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Medical Device Sterilization

Bioburden reduction of consumer goods:

  • cosmetics
  • spices
  • herbs
  • toiletries
  • pharmaceutical raw materials
  • food packaging
  • dyes and colorants

Industrial products processing



Background radiation measured on the exterior wall of Scandinavian Clinics Estonia OÜ´s plant (μSv/h):

(Natural background radiation in Estonia may reach up to 0,3 μSv/h)



Survey area: 0,12 µSv/h;
Control area: 0,10 µSv/h;
Pool: 0,08 µSv/h


Industrial irradiation has been used during the last 50 years for a multitude of purposes. The main application is for industrial sterilisation of medical supplies and this method is worldwide the preferred method of irradiation. Other methods are gas (ethylene oxide) and steam (autoclaves).

Outside the medical field, irradiation may be used to increase shelf life of food by decontamination. Irradiation is also used for cross-linking of certain plastics, mainly polyethylene, thereby introducing resistance against heat, mechanical wear and biodegrading. Even the difficult waste problem of handling Teflon (PTFE) scrap, mainly from cable manufacturers, by converting it to a powder, reusable in the smear industry, is a field for irradiation.

Irradiation plants are sophisticated both in technique and from a quality assurance point of view. Quality and safety of the plant are in every country controlled by the National Radiation Protection Board or a similar agency.

If there is a choice most medical device manufacturers will usually prefer irradiation. 55% of the world production of medical disposables is today sterilised by irradiation.

The main argument for irradiation is that the beam penetrates the material, so there is no need for a design that allows for gas or steam penetration. Another factor is that irradiation does not create toxic residuals. A very important parameter is the freedom of choice regarding packing material. It means that plastics (polyethylene) can be used. This is not possible for EtO and steam. Irradiation is also a cold process compared to the alternatives, i.e. no thermal problems for the materials involved. If PTFE (teflon) is used, the choice must be EtO or steam, because this material will be converted to a powder in an ionizing field.



Industrial irradiation is a very effective way of sterilisation of goods for primarily medical or laboratory use. An irradiation facility is mainly composed of:

  • a chamber made of thick (2 m) concrete walls that prevent radiation to escape to the environment.
  • a source of ionizing radiation inside the chamber.
  • a warehouse where irradiated products are physically separated from non-irradiated products.
  • a conveyor belt that takes the products from the untreated area of the warehouse to the irradiation chamber, and then from the irradiation chamber to the treated area of the warehouse.
  • computerized systems to control the time of exposure of the products to the source of radiation.
  • safety systems that ensure that nobody can enter the irradiation chamber when it is in operation and that nobody can start operation when there is someone inside the chamber.
  • various auxiliary systems for ventilation, supply of compressed air and preservation of water quality where there is a pool.

The source of radiation can be,

  • Cobalt-60, a man-made radioisotope-emitting gamma rays.
  • an electron beam machine producing electrons of high energy (beta rays).

The emission of gamma rays by Cobalt-60 cannot be stopped. In gamma irradiators, the operation is stopped by lowering the source into a 6-7 meter deep pool where there is a 2 meter deep layer of water on top of the source. The water cover absorbs the radiation making entry into the chamber perfectly safe. Gamma allows treating dense and thick products. The exposure time is generally counted in hours and minutes. The half life of cobalt is 5.25 years. This means that the activity in 150 years will reach 2.7 mCi., which is the limit for classification as a non-radioactive material. The residual will be a stable Nickel isotope.

Radioactivity means that instable atoms fall apart and thereby emit ionising radiation. Co60 is one example of a radioactive isotope that emits so called gamma rays (= high energy photons). The activity is measured in amount of broken atoms per second. This unit is called Bequerel (Bq). Note that it is a very small unit. A normal load of a reasonably big gamma plant, measured in the older unit, could be 500 kCi ( kilo Curie). It will correspond to 1.85*1016 Bq. This big amount of zeroes makes Bequerel not a very practical unit. Curie is therefore the prevailing unit in the business.

Electron beams (beta rays) are produced by an electrically powered accelerator and can be interrupted at will. They are less penetrating than gamma radiation and do not allow the uniform processing of dense or thick packages. The exposure time is counted in seconds.



Radiation creates free radicals which are highly active chemical compounds. These radicals cut the DNA spiral. The aim is to create multiple cuts so that the cell is unable to activate its repair mechanisms.

One example of this is the background radiation on the earth, which is the most important mutation factor known. At the same time radiation may produce new variations of life. The multiple cuts in combination with the repair mechanisms may create new variants of life. Most of these new forms of life are not viable, but now and then something new and viable is created. Without background radiation there would be no life on earth and no continued evolution of life!



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.



