thermally activated reversible shape switch of polymer particles

Journal ofMaterials Chemistry B

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Thermally activa

Key Laboratory of Advanced Technologies of

of Materials Science and Engineering, So

610031, P. R. China. E-mail: shaobingzhou@

† Electronic supplementary informa10.1039/c4tb01155d

Cite this: J. Mater. Chem. B, 2014, 2,6855

Received 14th July 2014Accepted 19th August 2014

DOI: 10.1039/c4tb01155d

www.rsc.org/MaterialsB

This journal is © The Royal Society of C

ted reversible shape switch ofpolymer particles†

Tao Gong, Kun Zhao, Wenxi Wang, Hongmei Chen, Lin Wang and Shaobing Zhou*

The particles that can reversibly switch shape in response to an environmental stimulus are preferable for

controlling the performance of drug carriers. In this work, we present a facile strategy towards the

design and fabrication of polymer particles that can reversibly switch their shape on the basis of a

biocompatible and biodegradable polymer network containing well-defined six-arm poly(ethylene

glycol)-poly(3-caprolactone) (6A PEG-PCL). These polymer particles have a capacity of reversibly

changing shape from spherical to elliptical either extracellularly or intracellularly with the cyclic heating

and cooling between 43 �C and 0 �C under a stress-free condition via a reversible two-way shape

memory effect (2W-SME) of a polymer matrix. This study of the shape-switching particles opens up

exciting possibilities for engineering dynamically shape-switching drug delivery carriers to either avoid or

promote phagocytosis.

Introduction

Polymeric particles have been widely used as carriers for drugdelivery and medical imaging because they have an ability ofdelivering their cargos preferentially into a targeted location,and in turn enhance the therapeutic and diagnostic efficacy.1–3

Particle shape has been recently recognized as an importantdesign parameter4 because it has a strong impact on the carrierperformances, such as phagocytic internalization,5–8 transportin the vasculature,9 blood circulation half-life,10,11 and targetingefficiency.12,13 The optimization of particle geometry is signi-cantly vital to enhance particle functions in biomedical appli-cations. The general approaches for synthesizing particles haveinvolved “bottom-up” chemical processes14–16 and “top-down”fabrication techniques.17,18 To date, the one-way shape memoryeffect of microparticles has received signicant attention.Andreas Lendlein et al. reported a strategy to prepare andevaluate microsized SMP model particles; processes for thepreparation of SMPMP-loaded water-soluble polymer lms withtailored mechanical properties were developed and applied forprogramming the SMP MP into a temporary ellipsoid shape bylm stretching.19,20 Biqiong Chen et al. reported that polymercomposites with excellent water-active shape-memory effects(SMEs) were successfully prepared using polyvinyl alcohol (PVA)submicron particles as the SME-activating phase and thermo-plastic polyurethane (TPU) as the resilient source and matrix.21

In this investigation, we present a facile strategy towards the

Materials, Ministry of Education, School

uthwest Jiaotong University, Chengdu,

swjtu.cn; [email protected]

tion (ESI) available. See DOI:

hemistry 2014

design and fabrication of polymer particles that can reversiblyswitch their shape on exposure to a thermo stimulus. Theshape-switching function is endowed by the reversible two-wayshape memory effect (2W-SME) of a polymer matrix. Thesechangeable shapes of the polymer particles are programmedfrom spherical shape to predesigned shapes using a modiedlm-stretching technique similar to the one described byChampion et al.5 Unlike other abovementioned approaches, ourstrategy to achieve various shapes of particles is a combinationof a simple processing technique with smart-responsivematerial.

Shape memory polymers (SMPs) are an exciting class ofsmart materials and present great potential applications inintelligent biomedical devices for their ability to change theirshape as demanded in response to a tailored environmentalstimulus.22–25 Among these stimuli-responsive SMPs, thermallyinduced SMPs are frequently investigated, and their shapechange usually displays a one-way character. They can onlychange their shape in a predened pattern from a singletemporary shape (B) to another permanent shape (A) by heatingthese materials above a characteristic temperature, commonlydened as the transition temperature (Ttrans).26–28 The shi ofthe dual shape is irreversible, and any one-way shape memorycycles going from the original shape to the previous temporaryshape always requires the application of an external mechanicalmanipulation.29 As a result, their applicability can be restrictedbecause some applications, such as the realization of actuators,articial muscles and controlled drug release on demand, areversible shape change can be highly required on cooling andheating.30,31

Most recently, smart materials with two-way shape memoryeffect (2W SME) have attracted considerable attention because

J. Mater. Chem. B, 2014, 2, 6855–6866 | 6855

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Journal of Materials Chemistry B Paper

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they are capable of undergoing reversible dimensional varia-tions between two distinguished geometries, when they areexposed to an external stimulus. Nowadays, semicrystallinepolymer networks have been reported as an interesting solutiontowards 2W SME because their synthesis is easy and cheap incontrast to liquid crystalline elastomers or other solutions.32–37

However, most of the reported 2W SMEs were realized under theapplication of a constant stress, when these materials weresubjected to cooling-heating cycles. If they are utilized in thebody, the constant stress cannot be conveniently carried out.Moreover, the biocompatibility should be seriously consideredif the material is designed for biomedical applications. Inaddition, along with the trends of miniaturization, small-scaleactuators for the use of SMEs will be required.38,39 Micrometer-sized carriers of switchable shape may allow a tailored bio-distribution and new concept of targeting in drug delivery,based on their shape dependent biorecognition.

