Packaging : ETO Sterilization and Packaging

[Sandeep Kumar Goyal, Rexam Packaging (India) Ltd]

Sterility is defined as complete freedom from all viable microorganisms. The process of sterilization is a probability function, because of the logarithmic order of microbial death and the less than absolute methods of confirming sterility.

Sterilization can be done by various methods like :

1. Steam sterilization

2. Radiation sterilization

3. E beam sterilization

4. Gaseous sterilization

5. Chemical sterilization

ETO sterilization, type of gaseous sterilization is also known as EO or ethylene oxide gas sterilization. Every sterilization method has its own limitations of destroying microorganisms. Major factors that affect the utility of sterilization method-1 are :

1. Its compatibility with the product, material or substance being sterilized.

2. Acceptability of the packaging.

3. Penetration of the agent to remote areas that may contain viable microorganisms.

4. High level of lethal activity resulting in the need for only low quantities of the sterilizing agent.

5. Relatively inexpensive.

6. High degree of safety and low toxicity.

7. Simplicity

8. Time required for the process; and

9. Adaptability to in-line processing.

Ethylene oxide has been used as an insecticide, pesticide and sterilizing medium for spices, gums and (latterly medical devices) since 1928. Today it is quite common to use this as a sterilant and many a times along with inert gas.

During World War II, work was conducted by the Chemical Corps of the US army and a number of papers were then published by Phillips and Kaye, thoroughly reviewing the use of ethylene oxide as a decontainant and fumigant. The method of action of ethylene oxide is a complex interaction of a number of important factors. The mechanism of action of ethylene oxide is commonly attributed to its alkylating properties. A hydrogen atom may be replaced by an alkyl group. Ethylene oxide may also react with a carboxylic acid to form a longer chain hydrocarbon and free a hydrogen radical. ETO sterilization depends on following factors :

1. Chamber temperature

2. Relative humidity

3. Time of exposure

4. Concentration of the gas

5. Physical and chemical nature of the environment in which the microbial contaminants are located and the type and the number of microorganisms during gaseous ETO sterilization.

Microbial destruction occurs through the alkylation primarily of tertiary nitrogen groups and phosphoric acid esters of nucleic acid moieties. The chemical acts by alkylating the proteins of microorganisms, thereby upsetting their equilibrium. If the process is applied correctly, this reaction is irreversible, and reanimation of the alkylated microorganisms is prevented. Other than a minimal concentration of the gas itself, humidity (water vapor) is the most critical factor in any ETO sterilization process. Ethylene oxide (EO) is suitable for sterilizing heat labile articles that will withstand temperatures of 50-60oC. It is a method, which requires careful control in respect of its explosive characteristics, toxicity and for monitoring the efficiency of the process.

Kaye and Phillips established the initial quantitaion of the role of moisture in gaseous sterilization with ETO3 in 1949. Their data showed that the action of gaseous ETO sterilization in killing air dried spores of B subtilis var niger at 28% RG is about four times rapid as it is at 65% RH and almost 10 times as rapid as at 97% RH. If the bioburden has been desiccated, held at dry environmental conditions or held under high vacuum for a period of time, sterilization with gaseous ETO is increasingly difficult to achieve.

Spores dried on porous materials, such as paper and cloth are easier to sterilize than are spores dried on hard nonporous objects such as glass, metal and plastics.

The presence of various salts in the suspending medium from which materials have been dried also makes the destruction of the spores more difficult. The inability to achieve sterilization at very low moisture levels may be due in part to poor gas penetration. It is more likely a blockage of the alkylating action of ETO which requires the presence of water to allow ionization of hydrogen from nucleic acid or protein molecules before they react with ETO.

The importance of relative humidity to ethylene oxide sterilization is such that humidity of less than 30% may cause failure of the process. The moisture content of the microbial cell is another important factor in gaseous sterilization. Gilbert et al demonstrated that excessive drying of bacterial cells will result in a nonuniform reaction to ethylene oxide, and to rehydrate, direct contact with water is necessary. In view of this property, moisture distribution and permeation should be controlled at approximately 50% relative humidity (40-70% is the optimum range) to minimize the potential for production of desiccated spores.

Disadvantages of ETO gas and residue One of its main disadvantages is the potential for toxic residues to be left in products and materials that have been sterilized. The three most comon toxic residues of importance are ethylene oxide and two of its reaction products, ethylene chlorohydrin and ethylene glycol. For example, polyethylene retain 2 mg ethylene oxide per gram. In general sense, ethylene oxide toxicity is basically equivalent to that of ammonia; however, its additional reactivity and mutagenecity require that elaborate safety precautions to be taken with the use of this sterilizing agent. Ethylene oxide is a toxic gas, which irritates the mucosa, causing acute pulmonary edema in high concentrations. It has also shown adverse reproductive and transplacental effects. It can also contribute to chromosomal damage and cancer incidence.

Selection of packaging materials

For effective sterilization, selection of packaging material also plays important role apart from sterilization parameters. This can be best done in consultation of packaging material suppliers. The following are keys in selecting a suitable packaging material for gas sterilization:

1. The packaging material must be permeable enough for ethylene oxide and moisture to enter the package (and air escape) and sterilize the contents within the desired cycle time: the penetration rate must be uniform. Productivity requirements make short cycles (high porosity) desirable.

2. The packaging material must be impermeable to bacteria and other contaminants.

3. The packaging material must not be deformed or porosity altered by pressure variations during vacuum cycles.

All plastic films used for wrapping should be evaluated on their ability to allow reasonable permeation of ethylene oxide gas, moisture, and air before and after sterilization. It has been observed that Medical Grade Paper on one side helps in faster aeration of EO Gas than laminate on both the sides. Permeability is one of the most important criteria. Not only must be sterilant be able to permeate the package, but the packaging material must have sufficient breathability to permit release of toxic residues (e.g. ethylene oxide residual gas). Additionally, the porosity and bond strength (the seal, or bond, between two packaging sub-strates) must be adequate enough to maintain package integrity.

For proper and safe ETO sterilization of medical devices, packaging materials and sterilization parameters go together. They need careful monitoring and selection.