Summary:
Medical products, such as surgical dressings, sutures, catheters and syringes must be sterilized before they can be used in medical procedures. This is frequently done using radiation. The following overview describes how this procedure is conducted as well as its advantages compared to other sterilization techniques and its disadvantages.
Nuclear and Radioactivity Physics
Radiation Sterilization
Medical products, such as surgical dressings, sutures, catheters and syringes must be sterilized before they can be used in medical procedures. This is frequently done using radiation.
*HOW DOES THIS PROCESS WORK"
Radiation sterilization involves exposure of products to gamma radiation from a cobalt-60 source for a predetermined amount of time. The step by step process is such:
Hermetically sealed medical products are packed in cardboard boxes.
Boxes are loaded in carriers.
Each carries travels at a controlled speed on an overhead monorail.
Enters the irradiation cell through a labyrinth.
Boxes are exposed to gamma rays from a Co-60
Five passes ensures that the products are exposed to the minimum recommended dose which is 25 kiloGray
The energy from the photons of gamma radiation is transferred to the electrons in the material producing highly active electrons and highly reactive free radicals. These species induce breaks in the DNA double helix of the bacteria, preventing replication and hence sterilizing the material.
*WHAT METHODS ARE USED TO PRODUCE THE RADIATION?
Cobalt-60 is produced by the process of artificial transmutation. The naturally occurring and stable isotope of cobalt-59 is irradiated with neutrons. Some of cobalt-59 nuclei absorb neutrons and this results in cobalt-60 being produced. The unstable cobalt-60 offloads some of its excess energy by releasing gamma radiation - high frequency electromagnetic radiation.
*ONCE TREATED, ARE THESE STERILIZED INSTRUMENTS RADIOACTIVE"
Once treated these sterilized instruments ARE NOT radioactive and are safe for use. This is because the energies given out by the decay of Cobalt 60 are insufficient to induce radioactivity in any material, irrespective of the length of exposure to the source.
*WHAT ADVANTAGES DOES RADIATION STERILIZATION HAVE COMPARED WITH OTHER TECHNIQUES"
The advantages of radiation sterilization over other techniques is the fact that the powerful gamma rays are able to penetrate through the products of any shape and size as well as any outside packaging. This means that the products can be individually pre-packed in sealed bags and sterilized fully packaged, ensuring that the product sterility is retained provided the package is undamaged. This gives radiation sterilization an advantage over the boiling and chemical methods, which are unsuitable for complex items with a lot of hard to reach and dense areas.
An advantage of radiation sterilization is the fact that it is a cold process, therefore, heat sensitive products that could be damaged using the dry heat method, can safely be sterilized using gamma radiation.
Autoclaving is probably the second best choice to radiation sterilization. Due to the special process involving high-pressured steam being let into a chamber with a vacuum, the steam is able to penetrate every part of the equipment, regardless of complexity. Unfortunately, repeated autoclaving is damaging to many plastic and rubber items. As such, for some materials gamma radiation is more advantageous.
Another benefit of radiation sterilization is the speed and cost. Since this is a continuous and fully automated process and only the timing of the exposure needs to be controlled, this is much more time effective that steam and chemical processes.
A further advantage is that products may be released for sale based on documentation that the desired dose of gamma radiation was delivered. Microbiological release testing generally is not required unless it is a local regulatory requirement.
Unfortunately, despite being highly effective, irradiation from a gamma ray source disrupts the polymer structure of some plastics causing hardening, embrittlement, odor generation, softening, decomposition, enhancement and reduction of chemical resistance, increase and decrease in melt temperature as well as discoloring, leaving it at a disadvantage to the more gentle sterilization methods.
When deciding on a sterilization method, one of the first considerations should be product compatibility. Now many products are radiation compatible and many manufactures are choosing this method due to its speed, cost efficiency, penetrative abilities and product material compatibility, however, despite radiation sterilization being one of the best choices for most products, some products are better off being sterilized using one of the other methods.
*WHY ARE MANY OF THESE PRODUCTS UNABLE TO BE STERILIZED USING STEAM OR HEAT"
Many of these products are unable to be sterilized using steam or heat. Heat because it has a long processing time and the high temperature limits the types of products and packaging materials compatible with this process. Steam is unsuitable for complex items due to the its inability to reach sealed off and highly dense components. A further problem is steam's ineffectiveness for products that are moisture sensitive or water resistant.
*WHAT ARE THE DISADVANTAGES OF STERILIZATION VIA CHAMICAL PROCESSES"
The disadvantages of sterilization via chemical processes are the facts that: some materials, such as plastics, absorb large amounts of chemicals, while others, such as stainless steel, absorb none. The more the chemical is absorbed by the material the longer is the aeration process. This is neither time, nor cost effective. Complex designs are hard to sterilize via chemical process since some parts of the device may have limited accessibility that is they will not be completely sterilized. Another concern is the fact that chemicals can not penetrate all packaging and only several millimeters of material. The only exception is ethylene oxide sterilization, however it is an expensive and complex process and requires thorough anti-pollution measures since the gas is very poisonous. The increasing cost of the gas and of the various engineering and environmental controls required to assure safe low residual products and low personnel exposure has raised the cost of ethylene oxide (EtO) sterilization which results in it being a very disadvantageous process.
Bibliography:
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