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Home | Alpha Telephone | Domain Names | Web Hosting | Get Traffic | xrEvidence | xrSoccer United States Patent
Medical stent with variable coil and related methods A medical stent includes a first section which includes a first material, defines a lumen, and includes a first coil completing more than one revolution. The first coil revolves about and is coaxial with an axis, expanding and opening as it revolves from the origin of the first coil. A second section of the stent includes a second material, defines a lumen, and includes a second coil completing at least one revolution. A third section defines a lumen and is located between the first and second sections. The third section includes a co-extrusion of the first and second materials. One of the first or second sections is harder than the other section.
Primary Examiner: Snow; Bruce E. What is claimed is: 1. A medical stent comprising: a first section defining a lumen and comprising a substantially planar, spiral first coil completing more than one revolution, wherein the first coil revolves in an ever increasing distance about an axis of the stent from an origin of the coil to an end of the coil; a second section defining a lumen and comprising a second coil, the second coil being generally perpendicular to the first coil; and a third section disposed between the first section and the second section, the third section generally extends along, and is substantially coaxial with, said axis. 2. The stent of claim 1, wherein the first coil is sized such that at least a portion of the first coil resides at the junction of a bladder and a ureter in a patient. 3. The stent of claim 1, wherein the first section is composed of a first material, the second section is composed of a second material, the second material being different than the first material. 4. The stent of claim 3, wherein the first material has a durometer value of about 70 to about 90 on a Shore A scale. 5. The stent of claim 3, wherein the second material has a durometer value of about 80 to about 95 on a Shore A scale. 6. The stent of claim 1, wherein the first section is composed of a first material, the second section is composed of a second material, the second material being softer than the first material. 7. The stent of claim 1, wherein the first section is composed of a first material, the second section is composed of a second material, the second material being different than the first material, the third section being composed of a coextrusion of the first material and the second material. 8. The stent of claim 1, wherein the second coil completes at least one revolution. 9. The stent of claim 1, wherein a cross-section of at least one of the first section, the second section, and the third section is substantially circular. 10. The stent of claim 1, wherein the first section is homogeneous and is composed of a first material, the second section is homogeneous and is composed of a second material, the second material being different than the first material. TECHNICAL FIELD The present invention relates to medical stents and related methods. More specifically, the invention relates to medical stents having one section which is softer than a section at the other end of the stent. BACKGROUND INFORMATION Fluid sometimes needs to be drained from a body. For example, urine formed in one or both kidneys might need to be drained into the bladder. One way to accomplish such drainage is to use a medical device that conveys the fluid (e.g., urine) through a lumen. Such devices include stents and catheters. Existing stents can be uncomfortable for the patient, especially when they reside in the ureter between the kidney and the bladder, can be difficult for a medical professional to place in a patient, or can allow urine from the bladder to move into the ureter towards the kidney. SUMMARY OF THE INVENTION The present invention provides medical stents for facilitating drainage of fluid and methods for placing such stents. For example, such stents can be placed in a ureter to facilitate drainage of fluid from a patient's kidney to a patient's bladder. Generally, stents according to the invention have a "softer" end and a "harder" end. The harder end generally resides in the patient's kidney while the softer end generally resides in the patient's bladder. The harder end can transition to the softer end in a transition section produced by, for example, a co-extrusion process where deposition of a first material is gradually ceased and deposition of a second is gradually increased. In general, the harder end is suited to retain the stent in the patient's kidney and/or facilitate placement in a patient, while the softer end is suited to increase patient comfort and/or retain the stent in the patient's bladder. Additionally, the softer end can inhibit movement of the stent in the bladder, minimize contact between the stent and the bladder, at least partially occlude the junction between the bladder and ureter in order to at least partially prevent retrograde urine flow from the bladder into the ureter both around the stent and through the stent, and/or otherwise minimize reflux of urine through the stent towards the kidney. Such stents also are useful in other situations