THE PROCESS

Unlike other methods of sterilization, such as gamma, EO sterilization process may be lengthy. Products are preconditioned prior to EO exposure. This preconditioning may occur in the sterilization chamber itself, or in a separate Preconditioning Room (or chamber). The preconditioning process consists of a temperature and humidity settings determined during the sterilization validation. After a set time the product is removed and placed into the sterilization chamber.

The properties that make EO such a good methylating agent also makes it extremely dangerous at ambient oxygen levels. To ensure an intrinsically safe environment for the EO a set of evacuations coupled with steam additions is executed. Adding nitrogen to the chamber is another way to reduce the danger as well as allowing EO to sit with the product at close to ambient pressures.

EO gas is then added and allowed to sit with the product. Historically, the amount of gas added was set to a particular weight. However, the evolution of the process has gradually gone to adding gas to a validated pressure (concentration of gas).

During this ‘sitting phase’ or ‘Gas Dwell Phase” the product absorbs EO gas. To maintain the gas concentration and offset the absorption - additional gas is added.

Following the Gas Dwell Phase a series of evacuations and air imbleeds occur. This helps in the removal of gas from the product. The product is then transferred to an Aeration Chamber (or room) where EO (and EO degradation products) dissipates safely from the product. Depending on the nature of the product/packaging and aeration conditions this gas dissipation period may last from several hours to weeks (and even months).

Important Standards

Many of the standards that have been developed to describe and guide the sterilization process have been mostly driven by AAMI and ISO. If your organization hasn’t done so already the following standards should be purchased. Depending on whether or not you will require EO sterilization or Gamma or both you can choose appropriately:

• Sterilization of healthcare products —Requirements for validation and routine control — Radiation sterilization, AAMI/ISO 11137
  
• Sterilization of healthcare products — Radiation Sterilization — Selection of a sterilization dose for single production batch, AAMI/ISO TIR No. 15844
  
• Medical devices — Validation and routine control of ethylene oxide sterilization, AAMI/ISO 11135
  
• Sterilization of medical devices Validation and routine control of ethylene oxide sterilization, EN550 (European standard)
  
• Biological evaluation of medical devices — Part 7: Ethylene oxide sterilization residues, AAMI/ISO 10993-7
  
• Sterilization of medical devices — Microbiological methods, Part 1: Estimation of population of microorganisms on products, AAMI/ISO 11737-1
  
• Sterilization of medical devices — Microbiological methods, Part 2: Tests of sterility performed in the validation of a sterilization process, AAMI/ISO 11737-2
  
• Biological Evaluation of Medical Devices—Part 7: Ethylene Oxide Sterilization
  
• ETO Residuals, ANSI/AAMI/ISO 10993-7
  
  

Ethylene Oxide Testing

Product Sterility Testing

• Product Sterility Testing is done to determine if any viable microorganisms remain on product following sterilization. Care should be taken when removing samples to prevent inadvertently contaminating product.
  
  
  

Ethylene Oxide Testing

• This test is used to determine the concentration of Ethylene Oxide as well as it’s two breakdown products: Ethylene Chlorohydrin and Ethylene Glycol. In recent years testing is done to the ANSI/AAMI/ISO 10993-7 standard. The upside to this method is that it strives to simulate the actual use of the product. Concentrations vary based on whether the device comes in contact with skin, blood/mucosa or areas that may come in contact with the fetus.
  
• Historical testing was done to determine the overall concentration and was not based on use. Acceptable levels using this method were posted by the FDA.
  
• When conducting EO testing it is important to record ambient levels of temperature and air circulation rates.
  
• When removing, samples should be placed under cold conditions and sent to the lab as soon as possible. The products are shipped under dry ice to reduce EO dissipation.
  
  
  

Bioburden Testing

• Bioburden testing should be a routine process. It is important to establish and maintain a schedule for retrieving non-sterilized samples. The samples should be taken from products that undergo the same manufacturing process as the rest of the products. It is usually customary to maintain a Statistical Process Chart. Any anomalous readings should be investigated and corrected.
  
• Identification of the organisms should be done at this time.
  
  
  

Fungistasis/Bacteriostasis Testing

• It is well known that certain product materials can cause microorganisms to remain ‘alive’ and yet keep them from reproducing. This test is used to determined the recovery percentage and will be used to correct Bioburden results.
  
  
  
  

Pyrogen Testing (also known as LAL testing)

• Pyrogen testing is used to ensure that the device is free from bacterial endotoxins. It is conducted for lot release purposes and testing to satisfy biocompatibility requirements. For lot-to-lot release Limulus amebocyte lysate (LAL) is often used.
  
  
  
  

Product Package Testing

• There are a number of tests that are conducted to evaluate the outer packaging of a device. One such test is Aging. Aging is done to determine the expiry date of products. Sometimes it may be convenient to do an accelerated aging test.
  
• Another important product packaging test is Burst Testing. Burst testing is especially important when using EO as a sterilizing method. The test involves measuring the force required to burst a package.
  
• The Peel test is used to measure the force required to peel back the packaging (usually Tyvek or paper, from plastic components).
  
  

Biological Indicator Testing

• Biological indicators are strips of bacteria – usually Bacillus subtillus, a bacterium found to be resistant to ethylene oxide. They are used to challenge the sterilization process and are used during validation and routine processing. It is recommended to present the test strips into ‘challenge packs’ to mimic (or surpass) the challenge posed by the worst locations (torturous paths) of the medical device.
  
  
  
  
  

Validation (Equipment Commissioning and Sterilization Validation)

Validation is a requirement for EO sterilization of medical devices. To effectively cover validation you need to demonstrate operation qualification and process qualification. Your EO sterilization provider should also be capable of demonstrating installation qualification of the equipment.

To demonstrate operation qualification of equipment your sterilization provider will often conduct “Empty chamber studies” whereby temperature probes are placed spread out throughout the chamber. A cycle is then run while the probes record the temperature at set intervals. This information is plotted and the spread is obtained to demonstrate the variation. Several factors go into determining the variation of temperatures throughout the chamber such as blower efficiency and whether or not the water jacket (used to heat the chamber) is functioning correctly. Many device manufacturers will often use this information to determine ‘cold spots’ within the chamber. Biological indicators are used at these locations to monitor the process during the actual process sterilization (with product).

Half Cycle

During the process qualification a load of medical devices is subjected to a sterilization cycle with only half the time required for Gas Dwell. During this time the product is seeded with biological indicators and temperature/humidity probes. The intent is to have all the biological indicators die following processing. The temperature probes are used to record the temperature within the product during this time. Though there isn’t any correct temperature/humidity distribution the intent is to obtain a representative description of how the cycle temperature is penetrating the product. An old ST27 standard once set the spread of the temperature within the product to a maximum of 18 degrees Fahrenheit. This spread factor is still used by many manufacturers. Some manufacturers will use the temperature readings to determine where to place biological indicators during routine processing. The idea being that a higher temperature adds to the killing effect of EO.