Radiation sterilization - Pharmaceutical Microbiology Third Semester PDF Notes

 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

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