Stainless Steel in Hygienic Applications ISSF STAINLESS STEEL for HYGIENIC APPLICATIONS - 2
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Stainless Steel in Hygienic Applications ISSF STAINLESS STEEL FOR HYGIENIC APPLICATIONS - 2 Contents 1 Background 2 Active antimicrobial solutions 2.1 Overview 2.2 Silver-containing coatings 2.3 Copper and its alloys 2.3.1 Re-contamination 2.3.2 Biofilm formation 3 Why statements about antimicrobial effects should be viewed with scepticism 4 Why stainless steel is a preferred option 5 Conclusions 6 References © 2015 International Stainless Steel Forum ISSF STAINLESS STEEL FOR HYGIENIC APPLICATIONS - 3 1 Background Active antimicrobial surfaces vs. standard stainless steels: Summary of There has been a discussion amongst experts arguments and consumers about antimicrobial properties of materials. It has long been known that copper [1] Comparisons between countries show that the forming in bacteria such as E. coli and silver are metals which can inhibit the growth occurrence of healthcare-related infections may be softer and less wear-resistant than of bacteria, viruses and fungi. Stainless steel, with multiple-resistant micro-organisms is stainless steel in contrast, is an inert material and, although determined by factors other than material are expensive its easy cleanability makes it a proven solution selection in touch surfaces. Stainless steel whenever sanitization is essential, it is not in itself Active antimicrobial surfaces in general has a long history of successful use in the bioactive. are not effective against all micro- most demanding medical applications such organisms. There are many pathogenic as implants and surgical instruments, which In some applications (e.g. touch surfaces in micro-organisms and some of them are less require sterile conditions hospitals), active, antimicrobial materials have sensitive than others to active surfaces. Only does not imply the risk of micro-organisms been proposed as a substitute for stainless steels proper cleaning and disinfection can remove forming new types of resistance [2]. Extensions of the idea into professional all relevant germs does not undergo changes over time; the kitchens and handrails in public transport efficiency of cleaning and disinfection have also been discussed. It is the purpose of are often overestimated in terms of remains the same over many years this paper to put the discussion about active, antibacterial efficiency because some has a harder surface than many other antimicrobial materials into perspective. A current assessment methods are unable to metallic alloys and makes fixtures less summary is provided of the reasons why stainless identify dormant cells susceptible to superficial damage such as steel is often the only viable option when the Silver-containing coatings in particular scratches and dents, in which biomass may highest levels of hygiene or sterile surfaces are wear off easily accumulate required. provide no visible indicators to show when is resistant to both corrosion and high the antimicrobial effect starts to fail temperatures. It can be sterilized by are expensive thermal process and (liquid or gaseous) Copper and its alloys chemical agents without corroding or loss of owe their antibacterial effect to the release mechanical or physical properties of metal ions; however, there are indications is passive so there is virtually no exchange of of detrimental effects of copper on the ions with the environment environment is a cost-effective high-performance have become associated with resistance material ISSF STAINLESS STEEL FOR HYGIENIC APPLICATIONS - 4 2 Active antimicrobial solutions Recently, the use of the oligodynamic effect of 2.1 Overview silver and copper has been re-considered in hygiene-critical applications: Certain metals have an oligodynamic effect: they release ions into the environment, which damage In the United States, silver-containing coatings the cell walls and cell membrane. Some research were developed into an industrial solution also suggests that the DNA is damaged and [4]. They can be applied to metallic and other prevented from replicating [3]. surfaces – among them to stainless steel. They are used in, for instance, ventilation ducts and While bacteria are affected by the oligodynamic refrigerators. effect, viruses are generally much less sensitive The copper industry has launched an to it. “Antimicrobial Copper” campaign [5]. It maintains that copper and its alloys (such as For hygienic purposes, silver and copper are used. bronze, brass, cupronickel and copper-nickel- Their effects have been known for centuries – long zinc alloys) can be a solution to one of the before micro-organisms were even discovered. most serious problems in the medical sector: The downside is that their surfaces oxidise, if hospital (or, more generally, healthcare) unprotected, requiring labour-intensive regular acquired (nosocomial) infections. Certain polishing. This is why silver in cutlery and brass bacteria have become resistant to the usual in hardware were to a large extent replaced antibiotics. The best-known