Sterilisation is the process by which living and dormant micro-organisms (bacteria, fungi, viruses) are eliminated from materials and objects. Micro-organisms can be as small as 0,2 µm, but are still extremely potent, causing diseases and infections, some of which can be fatal.

The aim of a disinfection is to reduce the number of pathogenic germs using chemicals to gain a safe level to avoid infections. Usually, human skin, large surfaces and surgical instruments are disinfected by wiping with cleaners and disinfectants, possibly based on alcohols, aldehydes, peroxides, chlorines, etc.

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.

Sterilisation

1 in 1,000,000

germs remain

compared to

Disinfection

10 in 1,000,000
germs remain

Understanding different medical device sterilisation methods is essential to selecting the best polymer materials for the device. The appropriate selection of the sterilisation method early in the product design stage is critical as the sterilisation method is an integral part of the production process of the final device, changing this later can be costly and time consuming.

Today, the main methods for medical device sterilisation are steam sterilisation, sterilisation with EtO, as well as sterilisation with gamma irradiation.

Steam sterilisation

This method is performed in an autoclave using moist heat under pressure.

The history

Steam 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 process

Steam 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 selection

The 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.

The history

Ethylene-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 process

EtO 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 selection

Most 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.

The history

This 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 process

The 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 selection

This 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.

The demand for sterile medical devices has been growing constantly over the past years driven by megatrends including the ageing population and increase in chronic diseases. The global sterilisation service market reached ca. 3bn USD in 2020 and is expected to grow further with a CAGR of ca. 5%. COVID-19 boosted this demand with the need for more sterile test systems as well as more personal protective equipment. Sterilisation providers have limited capacities, and further regulatory restrictions or environmental controls might be ahead – especially concerning gamma and EtO sterilisation. With the current challenges, also in worldwide logistics, supply security is in an alarming state. Increased requirements and re-certifications for certain medical devices required under the new MDR pour even more “-oil into this fire-”.

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-beam

The 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-ray

X-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 Peroxide

The 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.

Selection of the medical device sterilisation method and of the device materials is complex as they have an impact on one another. And today, no one-size-fits-all solution has been found. For both sterilisation and disinfection processes, extensive testing must be conducted to verify the desired performance and suitability of each component.

Based in Hamburg / Germany, Jana Zietzling is the Head of Product Management Medical at IMCD. She has spent the last decade dedicated to the sales and marketing of medical polymers in Europe. Every day, she is passionate about saving and improving patient lives by supporting our customers in developing and producing various medical devices.