side-view optical images of a 1 mm thick bimorph structure at
two temperatures below (pink) and above (colourless) the spin
transition temperature. As expected, for increasing aspect
ratios, the bending amplitude also increased. The tip
displacement for a thin (0.24 mm) sample upon heating and
cooling between 15 and 120 °C is depicted in Figure 5b. One can
note the brusque movement associated with the spin transition.
The tip displacement for this 30-mm-long bimorph is ca. 4.5
mm, denoting a displacement (D)-to-length (L) ratio of D/L =
0.15. This effect was successfully repeated during twenty
thermal cycles for both aspect ratios. In order to use the
classical bilayer model to determine the mechanical and
actuating properties of our composite, a simple rectangular
bilayer object was fabricated (Fig S6). Using the classical
Timoshenko model for bilayer actuators, the total strain of the
active layer caused by the SCO phenomenon was calculated.
The linear strain of the active layer (ΔL/L) is 0.33 %. Using this
value, the volumetric work density of the actuator could be
estimated as W/V = 1.5 mJ·cm
. This value determines the
maximum amount of work per unit volume that an actuator can
perform. (Further details on these calculations can be found on
the SI.) We have also included in the SI a table comparing
different bending type polymer-based, soft actuators. The
actuating performance of the present 3D printed actuator falls
between that of different systems, but remains perfectible even
in comparison with other SCO-based systems. In particular, the
use of 3D resins with higher Young’s modulus and SCO
compounds with lower transition temperature could bring
Fig. 5. (a) Colour change and associated bending of a bimorph actuator (850 µm active
layer and 150 µm inactive layer) upon the SCO. (b) Actuation cycle of a bimorph actuator
(150 µm active layer and 90 µm inactive layer) upon heating and cooling.
Using a stereolithographic approach in conjunction with spin
crossover-polymer composites, we have 3D printed various
stimuli-responsive mono- and bimorph architectures with sizes
up to several cm and structural details down to the 80
The objects display good thermal and mechanical properties
and afford for reversible mechanical actuation generated by the
volume change accompanying the spin crossover phenomenon.
The fabrication process developed here is straightforward,
versatile and enables the creation of arbitrary planar and three-
dimensional geometries, which are otherwise not accessible
using spin crossover complexes. This work widens the restricted
choice of materials for 4D printing and opens up prospects for a
range of applications including grippers, moving parts in
microfluidics, drug delivery, adaptive optics, and so forth.
We thank financial support from the Federal University of
Toulouse/ Région Occitanie (PhD grant of MPB), the Agence
Nationale de la Recherche (ANR-19-CE09-0008-01), the French
RENATECH network, supported as part of the MultiFAB project
funded by FEDER European Regional Funds and Région
Occitanie (Grant No.16007407/MP0011594) and the European
Commission (H2020-MSCA-RISE-2016, SPINSWITCH, No.734322
and H2020-NMBP-PILOTS-2017, HoliFAB, No.760927).
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