MATERIALS | MEDICAL TUBING
An extrusion line to extrude these products needs careful control – and will include, for instance, close measurement of eccentricity, dimensions and the presence of defects such as pin holes.
Pushing ahead Steve Maxson, vice president of sales for vascular technologies at Spectrum Plastics, explained the rigours of producing low profile delivery systems – such as catheter shafts – that are pushed through long pathways in the body. “Non-reinforced catheter shafts are generally
Above: Raumedic has developed a number of special materials for catheters
molecular weight loss, even at low shear rates,” said Dillon. “PLLA’s properties are both tempera- ture- and strain rate-dependent.”
Wire in the tube Medical tubing is a precision product, and the most modern products need to do more than simply transport liquids. By co-extruding tubes with wires – or special coatings – manufacturers can make more sophisticated products. “Wire inlays can produce smart tubes with functional integration,” said Daniel Riechelmann, product manager for application technology at Raumedic in Germany. Examples include catheters with integrated functions. These can measure patient parameters – such as temperature, oxygen content and pH – and transmit the data wirelessly. Similarly, wires can add mechanical strength to tubing – raising stiffness, in order to resist kinking and bending. This is useful for products such as endoscopes and cannulas, he said. Both conductive and non-conductive materials can be co-extruded. A range of different wire materials can be incorporated, from stainless steel and copper, to platinum-iridium and special material like Nitinol, said Riechelmann.
Exgineering says that transitional extrusion integrates dissimilar materials more
smoothly, which can improve quality and lower cost
14 Transition extruded component
flimsy, and require a continuous braid embedded into catheter tubing,” he said. “This provides torquability and pushability to advance the device, while retaining flexibility and kink resistance to navigate the tortuous anatomy.” A braided catheter shaft might involve five layers
of material, including the core, liner, braiding, jacket and outer layer. These need to bond with one another, while the overall structure needs to be strong – with a low friction outer layer. There are several ways of addressing the challenges. In one case, Maxson said that the outer PTFE layer was delaminating from an internal polyimide (PI) layer. This is because thin-wall PTFE coatings have weak tensile strength, while PI has a high tensile modulus. The answer was to blend the two polymers, to
create a material with good mechanical properties and a low coefficient of friction. Another way of adapting properties is to vary the
braiding density along the tube’s length. So, close braiding helps to retain flexibility, while a shallower braid angle makes that part of the tube stiffer.
Transition period Stephen Davis, a scientist at Switzerland-based Exgineering, explained how ‘transitional extru- sions’ could help to improve the quality – and lower the cost – of extruded medical devices. Transitional extrusion allows the ‘blending’ of
Traditional assembly
PIPE & PROFILE EXTRUSION | September 2019
www.pipeandprofile.com
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