Journal of Materials Chemistry BMaterials for biology and medicinewww.rsc.org/MaterialsB

ISSN 2050-750X

COMMUNICATIONWei Hong, Zi Liang Wu et al.Programmed planar-to-helical shape transformations of composite hydrogels with bioinspired layered fibrous structures

Volume 4 Number 44 28 November 2016 Pages 7043–7170

This journal is©The Royal Society of Chemistry 2016 J. Mater. Chem. B, 2016, 4, 7075--7079 | 7075

Cite this: J.Mater. Chem. B, 2016,

4, 7075

Programmed planar-to-helical shapetransformations of composite hydrogels withbioinspired layered fibrous structures†

Zhi Jian Wang,a Chao Nan Zhu,a Wei Hong,*b Zi Liang Wu*a and Qiang Zhenga

Self-shaping materials have attracted tremendous interest due to

their promising applications in soft robotics, and flexible electro-

nics, etc. In this field, a crucial issue is how to construct complex yet

elaborate structures in active materials. Here, we present the fabrication

of composite hydrogels with both in-plane and out-of-plane struc-

tural gradients by multi-step photolithography and the resulting

controllable deformations. A patterned gel with a layered fibrous

structure like bean pod is developed, which shows programmed

deformations from a flat shape to a twisted helix. The parameters of

the helix can be deliberately tuned. This approach enables patterning

different responsive polymers in specific regions of composite gels,

leading to multiple shape transformations under stimulations. The

controllability of intricate structures, together with tunable responses

of localized gels, facilitates the generation of complex internal

stresses and three-dimensional deformations of composite gels

toward specific applications.

Adaptive materials with programmable deformations haveattracted increasing attention due to their promising applicationsin actuators, morphing spacecrafts, flexible electronics, bio-medical devices, etc.1 To develop such materials and devices,natural organisms with smart deformations have been a sourceof inspiration.2 For example, the leaves of a Venus flytrap snapshut to catch insects via a controlled fluid flow between the innerand outer surfaces of a leaf.2a Wheat awns, pinecones, and beanpods show hydration-triggered deformations, which are crucialfor seed dispersal.2c,d These deformations of plant organs rangingfrom bending to twisting are closely related to the specific fibrousand gradient structures. The stiff cellulose fibrils at specificorientations lead to localized anisotropic swelling or contraction,

meanwhile a gradient structure facilitates the buildup of internalstress, which promotes bending deformations. In the bean pod ofBauhinia variegata, there is a bilayer structure of cellulose fibrilswith orientations �451 to the long axis of the pod.2c After desicca-tion, each layer shrinks perpendicularly to the fiber orientation,resulting in a spontaneous saddle-like curvature. As a consequence,the bean pod opens its valves by changing shape to form twotwisted helices with opposite handedness.

Inspired by the smart deformations of natural systems, manyefforts have been made to develop artificial materials withbiomimetic structures and programmable deformations.3,4 Toconstruct a system with controllable shape changing, hydrogelis an ideal material due to its drastic changes in volume underexternal stimuli and its beneficial open network that forms anintegrated structure by post reaction in a preformed matrix. Thecentral issue when achieving programmed deformations of gelsis how to construct an elaborate heterogeneous structure, whichis closely related to the tunable swelling mismatch and internalstress. In a seminal work, Hu et al. developed gels with a bilayerstructure, in which a responsive gel sheet is bound to a non-responsive one.5 Under stimulation, the gels undergo differentialswelling or shrinking, leading to bending or folding deformationsin the composite gels.6 In recent years, researchers found that anin-plane heterogeneous structure also leads to programmed internalstresses and three-dimensional (3D) deformations of hydrogels.7–9

For example, planar gels patterned with different compositionsor cross-linking densities deform into 3D shapes.8–10 However,the gels should be very thin, especially when the driven force isnot very large; this is because thinner gels favor buckling/bendingat larger amplitudes.7b,11 Combining the two typical strategies toconstruct composites with both in-plane and out-of-plane structuralgradients should fuse their superiorities and afford complicated yetprogrammable 3D deformations, which are not attainable by eitherconventional strategy.

