Progress in Polymer Science

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Progress in Polymer Science 49–50 (2015) 3–33

Contents lists available at ScienceDirect

Progress in Polymer Science

journa l h om epa ge: www.elsev ier .com/ locate /ppolysc i

ditorial/Preface

hape memory polymers: Past, present and futureevelopments

artin D. Hagera,b, Stefan Bodea,b, Christine Webera,b, Ulrich S. Schuberta,b,∗

Laboratory for Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena,ermanyJena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany

r t i c l e i n f o

rticle history:vailable online 14 April 2015

a b s t r a c t

Shape memory polymers (SMPs) represent a highly interesting class of materials. As onerepresentative of the intelligent polymeric systems, these materials gained significant inter-est in recent years. Consequently, the variety of materials investigated virtually exploded

eywords:hape memory polymerstimuli responsive polymershape changing materials

and several promising shape memory effects have been developed. The present review willprovide a short overview on the history of SMPs as well as the current developments andconcepts for shape memory polymers. Additionally, future developments in this field willbe discussed.

wo-way shape memory effect© 2015 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43. History of shape memory polymers (SMPs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64. Design of shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1. General design principles of shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.2. Shape memory polymers based on a melting transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.3. Shape memory polymers based on a glass transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.4. Triple and multiple shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.5. Shape memory polymers based on reversible bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.5.1. Covalent bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.5.2. Supramolecular interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.6. Photo-induced shape memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.7. Other stimuli for shape memory polymers . . . . . . . . . . . . . . . . .4.8. Reversible shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . .

Abbreviations: ADMET, acyclic diene metathesis; EOC, ethylene-1-octene coensity polyethylene; IPN, interpenetrating network; OCL, oligo-�-caprolactone;

olyetheretherketone; PIPS, polymerization-induced phase separation; PP, polypME, shape memory effect; SMP, shape memory polymer; SWNT, single wall caeight polyethylene.∗ Corresponding author at: Laboratory for Organic and Macromolecular Chem

ena, Germany.E-mail address: [email protected] (U.S. Schubert).

http://dx.doi.org/10.1016/j.progpolymsci.2015.04.002079-6700/© 2015 Elsevier Ltd. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

polymer; LC, liquid crystal; LDPE, low density polyethylene; HDPE, highPA, polyamide; PCL, poly-�-caprolactone; PEO, polyethylene oxide; PEEK,ropylene; PS, polystyrene; SMASH, shape memory assisted self-healing;rbon nanotube; RT, room temperature; UHMWPE, ultra high molecular

istry (IOMC), Friedrich Schiller University Jena, Humboldtstr. 10, 07743

dx.doi.org/10.1016/j.progpolymsci.2015.04.002

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4 M.D. Hager et al. / Progress in Polymer Science 49–50 (2015) 3–33

5. Applications of shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.1. Industrial applications of shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.2. Biomedical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215.3. Self-healing materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.4. Shape memory polymers for aerospace applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6. Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24But where will the journey of SMPs take us in the long term? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1. Introduction

Due to the outstanding properties of natural materialschemists and material scientists often follow the princi-ples that nature has developed over millions of years forthe design of new synthetic materials (“bioinspired mate-rials”). In this context, one of the most inspiring features isthe ability to change a specific property upon an externaltrigger [1]. Hereby nature developed different mechanismsresulting in stimuli-responsive materials, which featureresponses within seconds to minutes up to years. Havingdeveloped perfectly designed structures that are able toexploit these mechanisms, natural materials (and therebythe living organisms) are able to adopt to the conditionsof their surrounding environment. The variety of the prop-erties that can be changed upon application of an externalstimulus is enormous, as is the nature of the applied stim-ulus as well. Variation of mechanical properties is onefeature, which for example can be found in sea cucum-bers. These animals are able to change the stiffness of theirskin by several orders of magnitude in case of danger (i.e.an attack by a predator) [2]. Several animals (e.g., octopus,squid, chameleon) can adopt their color, i.e. their appear-ance, according to their need (i.e. for camouflage, to warnpredators or conspecifics as well as to impress the othersex). In contrast to these fast processes, the growth of plantsis influenced by external influences in the timeframe ofyears (e.g., wind-bend trees or plants growing toward thelight) [3]. In addition to these macroscopic changes, naturalmaterials show a stimuli-responsiveness on the level of sin-gle molecules. For instance, proteins adopt different shapesdepending on the conditions used (e.g., temperature, pH-value, salt concentration, etc.). These few examples alreadydemonstrate impressively the unique properties of thesematerials developed by evolution.

Simplifying nature’s inspiring examples, many stimuli-responsive polymer systems have been developed byresearchers over the last decades [4–8]. The variety ofdifferent stimuli, which can be applied, cover a widerange (see Fig. 1). In response, the polymeric material willfeature dimensional changes. One specific kind of stimuli-responsive materials are shape-memory materials. Thesematerials respond with a change of their shape toward aspecific stimulus (mainly temperature), i.e. they will trans-form from a temporary shape to a permanent shape. Theinteresting feature of this behavior is that the original shape

materials are rather scarce. For instance, spider draglines,which are already known for their outstanding strengthand ductility, show a shape memory effect (SME) [9].

Shape memory polymers (SMP) – as well as their inor-ganic counterparts – received great attention throughoutthe last years, which has given rise to many interest-ing developments in both industry as well as academia[10–19]. The present review will focus on the past devel-opments of SMPs. Moreover, the different underlyingchemistries of SMPs will be discussed in detail. The lastparts of the review will focus on different applications ofSMPs as well as potential future developments in this inter-esting field.

2. Definitions

Shape memory polymers are considered as polymerswhich are able to “memorize” a permanent shape and thatcan be manipulated in a way that a certain temporary shapewill be “fixed” under appropriate conditions. Subsequently,a trigger (e.g., heat or light) will lead to a transformation ofthe temporary shape to the memorized permanent shape(see Fig. 2) [13].

As demonstrated for a SMP that responds to tempera-ture in Fig. 2, the construction of any kind of device shapeSMPs includes an initial processing step (such as extrusion,spinning, pressing, etc.), which is needed to determine thepermanent shape above the “switch temperature” TSW. Thesubsequent programming of the temporary shape requiresexternal mechanical force, which is applied to the materialbelow TSW. In case the formed device is exposed to temper-atures above TSW during usage, it will transform back to itspermanent shape. In principle, this cycle of programmingand restoration of the original shape can be repeated manytimes (see below).

Considering the application of SMP devices in real-life, one major drawback of the classical simple SME asdescribed above is the missing reversibility of the shapechange (“one way SME”). Once the permanent shape isrecovered, a new programming step is required to rebuildthe temporary shape (see Fig. 3). The fact that this stepwould have to be performed by the customer (the user)of the SMP device alone has often hampered the transferof fascinating materials into commercial products. A moresuitable shape memory device would be able to change itsshape reversibly many times without the necessity to apply

is “memorized” by the material and is reformed after defor-mation without additional mechanical efforts. Despitethe large variety of different stimuli-responsive naturalmaterials, the examples for natural shape memory

an external re-shaping process by the user. This means, thetemporary shape would have to reform by itself as soon asthe stimulus, which triggers the shaping back to the per-manent phase, is terminated (“two way SME”). Despite the

M.D. Hager et al. / Progress in Polymer Science 49–50 (2015) 3–33 5

Fig. 1. Schematic representation of dimensional changes of polymeric systems upon different stimuli and their resulting response.Ref. [8], Copyright 2010. Reproduced with permission from Elsevier Science Ltd.

Fig. 2. Schematic representation of the basic principle of the shape-memory effect in polymers.

Fig. 3. Basic definitions involving the terms shape memory effect (SME). Left: One-way and two-way SME. Right: Dual and triple SME. Each transition canrepresent a one-way or two-way SME, respectively. If more than three shapes can be realized, the nomenclature proceeds accordingly (quadruple, etc.).

Polyme
6 M.D. Hager et al. / Progress in
enormous increase in knowledge about the design princi-ples for SMPs (see below) systems revealing two-way SMEare still rare (see 4.8).

The SME as described so far is a dual SME, i.e. one tem-porary shape is transformed into the permanent shape. Incontrast, triple shape memory polymers feature two tem-porary shapes (A and B in Fig. 3) besides their permanentone (C in Fig. 3) [20]. First, the temporary shape B has tobe programmed, followed by the programming of the tem-porary shape A. The appropriate stimulus will lead to thetransformation from the second temporary shape to thefirst one (A → B). Subsequently, a second trigger starts theregeneration of the permanent shape C. Basically each ofthese transitions could be based on a one- or two-way SME,as indicated by the dotted arrows in Fig. 3 (right). In princi-ple, also multiple (n) SMPs can be designed, featuring onepermanent shape and (n − 1) temporary shapes [10]. Thedesign principles of such materials will be elucidated inSection 4.4.

3. History of shape memory polymers (SMPs)

The research on SMPs has developed to an interestingand vital research field in the last two decades. However,the development (or fortuitous discovery) of SMPs hasstarted rather early. Going back to the 1940s, Vernon et al.described the discovery of “shape memory” in an US patenton dental materials (methacrylic ester resin) [21]. The nextimportant milestone in the development of SMPs was theutilization of heat shrinkable polyethylenes (PE) (e.g., filmsof tubings) in the 1960s [22,23]. Again 20 years later, thefield, like we know it today, began to grow (in particular,in Japan and the USA). The application driven industrialresearch was now complemented by academic research,which started to investigate the underlying mechanismsand the design principles of these materials.

4. Design of shape memory polymers

4.1. General design principles of shape memory polymers

Generally, shape memory polymers possess at least twodifferent “phases”: A stable network and a second phase,which can be influenced by the external trigger. The formerphase stabilizes the whole SMP and is responsible to retainthe original shape, i.e. the deformation of this phase is thedriving force for the shape recovery. This stable phase canbe achieved by the introduction of chemical crosslinks,crystalline phases or interpenetrating networks (see Fig. 4)[10–15,24]. The second phase temporarily fixes the tem-porary shape by crystallization (i.e. a melting transitionwill lead to the shape recovery), a glass transition, a transi-tion between different liquid crystalline phases, reversiblecovalent or non-covalent bonds (e.g., photodimerizationof coumarine, Diels–Alder reactions, supramolecular inter-actions). Besides, other stimuli-responsive (e.g., switching
by redox reactions) domains capable of a segmentalrearrangement can be applied as well.
The most important transitions, which are appliedfor SMPs are the two thermal transitions of a distinct

r Science 49–50 (2015) 3–33

block/component of the SMP: The melting temperatureand the glass transition temperature, respectively (seeFig. 5). The melting transition can be utilized in chemicallycrosslinked rubbers, in semicrystalline polymeric networksas well as in physically crosslinked polymers. The latter rep-resent (multi)block copolymers with a low melting phase,which is responsible for the switching, and a high meltingphase, which constitutes the permanent network. In a simi-lar fashion, the glass transition can be utilized in chemicallycrosslinked thermosets as well as physically crosslinkedthermoplastics.

