Radiation Sterilisation in Medicine
Aworld-wide and increasing market exists today for single-use sterile medical devices, ranging from simple gauze swabs or
surgical drapes up in complexity and cost to implanted medical devices such as knee joints or cardiac stents. It is a
strongly-regulated market, where international standards are applied rigorously. In some sectors, such as the American market,
the US Food and Drug Administration (FDA) pursues yet higher standards, and organisations exporting to the USA can expect
regular visits from visiting FDA inspectors. Similar high standards are applied for example in Europe, Japan and in Russia.
In ensuring a device is sterile, the first question to be asked is, what do we mean by “sterile”? What is “sterility”? Once
we have established what this means (for example, that the level of contamination of a device is such that only one device in
one million might have a viable microorganism living on it – this may then be identified with a Sterility Assurance Level (SAL)
of 10(-6)) we can then ask the question, how do we measure it?
The technology behind gamma-ray irradiation basically involves passing the product along validated paths past a large
cobalt-60 source, with activity of typically more than a MegaCurie or on a conveyor past a high-energy (5 or 10 MeV, usually)
electron beam of power 10 kW or more. The measurement of the absorbed dose itself requires significant Quality Assurance (QA)
including a significant amount of effort in the calibration, and only a few laboratories worldwide offer traceability to
primary standards at these high dose levels; NIST in the USA is one, and NPL in the UK another. Most other labs offer
traceability via these laboratories.
In terms of international Quality Assurance and regulatory requirements, ISO 11137 applies particularly to the radiation
sterilisation of medical devices and describes how a bioburden once determined can be linked to a particular dose designed to
achieve a given SAL and then how that dose is verified. Linked standards include ISO 11737 (on microbiological methods,
measuring the bioburden of devices as well as tests of sterility) and ISO 13485, which describes the minimum requirements for a
quality system under GMP.
The growing market for sterilised medical devices will ensure a bright future for organisations in this field, wherever they
are located geographically.
Dr. Mark Bailey
National Physical Laboratory, Teddington, United Kingdom
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