Expansion-contraction of photoresponsive

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ARTICLE

Received 13 Jun 2012 | Accepted 12 Nov 2012 | Published 11 Dec 2012

Yoshinori Takashima1, Shogo Hatanaka1, Miyuki Otsubo1, Masaki Nakahata1, Takahiro Kakuta1, Akihito Hashidzume1, Hiroyasu Yamaguchi1 & Akira Harada1

The development of stimulus-responsive polymeric materials is of great importance, especially for the development of remotely manipulated materials not in direct contact with an actuator. Here we design a photoresponsive supramolecular actuator by integrating hostguest interactions and photoswitching ability in a hydrogel. A photoresponsive supra-molecular hydrogel with a-cyclodextrin as a host molecule and an azobenzene derivative as a photoresponsive guest molecule exhibits reversible macroscopic deformations in both size and shape when irradiated by ultraviolet light at 365 nm or visible light at 430 nm. The deformation of the supramolecular hydrogel depends on the incident direction. The selectivity of the incident direction allows plate-shaped hydrogels to bend in water. Irradiating with visible light immediately restores the deformed hydrogel. A light-driven supramolecular actuator with a-cyclodextrin and azobenzene stems from the formation and dissociation of an inclusion complex by ultraviolet or visible light irradiation.

DOI: 10.1038/ncomms2280 OPEN

Expansioncontraction of photoresponsive articial muscle regulated by hostguest interactions

1 Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan. Correspondence and requests for materials should be addressed to A.H. (email: mailto:[email protected]

Web End [email protected] ).

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2280

The construction of actuators, which are reminiscent of articial muscles, is an important target in elds ranging from medicine to physics, materials science and materials

engineering. One research topic to realize muscle-like movements in actuators is converting input energies (electric, thermal, charge, photo energies) into visualized movements (deformation, transformation, pressure, and so on)13. Many attempts have been made to realize organic, inorganic, electrostrictive and piezoelectric materials47. Polymer-based actuators (polymer gel811, liquid crystalline elastomers1223, conjugated polymers24 and carbon nanotubes2528) show reversible shape deformations in response to external stimuli. However, there are no examples of articial muscles in which polymeric materials are able to expand and contract owing to stimulus-responsive hostguest interactions. If such systems are realized, they can be used not only to conrm the mechanism for biological movement but also to realize soft robotics.

Previously, we have reported that stimulus-responsive supra-molecular polymers are formed by mixing an aqueous solution of a host polymer containing cyclodextrin (CD) with that of a guest polymer containing azobenzene (Azo)2932 or ferrocene33. An external stimulus induces a solgel phase transition in these hydrogels. Formation of inclusion complexes acts as crosslink points for the polymers to yield supramolecular hydrogels, whereas decomposition of inclusion complexes yields the sol state. We hypothesized that if covalent bonds partly crosslink polymer chains, then external stimuli would induce an expansioncontraction behaviour, and not a solgel phase transition. However, to our knowledge, there are no reports on an external stimulus-responsive hostguest gel with expansion contraction properties. Theoretically, the ionic strength and crosslink densities (effective network chain) have important roles in the expansioncontraction ability of hydrogels34,35. Although previous papers have altered the expansioncontraction properties of hydrogels via ionic strengths, changing these properties using the crosslink density in a hostguest complex has yet to be reported.

We selected Azo compounds as guest molecules because the association constant of a-cyclodextrin (aCD) for trans-azobenzene (trans-Azo) is larger than that for cis-azobenzene (cis-Azo) (trans-Azo; Ka 12,000 M 1, cis-Azo; Ka 4.1 M 1)30,31; Azo

affects the photoinduced deformation and remote controllability. Herein, we report supramolecular materials with expansion and contraction abilities constructed by hostguest polymers. Using supramolecular hydrogels, which exhibit an expansion contraction behaviour that depends on the photostimulus, we successfully prepared a photostimulus-responsive supramolecular actuator reminiscent of a natural muscle.

ResultsPreparation of an expansioncontraction gel. Initially we prepared a hostguest gel (aCDAzo gel) with aCD and Azo (Supplementary Methods). aCDAzo gel is synthesized by radical copolymerization of a mixture of aCD-modied acrylamide (aCDAAm), azobenzene acrylamide (AzoAAm), methylene bisacrylamide (MBAAm) and acrylamide (AAm) in dimethyl sulphoxide (DMSO). The mole percentage contents (x) of aCD

AAm and AzoAAm units are x 13 mol%. The polymer chains

in aCDAzo gel are crosslinked with MBAAm (the mole percentage content of MBAAm: y 2 and 4 mol%). Figure 1a depicts the

chemical structures of aCDAzo gel(x, y), aCD gel(1, 2) (without the AzoAAm unit), Azo gel(1, 2) (without the aCDAAm unit) and AAm gel(0, 2) (without aCDAAm and AzoAAm units).

