WO2003045666A1

WO2003045666A1 - Method for treating expandable polymer materials and products produced therefrom

Info

Publication number: WO2003045666A1
Application number: PCT/US2001/043402
Authority: WO
Inventors: Roger Labrecque; Joseph Ferraro; Tom Swanick; Paul Martakos
Original assignee: Atrium Medical Corporation
Priority date: 2001-11-21
Filing date: 2001-11-21
Publication date: 2003-06-05
Also published as: AU2002225660A1

Abstract

The invention is directed to methods involving rewetting of expandable polymers with a wettable liquid to allow for enhanced expansion at or below room temperature without breakage, and in some cases, allows one to achieve a greater expansion ratio than that allowed at elevated temperatures using known methods. The present invention also allows one to achieve material with unique properties and handling characteristics. These properties included decreased material thickness, increased density, an altered node/fibril morphology, and a more consistent web in the case of flat material. This method is not limited to room temperature conditions and can be applied whenever the expandable polymer material is wet with a wettable liquid, and the expansion is performed at a temperature preferably at a temperature preferably below the vaporization or boiling points of that liquid.

Description

METHODFORTREATINGEXPANDABLEPOLYMERMATERIALS ANDPRODUCTSPRODUCED THEREFROM

Technical Field

The present invention relates generally to materials and processing of materials.

More specifically, the present invention is directed to expandable polymers and methods

for processing of expandable polymers.

Background

A conventional method of forming an article made of an expandable polymer,

such as PTFE, is to blend a powdered resin with a wettable liquid, such as a lubricant or

extrusion aid, and compress the combination under relatively low pressure into a

preformed billet. A wettable liquid is mixed with the powdered resin to control the

degree of material shear that occurs during subsequent extrusion and to prevent

excessive shear, which can damage the material.

Using a ram-type extruder, the billet is extruded through a die having a desired

cross-section. Next, the wettable liquid is removed from the extruded material by drying

or by another extraction method. The dried extruded material is then stretched in one or

more directions at an elevated temperature below the crystalline melting point of the

resin. In the case of PTFE, this results in the material taking on a microstructure

characterized by elongated nodes interconnected by fibrils. Typically, the nodes are

oriented with their elongated axis perpendicular to the direction of stretching. According to conventional methods, there is a direct relationship between

temperature and maximum expansion ratio while maintaining material uniformity and

without breakage of the material. At low expansion temperatures, the material shows

inconsistencies, is weak, and often breaks. Typically, heating well above room

temperature is required to prevent the expandable polymer material from breaking and to

ensure uniform material thickness after expansion.

US Patent 4,187,390 describes a method of forming porous PTFE that requires

stretching at elevated temperatures. Material expanded at lower temperatures often

fractures or results in weak material.

US Patent 5,552, 100 describes a method of forming thin porous fluoropolymer

films by post-sinter stretching the material to a final thickness less than 0.002 inches.

The conventional manufacturing of films having thicknesses below 0.002 inches during

pre-sinter expansion often result in breaking or tearing of the film.

Therefore, a need exists for a method providing substantial expansion of

expandable polymers, without need for heating, to create uniform material with alternate

polymer morphologies. Furthermore, the ability to decrease thickness, increase strength,

uniformity and density of expandable polymers is desirable in many applications.

Summary

The present invention is directed generally to methods for treating expandable

polymers and products produced therefrom. More particularly, the invention relates to

methods for forming an article from an expandable polymer that has been stretched

involving the steps of re wetting the expandable polymer with a wettable liquid to form a wetted material, and stretching the wetted material. Preferably, the wettable liquid is

later removed.

According to another aspect of the invention, an article is formed by rewetting an

expandable polymer and then stretching the expandable polymer.

Expandable polymer articles formed in accordance with the processes of the

invention have characteristics, such as uniformity, porosity, density, node size,

thickness, fibril density and permeability not attainable from conventional methods.

These and other features, aspects, and advantages of the present invention will

become better understood with reference to the following description and appended

claims.

Brief Description of the Drawings

The subject matter regarded as the invention is particularly pointed out and

distinctly claimed in the concluding portion of the specification. The invention,

however, both as to organization and method of practice, together with further objects

and advantages thereof, is best understood by reference to the following illustrative

descriptions taken in conjunction with the accompanying drawings in which like

numerals refer to like elements.

Figure 1A illustrates an exemplary method of a first embodiment of the

invention;

Figure IB illustrates exemplary variations of a first embodiment of the invention;

Figure 2 A illustrates an exemplary method of a second embodiment of the

invention; Figure 2B illustrates exemplary variations of a second embodiment of the

invention;

Figure 3 provides a scanning electron micrograph (SEM) of Material A;

Figure 4 provides an SEM of Material B;

Figure 5 provides an SEM of Material D of the invention;

Figure 6 provides an SEM of Material F;

Figure 7 provides an SEM of Material G;

Figure 8 provides an SEM of Material H of the invention;

Figure 9 provides an SEM of Material I of the invention;

Figure 10 provides an SEM of Material J of the invention;

Figure 11 provides an SEM of Material K of the invention;

Figure 12 provides an SEM of Material L;

Figure 13 provides an SEM of Material M;

Figure 14 provides an SEM of Material N of the invention;

