Table I. (Click to See Full Table) A sampling of time-temperature relationships for steam sterilization.
Despite its use of high temperatures, steam is a simple and inexpensive sterilization method with many benefits. It yields little waste (entropy is its only by-product). It is also efficacious in terms of its ability to kill microbial organisms.
With the growing complexity of medical device and drug combinations, it is essential to consider steam. These combination devices, such as drugs in syringes, drugs on catheters, and drug-eluting stents, often require steam sterilization because EtO cannot sterilize liquids and irradiation breaks down many drug compounds.
Risks and Challenges
Steam sterilization has long been used in hospitals as well as in the pharmaceutical, aseptic processing, and food industries. In many ways, it has been a victim of its own successes. For example, steam is most often characterized by its overkill. It uses extremely high temperatures to inactivate highly resistant nonpathogenic thermophile spores and, more recently, extremely resistant prions, that other sterilization methods cannot destroy. Its simplicity and low capital cost make it an inexpensive, attractive, and viable sterilization method. Typical steam sterilization equipment costs less than one-third as much as an EtO chamber system and controller. It costs less than one-fourth as much as gamma or E-beam equipment and facilities.
Sterilization exposure times can range from as short as 3 minutes at 134°C to as long as 3 hours at 101°–111°C, depending upon the bioburden (see Table I). Hospitals use steam at 134°C for 3 minutes for flash sterilization in emergency situations and 121°C for 15 minutes on a routine basis.
Design Considerations for Steam
Table II. (Click to See Full Table) A sampling of time-temperature relationships for steam sterilization.
In order to consider steam as a sterilization method, manufacturers must account for the influence and effects of temperature when they are designing devices (see Tables II and III). As devices become more complicated and sophisticated, there is an urgent need for plastics that are more heat stable. There is also a need for a reasonable bioburden approach without concern for thermophiles, thermotolerant spores, and anaerobes.
If a bioburden approach were applied to steam sterilization, as it is toward radiation methods (without looking at thermophiles, thermotolerant spores, and anaerobes), low-temperature steam (e.g., 90°–100°C) would likely be sufficient to achieve 10–6 sterility assurance for ultraclean (low bioburden), environmentally controlled manufactured devices. With compatible adjuncts (e.g., pH <4.5, formaldehyde), steam cycles could be as low as 65°–75°C. A combination of steam (for heating) and dry heat (for inactivation) may provide another means of effectively sterilizing materials that are otherwise difficult to sterilize.
Table III. (Click to See Full Table) Materials that can be sterilized using dry heat.
The number of healthcare products that can be steam sterilized has always been high. Healthcare products that can be steam sterilized include drugs, fluids, surgical instruments, metal containers, implantables, and reusables. By contrast, the use of steam for presterilizing medical devices or for sterilizing disposables or devices for use in controlled environments is relatively small compared with EtO and radiation methods. Steam sterilization, however, is being used more frequently to sterilize combination products. Steam is sometimes the only means to effectively terminally sterilize combination devices without adversely affecting the drugs incorporated into them.
Processing and Materials
Steam sterilization is generally carried out at 121°C (250°F) for 15 minutes or at 134°C for 3–4 minutes. Temperatures can be reduced to 115°C, and even as low as 105°C, depending upon the bioburden, integrity, heat resistance, and characteristics of the material being sterilized. Low-temperature steam processes (65°–80°C) have been used (e.g., steam-formaldehyde); however, other combinations could also be used. Now that ethylene glycol has been deemed no longer a significant toxic residue, a combination of steam and EtO is an option for sterilization of pyronema domestication on cotton sponges. Steam-propylene oxide, which is less effective and less toxic, should also be considered. Although steam-formaldehyde is not used in the United States because of safety concerns, it is used in Europe, India, and elsewhere.
Steam (water vapor) is a ubiquitous compound. Steam delivers high heat condensation, and it is an activating agent. Before a dormant spore can begin germination and outgrowth, it must be activated. However, at higher temperatures, steam becomes sporicidal. Sterilization, by definition, destroys or eliminates resistant microbes, including bacterial spores such as anthrax.
More-resistant microorganisms (e.g., prions) cannot be eliminated using most standard methods. Extended and high-steam sterilization, however, can at least reduce the effectiveness of these organisms. Using the classical definition of sterilization, it is an absolute criterion. A method has to be capable of destroying or eliminating all forms of life. In practice, however, sterilization is best defined as a process that is capable of delivering a certain probability that an exposed or treated product or material is free from viable microorganisms, including resistant microbial spores, such as Bacillus anthracis and smallpox, and prions.
No sterilization method sterilizes all healthcare products and materials without some damage or destruction.
Consequently, sterilization methods must be selected after much consideration, evaluation, and review of their parameters and effects.
Heat, for example, can damage some materials. It can melt acrylics and styrene, distort PVC, and corrode some metals. A product that is wet after steam sterilization can also be a problem. Moisture also can adversely affect electronics and can cloud some materials or leave water mark stains on them. Wet products or packages can be a source for recontamination. To alleviate issues caused by moisture, changes to the loading and processing procedures may be needed or a drying step may be required to remove moisture and dry the product.
Sterilization agents that kill all microorganisms are not without complications and limitations. Heat can seriously deform, melt, or degrade parts. Radiation can alter, cross-link, deteriorate, discolor, offgas, and damage some materials. Chemical sterilants can leave toxic residues. Many sterilization methods may not penetrate certain plastics and mated surfaces. Steam is the only means of sterilizing prions with high-intensity temperature. It is a viable option for sterilizing and decontaminating neurosurgery and ophthalmic instruments, for example.
Government Use of Sterilization
More than three decades ago, the U.S. Department of Defense (DoD) took on the task of improving the war against germs with improved sterilizers for its field hospital units. Originally, the military used steam autoclave systems. For the DoD evaluation, the initial principal candidates evaluated were steam, steam-EtO, steam-formaldehyde, and electron-beam (E-beam) irradiation. Steam-EtO processes have been used for sterilization of cotton sponges and salt-encrusted spores.
In 2003, large x-ray machines were used to eliminate possible anthrax germs sent through the U.S. mail. X-ray systems, however, may not be effective against small viruses or prions. These machines are sufficient for treating paper and cellulose materials, but their ionizing irradiation can degrade materials and thus possibly start fires. In addition, they may be unable to inactivate some small germ viruses. X-ray irradiation can destroy drugs and high-tech electronics. It also produces diminutive toxic offgassings, and thus poststerilization aeration of the mail was required.
The Future of Steam
For combination products, steam or steam combined with other methods should be considered. For example, dialyzers can be steam sterilized in place on carousels and released via process control or parametric release on a routine basis. Dialyzers can also be sterilized with water at high temperatures. Some sutures can be steam sterilized, and some polypropylene films and Tyvek (spunbonded polyolefin) packages can be autoclaved. Some plastic containers and syringes that contain liquids can also be steam sterilized. These include containers and syringes made from materials such as high-density polyethylene, polyvinyl chloride (PVC), and polyallomer (copolymer of propylene and polyethylene). Items such as syringes, catheters, or drug-coated stents can be steam sterilized at low temperatures (i.e., <121°C).
Improvements to plastics, such as copolymerization and the addition of heat stabilizers, are making them more suitable for steam sterilization. The reduction of sterilization temperatures also helps make plastics more suitable for steam. The consideration of steam sterilization is also important in the context of developments in sterilization technology. Improvements in computer controls, microprocessors, monitoring devices, biochemical and chemical indicators, and the integration of lethality for parametric release all affect the viability of steam sterilization. Because of environmental considerations, some contract facilities in Europe are replacing EtO sterilizers with autoclaves as an acceptable alternative.
A Look at Dry Heat
Dry heat processes cannot produce heat as effectively as steam methods can. However, dry heat at lower temperatures has been found to be effective for materials and electronics that cannot be sterilized using steam or irradiation (see Table III). Given that steam can heat 12 times faster than dry heat, steam can be used to heat packaged products, which can then be sterilized with dry heat. Dry heat can sterilize electrical components without damaging them, and it can sterilize metals without producing corrosion.
Given sufficient time, dry heat can penetrate surfaces that steam and chemicals cannot. Considering the EtO process overall—including preconditioning, sterilization, and poststerilization aeration—dry heat times of 4–7 days would be equivalent to the overall EtO process release. Sterilization at lower temperatures (e.g., 105°–135°C) allows even more materials (polypropylene, high-density polyethylene, polysulfone, etc.) and items (instruments, nonaqueous embolics, etc.) to be sterilized by dry heat.
There are many materials that can be damaged by low-temperature dry heat, including acrylonitrile butadiene styrene, acrylics, styrene, polyethylene, and PVC. However, unlike radiation, dry heat is a possible option for heating materials such as acetal, polypropylene, and Teflon.
Polyurethane can be hydrolytically attacked by steam but not by low-temperature dry heat.
Dry heat processing uses no other agent (e.g., steam or chemical humidification) as a means of sterilization. Therefore, it should be a good candidate for process control and parametric release. The same principles of calculating lethality by steam sterilization apply to dry heat.
The combination of steam and dry heat may provide a means to effectively sterilize products that might not otherwise be sterilizable by either method alone. And, if a product might be contacted by both moist and dry heat, it may be beneficial to combine both methods in sequence. Using steam followed by dry heat would enable the dry heat to dry the load after the moist heat had destroyed most microorganisms. Spores resistant to moist heat are not typically resistant to dry heat and vice versa, so some sterilization may occur with time in dry heat after steam sterilization.
While steam has rarely been used in the medical device industry, many plastic devices sterilized by EtO and radiation may be compatible with lower-temperature steam. Many factors are pointing to steam as a viable alternative to the more-traditional methods of EtO and E-beam. The continuing development of combination products and increasing microbial challenges (e.g., prions) are two of the biggest factors.
Joslyn, Larry. “Sterilization by Heat,” in Disinfection, Sterilization, and Preservation, ed. Seymour Block, 5th ed., 669–728. Philadelphia: Lippincott Williams & Wilkins, 2001.
Perkins, John. Principles and Methods of Sterilization in Health Science. Springfield, IL: Charles Thomas, 1970.
Pflug, Irving et al. “Principles of Thermal Destruction of Microorganisms,” in Disinfection, Sterilization, and Preservation, ed. Seymour Block, 5th ed., 662–663. Philadelphia: Lippincott Williams & Wilkins, 2001.
Validation of Steam Sterilization Cycles, Technical Monograph #1. Philadelphia: Parenteral Drug Association, 1978.
Wayne Rogers is a sterilization consultant based in Temecula, CA, and can be e-mailed at [email protected].