such as biliary drainage or, generally, where fluid is drained from one body structure to another body structure or out of the body. In one embodiment, a medical stent can include a first section defining a lumen and including a first coil completing more than one revolution, a second section defining a lumen and including a second coil completing at least one revolution, and a third section defining a lumen and located between the first and second sections. The first section can include a first material having a first durometer value, and the second section can include a second material having a second durometer value. The second durometer value can be greater than the first durometer value, and at least a portion of the third section can include a co-extrusion of the first and second materials. The first coil can revolve about and be coaxial with an axis. A distance from a first point to the axis, the first point being at the center of a first cross-section of the first coil and on a line normal to the axis, can be less than a distance from a second point to the axis, the second point being at the center of a second cross-section of the first coil and on a line normal to the axis and the first point being closer to an origin of the first coil than the second point. The third section can be adjacent the origin of the first coil. The embodiment described above, or those described below, can have any of the following features. The axis can generally extend along the third section. The second coil can be offset from the axis. The third section can include a shaft. The second coil can be generally perpendicular to the first coil. The first material can be ethylene vinyl acetate. The first material can have a durometer value of about 70 to about 90 on a Shore A scale. The second material can have a durometer value of about 80 to about 95 on a Shore A scale. A cross-section of the lumen in at least one of the first, second, and third sections can be circular. A cross-section of at least one of the first, second, and third sections can be circular. At least one of the first, second, and third section can include a radiopaque material. The second coil can have an outer diameter of at least about 2.0 cm. The first coil can be sized and/or shaped such that at least a portion of the first coil resides at the junction of a bladder and a ureter in a patient. The first coil can be a spiral. In another aspect of the invention, a medical stent can include a first section defining a lumen and including a substantially planar first coil completing more than one revolution, a second section defining a lumen and comprising a second coil completing at least one revolution, and a third section defining a lumen and located between the first and second sections. The first section can include a first material having a first durometer value, and the second section can include a second material having a second durometer value. The second durometer value can be greater than the first durometer value. At least a portion of the third section can include a co-extrusion of the first and second materials. The second coil can be generally perpendicular to the first coil. In another aspect of the invention, a method for placing a medical stent includes inserting a medial stent, including any of the stents described above or below with any of the features described above or below, into a ureter. At least a portion of the first coil can reside at the junction of a bladder and a ureter in a patient. At least a portion of the first coil can at least partially occlude the junction. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating principles of the invention. FIG. 1 is a schematic rendering of a stent according to the invention. FIG. 2A is a schematic enlarged side view of one end of the stent of FIG. 1. FIG. 2B is a schematic end-on view of the stent of FIG. 1. FIG. 3 is a schematic cross section of the stent of FIG. 1. FIG. 4 is a table showing examples of measurements of portions of the stent of FIG. 1. FIG. 5 is a schematic rendering of an alternative embodiment of one coil of a stent according to the invention. FIG. 6 is a schematic end-on view of the coil of FIG. 5. FIG. 7 is a schematic rendering of an alternate embodiment of a stent according to the invention having a similar coil to that in FIG. 1 at one end and a different coil from that in FIG. 1 at the opposite end. FIG. 8 is a schematic end-on view of the stent of FIG. 7. FIG. 9 is an image of a cross section of the embodiment of FIG. 1 taken along section line 9--9. FIG. 10 is an image of a cross section of the embodiment of FIG. 1 taken along section line 10--10. FIG. 11 is an image of a cross section of the embodiment of FIG. 1 taken along section line 11--11. FIG. 12 is an image of a cross section of the embodiment of FIG. 1 taken along section line 12--12. FIG. 13 is an image of a cross section of the embodiment of FIG. 1 taken along section line 13--13. FIG. 14 is a schematic rendering of an alternative embodiment of a stent according to the invention. FIG. 15 is a schematic top view of the embodiment of FIG. 14. FIG. 16 is a schematic end-on view of the embodiment of FIG. 14. FIG. 17 is a schematic enlarged end-on view of the embodiment of FIG. 14. FIG. 18 is a schematic view of a proximal