Methods of making high flexibility stent-grafts and stent-grafts

Abstract

The present invention relates to a method of making a high flexibility stent graft (100), the method comprising: providing a stent graft having first and second longitudinal ends and a lumen extending longitudinally therethrough, wherein the stent graft comprises a base stent (10) having a first cladding material (14) disposed inside the base stent to form a lumen lining, and a second cladding material (12) disposed outside the base stent, locally removing the second cladding material without penetrating the first cladding material to increase flexibility of the stent graft.

Description

Technical Field

[0001] The present invention relates to a method for preparing a high-flexibility stent graft and also relates to a stent graft prepared by using the method. Background Art

[0002] Coated stents, also known as stent grafts, are used in a variety of surgical applications, such as endovascular aneurysm repair, treatment of vascular graft stenosis, and hemodialysis fistulas. These stent grafts typically include a stent (often referred to as a "base stent") that is tubular in shape with a lumen extending between two longitudinal ends.

[0003] The interior of the base stent, i.e., the side facing the lumen, is coated with a first layer of graft material, such as ePTFE. The exterior of the base stent is coated with a second layer of graft material (also typically ePTFE). Other biocompatible graft materials, such as FEP, can also be used. In such stent grafts, the base stent, typically made of a shape memory alloy such as Nitinol, determines the overall shape of the stent graft itself. The first and second layers of graft material give the stent graft its overall tubular shape, allowing blood to flow through it without significant leakage through the walls of the stent graft.

[0004] Stent grafts (and stents) are generally required to be quite flexible. This is because they need to be implanted into the patient's body through often tortuous blood vessels, so it is desirable for the stent to be flexible during delivery (i.e., before expansion). However, even after delivery, when the stent graft is placed in the patient's intended location, high flexibility is required because it improves the stent graft's compliance with the patient's body: a stent graft that is too stiff will stiffen the blood vessel in which it is implanted. In addition, such a stent graft can cause complications at the junction between the stent graft and the blood vessel. With a softer stent graft, the blood vessel will not react as strongly to the implantation of the stent graft, and the stent graft will also be more easily adapted to the anatomy of the blood vessel.

[0005] Additionally, it is often desirable to reduce the delivery profile of stent grafts so that they can be delivered through narrow blood vessels. On the other hand, stent grafts should be made of a material that is strong enough to prevent collapse after implantation.

[0006] To achieve these goals, attempts have been made to reduce the wall thickness of the base stent or to reduce the thickness of the ePTFE coating, which can lead to greater flexibility and a reduced delivery profile. Alternatively, a softer or thinner coating material may be used, or the outer ePTFE layer may be formed not as a single, integral layer, but as a ribbon wound helically around the base stent with gaps between each winding helix. Attempts have also been made to make the base stent itself more flexible.

[0007] But this setup is difficult to assemble because the secondary covering needs to be precisely aligned relative to the base stent. If misalignment occurs, the stent graft typically needs to be discarded because it would be unacceptable to implant it in the patient.

[0008] Furthermore, the preparation process for partially coated stent grafts is quite complex. In prior art methods for assembling such stent grafts, as a first step, an inner ePTFE layer, serving as the first coating material, is placed on an assembly mandrel. Subsequently, the base stent is placed on top of the inner ePTFE layer. This operation must ensure that the stent framework retains its shape. However, this is often difficult to achieve in practice, as the base stent is typically highly flexible and can easily deform, misalign, twist, stretch, and / or compress. Consequently, this placement process is difficult to implement and requires considerable skill.

[0009] Next, the outer ePTFE layer, forming the secondary covering material, must be placed on the base stent. It is important to note that the base stent cannot be misaligned or deformed during the placement of the outer ePTFE layer. This step also requires a high degree of care and skill, given the need to avoid any deformation of the base stent.

[0010] In this regard, it should be noted that in order to provide the outer ePTFE layer, a single closed outer layer need not be applied. Instead, individual ePTFE threads or ribbons can be applied that can be wrapped around the base stent. These ribbons / threads allow for movement of the base stent portion during stent graft delivery, which provides improved bending flexibility. On the other hand, this step of applying the outer ePTFE layer requires the use of complex wrapping equipment to ensure that the ribbons / threads do not fall into the gaps of the stent frame during their wrapping, as this would disrupt the structure of the base stent.

