Solvent-Free Photoresponsive Artificial Muscles Rapidly Driven by Molecular Machines

Article abstract of DOI:10.1021/jacs.8b11351

We prepared photoresponsive actuators as both hydrogels and dry gels consisting of 4-arm poly(ethylene glycol) (PEG) cross-linked by a [c2]daisy chain, which is a double-threaded [2]rotaxane dimer with α-cyclodextrin (αCD) and stilbene. The obtained gels

Full text of DOI:10.1021/jacs.8b11351

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Article
Solvent-Free Photoresponsive Artificial
Muscles Rapidly Driven by Molecular Machines
Shinji Ikejiri, Yoshinori Takashima, Motofumi Osaki, Hiroyasu Yamaguchi, and Akira Harada
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11351 • Publication Date (Web): 11 Nov 2018
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Solvent-Free Photoresponsive Artificial Muscles Rapidly Driven by
Molecular Machines
Shinji Ikejiri,† Yoshinori Takashima,‡†* Motofumi Osaki,† Hiroyasu Yamaguchi,† and Akira Harada†§*
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†Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
‡Institute for Advanced Co-Creation Studies, Osaka University, Toyonaka, Osaka 560-0043, Japan
§JST-ImPACT, 5-7, Chiyoda-ku, Tokyo 100-8914, Japan
ABSTRACT: We prepared photoresponsive actuators as both hydrogels and dry gels consisting of 4-arm poly(ethylene glycol)
(PEG) cross-linked by a [c2]daisy chain, which is a double-threaded [2]rotaxane dimer with α-cyclodextrin (αCD) and stilbene. The
obtained gels showed fast and large deformation triggered by UV irradiation in both wet and dry states. The UV/Vis spectroscopy
results, NMR measurements and tensile tests on the gels revealed that the actuation is driven by photoisomerization of the stilbene
unit in the [c2]daisy chain. The responsiveness of these gels depends on the molecular weight of the 4-arm PEG. These results suggest
that αCD recognizes trans-stilbene prior to UV irradiation to maintain the length of the PEG chain in the polymer network and that
photoisomerization allows αCD to leave the cis-stilbene moiety and move onto the PEG chain because the association constant of
αCD with cis-stilbene is quite low. Thus, the sliding motion of the αCD unit shrinks the [c2]daisy chain, leading to the contraction
of the gels. In both wet and dry states, these actuations are repeatable through reversible photoisomerization of the stilbene moiety
using different wavelengths of UV-light irradiation and can be used to perform bending and lifting actions (for 15 times heavier
weight compared to the dry gel).
Introduction
Macroscopic supramolecular actuators using molecular ma-
chines still attractive and challenging topics. To realize artifi-
cial actuators, an organized polymeric material cross-linked
with functional bridging structures is desired. In recent pro-
gresses, Giuseppone and colleagues developed not only light-
driven contraction of a macroscopic gel with a rotary molecular
machine similar to a twisting bobbin reel but also a pH-trig-
gered muscle-like system.36,37 Previously, we developed stim-
uli-responsive materials by using reversible host-guest interac-
tions or [c2]daisy chain that respond to photostimuli38,39 or re-
dox reagents40. The azobenzene(Azo)-based [c2]daisy chain
material (the αCD-Azo gel)38 showed tough mechanical prop-
erties. However, the motions of our actuators were very slow
due to slow photoisomerization and low swelling pressure of
the gels. Additionally, the αCD-Azo dry gel did not achieve
reversible deformation by irradiation with ultra violet (UV) and
visible light (Vis).
