(Image courtesy of the CDC.)
2020 certainly turned out to be a year where lives changed dramatically due to the COVID-19 pandemic. As they say, though, “necessity is the mother of invention.” The most obvious evidence of this has of course been the vaccines, which utilize the revolutionary mRNA process to overcome the coronavirus. There have been innovations in many other fields in response to the pandemic as well, including disinfecting drones, transparent face masks, and anti-COVID clothing.
Another avenue in which research has been occurring is antimicrobial coatings. These coatings utilize a chemical agent on the requisite surface to hinder the growth of microorganisms that are a cause for diseases. That’s not all—they also increase the durability and corrosion resistance of the surface.
During the peak period of the pandemic, scientists highlighted that viruses and bacteria can be transmitted via everyday surfaces too, and not just via coughs and sneezes. This caused many people to wipe down their groceries too! To counter this constant need for disinfection, antimicrobial coatings are the best option to reduce the growth of germs on surfaces and minimize their spread.
According to a report launched by Lux Research, antimicrobial coatings have not been as widely adopted considering their benefits. While they are utilized in sectors such as health care, it has been difficult to quantify their benefits on human health. However, research associate Tiffany Hua believes that COVID-19 has driven substantial interest in antimicrobial coatings and is proving to be a catalyst for antimicrobial research and funding. She expects this interest to spike even further and become a trend of continued interest in the materials and coatings industry.
Each microbial coating offers different advantages and challenges. “When considering the wide range of solutions used as preventive measures against COVID-19, it is important to understand the limitations of these technologies,” explained Hua. The Lux Report looks at these limitations in more detail to provide a framework for users to choose a coating that’s appropriate to their requirements.
Every Cloud Has a Silver Lining
Metallic antimicrobial agents like silver and copper are known to be effective against both bacteria and viruses, and they already have various applications.
Throughout history, silver has been well-known for its oligodynamic effect (biocidal effect of metals) against bacteria and viruses. The earliest recorded use of silver for therapeutic purposes dates back to the Han Dynasty in China circa 1500 B.C.E. There are archaeological discoveries of silver vessels and plates indicating their frequent use in the Phoenician, Macedonian and Persian empires. It may also have been the explanation behind the term “blue blood” to describe the aristocracy, as they developed bluish skin discolorations knows as argyria due to frequent and prolonged use of silver utensils.
Silver’s action on a bacterial cell. (1) Silver can perforate the peptidoglycan cell wall. (2) Silver inhibits the cell respiration cycle. (3) Metabolic pathways are also inhibited when they come in contact with silver. (4) The replication cycle of the cell is disrupted by silver particles via interaction with DNA. (Image courtesy of Sim et al.)
There are three known mechanisms through which silver acts on microbes. Firstly, silver cations can form pores and puncture the bacterial cell wall by reacting with the peptidoglycan component. Secondly, silver ions can enter the bacterial cell, both inhibiting cellular respiration and disrupting metabolic pathways, resulting in generation of reactive oxygen species. Lastly, once in the cell, silver can also disrupt DNA and its replication cycle.
Accordingly, the major use of silver is found in the medical industry, such as topical antibiotics for burns to prevent infections, dressings for wounds, endotracheal tubes and catheters. It is also now commonly incorporated into consumer products because they can be counted upon to provide an extra antibacterial boost.
Examples of silver usage in consumer products. (Image courtesy Beiersdorf AG.)
Likewise, studies have shown copper to be effective against a long list of microbes, and like silver it has a similarly ancient history. Between 2600 and 2200 B.C., copper was used to sterilize chest wounds and drinking water. Egyptian and Babylonian soldiers would similarly put the shavings from their bronze swords (made from copper and tin) into their open wounds to diminish infections. A more contemporary use of copper: in New York City’s Grand Central Station, the grand staircase is flanked by copper handrails that have antimicrobial properties, according to Michael Schmidt, professor of microbiology and immunology at the Medical University of South Carolina.
A 2016 clinical trial at Sentara Leigh Hospital in Virginia found that copper oxide surfaces led to a 78 percent reduction in drug-resistant microbes. Another clinical trial carried out that same year in Iowa demonstrated similar results. Last fall, Schmidt published his latest research about a two-year study that concluded with the finding that copper beds inside the ICU of a hospital in Indiana harbored an average of 95 percent fewer bacteria.
Representation of copper’s action against microbes. (A) Copper dissolves from the copper surface and causes cell damage. (B) The cell membrane ruptures because of copper and other stress phenomena, leading to loss of membrane potential and cytoplasmic content. (C) Copper ions induce the generation of reactive oxygen species, which cause further cell damage. (D) Genomic and plasmid DNA becomes degraded. (Image courtesy of Grass et al.)
Nevertheless, ensuring the effectiveness of these metallic coatings when dispersed in coating matrices still poses challenges. Additionally, while useful as a passive cleaner, they commonly underperform when compared to commercially available cleaners in terms of bacteria removal.