Figure 15 provides an SEM of Material O of the invention;

Figure 16 provides an SEM of an exterior of Sample P;

Figure 17 provides an SEM of an interior of Sample P;

Figure 18 provides an SEM of a cross-section of Sample P;

Figure 19 provides an SEM of an exterior of Sample Q of the invention;

Figure 20 provides an SEM of an interior of Sample Q of the invention;

Figure 21 provides an SEM of a cross-section of Sample Q of the invention;

Figure 22 provides an SEM of an exterior of Sample R of the invention;

Figure 23 provides an SEM of an interior of Sample R of the invention;

Figure 24 provides an SEM of a cross-section of Sample R of the invention; Figure 25 provides an SEM of an exterior of Sample S of the invention;

Figure 26 provides an SEM of an interior of Sample S of the invention;

Figure 27 provides an SEM of a cross-section of Sample S of the invention;

Figure 28 provides an SEM of Material X; and

Figure 29 provides an SEM of Material Y of the invention.

Detailed Description

The present invention provides a means to expand expandable polymers at or

below room temperature without breakage and maintaining a substantially uniform

material, and allows one to achieve a greater expansion ratio than that allowed at

elevated temperatures using known methods.

Polymers with ordered microstructures, often referred to as highly crystalline,

have the fundamental ability to expand into another shape and size. Fluoropolymers and

polyolefins are polymers suitable for expansion processes. Fluoropolymers include

homopolymers of polytetrafluoroethylene (PTFE), and copolymers of

polytetrafluoroethylene in which the comonomer is ethylene, chlorotrifluoroethylene,

perfluoroalkoxytetrafluoroethylene, and fluorinated propylene. Polyolefins include

polypropylene and polyethylene.

The concept of contact angle and its' equilibrium is valuable because it can be

used to define wettability. When a liquid wets a solid, it spreads freely over the surface

at a rate depending on the liquid viscosity and surface tension, and solid surface

roughness, porosity, and chemistry. The tendency for the liquid to spread increases as

contact angle decreases so contact angle is a useful inverse measure of wettability. Contact angle is the angle measured which the liquid makes with a solid. The contact

angle of a liquid is a result of the thermodynamic equilibrium of a drop on a solid

surface. Solids, liquids, and gases, exist in equilibrium. At the interface between a

liquid and solid, the interfacial monolayer of the liquid is attracted by the bulk liquid and

gas from one side and from the other side by the intermolecular forces which interact

between the solid and liquid. A porous material is said to be "wet" when the voids of

the material are at least partially occupied by a given fluid.

In accordance with the invention, multi-directional expansion of an expandable

polymer (sintered or unsintered) can occur at room temperature provided the material is

rewet with a wettable liquid before or during the expansion step. Rewetting involves the

application of wettable liquid after completion of activities for which wettable liquid

may be used, such as extrusion. Removal of any previous wettable liquid is not required

before rewetting. This method is not limited to room temperature conditions and can be

applied whenever the expandable polymer is rewet with the wettable liquid. Ideally, the

expansion is performed at a temperature below the vaporization or boiling points of the

wettable liquid.

Wet stretch is defined herein as the expansion or deformation of an expandable

polymer in one or more directions when the material is wet with a wettable liquid before

or during the expansion step. Wet stretching of an expandable polymer resin, such as

PTFE, can provide: modified processability, material structures which differ from those

made from conventionally processed resin, for example, decreased thickness, increased

density, and product uniformity. The overall feel of the product is typically enhanced,

due to increased smoothness. Major differences can be seen in the structures according to the invention

compared to conventionally processed material. The invention typically provides

increased density and improved strength, allowing products to be thinner than those

made from conventional methods.

The extrudate may also be sintered, after stretching or before stretching, by

heating it to a temperature above its crystalline melting point while being maintained in

a stretched condition. This can be considered an amorphous locking process for

permanently "locking-in" the microstructure in the expanded or stretched configuration.

The methods of the invention can simultaneously provide greatly reduced sintering times

and improved product structure over conventional methods. The present invention does

not require sintering for certain applications, including endovascular, filtration, and the

like, as is typically required by conventional methods.

The ability to increase the amount of expansion in either sintered or unsintered

expandable polymers has a wide variety of applications in medical, industrial, and

consumer products. For example: laminate structures with varying properties for filters

and membranes; medical implants with tailored porosities to control body fluid leakage

and tissue ingrowth; radially expandable PTFE with reduced expansion force and/or

increased expansion ratios for endovascular applications.

A variety of forms and sizes are included in the scope of the invention. For

example, flat sheets, hollow tubes, and solid rods, can be manufactured and utilized in

many applications. Furthermore, the invention is applicable to any structures formable

by conventional expandable polymer methods. Expanded PTFE material is characterized by lengthwise-oriented fibrils

interrupted by transverse nodes. The pore size in microns is typically determined by

measuring fiber length between the nodes (internodal distance). To compute fibril

length, the material is viewed under sufficient magnification. A fibril length is measured

from one edge of one node to the edge of an adjacent node. Fibril lengths are measured

from the sample to compute a mean fibril length.

