Packaging and dose determination.
Although gamma radiation is commonly used to sterilize plastics, not all types of plastics can be treated at a sufficient
dose to achieve sterilization without degrading the plastic (20). The assessment of degradation is on-going and should be
undertaken throughout the shelf life of the material (the application of ionizing radiation causes the excitation of polymer
molecules, where, over time, the adsorbed dose can result in changes to the physical or chemical properties of the polymers).
The plastic material to be irradiated is normally referred to as the "product." Most products are placed into an outer packaging
in order to protect the irradiated product and to keep the product sterile once sterilized. The product remains sterile provided
that the outer packaging remains intact. Occasionally, large or complex products cannot be tested in their entirety and a
staged sterilization process is required. In such circumstances, thought should be given to the conditions under which final
assembly will take place in order to avoid contamination from the environment or from personnel.
Given the range of different types of single-use sterilized disposable products being developed, and the range of different
packaging configurations, the required gamma radiation dose to achieve sterilization or to protect the product from degradation
will vary considerably. There is also considerable variation with types of plastic. For example, a relatively low dose of
radiation is required to sterilize polypropylene when compared with polystyrene. Furthermore, the assessment of the dose is
more straightforward for small items, such as a plastic container, and more complex for single-use systems. Single-use systems
have variables including tubing length, different numbers and types of filters, and differences in the design of containers,
bags, and valves, which make the determination of the irradiation dose more complicated. Considering these factors, a common
radiation dose used for plastics is in the range 15–25 kGy (21).
For the process of sterilization, the wrapped product is normally packed into a special container, typically manufactured
from aluminum, called a tote. A tote has fixed internal dimensions and is designed to transport product through the radiation
process. The weight and dimensions of the tote must be accounted for when establishing the radiation dose.
The dose determination is the key validation step when using gamma radiation. The dose is the amount of gamma radiation absorbed
by an item undergoing sterilization. This is normally set as a range, where a minimum and maximum dose is established. The
minimum dose is established as the point where sterilization occurs, and the maximum dose is the point beyond which the product
is no longer compatible with the sterilization process. In general, the higher the dose rate, the lower the adverse effects
upon polymer products. This is mainly due to the diffusion of oxygen during the irradiation process.
To validate a load, there are three aspects to consider: establishing the dose range, measuring the effectiveness of the sterilization,
and dose mapping.
Irradiation validation is designed to set the dose range. The primary focus is to determine if the irradiation process damages
the packaging material or the product to be sterilized. Damage is assessed by calculating the maximum dose. This assessment
is examined through stability trials, whereby samples are held at under defined storage conditions (i.e., temperature and
relative humidity) and examined at periodic intervals for discoloration, brittleness, and other damage.
Ionizing radiation generates free radicals in plastic polymers leading to degradation from chain scission (i.e., changes in
molecular weight) or alterations to cross-linking. Potential radiation effects on some materials include embrittlement (i.e.,
change to material hardness), discoloration (i.e., often yellowing caused by surface oxidation), unpleasant odor (i.e., from
volatile material formed by reactions from within the polymers), or lack of functionality due to a compromised physical trait,
such as tensile strength (22).