Ethylene Oxide (EtO) sterilization is mainly used to sterilize medical and pharmaceutical products that cannot support conventional high temperature steam sterilization - such as devices that incorporate electronic components, plastic packaging or plastic containers.
EtO gas infiltrates packages as well as products themselves to kill micro organisms that are left during production or packaging processes. This gas, mixed with air at a ratio of at least 3% EtO gas, forms an explosive mixture. Pure EtO gas boiling point is 10.73 ºC at atmospheric pressure. Most of the time, it is mixed with Nitrogen or CO2. This explosive condition requires Intrinsic Safe material (ATEX) zoning, for security of people as well as security of the process itself.
Safety of personnel is an important issue due to the harmful effect of EtO on humans. Polluted areas need to be alarmed using gas detectors set up at different locations to monitor any leak. Visual and audio alarm systems need to be provided. The system must inform any operator when a sterilization cell contains EtO.
When this toxic gas is removed from the room it needs to be treated using thermal burners, scrubbers or oxidation for environmental protection or be transported to an alternate facility for treatment.
EtO Sterilization process:
Most EtO sterilization lines involve three different stages. These can be separated into three different cells depending on the size or amount of devices to treat:
- Pre Conditioning
When cells are separated, automated loading/unloading systems are required. These save operator time as well as giving protection from exposure to a polluted environment, which could be detrimental to health.
Pre Conditioning Stage
First, products need to go through a pre conditioning phase to make micro organisms grow. The batch load goes through a dwell time under a controlled environment of:
Then the load goes through a long and complex sterilization cycle. Requirements of such a system are:
- Accurate temperature control.
- Availability of the control system.
- Accurate pressure and vacuum control.
- Easy displays of process phases
- Dedicated customer recipes.
- Auto batching release through tolerance tests.
- Security interlocks between actuators.
- Shut down strategies.
- Audit Trail facilities – Trending.
- 21CFR Part11
During this cycle, accurate temperature control is important and a heating jacket is used. As the overall duration of this cycle is around 60 hours, high availability of the system is vital and system redundancy is required. Doubling sensors, actuators and controllers as well as changeover facilities on these components, helps to ensure the product is sterilized even on hardware or software failure.
After the doors have been shut down and sealed correctly, the cycle can be started either manually or automatically. If any problem with door sealing is detected the cycle is interlocked and cannot start. Security interlocks are also used between air and EtO valves.
Once the cycle is started, easy to use displays are required to show:
- The actual phase of sterilization
- All the key set points and tolerances as loaded by the recipe
- All the key process values for the auto batch release facility
Control of vacuum and pressure is also required. Due to the toxic effect of EtO, water ring rotary pumps are used. The vacuum process needs to perform the emergency evacuation phase for a fast evacuation of gas.
The sterilization phases are:
- Cycle start delay to enable the system to start in stable conditions
- General cell temperature check
- Initial vacuum phase
- Leak rate test
- First flush
- Second flush
- DEC (Dynamic Environmental Conditioning)
- EtO gas injection
- Sterilization dwell time period under EtO
- Post dwell vacuum level
- First wash
- Second wash
- Final air admission
- Final chamber re-evacuation delay
During execution of these phases a batch report is generated. This report will include: tolerance checks, phase changes, alarms, events and critical process values. A key feature of the system is “auto batch” release. During the sterilization cycle if any abnormal condition occurs, the batch will be automatically stopped and condition(s) causing the stoppage will be identified. With this “auto batch” release facility operators do not have to wait until the end of the cycle and spend time going through the batch report to understand why it went wrong. With this feature, provided that batch is completed satisfactory it will be automatically forwarded to the degassing room without human check of tolerance, process values and alarms.
For each batch the operator selects appropriate product recipe. After recipe has been downloaded, the operator is given the opportunity to check if values are correct for this particular batch before starting the cycle.
When the batch is over an automatic print of the report can be performed. Batch logged files are also archived electronically for future review. Batch logged files could be searched by the following:
- Batch ID
- Customer name
- Product type
- Start and stop time
Finally, products need to go through a degassing phase to remove any particle of EtO. The batch load goes over a dwell time under a temperature controlled environment