Complex extrusions open up design, quality and performance possibilities for medical tubing products, such as catheters, wound drains and hemodialysis tubing.
Dan Sanchez, Trelleborg Healthcare & Medical
Silicone is a proven material of choice for medical devices because of its purity and biocompatibility. It is also highly customizable, allowing it to be optimized for a wide range of devices that require radiopacity, conductivity and physical properties such as high tensile strength.
HCR silicone’s unique green strength — the strength of rubber in its unvulcanized state — allows for highly complex geometries in continuous extrusion processes, setting it apart from other types of polymers, such as polyurethanes, thermoplastics and room temperature vulcanizing (RTV) silicones. Used with a geometric transition extrusion process, such as Trelleborg’s GeoTrans, HCR extrusions can change cross-sections dramatically, opening up new possibilities in a wide range of medical devices, including wound drains, catheters and hemodialysis tubing.
Device examples
With geometric transition extrusion, tool components can be moved during the extrusion process to change tubing geometry. For example, a tube can transition from single to multiple lumens or split from a multi-lumen tube into two or three single lumen tubes. Extruding different geometries in one process eliminates assembled joints that may be weak or create internal misalignments where fluids can become turbulent or stagnate. This, in turn, reduces labor costs and improves the overall quality of the product.
Additional examples of geometric transitions:
- A multiple lumen tube can have one or more lumen stops and restarts, eliminating the need for secondary operations, such as backfilling a lumen after filling a catheter balloon.
- Instead of a wound drain comprising three separate pieces¾an extruded tube, a complex cross-section extrusion, and a molded hub as a connecting piece¾ the drain can be created as a single extrusion with two or more distinct geometric cross-sections and a smooth, integrated transition where the hub had been.
- Off-ratio bump tubing, for applications requiring a variable outer diameter with either a constant or variable inner diameter, can be created with very short transitions (fractions of an inch). (Figure 3)
Design considerations
The first aspect for consideration in the design of geometric transition extrusion is the hardness required. Although HCR silicones are available from approximately 20 Shore A to 80 Shore A, the extremes of the hardness spectrum are challenging for highly technical extrusions.
The ideal target for GeoTrans extrusions is between 50 Shore A and 70 Shore A. The green properties of HCR silicone materials are ideal to facilitate the control of the material and forming of geometries in the critical transition areas. There is also greater availability of suitable materials from silicone suppliers in this hardness range.
Size, combined with the expected cross-sections, is the second consideration. The GeoTrans extrusion process has been used to manufacture products between 7 Fr and 24 Fr (on the catheter scale). This size range is ideal to minimize initial and maintenance costs. Reducing the complexity of the cross-sections also speeds time to market.
The third design consideration is tolerance targets. Transitioning from one cross-section to another creates pressure differentials within the raw silicone as it flows, causing variations in size due to tapering along the length of the tube. Therefore, with complex extrusions, one section can be manufactured to very tight tolerances and the others would need to have more flexibility in dimensional tolerances.
To eliminate further tolerance and measurement correlation issues, it’s important to define where the extrusion will be cut for dimensional inspection. Close collaboration between designer and manufacturer is critical to ensure that the final specifications and tolerances support design for manufacturability.
Benefits
The benefits of geometric transition extrusion include reduced processing time and cost, lower risk of mechanical failure, less validation cost, fewer regulatory issues and overall improvement to performance and quality.
The tooling costs for this process are considerably higher than traditional extrusion. Despite this, for some devices, the total cost of ownership is reduced by eliminating one or more components, secondary processes, and/or assembly steps.
Device designers and manufacturers are increasingly inclined to pursue a solution with this method because the strength improvement dramatically enhances the device’s longevity and robustness. For example, eliminating secondary bonds can greatly increase a device’s ability to withstand cycle loading, reducing the risk of failure and increasing device life expectancy.
Processing time can be significantly reduced when devices are redesigned to take advantage of this technology. Although running a simple extrusion is faster than running a geometric transition extrusion, assembly time is often cut in half. For instance, the production volume of many long-term implants does not justify complex, automated assembly. However, a redesign to include a geometric transition can eliminate the need for manual assembly of a portion of the device. Additionally, the design validation process may be significantly shorter, with fewer components and assembly processes.
An excellent example is bifurcated tubing, which traditionally has four components: a two-lumen tube extruded and cut to length, two single-lumen tubes extruded and cut to length, and a molded hub. In the past, each of these pieces had to be bonded together in a secondary step, after which each bond and secondary process step had to be tested. In contrast, using geometric transitioning, one extrusion process can produce the bifurcation, cut the extrusion to length, and stack complex extrusions in bundles, minimizing the secondary processes required during the final device assembly.
Reducing the risk that a device will run afoul of a U.S. or international regulation is top of mind for designers these days. The more components and secondary processes involved with a design, the greater the risk. Thus, medical device manufacturers are seeing benefits from partnering closely with component suppliers to identify how sophisticated technologies like GeoTrans can contribute to risk mitigation.
Indeed, the redesign of catheters, drains and tubing often comes from a collaboration between the manufacturer and a component supplier experienced in geometric transitioning, with a variety of devices being evaluated to see whether this method could be used to eliminate components.
Dan Sanchez is a product manager at Trelleborg Healthcare & Medical. He has been working closely with customers on GeoTrans projects for 21 years. For more information on GeoTrans, visit www.tss.trelleborg.com/healthcare.
The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Tubing + Extrusion or its employees.
