Forms of silver and their medical use

Silver offers a unique way of fighting bacteria, and this is very useful today in a large and growing number of medical applications. With the increasing efforts to promote healing and reduce infection, products such as medical uniforms, surgical sutures, hospital drains, catheters and wound dressings are beginning to contain silver in some form or another[1] Let’s take a closer look at some of them.

Silver Salts

In chemistry there are acids and bases. Since these terms are defined differently depending on the theory, they are defined in this book on the basis of their properties. An acid generally tastes acidic and has a pH value of less than seven. Strong, inorganic acids that fall on metals can produce hydrogen gas. A base generally tastes bitter, has a pH of more than seven, and feels slippery. When an acid and a base combine, a salt can be formed.

There are a variety of silver salts. Some of them are used for antimicrobial/antibacterial purposes:

  • Silver chloride: a component of bandages and wound healing products
  • Silver fluoride: a preventive agent against dental caries
  • Silver nitrate: a wart treatment
  • Silver sulfadiazine: the strongest topical dressing for burns [2]
  • Silver sulphate: a component of topical creams and bandages

Microbe Wiki, a student-run information project at Kenyon College in Ohio, notes that one advantage of the salt form of silver is that it “effectively delivers a large amount of silver ions at once” [3].

Silver nanoparticles (AgNP)

Nanoparticles (NP) are very small metal bodies. Their dimensions are measured in nanometers (nm), where 1 nm is equal to 3.93700787 by 10-8 inches; one nanometer is about 400 million times smaller than one inch. Depending on its size, a nanoparticle can contain several hundred to several tens of thousands of silver atoms. At present, silver nanoparticles are considered perhaps the best form for internal antimicrobial purposes. The Microbial Wiki suggests that this may be due to “their simple synthesis and highly effective observed antibacterial activity”[4]. Research points to the size and shape of the nanoparticles as reasons for this increased efficacy. Some studies suggest that due to the smaller particle size, the nanoparticles dissolve more easily in liquid environments and pass more easily through cell membranes and cell walls. Nanoparticles come in a variety of forms, including spheres, platelets, needles and wires. Obviously, the transport of spheres through the body is far less problematic than with the other forms. The fewer facets (sides) the particle has, the better its chances of preventing bacterial growth. In addition, nanocrystalline silver dressings have been shown to be anti-inflammatory [5].

Silver nanoparticles are basically spheres consisting of hundreds or thousands of atoms in a metallic state, as explained above. When these are brought into a liquid environment, a small fraction of the atoms leave the particle and, after their interaction with oxidizing species (oxygen in the external environment), they are converted into silver ions (Ag+). The silver ion is the species responsible for the antibacterial activity. The metallic silver itself does not provide antibacterial activity, but it acts as a reservoir and ensures a slow release of silver ions.

Silver powder

Silver powders are aggregates of very large silver crystals. They have been used with good results in the treatment of wounds caused by pressure, sores and burns. The powder is generally placed in or on the wound, either directly or as a dispersion in an acceptable cream, and then covered with a dressing.