Fluid transfer tubing

Bill Schoenherr and Christophe Sene, from Dow Corning, offer some thoughts on tubing selection for fluid transfer

It would seem that selecting tubing for fluid transfer should be an easy task, after all, tubing provides a pathway for materials to move from one tank to another, or from a tank to a filling machine. However, many factors can complicate the selection process: material, performance properties, change control, and regulatory compliance are but a few. Choose the wrong tubing and there is risk of contaminating a critical drug or intermediate, and safety becomes an immediate concern along with monetary and productivity issues.

Tubing used in pharmaceutical and biotechnology manufacturing is typically made of silicone or organic materials, but whatever the composition, it is critical that the tubing material be easily sterilised and have a known extractables profile. The need for sterility is obvious. However, it is equally important that the sterilisation process not change or degrade the material, because any alteration could impact tubing performance and extractables.

Extractables are a concern because they can react with or contaminate a processed material. Changing the fluid is considered an adulteration and must be well characterised from a composition and toxicological standpoint. Plastisers or additives such as phthalates are often used in organic materials to stabilise tubing or add flexibility, and they can easily cause contamination. In contrast to organic materials, silicone has become the material of choice for pharmaceutical processing because of its ease of sterilisation, and very low level of extractables.

But there are significant differences even among silicone materials. Silicone elastomers used in the manufacture of tubing can be cured with one of two chemical mechanisms: free-radical or addition cure. The free-radical mechanism uses a peroxide catalyst that decomposes under high temperature conditions, initiating a crosslinking reaction. The fate of peroxide decomposition byproducts such as chlorophenyl compounds is a source of concern in this process. To reduce the potential for extraction and contamination, the tubing must be fully post-cured in a high temperature oven to volatilise any potentially harmful byproducts. A chief issue is whether this post-cure step can be effectively and reliably implemented to ensure the complete removal of peroxide byproducts.

In the case of silicone elastomers crosslinked with an addition cure mechanism, a reaction occurs between vinyl and hydrogen functional groups on the silicone polymer. A platinum complex catalyses this curing mechanism, and because it is an addition reaction, there are no byproducts. Compared with peroxide curing, a significantly smaller amount of platinum is used (ppm versus %). Extraction studies confirm that all the platinum catalyst is trapped in the elastomer matrix, and as a result, no post-cure step is needed. This makes addition cured silicone elastomers a better choice for tubing used in pharmaceutical and biotechnology applications. For this reason, Dow Corning supplies tubing products made only from addition-cured silicone elastomer.

Material type and curing process are perhaps the most basic factors to consider when selecting tubing for pharmaceutical and biotechnology manufacturing processes. With this foundation, drug and tubing manufacturers can work to optimise other factors for safety and productivity in individual fluid transfer applications.