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An important consideration in polymer selection for medical applications is the method of sterilisation that will be used. One of the biggest problems is that a particular medical device may contain a variety of polymers, selected for their individual properties. The result could be that the complete device cannot be effectively sterilised. Therefore, sterilisation techniques need to be considered at the very outset of the project when material selections are made.
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There are a variety of sterilisation methods that may be used and it is important to ensure that the chosen polymer is suited to the selected process. (See Table).
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This is a widely used method using steam for sterilisation. Temperatures used vary from 121C to 143C, as do the cycles. Polymers that may be okay for autoclaving at 121C may not be suitable at higher temperatures so it is not always possible to say a polymer can be autoclaved.
There is also a cumulative effect on polymers so it is necessary to know the duration and the number of anticipated cycles. Depending on the device, it may also be necessary to support it whilst autoclaving to prevent distortion. This method is usually used for repeated use instruments, etc rather than medical devices as the latter are often pre-packaged.
See Table showing polymer suitability for different sterilisation techniques.
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There are two types of irradiation process – gamma rays and electron beam (E-beam). Dosages are measured in Megarad (Mrad) and doses of 2.5 Mrad will sterilise clean articles in air. However, this is the minimum dosage and the actual delivered dosage is often much higher. As with autoclaving, the effects of radiation are cumulative so this needs to be borne in mind. This method will work for items that are packaged.
Irradiation will result in changes to impact strength, tensile strength and elongation at break of polymers due to changes to their structure. Changes will depend on the base polymer and any additives used and may not be immediately apparent. As the effects of irradiation can continue long after the process has been completed, there may be a lapse of time before any changes become apparent. Many plastics will discolour after irradiation, although this may fade over time.
See Table showing polymer suitability for different sterilisation techniques.
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Ethylene Oxide may be used in certain circumstances. The majority of plastics are unaffected by this method although some can absorb ethylene oxide so need a further treatment to eliminate it before use.
See Table showing polymer suitability for different sterilisation techniques.
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Normally, when selecting a polymer for a particular application the main consideration is the performance required from the part. It is then a case of matching the material properties to those requirements. For medical applications things become a lot more complicated. Not only does biocompatibility need to be considered but so does the sterilisation method that is to be used. This makes material selection particularly difficult and often only a limited choice is available.
Design becomes increasingly important. It may not be possible to select a polymer that fully meets the performance properties required, so parts may need to be designed to compensate for this.
It must also be remembered that gaining approval for medical applications takes longer and is more costly than for non-medical products. Understandably, there is a considerable amount of form-filling needed, even at the beginning when trying to obtain data on suitable materials.
Providing as much information as possible on the application of the device (see Medical Standards Classifications) will help to speed the process.
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