The antimicrobial properties of silver ions have been known since ancient times, and several studies have explored the mechanisms of action that make them effective against such a broad range of microorganisms.1 This includes Gram-positive and Gram-negative bacteria,2, 3 fungi and yeasts,3 and viruses.4–6 Silver ions work in a number of ways, most noticeably by disrupting essential cell transport, i.e., the movement of nutrients, metabolites and waste in, out and around the cell; protein synthesis;7 and reproduction by interfering with the microorganism’s DNA and RNA.8, 9 This effectively starves or suffocates the microorganism, preventing its replication and resulting in inert stasis or death.
However, some microorganisms have a greater tolerance of silver than others, and even show a degree of resistance. Silver ions are non-selective and as noted, affect the performance of a number of critical physical functions within microorganisms. Therefore, in order to become resistant to silver ions, an untreated microorganism would need to mutate several critical functions simultaneously in a single generation through spontaneous mutation. The likelihood of this is very low;10–13 to date, the emergence of any silver-sensitive pathogen made resistant from the clinical use of silver has not been reported.
Further, one can conclude from the vast body of scientific work on this topic1, 4, 5, 14–17 that it is indeed the silver ion that is the antimicrobial agent and not metallic silver itself or silver salts, oxides or complexes. There are always some ions present in equilibrium with the stable silver form;9 the more stable the form, the lower its associated ionic concentration and, in the case of silver, the lower its antimicrobial activity. It is only the ions in equilibrium with these stable forms that are active.1 No lasting disinfection can therefore take place unless there is a sufficient and continuous replenishment of the active silver ions. This is illustrated by the use of silver nitrate compresses on wounds, where treatment requires frequent application of the solution—as often as every two hours—to sustain an effect.
Silver ions, however, are reactive and short-lived because they form complexes with other molecules.9, 18, 19 As such, the antimicrobial potency of silver can only truly be unlocked with a suitable method of delivering free silver ions in a controlled and sustained manner. This author’s interest to incorporate the antimicrobial potency of silver into medical devices, primarily for the wound care and urology markets, led to exploring the potential of water-soluble phosphate glasses, also known as phosphate polymers, as a controlled delivery platform for the silver ions.