**6. Gamma sterilization of medical devices**

When radiation is used for the sterilization of medical devices, the compatibility of all of the components has to be considered. Ionizing radiation not only kills microorganisms but also affects material properties. Medical devices are made of many different materials, some of which are metals, but most are non-metals, such as formed polymers, composite structures and even ceramics. Radiation itself does not directly affect metals since sterilization energies are safely below any activation thresholds. Metals, such as those used in orthopaedic implants, are virtually unchanged by the radiation sterilization process. Nevertheless, it has to be kept in mind that some types of polymers when irradiated in contact with a metal can cause some corrosion of the metal or surface discolouration. This is generally caused from by products released by some polymers during irradiation.

Polymer devices subjected to irradiation sterilization will inevitably be affected by the radiation and the environment used during sterilization, and will experience changes in the polymer structure such as chain scission and crosslinking (Schnabel, 1981). For some polymers both processes coexist and either one may be predominant depending not only upon the chemical structure of the polymer, but also upon the conditions of irradiation is performed like temperature, environment, dose rate, etc. The crosslinking and main scissions that take place during irradiation may lead to sharp changes in physical properties of the polymers. These effects will lead to changes in the tensile strength, elongation at break and impact strength. The exact changes seen will depend both on the basic polymer and any

Sterilization by Gamma Irradiation 189

scission results from a macroradical that itself is radiolysis product of a lateral bond as shown in the Figure 8 (reaction I ) (Guillet, 1985). The volatile products like HCOOCH3, CO, CO2, HCOCH3 and CH4, can be accounted for by the subsequent reactions of the carbomethoxy radical (B radical)**.** The formation of C radical is the basic reason for the radiation-induced degradation of PMMA. Under air atmosphere the C radical undergoes the chain oxidation process forming the peroxyl free radical (D). Once D radical is formed in PMMA, it can abstract hydrogen from PMMA chains to form hydroperoxide. The hydroperoxide decomposes slowly but steadily at room temperature to generate new oxidative products, which induce further degradation. In addition, it is believed that the free radical A, peroxyl radical (B) and the hydroperoxides are the main substances, which induce

the changes in PMMA properties when it is gamma irradiated (Schnabel, 1981).

Fig. 7. Effects of gamma irradiation on PVC molecule

1988).

Polycarbonate (PC) fills an important niche as one of the most popular engineering resins in the medical device market. Bisphenol-A polycarbonate has been commercially available since the 1960s, and its use in medical devices dates from approximately that time. Possessing a broad range of physical properties that enable it to replace glass or metal in many products, polycarbonate offers an unusual combination of strength, rigidity, and toughness that helps prevent potentially life-threatening material failures. In addition, it provides glasslike clarity, a critical characteristic for clinical and diagnostic a setting in which visibility of tissues, blood, and other fluids is required because biocompatibility is essential for any material used in direct or indirect contact with patients (Freitag et al.,

additives used. The changes in mechanical properties may not be immediately apparent and there can be some time delay in their development. One visible side effect of irradiation sterilization is that many plastics will discolor or yellow as a result of the processing. Irradiated devices are completely safe to handle and can be released and used immediately after sterilization.

Many polymers are resistant to radiation at doses of up to around 25 kGy, the actual doses used will be higher than this to achieve sterilization, however complete sterilization and radiation damage of some magnitude will inevitably occur. The effect of radiation is cumulative and for items that must be repeatedly sterilized the total dosage can rise rapidly. For these items records need to be kept to insure that safe limits are not exceeded. Irradiation is very effective for fully packaged and sealed single-use items where only one radiation dose is required.
