How to Select Tubing for Bioprocess Applications

Introduction

Single-use and multi-use polymer tubing is often required in pharmaceutical and biopharmaceutical processing applications to convey critical fluids in a closed, sanitary, and repeatable manner. Fortunately, tubing technology has evolved significantly to meet the unique needs found in these industries. If you are evaluating tubing for use in a bioprocess application or would like to learn more about some of the most common tubing materials, this article is for you.

What is Bioprocess Tubing?

Bioprocess tubing (i.e. biopharma or pharmaceutical tubing) is a specialized category of tubing designed to meet the rigorous requirements of biopharmaceutical and pharmaceutical processing applications. This includes the production of various drugs and vaccines in any stage of the drug-development process from discovery and development all the way to commercial manufacturing. Bioprocess tubing is used to convey critical fluids with maximum control of parameters that affect the final product and can be characterized by aspects such as extractables, spallation, chemical resistance, sterility response, extrusion environmental controls, biocompatibility, mechanical properties, and quality controls of the tubing manufacturer.

Bioprocess tubing is sometimes confused with tubing for “medical” or other life sciences applications. Although the tubes may be made with similar materials, or even have similar features, medical tubing, for example, is subject to differing requirements that are not considered in this article. Furthermore, the term “tubing” is typically used in the industry to refer to a non-metallic, commonly single-use, flexible annulus, whereas “process hose” is typically reserved in the industry for multi-use, higher-pressure composite hoses which commonly have crimped metal ends. Process hoses are not discussed in detail in this article.

Factors to Consider When Choosing Bioprocess Tubing

How will the ends be connected?

There are several options when considering how to connect a tube from one part of your process to another. It may be important to consider that the fitting/fastener system can be a limiting factor when selecting a tube, and even more so when retrofitting an existing system (i.e. the new tube must often be compatible with the existing connections). For example, for someone using a hose barb fitting/stainless fastener ring in an existing process, the new tube would ideally be compatible with the existing fitting/fastener system to minimize re-validation efforts and re-engineering. Some common fitting styles in the industry include single or multi-barbed fittings, insert molding, push-to-connect fittings, compression fittings, and other application-specific connections. Some common fasteners include steel crimp rings, cable ties, compression-style collets and over-molding over fittings.

Chemical Compatibility

Another factor to consider when selecting tubing for bioprocess applications is the compatibility of the tubing material with the media being processed. Some manufacturers offer general chemical compatibility data which often includes common chemicals seen in the industry. One method commonly used by manufacturers to generate objective chemical compatibility data is “swell testing”, which typically measures the change in volume or weight over time in the presence of particular chemicals. Although this can be useful for the early stages of material selection, it does not necessarily provide information about extracted compounds, or meaningful data about the change in relevant mechanical properties.

The temperature, pressure and the presence of other chemicals in a process system can have a significant influence on the compatibility of a material, so testing should ultimately be conducted to represent your actual process fluid and conditions. For example, if your process failure condition involves a particular volume/time or spallation allowance, you may find that further testing is necessary to accurately characterize and minimize risk in your process. As such, a key take-away is that you typically should not rely solely on generalized compatibility data, and should determine chemical compatibility based on your unique process fluid and conditions.

Temperature Resistance

Tubing used in bioprocess applications must withstand varying temperature conditions without compromising in terms of structural integrity and without unacceptably increasing the leaching of contaminants into the system. Two extreme examples are the freezing of tube assemblies around -80° C and depyrogenation which can exceed temperatures of 250° C.

For extreme cold temperatures, users risk shattering the tube while handling it in the frozen state or while thawing. In addition, when it comes to TPE tubing that will be aseptically welded, one will want to ensure that proper testing has been conducted to ensure that welding performance will not be affected following the change of state. Low temperatures can also have a potentially negative affect on peristaltic pumping and can even cause spallation beyond acceptable limits.