Nodes and fibrils may be further characterized by their relative geometry. That

is, nodes by length, width, and height; and fibrils, by diameter and length. It is the

relative geometry of nodes to fibrils, as well as, internodal distance and fibril density that

determines porosity and permeability of porous PTFE. The physical space between

connecting nodes is composed of solid thread like PTFE fibers called fibrils in

conjunction with a gaseous void volume. Fibril density refers to the relative volume

consumed by fibrils between the nodes.

Permeability or hydraulic conductivity is related to material porosity.

Permeability to fluid flow can be determined by measuring the amount of pressure

required for water to permeate the pores of the material. Water entry pressure (WEP) is

a good measuring technique to assess this trait because it closely mimics the permeation

process at the liquid/solid interface. WEP is defined as the pressure value necessary to

push water into the pores of a synthetic tubular substrate and can be classified as: High

(>400 mm Hg), Medium (200-400 mm Hg), and Low (<200 mm Hg). To compute

WEP, the material is subjected to an incrementally increasing water pressure until small

beads of water appear on the surface. Machine direction (MD) refers to the direction in which the polymeric material

travels through the processing machine. Transverse direction (TD) refers to the direction

that is perpendicular to the MD. Longitudinal Tensile Strength (LTS) is measured in

pounds per square inch by dividing the tensile force applied to the material by the cross-

sectional area of the material. Radial Tensile Strength (RTS) is also measured in pounds

per square inch. RTS is obtained by dividing the radial expansion force applied to the

material by the cross-sectional area of the material. Cross-sectional area is the amount

of material subjected to a controlled strain during tensile testing defined as the sample

width multiplied by its thickness.

Suture Retention strength (SRT), measured in pounds, indicates the amount of

force needed to pull out sutures from the polymeric material.

The invention will now be described with reference to two exemplary

embodiments. Cylinders, tubes, sheets, or other shapes can be created by either of these

embodiments.

Both embodiments involve the use of expandable polymers. Although

expandable polymer material may be prepared in a variety of ways, one method involves

the use of wettable liquid to aid an initial extrusion process. A wettable liquid is capable

of entering the pores of the expandable polymer resin. The invention is not limited to

expandable polymers prepared by extrusion, or by the use of a wettable liquid for

extrusion.

By way of example, an expandable polymer resin, such as PTFE resin (Fluon

CD- 123 obtained from ICI Americas), may be blended with a first wettable liquid, such

as ISOPAR-H odorless solvent (produced by EXXON Corporation), to form a lubricated

powder. The wettable liquid may be mixed with the resin to control the degree of material shear that occurs during subsequent extrusion and to prevent excessive shear,

which can damage the material. By application of pressure, the lubricated powder may

then be preformed into a billet, typically shaped like a large cylinder.

Using a ram-type extruder, the billet may be extruded through a die having a

desired cross-section, typically a circle, thereby forming a cylinder. A variety of shapes

may be formed by extrusion, such as a solid or hollow cylinder, a flat sheet, a rectangle

and the like.

The first embodiment of the invention is described with reference to Figure 1.

An expandable polymer is rewet, step 110, with a second wettable liquid such as

ISOPAR-H. Rewetting may be performed by exposing the expandable polymer to the

second wettable liquid, such as by submerging or soaking the expandable polymer in the

second wettable liquid, spraying the second wettable liquid, or rubbing the second

wettable liquid into the expandable polymer. As described above, rewetting involves the

before rewetting.

Use of the second wettable liquid results in substantially uniform material with

increased density and substantially altered node structure. Ideally, the second wettable

liquid completely saturates the expandable polymer. As with all embodiments described

herein, elevated temperature or pressure above ambient conditions may be used in

conjunction with the application of a wettable liquid to reduce the time necessary for

saturation or aid in saturation of the expandable polymer. Stretching, step 120, is then performed, preferably at a temperature below a

boiling point of the second wettable liquid. Stretching can be performed in more than

one direction. Stretching is typically performed, in the case of a cylinder, by applying

tensile force to the ends of the cylinder. In the case of a flat sheet, stretching is typically

performed in the machine direction. Alternatively, or in addition, stretching may be

performed in the radial or transverse direction to a cylinder or flat sheet, respectively.

For example, in the case of a hollow cylinder, a mandrel may be used to radially stretch

the hollow polymer cylinder. Tensile force may be applied to stretch the cylinder

simultaneously with the use of a mandrel or at a different time. Within the scope of the

invention, a combination of various stretching may be combined or applied in

succession.

As with all embodiments described herein, heat may also be applied to the

expandable polymer prior to or during stretching. It is preferable to keep the

temperature of the expandable polymer below a boiling point of the second wettable

liquid to inhibit loss of the second wettable liquid.

Although ISOPAR-H is used as the first and second wettable liquids in this

embodiment, other permeating liquids are within the scope of the invention and can be

considered interchangeable with ISOPAR-H or other wettable liquids. As an example,

polyethylene glycol is preferred for in vivo applications because it is a biocompatible

liquid. Naphtha is another example of a wettable liquid that may be used within the

scope of the invention. Alcohol and water may also be used in combination. It is also

within the scope of the invention to use one wettable liquid during the initial extrusion

process and another wettable liquid for rewetting. Also, a combination of liquids may be

used during either extrusion or rewetting. Optionally, further steps of the preferred embodiment of the invention may

include removing the second wettable liquid, step 130. Although removal can be

accomplished at room temperature, heating to an elevated temperature is preferred to

accelerate removal of the second wettable liquid. Optional heating, step 140, to, for

example, 320° C, can be performed following removal of the second wettable liquid, step

130. Heating, step 140, may optionally be sufficient to cause sintering, typically at

about 360°C, thereby locking in the microstructure. Alternatively, sintering, step 150,

may be conducted after heating.