germ of this type by stainless steel when this material became is the methicillin-resistant staphylococcus commonly available in the early 20th century. aureus (MRSA). Experts hold the view that infections caused by “superbugs” (e.g. MRSA) in hospitals cost more lives than road accidents. ISSF STAINLESS STEEL FOR HYGIENIC APPLICATIONS - 5 2.2 Silver-containing coatings 2.3 Copper and its alloys Silver-containing coatings have been applied to Copper is known to have antimicrobial effects. various materials. These include a silver/zinc- Summary: silver-containing Copper ions can penetrate cell walls and containing zeolite matrix, which is applied to coatings obstructs cell metabolism [8]. Studies show that stainless steel and other materials. It is reported the colonization of touch surfaces in hospitals to reduce colonies of microbes by 85.5–99.9 % are expensive can be reduced if copper or copper containing after 4 hours and virtually 100 % after 24 hours. wear off easily alloys are used for touch plates, light switches, Applications can be found in ventilation systems, show no visible indicator when their effect door handles, water taps and others. In some where the coating contributes to reducing the fades of these studies even a limited presence of proliferation of germs and fungi [6]. copper-containing materials in the environment is reported to reduce the number of hospital- However, for such coatings to be efficient, acquired infections strongly [9]. a relatively high concentration of 39–78 µg/ ml of silver ions in the zeolite is required [6]. However, these results need to be interpreted Nevertheless, such coatings can wear off carefully. Direct contact with the patient or especially if the coating is applied wet, i.e. common materials such as textiles, plastics, by spray painting. Powder coating produces chinaware or glass should be more relevant for somewhat more durable layers. the transfer of micro-organisms than fixtures and fittings. Artefacts in an experimental setting could Worn and damaged coatings have a negative explain why sometimes surprisingly strong effects effect on the surface topography of the material. are shown: when medical and cleaning staff They can form cracks and crevices, which make become aware that they are in an experimental it more prone to the adhesion and retention of situation, they may observe the cleaning and organic matter. They have obvious limitations on disinfection regimes more strictly than in their surfaces which are exposed to wear and tear [7]. normal day-to-day work. The reported reduction Over time, the silver is depleted. The silver ion in nosocomial infections may therefore partly release may also be inhibited in environments that be attributable to variables that are related to contain sulphur and chloride ions [6]. Against the human behaviour rather than the surface contact background of volatile silver prices, the treatment material. can be expensive. The surface topography of materials has a strong influence on their cleanability. Hard surfaces are ISSF STAINLESS STEEL FOR HYGIENIC APPLICATIONS - 6 less susceptible to scratches and dents which In applications outside of the medical sector, Summary: can form microscopic recesses in which deposits copper release is often unwanted because of its may accumulate and become difficult to wipe environmental effects [16]. The release of copper Release of copper ions can make away. Stainless steel grade 1.4301 (304) has a from roofs is a case in point. The release rate of pathogenic micro-organisms resistant to typical hardness of 88 (Rockwell B-scale), while copper has been found to be between 1.3 and 2.0 copper. the equivalent value is 10 for cold-rolled copper g/m² per year [17], [18]: The toxic or even lethal effect on certain (1/8 hard), 42 for Commercial Bronze (1/4 hard) cells is not limited to pathogen germs and between 55 and 65 for various types of brass In the Netherlands, the use of copper in but can also affect other useful micro- [10]. In contact with water, copper can release roofs and rainwater systems can locally be organisms. ions. The anti-bacterial efficiency of copper restricted or measures can become necessary depends on the presence of humidity. Biofilms to reduce copper emissions. The maximum that naturally occur on frequently touched admissible risk level for copper in surface surfaces can, however, act as a barrier between water is 3.8 µg/l and new installations must the copper and the micro-organisms and strongly not add more than 10 % of this level to the pre- reduce ion migration. There are indications that existing charge [19]. copper surfaces, despite regular cleaning, have a In Sweden, the environmental programme of tendency to build up layers of biomass that reduce the city of Stockholm for 2012–2015 says that the release of copper ions significantly over time (PVC and) copper piping should be avoided if [11]. suitable alternatives are available. In roofs and facades, copper, zinc and their alloys should It should also be borne in mind that the effect be avoided unless runoff-water is treated [20]. of copper can be detrimental.