Composites: Part A 100 (2017) 20–30

Contents lists available at ScienceDirect

Composites: Part A

journal homepage: www.elsevier.com/locate/compositesa

Review Stimulus methods of multi-functional shape memory : A review ⇑ ⇑ Tianzhen Liu a, Tianyang Zhou a, Yongtao Yao a, Fenghua Zhang a, Liwu Liu b, Yanju Liu b, , Jinsong Leng a, a Centre for Composite Materials and Structures, Harbin Institute of Technology (HIT), No. 2 YiKuang Street, PO Box 3011, Harbin 150080, People’s Republic of China b Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT), No. 92 West Dazhi Street, PO Box 301, Harbin 150001, People’s Republic of China article info abstract

Article history: This review is focused on the most recent research on multifunctional shape memory polymer nanocom- Received 19 February 2017 posites reinforced by various . Different multifunctional shape memory nanocomposites Received in revised form 22 April 2017 responsive to different kinds of stimulation methods, including thermal responsive, electro-activated, Accepted 28 April 2017 alternating magnetic field responsive, light sensitive and water induced SMPs, are discussed separately. Available online 2 May 2017 This review offers a comprehensive discussion on the mechanism, advantages and disadvantages of each actuation methods. In addition to presenting the micro- and macro- morphology and mechanical prop- Keywords: erties of shape memory polymer nanocomposites, this review demonstrates the shape memory perfor- Shape memory polymer mance and the potential applications of multifunctional shape memory polymer nanocomposites Nanocomposites Multifunctional under different stimulation methods. Actuation Ó 2017 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 20 2. Thermo-responsive SMP nanocomposites...... 22 3. Electro-activated SMP nanocomposites ...... 22 4. Magnetic field actuated SMP nanocomposites ...... 24 5. Light-responsive SMP nanocomposites ...... 24 6. Water-induced SMP nanocomposites ...... 27 7. Conclusions and outlook ...... 28 Acknowledgements ...... 28 Reference ...... 28

1. Introduction effectively. In general, SMAs are divided into two categories with respect to the stimulus methods, thermos-responsive SMAs trig- Shape memory materials (SMMs) have the capability to respond gered only by heating and magneto-responsive SMAs activated to certain stimuli and to deform from a temporary shape to the by static or variable magnetic field as well. As a type of smart original shape. Currently, among all types of SMMs developed, materials with unique characteristics, SMAs are widely used in var- shape memory alloys (SMAs) and shape memory (SMPs) ious areas especially in medical and industrial fields, specific are the most extensively studied and being applied broadly and examples in which include dental wires, arterial stents, fire secu- rity systems and helicopter blades [1–5]. Since the discover of

⇑ Corresponding authors at: Department of Astronautical Science and Mechanics, SMPs in 1980, SMPs offer more stimulation methods, such as heat- Harbin Institute of Technology (HIT), No. 92 West Dazhi Street, PO Box 301, Harbin ing, electrical current, alternating magnetic fields, light exposure 150001, People’s Republic of China (Y. Liu) and Centre for Composite Materials and and water immersion [6–8], which result from the underlying large Structures, Harbin Institute of Technology (HIT), No. 2 YiKuang Street, PO Box 3011, extensibility due to the intrinsically elastic polymeric networks. Harbin 150080, People’s Republic of China (J. Leng). SMPs possess special characteristics, such as the density range of E-mail addresses: [email protected] (Y. Liu), [email protected] (J. Leng). http://dx.doi.org/10.1016/j.compositesa.2017.04.022 1359-835X/Ó 2017 Elsevier Ltd. All rights reserved. T. Liu et al. / Composites: Part A 100 (2017) 20–30 21

900–1100 kg/m3, up to 800% extent of deformation, recovery potential manufacture of immediate engineering productions speeds from less than 1 s to several minutes, the Young’s modulus [26]. Combining 3D printing technique, the potential applications of 0.1–10 MPa above transition temperature, the Young’s modulus of the printed 4D devices based on SMP and their composites could of 0.01–3 GPa below transition temperature, stress of 1–3 MPa be broadly extended. For example, sequential self-folding struc- required for deformation and generated during recovery [9–13]. tures were obtained via 3D printed technique with digital shape In addition to the above mentioned special properties of SMPs, memory polymers, which can behave differently to specified they are easy processing, low cost, potentially biocompatible and shapes when exposed to thermal stimulus. A simplified reduced- biodegradable, able to bear large deformation and can be produced order model and metric were used to predict self-collision [27]. into many structures [9,14]. Due to these advantages, SMP and Qi et al. designed and fabricated active origami with 4D printing their composites, in which polymers matrix systems act as binder concept, which signified the time-dependent shape change after or matrix to make incorporated particulates, fibers or other rein- printing. A series of active origami components, such as box, pyra- forcements placed properly, have multifarious applications. In mid, and airplanes, were demonstrated with the guidance of a the- aerospace engineering, SMPs could be used for space environment oretical model, which was developed for valid selection of design evaluation and related testing [15], and space-deployable compo- parameters, as shown in Fig. 1 [28]. nents such as hinges [16], antennas [17] and rover wheels [18]. Nanoparticles are typically in the range of 1–100 nm in size, In biomedical field, SMPs were made into medical cast [19], artifi- which behave as a whole unit from the perspective of transport cial muscles [20], endovascular thrombectomy device [21], and and properties [29]. In addition, the properties of nanoparticles devices to prevent aneurysm rupture and cardiovascular stent are quite size-related, varying from those of either fine particles [22,23] which are feasible. As for textile, breathable clothing made or bulk materials. Recently, due to the potential that nanotechnol- of SMP can provide better comfort under various temperature and ogy holds, there has been a strong emphasis on the development of moisture [24,25]. By creating a functional gradient varying the nanocomposites. SMP nanocomposites play an important part in temperature (Tg), Qi et al. achieved spontaneous the field of SMP composites. Polymeric nanocomposites could be and sequential shape changing properties [27]. Such shape simply interpreted as a novel class of composite where there is changing sequence can be controlled precisely by created SMP at least one dimension of the component which is of the order of components with properly assigned spatial variation of the ther- nanometer in polymeric system [30]. Nanoscale materials have a modynamical property distribution. Meanwhile, with the develop- large surface area which is beneficial for different chemical and ment of 3D printing technique, the application of 3D printing physical interactions between polymer matrix and fil- technique has become a hot research topic in recent years, accord- lers. In addition, with the help of high aspect ratio of certain ing to the virtue of high resolution, large design freedom and nanoparticles, importing nano-scale fillers could reduce the level

