A nanoproduct made from silver and calcium phosphate and developed by ETH Zurich researchers is lethal to bacteria. Its special feature is that the bacteria themselves invoke and dispense this disinfectant effect.
The fact that silver is an antiseptic and thus a disinfectant has been known for about 3000 years. This is why well-to-do households used silver cutlery, which has an antibacterial effect, while poorer people put silver coins in their milk jugs.
Silver was used medicinally for around two hundred years before it was replaced by antibiotics, and, for a long time, practically its only remaining use was in alternative medicine. The noble metal has undergone a kind of renaissance in medicine since the start of the nanotechnology era. Medical instruments, artificial limbs, hospital furniture or even hospital linen are lined, sheathed or fortified with it.
Nutrient substrate activates the mechanism
However, until now it has been impossible to use the noble metal in a specific, controlled amount. The research group led by Wendelin Stark, Assistant Professor at the Institute for Chemical and Bioengineering of ETH Zurich, has now developed a plastics film coated with silver and calcium phosphate that fulfils these conditions and, in addition, is self-disinfecting.
For example, the combination of the two substances has an effect on the bacterium Escherichia coli that is up to 1000 times more lethal than conventional silicon-based silver preparations. One decisive factor appears to be that the bacteria use the calcium substrate for their metabolism. The calcium phosphate particles, 20 to 50 nanometres in size, are absorbed by the micro-organisms as food and are thereby disintegrated. This releases thousands of tiny silver particles measuring 1 to 2 nanometres which the researchers had coated onto the calcium nutrient.
According to current knowledge, silver nanoparticles have multiple effects on bacteria: they suppress the cell’s nutrient transport, attack the cell membrane and interfere with cell division and thus with the reproduction of the germs. Experiments with the carrier substances calcium phosphate and silicon dioxide, each coated with silver, showed different effects on various bacterial strains in the test.
The calcium phosphate substrate had an efficiency factor of up to 1000 times stronger than silicon dioxide. Within 24 hours, less than one bacterium out of 100,000 to 1,000,000 bacteria survived. However, according to the researchers, since the consumption of the organic calcium phosphate also nourishes the bacteria – without the effect of the silver, they would multiply thousand-fold in 24 hours – the silver must fight not only the bacteria that already exist but also those that would newly form. Wendelin Stark says, “This makes the effect even more astonishing.”
Reducing the risk of infection
The new product has enabled Stark’s group to successfully develop a preparation that is effective against a series of pathogenic bacteria and which becomes active in a targeted manner and in the correct dose only if a bacterium is present. The silver adhering to the calcium phosphate is only released in a quantity corresponding to the amount of calcium phosphate consumed by the bacterium. This saves costs, is efficient and is less stressful for the human body. The product is already being manufactured by the Perlen Converting AG Company in Perlen near Lucerne, which was involved in the development.
This involves coating a film with nanoparticles of silver and calcium phosphate. The film can be used in hospitals, for example, hotspots for germ transmission. Door handles, beds or sanitary equipment onto which the self-disinfecting film is stuck could protect patients from dreaded and dangerous hospital pathogens that can lead to complications, for example after surgical operations. It must be renewed from time to time because the bacteria consume and use up the calcium, so the film is not effective indefinitely.
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