The following is a summary of the most important applications of irradiation sterilization:

  • Sterilization of medical consumables and certain pharmaceuticals
  • Cold pasteurization of food
  • Cold pasteurization of pet food
  • Bio burden reduction of consumer goods. E.g.
    • cosmetics and toiletries
    • pharmaceutical raw material
    • food packaging
    • dyes and colorants
  • Disinfections of hospital waste
  • Degradation of toxic wastes
  • Degradation of selected polymer resins
  • Depolymerization of cellulose
  • Radiation doping of semiconductors
  • Coloring of glass and gemstones
  • Crosslinking of plastics, mainly PE
    • by introduction of the memory effect into heat shrink and stretch wrap materials
    • elimination of monomer residues from polymers
    • generation of foams for protective clothing
    • improving of green strength in rubber



Food irradiation is today allowed in many countries all over the world. Some countries however, in particular in Europe, still have severe restrictions. There is, at least in Europe, some customer resistance, which has resulted in a lack of action from authorities.

There is however a continuous effort in particular from the US government to promote the technique. There are also signs that American consumer resistance is weakening. It is likely that the development in Europe in a few years will be the same as in the US. Irradiation of chicken meat would for instance totally solve the campylobacter problem, which would be very positive from a public health point of view.

Notes from a conference concerning irradiation in Chicago September 2003

The industry has talked about this application during the last 50 years and has had some wishful thinking. Now it seems that it has started to take off in USA. (It also took 50 years to get pasteurisation to be a commonly accepted procedure).

  • Raw eggs often contain Salmonella and Campylobacter. It is shown that such a low dose as 1.5 kGy guarantees egg sanitation
  • The most used fumigant is Methyl Bromide, which is considered toxic and which therefore should be substituted by something else. Irradiation is technically the best alternative.
  • In spite of that EU has a restricted attitude towards irradiation of food, they have together with the Italian government - financed a study to define procedures for the safety and wholesomeness of fresh products relevant for the Italian food industry. One report talks about the treatment of chestnuts.
  • The government in USA has noted that there are more than 5000 deaths per year due to food poisoning. Irradiation will drastically reduce this number, if applied. All laws for irradiation are in place, so no excuse. The manufacturer has a strict responsibility, i.e. he must use the best possible process available.
  • In the last 2 years around 10.000 retailers have started to sell irradiated food, esp. ground meat.
  • One retail chain has already irradiated 2000 tons of ground meat.
  • Certain exotic fruits, like papaya, are sold in 2000-3000 shops.
  • Irradiated meat is served in 2000-3000 restaurants.
  • Fresh chicken will increase its shelf life with 25 days if irradiated
  • The cost for irradiation of 1 kg is 0.1 USD at doses of 1.5 3 kGy.
  • One cannot register any important negative consumer reactions.
  • An unexpected positive thing is that the sick days for restaurant workers have decreased significantly.
  • However, one cannot compare directly with Europe. In USA the law system is strange (many call it absurd) and if a lawsuit can be avoided by changing to irradiated ground beef so. In other words, the development is lawyer driven. The law cases are doubled each year. We suspect that we will see some substantial steps in Europe not earlier than 5 years from now. 
  • One important food address:




In water, light travels about 25% slower than it does in a vacuum and it is possible for an energetic particle to travel faster than light. (This is not possible in the vacuum where light is the fastest things there is!). When a particle travels faster than the speed of light in water, it produces a shock wave that is the equivalent of a sonic boom made by a jet travelling faster than the speed of sound in air. This shock wave takes the form of blue light called Cherenkov light, after the Russian physicist Pavel Cherenkov,wall of the detector.

