Patent Publication Number: US-2015073531-A1

Title: Drug eluting depot stent with enhanced fatigue life

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The instant disclosure relates to a drug eluting depot stent for implant into human blood vessels. 
     2. Description of Related Art 
     Typically, stents are clinically used for treatment of vascular diseases, specifically to treat stenosis or vessel occlusion, to be implanted into narrowed or diseased blood vessels via various catheter techniques. 
     In principle, stent can provide support to the periphery of the blood vessels such that the blood vessels will not collapse. However, in reality, after the stent is implanted in the blood vessels, cell proliferation on the inner walls of the blood vessels may occur, which may clog the blood vessels once again. 
     In order to prevent the blood vessels from restenosis, current drug eluting stents in the market attempts to elute drugs which inhibit cell proliferation and reduce the chances of further re-clogging. However, the structural strength and the service life of the drug eluting depot stents are typically lower in comparison with the conventional drug eluting stents without drug reservoirs, which limits their respective usage. 
     To address the above issues, the inventor strives via associated experience and research to present the instant disclosure, which can effectively improve the limitation described above. 
     SUMMARY OF THE INVENTION 
     The object of the instant disclosure is to provide a drug eluting depot stent which has openings of various cross-sectional shapes and sizes for drug retention, and more uniform stresses across the stent in order to prolong the service life of the stent. 
     In order to achieve the aforementioned objects, according to an embodiment of the instant disclosure, a drug eluting depot stent is provided which includes two free ends and a tubular body. The tubular body has a longitudinal axis and a plurality of rings connected via connectors. Each ring is defined by a plurality of wave-like or undulating structures. Each undulating structure comprises a bar arm and crowns. Each bar arm has a first end portion, a second end portion opposing to the first end portion and a mid-section located between the first end portion and the second end portion. The bar arm has at least one perforated pattern on a surface thereof such that the first end portion is arranged with a perforated pattern on a surface thereof to be defined as a first opened region, the mid-section is arranged with a perforated pattern on a surface thereof to be defined as a second opened region, and the second end portion is arranged with a perforated pattern on a surface thereof to be defined as a third opened region. A surface area ratio between the surface area of the first opened region and the surface area of the first end portion is a first opening ratio, a surface area ratio between the surface area of the second opened region and the surface area of the mid-section is a second opening ratio, a surface area ratio between the surface area of the third opened region and the surface area of the second end portion is a third opening ratio, the second opening ratio is larger than the first opening ratio, and the second opening ratio is larger than the third opening ratio. Moreover, the crown is connected to the end portion of the bar arm. 
     The stent of the instant disclosure has a perforated pattern formed on the surface of the bar arm for drug retention. The perforated pattern provides higher structural strength in two end portions of the bar arm relatively to the mid-section of the bar arm. After the stent is implanted in blood vessels, the bar arm can share the loading that is originally concentrated on the crowns such that the overall stresses are more uniformly distributed across the stent as a whole. Thus, the fatigue life of the stent is enhanced. 
     In order to further understand the instant disclosure, the following embodiments and illustrations are provided. However, the detailed description and drawings are merely illustrative of the disclosure, rather than limiting the scope being defined by the appended claims and equivalents thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view illustrating portions of a drug eluting depot stent in accordance with an embodiment of the instant disclosure; 
         FIG. 1B  is a two-dimensional view of  FIG. 1A  illustrating the stent in accordance with the instant disclosure; 
         FIG. 1C  is a cross-sectional view of  FIG. 1B  along the axis A-A in accordance with the instant disclosure; 
         FIG. 2  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure; 
         FIG. 3  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure; 
         FIG. 4  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure; 
         FIG. 5  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure; 
         FIG. 6  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure; and 
         FIG. 7  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The aforementioned illustrations and detailed descriptions are exemplarities for the purpose of further explaining the scope of the instant disclosure. Other objectives and advantages related to the instant disclosure will be illustrated in the subsequent descriptions and appended drawings. 
     When a vascular stent is implanted into a blood vessel, the stent is positioned in a pre-determined location via a catheter. The diameter of the stent is expanded via balloon angioplasty or self-expanded via nitinol materials to dilate the narrowed segment of a blood vessel, such that preferred blood flow is restored there through. The instant embodiment furthers discloses the stent before expansion as follow. 