In the case of extreme high temperatures, one must ensure that the tube’s pressure rating is not degraded to a point where it might fail during normal operation. It is important to consider that pressure ratings provided by tubing manufacturers may represent ideal conditions and that a pressure rating typically falls as the temperature of the tube increases. For this reason, some manufacturers may provide more detailed analyses such as a graph of pressure rating over temperature, rather than a single pressure rating value. High temperatures can also affect the leachability of the material and one may find that contaminants are more readily extracted during or after exposure to high temperatures. Last, when it comes to extreme heat, the tubing can actually melt, burn or degrade irreversibly during the process if the correct material is not chosen.

Leachables & Extractables

Extraction testing is commonly carried out in the biopharma industry to characterize the possible contaminants that could “leach” out into a process fluid. Testing protocols can either be custom (based on individual needs) or standardized [e.g. refer to USP <665> (currently in draft) or BPOG guidance documents]. Although standardized protocols are valuable for relative comparison, users should ultimately also test the materials following exposure to simulated or actual process conditions. Standardized tests typically use extraction solvents like an ethanol/water mixture, WFI, 0.1M phosphoric acid, or 0.5N NaOh. Although these compounds can represent a large number of possible conditions including a high and low pH scenario, actual leachable amounts may differ depending on the actual process media and parameters.

Furthermore, it can be critical to make sure the testing accounts for the sterilization method used (e.g. autoclave, gamma irradiation, UV, etc.). If your tubing will be gamma irradiated for example, it would be more representative to test a gamma-irradiated sample vs. an autoclaved sample.

Pressure-Handling Capability

The tubing’s ability to handle pressure is another crucial factor to consider for ensuring safety and integrity of the process system. Pressure ratings provided by tubing manufacturers will generally be calculated with a “safety factor” such as 3:1 or 4:1 (i.e. a safety factor of 4:1 would indicate that the “burst” pressure is 4 times higher than the reported pressure rating). Due to the critical nature of process safety and wide range of possible process conditions, further testing, beyond a manufacturer supplied recommendation, may be necessary. Also, flexible/elastomeric bioprocess tubes typically have a lower pressure rating than rigid tubing or composite process hoses. As such, to avoid over pressurizing a line, one should consider the maximum pressure of the system in combination with the statistical significance of provided pressure data. For example, is pressure testing conducted on each lot or has it been conducted using a statistically significant number of tests?

Finally, when selecting a tubing material, one should consider that temperature, concentricity, chemical compatibility, additional stresses and other factors can have an adverse affect on the pressure rating. As such, testing a tube exposed to your unique process conditions may be preferred vs. reliance solely on generalized pressure data.

Particle Generation (Spallation)

During peristaltic pumping, flexible tubes may shed particles into the process fluid over time. As such, one should have a full understanding of the number, size and nature of any particles expected to shed in to the process system. When evaluating the number and size of particles generated, USP <788> is one example of a guideline that might be considered (i.e. for evaluating the number of ≥10µm particles and ≥25µm particles shed in to a system). If a tubing manufacturer cannot provide spallation data to guide your decision, it may sometimes be necessary to test the tube in house or work with the manufacturer to conduct further testing. Chemical analysis of shed particles may also be beneficial for adequately characterizing the risk to your process and/or product.

Extrusion Environmental Controls

When evaluating and/or auditing a tubing supplier, it can often be important to understand the environmental conditions and controls in place where the tube is manufactured. For Bioprocess tubing, cleanroom conditions are often desired for controlling the number and size of particles in the environment. However, this is only part of the equation as the manufacturer should also ideally demonstrate control of the bioburden levels in the room and on the finished product. Furthermore, in the case of thermoplastic tubes (e.g. TPE or TPV), they are often extruded into a cooling-water bath, which can be an additional source of contamination. As such, a tubing manufacturer should manage the purity of the chilling water to ensure the finished product does not harbor unacceptable levels of bioburden or other contaminants.

