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Sterilization: How to Validate Novel and Non-Traditional Processes

This article summarizes the process for validation of novel and non-traditional sterilization processes, and is intended for those who are already familiar with traditional sterilization methods, such as gamma irradiation and ethylene oxide sterilization.

Many devices are sensitive to the heat of steam sterilization and ethylene oxide sterilization, while gamma and e-beam sterilization cause degradation of certain plastics and adhesives. For example, when ultra-high molecular weight polyethylene (UHMWPE) is exposed to gamma irradiation in the presence of oxygen, super-oxides are formed and become trapped in the plastic. Superoxides are responsible for oxidative degradation of UHMWPE implants over time, and results in premature failure. This is why companies developed sterilization processes based upon hydrogen peroxide, nitrogen dioxide, peracetic acid combined with hydrogen peroxide and low-temperature steam combined with formaldehyde, pulsed light, UV light and microwaves. In addition to the development of new sterilization processes, the standards for sterilization process validation have been updated, as well. For example,

IS0 14937 was initially released in 2000 (current version is 2009) to provide guidance on:

  1. the characterization of sterilizing agents,
  2. sterilization process validation, and
  3. the control of the sterilization process.

Material Compatibility

Numerous material compatibility charts have been published for gamma and ethylene oxide sterilization, but new polymers present new challenges to sterilization validation. If a new polymer material meets the performance criteria for strength and biocompatibility, then evaluating compatibility with various sterilization processes is usually the next step. Some companies rule out certain plastics because the materials are not compatible with a preferred sterilization process (e.g., gamma sterilization). However, many new polymers outperform traditional selections, though these polymers are frequently ruled out due to a lack of compatibility with gamma sterilization, or the material may exhibit excessively high ethylene oxide residuals after sterilization. Evaluation of new materials for non-traditional sterilization processes should be considered prior to eliminating a material from the list of design options.

Most of the non-traditional processes involve gas sterilants. If gas sterilants cause deterioration, the material is not compatible. Other gases may be absorbed by the plastic. For example, nylon absorbs hydrogen peroxide gas. Gas absorption by nylon can also complicate the design of primary packaging, because nylon is often selected as a reinforcing layer. If the gas is absorbed by the polymer, this can have several different effects:

  1. Gas residuals may be excessively high and result in failed biocompatibility testing
  2. Amount of gas remaining after absorption may be insufficient to sterilize the device
  3. Variations in sterilizer loading density may create more pronounced variation in sterilization effectiveness

In the case of light sterilization processes, such as UV light, the material must either be transparent to the light, or all surfaces of the device must be in the direct line of sight for the light source(s). Transparency varies significantly based upon the wavelength(s) of light, the clarity of the plastic (e.g., air bubbles) and the surface finish of finished component. For a light sterilization process, fixture design is important as well. Usually conveyors are needed to pass the devices in front of the light source without compromising operator safety. You may also find that dyes and colorants may change color when exposed to certain light sources.

Microorganism Resistance & Process Challenge Devices

One of the tools you will need to conduct sterilization validation is Biological indicators (BIs). Spore test strips and ampules are examples of BIs used for sterilization validation. However, you are required to use an organism that is resistant to the method of sterilization. For example, steam sterilization processes use a species of Bacillus spores that is resistant to heat. The microorganisms that are naturally occurring should not exhibit resistance to the sterilization method, and therefore the BIs can be used as a process control to ensure that the process was effective in killing all of the naturally occurring population. These BIs are used as an extra safety measure—not as an alternative to cleaning product prior to sterilization.