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* cited by examiner
FABRICATING A STENT FROM A BLOW
BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to methods of fabricating a stent
from a blow molded tube.
2. Description of the State of the Art
This invention relates to the fabrication of a stent, from an 10 expanded tube. Stents function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. A "lumen" refers to a cavity of a tubular organ such as a blood 15 vessel.
A stent has a cylindrical shape and includes a pattern with a number of interconnecting structural elements or struts. Some stents are designed so that they may be radially compressed (crimped) and radially expanded (to allow deploy- 20 ment). A stent can be fabricated from a tube that has been laser cut to form a stent pattern.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must withstand structural loads, namely radial compressive forces, imposed on the stent as it 25 supports the walls of a vessel. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, is due to strength and rigidity around a circumferential direction of the stent. Radial strength and rigidity, therefore, may also be described 30 as, hoop or circumferential strength and rigidity. Once expanded, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including cyclic loading, which is induced by a beating heart. 35
SUMMARY OF THE INVENTION
Various embodiments of the present invention include a method for fabricating a stent, the method comprising: posi- 40 tioning a polymeric tube inside a tubular mold, wherein a high thermally conductive element covers at least a portion of the outer surface of the mold, the high thermally conductive element having a thermal conductivity that is greater than that of the mold; heating at least a portion of the mold; allowing 45 the tube to radially expand within the mold; and fabricating a stent from the radially expanded tube.
Further embodiments of the present invention include a method for fabricating a stent, the method comprising: positioning a polymeric tube inside a mold, the mold having an 50 inner layer and an outer element, wherein the inner layer comprises a glass and the outer element comprises a high thermally conductive material having a thermal conductivity at least 100 times greater than the inner layer; heating at least a portion of the mold; radially expanding the tube against the 55 mold; and fabricating a stent from the radially expanded tube.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a stent. go FIG. 2 depicts a polymeric tube for use in fabricating a stent.
FIG. 3(a) depicts an axial, cross sectional view of a blow molding apparatus prior to radially expanding a polymeric tube. 65
FIG. 3(b) depicts a radial, cross sectional view of the blow molding apparatus of FIG. 3(a).
FIG. 4 depicts an axial, cross sectional view of the blow molding apparatus of FIG. 3(a) after radially expanding the tube.
FIG. 5 depicts a side view of a mesh of high thermal conductivity that may be fitted over the exterior of a mold.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the present invention may be applied to stents and, more generally, to implantable medical devices such as, but not limited to, self-expandable stents, balloon-expandable stents, stent-grafts, vascular grafts, or generally, tubular implantable medical devices.
A stent can have virtually any structural pattern that is compatible with a bodily lumen in which it is implanted. Typically, a stent is composed of a pattern or network of circumferential and longitudinally extending interconnecting structural elements or struts. In general, the struts are arranged in patterns, which are designed to contact the lumen walls of a vessel and to maintain vascular patency. A myriad of strut patterns are known in the art for achieving particular design goals. A few of the more important design characteristics of stents are radial or hoop strength, expansion ratio or coverage area, and longitudinal flexibility. The present invention is applicable to virtually any stent design and is, therefore, not limited to any particular stent design or pattern. One embodiment of a stent pattern may include cylindrical rings composed of struts. The cylindrical rings may be connected by connecting struts.
In some embodiments, a stent of the present invention may be formed from a tube by laser cutting the pattern of struts in the tube. The stent may also be formed by laser cutting a polymeric sheet, rolling the pattern into the shape of the cylindrical stent, and providing a longitudinal weld to form the stent. Other methods of forming stents are well known and include chemically etching a polymeric sheet and rolling and then welding it to form the stent.
FIG. 1 depicts an exemplary stent 100 with struts 110 that form cylindrical rings 115 which are connected by linking struts 120. The cross-section of the struts in stent 100 is rectangular-shaped. The cross-section of struts is not limited to what has been illustrated, and therefore, other cross-sectional shapes are applicable with embodiments of the present invention. The pattern should not be limited to what has been illustrated as other stent patterns are easily applicable with embodiments of the present invention.
As indicated above, it is important for a stent to have high radial strength so that once it is deployed from the crimped state, it can support a lumen. In general, deforming a polymer construct can strengthen the polymer of the construct along an axis of deformation. In some embodiments of fabricating a stent from a polymer tube, the polymer tube can be radially expanded and the stent can be fabricated from the polymer tube in its expanded state. FIG. 2 depicts an exemplary polymer tube 200 for use in forming a stent. Polymer tube 200 has a longitudinal axis 235 and an inner diameter 210 and outer diameter 205 and thickness 211. Polymer tube 200 can be radially deformed by applying stress in the radial direction, which strengthens tube 200 in circumferential direction 240, thereby increasing the tube's radial strength. Strength in the axial direction can also be increased by axial deformation. The uniformity of the radial expansion impacts the concentricity of the expanded tube and the uniformity of expanded thickness 211. These properties are important to the mechanical stability of the stent fabricated from the tube.
The embodiments disclosed herein relate to fabricating a polymeric stent as depicted in FIG. 1 that includes methods