Further improvement of air filtration efficiency of cellulose filters coated with nanofibers via inclusion of electrostatically active nanoparticles

Next-generation air filtration nanotechnology for improved indoor air quality

Indoor air quality (IAQ) significantly affects human health, with pollutants such as organic, inorganic substances, and biological contaminants contributing to various respiratory, neurological, and immunological diseases. In this review, we highlighted the need for advanced air filtration technologies to mitigate these pollutants, which are emitted from household products, building materials, combustion processes, and bioaerosols. While traditional HVAC systems and mechanical filtration methods have been effective, they are often energy-intensive and limited in their ability to capture specific pollutants. To address these limitations, nanotechnology-based air filtration technologies, particularly those utilizing electrospinning processes, offer promising alternatives. This review classifies pollutants and details the working principles of next-generation filters, focusing on passive, self-powered, and externally powered mechanisms. These advanced filters achieve high filtration efficiency with minimal pressure drop, enhanced pollutant capture, and in some cases, health monitoring capabilities. This review emphasizes the significance of ongoing research into eco-friendly and sustainable filtration systems to enhance IAQ and minimize health risks linked to long-term exposure to indoor air pollutants.

Gas transport into cellulose materials for highly porous structure

To solve the low thermal stability of polyolefin membranes, our group developed porous polymers using cellulose acetate (CA) material. The formation of pores in CA involved creating plasticized regions within the CA membrane using additives. By applying gas pressure to these regions, a CA/glycolic acid membrane could be prepared with a small average pore size and high porosity. According to the porosimeter measurement, the average pore size of the membrane was 150 nm, and the porosity was 77%. SEM observations of the surface and cross-section of the CA/glycolic acid membrane confirmed the abundant distribution of fine pores. Furthermore, IR analysis revealed the removal of glycolic acid from the membrane after pore formation, indicating its interaction with CA during the process of gas permeation. Additionally, TGA analysis demonstrated a decrease in thermal stability due to the formation of numerous pores after gas permeation.

A Novel Vision of Reinforcing Nanofibrous Masks with Metal Nanoparticles: Antiviral Mechanisms Investigation

Prevention of spreading viral respiratory disease, especially in case of a pandemic such as coronavirus disease of 2019 (COVID-19), has been proved impossible without considering obligatory face mask-wearing protocols for both healthy and contaminated populations. The widespread application of face masks for long hours and almost everywhere increases the risks of bacterial growth in the warm and humid environment inside the mask. On the other hand, in the absence of antiviral agents on the surface of the mask, the virus may have a chance to stay alive and be carried to different places or even put the wearers at risk of contamination when touching or disposing the masks. In this article, the antiviral activity and mechanism of action of some of the potent metal and metal oxide nanoparticles in the role of promising virucidal agents have been reviewed, and incorporation of them in an electrospun nanofibrous structure has been considered an applicable method for the fabrication of innovative respiratory protecting materials with upgraded safety levels.

Graphical Abstract

Smoke filtration performances of membranes produced from commercial PVA and recycled PET by electrospinning method and ANN modeling

Plastic waste and air pollution are becoming a great concern due to their adverse effect on human health and the environment. There is increasing number of evidence showing that recycling plastic and filtering harmful air pollutants are one of the most effective and promising way to eliminate their hazard on the environment. In this purpose, we developed eco-friendly filtration materials from recycled PET by electrospinning method to be used in air filtration and compared them with conventional PVA membranes. Filtration efficiency of prepared membranes were tested homemade membrane system using cigarette smoke source. Characterization results and smoke filtration performance of recycled PET and PVA membranes before and after smoke filtration were examined. The results demonstrated that the removal efficiencies of PVA-5 wt.%, PVA-10 wt.%, and PVA-15 wt.% were 4.11%, 11.32%, and 12.14%, respectively. A similar trend was also observed in recycled PET-5 wt.%, PET-10 wt.%, and PET-15 wt.% membranes with 4.32%, 10.79%, and 11.68% of filtration efficiency, respectively. Based on this result, using recycled PET can be an alternative way to produce a higher value product compared to traditional polymer membranes used commercially. This result is also supported by the neural network model of this study.

