**3. Gamma sterilization**

#### **3.1 General aspects**

Gamma rays are generally used for the sterilization of gaseous, liquid, solid materials, homogeneous and heterogeneous systems and medical devices, such as syringes, needles, cannulas, etc. Gamma irradiation is a physical means of decontamination, because it kills bacteria by breaking down bacterial DNA, inhibiting bacterial division. Energy of gamma rays passes through hive equipment, disrupting the pathogens that cause contamination. These photon-induced changes at the molecular level cause the death of contaminating organisms or render such organisms incapable of reproduction. The gamma irradiation process does not create residuals or impart radioactivity in the processed hive equipment. Complete penetration can be achieved depending on the thickness of the material. It supplies energy saving and it needs no chemical or heat dependence. Depending on the radiation protection rules, the main radioactive source has to be shielded for the safety of the operators. Storage of is needed depending on emitting gamma rays continuously

The first aspect to consider when sterilizing with gamma is product tolerance to the radiation. During use of this type of radiation, high-energy photons bombard the product,

Sterilization by Gamma Irradiation 175

pre-sterilized product. Radiation sterilization, as a physical cold process, has been widely used in many developed and developing countries for the sterilization of health care products. Earlier, a minimum dose of 25 kGy was routinely applied for many medical devices, pharmaceutical products and biological tissues. Now, as recommended by the International Organization for Standardization (ISO), the sterilization dose must be set for each type of product depending on its bioburden. Generally, the determination of sterilization dose is the responsibility of the principal manufacturer of the medical product,

The lethal effect of ionizing radiation on microorganisms, as measured by the loss by cells of colony-forming ability in nutrient medium, has been the subject of detailed study. Much progress has been made towards identification of the mechanism of inactivation, but there still remains considerable doubt as to the nature of the critical lesions involved, although it seems certain that lethality is primarily the consequence of genetic damage. Many hypotheses have been proposed and tested regarding the mechanism of cell damage by radiation. Some scientists proposed the mechanism thought 'radiotoxins' that are the toxic substances produced in the irradiated cells responsible for lethal effect. Others proposed that radiation was directly damaging the cellular membranes. In addition, radiation effects on enzymes or on energy metabolism were postulated. The effect on the cytoplasmic membrane appears to play an additional role in some circumstances (Greez et al, 1983).

It is now universally accepted that the deoxyribonucleic acid (DNA) in the chromosomes represents the most critical 'target' for ionizing radiation because it is responsible for

A DNA strand is composed of a series of nucleotides containing a purine (adenine, guanine) or a pyrimidine base (cytosine, thymine), a sugar (deoxyribose) bond to the base and a phosphate connected to the sugar. The nucleotides are joined by phosphodiester bonds

who must have access to a well qualified microbiology laboratory.

Fig. 2. Effect of gamma rays on water molecules

inhibition of cell division.

causing electron displacement within. These reactions, in turn, generate free radicals, which aid in breaking chemical bonds. Disrupting microbial DNA renders any organisms that survive the process nonviable or unable.

Gamma radiation does have some significant advantages over other methods of producing sterile product. These benefits include: better assurance of product sterility than filtration and aseptic processing; no residue like EtO leaves behind; more penetrating than E-beam; low-temperature process and simple validation process.

Process validation may be defined as the documented procedure for obtaining, recording and interpreting the results required to establish that a process will consistently yield product complying with a predetermined specification. For sterilization, process validation is essential, since sterilization is one of those special processes for which efficacy cannot be verified by retrospective inspection and testing of the product. Process validation consists of: i. installation qualification of the facility; ii. operational qualification of the facility and iii. performance qualification of the facility (ISO 14937, 2000)

Radiation sterilization of medical products also is currently regulated by two standards, EN 552 (1994) and ISO 11137 (1995). These standards will be harmonized in the very near future into ISO 11137 (2006) part 1, part 2 and part 3. Currently, all three parts of ISO 11137 (2006) are at the Final Draft International Standard Stage (FDIS). These three documents are now published. All sterilization standards consider 'dose' as a key parameter in order to determine if a product is sterile. However, measurement of dose is not a trivial task and a commercial dosimetry system consists of dosimeters, readout equipment and procedure for its use. Dosimeters may be films, small plastic blocks, fluids or pellets where there is a known and reproducible response to radiation dose. The dosimetry system must be calibrated, and the calibration must be traceable to a national standard. ISO/ASTM standard 51261 gives guidelines for calibration procedures.
