Page 56 - IJB-7-1

Additive Manufacture of Emulsion Inks to Produce Respiratory Protective Filters
This highlights that in addition to sample thickness, A redox-initiated polymerization of a poly(methyl
other factors, such as porosity and interconnectivity of methacrylate)-based polyHIPE filter can be cured in situ
the polyHIPE, are also required for improving filtering within a tubular mold and used within 20 min. This is
efficiency. useful if the housing material is opaque to light or heat
There is trade-off between the polyHIPE pore size/ sensitive as the reaction can occur at room temperature .
[24]
interconnectivity and the air flow resistance through the A similar polymerization method has been used for the
material. Smaller pores not only increase the strength polyHIPE-based chromatography protein purification .
[65]
of filtration but also the air flow resistance, which Low porosity emulsion templated foams still retain
is recorded as an increased pressure drop . A more some permeability , but it lacks the interconnectivity
[62]
[40]
permeable filter will have a lower pressure drop, but at level obtained using high volumes of water in the initial
a cost of filtration efficiency. This is the same for fibrous emulsion.
masks in which the air flow is typically perpendicular PolyHIPEs created from a W/O emulsion are
to the packed fibres. Increasing the amount of fibrous hydrophobic because a water immiscible monomer
material improves particle capture, but causes more air is used in the initial emulsion. This hydrophobicity
flow resistance . Ideally, the filter should have a low has led to applications such as oil or organic solvent
[63]
pressure drop and high collection efficiency . Typical removal from water where recycled polystyrene is one
[13]
filters are porous materials that allow air transfer while of the monomers . Hydrophobicity can be increased
[68]
trapping airborne particles; there are several types of by adding Fe O to the initial monomer to create
4
3
trapping mechanisms . A typical filter example is superhydrophobic foams (contact angle over 150°) to
[13]
Nylon6 (N6)-based non-woven fibrous mesh made by remove oil or organic solvent pollutants from water . A
[69]
electrospinning . poly(dicyclopentadiene)-based polyHIPE oxidizes in air
[63]
to produce reactive peroxy species that can be used to
6. PolyHIPE filter applications either decontaminate nerve agents or produce a self-
[70]
[71]
To create micron-sized pores within the polyHIPE, decontaminating air filter .
the initial emulsion can be mixed using a high-speed A poly(styrene-co-DVB) polyHIPE column with
homogenizer (25,000 rpm), resulting in average pore mean interconnects 0.57–0.59 µm can separate 52 nm
diameters between 0.6 and 4.5 µm, an increase in particles from 155 nm ones through a difference in retention
[72]
interconnectivity was observed when increasing the time . For other aqueous-based filter applications, the
surfactant from 5 to 15wt%, and the porosity from 75 surface of the polyHIPE can be sulfonated to create a more
to 90% . A styrene/divinylbenzene (St-DVB)-based hydrophilic surface and subsequently used to filter 1–11
[27]
fibrous polyHIPE with pore sizes ranging from 2 to 7 µm µm particles of calcium carbonate (aragonite) dispersed
[73]
and interconnect windows down to 1 µm was produced in water . For microfiltration applications, the outer
by tuning both the surfactant and mixing methods . surface of polyHIPEs can also be functionalized in situ by
[29]
The polyHIPE structure has been used for the adding a hydrophilic monomer, such as sodium acrylate,
chromatographic separation of proteins because of its to the emulsion droplet phase to filter out microalgae .
[74]
tunable porosity and flexible polymer choice. Glycidyl PolyHIPE scaffolds coated with iron hydroxides can
[75]
methacrylate (GMA) is a popular monomer for this remove arsenic from contaminated water . The use of
application because it can be chemically functionalized pickering based polyHIPEs for the decontamination of
through the epoxy groups . A list can be found in pollutants water has been reviewed in detail elsewhere .
[76]
[64]
Table 2. When used with the cross-linker ethylene To create a simple polyHIPE sheet, an open pored
glycol dimethacrylate (EGDMA), polyHIPEs created surface is critical and care is needed to maintain the
from these monomers are more hydrophilic compared surface porosity when curing the HIPE into a mold to
to their styrene/DVB counterparts. Poly(GMA-co- create a porous membrane. The use of either a hydrophilic
EGDMA)-based polyHIPEs have been reported to have or hydrophobic mold material affects the surface porosity
1–10 µm pores with submicron interconnects for protein of the polyHIPE at the HIPE/mold interface [43,73,77-80] . An
separation . They can be blended with an elastomer open or closed pored surface is created by preferential
[64]
chemically modified with diethylamine to get weak anion wetting of the mold surface with either the aqueous or
exchange supports in the flexible membrane for protein monomeric (oil) phase of the HIPE and is postulated to
purification by ion exchange chromatography with 3–10 relate to the surfactant orientation at the mold surface .
[73]
µm porosity and 1–3 µm interconnects . A similar When polymerizing a W/O emulsions such as St-DVB in
[65]
polyHIPE polymer blend has been continuously reused water, the emulsion can destabilize against a PVC mold,
over 300 times , and grafted GMA brushes have been have a closed pored surface skin against a PP mold and
[66]
used for chromatographic separation of proteins in an an open porosity against a PTFE one . The amount of
[43]
epoxy-based monoliths . open surface porosity can be fine-tuned by varying the
[67]
52 International Journal of Bioprinting (2021)–Volume 7, Issue 1