Shape Memory Polymers Containing Higher Acrylate Content Display Increased Endothelial Cell

16 Pages·2017·4.27 MB·English

polymers
Article
Shape Memory Polymers Containing Higher Acrylate
Content Display Increased Endothelial
Cell Attachment
TinaGovindarajan ID andRobinShandas*
DepartmentofBioengineering,UniversityofColoradoatDenver|AnschutzMedicalCampus,
Aurora,CO80045,USA;[email protected]
* Correspondence:[email protected];Tel.:[phone]
Received:6September2017;Accepted:31October2017;Published:3November2017
Abstract: Shape Memory Polymers (SMPs) are smart materials that can recall their shape upon
theapplicationofastimulus,whichmakesthemappealingmaterialsforavarietyofapplications,
especiallyinbiomedicaldevices. MostpriorSMPresearchhasfocusedontuningbulkproperties;
studying surface effects of SMPs may extend the use of these materials to blood-contacting
applications,suchascardiovascularstents,wheresurfacesthatsupportrapidendothelializationhave
beencorrelatedtostentsuccess.Here,weevaluateendothelialattachmentontothesurfacesofafamily
ofSMPspreviouslydevelopedinourgroupthathaveshownpromiseforbiomedicaldevices. Nine
SMPformulationscontainingvaryingamountsoftert-Butylacrylate(tBA)andPoly(ethyleneglycol)
dimethacrylate (PEGDMA) were analyzed for endothelial cell attachment. Dynamic mechanical
analysis(DMA),contactanglestudies,andatomicforcemicroscopy(AFM)wereusedtoverifybulk
andsurfacepropertiesoftheSMPs. Humanumbilicalveinendothelialcell(HUVEC)attachmentand
viabilitywasverifiedusingfluorescentmethods. EndothelialcellspreferentiallyattachedtoSMPs
withhighertBAcontent,whichhaverougher,morehydrophobicsurfaces. HUVECsalsodisplayed
anincreasedmetabolicactivityonthesehightBASMPsoverthecourseofthestudy. Thisclassof
SMPsmaybepromisingcandidatesfornextgenerationblood-contactingdevices.
Keywords: shapememorypolymer;acrylate;cardiovascular;stents;surfaceproperties;endothelial
cells
1. Introduction
Cardiovasculardisease,includingatherosclerosisandrelateddiseases,isoneofthemaincauses
of death worldwide [1]. Atherosclerosis is typically a result of a localized inflammatory response
andcanbecharacterizedbyplaqueformationinbloodvessels[1–3]. Thisplaque,whichmayconsist
offat,cholesterol,calcium,bloodcomponents,etc.,limitsthebloodflowthroughthebloodvessel,
potentiallyleadingtoanacuteischemiccondition[4]. Traditionally,surgicalinterventionwasusedto
openoccludedvessels,butlessinvasivemethodshavebecomemoreprevalent[5]. Balloonangioplasty
expandedoccludedarteries, butre-obstructionofexpandedvesselsoccurredrelativelyfrequently,
whichrequiredre-interventionatalatertime[6,7].
Stents,orexpandabletubesthatareusedtoscaffoldnarrowand/orweakenedarteries,provide
supporttobloodvesselsthathavebeenre-opened, restoringthebloodflowtoorgansandtissues
downstream [8]. Stents have saved many lives, but limitations of current stents continue to drive
researcheffortstofurtherimprovetheirinvivofunctionality[9]. Twoprimarylimitations,restenosis,
andthrombosis,arisefromalackofcompletecompatibilitybetweenthesurfaceofthestentmaterial
andthesurroundingphysiologicalenvironment[1]. Effortstotackletheselimitationshavefocused
Polymers2017,9,572;doi:10.3390/polym9110572 www.mdpi.com/journal/polymers
Polymers2017,9,572 2of16
on two areas: limiting local cell and tissue growth through drug elution; and, improving surface
biocompatibilityofthestentmaterial.
Stentmaterialshaveagreaterchanceofsurvivalinvivoifendothelializationofthedeviceoccurs
soon after device implantation [10]. Rapid endothelialization is often characterized as significant
cellpresencewithin24hofimplantation,withfullcellularconfluenceachievedafterthreetoseven
days[11].Althoughdrugelutingstents(DES)havedecreasedtheincidenceofrestenosis,theytypically
donotachieverapidendothelialization,whichmaylimittheirlongtermutility[6,12–14]. Assuch,
focushasturnedtomodifyingthesurfacecharacteristicsofstentstopromotenaturalendothelialization.
Avarietyofsurfacemodificationtechniques,includingphysical,chemical,andbiologicalmethods
havebeenevaluatedonstentmaterials,includingmetalsandpolymers[13,15–19].
Endothelializationofmaterialsforthesuccessfulintegrationofimplantedbiomedicaldeviceswas
studiedasearlyasthe1970s[20,21]. Endothelializationmayoccurbybindingcirculatingendothelial
progenitor cells or through endothelial cell migration from adjoining endothelium [10,22]. Once
endothelialcellsattach,usuallywithinthefirst24hafterdeviceimplantation,healthycellsproliferate,
forming and retaining a permanent endothelial barrier on the surface of the device, resulting in
reducedriskoflong-termdevicerejection. Thus,ifsurfacecharacteristicsofthedeviceallowforquick
recruitmentandproliferationoftheendothelialliningafterimplantation,thechancesofpost-implant
problemsshoulddecrease[23]. Rapidendothelializationalsoincreaseshemocompatibility,another
imperative for successful integration of a cardiovascular stent [24]. The potential for surface
modificationtoenhancebiocompatibilityhasledtoanincreasedinterestinendothelializationstudies,
particularlyforcardiovascularimplants[23].
