Review on polymeric, inorganic, and composite materials for air filters: From processing to properties [original]

Review on Polymeric, Inorganic, and Composite Materials

for Air Filters: From Processing to Properties

Laura M. Henning,* Amanmyrat Abdullayev, Cekdar Vakifahmetoglu, Ulla Simon,

Hiba Bensalah, Aleksander Gurlo, and Maged F. Bekheet

1. Introduction

Air pollution causes an estimated seven million premature

deaths worldwide every year, i.e., one in every nine deaths,

according to the World Health Organization (WHO). More than

80% of people in urban areas, most severely in low- and middle-

income countries, are exposed to air pollution levels that exceed

WHO guideline limits.

[1]

Hence, they suffer elevated risks for

cardiovascular and respiratory illnesses. In addition of being a

threat to human health, air pollution is

also closely interlinked to climate change

and problems associated therewith.

[2]

Undoubtedly and above all, it is most desir-

able to develop and apply techniques gen-

erating fewer air pollutants to preserve

human and environmental health. Until

that is accomplished, and for indoor areas

in which sufﬁcient reduction cannot be

achieved easily, e.g., airborne pathogens

such as the current severe acute respiratory

syndrome coronavirus 2 (SARS-CoV-2)

provoking the Coronavirus disease 2019

(COVID-19), air ﬁlters are a trusted and

proven instrument to achieve improved

or innocuous air qualities.

[3]

Nonmedical

cloth face masks, also known as commu-

nity masks, were found to reduce the trans-

mission of SARS-CoV-2, but do not reach

the performance and protection provided by ﬁltering facepiece

respirators.

[4]

In everyday life, air ﬁlters are mostly used to ﬁltrate cabin air

in vehicles such as cars and airplanes. They are also used in clean

rooms and other air-handling installations, for example, in ware-

houses or laboratories. Other common applications include gas

turbines, compressors, and engines. To be used as personal pro-

tective equipment, elastic bands are attached to the produced ﬁl-

ter material to form surgical or face masks. This is of particular

importance for people working in areas potentially harmful to

health, such as medical institutions or on construction sites.

For special applications or long-time usage, the ﬁlter material

can be ﬁt into reusable rubber masks, e.g., made from silicone,

yielding professional gas masks.

The ﬁrst modern air-purifying respirator with a ﬁlter made of

moistened wool was invented by Lewis Haslett, in 1849.

[5]

today, air ﬁltration was already improved signiﬁcantly with

respect to performance, sustainability, and multifunction. A gen-

eral overview of different air ﬁltration technologies for indoor

building ventilation is available.

[6]

Furthermore, the progress

on particulate matter (PM) ﬁltration and face masks was more

recently reviewed, emphasizing different materials and their per-

formance.

[7–9]

It becomes apparent that the majority of modern

air ﬁlters is based on synthetic, polymeric materials. Polymers

are most commonly processed into ﬁbrous structures at moder-

ate temperatures by spinning techniques, with electrospinning

leading the way. The production of air ﬁlters from polymers

and biopolymers by the electrospinning process was recently dis-

cussed in several review articles.

[10–13]

However, polymer-based

L. M. Henning, A. Abdullayev, Dr. U. Simon, Dr. H. Bensalah,

Prof. A. Gurlo, Dr. M. F. Bekheet

Fachgebiet Keramische Werkstoffe/Chair of Advanced Ceramic Materials,

Institut für Werkstoffwissenschaften und -technologien

Fakultät III

Technische Universität Berlin

Hardenbergstr. 40, 10623 Berlin, Germany

E-mail: laura.m.henning@ceramics.tu-berlin.de

Prof. C. Vakifahmetoglu

Department of Materials Science and Engineering

Izmir Institute of Technology

35430 Urla, Izmir, Turkey

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/aesr.202100005.

lished by Wiley-VCH GmbH. This is an open access article under the terms

of the Creative Commons Attribution License, which permits use,

distribution and reproduction in any medium, provided the original

work is properly cited.