section of the embodiment of FIG. 14. FIG. 19 is a schematic rendering of one system used to manufacture stents according to the invention. FIG. 20 is a table containing inner and outer diameter sizes for certain embodiments of the invention. FIG. 21 is a schematic rendering of the stent of FIG. 1 in a kidney, ureter, and bladder. DESCRIPTION The present invention provides medical stents for facilitating drainage of fluid and methods for placing such stents. For example, such stents are placed in a ureter to facilitate drainage of fluid from a patient's kidney to a patient's bladder. Generally, stents according to the invention have a "softer" end and a "harder" end. The harder end generally resides in the patient's kidney while the softer end generally resides in the patient's bladder. The harder end can transition to the softer end in a transition section produced by, for example, a co-extrusion process where deposition of a first material is gradually ceased and deposition of a second is gradually increased. As used herein, the terms "hard" and "soft," and various grammatical forms thereof, are general terms meant generally to refer to a difference in properties, including, but not limited to, (1) a difference in the durometer value of all or some of the material(s) used to construct a stent (for example, a higher durometer value of one material used in constructing a section of a stent, even if one or more other materials are also used to construct that same section of stent, can mean "hard" and a lower durometer value of one material used in constructing another section of a stent, even if one or more other materials are also used to construct that same section of stent, can mean "soft"), (2) a difference in the retention strengths of the coils on either end of a stent (for example, a higher retention strength can mean "hard" and a lower retention strength can mean "soft"), (3) a difference in stiffness (for example, a more stiff material/section of stent can be "hard" and a less stiff material/section of stent can be "soft"), or other differences between material(s) used to construct a stent or between sections of a stent that those skilled in the art would consider "hard" and/or "soft." Ureteral stents can be made from a higher durometer material to facilitate placement and retention in the body. However, these firmer stents may contribute to some patient discomfort issues. Ureteral stents also can be made from a lower durometer material in an effort to enhance patient comfort. However, these softer stents may be difficult to place and may migrate once placed in the patient's body. Stents according to the invention are harder end at one end and softer at the other end. This construction is desirable because, in general, the harder end is suited for placing the stent in the patient's kidney and/or to retain the stent in the patient's kidney, while the softer end is suited to increase patient comfort and/or, to a degree, retain the stent in the patient's bladder. Moreover, stents according to the invention can have a coil at the end of the stent to reside in the kidney that is of a size and/or shape that enhances retention of the stent in the kidney. Also, stents according to the invention can have a coil at the end of the stent to reside in the bladder that inhibits motion of the stent within the bladder, that enhances patient comfort by reducing, for example, contact between the stent and the neck of the bladder and/or the floor of the bladder, that at least partially occludes the junction between the bladder and ureter to at least partially prevent urine from entering the ureter from the bladder either through or around the stent, and/or that otherwise minimizes reflux of urine through the stent towards the kidney. Accordingly, stents according to the invention are designed to incorporate multiple desirable features into a single stent and can include any combination of these features. Referring to FIGS. 1, 2A, and 2B, a schematic representation of one embodiment of a stent 10 according to the invention is shown. Generally, the stent 10 has three sections 20, 22, 24. A first section 24 is located at the proximal end (as used herein, proximal refers to the end of a stent closest a medical professional when placing a stent in a patient) of the stent 10. A second section 20 is located at the distal end (as used herein, distal refers to the end of a stent furthest from a medical professional when placing a stent in a patient) of the stent 10. A third section 22 is located between the first 24 and second sections 20 and is generally in the form of a shaft. The location of the sections 20, 22, 24 as shown in FIG. 1 is approximate, emphasis instead being placed on illustrating the principles of the invention. The first section 24 has a first coil 14 that makes more than one revolution. The first coil 14 revolves about an axis 23 and is coaxial with the axis 23. The axis 23 is shown extending generally along the third section 22 of the stent 10. Although stents according to the invention typically are flexible, the stents can be placed in a position in which the shaft of the stent is generally linear to form an axis. In this embodiment, the first coil 14 makes more than two revolutions about the axis 23. However, alternate