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Catch some rays with gamma sterilisation

Gamma rays form part of a standard process for the sterilisation and decontamination of medical devices, but although the technology is advantageous and well established, the process can be complicated. Dr Blayne Welk, urologist at the Schulich School of Medicine and Dentistry in Ontario, talks to Medical Device Developments about the process.

Disposable medical devices – including syringes, implants, cannulas (flexible tubes) and intravenous sets – are required to be sterile. Given that plastics cannot be subjected to sterilisation by heat, the primary way in which plastic medical devices are sterilised is by gamma irradiation (electromagnetic irradiation). Alternative sterilisation processes include electron-beam irradiation, ion beams and ethylene oxide gas, but the advantage of using gamma irradiation is that it has a high penetration capability. This means that products that are relatively dense or moderately sealed can be sterilised in a fairly straightforward way. However, although gamma irradiation is a well-established process, various factors still need to be weighed up. These include microorganisms that are typically on the device during manufacture (this natural load can be reduced substantially when devices are assembled in clean rooms), the type of polymer used to manufacture the device and the selection of appropriate process controls.

Irradiation is the process by which an object is exposed to radiation. Gamma irradiation sterilises materials through energy from photons of gamma radiation (provided by a radioisotope) being transferred to the electrons in the target material. This creates highly active electrons (a process called ionisation) and highly active free radicals. These physical elements are capable of breaking the DNA within microorganisms and spores, which destroys them as well as preventing them from replicating, thus causing sterilisation.

Cobalt-60

The source of gamma irradiation used at most facilities is a radioisotope called Cobalt-60, which is specially manufactured for use by irradiation plants. Cobalt-60 is normally derived as pellets and placed into stainless steel tubes known as pencils. The pencils are, in turn, housed in a reinforced concrete structure called a cell (usually 2m thick) for shielding from gamma rays.

The Cobalt-60 pencils within the cell are held in a source rack, which has two operating positions. These are the storage position where the rack is either immersed in water or sometimes deep within concrete, and the operational position whereby the rack is raised. It is when in the operational position that sterilisation takes place. The storage position is needed because Cobalt-60 cannot be turned off; it continues to emit radiation.

"The source of gamma irradiation used at most facilities is a radioisotope called Cobalt-60."

Because most medical devices are constructed from plastic, there are inherent degradation risks in any sterilisation process.

Sterilisation by gamma radiation is a cold process and does not require any special physical changes like heat or pressure for the sterilisation process to be activated. The process involves the product being placed into special containers called totes, usually constructed from aluminium. The amount of product that can go into a tote is established during validation, which is why it is important to check the dimensions and weight of the product to be sterilised prior to beginning the process.

For the sterilisation process, the Cobalt-60 source is raised and the tote containing the product is moved around the source. The movement occurs at various heights and involves the rotation of the tote so that different sides of it are exposed. This ensures adequate penetration by the radiation and that the required dose is achieved. The speed at which this takes place depends upon the dose required, which is established during validation. Because Cobalt-60 has a relatively short half-life (less than five and a half years), the rate of decay varies, and the process speed must be checked every day to ensure the product in each cycle receives the correct dose.

Proving sterilisation

Sterilisation is demonstrated using devices called dosimeters, which measure the theoretical radiation dose received by the product. Dosimeters are plastic devices that contain a complex dye. After exposure to gamma irradiation, the dye alters: the greater the level of irradiation, the darker the dye becomes.

Dosimeters are placed in the tote and on the packaging of the product. At the end of the sterilisation cycle, the dose is checked by testing the dosimeters. This is undertaken by measuring their optical density – with a spectrophotometer – and the material thickness – with a microscope. The relationship between these two variables allows the dose calculation to be made.

To provide a visual confirmation of sterilisation, it is common to use chemical indicator labels on the outer packaging of medical devices. When sterilisation is complete, the indicator labels change colour (often from yellow to red). While such labels provide a rapid way of assessing effective sterilisation, they are not a substitute for dosimeters.

The importance of validation

Before any sterilisation process can be carried out on a medical device, it requires validating. This is in order to establish process parameters that can be replicated for routine sterilisation and to show that the sterilisation cycles can achieve a sterility assurance level of 1×10-6. This is a theoretical concept where it is assumed that, in terms of probability, no more than one item sterilised out of one million would contain one or more microorganisms after the completion of the sterilisation process.

Validation involves determining the dose required to be absorbed by the product in order to sterilise it. This is called the dose profile, and is assessed by dose mapping. For medical devices, the validation is usually performed in triplicate. The unit of absorbed dose is the Gray (Gy), typically measured as kiloGrays (kGy).

Sterilisation is demonstrated using devices called dosimeters, which measure the theoretical radiation dose received by the product.

The Gy is defined as the absorption of 1J of ionising radiation by 1kg of matter. The dose required by medical devices varies, and a careful balance needs to be struck: the dose must be sufficient to ensure sterilisation, but not so great that the material is damaged (such as becoming brittle or discoloured) or that a chemical reaction takes place (which can cause a chemical substance to leach out of the material).

Because most medical devices are constructed from plastic, there are inherent degradation risks in any sterilisation process, in any of the polymer’s properties such as tensile strength, colour, shape or molecular weight. For critical items, long-term stability tests are often undertaken to study the material over its shelf-life and across the typical storage temperatures. This involves physicochemical testing. A common radiation dose used for plastics is 15-25kGy, although it can be higher for denser materials.

The dose profile is based on assessing the absorbed dose, taking into account the density of the medical device, the pack size, the amount of wrapping around the product, the dose rate and the exposure time. This will vary between sterilisation plants because each plant design will be different, meaning validation is not transferrable between plants and must be repeated if a different sterilisation plant is selected.

High standards

It is important that sterilisation facilities operate to standards. The most common standard adopted for gamma irradiation is ISO 11137-1:2006, developed in association with the Association for the Advancement of Medical Instrumentation.

Standards are important for the manufacturers of medical devices to know that processes are being undertaken to a recognisable guideline and that there is a level of consistency between different sterilisation plants. The adoption of a standard also provides the manufacturer with a template with which to construct an audit of the plant.

Gamma irradiation has distinct advantages and is one of the most effective sterilisation methods available if the validation is sufficiently robust. If it is not, however, the sterilisation simply will not work or the medical device itself will be at risk from material degradation.

Five facts about Cobalt-60

  1. Cobalt-60 is a synthetic radioisotope of cobalt, made by the nuclear industry.
  2. It has a half-life of 5.27 years, after which it becomes stable isotope Nickel-60.
  3. It sterilises, but the energies given out are insufficient to induce radioactivity, thus rendering the product sterile and safe.
  4. It emits two different types of gamma rays, of different wavelengths.
  5. When placed in water, it creates a blue glow, caused by a light emission from atomic particles passing through water.

Gamma radiation explained

  • Gamma radiation is a type of electromagnetic radiation with a high frequency.
  • It occurs as an element decays.
  • It occurs naturally on Earth.
  • Gamma rays are also produced from exploding stars. They travel to Earth across vast distances and are absorbed by the atmosphere.
  • Gamma radiation can be created artificially, such as by nuclear fission.