Feature Article

Tubing with advanced functionality
Multicomponent technology performed under cleanroom conditions enables the development of innovative new medical products. The technique can be economically advantageous and, when combined with coextrusion technology, can produce tubing with advanced functionality. Combining different polymers and integrating additional functions, as well as streamlining postproduction and assembly processes, frees product design from the constraints of conventional methods and fosters innovation. From an end-use perspective, minimally invasive surgical and diagnostic applications are driving demand for miniaturised tubing and furthering innovation among suppliers and OEMs.

The use of multilayer extrusion to produce foil for food packaging, for example, does not present any particular tooling challenges, but the same cannot be said of medical applications. Coextruding polymers for the fabrication of miniaturised medical tubing is still fairly new ground for process engineers in many cases.

Advanced microextrusion systems can use up to three different polymers to manufacture multilayer tubing for various applications. Tubing with inner diameters that measure approximately 0.1 mm (100 μm) and wall thicknesses on the order of 0.05 mm (50 μm) can be extruded. Microextruders suited for these applications achieve flow rates as low as 50 g/h.

In theory, there are no limits to the range of polymers that can be co-extruded. Thermoplastics such as polyurethanes, polyamides, polyolefins, thermoplastic elastomers and, in some cases, soft PVC are of particular interest, however, because they have a history of use in medical technology and pharmaceutical applications. High-temperature thermoplastics such as PEI or PEEK also can be processed by microextrusion. These materials can replace metals in some cases, because of their exceptionally good mechanical properties. It is also possible to print on the high-temperature materials if, for example, measurement marks are required on a catheter.

When developing a catheter, a number of elements must be considered to achieve an optimally designed product that meets qualitative and economic benchmarks and satisfies user needs. Even during the early stages of product design, the manufacturing techniques and materials selection processes related to the individual catheter components should be top of mind, as they can have a significant impact on the quality and cost of the finished product.

Given the multistage processes inherent in series production, it is recommended that the design of individual components be optimised in relation to the anticipated volume requirements. This will help ensure that manufacturing and assembly processes—be they manual, semiautomated or fully automated—are conducted in the most economical manner. This also benefits process reliability.

Materials selection for catheter components must take into account chemical and mechanical properties, biocompatibility and, if applicable, haemocompatibility. Additional manufacturing steps such as component joining and sterilisation also must be considered during the materials selection phase.

The following example of a microdialysis catheter underscores some of the aforementioned points.

Developing a microdialysis catheter
Microdialysis renders obsolete the conventional technology of drawing blood samples from patients and creates new possibilities for continuous, quantitative blood analysis. In microdialysis, the sample is diffused from the blood by means of a very thin, semipermeable membrane. The sample is then identified by using conventional analytical methods. A physio-logical saline solution is generally used as the rinsing media. Diffusion takes place in the same way as it does in dialysis.

The technology depends on the fabrication of a two-lumen catheter that is made from polyamide or polyurethane. Small segments of the dialysis membrane serve as windows in the lumens, which measure 0.3 mm in diameter. Delicate and precise perforations at the tip of the catheter ensure contact with the blood.

Two lumens connect at the tip of the catheter, allowing the saline solution to be fed back into the device after the substances being analysed have been absorbed. Continuous measurement is possible without drawing large quantities of blood from the patient.

The microcatheter is supplied with an in-dwelling venous cannula, which is commercially available. The catheter system is completed with moulded components and a final fabrication step that maintains the delicate structure of the catheter tubing whilst connecting it with the analysis device.
 

Thomas Knechtel
is Head of Business Team M&S Assembly/Catheters
Raumedic AG, Hermann-Staudinger-Str. 2, 95233, Helmbrechts, Germany
tel. +49 9252 3590
e-mail: info@raumedic.com
www.raumedic.com