Shapememorypolymers(SMPs)areoneclassofmaterialsthatarebeingconsideredforusein
implanted,blood-contactingdevices[25]. SMPsaresmartmaterialsthatrecovertheiroriginalshape
upontheapplicationofanexternalstimulus[26–33]. Thesesmartplasticsareinitiallyfabricatedinto
their permanent shape and are then deformed and fixed into a temporary shape. These materials
recovertheiroriginalshapeuponexposuretoastimulussuchasheat,light,humidity,electrical,or
magneticfields,amongothers[28,31,32,34,35]. Theirabilitytorecoverfromlargedeformationsmakes
SMPsappealingasmaterialsforbiomedicaldevices,sincesuchrecoveryallowsfortheimplantation
ofthesedevicesusingminimallyinvasivetechniques.
ProgressinSMPresearchisnotlimitedtobiomedicalapplications. Developmentsininformation
carriers for one-time identification, aerospace applications, smart textiles, polymer actuators, and
sensors,activeassembly/disassemblyarealsonotableapplicationsofSMPs[36–42]. Highandlow
temperatureSMPsarebeingdevelopedforextremeenvironments,suchasjetpropulsionandaerospace
applications [43,44]. Smart textile applications using SMPs range from aesthetic improvements,
such as appeal, color changing capabilities, and soft display to functional applications, such as
comfort,controlleddrugrelease,woundmonitoring,emotionsensing,extremeenvironmentprotection,
etc.[37]. SMPactuatorsmaybeemployedinadjustablerotationrateheatenginesorself-regulating
sunprotectorsforbuildings[39]. Activeassemblyanddisassemblyshouldsimplifyandautomate
theprocessingprocedures,resultinginhighspeed,low-costdisassembly,renderingpartsusefulfor
additionallifecycles[41].
Sincetheshapememoryeffect,whichdrivestheshapememorycapabilitiesinSMPs,isaresult
ofpolymerstructureandprocessing[24,26],priorworkinbiomaterialshasfocusedontuningbulk
propertiestomeettherequirementsofvariousmedicalapplications[27,29,30]. Previousworkfrom
ourgrouphasfocusedprimarilyonthermomechanicalproperties,suchasshaperecovery,theshape
memory effect (SME), and modulus, as well as cytotoxicity, but the surface compatibility of these
acrylate-basedSMPshasnotbeeninvestigated[25,29,45,46]. Inaddition,therehavebeenveryfew
studiesevaluatingthesurfacecharacteristicsofSMPsinthecontextofendothelialgrowth[34,35].
Thepurposeofthisstudywastoexaminetherelationshipbetweenpolymercharacteristicsof
awell-studiedacrylate-basedSMPandendothelialcellattachmentandgrowth. Thiswouldrepresent
the first step in evaluating the potential for these materials for blood-contacting devices. Various
Polymers2017,9,572 3of16
Polymers 2017, 9, 572 3 of 17
compositions of SMP containing different weight percent ratios of tert-butyl acrylate (tBA) and
poly(ethyleneglycol)dimethacrylate(PEGDMA)weretestedforendothelialcellattachmentinvitro.
The rapid endothelialization target for this study was high live cell presence after 24 h and complete
Therapidendothelializationtargetforthisstudywashighlivecellpresenceafter24handcomplete
cell sheet formation 72 h after cell seeding.
cellsheetformation72haftercellseeding.
2. Materials and Methods
2. MaterialsandMethods
22..11.. SSyynntthheessiiss
SShhaappee mmeemmoorryy ppoollyymmeerrss wweerree ffoorrmmuullaatteedd uussiinngg tteerrtt--bbuuttyyll aaccrryyllaattee ((ttBBAA)) aanndd ppoolyly((eetthhyylleennee
ggllyyccooll)) ddiimmeetthhaaccrryyllaattee ((PPEEGGDDMMAA)),, wwiitthh aavveerraaggee mmoolleeccuullaarr wweeiigghhttss ((MMnn)) ooff 555500,, 775500,, aanndd 11000000 wwiitthh
ppoollyymmeerriizzaattiioonn bbeeiinngg ffaacciililtitaatetedd bbyy pphhotootioniintiiatitaotro 2r,22—,2—dimdiemtheothxoyx-2y--p2h-penhyelnaycelatocepthoepnhoenneo (nDeM(DPMA)P. AA)ll.
Aofll tohfet hperopdruodctusc wtsewree roebotabitnaeinde dfrofrmom SiSgimgma-aA-Aldlrdircihc h(S(tS.t .LLoouuisi,s ,MMOO, ,UUSSAA)), ,eexxcceepptt ffoorr PPEEGGDDMMAA11000000,,
wwhhiicchh wwaass oobbttaaiinneedd ffrroomm PPoollyysscciieenncceess ((WWaarrrriinnggttoonn,, PPAA,, UUSSAA)) aanndd wweerree uusseedd aass rreecceeiivveedd.. AA ttoottaall ooff
nniinnee ppoollyymmeerr ssoolluuttiioonnss wweerree pprreeppaarreedd ffrroomm tthheessee mmoonnoommeerr ccoommppoonneennttss ((FFigiguurree 11).) .

FFiigguurree 11.. SShhaappee MMeemmoorryy PPoollyymmeerrss ((SSMMPP)) ffoorrmmuullaattiioonn mmaattrriixx ooff tthhee nniinnee ffoorrmmuullaattiioonnss uusseedd..
Monomer mixtures were injected into molds composed of standard microscope slides (Thermo
Monomermixtureswereinjectedintomoldscomposedofstandardmicroscopeslides(Thermo
Fisher Scientific, Waltham, MA, USA), separated by a 1.33 mm silicone spacer (Mcmaster-Carr,
Fisher Scientific, Waltham, MA, USA), separated by a 1.33 mm silicone spacer (Mcmaster-Carr,
Elmhurst, IL, USA), and cured under ultraviolet (UV) radiation of wavelength = 365nm for 20 min,
Elmhurst,IL,USA),andcuredunderultraviolet(UV)radiationofwavelength=365nmfor20min,
similar to previous methods [29]. The samples were then removed from the molds and post-cured in
similartopreviousmethods[29]. Thesampleswerethenremovedfromthemoldsandpost-curedin
an oven at 75 °C overnight, similar to methods done previously [25].
anovenat75◦Covernight,similartomethodsdonepreviously[25].
