Tailored One-Way and Two-Way Shape Memory Capabilities of Poly(ε-Caprolactone)-Based Systems for...

Tailored One-Way and Two-Way Shape MemoryCapabilities of Poly(e-Caprolactone)-Based Systems

for Biomedical ApplicationsStefano Pandini, Theonis Ricco, Alberto Borboni, Ileana Bodini, David Vetturi, Danilo Cambiaghi, Maurizio Toselli, Katia Paderni,

Massimo Messori, Francesco Pilati, Federica Chiellini, and Cristina Bartoli

(Submitted September 26, 2013; in revised form January 21, 2014)

This paper investigates the shape memory capabilities of semicrystalline networks, focusing the attention onpoly(e-caprolactone) (PCL) systems, a class of materials that allows to satisfy important requirements fortheir applications as biomedical devices, such as the good biocompatibility, the fast recovery of large‘‘temporary’’ shape configurations, and the easy tailoring of the transformation temperatures. The mate-rials were prepared with various crosslink densities and crosslinking methodologies; in particular, beside athermal crosslinking based on reactive methacrylic end groups, a novel type of covalently crosslinkedsemicrystalline systems was prepared by a sol-gel approach from alkoxysilane-terminated PCL precursors,so as to avoid potentially toxic additives typically used for free-radical thermal curing. The materials weresubjected to biological tests, to study their ability in sustaining cell adhesion and proliferation, and tothermal characterizations, to evaluate the possibility to tailor their melting and crystallization tempera-tures. The one-way shape memory (i.e., the possibility to set the material in a given configuration and torecover its pristine shape) and the two-way shape memory response (i.e., the triggered change between twodistinguished shapes on the application of an on-off stimulus) were studied by applying optimized thermo-mechanical cyclic histories. The ability to fix the applied shape and to recover the original one on theapplication of heating (i.e., the one-way effect) was evaluated on tensile bars; further, to investigate apotential application as self-expandable stents, isothermal shape memory experiments were carried out alsoon tubular specimens, previously folded in a temporary compact configuration. The two-way response wasstudied through the application of a constant load and of a heating/cooling cycle from above melting tobelow the crystallization temperature, leading to a reversible elongation/contraction effect, involvingmaximum strain changes up to about 80%, whose extent may be controlled through the crosslink density.

Keywords advanced characterization testing, biomaterial, creepand stress rupture, mechanical

1. Introduction

Shape memory polymers (SMPs) are a class of smartmaterials, which may undergo significant shape variations onthe application of an environmental stimulus, usually consist-ing in a change of temperature (Ref 1). SMPs presentadvantages, such as processing at a low costs, the ability torecover very large strains (potentially about 100 times largerthan those typical of shape memory alloys (Ref 2)), and a

relatively easily tailoring of the temperatures that trigger theshape memory effect (Ref 3). These peculiar shape-shiftingfeatures allow SMPs to be potentially employed in variousapplications (Ref 4), ranging from aerospace structures to smarttextiles, sensors and actuators, and particular interest wasreserved to exploit the shape memory effect for biomedicalapplications and minimally invasive surgery devices (Ref 5),such as, for example, self-tightening suture knots (Ref 2) orself-expandable stents (Ref 6, 7). These systems offer in factthe possibility of a soft actuation, thanks to the reducedcompliance mismatch between the host tissue walls and thedevice, the possibility to trigger the actuation at the humanbody temperature (Ref 4, 6, 7) or slightly above it (Ref 8), and,if needed, a biodegradable behavior that would permit to avoidlater removal of the implants (Ref 4, 6, 7); a further desiredrequirement is to have transformation times shorter than aminute (Ref 7), to facilitate the delivery of the device and toprevent its migration.

The shape memory response of the SMPs may varydepending on their molecular architecture (i.e., crosslink typeand density) and on the process at the basis of the shaperecovery (i.e., glass transition or melting). The behavior ofsemicrystalline networks based on poly(e-caprolactone) haspeculiarities that may fulfill some of the requirements for theirapplication in the biomedical field: (i) they are well known asbiomedical and biodegradable systems (Ref 9); (ii) their

This article is an invited paper selected from presentations at theInternational Conference on Shape Memory and SuperelasticTechnologies 2013, held May 20-24, 2013, in Prague, CzechRepublic, and has been expanded from the original presentation.

