US20030236568A1

US20030236568A1 - Multi-lobed frame based unidirectional flow prosthetic implant

Info

Publication number: US20030236568A1
Application number: US10/431,967
Authority: US
Inventors: Hikmat Hojeibane; David Majercak
Original assignee: Cordis Corp
Current assignee: Cordis Corp
Priority date: 2002-05-10
Filing date: 2003-05-08
Publication date: 2003-12-25
Also published as: US7758632B2; AU2009200209A1; AU2003230363A1; AU2009200209B2; CA2485285A1; CA2501712C; DE60332938D1; JP2005525172A; WO2004034933A9; US20030225447A1; MXPA04011147A; ATE470406T1; WO2003094799A1; EP1667604A2; WO2004034933A2; EP1667604B1; JP2005525169A; EP2149350A2; MXPA04011144A; AU2003225291A1

Abstract

The present invention relates to a medical device, and in particular, to a stent-based valve. The valve has a radially expandable structural frame having a substantially cylindrical configuration with first and second open ends and a longitudinal axis extending there between. The structural frame is formed from a lattice of interconnected elements and has a plurality of distal crowns. A biocompatible membrane assembly maintaining a substantially cylindrical shape about the longitudinal axis is attached to the structural frame such that the structural frame supports the biocompatible membrane assembly in a slack condition between the distal crowns.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application Serial No. 60/379,604, filed May 10, 2002.[0001]

FIELD OF THE INVENTION

The present invention relates to a medical device, and more particularly to a multi-lobed frame based unidirectional flow prosthetic valve, and the method for fabricating such valve.

BACKGROUND OF RELATED ART

The human body has numerous biological valves that control fluid flow through body lumens and vessels. For example the circulatory system has various heart valves that allow the heart to act as a pump by controlling the flow of blood through the heart chambers, veins, and aorta. In addition, the venous system has numerous venous valves that help control the flow of blood back to the heart, particularly from the lower extremities.

These valves can become incompetent or damaged by disease, for example, phlebitis, injury, or the result of an inherited malformation. Heart valves are subject to disorders, such as mitral stenosis, mitral regurgitation, aortic stenosis, aortic regurgitation, mitral valve prolapse and tricuspid stenosis. These disorder are potentially life threatening. Similarly, incompetent or damaged venous valves usually leak, allowing the blood to improperly flow back down through veins away from the heart (regurgitation reflux or retrograde blood flow). Blood can then stagnate in sections of certain veins, and in particular, the veins in the lower extremities. This stagnation of blood raises blood pressure and dilates the veins and venous valves. The dilation of one vein may in turn disrupt the proper function of other venous valves in a cascading manner, leading to chronic venous insufficiency.

Numerous therapies have been advanced to treat symptoms and to correct incompetent valves. Less invasive procedures include compression, elevation and wound care. However, these treatments tend to be somewhat expensive and are not curative. Other procedures involve surgical intervention to repair, reconstruct or replace the incompetent or damaged valves, particularly heart valves.

Surgical procedures for incompetent or damaged venous valves include valvuloplasty, transplantation, and transposition of veins. However, these surgical procedures provide somewhat limited results. The leaflets of some venous valves are generally thin, and once the valve becomes incompetent or destroyed, any repair provides only marginal relief.

As an alternative to surgical intervention, drug therapy to correct valvular incompetence has been utilized. Currently, however, there are no effective drug therapies available.

Other means and methods for treating and/or correcting damaged or incompetent valves include utilizing xenograft valve transplantation (monocusp bovine pericardium), prosthetic/bioprosthetic heart valves and vascular grafts, and artificial venous valves. These means have all had somewhat limited results.

What is needed is an artificial endovascular valve for the replacement of incompetent biological human valves, particularly heart and venous valves. These valves may also find use in artificial hearts and artificial heart assist pumps used in conjunction with heart transplants.

SUMMARY OF THE INVENTION

The present invention relates to a medical device, and in particular, to a frame-based valve. One embodiment of the invention comprises a radially expandable structural frame having a plurality of distal crowns or lobes. The structural frame is formed from a lattice of interconnected elements, and has a substantially cylindrical configuration with first and second open ends and a longitudinal axis extending there between. A tubular biocompatible membrane is coaxially disposed over at least a portion of the structural frame such that the structural frame supports the biocompatible membrane assembly in a slack condition between the distal crowns. The prosthetic valve may further have a valve strut attached to at least one of the distal crowns that extends in a distal direction substantially parallel to the longitudinal axis. The biocompatible membrane assembly may also extend in a distal direction past the distal crowns.

In another embodiment of the invention, the prosthetic valve comprises a radially expandable structural frame having a plurality of articulating distal crowns. The structural frame is formed from a lattice of interconnected elements, and has a substantially cylindrical configuration with first and second open ends and a longitudinal axis extending there between. A tubular biocompatible membrane is coaxially disposed over at least a portion of the structural frame such that the structural frame supports the biocompatible membrane assembly in a slack condition between the distal crowns.

In still another embodiment of the invention the prosthetic valve comprises a substantially cylindrical structural frame that has a hoop structure with a plurality of distal crowns. A substantially cylindrical biocompatible membrane assembly is attached to the structural frame such that the structural frame supports the biocompatible membrane assembly in a slack condition between the distal crowns.

