(Spotlight on Nanowerk) Triggered by the COVID-19 pandemic, the surge in demand for surgical masks and respirators has led to a global manufacturing rush. However, not all mask filter materials are suitable for all pollutants and pathogens, as contaminants in the breathing air differ significantly in size. For example, the SARS-CoV-2 virus ranges in size from 60 to 140 nm, smaller than bacteria, dust, pollen, and other larger viruses.
This means that for masks to be effective against a particular pathogen or pollutant, the pore size of their filter material must be smaller than the size of the airborne droplets that contain the pathogen or pollutant against. which it is intended to be used.
While face masks for pollutants are intended to protect the wearer airborne particles, the primary role of face masks during the COVID-19 pandemic was to protect others droplets and particles that the wearer of the mask breathes in, coughs or sneezes (to ensure the wearer is protected from airborne viruses, the eyes should also be covered as viruses can enter the body by all the mucous membranes).
Face masks and respirators, reusable or disposable, offer different levels of protection to users. Typically, masks don’t fit snugly while respirators do. Reusable masks include full-facepiece or half-facepiece respirators for industrial use with attached cartridge filters and homemade or commercial cloth masks; disposable masks include most surgical masks.
A 3-layer surgical mask has been commonly used in the COVID-19 pandemic. This mask is made of 3 different layers of non-woven fabric, with each layer having a specific function, as shown below.
The outermost layer is impermeable and helps repel fluids such as mucosalivary droplets. The middle layer is the filter, which prevents particles or pathogens above a certain size from entering in either direction. The innermost layer is made of absorbent materials to trap the user’s mucosalivary droplets. This layer also absorbs moisture from exhaled air, improving comfort.
Together, these 3 layers effectively protect both the user and those around them by limiting the penetration of particles and pathogens in both directions.
Various parameters regulate filtration efficiency, such as fiber diameter, porosity and filter thickness. If the goal is to filter viruses, the filtration material must be able to capture nanoscale particles.
Manufacturers have developed different styles of masks and air filtration materials using various nanomaterials – nanofibers, nanoparticles, graphene, metal-organic frameworks.
For nanoscale filtration, various types of electrospun nanofibers (electrospinning typically generates fibers with a diameter between 50 and 100 nm) are by far the most widely used filter material. Filtering with nanofiber membranes is essentially achieved through five sets of mechanisms: interception; inertial impact; diffusion; gravitational settling; and electrostatic attraction.
Generally, all of the collection mechanisms illustrated above, except electrostatic attraction, refer to mechanical filters and are affected by particle size and velocity. Interception and inertial impaction are commonly known to be predominant combining mechanisms for macro and microparticles (>0.3 µm), while diffusion is predominant for nanoparticles (
A interception occurs when particles follow the airflow around the fiber and come into contact with the fiber surface, and settle on it due to van der Waals forces. Inertial shock occurs when the particle changes its streamline direction near a filter fiber and impacts the fiber due to inertia. Particles smaller than 0.3 µm are mainly affected by diffusion where they move through the streamlines (Brownian motion) until they come into contact with the fiber, due to the random movements of the air molecules. In
gravitational settling, and due to gravity, large particles can settle in slow-moving air currents. the electrostatic attraction works through the Columbia attraction, where charged particles are attracted to oppositely charged fibers.
Metal nanoparticles and their compounds have garnered much attention as a potent antimicrobial agent due to their high surface-to-volume ratios compared to their bulky counterparts. Nanoparticles of silver silver, copper, titanium dioxide, zinc aluminum oxide, and aluminum oxide compounds have been incorporated into various filters for their antimicrobial properties. A synergistic antimicrobial performance is also revealed via their association with other biocidal agents, such as carbon nanotubes.
Four main phases of interaction have been identified as to how these metallic nanoparticles exhibit antiviral properties:
1) The nanoparticles attach to the virus, which prevents the virus from attaching to a potential host cell.
2) The airflow causes a slight ionization of the layer of metallic nanoparticles on the surface. When the nanoparticle comes into contact with bacteria or viruses, it can rapidly oxidize the core material of bacteria or viruses by stimulating the generation of reactive oxygen species to achieve the inactivation effect.
3) Upon contact, metallic nanoparticles can adhere to the membrane walls of microorganisms, causing the denaturation and deactivation of specific proteins on the surface of bacteria or viruses, followed by apoptosis.
4) They indirectly destroy the virus by activating the immune response of infected cells by simulating their nucleus – this inhibits the spread of the virus.
Graphene is another promising nanomaterial in the fight against airborne pathogens. The substantial surface-to-volume ratio of graphene provides the highest ligand contact surface that can interact with the charged residue of virions to block microorganisms.
Researchers have succeeded in producing graphene antibacterial face masks with 80% efficiency, which can be increased to almost 100% with exposure to sunlight for about 10 minutes.
However, the use of graphene in face masks is controversial due to the potential health risks of inhaled graphene particles. For example, Health Canada, the Government of Canada department responsible for national health policy, has issued an advisory to Canadians not to use face masks containing graphene.
Organometallic structures (MOFs)
Metalorganic frameworks (MOFs) are a class of porous crystalline materials composed of transition metal cations and coordinately bonded organic multidentate linkers. With high porosity, tunable pore size, rich functionality and good thermal stability, MOFs show great promise as suitable candidates for air pollution filtration.
Already, researchers have shown that integrating MOFs into nanofibers results in superior wind resistance capability without film failure.
Researchers also demonstrated the photocatalytic bactericidal properties of a series of metal-organic frameworks (MOFs) and their potentials in air pollution control and personal protection.
As of this writing, there are no commercial MOF masks on the market.
Scientists believe that quantum dots have broad prospects as a potential antiviral mask material for their antiviral effect. Although the use of quantum dots as antiviral agents has been demonstrated, their applications for mask production have not yet resulted in commercial mask products.
Global Database of Nanotechnology Face Masks
For reference, we have compiled a worldwide database of nanotechnology face masks which currently contains 38 commercial products. They include mask filters based on nanofibers, nanoparticles and graphene.
This database is a work in progress. Please let us know if we missed a product.
In conclusion, since the start of the COVID-19 pandemic in 2020, researchers have intensified their efforts to improve the performance of antiviral face masks by adding various features such as metallic nanoparticles and plant extracts to inactivate pathogens, using graphene to make photothermal and superhydrophobic masks, and even using triboelectric nanogenerators to extend mask life.
For an in-depth, in-depth review of masks for COVID-19, you might want to read this review article.
Michael is the author of three books published by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny and Nanoengineering: The Skills and Tools Making Technology Invisible Copyright ©
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