Medical device balloons with improved strength properties and processes for producing the same

Abstract

A tubular parison for forming a medical device balloon. The parison is formed of a polymeric material, for instance a thermoplastic elastomer. The parison has an elongation at break which is not more than 80% of the elongation of the bulk polymeric material. The elongation of the parison is controlled by altering extrusion conditions. Balloons prepared from the parisons provide higher wall strength and/or higher inflation durability than balloons prepared from conventional parisons of the same material.

Claims

1. A medical device balloon formed of a thermoplastic elastomer polymeric material and having a tensile wall strength in excess of 34,000 psi in pre-sterilized condition.

2. A medical device balloon as in claim 1 wherein said tensile wall strength in excess of 37,000 psi in pre-sterilized condition.

3. A medical device balloon as in claim 1 wherein the polymeric material comprises a polyamide/polyether/polyester, a polyester/polyether block copolymer, a polyurethane block copolymer or a mixture thereof.

4. A medical device balloon as in claim 1 wherein the polymeric material is a polyamide/polyether/polyester.

5. A medical device balloon as in claim 1 formed with a single layer of said polymeric material.

6. A medical device balloon as in claim 1 comprising of a plurality of layers of said polymeric material.

7. A medical device comprising a balloon as in claim 1 mounted on a catheter.

8. A medical device as in claim 7 further comprising a stent mounted on the catheter.

9. A medical device balloon formed of a thermoplastic elastomer polymeric material and having a tensile wall strength, in post-sterilized condition, of 32,000 psi or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) All published documents, including all US patent documents, mentioned anywhere in this application are hereby expressly incorporated herein by reference in their entirety. Any copending patent applications, mentioned anywhere in this application are also hereby expressly incorporated herein by reference in their entirety.

(2) It has been found that the distention and the burst pressure of a balloon are affected by the elongation properties of the extruded parison, as well as by the hoop ratio and the tube wall thickness. It is believed the elongation affects the balloon properties through its effect on the balloon wall thickness. Thus, for a given hoop ratio and tube size, as parison elongation decreases, the balloon wall thickness increases, the balloon distention decreases and the burst pressure increases.

(3) Thus, while an increase in the hoop strength and modulus comes at the expense of thinner balloon walls, which can increase distention and decrease burst pressure, it is also possible to extrude tubes with lower elongation to break. This allows one to provide even stronger walls than were previously been obtained with a given polymer. Alternatively, the invention can allow one to thicken the balloon wall, while affecting the hoop strength and distension very little, thereby obtaining a balloon which is more suited to stent or other surgical device delivery operations.

(4) In one aspect the invention involves modifying the parison processing so as to provide the parison material with an elongation which is not more than 80% of the elongation of the bulk material. In particular, when 3 inch length of the extruded tube is stretched until it breaks, the length of the tube when it breaks will correspond to a percentage increase which is not more than 80% of the elongation value obtained by determining elongation of the bulk material per ASTM D-638. In some embodiments the parison is processed so as to provide the parison material with an elongation which is not more than 70% of the elongation of the bulk material, and in still others the parison elongation is less than 60% of the elongation of the bulk material.

(5) The parison processing techniques described herein, alone or in combination can provide balloon wall strength improvements of as much as 10-25% over those obtainable in their absence, for non-sterilized balloons. Sterilization, depending on the technique chosen, may reduce this benefit somewhat. The invention may be used with any known balloon materials, however high strength thermoplastic elastomers are preferred, especially polyamide/polyether block copolymers, including polyamide/polyether/polyesters such as sold under the PEBAX trademark, in particular PEBAX 7033 and PEBAX 7233; polyester/polyether block copolymers such as sold under the HYTREL and ARNITEL trademarks, in particular ARNITEL EM 740 and HYTREL 8238; and polyurethane block copolymers such as PELLETHANE 2363-75D.

(6) The parison may be extruded as a single layer or in multiple layers, for instance 3, 5, 7, or even more alternating layers of PEBAX 7033 and Pebax 7233. Blends of such polymers may also be used.

