The options for sterilisation
Isotron reviews the options for sterilising packaging, equipment and cleaning materials prior to cleanroom operation
The use of sterilisation and decontamination techniques, such as ionising radiation, moist/dry heat, ethylene oxide gas and chemicals, to assist in control of contamination in cleanrooms is growing. Many process items introduced into cleanrooms, such as packaging and raw materials, are routinely decontaminated prior to use to ensure that they do not contaminate the cleanroom.
In addition, garments and the supporting paraphernalia for manufacturing processes and cleaning equipment, such as wipes and mop heads, need to be decontaminated or sterilised before they are introduced. Such items are often sterilised elsewhere and then brought into the cleanroom in sterile packaging. Clearly, cleanrooms used for the manufacture of terminally sterilised products will be managed in a different manner from cleanrooms used for aseptic manufacture.
Different meanings The term sterile is an absolute, yet it can mean different things to different people. For example, medical devices sold within the EU and labelled sterile must achieve a Sterility Assurance Level (SAL) of 10-6: a probability of no more than one non-sterile item in a million units. Terminal sterilisation processes (such as those above) will achieve this, provided that product bioburden is low and under control. However, aseptic manufacturing processes will be challenged to approach these SALs. It is important, therefore, when bringing items into the cleanroom, to appreciate the sources of potential contamination and the challenges they present to achieving a desired SAL. The techniques described achieve sterilisation by the inactivation or destruction of viable micro-organisms. The mechanism depends on the chosen method, and susceptibility depends on the nature and number of contaminating microflora (see Table 1). The three ionising radiation techniques – gamma, electron beam and X-ray – normally take place within large radiation cells with two metre-thick concrete walls and ceilings in order to protect workers from the radiation emitted during these processes. Due to the large size and high capital cost of this type of plant, they tend to be run by contract service providers, such as Isotron, with customers sending products to the plant for treatment. Traditionally, radiation techniques have been used for terminal sterilisation of single-use items, such as medical devices, and preparatory treatment of packaging or raw materials. Gamma irradiation can be an integral part of the decontamination process, with items such as garments being returned to the cleaning company after use, where they are decontaminated, cleaned and then radiation sterilised. Garments can go through this process up to 50 times. In each process the key parameter is the absorbed radiation dose, measured in kiloGrays (kGy), where one Gray is equal to one joule per kilogram of product.
Gamma irradiation The radioisotope cobalt 60 is the predominant source of radiation in gamma plants. It is manufactured from naturally-occurring cobalt (cobalt 59) using a process of neutron bombardment over a number of years. The radioactive cobalt 60 is then encapsulated in multiple-layer stainless steel pencils, which in turn are housed in a 3D array called a source rack. During operation, the source rack is located in the middle of the radiation cell. For storage in a safe position, the source rack is lowered into a seven metre-deep pond. This is called a wet source and the five to six metres of water above the rack allow safe entry into the irradiation cell for inspection and maintenance. In some plants a concrete-lined pit (dry source) is used. The characteristic blue glow of gamma sources in pools is due to the "Cherenkov effect" of a charged particle passing through a transparent non-conducting liquid. The sterilisation process involves loading product, typically in its final packaged form, on to pallets or aluminium totes outside the cell, which are then transported into the plant and around the source via conveyor systems. Once inside, the product passes around the source rack in a defined fashion to deliver the intended dose. It is important to note that the product does not become radioactive during or after processing.
Electron beam systems In contrast, electron beam systems use sophisticated acceleration processes to generate high-energy beams of electrons. These electrons are then scanned back and forth at frequencies of 50-100Hz to produce a curtain of electrons that the product then passes through. Here, too, the product is loaded onto conveyor systems outside the cell and unloaded after the process has been completed. A principal difference between gamma and electron beam systems is in dose rate, which is significantly higher in electron beam plants. Consequently, cycle times are shorter, which can reduce the adverse impact of radiation on some materials, particularly with respect to oxidative changes. The drawback, however, is that the penetration of electrons is relatively poor compared with gamma photons. Consequently this technology is favoured for low-density products and in carefully controlled packaging configurations. Application of X-rays as a sterilisation technology is not yet well established commercially. It involves generation of X-rays through the impact of high-energy electrons into a metal (tantalum) target. The X-rays have equivalent properties to gamma radiation and can be directed into the product. The benefit is that the high penetration characteristics of gamma are obtained without the need to manage large quantities of isotopes. Currently the efficiency of conversation from electrons to X-rays is poor and the process is relatively costly.
