INSTRUMENT STERILISATION AND DECONTAMINATION
e-beam and X-ray sterilisation a similar exposure cell configuration is used. However, when needed, power is applied to the e-beam or X-ray generator; the goods then pass underneath the e-beam/X-ray on a conveyor system, which ensures that the sterilising dose is achieved. Unlike gamma there is no active source, so no need for storage of radioactive materials.
n Chemical methods: Low temperature – oxidising agent sterilisation
Figure 2: An instrument trolley going into a steriliser chamber.
recent innovation is the development of soft X-ray sterilisation. This is similar to e-beam, but instead low energy X-rays are generated and beamed onto loads in order to effect sterilisation. Such sterilisers are used for the mass production of single-use medical devices made from polymeric plastic materials, such as syringes and catheters, and surgical drapes and gowns, as used in operating theatres. Boxes of devices are loaded into transport ‘totes’, and moved into the exposure cell ‘en-masse’ for irradiation. It is important to understand that such processes do not induce radioactivity in the irradiated products. However, in some types of plastics, the radiation can induce degradative processes which result in fracture or formation of powdery deposits during storage (a ‘cascade degradative reaction’).
Sterilisers In gamma irradiation, the Cobalt 60 source is stored in a deep water pool, which attenuates the radiation to safe levels. When required the Cobalt 60 source is automatically raised, and goods which require sterilisation are exposed to the radiation to the required dose level. Exposure takes place in a concrete ‘block house’ – which is designed to absorb the radiation, thereby preventing escape into the environment, and exposure of operators, which would be fatal. A conveyor system moves the load items into the exposure ‘cell’, where irradiation takes place, and then transports the sterilised load out through tortuous paths which prevent escape of radiation. For
Sterilisation processes using strongly oxidising chemical agents to 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 sterilisation processes which use oxidising agents such as peracetic acid, ozone, and hydrogen peroxide – the last of the three being the most
popular and commonly used. A number of sterilisers 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 sterilisation cycle, resulting in them often being called ‘plasma sterilisers’, although they are, in fact, vaporised hydrogen peroxide sterilisers (VH2O2). 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 VH2O2 process are time, temperature, and hydrogen peroxide vapour concentration. Recent research suggests that the ratio between the H2
O2 and H2 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.
A ‘complex sequence’ The sterilisation 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 °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 VH2O2 is then allowed to permeate into load items, forming what is thought to be a microlayer of liquid on the surfaces which need to be sterilised. 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 VH2O2 process, and each steriliser manufacturer will have designed its own process, and sterilisers will often have several processes of different complexity programmed into each machine. Manufacturers will provide information about the medical devices which can be processed. Generally, simple devices will be able to be processed in shorter cycles (ca 30 mins), compared with more complex devices such as endoscopes, which will require a much longer processing time (ca 3 hours).
Figure 3: A vaporised hydrogen peroxide steriliser which uses a plasma phase in healthcare settings.
October 2022 Health Estate Journal 37
Image used courtesy of Getinge
Image used courtesy of 3M
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