2.2. Polymerization
2.2. Polymerization
SSaammpplleess wweerree ppoosstt--pprroocceesssseedd aatt 7755 °◦CC inin aann oovvenen ovoevrenringihgth tprpioriro trot ousues ien icnhcahraacrtaecrtiezraitziaotnio onr
cell attachment studies. Post-processing steps including annealing, which generated consistent
orcellattachmentstudies. Post-processingstepsincludingannealing, whichgeneratedconsistent
physical properties and reduced material defects. Complete conversion of monomers was verified
physicalpropertiesandreducedmaterialdefects. Completeconversionofmonomerswasverified
using Fourier transform infrared spectroscopy (FTIR, Nicolet Thermo Fisher Scientific, Waltham,
usingFouriertransforminfraredspectroscopy(FTIR,NicoletThermoFisherScientific,Waltham,MA,
MA, USA), as has been done previously [45]. FTIR samples were fabricated under similar conditions
USA),ashasbeendonepreviously[45]. FTIRsampleswerefabricatedundersimilarconditionsto
to those described above, but were made thinner, approximately 0.005 mm, to allow for the IR signal
thosedescribedabove,butweremadethinner,approximately0.005mm,toallowfortheIRsignalto
to penetrate the sample for FTIR analysis.
penetratethesampleforFTIRanalysis.
22..33.. DDyynnaammiicc MMeecchhaanniiccaall AAnnaallyyssiiss
DDyynnaammiicc MMeecchhaanniiccaall AAnnaallyyssiiss ((DDMMAA)) wwaass ppeerrffoorrmmeedd uussiinngg aa TTAA QQ880000 DDMMAA ((TTAA IInnssttrruummeennttss,,
NNeeww CCaasstltele,,D DEE,U, USASA)a) nadndw awsauss eudsetdo vtoe rvifeyritfhye tghlea sgsltarsasn tsritainosnittieomn pteermatpuerrea(tTugre) o(Tftgh) eofv athrieo uvsarSiMouPs
fSoMrmPu floartmiounlsa[t4io7n,4s8 []4.7A,4ll8o].f Athlle osfa tmhep lseasmwpelrees cwuetrien tcouts pinetcoim speencsimweitnhs dwimithe ndsiimonesnsoifo2n0s mofm 20× m5mm ×m 5
mm × 1 mm for testing. Each sample was equilibrated to 0 °C for 1 min and heated to 100 °C at a rate
of 3 °C/min. Testing was conducted at a frequency of 1.0 Hz and cyclic strain control at 0.1% strain.
Polymers2017,9,572 4of16
×1mmfortesting. Eachsamplewasequilibratedto0◦Cfor1minandheatedto100◦Catarateof
3◦C/min. Testingwasconductedatafrequencyof1.0Hzandcyclicstraincontrolat0.1%strain.
2.4. ContactAngle
ARamé-Hartgoniometer(Ramé-Hart,Succasunna,NJ,USA)wasusedtoobtaincontactangle
measurements and wettability of each SMP sample [49]. The wettability of each formulation was
measuredbyapplyingwaterdropletstoeachsurfaceandmeasuringtheanglethatformedbetween
the water droplet and the surface of the sample. Measurements were taken 10 s after the 5 µL
water droplet was introduced to the surface of the SMP to maintain consistency. Contact angles
weremeasuredusingDROPImageAdvancedcomputersoftware(Ramé-Hart,Succasunna,NJ,USA).
ThreedifferentsampleswereanalyzedperSMPformulation. FivedropswereappliedtoeachSMP
samplesurfaceandatleastfivemeasurementsweretakenperdrop.
2.5. AtomicForceMicroscopy(AFM)
Surfacetopography,ameasureofthesurfaceroughnessofeachSMPformulation,wasobtained
using atomic force microscopy (AFM) [50]. SMP fabrication molds were made using new glass
microscope slides, as done previously, which were cleaned using detergent and diH O, followed
2
byethanolandacetone,andafinalrinseusingdiH Otoremoveanysurfaceartifactsontheglass.
2
ThesemeasuresweretakentoensurethatthesurfacefeaturesdetectedbytheAFMwerearesultof
the changes in weight percent or molecular weight of the PEGDMA. Images were obtained using
a NanoSurf easyScan 2 (Nanomaterials Characterization Facility, University of Colorado, Boulder,
CO,USA).Imagepost-processingwascompletedusingGwyddionopensourcesoftware(Gwyddion,
Brno,CzechRepublic). Therootmeansquareroughnesscoefficient,R ,measuredbythestandard
q
deviationofthedistributionofsurfaceheights,alsoobtainedfromGwyddion,providedquantitative
informationofthesamplesurface[51].
2.6. CellCulture
Humanumbilicalveinendothelialcells(HUVECs),obtainedfromtheendotheliumoftheumbilical
vein,areacommoncellmodelforangiogenesisandre-endothelializationstudies. HUVECsarealso
robust, making them a favorable cell type for use in such studies, and as a result, HUVECs were
thechosencellmodelforthisre-endothelializationstudy[23].
Prior to cell culture experiments, human umbilical vein endothelial cells (HUVECS) (Lonza,
Walkersville, MD, USA) were seeded in T-75 flasks using complete growth medium: EBM-2 Cell
CultureBulletKit(Lonza,Walkersville,MD,USA).HUVECsweremaintainedinconditionsof37◦C
and5%CO inahumidifiedincubator. CellswerewashedwithHEPES,1M,buffer(LifeTechnologies,
2
Carlsbad,CA,USA)priortothechangingofthemedia. Mediawaschangedeverytwotothreedays,
and cells were passaged at 80–90% confluence. Cell passages two through six were used for cell
seedingonSMPsubstrates. Alloftheexperimentswereconductedintriplicate.
2.7. LightMicroscopy
To monitor general cell health and ensure that there were no signs of contamination present,
HUVECs and HUVEC-SMP samples were observed under an inverted light microscope (Nikon,
Melville,NY,USA)daily.
