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Polymer 54 (2013) 2199e2221

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Polymer

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Feature article

A review of stimuli-responsive shape memory polymer composites

Harper Meng a,b, Guoqiang Li a,b,*aDepartment of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, USAbDepartment of Mechanical Engineering, Southern University, Baton Rouge, LA 70813, USA

a r t i c l e i n f o

Article history:Received 14 December 2012Received in revised form31 January 2013Accepted 11 February 2013Available online 1 March 2013

Keywords:Stimuli-responsive polymerShape memory polymerSmart composite

Abbreviations: CB, carbon black; CD, cyclodextrin;carbon nanopaper; CNT, carbon nanotube; IPN, interpMSME, multiple-shape memory effect; MSMP, multiMWCNT, multi-walled carbon nanotube; PCLDMA,crylate; PEGMA, poly(ethylene glycol)mono-methyletshape memory alloy; SME, shape memory effect; SMSMPC, shape memory polymer composite.* Corresponding author. Department of Mechanica

Louisiana State University, Baton Rouge, LA 70803, Ufax: þ1 225 578 5924.

E-mail address: [email protected] (G. Li).

0032-3861� 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2013.02.023

Open access under CC BY

a b s t r a c t

The past decade has witnessed remarkable advances in stimuli-responsive shape memory polymers(SMPs) with potential applications in biomedical devices, aerospace, textiles, civil engineering, bionicsengineering, energy, electronic engineering, and household products. Shape memory polymer com-posites (SMPCs) have further enhanced and broadened the applications of shape memory polymers. Inaddition to reinforcement, SMPCs can enable or enhance athermal stimuli-active effects, novel shapememory effect, and new functions. Many athermal stimuli-responsive effects have been achieved such aselectroactive effect, magnetic-active effect, water-active effect, and photoactive effect. The typical ex-amples of novel shape memory effects are multiple-shape memory effect, spatially controlled shapememory effect, and two-way shape memory effect. In addition, new functions of SMPCs have beenobserved and systemically studied such as stimuli-memory effect and self-healing. This feature articlepresents an up-to-date review on these versatile SMPCs. The various methods to fabricate these SMPCsand the performances of the SMPCs are discussed. The potential directions for future advancement in thisfield are also discussed.

� 2013 Elsevier Ltd. Open access under CC BY-NC-ND license.

1. Introduction

Stimuli-responsive polymers significantly change their prop-erties such as shape, mechanical properties, phase separation,surface, permeability, optical properties, and electrical propertiesupon small variation of environmental conditions such as tem-perature, electric field, pH, light, magnetic field, electrical field,sonic field, solvent, ions, enzymes, and glucose [1e7]. Since 2010,three special and themed issues have been devoted to these smartmaterials: “Stimuli-responsive materials” in Progress in PolymerScience (2010, Vol. 35, issue 1-2); “Stimuli-sensitive polymers” inAdvanced Materials (2010, Vol. 22, issue 31); and “Actively mov-ing polymers” in Journal of Materials Chemistry (2010, Vol. 20,issue 17).

CNF, carbon nanofiber; CNP,enetrating polymer network;ple-shape memory polymer;poly(caprolactone) dimetha-her-monomethacrylate; SMA,P, shape memory polymer;

l and Industrial Engineering,SA. Tel.: þ1 225 578 5302;

-NC-ND license.

Shape memory polymers (SMPs) as a type of important stimuli-responsive polymers, can recover their original (or permanent)shape upon exposure to external stimuli [8e14]. Many review pa-pers have been published on various SMPs especially thermal-responsive SMPs [15e35]. Since the report of the first SMP, shapememory polynorbornene (high macromolecular weight withabundant physical cross-linking), numerous SMPs based ondifferent structures have been developed in labs and several typicalones have been commercialized in large scales such as poly-urethane (DiAPLEX, SMP Technologies Inc., originally from Mitsu-bishi Heavy Industries), polystyrene based SMP (Veriflex�,Verilyte�, Veritex�, Cornerstone Research Group, Inc.), aliphaticpolyurethane (Tecoflex�, Lubrizol Advanced Materials), epoxybased SMP (TEMBO�, Composite Technology Development, Inc.),and UV curable polyurethane (NOA-63, Norland Products Inc.).

A stable polymer network and a reversible switching transitionof the polymer are the two pre-requites for the shape memory ef-fect (SME) (see Fig. 1). The stable network of SMPs determines theoriginal shape, which can be formed by molecule entanglement,crystalline phase, chemical cross-linking, or interpenetratednetwork [36e42]. The lock in the network represents the reversibleswitching transition responsible for fixing the temporary shape,which can be crystallization/melting transition [38,40,43e45],vitrification/glass transition [25,35,38,43e82], liquid crystalanisotropic/isotropic transition [83e89], reversible molecule

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Fig. 1. Various molecular structures of SMPs. A stable network and a reversible switching transition are the prerequisites for the SMPs to show SME. The stable network can bemolecule entanglement, chemical cross-linking, crystallization, and IPN; the reversible switching transition can be crystallizationemelting transition, vitrificationeglass transition,anisotropiceisotropic transition, reversible chemical cross-linking, and associationedisassociation of supramolecular structures.

H. Meng, G. Li / Polymer 54 (2013) 2199e22212200

cross-linking, and supramolecular association/disassociation.Typical reversible molecule cross-linking reactions as switchingtransitions include photodimerization [8,90e94], DielseAlder re-action [95e103], and oxidation/redox reaction of mercapto group[104]. Typical switching transition per supramolecular association/disassociation includes hydrogen bonding [105e113], self-assemblymetaleligand coordination [114,115], and self-assembly of b-CD[116e121]. In addition to the above reversible switches, otherstimuli which can significantly change the mobility of the SMPmayalso trigger the SME, such as moisture, water/solvent, ions, pres-sure, light, pH, etc. [24].

SMPs can be used widely in many areas such as biomedicaldevices, aerospace, textiles, energy, bionics engineering, electronicengineering, civil engineering, and household products. It is diffi-cult to extensively elaborate all the applications. Table 1 summa-rizes the typical applications of SMPs. For detailed applicationstrategies, reader could go to the specified references.

The first research of shapememory polymer composites (SMPCs)may be reinforcement of SMPs. SMPs have intrinsic low mechanicalstrength and shape recovery stress, which have largely restricted theapplications of SMPs. A small amount of reinforcing fillers is able toimprove the mechanical performance and shape recovery stress ofSMPs. In addition to reinforcement effect, SMPCs can enable orenhance athermal stimuli-active effects, novel SME, and new func-tions as shown in Fig. 2. The examples of athermal stimuli-responsive effects include electroactive effect, magnetic-active ef-fect, water-active effect, and photoactive effect. The novel SMEsincludemultiple-shapememory effect, spatially controlled SME, andtwo-way SME. The two examples of new functions are stimuli-memory effect such as magnetic field-memory effect, and self-healing effect of SMPCs such as thermoplastic particles filled SMPs.

This paper is intended to present a comprehensive review of therecent progress of these various SMPCs in terms of the reinforce-ment, athermal stimuli-responsive effect, novel SME, and the newfunctions. The various methods and recent progress to fabricatethese SMPCs are presented. Though researchers are now able todesign various SMPCs with different properties for various

applications, there are still many challenges to overcome. Theresearch challenges in this field are discussed and future researchneeds are identified. By comprehensively summarizing the researchin this area, we hope this paper can not only help interdisciplinaryreaders to understand the state-of-the-art in this area but also shedsome light on future research directions in this research field.

2. Reinforcement of shape memory polymers (SMPs)

Although outstanding in many aspects as compared with SMAs,SMPs have intrinsic low mechanical strength and shape recoverystress. These problems have largely restricted the applications ofSMPs. Reinforcing fillers are able to improve the mechanical per-formance and shape recovery stress of SMPs through physicalblending, in-situ polymerization and chemical cross-linking[15,48,180,182,283e290].

2.1. Reinforcement by microfibers, fabrics, and mats

Microfiber, fabrics and mats made of carbon fibers, glass fibersand Kevlar fibers have high elastic modulus and high strength, andcan remarkably increase the mechanical strength of SMPs [291].These composites, in the fiber (axial) direction, can bearmuch highermechanical load; while in the transverse direction, the SME can bemostly maintained [292]. These reinforced SMPCs are mostlyproposed to be used for spacecraft self-deployable structures[190,292e295] and vibration control structures [296e301]. Fewupdated research was reported in the last five years on these SMPCs,which may be partially because the microfiber, fabric and mat canlower the SME of the SMP matrix, especially in the reinforcementdirection.

2.2. Reinforcement by carbon nanotubes (CNTs) or carbonnanofibers (CNFs)

One of the main objectives of incorporating CNTs or CNFs intoSMP is to achieve electroactive SME by Joule heating in intrinsic

Table 1Present and potential applications of SMPs.

Applications Ref. Applications Ref.

