Ethylene oxide is a colorless, flammable gas that reacts with water, organic and inorganic acids (such as amino acids, which make DNA), alcohols, phenols, and most of all amines. When it reacts with these different chemicals it undergoes a process called alkylation. Alkylation is a chemical reaction where a hydrogen atom is replaced with a hydroxyethyl radical. This process causes the amino acids that are found in proteins to change their shape. These proteins typically penetrate the cell membrane, forming a channel between the interior of the cell and the outside of the cell. When these proteins change their shape it causes a breach in the cell membrane, killing (lyzing) the microorganism.
The excellent microbicidal activity of ETO has been demonstrated during the past decades. ETO inactivates all microorganisms although bacterial spores (especially B. atrophaeus) are more resistant than other microorganisms. This means that when the conditions are no longer optimal for growth they will be able to form a spore structure. Environmental changes that may trigger sporulation can include temperature changes, humidity changes, a lack of nutrients, or chemical changes. By forming a spore the bacteria can protect themselves from the harsh environment until the conditions improve. For this reason B. atrophaeus is the recommended biological indicator for effectiveness validation of sterilization by EtO.
Today, with the evolution of complex medical devices, whether single use or reusable, the use of Ethylene Oxide (EtO) became the most common modality for sterilization process. The development of sterilization process with EtO serves as an ideal solution for variety of compounds components such as synthetic and thermoplastic polymers, fabric, electronics, optical components and other compounds that exhibit malformation and degradation at high temperature exposure during steam power process. The process is as well more readily available in comparison to irradiation due to cost effectiveness.
The effectiveness of sterilization process with EtO is influenced by concentration, temperature, humidity end exposure duration. These parameters are optimized according to the properties and characteristics of the product being sterilized. Products are packed at their final breathable package which enables the penetration of EtO during sterilization process and act as microbial barrier preventing penetration of organisms to the product following sterilization during its shelf life. Packed products are sterilized in a dedicated chamber capable of controlling the sterilization process parameters. Normally a sterilization process will include 5 stages:
Conditioning and Humidification – bringing the chamber and product load to defined temperature and relative humidity environment. Humidifying the chamber volume prior to gas introduction is necessary since EtO is carried to reactive sites by water molecules.
EtO introduction and exposure – Product load is exposed to EtO while predefined parameters are met, and usually at least 1 hour at temperature between 50-60° C or at least 3 hour at lower temperatures.
EtO evacuation – the gas present in the chamber is evacuated using closed pressure controlled air flow system. EtO is evacuated to and external catalytic converter which converts the toxic gas into water molecules and CO2, or to a water scrubber which produces water and Ethylene Glycol.
Following a sterilization process the product load ill undergo aeration to aerate EtO residuals absorbed by the exposed product. Modern sterilization chambers are capable of executing the aeration process as continuance of the sterilization process to eliminate risk of exposure to EtO while product is transferred to another chamber.
Sources: Center for Desease Control, USA. Westphal et al and Driks A.: PNAS March 18, 2003 (100)6: 3461-3466