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ARTICLE IN PRESSG ModelCR-112220; No. of Pages 15

Coordination Chemistry Reviews xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Coordination Chemistry Reviews

j ourna l h om epage: www.elsev ier .com/ locate /ccr

eview

ecent developments in shape memory polymer nanocomposites:ctuation methods and mechanisms

enxin Wanga, Yanju Liub, Jinsong Lenga,∗

Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, PR ChinaDepartment of Astronautical Science and Mechanics, Harbin Institute of Technology, Harbin 150080, PR China

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002. Chemo-responsive shape memory polymer nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.1. Nanostructure/microstructure shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2. Nanocellulose/shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.3. Carbon nanomaterials/shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.4. Bioactive glass nanoparticles/shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3. Electrically resistive Joule heating activated shape memory polymer nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.1. Graphene filled shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .003.2. Carbon nanotube and nanofiber filled shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.3. Metal nanoparticles filled shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .00

4. Light-activated shape memory polymer nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.1. Nanogold/shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .004.2. Nanocarbon/shape memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5. Microwave heating triggered shape memory polymer nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 006. Magnetically sensitive shape memory polymer nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 007. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Conflict of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .00References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

r t i c l e i n f o

rticle history:eceived 7 December 2015ccepted 20 March 2016vailable online xxx

eywords:unctional nanomaterialhape memory polymer

a b s t r a c t

Research on shape memory polymer is developing promptly over the past three decades, holding thepromise to apply in micro actuators, medicine, basic science, biological science, deployable structure,anti-counterfeit technology, sensor and automobile. Actuation methods and mechanisms are parts of theimportant developments in shape memory polymer. Shape memory polymer nanocomposites are usuallyconstructed by functional nanomaterials such as mesoporous materials, hierarchical porous materials,nanocomposite membranes, nanotubes and nanowires. Numerous studies of shape memory polymernanocomposite have been performed to interpret experimental data and provide rational design. This

ctuation methodechanismesign concept

review focuses on state-of-the-art actuation methods such as chemo-responsive, electrically resistiveJoule heating activated, light-activated, microwave heating triggered, and magnetically sensitive shapememory polymer nanocomposites in most recent studies. Specifically, emphases are placed on the actu-ation mechanism, the advanced design concept, and the potential application of novel shape memorypolymer nanocomposite probabilities to a wide variety of areas.

© 2016 Published by Elsevier B.V.

Please cite this article in press as: W. Wang, et al., Coord. Chem. Rev. (

∗ Corresponding author. Tel.: +86 451 86403269; fax: +86 451 86402328.E-mail addresses: [email protected] (W. Wang), yj [email protected] (Y. Liu), le

ttp://dx.doi.org/10.1016/j.ccr.2016.03.007010-8545/© 2016 Published by Elsevier B.V.

2016), http://dx.doi.org/10.1016/j.ccr.2016.03.007

[email protected] (J. Leng).

dx.doi.org/10.1016/j.ccr.2016.03.007

dx.doi.org/10.1016/j.ccr.2016.03.007

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dx.doi.org/10.1016/j.ccr.2016.03.007

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ARTICLECR-112220; No. of Pages 15

W. Wang et al. / Coordination C

. Introduction

Intelligent materials are important parts of contempo-ary chemistry research. As emerging intelligent materials,hape memory polymers (SMP) have the unique ability tondergo chemo-responsive, electrically activated, light-activated,icrowave heating triggered, magnetically sensitive, and ther-ally triggered shape changes [1–5]. The molecular chains have

n apparent coil/stretch transition with sliding when energy isonsumed through an applied load which can be best explainedhrough SMP’s entropic elasticity property [4,6]. There is morenergy storage in thermoset polymers than thermoplastics throughhe crosslinking bonds restricting sliding which freeze more energyn the temporary shape. Upon reheating, the SMP molecular chainsoil due to entropy spontaneously increasing and the energy storeds released, so the polymer shows more macroscopic shape change.n a typical shape memory process, cycles of deformation, shapexation, and shape recovery occur where the permanent shape isept and recovered repeatedly from the temporary shape. Whenhe orientations of polymer chains are changed, the free volumend distance between the chains are decreased due to defor-ation which are enhanced by multiple mechanisms including-bonding and polar–polar interactions [1]. Subsequently, theicro-Brownian movements of the polymer chains are frozen and

ransformed to a rigid state during cooling to resist the recoil-ng of the polymer chains with the shape fixity. The chemical orhysical crosslinking points and crystalline domains or stiff chainsf the SMP network are responsible for the permanent shape ofhe SMP chain network. The crystalline, amorphous phases, liquidrystal phases, supramolecular entities, hydrogen-bonded polymeretworks, and light-reversible coupling groups act as switches tohange the structure of the SMP chain network which ultimatelyrovide the ability of primary recovery and energy absorption7–10]. The glass/rubber transition and the crystalline/meltingransition are often utilized as switching transitions of SMP. Accord-ng to the shape memory effect (SME) mechanism, the SMP iseformed into a temporary shape under extra force with freezingrownian motion and stress. The SMP can recover to its permanenthape through releasing the frozen stress when it is exposed to aeating source or other external stimulus [11]. Although previousfforts have elucidated several SMP actuation methods, new SMPanocomposites have had few breakthroughs in their actuationechanisms. Currently, it is fundamentally significant to provide a

eference model that describes the interconnectivity of actuationethods and mechanisms toward developing novel SMP nanocom-

osite systems.Although SMPs exhibit the facile capability to change their

hape, they also exhibit limited mechanical properties like lowlastic modulus, stiffness, strength, and lack of functions [2]. Func-ional fillers have become a popular strategy for overcoming theirnnate deficiencies. The special functionality of mesoporous, bioac-ive nanoparticles, nanotubes, nanocellulose, and nanocomposite

embranes provide a means for simple infiltrate composite nano-echnology. The functional elements within SMP nanocompositesot only provide an opportunity to improve the strength and mod-lus of the material, but can also be utilized to impart the materialsith stimuli-responsive characteristics.

The directly thermal-induced SMP is closely related to ther-al transitions of the reversible phase in the SMP. The external

eat sources are usually used to stimulate the SMP, which is dif-cult to control for slow heat transfer and response. This review

s primarily interesting in shape memory properties, actuation


ethods, and mechanisms of indirectly induced SMP nanocom-osites as outlined in Fig. 1. The basic elements of the SME haveeen briefly introduced since different SMP nanocomposites fea-ure different actuation mechanisms which are directly correlated

PRESStry Reviews xxx (2016) xxx–xxx

to the rational design of novel SMP nanocomposite systems. Con-sequently, we review chemo-responsive, electrically resistive Jouleheating activated, light-activated, microwave heating triggered andmagnetically sensitive SMP nanocomposites, respectively. Thesesections describe the design principle, fabrication, shape memoryproperties, and corresponding actuation mechanisms of the SMPnanocomposites. Finally, rational designing and relevant potentialapplication of SMP nanocomposites are summarized.

