It's an Election Year.... Are You Making the Best Choices in Single-Use Tubing

Tubing is a crucial part of any single-use system and is required for secure fluid transfer. However, it also presents a significant contact area that can potentially contribute to adsorption, leachables, or particulates, so selection of appropriate tubing for each application is important. The best starting point is to consider the process fluid and conditions to which the fluid transfer tubing will be exposed. In this column, I will also consider the potential impact on product, the types of tubing available to choose from, and other considerations in making a selection.

Jerold Martin

The recommendations below are based on my experience with single-use systems, along with suggestions by technical experts from three tubing suppliers: Steve Wilkowski of Dow Corning, John Stover of NewAge Industries, and Christopher Shields of Saint-Gobain.

IMPACT OF FLUID AND PROCESS CONDITIONS

The first factor to consider when selecting tubing is the nature of the fluid you want to transfer. In single-use bioprocessing, fluids range from proteins or other biological solutions to strong pH adjusters and buffers. Biologicals may be sensitive to ultraviolet light or oxygen, and require opaque or low gas permeability tubing. Acids and bases must be used with chemically compatible tubing. In all cases, it is necessary to select tubing grades with low extractables to ensure minimum levels of leachables.

Once biological stability and chemical compatibility requirements are defined, processing conditions should be considered, including process temperature, pressure, time, and sterilization conditions prior to use. Single-use bioprocesses are generally conducted at temperatures below 40 °C, so most tubing options are compatible. If tube welding and/or sealing is required on the system then meltable tubing will be needed, whereas the use of only quick-connects or sterile connectors eliminates this temperature requirement. Similarly, if the tubing is to be autoclaved, it should be resistant to at least 125 °C.

The next process condition to consider is pressure. Although the lowpressure limits of single-use polymerfilm biocontainers generally limit pressure requirements for system tubing, tubing used upstream of sterilizing filters that are integrity tested prior to use is an exception. Depending on the grade of filter and integrity test method, test pressures can range from 40 psi (2760 mbar) to 70 psi (4825 mbar), or even higher with some mycoplasma or virus filters. To accommodate this, high durometer, braided or otherwise reinforced tubing must be incorporated upstream of the filters to ensure suitable pressure rating for integrity testing. Another exception is where peristaltic pumping is used to drive the process fluid. Tubing selection must consider the strength of the tubing to provide consistent flow and low generation of particulates. Tubing grades recommended for peristaltic pumping are typically qualified for much longer service than would be applied in most single-use applications, but failure to consider pumping suitability after sterilization can lead to significant problems.

Sterilization conditions must be considered when selecting tubing. Most single-use applications are gamma irradiated for sterilization and consequently require gamma stable tubing (generally up to 50 kGy dosage), but it is not uncommon to apply autoclave sterilization for some tubing manifolds and filtration systems, especially during process development and early phase clinical batch manufacturing. The desired sterilization method must be considered in tubing selection. Information on gamma sterilization is provided in the BPSA Guide to Irradiation and Sterilization (1).

IMPACT OF TUBING ON PROCESS FLUIDS

The primary concern for the tubing's impact on process fluids are biocompatibility, leachables, adsorption–absorption, and permeability of gas and light (particularly ultraviolet light). Biocompatibility is typically assessed by applying the USP Biological Reactivity Tests to the tubing material after a "worst case" sterilization process, either in vivo (<87>, cytotoxicity) (2), or in vitro for Class VI (<88>, implantable) plastics (3). Other standard tests may include pyrogenicity. Recommended standard reference tests for tubing are described in the BPSA component quality test matrices Guide (4).

Potential leachables are initially assessed by considering extractables determined under exaggerated process conditions, such as higher temperatures, more aggressive solvents, and longer contact times. The maximum sterilization process conditions should also be included (typically >125 °C steam autoclave or 50 kGy gamma irradiation) because these factors can increase the level of process leachables from some tubing. Similarly, the impact of heat welding on leachables should also be included in extractables assessments. For more information on determination of tubing extractables data, see the 2008 and 2010 BPSA extractables guides (5, 6).

