturally derived or genetically engineered proteins, polysaccharides, glycoproteins, or lipoproteins.  
         [0037]    Matrix formulations may be formulated by mixing one or more therapeutic agents with the polymer. The therapeutic agent, may be present as a liquid, a finely divided solid, or any other appropriate physical form. Typically, but optionally, the matrix will include one or more additives, such as diluents, carriers, excipients, stabilizers or the like.  
         [0038]    The amount of therapeutic agent will depend on the particular drug being employed and medical condition being treated. Typically, the amount of drug represents about 0.001 percent to about 70 percent, more typically about 0.001 percent to about 50 percent, most typically about 0.001 percent to about 20 percent by weight of the matrix. The quantity and type of polymer incorporated into the drug delivery matrix will vary depending on the release profile desired and the amount of drug employed.  
         [0039]    Upon contact with body fluids, the polymer coating undergoes gradual degradation (mainly through hydrolysis) or absorption with concomitant release of the dispersed drug for a sustained or extended period. This can result in prolonged delivery (over, say 1 to 5,000 hours, preferably 2 to 800 hours) of effective amounts (say, 0.0001 mg/kg/hour to 10 mg/kg/hour) of the drug. This dosage form can be administered as is necessary depending on the subject being treated, the severity of the affliction, the judgment of the prescribing physician, and the like. Following this or similar procedures, those skilled in the art will be able to prepare a variety of formulations.  
         [0040]    The stents  10  of the present invention when made from the coated filament  100  may be manufactured in the following manner using a winding process. A filament  100  is wound about a mandrel by heating the filament  100  and then coiling it around the mandrel. The assembly of the mandrel and the coil are annealed under constraint and then the mandrel is removed. The pitch and diameter of the coils are selected to provide the desired size and shape of stent. If desired, the filament  100  may be wound about the mandrel without heat, for example immediately upon entering a coating bath or melt bath, or the uncoated flexible member  110  can be wound about a mandrel, and then the coating can be applied in a conventional manner, and cured as necessary.  
         [0041]    The stents of the present invention may be utilized in the following manner in urethral stent placement procedures as illustrated in FIGS. 1, 2,  5 ,  6 ,  7  and  8 . Initially a stent  10  is placed upon the distal end of an applicator instrument  200 . Instrument  200  is seen to have handle  250  having grip  255 . At the top  257  of the handle  250  is mounted the shaft retention member  290 . Retention member  290  is seen to have longitudinal passageway  292 , front  295  and back  296 . The mounting tube  240  is seen to have distal end  242  and proximal end  244 . Mounting tube  240  is seen to have passage  248 . The proximal end  244  of tube  240  is seen to be mounted in passage way  292  such that the inner passageway  248  is in communication with passageway  292 . Slidably mounted in passageway  248  is the applicator tube  220 . Tube  220  has distal end  222 , proximal end  224 , and passageway  226 . Mounted to the proximal end  224  of tube  220  is the mounting block  300 , which is affixed to end  224  by pin  309 . Mounted to the bottom of block  300  is rack gear member  330  having gear teeth  335 . Contained in handle  250  is the cavity  350  for receiving pinion gear member  270 , having teeth  275 . Pinion gear member  270  is pivotally mounted in cavity  350  by pivot pins  265 . Teeth  275  mesh with and are engaged by teeth  335 . Extending out from pinion gear member  270  on the opposite side of pins  265  is the actuation trigger  280 . Actuation of trigger  280  will move tube  220  proximally and distally with respect to tube  240 . Actuating the trigger  280  will allow the stent  10  to be released from the tubes  220  and  240 .  
         [0042]    The stent and distal end of the instrument  200  are inserted into the urethra  410  through the meatus  400  of the patient&#39;s penis as seen in FIGS. 8 and 9. The distal end of the instrument  200  and the stent  10  are manipulated through the urethra  410  such that the prostatic section of the stent is located within the prostatic urethra  411  and the distal end of the stent is distal to the external sphincter  430 , thereby providing an open passage for urine from bladder  450  through the lumen of the urethra. Then, the application instrument  200  is withdrawn from the urethra  410  by engaging trigger  280  and pulling distally on the instrument, thereby completing the procedure and providing for an implanted stent  10  which allows for patency of the urethral lumen  410 . As seen in FIG. 9, the stent  10  after having been in place for the appropriate period of time has been converted into a state wherein it is substantially a soft, flexible filament, and is readily passed from the urethra  410  out of the patient&#39;s body with the urine flow, or is manually pulled out of the lumen. It will be appreciated by those skilled in the art that placement for other types of body lumens could be done in a similar manner, with modification as required by the unique characteristics of the lumen or of the surgical emplacement procedure.  
