Liquid crystal elastomer actuator with serpentine locomotion

Article abstract of DOI:10.1039/d0cc02823a

A snake-mimic soft actuator composed of a bilayered liquid crystal elastomer ribbon and two serrated feet is reported in this work. Under repeated on/off near-infrared light irradiation, this actuator can move forward relying on a reversible shape morphin

Full text of DOI:10.1039/d0cc02823a

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Liquid Crystal Elastomer Actuator with Serpentine Locomotion
Received 00th January 20xx,
Accepted 00th January 20xx
Meng Wang, Xin-Bao Hu, Bo Zuo, Shuai Huang, Xu-Man Chen and Hong Yang*
DOI: 10.1039/x0xx00000x
A snake-mimic soft actuator composed of a bilayered liquid crystal state induced by the LC-to-isotropic phase transition and have
elastomer ribbon and two serrated feet is reported in this work. widespread applications in soft actuators.23-39 To allow such a
Under repeating on/off near-infrared light irradiation, this snake-mimic device to achieve the desired serpentine
actuator can move forward relying on a reversible shape morphing locomotion, the LCE-based soft actuator should have one S-
between S-curve structure and reverse S-curve structure, which is shaped structure in one permanent state (State 1) and the
similar to the serpentine locomotion of snakes.
reverse S-shaped structure in another permanent state (State
2). Under the on-off application of external stimuli, this LCE
Soft-bodied animals, such as octopuses, worms and snakes, actuator will reversibly and repeatedly deform from
can regularly deform their shapes to realize efficient permanent state 1 to permanent state 2, to simulate the
locomotion to adapt to different natural environments, which serpentine locomotion as shown in Fig. 1b.
have attracted scientific attentions in developing a series of
bionic soft actuators,1-10 such as caterpillar-like walking
robots,11-14 flytrap mimics,15 octopus-inspired robots,16,17 etc.
As the representative soft-bodied reptile, snake has four basic
locomotion modes: serpentine, sidewinding, caterpillar and
concertina.18,19 Among them, serpentine locomotion is an S-
curve movement, which can be simplified into a reversible
shape morphing between S-shaped structure and reverse S-
Fig. 1 (a) Cartoon illustration of the serpentine locomotion of snakes. (b)
Schematic diagram of the S-shaped locomotion model.
shaped structure, as shown in Fig. 1a. In such a locomotion
mode, snake contracts its muscles, thrusts its body from side
to side, and creates a series of S-shaped curves, which
Based on this basic design logic, two strategies were
adopted to fabricate snake-mimic soft actuators. The first
strategy as shown in Fig. 2a, was very straightforward, we built
an S-shaped structure (State 1) in the polydomain LCE sample
obtained from the pre-crosslinking stage, performed
mechanical programming to reverse the S-shaped structure,
and solidified this permanent state 2 during the second
crosslinking stage. Meanwhile, in order to achieve a remote
control of the actuators, we endowed the LCE sample with
photo-thermal conversion effect so that a light stimulus would
trigger the LC-to-isotropic phase transition and the consequent
shape morphing between two permanent states.
According to strategy 1, we tried several LCE systems,
including polysiloxane LCEs,40,41 polyacrylate LCEs42 and thiol-
ene LCEs,43,44 which all failed in providing wave-shaped pre-
crosslinked LCE samples in a wavy-bottom mould, possibly due
to the low viscosities of these LCE systems. Alternatively, a
two-step acyclic diene metathesis (ADMET) in-situ
polymerization/crosslinking approach45 successfully prepared a
single-layered S-shaped LCE film, named as LCE0 (1.3 cm long ×
generate asymmetrical friction force between the ventral
scales and the rough ground, and further propel the snake
forward. Although some previously synthesized actuators have
mimicked the sidewinding,20,21 concertina22 and caterpillar22
motions, there were barely any reports of snake-like soft
actuators focusing on serpentine locomotion.
The key challenge in the fabrication of serpentine-type
soft actuators is how to achieve a reversible transformation
from one S-shaped motion to the reverse S-shaped motion.