[0011] Subsequently, tape and other layers are wrapped around the entire assembled stent graft structure before sintering to prevent displacement of the stent graft structure during sintering. Similarly, it is necessary to ensure that the base stent and the internal arrangement of the inner and outer ePTFE layers do not move relative to each other. This arrangement is then heat-treated to facilitate sintering. The stent graft is then unpacked and the stent graft ends are trimmed. It is important to exercise extreme caution during all of these assembly steps, as the outer ePTFE layer could potentially migrate uncontrollably into the free space within the stent, which is necessary to ensure a flexible stent arrangement. If such undesirable movement occurs, the stent graft itself is typically unfit for implantation and must be discarded. Summary of the Invention

[0012] The present inventors have recognized that there are several ways to optimize the aforementioned method. Specifically, they have recognized that the manner in which the base stent is coated can be improved, thereby simplifying the overall assembly process. Therefore, one object of the present invention is to provide a method for preparing a stent graft that produces a highly flexible stent graft, is easier to implement, and reduces the amount of defective stents that need to be discarded.

[0013] The invention is defined by the independent method claim 1 and the independent stent graft claim 12. Specific embodiments are defined in their respective dependent claims.

[0014] According to the present invention, a method for preparing a highly flexible stent graft includes providing a stent graft having first and second longitudinal ends and a lumen extending longitudinally therethrough. The stent graft includes a base stent, which can be made of a material such as a nickel-titanium alloy. In various embodiments, the base stent is self-expanding. In some embodiments, the base stent self-expands at near body temperature (i.e., approximately 37° C.). It is also contemplated that a balloon-expandable base stent may be used in place of a self-expanding base stent.

[0015] The base stent has a first coating material disposed within it to provide an inner lining for the lumen, allowing blood to flow therethrough. The base stent has a second coating material disposed on its exterior. The first and second coating materials may be, for example, ePTFE. However, other biocompatible coating materials such as FEP are also contemplated. Subsequently, the second coating material is partially removed (in some embodiments, by heat removal) without penetrating the first coating material. This partial removal increases the flexibility of the stent graft.

[0016] By initially providing a stent graft that is coated both internally and externally and subsequently removing only the second outer coating material, a highly flexible stent graft is provided. This is because the material thickness of the stent graft is reduced due to the at least partial removal of the second coating material. Furthermore, because the final coating shape provided by the second coating material is achieved through partial removal, a complex procedure for applying the second coating material already in its final shape to the base stent is not required. This simplifies the production process.

[0017] Furthermore, the present method for preparing a stent graft can be implemented on a fully coated stent graft prepared using a conventional method. Only after the application of the secondary coating material does the method need to be modified, as the secondary coating material must then be partially removed. Thus, for the essential portion of the method, the established and well-characterized method for preparing such stent grafts can be applied, with only the secondary coating material removal step added to complete the process.

[0018] Using the above method, a high degree of flexibility in the design of the outer surface can be achieved - spirals with different pitches can be provided (these spirals can be consistent or inconsistent with the spirals of the base stent), and the degree of coating can be adjusted quite flexibly. In addition, there is no restriction on the spiral design of the second coating material - for example, the second coating material can be removed to produce discrete, approximately circular holes in the second coating material. The second coating material can also be made to cover the longitudinal and / or circumferential connectors of the base stent. By carefully controlling the removal of the second coating material, it is also possible, for example, to write the name of the manufacturer and brand on the stent graft. In general, using CAD / CAM or finite element analysis, the desired stent graft outer surface shape can be obtained so that it has specific mechanical properties, and then the second coating material is removed accordingly.

[0019] In various embodiments, the second coating material is removed by applying heat. The application of heat can be implemented using a variety of methods. Although the application of heat using a laser will be discussed later, it is also conceivable to provide heat by thermal conduction, for example by placing a hot object next to the stent graft. Other alternatives are also conceivable, such as a concentrated stream of hot air. In addition to applying heat, a mechanical removal method can also be used to remove the second coating material, such as by applying a grinding or abrasive technique. The application of heat causes the second coating material to locally evaporate.

[0020] It should be noted that, while the first and second covering materials are provided in the form of tubes or sheets in various embodiments, in other embodiments, electrospinning can also be used to provide them. In addition, while ePTFE (expanded polytetrafluoroethylene) is used in some embodiments, other materials such as PET (polyethylene terephthalate, sometimes sold under the trade name "Dacron") can also be used. More generally, the material composition of the first and second covering materials is not limited, and regardless of the materials used for the first and second covering materials, flexibility will be improved.

