Plasma sterilisation: not just a bacterial problem!

5. April 2012

Safe sterilisation of medical devices is necessary in order to prevent human-to-human transmission. Due to the rapid development of medical devices, sterilisation procedures have to meet high demands.

Whether the death of two premature babies who died a short time ago in Klinikum Bremen Mitte – the same NICU which was subject to criticism after deaths occurred there last year – was due to shortcomings in hygiene procedures is still unclear. It is known, however, that in many hospitals – not only in Bremen – hygiene scandals arise periodically: improperly sterilised surgical instruments often are the trigger. Aside from weaknesses in organisational processes, which are supposed to ensure the highest standards of hygiene, there are also weaknesses in the sterilisation process itself.

Coming to grips with bacteria using gas mixture

Because increasingly heat-sensitive materials are used in the manufacture of medical devices, it is not possible to use a single method of sterilisation for all materials and applications. At the same time the complex physical forms of goods to be sterilised, with features such as indentations, narrow openings, etc., demand methods which act on every point equally well. One gentle and easy way to sterilise medical materials is plasma sterilisation using low pressure (low-pressure inductively coupled plasma, or ICP). Gas mixtures employed here are made up of the backgound gas argon (Ar) and nitrogen (N2), oxygen (O2) or hydrogen (H2). A gas mixture is introduced into a plasma reactor and electrically excited by supplying energy. This releases electrons, ions, atoms, radicals and UV-emitting photons, which interact with biological species or molecules on the object to be sterilised; the molecules or cells are inactivated. The advantages of the method are especially that sterilisation at low temperatures, ie below 70° or even below 60° C, is possible and at the same time no or only particular desired changes take place in the material to be sterilised.

By way of using plasma generation and the gases selected, researchers can virtually tailor plasmas according on use. Prof. Dr. Achim von Keudell, head of the Working Group on Reactive Plasmas at Ruhr-University Bochum, explains: “The plasmas always have to be adapted to the object that needs to be sterilised. This is somewhat complicated compared to conventional processes, which proceeds rather more like a ‘dish washer’ “. There is still a drawback here or a limitation in the method, which makes it difficult to use in daily practice. The Ruhr-University Bochum has, in collaboration with a company, developed a plasmasterilisator, which is currently being tested in England.

Another more recent method is Electron Beam Treatment (E-beam), in which a concentrated, highly charged electron current acts on the object to be sterilised. It is a form of ionising energy being used here, which shows best results with uniformly low-density packaged products. The efficiency and oxidation effect on the material to be sterilised is comparable to that of gamma radiation sterilisation.

What’s otherwise at hand

Four different methods of sterilisation are currently in widespread use in the field of medicine. Depending on the object to be sterilised (heat resistance, moisture resistance, etc.), geometry (flat material, voids, etc.) and the efficiency of the procedure (operation time, risk of the method), the suitable method is respectively selected. Steam sterilisation at 121° C and 2 bar in an autoclave is only suitable for temperature and humidity-resistant materials; otherwise, for example with plastics, softening agents can diffuse and the material can become brittle. Using radiation sterilisation based on gamma-rays from a cobalt-60 source, changes for instance in the material properties of ultrahigh molecular weight polyethylene (UHMWPE) can occur. Hydrogen peroxide gas plasma sterilisation, in the narrowest sense, is not plasma sterilisation: the plasma is only used for dissociation of the hydrogen peroxide used. The actual sterilisation is based on the oxidising effect of oxygen, which can also lead to changes in materials. Controversial, but widely used, is ethylene oxide (ETO) sterilisation. The gas is a carcinogen and mutagen; even at relatively low concentrations in the air it has deadly effects on humans. Although it has good microbicidal effects, ethylene oxide sterilisation, owing to basic safety rules – especially with regard to gas release times – lasts up to several days.

Snuffing prions out

The existing procedures have another big disadvantage: If, after the mechanical cleaning of surgical instruments and similar devices that come into direct contact with human tissue, there remain tiny invisible residues of bacteria or other pathogenic germs on the tools, it can’t be automatically assumed that they are totally removed during the subsequent sterilisation. Taken as a whole, a sterilisation process is desirable that: in a comparatively short time (few minutes) is flexibly employable on materials subject to controllable property changes; takes up little space; and inactivates bacteria and viruses, as well as fungi, pyrogenic, endotoxins and prions, with certainty. It is possible that plasma sterilisation is developing here as the treatment of choice. In contrast to other sterilisation methods, success has been had by using plasma sterilisation in deactivating prions. The prion protein that triggers Creutzfeldt-Jakob disease is resistant to proteases and the aforementioned conventional sterilisation procedures. For dealing with bacteria, a combination of gas mixture and UV irradiation at wavelengths around 200-250nm is appropriate, even persistent endospores can be broken using the appropriate settings.

Even if, over the next few years, the requisites for the widespread use of plasma sterilisation have to be established or further improved, the procedure appears, despite its complexity, to present an opportunity for a variety of different materials to be reliably made sterile.

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