1. IntroductionShape memory (SM) biodegradable polymers (SMPs)and related composites are emerging smart materi-als in different applications. SMPs may adopt one(dual-shape), two (triple-shape) or several (multi-shape) stable temporary shapes and recover theirpermanent shape (or other temporary shapes in caseof multi-shape versions) upon the action of an exter-nal stimulus. The external stimulus may be temper-ature, pH, water, light irradiation, redox conditionetc. In most cases, however, the SMPs are thermallyactivated. The ‘switching’ or transformation temper-ature (Ttrans), enabling the material to return to itspermanent shape, is either linked with the glasstransition (Tg) or with the melting temperature (Tm).

Thus, SMPs are often subdivided based on theirswitch types into Tg- or Tm-based SMPs. As reversible‘switches’ other mechanisms such as liquid crystal-lization and related transitions, supermolecularassembly/disassembly, irradiation-induced reversiblenetwork formation, formation and disruption of a per-colation network, may also serve [1]. The permanentshape is guaranteed by physical or chemical networkstructures. The latter may be both on molecular andsupramolecular levels. Linkages of these networksare termed net points. The temporary shape is cre-ated by mechanical deformation above Ttrans. In somecases the deformation temperature may be belowTtrans values (i.e. Tg- and Tm-linked temperatures) [2].The macroscopic deformation applied is translated

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Biodegradable polyester-based shape memory polymers:Concepts of (supra)molecular architecturingJ. Karger-Kocsis1,2*, S. Kéki3

1Department of Polymer Engineering, Faculty of Mechanical Engineering, Budapest University of Technology andEconomics, M!egyetem rkp. 3., H-1111 Budapest, Hungary

2MTA–BME Research Group for Composite Science and Technology, M!egyetem rkp. 3., H-1111 Budapest, Hungary3Department of Applied Chemistry, University of Debrecen, Egyetem tér 1, H-4032 Debrecen, Hungary

Received 19 December 2013; accepted in revised form 25 January 2014

Abstract. Shape memory polymers (SMPs) are capable of memorizing one or more temporary shapes and recovering to thepermanent shape upon an external stimulus that is usually heat. Biodegradable polymers are an emerging family within theSMPs. This minireview delivers an overlook on actual concepts of molecular and supramolecular architectures which arefollowed to tailor the shape memory (SM) properties of biodegradable polyesters. Because the underlying switching mech-anisms of SM actions is either related to the glass transition (Tg) or melting temperatures (Tm), the related SMPs are classi-fied as Tg- or Tm-activated ones. For fixing of the permanent shape various physical and chemical networks serve, whichwere also introduced and discussed. Beside of the structure developments in one-way, also those in two-way SM polyesterswere considered. Adjustment of the switching temperature to that of the human body, acceleration of the shape recovery,enhancement of the recovery stress, controlled degradation, and recycling aspects were concluded as main targets for thefuture development of SM systems with biodegradable polyesters.

Keywords: polymer synthesis, molecular engineering, smart polymers, biopolymers, biodegradable polymers

eXPRESS Polymer Letters Vol.8, No.6 (2014) 397–412Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2014.44

*Corresponding author, e-mail: karger@pt.bme.hu© BME-PT

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to conformational changes of the molecular chains,molecular segments of the networks or macroscopicdeformation of the ‘switching’ phase. Temporaryshape is fixed by maintaining the deformation viacooling causing vitrification and crystallization inTg- and Tm-based SMP variants, respectively. Thedeformation energy, stored by this way, is releasedwhen the material is unloaded and reheated aboveTtrans. Note that Ttrans " Tg or Tm. Heating induces con-formational rearrangements of the molecular chainsand segments of the networks, melting of the crys-tals formed, release the constraints on macroscopi-cally deformed phases, respectively, through whichthe permanent shape is restored. All what is describedabove is related to one-way SMPs. This means thatthe external stimulus activates only the change fromthe temporary to the permanent shape (dual-shapevariant) or from one temporary to the other one inmulti-shape SMPs. There are, however, two-waySMPs featuring a reversible shape change betweenthe permanent and temporary shapes upon externalstimulus.Many works have been dealing with different SMPs,and the related knowledge is summarized in reviewsand books [3–7]. SM properties are typically quan-tified by the shape fixity (Rf) and shape recoveryratios (Rr). Rf means the extent of fixing of the exter-nally applied deformation in the temporary shape.Its value is 100% when the applied deformation, setabove Ttrans, is fully kept below Ttrans in the tempo-rary shape. Rr is the percentage of the recovery of theoriginal shape when the material is reheated aboveTtrans. Rr = 100% when the original shape of thematerial is fully restored. SM properties are usuallydetermined in cyclic (one or more) thermomechani-cal tests performed under stress- or strain-con-trolled conditions. A scheme of shape programmingand recovery in a thermomechanical test is given inthree-dimensional (3D) stress-deformation-temper-ature plot in Figure 1. Beside of Rf and Rr, furtherSM characteristics, such as the temperature intervalof recovery, recovery rate and recovery force, canbe measured. An excellent overview on the quan-tification of SM behavior in cyclic thermomechani-cal tests was compiled by Sauter et al. [8].Environmental concerns and public interest for sus-tainable growth fuel the research and development(R&D) works to produce and use biodegradable andbiobased polymers. Biodegradable means that thepolymers decompose to carbon dioxide, water and

biomass under aerobic, while the decompositionproducts contain methane after decomposition inanaerobic conditions. Biobased polymers originatefrom biomass, synthesized by microorganisms orproduced by biotechnological routes from monomersgot from renewable resources. Biodegradable poly-mers can be produced from petrochemical sources,as well. Poly(#-caprolactone) (PCL), polyester -amides, aliphatic- aromatic copolyesters belong tothis category. Major part of biodegradable polymersare polyesters, and in particular, aliphatic poly-esters. Monomers of the latter can be synthesizedfrom renewable resources.Biodegradable polymers, and especially polyesters,play a key role in medical applications due to theirbiodegradability and versatile syntheses resulting intailored properties. SM biodegradable polyestersare predestinated for medical use because the related‘SM devices’ (e.g. sutures, catheters, stents) shouldbe present only temporarily in the human body.Their versatile synthesis is the premise of adjustingTtrans to the temperature of the body.Next we shall give an overview on SMP systemswhich are composed fully or partly from biodegrad-able polyesters. This overlook is aimed at introduc-ing the basic strategies of tailoring the (supra)mole-cular structures to meet the required performance.

