MATERIALS | MEDICAL TUBING


A third case study also used FTIR, to understand


why there was discoloration of an endoscope sheath – which was blackening in specific regions, after completion of diagnostic procedures. FTIR was used to analyse the different regions, and found varying amine/amide content in the sheaths. “The latex that stained easily showed higher


protein content than the latex that did not stain,” said Davis.


Above: Canadian scientists are making biodegradable medical tubing derived from kelp


modulus. Annealing Kepstan helped to give it a higher stiffness than Peek. “There are options beyond Peek – and multiple options to consider,” said Jurgaitis – adding that different materials were suitable for different applications. Hazel Davis, a PhD student at the University of Massachusetts Amherst, explained her project to identify appropriate materials and processes for medical extrusion. Part of this involved characteris- ing intentional (and unintentional) changes to materials that could lead to medical tubing failure. One part looked at varying the moisture content


of polyurethane. This involved measuring moisture loss in different ways: the weight loss method (weighing the sample at two different tempera- tures); and the titration method (which detects and quantifies volatile components). Samples were assessed for viscosity, using rheol-


ogy measurements, and tested in a falling dart test. Davis found that moisture levels can be affected


by several factors, including the specific chemistry of the polymer. In ‘condensation polymers’ (such as amides and esters), water content is critical. Polyurethanes can also depolymerise in the presence of excess water. A second study looked at blooming/tackiness of polyether block amides (PBAs) – and whether material is being transferred from the tube. Also, Davis wanted to know how tackiness affected coatings – and how tackiness can be monitored. Here, Fourier-Transform Infrared (FTIR) spectros- copy was used to determine a chemical ‘finger- print’ for the material. This detects changes in chemical structure – which helps to assess degrada- tion and identify contaminants. A residue was transferred from the tubing to a crystal used in the FTIR process – and it matched the surface of the tube, proving that there was material transfer.


14 PIPE & PROFILE EXTRUSION | May 2021


Biodegradable tube While PVC tubing has proved critical in the medical environment, it has a high environmental impact – with most medical tubing either incinerated or landfilled. One potential way around this is to make medical tubing from biodegradable materials. One approach, developed at the University of


British Columbia in Canada, is to use 3D printing to create tubing from three ‘natural’ materials: brown algae, wood pulp and water. The researchers have looked into the feasibility


of creating hydrogel products derived from alginate under inertial conditions. “Alginate is a derivative of brown kelp and has been widely used in tissue engineering due to its biocompatibility,” said Jordan McKenzie, a research associate at the university. One important consideration was to create the


tubes at “commercial speeds” – in this case, 12 cm/ second for a tube with an outside diameter of about 1cm.


“Our ‘flow-focusing’ design creates strong composites by reinforcement with wood pulp fibre, to enhance stiffness and breaking strength,” said McKenzie. The team has managed to make several shapes, including straight tubing, tapered tubing and bump tubing. It has also benchmarked blood compatibility of the hydrogel tubing using a lab-scale pumping system and human blood. Results showed a six-fold reduction in platelet adhesion, compared to uncoated PVC tubing. The hydrogel was tough enough to circulate blood for 200 hours an internal pressure above 2MPa. The team is continuing to look at reinforcing agents to strengthen the tube for high pressure applications.


PVC alternatives


While medical tube manufacturers may be keen to replace PVC, this is easier said than done. Giuseppe Fiandaca, CEO of Polyneers, said the despite the desire to replace PVC in medical devices, is still accounts for around 40% of material usage.


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