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Sherborne and Claeyssens

fine salt crystals can be combined with a DLP printer microfluidic channels with 128 cross-junctions [132] which
and photocurable ink, although the samples had limited can be adapted to a parallelized network of channels
interconnectivity [120] . Similarly, monodisperse particles of within a coaxial annular world-to-chip interface [133] .
wax or polycaprolactone can be used within a 3D-printed Membrane-based emulsification is one such way
silk to create porosity [121] . to create a scalable emulsion templating process. A
monodisperse foam can potentially be continuously made
10. Emulsion reproducibility and scalability using a dispersion cell, and this has recently been used

Commercial virus filtration membranes that use size for emulsion templating where the air is the dispersed
[134]
exclusion as the primary filtering mechanism require a phase . Furthermore, other studies about membrane-
high degree of control over the pore size distribution; based emulsions demonstrated the creation of a range of
[135]
where a larger pore sizes reduces the filters ability to micro- and nanoemulsions with tunable droplet sizes
retain the virus [122] . To create a reproducible filter using and these techniques were used to produce porous
[136]
emulsion templating, every mixing aspect of the initial particles and materials .
emulsion has to be controlled. The same mixing speed 11. Challenges and future perspectives
can be used; however, using mechanical mixing to break
up the droplet phase creates a broad distribution of pore In this review, the current research on the AM of emulsion-
sizes. These include overhead stirrers (320–1260 rpm) based inks that produce porous foams of various materials
[26]
and high-speed homogenizers (25,000 rpm) . A syringe and sizes is discussed. The versatility of emulsion
[27]
pump can be used to add the droplet phase during mixing templating in the manufacture of porous materials and its
to create a more uniform droplet breakup to increase use in 3D printing indicate that there are many potential
reproducibility between emulsions . However, this will crossovers that could adapt this technology to new
[28]
still produce a polydisperse droplet size distribution. applications, such as a respirator filter against bacterial
Depending on the difference between the smallest and and viral infections of the respiratory tract. Nevertheless,
largest pores, this may be sufficient for an antiviral filter. the AM aspects of this technique are still in its infancy.
Many large-scale emulsification techniques that are used Therefore, precautions should be taken when choosing a
in industries, such as the pharmaceuticals and cosmetics, specific emulsion-based AM technique. Specifically, an
could potentially be adapted [123] . aerosol filter is part of the RPE that is used prevent the
Emulsification techniques using membrane, transmission of COVID-19.
microchannel or microfluidic-based devices can create an Emulsion templating is a versatile manufacturing
unprecedented level of control over emulsion droplets as technique. The porosity, interconnectivity, surface
well as particle synthesis [124] . A microfluidic device can chemistry, and material choice are all independent
create droplets one by one to produce a highly ordered variables that can be varied depending on the desired
monodisperse polyHIPE with precise control over pore filter requirements. A 3D-printed polyHIPE-based
size and interconnectivity throughout the structure [125,126] respirator needs to adhere to the strict strict classification
(Figure 4), especially when combined with the control requirements before being categorized as FFP1, FFP2,
of the locus of initiation [127] . Microfluidic-based devices or FFP3. These requirements include being permeable
can be adapted to produce bubbles of air as the dispersed to air while adhering to the required standards of air
phase template that produces a foamed styrene-in-water flow resistance and retaining high filter efficiency. The
emulsion for creating a porous material [128] . In addition, European standards EN 149:2001 + A1:2009 set specific
when using a valve-based flow-focusing junction (vFF) criteria for this, including particle penetration determined
within a microfluidic device, the air bubble size can be using a NaCl aerosol generator.
adjusted in real time to produce a porous gradient ranging PolyHIPE-based filters can be used to remove
from 80 to 800 µm pores, and this method has been used airborne particulates, such as 2.5 µm hydrocarbon
to produce nanohydroxyapatite particle-loaded gelatin- fragments from the exhaust fumes, and as a water filter
based foams that were 3D printed and then sintered to which can remove 1–11 µm suspended particulates.
produce a porous ceramic [129] . This valve-based approach PolyHIPE publications relating to specific virus binding
can also be used for W/O emulsions [130] . and antibacterial feature are limited but they highlight
The logistics, potential benefits, and limitations the importance of surface functionalization that leads to
regarding the upscaling of both emulsion- and foam- efficient trapping of virus-laden particles by the polymer
based templating methods to produce porous polymers surface. We foresee the inherent hydrophobicity of the
have been reviewed in detail by Stubenrauch et al. [131] . polyHIPE created from water-in-oil emulsions being
The limitation of microfluidic setups is that droplets are advantageous in preventing virus-containing airborne
made one by one. Scaled-up production of monodisperse water droplets from penetrating the material. Furthermore,
droplets is achievable using a circularly arranged the tunable mechanical properties, manufacture flexibility,

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