2.8. Live/DeadAssay
SMPsamplesweresubmergedingrowthmediaandwereequilibratedto37◦Cfor24hpriorto
cellseeding. HUVECswerethenplatedon1cmdiameterSMPsubstratesin24-wellplatesandwere
allowedtoattach.Cellswereseededataseedingdensityof1×105cells/mLperwell.Dailymonitoring
ofcell-adherentSMPsusingtransmissionmicroscopyallowedforthequalitativeassessmentofproper
Polymers2017,9,572 5of16
cellgrowthandtheabsenceofcontamination. Cellviabilitywasquantitativelyassessedattwotime
points,approximately24hafterplatingandagainatapproximately72h. Completecellmediumwas
changeddailytoensurethatcellsreceivedconsistentnourishmentduringthestudy. TheLive/Dead
CellImagingKit(488/570)(LifeTechnologies,Carlsbad,CA,USA)wasusedtoassessendothelial
cellattachmentandviabilitythroughfluorescentstaining. Livecells,whichwereactivelyattached
tothesubstrate,emitgreenfluorescence,whiledeadcellsfluorescered. Imageswereobtainedusing
anEVOSFLCellImagingSystem(LifeTechnologies,Carlsbad,CA,USA).Atleastthreeimagesfrom
threereplicateexperimentswereusedforcellattachmentcountingusingImageJsoftware(NIH).
2.9. CellMetabolism
PrestoBlue®CellViabilityReagent(LifeTechnologies,Carlsbad,CA,USA),aplate-basedresazurin
assay,wasaddedtocell-substratesamplesandlefttoincubatefor2h. Thiscellviabilityreagent,when
addedtocells,exploitsthereducingpowerofcellstoquantitativelymeasurecellmetabolism. This
alsoprovidesanindirectmeasurementofcellproliferationandcytotoxicity[52]. Mediawasremoved
from samples after the two-hour incubation period and fluorescence was measured on a Synergy
2microplatereader(BioTek,Winooski,VT,USA).
2.10. StatisticalAnalysis
The data were expressed as mean ± standard deviation. Statistical analysis was performed
usingMATLAB(MathWorks, Natick, MA,USA),andsignificancewasdeterminedusingANOVA
(with α-level of significance set to 0.05). The Tukey’s Honest Significant Difference Test assessed
thesignificancebetweenindividualsamplesifANOVAdeterminedsignificanceofthesampleset.
3. Results
3.1. DynamicMechanicalAnalysis
SelectbulkpropertiesofSMPformulationswerecharacterizedusingstoragemodulusandtan
delta(δ)curvedata, obtainedfromdynamicmechanicalanalysis(DMA)[47]. Theplateausabove
andbelowT onthestoragemoduluscurvesrepresenttheglassyandrubberymoduli,respectively.
g
AsshowninFigure2A–C,theglassyregionsareabsentformostformulationscontaining50weight
percent tBA or less. Rubbery modulus increases with an increasing PEGDMA content, as shown
previously[25].
The peak of the tan delta curve was used to determine the activation temperature or glass
transitiontemperature,T ,whichisthetemperatureatwhichthematerialcanrevertfromitstemporary
g
shape back to its permanent shape. The onset of shape recovery, T , which is the beginning of
onset
theshaperecoverytransition,aswellastheT range,wascalculatedusingthemethodsdescribed
g
previouslyandaredisplayedinTable1[25]. DMAdataforsomeofthesampleshasbeenanalyzedby
ourgroupinpreviousexperiments;ourdataagreedwithpriorresults[25,29,48,53].
Table1.Tg,TonsetandTgrangeforSMPFormulations.
Formulation Tg(◦C) Tonset(◦C) TgRange(◦C)
20:80tBA:PEGDMA1000 6±2 - -
20:80tBA:PEGDMA750 11±1 - -
20:80tBA:PEGDMA550 25±1 15±2 19±5
50:50tBA:PEGDMA1000 10±1 8±3 3±5
50:50tBA:PEGDMA750 19±2 12±1 13±4
50:50tBA:PEGDMA550 44±1 25±3 38±7
80:20tBA:PEGDMA1000 44±1 26±3 37±4
80:20tBA:PEGDMA750 52±1 35±1 32±1
80:20tBA:PEGDMA550 60±3 47±3 24±2
Polymers2017,9,572 6of16
Polymers 2017, 9, 572 6 of 17

Figure 2. (A) Storage Modulus (MPa) of tert-Butyl acrylate (tBA): Poly(ethylene glycol)
Figure2.(A)StorageModulus(MPa)oftert-Butylacrylate(tBA):Poly(ethyleneglycol)dimethacrylate
dimethacrylate (PEGDMA)550; (B) Storage Modulus (MPa) of tBA:PEGDMA750; (C) Storage
(PEGDMA)550; (B) Storage Modulus (MPa) of tBA:PEGDMA750; (C) Storage Modulus (MPa) of
Modulus (MPa) of tBA:PEGDMA1000. Glassy regions are absent for the formulations containing 50
tBA:PEGDMA1000. Glassyregionsareabsentfortheformulationscontaining50wt%tBAorless;
wt % tBA or less; (D) Tan delta of tBA:PEGDMA550; (E) Tan delta of tBA:PEGDMA750; (F) Tan delta
(D)TandeltaoftBA:PEGDMA550;(E)TandeltaoftBA:PEGDMA750;(F)TandeltaoftBA:PEGDMA1000.
of tBA:PEGDMA1000. Tg increases with increasing tBA content and decreases with increasing
TgincreaseswithincreasingtBAcontentanddecreaseswithincreasingPEGDMAMW.
PEGDMA MW.
Increasedmonomer(tBA)contentresultedinincreasedT ,so
Increased monomer (tBA) content resulted in increased Tgg, so
(Tg80:20(wTtg %80t:2B0A w:PtE %G DtBMAA:P)E>G(DTMg5A0):5 >0 (wTgt 5%0:t5B0A w:Pt E%G tDBMA:APE)G>D(TMgA20): 8>0 (Twgt 2%0:8tB0 Aw:tP %EG DMA)
tBA:PEGDMA)
foragivencrosslinker.
for a given crosslinker.