Light at the End of the Tunnel
Light-activated, photocatalytic coatings are on the rise, thanks to their self-cleaning functionality and effectiveness against viruses. These coatings use materials like nano-titanium dioxide (TiO2) that absorb ultraviolet (UV) light and produce reactive radicals for breaking down organic compounds and pollutants on surfaces.
“Photocatalytic coating developers have historically targeted the elimination of pollution and smog, but COVID-19 has driven more use in antimicrobial applications, as they can be effective against both bacteria and viruses,” Hua noted.
Nano-TiO2 surfaces (coated with a 10-20 nm layer of TiO2) are commercially available and have been used in water and air purification, self-cleaning glass, concrete products, and a variety of coatings applications.
How photocatalysis works: Light-energized titanium dioxide attracts water and oxygen in the air and splits it to form two powerful oxidizing species (hydroxyl radical OH- and Oxygen Hole O2-). Due to super hydrophilicity after the split, water is very strongly attracted to the titanium dioxide surface. This results in a vastly reduced contact angle of water as it hugs the surface, not allowing droplets to form. (Image courtesy of Photocatalyst Coatings.)
These coatings are greatly beneficial for building maintenance—especially for skyscrapers—as they decrease the need for costly surface cleaning. Various types of surfaces, both external and interior, can be covered with TiO2 to make them self-cleaning under sunlight as well as room light, thus allowing them to have widespread applications to create environmentally clean areas within their proximity. They can also be used to curb the spread of viruses, and can consequently be used in medical facilities.
Food packaging is essential in preserving foods going to market and extending their shelf life, but it can actually become a liability if the packaged food itself becomes contaminated. There have been many outbreaks of foodborne illnesses in the past few years, such as E.coli and salmonella, leading to product recalls. This is where bio-based antimicrobial technologies are gaining attention, utilizing natural extracts and bio-based solutions such as polysaccharides or chitosan to impart antimicrobial and antifungal properties. Bio-based additives have the advantage of often having low toxicity to humans, and many are already generally considered safe by the Food and Drug Administration (FDA).
Food packaging currently consumes the majority of bio-based antimicrobial technologies, but there are some studies already being commissioned to expand their application to other sectors. For example, chitosan is being used to develop fabrics that are flame-retardant in addition to being antimicrobial.
Another application being studied is to reduce cross-contamination of fresh produce during harvesting. Pathogens from the soil, humans or the environment can persist on harvesting equipment and can lead to contamination at the food source. Therefore, a bio-based coating on the harvesting equipment would not only be chemically safe for food but would also minimize the chances of contamination.
Fighting Fire with Fire
Antimicrobial enzymes are abundant in nature, acting as the self-defense mechanisms of living organisms against infection by bacteria and fungi. There is research ongoing to take advantage of these properties as antimicrobial coatings, as some can also produce reactive oxygen species that kill bacteria under light and can produce longer-lasting antimicrobial coatings, thus mimicking the behavior of photocatalytic coatings. However, the potential of these enzymes is still far from being realized as cost and scalability are ongoing issues.
There are other methods on the market that are in the nascent stages but definitely worth keeping an eye on.
As discussed above, there has been an increase in interest for natural antimicrobials in food products. However, they are prone to degradation during food storage and processing, which reduces their effectiveness. One promising alternative is the microencapsulation of natural compounds. This involves producing microparticles, which protect the encapsulated active substances. In other words, the material to be protected is embedded inside another material or system known as wall material. This results in increased microbial stability and activity as compared to unprotected microbial particles during storage.
There are also many studies working on disinfectant stabilization. Disinfectants such as chlorine are ubiquitous in cutting down microbial risks in drinking water. However, one of the key challenges has always been the maintenance of stable concentrations of disinfectant residuals and the control of disinfection by-products that may form as a consequence of residual decay processes. As a result, a side effect of water disinfection using chlorine is an increase in toxicity levels, which can be harmful over time. Consequently, many companies are developing disinfectants that are more stable and do not break down into toxic components.
Antimicrobial coatings still have a long way to go, as their efficacy and durability are still major challenges to overcome.
“These technologies lack certainty and documentation around performance while still struggling to prove that their incorporation leads to better health outcomes,” continued Hua. “Regulatory approval is another challenge, as new disinfectant and antimicrobial solutions must have EPA and FDA approval to make effectiveness claims. There are also health and environmental concerns that need to be addressed and have increased regulation and oversight. With a surge in research and funding, there will be less concern over performance and regulation.”
Nonetheless, Lux Research anticipates major growth of antimicrobial coatings within the transportation and medical industries, along with deployment in public spaces. Over the next couple of years, more opportunities will arise for applications within the apparel, food and packaging industries. In the longer term, automotive manufacturers and other consumer goods companies are also expected to incorporate them in their products.