Superwetting Electrospun PDMS/PMMA Membrane for PM2.5 Capture and Microdroplet Transfer

Efficient acquiring and removal of a hazardous particulate matter (PM) have significant effects on human health. Here, we illustrate the fabrication of a superwetting electrospun polydimethylsiloxane/polymethyl methacrylate (PDMS/PMMA) membrane (EPPM) with multifunctional performance for PM_2.5 capture and microdroplet transfer, where PMMA was added as a carrier polymer to the superhydrophobic PDMS, which has very low cohesive energy density. The obtained EPPM, which is composed of special bead-on-string fibers with a mean fiber diameter of 350 nm, shows a porous structure with an aperture of 7.87 μm (calculated by the bubble pressure method) and superb thermostability (up to 325 °C). The EPPM possesses an excellent PM_2.5 purification efficiency of nearly up to 100% at a very low pressure drop (70 Pa, <0.07% of the atmospheric pressure) under the condition of high humidity (96 ± 3%), which is greatly advantageous over those hydrophilic filters frequently suffering the drawbacks of low efficiency or total invalidation in humid environments. In addition, benefitting from the superhydrophobic and strong adhesive properties of the membrane surface, the EPPM could complete the trace aqueous sample analysis such as "robotic hand" from superhydrophobic to hydrophilic surfaces without any contamination or loss and hold a high contact angle of 161.6° for water. Altogether, the EPPM may have technological advantages as a kind of novel fibrous filter in diverse environmental applications, including PM_2.5 capture, separation, microdroplet transfer, and so on.

Needleless electrospun phytochemicals encapsulated nanofibre based 3-ply biodegradable mask for combating COVID-19 pandemic

The emergence of COVID-19 pandemic has severely affected human health and world economies. According to WHO guidelines, continuous use of face mask is mandatory for personal protection for restricting the spread of bacteria and virus. Here, we report a 3-ply cotton-PLA-cotton layered biodegradable face-mask containing encapsulated phytochemicals in the inner-filtration layer. The nano-fibrous PLA filtration layer was fabricated using needleless electrospinning of PLA & phytochemical-based herbal-extracts. This 3-layred face mask exhibits enhanced air permeability with a differential pressure of 35.78 Pa/cm² and superior bacterial filtration efficiency of 97.9% compared to conventional face masks. Close-packed mesh structure of the nano-fibrous mat results in effective adsorption of particulate matter, aerosol particles, and bacterial targets deep inside the filtration layer. The outer hydrophobic layer of mask exhibited effective blood splash resistance up to a distance of 30 cm, ensuring its utilization for medical practices. Computational analysis of constituent phytochemicals using the LibDock algorithm predicted inhibitory activity of chemicals against the protein structured bacterial sites. The computational analysis projected superior performance of phytochemicals considering the presence of stearic acid, oleic acid, linoleic acid, and Arachidic acid exhibiting structural complementarity to inhibit targeted bacterial interface. Natural cotton fibers and PLA bio-polymer demonstrated promising biodegradable characteristics in the presence of in-house cow-dung based biodegradation slurry. Addition of jaggery to the slurry elevated the biodegradation performance, resulting in increment of change of weight from 07% to 12%. The improved performance was attributed to the increased sucrose content in biodegradation slurry, elevating the bacterial growth in the slurry. An innovative face mask has shown promising results for utilization in day-to-day life and medical frontline workers, considering the post-pandemic environmental impacts.

