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Various shape memory effects of stimuli-responsive shape memory polymers

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2013 Smart Mater. Struct. 22 093001

(http://iopscience.iop.org/0964-1726/22/9/093001)

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IOP PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 22 (2013) 093001 (23pp) doi:10.1088/0964-1726/22/9/093001

TOPICAL REVIEW

Various shape memory effects ofstimuli-responsive shape memorypolymers

Harper Meng1, Habib Mohamadian1, Michael Stubblefield1,Dwayne Jerro1, Samuel Ibekwe1, Su-Seng Pang2 and Guoqiang Li1,2

1 Department of Mechanical Engineering, Southern University, Baton Rouge, LA 70813, USA2 Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge,LA 70803, USA

E-mail: lguoqi1@lsu.edu

Received 25 March 2013, in final form 17 July 2013Published 13 August 2013Online at stacks.iop.org/SMS/22/093001

AbstractOne-step dual-shape memory polymers (SMPs) recover their original (permanent) shape uponsmall variation of environmental conditions such as temperature, electric field, light, magneticfield, and solvent/chemicals. For advanced applications such as aerospace and medicaldevices, complicated, multiple-step, spatially controllable, and two-way shape memory effects(SMEs) are required. In the past decade, researchers have devoted great effort to improve theversatility of the SME of SMPs to meet the needs of advanced applications. This paper isintended to review the up-to-date research endeavors on advanced SMEs. The problems facingthe various SMPs are discussed. The challenges and opportunities for future research arediscussed.

(Some figures may appear in colour only in the online journal)

Contents

1. Introduction 2

2. One-step SME 3

3. Stepwise SME 4

3.1. Stepwise SME with well-separated thermaltransitions 4

3.2. Stepwise SME with a broad switching transitionrange 6

3.3. Stepwise SME by selective stimulation 8

4. Two-way shape-changing effect (SCE) 9

4.1. Semicrystalline SMPs under constant stress 10

4.2. Thermal-responsive liquid crystal elastomers 10

4.3. Hybrid materials 10

4.4. Azobenzene-based liquid crystal elastomers 11

4.5. Azobenzenes/inorganic nanoporous membranes12

4.6. Anthracene-based polymers 134.7. Coumarin-based polymers 134.8. Cinnamate-based polymers 134.9. Photoactive effect based on photothermal effect 144.10. Photoactive effect based on addition–fragmentation

chain transfer reaction 144.11. Summary remarks 15

5. Future research trends 155.1. Fabrication of stepwise, controllable, spatial,

predictable, and two-way SMP 155.2. Modeling of stepwise, spatial, controllable, and

two-way SME 165.3. High-performance SMPs 165.4. Athermal-stimuli triggered SME 16

10964-1726/13/093001+23$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Smart Mater. Struct. 22 (2013) 093001 Topical Review

5.5. Complicated, stepwise, spatial, controllable,and two-way SMP composites 17

5.6. Microminiaturization of SMPs 175.7. Application-oriented research 17

6. Conclusions 17Acknowledgments 17References 17

Abbreviations

9-AC 9-anthracene carboxylic acidbOCL Branched oligo(ε-caprolactone)CD CyclodextrinCHMA CyclohexylmethacrylateCNT Carbon nanotubeIPN Interpenetrating polymer networkNIPA N-isopropylacrylamidePCL Poly(caprolactone)PCLDMA Poly(caprolactone)dimethacrylatePEGMA Poly(ethylene glycol)mono-methylether-mono-

methacrylatePOSS Polyhedral oligomeric silsesquioxaneSCE Shape-changing effectSCP Shape-changing polymerSME Shape memory effectSMP Shape memory polymerSMPU Shape memory polyurethaneTg Glass transition temperatureTm Melting transition temperatureXbOCL Cross-linked branched oligo(ε-caprolactone)

1. Introduction

As one of the major driving forces for industries, newmaterials lay a solid foundation for novel products.The last decade has seen remarkable advances in smartmaterials, which will come to play a significant rolein all areas of human life. Stimuli-responsive shapememory polymers (SMPs) change their shapes in additionto other properties such as mechanical properties [1],phase separation [2], surface [3], permeability [4], opticalproperties [5], and electrical properties [6], upon smallvariation of environmental conditions such as temperature [7],electric field [8], light [9], magnetic field [10], pH value [11],sonic field [12], solvent ions [13], specific antigen–antibodyinteractions [14], enzymes [15], and glucose [16]. Such smartfunctions are intrinsic to the stimuli-responsive SMPs. Thesesmart materials have significant advantages over mechano-electronic systems in that they do not rely on complicatedsense–response structures of feedback systems [17].

The originally studied SMPs were one-step SMPs mostlywith a glass transition or melting transition as the switchingtransition. These SMPs with one-step shape memory effect(SME) are relatively simple; they can only ‘memorize’their original (or permanent) shape and recover from atemporary shape to their original (or permanent) shape understimulation [18–20]. They are dual-shape memory polymers.

For complicated applications, multiple-step/multiple-shapememory effect, and even two-way SME are required.

In the past decade, researchers have devoted greateffort to improve the versatility of the SME for SMPsto meet the needs of complicated applications, as shownin figure 1. SMPs with one-step SME are programmedthermomechanically [21–24] to achieve and fix a temporaryshape. Later, stimuli which can induce the reversibleswitching transition trigger the recovery of the polymerto its original (permanent) shape. Different from one-stepSME, stepwise SMPs after programming can recover moreshapes in addition to their original permanent shape in astepwise manner [25]. The multiple temporary shapes ofSMPs are stable until an external stimulus is applied. StepwiseSME can result from ‘multiple’ switching transitions ofthe SMPs or selective heating of SMPs. Stepwise SMPsusually require a multiple-step programming process to createthe stepwise SME. For example, Behl and Lendlein [25]introduced the basic mechanism and an appropriate two-step programming process to create two-step SME of amulti-phase polymer network. Two-way SMPs which arealso called shape-changing polymers (SCPs) exhibit two-wayshape-changing effect (SCE). Lendlein [26] first defined SCPsin the ‘Editorial’ of the themed issue ‘Actively MovingPolymers’. He classified actively moving polymers intoSMPs and SCPs. Most SCPs change their shape uponstimulation without the requirement of predeformation orprogramming (certain SCPs also need pre-extension, such astwo-way semicrystalline SMPs) [27–30]. They may recovertheir original shape upon removal of the stimulation, orchanging the amplitude or direction of the stimulation. Thebasic moving of stimuli-responsive polymers can be simplemotions such as elongation and bending determined bythe programming (for one-way SMPs), materials structures,and stimulation. Based on these combined motions, wellcontrolled, stepwise, spatial and complicated moving effectscan be expected.

To make the terminology clear, the various SMEs aredefined as follows. ‘One-step SME’ is a one-step process; onlyone permanent shape can be remembered or recovered duringthe shape recovery process. One-step SME corresponds todual shape (one permanent shape and one temporary shape).‘Stepwise SME’ implies that the shape recovery process isa multiple-step process; in addition to the permanent shape,more temporary shapes can be ‘remembered’ or recoveredduring the stepwise shape recovery process. ‘Stepwise SME’is also referred to as ‘multiple SME’ in many referencesbecause during the shape recovery process, the SMP canundergo more than two distinguished shapes. The ‘StepwiseSME’ can be a result of ‘multiple’ switching transitions ofthe SMPs or selective heating SMPs. One-step and stepwiseSMEs are both one-way SME. ‘SCE’ also called ‘two-waySME or SCE’ is reversible; usually external stress is notrequired to program the polymer.

This paper is intended to review the up-to-date researchendeavors on the different SMEs. The research endeavorsto achieve the complicated and advanced stimuli-responsivemoving effect are summarized in terms of macromolecular

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Figure 1. Schematic representation of stimuli-responsive SMEs which include one-step SME, stepwise SME, and two-way SCE. Based onthe simple motions of bending and shrinkage, well-controlled, stepwise, spatial and complicated moving effects are expected to be achievedby proper materials design, stimulus design, and programming design.

design, macrostructure design, and stimuli design. Thechallenges in this field, such as in design and applicationare discussed. In addition, having introduced the presentachievements in the various stimuli-responsive movingpolymers, future directions for this promising area arediscussed.

2. One-step SME

A polymer stable network and a reversible switchingtransition of the polymer are the two prerequisites for theone-step (dual-shape) memory effect. Figure 2 schematicallyshows the molecular structures of SMPs. The cross-linkingstructure determines the original (or permanent) shape,which can be formed by molecule entanglement [31],crystalline phase [32], chemical cross-linking [33–41],interpenetrated network [42], or even cyclodextrin (CD)polymer inclusion [43, 44]. The yellow and blue blocks inthe network represents the reversible switching structures.During the thermomechanical cycle, first, the one-step SMPsare deformed. If these reversible structures are cross-linked,the deformed polymer cannot recover because of the lowmobility of the macromolecules upon removal of the external

load. Simultaneously, the internal stress produced duringdeformation is stored in the polymer network because of thelow mobility of the macromolecules. Finally, upon stimulationwhich can cleave the reversible structure, the macromoleculesobtain enough mobility; therefore, the polymer recovers itsoriginal shape as a result of releasing the internal stress [45].

Many reversible switching transitions have been suc-cessfully employed as the switching transition of SMPssuch as crystallization/melting transition [46–48], vitri-fication/glass transition [16, 34, 39, 42, 49–78], liq-uid crystal anisotropic/isotropic transition [79–85], re-versible molecule cross-linking reactions, and supramolecularassociation/disassociation. The typical reversible moleculecross-linking reaction used as switching transitions includephotodimerization [9, 86–89], Diels–Alder reaction [90–98],and oxidation/redox reaction of the mercapto group [99]. Thetypical supramolecular reversible association/disassociationwhich act as switching transitions are hydrogen bond-ing [100–108], self-assembly metal–ligand coordination[109, 110], and self-assembly of β-CD [11, 111–115]. Inaddition to the stimuli to directly trigger the switchingtransition, any stimuli which can significantly change themobility of the polymer may trigger the SME, such as

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Figure 2. Molecular structures of SMPs. A stable network and a reversible switching transition are the prerequisites for the SMPs to showSME.

electric field [116–128], magnetic field [129–140], water/solvent/chemical [45, 141–149], light [150–152], and pH [45].

For more details of one-step dual-SMPs, see the reviewpapers [26, 45, 153–167]. Numerous one-step dual-SMPshave been fabricated in labs and several are commer-cially available such as DiAPLEX, Veriflex R©, VerilyteTM,VeritexTM, Tecoflex R©, and TEMBO R©. One-step SMPscan be applied in medical devices [152, 159, 168–181],textiles [160, 182–189], aerospace [190–193], civil engineer-ing [194–200], organic photovoltaic cells [201], shrinkablepackages [202], sensor and actuators [203–206], self-healing [21–23, 77, 196, 207–219], self-deployable structuresin spacecraft [51, 166, 220], MEMs [221–224], self-peelingreversible adhesive [225], and data storage media [226]. Amore complete list of the applications of SMPs can be foundin table 1 in [166].

3. Stepwise SME

3.1. Stepwise SME with well-separated thermal transitions

Different from one-step SME which can only remember itssingle original permanent shape, stepwise SMEs (multipleSMEs) can remember more shapes in addition to its originalshape. Most stepwise SME polymers either have more thanone switching thermal transition or have a single switchingtransition with a broad thermal transition range. It can beeasily understood that if a SMP has well-separated thermaltransitions as the switching transitions, the SMP may showstepwise multiple SME after a multi-step programmingprocess. In addition to thermal transitions, any other switchingtransition listed in figure 2 may act as a switching transitionfor multi-step SME. Proper programming is normally requiredto achieve the multi-step SME [25, 27, 170, 227–229].Stepwise SMPs have many advanced application potentialssuch as in assembly, morphing aircraft, material packaging,releasable fasteners, and many medical applications [25, 27,170, 227–230].

3.1.1. Neat SMPs. Grafting and blocking copoly-mers of different soft segments may induce more thanone well-separated multiple phase in a single SMP.The multiple phases may provide more than one well-separated thermal transition as the shape memory switch-ing transition. Bellin et al [227] first reported a SMPwith two-step triple-SME by copolymerizing poly(ethyleneglycol)mono-methylether-monomethacrylate (PEGMA) withpoly(caprolactone)dimethacrylate (PCLDMA). At a suitablecomposition, the polymer had crystalline PCL domainsand crystalline polyethylene glycol domains, which act asthe two switching phases. By copolymerizing PCLDMAwith cyclohexymethacrylate (CHMA), Bellin et al [231]prepared another SMP showing triple-SME. The meltingtransition of the PCL domains and the glass transition of thecyclohexymethacrylate domains acted as the two switchingtransitions of the triple-SMEs. Chen et al [232] obtained twoseparated melting transitions as the switches in shape memorypolyurethane (SMPU) using two polyols PCL-10000 andpoly(tetramethylene glycol)-2900 with different molecularweights as the soft segment. Paderni et al [233] demonstratedthat the triple-SME can also be triggered by alternatingmagnetic field through non-contact activation in a stepwiseincreasing alternating magnetic field, if iron (III) oxideparticles are added to the polymer [233, 234].

One phase in SMPs from the same soft segmentsmay show two thermal transitions at two temperatures. Forexample, Pretsch [229] observed two-step SME in SMPUby employing the glass transition around −50 ◦C and themelting temperature of soft segment above the ambienttemperature as the two switching transitions. Similarly, Botheet al [235] prepared a star-shaped POSS-polycaprolactone(PCL) polyurethane network. The glass transition at around−40 ◦C and the melting transition at around 50 ◦C of thePCL were employed as the well-separated switching transitionfor the two-step SME. One of the disadvantages of theabove two-step SMPs is that the first switching transitiontemperature is very low, i.e., −40 to −50 ◦C, which is notsuitable for most applications.