1 Aneurysm occlusion devices [122e125] 34 Orthodontic [126e129]2 Assembly/disassembly tools [32,33] 35 Orthopedic cast [130e132]3 Bio-MEMs [133] 36 Orthopedics Morphix� suture anchor [134,135]4 Bone defect fillers [136,137] 37 Packaging [138e140]5 Cardiac valve repair [141,142] 38 Pharyngeal mucosa reconstruction [143]6 Cells manipulating and capturing [144,145] 39 Phase change fabrics [146e148]7 Chemical feeding in chemical reactions [149,150] 40 Physiological monitoring [151]8 Clot removal devices [152e154] 41 Post-surgical treatment of mitral insufficiency [155,156]9 Controlled drug release [157e164] 42 Pressure garments [165]10 Crease and pattern retention finishing [166] 43 Recordable and erasable memories [167e170]11 Damping fabrics [171,172] 44 Study of cell proliferation [173e176]12 Damping materials [177] 45 Selective desalination material [178]13 Deodorant fabrics [179] 46 Self-healing [180e189]14 Electroactive shape memory hinge [190] 47 Self-peeling dry adhesive [191]15 Embolic devices [123] 48 Shape changing nanofibers [192]16 Endoscopic surgery suture [193e197] 49 Shape memory fibers [10,198e207]17 Erasable Braille [208e210] 50 Smart mandrels for composite tooling [211]18 Fashion design [212e215] 51 Shape memory neuronal probe [216]19 Flexible light-emitting diodes [217] 52 Skin-care products [133]20 Hair treatment [218,219] 53 Soft lithography [220]21 Heat and moisture management [221e227] 54 Surface wetting [170,228e233]22 Heat shrinkable packages for electronics [234] 55 Microfluidic devices [235]23 Hot shrinkage micro-tags [236] 56 Surface wrinkle and micro-patterns [237e240]24 Kidney dialysis needles [241,242] 57 Switchable information carriers [243e245]25 Light-modulators and display devices [246] 58 Temperature sensors [78,243]26 Measuring tools in complex cavities [236] 59 Toys [247]27 Memory foam mattress, pillow and insoles [248] 60 Treatment of esophageal stenosis [249,250]28 MEMs applications [251e257] 61 Vascular stents [258e262]29 Microtweezers in medicine [90] 62 Surgery inside living cells [263]30 Micro-valves in microdevices [8,91,264,265] 63 Vehicle active air dams and other aerodynamic

surfaces[266,267]

31 Morphing of aircraft wings and helicopterrotor blades

[268e270] 64 Weight reducing agent [13]

32 Novel McKibben artificial muscles [271] 65 Wound dressing [272]33 Ophthalmic applications [273e277] 66 Wrinkle free finishing of cotton fabrics [278e282]

H. Meng, G. Li / Polymer 54 (2013) 2199e2221 2201

thermal-responsive SMPs [298,302e310]. The strategies to improvethe electroactive SME of SMPCs can be seen in Section 3.

The biggest challenge of CNT/SMP composites is to disperse theCNTs uniformly in SMPmatrix. Manymethods have been used suchas melt mixing [52], melt mixing followed by in-situ polymeriza-tion [301], chaotic mixing followed by in-situ polymerization [311],solution mixing followed by casting [312], solution mixing assistedwith ultrasonic distribution [306,313], and chemical functionali-zation of CNTs followed by cross-linking reaction.

Chemical functionalization of CNTs is one of the most effectivemethods because chemical functionalization can increase thecompatibility between the CNTs and the SMP matrix. For example,Amirian et al. [314], Bartholome et al. [315] and Viry et al. [316]functionalized CNTs and prepared biodegradable MWCNT/poly

Fig. 2. In addition to reinforcement research, SMPCs research has been focused onSMPCs with athermal stimuli-active effect, SMPCs with novel SME, and SMPCs withnew functions.

(L-lactide-co-ε-caprolactone) SMPCs. The functionalized MWCNTslead to the best reinforcing effect. Furthermore, after the chemicaltreatment, the born eOH groups in CNTs may react with the SMPmatrix such as the isocynate of SMPU monomers to form an addi-tional cross-linking network to improve the shape memoryproperties.

Among all the outstanding achievements, Miaudet et al. [290]achieved the highest recovery stress of SMPCs. The maximalstress generated by a CNT/polyvinyl alcohol fiber reached 150 MPawhich is two orders of magnitude higher than that of neat SMPs.The CNT/polyvinyl alcohol fiber was prepared using a specialcoagulation spinning technique. Surfactant stabilized CNTs wereinjected in the co-flowing stream of the coagulating polyvinylalcohol solution which enabled an extremely high content of CNTsto be incorporated into the polyvinyl alcohol fiber.

2.3. Reinforcement by exfoliated nanoclay

The recent research of exfoliated nanoclay/SMP focused onmodification of exfoliated nanoclay to enable its reaction with theSMP matrix. The modified organic-exfoliated nanoclay can signifi-cantly improve the shape recovery ratios, shape recovery stress andmechanical properties. The nanoclay may play its role as an addi-tional hard phase contributing to shape memory properties [317].For example, Cao and Jana [318,319] employed the eH2CH2OHgroup of the quaternary ammonium ions in organoclay to reactwith isocyanine eNCO. This reaction adds an additional networkcontributing to the SME. Chung et al. [320] used surface-modifiedmonmorillonite to chemically bond to SMPU. The modified mon-morillonite has methyl tallow bis-2-hydroxyethyl ammoniumgroup which can be chemically bonded with polyurethane.


Heat treatment of the nanoclay can improve the compatibility ofthe claywith the SMPmatrix. The loss of moisture andmost surfacehydroxyl groups as a result of the heat treatment can provide abetter interfacial bonding between the SMP and clay interface. Xuet al. [321] heat treated a nanofabric type clay attapulgite at 850 �Cto reinforce SMPU. The mechanical properties and SME of thenanocomposites were remarkably improved.

2.4. Reinforcement by SiC or SiO2 particles

Research at the early 90s showed that nano Si-based inorganicparticles deteriorated the SME of SMPs even at very low loading. Liuet al. [322] and Gall et al. [323] observed that nano SiC damaged theSME of shape memory epoxy, though it could improve the elasticmodulus and recovery stress of the epoxy. Gunes et al. [52,324] alsoobserved negative effect of SiC on the SME of SMPUs, which theyascribed as due to the dramatically decreased soft segment crys-tallinity of the SMPU.

Recent research showed some positive effect of celite particleson the SME of SMPs. Zhang et al. [325], using abundant surfacehydroxyl of silica, initiated epsilon-caprolactone on the particles.Uniform SiO2@PCL macroparticles were formed. After cross-linkingof SiO2@PCL by 4,40-methylenediphenyl diisocyanate, the SiO2/SMPwas prepared. Compared to traditional silica-based filler-filledSMPs, the SiO2/SMP exhibited excellent mechanical strength, highstrain and good shape memory properties due to the introductionof SiO2 as a cross-linking agent.

Celite, which is primarily composed of silica and alumina, hassurface hydroxyl groups. It can be coupled with SMPU chains. Parket al. [326] prepared celite/SMPU with celite acting as a cross-linking agent. The celite improved the shape memory perfor-mance and mechanical properties of SMPUs. Lee et al. [327] func-tionalized silicas with allyl isocyanate groups andmadewaterbornesilica/polyurethane nanocomposites using UV. It was demonstratedthat the functionalized silica particles had the function of bothreinforcing fillers and stress relaxation retarders.

2.5. Reinforcement by cellulose nanowhiskers

Cellulose nanowhiskers have been popularly used as the rein-forcement fillers of polymer composites. They have polar groupswhich can interact with polyurethane. Auad et al. [53,328] rein-forced SMPU with cellulose crystals by suspension casting. A smallamount of well-dispersed cellulose nanowhiskers remarkablyimproved the stiffness of SMP, while no obvious decrease in shapememory properties was observed.

2.6. Summary remarks

Most of the fillers can significantly improve the elastic modulusand recovery stress of SMPs. Microfibers are superior to micro- andnano-sized particles to reinforce SMPs. Rod-shaped clay nanofillershave better reinforcement effect than spherical nanoparticles as aresult of their high aspect ratios. Fillers which can have chemicalbondingwith SMP chains aremore effective in improving the shapememory performance and the mechanical properties of SMPs.Fillers performing the function of cross-linking agents not onlyreinforce the SMP, but also improve the shape memory properties.Fillers which improve the thermal conductivity of the polymermatrix [329e331], can have positive effect on the SME of SMPs.Many researchers also studied the influence of fillers on the ther-mal expansion of SMPs. This research is necessary if exact shapecontrol is required during the thermomechanical process [332e336]. All research shows that inorganic fillers can more or lessimprove the thermal stability of SMPs [17,317,337,338].

3. Athermal stimuli-active effect

3.1. Electroactive SMP composites (SMPCs) with improved electricalconductivity

One of themajor accomplishments of SMPCs in the past decade iselectroactivity using a low voltage. A certain level of electrical con-ductivity can be reached if the SMPs are filled with enough electricalconductive ingredients. If a voltage is applied to the conductiveSMPCs, resistive joule heating may heat the SMPCs to trigger theSME [311,339e346]. The internal resistive joule heating method byelectricity, as compared with direct external-heating, has many ad-vantages such as convenient, uniform heating, and remotelycontrollable. The conductive fillers can also improve the thermalconductivity of SMP which contribute to a fast response. This elec-tricity triggered SME is especially useful for applications wheredirect heating is not possible such as self-deployable aerospacestructures, implanted biomedical devices, actuators and sensors.

Various strategies have been studied to improve the electricalconductivity of the conductive filler-filled SMPs such as filler surfacetreatment, in-situ polymerization, forming conductive metallicparticles chains, using high aspect ratio particles, aligning the par-ticles, using continuous conductive fibers or mat, and using hybridfillers with synergic effect [307e310,316,347]. The conductive fillerswhich have been used include CNTs, polypyrrole, CB, Ni powders, Ninanorods, short carbon fibers, and CNF mats [27,308e310,339,341e345,347e362].

3.1.1. Carbon black (CB)/SMPA large amount of CB fillers, although not as effective as high

aspect ratio fillers to improve the electrical conductivity, can be filledinto SMPs up to 40 wt%. Le et al. [363] studied the electroactive SMEof CB filled highly branched ethylene-1-octene copolymer. It wasfound that increasing the mixing time could increase the electro-active SME because it could improve the homogenous distribution ofthe CBs.

Short carbon fibers and CBs have good synergic effect onimproving the electrical conductivity of the composite with shortcarbon fibers as conductive bridge to connect CBs. Leng et al.[309,310] demonstrated that the coexistence of CBs and short car-bon fibers helped each other, resulting in significant improvementof electrical conductivity.