Preparation, filtration, and photocatalytic properties of PAN@g-C3N4 fibrous membranes by electrospinning

Particulate matter and formaldehyde (HCHO) in closed indoor environments are seriously harmful to human health; hence, techniques for the improvement of air quality have attracted significant attention. PAN@g-C₃N₄ fibrous membranes with high efficiency, low resistance, and photocatalytic activity were prepared by electrospinning with polyacrylonitrile (PAN) and graphite carbon nitride (g-C₃N₄), followed by the high-temperature polycondensation of melamine. The addition of g-C₃N₄ to the nanofibrous membrane effectively improved the filtration efficiency of PM2.5. When the amount of added g-C₃N₄ was 3 wt%, the filtration efficiency of PM2.5 was 99.76 ± 0.3%, the filtration efficiency was stable for 24 hours at a continuous high concentration, and the filtration cycle stability was good. As a photocatalytic material, g-C₃N₄ causes the photocatalytic degradation of HCHO, and thus, significantly improves the filtration efficiency of the nanofibrous membrane to HCHO. When the amount of added g-C₃N₄ was 3 wt%, the filtration efficiency of the nanofibrous membrane to HCHO reached 78.0 ± 1.8%. The mechanism of catalytic degradation showed that the PAN fibres first adsorbed and intercepted the HCHO molecules. Under simulated sunlight irradiation, the photogenerated holes generated by the g-C₃N₄ nanosheets in the fibres oxidised and decomposed the adsorbed HCHO molecules. This study has broad application potential for high-efficiency filters to improve indoor air quality.

Particulate matter and formaldehyde (HCHO) in closed indoor environments are seriously harmful to human health; hence, techniques for the improvement of air quality have attracted significant attention.

Development of Filter Media by Electrospinning for Air Filtration of Nanoparticles from PET Bottles

Air pollution and solid pollution are considered global problems, and endanger human health mainly due to the emission of fine particulate matter released into the atmosphere and improper disposal of post-consumer plastic bottles. Therefore, it is urgent to develop filter media to effectively protect the public. The properties of plastics make them potential candidates for nanofiber mat formers due to their attractive structural and mechanical characteristics. This work aims to produce and evaluate novel PET electrospun fibers dispensed with the use of support materials to be used as filter media to remove nanoparticles from the air. The electrospinning process was carried out by changing the concentration of the polymer solution, the needle diameter, and the electrospinning processing time at two rotation speeds. The average diameters of the micro- and nanofibers of the filter media produced ranged from 3.25 μm to 0.65 μm and it was possible to conclude that, as the size of the fibers decreased, the mechanical strength increased from 3.2 to 4.5 MPa. In filtration tests, a collection efficiency of up to 99% with low-pressure drops (19.4 Pa) was obtained for nanoparticles, demonstrating high quality factor filter media, which could be applicable in gas filtration.

Biodegradable CA/CPB electrospun nanofibers for efficient retention of airborne nanoparticles

The increase of the industrialization process brought the growth of pollutant emissions into the atmosphere. At the same time, the demand for advances in aerosol filtration is evolving towards more sustainable technologies. Electrospinning is gaining notoriety, once it enables to produce polymeric nanofibers with different additives and also the obtaining of small pore sizes and fiber diameters; desirable features for air filtration materials. Therefore, this work aims to evaluate the filtration performance of cellulose acetate (CA) nanofibers and cationic surfactant cetylpyridinium bromide (CPB) produced by electrospinning technique for retention of aerosol nanoparticles. The pressure drop and collection efficiency measurements of sodium chloride (NaCl) aerosol particles (diameters from 7 to 300 nm) were performed using Scanning Mobility Particle Sizer (SMPS). The average diameter of the electrospun nanofibers used was 239 nm, ranging from 113 to 398 nm. Experimental results indicated that the nanofibers showed good permeability (10^-11 m²) and high-efficiency filtration for aerosol nanoparticles (about 100 %), which can include black carbon (BC) and the new coronavirus. The pressure drop was 1.8 kPa at 1.6 cm s^-1, which is similar to reported for some high-efficiency nanofiber filters. In addition, it also retains BC particles present in air, which was about 90 % for 375 nm and about 60 % for the 880 nm wavelength. Finally, this research provided information for future designs of indoor air filters and filter media for facial masks with renewable, non-toxic biodegradable, and potential antibacterial characteristics.

