Fabrication of air filters with advanced filtration performance for removal of viral aerosols and control the spread of COVID-19

Abstract

COVID-19 is caused via the SARS-CoV-2 virus, a lipid-based enveloped virus with spike-like projections. At present, the global epidemic of COVID-19 continues and waves of SARS-CoV-2, the mutant Delta and Omicron variant which are associated with enhanced transmissibility and evasion to vaccine-induced immunity have increased hospitalization and mortality, the biggest challenge we face is whether we will be able to overcome this virus? On the other side, warm seasons and heat have increased the need for proper ventilation systems to trap contaminants containing the virus. Besides, heat and sweating accelerate the growth of microorganisms. For example, medical staff that is in the front line use masks for a long time, and their facial sweat causes microbes to grow on the mask. Nowadays, efficient air filters with anti-viral and antimicrobial properties have received a lot of attention, and are used to make ventilation systems or medical masks. A wide range of materials plays an important role in the production of efficient air filters. For example, metals, metal oxides, or antimicrobial metal species that have anti-viral and antimicrobial properties, including Ag, ZnO, TiO₂, CuO, and Cu played a role in this regard. Carbon nanomaterials such as carbon nanotubes, graphene, or derivatives have also shown their role well. In addition, natural materials such as biopolymers such as alginate, and herbal extracts are employed to prepare effective air filters. In this review, we summarized the utilization of diverse materials in the preparation of efficient air filters to apply in the preparation of medical masks and ventilation systems. In the first part, the employing metal and metal oxides is examined, and the second part summarizes the application of carbon materials for the fabrication of air filters. After examination of the performance of natural materials, challenges and progress visions are discussed.

Keywords: Antimicrobial air filters; Carbon nanomaterials; Medical masks; Metallic materials; Natural materials; SARS-CoV-2.

Graphical abstract

Fig. 1

Sources and pathways of transmission of novel coronavirus through several routes of air medium containing solid particulate matters, gaseous hazards in form of aerosol leading to indoor in-house air pollution and outdoor ambient air pollution are depicted (Reprinted with permission from [14], Copyright 2021 Elsevier).

Fig. 2

a: Four types of particulates filtration mechanisms. The representative PM filtration mechanisms: impaction, interception, diffusion, electrostatic attraction, b: Viral load adhered to the surface of aerosol particulates severely affecting human respiratory tract and system (Reprinted with permission from [12,14], Copyright 2021 Elsevier).

Fig. 3

Schematics showing PA6@Ag ENM as an air filter membrane with antibacterial and antiviral property (Reprinted with permission from [25], Copyright 2021 Elsevier).

Fig. 4

a: Schematic illustration of the preparation of BC/AgNW filter with antibacterial activity for highly efficient PMs removal, b: The photographs of the BC air filter paper before and after filtration test, c: Schematic illustration the as-prepared air filter paper for anti-bacteria. The filter material can effectively kill bacteria (Reprinted with permission from [34], Copyright 2021 Wiley).

Fig. 5

A: SEM images of the PVDF/PS1/2 membrane (a) before and (b) after filtration, B: Antibacterial performance comparison between E-Ag/Zn@cotton fabric-based mask (E-SCM) and commercial mask (CM, N95 medical protective mask),C: Schematic representation of the air cleaning system (Reprinted with permission from [36], Copyright 2022 Elsevier).

Fig. 6

A: a) The SEM image of the surface of the TiO₂NWs filter overlapped with the schematic representations of the example germs to be filtered out. b) Schematic illustration of photocatalytical processes leading to ROS generation at the humid surface of TiO₂NWs. The resulting photogenerated ROS inactivate all the microbial targets in its proximity, B: The reusable protective mask designed around the TiO₂NWs-based filter paper. a) Photo of the mask prototype in which the TiO₂NWs filter paper is attached to a 3D-printed plastic frame. b) Photo of the mask prototype during its disinfection under 365 nm UV illumination. The filtered germs are inactivated by ROS, formed in the photocatalytic reaction on the surface of the filter material. c) Photo of the reusable protective mask prototype in real conditions (courtesy of Swoxid S.A.) (Reprinted with permission from [38], Copyright 2020 Wiley).

