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

C D

Figure 4. (A). A computer-generated schematic representation of the microfluidic monodisperse droplet formation. (B) Optical microscope
picture of the closely packed droplets in the emulsion (scale bar 200 µm). (C and D) Scanning electron microscope image of the polymerized
high internal phase emulsions created using this microfluidic system showing the surface and a fractured section (scale bar 100 µm). Images
were adapted from Costantini et al. [125] ,respectively, under the Creative Commons License.

increased temperature resistance, and potential reusability porosity gradient within the filter. All of which could be
are some of its strengths. used to create a complex internal geometry that controls the
A scalable emulsification process is required to produce air movement through the filter. Nevertheless, this is only
reproducible emulsions with high control over porosity and viable for niche applications currently because 3D printing
interconnectivity between batches. There are many industrial emulsion inks are a time-consuming process. Its strength lies
emulsification tools that can meet this need. Emulsion in its customizability or complex bespoke applications that
stability needs to be considered if the printable emulsion is cannot be made by the traditional manufacturing techniques.
to be stored for an extended period. Emulsion destabilization For an aerosol-based filter application that
can cause larger droplets to form at the expense of smaller only requires a porous membrane or column, bulk
ones. 3D printing of the emulsion will need to preserve the polymerization is preferable over AM. Pouring the
initial droplet size, not cause destabilization, and minimize emulsion into a membrane or mold should suffice, and
the time between the creation of emulsion and its subsequent specific mold materials can be chosen to prevent the
polymerization into the filter material. formation of a surface skin on the polyHIPE surface. This
From an AM perspective, an extrusion-based 3D would also be the simplest method for producing a filter
printing is recommended as a viable printing technique that can be brought to the market and can be incorporated
for filter applications because it maintains an open outer into the current RPE production using 3D printing. UV or
porosity. A micro- or nanoemulsion with tightly packed redox based cured emulsions could be the most suitable
water droplets has high viscosity so this type of emulsion for the industrial sector because of their fast curing times.
is not suitable for stereolithography-based 3D printing as Acknowledgements
the emulsion cannot spread over the surface for layering.
Furthermore, this technique has the capability to extrude We would like to acknowledge funding from the
multiple emulsions on the same print, giving user control Engineering and Physical Sciences Research Council
over multiple materials, surface functionalities, and a (Grant no. EP/R511754/1 and EP/L505055/1).
58 International Journal of Bioprinting (2021)–Volume 7, Issue 1