2. One-way SMP (1W-SMP)It is intuitive that to control the SM properties atten-tion should be focused on the manipulation of theswitching or fixing structures, or on both. The goalsbehind may be different: broadening or narrowingthe temperature interval of the switching transition,reduction of the switch temperature (i.e. Ttrans),enhancing the recovery force and speed, creation of

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Figure 1. Single SM cycles of one-way (1W) SMPs in 3Dplots, schematically. This figure also shows thereading of the Rf and Rr values

additional physical network, substitution of thephysical network by a chemical one, generation of atemporary networks etc. Next we shall introducethe actual (supra)molecular design concepts for Tg-and Tm-based SM biodegradable polyesters. This sur-vey concentrates on general strategies, which canbe adapted to other SM polymer systems, rather todeliver a detailed listing of the related develop-ments.

2.1. Molecular structure2.1.1. Tg-based systemsThe working principle of Tg-based SMPs is depictedschematically in Figure 2. As net points, remainingunaffected during deformation, molecular entangle-ments and entanglements along with crystallinedomains act in linear (i.e. thermoplastic) amorphousand semicrystalline systems, respectively. The netpoints are structural units of the above physical net-works.Semicrystalline poly(L-lactic acid) (PLLA or PLA)exhibits Tg and Tm in the ranges of 60–70 and 150–170°C, respectively. In its uncrosslinked form thecrystallites and entanglements act as net points.They can ensure, however, only moderate Rf and Rrdata. Variation of the molecular weight (MW), that

affects both entanglement and crystallization, yieldedlimited success [9]. Incorporation of hydroxyapatitenanoparticles up to 50 wt% into poly(D,L-lactide)(PDLLA) resulted in improved Rr. The initial Rrvalue of PDLLA was enhanced from ca. 80 to 98%upon filling. This was attributed to H-bondingbetween the matrix and nanoparticles creating netpoints in an additional physical network-like struc-ture [10, 11]. Accordingly, a more efficient networkshould be configured. Further, Ttrans usually higherthan Tg, is too high for biomedical applications andthus its reduction is essential. These two aspects aretargets of the ongoing research. Tg is traditionallyreduced by plasticizers. For this purpose water [12]and other plasticizers [13] were tried. To avoid themigration of the plasticizer, especially when addedin high amount, it is straightforward to crosslink thePLA [13]. So, here we already see an example for thestrategy to influence both the switches and net pointssimultaneously. This approach is quite frequentbecause manipulation of the switch phase is oftenaccompanied with a change in the net points andvice verse. This change is not always wanted butappears as a necessary ‘byproduct’ of the designstrategy followed. Just one example to underline thelatter: the crystalline phase may diminish with

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Figure 2. Molecular mechanisms of the thermally-induced 1W-SM effect in amorphous linear (thermoplastic) andcrosslinked (thermoset) polymers. Note: temporary shape is created by tensile deformation

crosslinking whereby even a Tm- based SMP can beconverted in a Tg-based one.Plasticization is, however, not the best tool to manip-ulate Tg. Plasticizer may bleed out with the time, andfacilitates the cold crystallization owing to Tg reduc-tion, as well. This substantially affects the SM prop-erties of Tg-based slow crystallizing polymers suchas PLA. A far better method is to reduce the Tg isthe synthesis of copolymers. Linear poly(L-lactide-co-#-caprolactone)s (PLACLs) were produced byLu et al. [14]. The lactide/lactone ratio was variedbetween 90/10 and 60/40 which was associatedwith a large change in the Tg (reduction from 54 to14°C) and Tm, as well. Here again Tg adjustmentwas accompanied with a notable change in Tm. Forshaping Ttrans = Tg + 15°C, whereas for shape fixingT = Tg –$15°C were selected. Rf decreased, whereasRr increased with increasing caprolactone contentof the copolymer. Copolymerization may work alsoin the other direction, i.e. toward Tg enhancement.Poly(trimethylene carbonate) (PTMC) itself is anamorphous elastomer having a Tg ~ –15°C, which isvery low for many SM applications. By copolymer-ization of TMC with D,L-lactides the Tg of theresulting copolymer (PTMC-DLLA) could be setbetween 11 and 44°C. As Ttrans (= 37°C)>Tg (= 22°C)was used for shaping (elongation to 150%), and fix-

ing occurred T = 0°C [15]. Several works weredevoted to synthesize terpolymers composed oftrimethylene carbonate, L-lactide and glycolide(PTMC-LLA-GA) [16].Crosslinking is the favored way to ‘stabilize’ thenetwork and its net points. Crosslinking usuallybrings further benefits with, such as high recoverystress and rate compared to the linear counterparts.The group of Lendlein functionalized oligomerscomposed of L-lactide and glycolide with UV-cur-able methacrylate end groups [17]. The comonomerratio and length of the chain segments in the corre-sponding polymer (PLLA-GA) were varied in theexperiments. The Tg values of the photocured ran-dom PLLA-GA copolymers were between Tg = 50–55°C. Instead of permanent crosslinks, introducedby various techniques (UV irradiation, high energyirradiation, peroxide), temporary ones can also beused. They may be – among others – photo- andthermoreversible ones. Diels-Alder type reactionsare favored for thermoresponsive SMPs. The beautyof this approach is that the thermoreversible reactionallows us to recycle the corresponding polymer viaremelting. As Diels-Alder reaction the cycloaddi-tion, occurring between furan and maleimidegroups, is preferred. This selection is due to the factthat the adduct forms and the retro Diels-Alder

