Biointerfaces Between Cells and Substrates in Three Dimensions - Biointerfaces: Where Material Meets Biology - page 139

6.2.6 Emulsion Templating and Alvetex s
Emulsion templating uses emulsion internal phase droplets to template
voids into a material polymerising in the emulsion external phase. Typi-
cally, water is used as the non-polymerising internal phase and organic
monomers are used as the continuous external phase. A non-water soluble
surfactant such as Span80 t is also included to create a stable emulsion.
When high internal phase emulsions (HIPEs) are employed, defined as
when the internal droplet phase occupies greater than 74% of the emulsion
volume, the resulting polymer (or polyHIPE) can display a highly open-cell
morphology that renders porosities above 90%. Cells are able to enter and
migrate around these voids to create extensive 3D organisation in the
material. Importantly, these materials are highly controllable, enabling
excellent scaffold reproducibility. 13 By manipulating the initial HIPE
characteristics (surfactant choice, temperature, introducing stabilising
salts) the distribution and diameter of the internal phase water droplets
can be controlled, thus controlling the final polymer morphology. This
morphological control provides a key advantage over many other materials
used for 3D cell growth.
When fabricating polyHIPE it is useful to keep the following points in
mind:

d n 3 r 4 n g | 3

The polyHIPE void diameter can be increased via de-stabilisation
of the initial HIPE. 13 Increasing emulsion temperature is one ap-
proach to emulsion de-stabilisation and thus increasing void dia-
meter. Another approach is to include small organic molecule
additives such as methanol or tetrahydrofuran into the emulsion
aqueous phase.
Functional polyHIPEs can be obtained by employing functional co-
monomers in the initial HIPE emulsion. 14 For example, penta-
fluorophenyl acrylate, a hydrophobic ester-containing acrylate, will
happily reside in the organic phase of styrene-water HIPEs to render
pendent ester functionality in the final polymer. 15 Alternatively,
hydrophilic functional co-monomers such as acrylic acid can be in-
cluded in the aqueous phase of styrene-water HIPEs to render pendent
acid functionality in the final polymer. 16
The choice of polymerisation initiator is an important consideration for
polyHIPE fabrication. For example, potassium persulfate (KPS) is typi-
cally dissolved in the aqueous phase of the HIPE and thus favours
polymerisation of those monomers close to the emulsion interface.
Conversely, azobisisobutyronitrile (AIBN) is more soluble in the organic
phase of the emulsion and thus favours polymerisation of those
monomers not at the emulsion interface.

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Recently, a polyHIPE derived from cross-linked polystyrene has been
developed as a 3D cell scaffold designed for routine in vitro use (Alvetex s ;