A further alternative of the first embodiment involves a second stretching step

160, after optional removing of the wettable liquid, step 130. As discussed above, this

second stretching step 160 may involve heating prior to or during stretching and may be

conducted in the machine direction, a transverse direction, or any combination or

sequential application thereof. Sintering, step 170, may optionally be performed after

the second stretching.

A further variation of the first embodiment of the invention includes calendaring,

step 180, before the rewetting step 110. Calendaring is preferably performed during the

creation of flat sheets after extrusion of the expandable polymer. Preferably, calendaring

rolls are operated at an elevated temperature, such as, for example, 130° F.

A second, preferred embodiment of the invention is shown in Figure 2. The

second embodiment of the invention differs from the first embodiment, at least in part,

by stretching of the expandable polymer before rewetting with a second wettable liquid.

According to an exemplary method of the second embodiment, an expandable

polymer is stretched, step 210. Because a second wettable liquid has not been applied, it

is preferable that stretching be performed with heat, for example in radiant heat oven set to approximately 705° F, thereby allowing greater stretch ratios. Rewetting, step 220, is

then performed by applying a second wettable liquid to the expandable polymer. As

discussed in relation to rewetting step 110 of the first embodiment, a wettable liquid may

be applied to the expandable polymer in a variety of ways.

Stretching, step 230, is performed as discussed in relation to the stretching step

120 of the first embodiment. Variations, including for example, heating and direction of

stretching, discussed in relation to stretching step 120 of the first embodiment are also

applicable to stretching step 230 of the second embodiment.

One variation of the second embodiment of the invention involves increased

tension on a take-up roller during processing. This will provide a variation in properties

of the resulting expandable polymer. Increased tension of the take-up roller will impart

some longitudinal orientation to the nodes of the expandable polymer and create a

thicker, less dense product than is typical without increased tension on the take-up roller.

Another variation of the second embodiment involves substituting the step of

stretching 230 with the application of pressure sufficient to change the structure of the

expandable polymer. By way of example, pressure may be applied by the use of rollers

to crush the expandable polymer.

The second embodiment of the invention may also include removal of the second

wettable liquid from the expandable polymer. Removal of the second wettable liquid,

step 240, is optionally performed after the stretching step 230. Further optional

variations include single or multi-directional stretching as discussed above in relation to

the stretching step 120 of the first embodiment, or sintering, step 250, after removal of

the second wettable liquid, step 240. Alternatively, a third stretching step 260 may be performed after removal of the

second wettable liquid, step 240.

A further variation of the second embodiment involves sintering, step 250, after

the second stretching step 230, or the removal of the second wettable liquid, step 240, or

stretching step 260.

Another variation of the second embodiment includes removal of the first

wettable liquid, step 270, after the stretching step 210. According to another variation,

cooling, step 280, may be performed prior to the rewetting step 220. Cooling, step 290,

may also be performed after optional removal of the first wettable liquid, step 270.

A further variation of the second embodiment includes removal of the second

wettable liquid, step 300, after the rewetting step 220. In this variation, the second

stretching step 230 is not conducted.

The second embodiment may also include the application of heat during the first

stretching step 210. The step of stretching without heat or use of a wettable liquid

typically cannot involve high stretch ratios without risk of tearing the expandable

polymer.

Further variations of the second embodiment include removal of the first

wettable liquid, step 310, prior to the first stretching step 210. Another variation

includes preliminary stretching of the expandable polymer, step 320, before the first

stretching step 210. Preliminary stretching, step 320, before the first stretching step 210

may be conducted prior to optional removal of the first wettable liquid, step 310, but the

invention is not so limited and stretching may be conducted without removal of the first

wettable liquid. A further variation of the second embodiment includes calendaring. As

discussed in relation to the first embodiment, calendaring is typically performed during

the creation of a flat sheet. The step of calendaring 330, 340 may be conducted shortly

after extrusion of the expandable polymer, such as before the step of preliminary

stretching 310, the step of removal of the first wettable liquid 310, or stretching step 210.

The embodiments and their variations described above are intended to be

< representative of the scope of the invention and not limiting. It is also intended to be

within the scope of the invention for variations of the embodiments to be applicable to

other embodiments. For example, variations of the second embodiment may be used in

combination with methods of the first embodiment.

The invention will now be described with respect to various examples involving

various forms, beginning with sheets and films.

Example # 1

Example 1 of the invention, Material D, involves flat material that is stretched in

the machine direction according to the second embodiment of the invention. This

example provides increased density of the material and an altered node structure from

those available by conventional methods.

PTFE resin (Fluon CD- 123 obtained from ICI Americas) was blended with

ISOPAR-H odorless lubricant solvent (produced by EXXON Corporation) at a level of

19.5% by weight to form a lubricated powder. The lubricated powder was then

compressed into a cylinder and ram extruded into a flat sheet 6 inches across and 0.040

inches thick. The flat sheet was then compressed through two heated rolls to form a film having a thickness of 5 mil. The lubricant was then removed from the film by passing

the film through a radiant heat oven set to 490°F to drive off the ISOPAR-H.

