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Infection Prevention
Expanding Options for Low-temperature Sterilization
Dan Mayworm
Publish Date: October 10, 2007   |  Tags:   Infection Prevention

Dan Mayworm In the past, decisions on how to sterilize medical supplies in your facility were simple:

  • For items that could withstand steam temperatures and pressures, there was gravity or pre-vacuum steam sterilization.
  • For packaged items requiring low-temperature sterilization, there was 100 percent ethylene oxide (EtO) or an EtO mix.

Dan Mayworm\

The matter got complicated when EtO was criticized for long sterilization times and potentially harmful vapors and residues. Then regulatory and environmental concerns sprung up about the use of chlorofluorocarbon (CFC) as a dilutent - which made the gas safer to handle.

The problems with EtO, actual and perceived, begat research into alternatives for low-temperature sterilization, especially oxidizing agents such as hydrogen peroxide, peracetic acid and ozone (see "The Evolution of EtO"). When I first wrote about low-temperature alternatives to EtO in 1995, there were only two gas plasma sterilizers and one ozone product with 510(k) clearances from the FDA for marketing to healthcare facilities. Now, there's one gas plasma product sold commercially to the healthcare market (Sterrad), and a new ozone player (TSO3) has emerged. The original ozone sterilizer (Ster-O3-Zone) never was sold commercially. Here's a look at what's happened with low-temperature technologies for packaged devices since the simpler days of sterilization.

Quick and effective: Gas plasma
The Sterrad system, by Advanced Sterilization Products is the surviving gas plasma system. The machine works by vaporizing an aqueous hydrogen peroxide (H2O2) solution in a deep vacuum chamber that contains the items to be sterilized. Free radicals and other biological chemical species responsible for the lethal effects that occur during the plasma phase are generated. (Because H2O2 is toxic, it exerts its own lethal effects.) Radiofrequency energy creates an electrical field that produces plasma; the H2O2 vapor is broken into reactive species that collide or react with the organisms and each other. They lose their energy and recombine to form oxygen, water and other non-toxic by-products. No aeration is required.

There are three Sterrad system sizes:

  • the "50," with a 45-minute cycle time and a 1.55-cubic foot chamber for multiple trays;
  • the "100S" (55 minutes, 3.53 cubic feet) and
  • the "200" (75 minutes, 5.3 cubic feet).

Internal temperatures run from between 104?F and 115?F to 131?F. Initially, the Sterrad system had difficulty sterilizing devices with small lumens, but it has improved. Visit www.sterrad.com to view the 1,400 items approved for sterilization with this system, including lengths and diameters of lumens.

The system can't sterilize linens or other cellulosic materials, powders, liquids and devices containing dead-end or lumens 400mm or longer. You must sterilize supplies in non-woven polypropylene CSR wraps or Tyvek pouches and proprietary-material trays that are compatible with all sterilization technologies, including the Sterrad, pre-vacuum steam and EtO machines.

What You Can"t Sterilize

Manufacturer TSO3 recommends you don't sterilize the following in its ozone machine:

  • Certain types of implants
  • Flexible endoscopes
  • Glass or plastic ampoules
  • Natural rubber or latex
  • Woven textiles
  • Copper and its alloys
  • Zinc
  • Nickel
  • Metal foils
  • Any solid barriers to penetration or hermetically sealed container
  • Container systems with cellulosic filters
  • Polyethylene
  • Single-use devices

- Dan Mayworm

Growing ozone options
In 1989, the FDA cleared for marketing an ozone sterilizer for healthcare applications developed by Life Support, Inc., of Erie, Pa. The system was sold to Ozone Sterilization Products of New York, and I never heard from them again. About the same time, Cyclopss Corporation of Salt Lake City was developing chambers from 1 cubic foot to 8 cubic feet for use with its Ster-O3-Zone system - but got stuck in an FDA black hole, says president and CEO Bill Stoddard. In 1995, the FDA devised regulations for medical sterilizers that Mr. Stoddard says were virtually impossible to meet. After 10 years, he says, Cyclopss is finally getting close - again - to selling its first medical sterilizers. The company has a prototype in final testing and will be working with Consolidated Stills and Sterilizers, Inc., in Boston to sell it.

One ozone sterilizer has been cleared by the FDA to be marketed to U.S. healthcare facilities. The 125L Ozone Sterilizer, developed and manufactured by TSO3, Inc., of Quebec City, Quebec, Canada, is aimed at hospital sterilization centers. Since gaining Health Canada licensure in May 2002, TSO3 has signed agreements with seven Canadian and six U.S. hospitals that call for several months of preclinical trials and two months of clinical trials. A first-generation 125L was installed at the Montreal Heart Institute in 1999; it's also being tested in Montreal's Sacre-Coeur Hospital and the Saint-Francois d'Assise Hospital Centre of the University of Quebec Hospital Centre.

