What does sterilization mean?
Microorganisms are present on all surfaces and some of them even survive in space. They can thus be difficult to get rid of, and since Louis Pasteur’s revolutionary discoveries in the 19th century, various methods have been developed to achieve sterility.
Sterilization of materials or instruments is achieved through processes that greatly reduce all living microorganisms, including their dormant stages such as spores.
AMB and sterilization
AMB does not provide in-house sterilization processes. On the other hand, we provide advice on the choice of plastic material in relation to the customer’s choice of sterilization process, and take care of the logistics.
An important prerequisite for a sterilization process is to minimize the number of viable microorganisms on the products already during production, the so-called bioburden. That is why production is located in clean rooms. AMB offers clean room production in two clean rooms class 7-8 including packaging.
Regardless of the method of sterilization, these must be reproducible and validable.
Process validation means that you ensure that a process consistently gives the same results and meets the specified specifications.
Is it possible to achieve complete sterility?
In practice, a 100% sterility cannot be achieved. Reference is therefore made to the SAL level (Sterility Assurance Level) which is based on probability calculations. For example, it is required that the residual content of reproducible microorganisms must not exceed SAL 10-6, ie that out of one million bacteria, only one may survive the sterilization process.
Compatibility of plastics with the sterilization process
Of course, the materials to be sterilized must withstand being exposed to the effects of the various treatments. They must be classified as suitable for the intended use and it is important that the mechanical properties are maintained. All sterilization processes affect the material properties. Material selection and packaging methods must therefore be considered already during the development and design process.
The American ISM has produced a table for the compatibility of different plastics with the most common sterilization processes.
What sterilization processes are there?
The sterilization is performed by physical processes (thermal or by radiation) or chemical processes.
There are many different methods that all have their specific uses. Since AMB works with plastic-based products, this article mainly concerns plastic-related sterilization processes.
4 different sterilization processes for plastic parts
A more commonly known method of sterilization is by steam in an autoclave (so-called autoclaving). Simply put, an autoclave is a pressure cooker. The air in the autoclave is completely replaced by water vapor.
Autoclaves are found in most laboratories and hospitals. They are often used in the daily work of objects and instruments that are used continuously but again must become sterile. The prerequisite for suitable materials is that they must be thermostable and resistant to moisture.
One of the advantages of autoclaving is the relatively harmless and simple handling.
The objects to be sterilized are heated in steam to 121 ° C at 2 bar pressure for 20 minutes, or to 134 ° C at 3 bar for 5 minutes. For infectious proteins (prions), heating to 134 ° C at 3 bar for 18 minutes is required, but longer times also occur.
After many autoclaving cycles, some materials may show color differences, usually a yellowing, which for the most part does not affect the function. For some materials it is stated how many cycles in the autoclave they can handle.
For metals, temperature is not a problem, but the high moisture content can cause corrosion. In general, for environmental, risk management and economic reasons, it is often better to choose materials that can be autoclaved.
Validation takes place according to ISO 17665.
2. Gas sterilization with formaldehyde or ethylene oxide
For heat-sensitive materials such as fiber optics and electronics – but also many plastics – alternative sterilization methods are needed. One such method is to use chemicals, either in liquid or gaseous form. In gas sterilization, substances such as formaldehyde or ethylene oxide are used.
Gas sterilization is carried out between 48 and 75 ° C and is therefore suitable for temperature-sensitive materials. The method is most often used for disposable items.
But here too, of course, the materials must withstand the chemical treatment. In addition, the chemical must be able to reach all surfaces to be sterilized and here there are some differences between the chemicals. Particularly complicated are narrow hoses and especially if one end is closed.
Prior to the process, it is important that the objects to be treated are clean and dry and that they are packed in special gas-permeable foils. After the treatment, the gas must leave the sterilization material, which takes many hours. As both gases are toxic, great safety is required in the process and handling.
Validation takes place according to ISO 11135 for ethylene oxide and ISO 25424 for formaldehyde.
3. Gas Sterilization with hydrogen peroxide plasma
Sterilization with hydrogen peroxide plasma works at low temperatures and is thus suitable for many plastics. Highly reactive hydroxyl radicals kill microorganisms at temperatures of only 45-50 ° C during cycles of 24-80 minutes, but variations occur. There is almost no risk of corrosion as the relative humidity is around 5%. There are also no toxic residues that require time for degassing.
However, there is still no validation standard for the process and therefore it is sometimes used ISO 14937.
4. Sterilization by radiation
Radiation sterilization is suitable for many types of plastic and uses gamma or beta rays (E-beam). For comparison, gamma radiation has high penetration and low radiation dose, while E-beam has high radiation dose and low penetration. There are no such radiation facilities in Sweden. The methods are mainly used for sterilization of disposable products in an industrial environment.
The radiation induces chain breakage in the DNA molecules of microorganisms, which makes it impossible for them to multiply and they die. The organisms then remain on the product.
Validation for both gamma and E-beam radiation takes place through ISO 11137.
The difference between gamma radiation and E-beam
Gamma radiation is an electromagnetic wave emanating from radioactive cobalt-60, while E-beam consists of highly accelerated electrons. These processes require expensive equipment and great safety. However, gamma radiation does not make the irradiated materials radioactive.
E-beam is similar to the gamma process in that electrons alter various chemical and molecular bonds in the exposed product, including the DNA of the microorganisms.
The radiation dose can reach the goods much faster than with gamma radiation, but the penetration of the electrons is more limited. E-beam technology is more suitable for low density products and uniform products. Usually the packaging unit for E-beam is individual boxes. The boxes are usually irradiated on one side and then rotated 180 degrees to expose the opposite side.
Benefits of radiation sterilization
- Relatively fast processes
- gamma: a few hours, pallet-wise (but many pallets can be handled simultaneously)
- e-beam: a few seconds, in boxes.
- Real-time (parametric) release based on dosimetric results allows immediate release after irradiation
- No chemicals needed, no risk of contamination
- “Cold” processes at low temperatures
- Suitable for complex geometries
- The goods are already packed in final packaging * before the process
*) Sterile barrier and packaging system according to ISO 11607 which is intended to prevent microorganisms from penetrating the packaging and also enable aseptic unpacking.
Disadvantages of radiation sterilization
The disadvantage of the radiation is that the facilities are very complex and expensive due to the potential risks associated with the processes.
The radiation supplies energy that triggers chemical reactions. Chemical bonds are broken down and depending on the material, this leads to crosslinking or reduction. Both radiation methods can change the appearance of the polymers used; often the color changes to yellow. In addition, the mechanical properties such as tensile and impact strength as well as extensibility can be changed.
Different plastic materials and radiation
Some plastics come (as partially mentioned above) when irradiated:
- to change color to yellow
- mainly to degenerate and the material properties to deteriorate
- mainly form crosslinks (crosslink) and the material properties improve
- remain neutral
Therefore, it is important to think about sterilization methods already in product development and consult with both material suppliers, tool manufacturers and injection molders.
Good to know when choosing materials for radiation
- Most medical plastics are radiation resistant
- Use materials with a high molecular weight and a narrow molecular weight distribution
- Aromatic materials are more durable than aliphatic materials
- Amorphous materials are more durable than semi-crystalline materials
- Higher levels of antioxidants improve radiation resistance
- Low density materials are more durable than high density materials
- Materials with small side groups are more durable
- The lower the crystallinity, the greater the radiation resistance
- Materials with low oxygen permeability are more radiation resistant
- Avoid materials such as acetal, polypropylene (unstabilized) or Teflon (PTFE)
Text: Jörg Hinz, AMB Industri AB.