Medical device manufacturers are being challenged by strong market demand for tubing that delivers increased functionality, lower profiles, and lower costs—pushing the limits of material behavior and manufacturing science. Next-generation balloon catheters are expected to deliver significantly higher burst pressures and better puncture resistance. They are also being designed to transport target-specific drug-polymer payloads or flexible microelectronic packages to various parts of the human body. These enhanced miniaturization and functionality requirements of medical devices are also creating substantial design and manufacturability challenges. For example, how can an R&D team combine a 35-atm balloon with a robust weld and corresponding shaft tube, within a folded balloon profile that is compatible with a 0.065 inch sheath?
OEM and end-user demands continue to push the limits of materials and manufacturing technology. However, with an integrated approach, these challenges can be met successfully. The combination of deep material knowledge, customized manufacturing equipment technologies, meticulous process control, and thorough quality-monitoring systems can achieve the very tight tolerances that are needed to produce next-generation balloons with higher burst pressures.
Tubing quality is affected by a number of factors, including raw material selection, material handling, and the many parameters of the tubing extrusion process. Careful monitoring, testing, and documentation of each step are essential for ensuring consistency and repeatability. Advances in technology and sophisticated instrumentation provide capabilities that weren’t available just a short time ago—such as being able to extrude long micro-extrusion runs with minimal variation in material properties over the length of the tubing, as verified by real-time monitoring of key dimensions, such outer diameter and wall thickness.
New equipment advances and process controls enable more efficient heating and cooling across the length of balloon molds, which result in more consistent wall thicknesses across the length of the balloon, higher burst pressures, and fewer defects. This becomes critical in a market with demands for longer and larger balloons. This is especially important for higher-strength/thinner-wall tubing. High-precision tools have also been developed that modify the surface of balloons to allow for easier attachment of electronics and drug-polymer combinations.
A keen understanding of material chemistry and behavior is absolutely crucial for forming high-quality balloons, especially as interventional products are being asked to do more through minimal access. Because physical properties and the morphological structure of polymers (for example, crystallinity and the orientation of molecular chains) often change under different cooling conditions, maintaining precise thermal gradients for heating and cooling is especially important. Polymers are in an amorphous state when they are injected into the mold. Therefore, the rate and length of cooling controls the degree of crystallinity of the final product, which, in turn, determines other key physical properties such as strength, flexibility, etc.
Even the smallest process variations can impact melt homogeneity and result in material degradation, which compromises the quality of the final product. Polymers can degrade due to excessive temperatures or high shear stress, causing points of weakness in the material. Although they are only tiny areas of slightly different chemical structure and behavior, they don’t bind as strongly with the surrounding, unaltered molecules, creating structural weaknesses that can hinder performance or even lead to product failure.
Another important characteristic of polymers is melt strength—the ability to be drawn out and still maintain its viscosity. Melt strength varies according to heat and pressure. Other important qualities include pushability and trackability, which allow the tubing to be navigated smoothly through some of the smallest vessels in the human body. Designing a flexible tip and proper shaft support are essential for success. (See Figure 1) Since each polymer in its molten state has slightly different physical and chemical properties, a single die head cannot process each polymer to the required tolerances. This is where expert knowledge of material science and machine design really come into play. For example, multilumen and co-extruded micro-tubing typically require custom-designed extrusion dies and tips. Several iterations are often required before the extrusion process is ready for production. Draw down ratios can be 10:1 or higher, depending on the selected material and complexity of the product design. Even though draw down ratios this high are considered by many to be stretching the limits for most thermoplastics, these ratios can be achieved with specially designed dies and tips and a carefully controlled, customized extrusion process.
To avoid material degradation, enough time must be taken, and enough shear stress applied, to completely melt the material. Residual solid particles create the false appearance of gels. However, these unmelted particles are mechanical defects in the material that alter the properties of the balloon. The production team must be vigilant to eliminate any low-velocity points in the head that may lead to material collecting in these dead spots in the resin flow channels as too much residence time and/or recrystallization of polymer can result in thermal degradation of the material. If too much shear stress is applied to the melt, significant mechano-chemical changes may result. These include chain scission, which leads to lower molecular weight or grafting reactions that may cause gel formation. Gelation and scission can also result in chemical reactions from acid/base catalyzed reactions and hydrolysis reactions. Operators and technicians must also be careful to avoid cross-contamination of feedstocks. The best ways to do this are by limiting material handling and adhering to best-practice cleaning procedures.