3.1.2. CNT/SMPCho et al. [342] first reported the electroactive SME of SMPs

filled with CNTs through joule heating. The CNTs were first treatedwith acid. Surface modification of the CNTs decreases the electricalconductivity of the composites because the acid treatment in-creases defects of the lattice structure of carbonecarbon bonds.

Heat treatment of CNT/SMP composites can remarkably improvethe composites conductivity as reported by Fei et al. [364]. Theyprepared CNTs/poly(methylmethacrylateco-butyl acrylate) by ul-trasound assisted in-situ polymerization. The composites weresubjected to a simple heating and cooling process. Themechanismofthe increased electrical conductivity was tentatively ascribed asfollows: in the original composite, there may be some internal re-sidual stresses or strains in the interface between polymer and CNTsas a result of thermal expansion mismatch and curing shrinkage;after the post thermal treatment, the interface contact area betweenthepolymermatrix andCNTsmay increases, and the thickness of theinterfacial polymer may decrease. The two effects reduce thetunneling resistance and thus significantly enhance the electricalconductivity.

Covalent linkages betweenCNTs and SMPs can effectively preventthe re-aggregation of the CNTswithin the polymermatrix. To achieve

Fig. 3. A schematic representation of the preparation of the chemical cross-linkingCNT/SMPU composites. Reproduced from Ref. [355] with permission.

Fig. 5. Series of photographs showing the SME of SMPC integrated with 1.8 g CNP.Reproduced from Ref. [345] with permission. Copyright (2010) of Institute of Physics,http://dx.doi.org/10.1088/0964-1726/19/7/075021.


the cross-linking between CNTs and SMPs, Jung et al. [355] modifiedCNTs using acid so that the CNTs could act as a cross-linking agentduring the in-situ polymerization step of the SMP. TheeOHgroups ofthe acid treated CNTs and the eNCO groups of the icosyanateterminated prepolymer form cross-linking structures as shown inFig. 3. The prepared SMPU showed outstanding shape memoryproperties and mechanical properties. In another similar research,Jung et al. [365] modified the sidewall of theMWCNTs using vacuumultraviolet light. The functional groups on the MWCNTs providedreactive sites to bind the MWCNTs strongly with SMPU.

3.1.3. Aligned conductive fillers/SMPTo construct physical conductive channels in SMPs, Yu et al.

[366] and Leng et al. [347] aligned Ni powders and CNTs, respec-tively in CB/SMP composites using magnetic field or electrical field.The chained Ni powders and CNTs can be seen in the SEM images inFig. 4. It was reported that compared with the samples withrandomly distributed conductive fillers, the electrical resistivitywas reduced by over 100 times.

Instead of Ni powders, filamentary nickel nanostrands with highaspect ratios can further improve the electrical conductivity of thecomposite. Lu et al. [367] filled filamentary nickel nanostrands andCNTs into shape memory epoxy. The aligned high aspect ratiosfillers further facilitated the actuation of the SMP nanocompositeeven though a low filler volume content and low electrical voltagewere used.

3.1.4. Carbon nanopaper (CNP)/SMP laminatesContinuous CNPs can provide remarkable electrical conductiv-

ity. They can be simply embedded in or layered on SMP films. Thismethod eliminates the laborious work of dispersing conductiveparticles in SMP matrix. Lu et al. [345] prepared CNP/shape mem-ory polystyrene composites. The CNP was achieved during thefiltration of CNTs. The composites were prepared by resin transfermolding. Nanopapers were attached on the mold before the resinwas injected into the mold. Fig. 5 shows the electroactive shape

Fig. 4. (a) Dispersion of CBs and chained Ni powder in SMP. Gray dots: CB; white dots: chainCNTs in SMP. Insets: zoom-in views of one CNT chain. Reproduced from Ref. [366] with pe

recovery of the prepared CNP/SMP layered composite. Luo andMather [354] simply immersed a CNP in uncured epoxy resin andcured the resin-impregnated CNP to fabricate electroactive SMPCs.Excellent electrical conductivity was also achieved and rapid elec-trical actuation was observed.

In the above CNP/SMP layered composite, although high elec-trical conductivity can be achieved on the CNPs, the neat SMP layeris still electrically insulative. To further improve the electroactiveSME and thermal conductivity, in their later research, Lu et al. [353]filled the SMP matrix with CNFs before the CNPs were laminated.The CNP/(CNF/SMP) structure significantly improved the electricalSME and the electrical response speed of the CNP/SMP laminates.

In the above CNP/(CNF/SMP) laminates, in the polymer layer, theCNFs are distributed randomly. Alignment of the electricalconductive fillers in the polymer matrix not only gives high speedelectrical actuation but also facilitates the transfer of heat from thenanopaper to the underlying SMPs. Therefore, Lu et al. [368]blended and vertically aligned the CNFs in the SMP layer using amagnetic field before CNP was laminated on the SMP. In anotherstudy, Lu et al. [369,370] vertically aligned electromagnetic nickelnanostrands in the SMP layer using a magnetic field in the sameCNP/SMP laminate as shown in Fig. 6.

3.1.5. Summary remarksDuring an electroactive thermomechanical shapememory cycle,

an SMPC is subjected to large strain and temperature variations.The temperature and strain change during the thermomechanicalprocess can significantly affect the electrical conductivity of theSMPC. However, this has not been systematically investigated. Yang[371] studied the influence of tensile strain on the electrical con-ductivity of CB/SMPU composites. It was found that the electricalconductivity decreased rapidly with increasing strain at a lower

ed Ni. Reproduced from Ref. [347] with permission; (b) Dispersion of CBs and chainedrmission.

http://dx.doi.org/10.1088/0964-1726/19/7/075021

Fig. 6. Schematic illustration of the nickel nanostrands being vertically aligned to helpresistive heating power transfer from the nanopaper to the underlying SMP. Repro-duced from Ref. [370] with permission. Copyright of the Royal Society of Chemistry.


strain range below 10%. Gunes et al. [372] studied the influence oftemperature on the electrical conductivity of CBs and CNFs filledSMPU composites, which showed that the influence of temperatureon the electrical conductivity of the SMPU was complicated.

To improve the electrical conductivity, high aspect ratios fillerssuch as CNFs, nickel nanorods, or even continuous CNPs arepreferred. These fibrous conductive fillers significantly affect thedeformability of SMPs. For specific applications, there may be anoptimal balance among the filler content, filler type, voltage to beapplied, mechanical properties and shape memory properties.

3.2. Magnetic-active SMPCs with improved magnetic properties

Thermomagnetically- and electromagnetically-induced SMEsare of great interest for medical applications because the embeddedSMPC devices can be later actuated by an externally applied mag-netic field by a non-contact remote mode. The promising applica-tions include smart implants or instruments, on demand drugdelivery, etc. [35,141,142,337,373e385].

The typical fillers used to achieve the magnetic-active SMEinclude metal particles, iron(III) oxide particles [379,386], ferro-magnetic particles [387], NdFeB particles [388], NieMneGa single-crystals [389] and nickel powders [390]. Because these materialsare originally proposed for biomedical applications, the SMP

Fig. 7. Schematic structure of Fe3O4@CD-M composite nanoparticles. Rep

matrixes are mostly biodegradable and biocompatible polymerssuch as poly(D,L-lactide), cross-linking poly(ε-caprolactone),poly(p-dioxanone)epoly(ε-caprolactone) copolymer, cross-linkingoligo(ε-caprolactone)dimethacrylate/butyl acrylate, and graftingpolymer poly(ε-caprolactone) diisocyanatoethyl methacrylate(PCLDIMA) and poly(ethylene glycol) mono-methylether-mono-methacrylate (PEGMA).

3.2.1. Surface coating of fillersTo obtain outstanding magnetic-active SME, a homogeneous

distribution and a large content of fillers in the SMP matrix arerequired [375,391]. To achieve homogenous dispersion and betterinterface heat transfer, fillers are normally treated to improve theircompatibility with the SMP matrix. Several chemicals have beenused to improve the compatibility between the fillers and thepolymer matrix based on different filler and SMP systems. Forexample, Weigel et al. [375] and Zhang et al. [392] coated silica onthe iron(III) oxide and Ni particles. Zhang et al. [390] coated a silanecoupling agent onto nickel powders. Yang et al. [393] coated Fe3O4nanoparticles with oleic acid.

3.2.2. Modification of SMP matrixEven though oleic acid coating can improve the dispersion of

Fe3O4 nanoparticles in epoxy matrix, the dispersion of a highcontent is still a challenge due to the dissimilar chemical structuresof the oleic acid and epoxy matrix. Puig et al. [394] modified theraw chemical of shape memory epoxy diglycidylether of bisphenolA with 20 wt% oleic acid. The content of the oleic acid-coatedmagnetite nanoparticles could be increased from under 1 wt% to8 wt% after the chemical modification of the epoxy.

3.2.3. Self-assembly of fillers in SMPAs shown in Fig. 7, Gong et al. [395] self-assembled Fe3O4 on the

cavity of eCD which functionalized MWCNTs. The Fe3O4-loadedMWCNT composite nanoparticles (Fe3O4@CD-M) were preparedfollowing two steps. (1) The raw MWCNTs were firstly graftedmaleic anhydride on their surface through a free radical reactionand later covalently modified by eCD through an esterificationreaction; (2) Fe3O4@CD-M composite nanoparticles were preparedby chemical co-precipitation of Fe2þ and Fe3þ ions on the surface ofthe eCD functionalized MWCNTs with an electrostatic self-

roduced from Ref. [395] with permission. Copyright (2012) Elsevier.


assembly approach using eCD as the depositional locus. Compos-ites were made using poly(caprolactone) as the matrix and theFe3O4@CD-M as the reinforced nanoparticles. Then nanofibers werefabricated from the composite by electrospinning. It was found thatthe Fe3O4 particles were aligned along the nanofiber axis. Thecomposite nanofibers showed an excellent SME, triggered by analternating magnetic field.