(7) Parison elongation may be controlled by varying one or more of the following extrusion parameters:

(8) Extrusion Temperature:

(9) The temperature at the extrusion head, die temperature, is lowered relative to the temperature in the extruder barrel. Heat loss begins even as the material is passing through the die head. The resulting tubing has a higher degree of crystallization. In general the die head temperature reduction should be about 5 to about 50° F., suitably 10° F. to 40° F., and preferably about 20-30° F. below the barrel temp.

(10) Draw Down Ratio:

(11) Die configuration, extruder pressure and/or line speeds can be adjusted to provide a cross-sectional area draw down ratio in excess of 5:1. Ratios as high as 17:1 have been employed, and even higher ratios may be advantageous because they reduce extruder pressure demands. Typically the draw down ratios will be in the range of about 8:1 to about 17:1.

(12) Quench Time:

(13) Decreasing the gap between the extrusion head and the cooling bath tank can also lower parison elongation by shortening the quench time. Quench time can also be shortened by increasing the line speed.

(14) Bath Temperature:

(15) Maintaining the cooling bath at a lower temperature also can lower the elongation of the parison.

(16) A surprising benefit of at least some embodiments of the invention is that balloons prepared from parisons of the invention have improved resistance to repeat inflation bursts versus controls utilizing the same polymer, but prepared using typical extrusion parameters for commercial balloons. The improvement may permit three times, or even more, the number of inflations to rated pressure, compared to the controls.

(17) The invention is illustrated by the following non-limiting examples.

EXAMPLES

(18) In the following examples the following abbreviations are used.

(19) TABLE-US-00001 Ex Example No. Alphabetic series are comparative, numeric series are invention examples. ID Internal diameter, as extruded. OD Outer diameter, as extruded. Die temp Extruder die zone temperature in degrees Fahrenheit. The extruder barrel was kept at 395° F. in these examples. Line speed Speed in feet/min of the puller. DDR Draw down ratio of the cross-sectional area from extrusion head opening, to final tube dimensions. DDR = [(Die ID).sup.2 − (Tip OD).sup.2]/ [(Tubing OD).sup.2 − (Tubing ID).sup.2] Elong @ Given as percentage elongation determined break on a 3″ long extruded tube which is stretched to break. Balloon Thickness in inches of the balloon double wall 2x wall as measured with a micrometer. Hoop Hoop ratio determined as balloon OD (mold diameter)/parison ID (as extruded). Distension The change in diameter as a % of start diameter for the stated ranges of 6:12 (6 atm to 12 atm) and 12:18 (12 atm to 18 atm) inflation pressure. Burst Pressure in psi at which the balloon burst Burst Wall strength at burst as calculated by the equation: strength T.sub.s = PD/2t where: T.sub.s is the wall tensile strength; P is the balloon burst pressure; D is the nominal diameter of the balloon; and t is the wall thickness.

(20) All values are averages of at least 6 balloons. Balloon blowing conditions used the same times, temperatures and sequences, except where indicated.

(21) All data is for balloons having a nominal diameter of 3.0 mm at 6 atm. The balloons were made from PEBAX 7033. The published elongation value for the bulk polymer, per ASTM D-638, is 400%. The balloons were made from conventionally extruded parisons using a very high hoop ratio and a step-wise dipping process similar to that described in Wang et al, Example 3, U.S. Pat. No. 5,714,110. A typical program is as follows:

(22) TABLE-US-00002 Program: bath at 95° C. (1) pressure to 100 psi tension to 50 g dip to D 8 seconds hold at D 6 seconds (2) pressure to 450 psi tension to 20 g dip to C 4 sec hold at C 6 seconds (3) pressure to 550 psi tension to 200 g dip to B 20 sec hold at B 6 seconds
where D, C and B are locations, as described in U.S. Pat. No. 5,714,110. The parison formation conditions and formed balloon results are described in Table 1. Die configuration was not varied between examples. Tank gaps, die temperatures and speeds were varied as needed to obtain parison elongation targets. Extruder pressure was not independently controlled and varied as a result of changing these conditions.

