by Dan Floyd, RM(NRCM), Sterilization Section Leader, Nelson Laboratories
Recently we wrote about considerations manufacturers need to know about radiation sterilization. Today we are covering Ethylene Oxide sterilization. Together they make up two of the most popular forms of sterilization for medical devices.
Ethylene Oxide is a gas at room temperature. Its reactivity and toxic nature makes it an ideal sterilant. It works by literally poisoning microorganisms by interacting with nucleic acids and other cellular processes. It has many industrial uses one of which is sterilization. It is a very efficient form of sterilization and has been used to sterilize medical devices since the late 1940’s giving it a long track record.
Ethylene Oxide sterilization is well known in the industry and is accepted as a sterilant by the FDA. It is a great form of sterilization for medical products that are heat sensitive or sensitive to radiation—such as polymers and plastics that can melt, become brittle or deteriorate.
There are four variables that need to be controlled when using ethylene oxide as a sterilant: Temperature, relative humidity, gas concentration, and exposure time. All are controlled in a vacuum tight sterilization chamber designed to hold loads ranging from a few medical device samples and packaging to several pallets for sterilization.
The optimum relative humidity is 30-35 percent at the spore level. The typical RH set point for industrial sterilization cycles is around 65 percent to assist with sterilant penetration through packaging. Humidity is also important to help facilitate the alkylation reaction. Packaging must be breathable so the gas and humidity can penetrate to the product.
Nelson Laboratories provides a wide range of ethylene oxide testing to help manufacturers determine the variables to properly sterilize their products. Once the sterilization parameters are defined, Nelson Labs can perform or assist with the validation process. The sterilization process is an overkill one, which means the validation determines the levels and exposure of the gas to the product necessary to kill microorganisms in half the time that will be used in the sterilization process.
Since Bacillus atrophaeus spores are accepted as the most resistant to ethylene oxide, it is used as the indicator organism. To validate the sterilization cycle, one million spores are placed at the most difficult location on a medical device to sterilize. The inoculated test samples are then put into the chamber along with the load of product to be sterilized. After the process the inoculated device is sterility tested to see if any living spores can be found. If not, it is considered a pass. The validation test must be done three times, often with the parameters set to deliver less lethality than the routine cycle. If all three cycles pass, then the variables are set properly to sterilize the product.
If there are any failures then some of the variables need to be changed and the validation process repeated. The most common variable to be changed is exposure time, but can also include heat, humidity, and the amount of gas. Increased exposure time can usually ensure sterilization takes place.
Keep in mind, additional validation cycles needs to take place for different configurations or product mixes to ensure sterility in those situations.
Nelson Labs provides the validation tests, but nearly as important is the fact the company can educate the manufacturers on the sterilization standards and assist in fixing failures—consulting for new manufacturers or for established manufacturers who are building in new processes.