embodiments can revolve to a greater or lesser extent about the axis 23 (for example, less than approximately two revolutions or more than approximately two revolutions). The first coil 14 begins at an origin 25. Typically, the origin of stents according to the invention is the location on the stent where the generally straight shaft connects to the coil. However, the origin can be slightly away from this location in certain embodiments. The first section 24 and third section 22 meet at the origin 25 in this embodiment. As the first coil 14 revolves about the axis 23, it opens outwardly. Thus, a point 17 that is located at the center of a cross-section of the first coil 14 closer to the origin 25 and along a line 17a normal to the axis 23 is closer to the axis 23 than is a second point 19 that is located at the center of a second cross-section of the first coil 14 further from the origin 25 and along a line 19a normal to the axis 23 (best seen in FIG. 2A). The following measurements provide one, non-limiting example of the possible size of the first coil 14. One revolution of the first coil 14 has a width C of about 0.1 cm, and two revolutions have a width D of about 0.2 cm. One of the turns of the coil 14 is measured to be at an angle E of about 75 degrees from the axis 23. A height F of one part of the coil 14 is about 0.75 cm, and at a location one revolution from measurement F, a height G of the coil 14 is about 1.5 cm. The second section 20 has a second coil 12 which also makes more than one revolution and also is offset from the axis 23 of the first coil 14 and the general axis of the stent 10. The second coil 12 has a tapered tip. Typically, a second coil is larger than coils in some other stents. For example, a coil can have a diameter of greater than about 1.5 cm, preferably greater than about 1.9 cm (including a diameter of about 2.0 cm), and more preferably greater than about 2.4 cm (including a diameter of about 2.5 cm). This size can enhance retention of a stent in a patient's kidney. Holes 16 (only some of the holes are labeled) in the outer surface of the stent 10 are located along the length of stent 10. These holes 16 allow the outside environment to communicate with a lumen inside the stent 10. The lumen 50 and the stent's outer diameter 52 can be shaped in cross-section as a circle (best seen in FIG. 3) or any other appropriate shape such as an oval or other oblong shape. The holes 16 can be placed in many configurations, one of which is shown in FIG. 1. In this configuration, the holes 16 are present in the first coil 14 in about its first revolution and not in about its second revolution. These holes 16 are approximately evenly spaced apart in the first coil 14 and are located along the length of the shaft at intervals of about 1.5 cm with one rotated 90 degrees from the next. In 4.8 French stents, about two to about four holes 16 are in the second coil 12 and in 6, 7, or 8 French stents, about three to about five holes 16 are on the second coil 12. These holes can be evenly spaced. A suture may be attached to the first section 24 for placing the stent 10 in a desired position as well as removing the stent 10. FIG. 4 provides non-limiting examples of sizes of various stents according to the invention. For example, a 4.8 French stent can have a length along portion A of the stent 10 of about 24, 26, or 28 mm and a length along portion B (the area in which the tip of the stent 10 tapers) of the stent 10 of about 4 mm and can have holes of about 0.26 inches. The third section 22 is formed from a coextrusion of the material(s) from which the first section 24 is made and the material(s) from which the second section 20 is made. As shown in FIG. 1, a transition section 15 (i.e., where the material(s) making up one portion of the stent transition to the material(s) making up another portion of the stent) in the third section 22 is closer to the first coil 14 than to the second coil 12. However, in alternative embodiments, the transition section can be located anywhere along the length of the stent. The transition section typically is located between the coils on either end of the stent and is about 2 cm long to about 10 cm long. However, the transition section can be any length. The first section 24 includes a first material having a first durometer. The second section 20 includes a second material having a second durometer, which is greater than the first durometer value. Accordingly, the first section is "softer" than the second section. The transition section 15 includes both the first and second materials, and the first and second materials are separate, distinct, and associated in an unsymmetrical, irregular configuration. In operation, the first coil 14 typically resides in the patient's bladder, and the second coil 12 typically resides in the patient's kidney (FIG. 21). An alternative embodiment of a first coil 114 to be placed in a patient's bladder is shown in FIGS. 5 and 6. The first coil 114 in this embodiment is in a generally spiral or funnel shape that expands as it revolves about an axis 123 from the origin 125 of the first coil 114. Again, a point 117 that is located at the center of a cross-section of the first coil 114 closer to the origin 125 and along a line normal to the axis 123 is closer to