Sterilization: Gas Plasma, Steam, and Washer-Decontamination


Gas Plasma, Steam, and Washer-Decontamination

By RaeAnn Slaybaugh

Many reprocessable devices are used in the cost-conscious, managed care hospital and clinical environment. There is a growing trend away from single-use, disposable devices and toward those that can be used more than once. To contain costs, most tertiary care hospitals still clean and sterilize critical surgical instruments in-house using more than one of the following sterilization processes.

Gas Plasma Sterilization

Historically, sterilization was done mainly through physical methods, especially using moist heat in the form of steam autoclaves as well as dry heat. Today, however, hospitals possess a greater number and diverse types of expensive, intricate instruments. Therefore, rapid turnaround times are a top priority while high heat wet or dry is not an option for all instruments. Using ethylene oxide (EO) gas has been one solution to sterilizing delicate instruments; however, due to the turnaround time in EO cycles for the aeration of loads to reduce toxic EO residuals, some facilities are choosing different methods. In addition, the composition of gaseous mixtures has changed. Until recently, EO gas was combined with chlorofluorocarbon stabilizing agent in a ratio of 12% EO mixed with 88% chlorofluorocarbon (referred to as 12/88 EO). Chlorofluorocarbons were phased out in December 1995 under provisions of the Clean Air Act. Some states began to require the use of EO abatement technology to reduce the amount of EO being released into ambient air by 90-99.9%.

With these changes, alternative technologies were developed. These include chlorine dioxide gas, ozone gas, peracetic acid liquid, and hydrogen peroxide gas plasma, all of which are designed to offer quicker turnaround times than EO. These methods also tend to use low concentrations of sterilizing agents, are free of toxic residual, and offer high compatibility with medical device materials.

One of the more recent low-temperature plasma sterilizers is STERRAD® System by Advanced Sterilization Products, a Johnson & Johnson Company (Irvine, Calif). STERRAD® uses hydrogen peroxide vapor and low-temperature gas plasma to sterilize most devices quickly with no toxic residues. Usually, the process takes about 75 minutes for wrapped and dry instruments and devices. Inside the chamber, a deep vacuum is drawn. Fifty-nine percent (nominal) aqueous hydrogen peroxide is vaporized into the chamber. The product is then enveloped in the hydrogen peroxide vapor. Following the diffusion of the gaseous hydrogen peroxide through the load, chamber pressure is reduced, allowing for the generation of low-temperature gas plasma. Radio frequency (RF) energy is applied to the chamber via an RF amplifier, inducing the plasma state. Reactive species are generated from the hydrogen peroxide in this state, reacting with materials and each other. Once the high-energy species have reacted, they recombine to form water vapor, oxygen, and other non-toxic byproducts.

Upon completion of sterilization, instruments are dry and wrapped/pouched for immediate use or sterile storage. Thus, recontamination risk is minimized, and since they remain sterile until their next use, time and money is saved by avoiding reprocessing instruments if the case in canceled or delayed. The STERRAD® system takes up minimal space and requires no venting or water hookup. The only utility requirement is electrical hookup.

Several new STERRAD® models have been developed and more are underway. The first of these, the STERRAD®100S System, is shown to improve load tolerance and provide faster kill in diffusion-restricted areas. It is also retrofitable to the original STERRAD®100 Sterilizer for faster cycle completion. The 100S System can run two different cycles--about 55 minutes long--as compared to about 75 minutes for the STERRAD®100. A second cycle is designed to sterilize flexible endoscopes in about 72 minutes. Both the short and long process has two identical half-cycles, each of which produces greater than 6 log reduction of Bacillus stearothermophilis spores. This translates to a direct determination of a Sterility Assurance Level (SAL) of 10-6. All of the new STERRAD® systems and the hydrogen peroxide gas plasma sterilization process are CE marked and are in compliance with ISO 14937.

