Heat is the means of terminal sterilization that is preferred by the regulatory authorities because of its relative simplicity and the high sterility assurance that it affords. However, a significant number of traditional pharmaceutical products and many recently developed biotechnology products are damaged by heat, as are many polymer-based medical devices and surgical implants.
NEW STERILIZATION TECHNOLOGIES
Heat is the means
of terminal sterilization that is preferred by the regulatory authorities because of its relative
simplicity and the high sterility
assurance that it affords.
However, a significant number
of traditional pharmaceutical products and many recently developed biotechnology products are damaged by heat, as are many polymer-based
medical devices and surgical implants; for such products alternative sterilization processes must be
adopted. Whilst
radiation is a viable option
for many dry materials, radiation-induced damage
is common in aqueous drug solutions, and gaseous methods
are also inappropriate
for liquids. Aseptic
manufacture from individually sterilized ingredients is a suitable
solution to the problem of making
sterile thermolabile products, but it affords a lower degree
of sterility assurance than steam
sterilization and is both time-consuming and expensive. For these reasons
alternative sterilization strategies have been developed in recent years.
Two processes that have progressed to the stage of commercial exploitation are those employing high-intensity light and low-temperature plasma. It must be stressed, however, that although the need to develop alternative
strategies for the terminal
sterilization of proteinor nucleic acid-containing biotechnology products is one of the stimuli for the investigation of new methods in general, these particular processes are unsuitable for
such products.
UV light
has long been known to have the potential to kill
all types of microorganisms, but its penetrating power is so poor that it has found practical application only in the decontamination of air (e.g.
in laminar-flow workstations and operating theatres), shallow layers of water and surfaces. UV light does
not penetrate metal
at all, nor glass to any useful degree,
but it will penetrate those polymers that do not contain unsaturated bonds or aromatic groups (e.g. polyethylene and polypropylene,
but not polystyrene, polycarbonate or polyvinyl chloride). High-intensity light sterilization is based on the
generation of short flashes of broad-wavelength light from a xenon lamp
that has an intensity almost
100 000 times that
of the sun; approximately 25% of the flash is UV
light. The procedure has been applied
to the sterilization of water and studied
as a means of terminal
sterilization for injectables in UV-transmitting plastic ampoules in a blow–fill–seal operation. Although pulsed
light is unlikely to be useful for
coloured solutions or those that contain solutes with
a high UV absorbance, it is likely
that the procedure will be readily
applicable not only to water but to some simple
solutions of organic
molecules, e.g. dextrose-saline injection.
Plasma is a gas or vapour
that has been
subjected to an electrical or magnetic field
which causes a substantial
proportion of the molecules to become ionized.
It is thus composed of a cloud of neutral species,
ions and electrons in which the numbers of positive and negatively
charged particles are
equal. Plasmas may be generated from many substances but those from chlorine, glutaraldehyde and hydrogen
peroxide have been shown to
possess the greatest antimicrobial activity.
Low-temperature plasma
is a method of sterilization that is applicable to most of the items
and materials for which
ethylene oxide is used, i.e. principally medical devices rather
than drugs; it cannot be used to sterilize liquids, powders
and certain fabrics.
Commercial plasma sterilizers, which
have been available since the early 1990s,
typically consist of a sterilization chamber of about 75 L; this
is evacuated, then
filled with hydrogen peroxide vapour which is subsequently converted
to a plasma by application of an electric
field. An alternative commercial plasma sterilizer utilizes alternating
cycles of peracetic acid vapour and a plasma
containing oxygen, hydrogen and an inert carrier gas. The cycle times are typically from 60 to 90 minutes
and the operating temperatures are less than 50 °C. Major benefits of plasma sterilization include
elimination of the requirement to remove toxic
gases at the end of the cycle
(in contrast to ethylene oxide
and LTSF processes); there is also no requirement for the treated device to be aired to remove
residual gas, and there is no significant corrosion or reduction in sharpness of exposed surgical instruments.
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