3.2.4. Summary remarksIn comparison with direct heating and electrical heating,

inducting heating by alternatingmagnetic field hasmore advantagesespecially formedical applications. In these cases, SMPC devicesmaybe implanted in which circumstance direct heating is not feasible.In addition, the magnetic-active SME makes selective heating ofspecific device areas of SMPCs possible [35,142,337,377,378].

Simple coating of coupling agents on the particles cannoteffectively improve the homogenous distribution and increase thewell-separated content of the fillers. Modification of the filler andthe matrix can significantly improve the compatibility between thefillers and the matrix. Chemical bonding between the fillers andmatrix will further improve the interface interaction. The re-lationships among the interface interaction, induction heatingperformance, as well as the influence on the mechanical propertiesand SME have been investigated systemically [375,380,391,396].Future research may systematically study relationship between theheat created by the hysteresis loss and the SME of the SMPCs so thatthe SME may be well controlled.

Despite the attractiveness of the non-contact magnetic-activeSME, there are still some challenges for practical applications. Firstof all, the frequency of the magnetic field should be in the clinicallysafety range (50e100 kHz) [397]. The switching transition tem-perature of the SMPCs should be in the range of human body saferange. Too high switching temperatures which require high trig-gering temperature will traumatize human body tissue. Furtherresearch is also necessary to design novel microstructures of SMPCsshowing outstanding magnetic field-induced SME which can bereally applied in practice.

3.3. Water-active SMPCs

Since first reported by Huang et al. [398] in 2005, water[9,21,399e406] or solvent-driven [407e409] shape recovery effectshave been observed in many SMPs having glass transition as theswitching transition. Water or solvent molecules may penetrateinto the amorphous areas of glass transition type SMPs. The wateror solvent molecules have plasticizing effect on the SMP molecules,which increase the flexibility of macromolecule. The glass transi-tion temperature of the SMPs may drop to a temperature below theroom temperature. These functions lead to the shape recovery of adeformed SMP. Recently, two types of cooling-induced SMPCs werealso reported based on novel shape memory mechanisms. The firstone is based on the cooling-induced phase transition of tin [30]. Thesecond type is based on the cooling-induced swelling and softeningof thermal-responsive hydrogels in water [410].

3.3.1. Hydrophilic filler/SMP compositesOne of the methods to obtain outstanding water-active SME is

filling a hydrophilic or water soluble ingredient in SMPs [21,404].Awater sensitive biodegradable stent made of chitosan films cross-linked with an epoxy compound is prepared following this strategyby Chen et al. [403]. One of the raw materials (chitosan and PEG-400) used in the fabrication of the developed stent is relativelyhydrophilic.

Cellulose nanowhisker is a hydrophilic filler used to preparewater-active SMPCs. If the cellulose nanowhisker/SMP is deformed

and immersed into water, the cellulose nanowhisker/SMP nano-composite recovers the original shape as a result of the enthalpyelasticity of the SMP network. Basing on this idea, Luo et al. [411]prepared cellulose nanowhisker/SMPU nanocomposites withoutstanding water-active SME. The cellulose nanowhisker inSMPUs did not affect the intrinsic thermal-active SME of the SMPUbut enabled the SMPU water-active SME.

3.3.2. Water-active cellulose nanowhisker/elastomerThe above mechanism of the water-active SME of the cellulose

nanowhisker composites indicates that an SMP matrix is notnecessary for the water-active. An elastic polymer network com-bined with cellulose nanowhisker should be enough to construct awater-active SMPC. Zhu et al. [412] prepared cellulose nanowhisker/polyurethane elastomer showingwater-active SMEwith rapid shapefixing (removal of the water from SMPs) and rapid shape recovery(penetration of water into SMPs) after a mild programming cycle.The water-active SME is tailor-able by changing the content of thecellulose nanowhiskers [413].

3.3.3. Summary remarksThe water-active cellulose nanowhisker/elastomer composites

have several advantages over water-active SMPU based on drop-ping glass transition temperature. To fix the temporary shape, thewater-active SMPUs have to be heated to a temperature above140 �C (even to 180 �C) to increase the glass transition temperatureabove the ambient temperature; while the fixity of the cellulosenanowhisker/polyurethane elastomer composite can be achievedusing a much lower temperature of 75 �C in a short time (10 min).The water-active SMPUs require hours to lower the glass transitiontemperature for recovery; while the water-responsive cellulosenanowhisker/elastomer composite needs only 10 min for recoverywhich is much faster. The elastomer matrix of the cellulose nano-whisker composite can be any elastomer or rubber materials. Inaddition to cellulose nanowhisker, other hydrophilic materials mayalso be used as the fillers for the water-active elastic polymercomposites.

3.4. Photoactive SMPCs

Non-contact infrared-light irradiation heating can increase thetemperature of SMPs to trigger shape recovery. Non-contactinfrared-light irradiation heating is safe in comparison with in-duction heating by electricity or alternating magnetic field[152,414e417]. The second type of photoactive SMPCs is based onreversile photochemical crosslinking. Some photochemicallyreactive groups are able to form photo-induced reversible cova-lent cross-linking in polymers. Photodimerization moieties suchas cinnamic acid, cinamylidene acetic acid, coumarin, anthraceneand their derivatives have been incorporated in polymer macro-molecules to act as the photoactive shape memory switches of thepolymers [8,90,265,418e420]. Another type of photoactive SMP isbased on the metaleligand coordinative interactions of metallo-supramolecular networks [115].

3.4.1. SMPCs based on photothermal effectMany researchers have successfully demonstrated the infrared-

light-active SME [152,414e417,421,422]. Fillers such as indocyaninegreen and Epolight 4121, gold particles [423,424], conductiveceramics, carbon blacks, nanotubes, and even ink [425] maybe incorporated in SMPs to improve the heating efficiency[149,311,416,426,427]. Different particles have significantlydifferent healing efficiency [311]. The effective selection of thewavelength of the driving light is a key step for good light-activeshape recovery performance [428]. Optical fibers may be


embedded in SMPs to transmit infrared-light to the SMPs. Becausedifferent SMPs may have different absorption peaks, the opticalfibers whose working band is at the suitable working frequencyshould be selected [428].

3.4.2. SMPs based on reversible photodimerizationThe research on reversible photodimerization-based SMPs has

been miainly focused on pure SMPs. The cinnamamide moieties inN,N-bis(2-hydroxyethyl) cinnamamide (BHECA) as shown inFig. 8 can undergo reversible photodimerization. Lendlein et al.[91,420] prepared two photo-responsive polymers with cinnamicacid and cinamylidene acetic acid terminal as the photoreversibleswitch. Sodhi and Rao [93] formulated a constitutive equation todescribe the mechanical behavior of these photoactive SMPsbased on the theory of multiple natural configurations. Beblo andWeiland [92] presented a chemical kinetic model to predict thethrough-thickness evolution of the elastic modulus of thesephotoactive polymers. To prepare photoactive SMPs forbiomedical applications, Wu et al. [94] prepared a biodegradableSMP composing of N,N-bis(2-hydroxyethyl) cinnamamide, poly-(L-lactic acid) and polycaprolactone as shown in Fig. 8. Thephotoactive cinnamamide is incorporated into the block copol-ymer through N,N-bis(2-hydroxyethyl) cinnamamide as thepedant groups of the block copolymer. The block copolymer iscoupled with aliphatic hexamethylene diisocyanate (HDI).

3.4.3. SMPCs based on reversible metaleligand coordinationMetaleligand coordination in metallo-supramolecular net-

works between monomeric units are reversible [429]. Yan et al.[115] used the combination of a heteroditopic monomer withbridging ligand and the metallic cross-linker to orthogonally self-assemble into an extended supramolecular polymer network,resulting in the gelation. Yan et al. [115] are trying to realize theSME of the supramolecular polymer network. Kumpfer and Rowan[114] reported the SME of ametallo-supramolecular polymerwith acovalently cross-linkable core. Addition of metal salts into a lowmolecular weight poly(butadiene end-capped with 4-oxy-2,6-bis(N-methylbenzimidazolyl)pyridine (�OMebip) ligands formedthe high molecular weight metallo-supramolecular polymers asshown in Fig. 9. The use of different metal salts can tailor the fixingbehavior of these materials. The polymer is also thermo-, photo-,and chemo-active because these stimuli can affect the metaleligand complexes by softening of the hard phase and increasingdecomplexation or rate of exchange.

3.4.4. Summary remarksSeveral types of photoactive SMPs have been developed which

are based on: (1) photoisomerizable molecules such as azo-benzenes [430e432], (2) photo-induced reversible dimerizationmolecules such as cinnamates [8,90,265,418e420], anthracene

Fig. 8. Synthesis of biodegradable multi-block polyesterurethane containing pendantphotoactive cinnamamide groups from biodegradable soft diol (polycaprolactone diol),biodegradable hard diol (poly-(L-lactic acid) diol), and N,N-bis(2-hydroxyethyl) cin-namamide (BHECA). Reproduced from Ref. [94] with permission. Copyright (2011)American Chemical Society.

[433] and coumarin [434], (3) addition-fragmentation chaintransfer reaction using allyl sulfides [435e437], (4) reversiblephoto-induced ionic dissociation such as trienphylmethane leuco[438,439], (5) photothermal effect [440], and (6) photoactivemetaleligand coordination bonds. Most of the research has beenfocused on pure photoactive polymers. Further research on pho-toactive polymer composites should be conducted to improve theproperties and obtain new functions of the polymers.

4. Novel shape memory effect (SME)

4.1. Multiple-shape memory polymer (MSMP) composites

Many SMPs with multiple-shape memory effect (MSME) havebeen developed [138,139,441e445] because of the significant ap-plications such as packaging and robot. Different from dual-shapeSMPs which can only remember its single original permanentshape, MSMP can remember one or more shapes in addition toits original permanent shape. MSMP composites either havemore than one reversible switching transitions or have a singlebroad switching transition acting as the switching transitions[80,85,445e448]. Normally multiple-step thermomechanical pro-gramming is required to achieve the MSME.

