49


Ethylene oxide gas sterilises through the alkylation of proteins, DNA, and RNA, which causes microorganisms to be non-viable [7]. This process occurs as an effect of the high reactivity of the compound; an attribute which leads to logistical complications. Due to a combination of its


toxicity and fl ammability, it is recommended that ethylene oxide is stored in a detached storage unit - with a 100m radius isolation zone established in case of a leak [12].


As with most chemical sterilants, Ethylene oxide is ineffective on prions and has the capacity to leave residue on sterilised surfaces. Its sterilisation time is also long - between 2 and 5 hours - and its processing time even longer; often at over 14 hours due to post-sterilisation vapour removal. However, it is highly penetrative, works at low temperatures (between 29- 65°C) and unreactive with most plastics.


option if materials are heat sensitive and can withstand its damaging effects. However, it is expensive to set up and reliant on the scale of its use to be cost-effective.


Chemical sterilisation, while fraught with toxicity, residue, and logistical concerns, along with ongoing consumable costs, has its part to play when the object being sterilised is incompatible with other sterilisation techniques – for example a heat and radiation sensitive solid material.


For objects that are not heat, moisture, and, to some degree pressure sensitive, steam sterilisation will often emerge as the standout choice; offering cost-effi ciency, effectiveness, and speed. This is also the case for waste materials, where their post- sterilisation condition is not of concern. As such, an autoclave remains essential equipment for almost all laboratories, while steam-heated effl uent decontamination systems (EDS) provide an excellent option for those with biologically active liquid waste to dispose of.


Through analysis of the material being sterilised, it is possible to fi nd a sterilisation technique that best fi ts the ideal model. A combination of speed, effi caciousness, materially compatibility, non-toxicity, adaptability, and ability to be monitored can all be assessed for the required situation, alongside the capacity to overcome any material resistance that may be found in the target material. Of course, cost-effectiveness will be a key factor in most organisations.


An effl uent decontamination system (EDS) which uses steam as a heat source for sterilisation


References


1. Schneider, P. (1994). Low-temperature sterilization alternatives in the 1990s. Tappi Journal;(United States), 77(1).


One further interesting advancement in this area is gas plasma sterilisation. Hydrogen peroxide gas plasma is created by exciting hydrogen peroxide vapour using an electrical fi eld, causing it to evolve free radicals that denature biological materials, as with the ionising radiation procedure. Hydrogen peroxide gas plasma has been shown effective against prions, works at low temperatures (37-44°C), and has a cycle time of less than 75 minutes [13]. However the process has poor penetrative abilities and the hydrogen peroxide it relies on is a toxic, explosive substance.


Heat


Heat sterilisation is the oldest and simplest kind of sterilisation, and takes two forms - dry and steam. Key benefi ts of heat sterilisation include its ability to destroy all biological material (including prions), while leaving no contaminants or residue [14]. Another is the low cost, especially with regard to consumables: at most, only electricity and water are required. Hazards related to heat sterilisation are also low, although there is a risk of burns from touching hot sterilised objects.


Heat sterilisation effects the outer surfaces of the object being sterilised fi rst, then spreads inwards until the entirety of a material has been sterilised. Heat denatures the proteins found in microorganisms and biological material, and as prions are proteins, they are also effected by heat sterilisation. The denaturing process alters hydrogen and disulphide bonds, and salt bridges, and alters the secondary, tertiary, and quaternary structure of proteins, rendering them invalid for biological processes [15].


Dry heat sterilisation involves creating an environment of 160°C to 170°C and maintaining it for between two and four hours. This combination of time and temperature mean sterilising heat sensitive material such as plastics via this method is unviable without material degradation [16]. Blades have traditionally been sterilised with dry heat, as concerns exist over steam dulling them. However, studies have shown stainless steel blades sterilised with steam display negligible blunting [17].


Steam sterilisation is a faster and less heat-intensive process than dry heat sterilisation. Steam transfers heat energy more effectively than hot air which means a lower temperature (121°C - 134°C) and quantity of time (3 to 15 minutes) can be utilised for sterilisation. The boiling point of water at atmospheric pressure is 100°C, which is too low a temperature for the sterilisation process. As such, steam has to be generated in a pressurised atmosphere, which raises water’s boiling point to the required temperature.


