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Progress in Materials Science 56 (2011) 1077–1135

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Progress in Materials Science

journa l homepage : www.e lsev ie r . com/ loca te /pmatsc i

Shape-memory polymers and their composites: Stimulusmethods and applications

Jinsong Leng a,⇑, Xin Lan a, Yanju Liu b, Shanyi Du a

a Center for Composite Materials and Structures, Harbin Institute of Technology (HIT), P.O. Box 3011, No. 2 YiKuang Street,Harbin 150080, PR Chinab Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT), P.O. Box 301, No. 92 West Dazhi Street,Harbin 150001, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 September 2009Received in revised form 1 December 2010Accepted 11 February 2011Available online 16 March 2011

0079-6425/$ - see front matter � 2011 Elsevier Ltdoi:10.1016/j.pmatsci.2011.03.001

⇑ Corresponding author. Tel./fax: +86 (0) 451 86E-mail address: [email protected] (J. Leng).

Shape-memory polymers (SMPs) undergo significant macroscopicdeformation upon the application of an external stimulus (e.g.,heat, electricity, light, magnetism, moisture and even a change inpH value). They have been widely researched since the 1980sand are an example of a promising smart material. This paper aimsto provide a comprehensive review of SMPs, encompassing a fun-damental understanding of the shape-memory effect, fabrication,modeling and characterization of SMPs, various actuation methodsand multifunctional properties of SMP composites, and potentialapplications for SMP structures. A definition of SMPs and their fun-damentals are first presented. Next, a description of their fabrica-tion, characterization and constitutive models of SMPs areintroduced. SMP composites, which act to improve a certain func-tion as functional materials or the general mechanical properties asstructural materials, are briefly discussed. Specially, the SMP com-posites can be developed into multifunctional materials actuatedby various methods, such as thermal-induced, electro-activated,light-induced, magnetic-actuated and solution-responsive SMPs.As smart materials, the applications of SMPs and their compositesreceive much interest, including deployable structures, morphingstructures, biomaterials, smart textiles and fabrics, SMP foams,automobile actuators and self-healing composite systems.

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1078 J. Leng et al. / Progress in Materials Science 56 (2011) 1077–1135

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10792. Fundamentals of shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1079

2.1. Background and definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10792.2. Comparison between shape-memory alloys and shape-memory polymers . . . . . . . . . . . . . . 10802.3. Architecture of shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10812.4. Stimulus methods of shape-memory polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10822.5. Categorization of shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1082

3. Fabrication, characterization and modeling of shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . 1083
3.1. Chemical architecture requirements of shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . 10833.2. Structural categorization of shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1085
3.2.1. Thermoplastic shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10853.2.2. Thermosetting shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1085

3.3. Synthesis of typical shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1086
3.3.1. Thermoplastic polyurethane shape-memory polymer . . . . . . . . . . . . . . . . . . . . . . . . . 10873.3.2. Thermosetting styrene-based shape-memory polymer . . . . . . . . . . . . . . . . . . . . . . . . 10873.3.3. Thermosetting epoxy shape-memory polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10883.3.4. Some novel shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1088
3.4. Characterization for shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1089
3.4.1. Chemical, thermal and mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10893.4.2. Shape memory performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1092
3.5. Constitutive models for shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093
3.5.1. Models based on viscoelasticity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10933.5.2. Models based on phase transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094
4. Stimulus methods of multi-functional shape-memory polymer composites . . . . . . . . . . . . . . . . . . . . 1095
4.1. Shape-memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095
4.1.1. Particle-filled shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10954.1.2. Fiber-reinforced shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1096

4.2. Electro-activated shape-memory polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1097
4.2.1. Shape-memory polymer filled with carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . 10974.2.2. Shape-memory polymer filled with carbon particles . . . . . . . . . . . . . . . . . . . . . . . . . . 10974.2.3. Shape-memory polymer filled with electromagnetic fillers. . . . . . . . . . . . . . . . . . . . . 10994.2.4. Shape-memory polymer filled with hybrid fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1101
4.3. Light-induced shape-memory polymers and their composites . . . . . . . . . . . . . . . . . . . . . . . . 1101
4.3.1. Intrinsically light-induced shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . 11014.3.2. Shape-memory polymers induced by radiant thermal energy of light . . . . . . . . . . . . 11024.3.3. Light-induced shape-memory polymer embedded with an optical fiber . . . . . . . . . . 1104
4.4. Solution-responsive shape-memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1105
4.4.1. Water-driven shape-memory effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11054.4.2. Solution driven shape-memory effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1106
4.5. Magnetic-responsive shape-memory polymer composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1107
5. Applications of shape-memory polymers and their composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1108
5.1. Deployable structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1108
5.1.1. Hinge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11085.1.2. Truss and boom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11095.1.3. Ground-based deployable mirror. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11115.1.4. Reflector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1111
5.2. Morphing structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1112
5.2.1. Folding wing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11135.2.2. Variable camber wing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113
5.3. Biomedicine and bioinspiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11145.4. MEMS and NEMS applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11195.5. Shape-memory polymer textiles and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11205.6. Shape-memory polymer foams and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11215.7. Automobile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11225.8. Self-healing composite system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11235.9. Other applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1124

6. Concluding remarks and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1124


J. Leng et al. / Progress in Materials Science 56 (2011) 1077–1135 1079

6.1. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11246.2. Key challenges and possible future directions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1126References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1126
1. Introduction

Since the discovery of the shape-memory polymers (SMPs) in the 1980s, international researchinterest into the shape-memory effect in polymers has been rapidly growing [1–7]. The SMPs arestimuli-responsive smart materials with the ability to undergo a large recoverable deformation uponthe application of an external stimulus [8]. While the reversible martensitic transformation is themechanism behind the shape-memory phenomenon in shape-memory alloys (SMAs) [9–12], theshape-memory phenomenon in SMPs stems from a dual-segment system: cross-links to determinethe permanent shape and switching segments with transition temperature (Ttrans) to fix the temporaryshape [13]. Below Ttrans, SMPs are stiff, while they will be relatively soft upon heating above Ttrans andconsequently they can be deformed into a desired temporary shape through applying an externalforce. When cooling and subsequently remove this external force, their temporary shape can bemaintained for long. Upon re-heating, their temporary deformed shape will automatically recoverthe original permanent shape.

SMPs can be activated not only by heat [14,15] and magnetism [16,17] similar to SMAs, but also byelectricity [18,19], light [20] moisture [21,22], even certain chemical stimulus (e.g., a change in pH va-lue [13]), etc. [23–25] SMPs present many potential technical advantages that surpass those of SMAsand shape-memory ceramics such as good shape recoverability (up to 400% recoverable strain), lowdensity, ease in processing and in tailoring of properties (e.g., transition temperature, stiffness, bio-degradability, and ease of functionally grading), programmability and controllability of recoverybehavior, and most importantly, low cost.

Based on the above advantages, SMP composites and some novel multi-functional SMPs have alsobeen developed. Furthermore, there is an increased activity in integrating SMPs with nanotechnologyto develop novel materials for the realistic applications. The latest developments in nanotechnologyenable SMPs to be better accommodate the requirements of a particular application in biomaterials,sensors, actuators or textiles. SMPs, SMP composites or SMP structures have been applied in formssuch as space deployable structures (e.g., hinges, trusses, mirrors and reflectors), morphing skins usedfor folding or variable camber wings in aircraft, biomedical and bioinspirational instruments (e.g., apolymer vascular stent with shape-memory as a drug delivery system, smart surgical sutures, and la-ser-activated SMP microactuator to remove a clot in a blood vessel), SMP textiles, automobile actua-tors and self-healing systems.

The present paper aims to provide a systematic and comprehensive analysis of SMPs, SMP compos-ites and their potential applications. Section 1 presents a brief introduction of SMPs, and the funda-mentals of SMPs are summarized in Section 2. Sections 3 review their synthesis, characterizationand modeling. Subsequently, in Section 4, the particles and fiber filled SMP composites are discussed,followed by a presentation of the various multi-functional SMP composites. Finally, relevant potentialapplications of SMPs or their composites are provided in Section 5. We conclude with future directionsin developing new SMPs and applications.

2. Fundamentals of shape-memory polymers

2.1. Background and definition

Shape-memory polymers (SMPs) are able to respond to a specific external stimulus by means ofcertain significantly macroscopic properties such as shape [26,27]. The basic molecular architectureof SMPs is a polymer network underlying active movement [28]. A SMP must consist of dual-segments,


Fig. 1. Schematic of shape-memory effect during a typical thermomechanical cycle.


one that is highly elastic and another that is able to reduce its stiffness upon a particular stimulus. Thelatter can be either molecular switches or stimulus sensitive domains. Upon exposure to a specificstimulus the switching/transition is triggered and strain energy stored in the temporary shape isreleased, which consequently results in the shape recovery [29,30].

SMPs and their composites can recover their original shapes after large deformation when sub-jected to an external stimulus, such as Joule heating, light, magnetism or moisture [31]. Among theseSMPs, the thermo-responsive SMPs are the most common [32–34]. At a macroscopic level, as illus-trated in Fig. 1, the typical thermomechanical cycle of a thermo-responsive SMP consists of the follow-ing steps [35,36]: (1) fabrication of the SMPs into an original shape; (2) heating the SMP above thethermal transition temperature (Ttrans) (either a glass transition temperature, Tg, or a melting temper-ature, Tm), and deformation of the SMP by applying an external force, cooling well below Ttrans, removalof the constraint to obtain a temporary pre-deformed shape; (3) when needed, heating of the pre-deformed SMP above Ttrans, and then recovery of the SMP towards its original shape (a recovered shape).

The first SMP was polynorborene with a Tg range of 35–40 �C developed by the French CdF ChimieCompany and commercialized by the Japan Nippon Zeon Company in 1984 [37,38]. However, the Tg ofthe polymer is very difficult to tailor due to its high molecular weight. Styrene butadiene based SMPswere developed by the Asahi Company. They presented a Tg of about 60–90 �C. However, their processability was also poor [37,39]. Recently, the Cornerstone Research Group (CRG), Inc. has developedsome commercialized thermosetting SMPs [40], including a styrene-based SMP (Veriflex�) with a Tg

ranging from 60 to 70 �C, a one part epoxy SMP with a Tg of about 90 �C, a two parts epoxy SMP witha Tg of about 104 �C, and a cyanate ester SMP with a high Tg range from 135 to 230 �C. Composite Tech-nology Development (CTD), Inc. has developed a thermosetting epoxy SMP (CTD-101 K) with a Tg ofabout 113 �C [41]. Moreover, a series of thermoplastic polyurethane SMPs is commercially availablefrom Mitsubishi Heavy Industry, with the Tg ranging from 40 to 55 �C.

As SMPs show a variety of novel properties, they have been widely researched since the 1980s. Todate, research activities regarding SMPs have been widely conducted in more than 60 institutes orcompanies all over the world. In the recent years, a variety of polymers has been synthesised withshape-memory effects and certain unique performance ability has been developed. Based on theshape-memory effect, some novel multi-functional SMPs or nano SMP composites have also beenproposed.

2.2. Comparison between shape-memory alloys and shape-memory polymers

Shape-memory alloys (SMAs) can deform at a low-temperature and then recover to their prior shapeupon heating above a particular temperature-related to the properties of alloy. SMAs mainly includethree types: nickel-titanium (NiTi) alloys, copper–zinc–aluminum–nickel and copper–aluminum–nickel. SMA changes from austenite to martensite upon cooling. The martensite phase exists at lowertemperatures and is the relatively soft and easily deformed phase of SMAs. Austenite is a stronger phaseof SMAs that occurs at higher temperatures. When the temperature of the SMA is cooled to below thetransition temperature (Ts), the shape-memory effect can be observed and the alloy is entirely com-posed of martensite. Upon re-heating above the transition temperature, the deformed shape will re-cover to its initial shape. The deformed martensite transformes into the cubic austenite phase [42].

SMAs show both one-way and two-way shape-memory effect. For one-way shape-memory effect,upon heating a SMA at deformed martensite phase, the shape changes to its original. However, whenthe metal cools again it will remain in the hot shape. With the one-way effect, cooling from high tem-peratures does not cause a macroscopic shape change. An external deformation is necessary to create


Table 1Main properties compared between typical SMA and SMP.

NiTi SMA Polystyrene SMP

Ts, �C Approx. 40–100 62Transformation strain, % Max 8 50–100Actuation stress, MPa Approx. 100 2–10Young’s Modulus above Ts, GPa Approx. 83 1.24Young’s Modulus below Ts, MPa Approx. 28–41 2–10Poisson’s Ratio 0.33 0.30Density, gms/cc 6.45 0.92

Fig. 2. Comparison of mechanical properties among some typical materials [44].


the low-temperature shape. For two-way shape-memory effect, when a SMA is cooled at hot austenitephase it will automatically deform without any external deformation. That is to say, the cool deformedshape and hot original shape can automatically transfer in the heating–cooling cycle.

On the other hand, the shape-memory effect of SMPs depends on the existence of separated phasesrelated to the coiled polymer structure and cross-links, etc. In the deformed shape below Tg, molecularchains of the SMPs are constrained by chemical or physical cross-link. When reheated again, these molec-ular chains will go back to the random coiled configuration. The shape-memory transformation dependson the mechanism by which polymer molecules transpose between the constrained and random entan-gled conformations [42]. SMPs only show one-way shape-memory effect. That is, the SMP at ‘‘soft’’ stagecan only deform with the help of external force rather than automatically deform by cooling.

The advantage of SMPs over SMAs relies mostly on their intrinsic properties such as lower cost,lower density, easier processing and larger attainable strains. SMPs exhibiting up to 200% strain havebeen reported compared to less than 10%, 1%, and 0.1% for SMA, shape-memory ceramics, and glasses,respectively [43]. Table 1 lists the main properties compared between NiTi SMA and polystyrene SMP.It reveals that Ts of NiTi SMA and polystyrene SMP are adjustable. As a polymer, polystyrene SMPshows a lower actuation force but a higher recoverable strain than those of NiTi SMAs. In addition,Fig. 2 shows some comparison of mechanical properties among some typical materials, where SMPsperform a large strain but a relatively low stress [44].