Stefano Pandini, Theonis Ricco, Alberto Borboni, Ileana Bodini,David Vetturi, Danilo Cambiaghi, Maurizio Toselli, Katia Paderni,Massimo Messori, Francesco Pilati, Federica Chiellini andCristina Bartoli, Department of Mechanical and IndustrialEngineering Brescia, University of Brescia, Brescia, Italy. Contact e-mail:[email protected].

JMEPEG �ASM InternationalDOI: 10.1007/s11665-014-1033-5 1059-9495/$19.00

Journal of Materials Engineering and Performance

thermo-mechanical properties can be easily tuned by varyingthe molecular architecture (Ref 10-12); and (iii) the shaperecovery taking place at the melting temperature occurs as afaster and sharp process (Ref 6), with respect to materials withshape memory triggered at Tg, whose self-actuation at thehuman body temperature is seen to require few minutes (Ref13, 14).

Although for these applications a single deployment (i.e., the‘‘one-way’’ shape memory effect) may be considered sufficient,recently also innovative features are demanded, such as the‘‘two-way’’ shape memory behavior, i.e., the triggered variationbetween two distinguished shapes on the application of an on-offstimulus, which may fulfill the requirements for the developmentof actuators or artificial muscles (Ref 15). Interestingly, semi-crystalline networks have been recently indicated as interestingcandidates for a cheap and easy achievement of such a behavior,allowing significant elongations under the applications ofmoderate levels of load and of heating/cooling cycles on athermal region that spans from above Tm to below Tc (Ref 15-17).This peculiar shape memory response of semicrystalline net-worksmay be achieved, as well as tailored, through the control ofthe crosslinked structure within the semicrystalline matrix,typically obtained by radical crosslinking, and recently devel-oped by applying strategies that allow to avoid the use ofpotentially toxic initiators, such as organic peroxides typicallyemployed in free-radical thermal or UV curing (Ref 11, 18).

In this work, we have explored the capabilities of covalentlycrosslinked semicrystalline systems with various moleculararchitectures, in order to have information concerning thepossibility to tailor their shape memory response. Two familiesof systems were investigated, one prepared by thermalcrosslinking in the presence of a free-radical initiator and theother by means of a safer sol-gel approach. The materialproperties were investigated in terms of biocompatibility and interms of the one- and two-way shape memory response underad-hoc thermo-mechanical histories.

2. Materials and Methods

2.1 Materials

The semicrystalline networks were prepared by crosslinkingreactions starting from hydroxyl-terminated poly(e-caprolac-tone) with different number-average molecular weights, rangingbetween 2200 and 13400 g mol�1 (see Table 1; valuesmeasuredby 1H NMR), later suitably modified to obtain methacrylate- ortrialkoxysilane-terminated PCL precursors. The former familywas crosslinked by a free-radical thermally activated process inthe presence of dicumyl peroxide as radical initiator and the latterby a sol-gel process carried out at room temperature throughhydrolysis and condensation reactions of alkoxysilane endgroups; a schematic representation of the material synthesis issketched in Scheme 1, and more details concerning the prepa-ration and the characterization of these materials are reported incompanion papers for both the free-radical (Ref 12) and sol-gel(Ref 11) processes. The obtained materials were coded asPCLMAx and PCLSGx, depending on the crosslinking reactions(MA: free-radical thermal-cured methacrylate-terminated PCL;SG: alkoxysilane-terminated PCL crosslinked by sol-gel reac-tions), and in which x depends on the molecular weight ofhydroxyl-terminated PCL precursors (see Table 1). The materi-als were prepared as disks, pouring the reactive mixture in Petri

dishes or aluminum plates; rectangular strips (overall length:15 mm; width: 0.7‚1.3 mm) were cut from the sheets for themechanical and the shape memory tests. Also, tubular specimenswere prepared, by the employ of silicon molds with a cavityhaving the shape of a tube (height = 40 mm, outer diame-ter = 25 mm, and thickness = 1 or 1.5 mm); for these speci-mens, special attention was taken to reduce the presence of airbubbles that may be trapped in the crosslinked materials,maintaining the melted methacrylate-terminated polymers underreduced pressure for several hours before the thermal curing forthe PCLMAx systems or covering the surface of the moldsexposed to the atmosphere with a plastic sheet to reduce thesolvent evaporation for PCLSGx ones.

