The extrusion and catheter finishing processes are inextricably linked, and balancing these processes can be a complex undertaking.
Paul Weafer, VistaMed, a Freudenberg Medical company
There are a number of variables that can ultimately affect the performance of a finished catheter – the most paramount being the properties of the raw material and the extrusion process.
Careful consideration of the extrusion process is essential to ensure consistency and repeatability. Including extrusion engineers in the development and decision-making process is integral. Collaborative communication between customer and supplier will enhance concept development and ultimately get devices to market faster. It is important to keep in mind that there is far more to tubing than dimensions. Establishing a “see it, say it, fix it together” culture is key to getting a tube from concept to production.
Control inputs and outputs
Manufacturing high-quality extrusions that are designed to be used in sophisticated catheter systems depends on tight control of the “inputs.” Initially, the inputs start with people, because people can influence all other inputs such as training, material handling, storage conditions, control of drying and equipment and tooling design. Having the right mix of people is a good starting point. The greater the control of inputs to the extrusion process, the higher the probability of achieving the desired output necessary to develop and manufacture high-quality extrusions.
“Outputs” begin with process stability, and then the spotlight turns to achieving the agreed upon customer specifications, such as critical dimensions, visual criteria and functional performance. The tolerance expectations from customers are constantly challenging the boundaries of extrusion capability. It has become more significant than ever for tubing manufacturers to maintain close control of their inputs and process.
Polymer science and material behavior
A good understanding of polymer science and material behavior is undeniably crucial for producing high-quality catheters and balloons. The morphological structure of the thermoplastic material can change with varying thermal conditions which, in turn, determines key physical properties such as strength and flexibility. The polymer exits the die head of an extruder in an amorphous state. The rate and length of the cooling downstream from an extruder determines the degree of crystallinity in the final product.
In some medical applications, such as balloon forming, it is critical that the extruded tubing is amorphous prior to the balloon forming process. Therefore, the cooling parameters and cooling method used are critical to ensuring that crystallization does not occur in the tube during the extrusion process. In other applications, such as the extrusion of PEEK tubing, it is critical that the PEEK tubing achieves a relatively high level of crystallinity during extrusion to ensure that the tubing utilizes the outstanding properties that PEEK possesses.
To produce high-quality balloons and catheter systems, it is fundamental to have all the extrusion inputs under control. A capable process with high-quality melt is critical to producing tubing for consistent balloon quality and desired performance. Additionally, a sophisticated extrusion process with precise in-line monitoring and control is also crucial to achieving high-quality balloon tubing. Small process variations can undoubtedly hinder the quality and performance of the final product. These variations potentially impact melt homogeneity and can result in variability in balloon tubing performance.
Polymers can degrade due to excessive temperatures or high shear stress causing deterioration in the material molecular weight which, in turn, compromises the material’s performance. An important first step in achieving optimal catheter performance is to choose the right materials for the catheter delivery system. In recent years leading-edge polymers have begun to replace traditional materials. These new polymers are being integrated into the design of next-generation catheters. For optimal performance, engineers need to consider the biological, physical and chemical characteristics of polymers and the growth of new break-through manufacturing processes.
Enhance performance and design
To enhance catheter performance and achieve optimum results, several features should be considered and incorporated where appropriate:
- Braid/coil reinforcement for strength, rigidity and torque control along the length of the catheter should balance the need for flexibility and kink resistance to navigate tortuous pathways.
- Use of hydrophilic coatings delivers high lubricity to achieve low insertion force or a reduction in friction for a specific delivery application.
- Soft tip and multi-durometer segments along the length of the catheter provide atraumatic entry and maneuverability.
- Radiopaque contrast at the tip and key segments offer better visibility for the physician to visualize accurate anatomical placement.
- Steerability and deflection help attain optimal navigation.
To achieve some of these attributes, a number of design aspects can affect performance and design:
- Varying material durometer along the outer jacket length will provide a number of flexibility options.
- Different levels of flexibility and kink resistance can be achieved by varying the pitch of the braid on the shaft. Braid patterns (e.g. diamond pattern) will effect flexibility and torque response. Wire options (e.g. flat or round) can also effect catheter performance; round wire will provide a more flexible shaft while flat wire will deliver a lower profile and less flexibility.
- Hybrid coil-braid designs can also offer the best balance between torque, flexibility and a thin wall solution.
- Laser profiled metal hypotubes with a spiral type design can assist in maximizing trackability.
Product differentiation is trending
Requests are on the rise for braided shafts with the lowest possible wall thickness while maintaining adequate levels of track and kink resistance. Achieving the best performance in one characteristic can often directly affect other characteristics. Applying a polymer jacket to a metal hypotube is a popular choice when high lubricity is a key design requirement, and choosing a unique color for the polymer jacket can also assist to differentiate a product. Controlled extrusion inputs combined with a robust extrusion process play a key role in achieving success with next-generation catheters.
Paul Weafer is R&D manager at VistaMed, a Freudenberg Medical company and extrusion and catheter provider to the medical device industry worldwide.