CA2199890C - Stents and stent-grafts having enhanced hoop strength and methods of making the same - Google Patents

Abstract

Stents are coated with a polymer such that the polymeric coating binds the crossover points of the wires, or, in the case of a zig-zag stent, binds adjacent zig-zags of wires without occluding the interstices of the stent lattice. Suitable polymers include polyurethane, polycarbonate urethane, polyurethane urea, silicone rubber, polyisobutylene copolymer (with styrene, etc.), polyolefin, polyester, glycolated polyester, polyamide, amorphous polyamide, combinations of the above and the like. Biodegradable polymers such as Polyisobutyrate, polyvalerate, polylactic acid, polyglycolic acid and combinations of these are also suitable.
The polymer can be reacted in place without a solvent, such as two component polyurethanes, or silicone rubbers, or the reacted polymer can be dissolved in an appropriate solvent, for example, dimethylacetamide for the polyurethanes, toluene for the polyolefins, or heptane for the silicone rubbers. In order to enhance bonding of the polymer to the stent wires, the metallic stent can be primed prior to coating. The hoop strength of a polymer coated stent is improved due to locking of the crossover points and preventing free motion of the stent wires relative to each other.

Description

o~~~~e~o

2 OF MAKING THE SAME

3

4 BACKGROUND OF THE INVENTION
6 1. Field of the Invention 7 The invention relates to self expanding stem s and stent-8 grafts. More particularly, the invention relates to stems and 9 stmt-grafts with the stmt having a polymeric coating which provides enhanced hoop strength as well as other benefits.

12 2. State of the Art 13 Transluminal prostheses are well known in the medical arts 14 for implantation in blood vessels, biliary ducts, or other similar organs of the living body. These prostheses are commonly known as 16 stems and are used to maintain, open, or dilate tubular 17 structures or to support tubular structures that are being 18 anastomosed. When biocompatible materials are used as a covering 19 or lining for the stmt, the prosthesis is called a stmt-graft.
If used specifically in blood vessels, the stmt-graft is known as 21 an endovascular graft. A stmt or stmt-graft may be introduced 22 into the body by stretching it longitudinally or compressing it 23 radially, until its diameter is reduced sufficiently so that it 24 can be fed into a catheter. The stmt-graft is delivered through ~~~9~~90 the catheter to the site of deployment and then released from the 2 catheter, whereupon it self-expands. Stent-grafts introduced in 3 this manner are known as endoluminal stmt-grafts.

A typical state of the art st mt, such as disclosed in U.S.
6 Patent Number 4,655,771 to Wallsten or in U.K. Patent Number 7 1,205,743 to Didcott, is shown herein in prior art Figures 1, 1a, 8 2, and 2a. Didcott and Wallsten disclose a tubular body st mt 10 9 composed of wire elements, e.g. 12, 13, each of which extends in a helical configuration with the centerline 14 of the stmt 10 as a 11 common axis. Half of the elements, e.g. 12, are wound in one 12 direction while the other half, e.g. 13, are wound in an opposite 13 direction. With this configuration, the diameter of the st mt is 14 changeable by axial movement of the ends 9, 11 of the stmt.
Typically, the crossing elements form a braid-like configuration 16 and are arranged so that the diameter of the st mt 10 is normally 17 expanded as shown in Figures 1 and la. The diameter may be 18 contracted by pulling the ends 9, 11 of the stmt 10 away from 19 each other as shown by the arrows 16, 18 in Figure 2. When the ends of the body are released, the diameter of the st mt 10 self-21 expands and draws the ends 9, 11 of the st mt closer to each 22 other. The contraction to stretching ratio and radial pressure of 23 stems can usually be determined from basic braid equations. A
24 thorough technical discussion of braid equations and the 1 mechanical properties of stem s is found in Jedweb, M.R. and 2 Clerc, C.O., "A Study of the Geometrical and Mechanical Properties 3 of a Self-Expanding Metallic Stent--Theory and Experiment", 4 ,journal of Applied Biomaterials: Vol. 4, pp. 77-85 (1993). In general, however, the contraction to stretching ratio is related 6 to the axially directed angle a between the crossing elements 12, 7 13 in the expanded state as shown in Figure 1. As explained in 8 Didcott, the greater the magnitude of the angle oc, the greater the 9 amount of axial extension will be required to contract the diameter of the stmt.