Accordingly, in this study, our concept for implementing athermally switched two-way reversible shape memory inmicrometer-sized spheres is a cross-linked network of abiocompatible star-shaped polymer, in which actuator domains(free PEG-PCL molecular chain) and shiing-geometry deter-mining the domains (the cross-linked PEG-PCLmolecular chain)coexist. Currently, biodegradable shape-memory poly(3-capro-lactone)-co-poly(ethylene glycol) networks are frequently adop-ted.40,41 This network is fabricated on the basis of well-denedsix-arm poly(ethylene glycol)-poly(3-caprolactone) (6A PEG-PCL)functionalized with vinyl end groups, and the predesignedarchitecture is easily achieved by photo crosslinking. In regardsto the polymer composition, PCL is frequently considered as aninteresting material for potential shape memory applications inthe biomedical eld, due to its biodegradability. However, thehigh crystallinity reduces its compatibility with so tissues andlowers its biodegradability.42 Six-arm poly(ethylene glycol) (6APEG) possesses excellent hydrophilicity and nontoxicity, and itscopolymerization with 3-caprolactone monomers can overcomethe drawbacks of PCL and reduce the transition temperature ofshape memory close to the body temperature. In contrast toother polymers with 2W SME, this polymer network is tailoredfor biomedical applications as a reversibly changeable microm-eter-sized carrier for drug delivery. To explore the SME on amicrometer scale, the spherical particles with an average diam-eter of 5 mm were prepared by an oil-in-water (o/w) emulsiontechnique. The particles with an ellipsoidal temporal shape wereachieved by PVA lm stretching at 60 �Cwith subsequent coolingto 0 �C. The reversible shape memory recovery between thespherical and ellipsoidal shape was realized with the cyclicheating and cooling between 43 �C and 0 �C. In particular,macrophages were used as a model cell for the investigation ofthe internalization of these particles and subsequent intracel-lular shape memory recovery.

Experimental sectionMaterials

3-Caprolactone (3-CL, 99.9%, Aldrich) was distilled over freshlypowdered CaH2 under a reduced pressure; 6-arm polyethylene

6856 | J. Mater. Chem. B, 2014, 2, 6855–6866

glycol (6A PEG, Mn z 6000) was purchased from LimingResearch Institute of Chemical Industry in Luoyang (China).Diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO), stan-nous chloride (99%, SnCl2) and polyvinyl alcohol (PVA 1788,average Mw 88 000, degree of hydrolysis % (m/m) 87.0–89.0)were purchased from Chengdu Kelong Chemical ReagentCompany (China). Nile red was purchased from Adamas. Allother chemicals and solvents were of reagent grade or better.

Fabrication of chemically crosslinked 6A PEG-PCL

6A PEG-PCL was rst synthesized via a ring-opening poly-merization of 3-CL with 6A PEG as the initiator in the presenceof SnCl2 as the catalyst.43 A typical experimental procedure forthe synthesis of the copolymers designated as 6A PEG-PCL isdescribed as follows: preweighed 3-CL, 6A PEG and SnCl2(1 wt%) were quickly added into a 50 mL bottom ask with astopcock, which was preheated to remove the moisture.Subsequently, the reaction system was degassed undervacuum for 4 h with continuous stirring to produce a well-mixed molten phase and treated at 140 �C for 6 h. Theobtained copolymer was then dissolved in dichloromethane.Aer suction ltration, the polymer solution precipitated inexcess cold mixed liquid (Vethyl alcohol : VH2O ¼ 5 : 1). The 6APEG-PCL in this mixture has been puried of residual oligomerand catalyst. To obtain the 6A PEG-PCL with vinyl end groups(6A PEG-PCL-AC), acryloyl chloride was then reacted with theend groups of the chains of 6A PEG-PCL in the presence oftriethylamine for the removal of hydrogen chloride from theproducts (Macryloyl chloride : Mtriethylamine : M6A PEG-PCL ¼ 6 : 6 : 1)1.The obtained 6A PEG-PCL-AC was again puried in excess coldmixed liquid (Vethyl alcohol : VH2O ¼ 5 : 1) as 6A PEG-PCL.Subsequently, preweighed 6A PEG-PCL-AC and 5 wt% TPO weredissolved in CH2Cl2 with stirring to ensure uniformmixing. Themixture was then cast on a mold made of polytetrauoro-ethylene (PTFE), and the mold was evaporated in an aeratorovernight and dried at room temperature (22 �C). Finally, thecompletely dried 6A PEG-PCL-AC/TPO was kept in an oven at60 �C to melt it. Photo-crosslinking of 6-arm PEG-PCL-AC/TPOwas initiated by a UV light (l ¼ 360 nm) generated from a high-intensity long wave UV lamp (100 W, intensity: 400 mW cm�2)for 30 min.

Characterization

FT-IR was recorded on a Nicolet 5700IR spectrometer. All thespectra were obtained by KBr plates in which dry polymer powerwas mixed with KBr at a weight ratio of 0.5–1%. The hydroxylnumbers were determined according to the Chinese NationalStandard GB 12008.3-89. The amount of reactive hydroxylgroups on 6A PEG, 6A PEG-PCL and 6A PEG-PCLAC weremeasured by the titration of excessive o-phthalic anhydridegroups. The molecular weights of 6A PEG, 6A PEG-PCL and 6APEG-PCLAC were determined using GPC (Waters 1515, USA)measurement, dimethylformamide (DMF)/LiBr was used as theeluent at a ow rate of 1 mL min�1 at 40 �C, and polymethylmethacrylate (PMMA) was used as the reference. 1H-NMRspectra were recorded with a Bruker AM-300 spectrometer.

This journal is © The Royal Society of Chemistry 2014


Scheme 1 Programming of spherical particles to their temporaryshapes.

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Tetramethylsilane (TMS) was used as the internal standard andCDCl3 was used as the solvent for 6A PEG-PCL. Differentialscanning calorimetry (DSC) was employed to measure thethermal properties of polymers on a TA instrument (Q100, USA).Nitrogen was used as the purge gas with a ow rate of 20 mLmin�1, and the rate of temperature increase was 0.5 �C min�1.Dynamic mechanical analysis (DMA) was carried out on aDMA983 analyzer (TA Instruments, USA), using a tensile reso-nant mode at a heating rate of 5 �C min�1 from �10 �C to 60 �Cat a frequency of 1 Hz. Gel fraction estimate was performedaccording to the previously reported method.44 In situ X-raydiffractometry (XRD) (Philips, X'Pert PRO, Netherlands) wasused to determine the crystallization property of crosslinked 6APEG-PCL networks. The scanning range was 5–40�. Thesesamples were heated from 25 �C to 60 �C, and cooled to 0 �C; inthe second heating process, the data was recorded when itreached the desired temperatures and isotherm for 1 min. Thewater contact angle (CA) was measured using a sessile dropmethod at room temperature with the contact angle equipment(DSA 100, Kruss, Germany).