Fig. 1. (A) 3D folding structures mimicking the USPS mailbox [27] (B) Active origami box and pyramid. The printed flat cross shape in (a) assembles itself into a desired box shape in (b) after the programming steps. The printed flat Ninja star shape plate in (c) assembles itself into a desired pyramid shape in (d) after the programming steps. (C) Active origami airplanes. A flat triangle sheet with three hinges in (a) assembles itself into an origami airplane with a 0° angle in the middle hinge that bends upward and 90° angles in the two side hinges that bend downward in (b). A flat triangle sheet with five hinges in (c) assembles itself into an origami airplane with two winglets in (d) [28]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 22 T. Liu et al. / Composites: Part A 100 (2017) 20–30 of defects resulting by fillers introduced into the system, compared with the introduction of micro scale fillers [31–34], thus higher mechanical properties and multifunction is achieved. The intro- duction of nano-scale fillers to SMPs can not only improve the mechanical properties, but also open up new possibilities for the integration of diverse stimulus methods, such as electro- activated, magnetic field actuated, light-responsive and water- induced. SMP nanocomposites are attracting more and more attentions due to their benefits. So far, common nano-fillers mainly include nanoparticles [35–40], nanotubes [41–45], nanofibers [46,47], nanowires [48], nanorods [49,50] and nanopaper [51]. Some con- ventional fabrication methods, such as resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin film infusion (RFI), and filament winding, are still effective choices for SMP nanocomposites fabrication [30,52]. Besides, new processes were developed constantly to fabricate multifunctional nanocom- posites. A manufacturing process with intercalation polymeriza- tion of e-caprolactam in the presence of expanded graphite was used to obtain electrically conductive nylon 6/graphite nanocom- posites, of which the electrical conductivity reached 10À4 S/cm with 2.0 vol% of graphite [53]. To actuate SMPs using magnetic

fields, Fe3O4 particles were incorporated into Nafion nonwoven nanofiber fabrics via method to fabricate compos- ite fibers [54]. Latex technology was selected to fabricate a novel Fig. 2. Molecular mechanism of dual-SME. Black dots: netpoints; blue lines: kind of vapor-grown carbon nanofiber (VGCNF)/epoxy shape mem- molecular chains of low mobility below Ttrans; red lines: molecular chains of high ory (SM) nanocomposites, which possessed excellent SM function- mobility above Ttrans [61]. (For interpretation of the references to colour in this ality and significantly improved yield strength and Young’s figure legend, the reader is referred to the web version of this article.) modulus with 1.0 wt% of VGCNF [55]. In this review, the performance of multifunctional SMP nanocomposites under various actuation methods and advantages in rapid development of thermo-responsive SMP based deployable and disadvantages of each actuation methods are demonstrated. space structures and medical devices. Lendlein et al. reported that The modification of nanoparticles is also presented in order to a series of degradable thermoplastic polymers, which served as achieve better interaction between the filler and the matrix. The structural concept to tailor macroscopic properties with different micro- and macro- morphology and mechanical properties of molecular parameters, were applied for potential biomedical appli- shape memory polymer nanocomposites are discussed. Finally, cations. They made it possible for the transplant of bulky devices in the potential applications of multifunctional shape memory poly- the body through small incisions and the automatic performance mer nanocomposites are given. of complex mechanical deformations, indicating potential influ- ence on the design of implants and surgical devices tomorrow [65]. 2. Thermo-responsive SMP nanocomposites 3. Electro-activated SMP nanocomposites Thermal actuation is one of the most common, basic and direct way to actuate SMPs, relying on the transmission of thermal In order to remotely control the actuation and to heat up SMPs, energy from external environment to the thermal sensitive SMP especially SMP bulks with less temperature gradient from surface via conduction, convection or thermal radiation in a direct contact to core. Since most SMPs have high electrical and thermal resis- way [56]. Traditionally thermal actuated SMPs require certain type tance, researchers brought up with embedding conductive fillers of media such as gas, liquid and solid to transfer the heat and as heating source inside of the SMPs which makes it easier to pass achieve fast actuation. For mechanisms of the thermosensitive electrical current and Joule heating. Joule heating could be induced materials, the recovery deformation is triggered by exceeding the by the current passing through the conductive ingredient network critical temperature. Particularly, two separated phases are neces- within an SMP. When the internal temperature is above the transi- sarily included in thermal actuated SMPs, one is to stabilize the tion temperature, the deformation recovery is triggered. Owing to permanent shape by acting as fixing phase, the other serves as the uniform heating and high recovery speed, electro-activated switch with lower thermal transition (Ttrans). Ttrans is the ther- SMP nanocomposites have a huge advantage over thermally sensi- mal transition of the switching segment phase. For T > Ttrans, the tive SMP [43,66]. Plenty of nanoparticles blended into SMPs has material possesses high elasticity due to the flexible chain seg- been systemic studied, such as metal particles [42,50,51], carbon ments in switching phase. On the contrary, the recovery of the black [43], carbon nanotubes (CNTs) [44–47,63,67,68], graphene polymer chains is prohibited by the limited flexibility of the chains and carbon nanofibers (CNFs) [55,64]. Nanoparticles introduce below Ttrans, which makes it possible to fix the temporary shape fewer defects in the material system and have better interface [57–60]. Almost half of the following actuation methods are actu- interaction with the matrix. ally based on thermal mechanics as shown in Fig. 2, namely elevat- Due to high mechanical properties [69,70] as well as good ther- ing the temperature of the filler to heat up the matrix [61]. mal [71,72] and electrical [73] performance, CNTs are the rising Incorporating nanoparticles in thermal responsive SMPs can star achieving multifunctionality of SMPs, and improving their increase the efficiency of heat transfer and raise its mechanical conductivity [74]. However, because of its high aspect ratio and performance. Lu et al. has thoroughly discussed research and Van der Waals interaction [75], CNTs often aggregate into bundles development on mechanical reinforcement and multifunctional- without proper dispersion methods, preventing the efficient trans- ization [62–64]. The capacity of shape memory effect has resulted fer of their superior properties to the matrix. Exactly, the Young’s T. Liu et al. / Composites: Part A 100 (2017) 20–30 23 modulus and strength of CNTs are up to 1 TPa and 60 GPa respec- Raja et al. improved the mechanical, electrical and thermal per- tively, but they can only lead to moderate increase in modulus and formance by modifying multi-walled carbon nanotube (MWCNT) even more limited enhancement in strength for CNTs reinforced and bonding MWCNT with polyurethane (PU)-polylactide (PLA) polymer composites [33,76–78]. Results reveal that at present blend [67] as well as PU-poly (vinylidene difluoride) (PVDF) blend the theoretical predictions of modulus and strength for CNTs rein- [68]. Mahapatra et al. optimized the amount of MWCNT to be 5 wt forced composites are 2 orders of magnitude higher than practical % in the easy synthesized hyperbranched polyurethane (HPU)- situations [32]. Both physical and chemical methods were adopted MWCNT composites through polymer assisted dispersion, and by researchers to mitigate this phenomenon. On one hand, some achieved high speed actuation while maintaining the mechanical researchers adopted ultrasonication and high-shear mixing, the properties [63], the results of which are shown in Fig. 3. resultant well dispersed system of which is hard to maintain. Other fillers can also improve the mechanical and electrical per- Leng’s group presented one way to solve this problem, that was formance. Rana succeeded in controlling the interface between the to create a self-supporting network-CNT nanopaper after mixing polymer and graphene for effective load transfer, based on the it with the matrix solvent, then SMP nanocomposites were suc- functionalized graphene sheets, which were used as crosslinkers cessfully fabricated via resin transfer molding process [79]. Fei to the prepolymer [80]. Luo et al. fabricated stretchable silver et al. discussed the second way to solve the bundling based on nanowire-shape memory polymer composites with flexibility and ultrasound assisted in situ polymerization, which resulted in high high electrical conductivity. Such experimental results and model- electrical conductivity and good shape memory properties [44]. ing simulation of electro-triggered shape recovery behaviors of the The above mentioned mechanical dispersion methods often com- composites were consistent [48]. promise the aspect ratio of CNT [45]. On the other hand, using sur- Some researchers mixed hybrid fillers in the matrix either to factant and polymer to functionalize the surface of CNT and bridge the larger particles to transfer heat and electric current bet- improve the mechanical behavior is commonly seen [46]. Usually ter or providing larger interface to enhance the mechanical load- in this process, the polymer or surfactant could influence either ing. The relative high electrical conductance and effective heat the thermal or the electrical conductivity [47]. transfer which means the electrical actuation of CNF/SMPs were