2.2 Biological Tests

A sample for both the PCLMA and PCLSG family wasappositely prepared for the biological tests, starting fromprecursors with molecular weight of 10000 and 3900 g mol�1,respectively. Hydroxyl-terminated PCLs were purified by adissolution-precipitation procedure to remove the impuritiesthat may be present in the polymers and, after crosslinkingreactions, PCLSG and PCLMA sheets were further purified viaSoxhlet extraction with THF. The two samples were then cut as100 mm diameter disks and sterilized under UV light for30 min on each side, covered with ethanol/water solution(70%) for 48 h, and washed with phosphate buffer. Thesterilized samples were then placed into a Cell Crown24 sampleholder (Scaffdex, Finland) in 24-well culture plates, seededwith mouse embryo fibroblast balb/3T3 clone A31 cells,obtained from American Type Culture Collection (ATCC),propagated as indicated by the supplier, at a concentration of29 104/well, and incubated at 37 �C in a 5% CO2-enrichedatmosphere. After 48 h from the seeding, samples weretransferred in a new plate, in order to evaluate the proliferationof only the cells grown onto disks. The proliferation rate wasmeasured at the 2nd and 7th day of culture by means of WST-1tetrazolium salts (Roche Applied Science, US). Cells grownonto tissue culture polystyrene were used as control. Morphol-ogy of grown cells was investigated by means of confocal laserscanning microscopy (CLSM) at the 7th day of culture. Cellswere stained with Phalloidin-Fluorescein isothiocyanate (FITC)and 4¢,6-diamidino-2-phenylindole (DAPI) as fluorescent mark-ers of cytoskeleton and nuclei, respectively, and visualizedusing a Nikon Eclipse TE2000 inverted microscope equippedwith an EZ-C1 confocal laser (Nikon, Japan).

2.3 Thermal and Mechanical Characterization

DSC tests were carried out by means of a DSC Q1000(Thermal Analysis Inc.) on thin slices of about 5 mg andemploying nitrogen as purge gas. The tests were carried out ona thermal program between �90 and 90 �C (1st heating run at10 �C min�1; cooling at 2 �C min�1; 2nd heating run at10 �C min�1). The melting temperature, Tm, was evaluated asthe endothermal peak temperature on the second heating scan,while the crystallization temperature, Tc, on the exothermalpeak temperature was measured on the cooling run.

The mechanical behavior of the material above Tm (atTm + 40 �C) was tested by measuring the stress versus strainrelationship under tensile configuration. A DMTA machine(DMA Q800) was employed to perform a tensile ramp untilfailure on the rectangular strips (length between grips:8‚12 mm) at a testing rate of 29 10�2 N min�1.


2.4 Shape Memory Characterization

The one-way shape memory behavior of the materials wasinvestigated under the application of a three-stage thermo-mechanical cycle, carried out by means of a DMA Q800, on therectangular bars, and consisting in: (i) deforming the specimenat Tm + 40 �C under a loading ramp at 29 10�2 N min�1 up to

a given stress level (200 kPa for the PCLMAx set of materials,and 600 kPa for the PCLSGx materials); (ii) cooling thespecimen at least 15 �C below Tc at 2 �C min�1 by keeping theattained strain constant, maintaining the material under theseconditions for about 10 min in order to allow crystallization,and finally unloading; and (iii) heating the deformed specimen

Scheme 1 Schematic representation of the crosslinked systems synthesis

Table 1 Mechanical (E tensile modulus, rb stress at break, and eb strain at break), structural (m crosslink density), ther-mal (Tc crystallization temperature and Tm melting temperature), and shape memory behavior of the PCL-based systems

