Apparatus and methods for bidirectional hyperelastic stent covers

An apparatus and method for a micro-patterned thin film Nitinol (TFN) that is used as a cover for an expandable stent structure, and has elongation/expansion properties that are configured to match the elongation/expansion properties of the expandable stent structure is presented.

INCORPORATION-BY-REFERENCE OF COMPUTER PROGRAM APPENDIX

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to endovascular stents, and more particularly to thin-film covers for endovascular stents and methods of fabricating the same.

2. Description of Related Art

Aneurysms can occur in the neurovasculature. An aneurysm is a spherical out-pouching of blood vessels formed from a localized weakness in the wall of an artery. Aneurysms can occasionally rupture and cause a life threatening hemorrhage. Postmortem examinations indicate that 10˜12 million people have brain aneurysms in the United States and 20˜50% will potentially rupture. Aneurysm rupture carries a high rate of morbidity and mortality. Current approaches to prevent aneurysms from rupturing include both surgical and transcatheter methods.

Recent advancements have provided covered stents that have a low profile and flexibility for use in the neurovasculature. The cover preferably comprises a porous, hydrophilic surface to prevent platelet adhesion.

In conventional film covered stents, the porous film used generally does not match the desired deformation ratio of the stent, preventing it from conforming to the stent deformation, which can result in undesired wrinkling or failure of the mesh material.

Accordingly, an object of the present invention is a micropatterned thin film nitinol (MTFN) covered stent that overcomes the problems associated with current-generation endovascular technologies.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the invention, a micro-patterned thin film Nitinol (TFN) sheet that is used as a cover for an expandable stent structure, and has elongation/expansion properties that are configured to match the elongation/expansion properties of the expandable stent structure.

Another aspect is a fabrication method/process for manufacturing a thin film Nitinol (TFN) stent cover by sputter deposition using a heated target and a novel lift-off micromachining technique.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates schematic diagram of a stent assembly10comprising an expandable stent12having a plurality of struts14, and covered with the Nitinol film16of the present invention. Stent assembly10is particularly suited for use and delivery as an implant for small vessels such as intracranial aneurysms. Nitinol film is porous with a series of very small micropores, withsurface adhesion properties and to allow flow of through the film while occluding the aneurysm without significant thrombus formation. The Nitinol film may be fabricated with any number of surface treatment methods, such as those disclosed in PCT International Publication No. WO 2010/102254 published on Sep. 10, 2010 and republished on Jan. 20, 2011, which publications are incorporated herein by reference in their entireties. The film16may be adhered to the stent12using conventional techniques, such as: suturing, laser welding, polymer adhesive, or the like.

Referring toFIG. 1, the expandable stent12generally will experience axial shortening (Ax) along the horizontal direction of the stent (e.g. axis H) in response to radial expansion (Rx) in the vertical or radial direction V. The film cover will also experience axial shortening (Ax) along the horizontal direction of the stent (e.g. axis H) in response to radial expansion (Rx) in the vertical or radial direction V. However, without the configuration or features of the film16of the present invention, the horizontal or axial shortening (Ax) of the film will likely experience axial shortening (Ax) of a different rate or magnitude than that experienced by the stent, causing potential for wrinkles, folding, kinking, and/or failure.

FIG. 2shows a schematic diagram of a preferred embodiment of the pore array of film16of the present invention (not to scale). Film16comprises a series of alternating, undulating struts22that define a plurality of pores18between them. In a preferred embodiment, pores18have a diamond shape pattern with a gradually curving apex or mid-curve24, and are dimensioned with a height h several times smaller than the width w, e.g. h of approximately 30 μm and width w of approximately 200 μm, with a strut wall thickness t and sheet thickness of approximately 5 μm. Since the covering film16is porous, the deviation between the deformation of the film and stent when stretched from a compressed configuration can be problematic, and thus the film16of the present invention provides the shape of the pores and the size of the bordering struts22to accommodate for this phenomenon.

FIG. 3shows preferred shape and sized characteristics for a single pore18of the thin-film cover16of the present invention.FIG. 3is illustrated in graph form, with dimensions extending from the mid-curve point24to define the pore shapes. The pore18shape ofFIG. 3delivers a shortening of approximately 41% in the horizontal direction when stretched approximately 400% in the vertical direction (horizontal and vertical directions defined as provided in the axis of the graph shown inFIG. 3). Functionally, in relation to the stent, horizontal shortening in the axial direction of the stent12, and vertical stretching occurs in the radial direction of the stent12. Preferred dimensions include the thickness t of the strut22(in plane) of approximately 0.005 mm, and width or thickness of the film (out of plane) of approximately 0.005 mm. A distinct feature of the configuration of cover16ofFIG. 3is the precise combination of strut22length (i.e. width w between mid-curve points24) and curvatures of the mid curve24defining the pore18shape (and corresponding ratio to height h). This pore18shape provides the correct ratio for commercial self-expanding woven stents used for endovascular procedures.

Referring toFIG. 3, it was noted that small deviations from the optimal design induce large changes in the deformation ratio.