     Please refer to  FIG. 1A  as the perspective view illustrating portions of a drug eluting depot stent in accordance with an embodiment of the instant disclosure. A stent  1  is made of metallic materials or bioabsorbable materials that are biocompatible such as stainless steel alloys, nickel titanium alloys, cobalt-chromium alloys, cobalt-nickel alloys, platinum chromium alloys, polylactic acids, L-lactic acids, polyglycolic acids, etc. The stent  1  has two free ends (not labeled) and a tubular body  10 . The tubular body  10  extends throughout and in between the two free ends while having a longitudinal axis O. The tubular body  10  includes a plurality of rings  100 . The rings  100  are connected in series along the longitudinal axis O to form the tubular body  10 . 
     Specifically, any two adjacent rings  100  are connected via at least a connector  101 . The connector  101  can be straight, curved, or any type of shape. In an embodiment, the rings  100  are formed by a plurality of wave-like or undulating structures. The undulating structure includes a bar arm  110 , a first crown  120 , and a second crown  121 . 
     Please refer to  FIG. 1B  as a two-dimensional local view of  FIG. 1A  illustrating the stent in accordance with the instant disclosure. The bar arm  110  has a first end portion  110   a , a second end portion  110   b  opposing to the first end portion  110   a , and a mid-section  110   c . The mid-section  110   c  is located between the first and the second end portions  110   a ,  110   b . The first end portion  110   a  of the bar arm  110  is connected to the first crown  120 , and the second end portion  110   b  is connected to the second crown  121 . The first and the second crowns  120 , 121  may have different curvatures. 
     Please refer to  FIGS. 1B and 1C .  FIG. 1C  is a cross-sectional view of  FIG. 1B  along the axis A-A in accordance with the instant disclosure. The bar arm  110  has a surface which includes an outer surface  111  and an inner surface  112  oppositely faced from the outer surface  111 . In the instant embodiment, the outer wall surface of the tubular body  10  is made of the outer surfaces  111  of the bar arm  110 , and the inner wall surface of the tubular body  10  is made of the inner surfaces  112  of the bar arm  110 . After the stent  1  is implanted into blood vessels, the outer wall surface of the tubular body  10 , or the outer surfaces  111  of the bar arm  110 , is in close contact with the periphery of the inner walls of the blood vessels. 
     Please refer to  FIG. 1B . In the instant embodiment, the bar arm  110  has a width that is substantially equivalent to a width W of the first crown  120  and also a width of the second crown  121 . The width of bar arm  110  is substantially uniform from the first end portion  110   a , across and to the second end portion  110   b . However, the width of the bar arm  100  is not necessarily the same throughout the first end portion  110   a  and the second end portion  110   b  in another embodiment. The bar arm  110  has a perforated pattern arranged on a surface thereof. Specifically, the bar arm  110  has at least one opening formed on the outer surface  111  and/or the inner surface  112  to retain drugs therein. The perforated pattern formed on a surface of the first end portion  110   a  is defined as a first opened region, the perforated pattern formed on a surface of the mid-section  110   c  is defined as a second opened region, and the perforated pattern formed on a surface of the second end portion  110   b  is defined as a third opened region. The perforated pattern respectively arranged or formed on the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b  can be the same or different from one another, thus the examples provided herein do not limit the shapes, sizes, or quantity of the perforated patterns. 
     The surface area of the first opened region on the first end portion  110   a  with respect to the surface area of the first end portion  110   a  is defined as a first opening ratio, the surface area of the second opened region on the mid-section  110   c  with respect to the surface area of the mid-section  110   c  is defined as a second opening ratio, and the surface area of the third opened region on the second end portion  110   b  with respect to the surface area of the second end portion  110   b  is defined as the third opening ratio. The second opening ratio is larger than the first opening ratio, and the second opening ratio is larger than the third opening ratio. 
     Specifically, the perforated pattern is a rhombic-shaped opened region  113  as shown in  FIG. 1B . The rhombic-shaped opened region  113  is a rhombic opening in the instant embodiment. In other words, the bar arm  110  is formed with a rhombic-shaped opening, which can be a through hole or a blind hole, on the surface of the bar arm  110 . In the instant embodiment, the rhombic opening is a through hole surrounded by the outer surface  111 . 
     In the instant embodiment, the bar arm  110  is divided into three regions defined by the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b . In other words, the bar arm  110  has a length L substantially divided into three portions defined by the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b . The length L of the bar arm  110  is a distance between the two end portions thereof. In other words, the first end portion  110   a  has a length L1, the mid-section  110   c  has a length L2, and the second end portion  110   b  has a length L3. All the lengths L1, L2, L3 are substantially ⅓ of the length L. 
