Engineers Give Silver a Healthier Future

Oct. 10, 2008
Modern technology enhances the metal’s age-old role in fighting infection
Throughout history, silver has had a long and well-recorded role in healing. The ancient civilizations of Greece and Rome used silver to control bodily infection  and prevent food spoilage. Almost Seven decades before the birth of Christ, Roman scholar Pliny the Elder recorded silver’s ability to heal wounds by preventing infection. In the fifth century B.C., Hippocrates — the father of medicine — also used it to heal wounds and to treat ulcers. In the same period, the King of Persia  used silver water containers to prevent contamination.Fourteen-hundred years later, the metal was a popular treatment in the Middle East for many ailments, including cardiovascular disease and bad breath. However, its importance really began to spread in the 16th century when Swiss-born physician, chemist and alchemist Theophrastus Philippus Aureolus Bombastus von Hohenheim — known as Paracelsus — used silver and other elements as medicines. Today he is often referred to as the father of toxicology.More recently, before the rise of antibiotics, silver compounds were used during World War I to prevent wounds from becoming infected. Today, it finds uses in catheters and wound dressings, particularly where burns are involved. Johnson & Johnson and Bristol-Myers Squibb, among others, have been active in developing silver-based treatments. Cycling footwear manufacturers now impregnate special socks with very thin pure silver linings as protection against painful fungal infection and unpleasant foot odor. Many floor coatings used by food processors and others are routinely cured with silver-based antimicrobial coatings to protect against a broad spectrum of potential bacteria and mold infections. However, the newest advance in silver’s long therapeutic history is from a group of chemical engineers at the Institute for Chemical and Bioengineering at Swiss Federal Institute of Technology (ETH) in Zurich. Building on their predecessor’s interest in silver, together with Swiss enthusiasm for all things small-scale, they have developed a plastic film coated with silver and calcium phosphate nanoparticles that is up to 1,000 times more effective at killing E coli bacteria than conventional methods.Currently more than 700 strains of E Coli have been identified, with many necessary for healthy digestive function. Others, however, are present in contaminated food and beverages and can cause gastrointestinal and urinary tract infections. “Within 24 hours of the plastic film being applied to a surface, less than one bacterium out of 1 million bacteria will survive,” says Wendelin Stark, a chemical engineer and leader of the project.Previously it had been impossible to apply silver in a targeted and measured way. However, by using a film and applying silver to the calcium phosphate, he believes the problem has been overcome.A recent article “Micro-organism-Triggered Release of Silver Nanoparticles from Biodegradable Oxide Carriers Allows Preparation of Self-Sterilizing Polymer Surfaces,” (Small,  Volume 4, Issue 6, June 2008, pp. 824-832) chemical engineers Stefan Loher, Oliver D Schneider, Tobias Maienfisch, Stefan Bokorny, Wendelin J Stark describe how to increase the antimicrobial activity of a silver-containing surface by two to three orders of magnitude. The use of 1–2-nm silver particles decorating the surface of 20–50-nm carrier particles consisting of a phosphate-based, biodegradable ceramic allows the triggered release of silver in the presence of a growing microorganism. This effect is based on the organism's requirements for mineral uptake during growth — creating a flux of calcium, phosphate and other ions to the organism. The growing micro-organism dissolves the carrier containing these nutrients and thereby releases the silver nanoparticles. So the polymer film only emits silver if bacteria are growing in the vicinity — a factor that can be assessed by the quantity of calcium phosphate they take up.The engineers then demonstrate the rapid self-sterilization of polymer surfaces containing silver on calcium phosphate nanoparticles using a series of human pathogens. Colony-forming units (viable bacteria or fungi counts) have been routinely reduced below detection limit and suggest application of these self-sterilizing surfaces in hospital environments, food and pharmaceutical processing, and personal care. “It saves money and is much more efficient. It also reduces the environmental impact of the process and we have developed a method that is easy to apply and could bring great benefits to patients in hospitals, as well as the food industry,” adds Stark.Although its identity remains a secret for the moment, a Swiss company is already working on techniques to scale-up the manufacturing of the film developed by Stark and his colleagues.

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