Recent Developments in Sterilization Technology

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published September 1998

STERILIZATION

Sterilization, as a specific discipline, has been with us for approximately 120 years, since the invention of the steam autoclave by Charles Chamberland in 1879.¹ Since that time, we have seen progressive refinement in steam sterilizers: from the early, manually operated equipment to modern microprocessor-controlled, automatic machines. Although the efficiency, reliability, and performance monitoring of modern equipment is continually improving, the fundamental process remains essentially the same.

Sterilization processes cannot be considered in isolation; rather, they are inextricably related to the product to be sterilized. They are also related to the packaging of the sterilized product. Except for the rare instances when the sterilizer can be located where the sterile goods are to be used, there is a need for the sterilized products to be packaged in a manner that will preserve their sterility during storage, handling, and transport. The majority of sterile goods produced in the medical device industry and in healthcare facilities are terminally sterilized—that is, they are sterilized already packaged. They may be packaged only in their primary packaging or in multiple layers of packaging such as a unit pack, shelf pack, and shipping carton.

The PureBright system (PurePulse Technologies, Inc.) uses intense pulses of light to kill a variety microorganisms, viruses, and spores. Photo: Purepulse Technologies, Inc.

For a product sterilized in its packaging, the packaging material must be compatible with the sterilization process. This requires both that the packaging tolerates the process without adverse effects on its performance characteristics, and that it permits the attainment of the specified sterilization conditions in the product to be sterilized.

There is no single sterilization process that is suitable for all medical products. The diversity in sterilization processes—and of operating systems within each process—has arisen as a consequence of the efforts made to optimize medical sterilization and to meet the differing needs imposed by the vast range of products to be sterilized.

Traditional Processes

The sterilization processes that have traditionally been used for medical products include steam, ethylene oxide (EtO), ionizing radiation (gamma or E-beam), low-temperature steam and formaldehyde, and dry heat (hot air). These methods can be divided into three categories, based on the nature of the sterilant and its reaction with microorganisms: physical processes (ionizing radiation, dry heat); physicochemical processes (steam, steam/formaldehyde); and chemical processes (EtO, glutaraldehyde).

Chemical and physicochemical processes depend on direct physical contact between molecules of the sterilant and the microorganism to be killed. In consequence, access must be available to the surfaces of the product to be sterilized and the packaging material (for terminally sterilized products) must be porous or permeable to these molecules. For example, in the case of steam sterilization of dry products, the air must be removed from the package and replaced with steam in direct contact with the product. This is necessary both to provide the required thermal energy by condensation of the steam and the required water for the protein hydrolysis reaction to occur. At the same temperature in the absence of water, the degradation of proteins would occur at the much slower rate characteristic of dry-heat processes.

Purely physical processes such as ionizing radiation may be used for product designs and packaging materials that are impermeable to gases as long as they are "transparent" to energy of the wavelengths employed in the sterilization process.

New Processes

What do we mean by a new sterilization process? New is often a marketing description for the latest outcome in a gradual development and refinement of an existing process. Continued development or refinement in one area allows, and sometimes requires, development in another. This advance may be driven by the product, the sterilization process, the packaging, economics, or other external forces.

Over the past two or three years, the development of the art of sterilization seems to have accelerated, with the introduction of several new processes. At least one reason for this is the potential decline in the use of EtO in hospitals. This is a result both of increased concern over the toxicity of residuals and of the need to eliminate the use of chlorofluorocarbons (CFCs), which had previously been employed to minimize the flammability and explosion risks of the EtO. Another prominent trend is the proliferation of various minimally invasive therapies, and the need for appropriate sterilization protocols for the equipment used in these procedures.

Medical instrumentation can be sterilized through a hydrogen peroxide gas plasma process with the Sterrad 100 system (Advanced Sterilization Products). Photo: Advanced Steilization Products

It is important to note that one must consider two very different fields of application for sterilization processes—industrial and hospital. Although the same level of sterility assurance should be provided in each case, the operational circumstances are sufficiently different that the two fields need to be considered separately. In industry, a sterilizer will typically be used to process virgin product with a known bioburden. The range of products will be limited and may be a single product type or single product family of closely related types. There will normally be good engineering and analytical laboratory support, and the process will be subjected to in-depth validation and routine monitoring. All of the personnel involved will be specifically trained in the process.

