Designing for Sterilisation
Frequently, sterilisation of medical devices can come as an afterthought in the design process. Often companies get away with this approach, but sometimes it can be a design disaster. Fabulously designed, clever products both hi-tech and low-tech have succumbed to a lack of forethought about sterilisation, costing time, money and failed launch targets or even being forced into design compromises that could have been avoided.
There is a range of sterilisation technologies available to device manufacturers: gamma irradiation, electron beam irradiation, ethylene oxide gas (EtO), steam, dry heat, gas plasma and X-ray sterilisation. Each technology has its pros and cons and it is the design, material choice and packaging options that influence the suitability of any particular sterilisation technology. This article considers gamma irradiation and EtO technologies.
Irradiation technologies are often viewed as quick and clean processes and for some seem to be the default choice for sterilisation. One of the main pitfalls can be material compatibility; a wide range of polymers within broad categories of thermoplastics, thermosets, rubbers and thermoplastic rubbers are used in medical device design covering a wider range of products. Ionising radiation generates free radicals in the polymers, which, depending on the dose and the polymer, can cause radiation effects on the material by chain scission or cross-linking. In some cases these can have some desirable effects, but in others the effects need to be neutralised. Stabilisers, in the form of antioxidants that mop up free radicals and peroxides and hydroperoxides, which can be formed when free radicals react with naturally present oxygen, can be added to polymers to protect them during the sterilisation process. Effects of radiation include:
- Changes in colour, typically yellowing, but sometimes materials turn black/opaque
- Changes in molecular weight, leading to changes in many mechanical properties such as strength or rigidity
- Increases to cross-linking density, leading to harder compounds, stiffness and flex cracking
- Formation of unpleasant odours
- Changes to extractables and leachables.
EtO technology is compatible with a wider range of polymers and materials, but brings its own challenges. The sterilisation process typically takes longer, with a seven day wait for biological indicator incubation unless using parametric release or a validated rapid biological indicator system, which requires extra inventory costs.
When designing products destined for EtO sterilisation consideration must be given to how the sterilant gas will access. One-way valves, long tubing, narrow lumens all make it more difficult for the gas to access all areas of the device and make it sterile. These challenges may be overcome with more aggressive cycles such as deeper vacuums, higher temperatures, longer gas dwell times or a combination of these. For example, long tubing sets can be sterilised with long gas dwells during which time the gas penetrates along the entire tubing length or can cross the tubing wall directly to deliver its kill, but this takes time. Materials impenetrable to EtO such as metals and glass are not suitable for EtO sterilisation because there is no pathway for the gas; for example products such as foil pouches cannot be sterilised this way.
EtO is highly toxic, a carcinogen and goes beyond flammable, it is explosive at just 3% in air, accordingly there are strict safety controls applied and medical devices with a source of a spark such as a battery may not be suitable. Where EtO gas is applied to a product, it must also be removed from the product and ISO 10993-7:2008/AC:2009 gives the current limits for residual gasses, tolerable contact limits and also special situations where different limits are applied to specific device types. It is worth noting not all world markets, notably Japan, accept the limits in ISO 10993-7.
There are of course many other considerations when choosing the best sterilisation technology such as location, capacity, expertise, cost of a sacrificial sterilisation load, costs of ongoing compliance, technical challenges to the process, bioburden and so on. If you don’t have the expertise in-house you are well advised to discuss these openly and honestly with your contract steriliser or a third party expert.
Peter Rose is Managing Director, High Edge Consulting,
BioCity Nottingham, Pennyfoot Street, Nottingham NG1 1GF, UK
tel. +44 (0)115 9216 200
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