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Table 1. Selected concepts and their realization for Tg-based SM biodegradable polyesters. Designation: – not reported

Material,composition

Temporary shape, ‘switching’ Permanent shape, ‘fixing’network, net points Concept, realization ReferenceTg

[°C]Ttrans[°C]

PLA (PLLA) 62–64 Tg + 15 crystalline domains + entanglements(physical network) various MW PLAs 9

PLA 49 < 227 entanglements(physical network)

simulation study: Tg manipulationby water uptake (< 2 wt%) 12

PLA ~ –50 – permanent crosslinks(chemical network)

elastic PLA gel with high amount(< 40 wt%) of plasticizer, cured byelectron beam. Swelling byplasticizer after crosslinking

13

PLACL 14–54 Tg + 15

crystalline domains + entanglements(physical network). At 40 wt%caprolactone content onlyentanglements

Tg reduction by copolymerizationthat reduced Tm, too. Copolymer athigh caprolactone contentamorphous

14

PTMCPTMC-DLLA

~ –1511–44

–Tg + 15

entanglements (physical network)entanglements (physical network)

not suited for SMTg increase via copolymerization 15

PTMC-LLA-GA 38–42 48 entanglements (physical network) terpolymerization through

compositional variations 16

PLLA-GA 50–55 – permanent crosslinks by UVirradiation (chemical network)

crosslinking of oligo(L-lactide-r-glycolide)dimethacrylate. Crosslinkdensity varied via the MW of themonomers and their ratio

17

PLA 50–65 60– 100 temporary crosslinks(thermoreversible chemical network)

furan-functionalized PLAcrosslinked with maleimidefunctional ‘linkers’ – Diels-Aldercycloaddition

18

reaction runs at rather low temperatures. Inoue etal. [18] reported that the Diels-Alder typecrosslinking of properly functionalized PLA yieldsgood SM properties. Recall that here Ttrans (= 60–100°C) is still linked with the Tg for PLA. The SMfunction was lost when the material was heated toT ~ 160°C where the retro reaction, disconnecting thecrosslinks, took place. The above strategies aresummarized in Table 1.

2.1.2. Tm-based systemsIn semicrystalline polymers Tm is linked with Ttrans.To use the melting/recrystallization transition insteadof the glass/rubbery one is beneficial since the for-mer transition is ‘sharper’ (occurring in a smallertemperature range) and faster than the latter one.Similar to Tg-based SMPs, chemical (covalent orthermoreversible) or physical crosslinking (throughphase segregated domains), and their combinations,may be the ‘net points’ which guarantee the perma-nent shape. The molecular mechanism, underlyingthe SM function in Tm-activated SMPs, is depictedschematically in Figure 3.Accordingly, the polymer is heated above its Tm anddeformed. The deformation induced temporaryshape is fixed by cooling during which crystalliza-tions takes place. Tm is never matched with the crys-tallization temperature (Tc) mostly due to the differ-ently long molecular chains (undercooling effect).Recovery occurs after heating above Tm followedby cooling below Tc. Figure 3 also highlights thatthe initial crystalline structure may be not be com-pletely restored after the SM cycle. This means areduction in the Rr value.

Unlike PLA, the ductility of PCL is very highwhich predestinates it for SM applications. The Tgand Tm of PCL are at about –50 and 60°C, respec-tively. In order to make use of its Tm as Ttrans PCLshould be crosslinked by a suitable manner. Other-wise only the chain entanglements serve as netpoints. On the other hand, Tm of PCL is rather highfor many biomedical applications and thus itsreduction is the other major goal of R&D works.

Physical networksA novel approach was recommended by Luo et al.[19] to ‘strengthen’ the physical network of PCLand thus improve its SM performance. The citedauthors prepared inclusion complexes between %-cyclodextrin and PCL. Through this host-guestcomplexation a peculiar physical network has beencreated with ‘naked’ PCL segments as ‘switches’and cyclodextrin-PCL inclusion complex domains asadditional net points. A seldom used crosslinkingtechnique is to create ionic clusters as net pointsthrough incorporation of ionic monomers in themain chain [20]. They can be, however, easily incor-porated into polyesters in polycondensation reac-tions. The ionic aggregates may restrict the crystal-lization of the main chain or its segments. Conse-quently, creation of ionomers may be a reasonableapproach to tune T8 of semicrystalline SMPs.

Chemical networksPermanent crosslinks in PCL were produced by sol-gel chemistry [21]. The related pathway containedthe alkoxysilane end grouping of an %,&-hydroxylterminated PCL, followed by hydrolysis and poly-

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Figure 3. Molecular mechanisms of the thermally-induced SM effect in semicrystalline linear (thermoplastic) andcrosslinked (thermoset) polymers. Note: temporary shape is created by tensile deformation