A prosthetic valve according to another embodiment of the invention has a radially expandable structural frame comprising a cylindrical hoop structure having a plurality of distal and proximal crowns, a proximal anchor, and one or more connecting members. The proximal anchor has a substantially cylindrical configuration and is formed from a lattice of interconnected elements. The one or more connecting members has a first and a second end, the first end of each connecting member is attached to the proximal anchor and the second end of each connecting member is attached to the hoop structure. A biocompatible membrane assembly is coaxially disposed over the structural frame and attached to the proximal anchor, such that the biocompatible membrane assembly extends distally along the one or more connecting members.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a prosthetic venous valve in the deployed state according to one embodiment of the present invention. [0014]
FIG. 2A shows a perspective view of the prosthetic venous valve structural frame in the deployed state [0015]
FIG. 2B shows a close-up perspective view of a loop having inner and outer radii according to one embodiment of the present invention. [0016]
FIG. 3A shows a perspective view of a prosthetic valve having two hoop structures according to another embodiment of the present invention. [0017]
FIG. 3B shows a perspective view of a structural frame having two hoop structures according to another embodiment of the present invention. [0018]
FIG. 3C shows a perspective view of a structural frame having two hoop structures attached with bridge members. [0019]
FIG. 3D shows a perspective view of a prosthetic venous valve having connecting members connected between the sinusoidal structure and proximal anchor according to one embodiment of the present invention. [0020]
FIG. 3E shows a perspective view of the prosthetic venous valve structural frame having connecting members connected between the sinusoidal structure and proximal anchor in a peak-to-peak configuration according to one embodiment of the present invention. [0021]
FIG. 4A is a perspective view illustrating one embodiment of the expanded (deployed) prosthetic venous valve assembly in the open position. [0022]
FIG. 4B is a section view illustrating one embodiment of the expanded (deployed) prosthetic venous valve assembly in the open position. [0023]
FIG. 5A is a perspective view illustrating one embodiment of the expanded (deployed) prosthetic venous valve assembly in the closed position. [0024]
FIG. 5B is a section view illustrating one embodiment of the expanded (deployed) prosthetic venous valve assembly in the closed position. [0025]
FIG. 6A is a perspective view of a prosthetic valve having flexible distal crowns capable of deflecting inward during retrograde blood flow. [0026]
FIG. 6B is a perspective view of a prosthetic valve according to an embodiment of the present invention. [0027]
FIG. 6C is a perspective view of a prosthetic valve having valve struts according to an embodiment of the present invention. [0028]
FIG. 6D is a perspective view illustrating a membrane limiting means according to one embodiment of the present invention. [0029]
FIG. 6E is a perspective view illustrating a membrane limiting means according to one embodiment of the present invention. [0030]
FIG. 6F is a perspective view illustrating a membrane limiting means according to one embodiment of the present invention. [0031]
FIG. 6G is a perspective view of a prosthetic valve having valve struts according to an embodiment of the present invention. [0032]
FIG. 7 is a flow diagram illustrating the steps to electro-statically spin a tubular membrane on a structural frame according to one embodiment of the present invention. [0033]
FIG. 8A is section view illustrating the expanded (deployed) prosthetic venous valve assembly in the open position after some post processing according to one embodiment of the present invention. [0034]
FIG. 8B shows a close-up section view illustrating a portion of the valve assembly after some post processing according to one embodiment of the present invention. [0035]
FIG. 9 is a flow diagram illustrating the steps to electro-statically spin a tubular membrane on a structural frame according to one embodiment of the present invention. [0036]
FIG. 10 is a flow diagram illustrating the steps to place a tubular membrane over a structural frame according to one embodiment of the present invention.[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The stent-based valves of the present invention provide a method for overcoming the difficulties associated with the treatment of valve insufficiency. Although stent based venous valves are disclosed to illustrate one embodiment of the present invention, one of ordinary skill in the art would understand that the disclosed invention can be equally applied to other locations and lumens in the body, such as, for example, coronary, vascular, non-vascular and peripheral vessels, ducts, and the like, including but not limited to cardiac valves, venous valves, valves in the esophagus and at the stomach, valves in the ureter and/or the vesica, valves in the biliary passages, valves in the lymphatic system and valves in the intestines. [0038]
In accordance with one aspect of the present invention, the prosthetic valve is designed to be percutaneously delivered through a body lumen to a target site by a delivery catheter. The target site may be, for example, a location in the venous system adjacent to an insufficient venous valve. Once deployed the prosthetic venous valve functions to assist or replace the incompetent or damaged natural valve by allowing normal blood flow (antegrade blood flow) and preventing or reducing backflow (retrograde blood flow). [0039]
A perspective view of a prosthetic venous valve in the expanded (deployed) state according to one embodiment of the present invention is shown in FIG. 1. The prosthetic [0040] venous valve 100 comprises a structural frame 101 and a biocompatible membrane assembly 102.
In one embodiment, the [0041] membrane assembly 102 is comprised of a tubular membrane, valve flaps and valve cusps. The flaps and cusps may be independent components attached to the tubular membrane to form the membrane assembly 102, but are preferably part of, and integrated into, the tubular membrane. In a preferred embodiment, the valve flaps and valve cusps are formed into the tubular membrane by processing techniques as will be discussed in greater detail below.
For clarity, a perspective view of the [0042] structural frame 101 according to one embodiment of the present invention is shown in FIG. 2A. The structural frame 101 consists of a stent based sinusoidal structure, having a single hoop section 200A with one or more proximal and distal crowns (lobes) 205, 206 respectively. In a preferred embodiment, at least three distal crowns 206 are utilized as illustrated. However, this configuration is not meant to limit the scope of the invention. Various other configurations having one or more distal crowns 206 may be used, and would be understood by one of skill in the art.
It should be noted that the terms proximal and distal are typically used to connote a direction or position relative to a human body. For example, the proximal end of a bone may be used to reference the end of the bone that is closer to the center of the body. Conversely, the term distal can be used to refer to the end of the bone farthest from the body. In the vasculature, proximal and distal are sometimes used to refer to the flow of blood to the heart, or away from the heart, respectively. Since the prosthetic valves described in this invention can be used in many different body lumens, including both the arterial and venous system, the use of the terms proximal and distal in this application are used to describe relative position in relation to the direction of fluid flow. For example, the use of the term proximal crown in the present application describes the upstream crown of [0043] structural frame 101 regardless of its orientation relative to the body. Conversely, the use of the term distal crown is used to describe the down stream crown on structural frame 101 regardless of its orientation relative to the body. Similarly, the use of the terms proximal and distal to connote a direction describe upstream (retrograde) or downstream (antegrade) respectively.
As previously disclosed, in one embodiment of the invention, the structural frame is a stent-based structure. This configuration facilitates the percutaneous delivery of the prosthetic [0044] venous valve 100 through the vascular system in a compressed state. Once properly located, the stent-based venous valve 100 may be deployed to the expanded state.
The sinusoidal stent based structural frame illustrated in FIG. 2A is shown having an S shaped pattern. This configuration is shown for the purpose of example, and is not meant to be construed as limiting the scope of the invention. One of ordinary skill in the art would understand that other stent geometries having similar crowns may be used. [0045]
The sinusoidal stent based [0046] structural frame 101 comprises a tubular configuration of structural elements having proximal and distal open ends and defining a longitudinal axis 106 extending there between. The structural frame 101 has a first diameter (not shown) for insertion into a patient and navigation through the vessels, and a second diameter D2 for deployment into the target area of a vessel, with the second diameter being greater than the first diameter. The structural frame 101, and thus the stent based venous valve 100, may be either a mechanical (balloon) or self-expanding stent based structure.
The [0047] structural frame 101 comprises at least one hoop structure 200A extending between the proximal and distal ends. The hoop structure 200A includes a plurality of longitudinally arranged strut members 208 and a plurality of loop members 210 connecting adjacent struts 208. Together, these strut members 208 and loop members 210 form the proximal and distal crowns 205, 206 respectively. Adjacent struts 208 are connected at opposite ends in a substantially S or Z shaped pattern so as to form a plurality of cells. As previously discussed, one of ordinary skill in the art would recognize that the pattern shaped by the struts is not a limiting factor, and other shaped patterns may be used. The plurality of loops 210 have a substantially semi-circular configuration, having an inner radii 212 and outer radii 214, and are substantially symmetric about their centers. The inner and outer radii 212, 214 respectively, are shown in a close-up perspective view illustrated in FIG. 2B.
The embodiment of the invention illustrated in FIGS. 1 and 2 show a [0048] structural frame 101 having a single hoop structure 200A. However, it should be understood that this configuration is not meant to be construed as a limiting feature, and other configurations having a plurality of hoop structures are also contemplated by the present invention.
FIGS. 3A through 3C illustrate a [0049] structural frame 101 having two hoop structures 200A and 200B according to another embodiment of the present invention. FIG. 3A shows a complete prosthetic valve 300 in the expanded (deployed) position, illustrating both the structural frame 101 and membrane assembly 102. For clarity, FIG. 3B is an illustration of just the structural frame 101 without the membrane assembly 102. In the illustrated embodiment, the hoop structures 200A and 200B are rigidly attached at complimentary points along their respective outer radii of the loops 210.
