Gamma sterilisation: which plastics can take it
An eye drop bottle must be sterile, and stay so. One way to achieve this is to pass gamma rays through it, without heating it. Its plastic still has to withstand the treatment.
By Tecmom editorial team · 2 min read
Sterilising without heat
Gamma rays, most often emitted by a cobalt-60 source, pass through packs and destroy the micro-organisms inside. The treatment is done cold, through cartons, on products already packed: that is its great advantage for materials that cannot take heat.
The dose is measured in kilograys. Usual sterilisation doses lie, according to the sterilisation supplier Nordion, between 20 and 50 kGy. The ISO 11137 series of standards governs how that dose is set and validated.
What radiation does to plastic
Radiation does not act only on microbes. It can break or link polymer chains, and react with additives. Depending on the material, this shows as yellowing, embrittlement, or nothing visible.
The differences are large. Nordion ranks PET among very tolerant materials. Conversely, it advises against unstabilised polypropylene, which handles usual doses poorly: it can yellow and turn brittle. Copolymers and radiation-stabilised grades behave much better.
Other routes exist
Radiation is not the only method. Steam, in an autoclave at 121 °C, suits materials that can take heat, such as polypropylene. Ethylene oxide, a gas, sterilises at low temperature but requires a phase to remove its residues. The choice of method and the choice of material are made together. See polypropylene.
For eye drops or a sterile product
The European Pharmacopoeia requires eye drops to be sterile. For their bottle, the sterilisation method, the material and its compatibility are part of the medicine's file; one is not changed without checking the other. See eye dropper bottles and pharmaceutical packaging.
Gamma rays leave no trace on the product, but they can leave one on its pack. Choosing the material together with the method ensures that the sterile bottle is as sound as the new one.
Glossary
- Gamma rays
- Highly penetrating radiation used to sterilise packed products cold.
- Kilogray
- Unit of absorbed radiation dose, written kGy.
- Stabilised grade
- A version of a material formulated to withstand radiation better.
- Ethylene oxide
- A sterilising gas used at low temperature.