Flexible core–shell 3D structures are essential
for the
development of soft sensors and actuators. Despite recent advancements
in 3D printing, the fabrication of flexible 3D objects with internal
architectures (such as channels and void spaces) remains challenging
with liquid precursors due to the difficulty to maintain the printed
structures. The difficulty of such fabrication is prominent especially
when low-viscosity polysiloxane resins are used. This study presents
a unique approach to applying direct ink writing (DIW) 3D printing
in a three-phase system to overcome this limitation. We performed
core–shell 3D printing using a low-viscosity commercial polysiloxane
resin (Ecoflex 10) as shell inks combined with a coaxially extruded
core liquid (Pluronic F127) in Bingham plastic microparticulate gels
(ethanol gel). In the process termed embedded core–shell 3D
printing (eCS3DP), we highlighted the dependence of the rheological
characteristics of the three fluids on the stability of the printed
core–shell filament. With the core liquid with a sufficiently
high concentration of Pluronic F127 (30 w/w%; σy =
158.5 Pa), the interfacial instability between the shell liquid and
core liquid was suppressed; the removal of the core liquid permitted
the fabrication of perfusable channels. We identified the printing
conditions to ensure lateral attachments of printed core–shell
filaments. Interestingly, judicious selection of the rheological properties
and flow rates of three phases allowed the formation of droplets consisting
of core liquids distributed along the printed filaments. eCS3DP offers
a simple route to fabricate 3D structures of a soft elastomeric matrix
with embedded channels and should serve as a useful tool for DIW-based
fabrication of flexible wearable devices and soft robotic components.