1H solid state NMR (1H magic angle spinning NMR (1H MASNMR)) and infrared spectroscopy characterized the chemical

structure of aCDAzo gels (Supplementary Figs S1 and S4), Azo gel(1, 2) (Supplementary Figs S2 and S4) and AAm gel(0, 2) (Supplementary Figs S3 and S4).aCDAzo gels feature three types of gels with hostguest units (x 1, 2 and 3 mol%) and crosslinking units (y 2 and 4 mol%).

After gelation in DMSO, rinsing with water replaces the absorbed DMSO in the aCDAzo gel(x, y). Figure 1b depicts the weight ratio of the gels upon substituting DMSO with water. The weight ratio of gel absorbed with DMSO is dened as 100%. Removing the absorbed DMSO signicantly decreases the weight ratio of aCD

Azo gel(x, y) with an increase in the mol% of the aCD and Azo unit. As shown in Fig. 1c, substituting DMSO with water causes aCDAzo gel(x, 4) to contract. In addition, the weights of aCD

Azo gel(2, 2) and (3, 2) decrease, reaching 101.1 and 7.34.7% of the initial weight, respectively. aCDAzo gel(x, 2) with 2 mol%

of MBAAm exhibits a greater contraction than aCDAzo gel(x, 4) with 4 mol% of MBAAm. On the other hand, the weight ratio of aCD gel(1, 2), Azo gel(1, 2) and AAm gel(0, 2) increase upon solvent manipulation, reaching 19711, 17925 and 18718% of their original weights, respectively (Fig. 1b).aCDAzo gel(x, y) contracts upon substituting DMSO with water because hostguest complexation forms crosslinks, which was conrmed using creep rupture measurements. Supplementary Figure S5 shows the stressstrain curves of aCDAzo gel(x, 2) with various amounts of hostguest units. The stress of aCDAzo gel(x,2) increases as aCD and the Azo units (x) increase. These results indicate that the formation of an inclusion complex between aCD and the Azo units causes aCDAzo gels to shrink owing to the increase in crosslinks.

Expansion of aCDAzo gel with competitive molecules. To demonstrate the complementary hostguest interaction between aCD and the Azo groups, aCDAzo gels were immersed in aqueous solutions of competitive guest or host molecules for 12 h. We chose diol derivatives as competitive guest molecules (for example, 1,4-butane diol (C4 diol), 1,5-pentane diol (C5 diol), 1,6-hexane diol (C6 diol) and 1,7-heptane diol (C7 diol))36. Flat plates of aCDAzo gels (size: 3 3 2 mm3) were immersed in

solutions with various concentrations of competitive guests or hosts.

Figure 2a and b show the weight ratio of aCDAzo gel(1, 2) or aCDAzo gel(1, 4) with competitive guests or competitive hosts. In addition, we investigated the inuence of the concentration of competitive molecules on the weight ratio of aCDAzo gels. After immersion in an aqueous solution of competitive guests for 12 h, the weight ratio of the aCDAzo gels depends on the association constant with aCD36 as well as the concentration of competitive guests. When aCDAzo gels(1, 2) are immersed in 100 and 1,000 mM of C7 diol aq., the weight ratio of aCDAzo gel(1, 2)

after 12 h are 1686.0 and 2179.5% of the original weight ratio, respectively (Fig. 2c). The weight ratio of aCDAzo gel(1, 2) is larger than that of aCDAzo gel(1, 4), indicating that the smaller the crosslink ratio of aCDAzo gels leads to the greater the change in volume. Immersing aCDAzo gels(1, 2) in 10 and 100 mM of aCD aq. leads to a 1944.2 and 2563.5% increase in the weight ratio, respectively (Fig. 2d). The weight ratio of aCDAzo gel(1, 2) immersed in aCD aq. is larger than those of bCD and gCD aq. owing to the low afnities of bCD and gCD for Azo derivatives36. The expansion of aCDAzo gels depends on the association constant of the competitive molecules with host or guest units on the polymer chains. These results indicate that aCDAzo gels shrink in water owing to the formation of an inclusion complex between aCD and the Azo units, and then competitive molecules decompose the inclusion complexes, which function as crosslinkers, to swell aCDAzo gels (Fig. 2e).