Fabrication of micrometer-sized particles

The spherical particles were fabricated with an oil-in-water (o/w)emulsion technique. First, 0.25 g 6A PEG-PCL-AC and 0.0125 gTPO were dissolved in 8 mL CH2Cl2 with Nile red, a red-uo-rescent dye that stains the polymer red. 0.5 g PVA was dissolvedin 100 mL deionized water containing 100 mL Span-80 as asurfactant, and the solution was used as the emulsion stabilizer.The PEG-PCL-AC/TPO/Nile red mixed solution was dropwiseadded to the PVA solution using a disposable syringe (21 gauge)with mild stirring (200 rpm) at room temperature. Aer thecomplete removal of the organic solvent in order to obtainevenly distributed crosslinked microparticles, (microparticleswill aggregate during centrifugation) the particles were alsophotocross linked as described above. Finally, the resultantparticles were rinsed ve times with distilled water andcollected by centrifugation.

Spherical particles programming to their temporary shapes

The shape changeable particles were fabricated using a modi-ed PVA lm stretching method as illustrated in the Scheme 1.First, the spherical particles (0.3 wt%) were dispersed into 5 mL5% PVA water solution at 22 �C with 2% glycerol as plasticizerfor the resultant PVA lm, and the mixed solution was pouredinto a mold with a size of 10� 10 cm (length� width) and driedat room temperature to acquire a PVA lm with thickness of 200mm. Second, the PVA lm was typically cut into sections of 10 �5 cm and pasted onto a metal plate. In cases of heat stretching,the lm was heated at 60 �C for 10 min, and then a presetsupport chip was inserted into the slot. The spherical particlesinside the PVA lm were stretched to ellipsoids with differentaspect ratios (AR) by controlling the tensile degree of the lm,which was in proportion to the height of the elevated supportchip. Finally, the metal plate and the deformed lm were cooledto 0 �C for 30 min to x the ellipsoidal temporary shape. Thelm was dissolved in water at 25 �C. The ellipsoids were


collected by centrifugation and rinsed ve times with distilledwater to remove excess of PVA from the surface of the particles.The particle size and distribution were determined with a laserdiffraction particle size analyzer (LA-9200, HORIBA, Japan). Themorphology was observed by confocal laser scanning micros-copy (CLSM, FV1000, Olympus, Japan).

Macroscopic and microcosmic reversible shape memory effect

Reversible shape memory cycle was performed under a typicalinvestigation procedure.45 Samples were thermally equilibratedon a DMA983 analyzer at 60 �C for 1 min before the data wererecorded. Stress was then increased from 0 to 0.5 MPa at a rateof 0.01 MPa per second and xed at 0.5 MPa, while temperaturecooled to 0 �C at a rate of 5 �C per minute. The temperature wasequilibrated to 0 �C for 1 min, and stress was isothermallyunloaded back to 0 MPa at a rate of 0.01 MPa per second.Finally, free-strain cyclic recovery was measured with the cyclicheating and cooling between 43 �C and 0 �C. The macroscopicshape memory effect in the initial ribbon-like shape were rstinvestigated by heating from 30 �C to 37 �C, 39 �C and 42 �C,respectively. The reversible shape memory recovery processes inthe forms of both ribbon and microsized sphere were investi-gated at an alternating temperature between 0 �C and 43 �Caccording to the DMA result. Aer 30 min, the system reachedequilibrium for each temperature change during the shapememory cycles.

In vitro cytotoxicity assay

The cytotoxicity of all the samples was further evaluated basedon Alamar blue assay.46 First, c-6A PCL and c-6A PEG-PCL weresterilized by UV for 4 h. The UV wavelength was 365 nm, and thepower of the UV lamp was 12 W. These samples were cut intosmall round akes with average diameters of approximately12 mm for all the samples, and they were then in vitro used forosteoblast cultures. The cells belonged to a normal cell line,which was from a neonatal rat's mandibular. They have beenpassaged to the third generation just before the experiments. Inbrief, the osteoblast cells were grown in RPMI medium1640 (Gibcos) with 10% fetal bovine serum (FBS). The cells witha density of 1.0 � 104 cells per well were cultured in 24 well

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plates in the abovementioned medium and maintained at 37 �Cin a humidied incubator under 5% CO2 and 95% air. At thepre-designed time points of 1, 3, and 5 days, the medium wascarefully removed and 300 mL Alamar blue solutions (10% Ala-mar blue, 80% media 199 (Gibcos) and 10% FBS; v/v) wereadded into each well and incubated for further 3 h at 37 �C, 5%CO2. Wells without the cells were used as the blank controls. Asample of 200 mL of reduced Alamar blue solution was pipettedinto a costar opaque black bottom 96-well plate (Sigma) andobserved at 570 (excitation)/600 (emission) in a ELISA micro-plate reader (Molecular Devices, Sunnyvale, CA). Results werethe mean � standard deviation of three experiments. The cellmorphology and growth on the surfaces of the meshes wereevaluated by uorescence microscopy (IX51,Olympus, Japan).

Phagocytosis and intracellular shape memory recovery

The mouse macrophage cell lines as model macrophages wereobtained from the Sichuan University (China). They werecultured in DMEMmedia containing L-glutamine and 10% fetalbovine serum (FBS). The macrophage cells were grown in a6-well slides, which were treated with the culture (BD Biosci-ences) at a concentration of 1 � 105 cells per mL and incubatedat 37 �C for 24 h. Non-attached cells were removed by aspiration;subsequently, 200 mL of the fresh media, containing 1 mgspherical or ellipsoidal micro-sized particles, was added intothe 6 cell culture plate and co-cultured with macrophages for 5,15, 30 and 60 min at 37 �C. For the investigation of intracellularshape memory recovery, aer the cells were treated with ellip-soidal particles for 30 min at 37 �C, they were subjected to cyclicheating and cooling between 43 �C and 0 �C. Finally the cellswere xed with 2.5% glutaraldehyde and washed with PBS.Images were acquired on a CLSM (FV1000, Olympus, Japan).