Here, we demonstrate the fabrication of composite gels withcomplex gradient structures by multi-step photolithography,which show programmed twisting, bending, and shape transfor-mations under specific stimulations. A representative structure,

a MOE Key Laboratory of Macromolecular Synthesis and Functionalization,

Department of Polymer Science and Engineering, Zhejiang University, Hangzhou,

310027, China. E-mail: wuziliang@zju.edu.cnb Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA.

E-mail: whong@iastate.edu

† Electronic supplementary information (ESI) available: Experimental details,photos of composite gels with 3D shapes, and movies of the shape transformations.See DOI: 10.1039/c6tb02178f

Received 25th August 2016,Accepted 19th September 2016

DOI: 10.1039/c6tb02178f

www.rsc.org/MaterialsB

Journal ofMaterials Chemistry B

COMMUNICATION

Publ

ishe

d on

20

Sept

embe

r 20

16. D

ownl

oade

d by

Iow

a St

ate

Uni

vers

ity o

n 27

/01/

2017

17:

45:4

6.

View Article OnlineView Journal | View Issue

7076 | J. Mater. Chem. B, 2016, 4, 7075--7079 This journal is©The Royal Society of Chemistry 2016

which mimics the bilayer structure of cellulose fibrils withdifferent orientations in a bean pod, is constructed in thepatterned hydrogel. In each layer, alternating parallel gel stripeshave different compositions, corresponding to different responses.Under stimulation, internal stresses are built simultaneously inthe plane of each layer and in the thickness direction, leadingto a local saddle-like curvature and twisting deformation of theintegrated gel. By tuning the dimensions of the gels and thecross angle of the layered fibrils, we can control the final 3Dconfigurations of the gels, ranging from rolls, cylindrical helix,to twisted helix. In addition, different kinds of polymers can bepatterned in different layers or regions, facilitating transforma-tions between different 3D shapes. These results demonstratethe deliberate controllability of the complex microstructure and3D deformations of hydrogels, which should be suitable for othersoft materials and facilitate the development of soft robotics,actuators, and flexible electronics, etc.

The layered fibrous composite gel is fabricated via three-stepphotopolymerization, as shown in Fig. 1. A precursor solution ispoured into a reaction cell consisting of two substrates separatedby a 1 mm spacer. Photopolymerization is guided by a mask withprinted black stripes, resulting in the formation of a patternedsingle network (SN) gel at the light-exposed regions. After open-ing the cell and removing the residual solution, the patterned gelis selectively left on the upper glass substrate. Following a similarprocess, another patterned SN gel is prepared. The two substrateswith patterned gels are assembled face to face with 2 mm spacingand followed by injection of another precursor solution into theinterspace. After the reagents diffuse into the gel matrix, the thirdpolymerization, which produces an integrated hydrogel with alocalized interpenetrating network (IPN) structure, is performedwithout a photo mask. If we use the mask with parallel blackstripes to pattern the poly(acrylic acid) (PAA) gels and injectN-isopropylacrylamide (NIPAm) precursor solution, a compo-site hydrogel with a layered fibrous structure is obtained, which

contains patterned PAA/PNIPAm IPN gel stripes and PNIPAmSN gel stripes (see Experimental details in the ESI†). The IPNgel stripes become whitish at room temperature due to theformation of a hydrogen bonded complex between PAA andPNIPAm.12

A gel strip with the biomimetic structure of the bean pod isextracted from the preformed bulk composite gel; the structuraldetails are shown in Fig. 2a. After swelling the composite gel inpure water, it shows a slightly twisted deformation. Variation inpH from 7 to 1 does not evidently influence the 3D configurationof the patterned gel. However, if the gel is transferred from theacidic solution to a basic one at pH = 9, it gradually deforms intoa compact, right-handed twisted helix with 2.5 turns after 30 min(Fig. 2b and c). The response speed, which is related to thediffusion of molecules in and out of the gel matrix, can beincreased by decreasing the gel thickness or increasing the gelporosity.8a,13 Since the deformation is triggered by stimuli-induced swelling mismatch and the resultant internal stress,the shape transformation is reversible when the pH is switchedbetween 9 and 1 (Fig. 2d, Movies S1 and S2, ESI†).

The mechanism of twisting deformation has been wellanalyzed by Sharon and his co-workers on the opening of chiralseed pods.2c When the layered composite sheet intends to bendin two opposite and perpendicular directions, it compromises asaddle-like configuration with a negative Gaussian curvature,leading to the twisting of the slender sheet. In our system, theactive gel is the PAA/PNIPAm IPN stripes, which have a differentswelling ratio, S, and modulus, E, at different pH values, whereasthe PNIPAm SN gel stripes are nonresponsive to pH (Table 1). AtpH = 9, the carboxylic acid group of PAA becomes deprotonated,and destructs the hydrogen bonds between the amide groups ofPNIPAm and the carboxyl groups of PAA;12c as a consequence, thePAA/PNIPAm gel swells and produces internal stress along thedirection of stripes in each layer. The orthogonal internal stressesin different layers deform the composite gel into a twisted helix.

Fig. 1 Schematic for the fabrication of composite hydrogels with a layered fibrous structure and programmed deformations. A precursor solution in thereaction cell (a) is photolithographically patterned to produce gel stripes at light-exposed regions (b). Two samples with patterned gel stripes adhered onthe substrate are assembled face to face (c) and followed by injecting another precursor solution into the interspace (d). After UV light irradiation,a composite gel is obtained (e). A slender gel strip with a layered fibrous structure is cut at a specific angle from the composite gel (f), which deforms intoa twisted helix under stimulation due to the different responses of gel stripes and the resultant internal stresses (g).

Communication Journal of Materials Chemistry B

Publ

ishe

d on

20

Sept

embe

r 20

16. D

ownl

oade

d by

Iow

a St

ate

Uni

vers

ity o

n 27

/01/

2017

17:

45:4

6.

View Article Online

This journal is©The Royal Society of Chemistry 2016 J. Mater. Chem. B, 2016, 4, 7075--7079 | 7077

At pH = 1, the ionization of PAA is depressed, leading tothe recovery of robust hydrogen bonds between PNIPAm andPAA, as well as the volume of IPN stripes; therefore, themismatch of S between SN and IPN gel stripes is minimized,and the composite gel relaxes to an approximately planarconfiguration.

The 3D shapes can be mediated by tuning the dimensions andpatterned structures of the gels (Fig. 3 and Fig. S1, ESI†). Asexpected, the turns of the twisted helix increase with the length ofthe composite gel, whereas the pitch remains constant (Fig. 3a).However, the width of the composite gel affects both the turnsand pitch of the twisted helix (Fig. 3b). As the width increasesfrom 0.2 to 0.6 cm, the turns slightly increase from 3 to 3.4;they quickly decrease to 2 when the width increases to 0.8 cm.A further increase in width up to 1.8 cm leads to a slight decreasein turns. The pitch of the helix shows an opposite trend. Thecutting angle, a, also affects the 3D configurations of the compo-site gels (Fig. 3c). For rectangular gel samples with relatively largeaspect ratios, the swelling mismatch in the gel stripes exhibitsdifferent effectiveness in bending the gel. In the lateral direction,the mismatch stress is largely relaxed at the free edges. Therefore,the swelling mismatch causes more significant bending (or a

smaller radius of curvature) when the activated stripes are parallelto the longitudinal direction of the sample. Such a difference givesrise to the variety of buckling patterns observed. First of all, whenthe stripes on one side of the gel are mostly along the longitudinaldirection and bend significantly, and those on the other side showalmost no bending, the gel deforms into a roll or a cylindricalhelix with zero Gaussian curvature, as shown in Fig. 3c at thecutting angle a close to 451. On the other hand, if the stripes oneither side of the gel form a similar angle with the longitudinaldirection, the gel will deform into a twisted helical shape witha non-zero Gaussian curvature, as in the cases of cutting anglesclose to 01 or 901. Moreover, the chirality of the helices formed isdetermined by the orientation of the extending stripes relative tothe longitudinal direction of a sample. When viewed atop theextending stripes, a right-handed helix is formed if the stripesform an acute angle on the clockwise side of the longitudinaldirection, as shown in Fig. 3c at a cutting angle a A (01, 451); anda left-handed helix if the stripes form an acute angle with thelongitudinal line on the counterclockwise side, a A (451, 901).Another factor that affects the configuration of the helix is thecross angle, b, between the pattern stripes at different layers(Fig. 3d). The most compact twisted helix with a pitch of 2.4 cmis formed when b = 451. At relatively large cross angles, the stripeson both sides are more aligned in the transverse direction of thegel sample, and are thus less effective in bending. Therefore, thehelix pitch is higher than b = 451. At relatively small cross angles,on the other hand, the stripes on two sides of the gel tend tobend the gel in opposite directions, even though the longitudinalalignment facilitates bending. As a result, the helices also exhibithigher pitches. A symmetric distribution about b was also

