Feature Article

The layout of a typical X-ray pallet processing facility shows how different products with varying sterilisation requirements can be combined during the same irradiation cycle.

Sourcing sterilisation methods for medical devices
Deciding on a sterilisation strategy for a medical product is not easy. Today, three main irradiation sterilisation technologies are available, all of which have advantages and drawbacks. They are summarised in Table I. The materials used to fabricate the medical device, ultimately, will determine the sterilisation method that is used: different materials react differently to radiation, and each type has its own tolerance level.

The physical properties of some plastic materials can degrade when they are treated with ionising radiation and exposed to ozone, which is produced by irradiating air in the treatment area. The degradation is more noticeable when the material is exposed to gamma rays than when X-ray or E-beam sterilisation methods are used, because of the lower dose rates and longer exposure times required by gamma rays. However, some radiation-resistant plastic compounds are now available that withstand gamma, X-ray and E-beam sterilisation.

The Dose Uniformity Ratio (DUR) is the ratio between the maximal and minimal dose that is required to effectively process a product. The DUR is not as crucial for materials that have a good tolerance to irradiation; devices made of materials that have a limited resistance to irradiation, however, will require an optimal DUR to prevent unacceptable levels of degradation.

X-ray sterilisation of medical products has been studied theoretically and in practice by the US National Institute of Standards and Technology, universities and accelerator manufacturers for more than 40 years. Commercial use began about 15 years ago, but the low output power of early accelerators hindered industrial uptake. That is changing with the recent introduction of high-power, high-energy accelerators.

Modern industrial accelerators have increased throughput, making X-ray sterilisation competitive with medium and large cobalt-60 facilities. Today there are X-ray sterilisation facilities in Europe, Japan and North America.

X-ray sterilisation enables increased penetration
Processing materials and commercial products with high-energy X-rays can produce beneficial changes that are similar to those obtained by the use of gamma rays emitted by cobalt-60 sources. Both X-rays and gamma rays are electromagnetic radiations with short wavelengths and high photon energies that can stimulate chemical reactions by creating ions and free radicals in irradiated materials.

Figure 1: The economics of X-ray sterilisation compared with gamma sterilisation.

A significant difference between X-rays and gamma rays, however, is the radiation’s angular distribution: nuclear gamma rays are emitted in all directions, whereas high-energy X-ray photons are concentrated in the direction of the product being sterilised. The narrow angular distribution of X-rays enables increased penetration of materials, because the most intense zone of emitted radiation is perpendicular to the surface of the target products. By contrast, the nearly isotropic radiation in an industrial gamma facility has a wide angular distribution. Consequently, gamma-ray emission is more divergent than high-energy X-ray emission, penetrating the products at larger angles from a perpendicular direction.

These properties partially explain why X-ray sterilised products have a significantly better DUR than gamma-sterilised products. The other key reason is the wider energy spectrum generated by accelerated electrons at energies higher than 5 MeV.

From a practical point of view, X-ray sterilisation systems irradiate full pallet loads by moving them continuously through the X-ray beam. The loads are irradiated from the side as they pass in front of a long, vertically oriented target and on opposite sides at both high and low elevations. A nearly uniform vertical dose is obtained. A virtual animation of the X-ray irradiation process is available for viewing on the IBA website: www.iba-industrial.com/animis.

Irradiation tests performed in a new X-ray facility located in Europe used a full pallet load measuring 100 x 120 x 180 cm with a homogeneous density of 0.15 g/cm3. Alanine dosimeters were placed on a horizontal grid at various heights inside the load to identify the maximal and minimal dose locations. The load was irradiated by multiple pass cycles, each cycle consisting of four passes in front of the X-ray target (both sides, top and bottom of the pallets).

Several irradiation tests were performed to identify optimal irradiation parameters. Following optimisation, a DUR value of 1.25 was achieved when processing full pallets of homogeneous products with densities similar to medical devices. This is a significantly better result than the typical 1.45 DUR obtained with the same pallet irradiated by a cobalt-60 source.
The DUR of the X-ray process permits pallet-based sterilisation of medical devices, which could only be processed previously by means of Gamma-based tote sterilisation.

The simulated cost per volume of sterilised medical devices (cost per m³ of sterilised products, for example) shows that, compared with gamma sterilisation, the economic advantages of X-ray sterilisation increase in relation to the production volume of the facility (Figure 1). The tipping point at which X-ray sterilisation becomes economically advantageous compared with gamma sterilisation is mainly a function of the price of cobalt-60 and the cost of the irradiation solution and electricity. These costs vary by project and by location. As a rule of thumb, however, X-ray sterilisation should be taken into consideration starting with 1.5 MCi capacity facilities.

X-ray sterilisation design strategies
When designing an X-ray sterilisation facility, various power strategies can be considered:

• Nonstop production. A nonstop sterilisation facility operates more than 8000 hours per year. With this configuration, initial investments are reduced since the required power is optimised for nonstop production. When more capacity is needed, X-ray generator power can be increased with minimal downtime.

• Operating only during off-peak hours. Under some circumstances, it may be feasible to operate an X-ray sterilisation facility only during off-peak hours (when electricity costs are reduced). It is possible to set up an X-ray sterilisation operation in this manner by installing an accelerator with more power than is required, allowing the manufacturer to treat the same volume in less time. This extra power is also available to react to surges in production volumes.

• Temporary power licenses. X-ray sterilisation technology allows the introduction of new cost models such as temporary power licenses. Depending on the facility and system installation, temporary power licenses can be purchased to manage a sudden temporary peak in production because of an unanticipated customer request or to get caught up after a production delay, for example. The cost of temporary licenses depends on additional power needs and duration.

An alternative to traditional medical device sterilisation technologies
Medical device manufacturers seeking a sterilisation solution have more options today. It’s not just about gamma, E-beam or EtO, anymore: X-ray sterilisation can be an attractive alternative, thanks to recent developments in high-energy, high-power electron accelerators. X-ray sterilisation facilities are operating successfully today in Europe, Japan and North America. The technology offers the capability to turn off the radiation source, to control the X-ray intensity and to process products on pallets whilst achieving a desirable DUR. Irradiation tests performed in the newest industrial X-ray facility have shown that a DUR of 1.25 can be achieved for pallet loads of low-density materials compared with a DUR of 1.45 for pallet loads in a cobalt-60 irradiation facility.

In our view, there will be a slow but steady migration from gamma to X-ray sterilisation driven by market demand for better sterilisation quality and in response to the increasing cost of cobalt-60, the regulatory constraints underpinning its use and its shrinking availability.

Philippe Dethier
is Marketing Manager at IBA,
3, chemin du Cyclotron, B-1348 Louvain-La-Neuve, Belgium
tel. +32 10 201 249
e-mail: philippe.dethier@iba-group.com
www.iba-industrial.com