The Principles of Medical Device Decontamination

Chemical Methods – Low Temperature – Oxidising Agents Sterilization

Sterilization processes using strongly oxidizing chemical agents bring about microbial inactivation by chemical oxidation of the molecules present within the microbial cell vital for life processes.

Figure 1 shows a number of sterilization processes which use oxidising agents such as peracetic acid, ozone and hydrogen peroxide, this latter process being the most popular and common in use. A number of sterilizers use hydrogen peroxide vapour or a combination with another chemical agent such as peracetic acid or ozone. Some utilise a “gas plasma” phase at some stage in the sterilization cycle resulting in them often being called “plasma sterilizers” although they are in fact vaporised hydrogen peroxide sterilizers (VH 2 O 2 ). The purpose of the plasma phase has been debated but essentially if used at the start of the process it will pre heat the load. If used at the end of the vapour exposure phase it will usually contribute to microbial inactivation but more importantly degrade the hydrogen peroxide vapour to oxygen and water. The process variables of a VH 2 O 2 process are time, temperature, hydrogen peroxide vapour concentration. Recent research suggests the ratio between the H 2 O 2 and H 2 O concentration, both of which are present in the vapour within the chamber, affects the rate of inactivation of microorganisms and so the water concentration may also be a process variable. The sterilization process is a complex sequence. In all cases once sealed the chamber is evacuated to a very low residual pressure. The chamber and load are typically allowed to heat to around 50 o C. Hydrogen Peroxide vapour is then introduced into the chamber. This is produced from an aqueous solution of hydrogen peroxide by a vaporiser system. The VH 2 O 2 is then allowed to permeate into load items forming what is thought to be a micro layer of liquid on the surfaces which need to be sterilized. After a suitable diffusion or exposure phase, air may be allowed into the chamber or a vacuum may be drawn or a plasma phase initiated. The sequence might then be repeated. The important point to note is that there is no standard VH 2 O 2 process and each sterilizer manufacturer will have designed their own process and sterilizers will often have several processes of different complexity programmed into each machine.

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