Fig. 3. (A) FE-SEM images of PU/PLA with pristine MWCNT (a) and PU/PLA with modified MWCNT (b) [67]; (B) FE–SEM results of pristine and modified CNT filled PU/PVDF nanocomposites [68]; (C) SEM images of HPU with 3, 5 and 7 wt% MWCNT loading respectively [63]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 24 T. Liu et al. / Composites: Part A 100 (2017) 20–30

Fig. 4. (A) TEM image of Ag-CNT [81] (B) SEM image of the CNTs grafted onto the carbon fibers [82]. greatly improved after CNTs grafted on the CNF for covalent linking mer network nanocomposites was precisely controlled. In Fig. 5, [82,83]. Meanwhile, Young’s modulus, yield stress and cyclic one-step and two-step shape memory creation procedure (SMCP) recovery rate could be greatly improved through melt mixing which resulted in triple-shape and dual-shape properties, respec- metal nanoparticles, decorated MWCNTs and PU [81]. The hybrid tively, were discussed on the shape memory properties [37]. Zhang fillers are presented in Fig. 4. et al. modified the surface of magnetite nanoparticles for better The electric triggering extends the potential application of dispersion with poly(L-lactides) (PLLA) matrix, after which the shape memory polymer technologically. SMP/nanopaper elastic modulus, tensile strength, elongation at break and the shape nanocomposites with high electrical conductivity and thermal sen- memory properties were all greatly enhanced [40]. sitivity can serve as sensors or self-deployable structures [79]. The Mohr et al. induced magnetic nanoparticles in shape memory composites skins and cold hibernated elastic memory (CHEM) polymers effect of composites, which demonstrated that magnetic foam cores are involved to make sandwich structures, which field actuated SMP were comparable to thermo-responsive SMP. In makes it possible to obtain high packaging ratios [16]. On account their work, a Fe2O3 core in silica matrix was incorporated into of the convenient control with electricity, there are prospects for polyetherurethane (TFX) and a biodegradable multiblock copoly- development in some medical applications, such as medical mer (PDC) with poly (p-dioxanone) as hard segment and poly (e- devices for some common intravascular diseases [44]. caprolactone) as soft segment separately [57].InFig. 6, an alternat- ing magnetic field with frequency of 258 kHz was applied to the system for heating [57]. Kumar et al. programmed 4. Magnetic field actuated SMP nanocomposites SMP with triple shape effect, using silica coated Fe2O3 for magnetic actuation [88]. Generally, the simple way to obtain magnetic responsive SMP And a selective triggering of shape recovery process under two composites is incorporating magnetic nanoparticles in thermoplas- different frequency of RF field under 13.56 MHz for the CNT/SMP tic SMPs. The magnetically induced recovery process is triggered part to recover and 296 kHz letting Fe3O4/SMP region to recover by inductive heating in an alternating magnetic field, the impor- are studied by He, which are shown in Fig. 7 [38]. tant feature of which is that the heat is generated inside the poly- Golbang and Kokabi used static field to deform the SMP mer itself, thus resulting in rapid temperature increase [56,57]. nanocomposites into a temporary shape and then applied alternat- Prior studies demonstrated that embedded magnetic particles ing magnetic field to actuate the nanocomposites (Fig. 8) [39]. could be adopted to generate inductive heating of smart hydrogels The functionality of magnetic field actuated shape-memory [84], ferrogels [85], and ferrofluids [86] in alternating magnetic polymer makes it possible to achieve the versatility of their poten- fields. tial technically. Magnetic responsive SMP multifunctional There are three mechanisms under inductive heating including nanocomposites have the potential in the usage of smart implants, eddy current losses, hysteresis losses and rotational losses. Hys- magneto-optical storage, biomedical sensing, flexible electronics teresis loss and eddy current losses play the major role in heating and controlled medical instrument [87]. micro sized particles and heat can be under relatively low fre- quency [87]. Here we mainly consider rotational losses under high alternating frequency due to the small size of magnetic nanoparti- 5. Light-responsive SMP nanocomposites cles [35].

