Claims:We Claim:
1. A low thickness flexible tubular stent for blood vessel obstruction application comprising struts of suitable material formed continuously in a zigzag pattern and extending in a circumferential direction to form plurality of ring-shaped stent cell units; said ring shaped stent cell stent units being placed adjacent to each other in the axial direction of the stent body from a proximal to distal end;
two of the said axially adjacent ring-shaped cell stent units being connected by short straight links in a manner suitable to form ring shaped closed cell stent units, said ring shaped closed cell stent units being formed at both the proximal and distal end of the stent body;
other said ring shaped stent cell units are interconnected with each other and also to the said closed cell stent units through long straight links in the stent axial direction in a manner suitable to form plurality of open cell stent units, thereby positioning the said plurality of interconnected open cell stent units axially along the length of the stent body between the said closed cell stent units at the proximal and distal end of the stent body;
the outer circumference of the said stent units are expandable in a radial direction from a contracted state to an expanded state thereof;
wherein,
the zigzag struts of the open cell stent units have a greater strut width in the peak/trough section than in the arm section; and
the arm section of the zigzag strut has a section with at least two opposite curvatures along its length.
2. The stent as claimed in claim 1, wherein the said short straight links in the closed cells stent units are provided offset from the centre of the cell units.
3. The stent as claimed in claim 1, wherein the curvatures in the arm section is around 170 degree.
4. The stent as claimed in claim 1, wherein the open cell stent units consist of unequal number of open cells and are axially positioned in a consecutive manner along the length of the stent.
5. The stent as claimed in claim 4, wherein the unequal number of open cells are 18 and 15 in two consecutive axially positioned open cell units.
6. The stent as clamed in claim 1, wherein the peak/trough section of the zig zag strut has a strut width in the range of 240 - 320 µm and preferably 250 - 300 µm.
7. The stent as claimed in claim 1, wherein the strut width of the arm section of the open cell stent units consisting 18 cells is in the range of 150-200 µm and preferably 160-190 µm, while strut width of the arm section of open cell stent units consisting 15 cells is in the range of 150-200 µm and preferably 170-190 µm.
8. The stent as claimed in claim 1, wherein the closed cell stent units have 18 number closed cells both at the proximal and distal end.
9. The stent as claimed in claim 1, wherein the width of the said short straight link is in the range of 230 -280 µm and preferably 240-270 µm, while the width of the said long straight link is in the range of 400-470 µm and preferably 410-460 µm.
10. The stent as claimed in any preceding claim, wherein the wall thickness of the strut is in the range of 150-400 µm, preferably 300-380 µm.
11. The stent as claimed in claim in any preceding claim, wherein the stent has same strut wall thickness at proximal and distal end ranging from 380µm to 420µm while having lesser thickness at the intermediate region ranging from 350µm to 400µm.
12. The stent as claimed in any preceding claim, wherein the struts have three variable thickness distributed in five equal regions in such way that proximal and distal end region have highest wall thickness ranging from 380µm to 420µm, the very next region of the proximal and distal end has lesser wall thickness ranging from 350µm to 400µm and the central region of the stent has the least thickness ranges from 320µm to 380µm.
13. The stent as claimed in any preceding claim, wherein the closed cell length is in the range of 8 - 12 mm.
14. The stent as claimed in any preceding claim, wherein said open cell length in 18 open cell containing unit is in the range of 3 - 6 mm, while the said open cell length in 15 open cell containing unit is in the rage of 3 - 6 mm.
15. The stent as claimed in claim 1, wherein the proximal and distal end closed cell units are capable of being flared at the time of deployment for proper hold with the inner lumen of the blood vessel.

Dated this 5th day of January, 2021.

(SOUMEN MUKHERJEE)
IN/PA - 214
Applicants’ Agent
for seenergi IPR
, Description:SELF-EXPANDING STENT

Field of Invention:
The present invention relates to self-expanding stent. The invention particularly relates to a self expanding venous stent with high radial strength and flexibility required for low thickness vein vasculature/ anatomy (compared to artery).