2.3. Architecture of shape-memory polymers

SMPs are typically dual-shape in nature, being comprised of an original shape and a deformed shape.This shape-memory feature is resulted from a combination of the polymer’s molecular architecture


Fig. 3. Time series photographs that show the recovery of a pure SMP (top row), and a glass-fiber-reinforced SMP (bottom row).


and a particular programming procedure that enables the formation of a temporary deformed shape[45]. The special chemical architecture consists of netpoints, and molecular switches that are sensitiveto an external stimulus. In this sense, SMPs can be considered as copolymers with a hard segmentacting as a fixed phase and a soft segment acting as a reversible phase [46].The fixed phase prevents‘free’ flow of the surrounding polymer chains upon the application of a stress. The reversible phase, onthe other hand, undergoes deformation in a shape-memory cycle and is responsible for elasticity [47].This phase acts as a ‘molecular switch’, freezing the deformed shape below the transition temperaturewhile releasing them towards the original shape at or above the transition temperature.

2.4. Stimulus methods of shape-memory polymers

The actuation methods of SMPs can be generally divided into heat, electricity, light, magnetism,moisture, etc. The shape-memory effect of thermal-responsive SMPs is directly triggered by Jouleheating from a source such as hot gas or hot water [48]. Fig. 3 shows an example of a thermally in-duced shape recovery process for a pure SMP (Fig. 3, top row) and a glass-fiber-reinforced SMP(Fig. 3, bottom row) in hot gas. Because it is not convenient to use hot water or gas to directly activatethe SMPs in most applications, the corresponding studies have received little attention. Alternatively,special functional fillers can be incorporated into SMPs to trigger shape-memory effect through elec-tricity, light, magnetism or moisture, etc. While the SMPs induced by these methods are still triggeredby Joule heat intrinsically, this heat is produced by various indirect methods. For the water or solventdriven SMPs, solvent molecules are diffused into the polymer sample and act as a plasticizers whenimmersed in water, leading to a reduction in the transition temperature and consequently resultingin shape recovery. This could also be classified as an indirectly thermal-induced actuation method.In particular, the intrinsically light-induced SMPs are produced by incorporating reversible photoreac-tive molecular switches; the stimulation is not considered to be related to any temperature effects andshould, therefore, be differentiated from the indirect actuation of the thermal-responsive shape-mem-ory effect. The various actuation methods of SMPs will be discussed in Section 4.

2.5. Categorization of shape-memory polymers

SMPs are considered to consist of netpoints and molecular switches. Netpoints can be achievedformed by covalent bonds or intermolecular interactions. Hence, they can be either of a chemical orphysical nature. Chemically cross-linked SMPs can be achieved through a suitable cross-linking chem-istry, which are referred to as thermosets. Physically cross-linked ones require a polymer morphologyconsisting of at least two segregated domains, which are referred to as thermoplastics.

The network chains of SMPs can be either amorphous or crystalline, and therefore the transitiontemperature (Ttrans) is either a glass transition temperature (Tg), or a melting temperature (Tm) [49].If the thermal transition is a glass transition, the micro Brownian motion of the network chains isfrozen at temperatures below Tg, and will be ‘‘switched’’ on at temperatures at or above Tg upon



reheating. When the thermal transition is a melting point, the switching segments crystallize attemperatures below Tm, and then recover their original shape at an elevated temperature at or aboveTm [50]. Detailed discussions will be provided in Section 3.

3. Fabrication, characterization and modeling of shape-memory polymers

The shape-memory behavior has been observed in several polymers with significantly differentchemical compositions. In this section, we discuss the structural requirements for the various chem-ical compositions of SMPs, structural categorization, and the synthesis of three typical SMPs (polyure-thane, styrene-based and epoxy SMPs). Subsequently, we discuss the main characterizationtechniques and the corresponding chemical, thermal and mechanical parameters. In addition, wideengineering applications demand effective and simple theoretical models that can explain and predictthe shape-memory effect of SMPs. Theoretical modeling of the mechanical constitutive behavior of anSMP proves difficult due to the large recoverable deformation capability of SMPs, and the heavydependence of polymer properties on both time and temperature. Generally, there are two approachesto developing a constitutive model for SMPs, which we apply here. One is based on the linear or non-linear viscoelastic theory [32,33], while the other is based on the micromechanical theorem of phasetransformation [51,52].

3.1. Chemical architecture requirements of shape-memory polymers

Like normal polymers, SMPs also possess three-dimensional molecular network-like architectures[53–58]. The network architectures are thought to be constructed through cross-linking net points,with polymer segments connecting adjacent net points. These strongly cross-linked architectures en-sure that the polymer can maintain a stable shape on the macroscopic level for enabling both the ori-ginal and recovered shapes [5]. The domain of the cross-linking netpoints can be either physically orchemically cross-linked structures. Physically cross-linked polymers (thermoplastics) exhibit a revers-ible nature (see Fig. 4a), meaning that they can be melted or dissolved in certain solutions. The inter-connection of the individual polymer chains in a physically cross-linked network is achieved by theformation of crystalline or glassy phases. For the chemically cross-linked polymers (thermosets)(see Fig. 4b), the individual polymer chains are connected by covalent bonds, which are more stablethan the physical cross-linking network and show an irreversible nature.[53,54].

The other basic requirement of an SMP is the formation of strong reversible interactions (secondarycross-links) between the polymer segments so that a macroscopic shape deformation, namely tempo-rary pre-deformed shape, can be fixed [29,53,59,60]. As shown in Fig. 5, when an SMP is heated above

Fig. 4. Schematics of the typical structures of a polymer network that is (a) physically cross-linked network; (b) chemicallycross-linked network (modified from [53]).


Fig. 5. Schematic of polymer networks during shape recovery.


Ttrans, a polymer chain between two network points is randomly coiled without deformation of theSMP. Upon applying an external stretching stress on the material, the polymer segments are elon-gated. It results in an orientation of most of the polymer chains and a dislocation of the network points(physically or chemically cross-linked net points). Upon cooling and maintaining the macroscopic de-formed shape, the secondary cross-links will form among the new orientational polymer segments.The fixation of this additional order in the network segments is the main principle underlying theshape-memory effect, and upon cooling, it can occur by effectively reducing the dynamics and flexi-bility of the segments. In suitable polymer architectures, these secondary cross-linked domains areformed by side chains or by chain segments linking two netpoints. The secondary cross-links canbe glassy or crystalline domains. Consequently, the transition temperature Ttrans of SMPs is either aTg or a Tm [29]. In both cases, the temporary stabilization is formed by the aggregation of the switchingsegments. Upon reheating, the shape-memory effect is realized by detaching the secondary cross-links, the strain energy will be released, and the macroscopic shape recovery of the SMPs will occur.The recovery shape of SMPs is determined by the primary cross-links and polymer segments when nosecondary cross-links exited.

Based on the chemical architectures and corresponding evolution in the shape-memory program-ming process, the polymer segments can fulfill two types of structural tasks: the hard segments and thesoft segments [47,53,61]. The hard segments form the ‘netpoints’ that link the soft segments, acting as afixed phase. The hard segments may be of physical or chemical nature, such as crystals, glassy do-mains, chain entanglements or chemical cross-links. The soft segments work as a reversible molecularswitch (reversible phase) and show a thermal transition at temperature Ttrans. They fix (‘‘freeze in’’) thetemporary pre-deformed shapes below Ttrans, while freezing the pre-deformed shapes towards the ori-ginal shapes at or above the transition temperature.



3.2. Structural categorization of shape-memory polymers

SMPs consist of netpoints and molecular switches. Netpoints are constructed with either physicalcross-links through physical intermolecular interactions or chemical cross-links through covalentbonds [50]. Thermal-responsive SMPs can be classified according to the nature of their permanent net-points and the thermal transition related to the switching domains into four different categories:[45,62] (I) physically cross-linked thermoplastics with Ttrans = Tg; (II) physically cross-linked thermo-plastics with Ttrans = Tm. (III) chemically cross-linked amorphous polymers (Ttrans = Tg); (IV) chemicallycross-linked semi-crystalline polymer networks (Ttrans = Tm).

3.2.1. Thermoplastic shape-memory polymersFor the physically cross-linked SMPs, the formation of a phase-segregated morphology is the fun-

damental mechanism behind the thermally induced shape-memory effect of these materials. Onephase provides the physical cross-links while another phase acts as a molecular switch [29]. Theycan be further classified into linear polymers [63,64], branched polymers [65] or a polymer complex.The linear SMPs consist of block copolymers and high molecular weight polymers [50].

Among thermoplastic SMPs, the polyurethane SMP performs many advantages when comparedwith other available SMPs, including higher shape recoverability (maximum recoverable strain>400%) [66], a wider range of shape recovery temperature (from �30 to 70 �C), better biocompatibilityand better processing ability [67,5].

3.2.2. Thermosetting shape-memory polymersFor the chemically cross-linked SMPs, there are two methods to synthesize covalently cross-linked

networks [29,53]. First of all, the polymer network can be synthesised by adding a multi-functionalcross-linker during the polymerization. The chemical, thermal and mechanical properties of the net-work can be adjusted by the choice of monomers, their functionality, and the cross-linker content. Thesecond method to obtain polymer networks is the subsequent cross-linking of a linear or branchedpolymer. The networks are formed based on many different polymer backbones, such as polystyrene,polyurethanes, and polyolfines. Covalently cross-linked SMPs possess chemically interconnectedstructures that determine the original macroscopic shape of SMPs. The switching segments of thechemically cross-linked SMPs are generally the network chains between netpoints, and a thermaltransition of the polymer segments is used as the shape-memory switch. The chemical, thermal,mechanical and shape-memory properties are determined by the reaction conditions, curing times,the type and length of network chains, and the cross-linking density.

Additionally, according to the thermal transition of the switching segment, SMPs can be dividedinto two categories. Either the transition temperature Ttrans is a melting temperature Tm or a glass tran-sition temperature Tg [50]. If the thermal transition belongs to a glass transition, the micro Brownianmotion of the network chains is frozen and the temporary shape is fixed at low-temperatures; corre-spondingly, the network chain segments are in the glassy state. The SMPs will remember the tempo-rary shape and store the strain energy. When heating at or above Tg, the micro Brownian motion willbe triggered and the ‘switch’ will be opened. In the case of glass transition, glass transitions alwaysextend over a broad temperature range. When the thermal transition is a melting point, the switchingsegments crystallized at low-temperature as a fixed segment to store the strain energy, and then re-cover the original shape at elevated temperatures at or above Tm. In the case of melting temperature,the transition presents a relatively sharp transition in most cases.

Compared with physically cross-linked SMPs, the chemically cross-linked SMPs often show lesscreep, thus the irreversible deformation during shape recovery is less. Chemically cross-linked SMPsusually show better chemical, thermal, mechanical and shape-memory properties than physicallycross-linked SMPs. Additionally, these properties can be adjusted by the controlling the cross-linkdensity, curing conditions and curing times.

The shape recovery ratio of the thermoplastic polyurethane SMP is usually in the range of 90–95%within 20 shape recovery cycles. The elastic modulus of the SMP is between 0.5 and 2.5 GPa at roomtemperature. Additionally, when exposed to air, it is very sensitive to moisture and therefore showsunstable mechanical properties. In contrast, the epoxy SMP exhibits better overall performance. The



shape recovery ratio can approach 98–100%, and the elastic modulus approaches 2–4.5 GPa. Inaddition, it performs better stabilization against moisture and space radiation. Because of therelatively poor thermal and mechanical properties, the thermoplastic SMPs (e.g., polyurethane SMP)are mostly researched and used as functional materials at a small scale, such as for biomaterialsand shape-memory polymer textiles. On the other hand, the thermosetting SMPs (e.g., styrene-basedSMP and epoxy SMP) are generally used for structural materials, such as space deployable structuresand automobile actuators.

3.3. Synthesis of typical shape-memory polymers

More than twenty types of SMPs have been synthesised and widely researched in the recent years.Fig. 6 presents the classification scheme for existing polymer networks that exhibit the shape-memoryeffect [50].

The physically cross-linked SMPs include linear polymers [50,63,68,69], branched polymers [65] ora polymer complex [70–72]. The shape-memory effect of linear polymer is due to the phase separationand the domain orientation. The typical physically cross-linked SMP is linear block copolymers, suchas polyurethanes. In polyesterurethanes, oligourethane segments are the hard elastic segments, whilepolyester serves as the switching segment [73,74]. The phase separation and the domain orientation ofpoly(e-caprolactone) based polyesterurethanes can be determined by Raman spectroscopy [75]. Asimilar effect is found for copolyester based ionomers obtained by the bulk polymerization of adipicacid and mixed monomers (bis(poly(oxyethylene)) sulfonated dimethyl fumarate and 1,4-butanediol)[76]. The polycarbonate segment is synthesised by the copolymerization of ethylene oxide in the pres-ence of CO2 catalyzed by a polymer supported bimetallic catalyst, which yields an aliphatic polycar-bonate diol. This macrodiol is further processed by the prepolymer method into a polyurethaneSMP [77].

Covalent netpoints can be obtained by cross-linking of linear or branched polymers as well as by(co)polymerization/poly(co)condensation of one or several monomers, whereby at least one has tobe at least tri-functional [45]. Depending on the synthesis strategy, cross-links can be created duringsynthesis or by post-processing. The shape recovery is triggered by controlled by the trans-vinylene

⎧⎪⎪⎨⎪⎪⎩

⎧⎪⎪⎨⎪⎪⎩

⎧⎪⎪⎨⎪⎪⎩

⎧⎪⎪⎨⎪⎪⎩

⎧⎪⎪⎨⎪⎪⎩

Fig. 6. Schematic of structural categorization of SMPs (modified from [50]).



content [78]. The other synthesis routes to obtain polymer networks involve the copolymerization ofmonofunctional monomers with low-molecular weight or oligomeric difunctional cross-linkers [79].The copolymerization of monofunctional monomers of low molecular weight with oligomeric difunc-tional cross-linkers enables the creation of AB copolymer networks with increased toughness and elas-ticity at room temperature [80,81]. To date, many kinds of SMPs have been developed throughchemical cross-linking, such as cross-linked polystyrene, epoxy, poly(vinyl chloride), polyethyleneand PE/poly(vinyl acetate) copolymer [2,50].

