Radially expandable polytetrafluoroethylene

This is a continuation of application Ser. No. 08/588,228 filed Jan. 18, 1996, now abandoned; which is a continuation application of Ser. No. 08/239,239, filed on May 6, 1994 (abandoned).

This invention relates to polytetrafluoroethylene (hereinafter PTFE) materials which, after being radially expanded, retain their the structural integrity.

More particularly, the invention relates to extruded, stretched, sintered tubular PTFE materials suited for use in the medical field as liners and covers for. expandable stents.

The use of expandable endovascular stents to open and support aortic blood vessels is well known in the art. Such stents, which are typically made from stainless steel, are thrombogenic and tend to occlude due to growth of tissue through the stent into the blood vessel. The length of such stents is also limited because of their rigidity. Consequently, liners and covers have been sought for use in conjunction with metallic stents in order to shield the stent and to extend the length of anatomy which can be treated with the stent. The development of acceptable stent liners or covers has been slow because the liners or covers preferably must (1) expand with the stent, (2) be non-thrombogenic, (3) be biocompatible, (4) be inert, (5) have a low profile with the ability to expand up to about four times its original dimension, (6) be expandable at low pressures of less than five to ten atmospheres to reduce the risk of injury to the patient, (7) retain its physical properties and structural strength after being expanded, (8) generally not alter its length after being expanded, (9) be impervious to blood at physiological pressures, (10) conform to host anatomy when expanded, (11) resist the growth of bodily tissue therethrough, (12) be able to carry radiopaque markings for location during fluoroscopy.

Paste-formed, extruded tubular PTFE products are well known, as are paste extrusion and paste forming manufacturing processes for producing such products. During such manufacturing processes, a PTFE resin is mixed with a liquid lubricant. A preformed resin—lubricant charge is then produced and extruded through an annular orifice to produce an unsintered PTFE tube. The extruded tube is heated to remove the lubricant and produce a porous, unsintered PTFE tube. The tube typically has a density of from 1.5 to about 1.75 gm/cc and accompanying porosities of 39% to 26%. If the unsintered tube is sintered by heating the tube to a temperature above its crystalline melting temperature, a nonporous tube results. See U.S. Pat. Nos. 3,953,566, 3,962,153, 4,110,392, 4,187,309, 4,283,448, 4,385,093, 4,478,665, 4,482,516, 4,877,661, and 5,026,513.

In the medical field, PTFE products are used as replacement veins and arteries. PTFE is inert, is non-thrombogenic, and has other characteristics desirable for a stent cover or liner. Commercially available PTFE medical tubular products have, however, significant radial strength and are not readily dilated. Conventional PTFE tubes typically have a high radial strength and rapidly lose their tensile strength and become weak and thin after being dilated by only small amounts.

Accordingly, it would be highly desirable to provide improved PTFE products which can be readily expanded and which, after being expanded, substantially retain their tensile strength and other physical properties which make the use of PTFE in the body desirable.

Therefore, it is a principal object of the invention to provide improved PTFE products which are amenable to use as liners and covers for expandable stents.

A further object of the invention is to provide improved tubular PTFE products which substantially retain their structural integrity after the products are radially expanded.

Another object of the invention is to extend the length of anatomy which can be treated with an expandable

I have discovered new PTFE products and a process and composition for producing the same. The new PTFE products can be significantly expanded to configurations which retain their structural integrity and which substantially retain their tensile strength and other desirable physical properties. As discussed in detail in the examples below, the new PTFE products have an unusually low REC (Radial Expansion Coefficient) and RER (Radial Expansion Ratio) which function to permit thin-walled PTFE tubes to expand about 50% to 400% before, the tubes lose their structural integrity and suffer a rapid decline in tensile strength.

The following examples are presented to illustrate the presently preferred embodiments of and practice of the invention and not by way of limitation of the scope of the invention.

Examples 1-17, below, concern expandable PTFE material formed as a result of stretching and subsequent sintering.