Heat methods The dry and moist heat methods tend to be batch processes with the product being loaded into processing chambers less than 2m3 in size. They generally employ parametric measurement of variables, such as time, temperature and pressure, to confirm sterilisation, although biological indicators (BIs) are sometimes used. Dry heat is an effective method where hot air is used to destroy organisms, primarily by oxidation. As dry air is a poor conductor of heat, temperatures of around 170°C are usually required for long periods of time. This technique tends to be used for items that can withstand high temperatures but require to be kept dry, such as glassware. Another benefit of this method is its ability to inactivate bacterial endotoxins. Moist heat (autoclaving) is one of the most commonly used sterilisation techniques where heat is employed to denature proteins in any contaminating micro-organisms. This typically involves exposure at 134°C for three minutes or 121°C for 15 minutes, both using pressures above atmospheric. Use of steam allows far better conduction of heat than a dry environment. consequently it is critical that the steam comes into contact with all the surfaces that need to be sterilised, and the sterilisation cycle characteristics reflect this requirement. With the high temperatures employed, many plastics are not suitable for sterilisation by this method. The technique is commonly used in pharmaceutical manufacture and hospitals for sterilisation of vials/ampoules and multi-use items such as metal instruments. Ethylene oxide has been used for many years for sterilising raw materials, packaging and single-use medical items. One of its key benefits is its broad material compatibility, so it is frequently the method of choice when dealing with polymers not compatible with irradiation processes (e.g. items containing PTFE). Typically, the ethylene oxide process handles batch volumes of 20-90m3 in specially designed and automated sterilisation chambers. The process takes 12-24 hours, including pre-conditioning, with a further 12 to 120 hours needed to remove residuals from the product. The process is controlled by monitoring process parameters and by the testing of BIs (typically Bacillus sp. spores). As these BIs can take three to seven days to test, the typical turnaround time for ethylene oxide is around seven days. Pressure to reduce this has led to the development of "rapid BI systems" capable of delivering results within hours, and of parametric release systems, which are gradually being applied. Hydrogen peroxide (H2O2) vapour has also been used for the sterilisation of equipment and, more recently, the decontamination of cleanrooms. Systems usually combine four stages of operation: a conditioning/heat-up stage to around 50°C; the gassing stage, in which the H2O2 solution is vaporised and circulated; a dwell stage, during which H2O2 condenses onto surfaces; then, once the H2O2 has been in contact for the necessary length of time, a final aeration stage to remove H2O2 from the atmosphere and pass it through catalytic converters to be destroyed before being exhausted. In this system the micro-organisms are killed by chemical oxidation. The main advantages are the lack of residuals and its speed of one to four hours. Gas plasma sterilisation is a patented process using a combination of H2O2 vapour and low-temperature gas plasma to rapidly sterilise without leaving toxic residues. Systems have capacities that range from 50-400 litres, with cycle times of around an hour, and they are particularly suited to heat- and moisture-sensitive instruments.
Plasma cloud The H2O2 evaporates in the chamber and disperses to kill bacteria before an electromagnetic field is created during the plasma discharge phase of the cycle. This produces a low-temperature plasma cloud containing free radicals and ultraviolet light. These activated components then recombine to form oxygen and water. After venting, instruments are ready for use as they do not need to be cooled or aerated. These are the main techniques available for sterilising or decontaminating items entering cleanrooms, and for microbiological control of the cleanroom environment itself. Some, such as heat (dry or moist) and chemical techniques, can be carried out in or near the cleanroom, while radiation techniques are usually offered by contractors from large centralised facilities. Choice of the most appropriate technique depends on variables such as material compatibility with the process, packaging, size, other components present, and cost. Different sterilisation techniques can be used in combination; however, the impact and interaction of these processes on target materials needs to be evaluated.