Most frequently used particles are Ni, Fe2O3 and magnetite While for the remote control, electro-magnetic interference nanoparticles, among which magnetite nanoparticles are the most could be inconvenient or harmful when we use electrical or mag- favorable. Foams illustrated the potential of shape memory tech- netic responsive SMPs, light induced SMPs can avoid these prob- nology. Owing to the low weight, and recovery force, shape mem- lems [89]. There are mainly two mechanisms in light induced ory polymer foams (SMP foams) are capable of tailoring material SMPs, photochemical reactions leading to deformation [90] and properties for application requirements as a kind of critical tech- employment of particles that can convert light to heat. nique. Remote induction of magnetic susceptor filler particles, In photochemical reactions, researchers introduced light- which were dispersed in the foam matrix, thermally activated sensitive groups such as cinnamic groups [91] or molecules [92] SMP foams. Their thermo-mechanical properties were maintained which would alter the structure of the crosslinked polymer net- with up to 10 wt% filler [36]. Kumar et al. used silica coated mag- work. The accumulation of structural alterations leads to an evolu- netite nanoparticles for better compatibility with polymer matrix tion of the polymer network, and even macroscopic deformation compared to uncoated fillers. The temperature of the grafted poly- subsequently. Consequently, the photomechanical shape-memory T. Liu et al. / Composites: Part A 100 (2017) 20–30 25

Fig. 5. (a) Schematic representations of the different shape-memory creation procedures applied for bending of the nanocomposites. Two-step programming methods: SMCP-2s-I, SMCP-2s-II as well as single-step programming procedure SMCP-1s. (b) Molecular mechanism for graft polymer network composites during shape-memory creation procedure where the color indicating the different phases of the polymer segments. Orange: amorphous PCL chain segments, Light blue: amorphous PEG chain segments, Red: crystalline PCL chain segments, Dark blue: crystalline PEG chain segments, grey: amorphous poly (methacrylate) chain segments [37]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Series of photographs showing the macroscopic shape-memory effect of TFX100 composite with 10 wt% particle content in an inductor [57]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) effect is achieved. Although it is hard to give clearly defined finite-deformation modeling framework, which integrated four mechanisms that describe the relationship between optical excita- coupled phenomena that lead to macroscopic photomechanical tion and mechanical behavior because of the differences from behavior: photophysics, photochemistry, chemomechanical cou- material to material, there are enough characteristics in common pling, and mechanical deformation, was developed to describe among them to permit the development of a generalized modeling the photomechanical response. The simulation results were in con- framework to describe the photomechanics. A three-dimensional cordance with the experimental data reported previously [90]. 26 T. Liu et al. / Composites: Part A 100 (2017) 20–30

Fig. 7. Conceptual illustration of various shape recovery routes from Temporary Shape #1 to the permanent shape. Five recovery routes are possible depending on the actuation sequences. RF1 and RF2 represent low and high radio frequencies, respectively. In all the five recovery routes, the last recovery step to the permanent shape is always achieved by direct heating since the central neat SMP region does not contain any filler [38].

Fig. 8. A schematic of: (a) magnetic induced deformation in a polymer matrix loaded with magnetic particles and (b) shape recovery measurement [39]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Since the functional groups and molecules are responsive only to altered by varying the loading of nanorod [50]. Zhang et al. intro- certain wave length, precise control on the deformation of the sys- duced gold nanoparticles (AuNPs) into PU based matrix for section tem can be achieved (see Fig. 9). [92,93]. However, the discussion control via light exposure and the shape recovery rate can be of mechanical properties of these kinds of SMPs are limited. adjusted through changing intensity and length of the exposure Others introduced photosensitive functional groups as molecu- [96]. The mechanical property is also impressive as shown in lar switches to convert light to heat and actuate the thermal Fig. 10. The same group later developed SMP nanocomposites responsive SMPs [89]. This method can absorb light from a broader based on gold nanoparticles possessing self-healing abilities and range, carbon black for example can absorb light wave from 500 to shape memory performance which can be triggered separately 4000 cmÀ1, resulting in faster actuation process [94]. CNTs are [97]. again adopted not only to increase the mechanical properties but The applications of light induced SMP multifunctional also to absorb infrared photons and release the stored strain nanocomposites are diverse. They can serve as optical sensors, energy [95]. Hribar et al. blended gold nanorod which has been microrobots, optical actuators, and other light-responsive medical proven to be biocompatible [49] into polymer to achieve light actu- devices. Especially in biomedical field, SMP nanocomposites con- ation and the glass transitional temperature change could be taining nanoparticles in vivo can be heated by near infrared which T. Liu et al. / Composites: Part A 100 (2017) 20–30 27

Fig. 9. Precise control of the bending direction of an LCE film by linearly polarized light [92]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. Spatially selective shape recovery process at room temperature by separate laser exposures on four sections of an AuNP-loaded film stretched to 100% deformation lifting a load 350 times its weight [96]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

can penetrate tissues well [98] and achieve thermal ablation of to penetrate the bulk. More work could be done on the transmit- tumors. The in vivo assessment was carried out on rats [49]. tance of the SMPs. The light-responsive SMPs open up new possibilities for surface patterning, light-driven actuators, photo-induced shape memory behavior, and photo-origami. An excellent eye-safe 442 nm light- 6. Water-induced SMP nanocomposites responsive SMP system based on the photoisomerization of azo- functionalities is demonstrated, whose shape recovery and fixity Water driven actuation of SMPs was first brought up in 2005 by properties show advantages in applications that require precise Huang [100]. The mechanisms have attracted broad attention since shape control [99]. However, most of the light-responsive SMP then. In general, water or solvent molecules could infiltrate into multifunctional composites are thin films. Since the SMP multi- the SMPs. Due to the plasticizing effect of water or solvent on SMPs functional nanocomposites are too thick, it will be difficult for light and increase of the flexibility of macromolecules, glass transition 28 T. Liu et al. / Composites: Part A 100 (2017) 20–30

trical actuation with conductive nanoparticles inside the SMP nanocomposites can heat the system from within. However, the electrode applying the current constrained the application in remote and spatial applications. Magnetic and light actuation method can achieve remote control over the SMP nanocomposites. Yet light actuation methods have the penetration problem to solve. The application of multifunctional SMP nanocomposites is unlimited, if future research can solve the following problems.

