Many of the sterilization issues concerning gamma and electron beam radiation are the same.

  • The mechanism by which living organisms are destroyed is the same.
  • Both enjoy parametric release after processing.
  • Both utilize the same dosimetry and indicator systems.

This point is supported by the fact that both technologies are governed by the same ISO standards when applied to medical device sterilization—ISO 11137 and ISO 13409. Both technologies have been proven effective at sterilization and are accepted worldwide.

Figure 1: Depth dose curves for two-sided gamma irradiation of a tote or carrier of many boxes of homogeneous material.There are, however, significant differences between the two forms of sterilization radiation processing. The medical device engineer or regulatory manager should evaluate which method is right for the product(s) in question and for the future of their company. Conversion from one technology to another is fairly straightforward and the guidelines for validation of the changeover are found in the ISO standards mentioned previously. The major differences between E-beam and gamma irradiation are explained in this article.

Gamma rays can penetrate farther into materials, occasionally colliding with an electron and giving the affected electrons enough energy to destroy the bioburden. The gamma ray may then initiate more high energy electrons as it travels through the medical device (Figure 1). Electron beams start out as very high energy electrons and destroy bioburden directly. In addition, these high energy electrons collide with other local electrons. These secondary electrons also have energy adequate to destroy bioburden. This results in an increase in the dose as the beam penetrates the medical device. Each succeeding collision reduces the energy of the resulting electrons until there is no penetrating power left in the beam (Figure 2).

Figure 2: Depth dose curves for two-sided E-beam irradiation of a box of homogenous material.Dose Rate or Processing Time
The dose rate is the biggest difference between the two technologies. Gamma radiation has high penetration and low dose rate. E-beam has high dose rate and low penetration. Either technology can give a reproducible sterilization dose. Gamma radiation is best suited to treating a large batch of many “totes” of boxes over a six to ten hour cycle. E-beam is a continuous process that can deliver a required dose in one to two minutes as individual boxes pass by the e-beam accelerator. The lower “dwell time” in the E-beam irradiation process is less stressful to materials. The E-beam process allows faster turnaround time and more flexibility in delivering a specific irradiation dose to small batches of product. E-beam can easily switch from decontaminating returned goods at 40 kGy to sterilizing a batch of radiation sensitive product at 15 kGy.

Material Effects
Much has been written about the effects of gamma and e-beam radiation on plastic materials. The general consensus is that e-beam is friendlier to a plastic medical device and its packaging than gamma. These differences are attributed to the longer dwell time in gamma irradiation cells during which ozone, other reactive compounds, and heat can accumulate. The short dwell time of E-beam has less severe effects on materials because the radiation by-products rapidly dissipate after the short exposure to irradiation. NUTEK has several comprehensive references available to its customers that compare changes in yellowing, embrittlement, and strength characteristics when subject polymers are exposed to varying doses from both forms of irradiation.

There are no direct environmental issues that affect the end users of contract sterilizer services. The gamma irradiator must cope with higher containment and reprocessing costs, however, that may be reflected in overall operating costs. With E-beam irradiation, when the power is turned off, the radiation stops.

Generally, the per box rate for sterilization irradiation is less using gamma when large batches of high density material are processed. However, the set-up costs and minimum fees for E-beam are usually significantly lower. This means that E-beam tends to be advantageous for small to midsize sterilization lots of medical devices. In addition, the quick turnaround in processing adds to the value of E-beam technology for the medical device manufacturer. Even large lots may be more economically processed using E-beam if the final packaging is designed to match the penetration power of the E-beam facility.