Not only can glass and melting transitions be used asthe switching transitions of stepwise SMPs, nematic–isotropic

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Figure 3. (a) The molecular structure of the terpolymer showing two-step triple-SME; (b) the states of the molecular morphology atdifferent temperatures and the two-step shape recovery of the liquid crystal polymer. (Reprinted with permission, copyright ofASC Publications 2010, doi:10.1021/ma101145r.)

transformations of nematic network can also be employed forthis purpose. Qin and Mather [27] developed a glass-forming,polydomain nematic network by cross-linking a thermotropicunsaturated polyester. A glass transition at about 80 ◦Cand nematic–isotropic transitions at 160 ◦C were employedas the two switching temperatures of the triple-SME.Similarly, Ahn et al [81] prepared a side-chain liquid crystalrandom terpolymer network. The molecular structure ofthe terpolymer is shown in figure 3(a). The acrylate endgroup forms cross-linking by curing at 120 ◦C. The polymernetwork had well-separated glass transition temperatureand clearing temperature for the triple-SME. Figure 3(b)shows the two-step shape recovery of the side-chain liquidcrystal polymer networks, corresponding to the states of themolecular morphology at different temperatures. The study byAhn and Kasi [236] on the stepwise liquid crystal polymersshows that a sufficient extent of motional decoupling betweenmesogen-rich and backbone-rich domains is the key factorfor the outstanding stepwise SME in side-chain liquid crystalpolymers.

3.1.2. Polymer blends. Without synthesizing a newpolymer, researchers can create new SMPs with tailorableshape memory properties by blending two polymers[60, 62, 65, 69, 237]. Similarly, by blending differentpolymers, researchers can easily achieve well-separated phasetransition temperatures in a single SMP. For example,Kolesov and Radusch [238] blended linear high-densitypolyethylene, ethylene-1-octene copolymers, and short-chainbranched polyethylenes, and created a SMP blend withmultiple crystallization and melting behavior. Two-step andthree-step shape recovery was observed after two- andaccordingly three-step programming of the blend. Cuevaset al [239] blended and cross-linked two semicrystallinepolymers poly(cyclooctene) and polyethylene and created twowell-separated crystalline domains in the blend. Outstandingtriple-SME was also observed.

3.1.3. Polymer laminates. The second facile methodto create multiple well-separated switching transition islaminating two SMPs which have well-separated phasetransition temperatures. For example, Xie et al [240] prepared

a stepwise SMP with a laminated structure from epoxy ofdifferent glass transition temperatures. The epoxy with a highTg was first cured and then an epoxy with a low Tg wascured on top of the first layer. Bae et al [241] prepared alaminate from SMPUs of different molecular weights withsilica particles acting as multifunctional cross-linkers. Thebilayer formed from the two films exhibited two undisturbedglass transitions and outstanding stepwise SME. One of thegreatest advantages of stepwise shape memory laminate isthat the stepwise SME is tailorable by simply changing thethickness of the two layers.

3.1.4. Other SMP hybrids. In fact, not only blends orlaminates, but any hybrid SMPs with multiple switchingtransitions, may show stepwise SME after proper thermome-chanical programming. For example, Luo and Mather [242]simply impregnated a PCL microfiber mat with shape memoryepoxy resin which was cured later. The elastic modulusprofiles indicated that the composite had two separatedthermal transitions, corresponding to the glass transition of theepoxy and the melting of the PCL mat, respectively. Figure 4shows the two-step triple-SME of the composite.

3.1.5. Programming of stepwise SMEs. Normally, to create astepwise SME, the polymer has to be subjected to a multi-stepprogramming process which corresponds to the multiplephase switching transitions of the SMP [194, 227, 228].During the multiple-step programming, multiple temporaryshapes are consequently fixed at a temperature below therespective switching transition temperatures as shown infigure 5(a) using external load. One-step programmingas shown in figures 5(b) and (c) with a simplifiedprogramming process is more feasible or preferable inmany applications. Several researchers have demonstratedthat multiple-step programming is not a prerequisite formost SMPs to show stepwise SME. The stepwise SMEcreated by one-step programming arises because, duringthe programming, the segments of all the different phasescan be oriented and fixed at the programming temperature.Using a high-temperature one-step programming processshown in figure 5(b), Behl et al [243] created two-steptriple-shape memory capability of a polymer network based

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Figure 4. The sequential recovery of the PCL fiber mat/epoxy composites from a temporary shape (A), to a temporary shape (B), and to apermanent shape (C). (Reprinted with permission, copyright of Wiley 2010, doi:10.1002/adfm.201000052.)

Figure 5. Multiple-step programming and one-step programmingof SMPs to create stepwise SME. (a) Multiple-step programming,(b) high-temperature one-step programming, (c) cold-temperatureone-step programming. Switching transition temperatures:Tn > Tn−1 > Tn−2 · · · > T2 > T1.

on PCL and poly(cyclohexyl methacrylate) at a temperaturebelow the switching transition temperature of both the PCLand poly(cyclohexyl methacrylate) phases. In comparisonwith the high-temperature one-step programming, the cold-temperature one-step programming shown in figure 5(c) doesnot need to heat the polymer to above the highest switchingtemperature. Zotzmann et al [244] showed that a cold-temperature one-step programming process at an ambienttemperature (which was below the switching temperaturesof a copolymer) could induce stepwise shape recovery. Itwas also found that the one-step cold-drawn programmingtemperature did not affect the shape recovery temperature andthe proportioning of the recovery in the two steps. To achieve

better shape controllability of the SMPs during the stepwiseSME, a multiple-step programming is preferred.

For applications such as sealant in expansion joint,SMP needs to be subjected to two-dimensional programming(tension in one direction and compression in the transversedirection) to solve the problem of interfacial debondingand sealant squeezing out of the channel problem insummer [197]. Li et al [194] shows that this simultaneous 2Dprogramming [245] can be replaced by hybrid two-step 1Dprogramming, i.e., tension programming in the longitudinaldirection followed by compression programming in thetransverse direction. They found that, depending on theprestrain level in the two directions, the thermosetting SMPshows some weak triple shape during shape recovery. Theyalso found that the recovery sequence depends more onthe prestrain level, not necessarily the reverse sequence ofprogramming [194].

3.1.6. Summary remarks. Stepwise SMPs with well-separated thermal transitions are capable of fixing morethan one temporary shape and recovering sequentially thetemporary shapes and eventually the permanent shape uponstimulation. Although this stepwise SME is especially usefulfor advanced applications such as smart packaging, robots,and implant devices, two limitations are associated withthis type of stepwise SME. First, to show outstandingstepwise SME, they require well-separated thermal transitionsin the single material. However, the thermal transitionsin a single polymer are limited and sometimes they arenot well separated. Although polymer blending, polymerlaminating, and any other SMP hybridization are simplemethods for creating a SMP composite with multipleswitching transitions, the quantity of well-separated switchingtransitions in the single-shape memory material is stilllimited. Second, the fracture strain or failure strain of theseshape memory materials should be high enough to allowmultiple distinguished temporary shapes to be fixed. Forexample, some thermosetting SMPs are brittle and cannotbe cold-drawing programmed to a high strain level [246].Since the shape fixity of most SMPs is not perfect, thetemporary shapes should have significantly different strainlevels; otherwise, the temporary shapes may significantlyaffect each other.

3.2. Stepwise SME with a broad switching transition range

3.2.1. SMPs with a broad glass transition range. In 2007,Miaudet et al [247] first observed the temperature memory

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effect of shape memory polyvinyl alcohol composites with abroad glass transition range. The composite showed fast shaperecovery speed at the temperature at which it was deformed.According to this observation, the broad glass transition ofSMPs may be assumed to be a consecutive distribution ofa number of glass transitions. In this way, the temperaturememory effect can be easily explained [248]. Based on thisassumption, stepwise SME may be achieved in a SMP with abroad glass transition range while not having well-separatedmultiple switching transitions.

Xie [248] demonstrated two-step, three-step, and four-step shape recovery effects of a SMP with only onebroad glass transition. The number of the multiple SMEwas determined by the steps of the programming process.Because this SME is mostly determined by the multiple-step programming, the programming and recovery heatingtechniques have significant impact on the stepwise shapememory of the polymer [249]. For one-step SME, Yakackiet al [250] have comprehensively studied the dramaticinfluence of deformation temperature, long-term storage, andrecovery mode (free or constrained) [251, 252]. It is expectedthat the programming process may have more influenceon stepwise SME, although it has not been systematicallyinvestigated.

Since the report of the first stepwise SME with abroad glass transition as the switching transition, manyresearchers [247, 248, 253–257] have found that most SMPswith a broad glass transition can show the stepwise SME.The broad switching transition temperatures of the stepwiseSMPs may be tuned for different applications [257, 258]. Forexample, Shao et al [258] prepared SMP with stepwise SMEsby copolymerizing two norbornene derivatives: one based oncholic acid and the other on triethylene glycol mono-methylether. The glass transition temperature of the copolymerscould be tuned over a wide temperature range from −58 to176 ◦C with a broad glass transition range over 20 ◦C.

3.2.2. SMPs with a broad melting transition range.Theoretically, SMPs with a broad melt switching temperaturerange can also show the stepwise SME as long as the meltingtransition is broad enough. Kratz et al [254] demonstratedthe temperature memory effect of a SMP network witha crystalline switching phase. Based on the theory insection 4.1, the stepwise SME may also be achieved in theSMP with a broad melting transition although it has not yetbeen reported.

3.2.3. SMPs with a broad glass and a melting switchingtransition. Although theoretically feasible, practically, itis impossible to achieve unlimited-step SME in a polymerwith a broad thermal transition. The maximum number ofsteps a SMP can achieve so far is three-step (quadric-SME)reported by Xie et al [248]. In further research, to achievea four-step quintuple-SME, Li et al [259] incorporatedanother additional melting transition into a polymer whichalready possessed a broad glass transition. They prepareda poly(methylmethacrylate)/polyethylene glycol semi-IPNwhich provides a broad glass transition temperature and an

Figure 6. The four-step quintuple-SME of thepolymer(methylmethacrylate)/polyethylene glycol semi-IPNs with abroad glass transition and an additional melting transition.(Reprinted with permission, copyright of RSC Publishing 2011,doi:10.1039/C1JM12496J.)

Figure 7. Demonstration of the gradient shape recovery process ofa SMP with linear gradient glass transition temperature. Thedirection of Tg gradient is from left to right. (Reprint withpermission, copyright of RSC Publishing 2011, doi:10.1039/c0sm00487a.)

additional melting temperature. The four-step quintuple-SMEin a single-shape memory cycle of the SMP is shown infigure 6.

3.2.4. SMPs with a broad gradient glass transition. Creatinga linear glass transition temperature gradient within one SMPleads to glass transition distribution throughout the polymer.The linear glass transition temperature gradient can allowconsequent recovery of the polymer in the linear gradientdirection. DiOrio et al [204] obtained linear glass transitiongradient in a SMP by UV precuring a thiol–ene-basedphoto-cross-linkable glassy thermoset and post-curing thethermoset on a temperature gradient plate under the sameUV source. Figure 7 shows the consequent shape recovery ofthe SMP from right to left. The glass transition temperaturegradient is marked by slicing along the sample to give 15‘fingers’ along the bottom edge with a black dot at the endof each ‘finger’. The shape recovery initiates at the left endand propagates to the right.

3.2.5. Summary remarks. The stepwise SME of SMPs witha broad switching transition is generally attributed to the stepby step freezing elastic energy (or macromolecules) duringcooling and releasing of elastic energy (or macromolecules)during heating of the SMPs. Sun and Huang [260] proposeda model to demonstrate the feasibility of the stepwiseSME. Then, using a multibranch constitutive model, Yuet al [261] quantitatively revealed the stepwise SME bycapturing the complex relaxation processes in SMPs. Thesimulation results suggest that the stepwise SME arises

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from the shifting of individual nonequilibrium branches (orrelaxation modes) between a shape memory active state anda shape memory inactive state at different programming andrecovery temperatures.

The molecular relaxation not only provides the stepwiseSME, but also determines that the stepwise SME cannot beperfect. This means that aging can significantly affect not onlythe stepwise shape recovery, but also the exact fixability ofindividual temporary shapes which is affected by the energy(or molecular) relaxation. The reason is similar to that agingthat affects one-step SME of SMPs after long-term use [38,262, 263]. The programming and recovery heating parameterssuch as deformation temperature, holding time, cooling time,and heating speed significantly affect and even remove thestepwise SME.

Although theoretically the broad switching transition ofSMPs may be regarded as a consecutive distribution of anunlimited number of switching transitions, practically, it is toodifficult to fabricate a SMP with the capability of more thanfour steps of SME.

3.3. Stepwise SME by selective stimulation

Selective heating of predeformed SMPs or homogeneousheating of predeformed structurally inhomogeneous SMPscan lead to spatially controllable stepwise SME. Polymersare thermal insulators, which makes it possible to raisethe temperature of SMPs in a local area while the otherpart of the polymer is not considerably affected. Therecovery only occurs at the place where it is heated.Heating another area will trigger the recovery of that area.Localizing heating of SMPs to achieve spatially controllablestepwise shape recovery does not require complicatedpolymer structure design and/or synthesis. Homogeneousheating of predeformed structurally inhomogeneous SMPs toachieve stepwise SME needs complicated polymer structuredesign. For example, for a multi-component SMP withdifferent switching transition temperatures at different areas,homogeneous heating of the polymer will trigger the recoveryof the different components at different temperatures.Therefore, controllable stepwise SME can be achieved [264].

3.3.1. Selective joule heating of conductive CNT/SMP.Spatially controllable stepwise SME has been demonstratedin many traditional one-step SMPs. If the SMP isfilled with electrical conductive fillers, selective heatinglocally using electricity can control the shape recoveryof the SMP. For example, Fei et al [265] prepared aCNT/poly(methylmethacrylate-co-butyl acrylate) compositewith good electrical conductivity. Figure 8 shows theschematic of the consequent spatially controlled shaperecovery of the SMP composite by selection of the placewhere the electrical voltage is applied or switching the poweron/off.

Figure 8. Schematic illustration of the electro-triggered spatiallyand temporally controlled spatial shape recovery of electricalconductive CNT/SMP composites. (Reprinted with permission,copyright of RSC Publishing 2012, doi:10.1039/C2SM07357A.)

