How are plastic-based medical devices sterilized?

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.

Safe sterilization

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

1. Autoclave

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.