The film was then stretched in the machine direction at a ratio of 10:1 in a radiant

oven set at 705°F to form a Material A. A scanning electron micrograph (SEM) of

Material A is shown in Figure 3. A control experiment was performed where Material A

was restretched in the machine direction at a ratio of 1.8:1 with the use of heat but with

no wettable liquid to form a Material B shown in Figure 4. A further experiment was

performed to stretch Material A to a ratio of 1.8: 1 in the machine direction without the

use of heat or wettable liquid to attempt to form a Material C. However, the sample

broke before reaching the ratio of 1.8 : 1. Materials A and B represent samples prepared

according to conventional methods. The attempt to form Material C demonstrates the

need for heat in conventional stretching methods.

According to Example 1 of the invention, Material A is rewet with the ISOPAR-

H wettable liquid and stretched in the machine direction at a ratio of 1.8:1 at room

temperature to form a Material D. Figure 5 shows the elongated nodes and alignment of

the fibrils of Material D.

As summarized in Table 1, the film produced by Example 1, Material D, is

thinner and has a higher density than a similarly-stretched film produced by a

conventional method, Material B. The node structure that is obtained by these two

methods differs drastically, as shown in Figures 4 and 5.

Figures 4 and 5 illustrate that by stretching the material wet, a substantially

different node structure is obtained than when conventional methods are used. Material

B, shown in Figure 4, is formed conventionally without wettable liquid and with heat

and has a structure where the nodes are comiected by fibrils that have a substantial amount of open space between them. Material D, shown in Figure 5, the film formed by

the method of Example 1 involving wettable liquid and no heat, consists of densely

packed fibrils and drawn out nodes. As shown in Table 1, a lower thickness and a higher

density are obtainable by using the method of Example 1.

Table 1

Example # 2

Example 2 of the invention involves a flat material that is stretched in the

transverse direction according to the second embodiment of the invention to provide a

thin, dense uniform material that has a low porosity. The same lubricated powder of

Example 1 is compressed into a cylinder and ram extruded into a flat sheet 6 inches

across and 0.040 inches thick. The flat sheet is then compressed through two heated

rolls to form a film having a thickness of 10 mil, twice the thickness of the film of

Example 1. The ISOPAR-H is then driven off by passing the film through a radiant

oven set to 490°F. The film is then stretched in the machine direction, in a radiant oven

set at 705°F at a ratio of 6: 1 to form a Material E.

Two control experiments were performed at the same transverse stretch ratio. A

first control sample, Material F, was created with heat and no wettable liquid; the other

sample, Material G, was created at room temperature also with no wettable liquid.

Material F was made by stretching Material E in the transverse direction, with

heat to a 12:1 stretch ratio. Material F has a star-like structure. Figure 6 shows a scanning electron micrograph (SEM) of Material F. Material F has a very inconsistent

thickness that ranged from 4.3 mil in the center to 1 mil at the edges. The average

density of this material is 0.319 g/cm³.

Material G, another control sample, was made like Material F, except that

stretching to a 12: 1 ratio was performed at room temperature. A scanning electron

micrograph (SEM) of Material G, is shown in Figure 7. Material G has long ordered

nodes with an internodal distance of about 15-30 microns. The density of Material G is

0.348 g/cm³, similar to the other control, Material F.

According to one variation of the invention, Material E is wet with the ISOPAR-

H wettable liquid and stretched in the transverse direction at a ratio of 12: 1 at room

temperature to form a Material H. Figure 8 is a scanning electron micrograph of

Material H. Material H has a node structure that has long drawn out nodes and very

small internodal distances between and including 0 to 10 microns. The thickness of

material H is consistently 0.5 mils, and the density of Material H is 1.228 g/cm³, which

is approximately four times higher than control materials, Materials F and G.

Material I, shown in Figure 9, is material that started out as Material H. The

wettable liquid was then removed and the material was stretched a second time. This

second stretch was in the transverse direction with heat at a ratio of 2: 1.

Another variation of the invention, Material J, involves increasing tension on the

take-up roller to stretch the material in the machine direction during stretching in the

transverse direction. Material J is very similar to Material H involving wetting Material

E with wettable liquid prior to transverse stretching at room temperature. Increased

tension on the take up roller imparts some longitudinal orientation to the nodes.

Material J was slightly thicker than Material H, 0.63 mils vs. 0.5 mils, and slightly less dense, 1.158 g/cm³ vs. 1.228 g/cm³. As shown in Figure 10, Material J has long wavy

nodes and was very consistent across the material.

Materials H and J were stretched with the wettable liquid and were much more

consistent in thickness and node structure than the two control samples, Materials F and

G. The node structure and internodal distance of the materials were also drastically

different. Materials H and J, processed by the inventive method, had a density four

times higher and were significantly thinner with a different overall feel, than Materials F

and G that were processed conventionally.