Contrast this with the situation found, all too often, in hospitals. A diverse range of reusable products will be processed after being subjected to a largely unvalidated cleaning process. The extent and nature of residual soiling and bioburden will be unknown. The process will be subject to minimal validation—certainly not covering the diversity of products processed—and only rarely will there be adequate support from an analytical laboratory. The expectation will be that the process should be simple to operate and safe for use by personnel with minimal training.

Chemical Processes (Gas/Liquid)

Whereas the most widely used traditional chemical processes were based on alkylating agents such as EtO and the various aldehydes, most of the new methods are oxidative processes based on "peroxy" compounds. These include sterilants based on compounds such as hydrogen peroxide, peracetic acid, peroxysulphates, chlorine dioxide, and ozone. For the most part, the microbicidal action of these chemicals has been recognized for many years.

Peracetic Acid. Peracetic acid is currently used in a number of sterilization processes. Examples include liquid systems such as the Steris machine (Steris Corp., Mentor, OH) for endoscopes, or use as a liquid sterilant in suitable disinfectors (e.g., Nu-Cidex from Johnson & Johnson Medical Ltd., Skipton, UK) for sterilization of thermolabile endoscopes. Vapor-phase generators employing peracetic acid are being sold for the decontamination, disinfection, or sterilization of products such as isolators.

The process itself is not new, as the bactericidal activity of peracetic acid was noted by Greenspan and MacKellar in 1951.² It was used in solution as a sterilization process as early as 1955, and, in the vapor phase, by Portner and Hoffman in 1968.³ Aerosolized peracetic acid for the sterilization of surgical instruments was considered by Werner and others in the early 1970s.

Peracetic acid is a colorless liquid with a pungent odor, miscible with water. Commercially available as a 35% or 40% solution, it is generally unstable, decomposing to give oxygen, acetic acid, and other degradation products, which include hydrogen peroxide and water.

Acetic acid and hydrogen peroxide are invariably present in low concentration. Peracetic acid is corrosive to certain materials and it is lachrymatory, an irritant, and a vesicant (causing blistering) on prolonged contact. Its use as a sterilant therefore relies on obtaining formulations that inhibit corrosion of sensitive materials and stabilize the solution to give it a usable shelf life.

The Steris system currently available for the sterilization of endoscopes is—like the glutaraldehyde system it is intended to replace—a wet system. This limits its applicability, since it becomes difficult to provide a system for packaging and storage of the sterile product. In common with any other liquid chemical system, water is used to flush out any residual chemical at the end of the process. Ensuring that this water is of suitable chemical purity and microbial quality to prevent recontamination of the processed goods requires thorough control. The vapor-phase process has found applications within industry for decontamination of environmental spaces, but there has been no move toward the development of a general-purpose peraceptic-acid sterilization process, and none seems likely.

Hydrogen Peroxide. Hydrogen peroxide as a 3% aqueous solution has long been used as an antiseptic. For example, hydrogen peroxide potentiated by ultraviolet light has been used in the production-line sterilization of commodity items such as cartons for food products. The use of hydrogen peroxide as a vapor-phase sterilant was developed by Amsco in the United States as the VHP system. This process was originally developed in several formats, including a cassette system for endoscopes, a freestanding system for environmental decontamination, a system for use in sterilizing lyophilizers and isolators, and a general-purpose unit for the sterilization of medical devices. (Following the recent acquisition of Amsco by Steris, it appears that the company will emphasize the development of peracetic acid for endoscope sterilization and hydrogen peroxide for environmental and general-purpose applications.)

The deep-vacuum hydrogen peroxide process operates in a manner analogous to gas sterilization processes.^4–6 Initial air removal allows for rapid diffusion, and humidity is controlled to optimize the microbicidal effect. The process is compatible with a wide range of materials, but traditional packaging is likely to interfere with the process because of its reaction with, or high absorption of, the hydrogen peroxide. Although the process is finding extensive use in the sterilization of lyophilizers, a general-purpose unit is not yet available.

