A living cell is a kind of polymer. Medical goods are usually partly some kind of polymer that will react on the free radicals. The following are some examples.

A polymer can be cross-linked and/or chain cut. The typical cross-linked polymer is polyethylene, which will be harder and tougher by increasing dose. In order to get an effect for industrial use 100-300 kGy is needed. Applications are a/o to shrink tubes, foam and heat cables. The easiest polymer to cut is PTFE (teflon). Scrap material can, as a consequence, be irradiated in order to get a powder that then can by used in the paint/smear industry. All other polymers are in between PE and PTFE. The relatively low dose needed for sterilization will normally not affect medical plastics. There are however some materials that are not suitable for radiation sterilization.

Many materials will turn yellow/brown, especially older types of PVC, by being irradiated. Today however, many plastics, i.e. PVC can be purchased in a form that will not darken by being irradiated. The coloring itself will not affect the material properties, although it will not be translucent. However, the medical society is since the 40s used to yellowish tones on plastics and the effect is judged as a visual proof of that the product is sterile. So, even if it is possible to use materials that are not changing color, it may not be wise to do so.

The most common materials are PE, PP, PVC, EVA, PS, PU, PC and silicone. All are possible to irradiate. With PP some caution is needed. The manufacturer must clearly state if radiation is possible or not. Normal bulk PP will heavily deteriorate after irradiation. As a general rule aromatic polymers (contain rings) are more resistant to radiation than aliphatic polymers. By addition of additives, for example an anti-oxidant, the negative effects can be limited or even eliminated.

Cellulose will be slightly weakened, because the polymer chains will be cut. For normal medical applications this reduction of strength and flexibility is not significant. Doses as small as 10 kGy will however also affect cellulose. A new application is to irradiate pulp before it is entered into the viscose plant. The pulp fibers will not only be cut in a nice way, they will also be far more sensitive to chemical treatment. This makes it possible to buy cheaper pulp qualities and to decrease the amount of sulphuric acid.

Another similar application, but now in the medical sphere, is in production of vaccine. Here the active molecules can be connected to a polysaccharide, which is acceptable to the metabolism, and increases the tolerance to irradiation. Irradiation is thus an effective tool for engineering of material properties.

Metals such as titanium and steel can be irradiated as all other materials. It is only a question of density. In beta, due to the high dose rate, an increase in temperature will sometimes be seen. Needle tops can show an increase of 50 degrees. In such cases welding effects between needle and cover may occur. This can be avoided by giving the dose in a fractionated form with some hours of waiting in between. To use fractional irradiation is normal in cancer treatment due to the limitation of the body to stand a high dose.

Oxygen is sometimes a problem. It is the case when the surface is sensitive to oxidation. A cure is then to use nitrogen as a barrier.

RADIATION EFFECTS ON MATERIALS

LEADING PLASTICS FOR MEDICAL DEVICES

Physical Properties versus Application ranked by annual usage

(Source: POLYETHYLENES, Brian J. Pellon, Director, Technical Services, Rexene Corp. Dallas; MDDI, April 1994)

DEPENDENCE ON ENVIRONMENTAL FACTORS

All packages contain air which when irradiated will be ionized creating ozone. A dose of 25 kGy will give a concentration of 100 PPM inside a normal package. The half-life of ozone is short, about 1 hour. The rate of ozone will therefore decrease to 1 PPM in 7 hours. Ozone is by itself a sterilizing agent.

Irradiation is basically a cold process. The increase in temperature is in gamma less than 5oC but can in beta be considerably higher especially if there are metals involved. Before routine production, the temperature parameter should therefore be checked.

Most chemical reactions work better in moisture. The radicals, produced in an irradiation process, are also chemicals. They can in moisture move more freely and the sterilizing effect will increase. On the other hand the microbes will also enjoy life better and subsequently multiply themselves. These two factors work against each other. Uncontrolled growth of microbes must be considered worse. Wet products, like eye rinsing water, should therefore be sterilized within a few hours from filling or be kept at low temperature (+4oC) until it is possible to perform the sterilization.

Certain pharmaceuticals will be unacceptably hurt by irradiation. In such cases it may be possible to irradiate under extremely low temperatures, down to -196oC (liquid nitrogen). Low temperature is however not the whole trick. A slow freezing rate is more important. But correctly made, it is possible to sterilize drugs such as insulin. Deep freeze temperatures can be maintained in the slow gamma process. Low temperature is a specialty of the speedy beta treatment.