As a medical balloon contract manufacturer for the past 15 years, we have accumulated substantial understanding of the issues linked to balloon quality and balloon production yields. Clearly, high balloon tubing quality is an essential precondition to high balloon quality and production yields. To meet our demand for high-quality tubing, we decided to develop our own extruded balloon tubing processes. During the last four years, the extruded balloon tubing process development has produced a 10- to 40-percent improvement in balloon production yields.
Tubing quality is one of the crucial elements that impacts medical balloon production yields and finished balloon performance. It is affected by a multitude of variables that encompass the raw material selection, material handling and the tubing extrusion process itself. Thorough understanding of the cause-and-effect relationship between the process parameters and process outcomes is essential to balloon tubing production. Operator’s expertise, meticulous monitoring, testing and documenting of each step of the process also play an important role. And finally, without a number of highly specialized machines and instruments, it would be virtually impossible to achieve the desired goals.
To meet the growing need for ever smaller and stronger high-end diagnostic and therapeutic devices—such as angioplasty balloon catheters, stent delivery catheters and a variety of other high-pressure balloon devices—manufacturers are demanding superior quality medical tubing from their suppliers.
Producing flawless tubing that meets extremely tight dimensional tolerances, as well as a number of mechanical properties such as tensile strength and ultimate elongation, is a complex task. It is further complicated by the fact that the supplier must be capable to consistently and repeatedly maintain such high quality on any subsequent production run of the same tubing size. The outcome is dependent on a number of output factors. To tightly control, keep track, document and ultimately store such a large number of process parameters, a fully instrumented, computerized extrusion line is required.
Any variation in tubing extrusion parameters can diminish the mechanical properties of the finished product. We have seen fluctuations as high as 300 percent in both tensile and elongation values for tubing produced from the same batch of resin by different extrusion vendors. Although this tubing met the dimensional and visual requirements of the customer, it was completely unusable for balloon production.
The concept of balloon tubing quality is widely misunderstood and poorly defined, both by balloon manufacturers and many tubing extruders. They tend to focus only on the dimensional and visual parameters, and possibly color, which are key specifications for more ordinary tubing such as shaft tubing or intravenous set tubing, where strength and performance under higher pressures are not required.
Balloon tubing dimensional tolerances are extremely tight. Tubing inside diameter, wall thickness and concentricity are carefully calculated to produce a balloon with desired performance. Dimensional requirements are supplemented by a specific range of tensile and elongation values. Visual inspection goes beyond the naked eye to microscopic 10x to 30x magnification.
Angioplasty balloons or other high-pressure dilation balloons are one of the most highly stressed components in the field of medical engineering. If improperly manufactured, balloons quickly can turn from being a life-saving device to a life-threatening one. To reduce this risk the medical balloons are subjected to rigorous inspection and testing. It is at this stage where most of the balloon flaws and defects come to light. As a result of the stretch-blow molding process that is used to form the balloons, even an invisible or hard-to-discern tubing defect is greatly magnified and may produce a visual blemish or create a weak spot within the super-thin balloon wall. Obviously, it is very costly to identify the problems that originated from poor balloon tubing quality and have the finished balloon rejected in the last step during production.
The best way to reduce or eliminate inferior physical properties and microscopic flaws in balloon tubing that lead to high scrap rates of finished balloons is to adhere to a very disciplined and methodical approach to the extrusion process. Homogenous composition of the balloon material is the key to production success. When polymers degrade due to an excessive temperature exposure or a high shear stress, physical properties are affected, often creating points of weakness in the material. Although they are only tiny alterations in chemical structure (essentially the same polymer, but with slightly different molecular weights and melting temperatures), they don’t bind as strongly with the surrounding, unaltered molecules—a situation that can lead to product failure.
Any number of factors can contribute to polymer degradation including temperature exposure and particulates.
