Archive October 2000 I, No. 10

How to Select the Proper Packaging for Low Temperature Sterilization

Outpatient surgical facilities are now hosting more and more complex procedures, and consequently, surgeons are using many new surgical instruments that can neither be sterilized with steam, due to their sensitivity to high temperatures, nor emersed in liquids. To handle these sensitive instruments, low-temperature terminal sterilization has taken on a larger role. In this article, I'll briefly explain the mechanism of low-temperature sterilization processes; namely, EtO and gas plasma sterilization, discuss some appropriate packaging choices for each, and tell you about some packaging options you should avoid when sterilizing with these two methods.

EtO Sterilization
Ethylene oxide (EtO), formaldehyde and glutaraldehyde, the most commonly used low-temperature sterilizing agents, interact with microorganisms by replacing available hydrogen atoms with alkyl groups. The alkylation injures and/or kills the organisms, but the process also requires long aeration or thorough rinsing to remove residuals that could harm the patient.

A number of facilities have used EtO for many years, to sterilize items that cannot withstand the high temperatures and humidity of steam. A very effective sterilant that is compatible with a wide range of materials, EtO also has excellent penetration characteristics and is available in equipment sizes from table-top to large floor loaders. Its main disadvantages are the long sterilization and aeration times it requires, which make it difficult to use when quick turnaround times are called for.

There are three reasons why facilities are searching for alternatives to EtO. First, it's become more expensive. One of its major ingredients, CFC-12, is being phased out, because it's been identified as a major cause of the depletion of the ozone layer. CFC-12's replacement, HCFC, is much more expensive to come by. Second, the safety of using EtO sterilizers has been called into question; breathing EtO vapors has been linked to serious health problems, and to use it safely, facilities have had to take expensive measures such as installing engineering controls, using ventilators, training staff, etc. Third, there's a growing need for faster turnaround time. More facilities are using complex, heat-sensitive equipment for minimally invasive surgery, and they need low-temperature alternatives that will sterilize a wide variety of equipment with quicker turnaround times than EtO can offer.

Gas Plasma Sterilization
Oxidizing sterilants such as chlorine dioxide, hydrogen peroxide, ozone, and peracedic acid are reactive and readily decompose into more unstable species called free radicals. These agents strip an electron from the microorganism, causing its demise. Oxidizing sterilants are more chemically reactive than alkylating sterilants, which makes them more effective at low concentrations. However, it's also more difficult to ensure that they penetrate to all locations requiring sterilization, such as long, narrow lumens.

Manufacturers are now developing many oxidizing sterilants for hospital use. In this article, I'll discuss only the STERRAD® hydrogen peroxide plasma system because this and EtO are the most commonly used agents.

Plasma is the fourth state of matter as distinguished from solid, liquid, or gas. It can be produced either through the action of very high temperatures or by strong electric or magnetic fields. The STERRAD system uses an aqueous solution of hydrogen peroxide (H2O2) that is vaporized in a deep vacuum chamber containing the items to be sterilized. The H2O2 is toxic and exerts its own lethal effects, but its real purpose is to serve as a precursor for the generation of the plasma. The system then introduces a radio frequency that creates an electrical field, which turns the H2O2 vapor into plasma. The plasma releases free radicals that collide or react with and kill harmful organisms.

Packaging Options
Because EtO and plasma sterilization use relatively low temperatures, you can employ a variety of packaging materials with them that you can't use with high- temperature steam.

When selecting packaging for EtO and gas plasma sterilization, remember the following:
- Packaging used for EtO gas sterilization must allow the EtO to penetrate and entrapped air to escape.
- Gas plasma sterilization requires packaging composed of only all-synthetic fabrics, because if the packaging includes any cellulosic fibers, it will absorb the sterilant and the cycle will abort.

The packaging options that meet these criteria and are appropriate for low-temperature sterilization are plastic peel pouches and plastic bags.

Plastic peel pouches: Typically made of Tyvek® on one side and a polyethylene/polyester lamination on the other side, these pouches are designed for EtO and gas plasma sterilization and are the only type of peel pouches that you should use for gas sterilization. You can obtain plastic peel pouches in just about the same sizes-in both preformed pouches and tubing-as paper/plastic pouches, and they are far superior in strength and moisture resistance. However, plastic peel pouches are comparatively expensive and you can only use them for low temperature sterilization; they are unsuitable for steam sterilization due to their low melting point.

To heat seal plastic peel pouches, use a thermal-impulse, narrow jaw-type heat sealer, a narrow jaw hot-bar sealer covered with Teflon, or a band or wheel type continuous sealer designed specifically for all plastic bags. The type of sealer you use must heat and cool the package while under pressure.

Plastic bags: Plastic bags fit into two categories: pre-sterilization and post-sterilization. You should only use the former for low-temperature sterilization.

Pre-sterilization plastic bags are polyethylene bags, which are a minimum of 3 mm thick and have a breathable patch (either circular or a strip) sealed to one side. Because both EtO and plasma sterilization systems draw a vacuum, pre-sterilization plastic bags need to allow the entrapped air to escape during the vacuum cycles. When used in combination with a single disposable non-woven wrap, pre-sterilization plastic bags are the ideal package for kits and trays that require low-temperature sterilization. You can seal them with the same type of sealer as all plastic peel pouches. This type of package is identical to single-use kit and tray packages furnished by device manufacturers and they offer superior sterile integrity. They are easy to seal and available in a wide variety of sizes. However, you can only use them for low temperature sterilization because they have a low melting point.

Post-sterilization plastic bags are commonly called "dust covers" or "sterility maintenance covers." Do not use them as the primary package for low-temperature sterilization; they are only useful to cover steam-sterilized packages that are cooled down to room temperature after sterilization.

Rigid Containers: Rigid containers are sealed, sterilizable boxes that serve as packages for surgical instrument sets in place of muslin or non-woven fabric CSR wraps. They are made of aluminum, plastic, or stainless steel and come in a wide variety of sizes and configurations. This is one packaging option that the manufacturers claim may be fine for low-temperature sterilization, but I recommend you stay away from them for all but pre-vacuum steam sterilization.

The merits of rigid containers are: Improved productivity over double CSR wrapping; good stackability; ease of use; and aseptic presentation. Their disadvantages are that they are expensive, require more storage space, and are really only suitable for pre-vacuum steam sterilization, although some container manufacturers will debate this. I recommend thoroughly testing any rigid container system before you attempt to use it in a gravity displacement sterilizer or any low-temperature sterilization system. Use your standard instrument sets and lots of biological indicators placed in all the hard-to-reach places within the container. There are very few, if any, situations where you can use them in low-temperature sterilization.

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