In an alternate embodiment shown in FIG. 3C, the [0050] hoop structures 200A, 200B may be attached with one or more bridge members 305. Each bridge member 305 comprises two ends 316A, 316B. One end 316A, 316B of each bridge 305 is attached to one loop on one hoop. Using hoop sections 200A and 200B for example, each bridge member 305 is connected at end 316A to loop 210 on hoop section 200A at a point 320. Similarly, the opposite end 316B of each bridge member 314 is connected to loop 210 on hoop sections 200B at a point 321.
In any of the above described configurations, the connections between the [0051] hoop structures 200A, 200B etc. may be made at every loop member 210 around the circumference of the structure; or alternatively, at a subset of the loop members 210 around the circumference of the structure. In other words, connected loop members 210 alternate with unconnected loop members in some defined pattern.
Depending on the location of the implanted valve, it may be desirable to attach the hoop structures to an anchor by means of one or more connecting members. This configuration may add to the stability of the implanted valve. The anchor may be in the form of another sinusoidal stent based structure, such as the structures depicted in FIGS. 1 through 3C. However, any radially expandable structural frame that can aid in anchoring prosthetic valve is contemplated by the present invention. These anchors may be located downstream (proximal) or upstream (distal) from the lobed valve. [0052]
FIG. 3D illustrates a prosthetic [0053] lobed valve 300 incorporating a proximal anchor and connecting members according to one embodiment of the invention. For clarity, FIG. 3E shows the valve 300 structural frame 101 with the membrane structure 102 removed. The illustrated valve 300 comprises a single hoop structure 200A having proximal and distal crowns 205, 206 respectively. The structural frame also has an anchor 315 proximal to the hoop structure 200A.
The [0054] anchor 315 illustrated in FIGS. 3D and 3E is structurally similar to the sinusoidal stent based structure comprising the hoop structure 200A. The anchor 315 comprises a tubular configuration of structural elements having proximal and distal open ends and defining a longitudinal axis 306 extending there between. The stent anchor 315 has a first diameter (not shown) for insertion into a patient and navigation through the vessels, and a second diameter D2 for deployment into the target area of a vessel, with the second diameter being greater than the first diameter. The stent anchor 315, and thus the stent based venous valve 300, may be either a mechanical (balloon) or self-expanding stent based structure.
The [0055] stent anchor 315 comprises at least one hoop structure 336 extending between the proximal and distal ends. The hoop structure 336 includes a plurality of longitudinally arranged strut members 338 and a plurality of loop members 340 connecting adjacent struts 338. As shown, the stent anchor 315 has three hoop structures.
[0056] Adjacent struts 338 are connected at opposite ends in a substantially S or Z shaped pattern so as to form a plurality of cells. As previously discussed, one of ordinary skill in the art would recognize that the pattern shaped by the struts is not a limiting factor, and other shaped patterns may be used. The plurality of loops 340 have a substantially semi-circular configuration, having an inner radii and outer radii, and are substantially symmetric about their centers.
The connecting [0057] member 310 may be connected to the hoop structure 200A (on the sinusoidal valve structure) and the proximal anchor 315 at various points along the structures. As illustrated in FIG. 3E, the connecting members 310 are connected between the proximal end of the hoop structure 200A and the distal end of the proximal anchor 315 at the inflection point of the loop members. This configuration creates a “Peak-to-Peak” connection bridging the outer radii of the inflection point of loop members 210 on the hoop structure 200A with the outer radii of the inflection point of the loop member 340 on the proximal anchor 315.
Preferably the connecting [0058] members 310 are connected to the inflection point of loop members 210, 340 oriented directly opposite one another, and are evenly spaced along the circumference of the tubular structures. This configuration facilitates the radial expansion of the prosthetic valve from the collapsed (delivered) state to the expanded (deployed) state, and provides a substantially symmetrical valve configuration.
Alternatively, the connecting [0059] members 310 may be connected between the hoop structure 200A and proximal anchor 315 to create a “Peak-to-Valley” connection between the loop members 210, 340 respectively (not shown). In this configuration the connecting members 310 are connected to the proximal end of the hoop structure 200A at the outer radii of the inflection point of loop member 210, and the inner radii of the inflection point of loop member 340 on the proximal end of the proximal anchor 315.
In a further embodiment (not shown), the connecting [0060] members 310 may be connected between the distal end of the hoop structure 200A and the proximal end of the proximal anchor 315 at the inflection point of the loop members 210, 340. This configuration creates a “Valley-to-Valley” connection bridging the inner radii of the inflection point of loop members 340 on the proximal anchor 315 with the inner radii of the inflection point of the loop member 210 on the hoop structure 200A.
In still a further embodiment (not shown), the connecting [0061] members 310 may be connected between the strut members 208 of the hoop structure 200A and the strut members 338 of the proximal anchor 315.
In any of the above described configurations, the connections between the connecting [0062] members 310 and the hoops 200 may be made at every inflection point around the circumference of the structure; or alternatively, at a subset of the inflection points around the circumference of the structure. In other words, connected inflection points alternate with unconnected inflection points in some defined pattern.
As earlier described, the connecting [0063] members 310 are attached between the sinusoidal stent based structure (having loop 200A in FIGS. 3D and 3E) and the proximal anchor 315 to further support the biocompatible membrane assembly 102 (not shown in FIG. 3E). In one embodiment, the connecting members 315 are substantially straight members, connecting the hoop structure 200A and proximal anchor 315 in a direction substantially parallel to the longitudinal axis 306. Although three connecting members 315 are shown in the illustrated embodiment, one of skill in the art would understand that one or more connecting members may be used.
Alternatively, the connecting [0064] members 315 may be twisted in a helical fashion as they extend from the hoop structure 200A to the proximal anchor 315 (not shown). Specifically, the connection points between the connecting members 315 and the hoop structure 200A, and the connecting members 105 and the proximal anchor 315, are rotationally phased 180 degrees from each other to provide the helical design.
Each connecting [0065] member 310 may also be biased inward slightly toward the longitudinal centerline 306, creating a structural frame 101 having an hour-glass shape with the minimum radius located substantially at the longitudinal midpoint along the connecting member 310 length (not shown). The proximal crowns 205 may similarly be biased inward. This configuration may assist the prosthetic valve 300 when closing by forming larger valve cusps.
The materials for the [0066] structural frame 101 should exhibit excellent corrosion resistance and biocompatibility. In addition, the material comprising the structural frame 101 should be sufficiently radiopaque and create minimal artifacts during MRI.
The present invention contemplates deployment of the prosthetic [0067] venous valve 100 by both assisted (mechanical) expansion, i.e. balloon expansion, and self-expansion means. In embodiments where the prosthetic venous valve 100 is deployed by mechanical (balloon) expansion, the structural frames 101 is made from materials that can be plastically deformed through the expansion of a mechanical assist device, such as by the inflation of a catheter based balloon. When the balloon is deflated, the frame 101 remains substantially in the expanded shape. Accordingly, the ideal material has a low yield stress (to make the frame 101 deformable at manageable balloon pressures), high elastic modulus (for minimal recoil), and is work hardened through expansion for high strength. The most widely used material for balloon expandable structures 101 is stainless steel, particularly 316L stainless steel. This material is particularly corrosion resistant with a low carbon content and additions of molybdenum and niobium. Fully annealed, stainless steel is easily deformable.
Alternative materials for mechanically expandable [0068] structural frames 101 that maintain similar characteristics to stainless steel include tantalum, platinum alloys, niobium alloys, and cobalt alloys. In addition other materials, such as polymers and bioabsorbable polymers may be used for the structural frames 101.
Where the prosthetic [0069] venous valve 100 is self-expanding, the materials comprising the structural frame 101 should exhibit large elastic strains. A suitable material possessing this characteristic is Nitinol, a Nickel-Titanium alloy that can recover elastic deformations of up to 10 percent. This unusually large elastic range is commonly known as superelasticity.
The disclosure of various materials comprising the structural frame should not be construed as limiting the scope of the invention. One of ordinary skill in the art would understand that other material possessing similar characteristics may also be used in the construction of the prosthetic [0070] venous valve 100. For example, bioabsorbable polymers, such as polydioxanone may also be used. Bioabsorbable materials absorb into the body after a period of time, leaving only the biocompatible membrane 102 in place. The period of time for the structural frame 101 to absorb may vary, but is typically sufficient to allow adequate tissue growth at the implant location to adhere to and anchor the biocompatible membrane 102.
The [0071] structural frame 101 may be fabricated using several different methods. Typically, the structural frame 101 is constructed from sheet, wire (round or flat) or tubing, but the method of fabrication generally depends on the raw material form used.
The [0072] structural frame 101 can be formed from wire using convention wire forming techniques, such as coiling, braiding, or knitting. By welding the wire at specific locations a closed-cell structure may be created. This allows for continuous production, i.e. the components of the structural frame 101 may be cut to length from a long wire mesh tube.
In addition, the complete frame structure may be cut from a solid tube or sheet of material, and thus the [0073] structural frame 101 would be considered a monolithic unit. Laser cutting, water-jet cutting and photochemical etching are all methods that can be employed to form the structural frame 101 from sheet and tube stock.
As discussed above, the disclosure of various methods for constructing the [0074] structural frame 101 should not be construed as limiting the scope of the invention. One of ordinary skill in the art would understand that other construction methods may be employed to form the structural frame 101 of the prosthetic venous valve 100.
The [0075] structural frame 101 is radially expandable and assists in securing the prosthetic valve 100 to the inside wall of a body vessel such as a vein. Once deployed in the desired location, radially expandable structural frame (and thus the prosthetic valve 100) will expand to an outside diameter slightly larger that the inside diameter of the native vessel (not shown) and remain substantially rigid in place, anchoring the valve assembly to the vessel.
