Will hygienic surfaces be the positive outcome of the Covid-19 crisis?

The single most important factor for the global Covid-19 pandemic is the extremely efficient transfer of the SARS-CoV-2 virus between individuals.  We have previously encountered outbreaks of virus having far higher fatality rates, but it is the rapid global spread of Covid-19 that is the main cause for the severe global lock down.

Transfer generally occurs via direct contact, airborne or via surfaces.  Although airborne transfer is crucial, the efficient transfer of SARS-CoV-2 also increases the focus on surfaces, and especially surfaces that are in contact with multiple individuals.  The current Covid-19 crisis highlights the importance of hygienic and antimicrobial surfaces, so the question arises whether now is the time for wide use of these in the general society? The benefits are obvious in relation to the Covid-19 pandemic, but the general profit of reduced transfer of bacteria, virus and fungi are significant.

First, let’s get technical and get some terminology sorted:

As an example of different hygienic surfaces from the same material, images from an electron microscope are shown below. The images show an AISI 316L stainless steel surface that have been mechanically- and electro-polished. Both surfaces are “real life surfaces”, graded for use in medical and pharmaceutical production, and produced by an industrial supplier to these industries.

The electropolished surface is so flat that we here show an image with a bit of debris and crystal grains as it would otherwise be flat grey. The surface after mechanical polish, however, is highly deformed. All these kinks and folds contain sites that are hard to clean, as compared to the electropolished surface. One would then think that everything is good if electropolished surfaces are used, but experience show that improper maintenance if electropolished surfaces can significantly increase the roughness over time, and hence deteriorate the hygienic properties.

Non-porous materials, such as typical metals and glass are easier to clean, compared to ex. wood or textile surfaces, which is why such materials are generally preferred for food, medical and pharmaceutical purposes, depending on the demands for hygiene in the production..

Finally, design features are another parameter for the hygienic properties, as poor design can lead to areas that are hard to clean.

An anti-microbial surface is a term that is used for surfaces that can inactivate microbial activity and growth, such as bacteria, virus and fungi. The anti-microbial effect can be obtained by various mechanisms, depending on what functionality is desired. It is important to note, that an antimicrobial surface is rarely functional towards all microbes, so it could be efficient at reducing certain (but not all) bacteria, while not having any efficacy towards ex. fungi or virus. Over the years, it has been an intensive research topic to produces anti-microbial surfaces. Mechanisms such as photocatalysis, release of biocides or surface functionalization have been explored intensively over the years.

Finally, the term “self-cleaning” surface is often encountered. Mostly, such surfaces refer to biomimicking of the Lotus leaf effect, where a hierarchical surface topography is used to alter the wetting properties. The result typically a surface where water is not able to wet the surface, and remains as a droplet on top, see the video below as an example. Although such surfaces are truly intriguing and hold a huge potential for fascinating applications, IPU does not recommend such surfaces for anti-bacterial or anti-virus applications.

Video example of of a superhydrophobic surface obtained by mimicking the Lotus leaf effect. The strategy behind using this method for self-cleaning surfaces is that fouling will not adhere to the surface. The surfaces are, however, generally very sensitive to wear.  

From our experience, many research and development activities have lacked the holistic approach to the subject. First of all, the functionality over time is an issue, as well as functionality as a function of fouling intensity. In one example, studies on an antifouling surface showed superb antibacterial functionality at first, but layers of dead bacterial then covered the surface, blocking for the functionality. Over time, biofilms then formed that fed on the dead bacteria.

Another example is photocatalytic surfaces, that can form extremely potent radicals that are able to oxidize species nearby, when exposed to light (normally UV-light). During the past couple of decades, this led to an explosion in scientific studies, trying to exploit these properties for antimicrobial use. Although the mechanism of photocatalysis certainly works, many uses failed, since they only considered a just a few idealized chemical reactions for what they tried to achieve. The true world, however, is far more complicated and unintended chemical reactions can lead to release of toxic substances, and uncontrolled behavior of the surface.

Anti-microbial surfaces often rely on highly interdisciplinary understanding of the (often complex) working mechanisms, and sometimes lack of knowledge from the manufacturer causes critical concern. As an example, we once encountered an interior coating for use in pharmaceutical production, that claimed to have antimicrobial functionality. The datasheet claimed that the biocide added to the coating was not consumed over time, and would not react with detergents used for cleaning. Since no data was given on the additive, it was difficult to evaluate the validity of this claim. However, the materials safety datasheet contained a reference to the authorization from environmental authorities, which then provided information on the substance used. This turned out to be a pesticide commonly used in agriculture, that would both be depleted from the coating over time, and would react with the detergents used by our customer. Although an antimicrobial coating would have been highly relevant for the particular use in that case, the product was never used due to the uncertainty and lack of tools for verification of the coating functionality over time.

The above critical examples show some of the issues with antimicrobial surfaces, and in our opinion they are illustrative of the difficulties encountered. However, it should be stressed that while the topic is not easy, it certainly holds very large potentials. By using a strategic approach for the use of hygienic and anti-microbial surfaces where material properties, manufacturing, processing, service and use parameters are balanced to the user requirements, a huge potential awaits our society post Covid-19, and we stand by to assist this.