Background of the Invention:
The blood vessel walls like arteries and veins are made up of tissues that may get damaged due to various causes leading to constriction and perforation of the vessels causing clinically relevant diseases like vessel perforation, ischemia, aneurysms, etc. To maintain patency/ structure of blood vessels, stenting in veins are performed such as inferior vena cava, iliac veins, femoral veins, and popliteal vein. Stents are generally used to keep the vessel lumen intact, open and to facilitate normal blood flow. Stent may be made up of stainless steel, cobalt chromium alloy, nitinol alloy, etc. Stent made up of nitinol alloy is widely used for making self-expanding stents, especially for peripheral vascular applications which include stenting in iliac, femoral popliteal and infra-popliteal arteries and veins. Stents implanted in the peripheral arteries and veins are exposed to high mechanical stresses from the surrounding environment, for example bending of the knee, twisting and compression during walking or running. Nitinol stents are able to handle these external forces better than other shape memory materials due to its characteristic properties of super elasticity, extreme stress and kink resistance. Nitinol stents are well suited for the tortuous vessel pathways of peripheral arteries and veins.
Based on the mechanical attributes, anatomy and the physiology of the vessel, the corresponding stent has to meet requirements for safe performance. The biological and mechanical response of a vessel to the stent is another fact that should be considered for long-term performance of the stent. Venous stents are designed for venous disease treatment, and in comparison to arterial stents, the venous stent has not been sufficiently investigated. The mechanical properties of venous stents are not necessarily same as arterial stent, thus, it is not always effective to treat a venous disease by arterial stent. The requirement of venous vasculature comprises of high radial force, crush resistance and excellent flexibility to sustain in the venous environment.
Conventionally, nitinol stent for venous vasculature exhibits greater wall thickness having desired strength and flexibility to treat venous vasculature. Moreover, stent comprises straight strut of the cells where stress is distributed only at one to two points which may affect the radial strength of stent. Due to greater wall thickness of stent, it requires longer duration for endothelial layer formation and thus relatively more time to heal at treatment site. Stent comprises only longer straight link may damage blood vessel during bending movement of the stent as crowns go outward and thus it may affect flexibility of the stent post implantation. The closed cells are connected with links which are located at the centre of the closed cells located at the proximal and distal end thereby hampering the flexibility and flaring capabilities of the stent. Figures 1-3 show the stent which are currently in use.
Therefore, there is a need for development of a stent which can overcome the drawbacks of the existing stents.

Object of the Invention:
It is the primary object of the invention to provide a stent for venous vasculature which can maintain patency of the blood vessel and regulate the blood flow.
It is another object of the present invention to provide a stent with desired radial strength and high flexibility with optimal strut width and reduced wall thickness.
It is yet another object of the present invention to provide a stent with desired radial strength and high flexibility by varying the strut design and width in various sections of the stent body.
It is another object of the present invention to provide a stent with desired radial strength and high flexibility by designing multiple load distribution points in the struts of the stent.
It is yet another object of the present invention to provide a stent with desired radial strength and high flexibility along with better flaring capability on deployment to hold a greater area of the blood vessel and to reduce the stent displacement possibility.
It is another object of the present invention to provide a stent with desired radial strength and high flexibility along with better flaring capability on deployment, for faster endothelial layer formation at the healing site.
It is a further object of the invention to provide a stent for venous vasculature, which is easily deployable.
The above and the other objects of the invention will be described in details with the help of the drawings in the following detailed description of the invention.

Brief Description of the Drawings:
Figure 1-3 show the stent of the prior art.
Figure 4 shows the stent according to an embodiment of the present invention.
Figure 4A shows the various dimension areas of the stent according to an embodiment of the present invention.
Figure 5 shows the various sections of the zig zag struts of the stent according to an embodiment of the present invention.
Figure 5A shows the various dimension areas of the zig zag struts of the stent according to an embodiment of the present invention.
Figure 6 shows closed cell units of the stent according to an embodiment of the present invention.
Figure 7 shows the stent according to an embodiment of the present invention in the contacted state within the catheter.
Figure 8 shows the stent according to an embodiment of the present invention in the partially deployed state in combination with the catheter.
Figure 9 shows the stent according to an embodiment of the present invention
in the fully deployed state in combination with the catheter.