3.3.2. Selective focused light heating of gold nanoparticle/SMPs. Gold nanoparticles have a high level of interactionwith light especially with infrared light. Focused lightabsorbed by the gold nanoparticles in gold nanoparticle/SMPcomposites can heat the polymer up locally and trigger theshape recovery locally. Since light can be easily manipulated,the spatial stepwise SME can be controlled by manipulatingthe light. Once the light is turned off, the heating caused bylight is halted; the shape recovery may be stopped at any stage.

Zhang et al [266] loaded PCL-surface-functionalizedgold nanoparticles in a branched oligo(ε-caprolactone)(bOCL) cross-linked with hexamethylene diisocyanate (re-ferred to as XbOCL). The chemical structure of the SMP(XbOCL) and polymer-functionalized gold nanoparticles areshown in figure 9(a). Figure 9(b) shows the stepwise shape re-covery process of the prestretched gold nanoparticles-loadedXbOCL film by separate laser exposures on four sections ofthe film.

3.3.3. Selective light heating of SMPs with printed patterns.If selected areas of a prestretched SMP are coated withlight-absorption materials such as dark ink or carbon black,unfocused light will only heat up the predefined areawhere it is coated. In this way, localized heating at thepredefined coated areas is possible without using focusedlight. Liu et al [264] demonstrated localized self-folding ofa prestretched shape memory film on which predefined hingepatterns were created using a desktop printer as shown infigure 10. The black ink pattern provided localized absorptionof light. The sheet under the predefined inked regions(i.e., hinges) recovers, and thereby causes the planar sheet tofold into a three-dimensional object.

3.3.4. Selective alternative magnetic field heating of multi-composites. Figure 11(a) shows a SMP multi-compositeconsisting of a Fe3O4/SMP region and a CNT/SMP regionseparated by the neat SMP prepared by He et al [267]. TheFe3O4 and CNT nanoparticles have different selective radiofrequency heating properties. The 13.56 kHz magnetic field

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Figure 9. (a) Chemical structures of the SMP (XbOCL) and polymer-functionalized gold nanoparticles used to prepare the compositematerial. PCL-SS-PCL: PCL disulfide [266]. (b) Photographs showing a spatially selective shape recovery process at room temperature byseparate laser exposures on four sections of a gold nanoparticles-loaded XbOCL film stretched to 100% deformation, with the film stepwiselifting a load 350 times its weight. (Reprinted with permission, copyright of RSC Publishing 2012, doi:10.1039/C1JM14615G.)

Figure 10. Photographs of 3D structures created by self-folding ofShrinky Dinks (shape memory polystyrene) patterned with adesktop printer. (Reprinted with permission, copyright of RSCPublishing 2012, doi:10.1039/C1SM06564E.)

only heats the filler particles Fe3O4; therefore, it only heats upand recovers the Fe3O4/SMP region. The 296 MHz magneticfield only heats up the filler CNTs; therefore, it only heatsup and recovers the CNT/SMP region. Finally, in an oven ata temperature above the switching transition of the SMP, theneat SMP region recovers. The well-controlled stepwise shaperecovery of the multi-composite is shown in figure 11(b).

3.3.5. High-intensity focused ultrasound-induced selectiveheating. High-intensity focused ultrasound (HIFU) wasoriginally used as an extracorporeal tool for the treatment oftumors based on its thermal effects. It can also be used to heatup SMPs locally since polymers can absorb the mechanicalenergy generated by viscous shearing oscillation exerted by

focused ultrasound. Li et al [12] demonstrated the HIFUtriggered shape recovery of a SMP as shown in figure 12by localized heating in the circled area. The shape recoveryprocess can be controlled by selecting the ultrasound exposuretime, intensity and the position of its action.

3.3.6. Summary remarks. Researchers have devotedtremendous effort to achieve complex and well-controlledshape recovery of SMPs [265]. Selective heating ofpredeformed SMPs or homogeneous heating of predeformedstructurally inhomogeneous SMPs makes it possible tospatially control the location and shape recovery of SMPson demand. During the shape recovery process, once theheating source such as focused light is removed, the shaperecovery process may be stopped at any shape. Therefore, newtemporary shapes may be obtained anytime once the heatingis removed. Spatially controlled SMPs provide versatile SMEfor broad advanced applications such as actuators and robotsin industry, aerospace, and medical devices.

4. Two-way shape-changing effect (SCE)

For one-way SMPs, thermomechanical programming has tobe employed to deform them and fix a temporary shape.Two-way SCPs change their geometry, provided that theyare exposed to external stimuli without the requirement fordeformation by external thermomechanical programming.They may recover their original shape if the stimulation isremoved, or the amplitude or direction of the stimulation

Figure 11. Spatially controlled shape recovery of the multi-composite. (a) The components of the multi-composite consisting of aFe3O4/SMP region and a CNT/SMP region separated by the neat SMP. (b) The spatial recovery of the multi-composite when subjected toradio frequency fields of 13.56 MHz and 296 kHz sequentially. (Reprinted with permission, copyright of Wiley 2011, doi:10.1002/adma.201100646.)

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Figure 12. Spatially and temporally controllable shape recovery of SMP using HIFU irradiation. (Reprinted with permission, copyright ofRSC publishing 2012, doi:10.1039/c2jm30848g.)

is changed. Many applications require two-way SCE suchas artificial muscles, actuators, and sensors where the shapechange has to be reversible.

4.1. Semicrystalline SMPs under constant stress

Many semicrystalline SMPs have been demonstrated to beable to show SCE if they are subjected to a constant tensionload. The reason for two-way SCE is that cooling-inducedcrystallization of semicrystalline polymer films under atensile load results in elongation; and subsequent heatingtriggers shape recovery leading to contraction. Chunget al [268] first revealed the SCE in cross-linked shapememory semicrystalline poly(cyclooctene). During coolingunder constant stress, the crystallites form in the loadingdirection, leading to extension of the SMP. When heated toa temperature above the melting transition of the polymer,the SMP recovers, causing contraction. The film showedthe highest two-way shape-changing strain at 44% with arecovery ratio of 85% under a stress of 0.4 MPa. Westbrooket al [269] developed a 1D constitutive model to describethe SCE of semicrystalline SMPs based on the fact thatthe deformation states within the individual stretch-inducedcrystallization phases formed at different times are different.

Inspired by this research, many researchers have ob-served the two-way SCE on many semicrystalline SMPsunder appropriate constant tensile load, such as semicrys-talline SMPU [270], polyhedral oligomeric silsesquiox-ane (POSS)/PCL networks [271], cross-linked PCL-basedpolyesterurethane [272], linear, three- and four-arm star PCLfunctionalized with methacrylate [30], and poly(ethylene-co-vinyl acetate) [28]. Zotzmann et al [273] achieved notonly two-way SCE but also two-step SME on a multi-phasepolymer network with two different crystalline phases at thetwo switching transitions. The constant tensile stress is thekey factor to achieve the two-way SCE. To obtain long-termstress for the semicrystalline SMP, Kang et al [274] coateda semicrystalline SMP fiber with elastomer, which can applytensile stress to the SMP fiber.

4.2. Thermal-responsive liquid crystal elastomers

The two-way SCE of liquid crystal elastomers results from thecoupling effect between the ordering changing of mesogenicmoieties and the elastic properties of the elastic network.

The original research on liquid crystal elastomers wasfocused on monodomain liquid crystal polymer becauseit was believed that monodomain liquid crystal polymercould show higher shape-changing amplitudes. Preparation ofliquid crystal monodomain requires two processes: synthesisof liquid crystalline elastomer and aligning uniformly themesogens over the whole sample. Later, Burke [275], Qin andMather [27] found that glass-forming polydomain nematicnetwork could also show SCE. A large shape-changing stainwas also observed on glass-forming polydomain nematicnetwork. This research simplifies the fabrication process oftwo-way shape change liquid crystal polymer because it doesnot require the alignment procedure as do monodomain liquidcrystal elastomers.

A maximum spontaneous elongation up to 500% hasbeen achieved in main-chain liquid crystal elastomer in afiber form by Ahir et al [276]. The large central block ismade of a main-chain nematic polymer which has largespontaneous elongation along the nematic direction. Tofurther improve the nematic structure orientation, Krauseet al [277] prepared nanofibers of main-chain liquidcrystal elastomers. Photoinduced cross-linking was formedduring the electro-spinning process. Exceptional mechanicalproperties with outstanding shape-changing effect wereobtained. Normally, in comparison with side-chain liquidcrystal elastomers, main-chain liquid crystal elastomers havehigher shape change. It is believed that combining a mesogenside chain and main chains in one elastomer can increase theshape change amplitude.

4.3. Hybrid materials

Laminates, layered materials or any hybrid materials madeof materials of different thermomechanical properties maychange their geometry upon changes in environmental tem-perature. The mechanism is that heating creates unbalancedmechanical stress. The unbalanced stress may arise fromdifferent coefficients of thermal expansion, different moduluschanges, and SME (if shape memory materials are used). Thelayered materials can be both SMPs, or a SMP layered withnon-SMP, or both non-SMPs.

Langer and Lendlein [278] in their patent first outlinedthe method of preparing SCPs using SMPs by a laminatemethod. Chen et al [279] prepared a shape changing materialby laminating a SMP with an elastic polymer. As well

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Figure 13. (a) Thermal-active SCE of the laminate scrolls;(b) schematic of the bimaterial cantilever bending upon incidentheat. Top: actual optical image, bottom: side view of themicrocantilever bending as temperature increases from 20 to 40 ◦C.(Reprinted with permission, copyright of ACS Publications 2006,doi:10.1021/nl0525305.)

as SMPs, shape memory alloys have also been used forthis purpose. Tobushi et al [280] fabricated a two-wayshape-changing laminate with shape memory alloys andSMPs. Ghosh et al [281] prepared a two-way shape changingbias system by embedding shape memory alloy (SMA) wiresin a SMP which had a glass switching transition betweenthe martensite transformation finish temperature and austenitephase transformation finishing temperature of the SMA.Actually, if any two dissimilar materials are laminated, theinterfacial stress will vary at the interface if the environmentaltemperature changes, which may drive the shape change ofthe laminate. These two-way shape memory laminates donot depend on the switching transition of SMPs. The maindisadvantages of these two-way shape memory laminatescompared with those made of SMPs are slow shape-changingspeed.

Two-way shape-changing laminates can be easilyminiaturized using advanced processing techniques such aschemical vapor deposition and photolithographic technique.For example, Kalaitzidou et al [282, 283] fabricated alayered tube composed of a polydimethylsiloxane filmand nanometer-thick gold which was coated on thepolydimethysiloxane film. Figure 13(a) shows the micro-shape and thermal-active two-way SCE of the compositerolls. Upon cooling, the polydimethysiloxane film shrinksmore than gold, which produces a compression at thepolydimethysiloxane side to fold the tube. Reversely, uponheating, the layered composite unfolds. Similarly, LeMieuxet al [284] made a bimorph actuator by coating plasma-polymerized polystyrene on one side of an atomic forcemicroscope silicon tip by a plasma enhanced chemical vapordeposition process. Figure 13(b) shows the schematic ofthe bimaterial cantilever and the actual optical image ofthe microcantilever bending. The actuator has ultrathermalsensitivity.

The principle of the thermal-active SCE of the laminatematerials can be extended to water-active SCE of laminatematerials if the laminated materials have a component of awater-solvable/swellable material such as hydrogels. In fact,any polymers whose elastic modulus can be changed by wateror any chemical may be used to prepare water/chemical-activetwo-way shape-changing laminates, provided that the coupled

Figure 14. Curved compositionally anisotropic microcylinders.(Reprinted with permission, copyright of Wiley 2012, doi:10.1002/anie.201105387.)

Figure 15. Photoactive shape-changing mechanism of liquidcrystal elastomers based on photoinduced trans–cis isomerization ofazobenzenes.

materials have good interface interaction. For example, Sahaet al [285] fabricated water-responsive microcylinders asshown in figure 14. The cylinders are comprised of awater-swellable hydrogel compartment as well as an inertcompartment and thus undergo substantial bending in water.The asymmetric expansion creates surface stresses resultingin significant and controllable bending of the microcylinders.The microcylinders are fabricated by electrohydrodynamicco-jetting followed by microsectioning [285–288].

4.4. Azobenzene-based liquid crystal elastomers

Azobenzenes and their derivatives undergo reversibletrans–cis isomerization upon the irradiation of light of suitablewavelength [289]. The isomerization causes the angle changeof the two isomers accompanied by significant change inmolecular length between 9.0 A (trans) and 5.5 A [290, 291]as shown in figure 15. If azobenzenes are coupled into liquidcrystal elastomers, significant photoinduced deformation canbe achieved. Due to the photoisomerization of azobenzene, theliquid crystal elastomers experience a reduction in alignmentorder and even liquid crystal to isotropic transition, leading tomacroscopic shape change [290, 292–296]. Different liquidcrystal elastomer molecular structures with azobenzene asdopants, chemically bonded pendants or photo-crosslinkershave been studied to improve the response speed anddeformation strain [215, 297–299]. In comparison withother photoactive molecules, azobenzenes can produce largergeometrical changes.

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Figure 16. (A) The setup of the light-induced oscillationexperiment. The polymer film is mounted vertically and oscillatesaround the horizontal plane of incidence of the laser beam; (B) theoscillation mechanism of the liquid crystal polymer. (Reprinted withpermission, copyright of RSC Publishing 2008, doi:10.1039/B805434G.)

Several outstanding review papers [300–305] have beenpublished regarding the synthesis, photomechanical effectand application strategies of azobenzene-based liquid crystalelastomers. Light, in comparison with other stimuli suchas electricity, can be easily manipulated in terms ofintensity, polarization, and phase [306]. Through propermolecular design, polymer structure design, and stimulusdesign, complex SCE such as twisting, bending [307],swimming [308, 309], rotation [310], inchworm walk andcontrolled vibration [294, 308] have been achieved inliquid crystal elastomers [311, 312] and their composites[86, 294, 300–303, 311, 313–317]. Recently, oscillationswere demonstrated in monodomain azobenzene liquid crystalpolymer cantilevers when the cantilever was sequentiallyexposed to the front and back surface of the sample asthe film inertially passed through the laser beam [294, 308]as shown in figure 16 [308]. The polymer was synthesizedby copolymerizing two azobenzene liquid crystal monomersin the aligned nematic phase. At the start, the azobenzenemesogens are aligned vertically. The azobenzene mesogenson the irradiated surface reorient due to the formation oftrans-azobenzene orthogonal to the polarization direction ofthe laser light. During the upstroke, the front surface of thesamples is exposed. During the down-stroke, the back surfaceis exposed. Therefore, controlled vibration of the cantilever isachieved. Spatial variation of the domain orientation utilizingcommand surfaces and photo-patterning (holographic andmasking) can enable more complicated moving effects.

Azobenzene-based photoactive SMP does not have tohave a liquid crystal structure. For example, Lee et al [85, 318]prepared a light-active linear azobenzene-functionalizedpolyimide fiber which was spun from an amorphous tosemicrystalline structure. As shown in figure 17 [85], expo-sure of the cantilever to blue-green irradiation (λ = 442 nm)produced polarization directed forward and reverse bending.The SCE effect is similar to that observed on glassyazobenzene liquid crystal network polymers [318, 319]. Thecrystallinity is the dominant factor influencing azobenzenephotoisomerization and the SCE.

Photoactive azobenzene-based polymers can be used forrecordable and erasable memories [86, 311, 315], biosepara-tion [155], microrobots or optical microtweezers [315, 320],remotely triggered drug delivery systems [321, 322],remote-controllable micro/nanoactuators and sensors [323],cardiovascular therapeutic device [324, 325], artificial mus-cles [313], energy harvesting [294], and display devices[294, 301].

4.5. Azobenzenes/inorganic nanoporous membranes

Photoactive polymers in the nanoscale have special functionssuch as photo-switchable nanoarchitectures [326, 327], andphotoactive polymer membranes with photo-regulated perme-ability [328]. Photoactive azobenzenes/inorganic nanoporousmembranes are made by immobilizing azobenzene derivativesto inorganic nanoporous membranes such as silica frame-works. The light-controlled conformation of the azoben-zene in the composites can change the pore size andcorrespondingly the transport behavior of the compositemembrane [329]. Continuous excitation at a wavelength,which both the trans and cis azobenzene absorb, can lead tocontinuous photoisomerization and continual dynamic wag-ging [330]. The azobenzenes attached to the interiors of theinorganic substrate pores templated by surfactant [331, 332]can act as both impellers and gatekeepers [333, 334].Azobenzenes/inorganic nanoporous composites can be usedas photoactive nanovalves [330, 335], drug delivery nanovehi-cles [336], microfluidic devices [337], photonic nanoparticledevices [338], and micro-optical-controlled soft micro-robots [339].

Figure 17. Top: molecular structure of the azobenzene-functionalized linear polyimides. Bottom: the photoactive SCE effect of theazobenzene-functionalized linear polyimides. (Reprinted with permission, copyright of Wiley 2012, doi:10.1002/anie.201200726.)

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Figure 18. (A) Photodimerization and dissociation reaction of 9-anthracene carboxylic acid; (B) repeated forth and back flexes of a singlenanorod made of 9-anthracene carboxylic acid (UV irradiation = panels b, d, f, h; without irradiation = panels a, c, e, g). (Reprinted withpermission, copyright of Wiley 2007, doi:10.1002/adma.200602741.)

Figure 19. (A) Molecular structure of the photoactive polymer withanthracene as the side group; (B) schematic illustration ofexperimental setup; (C) photoactive bending of the fiber (diameterof 200 µm) upon irradiation with UV of 365 nm for 30 min atdifferent temperatures. (Reprinted with permission, copyright of theChemical Society of Japan 2010, doi:10.1246/cl.2010.234.)

4.6. Anthracene-based polymers

Anthracene and its derivatives can undergo 4+ 4 dimerizationupon irradiation of UV light. Figure 18(A) shows thephotodimerization of 9-anthracene carboxylic acid (9-AC)under UV light of proper wavelength. Al-Kaysi andBardeen [340] demonstrated the large reversible SCE of acrystal nanorod made of 9-AC compound material using UVlight as shown in figure 18(B).

Kondo et al [341] incorporated the anthracene into apolymer with anthracene as the side group and made a

photo-responsive fiber from the polymer. Figure 19(A) showsthe molecular structure of the polymer. Under UV irradiation,the bending angle of the fiber increases with increasingtemperature as shown in figure 19(C), which is related to themodulus decrease of the fiber.

4.7. Coumarin-based polymers

Coumarin and its derivatives undergo photoinduced reversibledimerization reaction upon photo-irradiation as shown infigure 20(a) [342]. The photochemical dimerization reactionin a polymer can cause migration of the photodimerizationmoieties and bending effect of the polymer. He et al [342]prepared a coumarin-based polymer by partially function-alizing poly(4-vinyl pyridine) with 7-(carboxymethoxy)-4-methylcoumarin. The coumarin moieties dimerization occursupon irradiation with UV light of wavelength above 310 nm.The reversible cleavage of the cyclobutane bridge happens onirradiation with UV light of wavelength below 260 nm. Thephotodimerization on the surface exposed to UV light leads tolarge bending toward the UV light as shown in figure 20(b).The degree of the photoactive bending of the polymer iscomparable with that of photoactive liquid crystal elastomermade of azobenzene-based polymers.

4.8. Cinnamate-based polymers

The UV reversible photodimerization of cinnamic acid groupas shown in figure 21 can lead to macroscopic shape changingof the polymer. Several types of one-step SMPs based on

Figure 20. (a) Reversible photodimerization of poly(4-vinyl pyridine) partially complexed with 7-(carboxymethoxy)-4-methylcoumarinthrough hydrogen bonding. (b) Schematic representation of the photoinduced bending mechanism. (Reprinted with permission, copyright ofRSC Publishing 2009, doi:10.1039/B814278E.)

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Figure 21. Photodimerization of the cinnamic acid group.

cinnamate have been reported. Two-way SCE of cinnamate-based polymers has not been studied intensively. Cinnamatemoieties may be incorporated into the polymer in the graftedbranches or in the main chain [86, 311, 320, 343–345].Lendlein et al [9] grafted cinnamic acid into the backboneof hydroxyethyl acrylate hydrogels. Wu et al [89] prepareda biodegradable polymer from soft diol(polycaprolactonediol), biodegradable hard diol (poly-(l-lactic acid) diol), andN,N-bis(2-hydroxyethyl) cinnamamide with cinnamamidegroups as the pendent groups. Both the cinnamate polymersshowed outstanding UV-light-induced one-step SME. Theone-step SMPs are first deformed and the temporary shapeis fixed by the photodimerization of the cinnamate moieties.Upon light irradiation of another proper wavelength, thereversible cross-linking cleaves, leading to the shape recoveryof the polymers [9, 87, 88, 345].

Shi et al [346] and Thi et al [347] developeda bio-copolymer poly[(3,4-dihydroxycinnamicacid)-co-(4-hydroxycinnamic acid)] with hyper-branched structure fromcinnamic acid. The copolymers formed nanoparticles byself-organization. They observed significant diameter changeand rapid recovery of the nanoparticles with whole cinnamategroups upon the photo-crosslinking and photo-cleavage of thecinnamate groups with UV irradiation.

4.9. Photoactive effect based on photothermal effect

CNTs [348, 349] absorb near-infrared irradiation. Ahirand Vaia [348, 350] filled CNTs into polydimethylsiloxaneelastomer and observed two-way SCE in the composite. Theexpansion or contraction of the composite depends on theextent to which the composite is strained. If the material isslightly pulled, it expands when it is exposed to infrared light.Conversely, if the material is subjected to a strain greaterthan 10%, it contracts under identical exposure to infrared

light. This process is completely reversible and persists afternumerous cycles. The mechanism of two-way SCE has notbeen fully revealed although it is believed to be related to thephotothermal effect.

The above materials are elastomer materials filled withnear-infrared irradiation fillers. Ugur et al [351] achievedthe near-infrared irradiation-induced SCE of a pure polymerfilm made of cross-linked polyarylamide nanofibers. Thecross-linked multi-block polyarylamide contained ‘coil-like’polyethylene oxide and ‘rod-like’ polyarylamide blocks asshown in figure 22. The polyarylamide self-assembles intofibrillar morphology on silicon substrates and the resultingfilms exhibit a high degree of optical anisotropy. Thereversible SCE can be repeated for numerous cycles. A studyon better understanding of the mechanisms is underway; it issuspected that the two-way SCE is related to the photothermaleffect of the highly aligned fibrillar structure.

4.10. Photoactive effect based on addition–fragmentationchain transfer reaction

Scott et al [352, 353] developed a light-active covalentlycross-linked network which underwent photo-cleavage inits backbone chain to allow chain re-arrangement. Themonomers used to create the cross-linking network areshown in figure 23. The switch mechanism of the reversiblebackbone cleavage is addition–fragmentation chain transfer.Reaction diffusion of radicals through the cross-linkedmatrix occurs initially by the reaction of a radical withan in-chain functionality, forming an intermediate, whichin turn fragments, reforming the initial functionality andradical [354]. Figure 23(D) shows the addition–fragmentationprocess through the polymer backbone of the photoactivenetwork [352] and figure 24 shows the photoinduced actuationaccordingly [352]. Irradiating the sample on the unirradiatedside allows elimination of the introduced stress and ashape change in the other direction. If the sample isirradiated on both sides in an alternating pattern, oscillatorySCE can be observed. Long et al [355] have proposeda photomechanics model for this light-activated polymerby integrating four coupled phenomena, i.e., photophysics,photochemistry, chemo-mechanical coupling, and mechanicaldeformation. The simulation results compared nicely with theexperiment results.

Figure 22. Chemical structure of ‘rod-like’ polyarylamide-block and ‘coil-like’ poly(ethylene oxide). (Reprinted with permission,copyright of Wiley 2012, doi:10.1002/adma.201104538.)

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Figure 23. Monomers used: (A) pentaerythritol tetra (3-mercaptopropionate) (PETMP); (B) 2-methylene-propane-1,3-di(thioethyl vinylether (MDTVE)); (C) 2-methyl-7-methylene-1,5-dithiacyclooctane (MDTO), and (D) addition–fragmentation mechanism through thepolymer backbone of the resultant polymer. (Reprinted with permission, copyright of Wiley 2006, doi:10.1002/adma.200600379.)

Figure 24. Photoinduced actuation. The sample was irradiated from the upper left at 365 nm, 160 mW cm−2, for different times:(A) t = 0 s; (B) t = 30 s; (C) t = 210 s; (D) t = 930 s. (Reprinted with permission, copyright of Wiley 2006, doi:10.1002/adma.200600379.)

4.11. Summary remarks

The mechanism of the SCE of semicrystalline polymersindicates that all the semicrystalline SMPs can exhibittwo-way SCE more or less under proper stress situation. Theproper tensile stress has to be applied to the polymer. Inaddition, the SCE of semicrystalline SMPs can only haveelongation/contraction. Due to the limitation of the crystallitesformation in the loaded direction, the length extension of thesemicrystalline SMP is not high. All of the above limitationscan significantly affect the applications of semicrystallinetwo-way SMPs.

Without a complex synthesis process, the laminating orhybridization method is one of the most promising methodsto prepare shape-changing materials. There are many optionsfor selecting the compounding materials. The compoundingmaterials are not limited to polymers. Metals can also beused for this purpose. The SCE of laminated or hybridmaterials is easily tailored by selecting suitable materialscouples, adjusting the thickness of the layers, and changingthe processing conditions.

Four main types of photo-responsive two-way SMPshave been developed, i.e., azobenzene-based polymers,anthracene-based polymers, coumarin-based polymers, andpolymers with addition–fragmentation chain transfer reaction.Complex movements of photoactive polymer films andlaminated films have been obtained from photo-responsivetwo-way SMPs especially with azobenzene based liquidcrystal elastomers, such as oscillating, twisting, swimming[308, 309, 356], rotation [310], and inchworm walk [314].So far, the photoactive SCE of these SCPs, except for theazobenzene-based polymers, has not been fully investigated.In addition, the photothermal-effect-induced two-way SCEshould be studied in detail because the photothermaleffect is common and easy to achieve. The research on

photothermal-effect-induced two-way SCE may lead to alarge number of novel two-way shape-changing materials.

Of the two-way SCPs, liquid crystal elastomers haveattracted a high level of research attention because theycan provide high shape-changing strain and high mechanicaloutput. Their promising applications as artificial muscles,soft actuators and nonlinear optical devices are still underinvestigation and some good results have been obtained[82, 357]. For practical applications, researchers [358–360]are investigating in more detail the properties of liquid crystalelastomers, such as chemical and mechanical fatigue, andenvironmental stability.

5. Future research trends

To meet the needs of various complicated applications,the past decade has witnessed significant advances onthe various new SMEs of SMPs such as stepwise SME,spatially controllable SME, and two-way SCEs. Throughmolecular structure design, material macrostructure design,and stimulation design, various shape-moving effects havebeen achieved. This paper presents the research progress onthese various complicated moving effects. The future outlookis discussed below.

5.1. Fabrication of stepwise, controllable, spatial,predictable, and two-way SMP

Most of the movements of present SMPs are simple shrinkageand bending. For more advanced applications, complexmovements with controllable steps, controllable strains,controllable deformation speed, and exact deformationstress are preferred. The moving effect of stimuli-activemoving polymers is affected by many factors such as thethermomechanical programming and recovery process in

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addition to the molecular structures. The localized stimulationmethod has provided a facile and efficient method to achievea well-controlled, actively moving process. Further studiesneed to be conducted to investigate the controllability of SMEby localizing stimulation. For many advanced applicationsof SMPs such as sensors and actuators, accuracy of shaperecovery is equally important. Although some effort has beenmade to understand the accuracy of SMPs [361], there is alack of systematic studies on the influences of environmentalconditions on the whole shape recovery process of activelymoving polymers.

5.2. Modeling of stepwise, spatial, controllable, and two-waySME

In recent years, modeling of SMPs has accelerated, motivatedby the need for an efficient design tool for increasinglysophisticated applications [24, 362–365]. The modeling ofSMPs falls into two main approaches: the viscoelasticapproach [24, 53, 218, 261, 263, 366–368, 373], and thephase change approach [217, 369–371]. Although a numberof models have been set up and used to explain and predictthe complicated SME of SMPs [372], each of them is onlyapplicable to a certain type of SMP such as thermosetor thermoplastic, thermal active or athermal active, anddual SME or stepwise SME. SMPs are versatile; a moregeneral model is urgently needed which can capture themost important behaviors of SME while being simple andapplicable.

Effort should be directed into accurate molecularsimulations and physics-based mathematical modeling toobtain the optimal actively moving effect and predicting theshape memory process. Through the effort of researchers andscientists, this goal will be achieved, although the complicatedstructures of various SMPs make it a tough task for theoreticalstudies [24, 218, 373–385, 355]. Recently, Nguyen has givenan excellent review on this topic [386].

5.3. High-performance SMPs

At present, one of the critical challenges facing SMPsfor some engineering applications is their low mechanicalstrength and low mechanical output. Due to the softnature of polymers, the mechanical output of SMPs isfar less than that of their metallic counterparts, althoughSMPs can have high recoverable strain up to severalhundred per cent. Even worse, the mechanical propertiesof thermal-responsive SMPs at a temperature above theswitching transition temperature drop significantly. Improvingthe molecular orientation of stimuli-active moving polymerscan significantly improve their mechanical properties. Somestudies have demonstrated the effectiveness of this method.Also, cold-drawing programming of semicrystalline SMPscan increase the recovery stress [218].

Recently, Nair et al [387] reported a two-stage reactivenon-stoichiometric thiol–acrylate SMP. After shape recovery,a second stage reaction by UV irradiation can polymerizeremaining acrylate functional groups to form a more stable

chemical cross-linking system. In this way, the SMP canmaintain a high mechanical strength at a temperature abovethe switching transition temperature. This strategy still doesnot improve the mechanical output or recovery stress of theSMP during the shape recovery process. Another researchdirection may be to explore novel intrinsically athermal-activeshape memory structures for which direct or indirect heatingis not required to trigger the SME. Stimulus does not heat thepolymer; instead it triggers molecular re-arrangement of thepolymer without decreasing the modulus.

Another problem of most present SMPs is that theycan only operate at low temperature, normally in therange of ambient temperature to 100 ◦C. The low ther-mal stability and low operating temperature have limitedSMPs’ wider structural applications such as in satellites,aerospace, and transportation. Aromatic polyimides havehigh thermal stability (>300 ◦C) and are intrinsically flameresistant. Koerner et al [388] studied the SME of anaerospace-grade polyimide CP2 which is derived from2,2-bis(phthalic anhydride)-1,1,1,3,3,3-hexafluoroisopropane(6FDA) and 1,3-bis(3-5 aminophenoxy)benzene (APB) [389].The polyimide CP2 was slightly cross-linked so that thepolyimide had proper thermomechanical properties for theSME. The CP2 had a narrow glass transition (<10 ◦C) at220 ◦C which could act as the switching temperature for SME.The polymer exhibited excellent high-temperature shapememory performance with excellent extensibility (>200%),and high room temperature modulus (2–3 GPa). The signif-icantly high glass transition also affords this polymer withlittle to no creep at room temperature. Yoonessi et al [390]prepared a polyimide from bis phenol A dianhydride and2,2-bis[4-(4-aminophenoxy)phenyl] propane. They studiedthe SME of the polyimide and the corresponding graphenepolyimide nanocomposites. Both the neat polyimide andpolyimide nanocomposites exhibited SME at 230 ◦C with theglass transition as the switching transition.

5.4. Athermal-stimuli triggered SME

Light as an energy source is environmentally friendly. As astimulus, it can be controlled remotely, instantly and precisely.Many studies have been conducted on photoactive movingpolymers. More research is needed for photoactive movingpolymers. Particularly, photoactive moving polymers mayplay an important role as environmentally friendly energyharvesting materials [300]. Some problems, such as fatigueresistance and environmental stability of photoactive movingpolymers, need to be further investigated [300].

Most of the present stimuli-responsive SMPs areresponsive to a specific stimulus in a certain range. Futureresearch will also include studying multiple-stimulus-activemoving polymers by combining different stimulus-activestructures into one single polymer. In this way, the SMPs maychange their properties to adapt to the overall environmentalconditions such as temperature, light, humidity, electricitysignals, and pH.

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5.5. Complicated, stepwise, spatial, controllable, andtwo-way SMP composites

Blending, in situ polymerization, grafting, and laminatingnot only improve the properties but also increase the SMEversatility of SMPs such as stepwise SME and two-way SCE.For SCPs based on cross-linking semicrystalline polymers,a tensile load is necessary. For SCPs based on thermal orphotoactive isomerization, high deformation strain cannot beproduced except in liquid crystal structures. However, forpractical applications, liquid crystal SCPs are still costly,in addition to their unstable shape-changing behavior. Itseems that the lamination method is one of the mostpromising methods to prepare shape-changing materials.First, the laminating process is not complicated. In addition,the properties of shape-changing laminates are tailorable bysimply changing the stacking sequence and ply thickness.

5.6. Microminiaturization of SMPs

Although SMPs have been studied intensively, reports onSMP micro- and nanoscale devices are few [391, 392].SMPs in microscale will have many applications such astetherless microgrippers, microvalves, data storage, and softrobots. The smart tetherless microgrippers will be able tocapture and release clusters of cells controlled by temperature,chemicals, and biological conditions. The present researchhas been mostly focused on creating surface wrinkles orpatterns on SMPs [3, 393–399]. These SMPs with controllablemicrowrinkles and patterns may be used as moleculardetection, optical devices, filters, and sorters. The futureresearch may be to microminiaturize the present various bulkSMPs and develop micro or nano SMP devices, in additionto investigating the SME of SMPs at a micro- or nanoscale.It is believed that research in this direction will boost moreadvanced application opportunities for SMPs.

5.7. Application-oriented research

SMPs have been studied for half a century. The researchon the molecular structures, mechanisms and principles hasbeen conducted intensively. Now, researchers have betterunderstanding of the SMEs of various SMPs. The researchis entering a stage of practical engineering design andapplications of various SMPs. SMPs have been proposed foruse in many areas. Numerous patents related to SMPs havebeen filed in the past 10 years. In the past 5 years, several newcompanies developing SMP products have been establishedand several SMP products have been commercialized byresearchers such as Yakacki et al [400] from Medshape.It is believed that the next 10 years will see an explosionin growth of SMP products. Although the applications ofone-step SMPs have been widely studied, the applicationsof stepwise, spatially controllable and two-way SCPs havenot been extensively explored. The next decade will seetremendous application-oriented research on complicated,stepwise, spatial, controllable, and two-way SMPs, in additionto research on the polymers themselves.

6. Conclusions

Traditional SMPs can fix a temporary shape and recover theiroriginal shape upon proper stimulation. In the past decade,researchers have devoted great effort toward achieving morecomplicated, stepwise, controllable, and two-way SMEs.These smart moving effects are as a response to environmentalsignals and intrinsic to these smart materials in that they donot require additional complex electrical feedback systemsfor the action. Therefore, they are real ‘Material Machines’or ‘Material Robots’.

This paper has presented the recent research endeavorson the various complicated SMEs. After a short review onone-step SMEs, the focus was on the research progress inSMPs with stepwise SME, including spatially controlledSME. The focus was then on reviews of the presentachievements in two-way SCPs. Finally, suggestions were putforward for future research.

Although amazing progress has been made on theversatile SMEs of SMPs, the research on these SMPsis far from complete. First, the understanding on moststepwise SME, spatial, controllable, and two-way SMEs isstill superficial. Some underlying mechanisms have not beenfully revealed. Second, many limitations of SMPs such aslow mechanical strength and low recovery stress, which haveexisted since the invention of the first SMP, are still waitingfor solutions. By discussing the problems facing variousSMEs, we hope that this review will inspire researchersto find solutions in the near future. With the tremendouscreativity and hard work of researchers, it is believed thatthe complicated, stepwise, spatially controllable, and two-waySMPs will find booming and innovative applications in theforeseeable future.

Acknowledgments

The research is funded by Cooperative AgreementNNX11AM17A between NASA and Louisiana Board ofRegents under contract NASA/LEQSF(2011–14)-Phase3-05,NSF under Grant number CMMI 0900064, NSF EPSCoRLA-SiGMA project under award No. EPS-1003897, NSFunder grant number HRD 0932300 and Louisiana BoR FundLEQSF(2010–2015)- LaSPACE by Cooperative AgreementNNX10AI40H.

References

[1] Beblo R, Joo J, Smyers B and Reich G 2012 J. Intell. Mater.Syst. Struct. 23 1987–2002

[2] Jagur-Grodzinski J 2010 Polym. Adv. Technol. 21 27–47[3] Fu C, Grimes A, Long M, Ferri C, Rich B, Ghosh S,

Ghosh S, Lee L, Gopinathan A and Khine M 2009 Adv.Mater. 21 4472–6

[4] Jeong H M, Ahn B K and Kim B K 2000 Polym. Int.49 1714–21

[5] Xu H, Yu C, Wang S, Malyarchuk V, Xie T and Rogers J A2013 Adv. Funct. Mater. 23 3299–306

[6] Leng J, Lan X, Lv H, Zhang D, Liu Y and Du S 2007Behavior and mechanics of multifunctional and compositematerials Proc. SPIE 6526 65262

17

Smart Mater. Struct. 22 (2013) 093001 Topical Review

[7] Lendlein A and Kelch S 2002 Angew. Chem. Int. Edn41 2034–57

[8] Cho J W, Kim J W, Jung Y C and Goo N S 2005 Macromol.Rapid Commun. 26 412–6

[9] Lendlein A, Jiang H, Junger O and Langer R 2005 Nature434 879–82

[10] Schmidt A M 2006 Macromol. Rapid Commun. 27 1168–72[11] Han X J, Dong Z Q, Fan M M, Liu Y, Li J H, Wang Y F,

Yuan Q J, L B J and Zhang S 2012 Macromol. RapidCommun. 33 1055–60

[12] Li G, Fei G, Xia H, Han J and Zhao Y 2012 J. Mater. Chem.22 7692–6

[13] Bai T, Han Y, Zhang P, Wang W and Liu W 2012 SoftMatter 8 6846–52

[14] Miyata T, Asami N and Uragami T 1999 Nature 399 766–9[15] Lee M R, Baek K H, Jin H J, Jung Y G and Shin I 2004

Angew. Chem. Int. Edn 43 1675–8[16] Chung Y C, Nguyen D K, Choi J W and Chun B C 2010

Fibers Polym. 11 952–9[17] Huang W M, Ding Z, Wang C C, Wei J, Zhao Y and

Purnawali H 2010 Mater. Today 13 54–61[18] Hu J L, Meng Q H, Zhu Y, Lu J and Zhuo H T 2007 US

Patent 11907012[19] Lendlein A and Langer R S 2004 US Patent 2004 004776[20] Marco D and Eckhouse S 2006 US Patent 20070156248[21] Li G and Nettles D 2010 Polymer 51 755–62[22] Nji J and Li G 2010 Polymer 51 6021–9[23] Li G and Uppu N 2010 Compos. Sci. Technol. 70 1419–27[24] Li G and Xu W 2011 J. Mech. Phys. Solids 59 1231–50[25] Behl M and Lendlein A 2010 J. Mater. Chem. 20 3335–45[26] Lendlein A 2010 J. Mater. Chem. 20 3332–4[27] Qin H and Mather P T 2009 Macromolecules 42 273–80[28] Li J J, Rodgers W R and Xie T 2011 Polymer 52 5320–5[29] Westbrook K K, Mather P T, Parakh V, Dunn M L, Ge Q,

Lee B M and Qi H J 2011 Smart Mater. Struct. 20 065010[30] Pandini S, Passera S, Messori M, Paderni K, Toselli M,

Gianoncelli A, Bontempi E and Ricco T 2012 Polymer53 1915–24

[31] Yang D, Huang W, Yu J, Jiang J, Zhang L and Xie M 2010Polymer 51 5100–6

[32] Meng Q, Hu J, Ho K, Ji F and Chen S 2009 J. Polym.Environ. 17 212–24

[33] Voit W, Ware T, Dasari R R, Smith P, Danz L, Simon D,Barlow S, Marder S R and Gall K 2010 Adv. Funct. Mater.20 162–71

[34] Kolesov I S, Kratz K, Lendlein A and Radusch H J 2009Polymer 50 5490–8

[35] Prima M A D, Galla K, McDowella D L, Guldberg R,Linb A, Sandersonc T, Campbelle D and Arzbergere S C2010 Mech. Mater. 42 315–25

[36] Nagata M and Mamoto Y 2009 J. Polym. Sci. A 47 2422–33[37] Ware T, Voit W and Gall K 2010 Radiat. Phys. Chem.

79 446–53[38] Ortega A M, Yakacki C M, Dixon S A, Likos R,

Greenberg A R and Gall K 2012 Soft Matter 8 3381–92[39] Tandon G P, Goecke K, Cable K and Baur J 2009 J. Intell.

Mater. Syst. Struct. 20 2127–43[40] Lutzen H, Gesing T M, Kim B K and Hartwig A 2012

Polymer 53 6089–95[41] Shumaker J A, McClung A J W and Baur J W 2012 Polymer

53 4637–42[42] Zhang S, Feng Y, Zhang L, Sun J, Xu X and Xu Y 2007

J. Polym. Sci. A 45 768–75[43] Zhang S, Yu Z, Govender T, Luo H and Li B 2008 Polymer

49 3205–10[44] Yu Z, Liu Y, Fan M, Meng X, Li B and Zhang S 2010

J. Polym. Sci. B 48 951–7[45] Huang W M, Yang B, Zhao Y and Ding Z 2010 J. Mater.

Chem. 20 3367–81

[46] Ji F L, Hu J L, Li T C and Wong Y W 2007 Polymer48 5133–45

[47] Zhu Y, Hu J L, Yeung K W, Liu Y Q and Liem H M 2006J. Appl. Polym. Sci. 100 4603–13

[48] Ji F L, Zhu Y, Hu J L, Liu Y, Yeung L Y and Ye G D 2006Smart Mater. Struct. 15 1547–54

[49] Zhu Y, Hu J, Choi K F, Yeung K W, Meng Q and Chen S2008 J. Appl. Polym. Sci. 107 599–609

[50] Mondal S and Hu J L 2007 J. Elastomers Plast. 39 81–91[51] Lake M S and Beavers F L 2002 Collection of Technical

Papers—AIAA/ASME/ASCE/AHS/ASC Structures,Structural Dynamics and Materials Conf. (Denver, CO)vol 3, pp 2050–63

[52] Rezanejad S and Kokabi M 2007 Eur. Polym. J. 43 2856–65[53] Khonakdar H A, Jafari S H, Rasouli S, Morshedian J and

Abedini H 2007 Macromol. Theory Simul. 16 43–52[54] Gunes I S, Cao F and Jana S C 2008 Polymer 49 2223–34[55] Auad M L, Contos V S, Nutt S, Aranguren M I and

Marcovich N E 2008 Polym. Int. 57 651–9[56] Fei P and Cavicchi K 2010 ACS Appl. Mater. Interfaces

2 2797–803[57] Nagata M and Yamamoto Y 2010 Macromol. Chem. Phys.

211 1826–35[58] Inomata K, Nakagawa K, Fukuda C, Nakada Y, Sugimoto H

and Nakanishi E 2010 Polymer 51 793–8[59] Zhang W, Chen L and Zhang Y 2009 Polymer 50 1311–5[60] Wang L S, Chen H C, Xiong Z C, Pang X B and Xiong C D

2010 Mater. Lett. 64 284–6[61] Mather P T, Liu C and Campo C J 2007 US Patent 7371799[62] Weiss R A, Izzo E and Mandelbaum S 2008 Macromolecules

41 2978–80[63] Behl M, Ridder U, Feng Y, Kelch S and Lendlein A 2009

Soft Matter 5 676–84[64] Li S, Lu L and Zeng W 2009 J. Appl. Polym. Sci. 112 3341–6[65] Zhang H, Wang H, Zhong W and Du Q 2009 Polymer

50 1596–601[66] Zhu G, Xu S, Wang J and Zhang L 2006 Radiat. Phys. Chem.

75 443–8[67] Kolesov I S and Radusch H J 2008 Express Polym. Lett.

2 461–73[68] Alteheld A, Feng Y, Kelch S and Lendlein A 2005 Angew.

Chem. Int. Edn 44 1188–92[69] Zhou J W, Schmidt A M and Ritter H 2010 Macromolecules

43 939–42[70] Knight P T, Lee K M, Chung T and Mather P T 2009

Macromolecules 42 6596–605[71] Chung Y C, Choi J H and Chun B C 2008 J. Mater. Sci.

43 6366–73[72] Xu J and Shi W 2006 Polymer 47 457–65[73] Alonso-Villanueva J, Cuevas J M, Laza J M, Vilas J L and

Leon L M 2010 J. Appl. Polym. Sci. 115 2440–7[74] Kunzelman J, Chung T, Mather P T and Weder C 2008

J. Mater. Chem. 18 1082–6[75] Voit W, Ware T and Gall K 2010 Polymer 51 3551–9[76] Sun H 1993 Macromolecules 26 5924–36[77] Nji J and Li G 2010 Smart Mater. Struct. 19 035007[78] Zhang P and Li G 2013 J. Polym. Sci. B 51 966–77[79] Rousseau I and Mather P T 2003 J. Am. Chem. Soc.

125 15300–1[80] Rousseau I A, Qin H and Mather P T 2005 Macromolecules

38 4103–13[81] Ahn S K, Deshmukh P and Kasi R M 2010 Macromolecules

43 7330–40[82] Ahn S K, Deshmukh P, Gopinadhan M, Osuji C O and

Kasi R M 2011 ACS Nano 5 3085–95[83] Hu J, Zhu Y, Huang H and Lu J 2012 Prog. Polym. Sci.

37 1720–63[84] Lee K M, Koerner H, Vaia R A, Bunning T J and White T J

2011 Soft Matter 7 4318–24

18

Smart Mater. Struct. 22 (2013) 093001 Topical Review

[85] Lee K M, Wang D H, Koerner H, Vaia R A, Tan L S andWhite T J 2012 Angew. Chem. Int. Edn 51 4117–21

[86] Yu Y and Ikeda T 2005 Macromol. Chem. Phys. 206 1705–8[87] Beblo R V and Weiland L M 2009 J. Appl. Mech. 76 011008[88] Sodhi J S and Rao I J 2010 Int. J. Eng. Sci. 48 1576–89[89] Wu L B, Jin C L and Sun X Y 2011 Biomacromolecules

12 235–41[90] Kloxin C J, Scott T F, Adzima B J and Bowman C N 2010

Macromolecules 43 2643–53[91] Ishida K, Nishiyama Y, Michimura Y, Oya N and Yoshie N

2010 Macromolecules 43 1011–5[92] Ishida K and Yoshie N 2008 Macromolecules 41 4753–7[93] Yoshie N, Watanabe M, Araki H and Ishida K 2010 Polym.

Degrad. Stab. 95 826–9[94] Defize T, Riva R, Raquez J-M, Dubois P, Jerome C and

Alexandre M 2011 Macromol. Rapid Commun. 32 1264–9[95] Inglis A J, Nebhani L, Altintas O, Schmidt F G and

Barner-Kowollik C 2010 Macromolecules 43 5515–20[96] Defize T, Riva R, Jerome C and Alexandre M 2012

Macromol. Chem. Phys. 213 187–97[97] Toncelli C, Reus D C D, Picchioni F and Broekhuis A A

2012 Macromol. Chem. Phys. 213 157–65[98] Zhang J, Niu Y, Huang C, Xiao L, Chen Z, Yang K and

Wang Y 2012 Polym. Chem. 3 1390–3[99] Aoki D, Teramoto Y and Nishio Y 2007 Biomacromolecules

8 3749–57[100] Capadona J, Shanmuganathan K, Tyler D, Rowan S and

Weder C 2008 Science 319 1370–4[101] Li J, Viveros J A, Wrue M H and Anthamatten M 2007 Adv.

Mater. 19 2851–5[102] Li J, Lewis C L, Chen D L and Anthamatten M 2011

Macromolecules 44 5336–43[103] Chen S, Hu J L, Zhuo H, Yuen C and Chan L 2010 Polymer

51 240–8[104] Chen S, Hu J, Yuen C and Chana L 2009 Mater. Lett.

63 1462–4[105] Chen S, Hu J, Yuen C and Chan L 2009 Polymer 50 4424–8[106] Jaudouin O, Robin J A, Lopez-Cuesta J M, Perrina D and

Imbert C 2011 Polym. Int. 61 495–510[107] Zhu Y, Hu J, Lu J, Yeung L and Yeung K 2008 Polym. Adv.

Technol. 19 1745–53[108] Liu G, He W, Peng Y and Xia H 2011 J. Polym. Res.

18 2109–17[109] Kumpfer J R and Rowan S J 2011 J. Am. Chem. Soc.

133 12866–74[110] Yan X, Xu D, Chi X, Chen J, Dong S, Ding X, Yu Y and

Huang F 2012 Adv. Mater. 24 362–9[111] Gann M J M, Higginbotham C L, Geever L M and

Nugent M J D 2009 Int. J. Pharm. 372 154–61[112] Tourrette A, Geyter N D, Jocic D, Morent R,

Warmoeskerken M M C G and Leys C 2009 Colloids Surf.A 352 126–35

[113] Liu B, Hu J and Meng Q 2009 J. Biomed. Mater. Res. B89B 1–8

[114] Chen J, Liu M, Liu H, Ma L, Gao C, Zhu S and Zhang S2010 Chem. Eng. J. 159 247–56

[115] Gras S L, Mahmud T, Rosengarten G, Mitchell A andKalantar-zadeh K 2007 ChemPhysChem 8 2036–50

[116] Lu H, Liu Y, Gou J, Leng J and Du S 2010 Smart Mater.Struct. 19 075021

[117] Lee H F and Yu H H 2011 Soft Matter 7 3801–7[118] Leng J, Lv H, Liu Y and Du S 2007 Appl. Phys. Lett.

91 144105[119] Yu K, Zhang Z, Liu Y and Leng J 2011 Appl. Phys. Lett.

98 074102[120] Jung Y C, Yoo H J, Kim Y A, Cho J W and Endo M 2010

Carbon 48 1598–603[121] Luo X and Mather P 2010 Soft Matter 6 2146–9[122] Lu H and Gou J 2012 Polym. Adv. Technol. 23 1529–35

[123] Lu H B, Liang F and Gou J H 2011 Soft Matter 7 7416–23[124] Gunes I S, Jimenez G A and Jana S C 2009 Carbon

47 981–97[125] Leng J, Lan X, Liu Y and Du S 2009 Smart Mater. Struct.

18 074003[126] Sahoo N G, Jung Y C, Yoo H J and Cho J W 2007 Compos.

Sci. Technol. 67 1920–9[127] Lu H, Liu Y and Leng J 2012 Macromol. Mater. Eng.

297 1138–47[128] Rogers N and Khan F 2013 Polym. Test. 32 71–7[129] Lu H, Gou J, Leng J and Du S 2011 Appl. Phys. Lett.

98 174105[130] Weigel T, Mohr R and Lendlein A 2009 Smart Mater. Struct.

18 025011[131] Cuevas J M, Alonso J, German L, Iturrondobeitia M,

Laza J M, Vilas J L and Leon L M 2009 Smart Mater.Struct. 18 075003

[132] Razzaq M Y, Behl M, Kratz K and Lendlein A 2009Material Research Society Symp. Proc. vol1140 p 1140-HH1105-1107

[133] Kumar U N, Kratz K, Wagermaier W, Behl M andLendlein A 2010 J. Mater. Chem. 20 3404–15

[134] Buckley P R, McKinley G H, Wilson T S, Small W,Benett W J, Bearinger J P, McElfresh M W andMaitland D J 2006 IEEE Trans. Biomed. Eng. 53 2075–83

[135] Kumar U N, Kratz K, Behl M and Lendlein A 2012 ExpressPolym. Lett. 6 26–40

[136] Golbang A and Kokabi M 2010 Adv. Mater. Res.123–125 999–1002

[137] Zeng M, Or S W and Chan H L W 2010 J. Appl. Phys.107 09A942

[138] Vialle G, Prima M D, Hocking E, Gall K, Garmestani H,Sanderson T and Arzberger S C 2009 Smart Mater. Struct.18 115014

[139] Zheng X, Zhou S, Xiao Y, Yu X, Li X and Wu P 2009Colloids Surf. B 71 67–72

[140] Yu K, Westbrook K K, Kao P H, Leng J S and Qi H J 2013J. Compos. Mater. 47 51–63

[141] Sun L and Huang W M 2010 Mater. Des. 31 2684–9[142] Du H and Zhang J 2010 Soft Matter 6 3370–6[143] Zhu Y, Hu J, Luo H, Young R J, Deng L, Zhang S, Fan Y and

Ye G 2012 Soft Matter 8 2509–17[144] Lu H B, Liu Y J, Leng J S and Du S Y 2009 Smart Mater.

Struct. 18 085003[145] Lu H, Liu Y, Leng J and Du S 2009 Smart Mater. Struct.

18 085003[146] Huang W M, Yang B, Li C, Chan Y S and An L 2010 Appl.

Phys. Lett. 97 056102[147] Lu H, Liu Y, Leng J and Du S 2010 Eur. Polym. J.

46 1908–14[148] Wang C C, Zhao Y, Purnawali H, Huang W M and Sun L

2012 React. Funct. Polym. 72 757–64[149] Zhao Y, Huang W M and Wang C C 2012 Nanosci.

Nanotechnol. Lett. 4 862–78[150] Leng J, Zhang D, Liu Y, Yu K and Lan X 2010 Appl. Phys.

Lett. 96 111905[151] Jung Y C, Kim H H, Kim Y A, Kim J H, Cho J W, Endo M

and Dresselhaus M S 2010 Macromolecules 43 6106–12[152] Liang J J, Xu Y F, Huang Y, Zhang L, Wang Y, Ma Y F,

Li F F, Guo T Y and Chen Y S 2009 J. Phys. Chem. C113 9921–7

[153] Mather P T, Luo X and Rousseau I A 2009 Annu. Rev. Mater.Res. 39 445–71

[154] Gunes I S and Jana S C 2008 J. Nanosci. Nanotechnol.8 1616–37

[155] Dietsch B and Tong T 2007 J. Adv. Mater. 39 3–12[156] Ratna D and Karger-Kocsis J 2008 J. Mater. Sci. 43 254–69[157] Rousseau I A and Xie T 2010 J. Mater. Chem. 20 3431–41

19

Smart Mater. Struct. 22 (2013) 093001 Topical Review

[158] Behl M, Zotzmann J and Lendlein A 2010 Shape MemoryPolymers vol 226, ed A Lendlein (Berlin: Springer)pp 1–40 (Shape Memory Polymer Composites)

[159] Yakacki C M and Gall K 2010 Shape Memory Polymersvol 226, ed A Lendlein (Berlin: Springer) pp 147–75(Shape Memory Polymers for Biomedical Applications)

[160] Meng Q and Hu J 2009 Composites A 40 1661–72[161] Madbouly S A and Lendlein A 2010 Shape Memory

Polymers vol 226, ed A Lendlein (Berlin: Springer)pp 41–95 (Shape Memory Polymer Composites)

[162] Liu Y, Lan X, Lu H and Leng J 2010 Int. J. Mod. Phys. B24 2351–6

[163] Liu Y, Lv H, Lan X, Leng J and Du S 2009 Compos. Sci.Technol. 69 2064–8

[164] Behl M, Razzaq M and Lendlein A 2010 Adv. Mater.22 3388–410

[165] Sun L, Huang W M, Ding Z, Zhao Y, Wang C C,Purnawali H and Tang C 2012 Mater. Des. 33 577–640

[166] Meng H and Li G 2013 Polymer 54 2199–221[167] Meng H and Li G 2013 J. Mater. Chem. A 1 7838–65[168] Small I W, Singhal P, Wilson T S and Maitland D J 2010

J. Mater. Chem. 20 3356–66[169] Mano J F 2008 Adv. Eng. Mater. 10 515–27[170] Lendlein A and Behl M 2008 Adv. Sci. Technol. 54 96–102[171] Small W, Wilson T S, Buckley P R, Benett W J, Loge J M

and Hartman J 2007 IEEE Trans. Biomed. Eng.54 1657–66

[172] Ortega J, Small W, Wilson T, Benett W, Loge J andMaitland D 2007 IEEE Trans. Biomed. Eng. 54 1722–4

[173] Wischke C, Neffe A T, Steuer S and Lendlein A 2009J. Control. Release 138 243–50

[174] Nagahama K, Ueda Y, Ouchi T and Ohya Y 2009Biomacromolecules 10 1789–94

[175] Yakacki C M, Shandas R, Safranski D, Ortega A M,Sassaman K and Gall K 2008 Adv. Funct. Mater.22 2428–35

[176] Madbouly S A and Lendlein A 2012 Macromol. Mater. Eng.297 1213–24

[177] Tang S, Zhang C Y, Huang M N, Luo Y F and Liang Z Q2013 Contraception 87 235–41

[178] Zhang D, Petersen K M and Grunlan M A 2013 ACS Appl.Mater. Interfaces 5 186–91

[179] Serrano M C and Ameer G A 2012 Macromol. Biosci.12 1156–71

[180] Ware T et al 2012 Macromol. Mater. Eng. 297 1193–202[181] Zhao Q, Behl M and Lendlein A 2013 Soft Matter 9 1744–55[182] Li Y K 2007 Masters Thesis Hong Kong Polytechnic

University[183] Mondal S, Hu J L and Zhu Y 2006 J. Membrane Sci.