2. Chemo-responsive shape memory polymernanocomposites

Chemo-responsive SME has the advantage of being struc-turally and chemically tunable with a variable recovery ratethat is independent of external heating which gives way to amore environmentally friendly and green energy saving approachthan the thermally activated SME method. The absorbed solventmolecules of the chemo-responsive SMP nanocomposites enableshape recovery at a significantly lower temperature with disrup-ting intermolecular hydrogen bonds, plasticizing, and reducing theglass transition (Tg) temperature.

2.1. Nanostructure/microstructure shape memory polymercomposites

Wang et al. have advocated the unique design concept toconstruct a novel sodium dodecyl sulfate/thermosetting epoxywater-driven nano/microstructure SMP composite and providedfurther clarification of the water-induced SME mechanism [6]. Asshown in Fig. 2, the composite was designed based on the chem-ical interaction of sodium dodecyl sulfate (SDS) with water upondissolution which consequently created 3D microvoids. The com-posite took advantage of SDS’ amphiphilic nature, stemming fromthe sulfate group at the end of the 12 carbon chain, to increase theinherent water solubility. Mechanical mixing of the SDS and epoxy-based shape memory polymer constructed the composite and thenproduced 3D microvoids which enhanced the composite’s specificsurface area with water solubility lowered the SDS concentrationin the composite [6]. The increased specific surface area benefittedfrom the water diffusivity which subsequently caused a physicalswelling effect (PSE) [12].

1 mm cast thin films of epoxy-based SMP and sodium dodecylsulfate/epoxy shape memory composite (SDS-ER) were evaluatedin the water-induced SME test, while each film was under standardtemperature and pressure (STP) separately in water and in atmo-sphere for an equal amount of time. Interestingly, only the SDS-ERexhibited good shape memory recovery properties in water, but theothers had no visible effect. The shape recovery ratio of SDS-ER wassmall during the first 30 min, where it started to gradually recoverand eventually exhibit macroscopically noticeable recovery. Inwater, the composite imbibed water molecules which migratedinto the interstitial space of the polymer chains and the bulk struc-ture resulting in the specimen’s dimensional change. Furthermore,there were different force conditions between the inside and out-side surfaces of the “U”-like shape composite as shown in Fig. 2.For this reason, the PSE of the inside surface was a part of shaperecovery outputting force. It gives inspiration to design the com-posites with nonlinear/non-uniform PSE of programmed regionsas the functionally graded SMP nanocomposite.

In the first step, the shape recovery rate was controlled by theslow diffusion process. In the middle stage, its large specific surface


area benefitted the diffusion of water molecules from the exter-nal surface into the bulk which subsequently improved the shaperecovery rate. Once the diffusion equilibrium condition was met,the diffusion of water molecules from the bulk to the solution was

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ARTICLE IN PRESSG ModelCCR-112220; No. of Pages 15

W. Wang et al. / Coordination Chemistry Reviews xxx (2016) xxx–xxx 3

Fig. 1. The actuation methods and mechanisms of shape memory polymer nanocomposites, in particular of chemo-responsive, electrically resistive Joule heating activated,light-activated, microwave heating triggered, and magnetically sensitive shape memory polymer nanocomposites.

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n par with the diffusion of the water molecules into the bulk. Con-equently, the water-induced shape recovery rate is limited by theiffusion of water molecules until equilibrium is met. These results

ndicated that SDS of the composite and water were decisive fac-ors for the water-induced SME. Additionally, the shape recoveryate strongly depended on the ambient temperature. The shapeecovery ratio of the SDS-ER in water at 35 ◦C was about 20 timesaster than the specimen at 20 ◦C [6]. The elastic macromoleculeragments of the SDS-ER discretionarily tangled together beforeeformation with high entropy. After pre-deformation, the tan-led molecular chain was in an orderly permutation, which was

metastable structure with low entropy [6,13]. According to ther-odynamics, a system’s entropy typically increases with increasing

emperature as demonstrated in the shape change of the SDS-ER.


DS increased the spacing between the crosslinking net points.he crosslink density of polymer networks influences its Tg. Theecreased Tg was also attributed to the crosslink density reductionith increasing SDS content [6]. The Tg of the composite can be

f sodium dodecyl sulfate/thermosetting epoxy shape memory polymer composites.

tuned by its SDS content, since the 12 carbon chain of SDS disturbedthe chain interactions in the epoxy-based SMP. Furthermore, thewater molecules plasticize the polymeric composites, increasingflexibility of the chains in SMP composite and decreasing its Tg [5].Therefore, the water-induced shape recovery rate of the compos-ites can be controlled by tuning the synergistic effect of water andheat [6]. Above all, the results suggest that chemical interaction andthe PSE are important influences on the water-induced SME.

Rahman et al. reported the nanotube embedded membranewhich was made of polyaniline nanotube embedded in nanoporous(polycarbonate) membrane and one end of the nanotube wasattached to a surface deposited thin polyaniline layer [14]. Thenanocomposite membrane kept rolled up in dry condition andgradually expanded with increasing humidity in sealed environ-


ment. It was worth reminding that the nanocomposite membraneactuation relied on the asymmetric absorption and desorption ofwater molecules. The open side faces of polymer nanotube effi-ciently absorbed water molecules and the surface deposited thin

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ARTICLE ING ModelCCR-112220; No. of Pages 15

4 W. Wang et al. / Coordination Chemis

Fig. 3. (a) SEM image of a typical polycarbonate membrane having 50 nm pore diam-eter. (b) Polyaniline nanotubes synthesized using 200 nm pore diameter membrane.Image was taken after removing the polycarbonate membrane. (c) Higher magnifi-cation image showing open end of the nanotubes. (d) Low magnification image fromtop, showing nanotubes and part of the surface deposited layer. (e) Cross-sectionalview clearly showed that nanotubes were attached to the surface deposited layer.(f) Schematic of the nanotubes embedded membrane (NEM) indicating nanotubesand surface deposited layer. (g) Schematic of the actuation motion of NEM withchanging humidity. NEM got unrolled with increasing humidity and rolled up withdecreasing humidity. In the rolled condition, the surface deposited layer remainsi

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olyaniline layer rapidly evaporated water molecules, in presencef humidity gradient (Fig. 3).

In addition, batch foaming, injection foaming and natural cel-ular material [15–17] also can be able to manufacture uniformorous structure solvent-driven SMP composite. The molecularhains of composites will be expanded or stretched to providepace for increasing the solvent molecules absorption rate, whichenefits solvent-driven SME due to plasticization. Cellulose aero-el is an ultra-lightweight and highly porous material. Jiang andsieh explored a self-assembled cellulose nanofibril aerogels withltralow density, superior porosity, super water absorption capac-


ty, high crystallinity and water-induced shape memory propertyia cyclic freezing–thawing without chemical crosslinking, whichould rapidly absorbed water at around 100 times their own weightithout noticeable changes in the volume or shape [18]. These


materials possess low density, high-volume expansion, and highthermal/electrical insulation, which have the potential appliedvalue in the clothing, sporting gear, and biomedical industries.

2.2. Nanocellulose/shape memory polymer composites

Nanocellulose as hydrophilic bulking agent is one of the mostabundant, biodegradable and renewable natural biopolymers, soit has attracted a great deal of interest and has been utilized asnanofiller in polymer nanocomposites. There are some researchesthat take advantage of nanocellulose to prepare water-inducedshape memory nanocomposites [18–25].

In Fig. 4, Zhu et al. proposed characteristic microstruc-ture details of the water-sensitive SME programming cellulosenanowhisker/thermoplastic polyurethane (CNW/TPU) [19]. TheCNW/TPU softened in the wet state due to water moleculesbreaking the hydrogen bonds between the nanowhiskers, whichcould deform into temporary shape. And the subsequent dry-ing at 75 ◦C, the shape was fixed by forming three-dimensionalhydrogen-bonded network between the nanowhiskers withoutwater molecules. In the recovery procedure, the wetting compos-ite recovered its original shape with decoupling the nanowhiskersnetwork. The reversible hydrogen bonding of the cellulosenanowhiskers is the key of this approach which provides the pos-sible applications in breathable clothing and medical devices.

Biodegradable and biocompatible SMP nanocomposites areattractive to use as minimally invasive medical devices. Wuet al. provided an approach to prepare enzymatically degrad-able cellulose nanocrystals/biodegradable poly(glycerol sebacateurethane) nanocomposites with water-induced SME [20]. Dagnonet al. fabricated a poly(vinyl acetate) based water-responsive SMPnanocomposite with carboxylated cellulose nanocrystals (CNCs) ofvarying charge densities via partial functionalization using differ-ent equivalent propylamine [21]. The results suggested that thecharges on the CNCs might enhance water diffusion through thenanocomposite without adversely affecting the modulus of eitherthe stiff or soft state. The nanocomposite membrane is able to offera way to enhance the water diffusion rate without significantlyimpacting its mechanical properties due to changing the chargedensity of the CNCs in the nanocomposite.

2.3. Carbon nanomaterials/shape memory polymer composites

As two-dimensional nanomaterial, grapheme oxide (GO) is acompound of carbon, oxygen, and hydrogen in variable ratios withplenty oxygenated functional groups [26,27], which is easy to dis-perse well among polymer in the molecule level through formingstrong hydrogen bonding interaction between components [28].

Qi et al. prepared a grapheme oxide reinforced polyvinyl alcohol(PVA/GO) nanocomposites. In Fig. 5, shape memory properties ofPVA/GO nanocomposites increased with forming additional phys-ical cross-linked points between GO sheets and a layer PVA of GOsurface in the composite. The hydroxyl groups of hydrophilic PVAexhibited high affinity with water molecules which could weakenthe primary hydrogen bonding of the PVA/GO nanocomposites [29].The combined effects of the plasticizing effect and the competitivehydrogen bonding are the two main reasons for the water-inducedshape recovery of PVA/GO nanocomposites. Besides that, carbonnanotubes (CNTs) are also effective reinforcing filler [30–32] forenhancing shape recovery force and speed due to their excellentmechanical properties as well as high thermal conductivity coeffi-cient [33].


Traditionally, the researches of solvent-induced [34] andmoisture-sensitive [35] SMP focus on polyurethane [36–39] andhydrogel [40–44] based SMP. Meanwhile, there are some effec-tive theoretical approaches such as phenomenological approach

dx.doi.org/10.1016/j.ccr.2016.03.007



Fig. 4. Proposed rapidly switchable water-sensitive shape-memory mechanism for the cellulose nanowhisker/thermoplastic polyurethane comprising a cellulosen

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45,46] and constitutive model [47] about studying the constitutiveelations and shape memory mechanisms of the chemo-responsiveME. The polymer network can gradually imbibe the certain solventolecules causing change in the volume and shape, until reach-

ng equilibrium condition in the particular environment. However,hese solvent-induced SMP in general exhibit lower strength andtiffness, which limits their applications. It is concluded that thehemical and physical crosslinking of SMP nanocomposites over-ome the weakness of thermal stability and exhibit excellent SME.

.4. Bioactive glass nanoparticles/shape memory polymeromposites

Bioactive glass nanoparticles (BG-NPs) have ability to inducepatite precipitation upon immersion in simulated body fluid48], which can be used in tissue engineering or other biomedi-al applications. Correia et al. synthesized chitosan/bioactive glassanoparticles (CHT/BG-NPs) scaffolds which combined the shapeemory properties of chitosan and the biomineralization ability of

G-NPs for application as shape memory bone implantable mate-ials [49,50]. Chitosan (CHT) is a linear polysaccharide which isne of the most widespread used natural polymers in self-healingnd biomedical application due to its unique properties, such asntibacterial activity, wound healing property, biocompatibilitynd biodegradability [51]. It is also worth noting that CHT can beriggered by hydration and recover the permanent shape at roomemperature attributed to its hydrophilicity and pore [52,53]. Inheir study, BG-NPs as reinforcing fillers increased the stiffness ofhe CHT/BG-NPs scaffolds. The temporary shape of the scaffolds wasxed by ethanol dehydration. The combined effect of solvent andon-solvent in miscible liquid pairs was used to control the swellingatio of polymer networks. Water molecules disrupted intermolec-


lar hydrogen bonds allowing large-scale segmental mobility uponhe occurrence of Tg temperature to recover the original shape ofre-deformed scaffolds. Furthermore, water content could controlhe molecular mobility of the polymeric structure. Therefore, the

water was used to induce shape recovery by hydration and ethanolwas used as a non-solvent to control the CHT/BG-NPs scaffoldsswelling capability.

Fig. 6A showed that the previously deformed CHT/BG-NPsscaffold was placed in the empty space of a fresh pig femurwith size between the permanent and temporary shape ofthe scaffold. Fig. 6B showed that the recovered scaffold fit-ted perfectly the geometrical contour of bone defect with goodmechanical fixation attributed to press-fitting effect. The scaf-fold combines biodegradability and shape memory capability forapplication in controlled drug release and minimally invasivesurgery.

3. Electrically resistive Joule heating activated shapememory polymer nanocomposites

The electrically driven method is more precise, convenient, andefficient than heat-triggered actuation. The electrical current pas-sage of SMP nanocomposites with sufficient conductivity is able togenerate resistive heating by electrical triggering. However, SMPis typically good electrical insulator, so it is important to establishsufficient electrical conductivity of SMP nanocomposite withoutadversely influencing the shape memory performance. Electri-cally resistive Joule heating activated SMP nanocomposites havepromising applications involving actively moving polymers withsignificant macroscopic deformation in a predefined manner. Thesenanocomposites can greatly improve the performance of the SMPand expand their potential applications through incorporation ofelectrically driven SMP composites utilizing graphene, CNTs, car-bon nanoparticles, carbon nanofibers, and metal nanoparticles asconducting filler.