It should be noted that, in this paper, the further studies in SMPs (e.g., various actuation methods ofmulti-functional SMPs or SMP composites (Section 4), and the corresponding potential applications(Section 5)) are generally concerned about three types of SMPs (polyurethane, styrene-based andepoxy SMPs). Consequently, the synthesis of these three types of SMPs is discussed emphatically inthis paper. Readers could refer to relevant literatures that discuss the synthesis of other types of SMPs[82–84], such as polyolefines SMP [85], polyethylene-poly(vinyl acetate) copolymers [86], polysilox-anes SMP [87], poly((meth)acrylates) SMP [88], polyethylenes SMP [89], poly(epsilon-caprolactone)SMP [90], polycyclooctene SMP [34,91], soybean oil-styrene–divinylbenzene SMP [92], stearyl acrylateSMP [93], poly(ethylene oxide-ethylene terephtalate) copolymers [94–96] and trans-poly(1,4-butadi-ene)-PS copolymer [97,98].

3.3.1. Thermoplastic polyurethane shape-memory polymerSince the development of the shape-memory segmented polyurethane (PU) copolymer by Hayashi

of Mitsubishi Heavy Industry, extensive studies on this materials have been carried out [99–108]. PUSMP consists of a two-phase structure: a hard segment and soft segment. The hard segments areformed either from a long chain macro diol with a higher thermal transition temperature, or from diis-ocyanates and chain extenders [53]. Typically, the linear polyurethane can be usually synthesisedusing the following process: through the reaction of difunctional, hydroxy-terminated oligoestersand -ethers with an excess of a low molecular weight diisocyanate, the isocyanate-terminated pre-polymers can be obtained [47,109]. The basic structure of the diisocyanate can be either aromaticor aliphatic; each type of diisocyanate has a different ability to form a semi-crystalline hard segment.Then low molecular weight diols or diamines are incorporated as chain extenders to further couplethese pre-polymers. In this way, the linear, phase-segregated polyurethane or polyurethane–ureablock copolymers are formed. As shown in Fig. 7, the phase separation morphology of the molecularstructure of such polyurethanes is obtained.

3.3.2. Thermosetting styrene-based shape-memory polymerStyrene-based SMP must have two-phases or exhibit a cross-linked structure to exhibit the shape-

memory effect. The variety of methods to polymerize styrene and the wide availability of possiblecomonomers enable these necessary features. Styrene can be polymerized by anionic, cationic, or freecontrolled radical polymerization methods. In this way, a huge variety of network architectures maybe designed by the proper choice of mechanism, initiator and comonomers [53]. Through cationicpolymerization, random copolymer networks formed from renewable natural oils with a high degreeof unsaturation, like soy bean oils, copolymerized with styrene and divinylbenzene are obtained [110].Styrene-based SMPs have been prepared through the cationic copolymerization of the following can-didate materials: regular soybean oil, low saturation soybean oil (LoSatSoy) and conjugated LoSatSoyoil with various alkene comonomers including styrene and divinylbenzene, norbornadiene, or dicyclo-pentadiene (comonomers used as cross-linking agents) initiated by boron trifluoride diethyl etherate

2 5 n

Fig. 7. Polyurethane with micro-phase separation structure (modified from [50]).



or related modified initiators [111,112]. By controlling the cross-link densities and the rigidity of thepolymer backbones, the SMP shows a tunable Tg, mechanical properties, and a good shape-memoryeffect. In addition, the materials show good reprogramming properties upon numerous shape recoverycycles, and excellent shape fixity and recovery ratios. The advantage of the soybean oil polymers lies inthe high degree of chemical control over the shape-memory characteristics. This makes these shape-memory materials particularly promising in applications.

A new shape-memory styrene copolymer has been developed for some particular applications,such as the contact lenses [113]. This new SMP is prepared from the reaction product of styrene, a vi-nyl compound other than styrene, a multi-functional cross-linking agent and an initiator. The SMP canemploy norbornene homopolymers and copolymers of norbornene and alkylated, cyano, alkoxylated,mono- or di-esterified imides, or carboxylic acid derivatives. In addition, the copolymer may also in-clude, as a comonomer, dimethaneoctahydronapthalene (DMON).

Leng et al. [78,114] have synthesised SMP based on styrene copolymer networks with different de-grees of cross-linking. The monomers of copolymer included styrene, a vinyl compound and a cross-linking agent (a difunctional monomer). During the process of synthesis, the inhibitor was removed bypassing the liquid monomers through an initiator removal column. Benzoyl peroxide (BPO) was usedas thermal initiator. The purified monomers (styrene and the vinyl compound) and the cross-linkingagent (difunctional monomer) were mixed in varying proportions with BPO at the room temperatureby stirring. The mixture was then polymerized between two casting glass plates, placed into an ovenand kept at 75 �C for 36 h. This prolonged curing time minimized shrinkage and the temperature waskept at 75 �C for 48 h in order to complete the reaction. The SMP, with Tg in the range from 35 to 55 �C,experienced good shape recovery performance. Moreover, the Tg can be adjusted through the alterna-tion of the gel content of the copolymer; the Tg increased with increasing gel content. The largestreversible strain of the SMP reached as high as 100% and the reversible strain of the copolymer de-creased with increasing gel content.

3.3.3. Thermosetting epoxy shape-memory polymerShape-memory behaviors can be observed in several polymers, including polyurethane-based poly-

mers [115,116], styrene-based polymers, epoxy polymer [117,118] and others. Controlling macro-scopic properties through variation of network parameters is suggested to play an important role inthe investigation of SMPs. For example, Kim [119] studied the effect of soft/hard segment phase sep-aration and soft segment crystallization on shape-memory property in polyurethane SMP. Hu and hercoworkers [116] reported that the molecule weight of polymer network influences the shape-memoryeffect in PU shape-memory films. Studies have also been performed on the relation of network struc-ture and shape recovery in copolymers of methyl methacyrlate and poly(ethyleneglycol)-dimethacry-late [120]. However, few works have been preformed to investigate the systematic tailoring ofthermomechanical properties in addition to the shape-memory response for polymers [121].

In particular, epoxy SMP is a high-performance thermosetting resin possessing a unique thermo-mechanical property together with excellent shape-memory effect. Leng et al. [121] have synthesisedthe epoxy SMP, from an epoxy resin, a hardener and another linear epoxy monomer. The epoxy resinwas mixed with the hardener at a 1:1 weight ratio. An active linear epoxy monomer was copolymer-ized with the polymer matrix to tailor the network. The monomer is composed of a long linear chain ofCAO bonds and two epoxy groups at the chain ends. With an increase in the linear monomer content,the Tg ranges from 37 to 96 �C, and a decrease in the rubber modulus is obtained from dynamicmechanical analysis.

3.3.4. Some novel shape-memory polymersSeveral researchers have reported the intrinsically light-induced deformation of polymers [122] or

gels [123,124], including bending [125], contraction [126] or volume changes [20]. Lendlein et al.[1,20,127] have described the light-induced SMPs that have no relation with thermal effect duringthe shape recovery, despite most of the SMPs are considered as directly or indirectly thermal-responsivepolymers. A grafted polymer has been used to design the light sensitive SMPs at a molecular scale[1,20]. Cinnamic acid (CA) molecules are grafted onto a permanent polymer network. The polymerarchitecture is formed by the copolymerization of n-butylacrylate, hydroxyethyl methacrylate,



ethylenegylcol-1-acrylate-2-CA, and poly (propylene glycol)-dimethacrylate) working as the cross-linker.To construct a permanent polymer network, it is made from BA with 3.0 wt.% poly(propyleneglycol)-dimethacrylate as the cross-linker, with 20 wt.% star-poly(ethylene glycol) end capped with(cinnamylidene acetic acid (CAA) terminal groups (SCAA). This SCAA molecule yields a photo-sensitiveinterpenetrating network that will change shape in response to light of a special wavelength light. Inthis case, cross-links of the amorphous polymer network determine the permanent shape [20].

Biodegradable SMPs are a promising field during the development of SMPs [2,13,55,128]. They canbe synthesised from a few monomers which have been already used in synthesis of exiting biodegrad-able materials [129–131]. A poly(e-caprolactone) (PCL)-based biodegradable SMP has been demon-strated its potential in medical applications [2,77,132]. The biodegradable SMP, is mainly based onpolyglycolide (PGA), poly(L-lactide) (PLLA) and PCL [133–138]. By selecting molecular weight of PCL,hard phase component and soft phase/hard phase compositions, the transition temperature of the bio-degradable SMP with PCL soft phase can be adjusted to meet the requirement for the biomedical orother applications [5].

Some other types of SMPs with novel functions also attract extensive interest. Mather et al. [139]developed a two-way reversible SMP in a semicrystalline network. In this polymer, the cooling-in-duced crystallization of cross-linked poly(cyclooctene) films under a tensile load results in a signifi-cant elongation and subsequent heating to melt the network reverses this elongation (contraction).In this way, the crystallization-induced elongation on cooling and melting-induced shrinkage on heat-ing yield a two-way shape-memory effect. Peng et al. [140] demonstrated a novel SMP with two tran-sition temperatures. For this SMP, a PMMA-PEG (poly(ethylene oxide)-poly(methyl methacrylate)(PMMA)) semi-interpenetrating network (semi-IPN) exhibits excellent shape-memory behaviors attwo transition temperatures, namely the Tm of the poly(ethylene glycol) (PEG) crystal and the Tg ofthe semi-IPN. In addition, Lendlein et al. [141–144] have demonstrated the triple-shape effect ofpolymer network with crystallizable network segments and grafted side chains. In this SMP, themulti-phased networks are synthesised by photopolymerization from PEG monomethyl ethermonomethacrylate and PCL dimethacrylate as cross-linker. The triple-shape effect is a general conceptthat requires the application of a two-step programming process for suitable polymers. As shown inFig. 8, the SMP is able to change from a first shape (A) to a second shape (B) and finally deform intoa third shape (C). The triple-shape SMP has great potential for various applications.

3.4. Characterization for shape-memory polymers

3.4.1. Chemical, thermal and mechanical propertiesThe main characterization techniques and the corresponding chemical, thermal and mechanical

parameters are discussed here. These include Fourier transform infrared spectroscopy, opticalmicroscopy, scanning electron microscopy, transmission electron microscopy, electrical conductivitymeasurements, and temperature distribution measurements detected by infrared camera, thermo-gravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, and mechanicalproperties testing.

3.4.1.1. Fourier transform infrared spectroscopy (FTIR). Fourier transform infrared spectroscopy (FTIR) isa measurement technique for collecting infrared spectra and is a powerful tool for identifying types ofchemical bonds (i.e., functional groups). The wavelength of light absorbed is characteristic of thechemical bond, which can be distinguished in this annotated spectrum. By interpreting the infraredabsorption spectrum, the chemical bonds can be determined. FTIR spectrometry is often used to inves-tigate the difference in structure of the networks in a molecule during the synthesis of an SMP[34,121]. In addition, Raman spectroscopy and X-ray diffraction (XRD) are generally used to determinethe corresponding chemical structures of SMPs [145].

3.4.1.2. Morphology detection. Optical microscopy, scanning electron microscopy (SEM) and transmis-sion electron microscopy (TEM) can be used to observe the surface morphology of SMPs or SMPs com-posites [26,46]. An optical microscope is a type of microscope that uses visible light and a system oflenses to magnify images of small samples; the resulting resolution is often at the micron level. SEM is


Fig. 8. Triple-shape effect of polymer network [141].


used for inspecting topographies of specimens at high magnifications [76]. The scattered electronsproduce an image with a depth of field that is typically 300–600 times better than that of an opticalmicroscope. Most SEMs have magnification ranges from 20� to 100,000�. TEM is a microscopy tech-nique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with thespecimen as it passes through, which is analogous to optical microscopy. Under favorable conditions,TEM can resolve detail at the magnitude of about 1 nm [146].

3.4.1.3. Electrical conductivity measurement. Some types of electrically conductive fillers may be incor-porated into SMPs to fabricate conductive SMP composites. In order to evaluate the electro-active per-formance, many researchers have used a variety of methods to measure the electrical conductivity[18,19,46]. If the electrical resistance ranges from 108 to 1011 Ohms, a digital high resistivity deter-miner should be employed. The Four-Point-Probes measurement, which eliminates the lead resis-tance, may also be used to determine the conductivity. During resistive heating, an infrared camera



can be employed to investigate the temperature distribution of the SMPs during the shape recoveryprocess.

3.4.1.4. Thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) is employed to investi-gate thermal stability by evaluating the change in mass as a function of temperature. During the shaperecovery process, the highest temperature should be lower than the onset temperature point ofdecomposition of SMPs. TGA or differential gravitational analysis (DTG) is used to determine the crit-ical temperature during the decomposition process of SMPs or SMP composites. TGA is a very usefultool for a first, simple and fast evaluation of the thermal stability properties before planning theirshape-memory effect [21].

3.4.1.5. Differential scanning calorimetry (DSC). Since differential scanning calorimetry (DSC) is used inmany different industries, its application in the plastics industry is widely accepted. It is used to char-acterize materials for their glass transition temperature, melting points and other material-reactioncharacteristics such as specific heat, percent crystallinity, and reaction kinetics [147]. For SMPs, inmost cases, DSC is used to determine the critical glass transition temperature, Tg, during the transitionprocess, which is often defined as the median point of the glass transition range in the heating ramp[46].

3.4.1.6. Dynamic mechanical analysis (DMA). Dynamic mechanical analysis (DMA) is used to monitorthe thermal and dynamic mechanical properties of the SMPs and their composites. In this technique,a strain or stress is applied to a sample at a set frequency and the response analyzed to obtain phaseangle and deformation data. These data allow the calculation of the complex modulus in Eq. (1) (e.g.,storage modulus and loss modulus), damping or tan delta (d) as well as viscosity data. The dynamicmechanical properties of a polymer are described in terms of a complex dynamic modulus [146]:

E� ¼ E0 þ iE00 ð1Þ

where E0 is the storage modulus and is a measurement of the recoverable strain energy; when defor-mation is small, it is approximately equal to the Young’s modulus. E00 denotes the loss modulus and isrelated to the hysteresial energy dissipation. The phase angle (d) is given by:

tan d ¼ E00=E0 ð2Þ

DMA allows the rapid scanning of the modulus and viscosity of an SMP as a function of tempera-ture or frequency [148]. In addition, DMA is very sensitive to the motions of polymer chains and,therefore, they are powerful tools for measuring the glass transitions in SMPs.

