A Comparison of 100% Ethylene Oxide Cycles and EO+ Cycles Within an EO Sterilization Processing Chamber

Background 

The ethylene oxide (EO) sterilization process consists of three phases: preconditioning, sterilization, and aeration.

This TechTip will discuss the similarities and differences of 100% ethylene oxide cycles and EO+ cycles during each phase while outlining process variables and considerations. 

What are 100% ethylene oxide cycles? 

100% ethylene oxide cycles utilize 100% EO gas injection operating in a vacuum using nitrogen as a diluent. 

What are EO+ cycles? 

EO+ cycles use a pre-mix of EO and carbon dioxide (CO2) gasses with 10-15% of the mixture being EO.

These cycles operate in both vacuum and positive pressure (above atmospheric pressure) dwell. 

What is preconditioning? 

Preconditioning is defined as “treatment of product, prior to the sterilization cycle, in a room or chamber to attain specified conditions for temperature and relative humidity.² ”

The process variables for preconditioning include at a minimum: 

• time 
• temperature 
• humidity 
• transfer time 

Both 100% EO cycles and EO+ cycles can deliver the same programmed preconditioning parameters. 

What are EO sterilization cycles? 

An EO sterilization cycle is defined as “treatment in a sealed chamber, which includes air removal, conditioning (if used), injection of ethylene oxide, inert gas (if used), exposure to ethylene oxide, removal of ethylene oxide and flushing (if used), and air/inert gas admission.¹ ” 

The process variables for an EO sterilization cycle include:

• exposure time
• temperature
• humidity
• EO concentration
• pressure 

The differences between 100% EO cycles and EO+ cycles are observed within the sterilization chamber.

Both 100% EO cycles and EO+ cycles can be programmed to deliver the same exposure time, temperature, humidity and EO concentration parameters.

The difference between the two systematic approaches/system designs may be summarized as follows: 

	100% EO process	EO+
EO sterilant supply	100% EO gas	10-15% EO gas
Pre-mixed diluent	None	CO2
In-cycle diluent	Nitrogen	None
Cycle pressure	Vacuum only	Vacuum and overpressure

After the initial vacuum to remove the air from the chamber, the 100% EO cycles require inert gas flush(es) to replace air with an inert gas, such as Nitrogen.

This is not required for positive pressure EO+ cycles as the injected EO/CO2 blend prior to dwell is already in a diluted concentration that falls below the 3% LEL (lower explosive limit) for EO gas.

Therefore, EO+ cycles can offer reduced process cycle times compared to 100% EO cycles that potentially require inert washes to bring them below LEL levels.

Due to the injection of 100% EO, a smaller injection is necessary to attain the desired EO concentration in the chamber during a 100% EO cycle, resulting in an operational pressure that is maintained under vacuum. 

After the EO exposure phase, the 100% EO cycle employs the use of a post EO exposure vacuum followed by inert gas flushes/washes to (1) reduce the level of EO in the chamber to below LEL prior to addition of air, and (2) assist in the removal of residual EO from the load.

After successfully removing the EO from the chamber, the inert gas is removed via another series of air washes before final air admission to atmospheric pressure.

The positive pressure EO+ cycles have a much larger EO/CO2 injection due to the blend of the EO/CO2 being of lower EO% concentration.

Therefore, to reach the desired operational EO concentration during the EO exposure dwell phase, a larger injection is necessary, which elevates the chamber pressure to above atmospheric pressure. 

The positive pressure EO+ cycles do not require inert washes either before or after EO exposure phase as the EO/CO2 blend used is already below the LEL value of EO.

The positive pressure EO+ cycles require only air washes before final air admission to atmospheric pressure, to remove residual EO from the load.

What is aeration?

Aeration is defined as “part of the sterilization process during which ethylene oxide and/or its reaction products desorb from the medical device until predetermined levels are reached.² ”

The aeration area is defined as “either a chamber or a room in which aeration occurs.¹ ” 

The process variables for aeration include at a minimum:

• Time 
• Temperature 

Both 100% EO cycles and EO+ cycles can be programmed to deliver the same aeration parameters. 

In both approaches, aeration temperatures are variable by way of system programming or equipment capability, dependent on product or Customer requirements.

Conclusion

 Based on the table below, qualification and release criteria are developed similarly among both EO sterilization approaches. 

• Both process variants are subject to IQ, OQ, and PQ in accordance with the ISO 11135 requirements and are capable of delivering equivalent lethality to the process
• Both cycle types can deliver a stable, controlled and monitored cycle, compliant with parametric release requirements
• Products being sterilized should be assessed to be unaffected by the pressures achieved in either process
• Due to the standardized method of Sustainable Ethylene Oxide® sterilization cycle development, both systems have proven to produce low levels of residuals after processing

Table Comparison 

100% EO Vacuum Dwell vs. EO+ Positive Pressure Dwell

Stage	100% EO	EO+ Positive Pressure (10-15% EO/ 85-90% CO2)	Comment
Preconditioning
Time	Same
Temperature	Same		Settings are product dependent based on sensitivity to temperature and humidity
Humidity	Same
Transfer Time	Same
Sterilizer
Chamber Temperature [Temperature]	Same		Settings are product dependent based on sensitivity to temperature and humidity
Depth of Vacuum [Pressure]	Same
Nitrogen Flush	Required	Not Required	N2 flushes required for safety
Conditioning [Humidity]	Same
EO Injection Pressure [Pressure]	Below atmospheric	Above atmospheric
Concentration [EO Concentration]	≤ 400 mg/L	≤ 400 mg/L
Sterilization Dwell Pressure [Pressure]	Vacuum	Positive pressure	Some products/packaging may be sensitive to positive pressure
	<101.3 kPa (Typically < 90 kPa required for safety)	>101.3 kPa
EO Dwell Time (hh:mm) [Exposure Time]	Based on validation results (Typically 03:00 – 06:	Based on validation results (Typically 03:00 – 06:00 )
Nitrogen Washes	Required	Air washes only	N2 flushes required for safety
Air Washes	Same
Aeration
Time	Same		Product dependent on meeting EO residual limits in ISO 10993-7
Temperature	Same
Qualification
Process Qualification	Same		ISO 11135
Parametric Release	Same		ISO 11135
Biological Indicators	Same		ISO 11138-1 & ISO 11138-2
EO Residual Limits	Same		ISO 10993-7

Graph Comparison

References: 

1. ISO 11135: Sterilization of health-care products – Ethylene oxide - Requirements for the development, validation and routine control of a sterilization process for medical devices
2. ISO 11139: Sterilization of health care products - Vocabulary - Terms used in sterilization and related equipment and process standards
3. ISO 11138: Sterilization of health care products — Biological indicators
4. ISO 10993-7: Biological evaluation of medical devices — Part 7: Ethylene oxide sterilization residuals
5. “Sustainable EO® Sterilization | STERIS AST.” STERIS, www.steris-ast.com/solutions/sustainable-eo-sterilization-services/