June 29, 2020

Maintain sterility with a single-use closed system

By Claire Jarmey-Swan, Cytiva

Single-use closed systems are rapidly becoming the bioprocessing tool of choice, especially when rapid scale-up responses to disease outbreaks are required. Offering the benefits of flexibility in process design, low setup costs compared to stainless steel facilities and reduced validation and sterilization regimes, optimal single-use bioprocessing systems can be replicated across the globe to enable fast drug production at a local level.

So just how are unit operations across upstream, downstream and formulation and filling processes connected to form a sterile closed system suited for monoclonal antibody, vaccine, or gene therapy production?

The answer lies in how fluid is transferred from one selected technology to another unit operation. For sterile fluid transfer such as bioreactor media or harvest or during formulation and filling, prevention of contaminant ingress is crucial. Without it the results could be devastating – an FDA 483 citing lack of sterility or worse still, the loss of expensive and patient-critical drug products.

In single-use this fluid transfer is managed though transfer lines or sets that comprise a tube line which connects one unit operation to another tube line from another unit operation. Process parameters such as fluid type, pH, temperature, and processing time need to be considered when designing these single-use fluid transfer sets. The need for step-up or step-down fluid transfer also needs to be considered as well as the mode of sterilization namely autoclave or gamma irradiation.

Selecting the optimum tubing for single-use falls into two main categories: silicone or thermoplastic elastomers (TPE). Historically silicone has been selected for its safe use for a broad range of bioprocess fluids, translucency, biocompatibility and low adsorption and particulates. In addition, platinum-cured silicone is preferred due to its lower extractables. If silicone tubing is selected, each tube is coupled using ‘sterile’ connectors, a connection device that is pre-assembled in the transfer set to maintain a sterile flow path when connected, even without the need for a laminar flow hood or isolator.

What affords the ‘sterile’ connector the ability to ensure sterility whilst making a connection, even in unclassified areas? Simply put, it is the unique design, either a membrane, film, or movable plastic component, that once integrated on the tubing and sterilized, continues to protect the port until ready for fluid transfer. This port protection is demonstrated through product verifications incorporating exposure of the protected port to high concentrations (106 colony forming units/mL) of micro-organisms such as Brevundimonas diminuta, coupling two connector halves, removing the port protection, and passing a broth through the connected tubes. The sign of success – no bacterial growth confirming sterility of the flow path.

If TPE tubing is selected, a choice between a sterile connector device or welding is available. Welding adds another complexity to the sterile connection process, requiring a well-maintained welder, skilled operators, and the availability of the welder at the time of connection need. In addition, only the same tubing diameters can the welded together and the flexibility of step-up or step-down fluid transfers is not possible, something that can be easily achieved with sterile connectors offering interconnectivity across different sizes.

Once the materials of construction of the fluid transfer sets have been decided, the design can be drawn, taking into consideration the length and internal diameter (ID) of the tubing required, the need for side branches required for sampling or filling of multiple bags, and once built, the additional testing required to verify that the transfer set is suited for its intended use – a closed single-use system. It is for these reasons that specialized integrators provide in-depth knowledge of components, established quality and continuity of supply, offering services to provide pre-assembled and pre-sterilized transfer sets ready for rapid implementation when time to market is vital.

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