DecreasingthemolecularweightofthePEGcomponentinPEGDMAalsoincreasedT ,which
Decreasing the molecular weight of the PEG component in PEGDMA also increased Tgg, which
indicates that the samples containing PEGDMA550 had a higher T than samples containing
indicates that the samples containing PEGDMA550 had a higher Tgg than samples containing
PEGDMA750, which in turn had higher T than samples containing PEGDMA1000, for a given
PEGDMA750, which in turn had higher Tg thgan samples containing PEGDMA1000, for a given weight
weightpercentratio(Figure2D–F).
percent ratio (Figure 2D–F).
3.2. ContactAngle
3.2. Contact Angle
ContactangleincreasedwithincreasingtBAcontentforagivencrosslinker(Figure3A–C).Thus,
Contact angle increased with increasing tBA content for a given crosslinker (Figure 3A–C). Thus,
(CA80:20wt%tBA:PEGDMA)>(CA50:50wt%tBA:PEGDMA)>(CA20:80wt%
(CA 80:20 wt % tBA:PEGDMA) > (CA 50:50 wt % tBA:PEGDMA) > (CA 20:80 wt
(1)
(1)
tBA:PEGDMAformulations).
% tBA:PEGDMA formulations).
SSppeecciifificcaallllyy,, wwaatteerr ccoonnttaacctt aanngglleess iinnccrreeaasseedd 1111––2233%% ffrroomm tthhee 2200::8800 wwtt %% ttBBAA:P:PEEGGDDMMAA
ffoorrmmuullaattiioonnss ttoo tthhee 8800::2200 wwtt %% ttBBAA::PPEEGGDDMMAA ffoorrmmuullaattiioonnss aanndd 77––2222%% bbeettwweeeenn tthhee 5500::5500 wwtt %%
ttBBAA::PPEEGGDDMMAA aanndd tthhee 8800::2200 wwtt %%t BtBAA:P:PEEGGDDMMAA ggrroouuppss.. AAddddiittiioonnaallllyy,, tthhee wweettttaabbiilliittyy ddeeccrreeaasseedd
wwiitthh iinnccrreeaassiinngg ccrroosssslliinnkkeerr lleennggtthh ffoorr aa ggiivveenn wweeiigghhtt ppeerrcceenntt ooff ccrroosssslliinnkkeerr,,i .ie.e.,.,s saammpplleess ccoonnttaaiinniinngg
PPEEGGDDMMAA11000000w weerreem moorreeh hyyddrroopphhoobbicict hthaannt hthoosseec coonntataininininggP PEEGGDDMMAA555500..
Polymers 2017, 9, 572 7 of 17
PPoollyymmeerrss 22001177,,9 9,,5 57722 77 ooff 1167

(A) (B) (C)
(A) (B) (C)
Figure 3. (A) Wettability of tBA:PEGDMA550; (B) Wettability of tBA:PEGDMA750; (C) Wettability of
Figure 3. (A) Wettability of tBA:PEGDMA550; (B) Wettability of tBA:PEGDMA750; (C) Wettability of
Figure3.(A)WettabilityoftBA:PEGDMA550;(B)WettabilityoftBA:PEGDMA750;(C)Wettabilityof
tBA:PEGDMA1000. Wettability decreases (increasing hydrophobicity) with increasing tBA content
tBA:PEGDMA1000. Wettability decreases (increasing hydrophobicity) with increasing tBA content
tBA:PEGDMA1000. Wettabilitydecreases(increasinghydrophobicity)withincreasingtBAcontent
and increasing crosslinker (PEGDMA) MW. Significance was determined using one-way ANOVA to
and increasing crosslinker (PEGDMA) MW. Significance was determined using one-way ANOVA to
andincreasingcrosslinker(PEGDMA)MW.Significancewasdeterminedusingone-wayANOVAto
determine significant differences between samples of a given crosslinker in addition to the Tukey’s
determine significant differences between samples of a given crosslinker in addition to the Tukey’s
determinesignificantdifferencesbetweensamplesofagivencrosslinkerinadditiontotheTukey’s
Honest Significance Difference Procedure between individual samples. * corresponds to p < 0.05, **
Honest Significance Difference Procedure between individual samples. * corresponds to p < 0.05, **
Honest Significance Difference Procedure between individual samples. * corresponds to p < 0.05,
corresponds to p < 0.01, *** corresponds to p < 0.001.
corresponds to p < 0.01, *** corresponds to p < 0.001.
**correspondstop<0.01,***correspondstop<0.001.
3.3. Atomic Force Microscopy (AFM)
3.3. Atomic Force Microscopy (AFM)
3.3. AtomicForceMicroscopy(AFM)
AFM imaging was used to assess the topographical features present on each SMP surface,
AFM imaging was used to assess the topographical features present on each SMP surface,
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roughness increased with increasing tBA content, so 80:20 wt % tBA:PEGDMA formulations were
irnocuregahseesdt wwihthilei ntchree a2s0i:n8g0 twBAt %co tnBtAen:Pt,EsGoD80M:2A0 wfotr%mutBlaAti:PonEsG wDMereA sfmoromouthlaetsito fnosrw ae greivreonu gcrhoessstlwinhkielre.
roughest while the 20:80 wt % tBA:PEGDMA formulations were smoothest for a given crosslinker.
tRhoeu2g0h:8n0esws tin%cretBasAe:dP E7G3–D95M%A bfeotrwmeuenla ttihoen s80w:2e0r ewstm %oo ttBhAes:tPEfoGrDaMgAiv egnrocurpos salnindk tehr.e 2R0o:8u0g hwnte s%s
Roughness increased 73–95% between the 80:20 wt % tBA:PEGDMA group and the 20:80 wt %
itnBcAre:PaEseGdD7M3–A95 g%robueptw aneedn inthcere8a0s:e2d0 2w3t–6%8%tB Afro:PmE GthDeM 50A:5g0r wout p%a ntBdAt:hPeE2G0D:8M0Aw tfo%rmtBuAla:PtiEonG DtoM thAe
tBA:PEGDMA group and increased 23–68% from the 50:50 wt % tBA:PEGDMA formulation to the
g8r0o:2u0p want d%in tcBreAa:sPeEdG2D3M–6A8% fofrrommulathtieon5.0 :A50ddwittio%natlBlyA, :PsEamGDplMesA cofonrtmainuilnatgi oPnEtGoDthMeA8100:2000 wwte%re
80:20 wt % tBA:PEGDMA formulation. Additionally, samples containing PEGDMA1000 were
trBoAu:gPhEeGr DthManA thfoorsme ucolanttioainn.iAngd dPiEtiGoDnaMllyA,5s5am0 fpolre sa cgoinvteani nwinegigPhEt GpeDrMceAnt1 r0a0t0iow. ererougherthanthose
rougher than those containing PEGDMA550 for a given weight percent ratio.
containingPEGDMA550foragivenweightpercentratio.