Summary of the Invention:
Accordingly, the present invention provides, a low thickness flexible tubular stent for blood vessel obstruction application comprising struts of suitable material formed continuously in a zigzag pattern and extending in a circumferential direction to form plurality of ring-shaped stent cell units. Said ring shaped stent cell stent units being placed adjacent to each other in the axial direction of the stent body from a proximal to distal end of the stent body.
Two of the said axially adjacent ring-shaped cell stent units being connected by short straight links in a manner suitable to form ring shaped closed cell stent units, said ring shaped closed cell stent units being formed at both the proximal and distal end of the stent body. Other said ring shaped stent cell units are interconnected with each other and also to the said closed cell stent units through long straight links in the stent axial direction in a manner suitable to form plurality of open cell stent units, thereby positioning the said plurality of interconnected open cell stent units axially along the length of the stent body between the said closed cell stent units at the proximal and distal end of the stent body. The outer circumference of the said stent units is expandable in a radial direction from a contracted state to an expanded state thereof. The zigzag struts of the open cell stent units have a greater strut width in the peak/trough section than in the arm section. The arm section of the zigzag strut has a section with at least two opposite curvatures along its length.
In an aspect, the short straight links in the closed cells stent units are located offset from the centre of the cell units.
In an aspect the curvatures in the arm section is around 170 degree.
In an aspect, the open cell stent units consist of unequal number of open cells and are axially positioned in a consecutive manner along the length of the stent. The unequal numbers of open cells are 18 and 15 in two consecutive axially positioned open cell units.
In an embodiment, the peak/trough section of the zig zag strut has a strut width in the range of 240 -320 µm and preferably 250 to 300µm.
In an embodiment, the strut width of the arm section of the open cell stent units consisting of 18 cells is in the range of 150-200 µm and preferably 160-190 µm, while that of the arm section of open cell stent units consisting of 15 cells is in the range of 150-200 µm and preferably 170-190 µm.
In an embodiment, the closed cell stent units have 18 number closed cells both at the proximal and distal end.
In an embodiment, the width of the said short straight link is in the range of 230 -280 µm and preferably 240 -270 µm, while the width of the said long straight link is in the range of 420 -460 and preferably 410 -460 µm.
In an embodiment, the wall thickness of the strut is in the range of 150-400 µm, preferably 300-380 µm.
Preferably, the stent has same strut wall thickness at proximal and distal end ranging from 380µm to 420µm while having lesser thickness at the intermediate region ranging from 350µm to 400µm.
In another embodiment, the struts have three variable thickness distributed in five equal regions in such way that proximal and distal end region have highest wall thickness ranging from 380µm to 420µm, the very next region of the proximal and distal end has lesser wall thickness ranging from 350µm to 400µm and the central region of the stent has the least thickness ranges from 320µm to 380µm.
In an embodiment, the closed cell length is in the range of 8 -12 mm.
In an embodiment, the open cell length in 18 open cell containing unit is in the range of 3 - 6 mm, while the said open cell length in 15 open cell containing unit is in the rage of 3 - 6 mm.
In an aspect, the proximal and distal end closed cell units are capable of being flared at the time of deployment for proper hold with the inner lumen of the blood vessel.

Detailed Description of the Invention
In order to achieve the desired objectives and to overcome the drawback of the prior art, the inventors have come up with a flexible stent for deployment at the blocked blood vessel specially in venous system, with desired thinner strut width without compromising the other desired characters of the stent such as radial strength, flexibility, ease of deployment, faster growth of tissue around the stent for patient stabilization etc.
The stent of the present invention is a hybrid design having open cells and closed cells geometry. The hybrid design consists of closed cells at the proximal and distal end connected with each other by short straight links and consecutive axially positioned open cells which are interlinked with each other as well as the closed cells by long straight link. Long straight link is interlinked with the cells in such a way that it helps to enhance flexibility of the stent without compromising structural integrity.
Open cells are distributed unequally which helps to achieve desired flexibility during bending without compromising structural integrity and to maintain smooth patency of the stent. This also prevent blood vessel from damage post implantation of stent.
The stent is formed by zigzag shape of strut having multiple inter radiuses which reduce the stress and helps to achieve good radial strength. Further, different strut width at different portions of the stent helps to achieve desired radial strength by distributing stress equally at every portion of the stent body.