3.4.1.7. Mechanical properties evaluation. The mechanical properties of SMPs under varying tempera-ture conditions are important parameters to evaluate the thermal, mechanical and shape-memoryperformance. The relevant tests include uniaxial tension tests, compression tests, three point bendingtests, relaxation tests, creep tests, and nanoscale indentations by atomic force microscopy (AFM). Niet al. [149] investigated an SMP filled with carbon nanotubes. The results indicate that CNT/SMP nano-composites exhibit a good shape-memory effect and their recovery stress with only 3.3% weight frac-tion of carbon nanotubes will reach almost twice of that bulk SMP. Yang et al. [150] studied nanoscaleindentation on polymer surfaces using AFM. With increasing molecular weight of cross-linker anddecreasing cross-linker concentration, the contact pressures decreased at a fixed maximum load.The results provide insight into the nanoscale response of these novel materials. Yang [151] have dem-onstrated that the polyurethane SMPs filled with carbon powders exhibited better mechanical andthermomechanical properties than pure SMP. In addition, many other researchers have conductedthe relevant mechanical tests to investigate the basic mechanical and thermomechanical performanceof these materials [15,31,32,36,152–154].


Fig. 9. Stress–strain–temperature relationship showing the loading path in the thermomechanical test.


3.4.2. Shape memory performanceThe typical programming of shape recovery process of thermo-responsive SMPs is introduced here

[151,154]. A cyclical thermomechanical test is the standard approach to characterize the mechanicaland shape-memory properties of SMPs. As shown in Fig. 9, a three-dimensional stress–strain–temper-ature relationship of thermo-responsive SMPs is illustrated in the following steps [32,155]: (a) at anelevated temperature Th (Th > Ttrans), the SMP specimen is deformed to a pre-determined maximumstrain (em) at a constant strain rate ð _eÞ under the external loading; (b) the sample is held at the strainem and then cooled well below Tg to a new temperature Tl (Tl < Ttrans); (c) after the constraint is releasedat Tl, the stress is reduced to zero, and a very small elastic strain is recovered (shape fixity) where ef isdefined as the pre-deformed remaining strain after the applied stress is fully removed; and (d) thesample is reheated from Tl to Th at a constant heating rate _T without applying any external loading(r = 0). The pre-deformed strain ef is recovered with only a very small irreversible strain ei left at Th

(shape recovery).As shown in Fig. 9, the cyclical thermomechanical tests mainly consist of two processes, namely

shape fixity and shape recovery [156]. Tobushi et al. [37,157] systematically investigated the mechan-ical properties of a polyurethane SMP, including the cyclic deformation properties at high tempera-tures, thermomechanical cycling properties, creep and stress relaxation. The results showed thatthe shape fixity with loading above Tg did not change under thermomechanical cycling, and the resid-ual strain recovered in the vicinity of Tg during the heating process. Gall et al. [155,158] demonstratedthat the SiC/SMP nanocomposite can generate higher recovery forces than those of a pure SMP spec-imen. Hu et al. [50,15] have also systematically reviewed and conducted an analysis of the shape-memory test.

For the practical application of SMPs, their shape recovery performance is extremely important andis generally evaluated using a bending test. During these tests, the main parameter is the angle of theSMP upon bending. The following descriptions illustrate the measurement of this angle and shaperecovery ratio during the bending program.

As shown in Fig. 10, the typical thermomechanical bending cycling for an SMP includes the follow-ing steps [36,159]: (1) the specimen in its original shape is kept in a water bath at Th; (2) the SMP isbent to a storage angle h0 around a mandrel with a radius of r in the soft rubbery state, and then theSMP is kept in cool water with the external constraint to ‘‘freeze’’ the elastic deformation energy (Stor-age); and (3) the SMP specimen fixed on the apparatus is immersed into another water bath at a ele-vated temperature, and then it recovers to an angle hN (Recovery). The method used to quantify theprecision of deployment is illustrated in Fig. 10, where, r denotes the radius of mandrel, t representsthe thickness of the SMP specimen, h0 is the original storage angle of the specimen in storage state


Fig. 10. Schematic illustration of setup for shape recovery performance test.


during the first bending cycle, S(x0, y0) is a point selected to determine h0. hN is the residual angle inrecovery state during the Nth thermomechanical bending cycle (N = 1, 2, 3 . . .). R(xN, yN) is a testingpoint in order to calculate hN:

hN ¼ ArcCotxN

yN

� �ðN ¼ 1;2;3 . . . ;0 6 hN 6 180�Þ; ð3Þ

The value of the shape recovery ratio is calculated by:

RN ¼h0 � hN

h0� 100% ðN ¼ 1;2;3 . . .Þ; ð4Þ

where RN denotes the shape recovery ratio of the Nth thermomechanical bending cycle. S(x0, y0) andR(xN, yN) can be measured by a vernier caliper. Finally, hN and RN are obtained through Eqs. (3) and (4).In the following tests, the radius r of mandrel and thickness t of the SMP specimen are 2 mm and3 mm, respectively.

3.5. Constitutive models for shape-memory polymers

In many practical applications, SMPs often undergo large three-dimensional deformations[160,161,162]. To date, an understanding of the thermomechanical behavior of SMPs is still limitedto the special cases of small one-dimensional deformation [51,52,163,164]. A constitutive model thatdescribes three-dimensional finite deformation, is useful for the development of SMP components.The finite element method based on the three-dimensional constitutive model is also important forengineering analysis and design of SMP components. In our brief review, we introduce two thermo-mechanical constitutive models for SMPs based on viscoelasticity [32,33,165] and based on phasetransition, respectively [35,166,167].

3.5.1. Models based on viscoelasticityModels based on linear viscoelasticity are usually used to describe rate-dependent behavior of

polymer on macroscopic level [51]. In these models, materials are assumed as the combinations of ele-ments, including springs, dashpots or frictional elements. Some constitutive models of the SMPs canbe viewed as an extension of viscoelastic models to describe the thermomechanical behaviors, such asChen model [168,169], Li model [170], Tobushi model [32,33], Abranhamson model [171] and Nguyenmodel [172].

As a typical constitutive model based on viscoelasticity, Tobushi et al. [32] established a linear one-dimensional constitutive equation for SMP on the basis of a standard linear viscoelastic model. In thisequation the stress–strain–temperature relationship of an SMP is expressed as



_e ¼ _rE þ r

l�e�eS

k þ a _T

eS ¼ Cðec � elÞ

(ð5Þ

where r, e and T denote stress, strain, and temperature, respectively. The dot denotes a time deriva-tive. eS is irrecoverable strain. The temperature-related parameters E, l, and k represent elastic mod-ulus, viscosity, and retardation time, respectively. The temperature-related C and el are related to theprocess of shape fixity. They are expressed by the same function of temperature, expressed as

x ¼ xg exp aTg

T� 1

� �� ðTl 6 T 6 ThÞ ð6Þ

where xg is the value of x at T = Tg. Tl and Th are the temperatures at the starting and finishing points ofthe glass transition from the glassy to rubbery state in SMP.

The shape-memory thermomechanical cycle of polyurethane-based SMP can be predicted by theconstitutive equation, Eq. (5), coupled with the material parameter function, Eq. (6).

In 2001, Tobushi et al. [33] developed a nonlinear constitutive equation based on the linear consti-tutive equation mentioned above. In this equation the stress–strain–temperature relationship of anSMP is expressed as

_e ¼ _rE þm r�ry

k

� �m�1 _rE þ r

lþ 1b

rrc� 1

� n� e�eS

k þ a _T

eS ¼ Sðec � epÞ

8<: ð7Þ

where ry and rc denote the elastic and viscous proportional limits of SMP. The shape-memory ther-momechanical cycle of an SMP was simulated by using the nonlinear constitutive equation, Eq. (3),and the material parameter function, Eq. (6).

3.5.2. Models based on phase transitionIn 2006, Liu et al. [35,166] developed a constitutive model under uniaxial loading conditions of

SMP. The model uses internal state variables based on experimental results and the molecular mech-anism of the shape-memory effect of SMP. According to this model there are two kinds of extremephases, frozen and active phases, in SMP at an arbitrary temperature. The fractions of frozen and activephases are defined as follows:

/f ¼Vfrz

V; /a ¼

Vact

V; /f þ /a ¼ 1 ð8Þ

where /f and /a denote the fractions of frozen and active phases in SMP, respectively. V, Vfrz, and Vact

stand for the total volume, the volume of the frozen phase and active phase, respectively.The strain of the SMP is composed of three parts: the stored strain, mechanical elastic strain, and

thermal expansion strain, expressed as

e ¼ es þ em þ eT ð9Þ

where e denotes a second order total strain tensor. es, em, and eT present second order stored strain,mechanical elastic strain, and thermal expansion strain, respectively. The stored strain of an SMP isexpressed as

es ¼Z 0

/f

eef ðxÞd/ ð10Þ

where eef denotes the entropic frozen strain. The mechanical elastic strain of an SMP is expressed as

em ¼ ½/f Si þ ð1� /f ÞSe� : r ð11Þ

where r denotes a second order stress tensor. Si is the elastic compliance fourth order tensor corre-sponding to the internal energetic deformation, while Se is the elastic compliance fourth order tensorcorresponding to the entropic deformation. The thermal expansion strain is expressed as



eT ¼Z T

T0

½/f af ðhÞ þ ð1� /f ÞaaðhÞ�dh

�I ð12Þ

where af and aa are the thermal expansion coefficients of the frozen phase and the active phase, and Iis the identity tensor.

In addition, Qi et al. [167] also conducted a comprehensive study of the thermomechanical behav-ior of the SMPs. In the 3D finite deformation constitutive model, it assumes that the glassy phaseformed during cooling has a different stress free configuration compared to the initial glassy phase.In this sense, the deformation storage process can be captured. Therefore, the SMPs can be assumedto consist of three phases: rubbery phase, initial glassy phase and frozen glassy phase. Recently, Chenand Lagoudas [51,52] developed a nonlinear constitutive model for SMPs to describe the thermome-chanical properties under large deformation. The model is based on the idea that developed in thework of Liu et al. [35] Due to the coexisting active and frozen phases of SMP and the transition be-tween them, they provide the underlying mechanisms for strain storage and recovery during ashape-memory cycle. Their model presents excellent agreement with the experimental results ofLiu et al. [35]. In addition, the similar models based on phase transition have been comprehensivelydeveloped by Barot and Rao [173–175].

4. Stimulus methods of multi-functional shape-memory polymer composites

The actuation of SMPs and their corresponding composites have been carried out controllably andwith programmability. They exhibit the shape-memory effect and have various responses to stimuli.In addition to thermal actuation, SMP composites filled with functional fillers can also be triggered byother external stimuli, such as electrical resistive heating, light or magnetic field. Moreover, the tradi-tional thermo-responsive SMPs can be driven by a solvent. SMP response to water or solution is due tothe plasticizing effect of the solution molecule on polymer materials.

These novel actuation approaches play a critical role in the development of multi-functional mate-rials that not only exhibit the shape-memory effect but also perform particular functions [78]. In thisway, the triggering and subsequent actions of SMPs are sensitive to a particular stimulus and SMPsmay be used as a particular sensor or actuator [176–179]. For instance, the electrically conductiveSMP composites filled with carbon particles could be used as a strain sensor, which is based on therelation between electrical conductivity and strain. This SMP composites could monitor its ownreal-time deformation by testing the evolution of conductivity when it recovered to its original shape.In addition, if light-induced SMPs are exposed to light of a particular wavelength, an instant reaction,or deformation, could result that would enable their use as both sensors and actuators. The applica-tions of these multi-functional SMPs and SMP composites may cover a broad range of fields from bio-medicine to micro-sensors and actuators, to MEMS/NEMS. They are proposed to provide cheaper,accurate and faster alternatives to devices already on the market.

As discussed in Section 2.4, most of the stimulus forms for SMPs (e.g., electricity, light, magnetismand moisture) intrinsically belong to thermal response. Actually, except for the intrinsically light-induced SMPs, all of the other SMPs and SMP composites belong to directly or indirectly thermal-responsive polymers. Consequently, the thermal-responsive SMPs and their composites will not bediscussed particularly in this section. The contents of this section discuss the stimulus methods,including electricity, light, solution and magnetism. The multi-function of SMPs or their compositeswill be introduced respectively in each part. When considering the composites, the mechanical prop-erties of particle-filled or fiber-reinforced SMPs will be briefly introduced.

4.1. Shape-memory polymer composites

4.1.1. Particle-filled shape-memory polymersThe thermomechanical behavior of SMPs can be designed either through modification of the molec-

ular structure of the polymer or through the addition of functional fillers into the polymer as a matrixfor multi-phase composites. The SMP composites can be classified as particle- or fiber- reinforceddepending on the type of filler they employ. SMP composites filled with particles (e.g., carbon black,



carbon nanotubes [180], carbon nanofibers, SiC, Ni, Fe3O4, clay [181,182], and short or continuous fi-bers may meet various requirements in practical applications [182,183]. In general, the SMP compos-ites filled with particles or short fiber develop some particular function, such as high electricalconductivity, magnetic-responsive performance, or high stiffness on the micro scale. Hence, this typeof SMP composites is studied as a multi-functional material.

Yang [151] investigated the thermomechanical properties of polyurethane SMP filled with carbonpowders. Their results demonstrate that the incorporation of carbon powders is an efficient means ofincreasing the recovery stress. In addition, Gall et al. [184,185] comprehensively investigated the ther-momechanics of SMPs and their composites, including a SiC reinforced epoxy SMP [155,158]. The SiC/SMP nanocomposites were found to have a higher elastic modulus and were capable of generatinghigher recovery forces in comparison with a pure SMP specimen.

