Several types of ionising radiation are currently being used to sterilise single-use medical devices, including gamma rays from radioactive nuclides, high-energy electrons and x-rays emitted by energetic electrons. All of these energy sources can eject electrons from atoms and molecules in irradiated materials, thereby producing charged ions that can kill or prevent the replication of micro-organisms. The choice of a radiation source for this application depends on practical factors, such as material thickness and density, absorbed dose uniformity, throughput rate and economics.
Gamma rays are electromagnetic photons with short wave-lengths and specific energies that emit in all directions. Cobalt-60, the preferred energy source for industrial irradiation facilities, emits two gamma rays with energies of 1.17 and 1.33 million electron volts (MeV). Gamma ray facilities consist of a large source rack surrounded by a multi-layered array of product carriers.
Many carriers are irradiated simultaneously to make efficient use of power. These carriers move in a shuffle-dwell manner to expose the products in many different locations around the source rack. Treatments may require several hours, because the radiation field is diffused over a large area and its intensity is relatively low.
High-energy electrons are atomic electrons that have been emitted into an evacuated space from a high-temperature cathode and acquire high kinetic energy from strong electric fields in a particle accelerator. Electron-beam facilities for medical device sterilisation typically have electron energies in the range of 3 to 12MeV.
The concentrated electron beam is spread across a wide product conveyor by scanning with a deflection magnet, which is similar to, but more powerful than, the magnet used in a computer monitor or a television tube. Treatment times are short, usually only a few seconds, because the high-intensity radiation field is concentrated within a small area. The penetration of the electron beam in irradiated materials is substantially less than with gamma rays or x-rays.
High-energy x-rays are electromagnetic photons emitted when high-energy electrons strike material. The efficiency of x-ray conversion increases with the electron energy and atomic number of the target material. Tantalum is the preferred material for large-area targets. The broad photon energy spectrum extends up to the maximum energy of incident electrons.
The x-rays are even more penetrating than the gamma rays from cobalt-60 sources when using electrons with 5MeV of kinetic energy for their production. In contrast to the isotropic emission of gamma rays, high-energy x-rays are concentrated in the forward direction of incident electron beams. This increases the intensity of radiation fields for the same emitted power and permits a reduction of product carriers in the treatment zone. It also facilitates the treatment of single-product carriers as large as full pallet loads. This feature can simplify the transition from one type of product to another, with a different density or a different dose requirement.
A typical facility for continuous x-ray sterilisation of medical products with low bulk densities at high production rates has three rows of product carriers in the treatment zone. The carriers rotate before making a second pass through the x-ray beam.
A variation of this design is the one-pallet rotating continuous system. There is only one rotating carrier in the treatment zone at any time. This version has only one row of product carriers in the treatment zone, with the central carrier rotating while it stands in the x-ray beam. This arrangement makes it easier to change the process conditions for different product requirements. The system is suitable for x-ray treatment of products with low, medium or high bulk densities. It also makes it easier to adapt to different product requirements, by being able to change the process conditions for each carrier without affecting the others.
A further innovation is the four-pallet rotating batch system (see Figure 1). This is a compact facility for the x-ray sterilisation of medical products with low bulk densities in batches. There are four product carriers – or pallets – on a large rotating platform in the treatment zone. The shielding doors are closed before the x-ray generator is energised.
DOSE UNIFORMITY RATIOS
Computer simulations using Monte Carlo codes have evaluated the capabilities of these facility concepts. The product carriers were assumed to be industrial pallets (1.0 x 1.2 x 1.8m). Figure 2 shows the dose uniformity ratios (DURs) of the different systems with 5MeV x-rays. These calculations cover a wide range of bulk densities from 0.05 to 0.8g/cm³ – although packages of single-use medical devices seldom exceed 0.³g/cm³. Higher densities, from 0.4 to 0.8g/cm³, are more applicable to other products, such as fresh foods.
The data show that any of these facilities would give sufficiently low DURs with full pallet loads and bulk densities below 0.3g/cm³. A four-pallet rotating batch facility has slightly lower DURs for densities below 0.2g/cm³, but this property is slightly higher at 0.3g/cm³. A single-pallet rotating continuous facility would have lower DURs than any of the other systems for densities above 0.2g/cm³.
Figure 3 illustrates the volume throughput rates in m³/h with 5MeV x-rays and 500kW electron beam power on the x-ray target. Emitted x-ray power is nearly as strong as the gamma ray power emitted by 3MCi of cobalt-60 sources. The data show that the three-row continuous system can potentially have the highest throughput rates for all densities.
The throughput rates of the four-pallet rotating batch system would be slightly below the three-row continuous system. The single-pallet rotating continuous system gives substantially lower throughput rates for all densities. Even so, the main advantage of the single-pallet rotating continuous system is obtaining lower DURs for products with higher densities in cases where higher-dose uniformity ratios obtained with other systems might not be acceptable.
It should be noted that all the throughput figures shown in the graphs take a 5MeV energy level into account. Considering that a 7MeV energy level would double all those throughputs without any alteration to the DUR. Indeed, discussions are going on at the ISO level to allow the use of 7.5MeV electron beams (under certain conditions) to produce x-rays.
The application of x-ray processing is increasing as a consequence of the development of high-power, high-energy electron accelerators and high-power x-ray targets. Several industrial irradiation facilities are now able to provide both electron beam and x-ray processing services. The use of this technique is expected to grow as its attractive features are more widely recognised.