     In the instant embodiment, the rhombic opening extends from the first end portion  110   a  to the second end portion  110   b , and the rhombic opening has a width at the center thereof which tapers towards the two end portions. In other words, the width of the rhombic opening enlarges from the first end portion  110   a  toward the mid-section  110   c  and reduces from the mid-section  110   c  to the second end portion  110   b.    
     If the first end portion  110   a  has a surface area A1 (meaning W×L1), the mid-section  110   c  has a surface area A2 (W×L2), and the second end portion  110   b  has a surface area A3 (W×L3), whereas the first opened region formed by the rhombic opening  113  on the surface of the first end portion  110   a  has a surface area of B1, the second opened region on the surface of the mid-section  110   c  has a surface area of B2, and the third opened region on the surface of the second end portion  110   b  has a surface area of B3, then A1, A2, A3, B1, B2, and B3 satisfy the following conditions: (B2/A2)&gt;(B1/A1), and (B2/A2)&gt;(B3/A3). As such, the structural strength of the mid-section of each bar arm  110  is relatively weaker than the two end portions. 
     Specifically, when the stent  1  is implanted into the blood vessels, the stent  1  will radially support the wall of the blood vessels. In terms of the stent, the loading on the crown is typically higher than the loading on the bar arm. As a result, majority of the cracks initiates from the crown of the stent. Moreover, if the drug reservoir or depot on the bar arm has a uniform width, the reservoir or depot can significantly reduce the average service life of the stent with respect to the stent without drug reservoirs. 
     However, the stent  1  in the instant embodiment has the second opening ratio of the mid-section  110   c  larger than the first opening ratio of the first end portion  110   a  and the third opening ratio of the second end portion  110   b  via variations of the perforated pattern. As such, the structure of the bar arm  110  gradually weakens from two end portions towards the center thereof, and the stress concentration on the crown  120  can be guided towards the bar arm  110 , such that fatigue life of the stent is increased. Although the stent  1  of the instant embodiment has the perforated pattern for drug retention, its fatigue life does not reduce but in contrast increases. 
     As shown in  FIG. 1C , the rhombic opening  113  can penetrate through the outer surface  111  to the inner surface  112 . When the rhombic opening  113  is a blind hole, the opening  113  can be formed on the outer surface  111  and/or the inner surface  112 . The opening  113  can be used to retain one or more types of drugs therein. In an embodiment, when the rhombic opening  113  is a through hole, the upper and lower half of the opening  113  can be used to retain different types of drugs. 
       FIG. 2  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure. In another embodiment, a plurality of discontinuous openings of various sizes and shapes are arranged in such way that the overall shape of the openings resembles the contour of a rhombus. In other words, the perforated pattern includes a plurality of openings of various sizes and shapes that are not interconnected to each other. 
       FIG. 3  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure. In the instant embodiment, the perforated pattern is an elliptical opening  114 . The elliptical opening  114  has a longitudinal axis which aligns with the first end portion  110   a  and the second end portion  110   b . The length of the longitudinal axis of the opening  114  can be less than the length L of the bar arm  110 , but is not limited herein. 
     Please refer to  FIG. 4  as a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure. In the instant embodiment, a plurality of discontinuous or non-interconnected openings having various sizes and shapes are arranged in such a way that the overall shape of the opening  114  resembles an ellipse. In other embodiment, the perforated pattern can also have a polygonal shape. The perforated pattern takes up a certain surface area on the outer surface  111  of the bar arm  110 , and the occupied surface area ratio of the perforated pattern at the mid-section  110   c  is respectively larger than the occupied surface area ratios of the perforated patterns at the two end portions. 
     In another embodiment, the bar arm  110  has a plurality of tiny holes arranged on a surface thereof to form the perforated pattern. Please refer to  FIG. 5  as a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure. In the instant embodiment, the perforated pattern includes a plurality of holes for example three holes  115   a ,  115   b ,  115   c , as shown in  FIG. 5 . Specifically, the bar arm  110  has the plurality of holes  115   a - 115   c , where the hole  115   a  is arranged on the surface of the first end portion  110   a , the hole  115   c  is arranged on the surface of the mid-section  110   c , and the hole  115   b  is arranged on the surface of the second end portion  110   b . In the instant embodiment, the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b  each has one hole  115 , however, the number of holes  115  on the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b  is not limited to the example provided herein. 