Carbon nanofibers have the potential to be used in nano-electronic devices, such as artificial mus-cles, as well as in electrochemical energy storage matrices. Carbon nanofibers usually vary from 5 to1000 lm in length and are between 50 and 300 nm in diameter. The Young’s modulus of these nanof-ibers can reach as high as 1.2 TPa and some samples have been reported to have tensile strengths up to64 GPa, compared to conventional carbon-fibers that have strengths equal to about 4.0–5.0 GPa. Thus,it is expected that composites filled with carbon nanofibers may present better performances whencompared to the conventional carbon-fiber-reinforced materials [186]. Consequently, carbon nanofi-bers with diameters of 24 nm and lengths 300 nm were used for reinforcement of SMPs by Manpreet[186]. They demonstrated that the presence of nanofibers in SMP leaded to significant reinforcementfor SMPs.

Pure SMPs or particle-reinforced SMPs are not suitable for use as structural materials; however,they are generally used as functional materials through the addition of some particular particle fillers.For instance, electrically conductive carbon black, carbon nanotubes, nickel powders and chopped car-bon-fibers are incorporated into SMPs to develop electro-active SMPs. Ferromagnetic Fe3O4-filledSMPs can lead to magnetism induced SMPs. Moreover, infrared optical fiber could be embedded intoSMPs to develop light-induced SMPs by transmitting the infrared light through the fiber to heat theSMP.

4.1.2. Fiber-reinforced shape-memory polymersIn general, since the improvement in their mechanical properties is quite limited and their strength

and stiffness remain low, SMP composite’s reinforced with particles or short fibers cannot be used asstructural materials [38]. In contrast, continuous fiber-reinforced SMPs offer a significant improved instrength, stiffness and resistance against relaxation and creep, thereby providing better mechanicalproperties. As both a functional and structural material, these SMPs show good potential for many ad-vanced applications. When used as actuator materials, they require no moving parts. Substantial inter-est has therefore been generated for the use of fiber-reinforced SMPs in deployable structuresincluding antennas, trusses and solar arrays.

In addition, most studies regarding SMP composites are focused on thermoplastic SMP resins suchas polyurethane SMP. However, the relatively poor thermal and mechanical properties (e.g., temper-ature, moisture and chemical resistance) of thermoplastic SMPs cannot fully meet practical require-ments. Thermosetting SMPs, however, show an improvement in the aforementioned properties andcan be widely used as functional or structural materials.

A carbon-fiber fabric reinforced polyurethane SMP was developed for industrial applications[152,187]. The bending recovery ratio of this SMP based laminates was larger than that of pureSMP sheet for any given recovery time. Epoxy SMP composites (Elastic Memory Composite, EMC)show great potential for light weight deployable spacecraft structures applications. Composite Tech-nology Development (CTD), Inc. firstly started to comprehensively researched the EMC and the rele-vant applications in deployable structures since the 1990s [188–191]. A study of the micro-mechanisms of deformation in EMC materials has been conducted by CTD and Gall et al. [192]. Theirstudies have investigated several interaction phenomena between the reinforcement fibers and theSMP resin in an epoxy EMC laminate. Due to the effects of microbuckling and the shift in the neu-tral-strain surface, the fiber-reinforced SMP composites laminate can achieve much larger compres-sive strains than traditional hard-resin composite. In addition, Leng et al. [36,171] developed a



fiber-reinforced thermosetting styrene-based SMP composite. Based on the high-strain capability andrelatively good mechanical properties, the fiber-reinforced SMP composites show potential for deploy-able structures applications [36,171,192,193].

The aforementioned carbon-fiber-reinforced SMP composites exhibit better mechanical and ther-momechanical properties than pure SMPs, and they can also be activated by loading a voltage onthe carbon-fibers embedded in the SMP matrix [36,171]. Hence, they could be somewhat describedas a multi-functional materials. In addition to carbon-fiber-reinforced SMPs, glass-fiber or Kevlar-fiber-reinforced SMPs are also an important type of shape-memory composite. Liang et al. [38]previously investigated glass and Kevlar-fiber-reinforced thermoplastic SMP composites. They foundan improvement in stiffness but a decrease in recoverable strain as a result of the incorporation ofthe reinforcement. Ohki et al. [194] investigated the mechanical and shape recovery properties ofthermoplastic SMP composites reinforced by chopped strand glass-fiber, especially for the relationshipbetween fiber weight fraction and recoverability. With the incorporation of 50 wt.% of glass-fiber, theyfound that the failure stress increased by 140% while the recovery rate decreased by 62%.

4.2. Electro-activated shape-memory polymer composites

In the past decade, much research has explored on the mechanism, shape recovery and electricalproperties of SMPs filled with conductive filler and their corresponding applications [195,196]. Thisreview discusses the most recent developments in undergoing electro-induced shape recovery andsummarizes the growth of electro-activate shape-memory polymer. Electricity as a stimulus enablesresistive actuation of SMPs filled with conductive fillers. In this way, external heating, which isunfavorable for many applications and is used to stimulate conventional shape-memory polymers,can be avoided. The electric triggering of SMP composites enlarges their technological potential[197,198,199].

4.2.1. Shape-memory polymer filled with carbon nanotubesSMPs are characterized by their remarkable recoverability and shape-memory effect; however

their mechanical properties such as strength and elastic modulus are low. Carbon nanotubes (CNT)based composites (e.g., CNT/PP [200,201], CNT/PVDF [202], CNT/nylon [203], CNT/polystyrene[204,205], CNT/epoxy [206,207], and CNT/PVA [208],) on the other hand, have remarkable mechanicalproperties with very high elastic modulus and electrical conductivity [209,210,155]. Based on theabove consideration, Goo et al. [18] reinforced shape-memory polyurethane (PU) with multi-walledcarbon nanotubes (MWNTs) of 10–20 nm diameter and 20 mm length. Surface-modification of theMWNTs in mixed solvents of nitric acid and sulfuric acid was found to further improve the mechanicalproperties of the composites. The mechanical properties of shape-memory composites were improvedby using acid treatment and sonication of the nanotubes at optimum conditions. In addition, the sur-face modified MWNT content increased the conductivity increased. Electro-active shape recovery wasobserved for the surface-modified MWNT composites with an energy conversion efficiency of 10.4%(Fig. 11). Hence, PU-MWNT composites may prove promising candidates for use as smart actuators[18].

4.2.2. Shape-memory polymer filled with carbon particlesLeng et al. [211,212] demonstrated that the thermosetting styrene-based SMP exhibited better

mechanical properties and moisture resistance than the thermoplastic polyurethane SMP. A new

Fig. 11. Electro-active shape recovery behavior of PU-MWNT composites. The pictured transition occurs within 10 s when aconstant voltage of 40 V is applied [18].


Fig. 12. Relationship between electrical resistivity and the content of carbon black for the SMP composite. Six samples weretested for each composition.

Fig. 13. Effect of tensile strain on electrical resistivity [151].


system of a thermosetting styrene-based SMP filled with nanocarbon powders has been presented[46,213]. Thermosetting polystyrene shape-memory polymer was reinforced with electrically conduc-tive carbon black particles (30 nm). It was found that thermal conductivity and storage modulus weresignificantly improved, while the glass transition temperature is slightly decreased. This SMP compos-ite has good electrical conductivity with a percolation threshold from approximately 3.0–5.0% in vol-ume fraction (see Fig. 12), which is associated with the high-structure of carbon black and itsagglomerations at nanoscale. The electrical resistivity was reduced slightly during resistive heating,and then increased after the applied voltage was removed. This is opposite to the conventional PTCeffect. The room temperature resistivity increases slightly after repeated heating.

Yang [151] investigated the effect of tensile strain on electrical resistivity. In practical applications,the conductive SMPs may be used as strain sensors based on the relation between electrical conduc-tivity and strain. As shown in Fig. 13, the electrical resistivity increases with an increase in strain. Theincrease of electrical resistivity is remarkable at the lower strain range below 10%. Based on the effectof strain on conductivity, the SMP can be used as a multi-functional material to fabricate strainsensors.


Fig. 14. Typical SEM images before (left column) and after (right column) five stretching (at 50% strain)-shape recovery cycles[214].


4.2.3. Shape-memory polymer filled with electromagnetic fillers

(1) Electro-active thermoplastic SMP composites filled with Ni powder chains

Electrically conductive powders, fibers and even nanowires/tubes have been utilized as fillers toimprove the electrical conductivity of polymers [214]. Although conductive fiber and nanowires/tubescan significantly enhance the stiffness and strength of polymers, their deformable strain is limited towithin a few percent. Since the recoverable strain in SMPs is normally on an order of one hundred per-cent, there is a potential problem of deformation compatibility [215,183]. As such, electrically conduc-tive powders, such as Ni powder, would be a better choice.

The electrical resistivity of a thermo-responsive polyurethane shape-memory polymer (SMP) filledwith micron-sized Ni powders is investigated [214]. By forming conductive Ni chains under a weakstatic magnetic field (0.03 T), the electrical conductivity of the SMP composite in the chain directioncan be significantly improved, which makes it more suitable for Joule heat induced shape recovery. Inaddition, Ni chains reinforce the SMP significantly but their influence on the glass transition temper-ature is about the same as that of the randomly distributed Ni powders. SEM images reveal that singlechains start to form at 1% volume fraction of Ni. With an increase in Ni content, multi-chains (bundles)result, and eventually no clear Ni chain can be recognized (Fig. 14, left column). In addition, after fivestretching-shape recovery cycles, the Ni chains still exist (Fig. 14, right column), which indicates thepossibility of using chained SMPs for cyclic actuation.

The results show that the electrical resistivity q of the chained samples in the chain direction is al-ways the lowest. For example, at 10% volume fraction of Ni powder, q of the random sample is


Fig. 15. Effect of volume fraction of Ni powder on electrical resistivity [214]. Right insert: illustration of setup for the resistivitymeasurement along the chain direction. Bottom-left insert: infrared image of temperature distribution in chained sample (10%Ni, 6 V).

Fig. 16. SEM images of conductive SMP with 10 vol.% of CB and 0.5 vol.% of chained Ni. Inset: zoom-in view of one Ni chain[184].


2.36 � 104 X � cm while that of the chained sample is 2.93 � 106 X � cm in the transverse directionand only 12.18 X � cm in the chain direction. However, at a high Ni content, q of all of the samples isclose. This is due to the fact that the Ni chains become unrecognizable at a high Ni content, as revealedin Fig. 14. At 10% volume fraction of Ni, the chained sample (with dimensions of 16 � 0.6 � 5mm3) canbe heated from room temperature (20 �C) to 55 �C by applying a voltage of 6 V (refer to infrared imagein Fig. 15, bottom-left insert), which is enough to trigger shape recovery.

(2) Electro-active thermoplastic SMP/CB composite filled with Ni powder chains

An approach to significantly reduce the electrical resistivity in a polyurethane SMP filled with ran-domly distributed carbon black (CB) is presented [184]. With an additional small amount of randomlydistributed Ni micro particles (0.5 vol.%) in the SMP/CB composite, its electrical resistivity is reducedonly slightly. However, if these Ni particles are aligned into chains (by applying a low magnetic fieldon the SMP/CB/Ni solution before curing), the drop in electrical resistivity becomes significant. Threetypes of thin films were fabricated using the same method as for the SMP/Ni composite, namely, SMP/CB/Ni (chained) (Fig. 16), SMP/CB/Ni (random), and SMP/CB (i.e., without Ni).

In order to demonstrate the shape recovery by Joule heating, three samples [SMP/CB/Ni (chained),SMP/CB/Ni (random) and SMP/CB], were bent by about 150�. A 30 V of power was applied. Fig. 17


Fig. 17. Sequence of shape recovery and temperature distribution. Top-left inset: dimensions of sample; middle-left inset: pre-bent shape. Bottom-left inset: temperature bar (in �C). Sample (a) 10 vol.% of CB only; Sample (b) 10 vol.% of CB, 0.5 vol.% ofrandomly distributed Ni; and Sample (c) 10 vol.% of CB, 0.5 vol.% of chained Ni. The tests were repeated more than five times oneach sample [184].


(right) presents four snapshots of each sample. It is clearly revealed that Sample (c) [SMP/CB/Ni(chained)] reaches the highest temperature, while the temperature of Sample (a) [SMP/CB] is the low-est. Hence, the shape recovery is small. Sample (b) [SMP/CB/Ni (random)] reaches around 65 �C, andthe shape recovery is not completed after 120 s.

4.2.4. Shape-memory polymer filled with hybrid fibersThe electrical and thermomechanical properties of styrene-based SMP composites containing CB

nanoparticle and short carbon-fiber (SCF) of 0.5–3 mm length and 7 lm diameter were investigated.Fig. 18 shows the typical relation between the resistivity of composites versus filler content. The data ob-tained for the SMP filled with micro carbon powder and the SMP filled with micro carbon powder and SCFwere compared to the results found in [216,217]. Through comparison, the composites containing nano-particles have a higher conductivity than those blended with micro-dimension conductive filler.

As shown in Fig. 19, the short fibers disperse randomly. There are many interconnections betweenthe fibers, which form the conductive networks that explain the excellent electrical conductivity of thecomposites filled with SCF. The two microstructural images in Fig. 19 show the formation of co-sup-porting conductive networks. Such conductive networks could improve the electrical properties ofSMP composites. It demonstrates that a distinct difference in the influence on conductivity of the com-posites results from the use of fibrous and particulate fillers.

4.3. Light-induced shape-memory polymers and their composites

4.3.1. Intrinsically light-induced shape-memory polymersIntrinsically light-induced SMPs have been produced by incorporating reversible photoreactive

molecular switches when light of a special wavenumber light is applied [20,177,179,218]. The stimu-lation is considered to have no relation with any temperature effects [127,219]. Therefore, it should bedifferentiated from the indirect actuation of the thermal-responsive shape-memory effect. As shown


Fig. 18. Resistivity of SMP matrix filled with MCP, CB, MCP/SCF and CB/SCF systems as a function of filler content [216].