The membrane assembly is formed from a flexible membrane-like biocompatible material that is affixed to the [0076] frame structure 101. The membrane must be strong enough to resist tearing under normal use, yet thin enough to provide the necessary flexibility that allows the biocompatible membrane assembly 102 to open and close satisfactorily.
FIGS. 4A and 4B are perspective and section views, respectively, illustrating one embodiment of the expanded (deployed) prosthetic [0077] venous valve assembly 100 in the open position. The membrane material may be a biological material, such as a vein or small intestine submucosa (SIS), but is preferably a synthetic material such as a polymer, for example an elastic or elastomeric polymer, including a fluoropolymer, fluoroelastomer, or a bioabsorbable material, such as a bioabsorbable polymer or bioabsorbable elastomer. Bioabsorbable materials may allow cells to grow and form a tissue membrane (or valve flaps) over the bioabsorbable membrane. The bioabsorbable membrane then absorbs into the body, leaving the tissue membrane and/or flaps in place to act as a new natural tissue valve.
To achieve the necessary flexibility and strength of the [0078] membrane assembly 102, the synthetic material may be reinforced with a fiber, such as an electro-statically spun (ESS) fiber, porous foam, such as ePTFE, or mesh. The flexible membrane like biocompatible material is formed into a tube (membrane tubular structure 400) placed over and around the structural frame 101. The membrane tubular structure 400 has a first (distal) and second (proximal) ends 401, 402 respectively, and preferably also has integrated valve flaps 403 and valve cusps 404. These components together comprise the membrane assembly 102.
The [0079] first end 401 of the membrane tubular structure 400 is located at and between the distal crowns 206. The second end 402 of the membrane tubular structure 400 is preferably located proximal to at least one half of the most proximal hoop structure, e.g. 200B in FIG. 3B. In one embodiment of the invention, the membrane structure 400 completely covers the proximal most hoop structure to the proximal crowns 205. This configuration allows the structural frame 101 to expand the membrane tubular structure 400 into the native vessel wall, anchoring the membrane tubular structure 400 in place, and providing adequate sealing against retrograde blood flow.
The [0080] distal end 401 of the membrane tubular structure 400 terminates with the valve flaps 403. The number of valve flaps 403 is directly proportional to the number of distal crowns 206 supporting the membrane tubular assembly 102. Preferably, the design of the valve flaps 403, and for that matter valve cusps 404, are such that the tubular membrane structure 400 between the distal crowns in not tightly drawn or taut. This “slack” facilitates closing the valve by allowing the valve cusps 404 to act as pockets that fill during retrograde flow. Conversely, during antegrade flow, the additional slack in the tubular membrane structure 400 is pushed to the vessel wall, allowing blood to flow through the valve leaflets.
The valve flaps [0081] 403 are sufficiently pliable and supple to easily open and close as the blood flow changes from antegrade to retrograde. When the valve flaps 403 close (during retrograde flow) the interior surfaces of the flaps 403 and/or membrane tubular structure 400 come into contact to prevent or adequately reduce retrograde blood flow.
As earlier disclosed, to facilitate closing the valve flaps [0082] 403 during retrograde blood flow, valve cusps 404 are formed into the membrane tubular structure 400. The valve cusps 404 are defined generally by the intersection of the distal crowns 206 and membrane tubular structure 400, and are preferably formed at least in part by the slack tubular membrane 400 between the distal crowns 206.
The use of the term “cusps” is not meant to limit the scope of this invention. Although the term “cusps” is often more aptly used to describe the valve members in semilunar valves, such as the aortic and pulmonary valves, this discussion refers to both the cusps of semilunar valves and the “leaflets” of venous and atrioventricular valves. Accordingly, it should be understood that the aspects discussed in relation to these valves could be applied to any type of mammalian valve, including heart valves, venous valves, peripheral valves, etc. [0083]
During retrograde flow, blood passes the leading edge of valve flaps [0084] 403 and enters the valve cusps 404. Since the membrane tubular structure 400 (and membrane assembly 102) are substantially sealed against the inner vessel wall by the structural frame 101, the valve cusps 404 form a substantially fluid tight chamber. As the valve cusps 404 fill, the membrane tubular structure 400 is directed inward until the interior surfaces of the membrane tubular structure 400 contact each other, particularly along the leading edges of valve flaps 403, closing the membrane assembly 102. FIGS. 5A and 5B show perspective and section views, respectively, illustrating one embodiment of the expanded (deployed) prosthetic venous valve assembly 100 in the closed position.
In another embodiment of the invention, the [0085] distal crowns 206 are flexible and capable of deflecting inward during retrograde blood flow, further assisting valve 100 when closing and opening. A perspective view illustrating an example of this embodiment is shown in FIG. 6A. As illustrated flexible distal crown 606 articulate inward in direction 608 to assist closing the valve 100. The flexible distal crowns 606 may pivot along a pivot line 610 as shown, or gradually bend inward along their length.
In a preferred embodiment of the invention, the [0086] membrane assembly 102 is normally configured in the open position, and only moves to the closed position upon retrograde blood flow. This configuration minimizes interference with blood flow (minimized blocking) and reduces turbulence at and through the valve. The flexible distal crowns 606 in this embodiment have an inferior radial stiffness, and provide a natural bias against the movement of the membrane assembly 102 to the closed position. This bias assists the valve flaps 403 and valve cusps 404 when returning to the open position.
Depending on the application, it may also be desired that the bias towards opening the membrane assembly [0087] 102 (against closing) be sufficiently high to commence opening the valve before antegrade blood flow begins, i.e. during a point in time when the blood flow is stagnant (there is neither antegrade nor retrograde blood flow), or when minimal retrograde flow is experienced.
In other applications, it may be desirable to have the valve assembly normally configured in the closed position, biased closed, and only open upon antegrade flow. [0088]
In a further embodiment, the [0089] valve membrane assembly 102 may extend past the distal end of the structural frame 101, i.e. past distal crowns 206, in a distal direction as shown in FIG. 6B. During retrograde blood flow, the extended section 602 of valve membrane 102 will collapse upon itself, thus limiting or preventing fluid flow back through the valve. In such embodiments, the valve membrane 102 distal the structural frame 101 (i.e. membrane section 602) is of sufficient rigidity to prevent the membrane 102 from collapsing in through the structural frame 101 and inverting. Rigidity may be provided by inserting structural elements 620 into the membrane assembly 102 as shown in FIG. 6B. Alternatively, the membrane assembly 102 may have ribs or thickened sections processed into the membrane to provide sufficient rigidity.
In another embodiment of the present invention, one or more valve struts may extend distally from the end of the [0090] structural frame 101 providing rigidity sufficient to support the valve membrane 102, particularly membrane section 602 from inverting. These valve struts may be an integral part of the structural frame 101, and made from similar material.
FIG. 6C illustrates a [0091] valve 100 having valve struts 630 according to one embodiment of the present invention. In the embodiment shown, the valve struts 630 extend from the distal end of the structural frame 101, in particular, from the outside radii 214 of the distal crown 206 comprising the hoop structure 200A. In an alternate embodiment, the valve strut 630 may extend from the inside radii 212 of the proximal crown 205 comprising the hoop structure 200A. This alternate embodiment is shown in FIG. 6G. Still other embodiments having different connection points would be understood by one of skill in the art.
Although three valve struts [0092] 630 are shown for illustrative purposes, this exemplary embodiment is not meant to limit the scope of the invention. One of skill in the art would understand that one or more valve struts may be used and still accomplish the general intent of the invention.
As earlier described, the [0093] membrane assembly 102 is made from a flexible membrane-like biocompatible material formed into the membrane tubular structure 400. The membrane 400 can be woven, non-woven (such as electrostatic spinning), mesh, knitted, film or porous film (such as foam).
The [0094] membrane assembly 102 may be fixedly attached to the structural frame by many different methods, including attachment resulting from radial pressure of the structural frame 101 against the membrane assembly 102, attachment by means of a binder, heat, or chemical bond, and/or attachment by mechanical means, such as welding, suturing or coating. Preferably some of the membrane assembly 102, such as distal end 402 of tubular membrane 400, is slideably attached to the structural frame 101, particularly along valve struts 630. Allowing the distal end 401 to slide along the valve struts 630 may allow or improve the opening and closing of the flaps 403. The sliding movement may also assist the cusps 404 when filling and emptying.
In some applications, excessive sliding movement of the [0095] membrane assembly 102 is undesirable. In these embodiments, a limiting device may be integrated into the prosthetic valve 100 to limit the sliding movement of the membrane assembly 102. Examples of limiting devices are shown in FIGS. 6D to 6F. In each embodiment a stop 600 (illustrated as stop 600A, 600B, and 600C in FIGS. 6D to 6F respectively) is integrated into the valve struts 630. The membrane assembly 102 is wrapped around the valve struts 630 and bonded to itself to form a loop collar 605. The loop collar 605 must be sized to inhibit the distal end 402 of the membrane assembly 102 from sliding past the stop 600. In FIG. 6D, the valve struts 630 has a thickened or “bulbous” section forming stop 600A. FIG. 6E illustrates an undulating stop 600B configuration. Similarly, FIG. 6F shows the stop 600C configured as a double bulbous section. It should be noted that the various configurations illustrated in FIGS. 6D through 6F are exemplary. One of ordinary skill in the art would understand that other configurations of stops may used.
In one embodiment of the invention the [0096] tubular membrane 400 is manufactured from a fiber reinforced elastomer, such as an elastomeric fluoropolymer. The elastomer allows the tubular membrane 400 to be extremely thin and elastic, while the fiber provides the necessary strength. One method used to produce this type of reinforced membrane valve is an Electro-Static Spinning (ESS) process. Alternatively, a reinforcing fiber may be would around the structural frame 101, and an ESS membrane formed over the reinforcing fiber and structural frame 101.