(1) Well dispersion of nanoparticles and the SMP matrix to avoid sinking and aggregating of nanoparticles. (2) Strong interface interaction between nanoparticles and SMP matrix for better heat transfer and electrical and mechanical performance. (3) Transparency of SMP to the light that’s used for actuation of SMP nanocomposites.

Fig. 11. Glass transition temperature vs fraction of moisture in weight [101]. Acknowledgements temperature drops upon addition of even a low amount of water. This work is supported by the National Natural Science Founda- When glass transition temperature approaches to the environment tion of China (Grant No. 11225211). temperature, the recovery process of water-induced SMPs is trig- gered. Huang et al. found that the glass transition temperature of the polyurethane SMP changed from 36 °Cto0°C approximately, Reference when the fraction of moisture reached about 4.5% in weight, as [1] Yang H, Fortier A, Horne K, Brostow W, Hagg Lobland HE. Shape memory shown in Fig. 11 [101]. Lu analyzed chemo-mechanical behavior metal alloys in the context of teaching smart materials. J Mater Educ 2016;38 of SMP in response to solvent [102]. Mendez suggested that the (3–4):149–56. dramatic decrease of elastic modulus upon exposure to water is [2] Liu Y, Zhao J, Zhao L, Li W, Hui Z, Xiang Y, et al. High Performance Shape the result of hydrogen bonding between water and cellulose nano- Memory Epoxy/Carbon Nanotube Nanocomposites. ACS Appl Mater Inter 2015;8(1):311–20. whiskers (CNWs) competing with those between CNWs, thus lead [3] Sun L, Huang WM, Ding Z, Zhao Y, Wang CC, Purnawali H, et al. Stimulus- to weakened network of CNWs [103]. responsive shape memory materials: a review. Mater Des 2012;33 Rapid switchable shape memory effect activated by water was (1):577–640. [4] Brostow W, Hagg Lobland HE. Materials: introduction and applications. John observed in a CNW and thermoplastic polyurethane network Wiley and Sons Ltd; 2017. [104]. Luo et al. achieved tunable shape recovery for cellulose [5] John S, Hariri M. Effect of shape memory alloy actuation on the dynamic nano-whisker (CNW) – shape memory polyurethane (SMPU) response of polymeric composite plates. Composites Part A 2008;39 (5):769–76. nanocomposites with stable water-active shape memory perfor- [6] Leng J, Lan X, Liu Y, Du S. Shape-memory polymers and their composites: mance [105]. They further pointed out that this type of water stimulus methods and applications. Prog Mater Sci 2011;56(7):1077–135. active shape memory performance can be controlled varying the [7] Liu Y, Lv H, Lan X, Leng J, Du S. Review of electro-active shape-memory polymer composite. Compos Sci Technol 2009;69(13):2064–8. CNW [106]. [8] Lan X, Liu Y, Lv H, Wang X, Leng J, Du S. Fiber reinforced shape-memory Water-induced shape memory polymer nanocomposites are polymer composite and its application in a deployable hinge. Smart Mater attracting more and more interest owing to their easy control of Struct 2009;18(2):024002. [9] Liu C, Qin H, Mather PT. Review of progress in shape-memory polymers. J the shape memory process. The most recent development of Mater Chem 2007;17(16):1543–58. water-induced SMP nanocomposites opens a door for more [10] Li W, Liu Y, Leng J. Selectively actuated multi-shape memory effect of a advanced applications. With the advantages of compatibility, polymer multicomposite. J Mater Chem A 2015;3(48):24532–9. water-induced SMP nanocomposites greatly benefit the develop- [11] Gu S, Yan B, Liu L, Ren J. Carbon nanotube-polyurethane shape memory nanocomposites with low trigger temperature. Eur Polym J ment of SMP nanocomposites in biomedical and sensors fields. 2013;49:3867–77. [12] García-Huete N, Axpe E, Cuevas JM, Mérida D, Laza JM, García JÁ, et al. In situ measurements of free volume during recovery process of a shape memory 7. Conclusions and outlook polymer. Polymer 2017;109:66–70. [13] Meng Q, Hu J. A review of shape memory polymer composites and blends. Composites Part A 2009;40(11):1661–72. In this paper, the various actuation methods of multifunctional [14] Xie T. Tunable polymer multi-shape memory effect. Nature 2010;464 SMP nanocomposites have been systematically discussed, includ- (7286):267–70. [15] Mondal S, Hu JL. Segmented shape memory polyurethane and its water vapor ing thermal, electrical, magnetic, light, and water actuation. Among transport properties. Des Monomers Polym 2012;9(6):527–50. these actuation methods, thermal actuation is the most basic of all. [16] Sokolowski W, Tan S, Pryor M. Lightweight shape memory self-deployable Electric field, magnetic field and light actuation methods are all structures for gossamer applications. Pasadena (CA): Jet Propulsion developed to realize the remote control and to meet special Laboratory, National Aeronautics and Space Administration; 2004. [17] Sokolowski W, Tan S, Willis P, Pryor M. Shape memory self-deployable requirement. Functional nanoparticles are introduced to these structures for solar sails. International Society for Optics and Photonics 2008. actuation methods and serve as energy converters which ulti- p. 72670K–72670K-14. mately present the stimulus in the form of heat. The other light [18] Li G, Xu T. Thermomechanical characterization of shape memory polymer– based self-healing syntactic foam sealant for expansion joints. J Transp Eng actuation method is based on light sensitive group and molecules. 2011;137(11):805–14. And water actuation method is based on the competition of hydro- [19] Sokolowski WM, Tan SC. Advanced self-deployable structures for space gen bonding. applications. J Spacecr Rockets 2007;44(4):750–4. [20] Rodriguez JN, Yu Y-J, Miller MW, Wilson TS, Hartman J, Clubb FJ, et al. Thermal and water actuation methods require certain media to Opacification of shape memory polymer foam designed for treatment of take effect due to the need for heat and mass transport, while elec- intracranial aneurysms. Ann Biomed Eng 2012;40(4):883–97. T. Liu et al. / Composites: Part A 100 (2017) 20–30 29