The following sections will provide a detailed overviewon the engineering of different classes of SMPs, whichwere all designed according to the above described criteria.For detailed information on SMP composites the reader isreferred to recent reviews [24,25].

4.2. Shape memory polymers based on a meltingtransition

One possibility for the fixation of the second phase isits crystallization. The melting of this phase will lead tothe shape recovery of the SMP. Often based on crosslinkedsemicrystalline networks or (multi)block copolymer sys-tems these materials feature a higher stiffness than otherSMP materials as well as a fast shape recovery [13].Most investigated materials of TM-type SMPs are based onpolyolefins (including the prime example polyethylene inheatshrinkable films), polyethers, or polyesters (in partic-ular poly(�-caprolactone)). All these “soft phases” reveal alow melting temperature, which still allows the existenceof a crystalline “hard phase” at the elevated switching tem-perature, which is particularly important for multiblockstructures. The crosslinking (by peroxides) of low densityPE (LDPE) results in polymeric materials, which feature aheat induced shrinkage [26]. The temperature, which isrequired to induce the shrinkage could be tuned by thestretch temperature. Higher stretch temperatures resultedin higher temperatures for the shrinkage. Interestingly, asimilar behavior could also be observed in an ethylene-1-octene copolymer, which shows only short chain branches[27]. The observed switching temperature could be tunedin a wide range (60–100 ◦C) depending on the degree ofbranching as well as the crosslinking density. Moreover,also ultra-high molecular weight PE (UHMWPE) was uti-lized as SMP. Fibers based on this material showed aswitching temperature of ca. 150 ◦C [28]. Polyolefins (e.g.,PE) are also interesting candidates for triple shape memorypolymers (vide infra) [29]. Natural rubber was also utilizedfrequently for the design of SMPs. Hereby two differentconcepts have been developed. The first concept is basedon the strain induced crystallization in weakly crosslinked(0.2%) natural rubber [30–32]. These systems can not onlybe triggered by temperature but also by transversal stress[31] and solvent vapors [33], respectively. The other strat-egy is based on crosslinked natural rubber with additives(e.g., fatty acids). A blend of natural rubber and stearic acid
shows an SME based on the melting/crystallization of thefatty acid [34].
Polyethers are another polymer class, which featuretypically low melting temperatures, which makes them


F rmines

pFsttda[w[sr

oPppitthmPcbbatosnbtoe–mpeo

ig. 4. General structure of SMPs consisting of a “hard phase”, which dete

romising materials for the switching block of SMPs.or instance, polyethyleneoxide (PEO) was utilized inegmented multiblock copolymers with poly(ethyleneerephthalate) (PET) hard blocks [35]. A higher content ofhe soft block resulted in higher switching temperaturesue to the better crystallization of the soft block. Addition-lly, PEO was applied as soft block in polyurethanes (PU)36,37,53], crosslinked polymethacrylate networks [38] asell as poly(p-dioxanone)-poly(ethylene glycol) networks

39]. Moreover, interpenetrating networks of linear andtar-shaped PEO with methylmethacrylate have been fab-icated as shape memory polymeric systems [40].

Low melting aliphatic polyesters are probably the mostften utilized soft block (in particular poly(�-caprolactone),CL). These soft blocks have also been applied inolyurethane systems [41–44]. Moreover, polyamides andolyaramides, respectively, can also be used as hard block

n combination with PCL [45,46]. Lower molar masses ofhe polyester results in lower switching temperatures, sohat switching temperatures (40 ◦C) in the region of theuman body temperature could be achieved [47]. Instead ofultiblock systems featuring a hard block also crosslinked

CL was applied as SMP [48]. Additionally, polymer blendsontaining PCL were investigated in the context of SMPs. Alend of the triblock copolymer poly(styrene-b-butadiene--styrene) (SBS) and PCL showed a SME with good recoverynd fixity between 30 and 70% PCL. The prerequisite forhis phenomenon is the formation of a continuous phasef the PCL [49]. Alternatively, oligo(�-caprolactone) (OCL)egments have been introduced into methacrylate basedetworks in order to obtain a SME (see Fig. 6) [50]. Theiodegradability of the soft PCL blocks opens the possibilityo design SMPs for biological applications (see also sectionn biomedical applications) [51,52]. The above selectedxamples – without claiming to be exhaustive (see Table 1)

showed that the most crucible part of a SMP based on the
elting transition as switching element is the soft block. In
articular the PCL based polymers revealed that the influ-nce of the hard block on the switching temperature isften negligible.

the shape and a “soft phase”, which can be triggered by external stimuli.

4.3. Shape memory polymers based on a glass transition

The above described TM-based SMPs typically featurea glass transition far below room temperature, preventingan application of the Tg as the switch temperature for theSME. Polymeric materials with a Tg above room tempera-ture can be utilized for Tg-based SMPs. Within this contexta large variety of different materials have been investi-gated. In comparison to the TM-based SMPs, the Tg-basedSMPs reveal a slower shape recovery due to the broad glasstransition (representing a second order phase transition)[10]. Consequently, these SMPs are not ideal for applica-tions where a sudden shape recovery is required. But onthe other hand, the slow recovery makes them interestingcandidates certain for biomedical applications [10].

Natural rubber as a SMP material was already exten-sively discussed as TM-based SMP. However, a differentcrosslinking chemistry as well as a higher density results ina changed behavior. Epoxidized natural rubber was curedwith zinc diacrylate in an oxa-Michael addition reaction[54]. An increasing crosslinking density resulted in higherglass transition temperatures and, therefore, also higherswitching temperatures. Epoxy – as a classical thermoset– were also utilized as SMPs (for a recent summary onshape memory epoxies the reader is referred to Ref. [55]).Epoxy based on oligo(bisphenol A) diglycidyl ether and Jeffamines showed a SME [56]; the glass transition could betuned by the thermoset composition between 31 and 93 ◦C.A higher crosslinking density resulted in a faster shaperecovery of these materials.

PUs containing a soft block with a glass transitioninstead of a semicrystalline soft bock (vide supra) canalso feature a SME. In contrast to the TM-based shapememory PUs, these systems are transparent and are moreprone to (bio)degradation due to the missing crystallinityof the switch segment. Lendlein and coworkers designed
copolyester-polyurethane SMPs, which are based onstar-shaped oligo[(rac-lactide)-co-glycolide] soft segments[57]. The glass transition could be tuned between 50 and65 ◦C. Moreover, specimen with complex forms based on


d area inty of Ch

Fig. 5. TM and Tg based SMPs (the shadeRef. [13], Copyright 20017. Adapted with permission from the Royal Socie

these materials showed a fast shape recovery (300 s) at70 ◦C. PUs with a thermoplastic elastomeric and shapememory behavior could be obtained by the polymeriza-tion of 1,3-butanediol and hexamethylene diisocyanatefor the soft block as well as 4,4′-bis-(6-hydroxy hex-oxy)biphenyl and tolylene 2,4-diisocyanate for the hardblock [58]. The glass transition temperature of the softsegment was around 30 ◦C. The utilization of crosslinkedpolyester type PUs enabled a tuning of the switching tem-perature between 0 and 60 ◦C [59]. Classical thermoplasticelastomers like PUs feature an excellent processability.However, the missing chemical crosslinks lead to inferiorproperties compared with covalently crosslinked poly-
mers. Latter materials show higher recovery stresses andhigher cyclic recoverable strains [17]. Therefore, crosslink-able PU systems have been investigated [60,61]. Thecrosslinking was achieved by the introduction of double
dicates the switching area of the SMP).emistry.

bond containing monomers and subsequent electron beamtreatment. The linear polymers could be processed likeclassical PUs, e.g., a stent was formed, which could becrosslinked subsequently using an e-beam.

Composites of polyesters based on polyols and sebacicacid with cellulose nanocrystals showed a SME based on thelow glass transition of the polyester (switching between 15and 45 ◦C) [62]. A higher degree of crosslinking was benefi-cial for both, the mechanical properties as well as the SME.High recovery and fixity (close to 100%) could be obtained.

Most (meth)acrylate-based covalently crosslinkedshape memory polymer networks are based on the glasstransition of the soft phase [63–66]. These materials are
also interesting for bioapplications due to their biocompat-ibility [64,65] or biodegradability [66]. The methacrylatenetwork structure was also utilized in shape memoryhydrogels [67].


late netwR Sons L

tSfi

TO

Fig. 6. Schematic representation of the SME effect of OCL–methacryef. [50], Copyright 2005. Reproduced with permission from John Wiley &

Most of the investigated Tg-type SMPs reveal transition
emperatures below 100 ◦C. Examples for high temperatureMPs are very scarce. A low crosslink density polyimideeatured switching temperatures at 220 ◦C [68,69]. Poly-mides with <1% crosslinks showed very fast recovery
able 1verview on selected examples of TM-type SMPs.

Material “Hard phase” “Soft phase”

LDPE Crosslinks LDPE

Ethylene-1-octene copolymers Crosslinks Ethylene-1-octenecopolymers

UHMWPE Entanglements,physicalcrosslinks

PE

Natural rubber Crosslinks Polyisoprene(strain inducedcrystals)

Natural rubber Crosslinks Polyisoprene/stearacid

PEO–PET multiblock copolymer PET PEO

PEO–PU PU PEO

PEO–PU PU PEO

PU PU Castor oilderived polyol

PMMA–PEG network PMMA network PEO

PPDO–PEG networks PPDO PEG

PMMA–PEG IPN PMMA PEG

PCL–PU PU PCL

PCL–PU PU PCL

PCL–PA PA PCL

Radiation crosslinked PCL Crosslinks PCL

PCL–SBS blend SBS PCL

PCL–methacrylate network Network OCL

PCL–PIBMD PIBMD PCL

orks (left); fast shape recovery of the network within 10 s at 50 ◦C.td.

within few seconds and shape fixity and recovery ratios
>99%. Due to their high switching temperature thesepolymeric materials have a very high room tempera-ture E-modulus (2000 GPa) combined with an excellentcreep resistance. A comparable behavior was observed
Switchingtemperature, TSW

Comments Reference

60–100 ◦C Shrinkage of polymer [26]60–100 ◦C [27]

145 ◦C Fibers [28]

0–45 ◦C Cold programming,stress induced SME

[31]

ic 75 ◦C 100% fixity and 95%recovery

[34]

45–55 ◦C TSW adjustable bypolymer composition

[35]

60 ◦C [53]60 ◦C [36]60 ◦C [44]

76 ◦C PEO star [38]70 ◦C [39]50 ◦C Interpenetrating

network[40]

65 ◦C [41,42]43–60 ◦C Switching tunable by

molar mass of PCL[47]

65 ◦C [45]55 ◦C [48]55 ◦C SME strongly

depending onmorphology

[49]

30–50 ◦C Switching temperatureadjustable by molarmass of OCL

[50]

60 ◦C Biodegradable [51]


Table 2Overview of selected examples of Tg-type SMPs.