  • Anthrax: The tragic events of September 11, 2001 has led to an increased level of security measures in society. Though not directly related, one such effort is to destroy anthrax spores ( sanitation) in mail and parcels. The only method on a large scale is irradiation by gamma.
  • Plastic stabilizers: Most plastics need stabilizers. These additives however often degrade, giving odour as a by-product. Common stabilizers are Irganox 1076 and Irgafos 168. These do not function well in PE. Irganox 1076 is however good for PS. Important to choose radiation resistent stabilizers.
  • Uranium: Graft polymerisation and crosslinking are attractive techniques for modification of the chemical and physical properties of conventional polymers. One unusual application developed by Japan Atomic Energy Research Institute is a cheap method to absorb uranium from seawater. Most of the worlds uranium is to find in seawater. By grafting of  PE-coated PP non-woven sheets a highly productive adsorbant was created.
  • Surgical mats: Crosslinked polysaccaride is highly effective as surgical operation mats. The mat is considered to disperse the body pressure and to maintain the best circulation of blood during operation. One commercial application is the Japanese Non-bedsore. An extra advantage is that the material is biodegradable.
  • Drug delivery: Gelatin is used for foods, films, glues, moisturizers, medical devices and many other products. The human body temperature of 37oC is however too high for maintaining of the necessary mechanical strength. The solution is to crosslink the gelatin by irradiation. An important application is within Drug Delivery Systems.
  • Insulin: It is important to protect insulin from the acid environment of the stomach before releasing in the small intestine. This is made possible by grafting a pH-sensitive hydrogel and use it as a drug carrier.
  • Beehives: In  honey business, one big problem is the Paenibacillus larvae, a devastating parasite in beehives. The only known cleaning possibility seems to be irradiation of the hives (without bees!).
  • Polypropylene: If you add 2-3% irradiated  PP to normal PP you might get 10-15% quicker biological detoriation.
  • Planting pots: Such pots may be in polypropylene. If irradiated they will detoriate quicker. Within a year or so. Important when the plants want to send out roots to a wider area.
  • Semiconductors: Irradiation can be used for artificial aging of certain electronic components.
  • NaCl: These crystals cannot be irradiated without colour changes in grey to black.
  • EtO: EtO used on PVC will give small amounts of ethylene chloride hydrine, which boils at 280oC. It will certainly not disappear during normal quarantine conditions.
  • Crosslinked PE: The penetration rate of O2 is 50-60% less in PE crosslinked by irradiation than if the crosslinking has been made by peroxide.
  • EVA, crosslinked at 125 kGy will give resistance to temperatures up to 130oC. Good for e.g. infusion bags. There will however be a small remain of acetic acid. Below the threshold of 1 ppm prescribed by the Pharmacopeia.
  • PET as a substitute for glass works well.



Ethylene Oxide (EtO) is an industrial chemical used in sterilizing medical items, fumigating spices, and manufacturing other chemicals. Pure EtO is a colorless gas at room temperature and a mobile, colorless liquid at -47oC. Sold as a mixture with either carbon dioxide or fluorocarbon 12, EtO has been licensed by the Environmental Protection Agency (EPA) in the US for use as an anti-microbial pesticide since the 1940s.

Ethylene oxide kills microorganisms by denaturing their proteins and subsequently modifying their molecular structure. This activity allows ethylene oxide to be effective as a sterilizing agent.

There are some hazards associated with EtO use. Acute inhalation of high levels of EtO has resulted in nausea, vomiting, neurological disorders, bronchitis, pulmonary edema, and emphysema. Skin and eye contact with solutions of EtO has caused irritation of the eyes and skin in humans. Tests involving acute exposure of animals have shown EtO to have relatively high toxicity from oral and inhalation exposures.

A short-term effect of EtO in humans is mainly central nervous system depression and irritation of the eyes and mucous membranes. Chronic (long-term) exposure to ethylene oxide in humans can cause irritation of the eyes, skin, and mucous membranes, and problems in the functioning of the brain and nerves. Some human data show an increase in the incidence of leukemia, stomach cancer, cancer of the pancreas, and Hodgkin's disease in workers exposed to EtO. EPA has classified EtO as a Group B1 hazard (probable human carcinogen).

EtO is not only present in sterilizers but also (in small concentrations) in the environment. Sources of environmental EtO include automobile exhaust and tobacco smoke.

Despite these drawbacks EtO is widely used for sterilization today because it is such an effective agent. There are however some practical limitations since EtO have difficulties penetrating different types of packaging material. In addition, the design of the product must allow the gas to enter into all cavities.

During the last 10 years EtO has become available as a steam under pressure mimicking the action of conventional steam sterilization. The future role of this modality remains to be seen.


An expert from the dialysis industry has currently ( 2003) claimed that EtO as a sterilizing agent is dead in the Western dialysis world due to hyper sensitation against EtO residuals. EtO is however still used for non-developed markets like Africa. The same person said that one way to express the danger of EtO is: The same exposition for an EtO worker as for a dialysis patient.



Steam sterilization is a time proven and economical process of killing microorganisms through the application of moist heat (saturated steam) under pressure. Heat damages the cells essential structures including the cytoplasmic membrane so that the cell is no longer viable.

The rate by which bacterial cells are thermally inactivated depends on the temperature and the time of heat to which they are exposed. In practical terms this means that it would take a longer time at lower temperatures to sterilize a population than at a high temperature. Additionally, the higher the concentration of organisms that need to be killed, the longer it will take to kill all of the cells in that population at the same temperature. In principle this mechanism is valid also for irradiation and EtO.

Furthermore steam cannot be used for goods with tight packing. It is much more suitable in a hospital setting where all packaging and wrapping can be penetrated by steam.