the axis 123 than is a second point 119 that is located at the center of a second cross-section of the first coil 114 further from the origin 125 and along a line normal to the axis 123. The first coil 114 in this embodiment makes more than three revolutions about the axis 123. The following measurements provide one, non-limiting example of the possible size of the coil 114. One revolution of the first coil 114 has a width H of about 0.33 cm, and three revolutions have a width I of about 0.99 cm. The turns of the coil 114 spread outward at an angle J of about 3 degrees relative to the axis 123. A height K of one part of the coil 114 is about 0.5 cm, and another height L of the coil 114 is about 1.5 cm. In a further alternative embodiment of a first coil 314 to be placed in a patient's bladder, a stent 310 is shown in FIGS. 7 and 8 that is substantially similar to that shown in FIG. 1 except for the first coil 314. The first coil 314 has a generally spiral or funnel shape that expands as it revolves about an axis 323 from the origin 325 of the first coil 314. As above, a point 317 that is located at the center of a cross-section of the first coil 314 closer to the origin 325 and along a line normal to the axis 323 is closer to the axis 323 than is a second point 319 that is located at the center of a second cross-section of the first coil 314 further from the origin 325 and along a line normal to the axis 323. The first coil 314 makes more than two revolutions about the axis 323, and the second coil 312 is generally perpendicular to the first coil 314. The following measurements provide one, non-limiting example of the possible size of the coil. One of the turns of the first coil 314 is measured to be an angle P of about 37 degrees from the axis 323. One revolution of the first coil 314 has a width Q of about 0.5 cm, and two revolutions have a width R of about 1 cm. The stent 10 of FIGS. 1, 2A, and 2B is a single piece and is sized to fit within a ureter. For example, two types of ethylene vinyl acetate ("EVA") can be extruded to form the stent. In a continuous process, the first section 24 is formed from one type of EVA; the third section 22, then, is formed by gradually ceasing the deposition of the first type of EVA and gradually increasing the deposition of a second type of EVA (creating the transition section 15 in the third section 22); and the other end of the stent, the second section 20, is formed from the second type of EVA after the first type of EVA has ceased being extruded. Each type of EVA has a different durometer value, with the first type of EVA having a durometer value that is less than the durometer value of the second type of EVA. The two materials in the third section 22 are separate, are distinct, and are associated with each other in an irregular configuration. Additionally, other materials may be mixed with the first and/or second types of the EVA prior to extrusion. For example, radiopaque materials, such as bismuth subcarbonate, and/or colorants can be added. The addition can occur at the site of manufacture or a supplier can supply the EVA already compounded with the radiopaque material alone or with the colorant alone or with both the radiopaque material and the colorant. Even if these materials are mixed, the fact that one EVA type has a durometer value less than the second EVA type can mean that the section of the stent formed from the first type of EVA is "softer" than the section of the stent formed from the second type of EVA. After extrusion, the curled portions are formed. For example, the extrusion can be placed on a mandrel, shaped in a particular form, and the extrusion can be formed into a desired shape by heating the extrusion while on the mandrel. Alternatively, the extrusion can be laid into a plate having a groove cut into it in the shape of the desired final product. The plate is heated from below (for example, with a heat lamp) to form the extrusion into a shape according to the configuration of the groove. Both coils can be formed at the same time using two adjacent plates, each with a groove for the coil at either end of the stent. The plates are heated at different temperatures, to the extent necessary, for example, if the two ends of the stent are made from different material(s), and can be heated for the same length of time. Additionally, after extrusion, holes can be bored into the stent by placing a nylon core inside the stent to prevent the stent from collapsing and drilling through the stent, for example, with a hollow sharpened bit. FIGS. 9 13 show a series of cross-sectional views taken along the length of the stent 10. The approximate position of these cross-sections are shown in FIG. 1. It should be understood that the position of these cross-sections is merely an example. In various embodiments, the transition section of the medical stent can be relatively short, or relatively long, depending upon the physical characteristics of the stent that are desired. Additionally, sections taken in various embodiments may look different than the representations shown in FIGS. 9 13, depending upon, for example, the length of the transition section, the materials being extruded, and the method of co-extrusion used to manufacture the stent. Thus, the cross-sections shown in FIG. 1 and