12 The ability of a stmt to withstand radial forces is known in 13 the art as "hoop strength". The hoop strength of both the 14 Wallsten and the Didcott stem s is relatively low. The Wallsten stent provides an improvement in hoop strength over Didcott by 16 virtue of the higher pitch angle (a > 90°). However, the higher 17 pitch angle of the Wallsten stmt renders the stmt more difficult 18 to place since substantial elongation is required to pull the 19 stmt down into a catheter introducer. Various designs have been advanced in efforts to increase stmt hoop strength. These 21 designs include the use of thicker wires, the use of more wires, 22 and the use of paired wires. However, there are limitations to 23 each of these designs. For example, if too many wires are used or 24 if the wire diameter is too large, the stmt will tend to demonstrate a taper on one end and a flare on the other end. This 2 is detrimental to stmt performance. Moreover, the use of 3 numerous and/or thick wires often results in wire jamming when the 4 stmt is drawn down. This requires a larger introducer catheter which renders it more difficult to place in distal and tortuous 6 vessels.

8 Apart from hoop strength, another problem with conventional 9 stem s is that the ends fray or become unbraided when they are cut. When this happens, it becomes difficult to load the stmt 11 into an introducer and it is possible for a stray wire end to 12 penetrate the wall of the introducer. Similarly, an unbraided 13 wire end can perforate the human vessel during or after placement.

Still another problem with the state of the art stems is 16 that during normal use, even without cutting the stmt, the ends 17 of the stmt tend to taper inward due to slippage of the wires and 18 loss of braid structure. The tapered ends of a stmt can perturb 19 flow through the lumen of the stmt and cause thrombosis. In addition, as the ends of an installed stmt taper inward, the 21 stmt can become dislodged and may even be washed downstream 22 through the vessel in which it was installed.

1 Yet, another problem with conventional Didcott or Wallsten 2 stem s is illustrated in prior art Figure 3. When a stmt 10 of 3 this type is deployed in a vessel 20 having a bend 22, the pitch 4 angle of the wired is increased in the portion of the stmt 10 which traverses the bend 22. Hence, the diameter of the st mt 10 6 at the center of the bend 22 is larger than the diameter of the 7 stmt 10 at its ends 9, 11, as the center of the stmt stretches 8 the vessel at the bend. This tends to alter the hemodynamics of 9 the vessel.
11 Still another problem associated with these aforementioned 12 stems is that the stmt will flex continuously with each bolus of 13 blood passing through the stmt. The flexion continues until the 14 stmt is totally ingrown with biological tissue. During flexion, the wires undergo a scissors-like activity at the crossover points 16 which can irritate tissue and adversely affect patency, especially 17 in small diameter vessels such as the coronary arteries.
18 Moreover, the points where the wires cross over each other are 19 subject to abrasion when the stmt is flexed in the vasculature.
Severe abrasion manifests as wear in the wires which can 21 ultimately lead to premature breakage of the wire components.

23 Another kind of (non-braided) stent is disclosed in European 24 Patent Publication No. 0312852 to Wiktor. A Wiktor-type st mt 30

- 5 -p~19~8g0 1 is shown in prior art Figure 4 in conjunction with a balloon 2 catheter 31. The stmt 30 is made of a single strand of zig-zag 3 filament 32 which is helically wrapped around a mandril. While 4 the filament 32 does not necessarily cross over itself, adjacent zig-zags, e.g. 34, 36, touch each other or come close to touching

6 each other. One of the disadvantages of the Wiktor-type st mt is

7 that the zig-zag wire tends to expand non-uniformly when expanded

8 in an artery by a balloon catheter. In addition, the non-braided

9 stmt can unfurl during maneuvering the balloon catheter in the vasculature which can cause placement problems as well as damage 11 to the endothelium. In addition, the hoop strength of the 12 Wiktor-type stent is relatively low.

14 Further disadvantages of conventional wire stems are that they are intrinsically thrombogenic and do not bind well to 16 surface coatings due to the inertness of the metallic oxide layers 17 on the wires.

21 It is therefore an object of the invention to provide a stmt 22 and a stmt-graft with improved hoop strength.

~1 It is also an object of the invention to provide a stmt and 2 a stem-graft which resist tapering and maintain flaring at the 3 ends.

