Cleanliness that is ?skin deep

?What the contamination control sector needs is something that makes the job of cleaning easier, and reduces the costs of cleaning and maintaining equipment such as HVAC systems. Even better, if it can contribute to reducing hospital acquired infections – that would be really good. “The trouble is there’s no such thing.” This was overheard recently at a conference; but is it true? Nano Hygiene Coatings believes it has something that does all that and more, with its recently launched 4MED coating.

4MED is an innovative hygiene coating with hydrophobic and oleophobic properties developed by Nano Hygiene Coatings. It combines the benefits of the easy-to-clean properties derived from nano chemical technology with an antimicrobial additive, actively preventing the growth of bacteria, such as Escherichia coli and MRSA. The combination of a hydrophobic/ oleophobic surface and active antimicrobial functionality has synergistic benefits that contribute to improved hygiene and infection control strategies.

Having a surface that prevents organisms adhering so that they can be removed in a benign and environmentally-friendly fashion is beneficial. Surfaces can act as reservoirs for many types of micro-organisms. While not all micro-organisms may be of significance to human health, or to food production facilities, reducing their growth will significantly reduce the risks associated with any harmful effects.

Do hydrophobic surfaces actually behave that way? They are probably best exemplified by the lotus flower. Hydro-phobic surface coatings mimic the properties of the lotus plant whose leaves are kept clean by millions of microscopic hairs, which trap air and repel water and dirt. A droplet on an inclined hydrophobic surface does not slide off; it rolls off. When the droplet rolls over contamination, the particle is removed from the surface. The force needed to remove a particle is very low due to the minimised contact area between the particle and the surface. As a result, the droplet cleans the surface.

At Nano Hygiene Coatings (NHC), we tested two substrates with a repeat soiling and cleaning test. We wanted to establish whether we could achieve the lotus flower’s hydrophobicity. Stainless steel and copper samples were soiled repeatedly by applying a solution containing bovine serum albumin (BSA) twice and then a mixed artificial organic soilant once. This soiling cycle was repeated three times and then followed by a final application of BSA.

Between each soilant event, the samples were cleaned for 10 seconds using light manual abrasion under running mains tap water (ca 10-15ºC at 0.5 litres minute-1). Of the two stainless steel samples tested, one was uncoated, whilst the other was coated with a NHC hydrophobic coating.

Reduced soiling

The results were interesting in that the appearance of the uncoated surfaces deteriorated significantly when subjected to soiling. Repeated soiling and washing cycles caused further deterioration of the surface appearance. The stainless steel became increasingly unattractive and the copper tarnished. Both uncoated copper and stainless steel retained significant organic deposits after the soiling/cleaning regime (see below).

In contrast, the coated samples maintained their appearance and retained very little soiling. They were both visually and microscopically cleaner than the uncoated surfaces.

The soiling cleaning cycles were repeated. Table 1 shows the results of a comparison of uncoated and coated stainless steel surfaces. The stainless steel samples were coated with NHC’s 4MED easy-clean, antimicrobial, hydrophobic coating. Antimicrobial activity was determined after the final soiling/ cleaning event using the method described in the Japanese Industrial Standard JIS Z2801: 2000, modified to include a 6-hour contact interval. The table shows that biologically significant activity was detected on the 4MED coated stainless steel sample, with the population of E. coli declining by four orders of magnitude over 24 hours – an effective kill rate of 99.99%.

Tests supporting the claims of any antimicrobial coating should be based on realistic exposure scenarios: that is to say, undertaken in the conditions in which they will be used, or simulating those conditions. 4MED has been tested simulating the effect of a surface being splashed by several contaminants: firstly, by a splash of contaminated water, and, secondly, by a splash of contaminated body/animal fluid.

Both tests were undertaken with a sample coated with 4MED, and a sample coated with a hydrophobic coating but without antimicrobial functionality. For the contaminated water test, a suspension of E. coli was prepared using the method described in JIS Z2801: 2000, standard but in sterile distilled water containing no nutrient broth. For the contaminated body/animal fluid test an E. coli inoculum was prepared in a vegetable oil/bovine serum albumin solution (oil/protein solution).

The results in table 2 show that 4MED achieves a fast rate of kill of the bacterial population, resulting in a reduction of ca 2.5 orders of magnitude in three hours and a reduction to below the limit of detection in six hours. The oil/protein test results in table 3 show that the bacterial population declined by ca 0.5 orders of magnitude within 12 hours and a statistically significant 2.5 orders of magnitude in 24 hours. The presence of protein-rich soiling agents has slowed down the antimicrobial coating but not prevented it from working.

Other metals

The contaminated water tests have also been conducted on other metal substrates, including chrome plate and aluminium with similar results: kill rates of 99.9% in 3-6 hours.

An antimicrobial coating that acts quickly and is effective against contaminated water and heavy soiling splashes has positive benefits. However, its performance should also be judged on its longevity. To determine how 4MED performs under these circumstances, a 4MED-coated substrate was artificially aged and the antimicrobial efficacy subsequently tested.

Test samples were produced using sterile distilled water contaminated with E. coli. The samples were processed in a dishwasher at 40ºC with no detergent (13 litres per cycle). Sampling points were at 60 dishwash cycles and 120 dishwash cycles.

The results in table 4 show that the 60 and 120 dishwash cycles did not have any statistically significant impact on the performance of 4MED antimicrobial efficacy. The populations were reduced by four orders of magnitude following six hours’ contact to below the limit of detection in 24 hours. The 4MED-coated surfaces show a statistically significant difference from the populations exposed to the hydrophobic, non-antimicrobial coating. The fastest kill rate is in the first three hours.

On uncoated surfaces, the appearance and visual appeal of the surface declines as deposits of dirt and organic deposits build up. Repeated washing and cleaning causes further deterioration of the surface appearance. However, coated hydrophobic/ oleophobic surfaces have no or little build-up of soil, which makes them an easy-clean solution.

This has a direct impact on the costs associated with cleaning. The combination of hydrophobicity with the antimicrobial functionality of 4MED produces synergistic benefits. By testing under simulated real-life conditions, the claims should satisfy the increasing demand of the authorities for testing under realistic exposure scenarios. The coating has been shown to be robust, durable, quick-acting and effective even after artificial ageing.

The implications for improved cleaning and hygiene strategies are clear. Such a coating makes the cleaning job easier. It reduces the growth of bacteria and contributes to the lowering of the risks associated with their harmful effects.