Ethylene oxide is a colorless, odorless, volatile, and toxic gas. (Credit: Sterigenics)

Single-use medical devices, pharmaceuticals, components, and packaging that need to be sterile must be treated with an appropriate and validated technique. It is also important to formally assess the potential effects of the sterilization process on product and packaging materials.

This article describes the most commonly used industrial sterilization techniques, which are radiation sterilization (gamma and electron beam) and gas sterilization (ethylene oxide), and their effect on commonly used materials.

Gamma Sterilization

Gamma sterilization uses a radioactive source, typically Cobalt-60 (60Co), which emits high energy gamma rays. Ionizing radiation can modify physical, chemical, and biological properties of materials. Currently, principal industrial applications of radiation are for sterilization of healthcare products (including pharmaceuticals), irradiation of food and materials modification (such as polymer cross-linking).

Gamma sterilization is a “cold” sterilization technique, where temperature is not a key parameter. Temperature may increase slightly in the product due to ionization, but gamma sterilization may be effective at ambient, refrigerated, or even frozen conditions. The key parameter is the dose received by the product. The dose is dependent on the presentation to the source and the time exposed to the gamma ray source.

Table 1 – Gamma sterilization compatibility with polymers. Note: Though some materials may be listed as “Not recommended,” there may be solutions that exist for improving the radiation compatibility for these polymers. For example, there are many ways to formulate polypropylene (PP) to improve radiation compatibility.

Gamma rays, emitted from 60Co, are pure energy, similar in many ways to microwaves and x-rays. Gamma rays delivered during radiation sterilization alter chemical bonds by interacting with the electrons at the atomic level. Although gamma rays are highly effective in reducing or eliminating microorganisms, they do not have sufficient energy to impart radioactivity on the device or component being sterilized.

The minimum dose required to sterilize a product is based on its bioburden (i.e., the microbiological contamination on the product) and the maximum acceptable dose is driven by the radiation tolerance, and stability, of the product.

Gamma sterilization may be performed on individual boxes, in irradiation containers usually referred to as totes, or on pallets.

Packaging for Gamma Radiation

Polymers classified by absorption and desorption characteristics.

Because there is no requirement for pressure or vacuum, gamma radiation eliminates the need for gas permeable packaging materials as required for EO processing. Packaging is developed and formulated for radiation stability. Tough, impermeable packaging materials provide a strong, long-term sterile barrier.

Materials compatibility. Gamma radiation is compatible with many plastics, all metals, and glass (subject to color change). Some polymers, however, are affected either by embrittlement, discoloration, or degradation.

Note that gamma rays generate free radicals that can further react and degrade the materials. The nature of the reactions of these free radicals depends on the nature of the plastics, the presence of oxygen, the absence/presence of additives (antioxidants may be added to limit the free radicals), the dose applied to the material, and other environmental factors. Some polymers that are quite resistant to temperature, chemicals, and acids, such as PTFE, may be extremely sensitive to radiation. Table 1, derived from AAMI TIR 17:2008, ranks polymers according to their relative resistance to radiation. 1

Electron beam (E-beam) Sterilization

Electron beam sterilization does not use a radioactive source and, instead, product is sterilized by exposure to a concentrated charged stream of accelerated electrons generated by an electron accelerator. Electron accelerators are capable of producing electron beams that are either pulsed or continuous. E-beam radiation is a form of ionizing energy that is generally characterized by its relatively low penetration and high dose rates.

In comparison, gamma radiation has high penetration and low dose rate, while E-beam has high dose rate and low penetration, but either technology can give a reproducible irradiation process.

E-beam irradiation is similar to gamma processing in that electrons alter various chemical and molecular bonds in the exposed product, including within the DNA of microorganisms.

The dose can be delivered to the product much faster than for gamma, but the penetration of the electrons is more limited than gamma rays. The technique is indicated more for low-density and uniform products. Typically, the irradiation container for E-beam processing is the individual product box. The boxes are typically irradiated on one side and then rotated 180 degrees to expose the opposite side.

Packaging for E-beam is quite comparable with that used for gamma and the compatibility with materials is also quite similar, and the information presented in Table 1 may also be applicable for the E-beam process.