Air-Filtering Masks for Respiratory Protection from PM2.5 and Pandemic Pathogens

Air-filtering masks, also known as respirators, protect wearers from inhaling fine particulate matter (PM2.5) in polluted air, as well as airborne pathogens during a pandemic, such as the ongoing COVID-19 pandemic. Fibrous medium, used as the filtration layer, is the most essential component of an air-filtering mask. This article presents an overview of the development of fibrous media for air filtration. We first synthesize the literature on several key factors that affect the filtration performance of fibrous media. We then concentrate on two major techniques for fabricating fibrous media, namely, meltblown and electrospinning. In addition, we underscore the importance of electret filters by reviewing various methods for imparting electrostatic charge on fibrous media. Finally, this article concludes with a perspective on the emerging research opportunities amid the COVID-19 crisis.

Structural design and environmental applications of electrospun nanofibers

Nanofibers have attracted extensive attention and been applied in various fields due to their high aspect ratio, high specific surface area, flexibility, structural abundance, etc. The electrospinning method is one of the most promising and effective ways to produce nanofibers. The electrospun nanofibers-based films and membranes have already been demonstrated to possess small pore sizes, larges specific surface area, and can be grafted with different functionalities to adapt to various purposes. The environmental applications of nanofibers are one of the essential application fields, and great achievements have been made in this field. To well summarize the development of nanofibers and their environmental applications, we review the nanofiber fabrication methods, advanced fiber structures, and their applications in the field of air filtration, heavy metal removal, and self-cleaning surface. We hope this review and summary can provide readers a comprehensive understanding of the structural design and environmental applications of electrospun nanofibers.

Advanced Design of Fiber-Based Particulate Filters: Materials, Morphology, and Construction of Fibrous Assembly

With increasing air pollution and sporadic outbreaks of epidemics, there is ramping attention on the filtration devices. The main constituents of airborne pollutants are particulate matters of solid particles, liquid aerosol, bioaerosol/bio-droplets, and gas/vapor. With the growing demand for high-performance filters, novel materials and functionalities are being developed applying advanced technologies. In this paper, recent developments of fiber-based particulate filters are reviewed, with a focus on the important performance parameters and material properties. Trends in technology and research activities are briefly reviewed, and the evaluative measures of filtration performance are reported. Recent studies on the advanced filter materials are reviewed in the aspect of polymers and the fabrication process of fibrous assembly. The characterization method including 3D modeling and simulation is also briefly introduced. Multifunctional filters such as antimicrobial filter and gas and particulate filters are briefly introduced, and efforts for developing environmentally sustainable filters are noted.

Ultrathin Nanofibrous Membranes Containing Insulating Microbeads for Highly Sensitive Flexible Pressure Sensors

Highly sensitive and flexible pressure sensors were developed based on dielectric membranes composed of insulating microbeads contained within polyvinylidene fluoride (PVDF) nanofibers. The membrane is fabricated using a simple electrospinning process. The presence of the microbeads enhances porosity, which in turn enhances the sensitivity (1.12 kPa^-1 for the range of 0-1 kPa) of the membrane when used as a pressure sensor. The microbeads are fixed in position and uniformly distributed throughout the nanofibers, resulting in a wide dynamic range (up to 40 kPa) without any sensitivity loss. The fluffy and nonsticky PVDF nanofiber features low hysteresis and ultrafast response times (∼10 ms). The sensor has also demonstrated reliable pressure detection over 10 000 loading cycles and 250 bending cycles at a 13 mm bending radius. These pressure sensors were successfully applied to detect heart rate and respiratory signals, and an array of sensors was fabricated and used to recognize spatial pressure distribution. The sensors described herein are ultrathin and ultralight, with a total thickness of less than 100 μm, including the electrodes. All of the materials comprising the sensors are flexible, making them suitable for on-body applications such as tactile sensors, electronic skins, and wearable healthcare devices.