3.1. Graphene filled shape memory polymer composites

Graphene is a unique two-dimensional structure carbon mate-rials possessing excellent electrical, thermal and mechanical

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6 W. Wang et al. / Coordination Chemistry Reviews xxx (2016) xxx–xxx

F (b) sch

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pmegwamtimririS

ig. 5. (a) Illustration of hydrogen bonding interaction among PVA, GO and water;


roperties [54,55]. Based on the electrical conductivity and highodulus of the reduced graphene oxide paper (RGOP), Wang

t al. used RGOP as functional layer to manufacture the reducedraphene oxide paper/epoxy-based shape memory compositehich exhibited excellent electric-induced SME [56]. RGOP acted

s conductive layer to generate resistive Joule heating and trans-it the heat to the polymer stimulating shape recovery of

he composite. The results showed that the storage modulusncreased for compositing with RGOP, which was supported by

icro/nano-mechanical tests [56]. It was worth noting that shapeecovery rate of the composites approximated 100% only tak-


ng 5 s under 6 V which was much lower voltage than previousesearches [57–64]. The temperature increased 74.81 ◦C compar-ng with the initial temperature at 1 s to activate the actuator.ubsequently, the actuator became more thorough. In Fig. 7, the

ematic representation of the shape-memory PVA/GO materials actuated by water.

uniform surface temperature distribution indicated the compos-ite possessed adequate electric heating performance. Furthermore,the shape recovery rate of the composites strongly dependedon the applied voltage which was decreased with increasingvoltage. The electrical actuation shape memory property of thecomposite can be controlled by tuning the synergistic effectof component ratio and applied voltage. This work suggestsfeasible design principles of electrically driven SMP compos-ites.

Valentini et al. reported a transferring method which wasable to transfer graphene nanoplatelets (GNPs) layer from


the glossy surface to polyurethane (PU) copolymer film [57].Applying a constant voltage of 10 V, the sample recovered theoriginal shape at least 20 s [57]. The results shown that theGNPs not only improved mechanical properties of PU film, but

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Fig. 6. (A) Illustration of CHT/BG-NPs recovery in a defect produced in a pig femur bone. (B) Representative images of tree orthogonal Micro CT slices (horizontal, frontal andv ows in

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lso generated resistive Joule heating transferred to the PUubstrate.

.2. Carbon nanotube and nanofiber filled shape memoryolymer composites

The CNT possesses fascinating properties such as high ther-al and electrical conductivity which encourages to utilizing

hem as functional filler in polymer nanocomposites [65–67].n the work of Wang et al, they developed a chemicallyross-linked polycyclooctene (PCO)-multiwalled carbon nanotubesMWCNTs)/polyethylene SMP nanocomposites with co-continuoustructure and selective distribution of fillers in PCO phase [59].he MWCNT was selectively distributed in the composite whichould observably reduce the fillers content for the same conduc-


ivity. The nanocomposites completely recovered with 150 V in min [59]. Alam et al. prepared an electroactive SMP nanocom-osite with epoxidized linseed oil plasticized polylactic acid andWCNTs. The 3 wt% MWCNTs-reinforced nanocomposite system

dicate the site of the filled bone defect.

finished 95% shape recovery within 45 s at 40 DC voltages [60].Sahoo et al. reported the polyurethane/multi-walled carbon nano-tubes (PU/MWNTs) composites which were added MWNTs andpolypyrrole (PPy) to enhance the conductivity of the electroactiveshape memory composites. PU containing 2.5% PPy-coated MWNTsshowed better mechanical, thermal, and electric properties, whichfinished 90–96% shape recovery within 20 s at 25 V [61].

The electrical alignment is a most convenient and efficientapproach to align electronic polarized single CNT along the direc-tion of the electric field [68,69]. The CNT tends to assemble intoconductive chains along the external electric field with electrodedistance in micrometer or even nanometer ranges [70]. The smalldistance between two CNTs leads to the strong Coulombic forcebetween oppositely charged ends forming the local attraction field,so that CNTs attract each other to assemble into conductive chains.


In Yu et al. study, the CNT was electrically induced into alignedchains in a SMP/carbon black (CB) composite which would act aslong-distance conductive channels [62]. The viscosity of the liq-uid polymer is the key influence factor for the dipole moment

dx.doi.org/10.1016/j.ccr.2016.03.007



Fig. 7. The design concept and SME mechanism schematic sketch of reduced graphene oxide paper/epoxy-based shape memory composite.Reproduced from Ref. [56] with permission from Elsevier 2015.

F P/CB,s

R

orScCatet

baeitnaa

ig. 8. Snap shots of shape recovery and temperature distributions. Sample A: SMhowed the sample dimension and experimental setup for Sample C.

eproduced from Ref. [62] with permission from AIP Publishing LLC 2011.

f CNTs [71]. In order to reduce viscosity of the shape memoryesin, a dimethylformamide solution 50 vol% was added into theMP matrix, which would be vaporized during the curing pro-ess. The electrical resistivity of the composite with well chainedNTs was reduced for more than 100 times comparing to the samemount randomly filled CNTs one. In Fig. 8, the fabricated conduc-ive SMP/CB/CNT could full recover within 75 s at 25 V direct currentlectric voltages due to the remarkable reduction in electrical resis-ivity [62].

In our other work, self-assembled carbon nanofiber (CNF) andoron nitride (BN) functionally graded nanopaper of electrical actu-tion shape memory nanocomposites was enabled to actuate bylectrically resistive Joule heating [63]. Here, BN was utilized tomprove heat transfer and electric-activated shape recovery of


he nanocomposite. The nanocomposite constituted with 0.16 ganopaper (0.08 g CNF and 0.08 g BN) had the 96.7% recovery ratiot voltage of 4.8 V in 80 s with the maximum temperature reachingpproximately 167.49 ◦C [63].

Sample B: SMP/CB/CNT (random), Sample C: SMP/CB/CNT (chained). Right figure

3.3. Metal nanoparticles filled shape memory polymer composites

Notably, some attempts have been made to inductively formconductive chains with applying magnetic field during curing thenanocomposite. For instance, Li et al. developed a type of chem-ically cross-linked poly(3-caprolactone) based SMP compositewith Fe3O4 nanoparticles decorated multiwalled carbon nano-tubes (Fe3O4@MWCNTs) as a magnetism and electricity responsivesource [64].

The nanocomposite exhibited multistage stimulus SME inalternating magnetic field, electric field, and temperature field,respectively. In this study, the average size of Fe3O4 magneticnanoparticles was about 20 nm which was suitable for keepingexcellent magnetic responsive performance. The most outside sur-


face of the carboxyl-modified MWCNT was covered by Fe3O4nanoparticles. On the one hand, the Fe3O4 layers made a greatcontribution to the magnetic properties. On the other hand, theMWCNT without loaded Fe3O4 contributed to the good electric

dx.doi.org/10.1016/j.ccr.2016.03.007



Fig. 9. A series of digital photographs showing the shape memory recovery process of the c-PCL/Fe O @MWCNTs nanocomposite film (A) in 34 ◦C, 37 ◦C and 41 ◦C water,r eld st

R 14.

ppatwrt

4

rrmpcrni

4

a(ZtAifmtstuchd

espectively; (B) in an alternating magnetic field with a frequency of 20 kHz and a fi


roperties. In Fig. 9, shape memory recovery processes of the com-osites were induced by hot water with different temperatures, anlternating magnetic field with frequency of 20 kHz, and an elec-ric field with applied voltage of 60 V, respectively [64]. Osteoblastsere cultured to evaluate the cytotoxicity of the composite. The

esults demonstrated that it had good biocompatibility, and poten-ially application in biomaterials [72].

. Light-activated shape memory polymer nanocomposites

The light-activated SMP is unique attributing to its unparalleledemote activation and spatial control without invading the mate-ial, local effect without compromising the property of the restaterial, and easy switching on-off feature. It usually employs the

hoto-reversible covalent cross-links, which is photo-responsiveinnamic acid type molecules as switches under appropriate lightadiation [2]. Commonly, functional fillers of light-activated SMPanocomposites absorb light energy and transform light energy

nto heat to perform SME [73–79].

.1. Nanogold/shape memory polymer composites

Gold nanoparticles (AuNPs) and nanorods (AuNRs) attract morettention for absorbing light at their surface plasmon resonanceSPR) wavelengths and thus generating heat release [80–83]. Inhang and Zhao study, they described a rational design strategyhat achieved the chemically cross-linking crystalline polymer withuNPs to exhibit both light-activated SME and fast optical heal-

ng capability based on the same localized heating effect arisingrom the SPR of AuNPs [82]. Herein, the stimuli-healable poly-

er benefited to prolonging the service lifetime and reducinghe cost due to repair damage (crack or fracture) with externaltimulus. However, the difficulty of constructing this material ishe structural incompatibility between two polymers. The SMP


sually possesses permanent network structure which restrictshain motion with a high modulus [1], but the SMP requires theigh chain mobility and inter diffusion [78,84–92]. This materialesign was based on that photothermally induced melting followed

3 4

rength of 6.8 kA m−1; (C) in the supplied voltage 60 V.

by recrystallization to optically heal (weld) thermoplastic crys-talline polymers [80,81,93–95] and the photothermal effect forlight-activated SME [73,96–98]. As shown in Fig. 10, poly(ethyleneoxide) (PEO20K) of both chain ends terminated with an acrylatemoiety was mixed with functionalized AuNPs, ammonium per-sulfate (APS), and N,N,N′,N′-tetramethylethylenediamine (TMDEA)before cross-linking reaction, which subsequently was cured atroom temperature and 60 ◦C. The results indicated that a lowermolecular weight PEO increased the network density leading toreduce the PEO crystallinity and difficultly achieve the optical heal-ing effect. Moreover, AuNPs were well dispersed in the cross-linkedPEO matrix displaying a SPR absorption band at ∼530 nm [82]. Thenanocomposite showed selectivity locally light-activated SEM inlaser irradiation (� = 532 nm, 7 W/cm2) for photoinduced heatingof the polymer to T > Tm of PEO [82]. Shape recovery took placequickly and only in the selected region with sufficient power laserirradiation. And then the process stopped almost immediately forfast cooling of the material to T < Tm once the laser turned off. Con-trol tests found that the samples without AuNPs could not recoverthe original shape under the same conditions, which suggested thatany possible heating effect resulting from laser exposure was notenough to activate the thermal phase transition of shape recov-ery. Same as the SME, the crosslinked PEO containing no AuNPscould not achieve optical healing effect under otherwise equalconditions. Surprisingly, the optical healing of the AuNPs compos-ite was effective, but the bulk heating could not lead to heal thecrack. The same phenomenon was also observed in a crosslinkedpolyethylene [99]. With heating the whole composite, the ther-mal expansion occurred freely throughout the sample leadingto outward expansion which prevented intimate contact, chainsinterdiffusion, and interlocked chain recrystallization of fracturesurfaces. By contrast, when the crack of the composite was selec-tively exposed in laser beam, the photo-induced heating occurredlocally resulting in confined-thermal inward expansion without


outward expansion. The localized expansion could generate hoopstress to push the fracture surfaces into contact benefiting chaininterdiffusion and recrystallization for sewing the crack withoutexternal force.

dx.doi.org/10.1016/j.ccr.2016.03.007



of croR

o(apealcitccmuTtanfrssnawTalcpp

itrTwbao

4

itoce

Fig. 10. Preparation procedure

eproduced from Ref. [82] with permission from American Chemical Society 2013.

Zhang et al. reported a unique photo-thermal effect which basedn chemically cross-linked poly(ethylene oxide) with small amount0.5 wt%) of AuNPs [100]. The controlled SPR absorption of AuNPsllowed for forming a temperature gradient in the polymer of tem-orary shape with anisotropic polymer chain relaxation and strainnergy release. The pre-tensile nanocomposite bent with a smallngle in the laser, when the AuNPs of near surface layer absorbedight first and released heat with chain relaxation of orientatedhains in the upper surface. The polymer chain relaxation occurredn deeper and deeper layers with heating downward along thehickness direction under continuous laser radiation. The strategyould make it possible to use a laser to release of strain energy forreating complex shape transformations and controlling delicateechanical work of SMP nanocomposites which were inaccessible

sually or cannot be matched by direct heating or other stimuli.hey also demonstrated that light polarization could be used to con-rol photothermal effect-based gold nanorods (AuNRs)/poly(vinyllcohol) (PVA) film in which anisotropic AuNRs was aligned in theanocomposite [101]. The temperature rise could be tuned by dif-

erent polarized light keeping all other conditions unchanged. Theesults indicated that AuNRs had excellent orientation along thetretching direction of the film. The polarization parallel with thetretching direction designated as 0◦ polarization angel. Longitudi-al absorption was observed while the transverse absorption wasbsent. And yet the polarized light was set to be perpendicularith the stretching direction designated as 90◦ polarization angle.

he longitudinal absorption disappeared, while only the transversebsorption was visible [101]. The key of the method is to chooseow Tm for semicrystalline or Tg for amorphous SMP matrix foronstructing orientation photoactive species/SMP film which hasossibility of using the polarized light to control the shape recoveryrocess.

Zheng et al. reported a light induced SMP micropillar (10 mmn diameter, 40 mm in height, and 20 mm in pillar-to-pillar dis-ance) in a hexagonal array containing 0.1–0.2 mol% AuNRs viaeplica molding from a poly(dimethylsiloxane) (PDMS) mold [102].he results suggested transmittance in the visible wavelength andater contact angle of the composite could be regulated by tuning

ent and straight of SMP micropillar array. The SMP micropillarrray film has potential application in light-guided smart windowsr light-tracking solar panels.

.2. Nanocarbon/shape memory polymer composites

Infrared light possesses wide emission spectra and unique heat-ng effect in a noncontact manner which might be a good alternative


o induced-heating. Lu et al. found that a unique synergistic effectf CNTs and boron nitride which could facilitate the thermallyonductive and accelerate the infrared light induced shape recov-ry behavior of the SMP nanocomposites [103]. The CNCs were

ss-linked PEO/AuNP composite.

modified with benzophenone group which had highly photoreac-tive and easily formed covalent bonds with each other and/or withsurrounding macromolecules in polymer matrix.