Table 2

Example # 3

Example 3 of the invention involves a flat material that is stretched in the

rolls to form a film having a thickness of 5 mil. A first wettable liquid was then removed by passing the film through an oven. The film was then stretched in the

machine direction at an elevated temperature to a ratio of 6: 1. The control sample,

labeled as Dry in Table 3, was then stretched in the transverse direction to the given

ratio. This transverse stretching was done at room temperature at a line speed of 5 feet

per minute. A sample according to the invention, labeled Wet in Table 3, was first

soaked in a second wettable liquid and then stretched to the same transverse stretch

ratios as the control sample.

Table 3

The thickness of the sample according to the invention was less than the control

sample at every stretch ratio. The density of the inventive sample was found to be much

higher than the control. The sample according to the invention was consistent at all

ratios tested and it did not rip. The control material was not consistent and it had a

tendency to rip. The overall look and feel of the materials was drastically different. The

dry processed material had a tendency to shrink and had noticeable striations in it.

The present invention is applicable to a wide variety of product configurations.

The following examples illustrate various embodiments of the invention involving tubes. Example # 4

Example 4 involves longitudinal stretching of a tube according to the first

embodiment of the invention. PTFE resin, Fluon CD-123, was blended with ISOPAR-H

odorless solvent at a level of 17% by weight. The lubricated powder was then

compressed into a cylinder and ram extruded into a 6mm-diameter tube. The tube was

then soaked in wettable liquid, ISOPAR-H, and stretched longitudinally at room

temperature. The tube could easily be stretched to a ratio of 3:1. However, an attempt to

stretch the 6 mm-diameter tube at room temperature to a ratio of 3:1 without wettable

liquid resulted in the tube breaking.

Example # 5

Example 5 involves stretching of tubes both radially and longitudinally according

to the second embodiment of the invention. A benefit of this example is a very thin,

high density tube with a small internodal distance. As in Example 4, PTFE resin, Fluon

CD-123, is blended with ISOPAR-H odorless solvent at a level of 17% by weight. The

lubricated powder is then compressed into a cylinder and ram extruded into a 2mm-

diameter tube. The ISOPAR-H is driven off in a convection oven at 250°F. The tube is

then expanded longitudinally at a rate of 5 inches/sec in a convection oven at a

temperature of 320°C.

The tube is then rewet with ISOPAR-H and stretched over a 19 mm mandrel. The

ISOPAR-H is driven off in a convection oven at 250°F. The tube is then sintered in a

convection oven at 360°C. The resulting tube, Material K shown in Figure 11, has a thickness of 0.5 mil and has an inner porosity of <1 μm. The density of Material K is

approximately 1.25 g/cm³.

Radial expansion over the 19 mm mandrel is not possible without the addition of

the wettable liquid. An attempt to put a non-sintered tube over the same mandrel when

it was not wet with wettable liquid resulted in a longitudinal split of the tube.

Example # 6

This example involves changing the node structure and density of an ePTFE tube

by stretching it longitudinally while it was wet with wettable liquid. As in Example 4,

PTFE resin, Fluon CD-123, is blended with ISOPAR-H odorless solvent at a level of

17% by weight. The lubricated powder is then compressed into a cylinder and ram

extruded into a 6mm-diameter tube.

Two control samples were prepared according to conventional methods using the

above extrudate. Material L, shown in Figure 12, was formed by stretching from 15" -

45" with heat and then sintering. Material M, shown in Figure 13, was formed by

stretching from 15" to 30" with heat and then stretching from 30" - 45" at room

temperature, without the use of wettable liquid, followed by sintering.

Material N, shown in Figure 14, was formed in accordance with the second

embodiment of the invention by first stretching from 15" - 30" with heat. It was then

wet with the wettable liquid and stretched from 30" - 45" at room temperature. The

wettable liquid was then removed and sintering was performed to create Material N.

Material O, shown in Figure 15, was created in accordance with the first embodiment of

the invention by wetting with a wettable liquid, then stretching from 15" - 20" at room temperature. The wettable liquid was then removed and stretching with heat

longitudinally to 45" was performed, followed by sintering.

Material N has a lower internodal distance than the conventional sample Material

L. When compared to Material N, Material M was found to have approximately the

same internodal distance but with smaller nodes. Material O has a very tight internodal

distance and very small nodes when compared to the conventional sample, Material L.

All three samples have a higher water entry pressure than the control with Material O

being the highest. The only other mechanical property that was changed was suture

retention strength, see Table 4.

Table 4

Example # 7

A further example of the invention involves a tube comprised of layers with

varying porosity. Variation in porosity can allow enhanced blood flow through a

vascular graft, while still enabling tissue to grow into the external surface of the graft. It

can also provide for selective filtration through the various pore size layers. Vascular

grafts prepared in a layered fashion consist of a highly stretched inner layer mounted on

a 6 mm mandrel that is wrapped with a tight porosity ePTFE film and covered with a high porosity outer layer. The resulting tube has a smooth, silky feel with a 10 mil wall

thickness. By use of the second embodiment of the present invention, the node structure

of one or more of the layers of the tube can be altered.

Sample P, shown in Figures 16-18, is an example of a vascular graft prepared

according to steps described in U.S. Patent No. 5,824,050, incorporated by reference

herein. A highly stretched and sintered graft is placed onto a mandrel where it is

wrapped, covered with a sintered cover and sintered together. Figure 16 illustrates the

structure of an exterior surface, and Figure 17 illustrates an interior surface. Figure 18

provides a cross-sectional view, showing the changing structure along the radius of the

graft.