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CH2

H2N HN

100-(2x+y)

CH CH2

HN O

r r r

CH2 CH

CH2 CH CH2

CH2

r r

CH2 CH

HN O

H2N HN

x x

O O

CH2

CDAzo gel(x, y)

CD gel(1, 2)

CH2 CH

HN O

CH2 CH2

CH2

CH2 CH2

CH2

H2N O

r r

97 2

H2N O O

Azo gel(1, 2)

b c

AAm gel(0, 2)

Replacement from DMSO to water (Initial weight ratio: 100%)

250

200

150

100

187% in DMSO

100%

in water

av. 52%

av. 20%

av. 9.9%

Weight ratio/%

CDAzo gel(1, 4)

CDAzo gel(2, 4)

CDAzo gel(3, 4)

in DMSO

in water

40%

20%

9.9%

197%

179%

10% 7.3%

52%

CDAzo gel(1, 2)

CDAzo gel(2, 2)

CDAzo gel(3, 2)

CDAzo gel(1, 4)

CDAzo gel(2, 4)

CDAzo gel(3, 4)

CD gel(1, 2)

AAm gel(0, 2)

Azo gel(1, 2)

Figure 1 | Polymer gels used to prepare photoresponsive supramolecular actuators and their expansioncontraction behaviour. (a) Chemical structures of aCDAzo gel, aCD gel (without the AzoAAm unit), Azo gel (without the aCDAAm unit) and AAm gel (without aCDAAm and AzoAAm units). x is the mole percentage of the host and guest units. y is the mole percentage of the crosslinking unit (MBAAm). (b) Weight ratio change in the gels upon replacing DMSO with water. Gels initially absorb DMSO. The weight ratio of gel absorbed with DMSO is dened as 100%. Error bars, standard deviation for 5 measurements. (c) Photographs of the volume change of aCDAzo gel (x, 4) upon replacing DMSO with water. Av., average.

Photoresponsive volume change of aCDAzo gels. We investigated the effects of photostimuli on the expansioncontraction behaviour of aCDAzo gels by irradiating at plates of aCDAzo gels (size: 56 56 23 mm3) immersed in water for an hour.

Photoirradiation with ultraviolet light (l 365 nm) isomerizes

the trans-Azo group into the cis-Azo group, whereas the reverse occurs with visible (Vis) light (l 430 nm)37. Figure 3a shows the

weight change of aCDAzo gels upon ultraviolet and Vis light irradiation. The light source located above the gels had a 300W

Xenon lamp with a mirror module and band-pass lters to irradiate at a suitable wavelength. Ultraviolet irradiation of aCD

Azo gels increases the weight of the hydrogels, whereas continuous irradiation of Vis light to the aCDAzo gels restores the initial weight and volume. These volume changes of aCDAzo gels are correlated with the inclusion complex formation between aCD and Azo units (Fig. 3b). The association constant of aCD for trans-Azo is larger than that for cis-Azo30,31. The difference in the association constants of aCD for the Azo

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CDAzo gel(1, 2) immersed inaq. of competitive molecules

300

250

300

CDAzo gel(1, 4) immersed inaq. of competitive molecules

250

Start

10 mM 100 mM 1,000 mM

Start 10 mM 100 mM 1,000 mM

200

Weight ratio / %

150

Weight ratio / %

150

100

C 4 diol

C 5 diol

C 6 diol

C 7 diol

-CD

C 4 diol

C 5 diol

C 6 diol

C 7 diol

-CD

c d

CDAzo gel(1, 2) (weight ratio: 100%)

CDAzo gel(1, 2) immersed in100 mM of C7 diol aq. (weight ratio: av. 168%)

CDAzo gel(1, 2) immersed in 1000 mM of C7 diol aq. (weight ratio: av. 217%)

CDAzo gel(1, 2) immersed in10 mM of -CD aq. (weight ratio: av. 194%)

CDAzo gel(1, 2) immersed in100 mM of -CD aq. (weight ratio: av. 256%)

CDAzo gel(1, 2) (weight ratio: 100%)

CDAzo gel(m,n) with competitive guests

CDAzo gel(m,n) with competitive hosts

= Competitive guest

CDAzo gel(m,n) absorbed water

= Competitive host

Figure 2 | Expansion of aCDAzo gels immersed in aqueous solutions of competitive molecules. (a) Weight ratio change of the aCDAzogel(1, 2) immersed in aqueous solutions of competitive molecules such as 1,4-butane diol (C4 diol), 1,5-pentane diol (C5 diol), 1,6-hexane diol (C6 diol), 1,7-heptane diol (C7 diol) and competitive hosts (CDs). Concentration of competitive molecules in water was changed from 0 to 1,000 mM. Data for 1,000 mM of aCD and gCD are not collected because they become saturated at 100 mM. Saturated concentration of bCD is 10 mM. Error bars, standard deviation for 5 measurements. (b) Weight ratio change of aCDAzo gel(1, 4) immersed in competitive molecules. Error bars, standard deviation for 5 measurements. (c) Photographs of the volume change of aCDAzo gel(1, 2) immersed in aqueous solutions of C7 diol. Scale bar, 5mm. (d) Photographs of the volume change of aCDAzo gel(1, 2) immersed in aqueous solutions of aCD. Scale bar, 5mm. (e) Schematic illustration of the expansion of aCDAzo gels immersed in aqueous solutions of competitive molecules. Azo unit on the polymer chain is ejected from the aCD cavity by competitive guests. Added

CDs competitively form inclusion complexes with the Azo units. Av., average.

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a b

150

CDAzo gel(1, 2)

Initial

Ultraviolet irradiation for 1 h

Vis irradiation for 1 h

140

Ultraviolet ( = 365 nm)

Vis( = 430 nm)

CDAzo gel(m, n)

130

Weight ratio / %

120

(weight ratio: 100%)

Expansion Shrink(weight ratio: av. 124%) (weight ratio: av. 104%)

110

100

Ultraviolet ( = 365 nm)

Vis( = 430 nm)

(1,4) (2,4) (3,4)

0 (1,2) (2,2) (3,2)

CDAzo gel(m, n)

CDAzo gel(m, n) absorbed water

Figure 3 | Photoresponsive weight change of aCDAzo gels in water. (a) Weight change of aCDAzo gels before and after photoirradiation with ultraviolet light (l 365 nm) and Vis light (l 430 nm). Error bars, standard deviation for 5 measurements. (b) Photographs of the volume change of

aCDAzo gel(1, 2) irradiated by ultraviolet and Vis light. Scale bar, 5 mm. (c) Schematic illustration of the expansioncontraction of aCDAzo gel irradiated by ultraviolet and Vis light. After ultraviolet irradiation, which induces isomerization from the trans- to cis-form, the complex between aCD and Azo units decomposes to expand aCDAzo gels. Vis irradiation causes isomerization from the cis- to trans-form, and complexation between aCD and the Azo units regenerates, shrinking the aCDAzo gel. Av., average.

isomers creates the expansioncontraction behaviour in aCD Azo gels upon ultraviolet and Vis light irradiation.

The weight ratio of aCDAzo gel(x, 2) with 2 mol% of MBAAm is larger than that of aCDAzo gel(x, 4) with 4 mol% of MBAAm, indicating that a smaller crosslinking ratio induces a larger volume change in the gel. Similarly, the weight ratio of aCDAzo gel(2, 2) is larger than that of aCDAzo gel(3, 2). The inside of the Azo unit of aCDAzo gel(3, 2) does not isomerize from the trans- to cis-form because the concentration of the Azo group is too high to optically transmit through the opposite side, meaning ultraviolet light is absorbed on the surface of aCDAzo gel(3, 2). On the other hand, the weight ratio of bCDAzo gel(1,2) does not change with the irradiation because the association constants of bCD with trans-Azo and cis-Azo are comparable.

These results indicate that the change in the photoresponsive volume of aCDAzo gels is due to the extensive complementarity between the aCD and trans-Azo units.

We used ultravioletVis spectroscopy (Supplementary Figs S6 and S7) to track the photoisomerization of the Azo group. As the irradiation time at l 365 nm increases, the intensity of pp*

transition band of the Azo group in aCDAzo gel(1, 2) decreases and an np* transition band appears. Conversely, as the Vis irradiation time at l 430 nm increases, cis-Azo recovers the

intensity of pp* transition band around 350 nm and the np* transition band disappears. These results indicate that ultraviolet irradiation causes the trans-Azo group of the aCDAzo gel to isomerize into the cis-form, whereas Vis irradiation causes the cis-Azo group to isomerize into the trans-form.

We characterized the trans- and cis-contents of the aCDAzo gels by calculating the integral value of the Azo unit with 1H MAS-NMR (Supplementary Fig. S8). Before ultraviolet irradiation, the isomer contents of aCDAzo gel is trans:cis 702.2:302.2, whereas afterward, the ratio changes

to trans:cis 51.2:951.2. However, the isomer contents

recovers to trans:cis 690.57:310.57 upon Vis light

irradiation (Supplementary Fig. S8). Consequently, the photoisomerization of the Azo unit is reversible even in the aCDAzo gel.

Figure 3c shows the proposed scheme for the expansion contraction behaviour of aCDAzo gels by photoirradiation.

Before photoirradiation, aCDAzo gels contract forcefully to form supramolecular noncovalent crosslinks between aCD and trans-Azo units through hostguest interactions. After ultraviolet irradiation (l 365 nm), the trans-form isomerizes into the

cis-form, decreasing the number of noncovalent crosslinks as the inclusion complexes between aCD and the Azo units decompose, causing the aCDAzo gels to expand. However, after subsequent Vis irradiation (l 430 nm) the trans-form recovers, increasing

the number of noncovalent crosslinks and forming inclusion complexes, which causes aCDAzo gels to contract. Thus, the expansioncontraction process of these supramolecular hydrogels depends on the wavelength.

Photoresponsive actuator of aCDAzo gels. We prepared photoresponsive actuators using the expansioncontraction ability of aCDAzo gels. Figure 4a shows a component drawing of an aCDAzo gel actuator where the plate gel is 20 10 12 mm3.

We chose aCDAzo gel(2, 2). Irradiating the plate gel with ultraviolet light (l 365 nm) from the left side bends the gel to

the right, whereas irradiating the bent gel with Vis light (l 430 nm) from the same side for an hour restores the initial

condition (Fig. 4b). Similarly, irradiating the plate gel from the right side causes the gel to bend to the left side, while irradiating with Vis light restores the initial state (Fig. 4c). This bending behaviour can be repeated for at least ve cycles with varying strains. These results demonstrate that aCDAzo gels bend in the opposite direction of the incident light.

In addition, we investigated the inuence of irradiation time (1, 5, 10, 20, 30 and 60 min.) on the amount of the exion angle in

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a b

Ultraviolet Irr.

Vis( = 430 nm)

Vis

Irr.

Xenon

light source

Rod lens

CDAzo gel(2, 2)

Ultraviolet ( = 365 nm)

20 mm

Expansion

Ultraviolet ( = 365 nm)

Ultraviolet Irr.

Vis

Irr.

1~2 mm

10 mm

e f

Clip

Initial state

60 Ultraviolet Dark Vis Dark

Time (min)

UVVisUVVisUVVisUVVisUVVisUVVis

Ultraviolet and Vis irradiation time (min)

Flexion angle ( ) /

Photo Irr.

After irr.

Lateral view

0 0 60 120 180 240

0 0 10 20 30 40 50 60

Ultraviolet irr. 4 min

Ultraviolet irr. 6 min

Ultraviolet irr. 15 min

Vis irr. 2 min

Vis irr. 3 min

Vis irr. 6 min

Start

Figure 4 | Photoresponsive actuator of aCDAzo gel in water. (a) Experimental devices and the size of aCDAzo gel(2, 2) in water.(b) Light irradiation from the left side of aCDAzo gel(2, 2) for an hour. After ultraviolet irradiation, aCDAzo gel(2, 2) bends to the right side. Subsequent irradiation with Vis light restores the initial state. (c) Light irradiation from the right side of aCDAzo gel(2, 2) for an hour. After ultraviolet irradiation, aCDAzo gel(2, 2) bends to the left side. Subsequent irradiation with Vis light restores the initial state. (d) Lateral view of aCDAzo gel(2, 2)

hung with a clip. Flexion angle (y) is dened here. (e) Plots of irradiation time versus the exion angle (y) in aCDAzo gel(2, 2). Blue and red areas denote ultraviolet irradiation and Vis irradiation, respectively. Blank area indicates dark storage without light irradiation. Flexion angle (y) is measured using snapshots. (f) The repeated experiment of aCDAzo gel(2, 2) irradiated with ultraviolet and Vis lights for 5 min. Plots show the correlation between irradiation time and exion angle (y). (g) Light irradiation from the left side of the ribbon-shaped aCDAzo gel(1, 2) for 15 min. After ultraviolet irradiation, aCDAzo gel(1, 2) forms a coil. Subsequent irradiation with Vis light restores the initial state. The colour prole of pictures under Vis light irradiation is adjusted to create clear images. The actual movie is shown in Supplementary Movie 8.

aCDAzo gel(2, 2) (Fig. 4e and Supplementary Figs S9 and S10). Figure 4d denes the exion angle (y), and Supplementary Movies 16 depict the ex behaviour of aCDAzo gel(2, 2). Figure 4e shows the exion angle of aCDAzo gel(2, 2) irradiated with ultraviolet and Vis lights for an hour. The exion angle (y)

becomes saturated after ultraviolet irradiation for about an hour, and does not signicantly decrease upon standing under its own weight for an hour in the dark. Conversely, irradiation with Vis light for an hour restores the bent gel to the initial state and the exion angle decreases. The Vis irradiation time required for the bent gel to return to a at gel is similar to the ultraviolet irradiation time. Figure 4f and Supplementary Movie 7 show the repeated experiment of aCDAzo gel(2, 2) irradiated with ultraviolet and Vis lights for 5 min. The gel plate clearly shows back-and-forth motion depending on the wavelength without

irradiation history. Moreover, to observe the deformation of the gel with irradiation, we prepare a ribbon-shaped aCDAzo gel(1,2). The ribbon-shaped gel turns to a coil by the irradiation of ultraviolet light (l 365 nm) from the left side (Fig. 4g and

Supplementary Movie 8). The coil-shaped gel returns to the ribbon-shaped gel by Vis light irradiation. The ribboncoil transition can be repeated at least ve cycles. These results indicate that photoisomerization of the Azo group is correlated to the ex behaviour of the aCDAzo gel plates. The plate or ribbon gels bend in the opposite direction of the incident light because the surface of the gel plate exposed to ultraviolet light expands in water, but the volume of the surface not exposed to ultraviolet light remains constant, suggesting that the strain deformation between the exposed and unexposed areas creates the ex behaviour of aCDAzo gels.

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DiscussionWe successfully prepared reversible expansioncontraction supramolecular hydrogels and a supramolecular actuator-like articial molecular muscle system consisting of aCDAzo gel.

Although microscale switching of supramolecular complexes by external stimuli is well known, achieving a macroscale mechanical change is difcult. Herein, we demonstrate that an intelligent supramolecular actuator could be formed using a main chain with a sufcient length and an adequate number of guest molecules to generate reversible crosslinks between aCD and the Azo units.

Although various stimuli and functional groups can be used in responsive materials with hostguest complexes, we chose to employ external photostimuli. Photoisomerization of the Azo group alters the volume of the supramolecular hydrogel by controlling the formation of an inclusion complex. Especially, the plate- and ribbon-like gels showed the shapememory properties controlled by photoirradiation.

Supramolecular hydrogels are important to realize soft machines like biological systems. These stimulus-responsive expansioncontraction properties are similar to that of muscle brils, such as sarcomere, which consists of actin laments. Moreover, photoresponsive materials have many general applications, including remotely controlled materials and medical devices. Currently, we aim to achieve a hydrogel system that moves faster and over a larger area. We believe that these stimulus-responsive stretching properties may eventually be used in stents and drug delivery carriers to selectively release drugs. aCDAzo gels may realize photoresponsive embolization application, where photoresponsive aCDAzo gels will be introduced into the vessels around a tumour using catheter techniques, and optical bres will provide the photostimuli. It is hypothesized that the introduced gels will embolize the blood stream in arbitrary vessel positions controlled by photostimuli using optical bres.

Methods

Materials. a-Cyclodextrin (aCD), bCD and gCD were obtained from Junsei Chemical Co., Ltd. Acetone, methanol, triethylamine, tetrahydrofuran (THF), dimethyl sulphoxide, azobisisobutyronitrile (AIBN), N,N0-methylenebis(acrylamide) and acrylamide were obtained from Nacalai Tesque Inc. Acryloyl chloride was obtained from Wako Pure Chemical Industries, Ltd. A highly porous synthetic resin (DIAION HP-20) used for column chromatography was purchased from Mitsubishi Chemical Co., Ltd. Water used for the preparation of the aqueous solutions (except for NMR measurements) was puried with a Millipore Elix 5 system. Other reagents were used without further purication.