Another new system is the STERRAD®50 Sterilization System introduced last year. A rectangular chamber and volume of 1.75ft3 means this is a smaller, faster low-temperature system than the original STERRAD®100. This makes it especially useful for minimally-invasive surgery common in ambulatory care settings and hospitals. With a 45-minute cycle time, the smaller, lower profile dimension fits point-of-care sterilization applications. Again, this system needs no venting, water hookup, or special electrical requirements and operates from a 110-120V outlet. One shelf measuring 24" by 16" is included while the overall chamber height is 7 inches. At this size, it processes three typical rigid endoscope sets in one cycle. It can sterilize effectively more than 95% of medical devices and surgical instruments tested by over 150 medical device manufacturers. These include drills, cameras, shavers, rigid lenses, microsurgical instruments, defibrillation paddles, cords, cables, power saws, and rigid telescopes.

A two-door model of the STERRAD® sterilizer was introduced last year in Japan, the STERRAD® 200. It offers twice the capacity and also comes in a single-door configuration. Currently, the STERRAD® 800 Sterilization System is under development.

According to experts, only a few disadvantages associated with gas plasma sterilization exist. These include the inability to process liquids, powders, or strong absorbers (cellulosics), and some lumen restrictions. Also, gas plasma sterilization is not recommended for liquids and other devices that can be damaged physically or changed by exposure to low pressure.

A number of design considerations and issues for hydrogen peroxide gas plasma units should be taken into account. For example, there is a risk of anodized aluminum fading following multiple exposures to hydrogen peroxide vapor. Also, a variety of adhesives has been evaluated and found to be compatible, but some (such as polysulfide-based adhesives) are not recommended.

Also, consider packaging. Devices are typically loaded into trays that have been validated especially for the process and tested to confirm good sterilization efficacy. The tray may also be double-wrapped in a standard nonwoven polypropylene wrap to provide a sterile barrier. Or, devices can be packaged in standard heat-sealable pouches to provide sterile packaging for storage until later use. For industrial or terminal sterilization, as well as for hospital applications, individual devices can be placed in heat-sealed pouches that allow adequate diffusion around the device. Paper products or cellulosics are not recommended, since large amounts of these materials can absorb and immobilize excessive amounts of hydrogen peroxide.

Low-temperature hydrogen peroxide gas plasma has also made its mark on device manufacturers for sterilizing temperature-sensitive polymeric materials. This method offers rapid turnaround time, in-house control of the sterilization process, and lower inventory requirements.


A washer/disinfector, which provides a variety of material loading options, including automation, is a good choice. This feature should be accompanied with a wide range of certified cleaning processes, followed by a thermal (hot water) disinfection process capable of killing vegetative microorganisms that can lead to nosocomial infections. Be aware, however, that these products do not kill bacterial spores.

While washer/decontaminators and washer/disinfectors are not certified or approved by the FDA for cleaning and disinfecting medical devices, a review and clearance is required prior to marketing a product with such claims.

The Castle 4656 washer-disinfector is designed to ensure a higher level of safety by providing clean, disinfected materials. Recommended for point-of-use decontamination, this model provides thermal (hot water) disinfection, capable of killing vegetative microorganisms that can lead to nosocomial infections. In the cold water pre-wash phase, gross debris is removed and followed by a wash phase. Various consumable products should be used throughout the disinfection process including enzyme in the pre-wash, neutral detergents in the wash, and an instrument lubrication in the final rinse.

Thermal disinfection includes exposure to moist heat at a pre-programmed time and temperate of one minute at 194º F (90ºC). Heated air is then forced into the chamber through each level of the wash rack and when direct injection racks are used, through the inside of tubular instruments and hoses. Items are dried thoroughly, both inside and out, reducing the labor it would take to dry them manually.

The 4656 wash carts accommodate four full-size instrument trays, eight medium-size trays or 16 half-size trays per cycle. Mixed loads of instruments and utensils can be processed, including up to 12 basins simultaneously with instruments.