Fig. 7

a: Schematic illustration of the synthesis of TiO₂@ PFOTES-CV nanoparticles (NPs),b: Schematic diagram of the aerosol deposition process for the VLA antimicrobial air filter, c: Schematic of the VLA inactivation mechanisms based on the production of ROS and ¹O₂. (Reprinted with permission from [40], Copyright 2021 American Chemical Society).

Fig. 8

Schematic illustration of 2 paradoxical effects on LAP-Cu²⁺-coated fabric. When the contaminated aerosol containing bacterial or proteinaceous pathogens contacts the fabric, the LAP can trap the pathogens (Spear), and the Cu²⁺ ions kill the bacteria (Shield) over the nanocoating without interfering air and vapor transmission. Finally, the aerosol is decontaminated (Reprinted with permission from [43], Copyright 2021 Elsevier).

Fig. 9

a: Schematic illustration of the materials used for the proposed LbL coating. Due to the LAP and Cu ions, the two functions that attacking proteinaceous contaminants are adsorbed onto LAP (Spear) and bacterial are killed by Cu²⁺ ions (Shield) are co-existing on filter, b:Schematic illustration of the protein-containing aerosol nebulizer system used to measure the protein-trapping efficiency of the fabric. To mimic human respiration situation (top), experimental model using nebulizer is designated (bottom) (Reprinted with permission from [43], Copyright 2021 Elsevier).

Fig. 10

(a) Schematic of the existing facemask that is not only exposed to PMs in air but also to bacteria and viruses from the saliva and sweat of humans. (b) Photograph and a cross-sectional SEM image of the hybrid air filter. Illustrations of (c) the multilayered structure of hybrid air filter composed of the air filtration, heating, and thermal insulation layers, and (d) the hybrid air filter that is installed inside of a commercial facemask. (e) Infrared image of the facemask equipped with the hybrid air filter, where the voltage of 3 V was applied to the thermal heating layer. The inset photograph is a real snapshot (Reprinted with permission from [44], Copyright 2021 American Chemical Society.

Fig. 11

A: (a) Diagram of the chemical solution process for conductive polyester/aluminum (PET/Al) filter fabrication. (b) Configuration of the electrostatic filtration device composed of a carbon fiber ionizer and two PET/Al filters. Electric fields are formed between the front filter and the ionizer as well as the back filter and the front filter. Inflowing particles are negatively charged by the ionizer, and are captured by Coulomb forces toward the front PET/Al filter. B: Schematic illustration of the formation mechanism of Al thin films on the filter fibers. C: Photograph of a 15 cm × 15 cm raw PET filter and Photograph of a 15 cm × 15 cm PET/Al filter (Reprinted with permission from [49], Copyright 2018 Elsevier).

Fig. 12

A: (a) Fabrication process of electrospinning nanofibers. (b) Components of electrospinning solutions B: Illustration of nanofibers before and after adsorption of (a and d) PAN nanofibers, (b and e) PAN/GO nanofibers, and (c and f) PAN/GO/PI-6 nanofibers, . C: (a) Schematic diagram of the equipment in the purification process and demonstration of the rigidity of the PAN/GO/PI nanofibrous membranes. (b,c) Demonstration of the flexibility of the PAN/GO/PI nanofibrous membranes (Reprinted with permission from [54], Copyright 2021 Elsevier).

Fig. 13

Schematic of bacteria capture, along with subsequent sterilization and depyrogenation by Joule-heating. (a) Schematic of air filtration with the LIG filter mounted on a vacuum filtration system with a backing PES test filter. Bacteria and endotoxins are suggested by the picture. (b) Schematic of filtration followed by (c), sterilization and depyrogenation through Joule-heating. (d) Schematic of the Joule-heating setup in which a potential is applied across the filter for Jouleheating. (e) Infrared image of a LIG filter that is Joule-heated to 380 °C. The scalebar is 2 cm. The shape of the LIG filter is denoted by black dotted lines (Reprinted with permission from [55], Copyright 2019 American Chemical Society).

