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Selecting materials for medical device sterilisation
By: Jana Zietzling
Matching the polymer performance with different medical device sterilisation techniques is not an easy task. Hygiene and cleanliness are essential in hospitals to provide a safe environment for the care and recovery of patients. The requirements for sterile medical devices in surgical theatres and wards are high and have become standard in our technologically developed world.
Defining medical device sterilisation
The difference in sterilisation and disinfection
After an optimal disinfection only 10 in one million germs remain on an object, whereas after optimal sterilisation only one in a million remain, according to DIN EN 556-1.
For devices which are disinfected, the material selection is critical as the more and more aggressive chemicals used in response to increased germ resistance are challenging for some polymers.
1 in 1,000,000
10 in 1,000,000
Designing for sterilisation methods
Today, the main methods for medical device sterilisation are steam sterilisation, sterilisation with EtO, as well as sterilisation with gamma irradiation.
Steam sterilisationThis method is performed in an autoclave using moist heat under pressure.
The historySteam sterilisation is the oldest method for medical device sterilisation used and remains the most common method in hospitals today. Around 1862, Louis Pasteur discovered that short-term heating of food and other products killed most micro-organisms. This was the basis for the steam sterilisation oven, the autoclave; invented by Charles Chamberland ca. 14 years later and still used today.
The processSteam sterilisation is carried out at temperatures between 110°C and 140°C at increased pressures. Germs can be reliably eliminated after about 20min at 121°C and a pressure of 2 bar in which proteins in the cells of the micro-organisms coagulate, removing all or almost all biological functions.
The material selectionThe elevated temperature applied during steam sterilisation challenges some polymer materials in terms of heat resistance. Consequently, only polymers that show a heat distortion temperature well above the steam sterilisation temperature could be considered as possible candidates.
These could be materials like polyolefins with high melting temperature and certain grades of COC (Cyclo-Olefinic Copolymer). In the case that multiple steam sterilisation cycles will be applied, with surgical instruments for example, materials such as PEEK could be considered.
Sterilisation with EtOThis method is a chemical sterilisation that involves the utilisation of gas.
The historyEthylene-Oxide is a colourless, flammable, explosive gas. Originally, Ethylene-oxide (EtO or EO) was used to protect food in storage against insects and rodents. In 1933, P.M. Gross and L.F. Dixon found out that EtO has an anti-microbial effect. Since then, EtO has been used for the sterilisation of food, with increased usage in other industrial segments, like pharmaceutical packaging.
The processEtO sterilisation has proven to be a stable and flexible method for a broad range of medical devices, gaining a market share of around 50% of all sterilisation methods. It is very effective in killing micro-organisms, and uses only comparably low temperatures around 50°C-60°C.
The material selectionMost medical polymers can be used for this sterilisation process as there are only limited requirements against heat or moisture. Items sterilised by this method need to be packed in gas-permeable packaging allowing the EtO gas to penetrate and dissipate after the process.
EtO gas can leave toxic, carcinogenic residues on sterilised devices, so special attention is needed to avoid potential physical and health hazards. This process of medical device sterilisation has been well optimised, but further regulations could be on the horizon, especially with an eye to stricter environmental impact rules.
Sterilisation with gamma irradiationThis method is a type of sterilisation with ionised radiation.
The historyThis method of medical device sterilisation was developed around 1940. A famous story says that Bruce Banner was injured by a gamma ray bomb and ever since transforms into the big, green Hulk when he becomes angry. Actually, gamma rays break the bonds of DNA, rendering micro-organisms harmless.
The processThe source of gamma rays is cobalt-60 which is produced by irradiating the naturally occurring metal cobalt 59 isotope with neutrons in a nuclear reactor. When radio-active cobalt 60 decays it emits gamma rays, which have a high penetration depth. This means they can easily pass-through packed products, often stacked on full pallets, at only a low dosage (typically 25kGy), without leaving them in a radio-active state.
The material selectionThis method is typically applied for medical disposables like catheters, pipette tips, and cannulas that do not have high requirements regarding temperature or barrier properties. Nevertheless, these high energy radiation methods can have certain effects on the polymer material such as yellowing or discolouration of the material.
Copolyesters suffer minimal colour shift and return almost to the original colour after a few days. For other transparent materials like PMMA, but also for oxidation sensitive polymers like polypropylene, stabilizing additives can help reduce or mask colour shift effects. In TPU, aromatic materials will show yellowing but in fact are more resistant to the radiation sterilisation than aliphatic materials which may suffer from a loss in molecular weight due to chain scission despite more stable appearance.
New approaches to medical device sterilisation
Existing, but currently less popular, sterilisation methods could be considered as alternatives in order to narrow the gap between supply and demand as well as to offer risk mitigation strategies for the supply chain. Recently, the FDA has encouraged the industry to develop new approaches such as supercritical carbon dioxide, nitrogen dioxide, vaporized hydrogen peroxide, vaporized hydrogen peroxide-ozone, and accelerator-based sterilisation, like e-beam and X-ray.
Interestingly, sterilisation by UV does not have sufficient energy to ionize particles as with gamma, e-beam, or X-ray, but is accepted for disinfection of air or contact lenses, for instance. Effectiveness is very much dependent on the material and application, though.
E-beamThe sterilisation by electron-beam is also a method with ionized radiation. While the before mentioned gamma rays are generated by the decay of Cobalt-60 – which would also apply to alpha and beta rays -, the e-beam is generated by normal electric current. These electrons are accelerated to reach almost the speed of light and achieve an energy which can penetrate many materials.
X-rayX-ray irradiation equipment consists of the e-beam system with a target of tantalum (or tungsten) positioned in front. In comparison to gamma radiation, X-ray is less harsh on the polymers due to a reduced oxidative effect.
Supercritical Carbon Dioxide
Sterilisation by CO₂ - this is Carbon Dioxide in a fluid state above critical temperature/pressure - is a process with mild conditions. Since 2006, CO₂ has been used for commercial cleaning of textiles for private clients and received the eco-label “Blue Angel” in 2007 as an environmentally friendly process.
Vaporized Hydrogen PeroxideThe VHP process is a low temperature and negative pressure sterilisation method using gaseous hydrogen peroxide. It is already used for cold aseptic filling of beverages like water, juices, etc. in PET bottles. During and after the sterilisation, only oxygen and water vapour are produced so the process is safe and environmentally friendly leaving no toxic residues.
Currently, some of our principals are carrying out corresponding tests of selected medical polymers with the aforementioned sterilisation methods, specific results can be provided upon request.