FIGS. 9 13 should be understood to illustrate both one embodiment of the invention and the general principle whereby the material(s) forming one section of the stent transition to the material(s) forming the other section of the stent. These figures show one material mixed with a colorant (for example, EVA and a colorant) (the darker portions of the cross-section) gradually increasing in abundance along the length of at least part of the stent and a second material not mixed with a colorant (for example, a second type of EVA) (the lighter portions of the cross-section) gradually decreasing in abundance along the length of at least part of the stent. Some of these views are indicative of the first and second materials being separate, distinct, and associating in an unsymmetrical, irregular configuration. In certain embodiments, the change in material composition can occur over any part of the shaft of the stent or all of the shaft of the stent. At least one of the materials can be ethylene vinyl acetate. Additionally, stents according to the invention can have several transition zones where materials change and/or can have more than two materials (or more than two mixtures of materials) that change along the length of the stent. For example, the shaft of a stent, or a portion thereof, may or may not be the same material(s) and/or the same durometer as either of the two coils. Moreover, each of the shaft and two coils can be formed from different material(s). In certain embodiments, the material(s) that make up the second section of the stent (the harder section of the stent) can extend at least half way down the shaft of the stent, and can extend even further, such that the transition section is closer to the first coil (the coil in the softer section of the stent) than to the second coil (the coil in the harder section of the stent). Such a configuration enhances the placement characteristics of a stent because the preponderance of hard material(s) makes the stent stiffer and easier for a medical profession to place. In many embodiments, the transition of material(s) does not occur in one of the coils such that each coil is formed from a single material (or a single mixture of materials). However, the transition can occur anywhere along the length of the stent. Also in some embodiments, the inner diameter of the stent is maximized but not so much as to adversely impact the stent's ability to be pushed over a guidewire. In an alternative embodiment, and referring to FIGS. 14, 15, 16, 17, and 18, a stent 210 is shown having a first section 224 located at the proximal end of the stent 210. A second section 220 is located at the distal end of the stent 210. A third section 222 is located between the first 224 and second sections 220 and is generally in the form of a shaft. The location of the sections 220, 222, 224 as shown in FIGS. 14 and 18 is approximate, emphasis instead being placed on illustrating the principles of the invention. The first section 224 has a first coil 214 that makes more than one revolution. The first coil 214 revolves about an axis 223 and is coaxial with the axis 223. The axis 223 is shown extending generally along the third section 222 of the stent 210. Although stents according to the invention typically are flexible, the stents can be placed in a position in which the shaft of the stent is generally linear to form an axis. In this embodiment, the first coil 214 makes more than two revolutions about the axis 223. However, alternate embodiments can revolve to a greater or lesser extent about the axis 223 (for example, less than approximately two revolutions or more than approximately two revolutions). The first coil 214 is attached to the shaft of the stent 210 at a neck 211. The neck is slightly curved and is set back from the axis 223. As the first coil 214 revolves about the axis 223, it revolves outwardly and substantially in a single plane. Accordingly, the first coil 214 is substantially planar. The following measurements provide one, non-limiting example of the possible size of the first coil 214 and the neck 211. The neck 211 has a length M of about 0.82 cm, and the greatest height N of the coil 214 is about 1.5 cm. The second section 220 has a second coil 212 which also makes more than one revolution and also is offset from the axis 223 of the first coil 214 and the general axis of the stent 210. The second coil 212 is generally perpendicular to the first coil 214 and has a tapered tip. Typically, a second coil is larger than coils in some other stents. For example, a coil can have a diameter of greater than about 1.5 cm, preferably greater than about 1.9 cm (including a diameter of about 2.0 cm), and more preferably greater than about 2.4 cm (including a diameter of about 2.5 cm). In this example the second coil has a diameter of about 2.5 cm. This size can enhance retention of a stent in a patient's kidney. Holes 216 in the outer surface of the stent 210 allow the outside environment to communicate with a lumen inside the stent 210. The holes 216 can be placed in many configurations. Holes 216 are shown in the first coil 214 in about its first revolution and are approximately evenly spaced apart. Holes also can be located in other parts of the stent 210. The lumen and the stent's outer diameter