Technical Data
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Sterilization of Plastics There are a variety of plastic materials and methods by which these plastic materials can be sterilized. However, not all plastics can be sterilized by every method. An understanding of sterilization methods, problems that can occur, and terms associated with sterilization is helpful in determining plastic and plastic ware capability and performance. The following is presented to assist in gaining that understanding. Exposure To Non-Sterile Conditions Causes Non-Sterility While temperature and time used to melt thermoplastics kills microorganisms, manufactured ware will not remain sterile unless it is made and maintained in a sterile environment. Plastic ware is not “sterile as manufactured“ since: ■ Ware is not blown with sterile air ■ Ware may be exposed to non-sterile conditions immediately after manufacture ■ Ware may contact non-sterile atmosphere, bags, boxes, personnel, etc. during packing after ware manufacture or during unpacking at the filling location ■ Low particulate does not mean sterile Producing ware under a shroud and using “particulate-free” or “low particulate” clean room bags does not result in sterile ware. These steps only reduce particulate in and on the ware to a lower level than would be present if ware were produced in an “unshrouded” production situation. In the future, molding may be performed in clean rooms and sterile conditions maintained after ware manufacture, however, until that time, ware cannot be represented as being sterile as molded. Until then, a secondary sterilization process must be performed. Terms Associated with Sterilization Bioburden This is the number of microorganisms (bacteria, virus, fungi, etc.) present. Microbiologists can test for these. When sterilizing ware, it is important to eliminate the bioburden to prevent futher microbical growth. Pyrogens A pyrogen, which means fever causing, is a remnant of bacteria that contains chemicals called endotoxins. Endotoxins can cause fever if injected into a mammal. Several tests exist to identify endotoxin contamination. Something may be sterile, but still have pyrogens on it. Glass can be sterilized and de-pyrogenated at the same time. Exposure to high temperature (600°F or higher) will kill microorganisms AND burn up endotoxins. The higher the temperature, the shorter the exposure time needed for de-pyrogenation. Most plastic ware is incapable of being exposed to these high temperatures. Therefore, plastic ware may be sterilized but, if it needs to be de-pyrogenated, it is usually washed with pyrogen free water. RNase and DNase Contaminating enzymes; RNase (which breaks down RNA), and DNase (which breaks down DNA), are the most critical substances influencing experimental work in molecular biology. These contaminants are one of the principle causes of failure in the manipulation and analysis of RNA and DNA in the laboratory. These enzymes come primarily from contact with skin (direct and indirect). Pipettors, lab benches, autoclaves, lab ware, doorknobs, etc. are all frequently handled without gloves. All of these items, and virtually everything in a lab setting, are contaminated with these enzymes after contact with skin. Wearing gloves only offers protection until a surface is contacted that has itself contacted skin, at which time the glove becomes contaminated. Because of the resiliency of these enzymes, maintaining a RNase / DNase – free lab is extremely difficult.
Steam autoclaving ware at 121°C for 20 minutes will destroy DNase, but will not destroy RNase. Baking ware in an oven at 300°C for 4 hours will destroy DNase and RNase. However, this method is not possible with most plastic items because of the high temperature. Alternatively, there are decontaminating cleansing solutions available in the marketplace that will destroy both of these enzymes immediately upon contact and can be used with most materials. The solution is simply sprayed onto the surface of the ware, which is then rinsed thoroughly with nuclease-free water. Sterilization Techniques Sterilization techniques are designed to kill microorganisms. There are varieties of sterilization methods, however the three basic approaches used to sterilize plastic ware are: ■ Ethylene Oxide (EtO) Exposure ■ Steam Autoclave ■ Radiation (gamma radiation, electron beam radiation) Tests should always be run on plastic ware to determine suitability for a given sterilization method. Ethylene Oxide Ethylene oxide (EtO) is a toxic, cancer causing gas. Technology and worker protection legislation allow continued EtO use. Most plastic can be EtO sterilized. EtO must contact the surfaces to be sterilized. There are several ways EtO sterilization can be accomplished. Pure EtO Empty ware in an open bag or ware in a sealed bag with a “breather” window, is placed in a chamber. Air is evacuated and moisture introduced (dry microorganisms are resistant to EtO sterilization). Pure EtO is flooded into the chamber. Chamber internal pressure is kept lower than external pressure to ensure gas will not leak. Exposure time varies depending on ware and bioburden. After exposure, the chamber is purged with filtered sterile air to eliminate residual EtO. Dilute EtO Since it is safer than pure EtO, a 10-15% mixture of EtO with inert gas is used. Empty ware in an open bag or ware in a sealed bag with a “breather window” is placed in a chamber. Air is evacuated, and moisture is introduced (dry microorganisms are resistant to EtO sterilization). Dilute EtO is flooded into the chamber and the chamber’s temperature increased up to 60°C (140°F). Exposure time of 4 to 24 hours varies depending on ware, bioburden, and sterilization parameters. After exposure, the chamber is purged with filtered sterile air to eliminate residual EtO. Most plastic ware is capable of being EtO sterilized. However, zinc stearate process aid, used in injection blow molding, can cause precipitants (particulate) to form in liquid products packaged in EtO sterilized ware. Therefore, only special LDPE grades and colorants that do not require zinc stearate for injection blow molded ware should be treated by EtO sterilization processes. Additionally, tests should always be run on plastic ware to determine suitability for a given sterilization method. Steam Autoclave Autoclaving can sterilize empty OR filled, sealed ware. The effect of temperature AND moisture kills microorganisms. Autoclaving involves exposing ware for a time to steam. The autoclave acts like a pressure cooker, allowing the steam temperature to get above the boiling point of water (100°C=212°F). Typically, autoclaving is done at 15 psi (pounds per square inch) steam being at 121°C (250°F). Autoclaving Empty Ware Empty ware must withstand autoclaving temperature for the exposure time. If it does not, parts will distort. Of the common plastics, polypropylene (PP) and polycarbonate (PC) have enough heat resistance to be autoclaved. Generally, PP homopolymer is slightly more heat resistant than PP copolymer. Also, there is a grade of a new transparent plastic material identified as a cyclic olefin copolymer (COC) that is capable of withstanding steam autoclave sterilization.
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