Highly efficient transparent air filter prepared by collecting-electrode-free bipolar electrospinning apparatus

Electrospinning technology has been used for a long time. A jet from a needle was formed by applying high voltage, and then the nanofibers are deposited onto a collecting electrode (usually metal) and the excess charge is conducted away to complete the electrospinning. Alternatively, it is also possible to prevent charge accumulation from hindering the progress of electrospinning by means of charge neutralization. A bipolar electrospinning technique (B-EEM) was developed to induce jets with different charges through a set of high-voltage power supplies of opposite polarity, and the two jets neutralize each other on the insulating mesh, thus completing the electrospinning process. There is no need for a collecting electrode to complete the electrospinning process. We have found that the filters produced by the new technology have better filtration efficiency while maintaining the same transparency in relative to the original technology, and this optimization is due to the distribution modification of the nanofibers on the mesh.

Electret nanofibrous membrane with enhanced filtration performance and wearing comfortability for face mask

Airborne particulate matter (PM) pollution has become a serious threat to human health, thus it is highly desired for a high-filtration-performance and good-wearing-comfort face mask. Herein, a highly breathable and thermal comfort filter medium consisting of electret polyethersulfone/barium titanate nanofibrous membrane (PES/BaTiO₃ NFM) integrated on a nonwoven polypropylene substrate was developed. Benefiting from the high porosity and optimized injection charge energy, the PES/BaTiO₃ membrane was endowed with a good air permeability of 743 mm s^-1, a modest water vapor permeability of 6.24 kg m^-2 d^-1, and an enhanced charge storage stability. In addition, the electret PES/BaTiO₃ NFM1.5 medium with a low basis weight of 4.32 g m^-2 still shows a high filtration efficiency of 99.99% and a low pressure drop of 67 Pa after being treated at 200 °C for 45 min, which is better than that of commercial media. Moreover, 3D simulation based on the characters of composite membrane was processed to graphically express the airflow distribution during the filtration process. Significantly, the NFM1.5 with a high infrared (IR) transmittance of 93.4% led to an effective radiative cooling to human body radiation. This multifunctional fibrous medium design may provide new insights into the development of environmental adaptive protection materials.

Electrostatically Active Polymer Hybrid Aerogels for Airborne Nanoparticle Filtration

The role of electrostatic force on separation of airborne nanoparticles is evaluated in this work by considering a hybrid monolithic aerogel of syndiotactic polystyrene (sPS) and polyvinylidene fluoride (PVDF). The sPS part accounts for open pore structures in the monolith, while the PVDF chains contribute spontaneous polarity for particle capture by the electrostatic force. The hybrid aerogels are fabricated by thermoreversible gelation of sPS from a solution with PVDF in tetrahydrofuran followed by supercritical drying of the gel. sPS is present as the δ-form clathrate crystalline phase and PVDF as α- and γ-form crystalline phases in the hybrid. The presence of PVDF induces significant static charges on the surfaces of hybrid aerogels. The filtration efficiency is determined by passing airborne NaCl nanoparticles with diameter in the range 25-150 nm through the filter media. The experimental data reveal that air permeability of the hybrid system (∼10^-10 m²) is close to that of sPS monoliths. The hybrid materials show filtration efficiency ≥99.999% in comparison to 98.889% observed for a sPS monolith with the same solid content.