In biological systems, molecular motors, such as myosin, ki-
nesin and dynein, efficiently convert ATP as chemical energy
source into linear sliding motions to perform macroscopic me-
chanical work.1 Myosin and actin filaments are alternatively
aligned in muscle cells to form sarcomere with a layered struc-
ture. The myosin head slides on the rail of the actin filaments,
leading to the muscle motion.2-4 The design and sliding motion
of sarcomere provide us useful information to realize artificial
linear motors. To design artificial linear molecular machine,
rotaxanes and the [c2]daisy chain molecules (double-threaded
[2]rotaxane dimers) are ideal structures. Especially, the use of
the [c2]daisy chain molecule is an useful approach due to sym-
metrical structure and stimuli-responsiveness.5-15 The [c2]daisy
chain molecule was discovered by Sauvage and coworker, who
realized an ~2 nm contractile co-conformational change in-
duced by transition-metal ion exchange.16 To create supramo-
lecular actuator based on [c2]daisy chain molecules, Stoddart
prepared poly([c2]daisy chain)s using crown ethers and showed
that the host and guest components move with respect to each
other.31-34 Grubbs also reported poly([c2]daisy chain)s with
crown ether-based [c2]daisy chains. The conformation of these
poly([c2]daisy chain)s were controlled by pH changes.35 Exter-
nal stimuli, such as chemical stimuli,16-21 pH changes,22-24 redox
reactions,25,26 and photoirradiation,27-30 adjusted the contraction
and expansion conformations of the [c2]daisy chain molecules.
Cyclodextrins (CDs) are also useful cyclic molecules to prepare
the [c2]daisy chain molecules. CD-based [c2]daisy chains also
demonstrated the conformational change using solvent polarity
changes20 or photostimuli27-30 in aqueous solutions.
To achieve fast macroscopic motions, dry material is ex-
pected to be better because slow diffusion process in wet gel
inhibits the response speed of CD-based polymeric actuators.
The development of a solvent-free molecular actuator showing
fast and reversible deformation with two different wavelength
is a challenge in the field of supramolecular polymeric materials.
Here, we assumed that a highly responsive actuator requires a
more sensitive cross-link that immediately deforms upon expo-
sure to light (UV-A and UV-C), and that an actuator with a fast
and sensitive response should work even in a dry state. We fo-
cused on a [c2]daisy chain composed of α-cyclodextrin (αCD)
and stilbene (Sti) as a molecular machine to perform polymeric
material actuation.
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complex, indicating that the photoisomerization is reversible.
Pseudo-first-order plots of the photoisomerization (Figure S5)
indicate that the isomerization rates are 0.11 s-1 (trans to cis)
and 0.12 s-1 (cis to trans). These values are 60 times faster than
those of the azobenzene(Azo)-based [c2]daisy chain complex
(αCD-Azo)2, which is our former precursor38. UV/Vis spectros-
copies also revealed that the trans-isomer of (αCD-Sti)2 was
converted to the cis-isomers in 83% yield, whereas (αCD-Azo)2
gave only 39% of the cis-isomer in the same conditions (Figure
S15). Moreover, quantum yields in the photoisomerization
were measured (Table S2), in which Sti showed 250 times
higher quantum yield than those of Azo. These results suggest
that Sti showing high reaction rate, conversion and quantum
yield in photoisomerization is suitable for the component of
molecular machine driven by photo-stimuli.
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As shown in Scheme 1, we prepared photoresponsive poly-
meric materials with the (αCD-Sti)2 complex as cross-linker.
First, the αCD-Sti molecule was dissolved in water to form the
(αCD-Sti)2 complex, and 4-arm PEG with amino groups at the
ends of the polymer chains (molecular weight (MW): 10,000)
was added to the solution. A polycondensation reaction be-
tween the carboxylic acid groups of the (αCD-Sti)2 complex and
the amino groups of PEG was carried out using 4-(4,6-di-
methoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride
(DMT-MM) as the condensation reagent to obtain a network
structure of PEG cross-linked by the (αCD-Sti)2 complex as a
transparent hydrogel (αCD-Sti hydrogel (10k)). A chemically
cross-linked hydrogel of 4-arm PEG and Sti (Sti hydrogel
(10k)) was also prepared in the same manner as a control. Char-
1
acterization of these materials was carried out by H and 13C
NMR, UV/Vis, and IR spectroscopies, as shown in Figures S6-
S9.
Scheme 1. Structure of the [c2]daisy chain prepared from αCD
and Sti and actuators with [c2]daisy chain cross-linking (αCD-Sti
hydrogel) and covalent bond cross-linking (Sti hydrogel).
The contracting and expanding behaviors of the hydrogels
were investigated. As shown in Figure 1a, the Xenon lamp was
used in the experiments. The radiation intensity (1.7 mW/cm2
for λ = 350 nm) is equivalent to that of light with λ = 365 nm,
which was used for isomerization of Azo in our former work38.
When the αCD-Sti hydrogel (10k) was irradiated by UV-A in
water, the volume of the αCD-Sti hydrogel (10k) decreased to
90%. Figure 1b indicates that the αCD-Sti hydrogel (10k) con-
tracted and desorbed water as a result of photoisomerization of
the (αCD-Sti)2 cross-linker. However, continuous UV-C irra-
diation increased the volume of the αCD-Sti hydrogel (10k) to
its original state. In contrast, the covalently cross-linked Sti hy-
drogel (10k) expanded upon UV-A irradiation and contracted
upon UV-C irradiation in water. Of course, the PEG gel with-
out the Sti moiety did not show any deformation upon UV-A or
UV-C irradiation (Figure 1c). These results suggest that the
(αCD-Sti)2 cross-linker upon photoirradiation changes the vol-
ume of the αCD-Sti hydrogel (10k) via a unique mechanism that
differs from that of the Sti hydrogel (10k).
Results and Discussion
We prepared αCD with tethered trans-Sti (αCD-Sti), which
was characterized by 1H, 13C and 2D NMR spectroscopies and
MALDI-TOF MS as shown in the Supporting Information (Fig-
ures S1-S4). Scheme 1 shows the proposed structure of the Sti-
based [c2]daisy chain complex (αCD-Sti)2, which is a precursor
for [c2]daisy chain cross-linking between poly(ethylene glycol)
(PEG) chains. As shown in Figure S3, the NMR measurements
revealed that the αCD tethered to trans-Sti and PEG chains
forms a uniform structure in D2O. In addition, the correlation
peaks between the inner proton of αCD and trans-Sti are ob-
served only in D2O, but not in DMSO-d6. These results support
that αCD includes trans-Sti moieties to form the (αCD-Sti)2
compl ex in water. Whereas the association constant (Ka) of
αCD with trans-Sti derivative is high (Ka = 1200 M-1), that of
αCD with cis-Sti derivative is low (Ka = 150 M-1) enough that
the [c2]daisy chain complex easily dissociates in an aqueous
solution.41 Therefore, the association/dissociation behaviors of
the (αCD-Sti)2 complex can be controlled by isomerization of
Sti, which is triggered by photoirradiation. A UV/Vis spectral
analysis (Figure S4) revealed the photoisomerization of the
(αCD-Sti)2 complex. The irradiation of an aqueous solution of
the (αCD-Sti)2 complex with UV-A (UV light by a Xenon lamp
with a band-pass filter, λ = 350 nm, radiation intensity: 1.7
mW/cm2) resulted in isomerization from the trans-isomer to the
cis-isomer within 20 seconds. Irradiation with UV-C (λ = 280
nm) resulted in cis to trans isomerization of the (αCD-Sti)2
Actuation in response to photostimuli was investigated in the
equipment shown in Figure 1d, and the gels were hung in the
air and irradiated with UV-A or UV-C. Here, we define the
flexion angle as shown in Figure 1d. When the αCD-Sti hydro-
gel (10k) was irradiated by UV-A in water, the αCD-Sti hydro-
gel (10k) bent toward the light source and increased the flexion
angle to 12.0 degrees within 7 seconds (Figures 1e and 1f).
