Silver has been known to have good anti-bacterial properties for centuries and is used in many types of wound dressings as well as other medical tools and equipment. Many household appliances and cleaning products use silver for its anti-bacterial properties. Ionic silver salt (ie. Ag+) is the most common form used, while silver nanoparticles (AgNP) consist of atomic silver (Ag0) that oxidise to silver oxide (Ag+) but this process can be altered by the chemical composition of the wound environment and lighting conditions.
How does it work?
Using colloidal silver in particular has formed the basis of many homeopathic treatments for a wide range of ailments. However studies over the last 20 years continue to elucidate many harmful effects of topical and oral/systemic treatments using silver. How silver eliminates bacteria is only partially understood and it most likely acts on the bacterial cell membrane, DNA, enzymes and proteins via multiple pathways. In many cases the research is showing that the healing benefits of a silver impregnated bandage is mostly and sometimes entirely due to the bandage and how it was engineered and not the added silver. Even more alarming is the damage silver can do to the cells it is exposed to with damage to DNA and bioaccumulation being the most noteworthy. It is also thought to form a reactive oxygen species. However, it should be noted that the antibacterial properties of silver also depend not only on the composition of the device it is in but the wound environment as well (biologically active molecules and compounds) which drive the release of Ag+ ions.
Toxicity of Silver
Studies into silver-containing wound dressings and devices detail impaired wound healing. Silver sulfadiazine has been shown to impair the healing of burns in studies involving animal wound healing and a meta-analysis of its use in children with burns high-lighted the fact that it was less effective at healing then non-silver alternatives. As well there is recent studies that connect the use of silver products to renal failure in burn patients. Other studies also indicate that silver-containing wound bandages delays skin healing over wounds.
There is detailed evidence that suggests that topical application of silver via dressings over wounds even results in skin cell cytotoxicity via DNA damage and pro-inflammatory effects correlating with the amount of silver absorbed by the skin.
One of the antimicrobial mechanisms, reactive oxidative species (ROS), of colloidal silver release silver ions (Ag+) which is what causes the toxicity in various organs. Once this ion (Ag+) is released it persistently accumulates in the body with 2 to 4% of the absorbed silver retained in tissues causing an irreversible bluish pigmentation called argyria as well as other issues such as kidney and liver damage. Silver can also bind to prescription antibiotics such as quinolones and tetracyclines causing them to be less effective.
How much is too much?
Recent WHO recommendations that are being proposed for human health are 0.3 micrograms per kg of body weight. As most colloidal silver is sold as 10 mg/L that means a single 15 ml dose contains 300 micrograms of silver and exceeds the recommended safe levels by a factor of 1000. These doses are determined in animal models and should be applied to animals as a matter of precaution.
So what other therapies work for topical wounds?
1) Manuka Honey has demonstrated effective antibacterial effects in topical wounds as well as oral and gastric health.
2) Hydrochlorous acid formed in anolyte water is showing some promising evidence of antimicrobial effects and may improve wound healing if used to flush topical wounds. It is extremely safe and breaks down to salt water over time.
3) Acetic acid (<2% in water) is inexpensive and readily available with some papers showing antimicrobial effects against Staphylococcus aureaus, Pseudomonas aeruginosa and other gram-negative bacteria. The draw-back is that higher concentrations sting when applied to wounds.
A couple of good references
Mao, BH., Chen, ZY., Wang, YJ. et al. Silver nanoparticles have lethal and sublethal adverse effects on development and longevity by inducing ROS-mediated stress responses. Sci Rep 8, 2445 (2018). https://doi.org/10.1038/s41598-018-20728-z
Nešporová, K., Pavlík, V., Šafránková, B. et al. Effects of wound dressings containing silver on skin and immune cells. Sci Rep 10, 15216 (2020). https://doi.org/10.1038/s41598-020-72249-3