Fig. 19. Morphology of short carbon fibers (left) and sectional observation (right) [216].


in Fig. 20, Lendlein and Langer et al. [1,20] reported a light-induced SMP. In the programming cycle,the polymer is firstly stretched, resulting in strained, coiled polymer segments. Subsequently, the net-work is irradiated with ultraviolet light with a wavelength >260 nm and new covalent netpoints arecreated to fix the strains, leading to the temporary shape. The permanent shape can be recovered uponirradiation with UV light of k < 260 nm through cleavage of the cross-links.

4.3.2. Shape-memory polymers induced by radiant thermal energy of lightIndirect actuation of the shape-memory effect has been realized by two typical strategies. One

method involves indirect heating, that is, magnetically induced SMPs filled with magnetic particleswith the inductive Joule heating achieved through an alternating magnetic field [17,220]. The otherpossibility is irradiation, that is, the thermally induced SMPs can be heated by illumination with radi-ant thermal energy of infrared light. As shown in Fig. 21, the laser-induced SMP has been demon-strated and subsequently considered for use as a medical device [221,222]. In such devices, heattransfer can be enhanced by incorporation of conductive fillers (e.g., carbon black and carbon nano-tubes) [223]. The irradiation by infrared light possesses a wide emission spectra and a unique heatingeffect, and the infrared light actuation method can be non-contact and non-medium and may, there-fore, dramatically expand the applications in smart structures [1,164].

Leng et al. [224] systematically investigated the relative performance of infrared light-inducedSMPs filled with carbon black. For the capability of absorbing infrared light, it is apparent that theabsorption spectrum of the SMP/CB composite not only covers a wider range but also exhibits higher


Fig. 20. The molecular mechanism responsible for the light-induced shape-memory effect in a grafted polymer network [20].

Fig. 21. Strain recovery under infrared radiation (exposure from the left) [1].


absorptivity than that of the pure SMP. Therefore, the presence of CB particles remarkably increasesthe capability to absorb infrared light. When a SMP and SMP/CB nanocomposite are exposed to infra-red light, most of the emitted energy is transmitted by the SMP, which is semitransparent, while mostof the emitted infrared light is absorbed by the nanocomposite since it is black and opaque. In addi-tion, the SMP and the nanocomposite have approximately equal reflection since their surfaces are ofsimilar smoothness. Therefore, the black CB absorbs infrared radiation much more notably than thepure SMP. Infrared light is unique in that it exhibits a notable heating effect. It demonstrated thatCB can absorb infrared radiation remarkably and produce a notable heating effect for wavenumbersbetween 500 and 3000 cm�1. In addition, the high capability to absorb infrared light in the nanocom-posite will result in more generated heat. In addition, Fig. 22 shows the curves of the shape recoveryangle as a function of time for the SMP and SMP/CB composite at 90 �C. Results show that the CB addi-tive not only increased the recovery force but also enhanced the capability to absorb infrared light for


0 50 100 150 200 250 300 350 400 450

0

30

60

90

120

150

180

210

SMP/CB composite SMP

Ang

le (º

)

Time (s)

1st 2nd 3rd 4th 5th

Fig. 22. Recovery angle as a function of time for five tests [224].

Fig. 23. Schematic of light-induced SMP actuator. (a) Treated optical fiber and (b) treated portion of the optical fiber embeddedinto the SMP [226].


the SMP/CB composite; as a result, the shape recovery ability and shape recovery speed for the com-posite are much quicker than for the SMP.

4.3.3. Light-induced shape-memory polymer embedded with an optical fiberThermally activated SMPs make it possible to initiate an original shape using a non-contact and

highly selective infrared laser stimulus. For light-induced SMPs, the infrared laser stimulation of theSMP is interesting for sensor and actuator systems as well as for medical applications. An infraredlight-induced SMP embedded with an optical fiber was proposed by Leng et al. [225,226]. Fig. 23 isthe schematic illustration, which shows the treated optical fiber that embedded into the SMP. Theoptical fiber was treated by an aqueous solution of sodium-hydroxide and the cladding was removed.In this way, the light may transmit from the side of the fiber, which provides a better method of heat-ing a large region. Furthermore, an optical fiber network may be embedded into the SMP for actuationas a fast response.

The results of IR spectra reveal a strong absorption peak about the value of 3000 cm�1, character-istic of the CAH bond of the benzene ring. Hence, the working length of the infrared laser is chosen at3–4 lm. Therefore, an optical fiber was embedded into the SMP for delivery of the laser light at awavelength of 3–4 lm for actuation. The free strain recovery investigation of a SMP thread embeddedwith an optical fiber was performed and the results are shown in Fig. 24.


Fig. 24. Shape recovery process of SMP [226].

Fig. 25. Water-driven actuation of a shape-memory effect. A shape-memory polyurethane with a circular temporary shape isimmersed into water. After 30 min, the recovery of the linear permanent shape begins [22].


4.4. Solution-responsive shape-memory polymers

4.4.1. Water-driven shape-memory effectThe shape-memory effect for shape-memory polyurethanes and their composites with carbon

nanotubes reinforcement has been triggered by inductively lowering Ttrans [227,21,228]. When im-mersed in water, solvent molecules diffuse into the polymer sample and act as plasticizers, leadingto a reduction in the transition temperature and resulting in shape recovery (Fig. 25). In the polyure-thane polymers and their composites, Tg is lowered from the transition temperature to ambient



temperatures or below. The experimental results reveal that the lowering of Tg depends on themoisture uptake, namely it indirectly depends on the immersion time. In time-dependent immersiontests, the water uptake can be adjusted between 0 and 4.5 wt.%, which coincides with a decrease in theTg of between 0 and 35 K. As the maximum moisture uptake is around 4.5 wt.% after 240 h, this SMP isstill considered to be a polymer and not a hydrogel. Another strategy for water-actuated SMPs hasbeen carried out in polyetherurethane polysilesquisiloxane block copolymers [22,229]. Here lowmolecular weight poly(ethylene glycol), or PEG, has been used as the polyether segment. Uponimmersion in water, the PEG segment is dissolved, leading to the disappearance of Tm, resulting inrecovery of the permanent shape.

4.4.2. Solution driven shape-memory effectThe mechanism behind the response of SMPs to solution (namely solvent or mixture) is that the

solution molecule has a plasticizing effect on polymeric materials and thus increases the flexibilityof the macromolecular chains. These two effects reduce the transition temperature of materials untilshape recovery occurs. This study of Leng et al. [230–233] is made in the context of and in reference tothe many reported theories explaining the mechanism of water-driven polyurethane SMP and aims toenlarge its extension.

(1) Solution driven shape-memory effect by chemical interaction

It is well known that phase transition often accompanies great changes in physical properties ofpolymeric materials, such as a large decrease in modulus on which the solution-responsive SMP isbased. The mechanism involves the interaction between polymeric macromolecules and micro-mole-cules of the absorbed solution [230,231]. There are three major explanations. First, the hydrogenbonding enlarges the flexibility of the polymeric chains. Secondly, the interaction resulting in a vol-ume change of the polymer destroys the modulus of the polymer as explained by continuum theoriesof rubber elasticity, the Mooney–Rivlin equation and volume change refinement theory [232,233].And, finally, when the temperature of the solution (or the ambient temperature) is lower than theglass transition temperature (Tg) of the polymer, the solution first intenerates the polymer until theTg is lowered to the temperature of the solution and following this, the solution has other effects onthe polymer. Thus, the actuation of SMPs can be triggered by their solutions. The styrene-based ther-mosetting SMP can recover from the pre-deformed ‘n’ shape sequentially after being immersed in N,N-Dimethyl formamide (DMF) by forming the hydrogen bonding shown in Fig. 26.

Solution-driven SMPs provide an alternative approach, making stimulation by heating unneces-sary. The actuation can be triggered upon immersion of the material into solution. Instead of heatingthe material to above its Tg, shape recovery can be achieved by means of reducing the Tg of the mate-rial itself upon immersion of the SMP into a solution that is, in principle, similar to the process used forwater-driven PU SMPs.

(2) Solution driven shape-memory effect by physical swelling effect

Work has been carried out by Leng et al. [232,233] to activate SMPs through the physical swellingeffect. Based on the free-volume theory, rubber elasticity theory and Mooney–Rivlin equation, thefeasibility of SMP activated through physical swelling is theoretically predicted. The swelling effect

Fig. 26. Shape recovery of a 2.88 mm diameter SMP wire in DMF. The wire was bent into an ‘‘n’’ like shape [230].


Fig. 27. Series of photographs showing the macroscopic shape-memory effect of SMP composite with 10 wt.% content ofnanoparticle (Fe2O3) [16].


occurs due to the interaction between macromolecules and solvent molecules, leading to an increasein the free-volume of polymeric chains (namely, the flexibility of the polymer chains is increasing) andresulting a decrease in Tg. Toluene solvent was selected for a styrene-based SMP driven by the physicalswelling effect. It is found that this solvent had an intensive swelling effect on the polymer. The flex-ibility of the macromolecular chains increased after being swollen. The increase in flexibility subse-quently led to a decrease in the transition temperature and resulted in the shape recoveryoccurring at a lower temperature. These phenomena all obey continuum theories of rubber elasticity,the Mooney–Rivlin equation and volume change refinement theory [234,235].

In summary, the response of polyurethane SMP to water or to solution (namely, a solvent or mix-ture) is due to the plasticizing effect of the solution molecule on the polymeric materials, which thenincreases the flexibility of the macromolecular chains. These two effects reduce the transition temper-ature of the materials until shape recovery occurs. This approach makes tremendous progress in theactuation of SMPs, and many results and achievements are based on it.

4.5. Magnetic-responsive shape-memory polymer composites

The magnetically induced shape recovery of SMP composites could be realized by incorporatingmagnetic nanoparticles (e.g., Fe2O3 and Fe3O4) in SMPs [16,17,236–240]. The shape-memory effectof the composites can be triggered by inductive Joule heating in an alternating magnetic field, i.e.,transforming electromagnetic energy from an external high frequency field to heat [241]. The magnet-ically induced SMPs can be remotely controlled to induce heating energy locally and selectively. Theyshow potential to be used in biomaterials, such as in tumor therapy (magnetic fluid hyperthermia) andas contactless magnetically actuated SMP microactuators for the removal of a clot in a blood vessel.

Fig. 28. (a) Schematic diagram of the LC resonant circuit-based HF generator used for induction heating experiments andsample positioning; (b) photo series demonstrating the shape-memory transition induced by exposure to an HF electromag-netic field [17].



Mohr et al. [16] investigated magnetically induced thermoplastic SMP composites filled with Fe2O3

nanoparticles. Iron oxide particles with average diameters at the nanoscale (20–30 nm) support apreferably homogeneous distribution within the SMP matrix. The shape recovery of SMP compositescould be induced by inductive heating in an alternating magnetic field (f = 258 kHz; H = 30 kA m�1).The maximum temperatures achieved by inductive heating in a specific magnetic field depend onsample geometry and nanoparticle content. As shown in Fig. 27, the magnetically induced shape-memory effect is exemplarily demonstrated.

In addition, Schmidt [17] incorporated surface-modified super-paramagnetic nanoparticles (Fe3O4,diameter: 11 nm) into a SMP matrix. The thermosetting SMP composite of oligo(e-caprolac-tone)dimethacrylate/butyl acrylate contains between 2 and 12 wt.% magnetite nanoparticles servingas nano-antennas for magnetic heating. The specific power loss of the particles is determined to be30 W g�1 at 300 kHz and 5.0 W. As shown in Fig. 28, a photo series presents the electromagneticallyinduced shape-memory effect of the sample. The rectangular strip (about 15 mm � 2 mm � 0.5 mm)was deformed to a helix temporary shape at 70 �C. It recovers to the original straight shape in about20 s with the application of an AC field of 300 kHz.

5. Applications of shape-memory polymers and their composites

As a novel kind of smart materials, SMPs currently cover a broad range of application areas rangingfrom outer space to automobiles. Recently, they are being developed and qualified especially fordeployable components and structures in aerospace [242]. The applications include hinges, trusses,booms, antennas, optical reflectors and morphing skins [24,243,244]. In addition, SMPs also presentadditional potential in the areas of biomedicine, smart textiles, self-healing composite systems andautomobile actuators. Additionally, there are many patents in relation to SMPs applications, such asgripper [245], intravascular delivery system [246], hood/seat assembly and tunable automotive brack-ets in vehicles [247–249].

5.1. Deployable structures

For the traditional aerospace deployable devices, the change of structural configuration in-orbit isaccomplished through the use of a mechanical hinge, stored energy devices or motor driven tools.There are some intrinsic drawbacks for the traditional deployment devices, such as complex assem-bling process, massive mechanisms, large volumes and undesired effects during deployment. In con-trast, the deployment devices fabricated using SMPs and their composites may overcome certaininherent disadvantages [250].

5.1.1. HingeLeng et al. [36] investigated a carbon-fiber-reinforced SMP composite that can be used in actively

deformable structures. In these structures [251], the flexural deformation is the main mode of the

Fig. 29. Shape-memory polymer composite hinge: (a) Illustration of the hinge (1: curved SMP-composite shell; 2, 3: fixture ofthe hinge) and (b) real scale hinge.


Fig. 30. Shape recovery process of SMP-composite hinge [36].

Fig. 31. Shape recovery process of a solar array prototype actuated by an SMP-composite hinge [36].


deformation in the structures with thin shells where the bending angle is almost larger than 90� butthe strain is often smaller than 10%. In this way, the shape recovery process of the structure concernsstructural deployment dynamics.

As shown in Fig. 29, an SMP-composite hinge was designed, which consists of two curved circularSMP-composite shells in the opposite directions. Fig. 30 shows the deployment process for theSMP-composite hinge. A voltage of 20 V is applied on the resistor heater embedded in each circularlaminate. The temperature of the hinge remains at about 80 �C after heating for 30 s. The wholedeployment process takes about 100 s. In addition, a deployment of a solar array prototype whichis actuated by a SMP-composite hinge is demonstrated (see Fig. 31).

During the launching of a spacecraft, the room inside the spacecraft is quite limited. Hence, thespacecraft needs lightweight, reliable and cost effective mechanisms for the deployment of radiators,solar arrays or other devices. Composite Technology Development (CTD), Inc. has developed epoxySMP composites reinforced by carbon-fiber (Elastic Memory Composite, EMC) material. The EMCmaterials show very high recoverable strains and high deployed stiffness- and strength-to-weight ra-tios. CTD has developed a deployable hinge fabricated with EMC materials (see Fig. 32). Recently, CTDperformed an extensive ground testing on an EMC deployable hinge that may be used for deployablespacecraft components [252].

