Catheters are common medical devices with a variety of different designs and applications but generally speaking are sterilized disposable devices made up of a long, hollow, flexible tube with elements at one or both ends.

Due to their long, narrow nature, the sterilization method is deeply intertwined with overall catheter design. Generally, long lumens as found in catheters would mean that gas sterilization (such as ethylene oxide—EtO) sterilization is not advised. However, that’s not the real story. Let’s find out more.

Radiation and EtO

Firstly, let’s talk about the theories behind radiation and EtO.

Radiation sterilization is a method that utilizes high powered gamma, X-ray, or electron beams to kill microbes. The main advantage of radiation is that it penetrates through device walls. Disadvantages relevant to catheters include the fact that the radiation can discolor and alter the properties of materials. Often, it makes plastic yellower and stiffer/ more brittle.

EtO is a gas that kills microbes on contact. The gas must be moved inside the sterile barrier through a membrane, make contact with every internal and external sterile surface of the device, then be sucked away. The main advantages of EtO are that it is relatively inexpensive, commonly available, low temperature (<63^oC), and it doesn’t alter the polymer properties as much as radiation for many materials.

Disadvantages relevant to catheters include residual chemicals and that the gas must reach every spot in the catheter to kill the microbes. Furthermore, the process includes long (hours) dwell time in warm and humid conditions, which can affect some materials.

In summary, radiation stiffens the material, and EtO is difficult to get into every internal surface. So, which is better for a long, flexible catheter? As always: it depends.

Radiation Cross-Polymerization

Now we know that radiation embrittles polymers. You can select materials that are less affected by radiation but even then, embrittlement is generally a bad thing. One approach is to attempt to account for these effects, which is possible because the phenomenon is fairly repeatable, and you can work backwards to choose a “softer” material that will stiffen to the desired amount during irradiation. You can do this by estimating the change in material properties using published data or test specific samples of your material pre- and post-irradiation and compare.

The challenge is that more than one material property (besides brittleness) is affected by sterilization, and some effects cannot be fully countered. Lastly, metal elements are less compatible with radiation due to shadowing and reflecting effects.

Gas Penetration

On the other hand, getting EtO down a small, coiled lumen isn’t trivial. The longer and narrower the tube, the longer the system will need to dwell in the gas environment to achieve complete coverage—which can stress the device if materials are not highly compatible with EtO—and the sterile barrier.

Further, EtO involves the gas contacting all portions that need to be sterile and then sucked away with a vacuum. This leaves some residual chemicals behind which can have negative health effects and must be kept under certain limits (see ANSI/AAMI/ISO 10993-7:2008/(R) 2012). Long, narrow lumens are therefore not only more difficult to fill with gas but also more likely to retain residuals, often requiring a longer evacuation time to attempt to meet limits.

Alternate Methods

There are a number of other sterilization methods in addition to the two noted above. Many are fluid methods which have the same weakness as EtO: The need to get a compound down a long tube.

Another method is dry heat which, like radiation, penetrates through walls. Dry heat could be a good choice when the chosen materials can withstand temperatures of 150-170^oC for upwards of an hour. For example, using heat-resistant silicone for the tubing might seem like a trivial solution, but the other factor to remember is the sterile barrier must also survive this temperature cycle, and many common types (PE, BON, Tyvek) are not compatible with those levels of heat. Therefore, by choosing dry heat you may be substituting one issue for another.

Choosing What’s Best

As with many things in life, there is no one-size-fits-all approach to sterilizing catheters. As we’ve seen, shorter and wider lumens that need to maintain material properties are more suitable for EtO, whereas long and narrow lumens that can still perform after embrittlement are more suitable for radiation. Dry heat is also an option, if the materials of both the device and its sterile packaging can withstand the stress.

In the real world, catheters are often effectively sterilized using either EtO or radiation. Designing for the chosen sterilization method as part of conceptualization, mitigating sterilization challenges applicable to the chosen method through design, and thorough testing are all keys to successfully designing sterilized catheters.

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