FIG. 4illustrates a schematic diagram of a covered stent assembly10disposed within a delivery catheter20. The covered stent10is introduced in the vessel20by crimping (compressing) it into a catheter and then, when released, the self expanding stent will bring the diameter of the assembly10to conform to the enclosing vessel20. During this process, the film16will substantially conform to the deformation of the stent12to avoid wrinkling, kinks, folds and/or failure. Because the stent12changes its length when crimped, so must the film in the same proportion. Thus, the film16is configured to deform to a prescribed ratio between the elongation in one direction and the shortening in the orthogonal direction.

In its collapsed form, the stent12and TFN16preferably fit through a catheter20having an ID of approximately 0.27 inches at most, and when extruded from the catheter, the stent needs to expand to a diameter of approximately 3.0 mm to approximately 3.5 mm (or more). This determines the ratio between the elongation in the vertical direction and the contraction in the horizontal direction (as defined inFIG. 3). Thus, the TFN16of the present invention achieves a desired ratio between radial elongation Rx(vertical direction V) and axial shortening Ax(horizontal direction H), while imposing stress and strain feasibility constraints. The method of solution is based on a gradient search. Sensitivities are calculated using finite differences. More particularly, the design problem is posed as follows:

It is appreciated that the parameters used determine the pore shape that may be varied to achieve the desired elongation ratio (e.g. end coordinates and slopes for the splines/struts22), length of the struts22that define the pore18, thickness t, etc. While the pore18configuration ofFIG. 3is a preferred embodiment using diamond shapes, it is appreciated that other shapes may be implemented to achieve the elongation ratio desired to be achieved.

In one optimization method, a gradient-based technique was used to minimize the distance between the existing ratio and the desired ratio, with sensitivities using finite differences, and commercial finite element code for structural analysis.

The numerical implementation of one optimization method was based on a finite element model for the struts22that define the pore shape18. The pore18is elongated in the vertical direction and the horizontal contraction is determined using the results predicted by the model. The derivatives for this contraction with respect to the design variables are estimated using finite differences using the same finite element analysis model. With this information, the negative of the gradient of the objective function was used as the direction of maximum descent to improve the design in that direction. The procedure was repeated until convergence.

Using the method described above, the optimization procedure above allows for a variety of different designs if needed. In this respect, other shapes can be used, for example hexagons (only the diamond shape design is shown here in detail). Also, the target ratio may also be changed as needed to accommodate other types of stents12.

Advantageously, when the film has a deformation ratio that allows it to conform to the stent deformation, unwanted kinks, folds or failures due to the crimping/expanding process is avoided. This in turn will allow a much improved behavior in the biological system.

Referring now toFIG. 5, the patterned thin film Nitinol (TFN) stent cover16detailed inFIG. 1throughFIG. 4above can be fabricated according to fabrication method/process100. In fabrication method100, the thin film is manufactured by sputter deposition using a heated target and a novel lift-off micromachining technique. In a preferred embodiment, the lift-off technique comprises the steps illustrated schematically inFIG. 5.

At first step102, positive photoresist52, e.g. AZ 5214, is spin coated onto a 4 inch Silicon (Si) wafer substrate50and then the substrate is soft baked at 110° C. for 90 seconds to drive off the excess solvent. To define desired patterns in the photoresist52, the substrate and the chromium mask with desired pattern are then exposed to UV light for 12 seconds. The exposed photoresist is developed using AZ400K and then hard baked at 120° C. for 120 seconds.

At second step104, the unexposed Si area of substrate50is used to create 50 micron deep trenches54using a Deep Reactive Ion Etching (DRIE) technique with an etch rate of approximately 10 microns per minute.

Subsequently at step106, the substrate50is chemically cleaned to remove all the photoresist residues52on the surface.

In the next step108, a 500 nm thick Copper (Cu) sacrificial layer56is deposited on the patterned substrate50using e-beam evaporation at a deposition rate of 0.5 nm per second. Following this, a 500 nm thick Silicon dioxide (SiO2) inhibition layer58is deposited using the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique onto the Cu layer at a deposition rate of 4 nm per second to prevent a reaction between Cu and the Nitinol film.

At step110, TFN layer60is then deposited using a DC magnetron sputtering process onto the substrate described above using a hot target technique in an Argon pressure of 6×10−3Torr. The target temperature of 650° C. is maintained during the film deposition to sustain the uniform stoichiometry throughout the film surface. Base vacuum of 7×10−8Torr, DC power of 300 Watts, and substrate to the target distance of 4 cm are used as other process parameters to obtain 5 micron thick Nitinol film60at a sputtering rate of 2 nm per second.

Finally at step112, the Cu sacrificial layer56is removed with ferric chloride solution to lift-off the TFN60with SiO2layer58from the substrate52. The SiO2layer58is then removed from the TFN60using a buffered oxide etchant (BOE: HF-based wet etchant). To finish off step112, the stand-alone patterned TFN60is crystallized at 500° C. for 120 min in a vacuum less than 2×10−7Torr.