DOI: 10.1002/aesr.202100005

Particulate and gaseous air pollutants pose a threat to human health and

contribute to climate change. By today, air ﬁlters, stationary and portable, are

markedly improved and can often provide innocuous air pollution levels. After

introducing the classiﬁcation and standards on air ﬁlters, the inﬂuence of the

processing route and its parameters on the resulting air ﬁlter properties and

consequently its performance are discussed. Numerous tools are presented to

adjust structural properties such as ﬁber or pore diameter, speciﬁc surface area,

surface charge, hydrophilicity, or photocatalytic activity to achieve the desired

performance in terms of high ﬁltration efﬁciencies, sufﬁcient mechanical stability,

regeneration eligibility, antimicrobial and optical properties. In particular, inor-

ganic and composite materials as well as nonﬁbrous structures are covered,

which are currently holding an outsider position in an air ﬁlter community

dominated by polymeric materials and ﬁbrous structures.

REVIEW

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air ﬁlters often lack sufﬁcient regeneration options and their

single-use does not reﬂect the spirit of the times. In contrast,

inorganic materials such as ceramics and metals are more easily

regenerable due to their higher resistance towards elevated tem-

peratures and harsh chemical conditions. Recently, researchers

provided insights into the removal of air pollutants by activated

carbon (AC) and carbon nanotubes (CNTs).

[14,15]

The utilization

of inorganic materials, on their own or in the form of composites,

and alternative processing techniques open up new possibilities

in enhancing modern air ﬁlters.

However, previous reviews mainly focused on ﬁbrous struc-

tures, polymeric materials, and electrospinning fabrication tech-

nology. Less or no attention was paid to the competitive

properties of nonﬁbrous structures, inorganic and composite

materials, and other processing routes. Our review aims to ﬁll

the gap by giving an overview of processing techniques and ﬁl-

tration mechanisms for both ﬁbrous and nonﬁbrous air ﬁlters

composed of polymeric, inorganic, and composite materials.

First, the classiﬁcation of air pollutants and their impact on

human beings and the environment will be introduced.

Afterward, the mechanisms of air ﬁltration, performance evalu-

ation, and worldwide classiﬁcation will be elucidated. In the main

part of this Review, the latest progress on processing techniques

of ﬁbrous, cellular, and granular air ﬁlters will be discussed, as

shown in Figure 1. Emphasis will be put on air ﬁlters for room

temperature applications and ﬁltration of PM. Processing param-

eters, possibilities, and limits will be described for polymeric,

composite, and inorganic materials. In the following, special

attention will be devoted to tailoring structural and material prop-

erties to improve the properties and performance of air ﬁlters.

Additional ﬁlter features such as antibacterial properties and ﬁl-

tration of gaseous pollutants will be exploited. All things consid-

ered, the reader will be provided with a well-resourced toolbox for

air ﬁlter understanding and performance optimization, consider-

ing sustainability and ﬁlter regeneration. Finally, remaining chal-

lenges and possible solutions within the progress of air ﬁlter

research will be addressed.

2. Air Pollutants and Their Impact

A variety of gaseous and particulate substances from both natural

and anthropogenic sources can cause air pollution, leading to

short- and long-term harm to human and environmental health,

as schematically shown in Figure 2. Air pollutants can be classi-

ﬁed according to their source, chemical composition, size, or

release mode. Common classiﬁcations are primary versus sec-

ondary, indoor versus outdoor, and gaseous versus particulate

pollutants.

[16]

Primary pollutants such as oxides of sulfur, nitro-

gen, and carbon, as well as PM from ash, smoke, and dust, are

directly emitted into the atmosphere from natural and anthropo-

genic sources. When primary pollutants react with atmospheric

constituents, secondary pollutants such as ground-level ozone,

nitrogen oxides, or acid rain are produced.

[17]

Air pollutants

can also be classiﬁed according to the place of emission. For

instance, combustion products and volatile organic compounds

(VOCs) released from indoor activities, e.g., cooking, smoking,

and heating, furniture, building materials, and paint, are called

indoor pollutants. In contrast, outdoor pollutants are predomi-

nantly generated from power plants, industries, and vehicles,

which emit large amounts of various pollutants. However, the

most common classiﬁcation of pollutants is gaseous versus

particulate.

The main gaseous pollutants are volatile oxides of nonmetallic

elements such as sulfur, nitrogen, and carbon, and VOCs. On the

contrary, particulate pollutants or PM refer to the solid particles,

liquid droplets, or mixtures of both, including aerosols formed

with viruses, bacteria, and spores. PM is usually further subclas-

siﬁed according to the size, denoted as PM

, whereby xdescribes

the maximum particle diameter in micrometer (μm). Established

classes are coarse PM

,ﬁne PM

2.5

, and ultraﬁne PM

0.1

. This

well-known ﬁlterable PM (FPM) is distinguished from condens-

able particulate matter (CPM).