Adsorption, where target molecules bind to tubing contact surfaces, is a primary concern with protein molecules, but the relative smallsurface area to volume ratio typically limits protein concentration losses to only highly dilute solutions. Absorption, where target molecules migrate into the tubing solid phase itself, are a greater concern with small molecules such as preservatives, which are generally limited to final formulations, but should be assessed for tubing used at that stage of the process. Low permeability tubing (for gases and light) can also provide reduced absorption of small molecules.

Although particles in tubing used in upstream processes (media, buffers and intermediates) upstream of filters have not been a major concern, applying single-use systems to aseptic vaccine manufacturing or in post-filtration formulation and filling of biopharmaceuticals has raised concerns about the potential impact of particles from tubing fill lines on final dosages. Extrusion of tubing is generally a low particle-generating process, but handling, cutting, joining, and environmental conditions for single-use system assembly can have an impact. Particle levels from tubing in systems used downstream of final filtration should be considered in critical applications.

TYPES OF TUBING FOR SELECTION

The most common types of tubing used in single-use applications are silicone and thermoplastic elastomers. Silicone tubing has a long history of safe use for a broad range of bioprocess fluids, and provides high temperature and gamma radiation stability. Other benefits include flexibility, translucency, biocompatibility, and smooth bore for low adsorption and particulates. Peroxide-cured grades have been successfully used in laboratory and clinical applications, but platinum-cured silicone is preferred for single-use systems because of lower extractables. Braided grades are available for high pressure applications. Silicone tubing cannot be thermo-welded and is limited to pre-assembled systems or connections using quick-connects or sterile connectors.

For tube welding and sealing applications, thermoplastic elastomer (TPE) tubing is required. These are proprietary formulations, commonly referred to under trade names, and have a good record of safe use and are also preferred where reduced adsorption, absorption or gas permeability is desired, though they do not have the transparency of silicone tubing. Other specialty formulations of tubing are also available.

Both silicone and thermoplastic elastomeric tubing are manufactured in numerous proprietary formulations, resulting in different extractables profiles and other properties. A case study was reported several years ago on the effect of tubing material on the bubble point values of downstream sterilizing-grade filters.(7) Poly(dimethlysiloxane) silicone oil leached from some grades of tubing (both silicone and thermoplastic elastomer) and was adsorbed on downstream filter surfaces. This reduced the critical surface wetting energy of the filter membranes, thus impacting post-use bubblepoint values, causing false test failures. Tubing used upstream of sterilizing filters, and infinal filling, should be considered for low silicone oil, along with other leachables.

It is common for new users of singleuse systems to consider choosing whatever grade of tubing they have experience with in their laboratory or clinicalbatch processes, but it is also important to consider the selection and sourcing of tubing by the single-use system integrator. It is also advantageous for integrators to limit their approved tubing sources, including inventory control, cost-efficiency, and generation of additional qualification studies (e.g., post irradiation extractables). Good integrators will also establish quality agreements for traceability and sourcing of raw materials (e.g., resins and elastomers), change notification protocols, and conduct supplier audits. Consequently, once your fluid and process requirements are defined, it can be beneficial to consider suitable grades among those already selected and qualified by the single-use system integrator.

Jerold Martin is the senior vice-president of global scientific affairs at Pall Life Sciences, Port Washington, NY, US. 516 801 9086, jerold_martin@pall.com. Jerold is also the chairman of the board and technology committee of the Bio-Process Systems Alliance.

REFERENCES

1. BPSA, Guide to Irradiation and Sterilization of Single-use Systems (2008).

2. USP 29–NF 24 General Chapter <87>, "Biological Reactivity Tests, in vitro," 2525.

3. USP 29–NF 24 General Chapter <88>, "Biological Reactivity Tests, in vivo," 2526.

4. BPSA, Component Quality Test Matrices Guide (2007).

5. BPSA, Guides to Extractables and Leachables from Single-use Systems (2008).

6. BPSA, Guides to Extractables and Leachables from Single-use Systems (2010).

7. B.K. Meyer and D. Vargas, PDA J. Pharm. Sci. Technol. 60 (4), 248–253 (2006).