3.4. Cell Viability
3.4. Cell Viability
3.4. CellViability
Cell viability, characterized as endothelial cell attachment on top of the SMP substrate, was
Cell viability, characterized as endothelial cell attachment on top of the SMP substrate, was
Cell viability, characterized as endothelial cell attachment on top of the SMP substrate, was
monitored using both light and fluorescence microscopy. Results for SMP formulations containing
monitored using both light and fluorescence microscopy. Results for SMP formulations containing
monitoredusingbothlightandfluorescencemicroscopy. ResultsforSMPformulationscontainingthe
the lowest amount of tBA (20 weight percent) are shown in Figure 5. These samples displayed little
the lowest amount of tBA (20 weight percent) are shown in Figure 5. These samples displayed little
lowestamountoftBA(20weightpercent)areshowninFigure5. Thesesamplesdisplayedlittleorno
or no live HUVEC presence 24 h after cell seeding, but the presence of dead cells was prevalent
or no live HUVEC presence 24 h after cell seeding, but the presence of dead cells was prevalent
liveHUVECpresence24haftercellseeding,butthepresenceofdeadcellswasprevalentindicating
indicating that few cells survived after 72 h.
indicating that few cells survived after 72 h.
thatfewcellssurvivedafter72h.

(A)
(A)
Figure4.Cont.
Polymers2017,9,572 8of16
Polymers 2017, 9, 572 8 of 17
Polymers 2017, 9, 572 8 of 17

(B) (C) (D)
(B) (C) (D)
Figure 4. (A) Representative Atomic Force Microscopy (AFM) images of tBA:PEGDMA550 samples.
Figure4.(A)RepresentativeAtomicForceMicroscopy(AFM)imagesoftBA:PEGDMA550samples.
3D FAigFuMre i4m. (aAg)e Rs edperepsiecnt tainticvree aAsteosm iinc Fsourrcfea cMe icrroousgcohpnye s(sA FaMs )t BimAa ginecs roefa tsBeAs. :PtBEAGD:PMEGAD55M0 Asa7m5p0 leasn. d
3D AFM images depict increases in surface roughness as tBA increases. tBA:PEGDMA750 and
3D AFM images depict increases in surface roughness as tBA increases. tBA:PEGDMA750 and
ttrttrBBoBBoAuAAAugttrg::::BBoPPPPhhAAuEEEEnngG::GGGeePPhssDDEEDDnssGGe MMMM((sRDDRsAAAA qMMq(5))151R 5AA050ooq0000)15ff; 0;0 05 ot(t (00BCBfCAA0; A A)t ()FFBC AR:R:MMPAP)oFo EER: MooPGiiGotmtmE oDmDmiGatma MMeDggmeaaaeeMAgAenssnae 1A1 ssnfsf00qo oq1 0sf0ull0uoqll000oaolau..l0r wwroae.ReR wr Rreooarao oooor ausouotst igiutsgmmmm ihghmmniheienlleiaeanaaelasnarnersnsr s st t sss (rrt(s qRqer(ReqReuunqnuqnq)ada)da)d rro o;r;oee; eff (f ( (B trtBrtBrBBooB)o))AuAu Au RRgRg:g::PoPPhohohoEEEoonnntGGGtteee smssDmDmDsss MeMMee(((aaRaRRAAnAnnqqq )7 7)7) ss 5s55gqqggq000euue;eu;;n an(n a(a(eDrDreeDrreerre)a ) )aa lrR rlRllrRooylloouyyoo uoui googtniig htntnmhc hnmcmrcnneeerreeeaaseeessanasaass enss ns(sse eR (q s(sssRwRuqqq )qwuaw uqi))troaaih eiftortro heheff
increasing tBA content and increasing PEGDMA MW. Significance was determined using one-way
increasingtBAcontentandincreasingPEGDMAMW.Significancewasdeterminedusingone-way
increasing tBA content and increasing PEGDMA MW. Significance was determined using one-way
ANOVA to determine significant differences between samples of a given crosslinker in addition to
ANAONVAOVtoAd teot edremteirnmeinseig snigifincifaincatndt idffiefrfeernecnecsesb ebtewtweeeenns saammpplleess ooff aa ggiivveenn ccrroosssslliinnkkeerr inin aadddditiitoino ntot o
the Tukey’s Honest Significance Difference Procedure between individual samples. * corresponds to
thetThuek Teuyk’seyH’so nHeosnteSsitg SniigfinciafincacnecDe iDffiefrfeernecneceP rPorcoecdeduurereb beetwtweeeenn iinnddiivviidduuaall ssaammpplleess.. ** ccoorrrerespspoonndds stot o
p < 0.05, ** corresponds to p < 0.01, *** corresponds to p < 0.001.
p<p0 .<0 50,.0*5*,c *o*r croersrpeosnpodnsdtos tpo <p 0<. 001.0,1*,* ***c* ocrorrersepspoonnddsst otop p< < 00..000011..