The ESS process can be used to form a tubular membrane on many different types of structural frames, including frames associated with stents, stent grafts, valves, including percutaneously delivered venous valve, AAA (Abdominal Aortic Aneurysm) devices, local drug delivery devices, and the like. The disclosure of the ESS process for forming the [0097] tubular membrane 400 on the structural frame of a stent-based venous valve is exemplary, and thus not meant to limit the scope of this invention.
FIG. 7 shows the steps for electro-statically spinning a reinforced tubular membrane onto a structural frame according to one embodiment of the present invention. The ESS process comprises first placing a transfer sheath over a spinning mandrel as shown in [0098] step 700. The transfer sheath is a thin material that is used to prevent the ESS spun fiber from adhering to the mandrel. In instances where the mandrel itself is not electrically conducting, the transfer sheet may also provide the necessary electrical conductivity to attract the ESS spun fiber.
In one embodiment of the invention, the transfer sheath comprises a thin polymer tube, preferably fluoropolymer, of such a thickness that it can be easily deformed, and preferably collapsed, so that it is capable of being withdrawn conveniently from the lumen of the [0099] structural frame 101 and/or membrane tubular structure 400. The use of a transfer sheath made of other fibrous or sheet materials, such as other polymer, polymeric or metallic materials is not excluded. Most preferably, the transfer sheath will be made of an ePTFE tube.
To enhance electrical conductivity and reduce the time it takes to build up the ESS layer, the ePTFE tube may be first coated with gold on at least a portion of the interior surface before placing the tube on the mandrel. This process may be completed by coating the inside of the tube, but is preferably done by coating the exterior of the ePTFE tube and then inverting the tube so that the gold coating is on the interior surface. The process may also be completed by inverting the tube so that the interior surface to be coated is exposed on exterior of the tube, coating the now exposed interior surface, and the inverting the tube so that the interior coated surface is back on the inside of the tube. [0100]
It should be noted that under certain circumstances it may not be necessary to use the transfer sheath. Such circumstances may include, for example, where the spinning mandrel is electro-statically conducting and has a surface or surface treatment that will prevent the ESS spun fiber from adhering to the mandrel. [0101]
In a preferred embodiment, the spinning mandrel is electrically conducting, and more preferably, is a metal coated with Teflon®. However, electrical conduction may not be essential. In such embodiments the spinning mandrel may be of any suitable material, including plastic material. Non-conductors may be used so long as the charge is capable of being transferred (i.e. bleed off) onto the transfer sheet or through the material itself. [0102]
The spinning mandrel may be hollow or solid, and preferably has a smooth surface to facilitate sliding between the transfer sheath and mandrel during removal. However, it may be desirable to maintain some degree of frictional resistance between the transfer sheath and mandrel to reduce slippage between the two components during the ESS process. [0103]
The valve [0104] structural frame 101 is then placed on the transfer sheath, step 710, and the ESS fiber is spun directly onto the valve structural frame 101 as shown in step 720. Preferably, the structural frame 101 is configured in the expanded or deployed state prior to placing the structural frame 101 on the spinning mandrel. This is generally the case when the structural frame 101 is of the self-expanding design. In other embodiments, such as balloon-expandable designs, the expansion mechanism may be integrated within the spinning mandrel to expand the structural frame during the spinning process.
The expandable mandrel may also be used for electro-statically spinning a fiber onto a self-expanding [0105] structural frame 101. In such instances, the self-expanding structural frame 101 is placed on the spinning mandrel in the expanded state, and the expansion mechanism on the expandable mandrel is mandrel activated to further radially expand the structural frame to a “super-expanded” state. ESS fiber is then spun directly onto the super-expanded structural frame 101. The larger diameter of the super-expanded structural frame 101 allows more material to be deposited on the structural frame, creating slack between the distal crowns, which may result in less post processing procedures. Post processing is described in step 760.
Electro-static spinning of a fiber is generally known in the art, and typically involves creating an electrical potential between a source component, i.e. the fiber or preferably a fiber forming liquid, and a downstream component, i.e. the spinning mandrel, transfer sheath or structural frame. The electrical potential causes the source component, typically the fiber forming liquid, to be attracted to, and thus move towards, the downstream component. [0106]
The electrical potential is created by providing an electrical charge to either the source or downstream component, and grounding the other component. Preferably, the source component will receive an electrical charge, while the downstream component is grounded. [0107]
Many different methods are known in the art for producing an electrical charge on a source component. In one embodiment, a fiber forming liquid is introduced into an electric field, whereby the fiber forming liquid is caused to produce a charged fiber. In another, more preferred embodiment, a device (introducer device) introducing the fiber forming liquid into the process is electrically charged, thus causing the fiber forming liquid to assume a like charge. [0108]
Several methods may be used to introduce the fiber forming liquid into the process, including spraying the fiber forming liquid from a nozzle, or injecting the fiber forming liquid from a needle, orifice or drip tube. In a preferred embodiment, the fiber forming liquid is sufficiently viscous to be extruded into the process with an extrusion device. [0109]
Once the fiber forming liquid is introduced into the process, it is hardened to form the ESS fiber. Hardening of the liquid into an ESS fiber may be accomplished, for example, by cooling the liquid until the fiber forming liquid will not lose its fibrous shape. Other methods for hardening the fiber may also include hardening by introducing a chemical hardener into the fiber forming liquid, or directing an air stream over the electrically drawn fiber forming liquid stream. In a preferred embodiment, a polymer is put into solution with a solvent to form a viscous fiber forming liquid. As the fiber forming liquid is drawn from the introducer device, the solvent comes out of solution forming the polymer fiber. [0110]
Various drying techniques may be applied to evaporate the solvent and bring the polymer out of solutions. Drying techniques may include, for example, applying heat or airflow to or over the coated fiber spun frame assembly. In addition, the solvent may dry naturally without applying artificial drying techniques. [0111]
The viscosity of the fiber forming liquid may be adjusted based on the material used for the source component, and the percent solids desired as the source component reaches the downstream component. Typical concentrations range from 2 to 100 percent. The choice of concentration depends on the material, its molecular weight, the solvent efficiency, and temperature. The concentration and temperature also control the diameter of the fiber. These viscosities will typically produce a fiber at the downstream component having percent solids in the range of about 95 percent to about 100 percent, and preferably over 99 percent. This is desirable in order to produce structures that contain entangled or point bonded fibers. Concentrations lower than 95 percent can be used if it is desired to allow filaments to fuse together into a sheet-like barrier structure. [0112]
The hardened fiber is then collected onto the structural frame. Collecting of the fiber involves attracting the ESS fiber to the downstream component (i.e. spinning mandrel, transfer sheath or structural frame) of the ESS system, while spinning the downstream component. In a preferred embodiment, where the source component is electrically charged, a downstream component is grounded to complete the electric potential between the source and downstream component, and thus attract the ESS fiber. In other embodiments, a downstream component may be electrically charged to attract the ESS fiber where the source component is grounded. In still other embodiments, various combinations of downstream components may be electrically charged to enhance electrical conductivity and reduce the time it takes to build up the ESS layer. [0113]
Particular ESS fibers suitable for this spinning process include fluoropolymers, such as a crystalline fluoropolymer with an 85/15% (weight/weight ratio) of vinylidene fluoride/hexafluoropropylene (VDF/HFP). Solvay Solef® 21508 and Kynarflex 2750-01 are two such examples. However, one of skill in the art would understand that any material possessing the desired characteristics may be used, including, for example: bioabsorbable polymers, such as polyglycolic acid, polylactic acid, poly (paradioxanone), polycaprolactone, poly (trimethylenecarbonate) and their copolymers; and semicrystalline bioelastomers, such as 60/40% (weight/weight ratio) of polylactic acid/polycaprolactone (PLA/PCL), 65/35 (weight/weight ratio) of polyglycolic acid/polycaprolactone (PGA/PCL), or nonabsorbable siliconized polyurethane, non-siliconized polyurethanes, siliconized polyureaurethane, including siliconized polyureaurethane end capped with silicone or fluorine end groups, or natural polymers in combination thereof. It should be noted that poly(trimethylenecarbonate) can not be spun as a homopolymer. [0114]
The spinning process should be continued until an ESS fiber tube, or fabric, is formed having a wall thickness of between 5 μm and 100 μm or more, preferably, approximately 20 μm. The ESS fiber spun [0115] structural frame 101 is then removed from the spinning mandrel, step 730, before the transfer sheath is removed from the fiber spun frame, step 740. Once this step is completed, the fiber spun structural frame is coated in a solution of polymer, such as fluoroelastomer, as shown in step 750.
Several different methods may be utilized to perform the coating process on the fiber spun structural frame, including spray coating with an air or airless sprayer, dip coating, chemical vapor deposition, plasma coating, coextrusion coating, spin coating and insert molding. In still another preferred embodiment, the fiber spun structural frame is first dip coated in a polymer solution, and then spun about its longitudinal axis to more evenly distribute the coating. In this embodiment, the fiber spun structural frame is not first removed from the spinning mandrel. Instead, the frame/mandrel assembly is dip coated and spun before removing the fiber spun structural frame from the spinning mandrel. Still other methods for coating the fiber spun structural frame would be obvious to one of skill in the art. [0116]
The coating process may act to encapsulate and attach at least a portion of the spun ESS reinforcement fiber to the [0117] structural frame 101. It should be noted that it in some embodiments of the invention, some movement between the membrane assembly 102 and the structural frame 101 is desired. Accordingly, not all of the ESS fiber spun structural frame may be coated.
The coating process may also remove some porosity of the membrane material. However, it may be desirable to maintain some porosity in particular embodiments to promote biological cell grown on and within the membrane tubular structure. [0118]
