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

C

Figure 1. Scanning electron microscope pictures showing an example of the polymerized high internal phase emulsions structure made with
different water ratios. The monomer-to-water ratios are (A) 1:9, (B) 1:20, and (C) 1:40. All samples contain 10 wt% surfactant relative to
the monomer and were mixed at 350 rpm.

This is because the distance between the monomers increase until around 45 wt%; an amount higher than this
reduces from a typical van der Waals distance (~3 will cause a gradual decline in the physical properties and
Å) to a covalent bond distance (~1.54 Å) during structural collapse . The surfactant is an amphiphilic
[34]
polymerization. This contraction creates interconnecting compound which stabilizes the emulsion by orientating
windows (holes) between adjacent water droplets [37] . itself at the interface between the two emulsion phases
In the HIPE emulsion, tightly packed water droplets with the hydrophobic part pointed toward the oil phase
deform into polyhedral shapes [38] . A thin coating of and the hydrophilic head group toward the aqueous
monomer and surfactant prevents them from merging phase . Surfactants are classified by their hydrophilic-
[41]
together. This barrier ruptures and breaks apart during lipophilic balance (HLB) number which relates to the
the polymerization reaction to create the connecting ratio between the hydrophilic and the lipophilic parts of
windows. Monomers such as methyl acrylate have the surfactant; low HLB (3–6) is oil soluble and used for
a high contraction level during polymerization, and water droplets in oil (W/O) emulsions, whereas high HLB
varying its amount in the emulsion can modify the (8–18) is water soluble for O/W emulsions . A stable
[41]
interconnectivity [30] . Furthermore, increasing the water emulsion can accommodate a thin barrier film between
volume ratio in the initial emulsion increases porosity adjacent water droplets, which is more prone to rupture
and openness by creating a thinner monomer film during polymerization.
surrounding water droplets [39] . Filter-based applications may require small pore
The surfactant has a profound effect on sizes and high surface area. This can be achieved by
emulsion stability and directly affects the polyHIPE either an increase in the surfactant or addition of a salt to
interconnectivity, permeability, and size of the pores in the emulsions droplet phase, as shown comprehensively
the polymer structure . A surfactant with a concentration by Williams et al. Typical polyHIPEs have a surface
[31]
[40]
below 5 wt% relative to the monomer in the initial area around 3–20 m g , but replacing some monomeric
2 −1
emulsion creates a polyHIPE with closed porosity, up continuous phase with a solvent can increase the area to
to 10 wt% creates small connecting windows which 829 m g depending on the solvent used . Solvents in
[33]
2 −1
50 International Journal of Bioprinting (2021)–Volume 7, Issue 1