Fig. 14

Schematic of the ultrathin hybrid coating and its functionalities. (a) One-step spray-coating process of the GO-PD coating on filters or masks. The blue dots represent airborne nanoparticles. (b) Sterilization by light irradiation is achieved due to the photothermal effect of GO. (c) The ultrathin hybrid coating has enhanced antimicrobial property after grafting cationic polymer brushes (Reprinted with permission from [56], Copyright 2021 American Chemical Society). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 15

A: Interaction of airborne virions and pollutant particulates with the electrostatically charged fibers of a graphene mask, B: A schematic representation for a) 3D-CAD model, b) slicing into g-code, c) 3D printer setup, and d) 3D printed mask prototype.C: Assembly of a 3D printed mask: a) various parts of 3D printed mask prototype and b) fabricated 3D printed mask with fG coated filters (Reprinted with permission from [57], Copyright 2021 Elsevier).

Fig. 16

The hybrid CNT filter. (a) The adapted direct spinning method using a collection bobbin covered with the polyester backing for the in-situ production of the CNT hybrid filter material. (b) An illustration showing the concept of the active hybrid CNT filter. The CNT filter can retain SARS-CoV-2 virions and aerosols containing them. The hybrid can be actively sterilized via resistive heating enabled by applying a potential between two electrodes. (c) Photographs showing (i) The upper layer of the hybrid that is made from a micrometer thin non-woven CNT mat. (ii) The lower layer is made from a porous polyester backing. (iii) The fine structure of the hybrid as revealed by a backlight (Reprinted with permission from [60], Copyright 2021 Elsevier).

Fig. 17

Biopolymer-based filtration materials fabricated from a variety of protein and polysaccharide sources (inserted cellulose, keratin, silk, chitin, and starch photo credits: pixabay.com). These unique surface chemistries and diverse molecular interactions aid filtration of various contaminants, including particulate matter (PM), bacteria, viruses, and smoke pollutants (O—H, HCHO, and CO) (Reprinted with permission from [21], Copyright 2021 American Chemical Society).

Fig. 18

a: (A) Schematics of fabricating air filters by electrospinning. (B) Schematic illustration of the filtration process of fibrous membranes, b: Schematic illustration of air flow pass through pure chitosan nanofiber filters and PDMS/PMMA-chitosan nanofiber filters under humidity conditions. c: Photographs of PDMS/PMMA-chitosan transparent air filters at different transparencies (Reprinted with permission from [63], Copyright 2019 Elsevier).

Fig. 19

Schematic illustration of fabrication of the multilayer membranes and application on the antibacterial air filtration (Reprinted with permission from [64], Copyright 2020 Elsevier).

Fig. 20

A: The schematic of alginate-based ionic polymer composites for air purification. a) Top: digital photograph. Bottom: local magnified image of Laminaria hyperborea. b) Real product image and structure of sodium alginate: heterogeneous blocks of α-L-guluronate (G-block) and β-D-mannuronate (M-block). c) Structures of conventional organic cations and the natural cation used in this work. d) Illustration of PM removal and sterilization of alginate-based ionic polymer composites. B: Schematic images of commercial mask, MF@mask, and [Ch] [Alg]@MF@mask worn by a man (Reprinted with permission from [68], Copyright 2020 Wiley).

Fig. 21

Inactivation mechanism of SARS-CoV-2 Delta variant and bacteriophage phi 6, in the synthesized negatively charged calcium alginate film: (a) calcium alginate structure in dry state according to the egg-box model; (b) calcium alginate in swollen state after being in contact with a viral aqueous solution; (c) enveloped RNA viruses: bacteriophage phi 6 and SARS-CoV-2 viral morphologies; (d) Negatively-charged calcium alginate interfering with SARS-CoV-2 Delta variant in viral aqueous solution (Reprinted from [69], Copyright 2021 Preprints, Open access).

Fig. 22

Preparation of TA-HF for influenza virus capture. Schematic illustration of TA-HF preparation and influenza virus capture based on interaction of TA with viral proteins (HA hemagglutinin, NA neuraminidase, M2 matrix-2). TA tannic acid, HF high-efficiency particulate air filter. And Evaluation of the virus capture efficiency of the bare HF and TA-HF by plaque reduction assay. MDCK cells were incubated with X31, which had been incubated with TA-HF, bare HF, or left untreated (virus control) (Reprinted with permission from [72], Copyright 2021 Nature).