Materialcode

Precursormolecularweight(g/mol)

Tensile tests at Tm + 40 �C DSC tests Shape memory behavior

E, MPam,

mol/cm3 rb, kPa eb, % Tc, �C Tm, �C

Appliedstress,kPa

One-way tests Two-way tests

Strainfixity, %

Strainrecovery,

%Ttrans,

�C

OverallCIE,

eCIE, %

Residualstrain,eres, %

PCLMA3 3900 3.5 4.09 10�4 440 20 0 38 200 97 100 37 3 3PCLMA6 6700 1.5 1.69 10�4 380 30 18 53 200 98 95 50 7 2PCLMA13 13400 0.3 3.69 10�5 >200 >120 24 60 200 99 94 55 63 10PCLSG2 2200 4.6 5.59 10�4 1080 30 �26 24 600 72 100 23 35 10PCLSG3 3900 2.1 2.49 10�4 730 50 8 41 600 99 99 43 24 4PCLSG6 6000 1.2 1.39 10�4 ÆÆÆ(a) ÆÆÆ(a) 18 51 ÆÆÆ(a) ÆÆÆ(a) ÆÆÆ(a) 48 ÆÆÆ(a) ÆÆÆ(a)PCLSG10 10000 1.3 1.49 10�4 850 140 25 55 600 100 98 53 88 11

(a)Values could not be measured due to the limited amount of tensile specimens


at 2 �C min�1 up to 80 �C (i.e., above Tm for all the materials)under the application of a moderate load of 0.01 N, whichallowed to track the specimen length during the heating ramp.

The one-way shape memory response of the materials wasalso evaluated on the tubular specimens, subjected to a similarmechanical history as that experienced by the tensile bars. Thespecimens were heated at about 80 �C and, by employing anad-hoc-designed crimping tool, folded into a six-arms star-shaped configuration, and later fixed by cooling the material inthis compacted configuration at about �17 �C for at least10 min. The folded specimens were than subjected to isother-mal recovery test by immersion into a water bath, previouslyheated at various temperatures between room temperature toabove Tm. The recovery process was monitored as a function oftime by means of a photo camera with a sampling frequency of1 frame/s, and the recovery was quantitatively described interms of an arbitrary shape parameter based on the section ofthe folded specimen, called

Shape recovery %ð Þ ¼ Ap;ext tð Þ=A0

� �� 100; ðEq 1Þ

where A0 and Ap,ext (t) are the areas comprised within the out-er perimeter before folding and at a given instant, t, duringunfolding, respectively; the areas were measured by means ofa digital image analysis software, ImageJ.

The tests to investigate the two-way shape memoryproperties were carried out by applying a similar thermo-mechanical heating-cooling history as in the one-way tests,performed in this case under constant non-zero stress condi-tions. The specimens were first heated above Tm and subjectedto the same stress level adopted in the one-way tests, and then,by maintaining the stress constant, the specimens were cooledunder fixed load below Tc and later reheated at Tm + 40 �C at aconstant heating rate of 2 �C min�1, while monitoring thestrain evolution.

3. Results and Discussion

3.1 Materials Properties

Semicrystalline networks were prepared by chemical cross-linking through free-radical thermal curing (PCLMAx), or viasol-gel chemistry (PCLSGx). The crosslinked structure pre-sents, in the case of PCLMAx systems, covalent C-C bonds,resulting from the reaction of the methacrylic end groups, asnetpoints, while in the case of the PCLSG systems, Si-O-Sisilica-like domains act as crosslinks. The average lengthbetween netpoints is varied so to obtain tighter (looser)network structures by starting from precursors with lower(higher) molecular weight. The macromolecular chains willtend to partly fold in crystalline domains, which lead, belowTm, to a stable coexistence of crystallized and amorphouschains, and to a crosslinked rubber above melting temperature.

Samples were obtained as disks and tubular specimens withreduced presence or complete absence of air bubbles. Theaverage thickness of sheets and tubes was of about 1.5 mm inthe case of the methacrylated systems, whereas for the PCLSGxsystems, the maximum thickness that allowed the obtainment oftubes with reduced amount of trapped bubbles was of about0.7 mm.