condensation reactions of the terminal alkoxysilanegroups. The created chemical crosslinks maintainedthe crystallization ability of PCL. The Tm and crys-tallization temperature (Tc) of PCL were, however,affected by the crosslinking degree. This is a furtherdemonstration that manipulation of the net pointsmay affect the switching phase (and vice versa).The resulting material showed both one- and two-way SM behaviors. In a companion paper the sameauthors [22] prepared peroxide crosslinked PCLwith different molecular architectures. In this casethe PCL was terminated by methacrylate and cross -linked in its melt phase. Polyhedral silsesquioxane(POSS) containing PCL with acrylate end groupswere also synthesized and photocrosslinked. Naren-dra Kumar et al. [23] elaborated a synthesis routefor producing thermally and magnetically activatedtriple-shape memory polymers using methacrylateend functionalized crystallizable PCL (Tm = 55°C)and polyethylene glycol (PEG) (Tm = 38°C). Thecopolymer was cured by peroxide in presence andabsence of silica coated magnetite nanoparticles.The outcome was a magnetically active SMP. Forprogramming of the two temporary shapes theabove Tm values were considered whereby adaptingone- or two-step shape programming procedures.Peroxide crosslinked poly(propylene sebacate),synthesized from biorenewable resources, exhibiteda Tm at about 50°C serving as Ttrans in the SM cycle.The value of Tm could be tuned by the peroxidecrosslinking and boehmite nanofillers contentyielding a temperature interval between 37 and51°C, which is close to the body temperature [24].Schmidt [25] produced thermosets from oligomeric#-caprolactone dimethacrylate and butyl acrylate byperoxide crosslinking. Into the network Fe3O4nanoparticles were also embedded to trigger the SMfunction by electromagnetic activation. The mag-netite particles in this case worked for the ‘remote’control of the temperature by transforming the elec-tromagnetic energy to heat. Tm of the PCL segments(43–49°C) was selected as Ttrans.UV-induced crosslinking is another possibility. #-caprolactone diol oligomers can easily be convertedinto the related dimethacrylates which can be effi-ciently photocured [26]. During their SM program-ming Tm of the PCL segments served as Ttrans. Thelatter could be changed between 30 and 50°C viathe MW of the PCL diol. The photosets showedexcellent SM properties in multiple cycles. Thus,

Rf = 86–97% and Rr = 92–97% data were measuredafter the 5th cycle. Crosslinkable functional groupsmay appear as end or side groups, and even in themain macromolecular chains. Garle et al. [27] mod-ified PCL homo- and copolymers by cinnamatecompound. The UV crosslinkable cinnamoyl sidegroups resulted in a crosslinked gel content of about70 wt%. SM transformation was governed by Tm ofthe PCL segments.The group of Nagata incorporated UV curable moi-eties directly into the main chain of the related poly-mers [28–31]. Thus, photocurable, biodegradablemultiblock SMPs were prepared by polycondensa-tion from PCL diol, PEG and 5-cinnamoyloxyisoph-thalic acid [28]. The latter compounds acted as UVcrosslinker without any photoinitiator. The semi-crystalline photosets exhibited Tg ~ –60°C, and Tmin the range of 35–47°C. Both Rf and Rr values werereported above 90% when selecting tensile defor-mation between 100 and 300% and Ttrans =37–60°C.A similar strategy was followed to produce pho-tocrosslinked PCL [29] and copolymers [30] com-posed of #-caprolactone and L-lactide whereby incor-porating coumarin groups in the man chain. Thecoumarin compound may participate in reversiblecrosslinking depending on the wavelength. Multi-block copolymers were also synthesized from PCLdiol and PLA diol via polycondensation with cin-namic acid compound [31]. Again, the latter wasresponsible for reversible photocuring that did notaffect the crystallization of PCL but reduced that ofPLA. Choosing Ttrans = 40–60°C, tensile elonga-tions 100–500% and fixing temperature 22°C, Rfand Rr data between 88 and 100% have been meas-ured. The authors emphasized that reversible pho-tocrosslinking may be a promising way to producelight-sensitive SMPs.Defize and coworkers [32, 33] used the Diels-Alderreaction to create temporary crosslinks. They synthe-sized star-shaped PCLs with furan, anthracene andmaleimide end functionalities, respectively. Theresulting SMPs showed excellent Rf and Rr values,even after 4 cycles, using Tm of the PCL as Ttrans(= 65°C). The outstanding SM behavior wasrestored after recycling of the corresponding sys-tem. Note that recycling means here the onset of theretro Diels-Alder reaction running at T " 105°C.The anthracene end functionalization, instead offuran, was foreseen to influence the kinetics of theadduct formation and its temperature stability.

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Apart from peroxide (free radical-induced) andphotocrosslinking, radiation crosslinking methodscan also be adapted. The Zhou and coworkers [34,35] used '-irradiation to crosslink PCL is presenceand absence of various sensitizers and studied theSM behavior as a function of radiation dose anddosage rate. Recovery of the related PCLs werestudied at T = 56°C. With increasing radiation dosethe recovery temperature could be decreased due toa similar decrease in Tm. Introduction of polyfunc-tional polyester acrylates proved to be suitableadditives to enhance the crosslinking degree of PCLupon '-irradiation [35].

CopolymersCopolymerization is the most widely used tech-nique to control Tm and also to tune the biodegrad-ability of polymers. Biobased polyesters composedof 1,3-propandiol, sebacic acid and itaconic acid invarious ratios showed excellent SM properties aftercrosslinking with peroxide. Ttrans could be tuned bythe composition between 12 and 54°C [36]. Bacter-ial synthesis may yield semicrystalline polyhydrox-yalkanoates of various compositions and various Tmdata. Ishida et al. [37] have shown how a bacterialPHA, namely poly(3-hydroxyoctanoate-co-3-hydrox-yundecanoate), can be crosslinked by POSS deriva-tive making use of UV-assisted thiol-ene coupling.The thiol compounds were tetrathiol crosslinkerand thiol functionalized POSS. The feasibility ofDiels-Alder coupling was shown on the example ofbiodegradable polyester by Ninh and Bettinger[38]. In this case hyperbranched poly(glycerol-co-sebacate) with pendant furan groups was coupledwith bifunctional maleimide crosslinker to producean elastomeric material. POSS moieties may serveas crosslinking sites, as well. This material is, how-ever, owing to its composition not a Tm- but a Tg-activated one.‘Graft’ and multiblock copolymers, both in linearand crosslinked forms, may have excellent SM prop-erties, as well. Linear, cyano functionalized poly-norbornene having long PCL side chains showedgood SM properties according to the report of Yanget al. [39]. This polynorbornene grafted by PCLalong with the cyano groups in the main chainshowed higher ductility and strength than theunmodified polynorbornene. Moreover, Tm of thephase separated PCL could be well used for Ttrans inSM tests.