Material “Hard phase” “Soft phase” Switchingtemperature, TSW

Comments Reference

Crosslinked natural rubber Crosslinks Polyisoprene ≈30–50 ◦C Also silicananocomposites

[54]

Epoxy Epoxy Jeff amine 31–93 ◦C [56]Polyurethane PU Copolyester 70 ◦C Biodegradable [57]Polyurethane Crystalline PU Amorphous PU 30 ◦C [58]Polyurethane PU Crosslinked

polyester0–60 ◦C [59]

Polyurethane Crosslinks PU 55 ◦C Crosslinked by e-beam [60,61]Polyester Nanocrystals Polyester 45 ◦C Nanocomposite [62]Polymethacrylate network Crosslinks Low Tg

methacrylaten.d. [63]

Polymethacrylate network Crosslinks PEGDMA 56–92 ◦C (Tg) Biocompatible [64]Polymethacrylate network Crosslinks PEGDA Below RT Biocompatible [65]Polymethacrylate network Crosslinks Polyester 70 ◦C Biodegradable [60]Polymethacrylate network Crosslinks Low Tg

methacrylate65 ◦C Hydrogel [67]

Polyimide Crosslinks Polyimide 220 ◦C Also SWNT compositesRT modulus: 2000 GPa

[68]

Polyaspartimide-urea Crosslinks Polyaspartimide-urea

150 ◦C [64]

PEEK Crystallinedomains

Amorphousdomains

>180 ◦C [71]

Sulfonated PEEK Ionic clusters Amorphousdomains

200 ◦C Also triple shapememory

[72]

Cyanate-based SMP Crosslinks Amorphousdomains

for polyaspartimide-urea [70]. The important engineer-ing polymer polyetheretherketone (PEEK) can be modifiedto reveal shape memory properties as well [71]. In con-trast to standard PEEK-based heat shrinkable tubing, whichrequires temperatures of around 340 ◦C to induce the SME,the authors could utilize the Tg of the material for the shaperecovery due to the programming of the PEEK. The intro-duction of ionic moieties into PEEK (by sulfonation) leadsto a SME at ca. 200 ◦C (depending on the correspondingmetal counter ions) [72]. Moreover, thermosetting shapememory cyanate polymers, which show a glass transitiontemperature above 250 ◦C, have been investigated [73].These materials featured a high thermal stability as well asexcellent fixity and recovery. Particularly these examplesof the high temperature SMPs show that the field, whichwas originally strongly driven by bio-related applications(i.e. low switching temperatures), starts to grow in fur-ther directions (see also applications of SMPs for aerospaceapplications) (Table 2).

4.4. Triple and multiple shape memory polymers

In contrast to the examples described above, triple SMPsfeature two temporary states (A and A′) as well as the per-manent shape [20,74]. The polymer will go from shape A′ toshape A induced by one trigger, whereas a second triggerinduces the change back to the original shape. The con-cept can also be extended to multiple shapes. There aretwo principal strategies for the design of triple SMPs: (a)
a very broad thermal transition, particularly a glass transi-tion temperature and (b) multiphase design, i.e. each phaseprovides a separate transition, which is responsible for onepart of the SME. In particular the latter strategy is rather
290 ◦C [73]

complicated due to the required complex design of thenew polymeric material. However, a better control over theSME and the required switching temperatures is possible.Since the former strategy requires “only” a broad glass tran-sition temperature also commercial polymers have beeninvestigated in this context. Even a perfluorosulphonic acidionomer (PFSA, i.e. Nafion) is capable of a SME [75–77]. Thetriple or quadruple SME is based on a broad glass transitiontemperature (see Fig. 7).

The approach of a broader Tg was also extended to inter-penetrating networks (IPNs) of poly(methyl methacrylate(PMMA) and PEO [78]. In combination with the meltingof the PEO at 35 ◦C and three transitions of the semi-IPN a quintuple SME could be achieved. Noteworthy, theSME could be tuned from a dual SME up to a quadrupleSME with three distinct switching temperatures, while stillshowing an excellent shape recovery. Although the fix-ity and recovery was quite low for the single steps (ca.60%) this approach is still remarkable (to the best of ourknowledge this approach represents the highest numberof temporary states realized so far). Additionally, a mul-tiple SME can also be achieved by tuning the local Tg

of a polymer film. A reversible Diels–Alder based poly-mer (poly(2,5-furandimethylenesuccinate-PFS crosslinkedwith a bismaleimide) was treated with an additionalcrosslinker in different concentrations [79]. By this man-ner, the local Tg could be adjusted between 16 and 50 ◦Cenabling the multiple SME of the polymer film.

The second – more sophisticated approach – is the
design of multiphase systems for multiple SMPs. Polymericnetworks featuring physical and chemical crosslinks havebeen utilized as triple SMPs. Lendlein and coworkers inves-tigated polymeric networks consisting of PCL segments and


Fig. 7. Quadruple-shape memory properties of PFSA. (a) Visual demonstration. S0: permanent shape; S1: first temporary shape (Td1: 140 ◦C); S2: secondtemporary shape (Td2: 107 ◦C); S3: third temporary shape (Td3: 68 ◦C); S2rec: recovered second temporary shape (Tr1: 68 ◦C); S1rec: recovered first tem-p C). (b) QTR ishing G

pr(locAspmoso

sgItp[pa

betn

orary shape (Tr2: 107 ◦C); S0rec: recovered permanent shape (Tr3: 140 ◦

d3 = Tr1 = 53 ◦C).ef. [75], Copyright 2010. Reproduced with permission from Nature Publ

oly(cyclohexyl methacrylate) (PCHMA) or PEO segments,espectively [80]. Both systems (former: PCHMA-PCLMACL); latter: PEO-PCL (CLEG)) were crosslinked cova-ently. The SME in the MACL system is based on the meltingf the PCL and the glass transition of the PCHMA. Inontrast, the CLEG system is based on two TMs (Fig. 8).djustment of the molar mass of the correspondingegments could be utilized to tune the transition tem-eratures. Noteworthy, the presence of the multiphaseorphology did not always result in a triple shape mem-

ry effect. CLEG networks with a high PEO content (>70%)howed two distinct transition temperatures; however,nly a dual SME could be observed.

The programming of these systems was based on a twotep procedure. For instance MACL networks were pro-rammed at a Thigh = 150 ◦C, Tmid = 70 ◦C and Tlow = –10 ◦C.n continuation of their work, Lendlein et al. could showhat these networks can also be programmed in a one-steprocedure (Thigh to Tlow) while still showing a triple SME81]. Moreover, a polyester network (urethane crosslinkedoly(�-caprolactone) and poly(�-pentadecalone)) couldlso be programmed in a one-step process [82].

The three systems by Lendlein and coworkers are
ased on one polymeric network. In contrast, Bowmant al. designed two interpenetrating networks based onhe selective thiol–Michael addition reaction [83]. Bothetworks (mercaptoacetate-vinyl sulfone as well as the
uantitative thermal mechanical cycle (Td1 = Tr3 = 140 ◦C, Td2 = Tr2 = 90 ◦C,

roup.

less reactive mercaptopropionate-acrylate system) featureseparate Tg’s (10 and 60 ◦C). The switching between thetwo temporary shapes and the permanent shape could berealized at 23 and 45 ◦C, respectively. Due to the pres-ence of two different networks, these systems were capableof holding their temporary shape at intermediate tem-peratures for an extended period of time. The polymernetwork approach was followed in a similar fashion usinga range of other polymeric materials, such as PCL/SM-PU[84], isocyanate polydopamine-graft-poly(�-caprolactone)[85], PMMA networks containing star-shaped PEO [40],maleated-PS-b-poly(ethylene-co-butylene)-b-PS [86] andpoly(vinyl alcohol)-graft-polyurethane [87].

Polymer composites contain different phases, which canbe utilized independently to generate multiple SMPs. Forinstance, a dual SMP can be combined with shape mem-ory fibers in order to obtain a triple SMP. In this context, aSMP epoxy matrix was reinforced by PCL fibers [88]. Thesecomposites possess two thermal transitions: The Tg of thematrix and the Tm of the PCL fiber, which will determinethe triple shape memory behavior.

The polymerization-induced phase separation (PIPS) isanother possible strategy to induce a multiphase mor-
phology within polymeric materials [89,90]. Mather andcoworkers utilized the PIPS to fabricate triple shape mem-ory polymer composites based on epoxy and PCL [91]. Twosystems based on PCL and PEO epoxy or poly(propylene


Fig. 8. (a) Schematic representation of the CLEG polymer network (red: PCL segments; blue: PEG side chains; gray: crosslinks); (b) elongation-temperaturediagram showing the recovery of shapes B and C in cyclic, thermomechanical experiments (third cycle) for CL(40)EG; (c and d) series of photographsillustrating the triple-shape effect of CL(50)EG (heating to 40 ◦C and 60 ◦C).

l Academ
Ref. [80], Copyright 2006. Reproduced with permission from the Nationalegend, the reader is referred to the web version of the article.)
glycol) (PPG) epoxy have been investigated. The former sys-tems showed two distinct melting temperatures; the latterone Tm (PCL) and one Tg (PPG). Particularly the PPG basedpolymer blend revealed a good fixity and recovery (>90%).

In general, block copolymers offer another possibility togenerate a nanophase separated polymer morphology [92],which can provide the two thermal transitions requiredfor the triple SME. Following this design, Gao et al. synthe-sized a gradient V-shape polymer (in analogy to a triblockcopolymer) [93]. The gradient in the composition causeda gradual shifting of the Tg resulting in a broad glasstransition temperature range of 20–103 ◦C. Consequently,these materials could be programmed for a quadrupleSME.

The structure of a classic bimetal strip was also trans-ferred to triple shape memory polymer composites [94].Both layers of the strip consisted of two different shapememory epoxies with a high and a low glass transitiontemperature, respectively. Both epoxies showed excellentshape fixity and recovery (close to 100%) as single layers.The properties of the bilayer system could be tuned by theratio of both layers. In the best case, recovery rates of >90%and fixities of 96% and 83%, respectively, could be obtained.

Similar to the “classical” SMP materials, polyolefines canalso be used for the fabrication of triple SMPs. A crosslinkedblend of polyethylene and polypropylene showed two tran-
sition temperatures, which originate from the two separatecomponents [29]. A similar behavior could be obtained forblends of HDPE, LDPE and ethylene-1-octene copolymerblends [95,96].
y of Sciences. (For interpretation of the references to color in this figure

The above described examples (see also Table 3) revealthe strong effort in recent years to design new triple andmultiple shape memory polymers. Several design princi-ples have been explored allowing the fabrication of a largevariety of different polymeric materials. Further devel-opments in this field will address other multiple SMPs,most probably increasing the number of possible structures(i.e. going beyond a quintuple SME). Moreover, the one-step programming of triple (and multiple) SMPs will gainincreasing importance, particularly, considering potentialapplications of these systems.

