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CYCLE BASIC PHASES

Load of product to be sterilized

Product heating by heated water circulating inside the jacket and by internal fans preventing

stratification which make heat exchange easier.

Pre -vacuum with value that can be automatically set up to 40 mbar (4 kPa) max.

Vacuum leak test of the chamber before gas inlet

Product conditioning by humidification up to a preset value

Inlet of preheated and vaporized gas up to selected pressure value

Sterilization with gas pressure constant holding for a presettime (up to 24 hours)

Flushing by gas evacuation through vacuum pump and sterileair inlet (pulsations) for a prefixed

number of times.

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The most efficient sterilizing process, based on the considerations listed above, is that of McDonald

(1962) and is shown on a pressure-time graph . The sequential stages are listed. Of minor importance is

the operating pressure which determines the EO gaseous concentration. What is most important is the

strategic placement of moisture under vacuum to provide the best dynamics for heat and moisture

permeation. This can best be accomplished under vacuum. Where the operating temperature is 54 C or

higher, the degree of vacuum is critical for highest efficiency. This would be 26 inches Hg vacuum or

higher based on the thermodynamic relationship of saturated moisture. Why is it so important to

strategically place moisture, that is to concentrate it near to but not directly on the sterilizing site Ernst

et al. (1970) presented four reasons why it is important to prehumidify under vacuum to strategically

place moisture prior to adding the sterilant gas:

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1. The number of water molecules in even a highly humidified environment is overwhelmed by the

greater number of EO molecules.

2. The diffusivity of EO far surpasses that of water vapor. If EO and water were equally dispersed, the

EO would permeate into and through materials preferentially, leaving moisture behind.

3. Water readily reacts with both EO and the diluents CO, and Freon 12, the latter presumably by

hydrogen bonding in the gaseous state, rendering the moisture ineffective as well as reducing the

efficiency of EO itself. Large molecular aggregates are formed by H-bonding which can sometimes be

seen as a vapor cloud which is heavier than air. This state also is conducive to the formation of toxic

residues and damaging polymers.

4. Molecular interference such as air pockets and expanded heat-sealed plastic bags prevents the

effective permeation of water vapor.

Molecular interactions can seriously interfere with diffusion of moisture and EO, as occurs by

attempting to sterilize such with pure EO under vacuum. Air in the lumen of coiled tubing is also a

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limiting condition, not so much for EO, but for the permeation of heat and moisture. Air is a great

barrier for diffusion of heat and moisture in EO, but not as severe as in steam sterilizers. Air stratification

can severely delay EO sterilization if the air strata is great enough in depth, such as might occur in

industrial sterilizers where no attempt is made to remove air. The delaying effect is directly related to

diffusion path length vertically. Thus, it would be most severe in industrial sterilizers having a large

vertical dimension (Ernst et al., 1970). The relative ease by which products sealed in various packaging

films are sterilizable has been determined by Doyle et al. (1970) to be in the following increasing order

of resistance: polyethylene and poly- vinyl chloride (the least resistant to achieve sterilization), nylon,

cello-phanelpolyethylene laminate, phenoxy, mylarlpolyethylene laminate (the most difficult to

permeate). Although increasing the thickness of the film is also a limiting factor, for polyethylene film,

increasing thickness from 1 to 4 mils (Kereluk et al., 1970) does not appreciably change the barrier effect

as much as the theoretical consideration may imply. HOW- ever, increasing the thickness of nylon andsome other difficult-to- permeate films does limit the permeation rate of the sterilizing gaseous

components and therefore affect the concomitant ease of sterilization.

1. Temperature setting. This is usually around 130 F (54 C). From Ernst and Shull(l962a), sterilizing

efficiency can be increased and exposure time reduced by increasing the temperature. Roughly,

sterilizing time may be reduced by one-half for every 30 C rise in temperature. However, the problem

of moiskurizing makes it necessary to compromise the temperature to prevent excessive wetting of

materials which would result at higher temperatures because of the necessity to maintain a

temperature-dependent relative humidity level of at least 35%. To increase temperature above 130F

would require decreasing relative humidity to prevent wetting. The overall effect of decreasing relative

humidity is greater in reducing sterilizing efficiency than the opposing effect of temperature.

2. Evacuation. The sterilizer is evacuated to about 27 inches Hg. This provides optimal conditions for

moisturizing and heating.

3. Humidifying. Generally steam is introduced to the evacuated chamber and controlled by some means

to provide a high level of relative humidity in the chamber (usually 45-85 yo). These conditions are

maintained for a period of time depending on size and density of load and other factors. This is called

the dwell (it is a soak period). Time is required for moisture to permeate depending on the driving

force also (20 min -2 hr). The rule is to provide moisture much in excess of the minimal 35 yo relative

humidity, but short of wetting materials. The higher relative humidity levels provide a driving force for

diffusion and also contain valuable elements of heat. The advantages of this strategic placement of

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moisture, referred to as pvehumidijication, have been discussed. It cannot be overemphasized,

however, that prehumidification in the sterilizer chamber has such distinct value and importance that it

cannot be replaced .