There are many possibilities to tailor the propertiesof SMPs via their block (segmented) architecture.Polyurethane (PU)-based systems are the mostwidely prepared and studied SMPs. Their develop-ment started in the 1990s by Hayashi, working forMitsubishi Heavy Industries in Nagoya, Japan [40],and they are still in the focus of both industrial andacademic interests [41]. This is, in particular, due tothe highly versatile PU chemistry. The reason whywe are mentioning them here is that the polyolsused for PU synthesis are typically biodegradablepolyester-based. The interested reader might havealready noticed that the #-caprolactone chemistryplays an important role in the reported polyester-related SMPs. PCL diols are traditional buildingblocks of PUs. Recently #-caprolactone based oligo -mers with various molecular architectures becameplatform chemicals for PU and polyester syntheses[42]. The interest behind this development is due tothe complete biodegradability, low Tg and relativelylow Tm of PCL. PCL-based PUs are segmented poly-mers with excellent SM behaviors. The reaction ofdiisocyanate with low MW diol chain extenders yieldsthe ‘hard’, whereas the PCL chains give the ‘soft’segments. Pioneering work in this field should becredited to Kim et al. [43]. It is generally acceptedthat the exceptional shape memory behavior of suchPUs is related to their phase segregated morphol-ogy. For thermoplastic version the hard phase isresponsible for memorizing the permanent, whereasthe soft one for the temporary shape and its fixing[41, 43, 44]. As a consequence, Ttrans is linked to theTm of PCL. In crosslinked PUs, the crosslinked net-work guarantees the permanent shape and thus tran-sitions related to the hard phase may also beinvolved in SM programming. The research on SMPUs has many similarities with that on SM poly-esters. Peculiar attention was paid at the copolymerarchitecture. Enormous efforts have been dedicatedto modify both soft and hard phases in order toimprove the SM performance of both linear andcrosslinked PU-based SMPs. For example, tripleshape memory PU systems were produced recentlyby generating intermolecular H-bonds between car-boxyl groups of the related PU chains and pyridinerings of cholesteryl isonicotinate mesogenic units.The SP PU itself contained PCL diol and thus oneof the switch temperatures was the Tm of PCL. Theother switch phase, having ca. 10°C higher temper-ature than Tm of PCL, was given by the domains

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hold together by H-bonding. The Rf and Rr data ofthe latter was lower than those triggered by Tm ofthe PCL [45]. Note that this concept falls into thecategory of supramolecular assembly – see later.Relevant strategies followed for Tm-based SMPsare listed in Table 2.

2.2. Supramolecular architecturingSupramolecular structures are scaled above molec-ular level. Their constituents may be highly differ-

ent nano- and micronsized entities, domains. Therelated structures involve nanodimensional molecu-lar ‘strings’ (in some interpenetrating networks(IPNs)), disperse or continuous phases in microme-ter range and even processing-induced higher orderstructures (skin-core or multilayer structures). TheSM function of the related systems is further on Tg-or Tm-based. Scheme of the morphology of therelated SMPs is given in Figure 4. The authors haveto underline here that the above definition differs

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Table 2. Selected concepts and their realization for Tm-based SM biodegradable polyesters. Designation: – not reported

Material,composition

Temporary shape, ‘switching’ Permanent shape, ‘fixing’network, net points Concept, realization ReferenceTm

[°C]Ttrans[°C]

PCL 59 90 entanglements + inclusioncomplexes (physical network)

host-guest complexation withcyclodextrin in solution 19

Poly(oxyethylene-block-butyleneadipate)

36–42 40 entanglements + ionic clusters(physical network)

ionomer copolymer formation viapolycondensation 20

PCL 25–57 Tm + 40 permanent crosslinks (chemical network)

crosslinking of suitable end groupfunctionalized PCLs through sol-gelchemistry and peroxide. Preparationof 1W- and 2W-SMPs

21, 22

PCL 20–40 (PEG),>50 (PCL) 40 and 70

permanent crosslinks – peroxide induced (chemical network)

conetwork formation using acrylatefunctionalized PCL and PEG. PCLand PEG segments crystallizable andexploited for triple-shape memoryfunction – triggered thermally andmagnetically

23

PCL 46–56 70permanent crosslinks – free radical induced (chemical network)

crosslinking of oligo(#-caprolactone)dimethacrylate; magneticallyactivated by incorporation of Fe3O4nanoparticles

25

PCL 44–55 70permanent crosslinks – UV irradiation (chemical network)

crosslinking of oligo(#-caprolactone)dimethacrylate in presence andabsence of photoinitiator

26

PCL 29–55 60permanent crosslinks by UVirradiation without initiator(chemical network)

caprolactone functionalized withcinnamate group; random and blockcopolymers synthesized; Tm tuning byMW of the caprolactone segments builtin the network

27

PCL, PCL-LLA 35–47 37–60permanent or temporarycrosslinks by UV irradiation(chemical network)

crosslinking by photosensitive groups,covalently built in bypolycondensation

28–31

PCL 44 65temporary crosslinks(thermoreversible chemicalnetwork)

Diels-Alder reaction of star-shapePCL with furan and maleimidegroups, effect of retro reaction on SMproperties studied

32, 33

PCL 55–56 52–55permanent crosslinks –'-irradiation (chemical network)

crosslinking by high energyirradiation, also in presence ofmultifunctional sensitizer

34, 35

Poly(propylenesebacate)copolymers

42–54 Tm + 20permanent crosslinks –peroxide induced (chemical network)

Tm depression by polycondensationvia varied composition 36

SM PUs ofdifferentcompositions andstructures

depends on theactual Tm of

thecrystallizing

segment(PLA-,

PCL-based)

broad rangeabove Tm

different physical andchemical networks

Tm reduction via crystallization inconfined space, MW change of thecrystallizable diols, multiblock andsupramolecular architectures…

41, 43–45

from that one often used for supramolecular materi-als and supramolecular chemistry. Supramolecularchemistry deals with compounds with non-covalentbonds and interactions. Recall that our above classi-fication covers a much broader range than that oneof supramolecular chemistry.