Here we report our effort on synthesizing a snake-mimic soft
actuator with serpentine locomotion, by using liquid crystal
elastomer (LCE) which as a classical kind of two-way shape
memory material, can reversibly undergo a large shape
deformation from one permanent state to anther permanent
School of Chemistry and Chemical Engineering, Jiangsu Province Hi-Tech Key
Laboratory for Bio-medical Research, State Key Laboratory of Bioelectronics,
Institute of Advanced Materials, Southeast University, Nanjing, 211189, China. E-
mail: yangh@seu.edu.cn
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See 0.27 cm wide, the thickness was ca. 180 μm). The detailed
DOI: 10.1039/x0xx00000x
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fabrication protocol is described in the supporting information. The flat strip was then pressed between two intermeshing
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Under NIR light irradiation, this S-shaped LCE ribbon exhibited wave-shaped transparent glass mouldDsOtI:o10.f1o0r3m9/D0thCeC02w8a2v3Ay
a reversible deformation from the wavy structure (state 2) to a structure. After polymerization under 365 nm ultraviolet (UV)
nearly flat state instead of the desired reverse wavy structure light for 5 min, a wavy film was removed from the mould and
(state 1). There are two possible reasons for this compromised cut into an S-shaped ribbon LCE1 (1.3 cm long × 0.27cm wide,
result: (1) due to gravity, the thickness of the LCE film the thickness was ca. 640 μm ). The outer layer LCE2 (0.65 cm
prepared at the pre-crosslinking stage was inhomogeneous; (2) long × 0.27 cm wide, the thickness was ca. 120 μm) was the
the shape of the LCE sample at the isotropic state deviated ADMET LCE system based on formula 1.45 Different from the
largely from the one of its polydomain mesogenic state (state preparation protocol of LCE1, the mechanical programming of
1).
LCE2 would have an extra uniaxial-stretching treatment before
We further optimized the strategy 1, by adopting one the curve-shape model-pressing, so that LCE2 would have a
extra LCE supporting layer which had larger contraction strain much larger contraction strain than LCE1. Eventually, the
than the main LCE layer and would contribute to reverse the S- bilayered LCE ribbon was obtained by gluing two arc-shaped
shaped structure beyond the LC-to-isotropic phase transition. LCE2 strips on the two opposite ridge sides of the S-shaped
Following this second strategy, we synthesized a dual-layer, LEC1 film. In such a design, when the temperature was raised
dual-composition LCE film. This bilayered LCE film was to above the clearing point, the bilayered LCE ribbon would
consisted of one main layer and two other outer layers stuck bend towards the more contracted LCE2 layers, forcing the
on the ridge sides of the main layer. The preparation protocols ridge sides to become the corrie sides, so that the original S-
of the main layer LCE1, the outer layer LCE2 and the bilayered shaped structure would turn to reverse S-curve.
LCE ribbon are schematically illustrated in Fig. 2b. For LCE1, a
thiol-acrylate LC mixture44 containing the nematic monomer bilayered LCE ribbon, the ultraviolet-visible (UV-vis) absorption
1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2- spectra of the NIR-absorbing dye YHD796C, LCE1 and LCE2
methylbenzene (RM257), the chain extender 1,2-bis(2- strips were recorded by a TU-2700 UV-Vis spectrophotometer.
mercaptoethoxy)ethane(DODT), the crosslinker As shown in Fig. 3a and Fig. S2, all the samples had a main
To investigate the optical absorption property of the
pentaerythritol tetra(3-mercaptopropionate) (PETMP), the absorption peak in the NIR absorption region centered at 796
photoinitiator Irgacure 651 and dipropylamine (DPA) catalyst nm, which indicated that the two LCE strips were both
(Formula 2, Fig. 2c), was poured into a flat-bottom PTFE mould sensitive to NIR light. The photothermal conversion capacities
to go through the pre-crosslinking stage.44 After removing of
from the mould, the obtained flat film was cut into thin strips.
the
two
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Fig. 2 Schematic illustration of the fabrication protocols based on (a) strategy 1 and (b) strategy 2. (c) The chemical compositions of the ADMET LCE system (Formula
1) and the thiol-acrylate LCE system (Formula 2) used in
thVisiew Article Ownolirnke.
DOI: 10.1039/D0CC02823A
forward wave motion trajectory, which indicated that the
reversible shape-morphing process and the continuous
forward movement of the actuator were simultaneous. To
investigate the snake- mimic actuator deforming and moving
quantitatively, the curve of the angle α between line l1 and line
l1 along with the time of NIR light irradiation was plotted in Fig.
4e. Here, l1 and l2 were tangents to the left end point and the
left arc midpoint of the bilayered LCE ribbon, respectively. It
could be seen that the angle α quickly changed from 72o to -
45o in 9 s, implying the accomplishment of the first
deformation from an S-curve to a reverse S-curve. After the
removal of NIR light, the angle α recovered to 72o in the next 6
s, owing to the recovery of crawler from the reverse S-shaped
motion to the original S-shaped motion. According to the
curvature radius formula: 1/r = (α × π ÷ 180) ÷ l, where l was
the length of the half left arc, the corresponding curve of the
bending curvature (1/r) with irradiation time was further
plotted in Fig. 4f. Similarly, the curvature changed between
3.14 cm-1 and -2.0 cm-1 in a stimulation cycle composed of 9 s
light on and 6 s light off. The change of data from positive to
negative sign indicated the inversion of the S-shaped motion.
