Antimicrobial surfaces

Definition

The term "antimicrobial" encompasses all active mechanisms that, when applied, kill bacteria, fungi, viruses and other microorganisms, inhibit their growth or counteract their colonisation of surfaces. Antimicrobial surfaces have one or more of the aforementioned properties, whereby their effect is limited to the surface.

Surfaces can be antimicrobial in three ways:

1. The material used is intrinsically antimicrobial and prevents colonisation or destroys microorganisms that have already settled. The basis for this is provided by protein structures on the surface that interact with microorganisms according to the lock-and-key principle and irreversibly destroy them.
2. The material used releases substances that destroy microorganisms or prevent colonisation. Silver ions or quaternised ammonium compounds, derivatives of ammonia with a positive charge, are most commonly used.
3. The surface prevents colonisation by microorganisms in response to an external stimulus, such as ozone or UV radiation. Here, singlet oxygen is formed, a high-energy form of normal atmospheric oxygen with an oxidising effect that destroys microorganisms.

Current and future applications

There is a very widely known Christian tradition of godparents giving their godchild a silver spoon as a christening gift. Probably very few of them are aware that this is an antimicrobial product, which will accompany the child throughout life in hygienically impeccable condition.

Today, antimicrobial surfaces have found their way into numerous industrial sectors and are playing an increasingly important role in public spaces. Antimicrobial surfaces have become an indispensable aspect of healthcare applications, be it for surgical instruments or to prevent the formation of biofilms on catheters, for example. Current practice is to coat these with antibiotics, which can cause resistance. New procedures apply droplets containing silver ions to the catheters, thus making antibiotics superfluous. Cosmetics have a long shelf life, thanks to the addition of preservatives. If these products are supplied in packaging with an antimicrobial coating, there is no need to use preservatives. For cosmetics in packaging with pumps, the head and spring are made out of silver to achieve the same goal. The use of antimicrobial materials for food packaging, on the other hand, plays only a minor role, as food does not have as long a shelf life as cosmetics and is packaged under steam sterilisation or in a modified atmosphere. The same applies to medicines. Antimicrobial coatings play an essential role in gastronomy for the disinfection of tables and menus, as well as in the shipping industry and generally for products used in water.

The coronavirus pandemic made us aware of a demand for more antimicrobial surfaces in public spaces. Conceivably, materials that function according to the lock-and-key principle or release singlet oxygen could be used for surfaces touched by lots of people: handrails and seats in public transport, banisters, door handles and buttons in lifts. Antimicrobial materials would thus become a physico-chemical line of defence in the fight against the spread of microorganisms.

Opportunities and challenges

To be prepared for the future, antimicrobial materials must become sustainable and affordable. Only then can they develop their full potential and, in the medium to long term, replace conventional disinfectants, as well as antibiotics in certain applications, and help prevent the emergence of multi-resistant germs. This opens up great opportunities for basic research. Medical technology, which is strongly represented in Switzerland, benefits from antimicrobial materials; this can become an incentive for the Swiss materials industry. If the Swiss medtech industry succeeds in exerting increasing pressure on plastics manufacturers and suppliers, the production of niche products in Switzerland is conceivable.

Technical progress is needed for the described applications and scenarios to become reality. One major challenge is the integration of materials that produce singlet oxygen, and of small droplets containing silver, into a polymer material like plastic. The plastic should retain properties such as flexibility and, at the same time, the antimicrobial substance should not get washed out – to maintain effectiveness on one hand and to avoid direct contact with the environment on the other.

Caution is necessary not only in the use, but also the disposal, of antimicrobial materials, so as to minimise environmental impact. The development of sustainable solutions is essential for future applications, especially so that acceptance in society can still be counted on. When using antimicrobial materials, it must be taken into account that they kill not only pathogenic but also desirable microorganisms.

In the coming years, there will be increasing pressure from authorities to replace toxic and non-specific quaternised ammonium compounds. The alternative is to be found in surfaces whose antimicrobial effect is unleashed by external stimuli. This requires new materials that have to go through an expensive and time-consuming approval process. In order for start-ups to remain involved in such development, a hub or consortium must be set up. Switzerland should make use of its reputation as a place of innovative technology, as well as the "Swissness" factor, so as to play a leading role in the establishment of such a hub.

Funding

The players in Switzerland are rather poorly networked and there is a lack of access to buyers and consumers. An initiative to bring research, industry and consumers closer together would probably pay off. One possibility would be to organise a symposium once a year, along the lines of the annual convention held by the American Cleaning Institute (ACI). This focuses exclusively on aspects of cleaning and disinfection, interconnects the players and represents their interests.

Further reading

• Linda Geddes, Imperial College London. Antimicrobial surfaces.
• Christine Ro, BBC Future. The surfaces that kill bacteria and viruses.
• Liljana Schmidt, Schurter GmbH. Input systems with antimicrobial surfaces.
• VDI Verein Deutscher Ingenieure. Antimicrobial surfaces for infection prevention.