2.2.1. BlendsBecause PLA is highly brittle, it has been blendedwith numerous polymers to improve its toughness.A ‘byproduct’ of this research was the observationthat some blends, in fact, showed SM feature. Laiand Lam [46] studied the SM performance of PLA/thermoplastic PU (TPU) blends at 70/30 and 50/50compositions. TPU was found in dispersed form at70/30 ratio, while a bicontinuous phase structure wasconcluded for the PLA/PU = 50/50. After deform-ing the specimens at Ttrans = 25, 80 and 120°C, therecovery was assessed in the temperature range T =20 to 160°C. Note that the selected Ttrans data arebelow and above of the Tg of the PLA (ca. 80°C).Rf, Rr and the recovery stress strongly depended onTtrans and recovery temperatures. With increasingTtrans Rf increased while an adverse trend wasobserved for Rr. Zhang et al. [47] demonstrated SM

behavior for PLA toughened by a polyamide-12based elastomer, which was incorporated up to30 wt%. For Ttrans of the tensile loaded specimensroom temperature was selected, which is in betweenthe Tg of the polyamide elastomer (Tg ~ –50°C) andthat of the PLA (Tg = 75°C). Recovery was trig-gered at temperatures above the Tg of PLA. TPUelastomer (Tg ~ –35°C) was blended with PLA in10 wt% with and without multiwall carbon nano -tubes (MWCNT) after various surface treatments[48]. The latter was introduced in 10 wt% to achieveelectroresponsive SM. For temporary shaping Tg ofPLA was considered. Rr decreased with increasingnumber of the electroactivated thermal cycles. Thiswas attributed to the formation of ‘frozen in’ crys-tals in the dispersed PLA phase. PLA/PCL blends inthe compositions range of 100/0 to 60/40 were pro-duced with and without additional MWCNT byAmirian et al. [49]. The phase segregated blendsexhibited two Tg and two Tm values. The latterincreased with increasing amount of MWCNT. ForTtrans = Tg(PLA) + 15°C, while for shape fixingTg(PLA) –$15°C were chosen. Rr was measured atT = 70°C where the melting of PCL is also involved.As a consequence, both Rf and Rr decreased with

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Figure 4. Scheme of the morphology changes during thermally-induced SM cycles in thermoplastic blends, semi and fullIPN structures, respectively. Note: temporary shape is created by tensile deformation

increasing PCL content of the blends. Rf was mar-ginally affected, while Rr went through a maximumas a function of the MWCNT content (0–3 wt%)during the tensile deformation SM tests. PCL workedas efficient switching phase also in styrenic thermo-plastic rubbers, such as styrene-butadiene-styreneblock copolymer [50]. Rf increased steeply beforeleveling off above 30 wt% PCL content. An oppo-site tendency, almost a mirrored picture of the Rfcourse, was found for Rr as a function of the PCLcontent that was varied in the whole compositionrange. This behavior was traced to the actual mor-phology of the blends. As long as the thermoplasticrubber phase remained continuous excellent Rf andRr data were measured.Crosslinking is a useful tool to improve the SMbehavior as already quoted. This technique has beenadapted for PLA/PEG blends investigated in therange of 100/0 to 70/30. Crosslinking occurred byadding blocked polyisocyanate. Tg of the amorphousphase, composed of both PLA and PEG, served toselect the Ttrans. Both Rf and Rr increased with increas-ing amount of the polyisocyanate crosslinker. Thecrosslinking reduced also the recovery time of thecorresponding blends [51].The above treatise makes clear that acceptable SMproperties can be achieved mostly by blend withbicontinuous phase structure (cf. Figure 4). The sta-bilization of the latter is, however, very challeng-ing. It is the right place to mention that to distin-guish between thermoset-containing semi IPN andIPN systems the term bicontinuous was used forthermoplastic blends.

2.2.2. Processing-induced structureRadjabian et al. [52] used spun PLA filament,wound in helical form, for SM testing. The filamentitself has a complex processing-induced supramole-cular structure which does not change in the SMcycle. Thus, Rr did not change with Ttrans (70–90°C),but remained still modest (~50%). By contrast, Rfdeceased with increasing Ttrans. Wang et al. [53]reinforced PLACL by in situ produced micro- andnanofibers from poly(glycolic acid) (PGA). Theauthors generated the PGA fibers in PLACL throughin line extrusion stretching. Note that this methodbasically follows the microfibrillar composite con-cept of Fakirov ([54] and references therein). Thelactide/lactone ratio of 82/18 resulted in an amor-phous PLACL with a Tg of 22°C. For the temporary

shape Ttrans = Tg + 15°C, and for its fixing T =Tg –$15°C were selected. The PGA phase supportedboth shape fixing (restraining the molecular chainmovement) and recovery (acting as additional netpoints).Du et al. [55] investigated effects of layered archi-tecture and blend morphology on the SM behaviorusing TPU and PCL. The TPU/PCL ratios set were75/25, 50/50 and 25/75, respectively. The layeredstructure was achieved by a special multilayer coex-trusion technique. The layer thickness varied withthe composition ratio whereby keeping the numberof layers and the overall thickness of the multilayerfilm as constants. Tm of PCL served as Ttrans(= 70°C) during shape creation that was fixed at T =21°C. Rf of the multilayer film was the higher thelower its PCL content was, and it changed as a func-tion of the thermomechanical cycles only margin-ally. The TPU/PCL blend at 50/50 ratio outperformedthe multilayer film with respect to Rf at the samePCL content. On the other hand, all other blendsshowed inferior Rf data to the multilayer film.