[18]

CPM is gaseous at fuel gas tem-

perature but forms a solid or liquid substance after dilution and

cooling, which belongs to harmful PM

2.5

. Frequent precursors of

CPM are VOCs and sulfur oxides.

The UN General Assembly listed air pollution within the ﬁve

main risks for noncommunicable diseases. The harmful effect of

air pollutants on human health strongly depends on the type of

Figure 1. Schematic illustration of this Review showing the dependence of

the air ﬁlter performance on the air ﬁlter properties, which are governed by

the choice of material and processing technique.

Figure 2. Schematic illustration of the main gaseous and particulate air

pollutants produced from selected outdoor and indoor sources and their

harmful effects on human and environmental health.

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pollutant, its amount, as well as the time of exposure and possible

accumulation in the body over time.

[19]

It is widely accepted that

air pollution causes respiratory diseases such as chronic obstruc-

tive pulmonary disease (COPD), asthma, and lung cancer. In that

respect, nitrogen dioxide and sulfur dioxide, in particular, were

found to be noticeably associated with respiratory diseases and

lung cancer mortality, respectively.

[20]

Furthermore, acute and

chronic cardiovascular diseases such as myocardial ischemia

and infarctions, heart failure, and strokes were found to be pro-

voked by air pollutants, predominantly by ﬁne PM

2.5

and ultra-

ﬁne PM

0.1

[21]

The PM inhaled into the pulmonary tree may harm the human

body via: i) instigation of systemic inﬂammation and/or oxidative

stress, ii) alterations in autonomic balance, and iii) direct actions

upon the vasculature of particle constituents. Other than that,

pathogens such as inﬂuenza or SARS viruses can spread by air-

borne or aerosol transmission, followed by disease outbreaks.

[22]

Without protection using air ﬁlters or other means, outbreaks

can possibly evolve into pandemics and endanger the health

and life of millions of people. Examples include the 1918 inﬂu-

enza pandemic, also known as Spanish ﬂu, or the current

COVID-19 pandemic.

Air pollution does not only cause harm to human health, but

also to the environment. Major damaging effects include i) acid

rain, which acidiﬁes water and soil environments, ii) haze from

PM dispersed in air, iii) ozone thinning in Earth’s protective

stratospheric layer, and iv) climate change, including global

warming induced by elevated emission of greenhouse pollutants,

such as carbon and nitrogen oxides and methane.

[23]

In addition,

climate change is expected to exacerbate further human health

impacts by increasing weather conditions that enhance air pol-

lution exposure, mainly to PM

2.5

and ozone.

[24,25]

To break the

cycle of air pollution and climate change, air pollution needs

to be restricted more drastically. In this regard, air ﬁlters provide

a suitable method for air puriﬁcation.

3. Fundamental Principles of Air Filters

In the following section, the mechanisms of air ﬁltration of par-

ticulate and gaseous pollutants will be introduced. Furthermore,

key factors in the evaluation of air ﬁlter performance will be

presented. An overview of the worldwide classiﬁcation and stan-

dardization of air ﬁlters according to these key factors will be pro-

vided. Furthermore, the inﬂuence of the model pollutant on the

air ﬁlter performance will be discussed.

3.1. Mechanisms of Air Filtration

Particulate and gaseous adsorbents signiﬁcantly differ in size

and chemical composition. Thus, they behave differently within

the air ﬂow and require different approaches for removal. In the

following, the underlying mechanisms for the removal of partic-

ulate and gaseous pollutants will be elucidated.

3.1.1. Filtration of Particulate Pollutants

Several mechanisms were reported to be involved in the ﬁltration

of PM, as shown in Figure 3a,b, namely sieving, gravity settling,

inertial impaction, interception, diffusion, and electrostatic

attraction.

[26,27]

Sieving takes place when the particles are too large to enter the

pores of the ﬁlters and are retained on the barrier. As sieving

requires adequately small pore sizes, usually in the range of

3–8 nm,

[28,29]

the corresponding pressure drop is commonly

in the range of a few hundreds or thousands of Pa. This is much

higher than that of other mechanisms, making sieving hardly

competitive as the main ﬁltration mechanism for air ﬁlters, com-

pare Section 4.

Particles larger than 5 μm experience a signiﬁcant net down-

ward motion and can be ﬁltered by gravity settling. This mecha-

nism is barely relevant for common air ﬁlters as such particles

often settle before reaching the ﬁlter surface. In contrast, small

particles with high inertia that can enter the ﬁlter material have

sufﬁcient momentum to break away from the air streamline and

collide with the ﬁlter medium, being separated by impaction.