In the next four irradiation cycles, the changes of angle and
curvature were consistent with those of the first cycle, which
indicated that the crawler could keep moving forward through
a regular morphing between the S-shaped motion and the
reverse S-shaped motion.
Fig. 3 (a) UV–vis spectra of YHD796C, LCE1 and LCE2 samples (conc = 0.15
mg/ml) dispersed in CH2Cl2. (b) Temperature profiles of LCE1 and LCE2 samples,
irradiated by 808 nm NIR light.
LCE samples were further studied by a thermal imager. As
shown in Fig. 3b, after irradiated under 808 nm NIR light for 4
s, Under the same irradiation condition, the surface
temperature of the LCE1 strip could reach to 70 oC in the first 4
s, which was higher than its Tiso (65.2 oC, Fig. S1a), and
o
continue to rise to 125 C in 10 s. When NIR-light was
removed, the temperature of the temperature in the next 20 s.
The mechanical properties of the two LCE samples were
measured by a dynamic mechanical analyzer (DMA Q850, TA
instrument). As shown in Fig. S3, beyond the LC-to-isotropic
phase transition temperature, the elastic modulus of the
o
supporting layer LCE2 (e.g. 1.3 MPa, 135 C) was much higher
than the modulus of the main layer LCE1 (e.g. 0.3 MPa, 80 oC),
which would help the supporting LCE layer to reverse the S-
shaped structure of the main LCE layer.
Encouraged by the above results, we fabricated a NIR
light-driven snake-mimic actuator, as schematically illustrated
in Fig. 4a. The snake-mimic robot consisted of an S-shaped
bilayered LCE ribbon and two serrated feet attached onto the
front and back ends of the LCE ribbon. The support feet with
inclined serrations5,11 could not only provide asymmetric
friction force for the serpentine robot, but also make the
ribbon actuator always stand on the ground in a side vertical
manner during locomotion. As shown in Fig. 4b,c and Video S1,
this S-shaped actuator constantly deformed between an S-
shaped motion and a reverse S-shaped motion, and moved
forward with a crawling velocity of ca. 0.13 mm/s under the
repeating on/off illumination cycles of 808 nm light (15 s per
cycle, including 9 s light on and 6 s light off), which was similar
to the serpentine locomotion of snakes. Meanwhile, it could
be observed that the longitudinal body length of the actuator
increased first and then decreased to the original length under
illumination. The same length change trend could also be
recorded after removing the NIR light. During these regular
length changing processes, benefitted from the support feet
with inclined serrations, the friction force to the left was
always greater than that to the right (f1 > f2),7,46 so the crawler
could always move forward to the right side during the NIR
light on/off cycles.
Fig. 4 (a) Schematic illustration of a snake-mimic soft actuator. (b) The images of the
serpentine robot moving under the repeating on/off illumination cycles of 808 nm light.
(c) The time-dependent motion distance of the snake-mimic actuator. (d) The real-time
position coordinate of the point A of the S-shaped LCE actuator. (e) The included angle
The real-time position coordinate of the point A of the S-
shaped LCE actuator was recorded and plotted in Fig. 4d.
Under repeating on/off illumination cycles of 808 nm light, the
location distribution of the point A presented a regular
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α vs NIR light illumination time diagram of the LCE actuator. (f) Curvature (1/r) as a
function of NIR light illumination time of the LCE actuator.
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In summary, a novel snake-mimic actuator composed of a
bilayered LCE ribbon and two serrated feet was designed and
fabricated in this work. The LCE actuator could realize a
reversible deformation from an S-shaped motion to a reverse
S-shaped motion caused by the different shape deformation
behaviours of the two LCE layers. Moreover, under repeating
on/off NIR light irradiation, this crawler robot could move
forward relying on the asymmetric friction, which was similar
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NIR-fuelled serpentine-type LCE robot would provide a new
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This research was supported by National Natural Science
Foundation of China (No.21971037, 51903048), Jiangsu
Provincial Natural Science Foundation of China (No.
BK20170024, BK20180406), and the Fundamental Research
Funds for the Central Universities (No. 2242020K40034). We
thank Dr. Hao Zeng (Tampere University of Technology, E-mail:
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Table of Contents Graphic
Liquid Crystal Elastomer Actuator with Serpentine Locomotion
Meng Wang, Xin-Bao Hu, Bo Zuo, Shuai Huang, Xu-Man Chen and Hong Yang*
In this manuscript, we describe a snake-mimic soft actuator with serpentine
locomotion, which can move forward relying on a reversible shape morphing between
S-curve and reverse S-curve structures.