It is another object of the invention to provide a st mt and 6 a stent-graft which exhibit little or no abrasion of wires in the 7 vasculature.

9 It is still another object of the invention to provide a st mt and a stmt-graft which maintain a substantially constant 11 diameter when installed in the bend of a vessel.

13 In accord with these objects which will be discussed in 14 detail below, the implantable stems and stent-grafts of the present invention include a conventional st mt which is coated 16 with a polymer such that the polymeric coating binds the crossover 17 points of the wires, or, in the case of a Wiktor-type stent, binds 18 adjacent zig-zags of wires without occluding the interstices of 19 the stem lattice. Suitable coating polymers include polyurethane, polycarbonate urethane, polyurethane urea, silicone 21 rubber, polyisobutylene copolymer (with styrene, etc.), 22 polyolefin, polyester, glycolated polyester, polyamide, amorphous 23 polyamide, combinations of the above and the like. Biodegradable 24 polymers such as polyisobutyrate, polyvalerate, polylactic acid, ~~199~gQ
1 polyglycolic acid and combinations of these are also suitable.
2 The major requirement for the polymer is that it can deform during 3 loading of the stmt into a catheter and that it has sufficient 4 rebound or memory to return substantially to its original shape after the stmt is deployed. The presently preferred polymer is 6 an aromatic polycarbonate urethane of Shore hardness 80A to 100D;
7 preferably Shore 55D to 75D. The polymer can be reacted in place 8 on the stmt without a solvent, such as two component 9 polyurethanes, or silicone rubbers, or the reacted polymer can be dissolved in an appropriate solvent, for example, 11 dimethylacetamide for the polyurethanes, toluene for the 12 polyolefins, or heptane for the silicone rubbers. The 13 concentration of solids to solvent is chosen for the specific 14 process used to apply the polymer to the stmt. For example, for spray coating, 5 to 10~ solids by weight is preferred, while 7~ to 16 13% solids is preferred for dip coating. Binary solvents such as 17 dimethyacetamide and tetrahydrofuran can be used to accelerate dry 18 times or spray build-up for polyurethanes.

In order to enhance bonding of the polymer to the stmt 21 wires, it may be desirable to prime the metallic stmt prior to 22 coating with polymer. Suitable primers include silane priming 23 agents such as aminoethyaminopropyltriacytoxysilane. In addition 24 to spraying and dipping, the polymer may be padded or spun onto _ g _ Q~'~9g$gQ
i the stmt (or primed stmt) and cured or dried to form the polymer 2 adhesive, care being taken to avoid occlusion of the interstices 3 of the stmt lattice. Alternatively, the polymeric coating can be 4 extruded onto the wire prior to fashioning the stmt and once complete, the polymer can be melt-adhered to adjacent components.

7 The hoop strength of a polymer coated stmt according to the 8 invention is improved due to locking of the crossover points (or 9 zig-zag points) and preventing free motion of the stent wires relative to each other. This also prevents sliding of the wires 11 and hence abrasion of the wires at the crossover points as well as 12 the scissor-like movement of the wires which causes irritation to 13 tissue and adversely affects patency, especially in small diameter 14 vessels such as coronary arteries. The polymer coating adds only a slight increase in wall thickness of the stmt wires while 16 increasing hoop strength significantly. The introduction profiles 17 of polymer coated stems according to the invention are not 18 appreciably increased. The polymeric coating also prevents ends 19 from fraying or unbraiding when the stmt is cut prior to deployment. In addition, the polymeric coating helps keep the 21 ends of the stmt flared to prevent the stmt from migrating after 22 deployment. This also provides a fluid dynamically favored 23 entrance for blood flow. Moreover, the polymeric coating also 24 maintains the st mt in a cylindrical configuration throughout its length thereby preventing the ballooning and tapering phenomenon when deployed in the bend of an artery.
The presence of a polymer on the surface of the stent also allows for the dispensing of drugs through the polymer which may take the form of a surface modification or a drug eluting reservoir. For example, an anticoagulant such as heparin or the like can be bound to the polymer surface and automatically dispensed from the stent after depolyment to prevent thrombosis. Alternatively, drugs such as antiinflammatory agents, steroids, or antimitotic drugs such as chemocompounds or radiomonic drugs can be eluted out of the stmt coating after deployment. These drugs may prevent intimal hyperplasia. Still alternatively, radioactive materials containing beta or gamma emitters can be embedded in the polymer with their actinic radiation interfering with DNA
replication thereby decreasing the incidence of hyperplasia.
Genetically engineered drugs such as growth factors and the like can also be eluted from the coating material.
Thus in one aspect, the invention provides: a) a radially and axially flexible substantially cylindrical body formed from a plurality of wire filaments having crossing points defining a lattice of interstices between wire filaments, said wire filaments being coated with a polycarbonate urethane polymer having a melting point of approximately 240°C substantially continuously alone substantially their entire lengths and at said crossing points so that said wire filaments are bound to each other by said polymer at said crossing points and said interstices of said lattice are not substantially occluded by said polymer; and b)