Ozone. The bactericidal and sporicidal effect of ozone has long been recognized. Its use as a sterilant, however, has been limited because of its instability, which precludes storing it ready for use, and because of the difficulty of generating pure ozone. Ozone is produced naturally by the effect of sunlight or ultraviolet light on oxygen, and also by electrical discharge. Recent technological advances have made the generation of ozone a more practicable proposition, and commercially available sterilizers have been developed.⁷ The Cyclops Co. has introduced a machine for sterilizing endoscopes that pumps humidified ozone through the unit. Advantages of the system are said to include freedom from long-term toxic residuals and ease of use, with only medical-grade oxygen and electrical connections required. Potential disadvantages include the reactivity of ozone with certain materials.

Chlorine Dioxide. Chlorine dioxide (ClO₂), which is a gas at temperatures above 11°C, was discovered by Sir Humphry Davey in 1811 and is another chemical that has long been known to have microbicidal properties and that has, like ozone, been used in water purification systems. The germicidal and sporicidal properties of chlorine dioxide have been recognized since 1936 (Leseurre) and 1949 (Ridenour et al.), respectively.^8,9

Many disinfection technologies employ chlorine dioxide in aqueous solution and, when necessary, use nitrogen or air purging to remove the traces of residual gas. The major problems with this technology have always been that chlorine dioxide gas cannot be safely liquefied or stored under pressure for transport and subsequent use (since under these conditions it is explosive), and that as an aqueous solution it is unstable and corrosive. Recent developments have seen the use of both gaseous and liquid chlorine dioxide systems.

Gaseous Systems. In the system developed by Johnson & Johnson, the chlorine dioxide is generated in situ by the action of chlorine on sodium chlorite. The chlorine is presented as 2% Cl₂ in N₂, in a cylinder filled initially to 2700 psig and then emptied to 300 psig; the chlorine accounts for a pressure of 60 psig. The generator employs a two-column system, with discharge of the chlorine into the first column pressure controlled and monitored, and output from the generator monitored by a fiber-optic UV absorption system. The working life of the column is limited to 70% of its theoretical capacity, as established by validation studies, in order to ensure that the conversion process will always take place effectively. The second column is used as a backup.

The sterilizer is operated at slightly above room temperature (32°C), which allows for good control over the process. The process uses a cycle analogous to that of EtO sterilizers, with a vacuum air-removal stage followed by a dynamic conditioning stage to humidify the chamber and load to an RH of about 70%. At the end of the conditioning phase, ClO₂ gas is admitted to give a concentration of 30 mg/L. This is then topped off by the addition of N₂ at pressures of 80 kPa. A total gas exposure time of about 60 minutes is standard. At the end of the cycle, the ClO₂ is removed using a four-pulse dynamic air exchange.

Advantages of this process compared with EtO are that—because ClO₂ does not have the chemical solubility of ethylene dioxide—there are no significant levels of residual sterilant within the product material, and that ClO₂ is not flammable in air at the concentrations employed.

Gaseous ClO₂ may be removed from the effluent airstream by scrubbing with Na₂S₂O₃. Residual levels for discharge to the atmosphere can be well below 1 ppm and are usually undetectable.

The gaseous chlorine dioxide system is currently being used in several medical applications, including the sterilization of contact lenses and the secondary sterilization of overwrapped foil suture packages.

Liquid Systems. Solutions of chlorine dioxide are also commercially available as liquid sterilants—under trade names such as Tristel and Medicide—and as such compete with glutaraldehyde and peracetic acid solutions. While the microbicidal efficacy of chlorine dioxide has long been recognized, there have been two problems associated with the use of liquid systems. First, the solutions are unstable, with the concentration of ClO₂ rapidly diminishing; second, because chlorine dioxide is highly oxidative, it is potentially corrosive to many materials. The development of usable solutions has therefore required formulations that incorporate stabilizing agents, usually based on boron components and anticorrosion compounds. These comprise a base solution and an activator which, when mixed, yield a solution of approximately 0.1% chlorine dioxide, with a 14-day shelf life. Solutions of this type are increasingly being used for the sterilization of fiber-optic endoscopes.

Physicochemical Processes

Plasmas. Plasma is the fourth state of matter, and as such is distinguished from solids, liquids, and gases. Plasmas are produced at very high temperatures, or at low temperatures in strong electromagnetic fields (the so-called "glow-discharge" plasmas). The plasma usually consists of a reactive cloud of ions, electrons, free radicals, and other neutral species.

