There is conclusive evidence, both academic and commercial, that metal atoms and ions can provide antimicrobial effects in paints, plastics, laminates and on the surface of other metals. Now that migratory, volatile organic biocides are viewed with suspicion because of safety, health and environmental effects, and many are, or will be, banned, the search for solid inorganic and polymeric direct contact inhibitors is vital. The most established ionic inhibitors are silver salts including silver nitrate (AgNO3). Other more complex silver based compounds are possible as are organometallics. Of course, it is not automatic that metal based biocides are safe to use: consider the recent ban on tributyltin oxide as the principal anti-fouling mechanism for ships and other immersed structures.
Slow silver ion release can provide long term continuous effective action and such systems have been shown to be highly effective in suppressing growth of a broad range of bacteria, mold and mildew. It is believed that positively charged monovalent silver is attracted to hygrogen ions and combines with them on the microbial sulphydryl groups. This interferes with electron transfer and respiration in some microorganisms. Non-ionic forms of silver can catalse the action of atomic oxygen with the sulphydryl which also inhibits respiration. It is possible that combinations of silver and photocatalytic TiO2 might offer enhanced synergistic action.
Other metals
Titanium in the form of anatase titanium dioxide (TiO2) is a prime contender because of its photocatalytic regeneration potential under UV lights and now, in development, grades regenerated by fluorescent lighting and even day light. TiO2 not only has a proven biocidal effect but may contribute to self-cleaning, anti-pollution and odour-destroying surfaces.
Some papers suggest a synergistic effect from a combination of TiO2 with silver and other metal oxides, eg. zinc and manganese. The bonus of the catalytic effect from some of these compounds or mixtures could be to activiate mild cleaning/disinfecting/sterilising precursors into more agggresive products at the surface where required. This process might be accelerated by the impingement of germicidal, ozone-generating, steady or pulsed UV.
An exciting step is the development, by DuPont, of silver-containing powder coatings. The growth of new applciations for powder is phenomenal and the current trend is UV cured types for temperature sensitive substrates such as wood and plastics. Metal hospital beds and equipment represent a large market for anti-bacterial powder coating applicators. Silver ions are estalished commercially in anti-microbial coatings for critical medical equipment for which catheters are a prime example. This trend is expected to increase.
We have mentioned silver and other metal-containing products for which examples are given in the abstracts in the news archive. Note that silver compounds are used even to enhance the antimicrobial properties of coated steel.
Copper is receiving increasing attention for anti-microbial surfaces as a reuslt of very positive promotion by the Copper Development Association.
Prospects for metal modified polymer films, as wall coatings and shrink film for pipes, etc, might be interesting, but that is a subject for a later time.
Formulation is all important. We need the minimum and most economic addition consistent with durability. This implies the importance of surface and colloid cemistry in order to ensure an economic continuous and continually active-rich layer at the surface or a reactivation or replenishment system. These must resist the aggressive attack by a combination of environmental conditions, cleaning regimes and possible high energy UV flooding.
Metallic particles are conductive and coatings able to conduct low voltage electricity might represent another weapon to counteract condensation and adhesive bio-films. Of course, it is ironic that metallic salts may be toxic to certain microorganisms since many may have been deposited by microbial activity.
Recent emphasis on various techniques to produce and disperse nano-particles should help to provide greater active surface area for a given addition weight, as should the plating of less expensive fillers.
It is possible that appropriate formulation/incorporation expertise lies within those skilled in the production of electrically conducting polymers. In those cases, a metal contact network (matrix) is essential and thus morphology is critical. Perhaps conductivity may represent a route to measuring the extent and uniformity of particle distribution.
The photographic and glass prodcution industries are skilled in physical vapour deposition (PVD) and chemical vapour desposition (CVD) methods to produce thin, continous films such as indium tin oxide. Major glass producers are amongst the first to commercialise self-cleaning, photoactive, TiO2-coated glass for automotive and architectural applications. Products are available from Pilkington, PPG and Saint-Gobain, as well as Japanese groups.
A sandwich assembly consisting of a layer of silver between two layers of TiO2 has been used to coat the interior of light bulbs by sputter coating to a thickness of about 180 Angstroms in order to significantly reduce power consumption, and triple the bulb life. There is also a need for anti-bacterial glass but this area will be covered in detail in future articles.
If we look hard enough at the anti-bacterial capacities of metals, we may find a silver lining!