Results and discussionsCharacterization of c-6A PEG-PCL

The cross-linked network, on the basis of chemically cross-linked six-arm poly(ethylene glycol)-poly(3-caprolactone) (c-6APEG-PCL) was achieved as schematically shown in Fig. 1A. Fromthe FT-IR spectrum of 6A PEG-PCL (Fig. 1B), we can nd that thecharacteristic peak of C–O–C at 1125 cm�1 belongs to the PEGchains, and the peak at 1730 cm�1 is related to O]C–O, indi-cating the successful ring-opening polymerization of 3-CL. Theappearance of a peak at 1640 cm�1 in the spectrum of 6A PEG-PCL-AC suggested that the acryloyl chloride reacted with theterminal hydroxy group of 6A PEG-PCL, which is the charac-teristic stretching vibration absorption peak of C]C. Theamount of hydroxyl group on 6A PEG and 6A PEG-PCL was 54 �3.2 mg KOH per g and 17.3 � 2.4 mg KOH per g, respectively.Aer graing acryloyl chloride, the amount of hydroxyl groupon 6A PEG-PCL was 3 � 1.2 mg KOH per g, indicating that thegraing ratio was 79 � 3%. To further conrm the results of thehydroxyl number, GPC measurements for these polymers wereperformed and the results are shown in Table S1 in ESI.†Remarkably, the graing ratio of acryloyl chloride and themolecular weight of PCL segments exert an effect on the

6858 | J. Mater. Chem. B, 2014, 2, 6855–6866

structure of the c-6A PEG-PCL, and the gel fraction should beproportional to the graing ratio and the crosslinking time. Inthis polymer network, we can regulate the content of acryloylchloride in the graing reaction and the crosslinking time tocontrol the proportion of the cross-linked PEG-PCL moleculechains and free PEG-PCL molecular chains.

Intermolecular transesterication reactions modify thesequences of co-polylactones and prevent the formation ofblock copolymers. The different types of intermolecular trans-esterication reactions broaden the molecular weight distribu-tion. Due to these side reactions, MWDs of the polymers areabove 1.3 and the product consists of a mixture of both linearand cyclic molecules.47–49 GPC measurements have shown thatboth linear and microgel could not be detected in the poly-merization mixtures up to the monomer conversion approach-ing 99.9%. Thus, if any linear oligomers are present in thesystem, they should constitute less than 0.1% of the linearpolymer.

To further conrm that 6A PEG-PCL-AC was successfullysynthesized, the structure was veried by 1H-NMR as shown inFig. 1C. In the 1H NMR spectrum, the peak around 3.55 ppmis attributed to the methylene hydrogen atoms (–CH2–O–)of the PEG blocks. The peaks at 2.25, 1.57, 1.32 and 4.02 ppmare all assigned to the methylene hydrogen atoms(–CH2–CH2–CH2–CH2–CH2–) of PCL units. 1H NMR was used todetermine the number of monomer units in each block, and theaverage repeated unit number of CL is 19 in the 6A-PEG-PCLpolymer. Here, we can nd three additional weak peaks at 5.8,6.2 and 6.4 ppm in the close-up image; they are assigned to thecarbon–carbon double bond hydrogen atoms (–CH]CH–)located at the end of the polymer chains. The graing ratio ofthe carbon–carbon double bond was also determined from thearea ratio of the 1H-NMR peaks at 5.8 ppm (attributed to theacryloyl chloride blocks) and 4.1 ppm (attributed to the PEGblocks); the value was calculated to be 77%. The meltingbehavior of c-6A PEG-PCL with a different molecular weight(Mw) of PCL was investigated using DSC analysis, as shown in qin the ESI.† From the curves, we can nd that with theincreasing of Mw of each PCL arm from 1000 g mol�1 to2000 g mol�1 and 3000 g mol�1, the Tm increases from 35 �C to41 �C and 51 �C, respectively. In contrast to the inuence of thecrosslinking on the thermal properties, we can see that the crosslinking makes the Tm to slightly decrease (Fig. 2A). The uncross-linked polymer shows the higher values of melting tempera-tures than the cross-linked networks. The Tm of c-6APEG-PCL(2000) is close to the physiological temperature, which isvery suitable for acting as a thermally induced SMP for biomed-ical applications. In fact, the Tm of c-6A PEG-PCL(1000) is closerto it, but it is lower. Considering the clinical application, weassume that it will be a little higher than the physiologicaltemperature; thus we have enough time to carry out the shapememory recovery process at the critical point. Consequently, allof the following investigations are focused on c-6APEG-PCL(2000). The further amplication of the melting peak ofc-6A PEG-PCL(2000) is shown in Fig. 2B, and we can nd that twoendothermic peaks appear in the DSC curve, demonstrating thatphase separation took place in the polymer network to form the



Fig. 1 Synthesis and thermal properties of the crosslinked network based on c-6A PEG-CL. (A) Scheme of the synthesis of c-6A PEG-PCLnetwork, (B) FT-IR spectra of 6-PEG, 6A PEG-PCL and 6A PEG-PCL-AC, (C) 1H NMR of 6A PEG-PCL-AC.


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cross-linking part (the previous peak) and a free segment (thesecond peak). The higher crystallinity with higher Tm2 is ascribedto the free PEG-PCL segment, and the lower one with lower Tm1 isfor the cross-linked PEG-PCL of this cross-linked network asshown in Fig. 1A. Because the material would be potentially usedas a biomaterial and it would come in contact with the water in


the body, the effect of water on the thermal property was furtherinvestigated. Fig. 2C displays the DSC curves of c-6A PEG-PCLunder dry and wet conditions, respectively. Aer c-6A PEG-PCLwas immersed in water for 1 h, the water content in the polymernetwork was 15.34 wt%, determined by thermal gravimetricanalysis (TGA) (Fig. S2 in ESI†). In Fig. 2C, we can nd that the

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Fig. 2 (A) DSC curves of 6A PEG, 6A-PEG-PCL and c-6A PEG-PCL(2000), (B) magnified view of c-6A PEG-PCL, (C) DSC curves of c-6A PEG-PCLunder dry and wet conditions, (D) DMA curves of storage modulus of c-6A PEG-PCL polymers with the Mw of each PCL arm of 1000 g mol�1,2000 g mol�1 and 3000 g mol�1 and c-6A PEG-PCL(2000) under wet conditions, respectively.


