Steam sterilisation

• It is non-toxic, inexpensive, rapidly microbicidal, sporicidal, and rapidly heats and penetrates fabrics, although it can lead to instrument corrosion.
• Recognised minimum exposure periods for sterilisation of wrapped healthcare supplies are 30 minutes at 121°C (250°F) in a gravity displacement steriliser or 4 minutes at 132°C (270°C) in a prevacuum steriliser.
• Gravity displacement autoclave: steam is lighter than air, forces air out the bottom of the chamber through the drain vent.
• High speed prevacuum sterilisers: similar to gravity displacement sterilisers except they are fitted with a vacuum pump (or ejector) to ensure air removal from the sterilising chamber and load before the steam is admitted. The advantage of using a vacuum pump is that there is nearly instantaneous steam penetration even into porous loads.
• Mode of action: moist heat destroys microorganisms by the irreversible coagulation and denaturation of enzymes and structural proteins.
• “Flash" steam sterilisation was originally defined by Underwood and Perkins as sterilisation of an unwrapped object at 132°C for 3 minutes at 27–28 lb of pressure in a gravity displacement steriliser.
• It is not recommended as a routine sterilisation method because of the absence of protective packaging following sterilisation, and possibility of contamination of processed items during transportation to the operating rooms.
• It is considered acceptable for processing cleaned patient care items that cannot be packaged, sterilised, and stored before use. It is also used when there is insufficient time to sterilise an item by the preferred package method.

Figure 1.High speed prevacuum sterilisers 

Ethylene oxide “gas” sterilisation

• Mode of action: microbicidal activity results from the alkylation of protein, DNA, and RNA. Alkylation, or the replacement of a hydrogen atom with an alkyl group, within cells prevents normal cellular metabolism and replication.
• The main disadvantages associated with ETO are the lengthy cycle time, the cost, and its potential hazards to patients and staff; the main advantage is that it can sterilise heat or moisture-sensitive medical equipment without deleterious effects on the material used in medical devices.

Ethylene oxide “gas” sterilisation

• Mode of action: microbicidal activity results from the alkylation of protein, DNA, and RNA. Alkylation, or the replacement of a hydrogen atom with an alkyl group, within cells prevents normal cellular metabolism and replication.
• The main disadvantages associated with ETO are the lengthy cycle time, the cost, and its potential hazards to patients and staff; the main advantage is that it can sterilise heat or moisture-sensitive medical equipment without deleterious effects on the material used in medical devices.

Hydrogen peroxide gas plasma

• Gas plasmas are generated in an enclosed chamber under deep vacuum using radio frequency or microwave energy to excite the gas molecules and produce charged particles, many of which are in the form of free radicals.
• Free radicals within a plasma field are capable of interacting with essential cell components (e.g. enzymes, nucleic acids) and thereby disrupt the metabolism of microorganisms.
• Uses: materials and devices that cannot tolerate high temperatures and humidity, such as some plastics, electrical devices, and corrosion-susceptible metal alloys.⁷