To enhance the performance of the stent, optimal strut width and reduced strut wall thickness of the stent help to achieve the desired radial strength to maintain patency in the blood vessel.
Closed cell design forms rings or crowns which are interlinked with the help of short straight link positioned at offset from the center of the cells. Due to offset link positioned at both ends of stent, the desired angle to form a flare shape is possible which helps the stent to contact more surface area of the blood vessel thereby improving the holding capacity thus preventing migration of the stent. This also avoids edge injury during deployment and post implantation.
The stent geometry helps to smoothly and easily load into the catheter without overlapping of strut and provides uniform crimping profile and thus helps in uniform self-expansion of stent without affecting structural integrity of the stent.
The embodiments of the invention are now more fully described with the reference to the drawings.
Figure 4 shows a low thickness flexible tubular stent (1) for blood vessel, specially venous obstruction removal application according to a preferred embodiment of the present invention.
For the sake of clarity and uniformity, as shown in Figures 1 and 4, the left side of the stent is termed as the proximal end while the right hand side of the stent is the distal end.
The stent (1) is made of struts (2) of suitable material such as nitinol and are formed continuously in a zigzag pattern extending in a circumferential direction to form plurality of ring-shaped stent cell units or crowns (3’, 3’’, 4’, 4’’). As can be seen in Figure 5 and as is known to a skilled person that the continuous zig zag pattern of the strut consists of sharp bend portions forming peak and trough (7, 8) and an arm section (9) extended between each said peak and trough.
The ring shaped stent cell units are placed adjacent to each other in the axial direction of the stent body from a proximal to distal end to form a tubular stent structure. The outer circumference of the said stent units is expandable in the radial direction from a contracted state to an expanded state.
Two of the said axially adjacent ring-shaped cell stent units (3’, 3’’) are connected by short links (5) suitably to form ring shaped closed cell stent units (3). The ring shaped closed cell stent units (3) are formed at both the proximal and distal end of the stent body.
As shown in Figure 6, the said closed cell stent units (3’, 3’’) are formed by connecting each adjacent peak and trough of the two axially adjacent zigzag struts through short links (5).
In an embodiment, as shown in Fig 6, the closed cells are interlinked with the help of short straight link (5) positioned slightly offset (13) from the center of the closed cells to achieve the desired angle of flare shape ranging from 165° to 175° at both the ends for desired holding capacity of the stent. Such an angle of flare also engages with greater contact surface area of the blood vessel thus preventing migration of stent from the treatment site.
In an embodiment, the closed cell stent units have 18 number of closed cells both at the proximal and distal end. However, the cell number may vary according to the diameter of the stent.
Diameter of the expanded stent ranges from 8mm to 25mm, more preferably from 10mm to 22mm. The expanded stent length ranges from 30mm to 200mm, more preferably 40mm to 160mm.
In an embodiment, the struts (2) of the closed cell units have uniform strut width.
The other said ring shaped stent cell units (4’, 4’’) are interconnected with each other and also to the said closed cell stent units (3) at the proximal and distal end of the stent through long straight links (6) in the stent axial direction in a manner suitable to form plurality of open cell stent units (4), thereby positioning the said plurality of interconnected open cell stent units (4’, 4’’) axially along the length of the stent body between the said closed cell stent units (3’, 3”) at the proximal and distal end of the stent body.
In one aspect of the invention, the said open cell stent units are formed by connecting a few of the selected adjacent peaks and trough of the two axially adjacent zigzag struts through long straight links (6), whereby each open cell units (4) consists of 18 or 15 i.e. unequal number of open cells (11, 12).
The open cell stent units with unequal number of cells are axially positioned in a consecutive manner along the length of the stent so that if one open cell unit (4’) consists of 18 cells (11), the next axially placed open cell unit (4”) consists of 15 cells (12). However, as would be clear to a skilled person, the number of units of open cells is not constant and may vary according to the varying circumference of the stent. For example, open cells may be distributed in unequal numbers ranging from 10 to 25 in the open cells depending on the stent diameter.
According to the invention, the zigzag patterned strut of the open cell stent units (4’, 4”) has different strut widths in the peak/trough (7, 8) section and the arm section (9), wherein the zigzag struts of the open cell stent units have a greater strut width in the peak/trough section than the arm section.