From the discussion above it will be appreciated that the invention can be embodied in various ways, including but not limited to the following:

1. A thin-film cover for a vascular implant, comprising: the thin-film sheet comprising a plurality of pores having a size and shape defined by adjacent struts; the thin-film sheet configured to be formed into a tubular cover disposed radially adjacent the vascular implant; the vascular implant comprising a tubular structure having a central axis and first compressed configuration and a second expanded configuration that is radially outward along the central axis from the first compressed configuration; the thin-film sheet configured to radially expand along with the vascular implant from the compressed configuration to the expanded configuration; wherein the thin-film sheet is configured to shorten in the axial direction along the central axis upon expansion from the compressed configuration to the expanded configuration; wherein shape and size of the pores of the thin-film sheet are configured such that the thin-film sheet shortens in the axial direction a pre-determined distance; said predetermined distance corresponding to axial shortening of the vascular implant.

2. A cover as in any of the previous embodiments, wherein the axial shortening of the thin-film sheet and vascular implant is a function of deformation resulting from the expansion from the compressed configuration to the expanded configuration.

3. A cover as recited in any of the previous embodiments, wherein the vascular implant comprises an expandable stent.

4. A cover as in any of the previous embodiments, wherein the adjacent struts comprise undulating, alternating struts configured to define individual pores having a radial height and axial width.

5. A cover as in any of the previous embodiments, wherein the axial width of the pores is several times larger than the radial height of the pores.

6. A cover as in any of the previous embodiments, wherein the individual pores are diamond-shaped.

7. A cover as in any of the previous embodiments: wherein the undulating struts are curved such that each said pore has a mid-curve point; and wherein an approximate 400% radial stretching of a pore at the mid-curve point results in an approximate 41% axial shortening of the pore.

8. A cover as in any of the previous embodiments, wherein the axial shortening of the thin-film sheet matches the axial shortening of the vascular implant.

9. A cover as in any of the previous embodiments, wherein the thin-film sheet comprises Nitinol.

10. A cover as in any of the previous embodiments, wherein the thin-film sheet has a thickness of approximately 0.005 mm.

11. A cover as in any of the previous embodiments, wherein the adjacent struts have a width of approximately 0.005 mm.

12. A vascular implant, comprising: an expandable stent; the expandable stent comprising a tubular structure having a central axis and first compressed configuration and a second expanded configuration that is radially outward along the central axis from the first compressed configuration; the expandable stent configured to be delivered to a target location in said compressed configuration and then expanded to the expanded configuration at the target location within the vessel; the expandable stent configured to shorten in the axial direction along the central axis upon expansion from the compressed configuration to the expanded configuration; and a thin-film sheet coupled to the expandable stent; the thin-film sheet configured to be formed into a tubular cover disposed radially adjacent to the expandable stent; the thin-film sheet comprising a plurality of pores having a size and shape defined by adjacent struts; the thin-film sheet configured to radially expand along with the expandable stent from the compressed configuration to the expanded configuration; wherein the thin-film sheet is configured to shorten in the axial direction along the central axis upon expansion from the compressed configuration to the expanded configuration; wherein shape and size of the pores of the thin-film sheet are configured such that the thin-film sheet shortens in the axial direction a pre-determined distance; said predetermined distance corresponding to axial shortening of the expandable stent.

13. An implant as in any of the previous embodiments, wherein the axial shortening of the thin-film sheet and expandable stent is a function of deformation resulting from the expansion of the from the compressed configuration to the expanded configuration.

14. An implant as in any of the previous embodiments, wherein the adjacent struts comprise undulating, alternating struts configured to define individual pores having a radial height and an axial width.

15. An implant as in any of the previous embodiments, wherein the axial width of the pores is several times larger than the radial height of the pores.

16. An implant as in any of the previous embodiments, wherein the individual pores are diamond-shaped.

17. An implant as in any of the previous embodiments: wherein the undulating struts are curved such that each said pore has a mid-curve point; and wherein an approximate 400% radial stretching of a pore at the mid-curve point results in an approximate 41% axial shortening of the pore.

18. An implant as in any of the previous embodiments, wherein the axial shortening of the thin-film sheet matches the axial shortening of the vascular implant.

19. An implant as in any of the previous embodiments, wherein the thin-film sheet comprises Nitinol.

20. An implant as in any of the previous embodiments, wherein the thin-film sheet has a thickness of approximately 0.005 mm.

21. An implant as in any of the previous embodiments, wherein the adjacent struts have a width of approximately 0.005 mm.

22. A method of fabricating a thin-film Nitinol (TFN) cover for a stent, comprising: spin coating a layer of positive photoresist on to a substrate; creating a plurality of trenches in the substrate to create a patterned substrate and removing remaining photoresist; depositing a sacrificial layer on the patterned substrate and an inhibition layer on the sacrificial layer; depositing a TFN layer on to the inhibition layer; lifting the TFN layer with inhibition layer from the substrate; and removing the inhibition layer from the TFN layer.

23. A method as in any of the previous embodiments, wherein the TFN layer is deposited using a DC magnetron sputtering process.

24. A method as in any of the previous embodiments, wherein the sacrificial layer comprises Cu.

25. A method as in any of the previous embodiments, wherein the inhibition layer comprises SiO2.

26. A method as in any of the previous embodiments, wherein the TFN layer is deposited as a 5 micron thick Nitinol film.