The plasma process seeks to produce a sterilizing effect using lower concentrations of sterilant—with a higher reactivity—than would be possible in a normal gas process.^10–12 Because the active species are only present when power is applied to the system and disappear quickly when the power is turned off, the very active species that act as the sterilant will not be present as a source of toxicity at the end of the process.

The precursor gas selected for plasma generation will determine which active species are present, and these may be expected to influence the comparative microbicidal activity of the system.

When a plasma contacts the surface of an item to be sterilized, the collisions between the active species and other molecules cause a significant proportion of the active species to return to the ground state. Packaging material can thus cause a serious depletion in the concentration of active species reaching the item to be sterilized, soiling on the surface may have a significant inhibitory effect, and the extent of diffusion into narrow lumens may be limited.

The Sterrad Process. The Sterrad process (Advanced Sterilization Products, Johnson & Johnson Medical Inc., Arlington, TX) is a plasma system that uses hydrogen peroxide as the source of the active species. The process seeks to overcome the inhibitory effect of packaging materials by using a gas-diffusion phase to allow gas to penetrate to all parts of the load before the plasma is created. The adequacy of this approach depends on the certainty with which one can ensure that the hydrogen peroxide gas diffuses to all parts of the load and that the nature and construction of load items will not inhibit subsequent plasma formation. Although this system is becoming widely adopted, there are still reservations about its use in hospitals—where product cleaning prior to sterilization may not have been well controlled, where inappropriate products may be processed, and where parametric release may be used without supporting evidence comparable to that required for other processes.

Steam. The inclusion of steam sterilization in the context of recent developments in sterilization technology may at first seem strange. However, there is a continuing evolution of the equipment, packaging, and monitoring systems used for the process. The publication of EN 554 has stimulated renewed interest in ensuring appropriate steam purity for product contact. One continues to see progressive refinement of the microprocessor-based control systems and, in particular, of secondary or supporting functions, such as providing users with a prompt when maintenance is required. Control systems are becoming much more user-friendly, with touch screen systems becoming commonplace. Improvements continue to be made in related steam sterilization supplies such as packaging materials and biological and chemical indicators.

Synergetic Processes

Psoralens and UVA (PUVA). An interesting example of the development of sterilization techniques for specific applications is the recently reported use of ultraviolet light in combination with psoralens to purge blood plasma and platelets of pathogenic organisms. Psoralens are naturally occurring substances found in a wide range of plants, in which their role is to fight infection from pathogenic fungi.

Irradiation of blood with UV light has been recognized as a method of treating otherwise intractable infections ever since its development in the 1930s for use with polio patients. It is reported that the fundamental effect of exposure to UVA is to stimulate the body's biochemical and physiological defenses.^13,14 Researchers have speculated that this is related to the low concentration of ozone produced from the oxygen circulating in the blood.

Ultraviolet blood-irradiation therapy is currently under investigation for the treatment of diseases such as HIV infection and hepatitis, and is the method of choice for the treatment of cutaneous T-cell lymphoma. The use of UV is also noted for its ability to inactivate viruses while preserving their antigenic properties for the preparation of vaccines.

The recent proliferation of novel blood-borne viruses has led to demands for better safety guarantees for blood products, and hence many methods of sterilization have been extensively examined.^15,16 It has become clear that most viruses are quite sensitive to UVB or to UVA when used with psoralens as photosensitizing agents. The psoralens form a labile bond with DNA and RNA which, upon exposure to UV light, becomes a firm bond. Recent work by Cerus Corp. (Concord, CA) appears to show that synthetic psoralens and UV irradiation can be used to destroy infectious agents such as HIV, hepatitis viruses, and toxemia-inducing bacteria. However, it has been thought that producing viral inactivation of sufficient magnitude was not feasible without causing intolerable damage to vital blood components—especially erythrocytes, in which hemoglobin blocks the penetration of UV light.

The absence of genetic material in platelets, however, means that these would remain unaffected by the PUVA mechanism, and it should therefore be possible to use the technique to sterilize plasma and platelets. This possibility is currently under investigation in clinical trials. Although the psoralens and dead microorganisms would remain in the product, it is considered unlikely that they would pose a risk, given the extensive clinical history of psoralens.