Biyani et al. reported a light-stimulated mechanically switch-able nanocomposite which was fabricated by incorporatingbenzophenone-derivatized cellulose nanocrystals (Bp-CNCs) intorubbery ethylene oxide/epichlorohydrin copolymer (EO-EPI)matrix [104]. They used a straightforward method to func-tionalize CNCs with benzophenone (Bp). The Bp-CNCs couldmitigate wettability of the composite in humid environments.The EO-EPI/Bp-CNCs nanocomposite exhibited multi-responsiveand mechanical switching characteristics. As mentioned earlier[18–24], the hydrogen-bonded of CNCs network in these mate-rials led them to exhibit water-responsive SME. Bp with reliablephotoactive moiety was usually used as photopatterning and pho-toinitiator [105–110]. In excited states, the UV light-activation Bppreferentially consumed nearby C H bonds to form a new C Cbond [111,112]. In Fig. 11, Bp-CNCs as light-activated mechanicallyadaptive materials could be triggered to form bonds among CNCs aswell as between CNCs and the surrounding rubbery EO-EPI matrixin light exposure [104]. When the Bp absorbed photon, one elec-tron promoted from the nonbonding n orbital to an antibonding �*orbital of its carbonyl group. This n–�* transition yielded a birad-icaloid triplet state where the electron-deficient oxygen n-orbitalwas electrophilic [113,114]. Therefore, it was presumably preferen-tial that the covalent bonds between Bp-CNCs eventually developedBp-CNCs networks. It is worthwhile to note that this approach canbe easily extended to use Bp-CNCs with more matrices.

The light-responsive SMP as a photomechanical material has thecapability to directly transfer light energy into mechanical workplayed an important role in smart actuators, artificial muscles, andmicroelectromechanical systems [116–118]. Yu et al. fabricated aphotoresponsive flexible grapheme oxide/polymer nanocompositefilm using a facile solution casting method [115]. The nanocom-posite film displayed fast, stable, and reversible photomechanicalbehavior attributed to the photothermal effect of graphene oxideand the SME of the unique programming polymer matrix. Theyimitated the wobbly man structure for achieving light-poweredtumbler movement of the film. The reversible tumbler movementwas only triggered and maintained by visible light without exter-nal force, which was attributed to the incorporation between thephotothermal effect of GO and the SME of vinylidenefluoride-hexafluoro propylene (PVDF-HFP). Here, GO as light absorbent andnanoscale thermal source triggered the SME of the film. Comparingto CNTs and AuNPs, GO has better visible light absorption and iseasier uniform dispersed in matrix. Furthermore, GO with unique


photothermal property and high thermal conductivity would ben-efit photomechanical properties of the fabricated actuators [119].Currently, the SMP composites could exhibit complex shape tran-sitions utilized the shape programming process. In this study, the

dx.doi.org/10.1016/j.ccr.2016.03.007



F nanocomposites and the architecture of these materials before (open jaw) and after UVe

R 014.

ndwfi

F

fi[c5lmeaicetfitwlttip

(CdmiaflIarTlfislsiwr

Fig. 12. Possible mechanism for the reversible photomechanical behavior of theGO/PVDF-HFP nanocomposite film. (A) DSC curves of pure PVDF-HFP and theGO/PVDF-HFP nanocomposite with a GO content of 3 wt%. (B) Scheme illustration ofthe microstructure of the nanocomposite film showing the two-way and one-way

ig. 11. Schematic representation of the process used to prepare EO-EPI/Bp-CNCs

xposure (closed jaw).


anocomposite film was programmed from a flat square to a cylin-er shape with F = 0.6 to imitate the nondownable doll. The factor Fas used to quantitatively characterize the bending degree of thelm as given by Eq. (1) [115]:

= L0 − L

L0(1)

Here L0 and L were the linear length of the nanocompositelm before and after the shape programming process, respectively115]. As expected, the nanocomposite film started the oscillationycle with frequency of 0.5 Hz in visible light irradiation (450 nm,4 mW/cm2). In the initial state, the film (F = 0.6) was in an equi-

ibrium state under the gravity and the bracing force with equalagnitude in opposite directions. During the unfolding process, the

quilibrium state of the film was unavoidable broken attributed tosymmetric shape of the unfolded film (F = 0.5) under visible-lightrradiation. As a result, the film rocked to the right with its gravityenter simultaneously shifting to the right side. When it reached thextreme right position, a left-rotation force moment was producedo enable the film recovering to its initial position [115]. Then, thelm would rock to the extreme left with its gravity center simul-aneously shifting to the right side for the motion inertia. In thisay, one tumbler cycle movement was achieved, when the action

ine of gravity and bracing force exhibited the displacement leadingo drive the film rotating toward the reverse side. The continuousumbler movement of the nanocomposite film could be kept exert-ng visible-light irradiation as an external stimulation in the wholerocess.

The PVDF-HFP copolymer had three crystal phases of CrPs1T1 = 40.4 ◦C) and CrPs2 (T2 = 75.5 ◦C) as switchable domains, whilerPs3 (T3 = 131.2 ◦C) acted as the physical crosslinked networksetermining the permanent shape. In Fig. 12A, the characteristicelting points of PVDF-HFP were not obviously influenced by the

ncorporation of GO [115]. GO could effectively absorb visible lightnd transfer it into thermal energy heating the nanocomposite filmrom 28 ◦C to 48 ◦C, which was higher than T1, but lower than T2,eading to the shape change from shape B to shape C (Fig. 12B).t was important that the crystal phase CrPs2 remained and acteds an internal skeleton to promise the melted crystal phase CrPs1ecovering its original state in the recrystallization process [120].herefore, shape C would recover to shape B with removing visibleight. If the crystal phases CrPs1 and CrPs2 of the nanocompositelm were melting, the film would directly change from shape B tohape A with the temperature rapidly increased higher than T2, butower than T3. Thus, the nanocomposite film could not recover to


hape B or shape C exhibiting reversible photomechanical behav-or, although the temperature decreased to 28 ◦C [115]. This work

ould have a significant impact on actuation devices such as softobots [121], inchworm walkers [122], motors [123], and smart

SME.

Reproduced from Ref. [115] with permission from American Chemical Society 2015.

curtains designed based on movement modes of photomechanicalmaterials.

5. Microwave heating triggered shape memory polymernanocomposites

Microwave is an electromagnetic energy ranging from approx-imately 1–100 GHz in frequency. In comparison with otherelectromagnetic waves, microwave possesses better penetrabilitywith the low attenuation degree during transportation. Therefore,microwave can be used as practical remote controllable, environ-mentally friendly, highly efficient, uniform, and rapid actuationmethod for SMP without temperature gradient, hysteresis effect,considerable energy loss, and preheating, which heats materials inmolecular level and meets the high requirements of uniform heat-


ing. Microwave heating triggered SMP nanocomposites offer anopportunity for potential applications in the biomedical treatmentssuch as tissue engineering and regenerative medicine. Accordingto the interaction with microwave, the materials may be divided

dx.doi.org/10.1016/j.ccr.2016.03.007


12 W. Wang et al. / Coordination Chemis

Fig. 13. Sequence of the shape recovery of a SMP composite with a nonuniformCNT concentration: (a) an experimental schematic, (b) infrared record of the shaper

R2

iTteraitee

wtCwgnncmuctirbam[ifitciF5bdscd

s

ecovery process.

eproduced from Ref. [127] with permission from the Royal Society of Chemistry013.

nto three groups, which are absorbers, insulators and conductors.he absorbing microwave materials are called dielectrics. There arewo basic properties of the materials to govern its interaction withlectromagnetic fields in the area of microwave heating. One is theeal part of dielectric (dielectric constant, ε′), which measures thebility of storing electrical potential energy. The other is the imag-nary part of dielectric (dielectric loss factor, ε′′), which measureshe efficiency of transforming electromagnetic energy into thermalnergy. The absorbed microwave energy is transferred into thermalnergy through molecular friction and collisions [124–126].