The model also has a programmable time and temperature interface. An operator can choose from six pre-programmed cycles or program up to 54 additional custom treatments. Safety features include fold-down load door(s), doors that interlock during operation, and HEPA-filtered drying air. The spray arms clean a variety of loads, such as surgical instruments, rigid tubular instruments (suitable), anesthesia materials, and glassware. The model also offers flexibility by providing the option of up to four pre-washes, up to four main washes, and up to three post rinses. True thermal disinfection renders goods safe to handle with a thorough cleaning followed by exposure to moist heat at 194ºC for a minimum of one minute. The user-friendly control panel has a eye-level mounted control panel for easy-to-read display and programming, and it also has passworld-protected cycle parameters. This model's wash cart accomodates for full-size instrument trays, eight medium-size trays, or 16 half-size instrument trays per cycle. Other features include three chemical pumps and a pure water valve. Options are as follows: steam or electric; single-door or pass-through door; drying or no drying; internal printer; upper load cart and lower roll-out wash cart or four-level lower wash cart; and direct injection racks for cleaning tubular instruments.

The Castle 8666 washer-disinfector ensures a higher level of safety by providing clean, disinfected materials for large jobs. Similar in process to the 4656, the 8666 features a larger chamber for more trays and instruments per cycle. A variety of packages and options can be added to the 8666, leading to full automation of central supply.

Flash Sterilization

An important question to ask when flash sterilizing: If you don't have time to do it right the first time, when will you have time to do it over? Experts agree that flash sterilization is safe and efficacious when all recommended practices are followed. Historically, flash sterilization was thought of as steam sterilization using the unwrapped method. Its original intention was for emergency need and immediate use of instruments that had been dropped on the floor or for instruments that were either forgotten or unanticipated. Today, AAMI standard ST37 refers to flash sterilization as steam sterilization of patient care items for immediate use.

Typical flash sterilization uses an abbreviated gravity displacement cycle with a 3-minute or 10-minute exposure phase (consult your sterilizer operating manual for guidelines in which to select). AAMI ST37 also includes prevacuum and steam-flash pressure-pulse cycles when referencing flash sterilization. With prevacuum and steam-flash pressure-pulse cycles, a single wrapper can be used to protect the processed instrument from contamination during the transfer from the sterilizer to the point of use. Some manufacturers also make available rigid container systems suitable for flash sterilization; however, it is important to ensure that they are specifically labeled for which flash cycle they have been validated and that the facility sterilizers can provide that cycle. For example, if the container specifies a prevacuum process only, you cannot process it in a sterilizer that only runs a gravity displacement flash cycle.

Regardless of the process used, proper sterilization depends on a number of factors. First, instruments must be clean and free of soil. They should then be opened and/or disassembled so that steam can reach all areas in need of sterilization. Porous and lumened items require special care due to the difficulty in removing air from the inside. Refer to the medical device manufacturers and the sterilizer manufacturers for guidance on selecting the appropriate cycle to use.

Positioning the item in the tray or container is an important part of the process. Air-removal, steam penetration, and drainage of condensate are all affected by proper positioning. Drainage of condensate is important since flash sterilization does not provide for drying of items, so positioning of items to reduce puddling of moisture is beneficial.

Once the cycle is complete, the item processed is to be transferred to the point of use. They are not to be stored for later use. Special precautions are necessary during this transfer to prevent re-contamination. Procedures for this transport process should be developed in conjunction with the department supervisor and the infection control department.

This article is not intended to be an all inclusive description of flash sterilization and should not be used as such. Refer to AAMI standard ST37 for more details on the flash sterilization process. Copies can be obtained directly from AAMI for a nominal cost. Contact AAMI at

