  4D printing empowers 3D printed structures made of hydrogels, liquid crystals
or shape memory polymers, with reversible morphing capabilities in response to
an external stimulus. To apply reversible shape-change to stiff lightweight
materials such as microfiber reinforced polymers, we developed a composite ink
that can be printed using direct-ink-writing (DIW), and that exhibits
multistability around its glass transition temperature. After curing at room
temperature, the flat print thermally morphs into a predefined shape upon
heating at an actuation temperature and cooling down. The sample can then
reversibly snap between multiple stable shapes when heated above its glass
transition temperature thanks to prestress-induced multistability. The key that
allows thermal morphing and prestress multistability is the microstructuring of
the 3D printed composites by shear-induced alignment of reinforcing
microfibers. This alignment leads to local anisotropy in thermomechanical
properties and the build-up of prestresses. Furthermore, the ink composition
can be tuned to generate shape-dependant reversible functional properties, such
as electrical conductivity. Based on finite element modelling and experimental
results, the method proposed here can be used for variety of compositions and
designs, for applications where stiffness, reconfigurability and
shape-dependent functionalities can be exploited.