280 427–32[184] Hu J, Meng H, Li G and Ibekwe S 2012 Smart Mater. Struct.

21 053001[185] Meng H and Hu J L 2010 J. Intell. Mater. Syst. Struct.

21 859–85[186] Meng Q, Hu J, Shen L, Hu Y and Han J 2009 J. Appl. Polym.

Sci. 113 2440–9[187] Meng Q, Hu J, Zhu Y, Lv J and Liu B 2009 Text. Res. J.

79 522–1533[188] Meng Q and Hu J 2008 Polym. Adv. Technol. 19 131–6[189] Meng Q H, Hu J L, Zhu Y, Lu J and Liu Y 2007 J. Appl.

Polym. Sci. 106 2515–23[190] Eddington D T and Beebe D J 2004 Adv. Drug Deliv. Rev.

56 199–210[191] Toensmeier P A 2005 Plast. Eng. 61 10–11[192] Hearon K, Smith S E, Maher C A, Wilson T S and

Maitland D J 2013 Radiat. Phys. Chem. 83 111–21[193] Chen Y J, Sun J, Liu Y J and Leng J S 2012 Smart Mater.

Struct. 21 094021

[194] Li G, King A, Xu T and Huang X 2013 J. Mater. Civ. Eng.25 393–402

[195] Voyiadjis G Z, Shojaei A, Li G and Kattan P 2012 Int. J.Damage Mech. 21 391–414

[196] Nji J and Li G 2012 Smart Mater. Struct. 21 025011[197] Li G and Xu T 2011 ASCE J. Transp. Eng. 137 805–14[198] Xu W and Li G 2011 ASME J. Appl. Mech. 78 061017[199] Xu T, Li G and Pang S S 2011 Composites A 42 1525–33[200] Xu T and Li G 2011 Mater. Sci. Eng. A 528 6804–11[201] Chen Z B, Kim Y Y and Krishnaswamy S 2012 J. Appl.

Phys. 112 124319[202] Charlesby A 1960 Atomic Radiation and Polymers

(New York: Pergamon) p 198[203] Kunzelman J, Chung T, Mather P and Weder C 2008

J. Mater. Chem. 18 1082–6[204] DiOrio A M, Luo X, Lee K M and Mather P T 2011 Soft

Matter 7 68–74[205] Hines L, Arabagi V and Sitti M 2012 IEEE Trans. Robot.

28 987–90[206] Rossiter J, Takashima K and Mukai T 2012 Smart Mater.

Struct. 21 112002[207] Xiao X, Xie T and Cheng Y T 2010 J. Mater. Chem.

20 3508–14[208] Li G, Ajisafe O and Meng H 2013 Polymer 54 920–8[209] Li G, Meng H and Hu J 2012 J. R. Soc. Interface 9 3279–87[210] Shojaei A and Li G 2013 Int. J. Plast. 42 31–49[211] John M and Li G 2010 Smart Mater. Struct. 19 075013[212] Li G, Ji G and Ouyang Z 2012 Int. J. Adhes. Adhes. 35 59–67[213] Ji G and Li G 2013 Mater. Des. 51 79–87[214] Li G and John M 2008 Compos. Sci. Technol. 68 3337–43[215] Oosten C L V, Corbett D, Davies D, Warner M,

Bastiaansen C W M and Broer D J 2008 Macromolecules41 8592–6

[216] Li G and Zhang P 2013 Polymer at press[217] Xu W and Li G 2010 Int. J. Solids Struct. 47 1306–16[218] Li G and Shojaei A 2012 Proc. R. Soc. A 468 2319–46[219] Shojaei A, Li G and Voyiadjis G Z 2013 ASME J. Appl.

Mech. 80 011014[220] Campbell D and Maji A 2006 J. Aerosp. Eng. 19 184–93[221] Yang J H, Chun B C, Chung Y C, Cho J W and Cho B G

2004 J. Appl. Polym. Sci. 94 302–7[222] Xia F, Zhu Y, Feng L and Jiang L 2009 Soft Matter 5 275–81[223] Athanassiou A, Lygeraki M, Pisignano D, Lakiotaki K,

Varda M and Mele E 2006 Langmuir 22 2329–33[224] Athanassiou A, Kalyva M, Lakiotaki K, Georgiou S and

Fotakis C 2005 Adv. Mater. 17 988–92[225] Xie T and Xiao X 2008 Chem. Mater. 20 2866–8[226] Wornyo E, May G and Gall K 2009 IEEE Trans. Semicond.

Manuf. 22 409–16[227] Bellin I, Kelch S and Lendlein A 2007 J. Mater. Chem.

17 2885–91[228] Behl M, Bellin I, Kelch S, Wagermaier W and Lendlein A

2008 Proc. Advances in Material Design for RegenerativeMedicine, Drug Delivery, and Targeting/Imaging(Warrendale, PA)

[229] Pretsch T 2010 Smart Mater. Struct. 19 015006[230] Bellin I, Kelch S, Langer R and Lendlein A 2006 Proc.

National Academy of Sciences. USA vol 103 pp 18043–7[231] Bellin I, Kelch S, Langer R and Lendlein A 2006 Proc. Natl

Acad. Sci. USA 103 18043–7[232] Chen S J, Hu J L, Yuen C W, Chan L and Zhuo H 2010

Polym. Adv. Technol. 21 377–80[233] Paderni K, Pandini S, Passera S, Pilati F, Toselli M and

Messori M 2012 J. Mater. Sci. 47 4354–62[234] Kumar U N, Kratz K, Behl M and Lendlein A 2009 Material

Research Society Symp. Proc. vol 1190 p 1190-NN03-21[235] Hu X, Zhou F, Hu S and Scullion T 2011 J. Test Eval.

39 177–83[236] Ahn S K and Kasi R M 2011 Adv. Funct. Mater. 21 4543–9

20

Smart Mater. Struct. 22 (2013) 093001 Topical Review

[237] Zhang H, Chen Z, Zheng Z, Zhu X M and Wang H T 2013Mater. Chem. Phys. 137 750–5

[238] Kolesov I S and Radusch H J 2008 Express Polym. Lett.2 461–73

[239] Cuevas J M, Rubio R, German L, Laza J M, Vilas J L,Rodriguez M and Leon L M 2012 Soft Matter 8 4928–35

[240] Xie T, Xiao X and Cheng Y-T 2009 Macromol. RapidCommun. 30 1823–7

[241] Bae C Y, Park J H, Kim E Y, Kang Y S and Kim B K 2011J. Mater. Chem. 21 11288–95

[242] Luo X and Mather P T 2010 Adv. Funct. Mater. 20 2649–56[243] Behl M, Bellin I, Kelch S, Wagermaier W and Lendlein A

2009 Adv. Funct. Mater. 19 102–8[244] Zotzmann J, Behl M, Feng Y and Lendlein A 2010 Adv.

Funct. Mater. 20 3583–94[245] Xu T and Li G 2011 Polymer 52 4571–80[246] Yakacki C, Willis S, Luders C and Gall K 2008 Adv. Eng.

Mater. 10 112–9[247] Miaudet P, Derre A, Maugey M, Zakri C, Piccione P M,

Inoubli R and Poulin P 2007 Science 318 1294–6[248] Xie T 2010 Nature 464 267–70[249] Li J J and Xie T 2011 Macromolecules 44 175–80[250] Yakacki C M, Ortega A M, Frick C P, Lakhera N, Xiao R

and Nguyen T D 2012 Macromol. Mater. Eng. 297 1160–6[251] Lakhera N, Yakacki C M, Nguyen T D and Frick C P 2012

J. Appl. Polym. Sci. 126 72–82[252] Yakacki C M, Nguyen T D, Likos R, Lamell R, Guigou D

and Gall K 2012 Polymer 52 4947–54[253] Xie T, Page K A and Eastman S A 2011 Adv. Funct. Mater.

21 2057–66[254] Kratz K, Madbouly S, Wagermaier W and Lendlein A 2011

Adv. Mater. 23 4058–62[255] Razzaq M Y, Behl M and Lendlein A 2012 Adv. Funct.

Mater. 22 184–91[256] Li J, Liu T, Pan Y, Xia S, Zheng Z H, Ding X B and

Peng Y X 2012 Macromol. Chem. Phys. 213 2246–52[257] Ware T, Hearon K, Lonnecker A, Wooley K L, Maitland D J

and Voit W 2012 Macromolecules 45 1062–9[258] Shao Y, Lavigueur C and Zhu X X 2012 Macromolecules

45 1924–30[259] Li J, Liu T, Xia S, Pan Y, Zheng Z, Ding X and Peng X 2011

J. Mater. Chem. 21 12213–7[260] Sun L and Huang W M 2010 Soft Matter 6 4403–6[261] Yu K, Xie T, Leng J, Ding Y and Qi H J 2012 Soft Matter

8 5687–95[262] Choi J, Ortega A M, Xiao R, Yakacki C M and Nguyen T D

2012 Polymer 53 2453–64[263] Nguyen T D, Yakacki C M, Brahmbhatt P D and

Chambers M L 2010 Adv. Mater. 22 3411–23[264] Liu Y, Boyles J K, Genzer J and Dickey M D 2012 Soft

Matter 8 1764–9[265] Fei G, Li G, Wu L and Xia H 2012 Soft Matter 8 5123–6[266] Zhang H, Xia H and Zhao Y 2012 J. Mater. Chem. 22 845–9[267] He Z W, Satarkar N, Xie T, Cheng Y T and Hilt J Z 2011

Adv. Mater. 23 3192–6[268] Chung T, Romo-Uribe A and Mather P T 2008

Macromolecules 41 184–92[269] Westbrook K K, Parakh V, Chung T, Mather P T, Wan L C,

Dunn M L and Qi H J 2010 Trans. ASME, J. Eng. Mater.Technol. 132 041010

[270] Hong S J, Yu W R and Youk J H 2010 Smart Mater. Struct.19 035022

[271] Lee K M, Knight P T, Chung T and Mather P T 2008Macromolecules 4 4730–8

[272] Raquez J M, Vanderstappen S, Meyer F, Verge P,Alexandre M, Thomassin J M, Jerome C and Dubois P2011 Chem.—Eur. J. 17 10135–43

[273] Zotzmann J, Behl M, Hofmann D and Lendlein A 2010 Adv.Mater. 22 3424–9

[274] Kang T H, Lee J M, Yu W R, Youk J H and Ryu H W 2012Smart Mater. Struct. 21 035028

[275] Burke K A and Mather P T 2010 J. Mater. Chem. 20 3449–57[276] Ahir S V, Tajbakhsh A R and Terentjev E M 2006 Adv.

Funct. Mater. 16 556–60[277] Krause S, Dersch R, Wendorff J H and Finkelmann H 2007

Macromol. Rapid Commun. 28 2062–8[278] Langer R S and Lendlein A 2002 US Patent 6388043 B1[279] Chen S, Hu J and Zhuo H 2010 Compos. Sci. Technol.

70 1437–43[280] Tobushi H, Hayashi S, Sugimoto Y and Date K 2009

Materials 2 1180–92[281] Ghosh P, Rao A and Srinivasa A R 2013 Mater. Des.

