Recent Centers for Disease Control and Prevention studies indicate that COVID-19 is spreading more commonly between individuals than between people and surfaces. That said, the CDC cites evidence that the virus can last up to multiple days on particular materials and argues that the “cleaning of visibly dirty surfaces followed by disinfection is a best practice measure for prevention of COVID-19 and other viral respiratory illnesses in households and community settings.”
As such, the novel coronavirus has inspired a renewed interest in antimicrobial coatings, an industry that was already valued at a billion dollars in 2016. Treating high-contact surfaces with these biocidal materials is an attractive way to reduce the viability of viruses and bacteria. However, this approach has raised concerns about ecotoxicological effects and the development of antibiotic-resistant microbes. According to a 2019 CDC report, “more than 2.8 million antibiotic-resistant infections occur in the U.S. each year, and more than 35,000 people die as a result.”
Those seeking alternatives to conventional antimicrobial coatings, which are made of potent disinfectants like quaternary ammonium and isothiazolinone, can look to less toxic antimicrobial examples found in nature. From soft-shell turtle shells to olive tree bark, the natural world is teeming with microbe-fighting strategies that do not involve harmful chemicals. Product manufacturers’ ability to emulate these approaches could lead to the production of nontoxic materials that also limit antibiotic resistance.
In a recent Biomaterials Science article, North Carolina State University bioengineering researchers Eunice Chee and Ashley Brown evaluate various natural methods for eliminating microbe viability on material surfaces. They classify emulation strategies according to three categories: surface mimicry, biomimetic functionalization, and biomimetic assemblies. Chee and Brown argue that adopting these biomimetic approaches can be effective in combating antibiotic-resistant bacteria in the medical field. Depending on the scalability of these strategies, they may also hold promise for high-contact surfaces throughout the designed environment.
Surface mimicry is the most straightforward approach, involving the physical manipulation of a material surface to impede microbe survival. Strategies include the creation of surface nanopillars, nanogrooves, and nanowrinkles—features named after their particular geometries. Nanopillars, present in insect wigs, discourage the physical attachment of microbes by inducing stress in the microbes' cell membranes, eventually resulting in their rupture. Nanogrooves, found in sharkskin, and nanowrinkles, found in crab shells, possess similar functionality. Scientists have recreated these tiny structures in silicon and hydrogel with measurable effects against microbial agents. For example, physicist Massimiliano Papi and his team at the Catholic University of the Sacred Heart, in Milan, employed laser printing to create nanowrinkles in graphene-oxide hydrogel, resulting in a biomimetic antimicrobial cloak.
Another fabrication method used to manipulate surface topology is reactive-ion etching, which allows the fine-tuning of antimicrobial efficacy in materials. Yet another approach is to grow nanostructures in aqueous solutions. For example, scientists at the Institute of Bioengineering and Nanotechnology of A*STAR in Singapore grew nanopillars of zinc oxide to create a nontoxic coating that kills 99.9% of surface germs. Ahrensburg, Germany–based Decorative Products makes the Airdal coating, a wipe- or spray-applied surface composed of transparent, amorphous glass. The ultra-thin, biocide-free coating lasts 12 months and is effective against microbes and fungi.
Biomimetic functionalization and biomimetic assemblies, the other two categories outlined by Chee and Brown, address more sophisticated approaches to emulating living organisms. Biomimetic functionalization involves the incorporation of nanometals, antimicrobial peptides (amino acid compounds), and bacteriophages (bacteria-killing viruses). Biomimetic assemblies mimic cellular systems and components, such as materials that emulate various immune functions in cells.
The Phillip B. Messersmith Research Group at Northwestern University has developed bio-mimicking antifouling coatings made of synthesized peptides that are not vulnerable to degradation like their naturally occurring counterparts. The passive, antiseptic surface treatment is intended for food and beverage containers, medical equipment, and healthcare environments. Hong Kong–based Filligent Limited offers BioFriend antimicrobial technology that traps and deactivates pathogens on the surfaces of various materials, including textiles, polymers, cardboard, and paper. The company’s BioMask received clearance from the U.S. Food & Drug Administration in 2011 as the world’s first antiviral face mask, which incorporates engineered compounds in an open weave where “pathogens are tricked into locking themselves,” rendering them inviable.
Biomimetic functionalization and assembly technologies are fascinating in that they emulate not only biological form but also performance. In contrast, surface mimicry merely requires copying the topology of an antimicrobial surface—albeit at a nanoscale. According to Chee and Brown, surface mimicry “allows for disruption of bacteria by mechanical means without the need for inclusion of additional antimicrobial substances, which can allow for safer methods of combating bacterial contamination.” Mimicking biological performance is a complicated endeavor in which various components, like peptides and nanometals, can be harmful to living cells, thus warranting further scrutiny. Assuming this challenge can be overcome, the creation of widespread pathogen-killing surfaces that are nontoxic and do not increase antibiotic resistance—whether via form- or performance-emulation—is a compelling vision.