(23) Table 1 provides an example of a balloon formed using conventional tube processing at a high hoop ratio.

(24) TABLE-US-00003 TABLE 1 Control Tube Tube Die Line Elong @ Balloon Distension Distension Burst Ex ID OD Temp Speed DDR break 2X wall Hoop 6:12 12:18 Burst Strength A .0177 .0321 395 24 3.5 367 .00116 6.9 5.6 4.4 301 31056

(25) The elongation at break of this parison corresponds to about 91% of the published value for the bulk polymer.

(26) Table 2 gives the results of the same balloon wall thickness made in accordance with the invention by increasing the DDR. The increased draw down ratio reduced the elongation of this tube to about 48% of the published elongation value.

(27) TABLE-US-00004 TABLE 2 High Draw Down Tube Tube Die Line Elong @ Balloon Distension Distension Burst Ex ID OD Temp Speed DDR break 2X wall Hoop 6:12 12:18 Burst Strength 1 .0176 .0310 395 50 12.1 190 0.00118 6.9 5.4 4.5 331 34411

(28) Table 3 shows extrusion parameters and balloon property results when, after extrusion, the parison was modified by one of the following steps before it was blow-formed into a balloon.

Example 2

(29) A freeze spray process was used to selectively reduce parison cone and waists as per Example 1 of U.S. Pat. No. 5,807,520.

Example 3

(30) Cones and waists were selectively reduced by a grinding and necking process which did not stretch the body-forming portion of the parison. Similar to Example 2, first paragraph of PCT/U.S. Pat. No. 01/26140, filed Aug. 22, 2001, corresponding to U.S. application Ser. No. 09/672330 filed Sep. 28, 2000.

Example 4

(31) the entire parison was stretched longitudinally at ambient temperature under internal pressurization to maintain ID at the extruded dimension (±4%) at a stretch ratio 3×, where × is starting length. See control in Example 1 of PCT/U.S. Pat. No. 01/26140.

(32) TABLE-US-00005 TABLE 3 Parison Modifications Tube Tube Die Line Elong @ Balloon Distension Distension Burst Ex ID OD Temp Speed DDR break 2X wall Hoop 6:12 12:18 Burst Strength 2 0.176 .0290 395 50 12:1 193 .00105 6.9 5.3 4.7 309 36101 3 .0176 .0290 395 50 12:1 193 .00098 6.9 4.8 4.8 297 37423 4 .0176 .0290 395 50 12:1 193 .00097 6.9 4.9 4.7 300 37577

(33) In examples 2-4, the burst pressure in all cases was comparable to the control balloon, but with thinner walls so the wall strength is much improved over the control balloon.

Example 5

(34) Balloons were made using PEBAX 7033 parisons stretched at ambient temperature at a stretch ratio of 1.5× and a hoop ratio of 7.0. Parisons, extruded to keep the parison elongation at break above 80% of the published elongation of the polymer, were used as controls. Parisons, extruded to provide a parison elongation at break of about 50% or less of the published elongation of the polymer, were prepared as invention examples. The balloons were inflated to 211 psi and deflated repeatedly. Four balloons were present in each group. The control balloon group, on average, failed at about 80 repeats. All of the balloons of the invention group survived 235 repeats without failure, at which point the test was discontinued.

(35) The above examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the claims, where the term “comprising” means “including, but not limited to”. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims. Further, the particular features presented in the dependent claims can be combined with each other in other manners within the scope of the invention such that the invention should be recognized as also specifically directed to other embodiments having any other possible combination of the features of the dependent claims. For instance, for purposes of claim publication, any dependent claim which follows should be taken as alternatively written in a multiple dependent form from all prior claims which possess all antecedents referenced in such dependent claim if such multiple dependent format is an accepted format within the jurisdiction (e.g. each claim depending directly from claim 1 should be alternatively taken as depending from all previous claims). In jurisdictions where multiple dependent claim formats are restricted, the following dependent claims should each be also taken as alternatively written in each singly dependent claim format which creates a dependency from a prior antecedent-possessing claim other than the specific claim listed in such dependent claim below.