The Principles of Medical Device Decontamination

Physical Methods – Low Temperature Sterilization process

The sterilization processes fitting into this category are almost exclusively used by the medical device industry. This would include processes which use ionising radiation to inactivate microorganisms. The process variables for all radiation processes is “absorbed dose” which is a function of radiation intensity and time of exposure. Gamma sterilization employs radiation emanating from the radioisotope, Cobalt 60. E-beam sterilization employs a highly energetic electron beam produced by a cathode ray array. The advantage of this type of process is that it is on/off and so no specialist storage facilities are required for the source. A recent innovation is the development of soft X ray sterilization. This is similar to e-beam but instead low energy X rays are generated and beamed onto loads in order to effect sterilization. Such sterilizers are used for the mass production of single use medical devices made from polymeric plastic materials such as syringes, catheters, surgical drapes and gowns such as is 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). Sterilizers 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 sterilization 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 sterilized load out through tortuous paths which prevent escape of radiation. For e-beam and X-ray sterilization a similar exposure cell configuration is used.

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