4.1.1. Filler/SMP with well-separated switching transitionsIt is easily understandable that if an SMP has well-separated

thermal transitions as the switching transitions, the SMP mayshow MSME after a multiple-step programming process. SMPCswith well-separated thermal transitions also show MSME. Poly-hedral oligomeric silsesquioxanes (POSS) is a good filler forbiodegrade SMPs [176,449e454]. Jeon et al. [455] studied the shapememory properties of norbornyl-POSS copolymers having eithercyclohexyl corner groups (CyPOSS) or cyclopentyl corner groups(CpPOSS). The POSS composites showed two stages of strain re-covery, a fast strain-recovery process related to the polynorbornenerelaxation in the norbornyl matrix, and a second slow strain-recovery process which was assumed to be related to the POSSrich domains. This two stages shape recovery is like the triple SMEof SMPs.

Berllin et al. [442] developed a triple-SMP by copolymerizationof poly(ethylene glycol)mono-methylether-monomethacrylate(PEGMA) with poly(caprolactone)dimethacrylate (PCLDMA). Thepolymer had crystalline poly(caprolactone) domains and crystal-line polyethylene glycol domains, which act as the two switchingphases. Then Kumar et al. [381,382] filled the triple-SMP with silicacoated nano iron(III) oxide particles. With the stepwise increases ofthe magnetic field strength from H ¼ 14.6 and 29.4 kA/m, triple-shape memory behavior was observed. Paderni et al. [384] pre-pared a triple-shape memory grafted polymer network containingcrystallizable poly(ethylene glycol) side chains and poly(-caprolactone) cross-links network. The polymer has two well-separated melting transitions. Then silica-coated nano iron(III)oxide particles were filled into the polymer; and the MSME by non-contact activation in a stepwise increasing alternating magneticfield was achieved [381,384].

4.1.2. Polymer blends with well-separated switching transitionsBlending polymer pairs with well-separated thermal transition

temperatures is a relatively easy method to prepare MSMPs. Thepolymer blends may be subject to post cross-linking to achieve astable network for the SME. The well-separated phase transitiontemperatures act as the multiple switching transitions. Kolesov andRadusch [67] blended and cross-linked linear high density poly-ethylene, ethylene-1-octene copolymers, and short-chain branchedpolyethylene to prepare SMPs with multiple crystallization and

Fig. 9. Synthesis of metal-chelating ligand end-capped poly(butadiene) macromonomer (1) films that contain different amounts of tetrathiol cross-linker and photoinitiator. Filmsare solution cast, and the dried films are photo-cross-linked to yield 2aec$Eu(NTf2)3 with a targeted 3, 8, and 14 cross-links/chain, respectively. The solution-cast un-cross-linkedfilms can be fixed into a variety of permanent shapes; for example, (a) a strip of un-cross-linked film is wrapped around a cylinder to give a spiral shape, and (b) irradiation with lowintensity UV light initiates photo-cross-linking to yield a film with a permanent spiral shape. Reproduced from Ref. [114] with permission. Copyright (2011) American ChemicalSociety.


melting behavior. Dual-shape and triple-shape memory wasobserved after two- and accordingly three-step programming ofthe blends. Cuevas et al. [456] blended and cross-linked twosemicrystalline polymers poly(cyclooctene) and polyethylene tocreate two well-separated crystalline domains in a single polymer.Outstanding multiple-shape recovery properties were observed.

4.1.3. SMP/SMP layered composites with well-separated switchingtransitions

Xie et al. [457] prepared shape memory epoxy with a layeredstructure showing MSME. The epoxy with a high Tg was first curedand then an epoxy with a low Tg was cured on top of the first layer.The two layers of epoxy have a strong interface as a result of thereaction among the residual epoxy or amine groups on the surfaceof the two layers. Bae et al. [458] prepared SMPU bi-layer films ofdifferent molecular weights with silica particles acting as multi-functional cross-links and reinforcing fillers. The bi-layer materialformed from the two films exhibited two undisturbed glass tran-sition. Triple SME was demonstrated.

4.1.4. Polymer/SMP hybrids with well-separated switchingtransitions

Filling a low melting or glass transition temperature polymerinto an SMP may add more switching transitions into the SMP toshow MSME. Luo and Mather [459] immersed electrospun poly-caprolactone nanofiber mat with a low melting transition into aglass transition type shape memory epoxy matrix. Fig. 10(I) showsthe elastic modulus profiles which indicate the two separatedthermal transitions of the composites. One is from the glass tran-sition of the epoxy and the other from the melting of the poly-caprolactone fibers. The sample was first deformed in a water bathat 80 �C to obtain an “M” shape and quenched the first temporaryshape in ice/water mixture at 0 �C. Then the sample was deformedat 40 �C and quenched in water again to fix the coiled secondtemporary shape. Fig. 10(II) shows the triple SME of the composite.

4.1.5. SMPCs with a single broad switching transitionMSMPs with well-separated thermal transitions can only

remember a limited number of temporary shapes, because thenumber of well-separated phase transition in a single polymerwithin the application temperature range is limited. Miaudet et al.[290] found the temperature-memory effect of shape memorypolyvinyl alcohol composites with a broad glass transition. Thebroad glass transition may be regarded as the consecutive distri-bution of a number of glass transitions. Based on this assumption,stepwise SME can be achieved in polymers without well-separatedswitching transitions. Theoretically, the temperature-memory ef-fect can also be found in SMPs with a broad melting transition.Kratz et al. [460] demonstrated the temperature-memory effect ofSMPs with crystalline phase switching transition [461,462]although stepwise SME was not studied.

Although theoretically feasible of unlimited-SME, quintupleSME cannot be achieved in a single polymer because practically themultiple shapes may affect each other during the thermomechan-ical process. To achieve quintuple SME, Li et al. [463] introduceda melting switching transition into an SMP which itself had abroad glass switching transition. The polymer poly(methyl meth-acrylate)/poly(ethylene glycol) semi-IPN not only has a broad glasstransition but also has a crystalline segment. Quintuple shape fix-ation and recovery in the single shape memory cycle are achievedas shown in Fig. 11.

4.1.6. Programming of multiple-SMPCsNormally, to create anMSME, the polymer has to be subjected to

a multiple-step programming process [441e443]. The temporaryshapes are fixed consequently at a temperature below the respec-tive switching transition temperatures. It was found that one-stepprogramming process at the temperature below all the switchingtransition temperatures (cold drawing) could also create the MSME[464e466]. For example, Kumar et al. [382] programmed a triple-SMP filled with silica-coated iron(III)oxide nanoparticles using

Fig. 10. (I) The elastic modulus of the epoxy/polycaprolactone composites with different epoxy compositions. (II) The sequential recovery of the epoxy/polycaprolactone composite(A) from a temporary shape, (B) to temporary shape b, and (C) to permanent shape c. Reproduced from Ref. [459] with permission. Copyright (2010) John Wiley and Sons.


the one-step method. Non-contact actuation of the triple-SMEusing an alternating magnetic field was achieved.

4.1.7. Summary remarksPolymer blending, laminating and hybriding are easy methods

to construct shape memory materials with MSME because they canincorporate well-separated switching thermal transitions into asingle material. The precise control of shape recovery is crucial fornumerous applications of MSMPs, such as sensors and actuators.Althoughmulti-step shape recovery effect can be achieved in shapememory materials with well-separated switching thermal transi-tions or with a broad switching transition temperature, precisecontrol and stabilization of the different temporary shapes duringthe shape recovery process is still a challenge. In addition, shaperecovery process is affected by many factors such as thermo-mechanical programming and molecular relaxation of the poly-mers. To precisely control the shape recovery process of MSMPs,more research should be done in the future.

4.2. Spatially controlled SMPCs

Polymers are thermal insulator, which make it possible to raisethe temperature of SMPs at a small area locally while the other partof the polymer is not affected. Therefore, it is possible to spatially

Fig. 11. The quintuple SME of poly(methyl methacrylate)/poly(ethylene glycol) semi-IPN wiwith permission.

control the location and recovery step of shape recovery becausethe recovery only occurs at the local place where it is heated.

4.2.1. Selective electrical heating of CNTs filled SMPsFei et al. [364] studied the controlled spatially electroactive

shape recovery of a CNT filled SMP by selecting the place where theelectrical voltage was applied as shown in Fig. 12. The heat gener-ated by electrical current could be accurately controlled. This makesit possible to spatially control the location of shape recovery. Therate of stress release and shape recovery is also controllable throughtuning the voltage applied on the polymer composites locally.

4.2.2. Selective light heating of gold nanoparticles filled SMPsGold nanoparticles between 60 and 200 nm in size have a strong

absorption of light especially infrared light. Heating of focused lightcan be absorbed by the gold nanoparticles to increase the tem-perature of gold nanoparticles/SMP composites locally to triggerthe spatially controlled SME [423]. Zhang et al. [424] demonstratedthe spatially controllable shape recovery of the gold nanoparticlesfilled SMP using focused laser light. Surface functionalized goldnanoparticles are loaded in semicrystalline shape memory poly(ε-caprolactone) macromolecules as shown in Fig. 13(a). The branchedoligo(ε-caprolactone) (bOCL) was cross-linked with hexamethylenediisocyanate, referred to as XbOCL. The shape recovery by the

th a broadened glass transition and a crystalline segment. Reproduced from Ref. [463]

Fig. 12. Schematic illustration of the electro-triggered spatially and temporallycontrolled shape recovery and the realization of multi-shapes on demand of CNTs-filled SMP. Reproduced from Ref. [364] with permission.