[21] Ortega JM, Hartman J, Rodriguez JN, Maitland DJ. Virtual treatment of basilar [52] Wei H, Zhang Q, Yao Y, Liu L, Liu Y, Leng J. Direct-write fabrication of 4D aneurysms using shape memory polymer foam. Ann Biomed Eng 2013;41 active shape-changing structures based on a shape memory polymer and its (4):725–43. nanocomposite. ACS Appl Mater Inter 2017;9(1):876. [22] Zhang F, Zhou T, Liu Y, Leng J. Microwave synthesis and actuation of shape [53] Pan YX, Yu ZZ, Ou YC, Hu GH. A new process of fabricating electrically memory polycaprolactone foams with high speed. Sci Rep 2015;5. conducting nylon 6/graphite nanocomposites via intercalation [23] Xu T, Li G. A shape memory polymer based syntactic foam with negative polymerization. J Polym Sci, Part B: Polym Phys 2000;38(12):1626–33. Poisson’s ratio. Mater Sci Eng 2011;528(22):6804–11. [54] Zhang FH, Zhang ZC, Luo CJ, Lin I, Liu Y, Leng J, et al. Remote, fast actuation of [24] Oh SH, Kang SG, Lee JH. Degradation behavior of hydrophilized PLGA scaffolds programmable multiple shape memory composites by magnetic fields. J prepared by melt-molding particulate-leaching method: comparison with Mater Chem C 2015;3(43):11290–3. control hydrophobic one. J Mater Sci – Mater Med 2006;17(2):131–7. [55] Dong Y, Ni Q-Q, Li L, Fu Y. Novel vapor-grown carbon nanofiber/epoxy shape [25] Quadrini F, Bellisario D, Santo L, Gaudio CD, Bianco A. Shape memory foams of memory nanocomposites prepared via latex technology. Mater Lett microbial polyester for biomedical applications. Polym-Plast Technol Eng 2014;132:206–9. 2013;52(6):599–602. [56] Heuchel M, Razzaq MY, Kratz K, Behl M, Lendlein A. Modeling the heat [26] Yu K, Ritchie A, Mao Y, Dunn ML, Qi HJ. Controlled sequential shape changing transfer in magneto-sensitive shape-memory polymer nanocomposites with components by 3D printing of shape memory polymer multimaterials. dynamically changing surface area to volume ratios. Polymer Procedia IUTAM 2015;12:193–203. 2015;65:215–22. [27] Mao Y, Yu K, Isakov MS, Wu J, Dunn ML, Qi HJ. Sequential self-folding [57] Mohr R, Kratz K, Weigel T, Luckagabor M, Moneke M, Lendlein A. Initiation of structures by 3D printed digital shape memory polymers. Sci Rep 2015;5. shape-memory effect by inductive heating of magnetic nanoparticles in [28] Ge Q, Dunn CK, Qi HJ, Dunn ML. Active origami by 4D printing. Smart Mater thermoplastic polymers. Proc Natl Acad Sci 2006;103(10):3540–5. Struct 2014;23(9):094007. [58] Yan B, Gu S, Zhang Y. Polylactide-based thermoplastic shape memory [29] Gall K, Dunn ML, Liu Y, Finch D, Lake M, Munshi NA. Shape memory polymer polymer nanocomposites. Eur Polym J 2013;49(2):366–78. nanocomposites. Acta Mater 2002;50(20):5115–26. [59] Babaahmadi M, Sabzi M, Mahdavinia GR, Keramati M. Preparation of [30] Hussain F, Hojjati M, Okamoto M, Gorga RE. Review article: polymer-matrix amorphous nanocomposites with quick heat triggered shape memory nanocomposites, processing, manufacturing, and application: an overview. J behavior. Polymer 2017;112:26–34. Compos Mater 2006;40(17):1511–75. [60] Guo J, Wang Z, Tong L, Lv H, Liang W. Shape memory and thermo-mechanical [31] Luo J-J, Daniel IM. Characterization and modeling of mechanical behavior of properties of shape memory polymer/carbon fiber composites. Composites polymer/clay nanocomposites. Compos Sci Technol 2003;63(11):1607–16. Part A 2015;76:162–71. [32] Ma W, Liu L, Zhang Z, Yang R, Liu G, Zhang T, et al. High-strength composite [61] Xie T. Recent advances in polymer shape memory. Polymer 2011;52 fibers: realizing true potential of carbon nanotubes in polymer matrix (22):4985–5000. through continuous reticulate architecture and molecular level couplings. [62] Lu H, Lei M, Yao Y, Yu K, Fu YQ. Shape memory polymer nanocomposites: Nano Lett 2009;9(8):2855–61. nano-reinforcement and multifunctionalization. Nanosci Nanotechnol Lett [33] Khan SU, Pothnis JR, Kim JK. Effects of carbon nanotube alignment on 2014;6(9):772–86. electrical and mechanical properties of epoxy nanocomposites. Composites [63] Mahapatra SS, Yadav SK, Yoo HJ, Ramasamy MS, Cho JW. Tailored and strong Part A 2013;49(3):26–34. electro-responsive shape memory actuation in carbon nanotube-reinforced [34] Vorobei AM, Pokrovskiy OI, Ustinovich KB, Parenago OO, Savilov SV, Lunin VV, hyperbranched polyurethane composites. Sens Actuators, B 2013;193 et al. Preparation of polymer – multi-walled carbon nanotube composites (3):384–90. with enhanced mechanical properties using supercritical antisolvent [64] Lu H, Liang F, Yao Y, Gou J, Hui D. Self-assembled multi-layered carbon precipitation. Polymer 2016;95:77–81. nanofiber nanopaper for significantly improving electrical actuation of shape [35] Razzaq MY, Anhalt M, Frormann L, Weidenfeller B. Thermal, electrical and memory polymer nanocomposite. Composites Part B 2014;59(3):191–5. magnetic studies of magnetite filled polyurethane shape memory polymers. [65] Lendlein A, Langer R. Biodegradable, elastic shape-memory polymers for Mater Sci Eng 2007;444(1):227–35. potential biomedical applications. Science 2002;296(5573):1673–6. [36] Vialle G, Di Prima M, Hocking E, Gall K, Garmestani H, Sanderson T, et al. [66] Liu T, Huang R, Qi X, Dong P, Fu Q. Facile preparation of rapidly electro-active Remote activation of nanomagnetite reinforced shape memory polymer shape memory thermoplastic polyurethane/polylactide blends via phase foam. Smart Mater Struct 2009;18(11):115014. morphology control and incorporation of conductive fillers. Polymer [37] Kumar UN, Kratz K, Behl M, Lendlein A. Shape-memory properties of 2017:28–35. magnetically active triple-shape nanocomposites based on a grafted [67] Raja M, Ryu SH, Shanmugharaj AM. Thermal, mechanical and electroactive polymer network with two crystallizable switching segments. Express shape memory properties of polyurethane (PU)/poly (lactic acid) (PLA)/CNT Polym Lett 2012;6(1):26–40. nanocomposites. Eur Polym J 2013;49(11):3492–500. [38] He Z, Satarkar N, Xie T, Cheng YT, Hilt JZ. Remote controlled multishape [68] Raja M, Ryu SH, Shanmugharaj AM. Influence of surface modified multiwalled polymer nanocomposites with selective radiofrequency actuations. Adv carbon nanotubes on the mechanical and electroactive shape memory Mater 2011;23(28):3192–6. properties of polyurethane (PU)/poly(vinylidene diflouride) (PVDF) [39] Golbang A, Kokabi M. Temporary shape development in shape memory composites. Surf A 2014;450(1):59–66. nanocomposites using magnetic force. Eur Polym J 2011;47(8):1709–19. [69] Pipes RB, Frankland SJV, Hubert P, Saether E. Self-consistent properties of [40] Zhang X, Lu X, Wang Z, Wang J, Sun Z. Biodegradable shape memory carbon nanotubes and hexagonal arrays as composite reinforcements. nanocomposites with thermal and magnetic field responsiveness. J Biomater Compos Sci Technol 2003;63(10):1349–58. Sci Polym Ed 2013;24(9):1057–70. [70] Yu M-F, Files BS, Arepalli S, Ruoff RS. Tensile loading of ropes of single wall [41] Stein IY, Wardle BL. Morphology and processing of aligned carbon nanotube carbon nanotubes and their mechanical properties. Phys Rev Lett 2000;84 carbon matrix nanocomposites. Carbon 2014;68:807–13. (24):5552. [42] Schmidt AM. Electromagnetic activation of shape memory polymer networks [71] Berber S, Kwon Y-K, Tománek D. Unusually high thermal conductivity of containing magnetic nanoparticles. Macromol Rapid Comm 2006;27 carbon nanotubes. Phys Rev Lett 2000;84(20):4613. (14):1168–72. [72] Zhou Q, Meng F, Liu Z, Shi S. The thermal conductivity of carbon nanotubes [43] Wei K, Zhu G, Tang Y, Li X. Electroactive shape-memory effects of hydro- with defects and intramolecular junctions. J Nanomater 2013;2013 epoxy|[sol]|carbon black composites. Polym J 2012;45(6):671–5. (2514103):12. [44] Fei G, Li G, Wu L, Xia H. A spatially and temporally controlled shape memory [73] Ebbesen TW, Lezec HJ, Hiura H, Bennett JW, Ghaemi HF, Thio T. Electrical- process for electrically conductive polymer–carbon nanotube composites. conductivity of individual carbon nanotubes. Nature 1996;382(6586):54–6. Soft Matter 2012;8(19):5123–6. [74] Wernik JM, Meguid SA. Recent developments in multifunctional [45] Tsang SC, Chen YK, Green MLH. Reduction of nitric oxide by arc vaporized nanocomposites using carbon nanotubes. Appl Mech Rev 2010;63(5):050801. carbons (AVC). Appl Catal B 1996;8(4):445–55. [75] Girifalco LA, Hodak M, Lee RS. Carbon nanotubes, buckyballs, ropes, and a [46] Cho JW, Kim JW, Yong CJ, Goo NS. Electroactive shape-memory polyurethane universal graphitic potential. Phys Rev B 2000;62(19):13104. composites incorporating carbon nanotubes. Macromol Rapid Comm 2005;26 [76] Wang Z, Liang Z, Wang B, Zhang C, Kramer L. Processing and property (5):412–6. investigation of single-walled carbon nanotube (SWNT) buckypaper/epoxy [47] Rastogi R, Kaushal R, Tripathi SK, Sharma AL, Kaur I, Bharadwaj LM. resin matrix nanocomposites. Composites Part A 2004;35(10):1225–32. Comparative study of carbon nanotube dispersion using surfactants. J [77] Srivastava S, Biswas A, Senapati S, Ray B, Rana D, Aswal VK, et al. Novel shape Interface Sci 2008;328(2):421–8. memory behaviour in IPDI based polyurethanes: Influence of nanoparticle. [48] Luo H, Li Z, Yi G, Zu X, Wang H, Wang Y, et al. Electro-responsive silver Polymer 2016. nanowire-shape memory polymer composites. Mater Lett 2014;134 [78] Tan L, Gan L, Hu J, Zhu Y, Han J. Functional shape memory composite (7):172–5. nanofibers with graphene oxide filler. Composites Part A 2015;76:115–23. [49] Dickerson EB, Dreaden EC, Huang X, Elsayed IH, Chu H, Pushpanketh S, et al. [79] Lu H, Liu Y, Gou JJ, Leng J, Du S. Surface coating of multi-walled carbon Gold nanorod assisted near-infrared plasmonic photothermal therapy (PPTT) nanotube nanopaper on shape-memory polymer for multifunctionalization. of squamous cell carcinoma in mice. Cancer Lett 2008;269(1):57–66. Compos Sci Technol 2011;71(11):1427–34. [50] Hribar KC, Metter RB, Ifkovits JL, Troxler T, Burdick JA. Light-induced [80] Rana S, Cho JW, Tan LP. Graphene-crosslinked polyurethane block temperature transitions in biodegradable polymer and nanorod composites. nanocomposites with enhanced mechanical, electrical, and shape memory Small 2009;5(16):1830–4. properties. RSC Adv 2013;3(33):13796–803. [51] Lu H, Liang F, Gou J. Nanopaper enabled shape-memory nanocomposite with [81] Raja M, Shanmugharaj AM, Ryu SH, Subha J. Influence of metal nanoparticle vertically aligned nickel nanostrand: controlled synthesis and electrical decorated CNTs on polyurethane based electro active shape memory actuation. Soft Matter 2011;7(16):7416–23. nanocomposite actuators. Mater Chem Phys 2011;129(3):925–31. 30 T. Liu et al. / Composites: Part A 100 (2017) 20–30