[0021] In one embodiment, the secondary covering material is removed to partially expose the underlying stent. That is, after the material is removed, the underlying stent is at least partially exposed. At these points where the underlying stent is partially exposed, the secondary covering material is completely removed. This results in a significantly increased flexibility of the stent graft. It also means that if heat is applied, the applied heat can be transferred to the underlying stent itself. Since the stent is typically made of metal and therefore has good thermal conductivity, this method of creating a highly flexible stent graft avoids applying unnecessary heat to the primary covering material. This prevents damage to the primary covering material, making this method more reliable.

[0022] In other embodiments, the laser thins the secondary cladding material only locally. That is, the material of the underlying stent is not exposed. This approach preserves the secondary cladding material without creating "holes" therein, which may be advantageous in certain applications.

[0023] In various embodiments, the second covering material has a thickness of 0.02-0.15 mm before partial removal. Such a thickness makes the stent graft highly flexible while also being elastic.

[0024] In some specific embodiments, when the thickness of the second covering material is 0.02-0.15 mm, the second covering material is locally thinned by at least 50% of its thickness, thereby ensuring that the thinned portion is thin enough to provide a highly flexible stent graft.

[0025] In one embodiment, heat is applied by a laser. Such a laser can be a polymer vaporization laser, such as a carbon dioxide laser. Lasers can be easily manipulated, which makes the method of making a highly flexible stent graft relatively easy to control.

[0026] In various embodiments, the laser is applied so as to glancingly impact the stent graft. That is, the laser is applied so as to be tangentially incident on the outer surface of the stent graft, in a manner comparable to a lathe in a machine shop. This method of applying the laser allows for precise control, as there is no need to focus the laser to control its application location. Instead, because the direction of laser propagation is typically very well-defined, and because the laser beam is typically extremely collimated, the spatial extent of the beam is also typically well-known. This means that the location at which the laser is applied to the secondary cladding material can be relatively easily controlled, which in turn means that the depth of removal of the secondary cladding material can be relatively easily controlled.

[0027] In an alternative embodiment, the laser is applied to impact the second covering material perpendicularly. In this way, the full laser power can be applied to the stent graft, which reduces the time required to remove the second covering material.

[0028] In another alternative embodiment, the laser is applied to the second covering material at an angle of 30-60 degrees relative to the longitudinal direction of the stent graft. In this manner of irradiating the second covering material with the laser, the evaporated second covering material is blown away from the stent graft by the evaporation gas, which improves the quality of the stent graft produced in this manner.

[0029] In various embodiments, the second coating material is removed, thereby cutting a spiral pattern in the second coating material, wherein the spiral extends axially along the stent graft. Having such a spiral pattern is particularly advantageous because it results in an increase in the flexibility of the entire stent graft along the entire length to which the spiral pattern is applied. Utilizing an annular pattern, the flexibility of the stent graft always has a certain degree of non-uniformity due to the rings being separated by portions of the second coating material. On the other hand, such rings may be beneficial for other mechanical advantages (e.g., device coupling, sensor space) of a medical procedure.

[0030] In a second aspect of the present invention, a stent graft for use in vascular surgery is provided. This stent graft comprises a base stent having a proximal end, a distal end, and a lumen extending longitudinally therethrough. A first coating material is provided within the base stent to form an inner lining of the lumen, thereby providing a tube that allows body fluids to flow through without significantly leaking to the outside. A second coating material is provided externally to the base stent and bonded to the first coating material and / or the base stent. The second coating material is provided only on a portion external to the base stent, thereby forming a helical structure. Such a stent graft is advantageous because the stent is highly flexible and uniform. The method of the present invention described above can be used to prepare a stent graft of this embodiment.

[0031] In various embodiments of the stent-graft of the present invention, an annular portion is provided at one longitudinal end of the stent-graft. This allows for the selective removal of the second covering material while retaining the first covering material, thereby leaving an uncovered portion on the underlying stent. In some embodiments, annular portions may be provided at both longitudinal ends. Such annular portions are advantageous in that they conserve material and, thereby, reduce the profile of the delivery catheter. They also make the stent-graft easier to load into the delivery catheter. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A stent graft prepared according to one embodiment of the present invention is presented.