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H2O molecules penetrating into the polymer network broadensthe melting transition region, suggesting that the aqueous envi-ronment brings an impact on the crystallization of this polymer.This broadmelting transition region can endow the polymer witha temperature memory function.50–52

Macroscopic reversible shape memory effect

Dynamic mechanical analysis (DMA) was performed for thepolymer to predict the shape memory property. Fig. 2D showsthe DMA curves of c-6A PEG-PCL networks with differentmolecular weights for each PCL arm under dry conditions andc-6A PEG-PCL (2000) under wet conditions. All of these poly-mers display a systematic decrease in the storage modulus (E0)with increasing temperature. The decrease of the molecularweight of PCL decreases the transition temperature (Ttrans) of c-6A PEG-PCL, which is in good agreement with the DSC analysis.In addition, the E0 values of all the specimens rst experienced aplatform over 100 MPa at low temperatures, and then droppedto a newmodulus plain of about 0.01 MPa at high temperatures.The tremendous change in E0 has been proven to have anobvious contribution to a good shape-memory effect.53

Comparing the DMA curves of c-6A PEG-PCL(2000) under thedry and wet conditions, it can be seen that there are more gaps

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in E0, which can be essentially attributed to the plasticizingeffect of H2O.54 Moreover, the polymer having a relatively broadtransition temperature range suggests that it may have a goodmultiple shape memory effect.55,56

According to the above DMA analysis, the temperature in therange of 30–43 �C can be selected to be the shape recoverytemperature. To demonstrate that the c-6A PEG-PCL has goodSME, a series of images of the macroscopic multiple shape-recovery process and temperature memory effect of the c-6APEG-PCL(2000) at 0 �C, 37 �C, 42 �C and 45 �C are given inFig. S3 in ESI.† It can be observed that the c-6A PEG-PCLpossesses a multiple shape memory effect in the temperaturerange of 30–43 �C. Moreover, we found that the c-6A PEG-PCLhas a temperature memory effect with a different deformationtemperature. Aer deformation above 45 �C, the shape recoverydoes not take place at body temperature. These results showthat the Ttrans and shape recovery ratio can be controlled by thedeformation temperature in a real application, such as intra-vascular stent and drug carrier.

As shown in Fig. 3A and Movie S1 in ESI†, the c-6A PEG-PCLribbon is programmed to form a ring shape at 60 �C, cooling to0 �C for xation of this shape, and subsequent heating to 43 �Cagain. Cooling back to 0 �C results in a hovering shape. Cyclicheating and cooling between 43 �C and 0 �C, reversibly switches




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the shape between the ring shape and arch shape. Heating to60 �C erases the shapes “ring” and “arch” from the memory ofthe polymer because of the fact that all crystalline domains weremelted. The effect of cross-linking degree derived from cross-linking time on SME was also studied (Fig. S4 in ESI†). We ndthat the polymer has an excellent reversible SME, when the gelfraction is in the range of 68–81%.

To further demonstrate that this polymer had good revers-ible SME, a cyclic tensile test was carried out. Fig. 3B shows thetypical strain–stress curves versus temperature change. First, thec-6A PEG-PCL is immersed in a water bath maintained at 60 �Cfor 1 h before the test because the c-6A PEG-PCL can bepotentially used in the body uid as biomaterials. The sampleswere then thermally equilibrated at 60 �C for 1 min, before the

Fig. 3 The reversible shape memory recovery behaviors of c-6A PEGreversible shape memory recovery process. (B) Consecutive reversible Sthe micro-sized particles (the stretching strain of PVA film z 200%) anparticles. The inset images are obtained by CLSM (Scale bar: 10 mm). Figthermal properties of the cross-linked network based on c-6A PEG-CL. (Aof 6-PEG, 6A PEG-PCL and 6A PEG-PCL-AC, (C) 1H NMR of 6A PEG-PC


data were recorded. Stress was then increased from 0 to 0.5 MPaat a rate of 0.05 MPa per second and xed at 0.5 MPa, whereasthe temperature cooled to 0 �C at a rate of 5 �C min�1. Thetemperature was equilibrated to 0 �C for 1 min, and stress wasisothermally unloaded back to 0 MPa at a rate of 0.5 MPa persecond (the shape xity ratio of c-6A PEG-PCL remained over90% (91.7%)). The temperature was then increased to 43 �C at arate of 5 �C min�1. Finally, the shape memory recovery wasrealized with the cyclic heating and cooling between 45 �C and0 �C. In Fig. 3B, the strain 3 is plotted against time for the rstreversibility cycle. We can nd that the changes of 3 are spon-taneous, without the action of an external force beginning fromthe second cycle. In the following alternant cycles, the samplereversibly switched between the elongated shape and half-

-PCL(2000). (A) Series of photographs macroscopically showing theME. (C) Fluorescence images showing the deformation and fixation ofd microscopically reversible shape memory recovery process of the. 1. Synthesis and thermal properties of the cross-Fig. 1. Synthesis and) Scheme of the synthesis of c-6A PEG-PCL network, (B) FT-IR spectraL-AC.

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Fig. 4 The relationship between the structure of the polymer and reversible SME. (A) and (B) In situ temperature XRD patterns of c-6A PEG-PCL(2000) under dry and wet conditions, respectively. The insets are the diffraction rings acquired from SAXS (the SAXS data measured at t �4100 s). (C) The crystallinity under dry and wet conditions versus determined temperatures. (D) The schematic molecular mechanism of thethermally induced reversible SME.


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recovered shape. Aer seven cycles, the temperature rises to60 �C, and the piece returns to its original shape. Cooling backagain to 0 �C, the reversible shape memory effect could notoccur because the molecular chains of c-6A PEG-PCL becameamorphous and subsequently the reversible shape memoryeffect disappeared.