The coating solution preferably comprises a polymer put into solution with a solvent. As the solvent evaporates, the polymer comes out of solution forming the coating layer. Accordingly, for the process to work properly, the solvent used in the coating solution should not dissolve or alter the ESS fibers being coated. By way of example, a coating solution of 60/40% VDF/HFP in methanol (methanol being the solvent) has been found to be a suitable solution for coating an ESS fiber comprised of 85/15% VDF/HFP. [0119]
In one embodiment of the invention, the polymer comprising the coating is Daikin's Dai-El G701BP, which is a 60/40% VDF/HFP. In addition, Daikin's Dai-El T630, a thermoplastic elastomer based on vinylidene fluoride/hexafluoropropylene/tetrafluoroethylene (VDF/HFP/TFE) can also be used. Again, one of ordinary skill in the art would understand that other materials having suitable characteristics may be used for the coating, for example, other polymers, such as siliconized polyurethane, including Polymer Technology Group's Pursil, Carbosil, Purspan and Purspan F. [0120]
The coating process may be repeated until the desired characteristics and thickness are achieved. For venous valves a thickness of between 12 μm and 100 μm and preferably between 25 μm and 50 μm has been found to be acceptable. [0121]
Once the coating process is complete some post processing of the membrane [0122] tubular structure 400 may take place to achieve particular desired characteristics or configurations. This may include creating the final form of the membrane assembly 102. The post processing step is shown as optional step 760 in FIG. 7.
The [0123] post processing step 760 may be used to form or shape, for example, a valve cusp, similar to cusp 404, in the membrane tubular structure 400. In addition, post processing may change the characteristics of the membrane tubular structure 400 by thickening or thinning the membrane in particular locations. Thickening the membrane may add rigidity and reinforcement to a particular area. Thinning the membrane may make the membrane more pliable, which is a desirable characteristic for the valve flaps 403. Still other post processing procedures may change the physical shape of the membrane tubular structure 400, for example, by forming the loop collar 605 along the distal edge of membrane tubular structure 400. The loop collar 605 may assist in controlling the movement (translational and circumferential) of the membrane assembly 102 along the valve struts 630. The loop collars 605 may also reduce fatigue and tear stresses in the membrane.
FIGS. 8A and 8B show an example of the result of a post processing step that forms a [0124] loop collar 605 according to one embodiment of the present invention. To achieve this result, the membrane tubular structure 400 is wrapped around at least one element of structural frame 101 (valve struts 630) and bonded to itself at bond point 800.
Another method for electro-statically spinning a tubular membrane onto a radially expandable structural frame according to another embodiment of the present invention is shown in FIG. 9. Although similar to the process described above, this alternative method provides an ESS spun membrane on the inside, as well as the outside of the structural frame. The inner and outer ESS spun membranes may mechanically adhere to each other, and in a sense encapsulated the structural frame. This configuration provides some additional features, including having a smoother interior surface that reduces turbulence, improves flow dynamics and lowers the chance of thrombosis formation. [0125]
Similar to the embodiment described earlier, the ESS process comprises first placing a transfer sheath over a spinning mandrel as shown in [0126] step 900. It should be noted that under certain circumstances it may not be necessary to use the transfer sheath. Such circumstances may include, for example, where the spinning mandrel is electro-statically conducting and has a surface or surface treatment that will prevent the ESS spun fiber from adhering to the mandrel.
An ESS fiber is then spun directly onto the transfer sheath creating an inner coat membrane as shown in [0127] step 910. The ESS process should continue until an ESS tube is formed having a wall thickness of between 2 μm and 50 μm or more, and preferably, approximately 20 μm. As previously stated, the inner coat membrane covers some or all of the interior surface of structural frame 101. The structural frame 101 is then radially expanded and placed over the inner coat membrane on the spinning mandrel as shown in step 920. Expansion of the structural frame 101 may be achieved by several different methods. One method includes taking advantage of the thermal and shape memory characteristics of particular materials. For example, shape memory materials, such as Nitinol, possess little or no recoil ability when cooled, but exhibit a high degree of memory, i.e. the ability to return to a configured shape, when heated. Cooling the Nitinol structural frame 101 before expansion allows the structural frame to remain in the expanded configuration until being heated. Accordingly, the Nitinol structural frame 101 can be cooled, expanded, and then placed over the inner coat membrane. Once in place, the structural frame can be heated to activate the Nitinol memory characteristics, causing the Nitinol structural frame 101 to contract to the pre-expansion size and configuration.
The [0128] structural frame 101 is sized such that when configured in the expanded or deployed state, it will fit tightly over the inner coat membrane on the spinning mandrel. To fit the structural frame 101 over the inner coat membrane, the structural frame 101 may have to be radially expanded (“super-expanded”) to a diameter slightly larger than the expanded deployed state to allow the structural frame 101 to fit over the inner coat membrane.
Once the [0129] structural frame 101 is placed over the inner coat membrane, another ESS fiber is spun directly onto the structural frame, as shown in step 930, to form a top-coat membrane. The ESS process should continue until the top-coat membrane tube is formed having a wall thickness of between 2 μm and 50 μm or more, and preferably, approximately 20 μm. The top-coat membrane may cover and adhere to the inner coat membrane through the interstitial spaces between the elements that comprise the structural frame 101.
As stated in an earlier described embodiment of the invention, the [0130] structural frame 101 is configured on the mandrel in the expanded deployed state prior to spinning the top-coat membrane. In other embodiments, it may be desirable to expand (super expand) the structural frame 101 on the spinning mandrel during or prior to the spinning process. This procedure may alter the configuration and properties of the spun membrane, resulting in less post processing of the membrane. Post processing is described in step 960.
The [0131] structural frame 101, with the inner coat and top coat membranes, is then removed from the spinning mandrel, as shown in step 940, and coated with a solution of highly elastic polymer as shown in step 950. As stated previously, the coating process may be achieved using several different coating methods, including spin coating, spray coating, dip coating, chemical vapor deposition, plasma coating, co-extrusion coating and insert molding.
As previously described, a representative elastomeric polymer is a fluoroelastomer. The coating process may be repeated until the desired characteristics and thickness are achieved. For a venous valve application, a thickness of between 12 μm and 100 μm, and preferably between 25 μm and 50 μm, has been found to be acceptable. [0132]
Once the coating process is complete, some post processing of the tubular membrane may take place, as shown as an [0133] optional step 960 in FIG. 9.
Although each of the above described ESS methods spin the fiber directly on to the structural frame, one of ordinary skill in the art would understand that a tubular membrane may also be spun separately, and then placed over the [0134] structural frame 101 by known methods.
Another, more preferred method for forming the membrane material over and around the [0135] structural frame 101 is shown in FIG. 10. As described earlier, this method is presented in the context of a prosthetic valve application. However, the method may be applied generally to any application where a micro-cellular foam or pourous material, particularly an ePTFE membrane, needs to be placed over and around a radially expandable structural frame. Exemplary structural frames may include stents, stents grafts, valves (including percutaneously delivered venous valves), AAA (Abdominal Aortic Aneurysm) devices, local drug delivery devices, and the like. Accordingly, the disclosed device is not meant to limit the scope of the inventive method.
In this embodiment, a tubular structure is fabricated from a polymer material that can be processed such that it exhibits an expanded cellular structure, preferably expanded Polytetrafluoroethylene (ePTFE). The ePTFE tubing is made by expanding Polytetrafluoroethylene (PTFE) tubing, under controlled conditions, as is well known in the art. This process alters the physical properties that make it satisfactory for use in medical devices. However, one of ordinary skill in the art would understand that other materials that possess the necessary characteristics could also be used. [0136]
The method comprises first placing a transfer sheath over a mandrel as shown in [0137] step 1000. As described earlier, the transfer sheath is a thin material that is used to prevent the tubing and coating from adhering to the mandrel. The transfer sheath may be made of sheet metal, metal foil, or polymer sheet, such as for example Polytetrafluoroethylene (PTFE) . Preferably, the transfer sheath will be made of a material that can be easily deformed, and preferably collapsed so that it can be withdrawn conveniently from the lumen of the tube once the process is complete.
The transfer sheath/mandrel combination are then coated in a solution of highly elastic polymer, such as fluoroelastomer, as shown in [0138] step 1010, to form an inner membrane. As stated previously, the coating may be applied using various methods, including, for example, spin coating, spray coating, dip coating, chemical vapor deposition, plasma coating, co-extrusion coating and insert molding.
In one embodiment of the invention, the coating solution comprises a polymer put into solution with a solvent, such as methanol. In addition, most solvents can be used with expanded Polytetrafluoroethylene (ePTFE). [0139]
In a preferred embodiment of the invention, the polymer comprising the coating includes Daikin's Dai-El T630, a thermoplastic elastomer based on vinylidene fluoride/hexafluoropropylene/tetrafluoroethylene (VDF/HFP/TFE) and blends thereof. Again, one of ordinary skill in the art would understand that other materials having suitable characteristics may be used for the coating, for example, other polymers, such as siliconized polyurethanes and blends thereof, including Polymer Technology Group's Pursil, Carbosil, Purspan and Purspan F. [0140]
The coating process should continue until the inner membrane achieves a wall thickness of between 6 μm and 100 μm or more, preferably between 12 μm to 25 μm. [0141]
In an alternate embodiment, a polymer tube, preferably an ePTFE tube, may be expanded and placed over the sheath/mandrel combination (step [0142] 1015), before being contracted (step 1020). Expansion may be by any suitable expansion means known in the art, including mechanical expansion, such as by means of a balloon expansion device or expandable cage, expansion by utilizing a tapered mandrel (i.e. sliding the polymer tube over a tapered mandrel of increasing diameter) , etc. In addition other means may be used in conjunction with the expansion means to assist placing the tube over the sheath mandrel combination. These assist means may include, for example, thermally expanding the tube with heat, or chemically expanding the tube with a solvent. These methods are known in the art.