4.5. Shape memory polymers based on reversible bonds

The phase, which fixes the temporary shape, can also bebased on reversible bonds. For this purpose, several exam-ples have been studied so far, which mainly rely on theapplication of reversible covalent bonds and supramolecu-lar interactions [16].

4.5.1. Covalent bondsThe number of utilized reversible covalent interactions

for the design of shape memory polymers is limited. Inparticular, the Diels–Alder reaction and its correspondingretro-Diels–Alder-reaction are well suitable. This concepthas been utilized in polymer science for many different
applications, e.g., for self-healing materials [97,98]. SinceSMPs have, to some extent, similar structural prerequi-sites, the reversibility of the systems has been exploitedto create SMPs as well (Table 4). Thus, Yamashiro et al.


Table 3Overview of selected examples of triple and multiple SMPs.

Material “Hard phase” “Soft phase” Switchingtemperature, TSW

Comments Reference

Nafion Physicalcrosslinks

Amorphous phase 53 ◦C140 ◦C

Triple andquadruple SME

[75]

PMMA/PEG Physicalcrosslinks

Amorphous phase 35 ◦C (Tm of PEG)50, 70, 110 ◦C

Interpenetratingnetwork. Up toquintuple SME

[78]

PFS Crosslinks PFS 25–65 ◦C Multiple SME bylocal Tg tuning

[79]

PCL/PCHMA Crosslinks PCLTM, PCHMA (Tg) 70 ◦C150 ◦C

One stepprogrammingpossible

[80]

PCL/PEG Crosslinks PCL, PEG 40 ◦C70 ◦C

[80]

PCL/PPD Crosslinks PCL, PPD 40–60 ◦C70–80 ◦C


[82]

Two IPN Crosslinks Mercaptoacetate-vinylsulfone; mercapto-propionate-acrylate

23 ◦C45 ◦C


[83]

PCL/PU Crosslinks PCL, SM–PU 40 ◦C65 ◦C

Goodcytocompatibility

[84]

Epoxy/PCL Crosslinks PEG epoxy or PPGepoxy, PCL

40 ◦C75 ◦C

PPG systemsshowed higherfixity and recovery

[91]

Poly(styrene-grad-n-butyl acrylate) Hard domains Soft domains 60 ◦C80 ◦C100 ◦C

Quadruple SME [93]

Bilayer epoxy Crosslinks Epoxy with high andlow Tg

56 ◦C90 ◦C

Bilayer structure [94]

Crosslinked PE and PP blends Crosslinks PEPP

130 ◦C175 ◦C

Two transitions [29]

EOC, HDPE, LDPE Crosslinks LDPEHDPEEOC

42 ◦C78 ◦C122 ◦C

Dual up toquadruple SME

[95]

Table 4Overview of selected examples of SMPs using reversible networks based on Diels–Alder cycloadditions.

Reversible Diels–Alder reaction Diene Dienophile Reference

O N

O

O

O

N

O

O

+

Furan-endfunctional PLA Low molar mass maleimides [99,100]Furan-endfunctional PCL–PU Low molar mass maleimides [101]Furan-endfunctionalPTMO/PPDOa

Low molar mass maleimides [106]

Furan-endfunctional PCL Maleimide-endfunctional PCL [102,103]Main-chain furan containingPolyester

Low molar mass maleimides [79,104]

Polyketone with pendant furanmoieties


PCL–PU with pendant furanmoieties


N

O

O

+

N

O

O Anthracene-endfunctional PCL Maleimide-endfunctional PCL [102]

a PTMO, poly(tetramethylene oxide); PPDO, poly(p-dioxanone).

Polyme
functionalized a poly(lactic acid) (PLA) with furan moieties[99,100]. The crosslinking with different bismaleimidesleads to polymer networks, which feature a SME due tothe possibility of the retro-Diels–Alder reaction at 160 ◦C.The utilization of PLA as the basic component makes thesenetworks also biodegradable. Another type of polymer wasutilized by Raquez and coworkers [101]. The authors syn-thesized two and four arm PCL star polymers, which bearfuran moieties as end groups. These systems were reactedwith N,N-phenylenedimaleimide resulting in shape mem-ory networks. This approach was further improved byAlexandre et al. [102,103]. They prepared four arm starpolymers of PCL, which were functionalized with furan,maleimide or anthracene moieties. Reactive extrusion wassubsequently applied to create crosslinked networks.

Beside the functionalization of end groups withDiels–Alder units it is also possible to prepare polymerswhich consist of furan moieties in the polymer chain. Forthis purpose, Yoshie and coworkers utilized a bio-basedfuran-diol, which was reacted in a polycondensation withsuccinic acid [104]. This polyester was further crosslinkedwith bismaleimides resulting in a shape memory polymerdue to the reversibility of the Diels–Alder moieties [79].Furthermore, the side chain functionalization with furanrepresents an opportunity to design shape memory poly-mers based on the Diels–Alder reaction. Thus, a polyketonwas functionalized with a furan, which could be further uti-lized for the crosslinking with multifunctional maleimides[105].

Due to the possibility to design self-healing mate-rials by application of functional moieties that canundergo Diels–Alder reactions it is also possible to com-bine self-healing and SME. Thus, Wang et al. preparedfuran-endfunctionalized poly(tetramethylene oxide) andpoly(dioxanone), which was further crosslinked by theaddition of a trismaleimide [106]. This system revealedSME, even after a damage occurs (Fig. 9). This basic conceptcould also be utilized by Du Prez and coworkers [107]. Theauthors prepared a PU by the polymerization of a furan-containing diol. The furan moieties were crosslinked bya bismaleimide. The shape memory effect enhances theself-healing effect by bringing the crack surfaces closertogether.

Beside the Diels–Alder unit it is also possible to utilizedisulfides as reversible covalent interactions in order todesign shape memory polymers. For this purpose, Nishioet al. functionalized cellulose with thiols and it could bedemonstrated that these materials revealed a shape recov-ery [108]. Furthermore Rowan and coworkers prepared apolydisulfide, which can self-heal under light irradiationand show shape memory under thermal treatment [109].

4.5.2. Supramolecular interactionsInstead of reversible covalent bonds, supramolecu-

lar interactions can also be utilized for the design of areversible network. For this purpose, several possibilities
exist: Hydrogen bonds, ionic interactions or metal ligandinteractions (Fig. 10). Mostly, hydrogen bonds are appliedfor the preparation of SMPs because the variation of thehydrogen bond donor and acceptor enables easy tuning of
r Science 49–50 (2015) 3–33

the binding strength and a straightforward design of mul-tifunctional materials [110].

The design of shape memory polymers based on hydro-gen bonds relies mostly on two binding motifs: Thepyridine moiety and the ureidopyrimidine unit. For the firstapproach, a pyridine could be installed in the side chainof a polyurethane. For this purpose, a pyridine-containingdiol was copolymerized with a diisocyanate and anotherdiol [111,112]. The pyridine moiety can form inter- orintramolecular hydrogen bonds to the urethane group. Thisinteraction can reversible be broken and, thus, interactsas the temporary network. The strain recovery is sensitiveto temperature as well as to the relative humidity due tothe replacement of the hydrogen bonds between the poly-mers by hydrogen bonds that are formed between waterand the pyridine moieties [113]. Furthermore, it is possibleto alter the properties of the material by the addition of anacid to the pyridine containing polyurethane, which resultsin the formation of hydrogen bonds between the pyridineand the carboxylic acid. In particular, Chen et al. added 4-hexadecyloxybenzoic acid to develop a polymer with both,shape memory as well as liquid crystalline properties [114].This approach can also be extended toward the introduc-tion of mesogenic units into the structure [115]. For thispurpose, Chen et al. prepared a polyurethane, which con-sists of a carboxylic acid in the side chain, and formedthe complex with cholesteryl isonicotinate via hydrogenbonds.

In order to increase the strength of the hydrogen inter-action, the ureidopyrimidine unit can be installed withinthe polymeric structures. Due to the quadruple-hydrogen-bonded dimer a strong interaction could be achieved[116–118]. Thus, this system is well-suitable for the designof shape memory polymers based on reversible hydro-gen bonds. For the functionalization of the polymer withthe ureidopyrimidine moiety in the side chain, the ure-idopyrimidine was functionalized with a polymerizablegroup [119,120]. Afterwards, it was copolymerized withacrylates and bisacrylates in order to obtain a doublenetwork structure that is held together by irreversiblecovalent as well as reversible supramolecular bonds. Theshape memory principle is solely based on the reversiblehydrogen bonds and is depicted in Fig. 11. Furthermore,Anthamatten et al. reported a system containing urei-dopyrimidine and a photo-crosslinkable group in the sidechain of poly((meth)acrylate) [121]. At first, a linear poly-mer was synthesized which was subsequently covalentlycrosslinked by UV-light irradiation. The ureidopyrimidineformed the reversible network structure and a benzophe-none group was utilized as covalent crosslinker.

Beside the installation of the ureidopyrimidine unit inthe side chain it is also possible to incorporate the structuralmotif into the main chain of the polymer. For this purpose,Meijer et al. synthesized a diisocyanate, which containsureidopyrimidine, and polymerized it with ˛,ω-amino-PEO[122]. This system featured a shape memory behavior acti-vated by temperature or by the addition of water due
to the possibility of additional hydrogen bonds. Further-more, the ureidopyrimidine unit can also be utilized forthe end group functionalization of PCL/poly(p-dioxanone)interpenetrating polymer networks [123]. In that case,


r co-netR hemica

Wmahmt

so

Fig. 9. The triple-shape memory effects of the initial Diels–Aldeef. [106], Copyright 2012. Reproduced with permission from The Royal C

ang et al. functionalized a four arm PCL star shaped poly-er with the hydrogen bond moieties and subsequently

dded a four arm poly(p-dioxanone) star polymer withydroxyl end groups. This system showed a triple shapeemory behavior due to the coexistence of an interpene-

rating network as well as hydrogen bonds.The basic principle to combine a semi interpenetrating

tructure and hydrogen bonds can also be applied usingther moieties beside the ureidopyrimidine. For this

Fig. 10. Exemplary binding motifs for the formation of reversi

work (TSEM1) and recovered Diels–Alder co-network (TSEM2).l Society.

purpose, Liu et al. prepared poly[(methyl methacrylate)-co-(vinyl-2-pyrrolidone)] (P(MMA-co-VP)/PEO withsemi-interpenetrating networks (semi-IPNs) structure[124,125].

The last example of a SME induced by hydrogen bonds
is the utilization of carboxylic acids, which can undergodimerization via hydrogen bonds. The most straightfor-ward way for the introduction of a carboxylic acid is thecopolymerization of (meth)acrylates with (meth)acrylic
ble networks in SMPs by supramolecular interactions.

16 M.D. Hager et al. / Progress in Polyme

Fig. 11. Cartoon of proposed shape-memory mechanism involvingthermo-reversible H-bonding. Colored side-groups represent H-bondinggroups in the hot (red) and cold (blue) states, and the darker lines repre-sent the lightly crosslinked covalent network.Ref. [119], Copyright 2007. Reproduced with permission from John Wiley
& Sons Ltd. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of the article.)
acid. This leads to a reversible crosslinking of poly-mer chains. The irreversible network structure could beobtained in two different ways: Esterification [126] or theutilization of bisacrylamides [127,128]. For the first case, ahydroxyl-containing monomer such as hydroxyethyl acry-late (HEA) was copolymerized and the esters were formedvia subsequent post-polymerization functionalization.For the second approach, N,N′-methylene-bis(acrylamide)(MBAA) was copolymerized with methacrylic acid andthe comonomer resulting in a direct formation of a poly-meric network. The structures contain an irreversible anda reversible network and, thus, can reveal a shape memorybehavior.

Beside hydrogen bonds, also other supramolecularinteractions can be used in order to create reversiblenetworks and, thus, to design a shape memory polymer. Inparticular, ionomers were investigated. The most-commonapproach is the neutralization of sulfonated polymerswith zinc salts [129–131]. For example, poly{ethylene-r-propylene-r-(5-ethylidene-2-norbornene)} was neutral-ized with zinc oleate and stearate, respectively. Theinter-chain interactions formed the permanent network,whereas the dipolar interactions between the dispersedphase of crystalline zinc salt and the ionomer itself resultedin the temporary network. Another approach is the inte-gration of the ionic groups via carboxylic acids. Here,copolymerization with acrylic acid or the preparation ofpoly(oxyethylene-b-butylene adipate) ionomers are com-mon principles for the design of ionomer-based shapememory polymers [132–134].

In addition, metal–ligand interactions provide a
reversibility which can be used for the design of shape-memory polymers [135]. Rowan et al. synthesized a lowmolar mass poly(butadiene), which was functionalizedwith 4-oxy-2,6-bis(N-methylbenzimidazolyl)pyridine
r Science 49–50 (2015) 3–33

as end groups [136]. The permanent covalent net-work was formed by a thiol-ene reaction withtetrafunctional thiol crosslinker, whereas the reversiblestructure could be achieved by the complexation ofthe ligand with europium ions. The resulting polymernetworks revealed shape memory behavior under severalstimuli: Temperature changes, light irradiation as wellas exposure to chemicals (e.g., methanol). All externalstimuli led to a softening of the metal–ligand interactionsand, thus, enable the shape memory behavior. This basicconcept was expanded by Xia et al., who prepared a2,6-bis(N-methylbenzimidazolyl)pyridine monomer andcopolymerized it with methyl methacrylate as well asbutyl acrylate [137]. After complexation with zinc ions, ashape memory behavior upon heating could be observed.

4.6. Photo-induced shape memory

Beside the application of several thermal transitions itis possible to use light irradiation for the design of SMPs.In many cases the exposure to light results in a heatingand, thus, to thermal transitions [138,139]. The examplesthat solely rely on this photo-thermal effect will not be dis-cussed in detail in this section, which focuses on chemicalreactions and changes in the chemical structure that areinduced by light irradiation (Fig. 12) [140]. These materialscan be classified as light-activated shape memory polymer(LASMP).

The most prominent example is the reversible photo-dimerization of cinnamic acid derivates. Leindlein et al.introduced these photo-switchable units as reversibleelements [141]. The cinnamic acid can dimerize underirradiation at wavelengths � > 260 nm to yield a four mem-bered ring, which is reopened by a simple alteration of thewavelength (� < 260 nm) of the irradiated light. The combi-nation with permanent crosslinks in other polymer-chainsegments results in a SME which is not based on the exter-nal heating (Fig. 13).

Nagata and coworkers prepared polyesters, which con-tain cinnamic acid in the main chain [142,143]. For thispurpose, a dicarboxylic acid of a cinnamic acid deriva-tive was polymerized with �,�-functional diols basedon PLA, PCL as well as PEO. The resulting (biodegrad-able) polyesters could be gelated by UV-light irradiationdue to the [2+2]-cycloaddition. Wu et al. prepared apolyesterurethane with a cinnamamide side group [144].Both, the PLA hard block as well as the PCL soft blockof the SMP represented biodegradable polymers. Duringlight exposure, a reversible crosslinking could be shown.A similar approach was studied by Ashby and coworkers[145]. Polyesterurethanes with a cinnamamide side groupwere end-functionalized with methacrylate moieties. Sub-sequent radical polymerization and UV-light irradiationresulted in a double network structure and, thus, shapememory behavior.

Coumarins can undergo a reversible [2+2] cycloaddi-tion and, thus, were applied for the design of materials
featuring a photo-induced SME [146,147]. Thus, Nagataand coworkers prepared PCL as well as PLA which bearcoumarin side groups [148,149]. The photo-crosslinkingand its reversibility were utilized for the design of several


le netw

sHhchdnb

rmezlpcpiittetir

FsR

Fig. 12. Exemplary binding motifs for the reversib

hape memory polymers. This basic idea was improved bye et al., who combined the dimerization of coumarin withydrogen bonds [150]. For this purpose, a coumarinontaining acid was applied that can additionally formydrogen bonds to a poly(4-vinylpyridine). After theimerization of the coumarin moieties, a light switchableetwork was obtained, which leads to the shape memoryehavior of the polymer.

Beside cycloadditions there are also photoisomerizationeactions which can be utilized for the design of shapeemory/shape changing polymers. The most prominent

xamples are azobenzenes and the spiropyran. The azoben-ene moiety can undergo a cis-trans isomerization duringight exposure, which results in a volume change of theolymer [151,152]. This effect can also be utilized in theontext of shape memory or reversible shape changing. Inarticular, the second approach is studied in detail. Thus,

t could be demonstrated that the very fast optomechan-cal response takes place in only a few milliseconds upo seconds [153,154]. The reversible shape change can beriggered by irradiation and can be monitored over sev-
ral cycles and hours [155]. In particular, the utilization ofhe azobenzene moiety in the side chain or as crosslinkers a common design principle of such systems [156]. Thiseaction and the resulting shape changing could even be
ig. 13. Light-induced shape-memory behavior based on reversible cinnamic acidhape.ef. [141], Copyright 2005. Reproduced with permission from the Nature Publish

ork formation utilized in photo-responsive SMPs.

utilized to perform photomechanical work [157]. Further-more, a selective irradiation can lead to the movement ofpolymeric structures. In fact, Ikeda and coworkers coulddemonstrate that an azobenzene-crosslinked polymer net-work moves forward if the polymer is illuminated at theright position [158–160]. Thus, several different shapes(e.g., fibers or continuous ring) were utilized. Beside thereversible shape changing it is also possible to use the pho-toisomerization of azobenzenes for the design of shapememory polymers. For this purpose, White et al. synthe-sized an azobenzene-containing diacrylate, which can actas a crosslinker in a polymeric material [161,162]. Theresulting polymer network is able to recover its originalshape after illumination. Remarkably, azobenzene moietieshave even been applied within liquid-crystal networks inorder to fabricate artificial cilia (for reviews on artificialcilia see Refs. [163,164]). The isomerization under illumi-nation (visible and UV light) resulted in a cilia-like motionof microstructured polymeric materials. Beside the pho-toisomerization of azobenzene it is also possible to utilizethe spiropyran isomerization reaction [165]. Thus, Liu and
coworkers could show the shape recovery of spiropyrandoped ethylene-vinyl acetate copolymers [166]. Due to theisomerization of spiropyran a plasticization of the polymercould be observed.
: (a) permanent shape; (b) temporary shape and (c) recovered permanent

ing Group.

Polyme
4.7. Other stimuli for shape memory polymers

The two major stimuli, which have been applied forSMPs are in particular the temperature and to a lesserextend light. However, also further stimuli can be uti-lized to trigger a transition (e.g., segmental rearrangement),which induces the shape memory effect. For more detailedinformation on the application of other stimuli see alsoRef. [167]. Classical glass transition SMPs, like crosslinkednatural rubber, have also been triggered by organicsolvents, which induce the rearrangement within the poly-mer [33]. Moreover, a supramolecular polymer based onEu(III) crosslinked Me-bip functionalized polybutadienewas utilized as chemo-responsive SMP [136]. The sol-vent methanol leads to a decrease of the stability of theEu(III) Me-bip complexes, which results in a decomplex-ation. Thus, the temporary crosslinks are broken and thephase separation becomes less prominent. Consequently,the original shape is recovered.

This effect can also be transferred from organic sol-vents to water, enabling a SME that is simply induced byhumidity [13]. Interestingly, ultrasound was also appliedto trigger the SME [168]. High intensity focused ultrasoundwas utilized to induce the SME in polyurethanes. Dur-ing the ultrasound treatment, the temperature within thepolymer increases, which leads consequently to the SME.Chemo-responsive SMPs are of particular interest [169].Besides the utilization of solvent or water as the trigger,also other small molecules have been applied. For instance,glucose has been utilized to induce the SME of supramolec-ular polymer network [170]. Latter network consists of�-cyclodextrin modified chitosan and a ferrocene modifiedbranched ethylene imine polymer. In addition to cova-lent crosslinks, this system features the supramolecularcrosslink of the host–guest complex of the cyclodextrin andthe ferrocene. Noteworthy, the interaction of both can beswitched by the redox state of the iron because an oxida-tion results in de-crosslinking of the supramolecular bonds.The oxidation, which induces the SME, was achieved bytreatment with Ce and glucose in combination with glucoseoxidase (GOD), respectively. Moreover, stimuli-responsivecopper-cross-linked hydrogels featured a redox-triggeredSME (see Fig. 14) [171].

The polymer network consisting of sodium (4-styrenesulfonate), 4-vinylpyridine, and poly(ethylene glycol)diacrylate was crosslinked with Cu(II), which binds withthe pyridine moieties. Reduction leads to a weakermetal–ligand interaction, resulting in a partial decom-plexation. The, thus, softer materials will be oxidizedsubsequently in air, thereby a fixing.

Instead of chemical oxidizing and reducing agents,respectively, also an electrochemical oxidation or reduc-tion by applying a voltage is thinkable. This approach couldbring these types of materials closer to an actual applica-tion because the need to apply special chemical substancesby the user of an SME device could be omitted.

4.8. Reversible shape memory polymers

The classical SME is a one-way effect, i.e. the per-manent shape is recovered after heating. A temporary

r Science 49–50 (2015) 3–33

shape can only be obtained after mechanical deformation.Materials featuring a reversible shape memory effect arecapable of switching several times between two differ-ent shapes upon an external stimulus without the needof additional mechanical (deformation) work [16,172]. Bythis manner, a great drawback of the standard SMPs –the missing reversibility – is overcome, which opens thedoor for different application fields (e.g., artificial muscles[153,173,174], shape-changing (nano)particles [175,176]and “self-peeling” reversible dry adhesives [177]). In orderto achieve the reversible SME, an internal “driving force”for the back transformation is required.