         [0043]    The following examples are illustrative of the principles and practice of the present invention, although not limited thereto.  
       EXAMPLE 1  
       [0044]    Manufacture of Removable Fiber by Extrusion Coating Process.  
         [0045]    A polydiaxanone (PDS) homopolymer was added to a nitrogen purged hopper of a ¾″ vertical single screw extruder with a 24:1 (Length:Diameter) standard screw. The temperature profile of the extruder was 250, 260, 270 and 275 degrees (F) from rear zone to die. The screw speed was 6.5 RPM and the adapt pressure was 1345 psi. A B&amp;H  30  cross head (B&amp;H Tool, Inc., San Marcos, Calif.) was employed with a 0.020-inch diameter guide (pressure tip) and a 0.048-inch diameter die. An 0.018-inch diameter non-degradabe polyester braided filament (sold under the tradename ETHIBOND, by Ethicon, Inc., Somerville, N.J.) was guided through the cross head. The fiber was coated with molten polydioxanone, chilled in a water trough, dried by a air wiper, taken off and sequentially spooled. The temperature of the water trough was 8° C. The take-off speed was 2.1 meter/min. The fiber, with the outer diameter of 0.044-inch, was stored in a nitrogen environment.  
       EXAMPLE 2  
       [0046]    Manufacture of Stent Using the Coated Filament  
         [0047]    A string was tied so that it created a small loop through the first hole C of the mandrel (see FIG. 10). Two metal posts (mm diameter 2×15 mm length) were inserted into the holes A and B.  
         [0048]    Posts were placed in holes A and B. The C-side end of the mandrel was clamped inside the collet of a winding motor. A 5-foot long coated filament was passed through the loop. The fiber was folded. The two free ends were held together and the folded fiber was stretched loosely so that the loop was positioned to be in the approximate center of the fiber. The folded filaments were loosely held together to make sure that the coils were packed adjacent to each other. The winding speed was between 20-30 RPM for the length of the Prostatic Section.  
         [0049]    Once the first post (B) was reached, the fiber was then bent over the post toward the distal section. Winding an additional 180° more formed the connector between posts (A) and (B). The filament was then positioned tangent to the mandrel and the distal loop was then coiled in a similar fashion as the Prostatic Section. An adjustable clamp was used to secure the filament to the mandrel. The assembly was stored under vacuum for 48 hours to allow it to dry prior to annealing.  
         [0050]    Prior to annealing, the posts (A) and (B) were removed from the mandrel. The entire assembly was suspended in an annealing oven and heated at 80° C. for 10 hours in a vacuum environment. The stent was removed from the mandrel, trimmed, and stored in nitrogen environment.  
       EXAMPLE 3  
       [0051]    A male patient is appropriately anesthetized and undergoes a prostrate thermal ablation procedure using conventional laser treatment devices. After successful completion of the surgical procedure, a stent  10  of the present invention is inserted into the patient&#39;s urethra and bladder in the following manner using an applicator  200 . The surgeon trims the Prostatic Section of the stent to size. The stent is placed at the end of the applicator. A conventional cystoscopic telescope is inserted into the lumen of the applicator. The stent and applicator are lubricated with a water soluble medical grade lubricant. A fluid reservoir is attached to the applier as in any standard cystoscopy procedure. The stent is placed in the prostatic urethra under direct visualization using the applicator. Once positioned correctly, the applicator is removed, leaving behind the stent in the prostatic urethra. In approximately 28 days after implantation, the outer coating degrades, thereby converting the stent into a soft, flexible filamentary structure that is removed from the urinary tract by grasping the end of the filament and pulling it from the lumen.  
         [0052]    The stents of the present invention provide many advantages over the stents of the prior art. The advantages include: rigidity (lumen patency) for a prescribed time; a degradation/absorption softening mechanism, whereby the stent softens into a readily passable/removable filament; biocompatibility; means to prevent migration; means to non-invasively monitor the stent and its position by X-ray, etc.  
         [0053]    Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the claimed invention.