can be shaped in cross-section as a circle or any other appropriate shape such as an oval or other oblong shape. A suture may be attached to the first section 224 for placing the stent 210 in a desired position as well as removing the stent 210. The third section 222 is formed from a coextrusion of the material(s) from which the first section 224 is made and the material(s) from which the second section 220 is made. A transition section 215 (i.e., where the material(s) making up one portion of the stent transition to the material(s) making up another portion of the stent) in the third section 222 is closer to the first coil 214 than to the second coil 212. However, in alternative embodiments, the transition section can be located anywhere along the length of the stent. The transition section typically is located between the coils on either end of the stent and is about 2 cm long to about 10 cm long. However, the transition section can be any length. The first section 224 includes a first material having a first durometer. The second section 220 includes a second material having a second durometer, which is greater than the first durometer value. Accordingly, the first section is "softer" than the second section. The transition section 215 includes both the first and second materials, and the first and second materials are separate, distinct, and associated in an unsymmetrical, irregular configuration. Interrupted layer extrusion techniques, gradient-type coextrusion techniques, or similar techniques can be used to produce any of the transition sections described above. Such extrusion techniques can be used instead of using joints or welds to bring together two ends of a stent, each end having a different physical property than the other end. Such joints or welds can fail during use of the stent and can be difficult to manufacture. Continuous material extrusion according to the invention enhances stent integrity while allowing for desired placement and drainage characteristics. Additionally, continuous extrusion products tend not to kink in the transition zone as might a stent with a butt-joint or a weld. In general, any type of thermoplastic polymer can be extruded such as a silicone, a polyurethane, or a polyolefin copolymer such as EVA. In general, in one embodiment of the invention, two types of EVA (at least one type of EVA can be mixed with a radiopaque material and at least one type of EVA can be mixed with a colorant) are extruded to form the stent. In a continuous process, one end of the stent is formed from one type of EVA (for example, the first section 24 in FIG. 1); an intermediate section (for example, the third section 22 in FIG. 1) containing a transition section (for example, the transition section 15 in FIG. 1), then, is formed by gradually ceasing the deposition of the first type of EVA and gradually increasing the deposition of a second type of EVA; and the other end of the stent is formed from the second type of EVA (for example, the second section 20 in FIG. 1) after the first type of EVA has ceased being extruded. Each type of EVA has a different durometer value. The mixing of the two types of EVA in the transition section produces a section in which the two materials are separate, are distinct, and are associated with each other in an irregular configuration. After extrusion, the curled portions are formed. In more detail and in one example of an extrusion technique as shown in FIG. 19, a gradient-type technique, a first pelletized type of EVA is placed in a first dryer 50 and a second pelletized type of EVA is placed in a second dryer 60. The dryers 50, 60 are hoppers to contain the pellets, and, to the extent necessary, to dry the pellets, and each dryer 50, 60 feeds the pellets to an extruder 52, 62. The two extruders 52, 62 melt the pellets, and each of the melted materials passes through a separate adapter 54, 64 to a separate melt pump 56, 66 (which are also referred to as a gear pumps). Each melt pump 56, 66 has a rotary gear which allows the melted materials to pass through the pump 56, 66. A computer 58 runs two servo motors 55, 65 that control the melt pumps 56, 66. The computer 58 controls the revolutions per minute as a function of the distance over which a point in the extruded product travels. There is a feedback loop between each melt pump 56, 66 and its related extruder 52, 62 such that when the pressure between the extruder 52, 62 and the melt pump 56, 66 is too high, the extruder 52, 62 shuts off. Each extruder 52, 62 is a slave to its respective melt pump 56, 66. The two separate lines, each containing a different EVA, come together at a cross-head 68. The cross-head 68 contains lumens that are separate from each other except for a relatively short distance in the cross-head 68. This distance is immediately adjacent a die and a tip where the extruded product exits the cross-head 68. The two materials only come together immediately adjacent to the die and the tip. The die dictates the outer diameter of the extruded product and the tip dictates the inner diameter of the product. The end of the tip is flush with the end of the die. Air is metered into a port that connects with the tip. Air from the tip pushes out the outer and inner diameters of the extruded product. Also, the tip is por |