A preliminary biological evaluation of one sample for eachfamily was performed to investigate their ability to sustain cell

adhesion and proliferation in the view of a potential biomedicalapplication. The results of the quantitative evaluation of cellproliferation, performed after 2 and 7 days of culture, arereported in Fig. 1a as cell proliferation with respect to thecontrol (i.e., the cells grown onto tissue culture polystyrene).The good cell proliferation observed during the culturing periodsuggests the suitability of both type of samples to sustain thebalb/3T3 clone A31 adhesion and proliferation. Results of cellmorphology investigations by CLSM presented in Fig. 1bconfirmed a significant proliferation of the cells on the surfaceof the samples, with a clear spreading and good morphology.The cells displayed both fusiform and stellate shapes withdendritic extensions from the cell membrane toward the disksurface. The cell architecture showed consistent F-actin orga-nization with great stress fibers stretched along the cytoplasm.

By changing the molecular architecture, it is possible toprovide a significant variation of both mechanical and thermalproperties of the materials. This result is clearly evidenced inTable 1, which reports the results of the materials tensilecharacterization above Tm, and those of the calorimetricanalysis. By increasing the molecular weight of the PCLprecursors, as a consequence of the lower crosslink density, therubbery modulus of the material decreases, allowing to obtain amore compliant behavior in the thermal region in which thematerial ‘‘programming’’ (i.e., the deformation leading to the‘‘temporary’’ shape configuration) is typically carried out. Theelastic modulus values span from a value of about 4.6 to0.3 MPa. From these values, the crosslink density, m, defined asmoles of effective network chains per unit volume, may beevaluated by applying the statistical Gaussian rubber theory,through the equation:

E Tð Þ ¼ 3m � R � T ; ðEq 2Þ

where E (T) is the material rubbery tensile modulus at a tem-perature T, R is the universal gas constant, and T is theabsolute temperature. As expected, the overall change of thenetwork density is of about one order of magnitude, rangingbetween 5.59 10�4 mol cm�3 for the material with thelowest initial molecular weight (PCLSG2) and3.69 10�5 mol cm�3 for the material PCLMA13, decreasingas the precursors molecular weight becomes higher. Further,larger elongation at break is found for less crosslinked net-works, and all the PCLSGx systems may sustain higher stres-ses than the PCLMAx; thus, the shape memory behavior ofthe former was tested at 600 kPa, while the latter at 200 kPa.

The crosslink density also has an important effect on thematerials transformation temperatures, showing that lowervalues of melting and crystallization temperature can be foundfor more crosslinked systems. Such an effect is regarded as aconsequence of an inhibition of the crystallites formation in thepresence of a tighter network, as confirmed by the fact that thematerials obtained from precursors with low molecular weight(2000-3000 g mol�1), the crystallization process may bepartially or completely suppressed. In order to enhance thecrystalline phase development in all samples, the cooling runswere carried out at 2 �C min�1; therefore, it has to be remarkedthat the Tc values of Table 1 are very sensitive to cooling rateand are here compared since being measured under the sameconditions and reported as indicative parameters to guide thefollowing shape memory testing protocols. The results areparticularly interesting since permitting a fine tuning of thetransformation temperatures by a proper choice of the precursor


molecular weight, and a Tm value close to the human bodytemperature (PCLMA3) or slightly higher than that (PCLSG3)can be easily attained.

3.2 One-Way Shape Memory Behavior

The one-way shape memory response of the variousmaterials was investigated by subjecting the materials to athermo-mechanical cycle, carried out under tensile conditionsand depicted as example in Fig. 2a for the system PCLMA13.The one-way shape memory characterization consists inproviding a fixed ‘‘temporary’’ shape on the specimen and inmeasuring its recovery ability under a thermal stimulus. Theprogramming step consists in heating the specimen above Tm(A fi B), applying a loading ramp up to a maximum stresslevel (B fi C), and cooling under fixed strain conditionsbelow Tc to finally unload the specimen (C fi D; this part ofthe curve is displayed as a dashed line, since it was not possibleto measure the stress evolution during this part of the test); byheating the specimen again under quasi stress-free conditionsup to the deformation temperature, the shape recovery ispromoted (D fi E).

