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Sherborne and Claeyssens
A B

C

Figure 2. Schematic (A) and 3D rendering (B and C) showing a projection and raster scanning-based 3D printing. (A) reproduced from
Ref. with permission from John Wiley and Sons, Licensed under Creative Common License. (B) and (C) modified from Journal of the
[92]
Mechanical Behavior of Biomedical Materials, Volume 54, Owen R, Sherbone C, Paterson T, et al. Emulsion templated scaffolds with
tunable mechanical properties for bone tissue engineering, pp 159-172, Copyright (2016), with permission from Elsevier .
[39]
a chromatography column [102] . Other DLP-based 3D the emulsion is curing against air; therefore, there is
printing techniques have used an oil-in-water emulsion- no surface contact with a material that can adversely
based ink consisting of droplets of photocurable ink affect the surface porosity by surface destabilization.
within an aqueous suspension [103] . This technique has also been used to create a composite
The 3D print speeds vary depending on the method polyHIPE for use as a biocatalytic flow reactor using an
used. The single line writing speeds of direct laser writing enzyme-laden hydrogel as the emulsion droplet phase [104] .
for printing polyHIPE are 1 – 5 mm/s, which is dependent Alternatively, an emulsion can simply be injected directly
[25]
on the laser power . With these speeds the authors into a void before bulk polymerization . Extrusion-
[92]
created a circular 13mm, 4 layered woodpile structure based printing of emulsion has been demonstrated to print
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in 13 minutes . Alternatively, the projection-based at a speed of 10 mm s with extrusion width of 0.6 mm
[39]
[93]
stereolithography can print a 17.25 mm surface area and using a modified RepRap style 3D printer . Modified
2
complex geometries simultaneously at a printing speed extrusion-based 3D printers have been used to print
of 2 vertical mm/h for a layer thickness of 25 µm [101] . emulsions that are cured on demand, with print speeds
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Stereolithography-based 3D printing setups are becoming tested up to 9 mm/s and layer heights of 100–300 µm [104] .
quite cost effective, after the major patent protecting the To create hydrophilic porous foams through material
technique (US5762856A, Hull) expired in 2015 and start- extrusion, an oil-in-water emulsion can be used with a
up companies started to produce low-cost setups. UV cure during extrusion. Some of the materials include
poly(ethylene glycol) diacrylate (PEGDA), alginate, and
9. Extrusion-based 3D printing hyaluronic acid with mineral oil as the dispersed phase
to increase emulsion viscosity [105] . However, the droplet
Through a manufacturing process known as material size was reported to increase overtime and the samples
extrusion (also called fused deposition modeling or are required to wash in DCM to remove the oil and
robocasting), a HIPE ink can be extruded through a subsequently lyophilized at −80°C for 24 h to solidify
syringe and polymerized using UV light to create a porous and dry.
polyHIPE-based structure (Figure 3). High emulsion Extrusion-based 3D printing combined with
viscosity is suitable for this 3D printing method as the emulsion templating can be used to create a porous ceramic
emulsion ink can maintain its structure post-extrusion that will be sintered at 1600°C for 2 h to form a solid [106] .
without unwanted spreading before polymerization, Nanoscale porosity of 100-900 nm can be achieved using
and the emulsion viscosity is tunable by altering the a nanodroplet-stabilized pickering emulsions which are
respective component amounts in the emulsion . No 3D printed through direct ink writing. It shows that 3D
[93]
surface skin is observed on the outer surface of extrusion- extrusion-based printing using nanoporosity emulsions
based 3D-printed polyHIPEs . This is expected as is achievable [107] . Furthermore, large 100 µm pores
[93]
International Journal of Bioprinting (2021)–Volume 7, Issue 1 55