Fig. 2 pH triggered reversible planar-to-helical shape transformation ofcomposite gels. (a) Schematic for the structural details of the as-preparedcomposite gel. (b) Digital photos to show the shape of the gel at differenttimes after switching the pH from 1 to 9. Scale bar: 1 cm. (c and d) Variationin the number of turns with the evolution time (c) and following therepetitive swelling of the gel in solutions with a pH of 1 and 9 (d).

Table 1 Swelling ratio in length, S, and Young’s modulus, E, of the SN andIPN gels

Gels

Sa E (kPa)

pH = 1 pH = 9 pH = 1 pH = 9

PAA/PNIPAm 0.99 � 0.01 1.31 � 0.05 7100 � 60 1900 � 40PNIPAm 1.02 � 0.01 1.05 � 0.02 97 � 8 87 � 12PAA 0.94 � 0.01 1.35 � 0.03 111 � 6 26 � 4P(VI-co-AAm) 1.67 � 0.06 1.15 � 0.03 9.3 � 1 43 � 2

a The swelling ratio in length was measured by S = D/D0, in which D and D0are the diameter of disc-shaped gels in the equilibrated and as-preparedstates, respectively.

Fig. 3 Influence of gel dimensions and microstructure on the final 3Dconfigurations of the composite gels. (a) Gel length; (b) gel width; (c)cutting angle; (d) cross angle between the stripes in different layers. Insetsare the representative images of gels with specific configurations; scalebar: 1 cm. The left-handed helices in (c) have a negative pitch.

Journal of Materials Chemistry B Communication

Publ

ishe

d on

20

Sept

embe

r 20

16. D

ownl

oade

d by

Iow

a St

ate

Uni

vers

ity o

n 27

/01/

2017

17:

45:4

6.

View Article Online

7078 | J. Mater. Chem. B, 2016, 4, 7075--7079 This journal is©The Royal Society of Chemistry 2016

observed. When b = 301 and 601, the gels form helices with asimilar pitch of B3 cm. When b = 15 and 751, the gels formhelices with a similar pitch of B4.4 cm. The width and spacingof the patterned stripes, as well as the composition and thicknessof the gels, should also affect the 3D shapes of the gels.

Another advantage of this method is patterning of differentpolymers in one integrated gel. The different responsive polymersenable selective actuation and specific 3D deformation. Therefore,the same patterned gel exhibits different configurations in differentconditions. As shown in Fig. 4, the bottom layer consists ofalternating stripes of the PAA gel and PNIPAm gel, while the upperlayer consists of alternating stripes of the poly(1-vinylimidazole-co-acrylamide) (P(VI-co-AAm)) gel and PNIPAm gel.14 The PAA gelswells in a basic solution (pH = 9), whereas the P(VI-co-AAm)gel swells in an acidic solution (pH = 1) (Table 1). Therefore,after swelling the gel in pH = 9, the swelling of PAA gel stripesdrives the deformation of the bottom layer into a right-handedcylindrical helix, of which the outside surface is the bottomlayer with PAA stripes. After switching the pH from 9 to 1, thePAA gel stripes relax, but the P(VI-co-AAm) gel stripes in theupper layer swell and drive the composite gel to deform. Whenthe P(VI-co-AAm) gel stripes in the upper layer pass with anangle, y, of �451 to the long axis of the composite strip, the gelswitches its chirality and deforms into a left-handed cylindricalhelix (Fig. 4a and Movie S3, ESI†). When the angle between thePAA and P(VI-co-AAm) stripes is relatively large, such as y = 901or 451, the composite gel deforms into rolls or cylindricalhelices, respectively (Fig. 4b and c, Movies S4 and S5, ESI†).In these shapes at pH = 1, the layer with the P(VI-co-AAm)stripes becomes the outside surface of the helices or rolls; thestructure is flipped but the chirality is maintained.

During the shape transformation between two right-handedcylindrical helices after switching the pH from 9 to 1 (Fig. 4c), weobserved a transient twisted helix structure (Movie S4, ESI†). At

pH = 1, the PAA gel stripes gradually contract to their originalstate, so that the internal stress in that layer weakens. At the sametime, the P(VI-co-AAm) gel stripes swell, and internal stress startsto build in the other layer. Therefore, a frustrated internal stressexists at the medium stage, in which a twisted helix formswith a mechanism identical to the patterned gel with the samePAA/PNIPAm IPN stripes (Fig. 2). After swelling for a long time,the internal stress in the layer with the PAA gel stripes weakensfurther, yet the internal stress in the layer with the P(VI-co-AAm)gel stripes enhances further and dominates the deformation ofthe integrated gel into a cylindrical helix. Although the handednessof the helix at pH = 1 is the same as that at pH = 9, the outsidesurfaces of the helices are different (Fig. 4c), indicating thedifferent driving forces (Fig. S2, ESI†).

We should note that the programmable deformations describedabove rely on the elaborate controllability of the gel structurewith both in-plane and out-of-plane gradients by patterningan isotropic gel network. Intricate structures have also been con-structed in soft materials by two-photon laser scanning lithographyand directed assembly of anisotropic units.15,16 The former methodenables precise fabrication of complex structures, which are crucialfor tissue engineering.14 However, the time required will dramati-cally increase with the dimension of the structures. The latterapproach can deliberately tune the local orientation of anisotropicplatelets by magnetic field or liquid crystalline mesogens by amodified substrate prior to gelation; the resultant materials showcontrollable 3D deformations.16,17 However, it is not easy to createcomplex in-plane structural gradients on a macroscopic scale usingthese techniques. Therefore, our method is advantageous inthe sophisticated fabrication of complex structures in isotropichydrogels toward programmable deformations.

In conclusion, we have demonstrated the construction of abiomimetic layered fibrous structure in hydrogels and the resul-tant controllable 3D deformations. As a representative example,we constructed parallel gel stripes in the two layers at�451 to thelong axis of the composite gel, which deform into a twisted helixunder stimulation, similar to the phenomenon found in theopening of chiral bean pods. By tuning the dimensions of thecomposite gel, the cutting angle, and the cross angle of patternedstripes within different layers, we can control the parameters ofthe twisted helices and even obtain cylindrical helices and rolls.In addition, shape transformations are observed in compositegels patterned with different polymers in the upper and bottomlayers under different stimulations. By multi-step photolitho-graphy, we can construct both in-plane and out-of-plane gradientstructures with different responsive polymers, which facilitatesthe control of internal stresses and 3D deformations of the gels.The construction of bioinspired heterogeneous structures of softmaterials should facilitate testing the deformation mechanismand designing other soft actuators and biomedical devices. Tofurther the development of bioinspired materials and soft actuators,efforts can be devoted to several aspects: (i) combining differenttypes of deformations via structural control, (ii) decreasing the sizeand increasing the resolution of the structure for micro-actuatorswith a faster response, and (iii) using contactless stimuli such aslight to actuate the deformation and locomotion.

Fig. 4 Shape transformations of composite hydrogels with differentresponsive polymers patterned in the upper and bottom layers. The green,brown, and gray stripes in the schemes correspond to PAA, P(VI-co-AAm),and PNIPAm gels. When pH = 9, the swelling of the PAA gel at the bottomlayer drives the composite gels to form a right-handed helix. When the pHis switched to 1, the swelling of the P(VI-co-AAm) gel at the upper layerdrives the deformation of gels into a left-handed helix (a), rolls (b), and aright-handed helix (c). Scale bar: 1 cm.

Communication Journal of Materials Chemistry B

Publ

ishe

d on

20

Sept

embe

r 20

16. D

ownl

oade

d by

Iow

a St

ate

Uni

vers

ity o

n 27

/01/

2017

17:

45:4

6.

View Article Online

This journal is©The Royal Society of Chemistry 2016 J. Mater. Chem. B, 2016, 4, 7075--7079 | 7079

Acknowledgements

This research was supported by the National Natural ScienceFoundation of China (51403184), the Scientific Research Foun-dation for the Returned Overseas Chinese Scholars (J20141135),the Natural Science Foundation of Zhejiang Province, China(Y14E030021), and the Fundamental Research Funds for theCentral Universities of China.

Notes and references

1 (a) Y. Liu, J. Genzer and M. D. Dickey, Prog. Polym. Sci., 2016,52, 79–106; (b) J. S. Randhawa, T. G. Leong, N. Bassik, B. R.Benson, M. T. Jochmans and D. H. Gracias, J. Am. Chem. Soc.,2008, 130, 17238–17239; (c) J. Kim, J. Yoon and R. C. Hayward,Nat. Mater., 2010, 9, 159–164.

2 (a) Y. Forterre, J. M. Skotheim, J. Dumais and L. Mahadevan,Nature, 2005, 433, 421–425; (b) P. Fratzl and F. G. Barth, Nature,2009, 462, 442–448; (c) S. Armon, E. Efrati, R. Kupferman andE. Sharon, Science, 2011, 333, 1726–1730; (d) M. J. Harrington,K. Razghandi, F. Ditsch, L. Guiducci, M. Rueggeberg,J. W. C. Dunlop, P. Fratzl, C. Neinhuis and I. Burgert, Nat.Commun., 2011, 2, 337; (e) Z. Liu, S. Swaddiwudhipong andW. Hong, Soft Matter, 2013, 9, 577–587.

3 (a) R. Kempaiah and Z. Nie, J. Mater. Chem. B, 2014, 2,2357–2368; (b) L. Ionov, Adv. Funct. Mater., 2013, 23,4555–4570; (c) A. R. Studart, Angew. Chem., Int. Ed., 2015, 54,3400–3416; (d) E. Palleau, D. Morales, M. D. Dickey andO. D. Velev, Nat. Commun., 2013, 4, 2257; (e) C. Ma, T. Li,Q. Zhao, X. Yang, J. Wu, Y. Luo and T. Xie, Adv. Mater., 2014, 26,5665–5669; ( f ) A. S. Gladman, E. A. Matsumoto, R. G. Nuzzo,L. Mahadevan and J. A. Lewis, Nat. Mater., 2016, 15, 413–418.

4 (a) K.-U. Jeong, J.-H. Jang, D.-Y. Kim, C. Nah, J. H. Lee,M.-H. Lee, H.-J. Sun, C.-L. Wang, S. Z. D. Cheng and E. L.Thomas, J. Mater. Chem., 2011, 21, 6824–6830; (b) M. Podgorski,D. P. Nair, S. Chatani, G. Berg and C. N. Bowman, ACS Appl.Mater. Interfaces, 2014, 6, 6111–6119.

5 Z. B. Hu, X. M. Zhang and Y. Li, Science, 1995, 269, 525–527.6 (a) G. Stoychev, S. Zakharchenko, S. Turcaud, J. W. C.

Dunlop and L. Ionov, ACS Nano, 2012, 6, 3925–3934;(b) N. Bassik, B. T. Abebe, K. E. Laflin and D. H. Gracias,Polymer, 2010, 51, 6093–6098; (c) T. S. Shim, S.-H. Kim,C.-J. Heo, H. C. Jeon and S.-M. Yang, Angew. Chem., Int. Ed.,2012, 51, 1420–1423; (d) S. Liu, G. Gao, Y. Xiao and J. Fu,J. Mater. Chem. B, 2016, 4, 3239–3246.

7 (a) Y. Klein, E. Efrati and E. Sharon, Science, 2007, 315,1115–1120; (b) E. Sharon and E. Efrati, Soft Matter, 2010, 6,

5693–5704; (c) S. Armon, H. Aharoni, M. Moshe andE. Sharon, Soft Matter, 2014, 10, 2733–2740.

8 (a) J. Kim, J. A. Hanna, M. Byun, C. D. Santangelo andR. C. Hayward, Science, 2012, 335, 1201–2105; (b) J. Kim,J. A. Hanna, R. C. Hayward and C. D. Santangelo, SoftMatter, 2012, 8, 2375–2381; (c) M. Byun, C. D. Santangeloand R. C. Hayward, Soft Matter, 2013, 9, 8264–8273.

9 (a) Z. L. Wu, M. Moshe, J. Greener, H. Therien-Aubin, Z. Nie,E. Sharon and E. Kumacheva, Nat. Commun., 2013, 4, 1586;(b) H. Therien-Aubin, Z. L. Wu, Z. Nie and E. Kumacheva,J. Am. Chem. Soc., 2013, 135, 4834–4839; (c) H. Therien-Aubin, M. Moshe, E. Sharon and E. Kumacheva, Soft Matter,2015, 11, 4600–4605.

10 (a) R. Takahashi, Z. L. Wu, M. Arifuzzaman, T. Nonoyama,T. Nakajima, T. Kurokawa and J. P. Gong, Nat. Commun.,2014, 5, 4490; (b) Y. Zhang and L. Ionov, Langmuir, 2015, 31,4552–4557.

11 Z. Wei, J. Zheng, J. Athas, C. Wang, S. R. Raghavan, T. Li andZ. Nie, Soft Matter, 2014, 10, 8157–8162.

12 (a) D. J. Eustace, D. B. Siano and E. N. Drake, J. Appl. Polym.Sci., 1988, 35, 707–716; (b) L.-Y. Chu, Y. Li, J.-H. Zhu andW.-M. Chen, Angew. Chem., Int. Ed., 2005, 44, 2124–2127;(c) S. Y. Yang and M. F. Rubner, J. Am. Chem. Soc., 2002, 124,2100–2101.

13 (a) R. Luo, J. Wu, N.-C. Dinh and C.-H. Chen, Adv. Funct.Mater., 2015, 25, 7272; (b) S. Jiang, F. Liu, A. Lerch, L. Ionovand S. Agarwal, Adv. Mater., 2015, 27, 4865.

14 Here we constructed PAA SN gel stripes instead of PAA/PNIPAm IPN ones in the bottom layer, because the PAA gelchanges its dimension and modulus with similar extent tothose of P(VI-co-AAm) gel. The modulus of PAA/PNIPAm IPNgel is much higher than that of P(VI-co-AAm) gel, whichhampers the deformation triggered by the swelling ofP(VI-co-AAm) gel stripes.

15 (a) M. B. Hahn, J. S. Miller and J. L. West, Adv. Mater., 2005,17, 2939–2942; (b) A. M. Kloxin, A. M. Kasko, C. N. Salinasand K. S. Anseth, Science, 2009, 324, 59–63.

16 (a) R. M. Erb, J. S. Sander, R. Grisch and A. R. Studart, Nat.Commun., 2013, 4, 1712; (b) J. J. Martin, B. E. Fiore andR. M. Erb, Nat. Commun., 2015, 6, 8641; (c) D. Kokkinis,M. Schaffner and A. R. Studart, Nat. Commun., 2015, 6, 8643.

17 (a) Y. Sawa, F. Ye, K. Urayara, T. Takigawa, V. Gimenez-Pinto, R. L. B. Selinger and J. V. Selinger, Proc. Natl. Acad.Sci. U. S. A., 2011, 108, 6364–6368; (b) K. M. Lee, T. J.Bunning and T. J. White, Adv. Mater., 2012, 24,2839–2843.

Journal of Materials Chemistry B Communication

Publ

ishe

d on

20

Sept

embe

r 20

16. D

ownl

oade

d by

Iow

a St

ate

Uni

vers

ity o

n 27

/01/

2017

17:

45:4

6.

View Article Online