Microscopic reversible shape memory effect

To explore the microscopic reversible shape memory effect, thispolymer was fabricated to spherical micrometer-sized particles

6862 | J. Mater. Chem. B, 2014, 2, 6855–6866

with an average size of 5 mm (Fig. S5 in ESI†). These sphericalparticles (permanent shape) were programmed to ellipsoids(temporary shape) through a modied PVA lm-stretchingtechnique as shown in Scheme 1. By changing the stretchingstrain (height of support chip), the ellipsoids with differentaspect ratios (AR) from 1 to 5 can be easily achieved. FromFig. S5,† we nd that the particle size and distribution of thedeformed particles are almost similar to the original particlesbecause of the fact that the volume of the particle remainsunchanged. The effect of stretching strength or the degree oftensile on the particle shape (AR ratio) is shown in Fig. S6 in




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ESI.† To clearly observe the morphology and the shape memoryprocess of these particles, they were stained with uorescentdye, Nile red. From the CLSM images in Fig. 3C, we can see thatthe particles adopt a well-spherical shape inside the PVA lmand turn to prolate ellipsoids, aer they were stretched at 60 �Cand immediately cooled to 0 �C. These particles were collectedby dissolving the PVA lm in water. The microscopic reversibleshapememory effect was also observed by CLSM (Fig. 3C). It canbe seen that the particles are capable of reversibly switchingtheir shape from spherical to ellipsoidal with the cyclic heatingand cooling between 43 �C and 0 �C.

The mechanism of reversible shape memory effect

From the abovementioned discussion, we know that this poly-mer network possesses an obvious reversible shape memoryeffect. To analyze the relationship between the structure of thispolymer and reversible SME, in situ temperature X-ray diffrac-tion (XRD) and small-angle X-ray scattering (SAXS) was carriedout. Fig. 4A and B show the XRD patterns of c-6A PEG-PCL lmunder dry and wet conditions, respectively. The water pene-trating into this polymer lm decreases the degree of

Fig. 5 Evaluation of in vitro cytotoxicity. (A) Analysis of water contact angmicroscopy images of cultured osteoblasts on c-6A PCL and c-6A PEG-Pgreen with calcein. (Scale bar: 10 mm).


crystallization at the same temperature, which is also consistentwith DSC analysis. The crystalline peaks at 2q ¼ 21.5� (110) and23.8� (200) are ascribed to the PCL segments, conrming itssemi-crystalline structure. Whereas for all temperatures, noobvious characteristic peaks exists corresponding to the PEG(19.24� and 23.42�). When the temperature was increased above40 �C, the crystallization behavior of PCL became worse. Underthe wet conditions, the degree of crystallization decreased fromabout 30% at 0 �C to 7% at 43 �C (Fig. 4C). Remarkably, aer thetemperature was increased to 60 �C, the molecular chains of c-6A PEG-PCL became amorphous, and subsequently the revers-ible shape memory effect disappeared.

The molecular mechanism of the thermally induced revers-ible shape memory effect of the c-6A PEG-PCL microsphere isschematically illustrated in Fig. 4D. In one-way SMPs, theswitching domains provide two functions at the same time.They enable active movement from the temporary shape B tothe original shape A by elastic recovery and determine thegeometry of the shape change by the temporary xation ofshape B.57 In the chemical structure of one-way shape memorypolymer, actuator domains (ADs) are lacking, which are capableof driving the permanent shape to return to the temporary

le of c-6A PCL and c-6A PEG-PCL. (B) Cell viability and (C) fluorescenceCL after day 1, day 3 and day 5, respectively. The live cells were stained

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Fig. 6 Phagocytosis and intracellular shapememory recovery of the shape-switching particles. (A) The association process of both spherical andellipsoidal particles with macrophages, observed with CLSM at 5 min, 15 min, 30 min and 60 min. These particles are stained with a fluorescentdye, Nile red. (B) Confocal images showing the intracellularly reversible shape memory recovery of the particles. (Scale bar: 10 mm).


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shape on cooling. In order to indicate the exact mechanism, theshapememory effect of c-6A PEG-PCL with a different ratio of 6APEG-PCL to 6A PEG-PCL-AC was carried out. To ensure that thec-6A PEG-PCL was completely cross-linked, photo-crosslinkingof 6A PEG-PCL/6-arm PEG-PCL-AC/TPO was initiated byexposing to UV light for 40 min. Fig. S8† shows the gel fraction,shape xed ratio and shape recovery ratio with a different ratioof 6A PEG-PCL to 6A PEG-PCL-AC. We can nd that 6A PEG-PCL-AC was cross-linked with each other under the UV light. Inaddition, as the content of 6A PEG-PCL-AC increased, the shapexed ratio and shape recovery ratio of c-6A PEG-PCL increased.In particular, only the samples with the ratio of 30 : 70 and20 : 80 (6A PEG-PCL–6A PEG-PCL-AC) show the reversible shapememory effect. Therefore, it becomes evident that this polymernetwork consists of two different units: the cross-linked PEG-PCL molecule chains and free PEG-PCL molecular chains. Thecross-linked PCL regions with lower melting temperature(Tm,AD) acting as the actuator domains (ADs) are responsible forthe actuation of the 2W SME, and the free PCL segmentsdetermine the shape shiing geometry due to the higherTm,SGD. Thus, a temperature-responsive “on-off” crystallinitytransition from the reversible crystallization and the melting oforiented ADs can be achieved under a stress-free condition, andsubsequently a 2W SME is realized. During the reversible shapememory recovery process, the free PEG-PCL molecular chaincan perform a role of skeleton and induce or regulate the shapechange of the cross-linked 6-arm molecules.32,37

Cytotoxicity analysis

The cytotoxicity of 6A PEG-PCL and its degradation productswas assessed with Alamar blue assay. The hydrophobic 6A PCLis selected as the control, which was synthesized by the

6864 | J. Mater. Chem. B, 2014, 2, 6855–6866

initiating of dipentaerythritol. The hydrophilicity of PCL wasgreatly improved with the introduction of 6A PEG (Fig. 5A).Fig. 5B and C show the cell viability and representative uo-rescence microscopy images of osteoblasts treated with the 6APCL, 6A PEG-PCL and the degradation products of the 6A PEG-PCL, respectively. We can nd that more than 85% of the cellsremained viable, and the cell viability of 6A PEG-PCL is betterthan that of 6A PCL due to the improvement of the hydrophi-licity. From Fig. 5C, it could be clearly found that the osteoblastswere elongated but attached and well spread on the polymersubstrates. Moreover, osteoblast cells were incubated with thedegradation products (6-hydroxycaproic acid and its oligomersand 6A PEG) for various incubation times. In Fig. 5B, it isobviously observed that all of the cell viability for degradationproducts aer the 1st day, 3rd day and 5th day was more than80%. Cell shape was also completely spread out, indicating thattheir growth is healthy. All these results strongly proved that thec-6A PEG-PCL has excellent cytocompatibility, suggesting that itis very suitable for biomedical applications.