Each system within a same material family was deformedby applying a same maximum level of stress, and in Fig. 2b,the evolution of the strain after unloading is represented as afunction of temperature for the three materials of thePCLMAx set. The thermally stimulated recovery processoccurs for all the materials in a similar manner, suggestingthat the applied strain (its value is represented by the fullsymbols in Fig. 2b) is almost completely maintained afterunloading and on a wide range of temperatures, with only aslight increase of strain due to thermal expansion. When thedeformed specimens are heated close to Tm, the shaperecovery is triggered, following a sigmoidal recovery processthat onsets at Tm�10 �C ‚ Tm�5 �C and displays a narrowdistribution around Tm.

These curves allowed to measure three specific levels ofstrain for the description of the one-way recovery behavior: theapplied strain, eapp; the level of strain following the unloadingprocess at low temperatures, eunload; the residual strain, eres,which represents the remaining strain after the recovery test.These values allow quantifying the strain fixity, i.e., thatamount of strain fixed after unloading, as:

Fig. 1 (a) Cell proliferation of balb/3T3 clone A31 cultured onto PCLMA- and PCLSG-based scaffolds, evaluated by WST-1 assay; (b) confo-cal microscopy images of merged stainings of balb/3T3 clone A31 cultured on PCLMA- and PCLSG-based disks at the 7th day of culture

Fig. 2 (a) Typical one-way shape memory cycle for the material PCLMA13 (A-B: heating at Tm + 40 �C; B-C: stress ramp up to 200 kPa;C-D: cooling under fixed strain and unloading; D-E: stress-free recovery; the segment C-D is an arbitrary dashed line due to the impossibility torecord the stress during this step); (b) stress-free recovery curves for the PCLMAx systems (full symbols represent the applied strain, eapp, main-tained during cooling before the unloading step)


Strain fixity %ð Þ ¼ eunload= eapp� �

� 100 ðEq 3Þ

and the strain recovery, i.e., the amount of the fixed strainrecovered under the application of heat, defined as:

Strain recovery %ð Þ ¼ eunload � eirrð Þ = eunload½ � � 100:ðEq 4Þ

The results, reported in Table 1, show for all the materials, withthe exception of system PCLSG2, the ability to fully fix theapplied strain and to finally restore from 94 to 100% of the origi-nal shape. The process of crystallization plays an important rolein achieving a fixed shape, and, although it is not known whetherat the end of cooling a complete crystallization is achieved, thedegree of crystallization developed is proved to be high enoughto fix the deformed shape; the lower fixity values of the systemPCLSG2 may be interpreted as a result of the slower crystalliza-tion process. Melting has a key role in activating the recoveryprocess, and the possibility to tune the Tm close to 37 �C (as forPCLMA3) allows to obtain full recovery close to the humanbody temperature. In order to better investigate the shape mem-ory capabilities of these systems for a potential employ as self-expandable stents, the kinetic of their deployment was investi-gated on tubular specimens, which, due to their outer diameter ofabout 25 mm, may be considered as pre-prototypal gastro-intes-tinal or esophageal stents. The tubes were folded in a six-armsstar-shaped configuration, by employing an ad-hoc-designedequipment, represented in Fig. 3a, capable of symmetricallybending the specimen at six load application points and to main-tain the deformed configuration during cooling. This allowed tofix the specimen shape in a temporary folded configuration forthe subsequent investigation concerning the material shape mem-ory ability. The shape memory tests were carried out by immers-ing the specimen in hot water at various temperatures, followingan isothermal testing methodology, so to provide a more precisedescription of the dependence on temperature and on time andto evaluate the time required for a full recovery under conditionssimilar to those in which the stent may be employed. The unfold-ing sequence was measured during the experiment (Fig. 3b), andthe shape recovery was evaluated as a function of time (Fig. 3c);both figures refer to the case of system PCLMA3, immersed inwater at 37 �C, showing the possibility to attain full recovery ona short time span of about 30 s.