Steam Sterilization

Full-size steam sterilizer options abound and offer a variety of features for ease of use, reliability, productivity, installation, and safety. The Amsco Century series of sterilizers from STERIS offers a full range of processing cycles, from standard unwrapped emergency flash to single-wrapped express cycles for transport through nonsterile areas--plus all standard wrapped gravity and prevac cycles--in one unit. Its design makes it ideal for use in surgical suites, central service departments, ambulatory surgeries, and satellite care facilities. A power vertical sliding door opens and closes with a foot pedal, freeing the hands. A touchscreen-operated control system means users only have to press start, and messages are complete (no codes), including help screen with programming prompts. The Century series also features door seals that never needs lubrication and offers a two-year warranty, and a temperature sensor installed in the waste to control the amount of cooling water used to condense steam exhausted from the chamber, minimizing water consumption. Automatic start up and shut down feature can be used to reduce utility consumption. The Century's control includes four preprogrammed and up to eight additional cycles; time remaining is clearly displayed. This feature allows the user to identify when the cycle will finish and the processed item is available. This is very helpful for a surgeon waiting for the item. An emergency manual exhaust enables the user to vent the chamber and remove the load in the event of a power outage. The entire front panel opens for easy access to piping and control components. The door stops closing if it meets any resistance; special nonheat-conducting material completely surrounds the door, protecting the operator from burns; and a liquid sensor will alarm, abort cycle, and vent the chamber if it senses any liquid buildup. The Century also includes an ink-on-paper printer; security access codes and other operating parameters; menu screens; and a countdown displayed on the cycle-in-progress screen.

Getinge/Castle recently introduced the Castle® Series 100HC Steam Sterilizers, with software that allows the cycles to be configured for specific processing needs and customized cycle names. To address a wide variety of possibilities, the 100HC is shipped from the factory with 19 pre-loaded cycles. During installation, the Getinge/Castle service representative will assign cycles as requested by the user and add custom cycle names, if desired.

Whether for the Surgical Suite or Central Supply, the Series 100HC Steam Sterilizers offer the greatest flexibility for all processing needs. Units are available in a wide variety of sizes and configurations in both gravity displacement and pre-vacuum models. Castle Series 100HC steam sterilizers offers a wide range of cycle functions to ensure that a facility is prepared for any standard steam sterilization application. The model offers a simplified control panel with configurable cycles and customizable cycle names and corrosion-resistant nickel-clad chamber cuts cleaning and maintenance time. This manually operated sterilizer also has vertical sliding doors and a foot pedal that is the power button to provide hands-free operation. Power outage protection and emergency chamber access allows for immediate load retrieval while burn guards and heat resistant door handles help improve safety. Load accessories include: racks and shelves; load cars; load baskets; and universal instrument trays.

The Millennium Steam Sterilization System from STERIS not only offers the same features found on the Century series, but also the Steam Flush Pressure Pulse (SFRPP) cycles. According to STERIS, the Millennium's SFPP cycles also save time each year on required leak testing associated with standard pre-vacuum cycles. Use of a Bowie-Dick test is not a requirement with the cycle according to AAMI standard ST8. These cycles maintain at or above atmospheric pressure throughout the conditioning phase, so air leaks cannot affect the sterilization process. Process controls ensure that conditioning and sterilizing the entire load is achieved cycle after cycle.

The Castle 233 chamber is two inches wider than comparable chambers. That means an operator can sterilize more loads in fewer cycles, saving time, utilities, and labor. The 26" chambers allows an operator to insert 24" containers sideways, not lenghtwise. Twenty-four inch chambers hold 24" containers only one way: lengthwise. Positive-pulsing preconditioning provides efficient air removal and sterility assurance for wrapped goods, 3-minute flash cycles and 10-minute flash cycles. Chamber cross section measures 26" X 36" in lengths of 37", 50", or 61".

The model has six pre-programmed cycles and a seven-day timer that allows for automatic turn on/off. An emergency chamber enables access for quick load retrieval during power outages. This sterilizer also has a high-water alarm, a one-touch cycle start, and a drain temperature control. Installation is easy as well since the sterilizer fits into the same footprint as a 42" X 36" sterilizer. Options for this model are as follows: single- or double-door design; optional water saver package; racks and shelves; load cars and transfer carriage; load baskets; and universal instrument trays.

The System 2S from STERIS provides safe, rapid steam sterilization for heat- and moisture-stable devices and materials. It also offers speed and flexibility--just like full-sized hospital steam sterilization systems--but costs less and is a smaller size. It is ideal for alternate care and outpatient facilities. Its compactness enables it to be installed close to the procedure rooms, eliminating excessive handling and transportation of surgical devices. Electric utility consumption is lower on the 2S as compared to other sterilizers, allowing easier installations to smaller facilities. The System 2S accommodates a variety of different instrument containers and trays as well as a wide range of sterile processing needs.

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