Choosing a Sterilisation Method: 'It Is Almost Impossible to Give a Rule of Thumb'
Medical device manufacturers need to take many factors into consideration when they choose their method of sterilisation. First and foremost, it has to be compatible not only with the product but also the packaging.
At the recent Medpack Conference in Luxembourg, I interviewed Reiner Eidenberger, managing director of contract sterilisation company Synergy Health Allershausen GmbH, about the selection criteria.
Thomas Klein: Each method of sterilisation has inherent advantages and shortcomings. Can you describe the strengths and weaknesses of heat, chemical, and radiation sterilisation?
Reiner Eidenberger: No sterilisation method suits all materials, products, and packaging.
Sterilisation by moist heat leaves no residue -- what's left is moisture and the product simply has to be dried after sterilisation -- but many single-use medical devices are made of thermoplastics, which do not withstand high temperatures. In addition to high temperatures, both the product and packaging have to sustain vacuum and pressure forces. Special attention must be paid to selection of the sterile packaging, which has to be permeable to moisture and air but also provide a barrier to microorganisms. For industrial sterilisation, this method is rather inadequate because of the small loading volume of the available sterilisers.
The most common chemical sterilisation method is fumigation by ethylene oxide (EO). This method is well suited to large volumes. As with moist heat sterilisation, packaging design requires special attention. The packaging must be permeable to EO, moisture, and air but act as a barrier to microorganisms. Like moist heat, EO is a surface sterilising medium. Hence, the product design has to be chosen carefully so that EO is able to get to each part of the product. This can be a challenge with very tight lumens, porous materials, and hermetically sealed cavities. After fumigation, the product has to be stored in a degassing area, where the product is flushed with fresh air or a mixture of nitrogen and fresh air to bring EO residuals below an acceptable value.
Validation of moist heat and EO sterilisation is rather complex. There are many process parameters such as time, temperature, vacuum, pressure, and moisture that have to be taken into consideration. For EO sterilisation, the question of gas concentration is also important.
Biological indicators have to be used with both sterilisation methods.
Validation of radiation sterilisation is much easier: no bio-indicators are necessary. Product can be released by checking the absorbed sterilisation dose, which is called parametric release. There are basically three technologies available: gamma radiation, accelerated electrons (E-beam), and X-ray. All three technologies are suitable for processing large volumes. Because of the good penetration -- especially with gamma and X-ray -- there are almost no restrictions on product and package design, and stacked pallets can be treated. Regarding E-beam sterilisation, only single boxes can be treated and they have to be re-stacked afterwards on a pallet. But, with every strength comes a weakness: Some plastic materials become brittle and most electronic parts or components do not withstand ionising radiation. Product, packaging, and the constituent materials have to be qualified, as with any other sterilisation method.
Klein: Choosing an appropriate sterilisation method depends on a lot of factors. Nonetheless, can you give us a rule of thumb for selecting a suitable sterilisation method depending on the product category and type of package?
Eidenberger: Taking into account the aforementioned strengths and weaknesses of the different sterilisation methods, it's almost impossible to give you a rule of thumb. But I could suggest the following: moist heat is a no-go and EO can be challenging for thermolabile materials, and the more complex a product is from a design perspective, the more I would prefer radiation sterilisation. Also, there are almost no restrictions regarding packaging design when radiation sterilisation is used.
Klein: Can you name a common mistake medical device manufacturers make when it comes to sterilisation? Is there a factor that is frequently overlooked?
Eidenberger: I do not want to talk about mistakes -- I prefer to talk about challenges. I would recommend that the sterilisation method be taken into consideration at an early stage, during the product design phase. Indeed, it is often overlooked that not all products, materials, and packaging are suitable for all sterilisation methods.
Thomas Klein, Associate Editor