The results of these experiments carried out at varioustemperatures are displayed in Fig. 4a for PCLMA13, andsimilar results were obtained for all the systems investigated.The isothermal recovery curves show that the response of thematerials strongly depends on the bath temperature. Inparticular, for heat treatments up to 10 min, three mainscenarios may be observed: (a) for Tbath up to Tm—15 �C,practically no shape variations are observed; (b) for tempera-tures between Tm—10 �C and Tm—5 �C, only a partialrecovery process is observed, taking place in the early1‚ 3 min and approaching a steady state for the remainingpart of the test; and (c) for recovery temperatures aboveTm—5 �C, a complete recovery process is observed, andaverage recovery times never exceeded about 2 min.

The effects of the recovery temperature evidenced by theisothermal tests may be conveniently represented, as in Fig. 4b,in terms of isochronal curves, which represent the shaperecovery achieved after a 10 min treatment as a function of thebath temperature. The results are consistent with those shownby the thermally stimulated recovery tests carried out in theDMA machine and allow to attempt at the evaluation of thetransformation temperature, Ttrans, defined as the minimumtemperature required to obtain full recovery; the values of Ttransare reported in Table 1 and are seen to be, on the average, 2 �Clower than the Tm values. The results confirm the possibility totailor the transformation temperatures of the expanding tubesand suggest the possibility to obtain potential self-expandingsystems operating in short time when directly exposed to thehuman body temperature, or heated at a slightly highertemperature.

3.3 Two-Way Shape Memory Behavior

The prepared semicrystalline networks have all displayedreversible two-way shape memory capabilities when subjectedto a constant load and heated and cooled on a region spanningfrom below Tc to above Tm. The typical response of the materialunder this thermo-mechanical cycle is shown in Fig. 5a for thesystem PCLMA13: after heating above Tm (A fi B), thetensile load is applied (B fi C), and the specimen attains acertain pre-stretch condition; while cooling under load, thespecimen undergoes a further significant elongation (C fi D),approaching an upper strain plateau value for a temperatureclose or slightly below Tc; under the same applied stress,

Fig. 3 (a) Equipment employed to crimp the tubular specimen in the star-shaped configuration; (b) self-deploying sequence for the systemPCLMA3 immersed in a water bath at Tbath = 37 �C; and c) shape recovery as a function of the time after immersion


heating the specimen above the melting temperature activates acontraction process toward the strain displayed before cooling(C fi E).

The possibility of a two-way shape memory effect is basedon a cooling-induced elongation (CIE), leading to the overallelongation eCIE and taking place when the material is cooledunder a fixed stress. The continuous elongation processpresents different slopes shown by the strain vs. temperaturecurve and ascribed in previous papers to different mechanisms:the high-temperature process is due to an entropic elasticityeffect (Ref 15, 17, 19), while the steeper one, taking place closeto the crystallization region, is ascribed to the nucleation of newcrystals, which tend to relax the stress and, in order to satisfythe constant stress conditions, promote a further stretching ofthe specimen (Ref 19, 20). Both these processes may have animportant contribution to the overall elongation and, as shownin Fig. 5a, they may lead to comparable elongations. Therecovery taking place on heating is seen to onset in correspon-dence to the melting region and finally leads to a non-fullrecovery, as shown by the residual strain eres, whose presence istypically found in literature and ascribed both to creep underthe fixed load and to slipping in the DMA clamp (Ref 15, 17).

