Silver - Applications - Medical

Medical

Silver ions and silver compounds show a toxic effect on some bacteria, viruses, algae and fungi, typical for heavy metals such as lead or mercury, but without the high toxicity to humans normally associated with these other metals. Its germicidal effects kill many microbial organisms in vitro, but testing and standardization of silver products is difficult.

Hippocrates, the "father of Western medicine", wrote that silver had beneficial healing and antidisease properties, and the Phoenicians stored water, wine, and vinegar in silver bottles to prevent spoiling. Its germicidal effects increased its value in utensils and as jewelry. The exact process of silver's germicidal effect is still not entirely understood, although theories exist. One of these is the oligodynamic effect, which explains the effect on microorganisms, but would not explain antiviral effects.

Silver is widely used in topical gels and impregnated into bandages because of its wide-spectrum antimicrobial activity. The antimicrobial properties of silver stem from the chemical properties of its ionized form, Ag+. This ion forms strong molecular bonds with other substances used by bacteria to respire, such as molecules containing sulfur, nitrogen, and oxygen. When the Ag+ ion forms a complex with these molecules, they are rendered unusable by the bacteria, depriving them of necessary compounds and eventually leading to their deaths.

Silver compounds were used to prevent infection in World War I before the advent of antibiotics. Silver nitrate solution use continued, then was largely replaced by silver sulfadiazine (SSD) cream, which generally became the "standard of care" for the antibacterial and antibiotic treatment of serious burns until the late 1990s. Now, other options, such as silver-coated dressings (activated silver dressings), are used in addition to SSD cream. However, the evidence for the effectiveness of such silver-treated dressings is mixed; although the evidence is promising, it is marred by the poor quality of the trials used to assess these products. Consequently, a systematic review by the Cochrane Collaboration (published in 2008) found insufficient evidence to recommend the use of silver-treated dressings to treat infected wounds.

Interest in silver has renewed as a broad-spectrum antimicrobial agent. One application has silver being used with alginate, a naturally occurring biopolymer derived from seaweed, in a range of products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims. In 2007, a company introduced a glass product they claimed had antibacterial properties by coating the glass with a thin layer of silver. In addition, in 2007 the U.S. Food and Drug Administration approved an endotracheal breathing tube with a fine coat of silver for use in mechanical ventilation, after studies found it reduced the risk of ventilator-associated pneumonia.

Another example uses the known enhanced antibacterial action of silver by applying an electric field. In 2009, the antibacterial action of silver electrodes was found to be greatly improved if the electrodes were covered with silver nanorods. The University of Missouri has found silver nanoparticles threaten benign bacteria which extract ammonia from sewage treatment systems. A serious concern is the eventual spread of the toxin into rivers, streams, lakes, and ultimately the oceans. A note of caution is sounded by Martin A. Philbert, professor of toxicology at the University of Michigan, Ann Arbor, "In the context of environmental health, the scientific community will have to pay close attention to those physicochemical properties of engineered nanomaterials that defeat or circumvent normal cellular processes and lend themselves to indiscriminate penetration of biological barriers, tissues, and cellular systems."

Silver is commonly used in catheters. Silver alloy catheters are more effective than standard catheters for reducing bacteriuria in adults having short-term catheterization in hospitals. This meta-analysis clarifies discrepant results among trials of silver-coated urinary catheters by revealing silver alloy catheters are significantly more effective in preventing urinary tract infections than are silver oxide catheters. Though silver alloy urinary catheters cost about $6 more than standard urinary catheters, they may be worth the extra cost, since catheter-related infection is a common cause of nosocomial infection and bacteremia.

Various silver compounds, devices to make homeopathic solutions, and colloidal silver suspensions are sold as remedies for numerous conditions. Although most colloidal silver preparations are harmless, cases where excessive consumption led to argyria over a period of months or years have been reported. High consumption doses of colloidal silver can result in coma, pleural edema, and hemolysis.