WHEATON Technical Data - page 19

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Technical Data, Plastic
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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