Fig. 14. (a) The schematic structure of the multi-composite SMP consisting of a Fe3O4/SMP region and a CNT/SMP region separated by the neat SMP. (b) Experimentaldemonstration of the multi-composite shape recovery. The sample was subjected tomagnetic strengths at the same radio frequency in the fields of 13.56 MHz and 296 kHzsequentially. Reproduced from Ref. [467] with permission.


localized photothermal effect of gold particles can be controlled bythe intensity of light which determines the amount of heat gener-ated by the gold nanoparticles. Once the light is turned off, theheating caused by the light is off. Therefore, the shape recovery canbe stopped at any stage. Fig. 13(b) shows the spatially selectiveshape recovery process by separate laser exposures on four sectionsof the gold nanoparticles-loaded XbOCL film.

4.2.3. Magnetic-active multi-composite SMPCsMore recently, He et al. [467] fabricated a multi-composite SMP

consisting of a Fe3O4/SMP region and a CNT/SMP region separatedby a pure SMP region as shown in Fig. 14(a). Upon remote actuationusing two different magnetic field of different radio frequencies,the triple-region composite was capable of controlled two-stepshape recovery as shown in Fig. 14(b). The Fe3O4/SMP and CNT/

Fig. 13. (a) Chemical structures of the SMP and polymer-functionalized gold nano-particles used to prepare the composite material and (b) photographs showing aspatially selective shape recovery process at room temperature by separate laser ex-posures on four sections of the gold nanoparticles filled SMP, with the film stepwiselifting a load 350 times of its weight. Reproduced from Ref. [424] with permission.

SMP regions have different radio frequency absorptions. When thedeformed composite sample is in 13.56 MHz RF field, only the CNT/SMP region is heated. This leads to the recovery of only the CNT/SMP region. Upon being subjected to subsequent 296 kHz RF field,the Fe3O4/SMP region is selectively heated leading to the recoveryof the Fe3O4/SMP region. Finally, the pure SMP region recovers in anoven.

4.2.4. Selective light heating of SMP films printed with black inkpatterns

Instead of using focused light, Liu et al. [425] printed pre-designed black ink pattern using a desktop printer on pre-stretched shape memory polystyrene films. The black ink patternprovides localized absorption of light, which heats the underlyingpolymer to trigger the shape recovery of the polymer immediateunder the pattern. The sheet under the predefined inked regionrecovers causing the planar sheet to fold into a three-dimensionalobject as shown in Fig. 15. This method removes the complexprocess of fabricating SMPCs and using focused light.

4.2.5. Summary remarksIn the past five years, researchers have put tremendous efforts

to achieve complicated and well-controlled shape recovery ofSMPs. Selective heating of SMPCs provides a facile and efficientmethod to achieve complicated and spatially controlled shaperecovery process. This research will inspire wide applicationpossibilities of the SMPs. More research may be conducted toinvestigate the real applications of these SMPCs such as actuator,sensors and robots.

4.3. Two-way SMPCs

Two-way SMPs change their shape upon suitable stimulationwithout the requirement for pre-deformation. They may recovertheir original shape by changing the amplitude or direction of thestimulation. Two-way SME has been studied systematically onliquid crystalline elastomers. The ordering of mesogenic moietiesand the elastic properties of liquid crystalline elastomers enable thetwo-way SME of liquid crystal elastomer [84,445,468e471].Thermal-active semicrystalline polymers with one single semi-crystalline phase [472e479], or with two semicrystalline phases[480] can also show two-way SME under constant stress. The un-derlying two-way shape memory mechanism is that cooling-induced crystallization of semicrystalline polymer films results inelongation under a tensile load; subsequent heating to melt thenetwork yields contraction (shape recovery).

4.3.1. Two-way semicrystalline SMP composites under stressThermal-active semicrystalline polymers showing two-way

SME have to be subjected to a stress. Recently, Westbrook et al.

Fig. 15. Photographs of 3D structures created by self-folding of shape memory polystyrene film (ShrinkyeDink) patterned with a desktop printer. (a) Single line patterned on thetop side of the ShrinkyeDink; (b) two lines patterned on either side of the ShrinkyeDink; (c) three lines patterned on alternating sides of the ShrinkyeDink; (d) rectangular box; (e)tetrahedral box; and (f) tetrahedral box with adjacent double hinges. Both tetrahedrons have a square bottom facet and equilateral triangles on the other facets. Reproduced fromRef. [425] with permission.


[481] fabricated a two-way shapememory cylinder by embedding apre-stretched semicrystalline SMP into an elastomeric matrix. Theelastomeric matrix could apply stress on the pre-stretched semi-crystalline SMP during the thermomechanical process. A trans-versal actuation of w10% of actuator length is achieved. The two-way SME of SMP/elastomer laminates is considered to be becauseof the combination of the shape memory properties of the SMP, theelasticity of the elastomer and the mechanical (such as modulus),and thermal properties (such as coefficient of thermal expansion)mismatch of the coupled materials.

4.3.2. Two-way SMP laminates with mismatch mechanicalproperties

Langer and Lendlein [482] in their patent opened the method ofpreparing two-way shape memory materials using the laminatingmethod. Chen et al. [483,484] prepared a two-way SMPC by lami-nating an SMP with an elastic polymer. The two-way SME wasascribed to the release of elastic strain of SMP layer upon heating,and the recovery of elastic strain induced by the bending force ofelastic polymer layer upon cooling. The modulus of SMP and SMAchanges significantly at round the switching or phase transitiontemperature. If they are laminated together, the unbalance of theinternal stress in the composites as a result of changing environ-mental temperature can produce the driving force for the two-waySMEs. Tobushi et al. [485] fabricated a two-way shape memorylaminate with SMAs and SMPs. In the laminates, the shape-memorized round SMA tapes faced in the opposite directions andwere sandwiched between the SMP sheets. The two-way SME wasdependent on the variation of the elastic modulus and mechanicalstrength of the two SMA tapes and SMP with changingtemperature.

4.3.3. Two-way photoactive SMPCsMany photoactive two-way SMPs have been developed based

on different mechanisms such as photo-induced reversible trans-cis isomerization of azobenzenes [486e490], photo-inducedreversible dimerization of anthracene [433] and coumarin [434],photothermal effect [440], photo-induced addition-fragmentationchain transfer reaction [435e437], and photo-induced reversiblemetaleligand coordination [114]. Two types of photoactive SMPCshave been reported.

4.3.3.1. Photo-responsive two-way SMPC membranes based on azo-benzenes. Azobenzenes and derivatives undergo reversible transecis isomerization on the irradiation of light of suitable wave-length [430e432]. The isomerization causes the angle change ofthe two isomers accompanied with the significant change in mo-lecular length between 9.0Å (trans) and 5.5 Å (cis) [430,491]. Since2001, it has been demonstrated that if azobenzenes are coupledinto liquid crystal elastomers, significant photo-induced deforma-tion can be achieved in the liquid crystal elastomers [167,169,486e490,492e501].

Photoactive azobenzene/inorganic nanoporous compositemembranes are a type of important photoactive nanocompositematerials which have broad applications. They are made byimmobilizing azobenzene derivatives to the inorganic nano-porous membrane such as silica framework. The light controlledconformation of the azobenzene in the composites can changethe pore size and correspondingly the transport behavior of thecomposite membrane [502e505]. Continuous excitation at awavelength can lead to the continuous photoisomerization andcontinual dynamic wagging of the tethered terminous [506]. Theazobenzenes attached to the interiors of the inorganic subtractpores templated by surfactant [507,508] can act as both impellersand gatekeepers [509,510]. These photoactive composite mem-branes have significant applications as photoactive nanovalves[506,508], drug delivery nanovehicles [511], microfluidic devices[512], photonic devices [513], and micro-optical-controlled softmicrorobots [514].

4.3.3.2. Photoactive two-way SMPCs based on photothermal effect.CNTs [515e517] absorb near infrared light and have been incor-porated into polymer systems to induce a mechanical response ofthe polymer. Ahir and Vaia [515,516] observed two-way SME on amulti-walled CNT/polydimethylsiloxane elastomer composite. Theexpansion or contraction of the composite depends on the extent towhich the composite is strained. If the material is slightly pulled, itexpands when it is exposed to infrared light. Conversely, if thematerial is subjected to a strain greater than 10%, it contracts underidentical exposure to infrared light.

The above materials are composite materials filled with nearinfrared irradiation fillers. Ugur et al. [440] achieved the nearinfrared irradiation actuation of a pure polymer film which was


made of cross-linked polyarylamide-based nanofibers. The cross-linked multi-block polyarylamide contained “coil-like” poly-ethylene oxide and “rod-like” polyarylamide blocks as shown inFig. 16. The polyarylamide could self-assemble into fibrillar mi-crostructures on the silicon substrates. This structure exhibited ahigh-degree of optical anisotropy. The study on understanding ofthe mechanisms is still underway. It is believed that the effect ishighly related to the photothermal effect of the near infrared lighton the highly aligned fibrillar structure.

4.3.4. Summary remarksCross-linking semicrystalline polymers can show two-way SME;

however, a tensile load is necessary. Two-way SMPs based onthermal or photoactive isomerization does not require an externalstress; however, they cannot produce high strain. Liquid crystaltwo-way shape memory elastomers are able to produce high strainup to 300%, however, they are still costly in addition to their un-stable shape changing behavior. The two-way shape memory ma-terials based on photothermal effect are still under study. Themechanism is still not completely understood.

The two-way SMPs prepared by the laminating method seemmore advantageous compared with other two-way SMPs. Two-wayshape memory laminates are easy to fabricate. Large shapechanging strain can be achieved and the shape changing strain canbe well controlled. The reversible shape changing strain can betailored by selecting suitable laminating materials, changing themodulus, adjusting the thickness, and changing deformation ratiosof the coupling materials. The two-way shape memory laminatesmay provide great promising applications because of the aboveadvantages.

5. New functions

5.1. Stimuli-memory effect of SMPCs

5.1.1. Temperature-memory effect of SMPCsTemperature-memory effect means the shape memory material

can memorize the temperatures at which it is programmed [243].Temperature-memory effect of SMAs has been widely studied.Miaudet et al. [290] first reported the temperature-memory effectof SMPs on shape memory polyvinyl alcohol composites with abroad glass transition. For SMPs with broad glass transition, thebroad glass transition may be regarded as the consecutive distri-bution of a number of glass transitions [461]. According to themechanism of SME, if the polymer is deformed at a temperatureabove the glass transition temperature and heated again to thetemperature, the polymer recovers. The “remembered” tempera-turemay not be the exact temperature that it is programmed; theremay be some quantitative relationship. Xie et al. [518] alsodemonstrated the temperature-memory effect of Nafion with abroad glass transition temperature. Theoretically, the temperature-memory effect can also be found in SMP with a melting transitiontemperature as the switching temperature as long as the meltingtransition is broad, which has been demonstrated by Kratz et al.[460].

Fig. 16. Chemical structure of “coil-like” polyarylamide block “coil-like” poly(ethyleneoxide).

5.1.2. Magnetic field-memory effect of SMPCsMagnetic field can be used to trigger the shape recovery of SMPs

filled with magnetic particles. The magnetic memory effect ofSMPCs means that magneto-sensitive materials can “remember”the magnetic field strength, at which they are deformed. Razzaqet al. [519] observed themagnetic memory properties of SMPs filledwith silica coated iron(III) oxide nanoparticles containing eithercrystalline or amorphous phase.

5.1.3. Summary remarksSMPs remember their stimuli by showing direct relations be-

tween the stimuli parameters and the recovery speed, recoverystress or recovery strain of the SMP. The temperature-memory ef-fect and magnetic field-memory effect suggest the possibility ofelectricity, light, humidity, ionic concentration, gas, and solventmemory effect, although not all of these have been demonstrated.For exact memory of the stimuli, SMPs still have some problemsbecause of the complexity of SMEwhich is affected bymany factorssuch as programming and triggering process parameters. Thestimuli-memory effect should be studied intensively because of itssignificant application potentials such as sensors.

5.2. Self-healing of SMPCs

SMPs have the intrinsic healing capability as a result of the SME.SMPCs can not only repair microscopic damage, but also healmacroscopic cracks in SMPs. Taking advantage of the SME, SMPfibers embedded in conventional thermoset engineering polymerscan impart healing capacity to the polymers, which may have aprofound impact on healable engineering structures.

5.2.1. SMPs with microscopic damage healing capabilitySMPs if deformed have the capability to recover their original

shape. This is a phenomenon of instinctive self-healing. If thedamage on the SMP is not severe like scratches, flaws or defor-mation which do not sacrifice the overall mechanical properties ofthe material, simply heating up to the transition temperature of thematerial will heal most of the damage through recovery of the SMP[520]. This type of damage is popular in daily life on plastic or metalsurfaces by scratching, sliding friction, etc. Xiao et al. [189]demonstrated the healing capability of shape memory epoxy sub-jected to microscopic damages as shown in Fig. 17. Upon heating toa temperature above the glass transition temperature of the shapememory epoxy, the scratch in microscale on the sample was healedcompletely or mostly. This system only works on microscopicdamages [521,522] while not work on macroscopic damage such asmacroscopic cracks because macroscopic crack may damage therecoverability of the polymer network [521,522].

5.2.2. Healing agents filled SMPs with macroscopic damage healingcapability

Like all the other self-healing systems such as microcapsule-based self-healing materials and microvascular-based self-healingmaterials [523e526], the above scheme only works for microscopicdamage [181,527,528]. Li [48,180e183,527,529] proposed a two-step healing scheme CTH (close-then-heal) for macroscopic dam-ages in SMPCs. The idea was demonstrated in an SMP filled withthermoplastic particles. As shown in Fig. 18, the composite was firstcompression programmed to reduce the volume at a temperatureabove the switching temperature of the SMP. The structural scalecrack is first closed by the constrained shape recovery (constrainedexpansion) of the SMP followed with healing of the closed crackusing thermoplastic particles pre-embedded in the SMP. Thehealing process is repeatable. Small thermoplastic contents as lowas 3% can heal structural-length scale cracks with high healing

Fig. 17. The surface images of the as-scratched (first column) and recovered (second column) samples. Reproduced from Ref. [189] with permission.


efficiency andwith low sacrifice of material load carrying capability[183]. The healing processes have been modeled systematically byLi’s research group [184,187].

5.2.3. SMP fibers filled conventional thermoset polymers withhealing capability

The above thermoplastic particle/SMP system requires an SMPas the matrix. To heal widely used conventional thermoset poly-mers in engineering structures without SME, Li et al. [185,530,531]proposed a new biomimetic self-healing system which includesusing SMP fibers as grid skeleton and thermoplastic particles as thehealing agent. The SMP fiber has superior toughness. The healingtakes two steps as shown in Fig. 19. Upon heating, constrainedshape recovery (shrinkage) of the pre-stretched SMP fibers closesmacrocracks in the polymer matrix which is similar to stitching acut in the human skin by suture; and bond the closed crack throughmelting and diffusing of thermoplastic particles at the crack inter-face. The self-healing process was reasonably repeatable [185,530].

For the SMP fiber/thermoplastic particle/epoxy compositeshealing system, Li and Shojaei [531] developed a micromechanicalmultiscale viscoplastic theory to predict the thermomechanicalbehaviors of the fiber. The proposed theory takes into account thestress-induced crystallization process and the evolution of themorphological texture based on the applied stresses. Using finiteelement analysis, Li and Shojaei [531] also demonstrated the

possibility of healing macroscopic cracks by localized heatingwithin a small area surrounding the cracked region, which wasvalidated by experiment [530]. Instead of continuous stimuli-responsive fiber grid skeleton, uniformly distributed short SMP fi-bers may have the same function to close structural-length scalecrack in the thermosetting polymer matrix [532].

5.2.4. SMA wires filled polymer with damage healing capabilityKirkby et al. [533,534] embedded SMAwires in epoxy matrix to

narrow down micro cracks (about 150 mm) by taking advantage ofthe shape recovery of the SMA wires. Huang et al. [535] embeddedSMA springs into SMPs as shown in Fig. 20(a) for the healing of SMPwith macroscopic damages. The SMP was made by mixing anelastomer with glue having a low melting temperature. Duringcracking, as shown in Fig. 20(b1), the composite material is pulleduntil fracture into two pieces. The SMA spring is also stretched butnot broken. A big gap is created on the SMP. Then electricity isapplied to the SMA spring to trigger the recovery of the SMA springto close the gap. In the mean time, the SMP with a low meltingtransition temperature is also heated and melted to bond the crack.

5.2.5. Summary remarksSMP has the intrinsic healing capability upon stimulation. How-

ever, it only works onminor damage because structural-length scaledamages may sacrifice the overall mechanical properties of the

Fig. 18. Schematic of the CTH scheme of the proposed healing thermoplastic particles filled SMP ((a)e(e) complete programming and closing and (f)e(h) ensure healing).Reproduced from Ref. [182] with permission.


polymer. Forwide-opened crack, constrained shape recovery of SMP,which has been properly programmed, has to be employed; other-wise, the cracked surface cannot be brought in touch. SMA wiresembedded in convention engineering materials are able to closemacroscopic crackswithout the requirmentof external confinement.However, because of the low recoverable strain of SMAwires, whichis below 8%, the capability for SMA wires to close macrocracks in

Fig. 19. (a) Schematic diagram of the two-step self-healing process of macroscopic damagesIn the first-step, heating the composite to a temperature (T1) above the glass transition tempe(about 3 mm in width). In the second-step, at the bonding temperature (T2), the thermoplastreal self-healing process of the composite. In the first-step, the macrocrack is closed by therparticles flow into the narrowed crack by capillary force, diffuse into the fractured epoxy matwith permission.

polymer composites is limited. Furthermore, SMA wires do notmechanically match the polymer matrix (mismatch in stiffness),particularly at the high recovery temperature.

SMP fiber/thermoplastic particles system is a better system toheal macroscopic damages in conventional engineering materials.The present limitation of this system is the low bonding strength oflow melting temperature thermoplastic particles. The melting

in the SRF (stimuli-responsive SMP fiber)/TP (thermoplastic particle)/epoxy composites.rature of the SMP fibers triggers the shape recovery of the fiber to close the macrocrackic particles melt and diffuse into the cracked matrix and bridge the crack plane. (b) Themal-induced shape recovery of the fiber. In the second-step, the molten thermoplasticrix, and form solid wedge when the sample is cooled down. Reproduced from Ref. [185]

Fig. 20. Self-healing of SMA spring/SMP. (a) SMP with an embedded SMA spring inside (illustration); (b1eb2) pulling SMP till fracture; (b3eb4) joule heating SMA coil for self-healing; (c) healed sample; (d) healed sample in bending. Reproduced from Ref. [535], with permission.


temperature of the thermoplastic particles has to be below thethermal resistance temperature of the SMP fiber; otherwise, theSMP fibers may lose most of their recovery stress at the thermo-plastic particle melting temperature. The future research will be:(1) increasing the thermal resistance temperature of the SMP fibers,and (2) using other healing agents which do not need heating toheal damages instead of thermoplastic particles.

6. Future outlook

The past decade has witnessed remarkable advances in stimuli-responsive SMPs with the potential applications in medical, aero-space, civil engineering, energy, and bionics areas. SMPCs bringnumerous possibilities of novel SMPs such as MSMP composites,two-way SMPCs, spatially controlled SMPCs, electroactive SMPCs,photoactive SMPCs, stimuli-memory effect, and self-healing SMPCs.While the field is rapidly progressing and many exciting achieve-ments have been made, some challenges are awaiting to beovercome.

6.1. Improve mechanical output

Although the recoverable strain of SMPs can be very bigcompared with SMAs, under a constrained state, the mechanicaloutput by SMPs upon stimulation is extremely low. More severely,at temperature above the switching temperature of SMPs, thestiffness of the polymer drops a few orders of magnitudes. SMPCscan have higher mechanical strength and recovery stress than theirneat counterparts. However, because of the polymeric nature, thelow mechanical strength and low mechanical output is still achallenge for SMPCs in many engineering applications. Fillers suchas particles or continuous fibers can significantly improve themechanical strength and recovery stress of SMPs. However, at mostcircumstances, it is at the expense of the SME, breaking strain andrecoverable strain of the SMP.

Recently, Nair et al. [536] reported a two-stage reactive non-stoichiometric thiol-acrylate SMP system. After shape recovery, asecond stage reaction can photo-polymerize remaining acrylatefunctional groups to form a more stable chemical cross-linkingsystem. In this way, even at a high temperature, the SMP cankeep its mechanical strength. This method can improve the stabilityof SMPs after triggering; it still cannot improve the mechanicaloutput of the polymer during the shape recovery process.

6.2. More complicated, controllable, multi-step movement

Materials or structures which can change their shapes in moviessuch as “Transformer” have attracted much interest of human be-ings. The past five years have seen the significant progress on thecomplicated and spatially controlled shape recovery of SMPCs.Lamination method is simple without the complicated synthesis

and dispersing of fillers into SMPs. The properties of shape memorylaminates are tailor-able by simply changing the stacking sequenceand thickness. Laminating methods can not only create newSMPs from conventional polymers without SME, but also createSMPs with two-way SME, multiple SME, and spatially controllableSME.

Most of the present recovery movement of shape memorylaminates is still simple. In the future, more complicatedmovementof SMPCs may be achieved such as worm walking, rotation or vi-bration. For more advanced applications high reliability, precisepredictability, controllable strains, controlled deformation speed,and exact deformation stress are preferred. It is optimistic that thisgoal will be achieved in the near future.

6.3. SMPCs inspired by nature nanomaterials

Nature mother has abundant smart nanocomposites in differentforms. Mimosa pudica collapses suddenly and dramatically whentouched; leaves of venus flytrap snap shut on doomed insect prey;codariocalyx motorius rotatesion leaflets under sunlight; etc. Thesestimuli-active effects are due to the movement of water into or outof cells which changes the cell shape and cell volume. Living or-ganisms such as sea cucumbers change their stiffness [105] uponstimulation because they can dynamically change the cross-linkingof the collagen fibrils. Chameleons change colors resulting from therearrangement of chromatophores. Squid, cuttlefish and octopuseschange colors upon stimulations because cephalopod chromato-phores are able to be spread under local muscular action andzebrafish chromatophores are able to be removed [537].

Inspired by these intelligent nanocomposite materials in nature,researchers can retro-engineer stimuli-responsive SMPs to mimicthe architectures of these intelligent systems. In this way, we notonly develop stimuli-responsive SMPs by mimic the intelligentnanocomposite materials in nature, but also understand in detailsand in depth of the activity of the intelligent nature materials.

6.4. Electro driven vs. other stimuli driven and multi-stimuli driven

Electricity as a stimulus for SMPs is convenient for many appli-cations. Now researchers are able to trigger an electroactive SMPusing a low voltage such as several volts. More research should beconducted on other stimuli such as light for SMPs. In comparisonwith electricity, light as an energy source is environmentallyfriendly. As a stimulus for SME, light can be controlled remotely,instantly and precisely. The intensity, polarization, and phase can beeasily modulated into complex patterns [538,539]. A few photo-active SMPCs have been developed [487,494]. Some properties, suchas fatigue resistance and biocompatibility of photoactive SMPs haveto be further investigated as pointed out by Ikeba in 2007 [486].

Most of the SMPCs are responsive only to one specific stimulussignal within a specific stimulus range. Three themed issues have


devoted to the various stimuli-responsive materials responsive todifferent stimulus: Progress in Polymer Science (2010, Vol. 35, issue1-2) [540e547], Advanced Materials (2010, Vol. 22, issue 31)[490,548e553], and Journal of Materials Chemistry (2010, Vol. 20,issue 17) [35,382,554e561]. SMPCs provide the possibility offabricating multi-stimuli-responsive SMPCs by an easier way. Tak-ing advantages of the versatile structures of SMPCs and combiningthe different stimulus-active structures into one single materialmay createmultiple-stimulus-responsive SMPCs. Filling conductivefillers into thermal-active SMPs, electroactive SMPs can be pre-pared; filling magnetic fillers into thermal-active SMPs, magnetic-active SMPs can be fabricated. Recently, utilizing a metallo-supramolecular polymer with a covalently cross-linkable core,Kumpfer and Rowan [114] prepared thermo-, photo-, and chemo-active SMPs because all the stimuli can result in softening of thehard phase and increased decomplexation or rate of exchange ofthe metaleligand complexes. In the future, SMPCs with multi-stimuli-responsive effect should be developed so that materialcan change their properties to adapt to the various environmentalparameters such as temperature, light, humidity, electricity, mag-netic field, and pH value.

6.5. Application-oriented research

Although a large number of papers have been publishedregarding the wide potential applications of SMPCs, many of thesepapers only demonstrate application possibilities of the SMPCs;commercialized products are still limited. This is partially because,for practical applications, there are more specific and stringentrequirements on the overall properties of the SMPCs. For example,for textile applications, sometimes the SMPCsmay have good shapefixity but do not have enough elasticity and water vapor perme-ability. For biomedical applications, the overall requirements onSMPCs for a specific application may include amplitude of shaperecovery strain, stiffness, recovery stress, safety range of the stim-ulation, biocompatibility, biodegradability speed, and safety of thedegradation products. Sometimes, the environmental stabilities,such as thermal stability and chemical stability of the SMPCs, arethe key issues limiting their applications. In the past 10 years, re-searchers and engineers have made great efforts to solve theseproblems. Although there are still challenges, we believe that thenext decade will see vast growth of the SMPC products.

6.6. Safety evaluation of SMPCs for biomedical applications

One of the significant driving forces for SMPCs research is theirbiomedical applications. The biological evaluation is mandatoryprior to their applications as biomaterials. Some types of fillersfilled into SMPs to achieve novel functions are toxic. For example,MWCNTs have high toxicity and hydrophobicity [562]; thereforeCNT/SMP have to be subjected to stringent evaluation beforebiomedical applications. Xiao et al. [563] studied the biocompati-bility of electroactive CNT/poly(ε-caprolactone) SMP. It was foundthat if the MWCNTs were pre-functionalized by acid-oxidationprocess and covalent grafting with poly(ethylene glycol), the pre-pared CNT/poly(ε-caprolactone) composite had goodbiocompatibility.

For medical applications, the SMPCs may be subjected to ster-ilizations. The sterilizations on the chemico-physical properties ofthe materials should be studied systematically. For examples,Nardo et al. [564] have reported the effects of sterilizations byplasma and ozone on chemico-physical, thermomechanical andshape memory performance of polyurethane foams. Yackacki et al.[565] have studied the properties of acrylate-based SMPs afterbeing subjected to various sterilization treatments. It was found

that plasma sterilization was the only method to elicit a severecytotoxic response, even at short cell test time points.

7. Conclusion

SMPCs can improve the properties and increase the versatility ofstimuli-responsive SMPs. A small amount of cross-linking micro/nano-sized fillers can significantly improve the mechanicalstrength and recovery stress of SMPs. SMPCs not only exhibitsignificantly improved mechanical and shape memory properties,but also exhibit novel properties not possessed by their purecounterparts such as indirect heating SME, multi (three/quadrate/quintuple)-SME, gradient shape recovery effect, spatially controlledshape recovery effect, two-way SME, and complicated movementSME. Traditional SMPs were usually triggered by direct heating.Now SMPCs can be triggered by other methods such as water im-mersion, electricity, magnetic field, and infrared light. A low con-tent of fillers can enable the indirect triggering of the SME. Thesestudies on SMPCs will further broaden the applications of SMPCssuch as deployable space structures, remoteeactive medical in-struments, biomedical devices, energy, textiles, engineering struc-tures, bionics engineering, electronic engineering, civilengineering, dry/wet adhesives and fasteners, and householdproducts. In the past decade, a large number of patents related toSMP products were filed. The authors believe in the next 10 years,many more SMP products will be coming out of labs and becommercialized.

Acknowledgment

This study was sponsored by NASA/EPSCoR under Grant Num-ber NASA/LEQSF (2011-14)-Phase3-05, NSF under Grant NumberCMMI-0900064, and Louisiana BoR Fund LEQSF(2010-2015)-LaSPACE and the support of NNX10AI40H.

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Harper Meng received his Ph.D. degree in Chemical En-gineering and Materials from The Hong Kong PolytechnicUniversity. He is working with Professor Guoqiang Li atboth Southern University and A&M College and LouisianaState University as a postdoctoral research associate. Hisresearch interests are in stimuli-responsive materials andlight-weight composites.

Guoqiang Li received his Ph.D. degree from SoutheastUniversity, M.S. degree from Beijing University of Tech-nology, and B.S. degree from Hebei University of Tech-nology, all in Civil Engineering. He joined Professor Su-Seng Pang’s group in the Department of Mechanical Engi-neering at Louisiana State University in 1997 as a postdoc-toral research associate, with research focus on polymercomposite materials. He was then promoted to ResearchAssistant Professor in 2000. In 2003, he was hired as atenure track Assistant Professor in the Department of Me-chanical Engineering at both Louisiana State Universityand Southern University (NSF Joint Faculty AppointmentProgram). He was promoted to Associate Professor withtenure in 2008, and promoted to Professor in 2012, at

both universities. His current research focuses on experimental testing and physics-based mathematical modeling of shape memory polymer composite materials andself-healing composites using shape memory polymers. He has coauthored 140 papersin refereed journals, 1 book, and 4 book chapters.

a review of stimuli-responsive shape memory polymer composites · shape memory polymers (smps) as a...

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