One design principle for such materials is based on liq-uid crystal elastomers (LCE) [178–180], which are knownin the liquid crystal (LC) field for their “reversible con-traction/extension” behavior [181], or in other words, areversible SME. The transition between the anisotropic andthe isotropic phase (i.e. the order of mesogens vanishes)leads generally to a contraction of the polymer [10]. In con-trast to a one-way SMP, the polymer will expand again,when the temperature is below the transition tempera-ture. Additionally, the uniform alignment of the mesogensis important for the effect.

Mather and coworkers synthesized a two-way SMPbased on a glass-forming nematic network [181]. A liq-uid crystalline monomer was polymerized by acyclic dienemetathesis (ADMET) and crosslinked subsequently. Thetransition from the nematic to the isotropic phase tookplace around 160 ◦C.

Polymer networks incorporating smectic LC monomershave been utilized to fabricate reversible SMPs as well[182]. Recently, Pei et al. presented a method to overcomethe limitations caused by the required macroscopic orien-tation [183]. The authors utilized a LCE with exchangeablecovalent bonds (epoxide with carboxylic acid). Due to thereversibility, an easy processing is possible and the LC moi-eties can be aligned. These systems were also capable of atriple SME based on the glass transition temperature andthe LC transition. Moreover, a highly reversible SME wasobserved at around 90 ◦C (see Fig. 15). Noteworthy, thereversibility allows a remolding of the polymers, which stillshowed the SME. Generally, the LCE based polymers aloneare limited to volume changes (i.e. contraction and expan-sion), in contrast to the “complex” shape changes of oneway SMPs (unbending, uncoiling, etc.). The fabrication ofa bilayer system with a LCE layer was utilized to expandthe possible shape transformations [184]. A polystyrene(PS) layer was combined with a nematic LCE layer. Thethickness of the PS layer was used to determine the shapechange: Thin PS layer for reversible wrinkling and a thickPS layer for reversible bending. More complex changes inthe geometry could be achieved by a pattering of the poly-mer film with actuator and passive domains. Within thiscontext, also photo-switchable SMPs (e.g., with azobenzenemoieties) have to be mentioned (see 4.6) [16].

A comparable behavior was also achieved withother polymer systems. For instance, cross-linked
poly(cyclooctene) featured tensile elongation uponcooling and tensile contraction on heating if a constantstress was applied [185]. Polymer samples, which wereelongated at 100 ◦C, showed an increase in strain upon


Fig. 14. (a) Oxidation of the Cu(I) doped hydrogel in air; (b) SME of the copper crosslinked hydrogels.Ref. [171], Copyright 2013. Reproduced with permission from the American Chemical Society.

F er consto CE actuaR Publish

cFShthipoawTmw

ig. 15. (a) Reversible thermal actuation of a polydomain (pd) xLCE undbtained by joining two original films together (see b); (c) pin-shaped xLef. [183], Copyright 2014. Reproduced with permission from the Nature

ooling. Heating back to 100 ◦C resulted in a stress release.urthermore, PCL is also capable to reveal the two-wayME under a constant stress [186]. The PCL specimen waseated to 65 ◦C. After cooling below the crystallizationemperature, the specimen expanded. Heating to theigher temperature resulted in a contraction again, reach-

ng nearly the original shape. Noteworthy, the reversiblerocess showed a quite large hysteresis (i.e. temperaturef contraction and expansion differ up to 70 ◦C). Lendleinnd coworkers extended this approach to triple SMPs,hich show a reversible SME over both transitions [187].

he effect was based on the introduction of two poly-ers segments (polypentadecalactone (PPL) and PCL),hich are capable of a melting induced expansion and a

ant stress (0.1 MPa here), and the same test on a remolded xLCE sampletor.ing Group.

crystallization induced contraction under constant stress.If the polymer sample is cooled from Thigh first the PPL willcrystallize; subsequently, the PCL at lower temperatures.Reheating of the polymer will induce the melting of thecorresponding segments in reverse order. The abovedescribed two-way SMEs only occur under constant stressof the polymer samples.

In contrast, reversible bidirectional SMPs do not requireany external stress [172]. Lendlein et al. fabricated apolyurethane copolyester network (PPL and PCL). The PDLdomains (high TM) determined the geometry, whereas
the lower melting PCL domain represented the actua-tor domain, responsible for the reversible shape shifting(by melting and crystallization) (see Fig. 16). Noteworthy,


MP (pro

& Sons

Fig. 16. (a) Schematic representation of the reversible bidirectional S40 mm × 4 mm × 0.4 mm from PPD-PCL(75)).Ref. [172], Copyright 2013. Reproduced with permission from John Wiley

the difference between both switching temperatures wasrather small: 11 ◦C (A → B: TSW = 27 ◦C; B → A TSW = 38 ◦C)[172]. Recently, Zhou et al. reported another polymer sys-tem, which is capable of a reversible bidirectional SME[188]. The authors utilized a poly(octylene adipate) (POA)elastomer as SMP. The reversible SME is based on a (par-tial) melting/crystallization of the crystalline domains ofthe semicrystalline elastomer. The shape of the poly-mer samples could be switched in a highly reversiblemanner by heating to 38 ◦C and cooling to 5 ◦C. Inter-estingly, the authors could also demonstrate a “one-wayreversibility”; for instance, the polymer sample featured acoil-rod-coil transition only by heating. The polymer wasreprogrammed in its temporary shape (i.e. rod). After cool-ing to 34 ◦C, which is below the melting temperature, therod was manually coiled and fixed at low temperatures.Heating of the sample induced first the transition (coilto rod at 34 ◦C) and the original shape (i.e. coil) could bereformed at 60 ◦C. As in the preceding example, a grip-per was fabricated, which was utilized to lift objects. Thegripping and the release can be triggered by temperature. Itshould be noted that these “soft” polymeric grippers revealan excellent weight-to-payload ratio (100!), which is muchhigher than that for industrial robotic grippers [188].

These selected examples (see Table 5) illustrate thegreat variety of reversible SMPs. Different materials havebeen utilized to achieve a reversibility of the SME. Addi-tionally, several definitions for a reversible SME have beencoined in recent years; however, these terms have not beenapplied consistently throughout the recent literature.

Typically, the two-way SME only occurs under con-stant load [185–187], the reversible bidirectional SME[172,188] is a “really” reversible SME (an external stressis not required) and LCEs show a reversible contrac-tion/extension as a basic SME [153,178–184].

5. Applications of shape memory polymers

The impressing property of SMP makes them interestingcandidates for several applications. Hereby, the SME can beutilized on surfaces (i.e. polymer films/coatings) in orderto tune the surface properties by changing the shape of

gramming left) and reversible SME; (b) experiment (polymer ribbon

Ltd.

the polymer surface. Moreover, bulk materials are of greatinterest where the polymeric material is able to changeits overall shape. Finally, the concept can be applied tonanomaterials featuring shape changing ability. The nextchapters summarize selected examples for applications ofSMPs: From industrial SMPs for commercial applications,biomedical applications (a field, which has been a strongbooster for the whole field of SMPs), self-healing mate-rials utilizing SME as well as applications in aerospace.Further (potential) applications of SMPs include artificialmuscles [173,174,179], switchable/recoverable optics (e.g.,gratings, holograms, tunable optical windows) (see Fig. 17)[189–192], toys (e.g., doll hair with SME) [193], tuning ofsurface structure (e.g., wrinkling) [194–196], reshape-ableproducts [197], automobile (e.g., seat assemblies, tunablevehicle structures, morphable automotive body molding)[25], self-peeling dry adhesive [177,198,199], shape mem-ory fibers [10], and their application in shape memorytextiles [10,200–202].

5.1. Industrial applications of shape memory polymers

Heat shrinkable polymer tubings, films etc., which couldbe considered as a “simple” SMP, are already widely usedtoday. Early developments of commercial SMPs includepolynorbornenes (Nippon Zeon Co. Japan) and styrenebutadiene copolymers (Ashai Japan) in the 1980s [203].One first major breakthrough was the discovery of ther-moplastic shape memory polyurethane, which overcamethe difficult processing of the former polymers [203]. As aconsequence, these materials found a wide range of appli-cations, for instance as safety tag [204] or as self-deployingchair [205].

Moreover, a recent development is based on apolystyrene thermoset by Cornerstone [206,207]. Accord-ing to their own statements, these materials are interestingfor morphing systems (see also aerospace applications).Additionally, SMPs are considered as fast repair and pro-
tection solution [208]. A SMP polymer plate can adjust tothe damaged area/surface (e.g., of a truck). These SMPs canalso be utilized for Smart MandrelsTM [209]. After the fil-ament winding, the mandrel can be easily removed to the


Table 5Overview on selected examples of reversible SMPs.

Material Principle Switching temperature, TSW Comments Reference

LCE Transition of nematic LC 160 ◦C [181]LCE Transition of smectic LC 90 ◦C [182]xLCE Transition of nematic LC 90 ◦C LCE with exchangeable

covalent bonds[183]

LCE Transition of nematic LC 75 ◦C Bilayer structure [184]PCO Crystallization under stress ca. 60 ◦C Under stress [185]PCL Crystallization under stress ca. 65 ◦C Under stress [186]Polymer network Crystallization under stress 40 and 71 ◦C Under stress

Rev. triple SME[187]

7 and 3 and 38

tmssafi

5

ufiobwtLblahtims

ancso

F

Polymer network Reversible crystallization/melting 2Poly(octylene adipate) Reversible crystallization/melting 5

hermally induced shrinkage. Moreover, CTD offers poly-ers under the tradename TEMBO® for deployable

tructures (see also Section 5.4) [207,210]. There are alsoome companies in the biomedical applications sector (seelso Section 5.2). For instance, MedShape solutions was therst company to offer a FDA approved SMP device [211].

.2. Biomedical applications

The shape memory effect is also well suitable for thetilization in biomedical applications. For this applicationeld, several requirements must be fulfilled and a rangef problems must be overcome [212]. As an example, oneasic issue is the heating of the SMP. Different strategiesere developed to allow the heating of the polymer inside

he human body. One possibility is the utilization of IR-asers [213,214]. These can heat the polymer inside theody at a precise position. An alternative way is the uti-

ization of magnetic nanoparticles [215–220]. In that case,n external magnetic field can be applied for the selectiveeating of the shape memory polymer, if these nanopar-icles are incorporated within the polymer. An alternatives to completely prevent the difficult heating required to

ake standard SMPs work is the utilization of light inducedhape memory polymers (see 4.6).