- 10 -a porous vascular graft attached to said body, said porous vascular graft comprising a spun polycarbonate urethane liner having a melting point of approximately 160°C.
In a further aspect, the invention provides a radially and axially flexible substantially cylindrical body formed from a plurality of wire filaments having crossing points defining a lattice of interstices between wire filaments, said wire filaments being coated with a polymer at said crossing points so that said wire filaments are bound to each other by said polymer at said crossing points and said interstices of said lattice are not substantially occluded by said polymer, wherein said wire filaments are coated by applying a polymeric solution containing a biodegradable mixture of polybuterate and polyvalerate dissolved in chloroform to the crossing points and allowing said polymeric solution to cure such that said crossing points are bound to each other by said polymer.
In another aspect, the invention comprises a radially and axially flexible substantially cylindrical body formed from a plurality of wire filaments having crossing points defining a lattice of interstices between wire filaments, said wire filaments being coated with a polymer at said crossing points so that said wire filaments are bound to each other by said polymer at said crossing points and said interstices of said lattice are not substantially occluded by said polymer, wherein said wire filaments are coated by dipping said substantially cylindrical body in a priming solution of 2%
aminopropylaminoethyltrimethoxysilane dissolved in a substantially 95%/5% mixture of ethanol and water, substantially drying the dipped cylindrical body, and then - l0a -applying a polymeric solution to said crossing points and allowing said polymeric solution to cure such that said crossing points are bound to each other by said polymer.
In another aspect, the invention also provides a method of making a prosthesis, comprising: a) obtaining a radially and axially flexible substantially cylindrical body formed from a plurality of filaments having crossing points defining a lattice of interstices between filaments; b) applying a polymeric solution of polycarbonate urethane polymer to the crossing points wherein said polymer has a melting point of approximately 240°C; and c) allowing the polymeric solution to cure such that crossing filaments are bound to each other by the polymer and the interstices of the lattice are not substantially occluded by the polymer.
In another aspect, the invention provides a method of making a prosthesis, comprising: a) obtaining a radially and axially flexible substantially cylindrical body formed from a plurality of filaments having crossing points defining a lattice of interstices between filaments; b) applying a polymeric solution to the crossing points wherein said polymeric solution contains a biodegradable mixture of polybuterate and polyvalerate dissolved in chloroform; and c) allowing the polymeric solution to cure such that crossing filaments are bound to each other by the polymer and interstices of the lattice are not substantially occluded by the polymer.
The methods described above may also comprise a step of further dipping said body into a priming solution of 2%
aminopropylaminoethyltrimethoxysilane dissolved in a - lOb -7.0238-.19 substantially 95%/5% mixture of ethanol and water before applying said polymeric solution.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.
- lOc -3 Figure 1 is a broken side elevation view of a prior art 4 braided st t expanded in a non-stressed position;
m 6 Figure 1a is a cross sectional view along line 1A-lA of 7 Figure 1;

9 Figure 2 is a broken side elevation view of a prior art stmt of Figures and 1a stretched and contracted;

11

12 Figure 2a is a cross sectional view along line 2A-2A of

13 Figure 2;

14 Figure 3 is a broken side elevation view of a prior art stmt 16 deployed in the bend of an artery;

18 Figure 4 is a view similar to Figure 1 of a prior art 19 undeployed ig-zag stmt on a balloon catheter;
z 21 Figure 5 is an enlarged broken side elevation view of a 22 portion of prior art braided stmt;
a 1 Figure 6 is a view similar to Figure 5 of a polymeric coated 2 braided st mt according to the invention;

4 Figure 7 is a view similar to Figure 4 of a polymeric coated zig-zag st mt according to the invention;

7 Figure 8 is a broken perspective view of a polymeric coated 8 braided stmt-graft according to the invention; and Figure 9 is a schematic sectional view of a polymeric coated 11 stmt according to the invention with conical inserts flaring the 12 ends of the stent.