Material Pre-Selection Criteria

When selecting a bioprocess tube, there is various criteria that might be valuable for aiding in your decision. For example, in terms of biocompatibility, USP Class VI or ISO 10993 (various parts) testing results could indicate a likelihood that the tube will not cause biocompatibility issues during clinical testing and in the commercial product. Furthermore, to enable a more compressive risk assessment, extractables testing should also be a factor in the selection process and should be relevant to your actual processing conditions. Animal-derived-ingredient-free (ADIF or ADCF) is another factor that can eliminate some cross-contamination concerns associated with some animal-derived ingredients. Where animal-derived ingredients are present, a user should ensure that the proper processing methods have been used to ensure that transmissible spongiform encephalopathies (TSE’s) such as bovine spongiform encephalopathy (BSE) are not a concern. In addition, California Proposition 65 compliance indicates that specific banned substances are either not present or adequately disclosed. A separate list of banned substances can also be found in the current Restriction of Hazardous Substances (RoHS) directive (generally intended as a regulation for electrical equipment).

Depending on your application, you may want to research whether any compounds are present that have a track record of inhibiting performance or causing adverse effects including some plasticizers, processing aids, or reaction byproducts, etc.. Some examples include phthalates such as DEHP, latex, peroxide-curing byproducts, animal byproducts, BPA and many other compounds. Also, when considering material testing data, it is important to evaluate any material testing based on the relevance to your actual use of the product. For example, biocompatibility or exctractables testing results based on an autoclaved sample may not be representative of would be seen from a gamma-irradiated sample. In general, although sometimes meaningful for comparison between two products, testing data, such as that for extractables and biocompatibiliy, will likely differ from what you see in your actual process. As such, testing tailored to your own unique conditions and requirements may be necessary.

Peristaltic-Pumping Capability

Performance in a peristaltic pump can often be crucial to the success and repeatability of a process. To provide an objective comparison, many peristaltic-pump-tubing manufacturers provide a “recommended pump life” or “pump life” value that can range from a few to thousands of hours. It is important to keep in mind that not all tube manufacturers use the same protocol when characterizing pump life. Some of the variables that may vary include RPM, occlusion, pressure (aka back pressure), temperature, or fluid composition. Additionally, this pump life can often be defined based on when the tube ruptures, which may not represent your actual failure condition. For example, you may find that after a particular duration in the pump, although a tube has not ruptured, the flow rate has dropped over time or the tube is creating excessive spallation. As such, further testing, with failure conditions defined based on your unique requirements, may be necessary.

Concentricity

Concentricity refers to the degree which the center of a tube’s inside diameter (ID) lines up with the center point of the outside diameter (OD). Concentricity is a critical feature for peristaltic-pump tubing as non-concentric tubing can cause abnormal and unpredictable pumping performance. In addition, the pressure rating of a tube can also be affected by a lack of concentricity as the tube’s wall is ultimately thinner on one side which influences the pressure rating.

It is important to note that being in specification in terms of ID and OD does not imply concentricity. As such, if you are using tubing in a peristaltic pump, the concentricity of the tube should also be considered and a proven method for measuring/controlling concentricity on the extrusion line can be crucial to the repeatable nature of your process.

Aseptic Welding & Sealing Performance

Aseptic transfer operations are commonly found in bioprocessing. With modern technology such as aseptic welding or aseptic-connector fittings, such transfers may sometimes be conducted in non-sterile room conditions. When it comes to aseptic welding, Thermoplastic Elastomer (TPE) tubing is commonly used and is designed for this purpose. Sealing is also commonly conducted and required to sever a process line which can then be joined with another sealed tube. Many aseptic welders and sealers have built in parameters that can handle common TPE tubes on the market. When selecting your TPE tubing, selecting welding or sealing equipment that is designed to handle a particular tube can often simplify the process.