Thus, this mechanism is called inertial impaction. In case the

particles do not have sufﬁcient inertia to depart from the gas

streamline, they can still be ﬁltered by collision with the ﬁlter

medium, i.e., by interception mechanism. Both inertial impac-

tion and interception mechanisms are more likely to occur with

higher internal tortuosity of the ﬁlter medium and are among the

most common ﬁlter mechanisms, especially for ﬁbrous ﬁlters.

Figure 3. Schematic representation of the major ﬁlter mechanisms sieving, gravity settling, inertial impaction, interception, diffusion, and electrostatic

attraction for particle matter a) past a single ﬁber and b) through a pore. Reproduced with permission.

[26]

most penetrating particle size for which a ﬁlter is least effective. c) Dependence of the particle size on the major ﬁlter mechanisms and the resulting total

ﬁlter efﬁciency, Reproduced with permission.

[230]

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The exploitation of these mechanisms comes along with low

pressure drop values, mostly below 100 Pa, compare Section 4.

Diffusion is based on the Brownian motion and migration of

particles smaller than 0.5 μm within the gaseous medium with

concentration gradients of particles from the regions of higher

concentration to those of lower local concentration, i.e., to the

surface of the ﬁlter medium. Electrostatic attraction, based on

the surface electrical charge of the particle and/ or ﬁber medium,

causes particles to divert from the streamline and settle on the

ﬁber medium. The electrical ﬁeld can also be externally imposed.

Electrostatic pollutant capture emerged as the most frequently

applied ﬁltration mechanism, as it conveniently allows to obtain

pressure drop values below 50 Pa down to values as low as

0–0.5 Pa, compare Section 5. Still, there usually exist particles

small enough to stay with the streamline, regardless of wall

surfaces or other obstacles. As a result, those cannot be withheld

by the ﬁlter medium. Among that, as shown in Figure 3c, differ-

ent mechanisms are dominant for different particle sizes. As a

result, for each ﬁlter, there exists a particle size, for which ﬁlters

are least effective, called the most penetrating particle size

(MPPS). The MPPS usually is in the particle size range of

0.04–0.4 μm. Research emphasis is mostly focused on removing

the particles within this size range, as they are hard to remove

and might pose a serious health threat due to their ability to enter

the lower respiratory tract.

In addition to the methods mentioned earlier, adsorption is

often proposed as one of the underlying ﬁltration mechanisms,

not specifying the exact nature of pollutant adhesion, compare

Section 4. As adsorption occurs at the surface, the available sur-

face area of the ﬁlter material is of most importance. In addition,

adsorption can be signiﬁcantly enhanced by functional surface

groups.

[30–32]

3.1.2. Filtration of Gaseous Pollutants

Chemical air ﬁlters, also called air puriﬁers, can be applied to

remove gaseous pollutants from the air via physisorption,

and/or chemisorption.

[33]

Physisorption is based on weak van

der Waals forces between the ﬁlter material and the pollutant.

van der Waals bonds are comparably weak, whereas a strong

bond between the ﬁlter material and pollutant can be formed dur-

ing chemisorption. Hence, molecules bond by physical adsorp-

tion can migrate about the surface and be easily removed from

the ﬁlter surface during regeneration. In contrast, molecules

bond by chemical sorption are restricted to speciﬁc surface sites

and can hardly be removed by reasonable energy demands.

3.2. Performance Evaluation

3.2.1. Model Pollutants

When determining the performance of an air ﬁlter, the size of

the model pollutant is not the only important factor to consider,

but so is its shape, surface charge, or ability to form stable aero-

sols. Air ﬁlter performance factors are most commonly deter-

mined using NaCl as a model pollutant because it forms

cubic particles with round edges. Furthermore, it is a low-cost

material, which is easy to handle and easily available. NaCl is well

suited to average ﬁltration efﬁciencies considering that spherical

particles like spherical polystyrene latex result in higher ﬁltration

efﬁciency compared with cubic particles like MgO of the same

aerodynamic size.

[34]

Incense smoke is another commonly used

model pollutant, which produces particles mostly smaller than

2.5 μm as well as gas products like CO, CO

,NO

,SO

, and

VOCs such as benzene, toluene, xylenes, aldehydes, and polycy-

clic aromatic hydrocarbons.

