Samples being loaded into Ethylene Oxide Joslyn vessel.Ethylene oxide and radiation sterilization make up two of the most common forms of sterilization for medical devices. As the device is prepared for release to market, knowing which type of sterilization to use for each product can save time and money.

Basic Differences
Ethylene oxide is a highly reactive gas sterilant. It is effective at low temperatures, unlike other sterilization modalities. Ethylene oxide sterilization validations are most commonly performed using the overkill method. This means that sterilization processes are designed to kill one million spores of the most resistant organism (MRO), Bacillus atrophaeus, in one half of the normal “routine” cycle time (full cycle).

Radiation differs from ethylene oxide in that it is a gamma wave, electron beam, or x-ray instead of a gas. The radiation sterilization process is bioburden based, as opposed to overkill. Manufacturers can determine the amount of radiation necessary to sterilize their product based on the product’s bioburden count. A bioburden test must be performed to determine the proper verification dose to be used for the test of sterility before a sterilization dose can be considered validated.

Compatible Materials
Ethylene oxide is compatible with most materials. Ethylene oxide gas is a contact sterilant, so complex devices having long lumens, mated surfaces, or other hard-to-kill designs may require longer exposure times to be effective. Ethylene oxide can be used to sterilize metal and glass surfaces, but it will not permeate through these materials. Ethylene oxide cannot sterilize or permeate liquids, so some lubricants may present a problem for sterilization.

A view of radiation sterilization from underwater.Radiation is also very compatible with most materials that are used in manufacturing medical devices. Recognize, however, that devices containing acetal, polypropylene, or Teflon will not be compatible. Radiation sterilization can cause discoloration of glass and plastic, which would be a visual problem, but it won’t ruin the device. Certain plastics may become brittle, while some polymers may become stronger. Devices with circuitry may also be damaged. Radiation penetrates very well and doesn’t have problems with liquids or metals.

When first designing a product, take into consideration sensitivities that the materials may have with the desired mode of sterilization (i.e., temperature, humidity, etc.). If it is known ahead of time which materials may be affected by the sterilization process, accommodations can be made during design.

Selection Method
First, focus on the material compatibility for the desired sterilization method. Many companies will choose the method they are most comfortable with because they have always used it; however, this may not be the best decision for the product. When unsure which sterilization is right for a product, the Association for the Advancement of Medical Instrumentation has a great resource document, “AAMI TIR 17:2008 - The Compatibility of Materials Subject to Sterilization.” This document was expanded to cover every type of modality that may be selected when sterilizing a device. Within the document, a search can be performed for the materials used in a product and their compatibility with a certain type of sterilization method can be determined.

The second important consideration for choosing the right sterilization method for a device is the timeframe for performing a validation. A radiation validation can be completed in approximately seven to eight weeks. An ethylene oxide process validation will take approximately 12 weeks. After the validation, there are also differences in maintaining these processes. To verify that the established radiation sterilization dose is still valid, quarterly dose audit testing must be performed. Medical device manufacturers need to make sure that this process is in place once their sterilization dose has been validated. An ethylene oxide sterilization validation requires an annual evaluation of the sterilization process and the device. Any changes to the device configuration, packaging, loading configuration, or sterilization process must be evaluated for impact to the validation.

If questions arise regarding whether or not a certain sterilization method will work well for a product being developed, check with a medical device testing consultant before moving forward.