Gas Transmission

Gasses generally pass through the sidewall of a tube to a magnitude which can be approximated by looking at gas transmission rates. For example, carbon dioxode and oxygen transmission rates are readily available in the literature for many compounds. For applications where gas transmission rate is critical, it may be necessary to test the tube material. Also, due to the proprietary nature of the unique compounds used for bioprocess tubing, it may be necessary to coordinate testing with the tubing manufacturer. Although this is a less common test in the industry, quantifying gas transmission rates of the actual tube material may be helpful in terms of assessing risk since tubing in the market commonly differs in composition vs. general material samples tested in scientific literature.

Packaging

When purchasing tubing in bulk (spools, rolls, or pre-cut pieces, etc.), it is often crucial to ensure the packaging is capable of preventing contamination from the outside environment. Many packaging films including polyethylene, nylon or other copolymer films are commonly empoloyed in the industry. You may sometimes encounter multilayer films where each film layer provides a different type of protection. For example, a moisture barrier layer and gas barrier layer may be combined to provide more comprehensive protection than each layer alone. The biourden levels and manufacturing conditions of the bags should also be considered with preference generally given to those manufactured in a cleanroom.

Care should be taken to ensure the seal of the bag has been validated if applicable and that the sealing equipment and procedures are sufficient to your requirements. Vacuum sealing also tends to be an effective visual indicator of seal integrity.

Tensile Strength

Tensile strength is based on a standardized “pull” test and can be insightful when comparing the relative strength of two materials. In particular, the relative burst pressure can be approximated by looking at two tubing materials with all else the same except for tensile strength. Barlow’s formula can also be used to produce a theoretical failure pressure but should not be used in place of actual pressure testing.

Common Tubing Materials for Bioprocess Applications

There are several materials commonly used for bioprocess tubing, each with its unique properties and benefits. Some of the most common materials include:

Platinum-Cured silicone: Platinum-cured silicone is a thermoset material that is generally a purer alternative to peroxide-cured silicone. In fact, the platinum-cured reaction mechanism has no byproducts whereas the reaction mechanism via peroxide-curing can lead to byproducts which tend to be volatile organic acids. Such volatile compounds can increase the leachable contaminants to levels that are not acceptable or not ideal for bioprocessing. Although post-curing can drive down the volatile compounds of peroxide-cured silicone, platinum-curing is generally preferred in bioprocessing due generally to its higher purity. Post curing of platinum-cured silicone can still be useful to ensure full crosslinking, which generally provides more consistent mechanical performance and lower leachables.

Silicone tubing tends to have the highest temperature rating vs. other common bioprocess tubes, but tends to kink easier and have a lower pressure rating. Also, silicone tends to be porous and allows a relatively high amount of gas exchange through the tube wall, although this is typically negligible for many processes. Compounds can also leach out of the external wall of the tube; for example, if pumping some alcohols, the odor can sometimes be detected outside of the tube. The gas exchange is a function of wall thickness and can be reduced by increasing the wall. However, a material like TPE can tend to be a better gas barrier when compared to silicone.

In terms of bioprocessing use, silicone tends to be sterilized by gamma irradiation, autoclave or UV, but can also be sterilized by CIP, ethylene oxide or other methods. Silicone can be over molded or insert molded to produce a variety of flexible end connections, such as sanitary (Tri-Clamp) connections with a built-in gasket, but these connections typically require a rigid “backing cup” or “backing ring” to provide a mounting surface for clamping.

Thermoplastic Elastomer (TPE): TPE materials are common for bioprocessing and are typically designed to accommodate aseptic-welding operations. A main component of TPE materials found in bioprocessing is generally a styrene-ethylene-butylene-styrene (SEBS) block copolymer. When it comes to peristaltic pumping, this is one area where TPE can tend to fall short in that it can generally be used only for short-duration transfers, such as pumping media into and out of a media bottle. Spallation should be considered if using TPE in a peristaltic pump as the materials typically degrade much faster in the pump vs. silicone or TPV and can introduce particles into the process fluid. Caution should be used when autoclaving TPE materials as autoclave temperatures can cause irreversible changes to the material. When autoclaving, TPE can tend to become hazy and increase in opacity (especially true for temperatures nearing 135°C).

