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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 .