Radiation Sterilization: Types, Mechanism, Applications

Radiation is currently used for sterilization and decontamination in the medical supplies (surgical supplies, vaccines, and drugs) and food industries. Two types of radiations are available; ionizing and non-ionizing.

Radiation sterilization is not widely used in food industries as people have concerns about radioactive contamination, production of toxic or carcinogenic products, change in nutritional value, and taste alteration, etc.

Ionizing radiations

Ionizing radiation is an excellent agent for sterilization/disinfection, it kills organisms without increasing the temperature; so aptly called cold sterilization. It destroys bacterial endospore and vegetative cells, both eukaryotic and prokaryotic; but not always effective against viruses.

Mechanism of Sterilization by Ionizing Radiations

When ionizing radiation collides with particles, they produce electrons (e⁻) and other reactive molecules such as hydroxyl radicals (•OH), and hydride radicals (H•). Each of these reactive molecules is capable of degrading and altering biopolymers such as DNA and protein. Breakage of DNA and degradation of enzymes lead to the death of the irradiated cells.

Several sources of ionizing radiation are available, including X-ray machines, cathode ray tubes (electron-beam radiation), and radioactive nuclides (sources of gamma/x-rays).

X-Rays

X-rays are lethal to microorganisms and higher forms of life but are rarely used in sterilization because their production is expensive and efficient utilization is difficult (since radiations are given off in all directions from the point of origin).

Gamma Rays

Gamma radiations are high-energy radiations emitted from certain radioisotopes such as Caesium-137 (¹³⁷Cs) and Cobalt-60 (⁶⁰Co), both relatively inexpensive bioproducts of nuclear fission. Gamma rays are similar to x-rays but are of shorter wavelength and higher energy. They are capable of great penetration into the matter, and they are lethal to all life, including microorganisms. Gamma rays are attractive for use in commercial sterilization of materials of considerable thickness or volume, eg., packaged food or medical devices.

Cathode Rays (Electron-Beam Radiation)

Cathode rays or electron-beams can sterilize materials at room temperature with brief exposure. They have limited penetrating power and are used for the sterilization of surgical supplies, drugs, and other materials.

Uses of Ionizing Radiations

The major method in use for radiation sterilization is gamma irradiation. Gamma radiation is used in the sterilization of;

• Disposables such as plastic syringes, infusion sets, catgut sutures, catheters, gloves, and adhesive dressings before use.
• Bone, tissue grafts, antibiotics, and hormones.
• Irradiation of food (permitted in some countries).

Advantages of Ionizing Radiations

1. High penetrating power: products can be processed in their fully sealed, final packaging thus limiting the risk of contamination following sterilization.
2. Rapidity of action: saves and efforts.
3. Temperature is not raised: compatible with temperature-sensitive materials, such as pharmaceuticals and biological samples.
4. Flexibility: can sterilize products of any phase (gaseous, liquid, or solid materials), density, size, or thickness.

Disadvantages

• Capital costs are high and specialized facilities are often needed e.g. for gamma irradiation.
• Use of gamma radiation requires handling and disposal of radioactive material.
• Not compatible with all materials and can cause breakdown of the packaging material and/or product. For example,  Common plastics such as polyvinyl chloride (PVC), acetal, and polytetrafluoroethylene (PTFE) are sensitive to gamma radiation.

Sterilization/disinfection control: The efficacy of ionizing radiation is tested using Bacillus pumilus.

Non-ionizing Radiation

Non-ionizing radiations are lethal but do not penetrate glass, dirt, films, or water; hence their use is restricted to disinfection of clean surfaces in operation theaters, laminar flow hoods, and water treatment. The recommended dose is 250-300 nm wavelength, given for 30 minutes.

Examples of non-ionizing radiation include infrared and ultraviolet radiation.

Infra-Red Radiation

Infra-red rays are low energy type electromagnetic rays, having wavelengths longer than those of visible light. They kill microorganisms by oxidation of molecules as a result of heat generated. Infra-red rays are used for the rapid mass sterilization of syringes and catheters.

Ultraviolet Light (UV) Sterilization

Sunlight is partly composed of UV light but most shorter wavelengths of light are filtered out by the ozone layer. There are three types of UV radiation; UVA, UVB, and UVC, classified according to their wavelength. Short-wavelength UVC is the most damaging type of UV radiation.

Mechanisms of UV Sterilization

Many cellular materials, including nucleic acids, absorb ultraviolet light. It causes the bonding of two adjacent pyrimidines, i.e., the formation of pyrimidine dimer, resulting in the inhibition of DNA replication. This leads to mutation and death of exposed organisms.

Uses of UV Sterilization

UV lights are useful for disinfecting surfaces, air, and water that do not absorb the UV rays. Certain types of UV lights can kill the flu (influenza) virus. Ultraviolet radiation is used for disinfecting enclosed areas such as bacterial laboratory, nurseries, inoculation hood, laminar flow, and operation theaters. For example, laboratory biological cabinets all come equipped with a “germicidal” UV light to decontaminate the surface after use.

While UV sterilization is ongoing, the area should be closed and UV lamps must be switched off immediately after use.

Effects of UV light in SARS-CoV-2 (COVID-19)

UV radiation kills viruses by chemically modifying their genetic material, DNA, and RNA. The most effective wavelength for inactivation, 260 nm, falls in the UVC range. Though we do not have much research regarding the effect of UVC in SARS-CoV-2, concentrated form of UVC is now on the front line in the fight against COVID-19. UVC light is being used to sterilize buses, UVC-emitting robots to sterilize hospital floors and even banks are using UV light to disinfect money.

Disadvantages

1. Damages skin and eyes: Conventional UV light can penetrate and damage skin and also cause cataracts.
2. Does not penetrate paper, glass, and cloth.

References and further readings

• Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
• Lytle, C. D., & Sagripanti, J.-L. (2005). Predicted Inactivation of Viruses of Relevance to Biodefense by Solar Radiation. Journal of Virology, 79(22), 14244.
• Tille, P. (2017). Bailey & Scott’s Diagnostic Microbiology (14 edition). Mosby.
• Willey, Joanne M, Sherwood, Linda M, & Woolverton, Christopher J. (2016). Prescott’s Microbiology (10 edition). McGraw-Hill Education.
• Radiation Sterilization. Stanford University. Retrieved April 24, 2020, f.