Objects that can handle a heat of 134°C or less can be sterilised with steam, but must be able to withstand a combination of temperature, moisture and pressure. For those which can, steam sterilisation forms a highly effective and penetrative form of sterilisation.


Summary


The fi nal conclusion for sterilisation isn’t straightforward. But it is possible to navigate through the complexities and fi nd the right option by maintaining a key focus on the objects in question and their required condition post-sterilisation.


Filtration forms an excellent option for those sterilising a limited number of fl uids – especially ones sensitive to heat, radiation, and chemical sterilisation. But decisions must be made in the knowledge that it is reliant on other forms of sterilisation to deactivate microorganisms from its mechanism.


Encapsulating multiple variants with different penetrative abilities, radiation is a good


2. Rutala, William A., and David J. Weber. “Clinical effectiveness of low-temperature sterilization technologies.” Infection Control & Hospital Epidemiology 19.10 (1998): 798-804.


3. Cavicchioli, Ricardo; Ostrowski, Martin (June 2003). Encyclopedia of Life Sciences. Nature Publishing Group. ISBN 9780470015902. Retrieved September 26, 2017.


4. Duda, V; Suzina, N; Polivtseva, V; Boronin, A (2012). “Ultramicrobacteria: Formation of the Concept and Contribution of Ultramicrobacteria to Biology”. Microbiology. 81 (4): 379–390. doi:10.1134/s0026261712040054.


5. Janssen, Peter; Schuhmann, Alexandra; Mörschel, Erhard; Rainey, Frederick (April 1997). “Novel anaerobic ultramicrobacteria belonging to the verrucomicrobiales lineage of bacterial descent isolated by dilution culture from anoxic rice paddy soil”. Applied and Environmental Microbiology. 63 (4): 1382–1388. PMC 168432.


6. 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;15(3):274-279. Published 2018 Jan 18. doi:10.7150/ijms.22644.


7. Qiu, Q.-Q & Sun, Wendell & Connor, J. (2011). Sterilization of Biomaterials of Synthetic and Biological Origin. Comprehensive Biomaterials. 4. 127-144. 10.1016/B978- 0-08-055294-1.00248-8.


8. Rutala, W. A., Weber, D. J., & Society for Healthcare Epidemiology of America. (2010). Guideline for disinfection and sterilization of prion-contaminated medical instruments. Infect Control Hosp Epidemiol, 31(2), 107-17.


9. Gominet, M., Vadrot, C., Austruy, G., & Darbord, J. C. (2007). Inactivation of prion infectivity by ionizing rays. Radiation Physics and Chemistry, 76(11-12), 1760-1762.


10. Simmons, A. (2012). Future trends for the sterilisation of biomaterials and medical devices. In Sterilisation of Biomaterials and Medical Devices (pp. 310-320). Woodhead Publishing.


11. United States Department of Labor, 2020. Ethylene Oxide. [online] Occupational Safety and Health Adminitrator. Available at: <https://www.osha.gov/ethylene-oxide> [Accessed 25 September 2020].


12. National Center for Biotechnology Information (2020). PubChem Compound Summary for CID 6354, Ethylene oxide. Retrieved September 23, 2020 from https:// pubchem.ncbi.nlm.nih.gov/compound/Ethylene-oxide. )


13. Cdc.gov (2020) Hydrogen Peroxide Gas Plasma | Disinfection & Sterilization Guidelines | Guidelines Library | Infection Control. Retrieved 27 September 2020, from https://www.cdc.gov/infectioncontrol/guidelines/disinfection/sterilization/hydrogen- peroxide-gas.html.


14. Rogers, W. J. (2012). Steam and dry heat sterilization of biomaterials and medical devices. In Sterilisation of Biomaterials and Medical Devices (pp. 20-55). Woodhead Publishing.


15. Mauer, L. (2003). PROTEIN| Heat Treatment for Food Proteins.


16. Vinny R. Sastri (2014). Material Requirements for Plastics Used in Medical Devices, in Plastics in Medical Devices (Second Edition).


17. Vendrell, R. J., Hayden, C. L., & Taloumis, L. J. (2002). Effect of steam versus dry- heat sterilization on the wear of orthodontic ligature-cutting pliers. American journal of orthodontics and dentofacial orthopedics, 121(5), 467-471.


Astell Autoclaves – www.astell.com Astell Effl uent Decontamination Systems – www.astellbio.com


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