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From: The eUpdate, 5.29.13

Anti-Bacterial Technology for Stainless Steel

Researchers have discovered a technique that could correct past attempts to make stainless steel resistant to bacteria

by Nick Goddard, MA, PhD, DIC, CEng, CPhys, MIMMM, MInstP

“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.

A number of researchers worldwide have tried to develop hybrid approaches producing surface coatings which combine the thickness of a polymer-based layer with the hardness of ion-beam implanted metal. These have tended to have the metal (typically silver rather than copper) retained within a ceramic matrix rather than a polymeric one. However, hard coatings tend to be brittle and this makes them liable to flake off when applied in a discrete thick layer. Scouring obviously exacerbates this tendency.

Recently, a new approach has been developed by an inter-disciplinary team of scientists at working in the Colleges of Engineering & Physical Science and Medicine & Dentistry at the University of Birmingham in the U.K. The researchers use a technique called active screen plasma (ASP) alloying. The object to be coated is placed inside a low pressure chamber with a metal lining. An electrical potential is applied between the object and the lining which causes a plasma (a high energy “soup” of ions and electrons) to form within the chamber. The team at Birmingham was led by Prof. Hanshan Dong in the School of Metallurgy & Materials.

The distinctive feature of ASP is that a mesh screen is placed between the wall of the chamber and the object to be coated. In the proprietary process developed at Birmingham, this screen is made from a mixture of stainless steel and copper and/or silver wires. As the plasma passes through the screen on its way to strike the target object it detaches a mixture of atoms from the steel, copper, and silver components of the mesh. These travel to the target where they are co-deposited into a single layer of mixed composition. Because the layer is built up progressively it can be much thicker than the limited penetration depth possible with ion beam implantation. And because the plasma behaves like a fluid, the ions being deposited do not travel in straight lines, allowing the coating of complex shapes.

A second distinctive feature of the Birmingham process is that organic gases containing carbon and/or nitrogen atoms are introduced into the ASP chamber. The gas molecules are broken apart by the plasma and some of the carbon/nitrogen atoms become incorporated in the new surface layer formed on the target. These interstitial atoms promote the formation of a stable S-phase in the basic stainless steel alloy (S-phase is also called expanded austenite—it is one particular geometric arrangement of the atoms in the metal lattice). This makes the surface layer harder than the underlying material (a similar effect is well known from the “case hardening” of low alloy steels).

The result is that a stainless steel surface treated by the ASP process is able to offer a unique combination of good wear resistance (superior to the untreated metal) promoted by the S phase component and long lasting anti-bacterial properties due to the nano-crystals of silver and/or copper. Tests at the University show that a stainless steel surface alloyed with 50 to 60 percent copper is 99 percent effective against E. coli NCTC 10418 and S. epidermidis NCTC 11047 within a six-hour testing contact time. This property remains intact after test instruments have been cleaned 120 times.

 

Dr. Goddard qualified as a metallurgist with degrees from Cambridge University and Imperial College, London. Since 2005, he has worked as an independent consultant advising start-up companies and public sector bodies on the commercialisation of materials and low carbon technologies. He can be reached at nick.goddard@technologygreenhouse.co.uk

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