Measurements. 1H NMR spectra were recorded at 500 MHz with a JEOL JNM-ECA 500 NMR spectrometer. Chemical shifts were referenced to the solvent values(d 2.49 ppm for DMSO and d 4.79 p.p.m. for HOD). 1H MAS-NMR spectra were

measured at 600 MHz on a VARIAN VNMRJ 600 NMR spectrometer with a sample spinning rate of 1.12B2 kHz and relaxation delay of 10 s at 30 1C. The solid-state 1H

FGMAS (Field Gradient Magic Angle Spinning) NMR spectra were recorded at 400 MHz with a JEOL JNM-ECA 400 NMR spectrometer. Sample spinning rate was 10 kHz. Chemical shifts were referenced to adamantane as an external standard(d 1.91 ppm). The infrared spectra were measured using a JASCO FT/IR-410

spectrometer. The ultravioletVis absorption spectra were recorded with a JASCO V-650 and a Hitachi U-4100 spectrometer in water with a 1-mm quartz cell at room temperature. Dynamic viscoelasticity and mechanical properties of the gel were measured using an Anton Paar MCR301 rheometer and mechanical tension testing system (Rheoner, RE-33005, Yamaden Ltd.), respectively.

Photoisomerization. Azo moieties were isomerized by photoirradiation using a 300 W Xenon lamp (Asahi spectra MAX-301) equipped with suitable mirror modules (ultraviolet mirror module, l 250385 nm; Vis mirror module,

l 385740 nm) as a function of irradiation wavelength. Moreover, to extract

specic wavelength, a band-pass lter (LX0365) for ultraviolet light and a bandpass lter (LX0430) for visible light were put on Xenon lamp. The intensities of transmitted ultraviolet light through the band-pass lters (LX0365) using a suitable mirror module is similar for that of Vis light (l 430 nm) using the band-pass

lter (LX0430). The distance between the sample cell and the lamp was xed at 10 cm.

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Acknowledgements

We thank S. Adachi (Osaka University) for his support with the 2D-NMR experiments. This work was nancially supported by the Core Research for Evolutional Science and Technology programme of the Japan Science and Technology Agency, Japan.

Author contributions

A. Harada and Y.T. conceived and directed the study, contributed to all experiments and wrote the paper. S.H., M.O., M.N. and T.K. performed syntheses, characterizations and

spectroscopic studies. A. Hashidzume and H.Y. contributed to the result disscussion.A.Harada oversaw the project as well as contributed to the execution of the experiments and interpretation of the results.

Additional information

Supplementary Information accompanies this paper on http://www.nature.com/naturecommunications

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Competing nancial interests: The authors declare no competing nancial interests.

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How to cite this article: Takashima, Y. et al. Expansioncontraction of photoresponsive articial muscle regulated by hostguest interactions. Nat. Commun. 3:1270 doi: 10.1038/ ncomms2280 (2012).

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/

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The development of stimulus-responsive polymeric materials is of great importance, especially for the development of remotely manipulated materials not in direct contact with an actuator. Here we design a photoresponsive supramolecular actuator by integrating host-guest interactions and photoswitching ability in a hydrogel. A photoresponsive supramolecular hydrogel with α-cyclodextrin as a host molecule and an azobenzene derivative as a photoresponsive guest molecule exhibits reversible macroscopic deformations in both size and shape when irradiated by ultraviolet light at 365 nm or visible light at 430 nm. The deformation of the supramolecular hydrogel depends on the incident direction. The selectivity of the incident direction allows plate-shaped hydrogels to bend in water. Irradiating with visible light immediately restores the deformed hydrogel. A light-driven supramolecular actuator with α-cyclodextrin and azobenzene stems from the formation and dissociation of an inclusion complex by ultraviolet or visible light irradiation.

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Title

Expansion-contraction of photoresponsive artificial muscle regulated by host-guest interactions

Author

Takashima, Yoshinori; Hatanaka, Shogo; Otsubo, Miyuki; Nakahata, Masaki; Kakuta, Takahiro; Hashidzume, Akihito; Yamaguchi, Hiroyasu; Harada, Akira

Pages

1270

Publication year

2012

Publication date

Dec 2012

Publisher

Nature Publishing Group

e-ISSN

20411723

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.1038/ncomms2280

ProQuest document ID

1247499097

Expansion-contraction of photoresponsive artificial muscle regulated by host-guest interactions

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