The antimicrobial properties of metals such as silver and copper were identified in ancient times and their antimicrobial action utilised by a broad range of civilisations up to the present day.
However, the use of silver and copper as antimicrobials in medical and public health applications waned following the proliferation of the antibiotic industry after World War II. In the late 20th Century increasing hygiene awareness coupled with the emergence of healthcare-associated, multi-drug resistant, disease causing bacteria prompted healthcare professionals to look for different solutions to hygiene problems, such as the incorporation of inorganic antimicrobials into plastics and other polymerised materials. In recent years there has been a significant development of polymer technology, which may benefit from antimicrobial protection. So what is the most effective way to protect plastics from microbes?
Synthetic antimicrobials such as triclosan gained worldwide attention during the 1960s and, although widely heralded for their broad-spectrum activity, have come under scrutiny from government bodies such as the FDA due to environmental and toxilogical questions, with some countries considering its use banned or further restricted. Naturally occurring antimicrobials have avoided much of the negative press associated with synthetics, and came to be regarded as the cleaner, safer and ‘natural’ alternative.
Within the category of common, naturally occurring antimicrobials, silver and copper are the most widely used, and are each well suited to different types of application. Copper is commonly used in solid alloy form in public health applications such as hospital railings, but is rarely used as an antimicrobial additive in other materials such as plastics due of environmental concerns over certain formulations, and its tendency to discolour materials.
Silver ion technology can be added to polymers in a number of ways, most commonly through the incorporation of a masterbatch via a dosing unit on the injection or extrusion machine, in the same way other colour or additive masterbatches would be introduced to the polymer melt. Raw additive powder can be mixed with virgin polymer before it is introduced into the hopper. Some machines (twin screw) will be able to mix the raw powder into the polymer melt. Once the polymer has been moulded the silver ions are situated in the polymer voids and the material is protected from colonisation from microbes. So where does antimicrobial plastic fit with the current situation in healthcare and other industries such a food preparation?
Inorganic antimicrobials utilise the oligodynamic effect for efficacy against microorganisms. The exact nature of this action is not fully understood, however evidence suggests inorganic ions act in a multitude of ways to damage microbial organisms, and this action occurs at low ionic concentrations. Upon contact with silver ions, microbial proteins and DNA are bound resulting in conformational changes, inhibiting cell growth; oxygen free radicals are formed within the cell, resulting in further cellular damage. The antimicrobial effect with regards to silver based antimicrobials is due to the silver ion, and this action is independent of the carrier.
Silver ion technology is particularly well suited for use in plastics for a number of reasons. When carefully dosed it should not affect the physical, mechanical or thermal properties of the plastic in a negative way, ensuring, for example, the appearance and tensile strength etc. remain. The multi modal non-specific action of silver ensures it is extremely unlikely microbes will develop resistance in the same way they can with drugs or molecular site-specific antimicrobials. The technology is thus suitable for widespread treatment of surfaces with a reduced concern of propagating resistant organisms. Incorporation of silver ion technology complements cleaning and hygiene methods in many environments, such as primary healthcare and public health, food preparation, medical devices and many more. Antimicrobial additives are a rapidly expanding field, with solutions being sought internationally for an increasing number of applications.