Contraction of the tube is typically done by reversing the method used to expand the tube. For example, where the tube is naturally elastic and expanded by a mechanical expansion means, removing the expansion means would allow the tube to contract towards it pre-expansion configuration. In addition the contraction of the tube may be enhanced by applying heat or chemicals (solvents). [0143]
Once the tube is expanded over the sheath/mandrel, the whole assembly may be coated with a solution of highly elastic polymer, such as fluoroelastomer as shown in [0144] step 1025 to form the inner membrane. The coating process is similar to that shown in step 1010 above, and may be achieved by any method known in the art capable of achieving the desired result, including spin coating, spray coating, dip coating, chemical vapor deposition, plasma coating, co-extrusion coating and insert molding.
The coating process described in [0145] step 1025 should continue until the inner membrane described in the alternate embodiment is coated with a polymer base having a wall thickness of between 6 μm and 100 μm or more, preferably between 12 μm to 25 μm.
The [0146] structural frame 101 is then radially expanded and positioned over the inner membrane as shown in step 1030. The structural frame 101 may be radially expanded using any know expansion means, including a balloon expansion device or frame expansion device. In one embodiment of the invention, the structural frame 101 is constructed from a shape memory alloy, such as Nitinol. As previously described, Nitinol characteristically holds a deformed shaped when cooled, and returns to its original shape when heated. Accordingly, it is possible to hold a Nitinol structural frame 101 in the radially expanded state by cooling the frame before the expansion means is removed. This will facilitate placement of the Nitinol structural frame over the inner membrane.
The [0147] structural frame 101 may then be radially contracted over the inner membrane, as shown in step 1040. It is desirable to maintain a slight interference fit between the structural frame 101 and the inner membrane. The method to radially contract the structural frame 101 may depend on the material and type of construction of the structural frame 101, and is not meant to limit the scope of the invention. As described above, a structural frame 101 constructed from a shape memory alloy, such as Nitinol, can be radially contracted (to the pre-expanded and cooled size) by heating. Depending on the material used, other methods that may also be employed to radially contract the structural frame include, simply removing the expansion means providing the radial expansion force, or applying a compressive force about the structural frame 101. Still other methods to radially contract the structural frame 101 would be obvious to one of skill in the art.
Once the [0148] structural frame 101 is contracted over the inner membrane, a second polymer tube, preferably an ePTFE tube, is expanded and placed over the structural frame, as shown in step 1050, forming an outer membrane. The tube is then contracted into position as shown in step 1060. As described earlier, the tube may be expanded by several different means, including mechanical, thermal, or chemical (solvents) expansion. Similarly, contraction of the tube may be accomplished by the methods described in step 1020.
In embodiments where two separate ePTFE tubes are used for the inner and outer membranes, as described in [0149] steps 1015 and 1050 respectively, each tube should have a wall thickness of between 25 μm and 50 μm before expansion; yielding a wall thickness of between 6 μm and 10 μm after expansion and placement. It should be noted that these membranes may or may not be bonded together. If only a single ePTFE tube is used for the outer membrane only, as described in step 1050 (not following alternate steps 1015 through 1025), the tube should have a wall thickness before expansion of between 50 μm and 100 μm; yielding a wall thickness after expansion of between 12 μm and 20 μm.
The inner and outer membranes combine to form a membrane structure. In the valve example described above, the membrane structure would represent membrane [0150] tubular structure 400, while the structural frame would represent the structural frame 101.
Once the membrane structure is formed, some or all of the assembly may be optionally coated with a solution of a highly elastic polymer, such as a elastomeric polymer, as shown in [0151] step 1070. The coating may be applied by any method known in the art, including spin coating, spray coating, dip coating, chemical vapor deposition, plasma coating, co-extrusion coating and insert molding.
As described earlier (see step [0152] 1010) the coating solution may be a fluoroelastomer. In one embodiment of the invention, the coating is Daikin G701BP, which is a 60/40% VDF/HFP. Again, one of ordinary skill in the art would understand that other materials having suitable characteristics might be used for the coating, for example, other polymers, such as siliconized polyurethane.
The coating process should continue until the coating achieves a wall thickness of between 6 μm and 100 μm or more, preferably between 12 μm to 25 μm. [0153]
Once the coating process is complete, some post processing of the membrane structure may take place to achieve particular desired characteristics or configurations. This post processing step is shown as [0154] optional step 1080 in FIG. 10.
By way of example, for valve applications, the [0155] post processing step 1080 may be used to form or shape valve cusps, similar to cusps 404, or valve flaps, such as flaps 403, in the membrane structure. In addition, post processing may change the characteristics of the membrane structure by thickening or thinning the membrane in particular locations. Thickening the membrane may add rigidity and reinforcement to a particular area. Thinning the membrane may make the membrane more pliable. Still other post processing procedures may change the physical shape of the membrane structure, for example, by forming the loop collar 605 along the distal edge of membrane assembly 102. The loop collar 605 may assist in controlling the translational and circumferential movement of the membrane assembly 102 along the valve struts 630. The loop collars 605 may also reduce fatigue and tear stresses in the membrane.
It is important to note that the local delivery of drug/drug combinations may be utilized to treat a wide variety of conditions utilizing any number of medical devices, or to enhance the function and/or life of the device. Medical devices that may benefit from this treatment include, for example, the frame based unidirectional flow prosthetic implant subject of the present invention. [0156]
Accordingly, in addition to the embodiments described above, therapeutic or pharmaceutic agents may be added to any component of the device during fabrication, including, for example, the ESS fiber, polymer or coating solution, membrane tube, structural frame or inner and outer membrane, to treat any number of conditions. In addition, therapeutic or pharmaceutic agents may be applied to the device, such as in the form of a drug or drug eluting layer, or surface treatment after the device has been formed. In a preferred embodiment, the therapeutic and pharmaceutic agents may include any one or more of the following: antiproliferative/antimitotic agents including natural products such as vinca alkaloids (i.e. vinblastine, vincristine, and vinorelbine), paclitaxel, epidipodophyllotoxins (i.e. etoposide, teniposide), antibiotics (dactinomycin (actinomycin D) daunorubicin, doxorubicin and idarubicin) anthracyclines, mitoxantrone, bleomycins, plicamycin (mithramycin) and mitomycin, enzymes (L-asparaginase which systemically metabolizes L-asparagine and deprives cells which do not have the capacity to synthesize their own asparagine); antiplatelet agents such as G(GP) ll[0157] _b/lll_ainhibitors and vitronectin receptor antagonists; antiproliferative/antimitotic alkylating agents such as nitrogen mustards (mechlorethamine, cyclophosphamide and analogs, melphalan, chlorambucil), ethylenimines and methylmelamines (hexamethylmelamine and thiotepa), alkyl sulfonates-busulfan, nirtosoureas (carmustine (BCNU) and analogs, streptozocin) , trazenes-dacarbazinine (DTIC); antiproliferative/antimitotic antimetabolites such as folic acid analogs (methotrexate), pyrimidine analogs (fluorouracil, floxuridine, and cytarabine), purine analogs and related inhibitors (mercaptopurine, thioguanine, pentostatin and 2-chlorodeoxyadenosine {cladribine}); platinum coordination complexes (cisplatin, carboplatin), procarbazine, hydroxyurea, mitotane, aminoglutethimide; hormones (i.e. estrogen); anticoagulants (heparin, synthetic heparin salts and other inhibitors of thrombin); fibrinolytic agents (such as tissue plasminogen activator, streptokinase and urokinase), aspirin, dipyridamole, ticlopidine, clopidogrel, abciximab; antimigratory; antisecretory (breveldin); anti-inflammatory: such as adrenocortical steroids (cortisol, cortisone, fludrocortisone, prednisone, prednisolone, 6α-methylprednisolone, triamcinolone, betamethasone, and dexamethasone), non-steroidal agents (salicylic acid derivatives i.e. aspirin; para-aminophenol derivatives i.e. acetominophen; indole and indene acetic acids (indomethacin, sulindac, and etodalac), heteroaryl acetic acids (tolmetin, diclofenac, and ketorolac), arylpropionic acids (ibuprofen and derivatives), anthranilic acids (mefenamic acid, and meclofenamic acid), enolic acids (piroxicam, tenoxicam, phenylbutazone, and oxyphenthatrazone) , nabumetone, gold compounds (auranofin, aurothioglucose, gold sodium thiomalate); immunosuppressives: (cyclosporine, tacrolimus (FK-506), sirolimus (rapamycin), azathioprine, mycophenolate mofetil); angiogenic agents: vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF); angiotensin receptor blockers; nitric oxide donors; anti-sense oligionucleotides and combinations thereof; cell cycle inhibitors, mTOR inhibitors, and growth factor receptor signal transduction kinase inhibitors; retenoids; cyclin/CDK inhibitors; HMG co-enzyme reductase inhibitors (statins); and protease inhibitors.
While a number of variations of the invention have been shown and described in detail, other modifications and methods of use contemplated within the scope of this invention will be readily apparent to those of skill in the art based upon this disclosure. It is contemplated that various combinations or subcombinations of the specific embodiments may be made and still fall within the scope of the invention. For example, the embodiments variously shown to be prosthetic “venous valves” may be modified to instead incorporate prosthetic “heart valves” and are also contemplated. Moreover, all assemblies described are believed useful when modified to treat other vessels or lumens in the body, in particular other regions of the body where fluid flow in a body vessel or lumen needs to be controlled or regulated. This may include, for example, the coronary, vascular, non-vascular and peripheral vessels and ducts. Accordingly, it should be understood that various applications, modifications and substitutions may be made of equivalents without departing from the spirit of the invention or the scope of the following claims. [0158]
The following claims are provided to illustrate examples of some beneficial aspects of the subject matter disclosed herein which are within the scope of the present invention. [0159]