Phagocytosis and subsequent intracellular shape memoryrecovery

Particle shape has an impact on phagocytic internalization.5–8

Internalization of the spherical and ellipsoidal micro-sizedparticles by macrophages was further observed with CLSM. Theellipsoidal particles with AR of 5 could not be taken up bymacrophages (data not shown). The images shown in Fig. 6Aexhibit that the phagocytosis has a strong dependence on thelocal particle shape. In 5 minutes, both the spherical andellipsoidal particles with AR of 1.6 are attached to the macro-phage membrane, and the ellipsoidal particles associate withthe membrane along the major axis of these particles. Aer




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15 minutes, we observed that the whole spherical particles weretaken up, while the ellipsoidal particle still remained on themembrane. Aer 60 minutes, 15% of the spherical particles(5–10 mm) were internalized by the macrophages, whereas only3% of the ellipsoidal particles were swallowed. The main reasonfor this is that the size of particles was too large (Fig. S9†). It isnot difficult to infer that particles with high AR exhibit thepotential to inhibit phagocytosis. The result is consistent with aprevious report.5 Thus, the phagocytic fate of the particleslargely depends on the polarizability. Thus, the ellipsoidalparticles used as drug carriers will prolong blood circulationbecause they can evade the phagocytosis by macrophagesduring delivery. Once the deformed particles arrive in targettissues, they can recover to spherical particles through a mildstimulus, and in turn be facilely internalized by targeting cells.By adjusting the aspect ratio (AR) of the shape-switching parti-cles, we can easily control the particles as the drug deliverycarriers to either avoid or promote the phagocytosis.

Subsequently, the intracellular shape memory recovery wasfurther investigated. From the uorescence microscopy images(Fig. 6B), we can clearly see that aer the ellipsoidal particleswith AR of 1.6 were swallowed into macrophages, themorphology of the particles can still alternatively switch fromthe spherical to ellipsoidal shape between 43 �C and 0 �C. Thisprocess is similar to that of the extracellular experiment. Theintracellular environment would not make a difference to thereversible shape memory behavior. Aer particles enter into acell, they will further encounter these compartments inside thecell, namely early endosomes, late endosomes and lysosomes.The intracellularly reversible shape memory recovery may openup a strategy towards investigating the intracellular behaviors ofthe carriers, such as intracellular drug release, the interactionbetween the carriers and compartments and nuclear drugdelivery.

Conclusions

In summary, we have originally developed one type of shape-switchable micrometer-sized particles based on a cross-linkedpolymer network including biocompatible six-arm poly-(ethylene glycol)-poly(3-caprolactone). These particles have theability of reversibly switching their shape both extracellularlyand intracellularly with the cyclic heating and cooling between43 �C and 0 �C via the reversible 2W SME of the polymer matrix.The molecular mechanism of the 2W SME can be ascribed to atemperature-responsive “on-off” crystallinity transition fromthe reversible crystallization and melting. By adjusting theaspect ratio (AR) of the shape-switching particles, we cancontrol the particles to be the drug delivery carriers to eitheravoid or promote phagocytosis. Therefore, the study provides anew concept of dynamically shape-switching carriers for drugdelivery.

Acknowledgements

This work was partially supported by National Basic ResearchProgram of China (973 Program, 2012CB933600), National


Natural Science Foundation of China (no. 30970723, 51173150,51373138) and Research Fund for the Doctoral Program ofHigher Education of China (20120184110029). T. Gong andK. Zhao equally contributed to this work.

Notes and references

1 R. Duncan, Nat. Rev. Drug Discovery, 2003, 2, 347–360.2 J. M. Karp and R. Langer, Curr. Opin. Biotechnol., 2007, 18,454–459.

3 D. Peer, J. M. Karp, S. Hong, O. C. Farokhzad, R. Margalit andR. Langer, Nat. Nanotechnol., 2007, 2, 751–760.

4 N. Doshi and S. Mitragotri, Adv. Funct. Mater., 2009, 19,3843–3854.

5 J. A. Champion and S. Mitragotri, Proc. Natl. Acad. Sci. U. S.A., 2006, 103, 4930–4934.

6 S. E. Gratton, P. A. Ropp, P. D. Pohlhaus, J. C. Lu,V. J. Madden, M. E. Napier and J. M. DeSimone, Proc. Natl.Acad. Sci. U. S. A., 2008, 105, 11613–11618.

7 X. Hu, J. Hu, J. Tian, Z. Ge, G. Zhang, K. Luo and S. Liu, J. Am.Chem. Soc., 2013, 135, 17617–17629.

8 X. Huang, X. Teng, D. Chen, F. Tang and J. He, Biomaterials,2010, 31, 438–448.

9 S. Muro, C. Garnacho, J. A. Champion, J. Leferovich,C. Gajewski, E. H. Schuchman, S. Mitragotri andV. R. Muzykantov, Mol. Ther., 2008, 16, 1450–1458.

10 T. Chen, X. Guo, X. Liu, S. Shi, J. Wang, C. Shi, Z. Qian andS. Zhou, Adv. Healthcare Mater., 2012, 1, 214–224.

11 Y. Geng, P. Dalhaimer, S. Cai, R. Tsai, M. Tewari, T. Minkoand D. E. Discher, Nat. Nanotechnol., 2007, 2, 249–255.

12 H. Meng, S. Yang, Z. Li, T. Xia, J. Chen, Z. Ji, H. Zhang,X. Wang, S. Lin and C. Huang, ACS Nano, 2011, 5, 4434–4447.

13 N. Doshi, B. Prabhakarpandian, A. Rea-Ramsey, K. Pant,S. Sundaram and S. Mitragotri, J. Controlled Release, 2010,146, 196–200.

14 L. E. Euliss, J. A. DuPont, S. Gratton and J. DeSimone, Chem.Soc. Rev., 2006, 35, 1095–1104.

15 J. A. Champion, Y. K. Katare and S. Mitragotri, Particleshape: a new design parameter for micro-and nanoscaledrug delivery carriers, J. Controlled Release, 2007, 121, 3–9.

