Round trip: Discussing coextrusion technology with Raumedic

Gert Walter, Business Unit Tubing, Raumedic, discusses coextrusion technology and its potential for opening up areas of medical applications.

Coextrusion and multilayer extrusion make it possible to use several different materials in one single tubing, providing a modern, effective and in practice, optimised solution that can be used in a range of medical applications. Polymer specialist Raumedic develops solutions by multilayer extrusion, making medical products from infusion tubing to filling tubing, safer and more efficient.

The role of coextrusion in medical engineering

The use of multicomponent technology under cleanroom conditions provides an excellent foundation for the development of new medical and pharmaceutical products. As well as improved cost, the objective is to achieve improved functionality in new tubing products with the help of coextrusion technology.

And while multilayer extrusion for the production of films in applications like food packaging no longer represents a significant challenge for modern tool and machine technology, coextrusion of multiple polymer layers in the production of micro-dimensional tubing for medical engineering is still uncharted territory in many cases.Nowadays, microextruders are able to produce multilayer tubing from up to four different polymer materials. The smallest achievable inner tubing diameter is about 100 µm, with a minimal wall thickness of approximately 50 µm. Microextruders can work at minimal material throughput rates, with an output of less than 30 grams per hour.

The advantages of coextrusion processes for medical engineering are obvious:

  • Application-specific distribution of layer thicknesses
  • Embedding of several colour stripes and x-ray contrast stripes
  • Integration of functional layers eg for light protection properties or gas barrier
  • Use of bonding agents for incompatible polymers against delamination

Suitable materials for multilayer tubing

In theory, any polymer can be used in coextrusion. In practice, however, those thermoplastics are used that have already proven their worth in other processing techniques in medical engineering and pharmaceuticals: polyurethanes, polyamides, polyolefins, thermoplastic elastomers, and to some extent soft PVC as well.

Multilayer tubing for medical applications

To protect light-sensitive solutions and ensure loss-free dosage of sensitive drugs, Raumedic has developed the products Rausorb, Rauinert and Rausonert. These three examples of medical engineering reflect the growing importance of multilayer extrusion.

Drug-compatible infusion tubing for highly sensitive drugs

For decades, soft PVC has proven its worth as an efficient and easy-to-process material for flexible infusion lines. Even today, well over 90% of all infusion tubing is made from soft PVC. With advances in the development of highly effective new drugs however, especially in oncology, an increasing number of problems have arisen involving drug compatibility with the PVC tubing material. Many highly-sensitive drugs are adsorbed on the tubing’s surface, with the result that only a fraction of the intended dose actually reaches the patient.

Conversely, ‘undesirable side effects’ may occur, if plasticizers and other additives are released from the PVC material by the infusion solution. This happens most often when the infusion solution contains fatty substances or lipid-like solubilizers.

Optimised application of soft PVC in multilayer tubing

In order to continue using soft PVC as a safe material in the production of multilayer tubing despite these challenges, Raumedic has developed Rauinert. The layering most commonly used with this product consists of an LDPE inner layer, an EVA bonding agent and a PVC outer layer. Polyethylene is chemically neutral in contact with the flow-through medium. The EVA middle layer serves as a bonding agent between the LDPE and PVC layers, since those two materials would not otherwise form a strong bond to one another in the coextrusion process. The outer layer made of soft PVC ensures that the manufacturer of the final infusion tubing sets is able to conduct all of its processes just as he would with any ordinary PVC tubing. This includes bonding, packaging and sterilisation, for example.

The market share of this type of highly-specialised multilayer tubing in infusion therapy is expected to grow in years to come. At the same time, new ideas are also being implemented. Compared with other production methods, multicomponent extrusion technology offers immense freedom in form and design, through its combination of multiple polymeric materials with the integration of additional functions.

Protecting light-sensitive pharmaceuticals is increasingly important

Pharmaceuticals that are activated by exposure to light, or that break down in a photochemical reaction are increasingly used for special therapies.

Substances like vitamin A and sodium nitroprusside take their activation energy from visible and invisible light in different ranges of wavelengths.

To provide the required protection for these substances, the development of black tubing seemed to solve the problem. This, however, makes it impossible to monitor the infusion solution. As a result, any gas bubbles, impurities or other problems cannot be detected when they occur.

Other solutions available on the market involve transparently coloured tubing, or windowed tubing made of a clear material including semi-circular segments of light-proof coextrusion materials embedded in the tubing wall. But these solutions are merely a compromise at best, since they do not comply with the applicable pharmacopoeias and relevant standards.

A solution for light-sensitive drugs

Multilayer tubing from the Rausorb line meets medical engineering requirements. The inner layer of this special tubing is physiologically harmless. The outer sheath is infused with light-absorbing substances that correspond to the spectrogram of each individual infusion solution.

With this technology, any chosen combination of wavelengths in the 220–800 nm range can be largely filtered out. Since each preparation is only sensitive to a very specific set of wavelengths, there are enough ranges remaining to allow for the production of transparent tubing that still blocks all but a negligible amount of light in the critical wavelength ranges. This makes it possible to develop tubing that is specific to individual drugs.

Loss-free dosing of light-sensitive solutions

For drugs that are both light-sensitive and PVC-incompatible, the Rausonert tubing line offers custom-tailored solutions – with regard to the requirements of later processing steps as well.

Inert inner tubing layers are coextruded with light-absorbing outer layers. The possible combinations of materials and dimensions are virtually unlimited.

Filling tubing for PVC-free infusion bags

As with food packaging technology, the trend towards the use of lightweight, flexible and unbreakable polymeric materials is developing in containers for infusion solutions, too.

For infusion bags, the first step was the use of PVC films, tubing and connectors containing plasticisers. Since the early 1990s, there has been an intensive search for alternative materials free of plasticisers and chlorine. For films, the industry quickly achieved adequate levels of quality that had already proven their value in the food industry. The films in question were multilayer films made from polypropylene or polyethylene/bonding agent/polyester that comply with the requirements for transparency and ability to be sterilised with water vapor at 250 °F (121 °C).

These PVC-free film bags require special filling tubing that is now produced in coextrusion technology. In developing these types of tubing, the goal was to provide an outer layer with good thermal weldability with all common films. At the same time, the inner layer should also provide excellent bonding to all common connector materials, such as polycarbonate, polypropylene or hard PVC, during the steam sterilisation process. Naturally, this combination of properties cannot be achieved in a single polymer formulation.

Construction of filling tubes for infusion bags

This special two-layer tubing is composed of an inner layer of ethylene-vinyl-acetate-copolymer (EVA) and an outer layer made of thermoplastic elastomer (TPE). The EVA provides excellent bonding to polycarbonate connectors, but must be cross-linked in order to maintain its shape at 250°F (121°C).

If polypropylene connectors are preferred, three-layer tubing with a soft PP/soft PP/TPE layering can be used. With this layering, the modified polypropylene in the inner layer provides good bonding to PP injection ports, while the flexibility or stiffness of the tube as a whole can be variably controlled through the formulation of the soft PP middle layer.