UV/Vis spectroscopies (Figure S10 and S11) revealed that the
αCD-Sti hydrogel (10k) showed the photo-isomerization in
55% yield ([trans-Sti]/[cis-Sti] = 45/55) within 40 seconds. In
contrast, the flexion angle of the Sti hydrogel (10k) indicated
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Figure 1. Experimental equipment and irradiation experimental setup of the αCD-Sti hydrogel(20k). Intensities of lights are summarized
in Table S1 in Supporting Information (a). Contraction and expansion of the αCD-Sti hydrogel(10k) (10 × 10 × 1 mm3) and the Sti hydro-
gel(10k) (10 × 10 × 1 mm3) irradiated by UV-A (λ = 350 nm, 30 sec) and UV-C (λ = 280 nm, 30 sec) (b). Volume change of the αCD-Sti
hydrogel(10k), the Sti hydrogel(10k) and the PEG hydrogel(10k) upon irradiation with the UV lights (c). Lateral views of the αCD-Sti
hydrogel(10k) hung with a clip. Flexion angle (θ) is defined here (d). Light irradiation from the left side of the αCD-Sti hydrogel(10k) for
seven seconds. After irradiation of UV-A, the αCD-Sti hydrogel(10k) bends to the left side. Subsequent irradiation with UV-C from the
left side restores the initial form. The Sti hydrogel(10k) bent to the opposite side by the photo-irradiations (e). Plots of the flexion angle (θ)
as a function of the irradiation time of the αCD-Sti hydrogel(10k) and Sti hydrogel(10k). The blue and purple areas denote irradiation of
UV-A and UV-C, respectively. The flexion angle (θ) is measured using snapshots from Supplementary Movie (f). Schematic illustrations
of the mechanisms for the deformation of the αCD-Sti hydrogel(10k) and the Sti hydrogel(10k) upon irradiation with UV-A and UV-C.
smaller bending to the other side. These results indicate that the
contraction and expansion of the hydrogels were induced on the
side near the light source and resulted in the bending behaviors.
The bending directions agree with the expanding/contracting
properties of the αCD-Sti hydrogel (10k) and Sti hydrogel (10k).
The initial speed of the bending motion was fast, i.e., 2.6 de-
gree/s in αCD-Sti hydrogel (10k). Although the actuation of the
αCD-Azo hydrogel (10k) took 3 hours to reach to 12.0 degrees,
the αCD-Sti hydrogel (10k) accomplished within 7 seconds,
which is 1600 times faster than that observed in our former
work.38 The αCD-Sti hydrogel (10k) returned to its original
state (flexion angle θ = 0°) upon UV-C irradiation, indicating
that isomerization of the Sti unit from the cis-isomer to the
trans-isomer leads to reverse deformation. The behavior of
αCD-Sti hydrogel (10k) upon photoirradiation in water is
shown in Movie S1. The Sti hydrogel (10k) also showed re-
versible deformation upon UV-A and UV-C irradiation, indi-
cating that the deformation is triggered by the isomerization of
the Sti unit.
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photo-irradiation (λ = 350 nm), intensity of the excimer’s peaks
decreased and that of monomer (λ = 408 nm) increased. This
result indicates that trans-Sti forms dimeric aggregation in wa-
ter and that the aggregations can be dissociated by the photo-
isomerization. The trans-Sti moieties is supposed to form ag-
gregations as cross-linking points in the Sti hydrogel (10k) (Fig-
ure 1h). When Sti was isomerized by UV-A irradiation, the cis-
isomers lost their planar symmetry and their aggregates in the
polymer network disassociated. Thus, the Sti hydrogel can be
expanded by UV-A irradiation.
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Dependency on MW during the deformation was investigated.
The αCD-Sti hydrogels with PEG compounds with various MW
were prepared (Figure 2a). The thermal properties of the hy-
drogels with higher MW of PEG were characterized by DSC
measurements (Figure S9). The contraction and expansion tests
in water showed that the αCD-Sti hydrogel (20k) decreased its
volume to 81% upon UV-A irradiation, whereas the volume of
the αCD-Sti hydrogel (10k) decreased to 90% upon UV-A irra-
diation. These results indicate that PEG with a larger MW re-
sulted in a larger contraction in the αCD-Sti hydrogels (Figure
2b). UV-C irradiation restored all of the αCD-Sti hydrogels to
their initial state, suggesting that the reversible property does
not depend on the MW. However, the covalently cross-linked
Sti hydrogels did not show a significant difference in their de-
formation upon UV-A irradiation with an increase in MW (Fig-
ure 2c). The amount of displacement in the length of the
[c2]daisy chain should increase as the PEG chain length in-
creases. These results also support the proposed mechanisms
shown in Figures 1g-1h.