Figure 5b shows the two-way shape memory response of thePCLMAx materials under a same level of stress in terms of thecooling-induced strain, DeCIE = e�epre-stretch, where e andepre-stretch represent the actual strain and the strain applied

during the load application (B fi C). The curves show thatunder a same level of load significantly different elongationaleffects may be displayed and that less crosslinked systems leadto larger CIEs. In particular, the most crosslinked system,whose strain level in the pre-stretch state is very moderate,presents practically no elongation effects at high temperatures,but only the elongation due to crystals formation; on the otherhand, the largely pre-stretched system PCLMA13 undergoesabout two-thirds of the elongational effect due to the rubberentropy process, and the remaining crystallization-inducedportion is still higher than the elongation shown by the lesscrosslinked systems. Crosslink density is thus a very powerfulparameter to control the elongational effect, allowing under asame level of load an increment of the CIE value of about 20times, and this result is interpreted as consequence of the largercompliance of less crosslinked structures, leading to larger eCIEportions in both the high-temperature process (due to the lowerrubbery modulus) and in the crystallization-induced process(due to the larger mobility of the remaining amorphous chainsduring the crystal formation). Further, by changing the valuesof Tc and Tm, also the transformation region may be tunedthrough the network density. The materials presented non-negligible levels of residual strain, as reported in Table 1;nevertheless, the recovery ability of the material may bequantified, with the exception of the less compliant systemPCLMA3, as a portion between 70 and 95% of the overall CIE.

Fig. 4 (a) Isothermal recovery curves for system PCLMA13 at various bath temperatures, Tbath; (b) isochronal recovery curves for the variousmaterials after a 10 min recovery treatment at various bath temperatures

Fig. 5 (a) Two-way shape memory cycle for the material PCLMA13 under a stress of 200 kPa (A-B: heating at Tm + 40 �C; B-C: stress appli-cation; C-D: cooling under fixed stress; D-E: heating under fixed stress); (b) two-way shape memory response for the PCLMAx systems under asame stress (200 kPa) in terms of cooling-induced elongation (thick line) and heating-induced contraction (dashed line) as a function of tempera-ture


Similar considerations can be provided also for the PCLSGxsystems, which could not be directly compared with the resultsof Fig. 5b, since subjected to a higher level of stress. As can beseen from the results of Table 1, the application of this stresslevels leads to deformational values between 20 and 90%,higher than those shown by the PCLMAx systems with asimilar network density suggesting the possibility to employalso stress as a tailoring variable to tune the elongational effect.It is anyway interesting to observe that, by considering the caseof the systems with the lowest crosslink densities, the systemPCLMA13 allows on the application of a smaller stress toachieve an elongation comparable to that shown by the systemPCLSG10 under an higher applied stress, confirming theimportant effect played by the crosslink density on controllingthe elongation capabilities.

4. Conclusions

A set of crosslinked PCL-based materials were preparedexploiting both traditional free-radical crosslinking reaction andsol-gel chemistry, and their structural features and shapememory characteristics were explored. By changing themolecular weight of the functionalized PCL precursors, thecrosslink density of the systems could be controlled, allowing afine tuning of the transformation temperatures (crystallizationand melting temperature). Both families, tested in biologicaltests by seeding UV-sterilized specimens with mouse embryofibroblasts, allowed to obtain, independently from the cross-linking method, materials with a lack of toxicity, as suggestedby the possibility to sustain proliferation and adhesion of thecells on the sample. The characterization of the one-way shapememory behavior revealed the possibility to fix the appliedshape, leading to a complete, or very high, recovery by heatingthe materials close to Tm, and it is shown that the changes incrosslink density mainly affect the transformation temperatures.The analysis of the shape memory behavior of tubularspecimens showed the possibility to obtain complete stress-free recovery in less than 1 minute, also at the human bodytemperature or at a slightly higher temperature. All thematerials displayed also interesting two-way shape memorycapabilities, consisting in a cyclic elongation-contraction cycle,involving strain levels up to 80%, with a moderately highdegree of reversibility. This effect is peculiar of the semicrys-talline crosslinked structure and is partly due to rubber entropyand partly to a crystallization-driven process. The level of stressand in particular the network density can be employed astailoring variable to control the elongation capabilities, thelatter term allowing to obtain very high elongation underreduced levels of stress.

Acknowledgments

The authors gratefully acknowledge INSTM (Firenze, Italy),Regione Lombardia, and the Department of Mechanical andIndustrial Engineering of the University of Brescia for providingfinancial support to this research, Mrs. Gloria Spagnoli for thesupport for the materials characterization, and Mr. Luca Dassa forthe design of the folding equipment.

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