“Stains” are discolorations that are visible to the human eye. However, it is well known that using alternative imaging techniques can reveal all sorts of contamination lurking unseen. Thus “stainless” steel which looks as if it has been cleaned to a high standard may nonetheless be playing host to a multitude of bacterial pathogens. And even if it looks clean, and is in fact clean, immediately after a periodic washing, how long will it remain free from bacteria which can accumulate invisible to the naked eye?
Given that visual inspection remains the first line of defence in many food preparation facilities and serveries, it is not surprising that there has been intense interest in developing a way of conferring inherently bactericidal properties on stainless steel surfaces and utensils—making them in effect “self-disinfecting.” And in view of recent high profile problems with hospital acquired superbug infections, the healthcare sector has a shared interest with the food safety community in the search for this anti-bacterial “holy grail.”
It has been known for over 100 years that silver and copper are powerful bactericides. Acting via the so-called oligodynamic effect, they disrupt key proteins essential not just to bacteria but also algae, molds, spores, fungi, and viruses. This was a significant benefit to the richer of our forebears who favoured these metals for making the utensils they used in the production and consumption of their food and drink (being “born with a silver spoon in your mouth” was of medical as well as social advantage). Further advantages presumably came from washing both their clothes and bodies in copper baths.
Stainless steel has now largely replaced silver and copper, certainly for industrial applications (one exception being whisky making). Alloying the stainless steel with relatively small quantities of silver or (more likely) copper might render it bactericidal but would be prohibitive on cost grounds. Furthermore, most of the valuable copper or silver atoms would be wasted as they would be locked away inside the body of the object whereas pathogens only have contact with the surface. The search for anti-bacterial stainless steel has therefore focused on providing a reservoir of bioavailable silver or copper atoms concentrated only at the metal surface.
Historically, this challenge has been approached either by the use of very thin implanted layers or by the deposition of soft polymer-based coatings. Atoms of one metal can be implanted into another by forming positively charged ions and then accelerating these ions using an electric field so that they smash into the target object and become embedded (a technique called ion beam implantation). However, the implanted ions only penetrate a very small distance into the surface—typically < 0.5µm (one or two hundred-thousandths of an inch). Furthermore, the ions travel to their target only in straight lines, which makes it difficult to coat complex 3D shapes, undercuts, or crevices.
Thicker layers can be achieved by dispersing metals in organic polymers via the use of covalently bonded organometallic complexes and then coating the object with a layer of the resulting polymer. A variant of this approach is the Agion technology being commercialised by Sciessent LLC in which microscopic “cassettes” of ionically bonded silver ions are introduced to the polymer in tiny pieces of microporous ceramic known as zeolites.
Neither of these approaches is well suited to the sort of regular scouring which is routinely used to clean items that come into contact with food. Regardless of whether it is a relatively hard ion beam implanted layer which is very thin (one hundred times thinner than a human hair) or a much thicker polymer layer which is very soft, the protective layer is relatively quickly worn away and the anti-bacterial properties are lost.