44 164–71[282] Simpson B, Nunnery G, Tannenbaum R and Kalaitzidou K

2010 J. Mater. Chem. 20 3496–501[283] Kalaitzidou K and Crosby A J 2008 Appl. Phys. Lett.

93 041910[284] LeMieux M C, McConney M E, Lin Y-H, Singamaneni S,

Jiang H, Bunning T J and Tsukruk V V 2006 Nano Lett.6 730–4

[285] Saha S, Copic D, Bhaskar S, Clay N, Donini A, Hart A J andLahann J 2012 Angew. Chem. Int. Edn 51 660–5

[286] Bhaskar S, Gibson C T, Yoshida M, Nandivada H, Deng X,Voelcker N H and Lahann J 2011 Small 7 812–9

[287] Bhaskar S, Hitt J, Chang S L and Lahann J 2009 Angew.Chem. Int. Edn 48 4580–93

[288] Bhaskar S and Lahann J 2009 J. Am. Chem. Soc. 131 6650–1[289] Finkelmann H, Kim S T, Munoz A, Palffy-Muhoray P and

Taheri B 2001 Adv. Mater. 13 1069–72[290] Yu Y, Nakano M, Shishido A, Shiono T and Ikeda T 2004

Chem. Mater. 16 1637–43[291] Tamesue S, Takashima Y, Yamaguchi H, Shinkai S and

Harada A 2010 Angew. Chem. 122 7623–6[292] Nakano M, Yu Y, Shishido A, Tsutsumi O, Kanazawa A,

Shiono T and Ikeda T 2003 Mol. Cryst. Liq. Cryst.398 1–9

[293] Jin L, Yan Y and Huo Y 2010 Int. J. Non-Linear Mech.45 370–81

[294] Serak S, Tabiryan N, Vergara R, White T J, Vaia R A andBunning T J 2010 Soft Matter 6 779–83

[295] Yu Y L and Ikeda T 2006 Angew. Chem. Int. Edn 45 5416–8[296] Iqbal D and Samiullah M H 2013 Materials 6 116–42[297] Mamiya J, Yoshitake A, Kondo M, Yu Y and Ikeda T 2008

J. Mater. Chem. 18 63–5[298] Domenici V, Ambrozic G, Copic M, Lebar A,

Drevensek-Olenik I, Umek P, Zalar B, Zupanc B andZigon M 2009 Polymer 50 4837–44

[299] Kawatsuki N and Uchida E 2007 Polymer 48 3066–73[300] Ikeda T, Mamiya J and Yu Y 2007 Angew. Chem. Int. Edn

46 506–28[301] Koerner H, White T J, Tabiryan N V, Bunning T J and

Vaia R A 2008 Mater. Today 11 34–42[302] Barrett C J, Mamiya J, Yager K G and Ikeda T 2007 Soft

Matter 3 1249–61[303] Schumers J M, Fustin C A and Gohy J F 2010 Macromol.

Rapid Commun. 31 1588–607[304] Russew M-M and Hecht S 2010 Adv. Mater. 22 3348–60[305] Pretsch T 2010 Polymers 2 120–58[306] White T J 2012 J. Polym. Sci. B 50 877–80[307] Yoshino T, Kondo M, Mamiya J, Kinoshita M, Yu Y and

Ikeda T 2010 Adv. Mater. 22 1361–3[308] White T J, Tabiryan N, Tondiglia V P, Serak S, Hrozhyk V,

Vaia R A and Bunning T J 2008 Soft Matter 4 1796–8[309] Camacho-Lopez M, Finkelmann H, Palffy-Muhoray P and

Shelley M 2004 Nature Mater. 3 307–10[310] Yamada M, Kondo M, Mamiya J, Yu Y, Kinoshita M,

Barrett C J and Ikeda T 2008 Angew. Chem. Int. Edn47 4986–8

21

Smart Mater. Struct. 22 (2013) 093001 Topical Review

[311] Jiang H Y, Kelch S and Lendlein A 2006 Adv. Mater.18 1471–5

[312] Beblo R V and Weiland L M 2010 Smart Mater. Struct.19 094012

[313] Li M and Keller P 2006 Phil. Trans. Math. Phys. Eng. Sci.364 2763–77

[314] Yamada M, Kondo M, Miyasato R, Naka Y, Mamiya J-I,Kinoshita M, Shishido A, Yu Y, Barrett C J and Iked T2009 J. Mater. Chem. 19 60–2

[315] Chen X, Zhang Y, Liu B, Zhang J, Wang H, Zhang W,Chen Q, Pei S and Jiang Z 2008 J. Mater. Chem.18 5019–26

[316] Lee M J, Jung D H and Han Y K 2006 Mol. Cryst. Liq. Cryst.444 41–50

[317] Sawaa Y, Ye F, Urayama K, Takigawa T, Gimenez-Pinto V,Selinger R L B and Selinger J V 2011 Proc. Natl Acad.Sci. 108 6364–8

[318] Lee K M, Tabiryan N V, Bunning T J and White T J 2012J. Mater. Chem. 22 691–8

[319] Lee K M, Koerner H, Vaia R, Bunning T and White T 2010Macromolecules 43 8185–90

[320] Ikeda T, Nakano M, Lei Y Y, Tsutsumi O and Kanazawa A2003 Adv. Mater. 15 201–5

[321] Small W IV, Wilson T S, Benett W J, Loge J M andMaitland D J 2005 Opt. Express 13 8204–13

[322] Yakacki C, Shandas R, Lanning C, Rech B, Eckstein A andGall K 2007 Biomaterials 28 2255–63

[323] Li M H, Keller P, Li B, Wang X G and Brunet M 2003 Adv.Mater. 15 569–72

[324] Small W, Buckley P, Wilson T, Benett W and Hartman J2007 IEEE Trans. Biomed. Eng. 54 1157–60

[325] Rickert D, Lendlein A, Peters I, Moses M A and Franke R2006 Eur. Arch. Oto-Rhino-Laryng. 263 215–22

[326] Yagai S and Kitamura A 2008 Chem. Soc. Rev. 37 1520–9[327] Sortino S 2008 Photochem. Photobiol. Sci. 7 911–24[328] He D M, Susanto H and Ulbricht M 2009 Prog. Polym. Sci.

34 62–98[329] Liu N, Dunphy D R, Atanassov P, Bunge S D, Chen Z,

Lopez G P, Boyle T J and Brinker C J 2004 Nano Lett.4 551–4

[330] Lu J, Choi E, Tamanoi F and Zink J I 2008 Small 4 421–6[331] Minoofar N, Dunn B S and Zink J I 2005 J. Am. Chem. Soc.

127 2656–65[332] Sierocki P, Maas H, Dragut P, Richardt G, Voegtle F,

Cola L D, Brouwer F and Zink J I 2006 J. Phys. Chem. B110 24390–8

[333] Klichko Y, Liong M, Choi E, Angelos S, Nel A E,Stoddart J F, Tamanoi F and Zinkw J I 2009 J. Am. Ceram.Soc. 92 S2–10

[334] Angelos S, Choi E, Volgtle F, Cola L D and Zink J I 2007J. Phys. Chem. C 111 6589–92

[335] Sierocki P, Maas H, Dragut P, Richardt G, Voegtle F,Cola L D, Brouwer F A M and Zink J I 2006 J. Phys.Chem. B 110 24390–8

[336] Angelos S, Choi E, Voegtle F, Cola L D and Zink J I 2007J. Phys. Chem. C 111 6589–92

[337] Yang Z, Herd G A, Clarke S M, Tajbakhsh A R,Terentjev E M and Huck W T S 2006 J. Am. Chem. Soc.128 1074–5

[338] Larpent C, Cannizzo C, Delgado A, Gouanve F, Sanghvi P,Gaillard C and Bacquet G 2008 Small 4 833–40

[339] Cheng F, Yin R, Zhang Y, Yen C C and Yu Y 2010 SoftMatter 6 3447–9

[340] Al-Kaysi R O and Bardeen C J 2007 Adv. Mater. 19 1276–80[341] Kondo M, Matsuda T, Fukae R and Kawatsuki N 2010

Chem. Lett. 39 234–5[342] He J, Zhao Y and Zhao Y 2009 Soft Matter 5 308–10[343] Thomsen D L, Keller P, Naciri J, Pink R, Jeon H, Shenoy D

and Ratna B R 2001 Macromolecules 34 5868–75

[344] Andreopoulos F M, Deible C R, Stauffer M T, Weber S G,Wagner W R, Beckman E J and Russell A J 1996 J. Am.Chem. Soc. 118 6235–40

[345] Andreopoulos F M, Beckman E J and Russell A J 1998Biomaterials 19 1343–52

[346] Shi D, Matsusaki M and Akashi M 2009 Macromol. Biosci.9 248–55

[347] Thi T H, Matsusaki M, Shi D, Kaneko T and Akashi M 2008J. Biomater. Sci., Polym. Edn 19 75–95

[348] Ahir S V and Terentjev E M 2005 Natural Mater. 4 491–5[349] Yang L, Setyowati K, Li A, Gong S and Chen J 2008 Adv.

Mater. 20 2271–5[350] Vaia R 2005 Nature Mater. 4 429–30[351] Ugur G, Chang J, Xiang S, Lin L and Lu J 2012 Adv. Mater.

24 2685–90[352] Scott T F, Draughon R B and Bowman C N 2006 Adv. Mater.

18 2128–32[353] Scott T F, Schneider A D, Cook W D and Bowman C N 2005

Science 308 1615–7[354] Sunder A and Mulhaupt R 1999 Macromol. Chem. Phys.

200 58–64[355] Long K N, Scott T F, Qi H J, Bowman C N and Dunn M L

2009 J. Mech. Phys. Solids 57 1103–21[356] Tabiryan N, Serak S, Dai X and Bunning T 2005 Opt.

Express 13 7442–8[357] Lee K M, Bunning T J and White T J 2012 Adv. Mater.

24 2839–43[358] Mahimwalla Z, Yager K G, Mamiya J, Shishido A,

Priimagi A and Barrett C J 2012 Polym. Bull.69 967–1006

[359] Garcia-Amoros J, Nonell S and Velasco D 2011 Chem.Commun. 47 4022–4

[360] Xu T and Li G 2011 Mater. Sci. Eng. A 528 7444–50[361] McClung A J W, Tandon G P and Baur J W 2012 Mech.

Time-Dep. Mater. 16 205–21[362] Srivastava V, Chester S A and Anand L 2010 J. Mech. Phys.

Solids 58 1100–24[363] Voyiadjis G Z, Shojaei A, Li G and Kattan P I 2012 Proc. R.

Soc. A 468 163–83[364] Voyiadjis G Z, Shojaei A and Li G 2011 Int. J. Plast.

27 1025–44[365] Voyiadjis G Z, Shojaei A and Li G 2012 Int. J. Plast.

28 21–45[366] Chen X and Nguyen T D 2011 Mech. Mater. 43 127–38[367] Baghani M, Naghdabadi R and Arghavani J 2012 J. Intell.

Mater. Syst. Struct. 24 21–32[368] Qi H J, Nguyen T D, Castro F, Yakacki C M and Shandas R

2008 J. Mech. Phys. Solids 56 1730–51[369] Chen Y and Lagoudas D C 2008 J. Mech. Phys. Solids

56 1752–65[370] Baghani M, Naghdabadi R, Arghavani J and Sohrabpour S

2012 Int. J. Plast. 35 13–30[371] Liu Y, Gall K, Dunn M L, Greenberg A R and Diani J 2006

Int. J. Plast. 22 279–313[372] Ghosh P, Reddy J N and Srinivasa A R 2013 Int. J. Solids

Struct. 50 595–608[373] Nguyen T D, Qi H J, Castro F and Long K N 2008 J. Mech.

Phys. Solids 56 2792–814[374] Chen Y and Lagoudas D C 2008 J. Mech. Phys. Solids

56 1766–78[375] Zhou B, Liu Y and Leng J 2010 Sci. China Phys. Mech.

Astron. 53 2266–73[376] Ghosh P and Srinivasa A R 2011 Int. J. Eng. Sci. 49 823–38[377] Bonner M, Oca H M D, Brown M and Ward I M 2010

Polymer 51 1432–6[378] Baghani M, Naghdabadi R, Arghavani J and Sohrabpour S

2012 J. Intell. Mater. Syst. Struct. 37 107–16

22

Smart Mater. Struct. 22 (2013) 093001 Topical Review

[379] Diani J, Gilormini P, Fredy C and Rousseau I 2012 Int. J.Solids Struct. 49 793–9

[380] Nishikawa M, Wakatsuki K, Yoshimura A and Takeda N2012 Composites A 43 165–73

[381] Luo X and Mather P T 2009 Macromolecules 42 7251–3[382] Ge Q, Luo X, Rodriguez E D, Zhang X, Mather P T,

Dunn M L and Qi H J 2012 J. Mech. Phys. Solids60 67–83

[383] Zhang C, Hu J, Chen S and Ji F 2010 J. Mol. Modeling16 1391–9

[384] Zhang C, Hu J, Ji F, Fan Y and Liu Y 2012 J. Mol. Modeling18 1263–71

[385] Diani J and Gall K 2007 Smart Mater. Struct. 16 1575–83[386] Nguyen T 2013 Polym. Rev. 53 130–52[387] Nair D P, Cramer N B, Gaipa J C, McBride M K,

Matherly E M, McLeod R R, Shandas R and Bowman C N2012 Adv. Funct. Mater. 22 1502–10

[388] Koerner H, Strong R J, Smith M L, Wang D H, Tan L-S,Lee K M, White T J and Vaia R A 2013 Polymer54 391–402

[389] Jacobs J D, Arlen M J, Wang D H, Ounaies Z, Berry R,Tan L-S, Garrett P H and Vaia R A 2010 Polymer51 3139–46

[390] Yoonessi M, Shi Y, Scheiman D A, Lebron-Colon M,Tigelaar D M, Weiss R A and Meador M A 2012 ACSNano 6 7644–55

[391] Higgins M J, Grosse W, Wagner K, Molino P J andWallace G G 2011 J. Phys. Chem. B 115 3371–8

[392] Nelson B A, King W P and Gall K 2005 Appl. Phys. Lett.86 103108

[393] Li J, An Y, Huang R, Jiang H and Xie T 2012 ACS Appl.Mater. Interfaces 4 598–603

[394] Wang Z, Hansen C, Ge Q, Maruf S H, Ahn D U, Qi H J andDing Y F 2011 Adv. Mater. 23 3669–73

[395] Liu X, Chakraborty A and Luo C 2010 J. Micromech.Microeng. 20 095025

[396] Gall K, Kreiner P, Turner D and Hulse M 2004 IEEE/ASMEJ. Microelectromech. Syst. 13 472–83

[397] Grimes A, Breslauer D, Long M, Pegan J, Lee L andKhine M 2008 Lab Chip 8 170–2

[398] Chen C, Breslauer D, Luna J, Grimes A, Chin W, Lee L andKhine M 2008 Lab Chip 8 622–4

[399] Zhao Y, Huang W M and Fu Y Q 2011 J. Micromech.Microeng. 21 067007

[400] Yakacki C 2013 MedShape www.medshape.com/

23