Microwave and Bactericide. Sterilization methods are being marketed that propose the use of microwaves in conjunction with a bactericidal solution—a modern version of the century-old process of heating with a bactericide. The technique is being promoted for use with dental instruments and relies on heating a solution of a quaternary ammonium compound (benzylkonium chloride) to approximately 100°C. At present, these processes are applicable only to unpackaged instruments.

Low-Temperature Steam and Formaldehyde. Low-temperature steam in combination with formaldehyde is another traditional process that has continued to evolve. It is an example of synergism in that it brings together steam at subatmospheric pressure and formaldehyde gas—neither of which is markedly sporicidal—to produce a highly efficient sporicidal effect.

The process has been in and out of fashion several times over its 100-year history. Concerns over the toxicity and carcinogenicity of formaldehyde have limited its acceptance in the United States, despite potential advantages over ethylene oxide. More recently, improved process control has allowed the production of sterilizers with negligible environmental emissions and very low product residual levels. Other developments have included the use of operating cycles at temperatures comparable to those employed for ethylene oxide instead of in the 70°–80°C range that was traditionally employed.

Physical Processes

Microwaves. The inherent advantage of microwave heating compared with other forms of heating lies in its lower power requirements. The interaction between microwaves and biological materials does not of itself appear to be lethal: rather, the lethality obtained is directly derived from the heating effect, which in turn depends on the composition of the microorganism being targeted, including its water content. Limitations related to the specifics of microwave reflectance, transmittance, and absorbance may limit applicability for device sterilization.

Pulsed-Light Systems. A novel sterilization method introduced in the past several years uses high-power electrical energy to produce intense pulses of light that are claimed to provide unique bactericidal effects.¹⁷ Called the PureBright system (PurePulse Technologies, San Diego), the technology rectifies and converts normal building ac to high-voltage dc and uses it to charge a capacitor, which is then discharged through a specially designed xenon lamp unit. The high-voltage, high-current pulse applied to the lamp causes it to emit an intense pulse of light, which typically lasts for a few hundred microseconds. The light produced by the lamp includes a broad spectrum of wavelengths, from ultraviolet to infrared, with an intensity some 20,000 times greater than sunlight.

The process is reported to be highly successful in killing microorganisms, viruses, and spores, as well as in deactivating enzymes. Its effectiveness depends in part on the ease with which the organisms to be killed can be directly illuminated. For example, organisms on porous surfaces or those suspended in turbid solutions will require higher treatment levels compared with those on smooth, continuous surfaces or transparent materials. Parametric release should be practicable, since the factors controlling the microbicidal activity can be directly, and continuously, monitored. These include both the energy output in the UV range and the lamp current, on which the intensity and spectrum of each flash depend. In use, the normal operating ranges for the system are from 0.1 to 3.0 J/cm² per flash, with total accumulated fluences of 0.1 to 12.0 J/cm². The number of lamps, their configuration, and the flash rate depend on the particular application. The economics of the process are encouraging, with costs as low as one cent per square meter of surface sterilized.

Potential applications include the surface sterilization of packaging materials for aseptic packaging or for bioburden reduction, and the terminal sterilization of parenterals packed in transparent plastic bags or bottles (e.g., from a blow, fill, seal machine). The photoproducts from treated substrates are reported to be generally similar to those induced by exposure to sunlight and similar to, but fewer than, those produced by thermal sterilization processes. When the process is used to sterilize the surface of opaque materials, any degradative effects would, of course, be restricted to the surface.

Validation of Sterilizer Processes

In the development of new sterilization methods, a key consideration is the data that are needed to demonstrate the efficiency of a process. Although in the United States there is a well-defined process for review of new types of hospital sterilizers, this is not the case worldwide. Furthermore, the FDA approval system does not apply to sterilizers for use in industry.

The lack of any universally accepted approach to validation or process approval for new sterilizers has led to considerable difficulty for both manufacturers and users as new processes have been introduced. There is also a need for a suitable European standard providing a means for presumption of conformity to the Medical Devices Directive for sterilizers, since these have become devices within the scope of the directive. These factors have stimulated work on a standard for Validation and Routine Control of Sterilization Processes—General Requirements, which is being developed within ISO TC 198 as a common international and European standard under the Vienna agreement. A draft for public comment is expected to be published some time this year. It is to be hoped that the standard will provide a format establishing a common standard for the acceptance or rejection of new sterilizing processes.