Users should ensure that their qualification testing accounts for any sterility methods that will be used, and that aseptic welding performance and the level of extractables are acceptable following the sterilization. TPV materials can be over-molded or insert molded to produce a wide range of possible rigid or flexible unitized end connections, such as sanitary (Tri-Clamp) connections and a wide variety of other connections.

Thermoplastic Vulcanizate (TPV): TPV, sometimes referred to commonly as “Santoprene,” is generally a more chemically resistant alternative to silicone and TPE, and typically offers a significantly longer peristaltic-pump life. Conveying chemicals containing methylene chloride or acetone are two examples where TPV may be preferred over silicone (not suitable for all applications). The tubing is opaque and is sometimes extruded with a polyolefin inner (process-fluid contact) layer. However, this inner layer may shorten the effective life of the tube as it can eventually crack, adding additional particles to the process fluid.

TPV materials can be over-molded or insert molded to produce a wide range of possible rigid or flexible unitized end connections, such as sanitary (Tri-Clamp) connections and a wide variety of other connections.

Co-Extrusions – Coextruded tubing has two or more layers and can include braiding or other reinforcements in between at least two of the layers. Co-extruded tubing is typically manufactured by a batch or multi-batch process, depending on the type of tubing being manufactured.  Some examples of co-extruded tubing include:

• Braided-silicone Hose – This is a common product found in the bioprocessing industry which can increase the pressure rating and kink resistance when compared to non-braided silicone tubing.
• Braided-TPE Hose – Similar to braided-silicone hose, braided TPE can provide a a higher pressure rating and may be beneficial for maintaining a TPE process-contact layer in your application. However, braided TPE hoses currently on the market are not intended for aseptic welding.
• Polyethylene lined TPV – A thin polyethylene inner wall can be extruded onto the inside surface of the tube for added chemical resistance and other benefits.
• Polyethylene-lined EVA – The combination of a thin polyethylene inner wall and an EVA outer wall can create an ultra-flexible, kink resistant, and chemically resistant tube with other unique properties.

Rigid or Semi-Rigid Tubing –  A non-flexible or “rigid” tube can be of value for many reasons including robustness, kink resistance, high pressure rating, and multi-use requirements. Depending on the wall thickness and additional monomers added to create desirable copolymer characteristics, the level of rigidity can be adjusted. A few examples of semi-rigid or rigid tube (a.k.a. pipe) include:

• Kynar and Kynar Flex – Kynar, manufactured by Arkema, is a line of polymers which are based on a polyvinylidene fluoride (PVDF) backbone. Kynar homopolymer materials tend to be rigid while Kynar-Flex is a more flexible, but highly kink-resistant co-polymer option. Both materials can be gamma irradiated and autoclaved for sterility management, although gamma irradiation can sometimes change the color of the material. Rigid, multi-use Kynar pipe is commonly cleaned in place.
• Polypropylene: Rigid Polypropylene pipe can typically be a more economical alternative for PVDF. Caution should be used when gamma irradiating polypropylene tubing or fittings as brittleness can occur and can even increase over time. For this reason, specialty “gamma stable” materials are often used for bioprocess applications. Autoclaving may be problematic when compared to higher temperature materials such as Kynar. Rigid, multi-use polypropylene pipe is commonly cleaned in place.

Conclusion

Selecting the correct tubing for bioprocess applications can be a complex task, but understanding the key factors involved can simplify the process and lead to safer, more efficient operations while enhancing the quality and safety of your final products. This article is intended as a brief introduction to many of the key factors to consider, but your selection should ultimately be based on a validation for use in your unique process application.  Additional resources and literature for aiding in tubing selection are widely available – the American Society of Mechanical Engineers BioProcess-Equipment (ASME-BPE) handbook, and the many technical guides from BioProcess Systems Alliance (BPSA)  are just two examples.

Author: Chris Ray, MBA – Founder: Virtual Alchemy

KYNAR and KYNAR FLEX are registered trademarks of Arkema, INC.

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