Figure 3 shows the results of the tensile tests on the hydrogels.
The Young’s modulus of the αCD-Sti hydrogel (10k) did not
increase largely before and after UV-A and UV-C irradiation
(Figure 3b). On the other hand, the Sti hydrogel (10k) showed
a much larger decrease in the Young’s modulus upon UV-A ir-
radiation (Figure 3e). The Young’s modulus is generally corre-
lated with the cross-linking density of a polymeric material.
These results suggest that the cross-linking density decreased in
the Sti hydrogel (10k) and support the deformation mechanism
of the Sti hydrogel (10k); i.e., assembled Sti’s cross-linking
points dissociate upon UV-A irradiation (Figure 1h). In con-
trast, the number of cross-linking units in the αCD-Sti hydrogel
(10k) should not increase or decrease in the deformation mech-
anism upon UV irradiation. The small change in the Young’s
modulus agrees with the deformation mechanism of the αCD-
Sti hydrogel (10k).
Figure 2. Dependence of molecular weight of PEGs in αCD-Sti hy-
drogels and Sti hydrogels. Structure of 4-arm PEG with molecular
weight (MW) of 10k, 15k and 20k (a). Volume change of the αCD-
Sti hydrogel(10k), (15k) and (20k) upon irradiation with the UV-A
and UV-C (b), and those of the Sti hydrogel(10k), (15k) and (20k)
(c).
Furthermore, the fracture energy of the αCD-Sti hydrogels
increased with increasing MW of PEG when the αCD-Sti hy-
drogels were subjected to UV-A irradiation (Figure 3c). The
shrunken [c2]daisy chain can be expanded through the sliding
motion of the αCD rings, so the UV-A-irradiated αCD-Sti hy-
drogels showed a larger fracture energy. The PEG with a larger
MW should provide longer sliding rails for the αCD rings, re-
sulting in the larger fracture energy of the αCD-Sti hydrogels.
Note that the Sti hydrogels (10k, 15k and 20k) did not show
significant increase in the fracture energy with UV-A irradia-
tion (Figure 3f).
We propose a mechanism for the contraction and expansion
of the αCD-Sti hydrogels (10k) in Figure 1g. Before UV-A ir-
radiation, the trans-Sti moiety in the [c2]daisy chain is included
by αCD to form the (αCD-Sti)2 complex. Therefore, the
[c2]daisy chain cross-linker is in its longest form. When the
αCD-Sti hydrogel (10k) was irradiated by UV-C irradiation, the
trans-Sti unit isomerized to the cis-form, and the αCD unit al-
lowed the cis-Sti moiety to slide onto the PEG chain, shrinking
the length of the [c2]daisy chain. This molecular motion is sup-
posed to result in the macroscopic contraction of the αCD-Sti
hydrogel(10k). The behavior of Sti hydrogel (10k) was studied
by fluorescence spectra of model Sti dye (stilbene dicarboxylic
acid) in water (Figure S16), which revealed that excimer’s
peaks were observed at 420 ~ 600 nm, indicating the presence
of exited dimeric aggregation of Sti residue. Through the
In the proposed mechanism, the swelling process is not re-
quired in the deformation of the αCD-Sti hydrogels, but the slid-
ing process of the (αCD-Sti)2 unit is essential factor to shrink
the αCD-Sti hydrogels as shown in the chain length dependency.