16 M. R. Langille, M. L. Personick, J. Zhang and C. A. Mirkin,J. Am. Chem. Soc., 2011, 133, 10414–10417.

17 T. J. Merkel, K. P. Herlihy, J. Nunes, R. M. Orgel, J. P. Rollandand J. M. DeSimone, Scalable, Langmuir, 2009, 26, 13086–13096.

18 S. Chen, J. G. Bomer, W. G. van der Wiel, E. T. Carlen andA. van den Berg, ACS Nano, 2009, 3, 3485–3492.

19 C. Wischke, M. Schossig and A. Lendlein, Small, 2014, 10,83–87.

20 C. Wischke and A. Lendlein, Langmuir, 2014, 30, 2820–2827.21 T. Wu, K. O'Kelly and B. Chen, Eur. Polym. J., 2014, 53, 230–

237.22 A. Lendlein and S. Kelch, Angew. Chem., Int. Ed., 2002, 41,

2034–2057.23 J. Leng, X. Lan, Y. Liu and S. Du, Prog. Mater. Sci., 2011, 56,

1077–1135.

J. Mater. Chem. B, 2014, 2, 6855–6866 | 6865



Publ

ishe

d on

19

Aug

ust 2

014.

Dow

nloa

ded

by U

nive

rsity

of

Con

nect

icut

on

29/1

0/20

14 1

1:51

:26.

View Article Online

24 J. Hu, Y. Zhu, H. Huang and J. Lu, Prog. Polym. Sci., 2012, 37,1720–1763.

25 A. Lendlein and R. Langer, Science, 2002, 296, 1673–1676.26 A. Garle, S. Kong, U. Ojha and B. M. Budhlall, ACS Appl.

Mater. Interfaces, 2012, 4, 645–657.27 J. Li, J. A. Viveros, M. H. Wrue and M. Anthamatten, Adv.

Mater., 2007, 19, 2851–2855.28 M. C. Serrano, L. Carbajal and G. A. Ameer, Adv. Mater., 2011,

23, 2211–2215.29 T. Xie, Polymer, 2011, 52, 4985–5000.30 D. L. Thomsen, P. Keller, J. Naciri, R. Pink, H. Jeon,

D. Shenoy and B. R. Ratna, Macromolecules, 2001, 34,5868–5875.

31 D. K. Shenoy, L. Thomsen III, A. Srinivasan, P. Keller andB. R. Ratna, Sens. Actuators, A, 2002, 96, 184–188.

32 M. Behl, K. Kratz, J. Zotzmann, U. Nochel and A. Lendlein,Adv. Mater., 2013, 25, 4466–4469.

33 H. Zhuo, J. Hu and S. Chen, J. Mater. Sci., 2011, 46, 3464–3469.

34 T. Chung, A. Romo-Uribe and P. T. Mather, Macromolecules,2008, 41, 184–192.

35 K. M. Lee, P. T. Knight, T. Chung and P. T. Mather,Macromolecules, 2008, 41, 4730–4738.

36 J. Li, W. R. Rodgers and T. Xie, Polymer, 2011, 52, 5320–5325.37 S. Pandini, S. Passera, M. Messori, K. Paderni, M. Toselli,

A. Gianoncelli, E. Bontempi and T. Ricco, Polymer, 2012,53, 1915–1924.

38 M. Ebara, K. Uto, N. Idota, J. M. Hoffman and T. Aoyagi, Adv.Mater., 2012, 24, 273–278.

39 K. A. Davis, K. A. Burke, P. T. Mather and J. H. Henderson,Biomaterials, 2011, 32, 2285–2293.

40 C. Min, W. Cui, J. Bei and S. Wang, Polym. Adv. Technol.,2005, 16, 608–615.

6866 | J. Mater. Chem. B, 2014, 2, 6855–6866

41 S. Neuss, I. Blomenkamp, R. Stainforth, D. Boltersdorf,M. Jansen, N. Butz, A. Perez-Bouza and R. Knuchel,Biomaterials, 2009, 30, 1697–1705.

42 S. Zhou, X. Deng and H. Yang, Biomaterials, 2003, 24, 3563–3570.

43 X. Yang, L. Wang, W. Wang, H. Chen, G. Yang and S. Zhou,ACS Appl. Mater. Interfaces, 2014, 6, 6545–6554.

44 X. Yu, S. Zhou, X. Zheng, T. Guo, Y. Xiao and B. Song,Nanotechnology, 2009, 20, 235702.

45 T. Gong, W. Li, H. Chen, L. Wang, S. Shao and S. Zhou, ActaBiomater., 2012, 8, 1248–1259.

46 S. Shao, S. Zhou, L. Li, J. Li, C. Luo, J. Wang, X. Li andJ. Weng, Biomaterials, 2011, 32, 2821–2833.

47 A. Duda, Z. Florjanczyk, A. Hofman, S. Slomkowski andS. Penczek, Macromolecules, 1990, 23, 1640–1646.

48 M.-H. Thibault and F.-G. Fontaine, Dalton Trans., 2010, 39,5688–5697.

49 M. Labet and W. Thielemans, Chem. Soc. Rev., 2009, 38,3484–3504.

50 T. Xie, Nature, 2010, 464, 267–270.51 T. Xie, K. A. Page and S. A. Eastman, Adv. Funct. Mater., 2011,

21, 2057–2066.52 P. Miaudet, A. Derre, M. Maugey, C. Zakri, P. M. Piccione,

R. Inoubli and P. Poulin, Science, 2007, 318, 1294–1296.53 S. Rezanejad and M. Kokabi, Eur. Polym. J., 2007, 43, 2856–

2865.54 R. M. Baker, J. H. Henderson and P. T. Mather, J. Mater.

Chem. B, 2013, 1, 4916–4920.55 M. Nagata and Y. Yamamoto, Macromol. Chem. Phys., 2010,

211, 1826–1835.56 M. Y. Razzaq, M. Behl, K. Kratz and A. Lendlein, Adv. Mater.,

2013, 25, 5730–5733.57 M. Behl, M. Y. Razzaq and A. Lendlein, Adv. Mater., 2010, 22,

3388–3410.



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