FiguFrigeu5r.eL 5iv. eL-iDvee-aDdeAadn aAlynsailsyosifsS oMf PSMfoPrm fourlmatuiolantsiownsit hwtihthe tlhowe leoswtweseti gwhetigphetr cpeenrtcoenftm oof nmomoneorm(2e0r w(2t0%
Figure 5. Live-Dead Analysis of SMP formulations with the lowest weight percent of monomer (20
tBAw).tT %h etsBeAs)a. mThpelsees ssahmopwlelsi tsthleotwo lnitotlee ntod ontoh eenlidaoltcheellliaatl tcaeclhl matteancthamnednth aanvde haahvieg ha hpirgehs epnrceeseonfcde eoaf d
wt % tBA). These samples show little to no endothelial cell attachment and have a high presence of
enddoethadel ieanldcoetlhlse.liSacl acleellbs.a Src=al4e0 b0aµr m= 4.00 µm.
dead endothelial cells. Scale bar = 400 µm.
SMP formulations containing equal weight percent monomer and crosslinking agent, 50:50 wt %
SMPformulationscontainingequalweightpercentmonomerandcrosslinkingagent,50:50wt%
SMP formulations containing equal weight percent monomer and crosslinking agent, 50:50 wt %
tBA:PEGDMA, displayed the greatest variability in endothelial cell viability (Figure 6). These
tBA:PEGDMA, displayed the greatest variability in endothelial cell viability (Figure 6). These
tBA:PEGDMA, displayed the greatest variability in endothelial cell viability (Figure 6). These
formulations showed endothelial cell presence 24 h after HUVEC introduction, but viability and cell
formulations showed endothelial cell presence 24 h after HUVEC introduction, but viability and
formatutalcahtimonens ts dheocwreeads eedn d72o thh aefltiearl cceellll ipnrterosednucceti 2o4n .h after HUVEC introduction, but viability and cell
cellattachmentdecreased72haftercellintroduction.
attachment decreased 72 h after cell introduction.
SMPswiththehighesttBAcontent,80weightpercent,showedthehighestamountofendothelial
cellattachment,displaying4–89%greaterendothelialcellpresence24haftercellintroductionand
33–100%increasedcellpresenceafter72hwhencomparedtotheotherformulations. Thesesamples
alsohadthehighestratiooflivecellstodeadcells(Figure7).
The80:20wt%tBA:PEGDMA1000sampleinitiallydisplayedlessendothelialcellattachment
whencomparedtotheotherformulationswith80weightpercentmonomer,butafter72h,cellpresence
increased,anindicationofhealthyendothelialcells. The80:20wt%tBA:PEGDMA750formulation
supportedcellattachment24hafterHUVECintroduction,andwasabletoretainmostcellsafter72h.
Polymers2017,9,572 9of16
Thefinalsample,80:20wt%tBA:PEGDMA550,displayedHUVECattachment24haftercellseeding,
andwasabletoretaincellattachment72hafterinitialintroduction. Allofthesamplescontaining
80:20wt%tBA:PEGDMAhadfewdeadcellspresent,ifany.
Polymers 2017, 9, 572 9 of 17
WefoundthatECattachmentoccurredonsamplescontainingatleast50weightpercenttBA,
asseeninFigure8A.However,eventhoughtheseformulationsdisplayedendothelialcellattachment,
thesamplesdidnotdisplaycellsheetformationafter72h.ThelargestratioofECcoverageonasample
comparedtothetissuecultureplatecontrol,whichdisplayedafullendothelialcellsheetafter72h,
isapPoplyrmoexrsi m201a7t,e 9l,y 5702. 4 ,asdisplayedinFigure8B. 9 of 17

Figure 6. Live-Dead Analysis of SMP formulations with equal weight percent monomer (tBA) and
crosslinker (PEGDMA). There are endothelial cells present on the surface of all samples regardless of
crosslinker length, but there is some variability based on the crosslinker used in the sample.
Specifically, both PEGDMA550 and PEGDMA750 samples seem to support more HUVEC attachment

as compared to the PEGDMA1000 sample. Scale bar = 400 µm.
FiguFirgeu6r.e L6i.v Lei-vDe-eDadeaAd nAanlyalsyissiso fofS MSMPPf oformrmuullaattiioonnss wwiitthh eeqquuaall wweeigighht tppeercrecnetn tmmonoonmomer e(rtB(AtB)A a)ndan d
croscsrSloMinsskPliesnr kw(ePirtE (hPG tEDhGeMD hAMig)A.hT)e.h sTte htrBeerAae r aceroeen netdnedonotth,t h8eel0il aiwallec ceieglllhlsst p pprreeesrsecenenntt too,n ns htthhoeew ssueurdrf aftachceee o hof ifaglahl lselassmta ampmlpeosle ursengrtea ogrdfa lreednslsde ososft hoeflial
ccerlol scasrtloitnsaskclheinrmkleeernn gtlt,e hnd,gibtshup,tl atbhyueitnr egthi s4ers–eo8 m9is%e s vgoamrreieaa btevilrai treyianbbdailositetyhd eoblnaiastleh dce ecollrn op strsheleisn ekcnrecoresu s2lsi4end khei nra ftutheseerd sca emilnlp itlneh.terS ospdaemucicpfitliceoa.n ll ya,nd
3b3o–t1hS0pP0eE%cGif iiDcnacMlrlyeA,a 5bs5oe0tdha cPneEdlGl PpDErMGesDAeM5n5cA0e a7an5fd0te Psra E7mG2pD hlMe wsAsh7ee5en0m scatoommspuplepasrp esoedret mtmo totohr seeu HoptpUhoVerrtE mfCoorarmtet aHuclUhamtVioEenCnst a.a tTtsahcceohsmmep esnaatrm edples
atlosotha hse acPdoEm tGhpDea rMheidAg th1oe0 ts0ht0e r sPaatEmiGop DolefM .lAiSvc1ea0 l0ce0eb lslaasr mt=op 4lde0e. 0aSdcµa mlcee. lblasr ( =F i4g0u0 rµem 7.) .
SMPs with the highest tBA content, 80 weight percent, showed the highest amount of endothelial
cell attachment, displaying 4–89% greater endothelial cell presence 24 h after cell introduction and
33–100% increased cell presence after 72 h when compared to the other formulations. These samples
also had the highest ratio of live cells to dead cells (Figure 7).