5.1.2. Truss and boomAn EMC boom has also been designed by CTD used on a micro-satellite. This extendable boom is

lightweight and can support a variety of tip payloads (see Fig. 33). It has been identified to support


Fig. 32. EMC flight hinge shown integrated onto ExpSA and packaged [252].

Fig. 33. The proposed three-longeron EMC tubular boom in both (a) a packaged (b) a deployed configuration without the outershroud [253].

Fig. 34. (a) EMC boom and (b) partially deployed model of solar arrays supported by the EMC boom (b) [253].


a micro-propulsion attitude control system. The EMC is the central element of the boom. To stow theboom, the longerons are in a z shape and, thus, the EMC are flattened and bent in both the equal- andopposite-sense in a pre-deformed shape [253].

A new generation of deployable structures experiment was proposed to deploy large and light-weight solar arrays. This type of structure has very little deployed structural depth over a very large


Fig. 35. Deployable ground-based mirror produced using EMC resin [255].

Fig. 36. An illustration of reflective mirror fabricated by SMP.


deployed area. To minimize the impacts to the spacecraft system, the structural mass and complexityshould be minimized. Based on the above considerations, the structure fabricated by EMC is proposedfor the deployed solar array and the actuation to deploy the solar array from its packaged configura-tion. As show in Fig. 34, the longeron booms fabricated by the thin film foldable EMC tubular are cur-rently being fabricated by CTD to meet the needs for the deployable structure [253,254]. Based on theexperimental demonstration, a detailed theoretical study is being put forward. Geometric nonlinearityshould be considered because of the large deformation of the longeron. In addition, the presence ofmicrobuckling of the composite leading to material nonlinearity should also be considered.

5.1.3. Ground-based deployable mirrorWith their unique properties, SMPs are suitable for the fabrication of thin, lightweight deployable

ground-based mirrors [255]. The mirror consists of a SMP composite substrate and the coated reflec-tive side of the composite-reflector (see Fig. 35). The reflective surfaces are mainly composed of elec-troplated nickel to provide high quality reflectance. The electroplated nickel metal surface, which isless than 30 lm thick, is adhered on the surface of the SMP-composite mirror. The substrate hasthe ability to be deformed for packaging and then achieve shape recovery upon Joule hearing froman external power supply. Similarly, Cornerstone Research Group (CRG), Inc. has proposed a deploy-able mirror fabricated by SMP composites, as show in Fig. 36. The support structure of the mirror con-sists of a honeycomb support structure fabricated by SMPs and a mirror surface, including carbonnanofibers, a reflective coating, fabrics, and microspheres.

5.1.4. Reflector

(1) Stiffeners for a flexible reflectorA new kind of solid surface deployable reflector was developed by Harris Corporation, which is

similar to a springback reflector. By enabling large-aperture antennas to be stowed within anexisting launch vehicle, the reflector is envisioned to significantly promote the ability of satellite


Fig. 37. Shape-memory polymer composite-reflector: (a) pre-deformed shape and (b) recovered shape [256].


communication systems. In addition, Harris Corporation is now considering the use of EMC materialsfor the system (see Fig. 37) to realize the full potential of the FPR design [256]. The stiffener around theedge of the antenna can be made by EMC materials. The antenna can be stowed in the spacecraft andthen its deployment can be triggered by the shape-memory effect to finally hold the surface of theantenna in a precision micro-system.

(2) Parabolic dish antenna reflector

The large-aperture inflatable antenna is one important kind of deployment reflector. However, thedeformation of the central part of inflatable antenna is mostly very large, which often makes it verydifficult to realize inflatable deployment. Recently, SMP composite materials have been envisionedfor the fabrication of the central part of the antenna to enable a large deformation (see Fig. 38 andFig. 39) [257]. In the design, both the dish and the supporting structure are made of the SMP compos-ite. Further research and development work are on-going to improve the properties of the SMP com-posite for this application.

5.2. Morphing structures

Flight vehicles are envisioned to be multi-functional so that they can perform more missions dur-ing a single flight, such as an efficient cruising and a high maneuverability mode [258,259]. When theairplane moves towards other portions of the flight envelope, its performance and efficiency may dete-riorate rapidly. To solve this problem, researchers have proposed to radically change the shape of theaircraft during flight [260]. By applying this kind of technology, both the efficiency and flight envelopecan be improved. This is because different shapes correspond to different trade-offs between

Fig. 38. SMP reflector (2-m diameter) for the hybrid inflatable antenna [257].


Fig. 39. SMP reflector (0.5-m diameter) in both deployed and packed configurations [257].

Fig. 40. z-shaped morphing wing produced by Lockheed Martin [261].


beneficial characteristics, such as speed, low energy consumption and maneuverability. For instance,the Defense Advanced Research Projects Agency (DARPA) is also developing morphing technology todemonstrate such radical shape changes. As illustrated in Fig. 40, Lockheed Martin is addressingtechnologies to achieve a z-shaped morphing change under the DARPA’s program fund [261].

During the development of morphing aircraft, finding a proper skin under certain criteria is crucial.Generally, a wing skin is necessary, especially for the wing of a morphing aircraft. Researchers focustheir works on investigating proper types of materials that are currently available to be used as a skinmaterial for a morphing wing. In this case, the SMPs show more advantages for this application. It be-comes flexible when heated to a certain degree, and then returns to a solid state when the stimulus isterminated. Since SMPs holds the ability to change its elastic modulus, they could potentially be usedin the mentioned concept designs.

5.2.1. Folding wingCornerstone Research Group (CRG) developed an improved SMP, which appears to be a prime can-

didate for seamless skin at the wing fold (see Fig. 41) [262]. As shown in Fig. 42, the feasibility ofmorphing SMP skin concepts in a 2-dimensional test jig has been demonstrated.

5.2.2. Variable camber wingA morphing concept of a variable camber wing was developed by Leng et al. [263,264], as shown in

Fig. 43. It comprises a flexible SMP skin, a metal sheet and a honeycomb structure. Metal sheet is usedto replace the traditional hinges to keep the surface smooth during the camber changing. Honeycomb,which is high-strain capable in one direction without dimensional change in the perpendicular


Fig. 41. Initial SMP prototype [262].

Fig. 42. SMP on a two dimensional test jig [262].

Fig. 43. Photograph of the original and morphing configurations of the variable camber wing.


in-plane axis, provides distributed support to the flexible skin. The flexible SMP skin is covered tocreate the smooth aerodynamic surface. The baseline airfoil is assumed to have an NACA 0020 profileand a chord of 150 mm.

5.3. Biomedicine and bioinspiration

SMPs show extensive interest in used for biomaterials and bioinspiration [265,266,243]. For in-stant, polyurethane SMP performs excellent biocompatibility, and it can be used for the deploymentof different clinical devices when contacted or implanted in the human body [267,268]. Recently,


Fig. 44. Schematic of the shape-memory effect prior to application (left) and right after reset (right) [269].

Fig. 45. Through increasing the temperature from 20 �C (A) to 40 �C (B) an initial shape change was induced followed by asecond shape change through increasing the temperature to 60 �C (C) [143].



Fig. 46. Concept of delivery of micro/nano device into a living cell for cell-level surgery/operation using thermo/moisture-responsive SMP [270].


Wache et al. [269] have conducted a feasibility study and preliminary development on a polymer vas-cular stent with an SMP as the drug delivery system (see Fig. 44). The field of applications of this poly-mer stent was demonstrated in pre-trials. The use of the SMP stent as a drug delivery system leads tosignificant reduction of restenosis and thrombosis. An improved biological tolerance in general is ex-pected when using biocompatible SMP materials.

The exciting SMPs can move from one shape to another in response to a stimulus. Thus, SMPs aredual-shape materials. A triple-shape polymer was recently reported in [141,143,144]. As shown inFig. 45, the SMP is able to change from an initial shape (A) to a second shape (B) and finally deformto a third shape (C). It may be inserted into the body, expanded at a target site, and be removed ata later point in time which may be necessary even with degradable materials.

The thermo/moisture-responsive polyurethane SMP sheds light for the possibility to fabricate mi-cro/nano devices for surgery/operation at the cellular level. Ikuta and Hayato [270,271] have devel-oped many polymer micro-machines, which are similar to the size of cells or even smaller and canbe triggered for operation by a laser beam outside the cell. However, there are tremendous difficultiesto deliver such machines into cells. The thermo/moisture-responsive polyurethane SMP offers a pos-sible solution. As illustrated in Fig. 46, a piece of original curved SMP is straightened and then insertedinto a living cell. Upon absorbing moisture inside the cell, the SMP recovers its original shape. As therecovery strain in solid or porous SMPs is on an order of hundred percent, it becomes possible to makecell or sub-cell sized machines using the thermo/moisture-responsive SMP and then deliver the ma-chines into living cells for operation controlled by an outside laser beam.

Biodegradable SMPs may be used in medical devices. The usefulness of SMPs in wound closure hasbeen recently investigated and a design of smart surgical sutures has been examined, where a tempo-rary shape is obtained by elongating the fiber under controlled stress (see Figs. 47 and 48). This suturecan be applied loosely in its temporary shape under elongated stress. When the temperature is raisedabove Tg, the suture will shrink and then tighten the knot, in which case it will apply an optimal force[13,29]. The biocompatibility and degradation products should be considered.

These sutures should be degradable and show gradual mass loss during degradation. The hydrolyz-able ester bonds are introduced into the polymers so that they would cleave under physiological con-ditions. In this way, the degradation kinetics could be controlled through the composition and relativemass content of the precursor macrodiols. The multi-block copolymers show linear mass loss in vitro(Fig. 49), resulting in a continuous release of degradation products [272].

A blood clot may cause an ischemic stroke, depriving the brain of oxygen and often resulting inpermanent disability. As an alternative to conventional clot-dissolving drug treatment, a laser-activated device for the mechanical removal of blood clots is also proposed (Fig. 50) [1,272]. The


Fig. 47. A fiber of a thermoplastic SMP is programmed by stretching about 200%. The knot tightened in 20 s when heated to40 �C [13].

Fig. 48. Degradable shape-memory suture for wound closure [13].

Fig. 49. Hydrolytic degradation of thermoplastic shape-memory elastomers in aqueous buffer solution (pH = 7) at 37 �C [272].


SMP microactuator, in its secondary straight rod form, could be inserted minimally by invasive surgeryinto the vascular occlusion. The microactuator, which is mounted on the end of an optical fiber, is thentransformed into its pre-deformed straight shape by laser heating. Once deployed into the corkscrew


Fig. 50. Depiction of removal of a clot in a blood vessel using the laser-activated SMP microactuator coupled to an optical fiber.(a) The temporary straight rod form. (b) The permanent corkscrew form by laser heating. (c) The deployed microactuator isretracted to capture the thrombus [272].

Fig. 51. Temperature distribution snapshots of the SMP actuator during the shape recovery process. (a) t = 2s; (b) t = 4s; (c)t = 6s; (d) t = 8s; (e) t = 10s; (f) t = 12s [226].


shape, the microactuator is retracted and captures the thrombus, enabling the mechanical removal ofthe thrombus.

Leng et al. [225,226] have also investigated the infrared light-induced SMP actuator. An infraredlight with a working wave band of 2–4 lm is coupled to the fiber in an SMP to study the realistic actu-ating effect of infrared laser (see Fig. 51). In Fig. 110a (t = 2s), the bright position is located at the end ofthe fiber, which indicates that the infrared laser is mainly refracted at that position. In the subsequentsnap shots, the actuator gets brighter globally. The temperature of the SMP around the treated opticalfiber rises to 21.4 �C, with an increase in temperature of 2.4 �C compared to the ambient temperature.In contrast, the temperature of the SMP at the end of the optical fiber increases to 48 �C, with an in-crease in temperature of 29 �C. It indicates that more light is refracted from the end of the optical fiberthan from the side of the optical fiber. This experimental phenomenon can be explained by the factthat the optical fiber is treated by chemical corrosion. Therefore, after removing the cladding, the sur-face of the core is uneven. Laser energy absorbed at the treated position of the optical fiber is in a rel-atively small amount so the increase of temperature is slow. However, no laser is refracted back at theend of the fiber and a high temperature is engendered for more laser-energy absorption. The temper-ature at the treated position of the fiber is shown in Fig. 51. It exhibits that the SMP actuator deployspromptly in about 12 s.



5.4. MEMS and NEMS applications

In recent years, it has been reported that the shape-memory effect in SMPs is not only a macro-scopic phenomenon, but also occurs even on the 10s of nm scale in bulk SMPs [273]. As such, SMPshave great potential for micron and even submicron scale actuation for MEMS and NEMS applications,micro/nano patterning [274,275], biomedical device [276,277], etc.

Micron-sized thermo/moisture-responsive PU SMP/Ni powder chains were fabricated and theirshape recovery was investigated as an indirect way to demonstrate the SME in micron/submicron-sized SMPs. Vertical protrusive SMP/Ni chains were fabricated by applying a vertical magnetic fieldduring curing of SMP/Ni mixture. Fig. 52 presents the morphology of the array of flattened chains atopthe 2 vol.% Ni sample and the morphology after subsequent heating for shape recovery. The zoomed-inview of a small chain reveals that in the flattened shape, the initially vertical chain was bent around90�. However, after heating, the bending angle was largely recovered, which reveals the shape recov-ery phenomenon in the chain and indirectly demonstrates the SME in micron/submicron-sized SMP.The shape-memory effect in an array of such chains (like a micro brush) may provide a simple mech-anism and convenient approach to achieve reversible surface morphology for a significant change inmany surface related properties such as friction and wetting ability.

Other micro-sized protrusion arrays can be realized by laser heating [278]. During the fabricationprocess, a pre-compressed SMP is heated by a local laser beam. The working principle of forming aprotrusive bump atop the SMP by laser heating is illustrated in Fig. 53. First, a polished SMP sampleis pre-compressed. A laser beam is then shone atop its surface, and the protrusive SMP arrays are pro-duced. Fig. 54 reveals a typical bump atop an SMP sample, which is pre-compressed to a maximum

Fig. 52. Typical shape-memory effect in protrusive chains (2 vol.% Ni).