Claims

What is claimed is:

1. A prosthetic valve comprising:

a radially expandable structural frame having a plurality of distal crowns, the structural frame being formed from a lattice of interconnected elements, and having a substantially cylindrical configuration with first and second open ends and a longitudinal axis extending there between;

a tubular biocompatible membrane coaxially disposed over at least a portion of the structural frame such that the structural frame supports the biocompatible membrane assembly in a slack condition between the distal crowns.

2. The prosthetic valve of claim 1 wherein the structural frame further comprises a valve strut attached to at least one of the distal crowns and extending in a distal direction substantially parallel to the longitudinal axis.

3. The prosthetic valve of claim 1 wherein the distal crowns are articulating.

4. The prosthetic valve of claim 1 wherein the biocompatible membrane assembly extends in a distal direction past the distal crowns.

5. A prosthetic valve comprising:

a tubular biocompatible membrane coaxially disposed over at least a portion of the structural frame such that the structural frame supports the biocompatible membrane assembly in a flexible condition between the distal crowns.

6. A prosthetic valve comprising:

a substantially cylindrical structural frame having a hoop structure, the hoop structure having a plurality of distal crowns;

a substantially cylindrical biocompatible membrane assembly coaxially disposed over the structural frame such that the structural frame supports the biocompatible membrane assembly in a slack condition between the distal crowns.

7. A prosthetic valve having a radially expandable structural frame comprising:

a cylindrical hoop structure having a plurality of distal and proximal crowns;

a proximal anchor formed from a lattice of interconnected elements and having a substantially cylindrical configurations;

one or more connecting members, the one or more connecting members having a first and a second end, the first end of each connecting member being attached to the proximal anchor and the second end of each connecting member being attached to the hoop structure; and

a substantially cylindrical biocompatible membrane assembly attached to the proximal anchor and extending distally along the one or more connecting members to the distal crowns, such that the distal crowns.