[82] Lu H, Yin W, Huang WM, Leng J. Self-assembled carboxylic acid- [95] Koerner H, Price G, Pearce NA, Alexander M, Vaia RA. Remotely actuated functionalized carbon nanotubes grafting onto carbon fiber for significantly polymer nanocomposites–stress-recovery of carbon-nanotube-filled improving electrical actuation of shape memory polymers. RSC Adv 2013;3 thermoplastic elastomers. Nat Mater 2004;3(2):115–20. (44):21484–8. [96] Zhang H, Xia H, Zhao Y. Optically triggered and spatially controllable shape- [83] Leng J, Lv H, Liu Y, Du S. Electroactivate shape-memory polymer filled with memory polymer–gold nanoparticle composite materials. J Mater Chem nanocarbon particles and short carbon fibers. Appl Phys Lett 2007;91(14). p. 2011;22(3):845–9. 144105–144105-3. [97] Zhang H, Zhao Y. Polymers with dual light-triggered functions of shape [84] Mitwalli AH, Denison TA, Jackson DK, Leeb SB, Tanaka T. Closed-loop feedback memory and healing using gold nanoparticles. ACS Appl Mater Inter 2013;5 control of magnetically-activated gels. J Intell Mater Syst Struct 1997;8 (24):13069–75. (7):596–604. [98] Weissleder R. A clearer vision for in vivo imaging. Nat Biotechnol 2001;19 [85] Lao LL, Ramanujan RV. Magnetic and hydrogel composite materials for (4):316–7. hyperthermia applications. J Mater Sci – Mater Med 2004;15(10):1061–4. [99] Lee KM, Koerner H, Vaia RA, Bunning TJ, White TJ. Light-activated shape [86] Ma M, Wu Y, Zhou J, Sun Y, Zhang Y, Gu N. Size dependence of specific power memory of glassy, azobenzene liquid crystalline polymer networks. Soft