CNTs, silicon carbide (SiC), ferroelectric nanoparticles, andater molecules usually used as microwave energy absorbers

o construct microwave-induced SMP nanocomposites [127–129].urrently, the interaction mechanism between CNT and microwaveas usually attributed to its impurities [130–133], generation of

as plasma [134], and dipolar polarization [135]. There are no sig-ificant temperature gradients within the CNT/SMP composites, soo residual stress generated during the microwave actuation of theomposites. In the study of Yu et al., the CNT with superb electrical,echanical, thermal, and strong microwave absorbing properties

sed as microwave energy absorber in CNT/styrene-based SMPomposites [127]. The CNT in the polymer matrix as the internalhermal sources transferred the absorbed electromagnetic energynto heating to induce shape recovery of the SMP composites. Theesults indicated that the CNT/SMP composites could successfullye induced by microwave radiation frequency of 2.45 GHz (40 W)nd 5 wt% CNT of the SMP composites could absorb 99.84% of theicrowave energy under microwave radiation frequency of 40 GHz

127]. The microwave absorption ratio of the CNT/SMP compositesncreased along with increasing the CNT content or the microwaverequency. In Fig. 13(a), the CNT/SMP part began to recover its orig-nal shape and constricted in length with heating above the Tg

emperature under microwave radiation, but the pure SMP partould not be heated which generated an inhomogeneous strainn the composite leading to the specimen wrap up. As shown inig. 13(b), the bending angle of the specimen was approximately0◦ in 30 s under microwave irradiation, subsequently decreasedy further exposure in the microwave field due to thermal con-uction from the composite part to the pure SMP part [127]. Theelective heating effect of nonuniform SMP in the microwave field


an be used in the rapid remote controllable actuation switch orevice.

As a functional material, SiC has many good properties,uch as strong microwave absorbing, low thermal expansion


coefficient, good chemical stability, and high temperature stabil-ity [124,136,137]. As mentioned above, SiC can strongly absorbmicrowave heated rapidly due to its dielectric loss factor. Duet al. prepared silicon carbide/poly(vinyl alcohol) based microwaveheating triggered SMP composites, in which SiC molecules asthe dielectric exhibited a dipole movement under the alternat-ing electric field to generate friction heat for increasing chainsegments flexibility and subsequently resulting to the polymerchains recover its original stress-free state at a molecular level[129]. Herein, microwave oven generally operated at a frequencyof 2.45 GHz as an alternating electric field for heating. Hydrophilicmodification of SiC had been done with silane coupling agent 3-triethoxysilylpropylamine to improve its dispersion in water andthe compatibility in PVA matrix. The SMP composites had goodthermal stability, mechanical properties, and microwave-inducedSME, which recovered original shape within 1 min under 600 Wmicrowaves.

The low viscosity, high solubility, high reactivity, and good com-patibility of SMP are able to gain through designing novel moleculararchitecture with appropriate hard and soft domains. Kalita andKarak developed an in situ polymerization technique for the syn-thesis of microwave actuated hyperbranched polyurethane/Fe3O4nanoparticles decorated multiwalled carbon nanotubes (Fe3O4-MWCNTs) thermosetting shape memory nanocomposites [138].Fe3O4 nanoparticles decorated MWCNTs had not only increasingdispersion but also absorbing microwave characteristics [139]. Inthis case, the saturation magnetization value of Fe3O4-MWCNTwas 0.23 emu/g and the tensile strength, scratch hardness, ther-mal stability, and shape memory properties of diglycidyl etherof bisphenol-A epoxy-based thermosetting nanocomposites wereimproved by increasing Fe3O4-MWCNT in the nanocomposite[138]. The cross-linking reactions among the OH/ C O of Fe3O4-MWCNTs, epoxy/hydroxyl groups of the epoxy resin, and free OHgroups of the hyperbranched polyurethane led to restrict molecu-lar chains movement and slippage, forming the three-dimensionalnetwork structures and increasing cross-linking density of thenanocomposite [138]. Based on the dose-dependent shape memorybehavior, the nanocomposite can be tuned as the requirement. Thisnanocomposite is a noncontact microwave-induced shape memorymaterial with good shape fixity and shape recovery ability.

6. Magnetically sensitive shape memory polymernanocomposites

The covalent integration of inorganic nanoparticles in polymermatrices can improve the shape memory and structural proper-ties of SMP composites. Magnetic nanoparticles have remarkableversatility, such as hyperthermia therapy [140,141], data stor-age [142,143], and microfluidics [144]. The magnetic properties ofmetal oxides such as Fe3O4 with sub-100 nm dimensions are thor-oughly different from those of the corresponding bulk materials[145,146]. When the magnetic particle size falls below a criti-cal limit, the magnetic nanoparticles exhibit super-paramagnetic[147]. Due to the features, the magnetic nanoparticles as inductiveheaters are considered to fabricate magnetically sensitive shape-memory polymer nanocomposites [148–150].

By adjusting the parameters of alternating magnetic field, themagnetically sensitive Fe3O4/SMP nanocomposite could recovertheir original shapes. Bai et al. used super-paramagnetic iron oxidenanoparticles (SPIONs) as macromolecular network crosslinkingpoints and magnetic-inductive heating elements to construct


norbornene-based magnetically sensitive nanocomposites withring-opening metathesis polymerization (ROMP) [151]. Traditionalapproach physically incorporated the iron oxide nanoparticles intothe polymer matrix. In this work, the complexation of carboxylate

dx.doi.org/10.1016/j.ccr.2016.03.007


W. Wang et al. / Coordination Chemist

Fig. 14. Schematic showing two approaches to obtain magnetically addressablesi

R

mfnHt5Fc

anffwprcTsNwfSm

bmstfiFd

pfinnipopnpa

hape-memory nanocomposites by simple embedding (left) by non-covalent tether-ng (right).

eproduced from Ref. [151] with permission from John Wiley & Sons 2014.

oieties on Fe3O4 surface was strong enough to disperse well andorm appropriately strong networks via specific non-covalent in theanocomposite with better magnetically sensitive SME (Fig. 14).erein, the Fe3O4 nanoparticles were synthetized by one-pot reac-

ion method with the oxidation of Fe(acac)3 in the presence of-norbornene-2-carboxylic acid [152]. The bulky surface groups ofe3O4 nanoparticles could prevent nanoparticle aggregation andopolymerize with polynorbornene.