2.2.3. Supramolecular networksCombination of crosslinkable resins with linear andcrosslinkable biodegradable polyesters may resultin various structures. Micron-scaled dispersion ofthe linear polyester in a crosslinked thermoset sys-tem is the usual prerequisite of toughness improve-ment of the latter. The dispersion is generated byphase separation upon curing. Such systems mayshow SM properties though this is not yet reportedfor systems with biodegradable polyesters. Far moreinteresting are, however, those systems which fea-ture conetwork, semi interpenetrating (semi IPN)and full IPN structures.

ConetworksConetworks are chemically crosslinked networks inwhich none of the constituents forms a continuousphase. In ideal case the conetwork is on molecularlevel. Its formation is, however, often accompaniedwith ‘homocrosslinking’ and the related domainsare on supramolecular level. That is the reason whywe are treating them here. Conetworks differmarkedly from the grafted IPN structure in whichboth phases are continuous and chemically coupled.Li et al. [56] prepared conetwork structured ther-mosets by the peroxide induced copolymerizationof methyl methacrylate and PCL dimethacrylate.

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The poly(methyl methacrylate)/CL ratio was variedbetween 80/20 and 20/80. The related systemsexhibited a single Tg that changed as a function ofcomposition in a broad range (Tg = 110…–20°C).The broad Tg relaxation may be used to memorizemultiple shapes via careful programming. Theauthors showed that the related conetwork mayshow quadruple SM, i.e. ‘remembering’ to threetemporary shapes. The prerequisite for that is thatthe energy stored in the partitioned Tg range duringtemporary shaping should be sufficient enough forshape fixing during cooling. Erden and Jana [57]modified SM PU with polybenzoxazine. The pre-cured benzoxazine with its phenolic hydroxylgroups can react with the polyisocyanate wherebyforming a conetwork with the polyurethane. Poly-benzoxazine appeared in the PU matrix in phasesegregated nanoscale domains. They can be treatedas net points of a second fixing phase in addition tothe hard segments of the PU. Cocrosslinking withbenzoxazine shifted the Tg of the correspondingsystems toward higher temperatures. At the sametime the recovery stress was doubled compared tothat of the reference PU. Enhancing the recoverystress is a very actual research direction to meet thedemand of sensors and actuators.Epoxy (EP)-PCL conetworks were produced byLützen et al. [58]. Crystalline PCL domains, over-taking the role of ‘switch’ phase, were covalentlyintegrated into the cationically polymerized EP net-work. The EP/PCL ratio has been varied between85/15 and 60/40. After deformation at Ttrans = 70°Cand fixing at T = 20°C, Rf of 100% was measured.

Semi interpenetrating networks (semi IPNs)Unlike conetworks, semi IPNs are composed of twocontinuous phases from which one is of thermo-plastic nature. It is intuitive that the related entan-gled structure should contribute to the onset of SMproperties. This was confirmed recently [59]. It isworth noting that entangling of the phases in semiand full IPN structures is never on molecular levelthough the materials may feature one single Tg [60].‘Full’ IPN denotes that both constituent continuousphases are crosslinked polymers. Semi IPNs may notonly have SM, but also self healing properties.Though the term ‘shape memory assisted self heal-ing’ was coined by the Rodriguez et al. [61], theconcept should be credited to Karger-Kocsis asquoted by Yuan et al. [62].

The group of Rodriguez et al. [61] produced semiIPNs containing high MW PCL as thermoplastic andtetra thiol crosslinked low MW PCL diacrylate asthermoset phase. The linear/network PCL ratio wasstudied between 0/100 and 80/20. At 200% tensiledeformation Rf increased slightly, whereas Rrdecreased sharply with increasing amount of thethermoplastic PCL. Self healing was demonstratedon partially broken double edge notched tensileloaded specimens making use of the essential workof fracture protocol [63]. Healing happened by wet-ting, diffusion and randomization of the linear PCLcomponent above its Tm, viz at T = 80°C.Quasi semi IPN structure can be produced by otherways. Such a structure is given when electrospunPCL nanofiber mat is infiltrated by EP followed bycuring of the latter. This material showed tripleshape behavior. For setting the two temporary shapesTm of PCL and Tg of EP served. Tg of EP was belowthe Tm of PCL [64]. Fej(s et al. [65] not only con-firmed this concept on the same material combina-tion but compared the SM behavior with that of a‘real’ semi IPN structured EP/PCL at the same com-position (cf. Figure 5). The latter was generated inone-pot synthesis via phase segregation. The stor-age modulus vs temperature traces of the PCLnanoweb containing EP and semi IPN structuredEP/PCL were similar (cf. Figure 6). The dynamicmechanical analysis (DMA) traces clearly showthat both EP systems have a bicontinuous phasestructure because their storage moduli do not dropat the Tg of EP which would happen for PCL dis-persed in the EP matrix. Instead of that, the moduli

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Figure 5. Etched (with methylene chloride) fracture surfaceof EP/PCL (= 77/23 wt%) displaying a semi IPNstructure. Note: the lacy structure is due to thepartially removed PCL phase

run in between those of the PCL and EP. The SMproperties of the EP/PCL with semi IPN structurewere somewhat better than that with the electrospunPCL nanofiber mat. Note that the above semi IPNversion should have self healing activity, whichwas, however, not yet tested.

