Choosing pump tubing successfully results in a system that delivers optimal performance and prevents redesign and troubleshooting later. For medical device applications, custom OEM peristaltic pump systems are common for fluid transfer. Designing a new medical device requires precise criteria for flow rate and pressure as well as regulatory compliance. More often than not, this necessitates a custom-designed pump system. (See Figure 1)
The tubing transporting fluid through these systems is just as important as the pump itself—from material formulation to tubing size to the impact of pump speed. To make a well-considered selection, remember the acronym STAMP. Each letter refers to a critical component of the selection process, as described below.
S = Size
While considering the inner diameter (ID), outer diameter (OD), and wall thickness, along with size, keep in mind cut lengths as well. ID, OD, and wall are significant factors in pressure and vacuum ratings. Tubing tolerances may vary by manufacturer and will affect reproducibility and repeatability. The component tolerances are also a concern for leak points and critical dimensions of the design. Understanding connection options will help ensure a proper fit and eliminate problems later in the process.
T = Temperature
Temperature impacts the tubing selection process greatly. Operating temperature is usually a consideration, but cleaning temperature, ambient temperature, and the complete temperature fluctuation should also figure into a selection decision.
Manufacturers and sellers may specify two different tubing temperature ranges. One range is for static applications in which the tubing is simply used as a transportation path for the fluid. The second is for more dynamic uses in which the tubing will be physically stressed in the occlusion bed of the pump, thus raising temperatures along with pressure.
Sudden changes or extreme temperatures can negatively affect performance and cause premature failures. This typically results in choosing a higher-cost material, but exploring multilayer products can help ease this cost while still meeting performance expectations.
A = Application
Understanding the application is the most complex part of the selection process. Users might not be sure what they are looking for or may be concerned about disclosing too much information about a breakthrough product. Yet, knowing how the tubing will be used is critical to making an optimal selection.
Regulatory concerns are at the top of the list, including knowing what documentation is required. USP, European Pharmacopeia, FDA, EU Food, USDA, 3A, ISO, and NSF all carry different compliance criteria, which warrant different material selection. Proper supporting documentation and certificates should be reviewed before placing an order, with all of the details agreed upon prior to the first shipment. Missing documentation will only delay the process and create unnecessary aggravation.
Knowing the dynamics of the application will help in selecting material for clarity, durometer (relative hardness), flexural fatigue or abrasion resistance, and peristaltic pump use. Materials are available to address each of these factors individually and multilayer options solve a combination of them.
For medical devices, determining sterilization method and chemical compatibility are necessary. What method is available for sterilizing the end assembly of the fluid path? A user’s options include gamma irradiation, ethylene oxide (EtO), and autoclaving. Some tubing formulations will withstand some methods better than others. (See Figure 2)
Silicone is a common tubing material used in pump systems for medical devices, yet other formulations may offer better chemical compatibility or physical characteristics. When silicone is exposed to a solution containing phosphoric acid, a popular cleansing agent, it can weaken the tubing. The manufacturing process of creating silicone tubing uses phosphoric acid. Even 0.5 percent is enough to weaken the tubing structure. Weighing the advantages of thermoplastic elastomer (TPE) formulations such as C-Flex®, Puri-Flex™, AdvantaFlex®, Pharmaprene®, or PharMed® BPT may produce a more desirable performance.
M = Media
Identifying the fluid or media being transported is necessary to minimize extractables, adsorption/absorption, and particulate entrapment. Various chemicals attack materials at different rates, so understanding the complete use of the product will enable appropriate selection. Often, cleaning agents are the most aggressive fluids used, but be aware of any chemical reactions that might be caused by changing fluids. Ambient chemicals present in the air surrounding the tubing product or dripping via leaks or condensation are sometimes overlooked. If the fluid is very aggressive, multilayer products can assist with barrier protection and decrease overall material cost.
Some materials can be produced at tighter tolerances, which optimizes pump accuracy, while others can be produced with a smoother inner surface for better flushing.
Safety is also paramount in material selection. Some requirements mandate viewing the fluid being conveyed. This means the material must have a sufficient level of clarity to see the fluid path and distinguish if any material is present. On the other hand, some medications are very susceptible to light, requiring a more opaque tubing formulation such as TPE.
Other options require identifying marks for various chemicals through the use of colored tubing products or specific marking text. This is a trend in efforts to further companies’ safety enhancements. Also, all tubing materials are permeable to gases. Ask suppliers for permeation rates.
P = Pressure
Vacuum and positive pressure both create stress on any tubing product. Ignoring these can create hazardous conditions during product use. Increased or decreased temperatures greatly reduce the pressure rating for materials and proper measures must be taken.
Changing the wall thickness can help increase pressure rating, as can reducing the overall OD/ID ratio. If this isn’t an option, multilayer products of various materials can help handle the desired pressure, as well as maintain the proper condition in the fluid path. Cleaning cycles can also incite extreme pressure conditions and must be considered as well.
In many medical device applications, the high pressure generated by peristaltic pumps is suitable for short cycles. For example, ablation devices have small tips that often require cooling to prevent surrounding tissue damage. The capillary-size fluid paths generate friction, demanding a pump that overcomes this friction and delivers enough flow to remove the generated heat. Medical device users should test all possible tubing materials for an appropriate life span with a suitable service factor. This will prevent catastrophic failure during a procedure.
Final Thoughts: Overmolding and Quality
In the pump setup for a medical device, there may be a need to install fittings in the fluid path. One way to accomplish this without adding a disruption in the fluid path is by “overmolding.” This process allows the joining of tubing into custom assemblies without a traditional fitting in the fluid path. Thus, the fluid path remains the same material, with very little area for “dead volume” in which contaminants can accumulate. Some tubing formulations allow for overmolding while others do not.
Underlying all of the above considerations is a somewhat obvious essential: quality is critical. Ask the tubing manufacturer about their quality process, take plant tours, and see how the product is designed and produced. Should something go wrong in the field, make sure your manufacturing partner has all the supporting quality documentation.
This article was written by Gregg Johnson, Senior Marketing Manager, Eric Nofziger, Category Manager, and Beth Dumey, Senior Marketing Communications Specialist, all at Cole-Parmer, Vernon Hills, IL. For more information, Click Here
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