16 The invention will be described with reference to several 17 examples in which a prior art stmt is coated with a polymer in 18 order to bind the wires of the stmt at the crossing points (or 19 zig-zag points) without occluding the interstices of the st mt lattice.

22 xa le 1 23 Referring to Figures 5 and 6, a stmt 50 is made of the 24 Didcott design as shown in prior art Figure 5. Each of the wires, ~~~~90 1 e.g. 52, 53, is approximately eight millimeters in diameter and 2 the wires are braided with a pitch angle of approximately 85°.
3 The stent 50 has an outward hoop force for 50~ compression of 0.11 4 lb, i.e., a radial load of 0.11 lbs is required to compress the stmt radially 50~.

7 According to a first method of the invention, a polycarbonate 8 urethane of Shore 55D is dissolved in dimethylacetamide at a 9 concentration of 5~ solids content by weight. The mixture is sprayed onto the stmt 50 and dried at 70°C for ten minutes to 11 create a coated stmt 150 as shown in Figure 6. Preferably, the 12 spraying and drying is repeated several times in order to build up 13 the surface coating of polycarbonate urethane, e.g. 156, on each 14 of the wires, e.g. 152, of the stmt 150. The load required to compress the stmt 150 by 50~ is tabulated below in Table 1 16 according to the number of coatings.

Number of spray coatings of 5~ Load (in lbs.) required to solids polycarbonate urethane compress the stmt 500 0 0.11 5 0.14 10 0.16

15 0.20 18 Table 1 a~~~~~a 1 The coated stmt 150 may be cut to size and loaded into a 2 catheter without the ends fraying. The stmt may then be 3 delivered to the site of injury in the vascular system and 4 deployed in the usual manner.
6 Example 2 7 A balloon expandable stent 30 of the Wiktor design (Figure 4) 8 is placed on a mandril with adjacent zig-zags, e.g. 34, 36, 9 touching each other. The st mt 30 is sprayed with polycarbonate urethane of Shore 75D hardness dissolved in dimethylacetamide at a 11 concentration of 10~ solids content by weight. The stmt is dried 12 and is sprayed and dried another five times. The resultant stmt 13 130, shown in Figure 7, is removed from the mandril and 14 demonstrates a uniform cylindrical outline with higher hoop strength than the uncoated stmt. The zig-zag wire comprising the

