Topic of the Month September 2011
Sports clubs, trams, public toilets: there is a risk of infection at any of the places that thousands of people frequent day in, day out. A slimy coating (“biofilm”) containing what are sometimes pathogenic bacteria and fungi that reproduce readily develops on door and grab handles or sanitary fittings as a result of repeated skin contact. A fleeting touch and the germs have already been passed on: unless hands are washed thoroughly, the germs get into the body and represent a particular danger to people with weakened immune systems – they face the threat of severe illnesses, from heavy diarrhoea to meningitis. So is it always necessary to reach for a disinfectant spray? A simpler solution is if the materials used have an integrated antiseptic, i.e. germicidal, function.
Hygiene is a serious problem, particularly in hospitals, where more and more extremely dangerous pathogens can be found, such as the multi-resistant bacterium Staphylococcus aureus (MRSA) that causes pneumonia and carditis and cannot be treated successfully with antibiotics any more. According to the European Centre for Disease Prevention and Control (ECDC), about 50,000 of the roughly three million infections that occur in European hospitals every year are fatal.
There are dangers everywhere. The U-shaped pipes (traps) located underneath the washbasins in patients’ rooms are an underestimated source of pathogens, for example. The magazine “Management & Krankenhaus” warns that the liquid retained in the trap contains up to ten billion living germs per millilitre. It is reported that aerosol formation leads to the transfer of them to the hands of hospital staff, where Pseudomonas aeruginosa – which causes urinary tract infections, pneumonia and meningitis – survives for longer than an hour. Enough time to be passed on to patients during care activities. Nosocomial infections (the term comes from the Greek words “nosos” = “illness” and “komein” = “care”) are also attributable to contaminated medical devices, drugs and food.
Killing germs and interrupting transmission channels is therefore the name of the game in hospital hygiene. All objects and surfaces on which biofilms can form – particularly when they are made of plastic, which has surfaces that are anything but smooth and which microorganisms find it easy to settle on – are candidates for physical and/or chemical disinfection measures, such as heating, irradiation, wet and dry sterilisation. It proves to be a problem to fight contamination by conventional means, because disinfectants containing chlorine are very hard on polymer construction and other materials such as drinking water pipes: the surface of the material corrodes, roughens and repels dirt less effectively, creating an environment in which bacteria can thrive particularly well. Since plastic is so versatile and economic that it can only be substituted by other materials to a very limited extent, it is not uncommon for different processes to be needed to guarantee freedom from germs.
• Antimicrobial metal coatings are one option. The Hygiene Institute at Leipzig University has found out that the survival rate of, for example, Escherichia coli (which as EHEC causes diarrhoeal disorders), Pseudomonas aeruginosa (see above), Staphylococcus aureus (see above) and Candida albicans (mucous membrane infections; pneumonia) is only half as high on chromium/nickel compounds we call stainless steel as it is on plastic. The mortality rate of Escherichia coli on stainless steel is apparently more than 99 per cent after two hours’ drying time. Other germs, on the other hand, such as the dangerous MRSA, survive on stainless steel for several days. Copper can deal with them better; in laboratory studies, MRSA died within two hours on high-density copper coatings. The germicidal properties of copper have, incidentally, been known since ancient times. The Greeks and Romans used copper vessels for drinking water to disinfect it and to keep it free from germs.
• What are known as sol-gel coatings can even be sprayed on or applied via immersion baths. Geometrically complex objects are easy to provide with antimicrobial surfaces in this way too.
• Compounding goes a stage further: plastics themselves are given antimicrobial properties while they are still being manufactured when additives are included in the process. For this purpose, germicidal substances are, for example, incorporated in polyethylene, polypropylene or polyamide, where they then have a disinfectant function thanks to the ongoing release of ions. Not only zinc oxide but also and above all silver are used in this context; the Chinese were already making acupuncture needles from the latter thousands of years ago. In addition to silver tableware and containers, silver coins were a popular disinfectant up to and including the 19th century, being put in water or milk. Modern processes are based on sterilising silver ions that diffuse from the material matrix to the surface and have an antimicrobial effect there: the cell membrane is destabilised, while metabolism and reproduction are disrupted at the same time, so that the microbe population is destroyed.
A distinction has to be made between two different methods of operation:
1. Contact sterilisation: the microorganisms die when they come into contact with the antimicrobial additive on the surface of the plastic object. The substance is not released; it remains firmly bonded to the surface.
2. Release sterilisation: the anti-microbial substance is released on the surface and migrates into the immediate surroundings as nanoparticles.
Since each kilogram of plastic contains a few hundred milligrams of silver at most, enhancement by the noble metal component remains inexpensive.
Polymers that in themselves already have antimicrobial properties, such as polyammonium salts or polyethylene glycol, manage without such additives. They are a convenient way to produce soft coatings for metals, glass and ceramics that are UV-resistant and can even provide protection against corrosion. Since there is a choice of high-gloss, matt, smooth or structured surfaces and the material can in addition have whatever colour is needed, decorative requirements can be met too.
Research scientists from the University of Massachusetts have adopted an innovative approach: they have developed polymers that imitate naturally antimicrobial substances (peptides), i.e. the compounds that form the first line of defence against infections in human beings and animals. The scientists found an ingenious way to do this: instead of imitating the peptides chemically, they just copied their surface structure. As soon as a microbe latches onto the plastic now, the latter perforates its cell membrane and the germ dies. The potential applications range from antiseptic kitchen tables to the equipment used in operating theatres.
Products made from practically all materials and used in practically all areas of life are being given better antimicrobial properties in the meantime. Measures taken to eliminate germs in the food industry, such as coatings for the inside of beverage cans, extend shelf life and help to economise on preservatives that can cause allergies. Making socks, shoe inlays, mattresses or upholstered furniture germ-free prevents unpleasant odour formation, for which bacteria are responsible that interact with the sweat.
The demand for antimicrobial plastic is probably strongest in the medical community. Wound dressings contain silver additives that are found in a similar form in the plastic containers used to store contact lenses too. Clinical studies have shown that catheters with a silver sulfadiazine coating can reduce vein infection rates by almost a factor of 5. In orthopaedic engineering, silver has recently been introduced as an antibacterial substance in the plastic shafts of prosthetic limbs, in order to counter odour formation and inflammation. According to the manufacturer, the technology reduces colonisation by the bacterial strains Staphylococcus aureus and Escherichia coli by 99.9 per cent.
Last but not least: a self-disinfecting film that can be stuck, for example, to door handles and sanitaryware is an effective way to prevent germs from being passed on in hospitals. It has been developed at the Institute for Chemistry and Bioengineering Sciences at Zurich Technical University (ETH) in Switzerland. The film is coated with silver and calcium phosphate; according to ETH, the combination of both substances is up to 1,000 times more deadly to Escherichia coli than conventional silver products. The background: calcium phosphate is a welcome food source for the bacteria, which consume it eagerly. When the substrate material breaks down, the integrated silver particles are released; they have a toxic effect on the bacteria, however, so that the latter can be depended on to eat themselves to death by consuming the calcium. In other words, the crucial feature of the process is that the bacteria initiate the germicidal effect themselves and in fact control it, because silver is only released to the extent that bacteria are present to consume the calcium phosphate.