[0033] Figure 2 A method for preparing a stent graft according to one embodiment of the present invention is presented. DETAILED DESCRIPTION

[0034] Reference below Figure 1 and 2 The method for preparing the stent graft of the present invention is described.

[0035] In a first step (step S100), an assembly mandrel 16 is provided. A first covering material 14, which may be ePTFE, is placed on the assembly mandrel 16. The ePTFE material can be provided in sheet form. The advantage of a sheet form rather than a strip form is that it eliminates the need for a winding process, which can require very high precision because it must ensure that adjacent windings are adjacent to each other on the mandrel without radially overlapping. It is also possible to use prefabricated tubular material instead of a sheet or strip material.

[0036] Subsequently (step S102), a base stent 10 is placed on the first covering material 14. In this step, it is important to ensure that the base stent 10 maintains its shape and does not stretch or deform. The base stent 10 can be made of a self-expanding material (e.g., nickel-titanium alloy) so that the base stent self-expands at approximately body temperature after being guided to the desired location and deployed by the delivery catheter.

[0037] Subsequently, in step S104, a second covering material 12 is placed on the base support 10. The second covering material 12 may also be ePTFE provided in sheet form. A single-layer tube may also be used for the second covering material 12. The thickness of the first and / or second covering materials 14, 12 is 0.02-0.15 mm.

[0038] Then, in step S106, the arrangement is packaged and subsequently sintered, so that the first and second covering materials 12, 14 and the base stent 10 are integrated to be bonded to each other. Subsequently, the stent graft is unpacked.

[0039] After step S106, in the next step S108, the second covering material 12 is partially removed so that discrete portions of the base stent are not covered by the second covering material. It is also conceivable that the second covering material 12 is only thinned, but not completely removed. The removal of the second covering material can be achieved by applying a laser (e.g., a carbon dioxide polymer evaporation laser 22). The carbon dioxide polymer evaporation laser 22 is applied to the outer layer of the stent graft 100 in a controlled manner, thereby locally removing the second covering material 12 by evaporating it. Figure 1 In the illustrated configuration, laser light 22 is applied in a grazing manner, such that, in the case of this diagram, the laser light propagates perpendicularly to the sheet on which the diagram is printed. This is, of course, merely exemplary, and, as mentioned above, other propagation angles are possible. Stent graft 100 is then removed from mandrel 16 and trimmed, if necessary.

[0040] This removal forms a gap 20 in the form of a spiral 21 in the second covering material 12. The spiral 21 terminates at either of the annular gaps 18 at the two longitudinal ends of the stent graft 100.

Claims

1. A method for preparing a highly flexible stent graft (100), the method comprising: - providing a stent graft having first and second longitudinal ends and a lumen extending longitudinally therethrough, the stent graft comprising a base stent (10) having a first covering material (14) disposed within the base stent to form a lumen lining and a second covering material (12) disposed outside the base stent, - Locally removing the second covering material without penetrating the first covering material to increase the flexibility of the stent graft, wherein the local removal is performed by applying heat to the second covering material (12), wherein the heat is applied by a laser (22), characterized in that the laser is applied to impact the second covering material at an angle of 30-60 degrees. The method according to claim 1 , wherein the second covering material is removed to partially expose the base support. The method according to claim 1 , wherein the second covering material is removed to locally thin the second covering material without locally exposing the base support.

4. The method according to any one of claims 1 to 3, wherein the thickness of the second coating material before removal is 0.02 to 0.15 mm. 5 . The method according to claim 4 , wherein the second cladding material is removed to be locally thinned without locally exposing the base support, wherein the second cladding material is locally thinned by at least 50% of its maximum thickness.

6. The method according to any one of claims 1 to 3, wherein the second covering material is removed, thereby cutting a helical pattern in the second covering material, wherein the helix (21) extends along the axial direction of the stent graft.

7. Stent grafts used in vascular surgery, including: - a base stent (10) having a proximal end and a distal end and a lumen extending longitudinally therethrough, - a first covering material (14) disposed inside the base stent (10) to form a lumen lining, - a second covering material (12) disposed outside the basic stent (10), wherein the second covering material (12) is only partially disposed outside the basic stent to form a spiral structure, and the stent graft is prepared by the method according to any one of claims 1 to 6.

8. The stent graft of claim 7, further comprising an uncovered annular portion (18) at one longitudinal end of the base stent.

9. The stent graft according to claim 7, further comprising uncovered annular portions (18) on both longitudinal ends of the base stent.

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