Page 60 - IJB-7-1

Additive Manufacture of Emulsion Inks to Produce Respiratory Protective Filters
A B C

D

Figure 3. (A) 3D schematic showing an extruded silica-chitosan-based emulsion and the theoretical pore distribution within the extruded
part and subsequent polymerized structure. Adapted from ref. with permission from The Royal Society of Chemistry. (B) 3D model
105
showing a cure-on-dispense 3D printing of a photocurable monomeric-based emulsion, (C and D) two pictures of polymerized high
internal phase emulsions based structures created using this device. (B-D) Images adapted from ref. [104] licensed under Creative Commons
Attribution License.

were also created using sacrificial polymer beads in the Extrusion-based 3D printing can be applied with
emulsion, highlighting the versatility of this method to non-polymerizable polymers dissolved in a solvent.
use porogens other than water for additional level of Here, the solvent acts as a porogen through polymer
porosity control. Emulsions stabilized with submicron precipitation-based 3D printing. This technique uses the
pickering particles can be used with a range of liquids and difference in polymer solubility between two mutually
applied to create both micro- and nano-sized emulsion miscible solvents to inject a solvent dissolved polymer
droplets [108] . SiO nanoparticles can stabilize a styrene- into a vat of a non-solvent to evoke rapid polymer
2
based W/O emulsion which holds its shape before being solidification in situ [113] . By varying the solvent/polymer
thermally polymerized in an oil bath [109] . Pickering ratio within this immersion precipitation 3D printing
emulsions using hydrophobized silica particle can create (ip3DP), tunable porous structures can be created from a
a 140–450 nm droplet size distribution [110] . However, range of dissolvable polymers [114] . Air-based drying using
when particle-stabilized emulsions are used for foam a similar solvent for dissolving has been reported using
templating, they typically have low interconnectivity as a pickering polyHIPE made from degradable polymers,
the particles hinder interconnecting window formation. such as poly(l-lactic acid) (PLLA) and PCL with
Nevertheless, adding a small amount of surfactant fixes hydrophobically modified silica nanoparticles (h-SiO ) to
2
this by orientating itself at the contact point between both stabilize the initial emulsion and increase its viscosity.
adjacent droplets, creating interconnectivity windows The polymers were dissolved in dichloromethane,
during polymerization [111] . and after printing, the solvent was evaporated to leave
Extrusion-based emulsion templating, including behind solid scaffolds, although the samples were
a variety of AM techniques, has numerous applications deformed during solvent evaporation [115] . The authors
in tissue engineering as the inherent interconnectivity reported in a follow-up study that this deformation can be
facilitates cell ingrowth . These include a 3D-printed avoided using hydrophobically modified hydroxyapatite
[54]
cure-on-dispense kaolin clay-loaded poly(ethylene (HAp) nanoparticles in the initial PCL-based pickering
glycol) diacrylate and methacrylate-based O/W HIPE (3 emulsion [116] .
µm average porosity with 0.75 µm interconnects) with an There are alternative routes, other than emulsion
ability to swell up to 11 times its size in buffer solution . templating, for producing porous structures. Air bubbles
[87]
A chitosan-modified silica nanoparticle for potential can be used as a porogen, either by directly printing a
drug release of 3D-printed materials has been reported, UV curable foamed monomer [117] or using a foam that
although the post-cross-linking with glutaraldehyde took sets by thermogelation after it has been printed [118] .
up to 48 h [112] . Extrusion-based printing also lends itself Alternatively, a blowing agent can be used within the
useful for printing multiple materials together, such as 3D-printed part, which is decomposed post-processing
a degradable UV curable HIPE combined with an outer to expand it into a porous material [119] . Furthermore,
thermoplastic extruded poly(ε-caprolactone) (PCL) or particles such as sieved salt can be used as a sacrificial
poly(lactic acid) (PLA) polymer . template for a monomer to cross-link around, 75 µm extra
[94]
56 International Journal of Bioprinting (2021)–Volume 7, Issue 1