Sample Q is a graft that is made with the same materials as Sample P, but the

highly stretched graft inner layer in an unsintered state is rewet with ISOPAR-H,. placed

on a mandrel where it is then wrapped with tight porosity ePTFE film. The ISOPAR-H

is then run off with heat at either 120°C for 10 minutes or 200°C for 3 minutes. A

sintered cover is prepared by stretching a second tube over a 10 mm mandrel. The

expanded tube is then placed over the wrapped inner layer. The entire assembly is then

sintered together. Rewetting results in the inner layer having a reduced porosity when

compared to Sample P, see Figures 19-21. Figure 19 shows an exterior of Sample Q,

while Figures 20 and 21 are interior and cross-sectional views of Sample Q, respectively.

Sample R is a graft that is constructed like Sample Q, except that the wrapped

inner layer is covered with a non-sintered cover that is wet with ISOPAR-H. The

ISOPAR-H is then run off with heat and the entire assembly is then sintered together.

See Figures 22-24 for views of an exterior, interior and cross-section of Sample R,

respectively. Sample S is a graft that is made with a sintered inner layer that is radially

stretched by placement on a mandrel, wrapped and then covered with a non-sintered

cover that is wet with ISOPAR-H. The ISOPAR-H is then run off with heat and the

entire assembly is then sintered together. See Figures 25-27 for an exterior, interior and

cross-section of Sample S, respectively.

With reference to Figures 17, 20 and 23, the samples that were made with the

non-sintered method for the inner layer, Samples Q (Figure 20) and R (Figure 23) have a

very tight inner porosity when compared to the method disclosed in U.S. Patent No.

5,824,050 (Figure 17). Samples R and S, made with the wet stretch method for the

cover, have higher tear strength values than samples using a sintered cover, Samples P

and Q.

Another embodiment of the invention involves the use of a crusher, such as a

roller, to crush the inner layer after radial stretching and before application of the ePTFE

film, thereby reducing the porosity of the inner layer. Samples T - W illustrate the

change in properties from the use of the crusher. Except for crushing, Samples T - W

are otherwise processed like Samples P - S, respectively. See Table 5 for comparison.

Table 5

The following example illustrates an embodiment of the invention involving the

application of pressure.

Example # 8

Example 8 of the invention involves a flat material that has pressure applied to it

according to the second embodiment of the invention to provide a thin, dense uniform

material that has a low porosity. The same lubricated powder of Example 1 is

inches thick. The flat sheet is then compressed through two heated rolls to form a film

having a thickness of 5 mil. A first wettable liquid was then removed by passing the

film through an oven. The film was then stretched in the machine direction at an

elevated temperature to a ratio of 7.5:1 to form Material X, shown in Figure 28. A roll

of Material X was then wet with the ISOPAR-H wettable liquid. Pressure was then

applied to the roll of wet material using a set of rollers to form Material Y, Shown in

Figure 29. Material Y has a much higher fibril density than Material X and has a denser

overall look to it. Material Y was thinner than Material X, 1.2 mil and 3.1 mil

respectively. The density of Material Y was 0.776 g/cm³ compared to Material X which

had a density of 0.375 g/cm³.

***

In addition to the embodiments and examples discussed above, wettable liquid

may be applied during initial wetting or rewetting by the use of increased or decreased temperature and/or pressure. Increased or decreased temperature and/or pressure can

reduce processing time and enhance the saturation of the expandable polymer.

The techniques of the present invention may be employed to create implantable

prosthetic devices that are adapted for delivery of bioactive materials. For example,

vascular grafts with multiple lumens may be created using the techniques described

herein. The physical structure components in such prosthetic devices is discussed in

further in detail U.S. Patent No. 5,411,550, entitled "Implantable Prosthetic Device for

the Delivery of a Bioactive Material," the contents of which are incorporated herein by

reference.

Expandable polymers of the present invention have wide ranging applications,

such as devices for in vivo implantation, prostheses intended for placement or

implantation to supplement or replace a segment of a natural biological blood vessel, and

supports for tissue repair, reinforcement or augmentation. Specific products include but

are not limited to heart valves, sutures, vascular access devices, vascular grafts, shunts

and catheters. Other products include single and multilayered membranes. Multilayered

membranes containing regions of selective porosity and chemistry are useful in the

medical diagnostic and the filtration industries. For example, some clinical diagnostic

test strips contain multilayer membranes with selective binding sites in each layer to

capture analytes from blood, serum, and the like, when the test liquid is flowing through

it.

According to additional aspects of the invention, expandable polymers may be

formed into sheets, grafts, electrical insulation and other known polymer applications.

These applications include among other devices, vascular grafts, endovascular liners and

grafts, prosthetic patches, vascular access devices, shunts, catheters, sutures or implantable tissue augmentation devices, such as those used in cosmetic surgery.

According to yet a further feature, the articles of manufacture include single and

multilayered membranes formed from sheets. Such membranes may be employed in

clinical diagnostic test strips or in filtration devices.

