Radiation sterilization
Learning
objectives
At the end of this lecture, the student will be able to:
– Classify the radiations used for sterilization
– Explain the design of gamma sterilizer and electron
accelerator
– List the applications of UV radiations
Radiation
Sterilization
Radiation kills germs that can cause disease and neutralizes
other harmful organisms.
Sterilization with ionizing radiation inactivates
microorganisms very efficiently and, when used for product wrapping, ensures
that healthcare products are safe and can be relied upon.
Types of
sterilizing radiations
• Non-ionizing
– Ultra-violet rays
• Ionizing
– Electron beam (particulate radiation)
– Gamma rays
Mechanism
of action
• Major target is the DNA of microbial cells
• Ionizing radiations (gamma-rays and electrons) cause
ionization and free radical production
• Ionizing radiations can also act indirectly, by the
interaction of radiation with other atoms or molecules in the cell or
surrounding the cell like water
• Non ionizing radiations (UV light) cause
excitationRadiation sterilization
Applications
• Sterilization of articles in the dried state; these
include surgical instruments, sutures, prostheses, unit-dose ointments, plastic
syringes and dry pharmaceutical products
Prostheses – an artificial body part, such as a limb, a
heart, or a breast implant
Limitations
• Undesirable changes can occur in irradiated preparations
• Certain glass or plastic (e.g. polypropylene, PTFE)
materials used for packaging or for medical devices can also suffer damage
• Radiolysis of water
Sterilizer
design and operation
Ultraviolet
irradiation
• The optimum wavelength for UV sterilization is around
260nm
• A suitable source for UV light in this region is a mercury
lamp giving peak emission levels at 254nm
• Sources are generally wall or ceiling-mounted for air
disinfection, or fixed to vessels for water treatment
• Operators present in an irradiated room should wear
appropriate protective clothing and eye shields
• Thymine dimers of DNA are formed when exposed to UV light
UV
radiation sterilization
• Ultraviolet light irradiation (UV, from a germicidal lamp)
is useful for sterilization of surfaces and some transparent objects
• Many objects that are transparent to visible light absorb
UV
• UV irradiation is routinely used to sterilize the
interiors of biological safety cabinets between uses
• It is ineffective in shaded areas, including areas under
dirt
• It also damages some plastics, such as polystyrene foam if
exposed for prolonged periods of time.
MOA of
ionizing radiation
• Radiation interacts with water, leading to the formation
of free radicals that can diffuse far enough to reach and damage DNA
• OH• radical is responsible for 90% of DNA damage
Gamma-ray
sterilizers
• Gamma-rays for sterilization are usually derived from a
cobalt-60 (60Co) source
• On disintegration emits radiation at two energy levels of
1.33 and 1.17MeV
• The isotope is held as pellets packed in metal rods
• The rods are replaced or re-arranged as the activity of
the source either drops or becomes unevenly distributed
• Source is housed within a reinforced concrete building
with walls some 2m thick, and it is only raised from a sunken water-filled tank
when required for use
• Articles being sterilized are passed through the
irradiation chamber on a conveyor belt or monorail system and move around the
raised source, the rate of passage regulating the dose absorbed
Electron
accelerators
Two types of electron accelerators
1. Electrostatic accelerator
– Producing electrons with maximum energies of 5MeV
– A high energy electron beam is generated by accelerating
electrons from a hot filament down an evacuated tube under high potential
difference
2. Microwave linear accelerator
– Producing electrons with maximum Energies of 10MeV
– Additional energy is imparted to this beam in a pulsed
manner by a synchronized travelling microwave
• Articles for treatment are limited to small packs
• Arranged on a horizontal conveyor belt
• The sterilizing dose is delivered more rapidly in an
electron accelerator than in a 60Co plant, with exposure times for
sterilization usually amounting to only a few seconds or minutes
Summary
• Radiations – ionizing and non-ionizing radiations
• Major target for radiations is the DNA of the microbial
cell
• Radiations are used for sterilization of articles in the
dried state; these include surgical instruments, sutures, prostheses, unit-dose
ointments, plastic syringes and dry pharmaceutical products