Thus, we hypothesized that the αCD-Sti hydrogels should work
even in the dry state, as the sliding process still would happen
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Figure 3. Tensile tests of the αCD-Sti hydrogels and the Sti hydrogels. The stress-strain curves of the αCD-Sti hydrogel(10k) (a) and the
Sti hydrogel(10k) (d) before and after irradiation of UV-A and UV-C. Changes in the Young’s modulus of the αCD-Sti hydrogels (b) and
the Sti hydrogels (e). Changes in the fracture energy of the αCD-Sti hydrogels (c) and the Sti hydrogels (f).
in dry gel. Here, the αCD-Sti hydrogels were dried via lyophi-
lization to obtain a dry gel. The αCD-Sti dry gels were trans-
parent and self-standing materials.
the αCD-Sti dry gel in this work realized reversible actuation in
air. Switching the wavelength of light (λ = 350/280 nm) can
repeatedly let the αCD-Sti dry gel bend and return to the origi-
nal shape as shown in Figure 4d and Movie S2. It is also im-
portant that the Sti residue is hydrophobic, compared to Azo.
The hydrophobic Sti residue can easily return to the αCD cavity.
That is, the hydrophobicity of Sti supports reversible motion of
the [c2]daisy chain.
The response of the αCD-Sti dry gel to photostimuli was in-
vestigated (Figures 4b and 4c). When the αCD-Sti dry gel (10k)
was subjected to UV-A irradiation, the αCD-Sti dry gel (10k)
bent toward the light source, and the flexion angle of the dry gel
increased to 6.0 degrees within 3 seconds, showing that the
αCD-Sti dry gel (10k) can work as a dry polymeric actuator.
That is, the αCD-Sti unit is a versatile material that can work in
both wet and dry states. The αCD-Sti dry gel (20k) quickly bent
to 26.0 degrees within 3 seconds, indicating that the degree of
deformation increases as the MW of PEG increases. Moreover,
when, the αCD-Sti dry gels were irradiated with UV-C, they
returned to their original state, leading to reverse actuation. The
photoirradiation of the αCD-Sti dry gel (20k) in air is shown in
Movie S2. At least 15 bending cycles could be performed with
the same dry gel using UV-A and UV-C irradiation. In contrast,
the covalently cross-linked Sti dry gels showed slight bending
to the opposite side of the light source. These results suggest
that the reversible, large and rapid deformations in both the wet
and dry states are due to the sliding motion of the (αCD-Sti)2
unit.
The dependence on PEG length during photoactuation is
summarized in Figure 4c. The maximum flexion angle of the
αCD-Sti dry gels increased to 11.5 degrees when the αCD-Sti
dry gel (20k) was employed. The longer PEG chains resulted
in the larger flexion angle of the αCD-Sti dry gel. The length
of the PEG chain, which the αCD unit can slide on, is supposed
to affect the degree of contraction. The hydrophobic property
of Sti should fully enhance the ability of the [c2]daisy chain
with higher MW to show the fast and reversible actuation de-
pending on the MW of PEGs.
Tensile tests were performed on the αCD-Sti dry gels and Sti
dry gels (Figure S13). When the αCD-Sti dry gels and Sti dry
gels were subjected to UV-A irradiation, the Young’s modulus
of the αCD -Sti dry gels did not change with UV-A irradiation,
but those of the Sti dry gels decreased upon UV-A irradiation.
The fracture energy of the αCD-Sti dry gels increased upon UV-
A irradiation. The fracture energy values of the Sti dry gels
were comparable upon UV-A irradiation. These dry state
In our former work38, the αCD-Azo dry gels did not return to
its original state by photo-stimulus. Bending actuation of the
αCD-Azo dry gels to arbitrary directions was performed by
changing sides of UV-irradiation (λ = 365 nm). On the contrary,
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Figure 4. Experimental equipment and irradiation experimental setup of the αCD-Sti dry gel (10k) (10 × 2 × 1 mm3) in air (a). Plots of the
flexion angle (θ) as a function of the irradiation time of the αCD-Sti dry gels (10k), (15k) and (20k) (b). The maximum flexion angles of
the αCD-Sti dry gels and Sti dry gels upon UV-A irradiation (c). Repeated tests of the αCD-Azo hydrogel (10k) bending upon exposure to
UV-A and UV-C irradiation (d). The energy conversion test of the αCD-Sti dry gel (20k). The energy conversion from light to mechanical
work was evaluated by using the values and equation shown in (e). Plot of the position of the weight as a function of the irradiation time
during the energy conversion testing of the αCD-Sti dry gel (20k).
results agree with those of the αCD-Sti hydrogels and Sti hy-
drogels, respectively. The mechanisms shown for the hydrogels
are also reasonable for the dry gels.