Fig. 53. Illustration of forming a protrusive bump atop an SMP using a laser: (a) original sample, (b) pre-compressed sample,and (c) sample after laser heating; a bump is formed [278].


Fig. 54. Optical profile and cross-section of a bump [278].

Fig. 55. Protrusion array atop a 20% pre-compressed sample at pulse energy of 2.28 mJ and shutter exposure time of 1 s. Thefocus of the microlens array (right top) is 795 lm. (Top) Optical microscopy profile. (Bottom) Taylor Hobson profile (3D andcross-sectional) [278].


strain of 50%, while Fig. 55 presents an array of protrusive bumps atop a 20% pre-compressed sample.It is significant that an array of bumps is formed [275].

5.5. Shape-memory polymer textiles and applications

Shape-memory fibers based on SMPs can be implemented to develop smart textiles that respond tothermal stimulus [14]. SMPs can be used for textiles, clothing and related products in the form ofshape-memory fibers, shape-memory yarns and shape-memory fabrics [279,280]. Hu et al.[14,37,99,101,102,109,281] at Hong Kong Polytechnic University have comprehensively investigatedshape-memory finishing chemicals and technologies for cotton fabrics, wool fabrics and garment fin-ishing. Fig. 56 shows a morphology of a shape-memory fiber detected by SEM. On the surface of a sin-gle filament of this shape-memory fiber, the grooves can be observed which are due to the traces after


Fig. 56. Scanning electron microscopy of shape-memory fibers. (a) View of multi-filament (500�); (b) view of an isolated singlefilament (4000�) [14].


the solution is extruded from the pinholes of the spinneret. The diameter of the single filament is mea-sured as about 16 lm. Furthermore, shape-memory finishing fabrics can be produced with coatingshape-memory emulsion or combining shape-memory films. When washed in hot water or dried attemperatures higher than Tg, their wrinkles will disappear and they can recover the original flat shape.It is also expected that the smart fibers may be used for novel fiber sensors or advanced medical usage[14].

5.6. Shape-memory polymer foams and applications

A technology called ‘‘cold hibernated elastic memory’’ (CHEM) based on SMPs received much atten-tion in recent years. This CHEM materials utilizes SMPs foam structure or sandwich structures made ofSMP foam cores and polymeric composite skins [282]. This SMP foam was proposed for space-boundstructural applications by Sokolowski of Jet Propulsion Laboratory, California [276,283–286]. This SMPfoam exhibits micron-size cells that are uniformly and evenly distributed within the cellular structureand, therefore, it is suitable for ultra-lightweight porous membrane or thin film space applications[287–289]. A study on the influence of long-term storage in cold hibernation on the recoverystrain–stress of a polyurethane SMP foam revealed its excellent stability [276,290]. As one of theseapplications, a spring-lock truss-element concept for large boom structures was proposed [289]. It in-volved a unique hybrid design of SMP foams and normal polymer composites. As shown in Fig. 57, thetruss-element consists of two carbon-fiber-reinforced polymer laminates separated by the SMP foams.

Fig. 57. Truss element based on SMP foam [289].


Fig. 58. A reversible attachment based on SMPs: (a) an alternative SMP based smart hook-and-loop attachment embodiment;(b) schematics of working process of the active hook-and-loop fastener [298].


In this way, the two material systems allow the truss-element to be stowed in a small volume andthen deployed without the use of complex mechanisms.

5.7. Automobile

SMPs have been used in automobile engineering, and many interesting products have been devel-oped. Some interesting applications of SMPs include seat assemblies, reconfigurable storage bins, en-ergy absorbing assemblies, tunable vehicle structures, hood assemblies, releasable fastener systems,airflow control devices, adaptive lens assemblies and morphable automotive body molding[247,291–297]. The reasons for using SMPs are due to their excellent advantages such as shape-mem-ory behavior, easy manufacturing, high deformed strain and low cost. That is why they have attracteda lot of attention in automobile engineering and have even been used to replace the traditional struc-tural materials, actuators or sensors.

As a typical example, SMPs are proposed to be used for the reversible attachments, as shown inFig. 58 [298]. In this embodiment (see Fig. 58a), one of the two surfaces to be engaged contains smarthooks, at least one portion of which is made from SMP materials. By actuating the hook and/or theloop, the on-demand remote engagement and disengagement of joints/attachments can be realized(see Fig. 58b). With a ‘‘memorized’’ hook shape, the release is effective and the pull-off force can be


Fig. 59. Self-healing system based on SMP [304].

Fig. 60. Flexural strength of epoxy composites before damage and after healing [304].


dramatically reduced by heating above the Tg. It can be used for a reversible lockdown system in thelockdown regions between the vehicle body and closure [295,299].

SMPs can also be used in an airflow control system to solve a long-time problem for automobiles.As we know, airflow over, under, around, and/or through a vehicle can affect many aspects of vehicleperformance, including vehicle drag, vehicle lift and down force, and cooling/heating exchange.Reduction to vehicle drag reduces the consumption of fuel. A vehicle airflow control system, whichcomprises an activation device made of SMP material, actively responses to the external activation sig-nal and alters the deflection angle accordingly [300–303]. Thus, the airflow is under control based onthe environmental changes.

5.8. Self-healing composite system

A healable composite system for use as primary load-bearing aircraft components has been devel-oped by Cornerstone Research Group (CRG), Inc. [304,305]. The composite system consists of piezo-electric structural health monitoring system and thermal activation systems based on SMPs (seeFig. 59). Upon damaging, the monitoring system will sense the location and magnitude of damage,send the corresponding signals to the controlling system, resistively heat the SMPs at the locationof damage, and finally the induced shape recovery of SMPs will heal the damage. As shown inFig. 60, the SMP composite system can recover 75–85% of flexural strength upon bending test.



5.9. Other applications

Referenced from [306], there is a list of potential applications for SMPs:

(I) SMPs packaging: packaging of thermal sensitive products; sensors/drug/food air deliverysystems.

(II) Deployable structures: thermally insulated deployable shelters, hubs; prefabricated walls/slabs;access denial barriers/walls; automatic disassembly of electronic products [307–310]; reusableshape-memory polymer mandrel [311].

(III) Recreation/sport products: tents and camping equipment; life jacket, floating wheels; waterand snow skis; surf and snow boards.

(IV) SMPs food equipment: dishes and meal containers; plates and coffee cups: hot/cold storage forfood.

(V) SMPs toys: toys that take the advantage of simple reversible compaction, deployment and rig-idization cycle: decoys with high fidelity features.

6. Concluding remarks and outlook

6.1. Concluding remarks

The present paper has systematically discussed various aspects of SMPs, including the fundamen-tals, fabrication, modeling and characterization of SMPs, SMP composites, stimulus methods ofmulti-functional SMPs, and the corresponding potential applications across a wide variety of fields.The following points can be concluded:

(I) The chemical architecture of SMPs consists of hard and soft segments. The hard segments deter-mine the original macroscopic shape and may be of physical (thermoplastic) or chemical (ther-mosetting) nature. The soft segments work as a reversible molecular switch and show a thermaltransition at temperature Ttrans. The temporary pre-deformed shapes are fixed below Ttrans,while they are freed towards the original shapes at or above Ttrans.

(II) The constitutive models to describe the thermomechanical properties of SMPs are mainly basedon viscoelasticity and the two-phase assumption based on phase transition. Exact models, espe-cially 2D and 3D models, need to be developed to describe and predict the shape-memory behav-ior of pure SMPs, particle-filled SMP composites, and fiber-reinforced SMP composites. Thefollowing parameters are proposed to be included in the future models: stress, strain, time,external stimulus parameters (e.g., temperature, electrical current intensity, strength of mag-netic field, some special chemical parameters, or even light intensity), and filler or reinforce-ment content.

(III) Particle-filled and fiber-reinforced SMP composites have been studied comprehensively todevelop some special functions or enhance the general mechanical properties for SMPs. Dueto the relatively poor mechanical properties and simple functions, fundamental research intoSMPs composites is highly demanded in order to meet the requirements of SMPs in variouspractical applications. Some novel fillers (e.g., carbon nanofibers, carbon nanotubes, nano clay,graphite selenium, graphene, or some other micro or nano materials) can result in high-performance SMPs with tailored properties for a particular application. In addition, orthotropicSMPs filled with particles may be proposed. SMPs with fillers (e.g., nickel particles or carbonnanotubes), if formed in special pattern inside a polymer matrix, can result in better performancealong certain directions than those with randomly distributed fillers.

(IV) SMPs can experience a large recoverable deformation upon various types of stimulus includ-ing heat, electricity, magnetism, light, moisture, or even some kind of chemical stimuli.However, the thermal-responsive SMPs are the most common ones, and most of the forms,like electricity, magnetism or even moisture, still intrinsically belong to thermal-responsivemethods. In addition to these actuation methods, hybrid actuations of SMPs may be developedin the future.



(V) SMPs and their composites receive much attention due to their potential applications inmany fields. It should be noted that the relatively poor mechanical properties are stillthe obstacle to the broad application of SMPs. Thus, the fabrication of SMP compositesmay be the main approach to overcoming this difficulty. In addition, insight into structuralperformance of SMPs, including structural fatigue, relaxation, creep and duration, need tobe obtained.

6.2. Key challenges and possible future directions

The advantages of SMPs include lower density, lower cost, easier processing, larger recoverablestrains, etc. However, some of the major drawbacks of SMPs are their relatively low modulus thatresults in heir lower recovery stress/forces, their long response time, their lower cycle life, andtheir weak material stability. SMPs show only one-way shape-memory effect, which is differentfrom that of SMAs. This is because most SMPs become much softer upon applying the correspond-ing stimuli. In order to obtain cyclic operation for SMPs, an automatic resetting system may bedeveloped [43].

Reliable thermomechanical properties of a SMP are required in the early design stage for engineers.Due to different processing techniques and procedures, the variation in properties (e.g., the shape fix-ity, shape recovery, switching and response temperature), recovery speed, and fill factor) provides us aconvenient way for tailoring a material for better performance, but in the mean time, brings the chal-lenge and/or uncertainty in design [312]. As such, first-hand experiments are most likely required toget reliable data for applications.

Considering the comparison between chemically cross-linked thermosets and physically cross-linked thermoplastics, it shows that the former SMPs are better candidate for realistic applications.The advantages of chemically cross-linked thermoset SMPs include excellent shape fixity and recoveryratio, higher transition temperature, better chemical and thermal stability.

Recently SMPs shows interest in making medical devices (see Section 5.3). One of the key chal-lenges in realizing SMP medical devices is the implementation of a safe and effective method of ther-mally actuating various SMP devices in vivo. One of the possible way is to load ferromagnetic particlesinto SMP to realize remote stimuli, namely by applying an alternating magnetic field to induce heating[313]. The drawback is that the required power is quite high but the efficiency is quite low.

Based on the previous results, the potential directions and applications of SMPs are proposed to bedeveloped in future research:

(I) Novel SMPs. Prospective SMPs may include: energy harvesting from solar energy (light-responsiveSMP), chemical energy (chemical-responsive SMP) or waste heat, SMPs with functionally gradedTg at different locations, two-way SMPs, and SMPs controlled in multi-step manners. Forinstance, the deformation of an SMP may be better controlled in a step-by-step manner thancurrent ones, which is achieved by having either two Tgs for different transitions (two-step)or functionally graded Tgs at different locations. Additionally, two-way SMPs may be developedto achieve cyclical deformation as is the case for shape-memory alloys.

(II) Stimulus of SMPs. Wireless and remote-controllable SMPs are proposed (magnetic-field actuatedSMP composites filled with ferroelectrics, electric-field actuated SMP composites filled withpizeoelectrics, intrinsically light-induced SMPs). High actuation force and fast actuation speedin the new SMPs are necessary. In thermo-responsive SMPs, a narrow transition temperaturerange, down to only a few degrees, could dramatically increase the actuation speed. Alternativestimuli, e.g., light (in particular, visible light), sound, chemicals or molecular stimuli, for actua-tion may be developed. Chemical-responsive SMPs can be designed to detect the change in envi-ronmental conditions and even make reactions automatically.

(III) Multi-functional SMPs. The following directions are proposed: self-healing or self-repairing com-posite systems, automatic chemical sensing and water cleaning after harmful chemicals basedSMPs. In addition to the shape-memory effect, built-in temperature sensors have been inte-grated into SMPs for temperature monitoring. SMPs with other types of sensing capabilitiesshould be very useful.



(IV) Applications of SMPs in biomedicine. Potential research interests may include: polymer vascularstents with shape-memory polymers as the drug delivery system, smart surgical suture, synthe-sis of protein-polymer conjugates for therapeutic, laser-activated SMP microactuators toremove clots in a blood vessel, implants for minimally invasive surgery, SMPs with excellentbiocompatibility and/or bio-degradability with adjustable degradation rates for intelligent med-ical devices.

(V) Other applications of SMPs. The following applications are prospective: space deployable struc-tures, such as for hinges, trusses, mirrors and reflectors, SMP skins used for morphing aircraftwings, automobile actuators such as hood/seat assemblies and tunable automotive brackets,smart textiles and fabrics, self-disassembling mobile phones, and shape-memory toys.

As a relatively new type of shape-memory material, the current development and application ofSMPs seemingly lag behind other smart materials. However, given their advantages and multi-func-tional nature, SMPs and their composites are expected to become one of the leading roles in the fieldof smart materials in near future.

Acknowledgements

The authors would like to express their hearty appreciation for the work of Dr. Weilong Yin, post-doctor Dr. Bo Zhou, Dr. Mr. Haibao Lu, Dr. Dawei Zhang, Dr. Xuelian Wu, and Mr. Kai Yu, as some partsof this paper are based on their research results. They are also very grateful to all of their colleagues fortheir support in the Smart Materials and Structures Group, Center for Composite Materials, HarbinInstitute of Technology, China.

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[313] Citable URI: http://hdl.handle.net/1721.1/35767.

http://hdl.handle.net/1721.1/35767

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