Ethylene Oxide
Ethylene oxide (EtO) is, for many products, an effective sterilization
agent. It is especially useful for custom procedure kits containing
unit dose drugs in hermetically sealed packages and for products
which discolor, distort, or otherwise degrade when processed
with radiation. The EtO sterilization process normally requires
a product conditioning step in which product is placed in a
highly humidified area for a specified period of time. This
process humidifies the product, allowing the sterilizing agent,
ethylene oxide gas, to penetrate more effectively. After pre-humidification,
products are exposed to the EtO by being placed in a chamber
for several hours. After exposure, products must be "aerated"
by sitting in another chamber in which residual gasses which
may have clung to the product are allowed to disperse. This
step may take up to several days, depending upon the product
and the amount of gas absorbed. After EtO processing, products
may not be released for use until a laboratory test on biological
indicators has been performed. This test may delay release of
product by an additional 3-7 days.
Ethylene Oxide - Processing Considerations
Each load of product which is sterilized with ethylene oxide
has a biological indicator or "spore" strip included in the
product. Upon removal from the EtO chamber, the product cannot
be released until a sterility test is performed upon the BI
strip to verify destruction of the indicator organism.Products
sterilized with ethylene oxide must be packaged in breathable
packaging to allow gasses to escape after EtO exposure. While
this type of packaging is readily available, it is quite costly.
The process of EtO sterilization requires careful control of
several parameters for every lot processed. These parameters
include time of exposure, humidity, temperature, pressure, EtO
gas concentration, and vacuum. Should an abnormal reading in
any of these parameters occur, effectiveness of the process
may be questioned. Probably the greatest concern for EtO users
and providers is evidence of EtO's potential health hazards
to plant employees, the environment, and patients. It is suspected
that EtO presents a hazard due to its potentially carcinogenic
properties. Furthermore, out-gassing of EtO from processing
facilities has come under close governmental scrutiny in recent
years. The resulting regulations have increased costs for EtO
processing providers, and those costs have been necessarily
passed along to EtO customers
Gamma Rays (Cobalt-60)
Gamma irradiation involves exposure of products in their final
shipping containers to a radioactive isotope known as Cobalt-60.
Cobalt-60 is processed almost exclusively worldwide by the Canadian
government and provided to gamma irradiation plants by Nordion
International, who processes and packages the isotope in cobalt
"pencils". The pencils are shipped, under extremely tight control
and under "hazardous materials" shipping regulations, to gamma
plants as needed.
The cobalt pencils are housed in "source racks" within the gamma
facility. The racks are placed in the gamma cell, and products
in their final shipping configurations travel through the cell
on a conveyor system. This process takes several hours to complete.
Cobalt-60 decays with time, and appropriate cycle timer setting
adjustments must be made at the gamma plant to account for the
current cobalt inventory quantity. When the cobalt source is
sufficiently decayed, the pencils must be replaced. This "re-sourcing"
procedure takes several days to complete, during which time
the affected gamma cell is rendered inoperable. Upon completion
of re-sourcing, dose mapping must be performed on all products
which will be irradiated in the cell.
The effect of gamma irradiation to the product microbial population
(bioburden) is the same as the effect of electron beam processing.
The fundamental difference between gamma and e-beam sterilization
is the manner in which the radiation energy is delivered to
the material being irradiated.
Cobalt-60 - Processing Considerations
The irradiation dose provided by a gamma plant is a function
of exposure time of the product to the irradiation source (Cobalt-60),
and is thus controlled by a timer setting. Products which can
be scheduled to run within the same timer setting are most convenient
for a gamma plant to process. The dose range which is most often
set in gamma plants is approximately 25 to 35 kilogray (kGy).
Products which require lower or higher doses than this range
frequently must wait until the timer setting is adjusted to
accommodate atypical doses. Gamma facilities cannot easily adjust
up or down to produce specific dose ranges, and frequently,
products which don't necessarily require a 25 kilogray minimum
to destroy their inherent bioburden are processed at the higher
rate anyway. Because products are exposed to the gamma source
for a time period of 4 to 8 hours, the possibility of product
degradation in the form of discoloration and/or embrittlement,
is increased over other forms of radiation sterilization such
as electron beam.
Other Sterilization Methods
Several other sterilization methods, such as gas plasma, steam,
and others are available and/or under development. At this writing,
however, these methods do not accommodate large volumes of product
packaged in final shipping configurations which are destined
for finished goods inventory at the completion of the sterilization
process.