16 stmt is maintained in a uniform cylindrical outline. The stmt

17 is deployed with a balloon catheter 31 which is expanded to beyond

18 the yield point of the polycarbonate urethane. When the st mt is

19 so deployed, it remains open in a uniform cylindrical outline.
21 Example 3 22 The Didcott-type stmt of Figure 5 is spray coated (once or 23 several times) with a biodegradable polymer comprised of a mixture 24 of 50~ polybuterate and 50$ polyvalerate dissolved in chloroform.

~~~~~~a 1- The resultant stent is rendered more rigid than the st mt of 2 Example 1, but softens appreciably after implantation in the body.

4 F~xam~ple 4 The coated stent 150 described in Example 1 and shown in 6 Figure 6 is attached to a vascular graft as described in Dereume 7 Belgium Patent No. 112,774 and shown in Figure 8 to form an 8 endoluminal graft 200. The vascular graft 202 may be applied to 9 the interior of the stmt or to the exterior of the stmt as shown in Figure 8. The endoluminal graft 200 is implanted in a tortuous 11 artery where it assumes the shape of the artery without the ends 12 becoming tapered or the center ballooning.

14 Example 5 The coated stmt 150 described in Example 1, with ten 16 coatings of polyurethane is immersed in a solution containing 5~
17 phospholipid in water. A thin layer of phospholipid is thereby 18 bound to the surface of the polymeric coating. A stmt made 19 according to this example was placed in the coronary artery of a dog and demonstrated little thrombus build-up due to the 21 hemocompatible nature of the phospholipid surface.

4~~~~~~
i Example 6 2 In a solution of dimethyl acetamide and 5$ polycarbonate 3 urethane of Shore 50D hardness, the drug 5-fluorouracil is added 4 (10~ weight of the drug to the weight of polycarbonate urethane).
The lacquer containing the drug is dip-coated onto a stmt and 6 allowed to dry. The drug eluting polymer-coated stmt according 7 to this example was implanted into the coronary artery of a dog 8 where the drug was slowly released. The eluted drug interfered 9 with the reproduction of DNA in the coronary artery thereby preventing intimal hyperplasia of the vessel.

12 ~ xa ple 7 13 A Didcott-type stmt such as that used in Example 1 is placed 14 on a mandril having two conical inserts as shown schematically in Figure 9. The conical inserts 302, 304 are forced into the ends 16 of the stmt 50 such that the ends are flared. The stmt is then 17 spray coated with 15 layers of polycarbonate urethane (5% solids) 18 and dried and removed from the mandril. The st mt demonstrates 19 flares on each end.
21 Example 8 22 A coated stmt 150 (Figure 6) made according to Example 1 23 with ten layers of polycarbonate urethane is placed on a mandril 24 having two conical inserts. The conical inserts are forced into ~~'~~~~9U
1 the ends of the stmt such that the ends are flared. The stmt 2 and mandril are then placed in an oven at 170° - 200°C where the 3 polycarbonate urethane is partially melted. The stmt and mandril 4 are then cooled to room temperature at which point the conical inserts are removed from the stmt. The stmt now demonstrates 6 flared ends with high hoop strength on the ends.

8 Example 9 9 An Elgiloy'''~ wire self-expanding stmt is primed by dipping it into a solution of 2~ aminopropylaminoethyltrimethoxysilane 11 dissolved in a 95/5$ ethanol-water mixture. The primed st mt is 12 then dried overnight at room temperature and placed on a rotating 13 mandril. The stmt is spray-coated and dried three times with a 14 solution containing 9 grams of polycarbonate urethane of 75D
durometer, 1 gram of polycarbonate urethane of 55D durometer, 75 16 grams of dimethylacetamide and 75 grams of tetrahydrofuran. The 17 dried stmt has a hoop strength four times higher than the initial 18 hoop strength of the uncoated stmt. In lieu of am ElgiloyTM wire, 19 the wire may be Phynox'1'x or 316 LV stainless steel.
21 Example 10 22 A porous spun polycarbonate urethane liner of melting point 23 160°C is made by spinning five hundred passes from a thirty 24 orifice spinneret of polymer onto a stainless steel mandril at '~~~~90 1 1,000 RPM with a wrap angle of 50°. The liner is cured in an oven 2 at 110°C overnight. A stent is dip-coated with 5~ polycarbonate 3 urethane of 75D hardness and with a melting point 240°C in 4 tetrahydrofuran and dried. The 75D-coated stmt is again dipped into another solution of polycarbonate urethane but of 80A
6 hardness and of 160°C melting point and dried. An additional ten 7 passes of fiber are spun over the liner and while wet, the 75D-8 and 80A-coated stmt is placed over the liner and the assembly 9 placed in an oven at 120°C where the wet outer layers on the liner are melted and bonds the stmt to the liner. The stmt-graft thus 11 formed has a higher hoop strength that without the 75D coating.

13 Example 11 14 The stmt-graft assembly of Example 10 is further reinforced by placing it back on the spinning machine where an additional one 16 hundred passes of fiber are spun over the stmt. While the fibers 17 are wet, a soft silicone roller is rolled over the st mt thereby 18 pressing the fibers through the picks of the stmt and bonding 19 them to the inner liner. The stmt-graft thus formed demonstrates a much better attachment of the liner to the stmt.

22 Example 12 23 Tantalum wire of 0.004" diameter is pulled through an 24 extruder die where a thin layer (0.001") of fluorinated ethylene (~, 1 propylene (FEP) polymer of melting point 420°C is extruded over 2 the wire. The wires are formed into a zig-zag pattern according 3 to Wiktor and then wound into a helical geometry with adjacent 4 zig-zags touching each other. The stmt is then heated to 420°C
where the FEP melts and when cooled adheres to adjacent zig-zags.