One hundred grams of FLUON CD123 resin produced by ICI Americas, Inc. was sifted through a No. 10 sieve and then blended at room temperature with twenty-five grams of ISOPAR M solvent produced by Exxon Corporation to produce a preform blend. Other lubricants well known in the art includes VM&P NAPHTHA (boiling point (bp) 118-130° C.), ISOPAR (Registered trademark of Exxon Corporation), ISOPAR 3 G (bp 159-174° C.), ISOPAR H (bp 176-189° C.), Low Odor Paraffin Solvent (bp 191-246° C.), and SHELLSOL (Trademark of Shell Oil) K.

The resulting preform blend was allowed to sit for over eight hours before being re-sifted through a No. 10 sieve. The lubricant level (LL) equals the weight of solvent used divided by the weight of resin used, which means the lubricant level utilized in this Example 1 was 25%. In the practice of the invention the lubricant level is normally in the range of 16% to 35%, and is presently preferably in the range of about 18% to 25%.

FIG. 1 is a radially expandable stent.

Referring now to FIG. 1, a preform charge 10 was created by compacting the preform blend under 200 to 400 psi for approximately one minute in a stainless steel cylinder containing a center shaft. The center shaft extended along the centerline X of the cylinder and was concentric with the cylinder. The resulting preform charge 10 was a hollow cylindrical mass having a doughnut shaped circular cross sectional area 13 , as shown in FIG. 1 . The cylindrical hollow mid-section 15 in the preform charge 10 was occupied by the center shaft. The preform charge 10 was then loaded into a cylindrical barrel in a ram extruder and was extruded into several individual lengths of cylindrical thin-walled tubing 11 at a reduction ratio (RR) of 125:1. The total length of tubing 11 produced from the preform charge 10 was about twenty feet. The extruded tubing 11 had a microstructure characterized by nodes interconnected by fibrils. The reduction ratio equals the ratio of the cross sesectional area 13 of the preform charge 10 to the cross sectional area 14 of the wall of the tubing 11 . In the practice of the invention, the RR is less than 200 or 300 to 1; preferably equal to or less than 125:1. The ratio of the RR to the LL in the practice of the invention is preferably less than five. In prior art preform blends the ratio of the RR to the LL is normally greater than five, and is typically nine or greater.

The solvent was removed from the extruded tubing by placing the tubes in a forced air circulation electric oven at 255 degrees C. for thirty minutes As used herein, the length of the tube after it is extruded and heated at 255 degrees C. to remove the solvent is termed the original length of the tube.

After being heated to 255 degrees C., each tube was heated to 290 degrees C. for five minutes and then stretched longitudinally at rate of about 100% per second to a length four times the original length of the tube. Each tube can, if desired, be stretched at a rate in the range of 5% to 500% per second and stretched to a length in the range of two to six times the original length of the tube.

The stretched porous tubes were then sintered at approximately 300 degrees C. for forty-five to 90 seconds. The sintering crystallized the PTFE and increased the strength of the porous tubes. During sintering each end of the tubes was restrained to prevent longitudinal shrinkage of the tube. The resulting stretched, sintered, porous tubes consisted essentially of highly crystalline PTFE polymer and had a microstructure characterized by nodes interconnected by fibrils.

The FLUON CD123 resin is a white free-flowing powder made by coagulation of an aqueous dispersion of polytetrafluoroethylene (PTFE). It is designed for paste extrusion with volatile hydrocarbon lubricants for applications in which opacity in the sintered article is not a problem. FLUON CD123 has a relatively high molecular weight. Unsintered extrudates exhibit good green strength.

As used herein, a tube retains its structural integrity after being radially expanded as long as the tube requires the application of an increased inflation pressure before the amount of radial expansion of the tube increases. If a tube continues to expand when the amount of inflation pressure decreases, then the tube has lost its structural integrity. When the Pmax of a tube is exceeded, the tube loses its structural integrity. However, the loss in structural integrity results in degradations of physical properties which are significantly less than those which occur in prior art PTFE tubes. For example, at a percent dilatation of about 300% in Table I below, the tube still retains about 70% to 75% of its pre-dilatation tensile strength. Also, in Table I below, Tube No. 1 loses its structural integrity at an inflation pressure greater than 6.5 atm (P max ). In Tables II and III below, Tubes No. 2 and 3, respectively also lose their structural integrity at an inflation pressure greater than 6.5 atm (Pmax).

The following results were obtained for the three Example 1 tubes which were tested: