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Chapter 6 : Chemical Sterilization
Basingstoke, Hampshire, United Kingdom
Category: Industrial Microbiology; Clinical Microbiology
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This chapter provides an overview on chemical sterilization. Sterilization is a validated process that ensures that a surface or product is free from viable microorganisms, and evidence should be provided to support such a designation of a process. Despite the wide variety of chemical biocides, only a limited number have actually been developed for use in sterilization processes. They include the epoxides (particularly ethylene oxide (EO)), formaldehyde, hydrogen peroxide-based systems, and other oxidizing-agent-based liquid and gaseous processes. These systems are primarily used as alternatives to physical sterilization methods, particularly due to material compatibility concerns, for example, as alternatives to steam for the sterilization of temperature-sensitive materials. The type and application, spectrum of activity, advantages, disadvantages, and mode of action of each is discussed. EO is one of the most widely used products for industrial sterilization. Low temperature steam-formaldehyde (LTSF) systems are used for medical, dental, and some industrial sterilization processes. High-temperature formaldehyde-alcohol sterilization is a process that combines the biocidal activities of heat with those of formaldehyde and alcohol. Hydrogen peroxide solutions are not generally used in sterilization processes, but gaseous hydrogen peroxide is rapidly sporicidal at much lower concentrations and is considered significantly less damaging to surfaces. A section discusses other oxidizing agent-based sterilization processes that have been described or that are widely used.
An example of a small EO sterilizer, showing the front-loading sterilizer chamber into which the load is placed (load not shown) and the insertion of an EO canister, which is used to deliver the gas during the sterilization process.
A typical EO sterilizer.
Typical EO sterilization processes. Vacuum processes (top), in which sterilization is conducted at pressures below atmospheric pressure, are generally applied with 100% EO, while pressurized cycles (bottom), in which sterilization is conducted above atmospheric pressure, use EO mixtures.
The sporicidal (B. atrophaeus) effects of EO concentrations at 60% relative humidity and 54°C.
A representation of a typical LTSF sterilization system.
A typical LTSF sterilization cycle.
An LTSF sterilizer. The sterilizer (with the door open) is shown on the left, with the liquid formalin delivery system on the right.
An example of the effect of the hydrogen peroxide gas concentration on sporicidal activity. Various gas concentrations were tested under atmospheric pressure with G. stearothermophilus spores.
A typical hydrogen peroxide gas sterilizer.
STERRAD hydrogen peroxide gas-plasma sterilizers. (Reprinted with permission from Advanced Sterilization Products, a Johnson & Johnson company.)
Typical hydrogen peroxide gas sterilization processes. In the cycle on the right, only single conditioning, sterilization, and aeration pulses, which can vary in number (N) depending on the application, are shown. Similarly, the gas-plasma cycles can have multiple-stage pulses (only a single pulse is shown); for example, the most widely used health care application (the STERRAD 100S) consists of two peroxide injections.
A SYSTEM 1 processor with STERIS 20 sterilant. (Reprinted with permission of STERIS Corporation.)
Electrolyzed water. (A) A typical electrolyzed-water generator. (B) Generation of electrolyzed water, with a simple depiction of the active species formed.
Examples of electrolyzed-water generators. Courtesy of Sterilox Technologies.
A 125-liter ozone sterilizer. (Reprinted with permission from TSO3.)
Typical ethylene oxide sterilization process conditions, based on FDA-approved cycles for hospital sterilizer applications a
Examples of standards and guidelines for EO sterilization applications
Examples of standards and guidelines for LTSF sterilization applications
Comparison of STERRAD hydrogen peroxide gas-plasma sterilization systems used in health care applications