According to the invention, as shown in Figure 5, the arm section (9) of the zigzag strut has a section (10) with at least two opposite curvatures along its length. In an aspect, as shown in Figure 5A, the said two opposite curvatures in the arm section has an angle (L) of around 170 degree. However, number of curvatures and angle may vary according to the size of the stent and the required strut length for achieving desired radial strength.
Figs. 4A, 5A and the following Table 1 provide various preferred dimensions of the various parts of the stent of the preferred embodiment of the present invention. The dimensions are applicable for a stent dia of 14, 16mm and stent length of 40, 60, 80, 100, 120, 160 mm.
Refer Figures 4A & 5A Stent design description Post Processing Broader range of Dimension Post Processing Range of Preferred Dimension
A. Strut Width Peak and Trough sections 240-320 µm 250-300 µm
B. Strut Width (18 cell Open/ Closed cell strut width) 150-200 µm 160-190 µm
C. Strut Width (15 cell open cell strut width) 150-200 µm 160-190 µm
D. Short Straight Link (Proximal) width 230 -280 µm 240 -270 µm
E. Long Straight Link width 400 -470 µm 410 -460 µm
F. Short Straight Link width (Distal) 230 -280 µm 240 -270 µm
G. Wall Thickness 150-400 µm 300-380 µm
H. Marker Cavity Dia 450-580 µm 470-560 µm
I. Closed Cell Length (Single Crown 18 cell) -- 8 -12 mm
J. Open Cell Length (Single Crown 18 cell) -- 3 - 6 mm
K. Open Cell Length (Single Crown 15 cell) -- 3 - 6 mm
L. Both Opposite Curvature Angle -- 170°
M. Flared shape Angle on Deployment -- 165°-175°

Table 1
In an aspect, the peak/trough section has a strut width 240 -320 µm, after electropolishing. More preferably, the range can vary between 250 -300 µm.
In an aspect, the strut width of the arm section (9) of the open cell stent units consisting of 18 cells (11) is 150-200 µm and more preferably from 160-190 µm and that of the arm section (9) of open cell stent units consisting of 15 cells (12) is 150-200 µm and more preferably ranges from 170-190 µm after electropolishing.
In a preferred embodiment, the width of the said short link (5) is 230-280 µm and more preferably the range can vary from 240 - 270 µm. The width of the said long link is in the range between 400 and 470 µm and more preferably between 410 and 460 µm, after electropolishing.
Therefore, in one embodiment, in respect of the whole stent the proximal and distal end located closed cell short straight link width is in the range from 230 - 280 µm and more preferably from 240µm to 270µm, while the intermediate open cell long straight link width is in the range from 400 - 470 µm and preferably from 410µm to 460µm.
This different link widths help to achieve desired flexibility of the stent and provide bending radius ranging from 30mm to 40mm for 100 mm length stent, for maintaining smooth patency of stent and prevent damage of blood vessel during bending movement of stent.
In an embodiment, said closed cell length (3, 3”) is in the range between 8 and 12 mm.
In an embodiment, the said open cell length in 18 open cell containing unit ranges from 3 to 6 mm and the said open cell length in 15 open cell containing unit ranges from 3 to 6 mm.
In an embodiment the wall thickness of the strut preferably ranges from 300 to 380 µm which is thinner than the presently available stent for venous obstruction removal application.
In one aspect, different wall thickness of the strut may be used in different areas of the stent.
In one embodiment stent has two variable wall thickness in which 60mm stent has same thickness at proximal and distal end ranging from 380µm to 420µm while having lesser thickness at the intermediate region ranging from 350µm to 400µm.
In another embodiment the stent may have three variable thicknesses in a 60mm stent that is distributed in five equal regions of 12mm in such way that proximal and distal end region have highest wall thickness ranging from 380µm to 420µm, the very next region of the proximal and distal end has lesser wall thickness ranging from 350µm to 400µm and the central region of the stent has the least thickness ranges from 320µm to 380µm.
This feature of the stent adds more flexibility at intermediate region and better holding capacity & strength at both ends of the stent. Thus, combining of strength and flexibility helps to maintain crushing ability due to high forces and enough flexibility to align and accommodate with implantation environment.