Yu et al. proposed a feasible and versatile method to prepare biodegradable crosslinked poly(�-caprolactone) (c-PCL)/Fe3O4anocomposite which exhibited an good shape memory per-

ormance in an alternating magnetic field of H = 6.8 kA/m at arequency f = 20 kHz [153]. Herein, radicals and free radical chainsere caught by sodium oleate on Fe3O4 surface. And then rigidarticles worked as obstacles to impede the proximity motion ofadicals and free radical chains. This suggests that this modifi-ation method might be fitted to modify other SMP in principle.he nanocomposite was activated from their frozen temporaryhape attributed to inductive heating of Fe3O4 for Brownian andéel relaxation. Razzaq et al. synthesized Fe3O4/polyurethane SMPhich induced by a magnetizing field of H = 4.4 kA/m at a frequency

= 50 Hz [154]. The results indicated that the magnetically sensitiveMP nanocomposite was possible to be stimulated by an electro-agnetic field of low frequency and low field strength.Fe3O4 nanocrystals can be stabilized by interacting with the car-

oxylic acid groups of oleic acid on the surface [155,156]. Modifiedagnetite (Fe3O4) easily realizes well dispersion of the nanometer

ize range in polymer matrix. Yang et al. used a facile methodo synthesize Fe3O4/polynorbornene-based alternating magneticeld triggered SMP nanocomposites [157]. The surface-modifiede3O4 nanoparticles decreased the shape recovery induction timeue to increase in thermal conductivity.

As previously mentioned, our group fabricated a SMP nanocom-osite exhibited multistage stimulus SME in alternating magneticeld, electric field, and temperature field, respectively [64]. Theanocomposite exhibited good shape memory property in alter-ating magnetic field (f = 20 kHz, H = 6.8 kA/m). Cai et al. utilized

n situ polymerization method to synthesize magnetically sensitiveolyurethane composites which could be filled with high contentf Fe3O4 nanoparticles due to the crosslinking of MWCNTs with


olyurethane prepolymer [158]. Generally, a high content of mag-etic nanoparticles in polymer matrix deteriorate its mechanicalroperties [159–161]. Herein, the nitric acid-MWCNTs were useds crosslinking agents to form network structures and improve the

PRESSry Reviews xxx (2016) xxx–xxx 13

mechanical properties of the composites even with high contentFe3O4 nanoparticles. The shape recovery rate of the nanocom-posite was over 95% in alternating magnetic field (f = 45 kHz,H = 29.7 kA/m) within 1 min. In a short time of the beginning, therewas no observed shape change of composites in alternating mag-netic field which was called magnetic field response time. At thelater stage, the temperatures reached Tg and the composites recov-ered to their initial shape quickly.

Here Razzaq and Lendlein reported a poly(u-pentadecalactone)networks nanocomposites with three arm oligo(u-pentadecalactone) (OPDL). OPDL coated magnetite nanoparticlesas multifunctional covalent netpoints at the nanoscale providedmechanical stability and magnetically sensitive shape memoryperformance [162]. They found that the performance of nanocom-posites could be controlled by magnetite nanoparticles contentaccording to their applicability which focused in the directionof mechanical strength or shape-memory actuation capability.A similar study of their group carried out detailed studies onelectrospraying of a 1,1,1,3,3,3-hexafluoro-2-propanol solutioncontaining a mixture of copolyetheresterurethane (PDC) andmagnetic Fe3O4 nanoparticles to prepare magnetically sensitivepolymer-based nanocomposite microparticles [163]. The electro-spraying technique benefited for fabricating magneto-sensitivepolymer-based nanocomposite more than entrapping process, sus-pension polymerization, emulsion polymerization, and dispersionpolymerization, because it neither required high temperaturesnor further drying step [164–168]. PDC was a copolyether-esterurethane composed of crystallizable oligo(p-dioxanone) aspermanent netpoints and crystallizable oligo(�-caprolactone)as switching segments to trigger SME which were linked via ahexamethylene-1,6-diurethane unit [169].

7. Conclusions

The SMP nanocomposites can be optimized depending onapplication requirements to probe more interesting behav-iors and study more challenging actuation problems. In thiscontext, nanocellulose, CNTs, BG-NPs, graphene, carbon nanopar-ticles, AuNPs, Fe3O4 nanoparticles, and SiC nanoparticles haveemerged as useful nanofillers. Besides conventional thermal stim-ulus, the chemo-responsive, electrically activated, light-activated,microwave triggered and magnetically sensitive SME can be used aspotential driving methods to stimulate the SMP nanocomposites.Emphasis was placed on the design principle, fabrication, shapememory properties, and corresponding mechanisms of the SMPnanocomposites.

Researches on chemo-responsive SME generally agree that theplasticizing effect of solvent molecules can increase the flexibil-ity of the hydrophilic SMP chains. In particular, the water-inducedSMP have SME in water or physiological media at body temper-ature without heat. A water molecule is good plasticizer, whichweaken hydrogen bonding between the polymer chains. Corre-spondingly, the chains attain greater mobility and increase infree volume, which decrease the Tg and stiffness. Adding func-tional nanomaterial and constructing nanostructure can largelyimprove chemo-responsive shape memory performance of SMP.The chemo-responsive SMP nanocomposites have the advantagesof biological compatibility, facile processing, and good mechanicalproperties, which have the potential to give value in humidity sen-sors, underwater deployable structures, ultralow-power devices,breathable clothing, sporting gear, and biomedical industry. In


some applications, electrical activation is a more convenient andefficient method than the external thermally triggered method.The research shows that higher content of conductive filler yieldslower resistivity in the nanocomposites. However, it is difficult to

dx.doi.org/10.1016/j.ccr.2016.03.007

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4 W. Wang et al. / Coordination C

dequately disperse the filler in polymer matrices increasing fillerontent. The inhomogeneously dispersed fillers in nanocompos-tes not only cause nonuniform recovery with localized heating,ut also lead to localized melting, and even present a fire haz-rd in extreme cases. These safety issues have been the primarybstacle in developing efficient electrically actuated shape memoryanocomposites. Comparing to other stimuli-driven methods, the

ight-driven method is a non-contact clean energy source that cane controlled remotely, instantly, and precisely. The photo-thermalffect of light-induced SMP nanocomposite not only possesseshe appealing features of remote activation, spatial control, andwitching capability, but also has the healing effect under certainonditions in precisely selected regions. Further developing light-nduced SMP nanocomposites will have a significant impact onctuation devices such as soft robots, inchworm walkers, motors,nd smart curtains based on movement modes of photomechani-al materials. As other applicability exemplification of functionallyraded SMP nanocomposites, its selective heating effect in theicrowave field and magnetic field can be used in the rapid remote

ontrollable actuation switch or device. It is expected to provideew perspective and effective approach for designing novel SMPanocomposites via understanding interrelationships between theesign principle, fabrication, shape memory properties, and corre-ponding mechanisms.

onflict of interest

Authors declare no competing financial interests.

cknowledgements

This work has been financially supported by the National Naturecience Foundation of China (Grant No. 11225211) and the Chinacholarship Council, for which we are very grateful.

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