They are vitally important, make therapy possible, and they have become an integral part in hospitals or home-care settings: medication and feeding pumps supply patients with essential medication and nutrition. It is not only the right electronic and mechanical components that are crucial factors, but also the choice of materials for the pump segment. As in so many other areas of medical engineering, silicone plays a prominent role in these devices.
What properties and specifications must a product have in order to find a lasting place in the healthcare sector? Many products must be biocompatible, and often, they must be skin- and blood-compatible as well. Compatibility with common sterilization processes such as gamma, ethylene oxide, or electron beam sterilization, is usually a key requirement. Many products should be resistant to heat and cold. Often they must display excellent storage stability while maintaining a consistent pump performance.
It is also essential that the pump works reliably and that it provides precise pumping over long periods of time. For the use in both hospitals and in homecare settings, this means always administering the right dose, whether it is delivering drugs or being used in feeding applications. Obviously, no substances should be released from the processed material. These so-called extractables can negatively affect drug formulations. For any given pump design, the list of product-specific requirements can be quite extensive, and these requirements must be met by the selected material and its processing methods.
Silicone is a perfect fit for the requirements of precise dosing and integrity when in contact with medications. Both its physical and mechanical properties and the high degree of chemical purity in its formulation are decisive factors in this respect. Its key properties include the following:
- Outstanding resistance to high temperatures up to approximately +250 °C and down to approximately –40 °C
- Very good aging resistance
- Excellent sealing function
- Low compression set
- High permanent elasticity
- High biocompatibility
- Excellent extractables profile (smallest amounts of extractables substances)
- Suitable for all common sterilization processes
- Highly particle-free
Among its mechanical properties, the exceptional resilience of silicone plays a significant role. It has a direct influence on the tubing segment’s pump performance over the entire product lifecycle. The molecular structure of silicone elastomers can be imagined as a “spaghetti and meatball” type of structure, with the meatballs signifying cross-links.
The elasticity is construed from the ability of the long chains to reconfigure themselves to distribute an applied stress. The covalent cross-linkages ensure that the elastomer will return to its original configuration when the stress is removed. This resilience enables the pump segment to very quickly return to its original geometry after mechanical press, and it provides for a long-lasting, high rate of repetition. The profile of the cross-section of the pump segment’s inner diameter remains quite close to the original geometry at first usage.
These properties mean that the developer can be sure that the pump will consistently and precisely dose and deliver the prescribed and programmed quantity of medication or nutritional solution. To ensure this functionality, the pump segments are subjected to a qualification and validation process per the regulation or standard appropriate for the application as part of the development phase. In many cases, each lot or batch manufactured can also be tested during the production phase. The segments are tested for their adherence to in-house pump performance specifications on a custom-built test stand that has the corresponding production pump built in.
Injection Molding vs. Extrusion
The pump segments available in today’s market are manufactured in two different ways: injection molded or extruded. However, medical OEMs are increasingly demanding improved precision in delivery rates. Injection molded tubing has been shown to provide more precision than extruded tubing, and so injection-molded variants are beginning to draw increased attention from pump developers. The diameter (both inside and outside) tolerance of injected molded tubing segments can be reduced by almost half, depending on dimensions and geometry, compared with extruded tubing.
The reduced diameter provided by the injection molded tubing has had a direct and positive influence on the dosing accuracy of individual pump segments in the range of 1 percent. Combined with the development of the appropriate silicone material formulation in terms of Shore hardness and the type and degree of cross-linking, extremely high dosing accuracies can be achieved for the pump system as a whole over the course of the product lifecycle. The silicone formulation is determined based on the pump system, its mechanism, part design, etc. The perfect raw material formulation has to be adjusted to every single pump system. The primary advantages of injected molded tubing over extruded tubing include the following:
- Provides for no component or internal assembly costs
- Eliminates finishing work, such as cutting and stamping processes
- Produces precision-shaped edges of the component with no sharp corners
- Creates very narrow tolerances, which enables very high dosing accuracy
- Includes a section for air bubble detection
- Provides integrated connectors, using the poka-yoke principle to eliminate product defects by preventing, correcting, or identifying human errors as they occur
- Enables a high degree of design freedom
- Provides for improved anticounterfeiting
- Enables the development of pumps that ensure maximum patient safety
Injection molding of silicones provides batch-to-batch stability, process repeatability, and direct injection, which eliminates waste. Injection molding often involves automated processes and systems, shortening cycle times and minimizing errors.
The intelligent use of silicone injection molding technology allows for a wide range of features to be integrated into a single part. The pump developer must assess the needed requirements of the application and determine which features can be integrated into the injection molded part. Previously, such features had to be provided by integration of a more complex component.
Medical pumps play a critical role in the care of patients in hospitals as well as in home-care settings. These pumps must work reliably and provide precise pumping over long periods of time. As OEMs demand improved delivery rates, they have looked to injection molded tubing, which has been shown to provide more precision and improved reliability.
While extrusion and injection molding each has its own advantages and disadvantages, injection molded tubing segments have met the need for the precision demanded of pump developers. Injection molded tubing provides smaller inside and outside diameters as well as other advantages compared with extruded tubing so that the pump system can achieve extremely high dosing accuracies over the course of the product lifecycle.
This article was written by Markus Rössler, product manager for Raumedic AG, Helmbrechts, Germany. For more information, click here.