In general there are three types of radiation: alpha (α), beta (β), and gamma (γ). When considering radiation options for sterilization, only beta and gamma radiation are applicable.
Beta radiation is used as an electron beam (e-beam) for sterilization and causes less damage to cells or materials. Gamma radiation is the more powerful option as it has the shortest wavelength (<0.10 nm) resulting in the highest energy and more destructivity. It is generally preferred due to its deep uniform penetration of materials which gives a better certainty for sterilization and independence from temperature and pressure.
Gamma radiation sterilization is the controlled exposure to ionizing radiation from an isotopic source. Typically Cobalt 60, an isotope to Cobalt 59 that emits gamma rays while it decays radioactively, is used as the source.
So how does exposure to gamma rays result in sterilization? Gamma rays are high energy photons that can go through the cell membrane and damage DNA via destroying the helix directly (damaging the deoxyribose) or disrupting the chemical bonds within DNA. This activity can result in single-strand or double-strand breaks of DNA and eliminate a cell’s ability to replicate itself. However, it is important to note that microorganism resistance to gamma radiation relies almost exclusively on its DNA repair enzymes and their repair capabilities. Deinococcus radiodurans is the most familiar radiation-resistant microorganism due to its rather extreme resistance to DNA damage.
When sterilizing medical devices, the radiation dose is entirely dependent on the bioburden of the device (check out our article on bioburden for more information). ISO 11137 “Sterilization of Health Care Products” part 2 “Establishing sterilization dose” provides a guidance table that has bioburden CFU and the corresponding radiation dose in kGy (kiloGrays). After determining the target dose, the device is appropriately irradiated and a dosimeter is used to determine the real dose (what was actually absorbed by the device). The real dose must not exceed 10% or be lower than 90% of the target dose from the table. The real dose is also termed the sterilization dose, as the target dose (from the table) is also called the verification dose.
It is also important to consider the type of material used to make the device being sterilized via gamma rays. Gamma radiation can cause changes to polymers that impact the functionality of the product. Exposure can cause chain scission, which reduces the tensile strength and reduces elongation capabilities. It can also cause cross-linking, which does increase tensile strength, it also reduces elongation. Different reactions occur depending on the polymer type: oxidation in polyethylene can cause it to crack, polycarbonate is yellowed from the production of chromophore in the polymer matrix, and polyurethane can result in the generation of a carcinogen known as 4,4’-methylenedianiline (diaminodiphenylmethane, MDA).
Sources:
Harrell CR, Djonov V, Fellabaum C, Volarevic V. Risks of Using Sterilization by Gamma Radiation: The Other Side of the Coin. Int J Med Sci. 2018 Jan 18;15(3):274-279. doi: 10.7150/ijms.22644. PMID: 29483819; PMCID: PMC5820857.
Centers for Disease Control and Prevention. (2016, September 18). Other Sterilization Methods. https://www.cdc.gov/infectioncontrol/guidelines/disinfection/sterilization/other-methods.html#:~:text=Sterilization%20by%20ionizing%20radiation%2C%20primarily,%2C%20pharmaceuticals%2C%20medical%20devices
Simmons A. 11 – Future trends for the sterilisation of biomaterials and medical devices. Sterilisation of Biomaterials and Medical Devices (2012), 310-320. Available Online 2014 Mar 27. https://doi.org/10.1533/9780857096265.310
Martin, J.M. (2012). Understanding Gamma Sterilization. BioPharm International-02-01-2012, 25(2). https://www.biopharminternational.com/view/understanding-gamma-sterilization
Susanto A, Komara I, Beatrix MT, Lukitowati F, Amaliya A, Hendiani I, Miranda A. Determination of the Sterilization Dose of Gamma-Ray Irradiation for Polyvinyl Alcohol-Collagen-Chitosan Composite Membrane as a Material for Periodontal Regenerative Surgery. Eur J Dent. 2023 Oct;17(4):1289-1293. doi: 10.1055/s-0043-1761186. Epub 2023 Jun 27. PMID: 37369235; PMCID: PMC10756808.