In an embodiment, the desired expanded diameter of the stent ranges from 8mm to 25mm, more preferably from 10mm to 22mm. The expanded stent length ranges from 30mm to 200mm, more preferably 40mm to 160mm.
Expanded stent has reduced wall thickness ranges from 150µm to 400µm, more preferably from 300µm to 380µm. Reduced wall thickness of stent beneficial towards tissue growth rate on stent over period of time.
In a preferred embodiment, close cells are formed in a diamond shape as shown in fig. 4 which helps in equal expansion of stent.
In an aspect, both ends of stent have arrangements to attach number of radiopaque markers (14) ranging from 2 to 15, more preferably from 4 to 10 which help to track and position the stent at treatment site under fluoroscopic process.
The marker hole diameter ranges from 450-580 µm, more preferably from 470-560 µm.
The process for manufacturing the stent comprise several processes such as laser cutting, grinding, honing, shape setting, sand-blasting and electro-polishing processes. To get the desired shape of the stent required design of stent which is cut from tube under laser cutting operation where hybrid design of the stent is formed by laser cutting machine. During this process, burrs are generated as the metal gets melt due to high frequency of the laser beam.
After laser cutting process, grinding process is performed to remove burrs which are generated during laser cutting operation. To perform grinding operation, several abrasive files were used as per the inner diameter of laser cut tube. After grinding process, honing process is performed to get smooth surface from inner diameter of the laser cut tube. This process is performed by applying fluids like glycol and Alconox® detergent powder i.e. a mixture of Sodium tripolyphosphate, Sodium Alkyl benzene Sulfonate and Tetrasodium Pyrophosphate which is anionic detergent used for manual and ultrasonic cleaning and is excellent replacement for corrosive acids and damaging solvents.
In another embodiment, grinding is performed in such a manner to achieve variable strut thickness at different regions of the stent. In order to achieve variable thicknesses at different regions of the stent variable size files ranging from 2.70mm to 3.75mm are used to achieve precise bore diameter as required. Masking is done to the desired area of stent followed by electro-polishing process over the surface of stent. The process is repeated until desired wall thickness is achieved.
After the grinding & honing process, desired shape of stent is achieved by performing multiple steps of shape setting process using mandrel or mold in high temperature oven or furnace. Further, open cells are equally adjusted with the help of needle and each strut is also adjusted to obtain desired shape of the stent by using different mandrels or mold. Flare shape of the stent at both ends is obtained with the help of taper ring which is assembled with the mandrel by Allen screw at both side of the stent. To check the length of the stent, gauge is prepared which comprises grooves at both ends where taper ring with the stent is placed to confirm the stent overall length. By assembling of the stent and taper ring over mandrel, shape setting process is performed to get the desired shape of the stent. Stent is dipped into Aluminum white sand at specific temperature for specified time for effective shape setting. Shape setting process enhances internal grain hardness and relieves stress. On the other hand, inappropriate temperature leads to the destruction of shape memory of stent.
Sand blasting is used to provide highly smooth surface. Devices such as stents, shunts, cages, etc. are commonly blasted to de-burr them and to remove the oxide layers from surfaces and also to remove pulse marks and striations left by laser machining, cut or remove laser slag. This also decreases the propensity for micro cracking and lightly texture the surface to improve adhesion characteristics during sand blasting. Aluminum oxide power is struck on the device surface at certain velocity. Due to abrasion, the micro cracks and burrs are removed. Highly smooth surface with the increase in surface texture of stent after sand blasting is obtained. After sandblasting, electro-polishing is done.
In the present invention, mixture of perchloric acid and acetic acid is used during the process. To achieve electro-polishing of a rough surface, the protruding parts of a surface must dissolve faster than the recesses. By performing electro-polishing process over the stent, smooth surface finish without any cracks is obtained. The oxide layer generated during sandblasting process is also removed. If the sand blasting process is not proper, it directly affects the stent property during electro-polishing process and also affects the stent mechanical properties.
After electro-polishing process, radiopaque markers are attached or integrated into the design of the stent. Tantalum markers of riveted, coined, ellipses, rectangular shape are attached into stent by laser welding process. The presence of laser cut nitinol marker in addition to tantalum marker offer enhancement in radiopacity.