The invention can be applied to other processes where stretching or expanding of

material is involved. It will thus be seen that the invention efficiently attains the objects

set forth above, including providing implantable devices having tailored porosity and/or

chemistry characteristics. Since certain changes may be made in the above constructions

and the described methods without departing from the scope of the invention, it is

intended that all matter contained in the above description or shown in the

accompanying drawings be interpreted as illustrative and not in a limiting sense. By

way of example, any known methods for varying the porosity and/or chemistry

characteristics of implantable prostheses, such as varying the lubrication level in the

blended pasted, viewed in combination with the disclosed methods are considered to be

within the scope of the present invention. Additionally, any methods for combining

resins, pastes, billets or extrudates, according to the methods of the invention, are also

considered to be within the scope of the present invention.

Having described the invention, what is claimed as new and protected by Letters

Patent is:

Claims

1. A method for forming an article, comprising the steps of:

mixing an expandable polymer resin with a first wettable liquid to form a

mixture; extruding said mixture to form an extruded article; and

rewetting said extruded article with a second wettable liquid to form a wetted

material.

2. The method of claim 1, wherein said step of rewetting occurs at a temperature

below the boiling temperature of said second wettable liquid.

3. The method of claim 1 , wherein said first and second wettable liquids are the

same composition.

4. The method of claim 1 , further comprising, the step of stretching said extruded

article, after said step of extruding.

5. The method of claim 4, further comprising the step of removal of said first

wettable liquid before said step of stretching said extruded article.

6. The method of claim 5, further comprising, the step of stretching said wetted

material, after said step of rewetting.

7. The method of claim 6, further comprising, after said step of stretching said

wetted material, the step of sintering.

8. The method of claim 6, further comprising the step of removing said second

wettable liquid from said wetted material to form a dried material.

9. The method of claim 8, further comprising the step of stretching said dried

material, after said step of removing said second wettable liquid.

10. The method of claim 9, further comprising, the step of sintering, after said final

step of stretching said dried material.

11. The method of claim 6, wherein said step of stretching said wetted material is

performed at a temperature less than 80°F.

12. The method of claim 6, wherein said wetted material is in the shape of a tube and

said step of stretching consists of stretching in a radial direction to increase a diameter of

said wetted material .

13. The method of claim 12, further comprising, after said step of stretching said

wetted material, the step of sintering.

14. The method of claim 6, further comprising, the step of calendaring said extruded

article, after said step of extruding.

15. The method of claim 14, further comprising, after said step of stretching said

wetted material, the steps of: removing said second wettable liquid from said wetted material to form a dried

material;

stretching said dried material; and

sintering.

16. The method of claim 14, wherein said step of stretching said wetted material

involves stretching in a machine direction.

17. The method of claim 16, further comprising, after said step of stretching said

wetted material, the step of sintering.

18. The method of claim 14, wherein said step of stretching said wetted material

involves stretching in a direction transverse to a machine direction.

19. The method of claim 18, further comprising, after said step of stretching said

wetted material, the step of sintering.

20. The method of claim 5, further comprising, the step of applying pressure, after

said step of rewetting.

21. The method of claim 20, further comprising, the step of sintering, after said step

of applying pressure.

22. The method of claim 20, wherein a structure of said wetted material is changed

by the application of pressure.

23. The method of claim 20, wherein a density of said wetted material is changed by

the application of pressure.

24. The method of claim 1 , further comprising, the step of stretching said wetted

material, after said step of rewetting.

25. The method of claim 24, wherein said step of stretching said wetted material is

performed at a temperature less than 80°F.

26. The method of claim 24, further comprising, after said step of stretching said

wetted material, the steps of:

removing said second wettable liquid from said wetted material to form a dried

material;

stretching said dried material; and

sintering.

27. The method of claim 1 , wherein said article is in the shape of a tube.

28. The method of claim 1 , wherein said article is in the shape of a flat sheet.

29. The method of claim 1, wherein said expandable polymer is a fluoropolymer.

30. The method of claim 1 , wherein said expandable polymer is a polyolefin.

31. A method of forming an article from an expandable polymer, comprising the

steps of: rewetting said expandable polymer with a wettable liquid to form a

wetted material; stretching said wetted material.

32. An expandable polymer, comprising: a microstructure of nodes interconnected by

fibrils; wherein said expandable polymer has a density of at least 0.7 g/cc.

33. The method of claim 32, wherein said expandable polymer is a fluoropolymer.

34. The expandable polymer of claim 32, wherein said expandable polymer has a

thickness equal to or less than 0.001 inches.

35. An expandable polymer, comprising: a plurality of strands forming a sheet

having a machine direction and a transverse direction; wherein said sheet has at

thickness of no more than 0.65 mil and a longitudinal tensile strength in a machine

direction of at least 4000 psi.

36. The method of claim 35, wherein said expandable polymer is a fluoropolymer.

37. A method for forming an article, comprising the steps of:

mixing an expandable polymer resin with a first wettable liquid to form a

mixture; extruding said mixture to form an extruded article;

calendaring said extruded article;

removal of said first wettable liquid from said extruded article to form a first

dried material;

stretching said first dried material;

rewetting said first dried material with a second wettable liquid to form a wetted

material;

applying pressure to said wetted material;

removal of said second wettable liquid to form a second dried material;

stretching said second dried material; and

sintering.