[35]

Thus, incense smoke exhibits

both polar functional groups such as C─O, C═O, and C─N,

and nonpolar functional groups like C─C, C─H, and C═C.

[36]

To represent organic matter and oil droplets, di-ethyl-hexyl-

sebacat (DEHS) is a frequently used model pollutant as it forms

a stable aerosol.

[37]

Depending on the temperature and relative

humidity, changes in the morphology or state of the test pollu-

tants may occur and should be considered.

[38]

Recently, a low-cost

optical measurement for testing the efﬁcacy of facemasks against

expelled droplets during the speech was presented and applied to

commercial masks and mask alternatives such as neck ﬂeece and

bandanas.

[39]

The evaluation of antimicrobial properties is usually ascer-

tained by aerosol ﬁltration and subsequent cell incubation or

in liquid media, which can be problematic due to safety concerns.

Instead, testing in liquid media can be carried out. Recently,

researchers presented a novel method of determining the antivi-

ral performance of air ﬁlters against airborne infectious viruses.

This allows deriving results for tests in an air medium from tests

in a liquid medium without the need for aerosolized viruses.

[40]

3.2.2. Key Performance Indicators

The removal or ﬁltration efﬁciency η, which is one of the most

important factors to evaluate the performance of any ﬁlter, is

deﬁned as the percentage of contaminant removed by the ﬁlter,

according to Equation (1), where c0and c1(μgm

3

) are the

upstream and downstream concentrations of the contaminant,

respectively.

ηð%Þ¼ðc0c1Þ

100 (1)

In contrast to the ﬁltration efﬁciency, which should be as high

as possible, the pressure drop ΔP, which represents the pressure

loss through the ﬁlter, should be remained as small as possible.

This reduces both energy demand and breathing resistance. The

pressure drop ΔP(Pa) is practically determined by measuring the

difference between upstream and downstream pressure P0and

P1(Pa), respectively, according to Equation (2).

ΔP¼P0P1(2)

The pressure drop ΔP(Pa) can be expressed by Darcy’s law,

according to Equation (3), where h,μ,U, and kare the ﬁlter thick-

ness (m), gas viscosity (Pa s), gas velocity at the surface of the

ﬁlter (m s

1

), and permeability (m

), respectively. As shown

in Equation (3), Darcy’s model represents a linear relationship

between the pressure drop and the gas velocity in porous media.

Accordingly, the gas velocity should be considered when compar-

ing the pressure drop values of different air ﬁlters. However, in

some cases, especially at high gas velocities and for complex pore

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structures, deviations from Darcy’s model can be observed. Thus,

other models, such as the Forchheimer model, can be used to

correlate this nonlinear relationship between pressure drop

and the gas velocity based on inertial effects.

[41]

The

Forchheimer model extends Darcy’s linear law by an additional

quadratic term βρU

, where βrepresents the inertial coefﬁcient

1

) and ρis the air density (kg m

3

), to account for the change

in pressure drop.

[42]

The Forchheimer number, which refers to

the ratio between the viscous and inertial effects, can be used as

criterion to decide for the applicability of the respective mod-

els.

[43]

ΔP

h¼μU

k(3)

Finally, the overall ﬁltration performance of a ﬁlter can be eval-

uated by calculating the quality factor, QF, according to

Equation (4), considering both efﬁciency and pressure drop fac-

tors. Thus, the QF is considered as a beneﬁt-to-cost ratio.

QF ¼lnð1ηÞ

ΔP(4)

3.3. Classiﬁcation and Standardization of Air Filters

Air ﬁlters can be classiﬁed according to their performance, appli-

cation, operation principle, or material composition. Commonly,

air ﬁlters are classiﬁed according to their efﬁciency based on reg-

ulations published by international organizations such as the

International Organization for Standardization (ISO), regional

organizations such as the European Committee for

Standardization (Comité Européen de Normalisation, CEN),

Europe’s Industry Association for Indoor Climate (HVAC),

Process Cooling, and Food Cold Chain Technologies

(EUROVENT), or national organizations such as the American

Society for Testing and Materials (ASTM), American Society

of Heating, Refrigeration and Air Conditioning Engineers

(ASHRAE), and from the China Association for

Standardization (CAS) and China Household Electric

Appliance Research Institute (CHEARI), among others. These

organizations do not only classify air ﬁlters, but also recommend

widely accepted regulations for air ﬁlter testing.