Fabrication, Polarization of Electrospun Polyvinylidene Fluoride Electret Fibers and Effect on Capturing Nanoscale Solid Aerosols

Electrospun polyvinylidene fluoride (PVDF) fiber mats with average fiber diameters (≈200 nm, ≈2000 nm) were fabricated by controlled electrospinning conditions. These fiber mats were polarized using a custom-made device to enhance the formation of the electret β-phase ferroelectric property of the fibers by simultaneous uniaxial stretching of the fiber mat and heating the mat to the Curie temperature of the PVDF polymer in a strong electric field of 2.5 kV/cm. Scanning electron microscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry and Brunauer-Emmett-Teller (BET) surface area analyses were performed to characterize both the internal and external morphologies of the fiber mat samples to study polarization-associated changes. MATLAB simulations revealed the changes in the paths of the electric fields and the magnetic flux inside the polarization field with inclusion of the ferroelectric fiber mats. Both polarized and unpolarized fiber mats were challenged as filters against NaCl particles with average particle diameters of about 150 nm using a TSI 8130 to study capture efficiencies and relative pressure drops. Twelve filter experiments were conducted on each sample at one month time intervals between experiments to evaluate the reduction of the polarization enhancement over time. The results showed negligible polarization loss for the 200-nm fiber sample. The polarized mats had the highest filter efficiencies and lowest pressure drops.

A novel preparation of anti-layered poly(vinylalcohol)–polyacrylonitrile (PVA/PAN) membrane for air filtration by electrospinning

Schematic for the preparation of PAN/PVA membrane deposited on the stainless steel mesh modified by dilute PVA solution.

Electrospinning of Grooved Polystyrene Fibers: Effect of Solvent Systems

Secondary surface texture is of great significance to morphological variety and further expands the application areas of electrospun nanofibers. This paper presents the possibility of directly electrospinning grooved polystyrene (PS) fibers using both single and binary solvent systems. Solvents were classified as low boiling point solvent (LBPS): dichloromethane (DCM), acetone (ACE), and tetrahydrofuran (THF); high boiling point solvent (HBPS): N,N-dimethylformamide (DMF) and cyclohexanone (CYCo); and non-solvent (NS): 1-butanol (BuOH). By the systematic selection and combination of these solvents at given parameters, we found that single solvent systems produced non-grooved fibers. LBPS/DMF solvent systems resulted in fibers with different grooved textures, while LBPS/CYCo led to fibers with double grooved texture. Grooved fibers can also be fabricated from LBPS/LBPS, NS/LBPS, and NS/HBPS systems under specific conditions. The results indicated that the difference of evaporation rate (DER) between the two solvents played a key role in the formation of grooved texture. The formation of this unique texture should be attributed to three separate mechanisms, namely void-based elongation, wrinkle-based elongation, and collapsed jet-based elongation. Our findings can serve as guidelines for the preparation of ultrafine fibers with grooved secondary texture.

Large scale poly(vinyl alcohol-co-ethylene)/TiO2hybrid nanofibrous filters with efficient fine particle filtration and repetitive-use performance

A high-yielding nanofiber-based filtration material with excellent performance was developed and provides a promising way to control the severe air pollution at present.

Electrospun regenerated cellulose nanofibrous membranes surface-grafted with polymer chains/brushes via the atom transfer radical polymerization method for catalase immobilization