Higher Burst Pressures
The push to make high-end diagnostic and therapeutic devices like angioplasty balloon catheters and stent delivery catheters smaller and stronger requires higher burst pressures.
Designs for larger and/or longer balloons with higher burst pressures put greater stress on the critical balloonshaft weld and the shaft tube. Engineering the perfect weld requires knowledge of material science, welding techniques, and clinical procedural goals. The final solution ultimately requires a balance between shaft tube wall thickness, balloon neck wall thickness, and weld length.
The push for thinner-walled, smaller-diameter shaft tubes means there is less material surface area for a high-quality bond. An adequate amount of same-family material must be available to melt together to create enough bond strength. Typically the weld program requires customized die and welding program to maximize weld integrity and repeatability moving from R&D to process engineering.
Higher burst pressure balloons must be compatible with an equal or smaller sheath size to be accepted by the market. A typical approach is to balance the balloon wall thickness with target burst pressure. Some projects require the testing of new materials to achieve the correct wall thickness and burst pressure. Balloon post-processing can also play a key role, including annealing the balloon, heat set processing to give the balloon better memory after it has been folded and wrapped so it can be removed easily, and customization of the balloon folding and wrapping machines to create consistent folds in long percutaneous transluminal angioplasty (PTA) balloons.
Another method for achieving highstrength balloons is placing one balloon inside another, with a slip layer between them. This provides a finished product with two optimized balloon inner layers, where the material molecules are most aligned and can deliver ultra-high pressures with small folded profiles. More innovative methods for achieving high burst pressure balloons are also being developed to respond to market unmet needs. (See Figure 2)
Extremely tight dimensional tolerances for tubing must be maintained for consistent balloon quality. Inside diameter, wall thickness, and concentricity are carefully calculated to produce a balloon with desired performance characteristics. Meeting these and other tolerances is absolutely essential to deliver high-quality balloons with consistent dimensions and high burst pressures. The best way to do this is by using advanced balloon extrusion processes with strict in-line monitoring and adjustment mechanisms. (See Figure 3)
Balloon tubing must also meet dimensional and performance standards established by medical device companies. This requires thorough inspection aided by magnification to look for scratches, bubbles, lines, foreign particles, fish eyes, etc., as tiny as 5/1000 inch in diameter that could impact performance. Finished balloons must also meet burst pressure requirements, fatigue cycle tests, and specified compliance during inflation. All these parameters must be within prescribed limits and must be consistently reproduced in all subsequent production runs.
Although balloons containing defects can be detected during inspection and scrapped, a balloon made from two different extrusion lots with high percent stress variation will look the same but have widely different burst pressures. It is all too common to hear high-level quality managers comment that they “can’t inspect quality” during design reviews—creating a challenging situation for R&D and process engineering, especially when project schedule pressures and cost constraints are also considered. To get 25,000 “good” balloons with a 65 percent yield, for example, a company needs to manufacture 38,000 balloons. The “all-in” cost can kill product margins on the back end, even when the purchasing department has found the lowest bid on the front end. To eliminate these product cost and performance issues, it is critical to conduct balloon catheter R&D with the highest-quality, most-consistent balloon tubing possible.
The balloon catheter has become such a common, reliable platform that many companies rely on it as their “goto” diagnostic and therapeutic tool for clinical applications throughout the human body. Designs and specifications are becoming increasingly complex— a look at some of the device procedural animations on many of the medical device websites show continuous progress toward minimally invasive and less disruptive clinical technology and procedures.
In the balloon catheter world, core competencies include balloon extrusion, balloon development, folding and wrapping, shaft tube extrusion, weld development, and performance testing. Excelling at this work requires state-of-the-art equipment, deep knowledge of material chemistry and behavior, very high precision and process control, highly skilled labor, and a very clean production environment. When all of these requirements are met, a wide range of micro-components and micro-tubing with extremely tight tolerances can be produced, such as complex parts with wall thickness to ± 0.001 inch.
This article was written by Mark Geiger, Vice President of Sales and Marketing, Interface Catheter Solutions, Laguna Niguel, CA. For more information, visit http://info.hotims.com/45607-160.