Page 112 - IJB-6-2

Scaffolds produced by combining porogen leaching and emulsion templating
ethyl acetate leaving a polyHIPE with ~100 µm rather than crystals is that it can be deformed to
macropores from the PMMA beads . While accommodate a high packing efficiency yielding a
[50]
this approach successfully generated multiscale much higher porosity. At high water volume ratios
porosity polyHIPEs, it is complex and time (>74%), droplets are forced to deform and become
consuming. This is due to the extended sintering polygonal but remain separated by a thin layer
time and the use of Soxhlet washing to remove the of the monomer/surfactant solution. This thin
PMMA and residual solvent; both essential steps monomer film is a precursor for interconnectivity
to form the macropores and minimize cytotoxicity. as the monomer contracts during polymerization
Furthermore, the 100 μm macropores introduced to create interconnecting holes between adjacent
here are still a limiting factor as vascularization pores . However, creating numerous large
[31]
requires larger macropores . In addition, as pores (>200 μm) solely by tailoring the emulsion
[6]
cellular penetration also depends on pore size and conditions to have large water droplets will
monolith thickness [51,52] , ingrowth can be further often result in limited pore interconnectivity.
improved by introducing larger macropores as This is because the large water droplets are
shown here. Importantly, our findings showing surrounded by a thick monomer film that resists
that an increased macroporosity increases cell the contraction forces that create interconnectivity
infiltration agree with those of Paljevac et al., during polymerization. On the other hand, smaller
which demonstrates its value in tissue engineering droplets of water have a higher surface area, so
and 3D cell culture . the monomer layer surrounding it will be thinner
[50]
Alternative work within our group has focused and more prone to the contraction forces that
on using stereolithography to introduce this create the interconnectivity . As shown here,
[25]
larger length-scale hierarchical porosity into the addition of a non-emulsion-based method is
polyHIPEs [8-11,13] , and various alternative additive ideal for creating large pores independent of the
manufacturing techniques have also been used by emulsion conditions.
other groups [33,35] . While these approaches produce Alginate beads with a size distribution between
structures with well-defined architectures; they are 275 and 780 μm were made by injecting an alginate
not without their limitations. Laser-based systems solution into a calcium chloride solution. The
are expensive to set up and control over the size distribution of the alginate beads represents
final structure requires time-consuming process the range that could be produced when using the
optimization. Furthermore, the emulsions scatter fastest injection speed on the mechanical syringe
light; therefore, for high resolution 3D printing the pump and forcing the alginate solution through
addition of light-absorbers is needed to control the a 30 G needle. Some of the alginate beads had a
polymerized region . Therefore, in applications slight non-spherical shape, which is most likely
[9]
where a multiscale porosity is beneficial, but the because of a combination of high injection speed,
final scaffold architecture does not have to be the viscosity of the injected alginate solution not
precisely defined, for example, initial 3D cell having enough time in flight to form a sphere,
culture investigations, a more straightforward and the fast gelation of these beads as they hit the
approach, such as porogen leaching may be calcium chloride solution. It required the maximum
desirable. injection speed of the pump (0.2195 mL/s through
In emulsion templating, the internal phase a 3 mL syringe) and small needle size to extrude
acts as a template for the continuous phase to the viscous alginate solution fast enough to
polymerize around. This is like porogen leaching produce the smallest droplets. However, this
in that the monomer is polymerized around occasionally caused some alginate solution to leak
something immiscible with it. The size of the past the syringe’s internal plastic seal during the
water droplets directly affects the pore size, as injection process. In turn, this would reduce the
does the size of the sugar/salt crystals in porogen internal syringe pressure and therefore the speed
leaching. The benefit of using a liquid porogen at which the alginate solution was being injected

108 International Journal of Bioprinting (2020)–Volume 6, Issue 2