In this study, an electrospun regenerated cellulose (RC) nanofibrous membrane with fiber diameters of ∼200-400 nm was prepared first; subsequently, 2-hydroxyethyl methacrylate (HEMA), 2-dimethylaminoethyl methacrylate (DMAEMA), and acrylic acid (AA) were selected as the monomers for surface grafting of polymer chains/brushes via the atom transfer radical polymerization (ATRP) method. Thereafter, four nanofibrous membranes (i.e., RC, RC-poly(HEMA), RC-poly(DMAEMA), and RC-poly(AA)) were explored as innovative supports for immobilization of an enzyme of bovine liver catalase (CAT). The amount/capacity, activity, stability, and reusability of immobilized catalase were evaluated, and the kinetic parameters (Vmax and Km) for immobilized and free catalase were determined. The results indicated that the respective amounts/capacities of immobilized catalase on RC-poly(HEMA) and RC-poly(DMAEMA) nanofibrous membranes reached 78 ± 3.5 and 67 ± 2.7 mg g(-1), which were considerably higher than the previously reported values. Meanwhile, compared to that of free CAT (i.e., 18 days), the half-life periods of RC-CAT, RC-poly(HEMA)-CAT, RC-poly(DMAEMA)-CAT, and RC-poly(AA)-CAT were 49, 58, 56, and 60 days, respectively, indicating that the storage stability of immobilized catalase was also significantly improved. Furthermore, the immobilized catalase exhibited substantially higher resistance to temperature variation (tested from 5 to 70 °C) and lower degree of sensitivity to pH value (tested from 4.0 and 10.0) than the free catalase. In particular, according to the kinetic parameters of Vmax and Km, the nanofibrous membranes of RC-poly(HEMA) (i.e., 5102 μmol mg(-1) min(-1) and 44.89 mM) and RC-poly(DMAEMA) (i.e., 4651 μmol mg(-1) min(-1) and 46.98 mM) had the most satisfactory biocompatibility with immobilized catalase. It was therefore concluded that the electrospun RC nanofibrous membranes surface-grafted with 3-dimensional nanolayers of polymer chains/brushes would be suitable/ideal as efficient supports for high-density and reusable enzyme immobilization.

Porous bead-on-string poly(lactic acid) fibrous membranes for air filtration

Porous bead-on-string poly(lactic acid) (PLA) nanofibrous membranes (NMs) were fabricated by electrospinning, and the formation mechanism of the membranes was determined in this study. The PLA fibrous morphology, including the fiber diameter, bead size, number of beads, and surface structure of the beads, could be closely controlled by regulating the solvent compositions and the concentrations of the PLA solutions. The filtration performance, which was evaluated by measuring the penetration of sodium chloride (NaCl) aerosol particles with an average diameter of 260nm, indicated that the filtration efficiency and pressure drop for the resultant PLA membranes could be manipulated by modifying the morphology of the fibers. Moderate bead size and quantity contribute to the low pressure drop, and small fiber diameters and nanopores on the beads were conducive to high filtration efficiency. Furthermore, the NM formed from a 5 wt% solution and a solvent mixture containing dichloromethane (DCM)/N,N-dimethylacetamide (DMAC) in a 10/1 ratio of PLA by weight exhibited excellent filtration efficiency (99.997%) and a low pressure drop (165.3 Pa), which are promising characteristics for the membranes' application as filters for respiratory protection, indoor air purification, and other filtration applications.

Air filtration in the free molecular flow regime: a review of high-efficiency particulate air filters based on carbon nanotubes

Air filtration in the free molecular flow (FMF) regime is important and challenging because a higher filtration efficiency and lower pressure drop are obtained when the fiber diameter is smaller than the gas mean free path in the FMF regime. In previous studies, FMF conditions have been obtained by increasing the gas mean free path through reducing the pressure and increasing the temperature. In the case of carbon nanotubes (CNTs) with nanoscale diameters, it is possible to filtrate in the FMF regime under normal conditions. This paper reviews recent progress in theoretical and experimental studies of air filtration in the FMF regime. Typical structure models of high-efficiency particulate (HEPA) air filters based on CNTs are introduced. The pressure drop in air filters operated in the FMF regime is less than that predicted by the conventional air filtration theory. The thinnest HEPA filters fabricated from single-walled CNT films have an extremely low pressure drop. CNT air filters with a gradient nanostructure are shown to give a much better filtration performance in dynamic filtration. CNT air filters with a hierarchical structure and an agglomerated CNT fluidized bed air filter are also introduced. Finally, the challenges and opportunities for the application of CNTs in air filtration are discussed.

Hierarchical carbon-nanotube/quartz-fiber films with gradient nanostructures for high efficiency and long service life air filters

“Gradient nano-structure filtration” for the simultaneous enhancement of filtration efficiency and service life of air filters.