The assignee for this patent application is
Reporters obtained the following quote from the background information supplied by the inventors: "The invention relates to the field of intravascular medical devices, and more particularly to an improved balloon for a catheter.
"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 a 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 or obstruction to be dilated. The dilatation catheter having an inflatable balloon on the distal portion thereof is then 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 or obstruction. 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 effect the dilatation without over-expanding the arterial 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the balloon is finally deflated, blood flow may resume through the dilated artery and the dilatation catheter can be withdrawn from the patient.
"In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, or obstructions that cannot be resolved by inflation of the balloon alone. These conditions often necessitate either another angioplasty procedure, or some other method of repairing, strengthening, or unblocking the dilated area. To reduce the restenosis rate and to strengthen or unblock 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.
"Catheter balloons are typically manufactured independently of the catheter itself and then secured to the catheter with an adhesive or other bonding method. In standard balloon manufacture, a polymer tube is blown biaxially under the action of axial tension, internal pressure, and heat within a mold. The polymer tube may either be simultaneously stretched in the radial and axial directions, or sequentially by first stretching axially and then radially. The starting dimensions of the polymer tube and the finished dimensions of the blow-molded balloon within the mold are a measure of the degree to which the polymeric material has been stretched and oriented during balloon blowing, and affect important characteristics of the finished balloon such as rupture pressure and compliance. The blow-up-ratio (BUR) refers generally to the ratio of the diameter of the blown balloon to the diameter of the undeformed polymer tube. Above a critical BUR for a selected polymer, the balloon blowing process becomes unstable and the polymer tubing will rupture or tear before a balloon is fully formed.
"In the standard blow molding process, an initiated air bubble in the polymer tube rapidly expands until the polymer tube forms a balloon that is eventually constrained by the mold wall. The hoop stress in the wall of the tubing, as it grows into a balloon, may be approximated by the expression:
"where P is the inflation pressure, R is the mean radius of the polymeric tube at any time during the inflation and .delta. is the wall thickness of the tubing. To form a balloon from the tubing, the inflation pressure P should be such that the wall hoop stress exceeds the material resistance (typically the yield stress) to stretching at the blowing temperature. Once a balloon begins to form from the tubing, it grows rapidly in size until it touches the mold wall. As the balloon expands, its radius R increases and its wall thickness 8 decreases. This results in a rapid increase in the wall hoop stress .sigma..sub.h during constant pressure blowing. If the wall hoop stress of the growing balloon exceeds the ultimate hoop strength of the material, rupture will occur. This phenomena limits the maximum attainable BUR for a given polymeric material forming the balloon layer.
"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 and stent delivery 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. The balloon compliance is chosen so that the balloon will have the required amount of expansion during inflation. Compliant balloons, for example balloons made from materials such as polyethylene, exhibit substantial stretching upon the 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. However, noncompliant balloons generally have relatively low flexibility and softness, making it challenging to provide a low compliance balloon with high flexibility and softness for enhanced catheter trackability. A compromise is typically struck between the competing considerations of softness/flexibility and noncompliance, which, as a result, has limited the degree to which the compliance of catheter balloons can be further lowered.
"As a balloon is formed by the process described above where an extruded tube is expanded into a mold cavity, the balloon's wall always exhibits a gradient in circumferential orientation of the polymer molecules within. Moreover, the highest degree of orientation occurs at the inner surface and the outer surface experiences the lowest degree of orientation. The gradient is nonlinear, and arises because the percent change in circumference at the inner surface is always greater than that at the outer surface.
"There have been attempts to develop methods to raise the overall degree of orientation within the wall of a balloon during expansion. One method employs extruded tubing containing multiple layers of different durometers materials, with the material possessing the highest elongation (typically the lowest durometer) as the innermost layer to enable an increase in the 'blow-up ratio' at the balloon's inner surface. Another method utilizes a two-stage expansion process, in which the extruded tubing is first expanded into a mold cavity of intermediate size before being subsequently expanded again into a final, larger mold. This so-called 'double-blow' method helps to make the initiation event during balloon expansion less severe and enables the processing of balloons possessing a greater overall BUR value at their innermost surface.
"When extruded tubing is expanded circumferentially into a balloon mold, invariably some degree of axial elongation also occurs. The actual degree of axial elongation can be calculated using measured values of initial, 'as-extruded' inner diameter (ID) and outer diameter (OD), as well as the final balloon OD, and final balloon double-wall thickness. The calculated value, known as the 'area draw-down ratio' (or ADDR) is the ratio of the extrusion's original cross-sectional area to the final balloon's cross-sectional area. The value of ADDR is mathematically equivalent to the ratio of the final length to the original length of the material which comprises the balloon's working length. Higher values of ADDR represent greater axial elongation during the balloon's formation.
"The current design and processing approach for balloons purposely involves imparting a substantial amount of axial elongation during balloon expansion. A target ADDR value of 3.0 is common, which means that the material within the wall of the balloon's working length is stretched to 3 times its original length. This amount of axial elongation must be accounted for in the design of the extruded tubing. The extrusion's ID is already made as small as possible by the need to maximize the blow-up ratio, so it is necessary to account for the high degree of axial stretch by increasing the extrusion's OD. Geometric calculations are routinely performed to ensure that the correct final wall thickness will result when a proposed extruded tube is expanded into the desired balloon mold size with an ADDR value of about 3.0. To help control the lot-to-lot variability in axial flow behavior of the extruded tube, a specification exists for the tube's as-extruded elongation to failure (nominally 225% elongation to failure, which translates to an ADDR value of 3.25). Additionally, the amount of externally applied tension may be varied in order to help control the lot-to-lot variability in the axial elongation and thus wall thickness."