Further, the stent is led towards loading process where it is loaded with the help of quill and crimping machine. By providing relative motion of both the stent and catheter, the stent is smoothly and uniformly loaded onto the delivery catheter ranging from 8F to 12F as shown in fig. 7 without affecting the stent’s structural integrity.
For deployment of the stent, as per the standard practice access site is prepared. Then, guide wire of 0.035” (0.89mm) is advanced to the location of lesion. If necessary, the treatment site is pre-dilated. After pre-dilation, the guide wire is kept for stent system advancement. The delivery system is placed over the guide wire and the delivery system is advanced as a single unit through the hemostatic valve of the introducer sheath. The stent is placed at the treatment site by advancement of delivery system till stent reaches the center region of the lesion as confirmed by stent marker position. Once the distal stent radiopaque markers start separating, the stent starts getting partially but uniformly deployed as shown in fig. 8. It is further led to full deployment as shown in fig.9 at the treatment site without affecting structural integrity. The blood vessel which gets narrowed/obstructed and treated with venous stent leads towards reconstruction of the blood vessel by maintaining patency and regulate the blood flow
The example of Venous Stent design trial is explained below:
Example 1:
According to prior art design the stent was laser cut with straight strut design having 60mm length and expanded to 16mm diameter and wall thickness of around 450µm. The straight strut comprises equal strut width around 180µm throughout the stent length. These straight struts are interlinked with the help of long straight link and having an equal link width around 230µm throughout the stent length. Further, at both ends of stent, closed cells comprised long straight link positioned at center of cell to achieve a flared angle at both ends around 174° which help in holding capacity and prevent migration of stent from treatment site.
Radial strength measured of straight strut design was around 52N. Moreover, stent was bent at a radius of around 18mm and further bending results in outward protrusion of cell towards outer surface of stent when bending to more than 18mm of stent geometry.
Example 2:
The stent was laser cut with zigzag strut design according to the present invention wherein the peak and trough portions have greater width than the arm section. Also, the arm section has the two opposite curvatures as additional load distribution points. These struts are inter-linked with the help of short and long straight links with different link width at proximal, intermediate and distal portion where proximal and distal closed cell link width is around 260µm, and middle open cell link width around 440µm. The stent is 60mm in length and capable of being expanded to 16mm diameter and wall thickness is around 350µm.
The radial strength of zigzag design stent according to the present invention was found to be 54N.
This zigzag stent geometry distributes stress equally because of more stress distribution points and is beneficial during compression force exerted on the stent. Moreover, when short straight link design stent was bend at a radius of around 22mm, there was no protrusion of cell towards outward surface of stent.
Further, at both ends of stent, closed cells comprise short straight link positioned at slightly offset from the centre of the cell for achieving a flare angle at both ends around 171°. This helps in good holding capacity and further prevents migration of stent from the treatment site. Due to offset position of link present at both ends, stent has more contact surface area post deployment at the proximal and distal ends and thus have greater holding capacity.
Due to reduced wall thickness, the stent has uniform crimping profile and smoothly loads into the 10F catheter of delivery system and can have uniform deployment without affecting stent structural integrity.
The aforesaid experimental data shows that the stent with straight strut design with higher wall thickness comprising equal strut and link width provides less satisfactory results in terms of radial strength and flexibility. As at both ends of the stent, close cells are interlinked and positioned at the center of the cells, this engages with less contact surface area of the blood vessel. The lower flare shape angle also directly affects the holding capacity of stent. Due to thicker wall thickness, there will be delay in formation of endothelial layer resulting in delay in healing post implantation of device.
The stent of the present invention comprising short straight link and different strut width provides desired results in terms of radial strength and flexibility even with reduced wall thickness. The holding capacity is also enhanced due to the offset position of short straight links in closed cell stent units at both proximal and distal end of the stent providing more contact surface area post deployment and prevents the migration of stent from its location. The reduced wall thickness also provides uniform crimping profile which helps in smoothly loading the stent into lower French catheter ranging from 8F to 12F. This leads towards uniform deployment of the stent at the treatment site without affecting stent structural integrity.
While the invention has been described with the help of preferred embodiments, several modification and improvements are possible without departing from the scope of the invention as described above and as defined in the claims herein below.