6 The cooled assembly is removed from the mandril and demonstrates a 7 uniform design with higher hoop strength than the uncoated stmt.
8 The zig-zag wire comprising the st mt is maintained uniform. The 9 stmt is then balloon expanded beyond the yield strength of the FEP where it remains open in a uniform manner.

12 There have been described and illustrated herein several 13 embodiments of implantable stems and st mt-grafts having wires 14 which are coated with a polymer at their crossing points or zig-zag vertices. While particular embodiments and examples of the 16 invention have been described, it is not intended that the 17 invention be limited thereto, as it is intended that the invention 18 be as broad in scope as the art will allow and that the 19 specification be read likewise. Thus, while particular conventional stem s have been disclosed in conjunction with the 21 method of the invention, it will be appreciated that other stems 22 could be subjected to the inventive methods disclosed herein.
23 Also, while specific examples of polymeric coatings have been 24 described, it will be recognized that other polymers having oz~~~~~a similar properties could be used with similar results obtained.
2 Furthermore, while specific methods of applying the coating have 3 been shown, such as dipping and spraying, other methods could be 4 used. For example, the polymer could be applied using electro-spraying where a potential difference is applied between the spray 6 nozzle and the stmt. Moreover, while particular examples have 7 been disclosed in reference to drug delivery via the polymeric 8 coating, it will be appreciated that other types of drugs could be 9 used as well. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the 11 provided invention without deviating from its spirit and scope as 12 so claimed.

- 20 -

Claims (6)

CLAIMS:

1. A prosthesis comprising:
a) a radially and axially flexible substantially cylindrical body formed from a plurality of wire filaments having crossing points defining a lattice of interstices between wire filaments, said wire filaments being coated with a polycarbonate urethane polymer having a melting point of approximately 240°C substantially continuously alone substantially their entire lengths and at said crossing points so that said wire filaments are bound to each other by said polymer at said crossing points and said interstices of said lattice are not substantially occluded by said polymer; and b) a porous vascular graft attached to said body, said porous vascular graft comprising a spun polycarbonate urethane liner having a melting point of approximately 160°C.

2. A prosthesis comprising:
a radially and axially flexible substantially cylindrical body formed from a plurality of wire filaments having crossing points defining a lattice of interstices between wire filaments, said wire filaments being coated with a polymer at said crossing points so that said wire filaments are bound to each other by said polymer at said crossing points and said interstices of said lattice are not substantially occluded by said polymer, wherein said wire filaments are coated by applying a polymeric solution containing a biodegradable mixture of polybuterate and polyvalerate dissolved in chloroform to the crossing points and allowing said polymeric solution to cure such that said crossing points are bound to each other by said polymer.

3. A prosthesis comprising:
a radially and axially flexible substantially cylindrical body formed from a plurality of wire filaments having crossing points defining a lattice of interstices between wire filaments, said wire filaments being coated with a polymer at said crossing points so that said wire filaments are bound to each other by said polymer at said crossing points and said interstices of said lattice are not substantially occluded by said polymer, wherein said wire filaments are coated by dipping said substantially cylindrical body in a priming solution of 2% aminopropylaminoethyltrime-thoxysilane dissolved in a substantially 95%/5% mixture of ethanol and water, substantially drying the dipped cylindrical body, and then applying a polymeric solution to said crossing points and allowing said polymeric solution to cure such that said crossing points are bound to each other by said polymer.

4. A method of making a prosthesis comprising:
a) obtaining a radially and axially flexible substantially cylindrical body formed from a plurality of filaments having crossing points defining a lattice of interstices between filaments;
b) applying a polymeric solution of polycarbonate urethane polymer to the crossing points wherein said polymer has a melting point of approximately 240°C; and c) allowing the polymeric solution to cure such that crossing filaments are bound to each other by the polymer and the interstices of the lattice are not substantially occluded by the polymer.

5. A method of making a prosthesis comprising:
a) obtaining a radially and axially flexible substantially cylindrical body formed from a plurality of filaments having crossing points defining a lattice of interstices between filaments;
b) applying a polymeric solution to the crossing points wherein said polymeric solution contains a biodegradable mixture of polybuterate and polyvalerate dissolved in chloroform; and c) allowing the polymeric solution to cure such that crossing filaments are bound to each other by the polymer and the interstices of the lattice are not substantially occluded by the polymer.

6. The method of claim 4 or 5, further comprising dipping said body into a priming solution of 2%
aminopropylaminoethyltrimethoxysilane dissolved in a substantially 95%/5% mixture of ethanol and water before applying said polymeric solution.