In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "The present invention enhances a catheter balloon's performance by improving compliance and rupture properties for a given wall thickness, or by maintaining targeted balloon performance levels while reducing wall thickness. In the former instance, a balloon possessing more robust properties are made with properties similar to more non-compliant balloons with little or no loss in flexibility or deliverability. In the latter instance, either a more robust or non-compliant balloon are made thinner and thus more flexible and deliverable. Additionally, the present invention reduces the compacted or folded balloon profiles by reducing the wall thickness of balloon tapers and shafts.
"The method of the invention purposely reduces the outer diameter of the as-extruded tube and correspondingly reduces the target value of ADDR during balloon expansion so that the resulting wall thickness is either unchanged (for improved balloon compliance and rupture performance without increasing wall thickness) or decreased (for comparable balloon compliance and rupture performance using thinner walls). Reducing the as-extruded tube's OD naturally raises the blow-up ratio at the outer surface of the final balloon. The blow-up ratio of underlying material within the balloon wall is also raised. The overall effect of reducing the as-extruded tube's OD is to shift the final balloon wall's BUR gradient upwards everywhere except at the innermost surface. It is further shown that a 10-20% increase in 'integrated average' BUR can be attained, in theory, by the present invention. It has been confirmed experimentally that improved balloon performance, notably in rupture strength and 'compliance modulus' results from the present invention.
"One approach for implementing the present invention is to use a small 1.sup.st-stage mold during the double-blow process. By thus limiting the extruded tube's radial growth during the 1.sup.st stage, the axial load imparted by the 1.sup.st stage inflation pressure is also minimized. This is because the axial load during balloon expansion is related to the inflation pressure multiplied by the cross-sectional area of the just-formed balloon, and that small 1.sup.st stage molds serve to minimize this cross-sectional area. By so limiting the axial elongation during the 1.sup.st stage expansion and then transferring to a 2.sup.nd stage mold of equivalent length, the ADDR of the final balloon after 2.sup.nd stage can be substantially reduced. Experiments of this nature on 3.0 mm balloons have led to final ADDR values of approximately 2.0, a significant reduction from the values near 3.0 associated with conventional balloon processing.
"A second approach is to controllably reduce the as-extruded tube's elongation to failure. All else being equal, extrusions possessing a high elongation to failure (say, 225% nominal, which translates to an ADDR of 3.25) naturally tend to axially elongate to a greater degree during balloon expansion than extrusions having lower elongation to failure. Thus, targeting an ADDR of 2.0 in the present invention would warrant extrusions possessing an elongation to failure more closely matched (say, 125 or 150% nominal, which translates to ADDR values of 2.25 or 2.5). In such manner, the extruded tubing would be axially deformed to an extent nearer its axial deformation limit and thus be more greatly oriented in the axial direction. The advantage of suitably reducing the as-extruded tube's elongation to failure, rather than controlling ADDR mainly through the judicious selection of 1.sup.st stage balloon mold diameter, is that single-stage balloon expansion is possible via the former approach and the resulting ADDR during expansion tends to be self-limited by the balloon tubing.
"The byproduct of reducing the as-extruded tubing's % elongation to failure is the increased likelihood of circumferential balloon ruptures, which may be addressed by modifying the balloon's post-expansion 'heat set' treatment so as to promote that material change which has been shown to enhance the material's axial strength. The 'enhanced heat set' approach offers the potential benefit of also increasing circumferential strength and thereby further enhancing balloon performance in terms of rupture strength and compliance modulus.
"In addition, to improving performance within a balloon's working length, this invention could be also used to reduce thickness within a balloon's tapers and shafts. Consequently, a balloon's folded profile could be improved by reducing wall thickness within its tapers, which often possess the greatest profile on the larger angioplasty balloon sizes. Further, the profile of its seals could be improved by reducing wall thickness within its shafts, particularly at the proximal seal where balloon shaft drilling is not presently performed.
"These and other advantages of the invention will become more apparent from the following detailed description of the invention and the accompanying exemplary drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
"FIG. 1 is an elevational view, partially in section, of an over-the-wire type stent delivery balloon catheter embodying features of the invention.
"FIGS. 2 and 3 are transverse cross sectional views of the catheter of FIG. 1, taken along lines 2-2 and 3-3, respectively.
"FIG. 4 illustrates the balloon catheter of FIG. 1 with the balloon inflated.
"FIG. 5 is a longitudinal cross sectional view of a balloon mold, with a multilayered balloon tubing in the mold prior to being radial expanded therein.
"FIGS. 6 and 7 depict a tubing material's initial, as extruded dimensions prior to expansion and the tubing material's final, as expanded dimensions.
"FIG. 8 is a plot of Blow Up Ratio as a function of position within the balloon wall after expansion.
"FIG. 9 is a plot of BUR and ADDR as a function of initial tube OD.
"FIG. 10 is a plot of average tensile break load versus average percent elongation for a variety of Pebax 72 balloon tubing lots purposely processed to have different average percent elongation values.
"FIG. 11 is a plot of heat treat temperature versus percentage increase in balloon break load."
For more information, see this patent application: Simpson, John A.;
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