For instance, according to the European standard EN 1822,

from which the current international standard ISO 29463 was

derived, air ﬁlters are classiﬁed as efﬁcient particulate air

(EPA) ﬁlters with efﬁciencies of 85–99.5%, high-efﬁciency partic-

ulate air (HEPA) ﬁlters with efﬁciencies of 99.95–99.995%, and

ultralow penetration air (ULPA) ﬁlters with efﬁciencies of

99.9995–99.999995%. Air ﬁlters are commonly tested with par-

ticle sizes of 0.3–0.5 μm. Concerning ﬁltering respirators, the

European ﬁltering face piece (FFP) classiﬁcation and the U.S.

National Institute for Occupational Safety and Health

(NIOSH)’s air ﬁltration rating are well-established standards.

The European standard EN 149 describes ﬁltering half masks

labeled as FFP1, FFP2, and FFP3, which ﬁlter at least 80%,

94%, and 99% of the test pollutants. Tests are conducted with

NaCl particles of 0.02–2μm, having a mean diameter of

0.6 μm and a concentration of 8 4mg m

3

, as well as with

parafﬁn oil, at 95 L min

1

. The breathing the resistance may

not exceed 210, 240, and 300 Pa for FFP1, FFP2, and FFP3 ﬁl-

tering half masks, respectively, for the ﬂow rate given. In the

U.S., as governed by Part 84 of Title 42 of the Code of

Federal Regulations (CFR), minimum efﬁciency levels of 95%,

99%, and 99.97% for particles ≥0.3 μm, made from NaCl or oily

dioctylphthalate (DOP), are assigned to respirator classes X95,

X99, and X100, respectively. Thereby, Xis replaced with Nfor

respirators not resistant to oil, i.e., particular ﬁltration only, R

for respirators resistant to oil, and Pfor oil-proof and reusable

respirators. More recently, the People’s Republic of China

updated the national standard GB 2626 for respiratory protection,

whose effective date was postponed to July 2021 by the

Standardization Administration of China (SAC) due to the

COVID-19 pandemic to ensure stable respiratory production.

The classiﬁcation contains the ﬁlter categories KN for nonoily

particles and KP for oily and nonoily particles with the sufﬁx

90, 95, or 100 that refer to the ﬁlter efﬁciencies.

4. Processing Technologies and Performance of

Fibrous Air Filters

4.1. Electrospinning

Electrospinning is a commonly used technique to produce ﬁbers

and ﬁber-based meshes by applying a high voltage to a polymeric

solution or melt, as shown in Figure 4a. During the electrospin-

ning process, the solution becomes electrically charged and over-

comes its surface tension, forming a Taylor cone at the needle

tip, which is subsequently stretched in a jet form toward a

grounded plate or rotary collector. On its way to the collector,

the jet dries, and the present solvent evaporates, resulting in

ﬁbers having diameters in the range of tens to a few hundred

nanometers.

[12]

Electrospinning allows for the production of con-

tinuous ﬁber assemblies such as nonwoven, aligned, and pat-

terned ﬁber meshes, as well as random 3D structures, evolved

from a long ﬁber overlaying on itself multiple times.

[44]

Exemplary structures for polymeric, inorganic, composite, and

metallized ﬁber structures are shown in Figure 4b-i.

Common polymeric building blocks for electrospinning

include polyacrylonitrile (PAN), polyvinylidene ﬂuoride

(PVDF), polyurethane (PU), polylactic acid (PLA), polybenzimi-

dazole (PBI), polyamide (PA), poly(methyl methacrylate)

(PMMA), polydimethylsiloxane (PDMS), and mixtures

thereof.

[45–54]

However, natural materials, including silk, wool,

cellulose, soy protein, gelatin, and chitosan, took on greater sig-

niﬁcance due to increasing awareness for sustainability.

[13,55–60]

Furthermore, expanded polystyrene (EPS) waste was successfully

used for the production of air ﬁlters by electrospinning.

[61]

addition to the material type and additives, which can be used

to functionalize the formed ﬁbers,

[12]

process variables such as

applied voltage, ﬂow rate, type of collector, and distance between

needle tip and collector were adjusted to tailor the ﬁnal ﬁber

properties such as diameter, length, porosity, pore size, and mor-

phology.

[62]

For instance, uniform polymeric PBI nanoﬁber ﬁl-

ters were obtained by electrospinning with a single solvent,

see Figure 4b,

[51]

whereas the usage of a mixed solvent system

allows producing polymer-rich and polymer-lean phases within

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