As a matter of fact, it is required for biomedicalpplications that the polymer itself is biocompatible and
on-toxic. In fact, the most-common structures that SMPsonsist of are biocompatible, and several investigationstudied this aspect in detail. Thus, PCL, PLA-polyurethanesr PEO based shape memory polymers were found to
ig. 17. Deformation and SME-based recovery of a hologram. Ref. [189], Copyrigh

8 ◦C Reversible without load [172]◦C Reversible without load [188]

be non-toxic [221–223]. Beside these academic studies, acommercially available shape memory polymer providedby DiAPLEX was investigated regarding its biocompati-bility. A general good biocompatibility was proven (lowcytotoxicity, low platelet activation, low cytokine activa-tion, low thrombogenicity and low in vivo inflammatoryresponse) [212,224–227].

If these basic requirements are fulfilled, shape memorypolymers can be applied in several biomedical applications,such as the repair of cardiac valves [228,229]. In that casethe annuloplasty procedure can be enhanced by the utiliza-tion of shape memory polymer rings. Beside the repair ofcardiac valves tissue engineering is a large application fieldof SMPs [230]. For this purpose, PCL was studied in detail,e.g., by cell seeding on the polymer [231]. Furthermore,the behavior of several cell lines on the same polymericmaterial was investigated [222]. SMPs are also used forEndovascular stroke treatment (clot removal). Maitlandand coworkers delivered a shape memory polymer througha catheter and activated it photothermally, which resultsin an easy removal of the clot and finally relieve of theischemia (Fig. 18) [232,233]. In order to enhance the SME,a shape memory alloy (nitinol) was encapsulated into thepolymeric matrix [234,235].

The same working principle can also be utilized for thedesign of vascular stents based on shape memory polymers.The state of art is the utilization of stents made of stain-
less steel or other metal containing materials (e.g., severalalloys). Several drawbacks of these stents, such as too highstiffness for navigation [236,237], can be overcome by theutilization of SMPs. In addition, it was shown that drug
t 2013. Reproduced with permission from John Wiley & Sons Ltd.


Fig. 18. Working principles of shape memory polymer based microactu-ator utilized for treating ischemic stroke: (a) guided catheter is pushedtoward/through the clot, (b) shape memory polymer is released and (c)
clot is removed and the ischemia is revealed.Ref. [232], Copyright 2002. Reproduced with permission from Wiley Peri-odicals Inc.
elution via the stent is easier to perform [238–242] andthat the healing after implantation can be enhanced by theutilization of a biodegradable SMP stent [243].

Beside these main biomedical topics SMPs are alsoapplied in some special fields [212] such as orthopedics[64], endoscopic surgery [244], orthodontics [245], kidneydialysis [246], photodynamic light therapy [247], the ther-apy of aneurysms [248–250] or neuroprosthetics [251]. Thedevelopment of new polymers and their biomedical inves-tigation of the current shape memory polymers will leadto more and more complex applications of these in themedical research.

5.3. Self-healing materials

The ability to repair a mechanical damage is a fasci-nating property of self-healing materials [98,252–254].Polymers with such properties could be designed inseveral ways. In principle two different pathways to obtainself-healing polymers can be distinguished: embeddingof healing agents (e.g., capsules or vascular systems) andintroduction of reversible interactions (e.g., reversiblecovalent bonds), respectively [255–257]. For the firstapproach a healing agent is embedded into the polymervia microcapsules or vascular networks [258–260]. Uponscratching the capsule is broken and a healing agent (e.g.,a monomer) is released and after a subsequent reaction(e.g., polymerization) the crack is closed. On the other
hand self-healing behavior can also be based on thepolymer material’s properties alone and, thus, on theutilization of special functionalities of the polymer. Forthis purpose, dynamic covalent bonds, e.g., Diels–Alder
r Science 49–50 (2015) 3–33

reactions [261–263] or hetero-Diels–Alder reactions[264], or non-covalent interactions, e.g., hydrogen bonds[265–267], ionic interactions [268–270] or metal–ligandinteractions [271–280], have been utilized. During theexposure to external energy the interactions are weakenedor the retro-Diels–Alder reaction takes place, which leadsto flexibility and, thus, to the ability to close the crack.

The SME can enhance the self-healing ability of poly-meric materials, which is referred to as shape-memoryassisted self-healing (SMASH). In that case, the SMEenhances the self-healing process by bringing the cracksurfaces closer together. For that purpose, two possibil-ities have been studied in detail. In the first approacha shape-memory system is embedded or combined witha self-healing material. The crack surfaces can be pulledcloser together due to the SME and, thus, a better self-healing, i.e. the healing of larger scratches and the healingin shorter times, can be obtained. The groups of White andManson combined shape-memory alloys with a capsule-based self-healing system [281]. For this purpose, the alloyand monomer containing microcapsules were embeddedinto a polymeric matrix [281,282]. The alloy wires did notinfluence the virgin fracture toughness, but increased theself-healing ability of the system. After a crack occurs,the shape-memory effect of the alloy brings the scratchsurfers closer together and the monomer, which is releasedfrom the microcapsules, closes the crack via polymeriza-tion. This basic system could be expanded by the additionalencapsulation of a solvent (i.e. ethyl phenylacetate) in themicrocapsules [283]. A full polymeric system was reportedby Li et al. [284,285]. The authors utilized polyurethanefibers instead of the alloy wires. Due to the SME of thepolyurethane a better self-healing ability of the systemcould be obtained.

The basic drawback of this system is the requirementthat the shape-memory part must be located in a positionclose to the crack. Otherwise the shape-memory behaviordoes not assist the self-healing process. In order to over-come this problem a bulk SMASH system was preparedby Mather et al. [286]. They designed an interpenetrat-ing network consisting of high molar mass linear PCL andreversibly crosslinked PCL. The latter was synthesized bya thiol-ene reaction of PCL-diacrylate with pentaerythritoltetrakis-(3-mercaptopropionate) and provides the shape-memory behavior. On the other hand, the high molar masslinear PCL can re-entangle after diffusion and, thus, is usedas the healing agent. The self-healing efficiencies of differ-ent mixtures were tested, showing that complete healingis possible if the content of linear PCL is at least 25 wt%(Fig. 19).

This SMASH strategy was extended by the utilization ofan epoxy, in which fibers of l-PCL were incorporated [287].This results in a coating, which was utilized as corrosioninhibitor. Furthermore, the group of Du Prez synthesizeda material based on polyurethane with PCL segments,which was crosslinked by furan-maleimide Diels–Alderunits [107]. During heating the reversible connections
break and enable the SME as well as the self-healingbehavior. This enhancement was classified as Diels–Aldershape-memory assisted self-healing (DASMASH). Li andcoworkers prepared composites of thermoplastic particles


F e (l-PCLtR an Chem

awSmoaidpitto

cmata

asipht

FpsR

ig. 19. Snapshots of crack closure and crack rebonding when the samplhe temperatures shown above (stereo micrographs scale bar: 500 �m).ef. [286], Copyright 2011. Reproduced with permission from the Americ

nd commercially available SMP [288–292]. The basicorking principle is depicted in Fig. 20. After a crack, the

ME leads to reduction of the crack size and the ther-oplastic can be molten, which results in a final closure

f the crack. Furthermore, the effect of the compositionnd the utilized basic shape-memory behavior was stud-ed in detail [293–296]. Beside that, Zhao and coworkersemonstrated a combined self-healing and shape-memoryolymer, which can be healed by light [297]. The light

nduces the surface plasmon resonance of gold nanopar-icles, which leads to a heating of the sample. Thus, bothhe SMP as well as the self-healing process could bebserved.

Furthermore, SMPs can be utilized as protectiveoatings for metal substrate. Terryn et al. prepared a shape-emory polyurethane, which consists of soft PCL segments

nd hard polyurethane segments [298–300]. A coating ofhis polymeric system decreased the corrosion process ofn aluminum substrate due to the SME.

Overall, the combination of SME with a self-healingbility represents a promising approach. In particular,elf-healing polymeric systems based on reversible
nteractions seem very straightforward, because bothroperties rely on the same (i.e. thermal) trigger. The SMEelps to overcome a crucial problem of these materials,he missing or low mobility, which allows also healing
ig. 20. General working principle of thermoplastic particles embedded into coerature of the shape-memory polymer foam and Tgp: glass transition temperaemi-crystalline thermoplastic particle).ef. [288], Copyright 2010. Reproduced with permission from Elsevier Science Ltd

50:n-PCL50) was unclamped from the Linkam tensile stage and heated to

ical Society.

of larger defects, compared to the classical approach. Thereversibility of the SME, i.e. the possibility to heal multipledamages, is one of the major challenges in this field.

5.4. Shape memory polymers for aerospace applications

Shape memory alloys are commercially available andfound a wide range of different applications [301]. How-ever, SMPs have one decisive advantage, particularlyconsidering applications in aerospace: Their weight. Theweight of the applied materials is one of the most cru-cial issues in the design of new systems in aerospace (e.g.,the newest Boeing 787 Dreamliner utilizes >50% polymercomposites in the primary structure). Consequently, enor-mous efforts have been undertaken in the development ofnew SMPs for aerospace applications. The field for potentialapplications in this sector is huge.

SMPs are of great interest for low-cost self-deployablestructures, e.g., solar arrays, solar sails, sunshields, or radarantennas [302,303]. Strategies for a mechanical deploy-ment and inflatable structures, respectively, proved tobe detrimental due to their high weight, high costs and
large required volume. In this context structures basedon the cold-hibernated elastic memory (CHEM) technol-ogy have been investigated [304]. The original structureis warmed above the Tg and rolled/deformed into the
mmercially available shape-memory polymer (Tgs: glass transition tem-ture for amorphous thermoplastic particle or melting temperature for

.


Fig. 21. Unfolding process of a self-deployable truss.Ref. [306], Copyright 2014. Reproduced with permission from Elsevier Ltd.

temporary shape. Upon cooling it is hibernated for stor-age. After the transport into space, the structure is heatedagain inducing the shape recovery. Subsequent coolingwill rigidize the structure. These systems would decreasethe mass of the total system considerably. For instance,the current inflatable sun shield of the international spacestation (ISIS) weights approx. 105 kg; the weight of a CHEMbased system would be decreased by a factor of 10 [304].Other potential applications for SMPs in space includedeployable panels (see Fig. 21), solar arrays and reflectorantennas [303,305,306]. First experiments under micro-gravity revealed that the SMPs show similar SME comparedto experiments on the ground [307].

In contrast to these self-deployable applications, the uti-lization of SMPs for morphing aircrafts is still in the futuretense. Morphing aircrafts can change the wing shape dur-ing flight to adopt to the current requirements [308]. Thisprinciple is bio-inspired: The wings of birds are far frombeing rigid. A Peregrine Falcon passing into fall flight isan impressive demonstration of this special ability. SMPcan be potentially applied to tune the structure on the sur-face of the wing and the overall shape of the wing itself,respectively [25,303].