Interpenetrating networks (IPNs)Full IPN structured thermosets are also suitableSMP systems. It was shown by Zhang et al. [66].The IPN structure was composed of crosslinked PU(PLGA-based diol crosslinked by polyisocyanate)and crosslinked PEG dimethacrylate (cured by UVirradiation). The content of the latter was variedbetween 0 and 50 wt%. The IPNs were amorphousand exhibited a rather broad Tg range (Tg between –23and 63°C) that served for selection of Ttrans. Both Rfand Rr data were reported over 93%. Kanazawa andKawano filed a patent on electron beam crosslinkedPLA/polystyrene having semi IPN or full IPNstructures [67].

3. Two-way SMP (2W-SMP)The 2W effect in bulk SMPs is linked with the pres-ence of a reversible melting/crystallization switch-ing segment. Accordingly, for 2W-SMPs Ttrans = Tm.This is, however, only a necessary but not sufficientprerequisite. 2W-SMPs exhibit two distinct featurescompared to 1W-SMPs. First, the shape changeoccurs between two temporary ones. Second, to trig-ger the reversible shape change a certain stress shouldbe steadily maintained. The preoriented chain seg-

ments crystallize upon cooling. This extends thespecimen due to the crystallization heat released.Upon heating, the crystallites melt and the materialcontracts, shrinks. It is obvious that in order to meetthe requirement, viz to return in the initial tempo-rary shape during heating, the related structureshould be crosslinked. In a linear system namelyviscous flow would occur excluding the SM func-tion. This, 2W-SMPs possess crosslinked (co)net-works. During their design the network deformabil-ity should be adjusted to the deformation (in thiscase stress)-induced crystallization capability of theswitch segments. This means that the MW of theswitching phase should be higher than that of themean MW between crosslinks. The SM cycle of a2W-SMP is given in Figure 7. PCL-based 2W-SMPswere already produced and tested by Pandini andcoworkers [21, 22].Recently, another strategy was proposed to produce2W-SM polymers. Behl et al. [68] suggested thatcreation of a special molecular conetwork (skele-ton) containing reversible crystallizing/melting seg-ments can overtake the job of external stress. Impor-tant prerequisites of this strategy are: presence ofnanoscaled crystalline domains which are keptaligned by the skeleton network (thereby overtak-ing the role of external loading) and a broad meltingrange of the related crystallites. In the reversibleshape change, i.e. extension upon cooling and con-traction upon heating, only a given population ofthese crystals takes part. The concept has been

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Figure 6. DMA traces (storage modulus vs temperature) ofthe EP with electrospun PCL nanoweb, semi IPNstructured EP/PCL and the constituting PCL andEP. Notes: the PCL content in the EP/PCL combi-nations was 23 wt%.

Figure 7. Single and multiple SM cycles of 2W-SMPs in 3Dplots, schematically. Note that reversible shapechange occurs between two temporary shapeswhereby maintaining the stress.

proven also for a conetwork composed of crys-talline PCL (exhibiting a very broad melting range)and poly(n-butyl acrylate) having the necessaryelasticity.

4. ConclusionsBiodegradable polyester-based SMPs are mostlyused in and developed for applications in the humanbody (surgical sutures, catheters and stents). That isthe reason why many R&D works addressed theadjustment of Ttrans, related to Tm or Tg, respectively,to the body temperature. The other aspect, usuallycovered in the related research, is the biodegradabil-ity. Note that the SM characteristics are markedlyreduced with the degradation time. This was shownon example of PCL-based systems degraded in vitro[69]. Ttrans adjustment to body temperature and con-trolled biodegradability will remain preferredresearch topics further on [70]. Research on the SMbehavior of bacterially synthesized aliphatic poly-esters will be intensified.There are, however, some general trends with SMPs.Nowadays considerable research efforts are under-taken to produce 1W multi-shape and 2W-SMP sys-tems. Besides, ensuring multi-functionality (such asshape memory combined with self healing) becomean emerging issue. Instead of direct thermal, manyworks are in progress to trigger the SM function byindirect heating via electric and magnetic fields.For this purpose suitable inorganic nanofillers areincorporated. The possibility of remote actuationwill be extended for the SM performance of nanos-tructures such as electrospun nanofiber structures[71]. Chemically-induced SM functions, especiallythose triggered by water swelling and dissolution,will be investigated. SM hybrid composites willbecome under spot of interest [72]. Apart from exper-imental works, modeling and simulation studieswill be increasingly performed. From the viewpointof (supra)molecular design the following tenden-cies can be predicted:Linear SMPs: creation of additional physical cross -links through host-guest complexation and ionicclustering. For their realization researcher will adaptachievements from other fields, for example forionic clustering the works on solid electrolytes mayshow the right directions [73]. Further attempts willbe made to stabilize the bicontinuous structures ofblends by adding nanofillers, polymeric coupling

agents and even by selective crosslinking. Note thatthe latter means a transition toward semi IPN.Crosslinked SMPs: conetworks, semi and full IPNswill become under spot of interest. They offer manybenefits, such as enhanced recovery stress, broad Tgrange, multiple Tg relaxations or Tm values (impor-tant for multi-shape programming). The non-poly-ester compounds in these crosslinked networks willbe derived from renewable resources. So, petro-based EP will be replaced by epoxy functionalizedplant oils [74]. Exploring various reversible cross -linking mechanisms, known under the heading of‘click chemistry’, seems to be a very promisingroute.

AcknowledgementsThe work reported here was supported by the HungarianResearch Fund (OTKA NK 83421) and by the TÁMOP-4.2.2.A-11/1/KONV-2012-0036 project co-financed by theEuropean Union and the European Social Fund. This workwas also supported by the European Union and the State ofHungary, co-financed by the European Social Fund, in theframework of TÁMOP-4.2.4.A/2-11/1-2012-0001 ‘NationalExcellence Program’ (S. K.). The authors thank M. Fej(s forher help in preparation of the artwork.

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