Material preparation is the first step in the extrusion process. All polymers used in balloon tubing production are hygroscopic—that is, they absorb moisture—and must be dried before being melted, compounded or extruded. If there is too much residual moisture in the polymer, the melt strength of the polymer decreases and makes the material properties less desirable for the blow molding process. It also is important that the moisture content is kept at the same level, otherwise constant quality and constant process control cannot be attained.
Critical physical properties of balloon tubing such as orientation of molecular chains and the degree of polymer crystallinity are affected by the extrusion process parameters, including tensile strength.
The extruder is the melting and pumping machine that converts solid pellets into a melt and forces the material through the die at a prescribed rate of speed. The proper screw design and size is essential for uniform melting and pumping without degrading the material.
Producing small-size balloon tubing requires small volumes of material to be delivered by the extrusion pump. However, many shops use larger, more general-purpose machines with much higher capacity screw pumps. These machines typically are designed for quarter-inch tubing or larger. To produce balloon tubing, these machines must run the screw at a much slower speed, at or below its RPM control limit, where it is more difficult to control the speed of the pump and thus the tubing dimensions and physical properties.
Specific thermal gradient needs to be maintained to melt the pellets at a rate that matches the extrusion speed. Operators are provided with real-time feedback on the processing parameters.
Dimensional, tensile and elongation test are performed at the beginning, middle and the end of the extrusion run to verify consistency within the run. If test results fall outside the specification range or flaws are present, adjustments are made and the process repeated until the correct combination of parameters is determined.
After leaving the extrusion head, the newly formed tubing enters a water bath to be cooled, another critical step in the manufacturing process. The physical properties and morphological structure of polymers often change under different cooling conditions. For balloon manufacturing, polymers must be in an amorphous state as they are formed into tubing. The rate and length of cooling controls the degree of crystallinity of the final product, which in turn determines other physical properties.
Only the highest-quality balloon tubing will predictably produce high-quality balloons with consistently high balloon production yields. However, finding that tubing material can be a challenge. Quality varies tremendously from lot to lot for a surprising number of vendors.
Before purchasing our own extruder and developing our own processes, we relied on outside producers for tubing, but rejected half the tubing material because of defects or difficulties in forming the balloons. One of our customers had very tight tensile and elongation requirements and we used the best tubing we could find, which still only resulted in a minimal balloon yield. Our vendors told us they couldn’t make it any better. After four years perfecting our own in-house extrusion system and methodology, we have experienced significant improvement in balloon production yields.
Balloon tubing must meet tough performance standards established by medical device companies. Medical balloons require thorough inspection aided by magnification to look for scratches, bubbles, lines, foreign particles, fish eyes, etc. as tiny as 5/1000 inch across. Finished balloons must also meet burst-pressure requirements, fatigue cycle tests as well as specific level of compliance while being inflated. All these parameters must be within prescribed limits and must be consistently reproduced in all subsequent production runs.
A finished balloon has extremely thin walls, up to five times thinner than a human hair. They must perform under high pressures, up to 20 to 30 atmospheres. Failure could result in serious health complications, such as a burst artery or fragments of balloon material within patient’s vasculature. Therefore it is absolutely essential that the desired tolerances and mechanical properties of the balloon tubing are achieved through a carefully controlled extrusion process. Only then can we reliably design, calculate and predict the behavior of newly developed balloons and maintain repeatable and consistent balloon production processes.
Joe Stupecky is founder and chief technology officer of Interface Catheter Solutions. Joe has more than 40 years of experience in mechanical engineering and design, operations and administration. Prior to founding Interface Associates in 1995, Joe founded Bicore Monitoring Systems in 1989, which he sold to Bear Medical in 1995. Previous to Bicore, Joe founded Alpha Technologies, a manufacturer of metabolic testing systems that was acquired by Beckman Instruments in 1985. Preceding this, he held various engineering and management positions. Joe holds a master’s degree in mechanical engineering, and is named as inventor on numerous patents.
Medical Balloon Tubing: Lessons Learned
Precise extrusion methodology is the secret to superior performance of medical balloon tubing.
By Josef Stupecky
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