6. Future directions

The research on SMP has resulted in a large varietyof fascinating systems. A wide range of potential applica-tions has been explored, and even more important severalcommercially available polymers have found first indus-trial applications. This fact will be a further driving forcefor further developments of SMPs and their applications.The SME of polymers is not a purely academic topic in the“ivory tower” of science.

But where will the journey of SMPs take us in the longterm?

Considering recent literature [10,13,17,309] there is stillmuch space for further developments. On the one sidenovel polymers and applications will be explored, which

r Science 49–50 (2015) 3–33

are still unimaginable today. However, on the other side,there is still the need for improvement of distinct proper-ties of current SMPs, which hamper further applications ofthese systems at the moment.

Based on the currently developed chemical synthesisstrategies [12] novel SMP structures will be developed.These novel systems will also provide deeper under-standing of structure–property-relationships of SMPs [10].However, it has to be noted that the current understand-ing of the SME mechanism and the design principle isquite good, particularly, if the “simpler” one-way dualshape polymers are considered. The more complex theSME behavior (e.g., multiple SME), the greater the demandfor further understanding. Another aspect of designingnew polymer structures is the tuning of the switchingtemperature. Many systems were inspired by biologicalapplications. Therefore most of the systems feature ratherlow TSW’s. However, in recent years reasonable effort hasbeen put into the development of high temperature SMPs[68–73]. The high switching temperature combined withan excellent thermal stability as well as good mechani-cal properties (e.g., high E-modulus) is the prerequisite formany applications (e.g., in aerospace). Moreover, tuningof the transition temperatures in triple as well as mul-tiple SMPs will come more into the focus. If a broadthermal transition (e.g., Tg) is utilized for the multipleSME, the range of the different TSW’s is limited inherently.Systems based on two separated phases offer generallya possible broader range; however, only few examplesshow switching temperatures with a difference larger than50 K [80,95]. Generally, tuning of the switching temperaturewas achieved in several systems, e.g., by the kind of theutilized monomers/segments, molar mass of the switch-ing segments. Future research will provide solutions fora “post-synthesis” tuning of the corresponding switch-ing temperatures in this field, which could be possible byadditional stress [32], solvent vapors [33] or by additionalswitchable units within in SMP itself.

Moreover, the variety of applicable stimuli for the SMEwill be extended further. In this respect, the present reviewwas mainly limiting itself to the thermal trigger as well aslight-induced SME. These both stimuli are abundant andcan be utilized in nearly every potential application. Themost applied stimulus is up to now still external heat (indi-rect). The possibilities for an internal heating of the SMPwill be expanded further. Current promising approachesinclude inductive heating [215], resistive heating (e.g.,by integrated wires, composite with conductive additive)[310], infrared irradiation [311] as well as photothermallyinduced heating. One possible aspect in photo-switchableSMPs is tuning of the wavelength required for switching.This can be achieved either by tuning of the correspondingphoto-switch or by energy transfer processes (shorter irra-diation wavelength, energy transfer from a dye moleculeto the switch) and upconversion processes, respectively(longer irradiation wavelength, upconversion on dye/metalcomplex, nanoparticle and transfer to switch). In particular,
latter approach could enable a photo-induced switching(NIR region) of SMPs for in vivo applications.
The responsiveness to certain non-thermal stimulimakes these SMPs also to interesting candidates for sensor

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ystems. Taking advantage of the already utilized multi-omain morphology of SMPs might also lead to SMPs whichan be triggered by different stimuli (e.g., one domain ishermally triggered, the other by light, etc.). Mechanicalorces are an interesting trigger for SMPs, because mostpotential) applications are as structural material [312].dditionally, one might think about autonomic SMPs (auto-omic aligned with the definition of self-healing materials98]). The desired SME is triggered during the usage of the

aterial without any external extra-intervention (which isommonly heating). This object seems still far from beingeached; however, SMPs for bioapplications could be con-idered as such materials. The body temperature triggershe SMP, when the material is utilized.

Another target of new polymer structures is the tuningf the possible shape transformations. As already describedn detail, multiple SMEs have been investigated. Herby,urther developments will try to increase the fixity asell as recovery of the single steps. However, also sim-le transformations between two different shapes coulde of interest. A naïve glace of SMP might suggest thathe permanent and temporary shape can be selected freelyithout any constraints. However, in reality, the perma-ent shape already determines the temporary shape to

large extend. Typically, the polymer sample is heatednd strained/coiled to the temporary shape, i.e. degree ofreedom in extension of the original shape. Upon heat-ng, the original shape is recovered, almost uniformly bycontraction” (see also LCEs). The present reversible sys-ems also follow this general trend. A kind of an “opposite”MP (heat induced expansion and contraction upon cool-ng) could be of interest. However, a new mechanism wille required, because the general trend is based on theolecular mechanism (e.g., crystallization induced expan-

ion). The synthesis of novel polymer structures will beot only of interest to achieve unprecedented proper-ies and possibilities of the SME, it will also be of greatnterest to obtain polymeric materials suitable for appli-ation/commercialization. This will focus on the one sideo improve the properties (or to meet the requirements forpplication): Higher fixity and recovery, better stability,igher cycle life, mechanical properties, etc. The mechani-al properties can be considered as one of the most crucialroperties. Most of the applied polymeric systems repre-ent rather soft materials, which limits the range of possiblepplications dramatically. Therefore, materials featuringetter mechanical properties while not losing the SME aref great interest. An inherent problem in the systems is theompulsory loss in the mechanical properties at the switch-ng temperature (the Tg or Tm of one phase is reached). As

consequence, the whole desired mechanical propertiesave to be provided by only one of the different phases ofhe SMP.

On the other side, the processability of currents SMPss well as their availability is a crucial issue for severalpplications. Many of the described polymer structures aredeal candidates for a scientific lab; however, a transforma-
ion of their synthesis/fabrication into a technical processs simply impossible. For instance, the requirement ofrosslinks, which is often met with chemical crosslinking,epresents a critical issue for industrial applications due to
r Science 49–50 (2015) 3–33 25

the limited processability. One issue will be to improve themechanical properties of physically crosslinked materials,which up to now, are almost always behind their chemi-cally crosslinked counterparts. Additionally, the potentialtonnages of some SMPs, particularly with complex archi-tecture, seems to be very limited. In turn, this will resultin a limited range of potential applications. Therefore itis encouraging that the research on commercial (largescale) polymers in the context of SMPs is increasing(e.g., Nafion, polyimides, natural rubber). Interestingly,even these “standard” materials showed fascinatingSMEs.

Tuning the polymer structure to increase the complexi-bility of the shape transformation will sooner or later reachits limits. However, already a “simple” pattering of thepolymer sample with active and inactive areas opens arange of new possibilities [184,313]. Application of specialprocessing technologies, such as inkjet printing, enablessuch patterning [314–316]. For instance the inkjet technol-ogy was applied to fabricate a pattern with SMP polymerson substrates (see Fig. 22) [313]. By this manner, com-plex folding geometries (e.g., folding of a cube) could beobtained. Control over the spatial distribution of the SMPallows for the fabrication of complex architectures. Addi-tionally, the control over alignment of the liquid crystals inLCEs (either by light of by surface guided assembly) opensup another possibility to tune the geometry of the resultingSME [317,318].

One special aspect of novel SMPs will be multifunc-tionality [11]. This represents materials which possess theSME as one central property, but also, another independentproperty (e.g., biocompatibility). Several potential applica-tions require this multifunctionality. The future challengewill be the integration of additional properties into SMPswithout any interference between both.

Having the desired SMP in hand, the programmabilityof the polymer is one crucial issue [10,17]. The polymericmaterials provide the platform which can be utilized bythe programming potentially in different ways. Dependingon the programming step, different SME can be observed.In particular, multiple SMPs are of great interest in thisfield. First investigations revealed that these polymers canalso be programmed in a one-step procedure without theneed of a tedious multi-step programming [80–82]. By thismanner, potential applications for such complex SMPs arebecoming more concrete due to the simpler applicabilityof these systems.

The future will also bring additional applications ofSMPs. Currently, three major trends within the numerouspotential applications are noticeable. Due to their excellentproperties and the commercial availability of one polymer,the applications of SMPs within the field of biomedicalapplications will increase. The field was one of the majordriving forces in the past and it will be a strong driv-ing force in the future. The properties of polymeric shapememory materials (e.g., tunability, soft material, switchingat body temperature, biocompatibility, biodegradability,
etc.) makes them superior in comparison to other shapememory materials, e.g., shape memory alloys. The mostpromising areas are self-deployable stents and minimalsurgery.


Fig. 22. Left: Inkjet printing of the SMP on the substrate. Right: (a) Layout of the self-folding cube with active areas (red-green) and passive areas; (b)Folding of the cube induced by heating.

hing LLC
Ref. [313], Copyright 2013. Reproduced with permission from AIP Publisreader is referred to the web version of the article.)
Another steadily growing field are shape memoryfibers for textiles. Different materials (e.g., shape memorypolyurethanes) are commercially available for the fabrica-tion of the required fibers. The SME will provide furtherfunctionality for such textiles, for instance, adjustment ofsize, shape, leading to a kind of “personalized” textiles.Additionally, applications in aerospace will reach the tech-nical readiness level for application. SMPs will find furthercommercial applications in the area of “consumer goods”(e.g., the enfolding chair, reshapeable handle of forks, etc.).The required materials are already available and are onlywaiting to be used. The only limit is the inventiveness ofthe engineers. The other potential applications (see above)will also boom, if the material basis is broadened and moreSMPs reach maturity.

Due to the large variety of already explored applicationsit is hard to see totally unprecedented applications of SMPs.One of such applications might be the storage of energy,which is currently in the focus, in particular for electro-chemical energy storage devices (e.g., redox flow batteries).Recently, Anthamatten et al. compared the stored elasticenergy density for different SMPs [319]. The best mate-rials can store over 1 MJ/m3 at strains over 100%. Thisenergy density (1 MJ/m3 = 0.28 kWh/m3) can now be com-pared with classical energy storage technologies (pumpedhydro: ca. 1 kWh/m3, compressed air: 1.3 kWh/m3, lithiumbatteries 200–350 kWh/m3). Astonishingly, SMPs are notfar away from the classical mechanical systems. A lithiumion battery still outperforms the polymer, but consider theprice, etc.

Interestingly, SMPs have never been optimized in thisdirection. However, many still unsolved challenges remain:How will the required switching temperature influencethe overall energy balance? How can the typically low

. (For interpretation of the references to color in this figure legend, the

recovery stress be increased? How can this principle beused technically?

Finally, shape memory polymers are one of the mostinteresting polymer classes within the field of functionalpolymers. They are one step further to man-made materialsshowing aspects of the outstanding properties of biologicalmaterials.

Acknowledgements

The authors thank the Deutsche Forschungsgemein-schaft (DFG, SPP 1568) for financial support. C.W.acknowledges the Carl Zeiss Foundation and S.B. is thankfulto the Fonds der Chemischen Industrie (FCI).

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