     In the instant embodiment, the width of the bar arm  110  across the first end portion  110   a  and the second end portion  110   b  are substantially uniform, however, the hole  115   c  has diameter respectively larger than the diameters of the holes  115   a ,  115   b . In other words, the two dimensional surface area of the outer surface  111  of the mid-section  110   c  occupied by the hole  115   c  is larger than that of the first end portion  110   a  occupied by the hole  115   a , and the two dimensional surface area of the outer surface  111  of the mid-section  110   c  which is occupied by the hole  115   c  is also larger than that of the second end portion  110   b  which is occupied by the hole  115   b . As a result, due to the distribution of the holes  115   a - 115   c , the structural strengths at two end portions (first and second end portions  110   a ,  110   b ) are stronger than that at the mid-section  110   c . The cross-sectional shapes of the holes  115   a - 115   c  are circles as shown in  FIG. 5 , but they can also be polygons, ellipses or other shapes in other embodiments. The two dimensional surface areas of the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b  respectively occupied by the holes  115   a - 115   c  are not limited to only the outer surface  111 , and may also refer to the inner surface  112 . 
       FIG. 6  is a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure. In the instant embodiment, the perforated pattern can have a plurality of discontinuous or non-interconnected tiny holes  116 . Specifically, the bar arm  110  has a plurality of holes  116  distributed across the outer surface  111 . The holes  116  have substantially similar diameters and have a larger distribution across the mid-section  110   c  than the two end portions (first and second end portions  110   a ,  110   b ). Moreover, the distribution of holes  116  is substantially similar across the two end portions  110   a ,  110   b.    
     Furthermore, the width of the bar arm  110  may vary, yet the second opening ratio of the mid-section  110   c  is still respectively larger than the opening ratios of the first and the second end portions  110   a ,  110   b . Please refer to  FIG. 7  as a two-dimensional view of the drug eluting depot stent in accordance with another embodiment of the instant disclosure. In the instant embodiment, the bar arm  110  has a first width W1 at the first and the second end portions  110   a ,  110   b , and a second width W2 at the mid-section  110   c . The first width W1 is substantially similar to the width W of the first crown  120 , and the second width W2 is smaller than the first width W1. In other words, the width of the bar arm  110  tapers from two end portions (first and second end portions  110   a ,  110   b ) towards the mid-section  110   c.    
     Moreover, in the instant embodiment, the perforated pattern also has a plurality of discontinuous or non-interconnected tiny holes  117  (as shown in  FIG. 7 ) distributed on the outer surface  111  of the bar arm  110 . The holes  117  have substantially similar diameters. In another embodiment, the holes  117  may have different diameters. In addition, the holes  117  are distributed along the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b . In the instant embodiment, the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b  each has one hole  117 , however, the number of holes  117  on the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b  is not limited to the example provided herein. 
     In the instant embodiment, the second width W2 of the bar arm  110  is smaller than the first width W1 such that the surface area A2 of the mid-section  110   c  is respectively smaller than the surface area A1 of the first end portion  110   a , and the surface area A3 of the second end portion  110   b . Although the holes  117  are distributed at the first end portion  110   a , the mid-section  110   c , and the second end portion  110   b , the diameters of the holes are substantially similar, however, the second opening ratio (B2/A2) are still the largest among all. 
     The perforated patterns on the outer surface  111  of the bar arm  110  in aforementioned embodiments are only exemplary and does not limit to the examples provided herein. The perforated pattern can also be formed on the inner surface  112  of the bar arm  110 . Alternatively, the perforated patterns can be blind holes separately formed on the outer surface  111  and the inner surface  112 . 
     As mentioned in previous embodiments, the first crown  120  and the second crown  121  usually do not have any openings on their respective surfaces. After the stent  1  radially expands, the first and second crowns  120 ,  121  typically have the maximum stresses on them. If the first and second crowns  120 ,  121  have the perforated pattern on their respective surfaces, the structure of the first and second crowns  120 ,  121  can be weakened such that the service life of the stent is substantially reduced. 
     In summary, the stent provided in accordance with the embodiments of the instant disclosure has at least an opening on the bar arm for drug retention. The structural strength of the bar arm gradually increases from the mid-section towards the two end portions due to the presence of one opening or multiple openings. As a result, when the stent is implanted into blood vessels, the bar arm shares more loadings with the crowns when the stent is radially expanded. In comparison to conventional stents, the stent of the instant disclosure has more uniform load distribution across its entirety, which enhances the fatigue life of the stent. Moreover, with the openings for drug retention on the bar arm of the stent, the service life of the stent is not reduced, but in contrast, is increased. 
     The figures and descriptions supra set forth illustrated the preferred embodiments of the instant disclosure; however, the characteristics of the instant disclosure are by no means restricted thereto. All changes, alternations, combinations or modifications conveniently considered by those skilled in the art are deemed to be encompassed within the scope of the instant disclosure delineated by the following claims.