Medical device formed of ultrahigh molecular weight polyolefin

Details Medical device formed of ultrahigh molecular weight polyolefin Medical device formed of ultrahigh molecular weight polyolefin

2000-11-14

2003-08-05

Kennedy, Sharon (Department: 3763)

Surgery

Means for introducing or removing material from body for...

Treating material introduced into or removed from body...

C428S036900

Reexamination Certificate

active

06602224

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention generally relates to medical devices, and particularly to balloon catheters, stent covers, and vascular grafts.
In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of an dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient's coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient's coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not overexpand the artery wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion. Stent covers on an inner or an outer surface of the stent have been used in, for example, the treatment of pseudo-aneurysms and perforated arteries, and to prevent prolapse of plaque. Similarly, vascular grafts comprising cylindrical tubes made from tissue or synthetic materials such as DACRON may be implanted in vessels to strengthen or repair the vessel, or used in an anastomosis procedure to connect vessels segments together.
In the design of catheter balloons, balloon characteristics such as strength, flexibility and compliance must be tailored to provide optimal performance for a particular application. Angioplasty balloons preferably have high strength for inflation at relatively high pressure, and high flexibility and softness for improved ability to track the tortuous anatomy and cross lesions in the uninflated state. The balloon compliance is chosen so that the balloon will have a desired amount of expansion during inflation. Compliant balloons, for example balloons made from materials such as polyethylene, exhibit substantial stretching upon application of internal pressure. Noncompliant balloons, for example balloons made from materials such as PET, exhibit relatively little stretching during inflation, and therefore provide controlled radial growth in response to an increase in inflation pressure within the working pressure range.
For many applications, intravascular catheter balloons should be substantially noncompliant once expanded to a working diameter. Further, catheter balloons should also be formed from relatively strong materials in order to withstand the pressures necessary for various procedures without failing. Typically, such characteristics require the use of a material that does not stretch appreciably, which consequently necessitates that the balloon material be folded around the catheter shaft prior to inflation. However, it can be desirable to employ balloons that are not folded prior to inflation, but which are instead expanded to the working diameter from a generally cylindrical or tubular shape having a nominal diameter that conforms to the catheter shaft. Such designs may be used for formed-in-place angioplasty balloons and stent delivery balloons. Prior art formed-in-place balloons have suffered from problems such as insufficient strength, poor control over expansion, and significantly complicated processing during catheter manufacturing.
It would be a significant advance to provide a catheter balloon, and other expandable members such as stent covers, and vascular grafts, with improved processing and expansion characteristics.
SUMMARY OF THE INVENTION
This invention is directed to medical devices, and particularly intracorporeal devices for therapeutic or diagnostic uses, having at least a component formed of ultrahigh molecular weight polyolefin (UHMW polyolefin). In a presently preferred embodiment, the UHMW polyolefin is an ultrahigh molecular weight polyethylene (UHMW polyethylene). A presently preferred embodiment is directed to UHMW polyolefin which is microporous, and having a node and fibril microstructure comprising nodes interconnected by fibrils.
One embodiment of the invention comprises an expandable member such as a balloon for an intraluminal catheter, formed at least in part of the UHMW polyolefin, such as UHMW polyethylene. In another embodiment of the invention, a stent delivery system comprising a balloon catheter and a stent mounted on the balloon has a component, such as the catheter balloon or a stent cover, which is formed at least in part of the UHMW polyolefin, such as UHMW polyethylene. Another embodiment of the invention comprises a vascular graft formed at least in part of the UHMW polyolefin, such as UHMW polyethylene. The terminology vascular graft as used herein should be understood to include grafts and endoluminal prostheses, such as those surgically attached to vessels, as for example in vascular bypass or anastomosis, or implanted within vessels, as for example in aneurysm repair or at the site of a balloon angioplasty or stent deployment. Although discussed below primarily in terms of a balloon catheter having a balloon formed of UHMW polyethylene, the invention should be understood to include other medical devices and particularly intracorporeal devices for a therapeutic or diagnostic purpose, such as stent covers and vascular grafts formed of UHMW polyolefin, such as UHMW polyethylene. Additionally, although discussed primarily in terms of UHMW polyethylene, it should be understood that the invention applies as well to UHMW polyolefins in general, and to other materials having a node and fibril microstructure such as polypropylene, nylon, and expanded polytetrafluoroethylene.
The UHMW polyethylene has a molecular weight which is higher than the molecular weight of high molecular weight polyethylenes, and which is about 2 million to about 10 million grams/mole, preferably about 3 million to about 6 million grams/mole. Unlike high molecular weight polyethylenes, which generally have a molecular weight of about 400,000 to about 600,000 grams/mole, the UHMW polyethylene is difficult to melt process. Balloons formed from this material exhibit compliant expansion at relatively low strains and exhibit substantially less compliance at higher strains.
The node and fibril structure of the UHMW polyethylene causes it to exhibit essentially compressible deformation at relatively small strains, with a low Young's modulus in tension for the compressed material. At high strains, the U

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