absorption of Fe3O4 particles in AC magnetic field. J Magn Magn Mater Matter 2011;7(9):4318–24. 2004;268(1):33–9. [100] Huang WM, Yang B, An L, Li C, Chan YS. Water-driven programmable [87] Razzaq MY, Behl M, Lendlein A. Memory-effects of magnetic nanocomposites. polyurethane shape memory polymer: demonstration and mechanism. Appl Nanoscale 2012;4(20):6181–95. Phys Lett 2005;86(11). p. 114105–114105-3. [88] Kumar UN, Kratz K, Wagermaier W, Behl M, Lendlein A. Non-contact [101] Yang B, Huang WM, Li C, Lee CM, Li L. On the effects of moisture in a actuation of triple-shape effect in multiphase polymer network polyurethane shape memory polymer. Smart Mater Struct 2003;13(1). p. nanocomposites in alternating magnetic field. J Mater Chem 2010;20 191–5. (20):3404–15. [102] Lu H. A simulation method to analyze chemo-mechanical behavior of [89] Leng J, Zhang D, Liu Y, Yu K, Lan X. Study on the activation of styrene-based swelling-induced shape-memory polymer in response to solvent. J Appl shape memory polymer by medium-infrared laser light. Appl Phys Lett Polym Sci 2012;123(2):1137–46. 2010;96(11). p. 111905–111905-3. [103] Mendez J, Annamalai PK, Eichhorn SJ, Rusli R, Rowan SJ, Foster EJ, et al. [90] Long KN, Scott TF, Qi HJ. Photomechanics of light-activated polymers. J Mech Bioinspired mechanically adaptive polymer nanocomposites with water- Phys Solids 2009;57(7):1103–21. activated shape-memory effect. Macromolecules 2011;44(17):6827–35. [91] Lendlein A, Jiang H, Jünger O, Langer R. Light-induced shape-memory [104] Zhu Y, Hu J, Luo H, Young RJ, Deng L, Zhang S, et al. Rapidly switchable water- polymers. Nature 2005;434(7035):879–82. sensitive shape-memory cellulose/elastomer nano-composites. Soft Matter [92] Yu Y, Nakano M, Ikeda T. Photomechanics: directed bending of a polymer film 2012;8(8):2509–17. by light. Nature 2003;425(6954). [105] Luo H, Hu J, Zhu Y, Zhang S, Fan Y, Ye G. Achieving shape memory: reversible [93] Wu L, Jin C, Sun X. Synthesis, properties, and light-induced shape memory behaviors of cellulose–PU blends in wet–dry cycles. J Appl Polym Sci effect of multiblock polyesterurethanes containing biodegradable segments 2012;125(1):657–65. and pendant cinnamamide groups. Biomacromol 2011;12(1):235–41. [106] Luo H, Hu J, Zhu Y. Tunable shape recovery of polymeric nano-composites. [94] Leng J, Wu X, Liu Y. Infrared light-active shape memory polymer filled with Mater Lett 2011;65(s 23-24):3583–5. nanocarbon particles. J Appl Polym Sci 2009;114(4):2455–60.