The Science of Sterilisation by Radiation by Adam Clough and Stacey Hasprey

Introduction

Radiations used in Sterilisation can be divided up into two groups, electro-magnetic waves and streams of particulate matter. The first group encompasses ultraviolet light and gamma rays and the second includes alpha and beta radiations.

UV light and gamma rays can both damage molecules that are vital to all living cells. As the waves pass through the matter they excite the electrons surrounding the nuclei. The electrons can then gather enough energy to escape from the atoms resulting in ionization. This then gives the effect of Sterilisation.

Ultraviolet light

The main method of using UV light in Sterilisation is by passing a low current at high voltages through mercury vapour in an evacuated tube made of borosilicate glass. The wavelength created is between 220-280nm but the vital wavelength required is 265nm which is thought to be most effective in killing microorganisms and will reduce populations of vegetative cells by up to 90%.

The intensity of UV radiation is shown as energy in an area, so microwatts/mm² is used. The intensity of UV radiation should be between 10-60microwatts/mm²

As a general rule UV light should not be relied on for Sterilisation of appliances alone, it should be done with other forms of sterilisation.

Ionizing radiations

This includes both high-speed electrons and gamma rays. The process of Sterilisation for commercial equipment must have certain factors associated with it to be a useful sterilisation method. It must:

·         Have good penetrating power

·         High sterilising efficiency

·         Have little to no damaging effect on materials

·         Be able to be produced efficiently

High-speed electrons are generated by using a machine known as a van de Graaff accelerator. The beam of high-speed electrons which is fired out of an highly evacuated tube is narrow and intense and it is used to irradiate equipment on a conveyor belt. The equipment that has become irradiated is now sterilized.

Cobolt is used as the source of gamma radiation. The equipment that is treated using gamma rays will have two types of gamma rays in succession because of the nature of cobolt. The energies produced from the two gamma rays are 1.33 MeV and 1.17 MeV.

Mode of Action

Ionizing radiations causes excitations, ionizations and free radical formations – where water is present. These are powerful oxidizing and reducing agents which damage essential molecules in living cells.

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Steriliaing Dose

The dose used is measure in kGy. The gray (Gy) is the SI equivalent to 100 rad. The recognised sterilising dose is 25kGy. The choice of the dose was based on an experiment whereby paper discs or pieces of plastic or foil were heavily contaminated with pathogens, non-pathogens, vegetable bacteria and spores. These were then exposed to different variations of radiation doses.

Within radiation the D10 value (decimal reduction dose) is the dose (measured in Gy) that reduces the number of viable organisms.

Sterilising Time

Depending on the size and density of the material which is to be irradiated, the beams or pulses of electrons from accelerators is able to deliver a dose in fractions that vary between 1 second to a few seconds. This differs from isotope sources, due to their diffuse and penetrating nature of their emissions; therefore the dose has to accumulate over several days.

 Advantages

The advantages of this type of Sterilisation are:

·         There is no significant temperature rise

·         There can be a continuous process because exposure is either short (machine generation) or large (isotope generation)

·         No aseptic handling is required

·         The method of Sterilisation is accurate and reliable

Undesirable effects and Disadvantages

The Disadvantages of radiation Sterilisation include:

·         Chemical decomposition (this can be immediate or can occur after storage)

·         Costs for capital and/or replacements are high

·         The precautions which have to take place in order to protect operators from harmful effects of ionizing radiation are elaborate and expensive

·         Alterations in colour, texture and solubility

Repeated irradiation can cause degradation with certain dressings and plastics