Skytron USA is marketing the 125L in the United States and showed the technology at the AORN Congress in April. The ozone sterilizers are being installed at U.S. pilot sites this summer, where the systems will be looked at for their practical workflow, labor and cost effects, says Ann Hewitt, the vice president of sales for TSO3.

"Yes, the process takes longer than gas plasma, but the throughput is about the same," says Ms. Hewitt. "So we're going to be looking at the impact on turnaround times so we'll have hard data on how it works in practice."

The pilot trials should end in late fall.

"Ozone sterilization technology is new and exciting; the potential is practically limitless," says Charles O. Hancock, BSEE, MBA, a sterilization expert who advises the Canadian and U.S. governments, instrument manufacturers and hospitals. "The reactions in the chamber are not simple oxidation. The chemistry is best described as providing active species effective in achieving an acceptable level of sterility assurance. Recent articles indicate a lack of understanding of what is taking place in the chamber because the reference information is wrong and out of date."

Explore your options
The Sterrad unit is the dominant low-temperature-sterilization player thanks to new chamber sizes and improved lumen claims. EtO is still useful for selected sites and applications when used by well-trained workers. Ozone is on the horizon and promises to provide competition to gas plasma once pilot studies are complete.

Several alternative liquid chemical processes are in the works; most demonstrate significant reductions in organisms - but material effects have not been desirable. Gaseous and vapor systems are being explored, but once again materials compatibility and packaging are problematic.

Regardless of the system, all sterilization is dependent on effective manual cleaning. The real challenge is to establish a reference for cleaning measurement - the visible-soil indicator is subjective and as such doesn't help anyone.

The Evolution of ETO

Because of its awesome penetrability and history of effectiveness against all known viruses, bacteria and fungi - not to mention its ability to kill even the most resistant bacterial spores - EtO continues to play a role in sterilization. Since CFCs have been legislated out of existence, and the only other dilutent, CO2, has manufacturing and operating problems, the only available EtO sterilizers are those that use 100 percent EtO. It's very effective, but explosive when mixed with oxygen and requires relatively long cycles.

EtO units manufactured by 3M (writeOutLink("www.3M.com/healthcare",1)) and Steris (www.steris.com) are small (4, 5 and 8 cubic feet) and simple to operate. As long as the personnel are thoroughly trained in operational and safety issues, EtO is a safe, efficient and cost-effective sterilant.

The EtO sterilization process has five phases: temperature/humidity conditioning, gas introduction, exposure, exhaust and purge/aerate.

The units' chambers draw a deep vacuum followed by a preconditioning phase that heats and humidifies the load. EtO is released into the chamber, usually from a small cartridge of liquefied gas, to an appropriate concentration level, then held for the required exposure time before purging the gas and aerating the contents. Because of the deep vacuum, the packaging you use must let the air escape and let the gas and humidity enter and leave. Cycle failures most often happen in the winter when the humidity levels can go into the single digits. Also, the EtO molecule is more active and therefore more effective at higher temperatures. Remember that aeration, which eliminates EtO residue from devices, depends entirely on the materials; metal instruments, for example, require little or no aeration while PVC and rubber products can take as long as 24 hours.

Another type of EtO sterilization, from Anderson Products, Inc. (writeOutLink("www.anpro.com",1)), uses proprietary gas cartridges placed in a plastic bag that becomes essentially the sterilization chamber.

To begin the process, you must choose the right co-extruded polyethylene/Surlyn bag, which comes in three sizes: No. 6 (2.12 cubic feet), No. 5 (0.88 cubic feet) and No. 4 (0.53 cubic feet). The items to be sterilized are placed in the bag with the EtO cartridge, the Humidichip, Dosimeter and any biological indicator appropriate for EtO sterilization (B. subtilis), and the bag is sealed. (The Humidichip maintains 60 percent relative humidity inside the bag; the Dosimeter is a proprietary chemical integrator providing visual assurance that conditions for sterilization are met.) You activate the EtO cartridge and place the bag in a modular chamber that controls temperature, exhaust and timing mechanisms. The gas elutes through the bag, then is exhausted from the chamber. Sterilization and aeration are accomplished in 16 hours.

Obviously, turnaround isn't quick, but the system could be useful if you have difficult-to-sterilize items that can be processed overnight. Operator training is necessary to ensure the process is safe and effective. Meticulous cleaning and drying of items before sterilization is essential.

- Dan Mayworm