Conclusion
We succeeded in preparing photoresponsive actuators as both
hydrogels and dry gels consisting of PEG molecules cross-
linked by [c2]daisy chains (αCD-Sti)2. The obtained gels expe-
rience fast and large deformations triggered by UV irradiation
in both wet and dry states. We investigated the details of the
contraction and expansion of the gels using spectroscopic anal-
yses and mechanical tests. The actuation was driven by pho-
toisomerization of the Sti moiety in the [c2]daisy chain with
high quantum yields. The responsiveness of these gels was
found to depend on the molecular weight of PEG. The αCD
ring stays on the trans-Sti unit before UV-A irradiation to main-
tain the length of the [c2]daisy chain in the polymer network.
After UV-A irradiation, photoisomerization of trans-Sti allows
the αCD ring to leave the cis-Sti moiety and move onto the PEG
chain. Thus, the sliding motion of αCD is supposed to shrink
the (αCD-Sti)2 unit, leading to the contraction of the gels. Alt-
hough reversible quick actuation in the dry state was not
achieved in our previous work38, we successfully accomplished
repeatable actuation under dry conditions through reversible
and rapid photoisomerization of the Sti moiety using UV-light
with different wavelengths (UV-A and UV-C). The gels can
perform fast bending motions and lift a weight 15 times heavier
than the dry gel. Currently, we are investigating new actuator
As shown in Figures 4e and 4f, an energy conversion from
light to mechanical work is performed. We used 36.5 mg of the
αCD-Sti dry gel (20k) for an energy conversion test as the αCD-
Sti dry gel (20k) showed the fastest and largest deformation.
When the αCD-Sti dry gel (20k) hung with a weight of 562.3
mg was irradiated with UV-A irradiation, the αCD-Sti dry gel
(20k) contracted and lifted the weight vertically by 3.2 mm
within 3 seconds (see Supporting Movie S3). The mechanical
work (W) produced by the αCD-Sti dry gel (20k), which is de-
termined by W = mgx (m: mass of the weight, g: acceleration of
gravity, x: distance the weight is lifted), was 17.8 μJ. This value
is 150 times higher than that obtained in our previous work (the
αCD-Azo dry gel).38 Additionally, the power (P = W/t, t: time)
generated by the αCD-Sti dry gel (20k) was 5.9 μW, which is
10 times higher than that of the [2]rotaxane-type actuator using
azobenzene that we recently reported.42 The αCD-Sti dry gel
(20k) performed the rapid and large mechanical work upon
photoirradiation.
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Supporting Information. The Supporting Information is available
free of charge on the ACS Publications website at DOI:
Corresponding Author
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*Institute for Advanced Co-Creation Studies,
*Department of Macromolecular Science, Graduate School of
Science, Osaka University, Toyonaka, Osaka 560–0043, Japan. E-
mail: takasima@chem.sci.osaka-u.ac.jp and
harada@chem.sci.osaka-u.ac.jp
ACKNOWLEDGMENT
This research was funded by the ImPACT Program of the Council
for Science, Technology and Innovation (Cabinet Office, Govern-
ment of Japan), a Grant-in-Aid for Scientific Research (B) (No.
18H02035) from MEXT of Japan, and a Grant-in-Aid from Iketani
Science and Technology Foundation, Research Grant Program of
the Asahi Glass Foundation (2015 and 2018).
ABBREVIATIONS
CD, cyclodextrin; Sti, stilbene; Azo, azobenzene; PEG, poly(eth-
ylene glycol)
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