Page 58 - IJB-7-1

Additive Manufacture of Emulsion Inks to Produce Respiratory Protective Filters
surfactant amount and droplet volume ratio of the initial viscosity . Since the emulsion is a liquid, its benefits
[95]
emulsion . A hydrophilic material is required to create and constraints are similar to those of the liquid-based
[77]
an open surface with W/O emulsions. This can include 3D printing . A review of the current progress of AM
[96]
hydrophilic glass surface [78-80] or alginate . coupled with emulsion templating is discussed below.
[81]
PolyHIPE-based bacteria filters can be surface-
coated with polypyrrole nanoparticles. These particles 8. Stereolithography of HIPE
act as nanoheater that can be heated to 180°C within 10 Raster scanning UV laser light across the top surface of a
s using near-infrared irradiation to kill trapped bacteria . vat of HIPE can be used to polymerize both simple porous
[82]
An approach like this could be beneficial for sterilizing polyHIPE shapes on top of a glass surface using an in-
a polyHIPE-based filter as the inner surface of current house built microstereolithography (µSL) rig . Later,
[92]
N95 filters can have a warm, wet microclimate which the group produced a woodpile structure of alternative
can contribute to bacterial growth (a strategy that could polyHIPE lines of 350 µm and used it as a porous scaffold
also work for viral infections) . Silver is an effective for tissue culture support [39,97,98] (Figure 2). The size of
[83]
antibacterial agent and can be coated on the surface the polymerized regions varied with write speed and UV
of a polystyrene sulfonate-based polyHIPE or a light power, and the droplet size determines the achievable
[84]
polystyrene-based polyHIPE . Alternatively, broad- minimum feature size. The internal microporosity of the
[85]
spectrum antimicrobial activity can be obtained by iodine- 3D-printed polyHIPE was preserved and comparable to a
releasing polyHIPEs made from kaolin-containing cross- bulk cured emulsion.
linked PEG-NaAA-PEG polyHIPEs . These can be 3D Light-based raster scanning of HIPEs requires
[86]
printed into a mesh for wound dressing as iodine is both careful control over light scattering to reduce both
[87]
antimicrobial and virucidal . Alternatively, antibacterial the overcuring and formation of a surface skin on the
[88]
properties of the polyHIPE surface can be acquired by polyHIPE . The refractive index mismatches between
[99]
dipping the polymer in antibiotics, such as ciprofloxacin the water and oil phase scatters light, and this results in
or tetracycline HCL, although freezing and lyophilizing the characteristic whiteness of the emulsion. This effect
are required afterward as reported in this case . However, is problematic during stereolithography as the emulsion
[89]
antibiotics do not work against viral infections. also scatters the polymerizing light outward from the
Electrospinning has also been combined with point of exposure, creating a gradually reduced cross-
emulsion templating to prevent cell migration in a tissue linked monomer gradient that decreases outward from
engineered scaffold . This hybrid manufacture approach polymerized polymer to liquid monomer. On washing,
[90]
has the potential to combine the strengths of fibrous and this partially polymerized polymer collapses on itself,
emulsion templated porous structures. Nanofibrous-based covering the polyHIPE with a surface skin. Overlapping
emulsion templated foams are fragile and can be difficult of two partially polymerized regions fully polymerizes
to scale. Nanofibrous porous syndiotactic polystyrene- the polymer causing overcuring and connecting polymer
based polyHIPE (average fibrous diameter of about bridges . To reduce the surface skin and increase
[99]
24 nm) can be used to remove airborne volatile organic printable resolution, a light absorber can be used to
compounds; however, the reported manufacture method control the light scattering . A surface skin is also
[99]
required extensive extraction through boiling in acetone found when the HIPE is polymerized against certain
and freeze-drying to produce the structures according to mold materials. [43,73,77-80] . Although a closed, pored outer
the choice of material . surface can act as a barrier to slow down drug release
[91]
7. AM of emulsions from a 3D-printed emulsion-based hydrogel [100] , it is
detrimental for filter-based applications.
Emulsions, in particular, HIPEs that contain photocurable A dynamic mask projection-based stereolithography
monomer can be used as an ink for AM techniques. approach can be used to 3D print the HIPE ink using a
Stereolithography and extrusion-based printing are 3D digital micromirror device (DMD) to produce complex
printing techniques that can be used to print with these 3D shapes in a layer-by-layer fashion [101] . A light absorber
emulsions. The conditions of the emulsion used in these is still required to improve resolution, and the authors did
techniques can control the porosity, while the print design not mention whether there was an outer surface skin. The
determines the bulk shape. The HIPE viscosity can be authors reported that this layer-by-layer-based printing
tailored for each technique: low speed mixing (350rpm) is sensitive to viscosity of the emulsion, which can be
creates a liquid suitable for stereolithography . While, lowered using a solvent such as toluene. Furthermore,
[92]
a highly viscous emulsion is advantageous for its shape DMD projection can be used to 3D print porous
retention during extrusion-based 3D printing. This can polyHIPE tubes in a continuous process . A functional
[92]
be achieved either through additives , high-speed polymer-based polyHIPE made of GMA was 3D printed
[93]
mixing (2500 rpm) , or other methods to change the using a digital light processing (DLP) printer to create
[94]
54 International Journal of Bioprinting (2021)–Volume 7, Issue 1