FiguFreig7u.rLe i7v. eLDiveea DdeAadn aAlnyasilyssoisf SoMf SPMfPo rfmorumlualtaiotinosnsw witihthh higighheesstt wweeiigghhtt ppeerrcceenntt ((8800 wwt t%%) )mmoonnomomere r
(tBA()t.BEAn)d. oEtnhdeolitahlelciealll caetltla acthtamchemnteinst iins dinicdaicteadtedb ybyth teheh higighhn nuummbbeerr ooff lliivviinngg cceellllss aanndd ththee loloww nnumumbebre r
ofdeoafd dceealdls ceplrles spernetseonnt tohne tshaem sapmlepsl.eSs.c aSlcealbea bra=r =4 04000µ µmm..

The 80:20 wt % tBA:PEGDMA1000 sample initially displayed less endothelial cell attachment
Cellmetabolismwasmeasureddailyfor72h,withresultsbeingdisplayedinFigure9. Resazurin,
wheFni gcuorme 7p. aLrievde Dtoe atdh Ae noatlhyesirs foof rSmMuPl afotiromnusl awtiiotnhs 8w0i thw heiigghhets tp werecigehntt pmerocnenotm (8e0r ,w bt u%t ) amftoenro 7m2e rh , cell
which is initially non-fluorescent, is reduced to a fluorescent resorufin when added to healthy
pres(etBnAce). Einncdroetahseelida,l caenll ainttdacichamtieonnt iso fin hdeicaalttehdy b ye nthdeo hthigehli naul mceblelsr .o fT lhivei n8g0 c:2el0l s wantd % th et BloAw:P nEuGmDbeMr A750
cells. Increases or decreases in reduction provide insight into cell health, such as metabolism and
formouf dlaetaido nc eslulsp ppreosretnetd o cne tlhl ea tstaamchpmlese. nStc a2l4e bha ar f=t e4r0 0H µUmV. EC introduction, and was able to retain most
cytotoxicity [52]. Samples containing 20 wt % tBA did not show any signs of resazurin reduction,
cells after 72 h. The final sample, 80:20 wt % tBA:PEGDMA550, displayed HUVEC attachment 24 h
afterT hceel l8 0se:2e0d iwntg ,% a ntBdA w:PaEs GaDblMe Ato1 0re0t0a isna mceplll ea titnaictihamllye ndti s7p2l ahy eadft elre sisn ietniadl oitnhterloidalu ccetilol na.t tAacllh mofe tnhte
wshamenp lceosm copnartaeidn itnog t8h0e: 2o0t hwetr % fo trBmAu:PlaEtGioDnsM wAi thha d8 0f ewwe digehatd pceerllcse pnrt emseonnt,o imf aenr,y .b ut after 72 h, cell
presence increased, an indication of healthy endothelial cells. The 80:20 wt % tBA:PEGDMA750
fo rmulation supported cell attachment 24 h after HUVEC introduction, and was able to retain most
cells after 72 h. The final sample, 80:20 wt % tBA:PEGDMA550, displayed HUVEC attachment 24 h
after cell seeding, and was able to retain cell attachment 72 h after initial introduction. All of the
samples containing 80:20 wt % tBA:PEGDMA had few dead cells present, if any.
PPoolylymmeerrss2 2001177,,9 9,,5 57722 1100o off1 167
We found that EC attachment occurred on samples containing at least 50 weight percent tBA, as
fsueretnh einr cFoignufirrem 8iAng. Hthoawteifvearn, yevceenll sthwoeurgehp trheesseen ftoormnuthlaetisoanms pdliessp,lathyeedc eelnlsdwotehreeliauln cheella latthtay,chdmyienngt,,
othrea lsraemadpyleds edaidd. nMoto sdtisopflathye cseallm sphleeest cfoonrmtaaintiionng a5f0tewr t72% ht.B TAhea nladrg8e0swt rta%tiot BoAf EsCh ocwoveedrasgigen osno fa
isnacmrepalsein cgomrepsaazreudri tno rtehdeu tcistsioune, cwulhtiucrhe mplaayteb ceoanntrionld, wichatiicohn doisfpalnayinecdr eaa fsueldl eennddootthheelliiaall cceelll lsphreeeste anfcteer,
a7n2d hc, oisn saepqpureonxtilmy,apteolsys i0b.4le, aces ldlpisrpollaifyeerda tiino nF.igure 8B.

(A) (B)
Figure 8. (A) cell count of human umbilical vein endothelial cell (HUVECs) present on each sample,
Figure8. (A)cellcountofhumanumbilicalveinendothelialcell(HUVECs)presentoneachsample,
scaled to size of the SMP sample. Living endothelial cells are present on sample containing at least 50
scaledtosizeoftheSMPsample.Livingendothelialcellsarepresentonsamplecontainingatleast50wt%
wt % tBA; (B) endothelial cell count of each SMP sample normalized to endothelial cell count of
tBA;(B)endothelialcellcountofeachSMPsamplenormalizedtoendothelialcellcountofcontrolsample
control sample (TCPS). While HUVECs attach to SMP surfaces, full coverage of SMP samples is yet
(TCPS).WhileHUVECsattachtoSMPsurfaces,fullcoverageofSMPsamplesisyettobeachieved.
Polymtoe rbs e2 0a1c7h, i9e,v 5e7d2 . 11 of 17
Cell metabolism was measured daily for 72 h, with results being displayed in Figure 9.
Resazurin, which is initially non-fluorescent, is reduced to a fluorescent resorufin when added to
healthy cells. Increases or decreases in reduction provide insight into cell health, such as metabolism
and cytotoxicity [52]. Samples containing 20 wt % tBA did not show any signs of resazurin reduction,
further confirming that if any cells were present on the samples, the cells were unhealthy, dying, or
already dead. Most of the samples containing 50 wt % tBA and 80 wt % tBA showed signs of
increasing resazurin reduction, which may be an indication of an increased endothelial cell presence,
and consequently, possible cell proliferation.

Figure 9. Cytocompatibility of SMPs. There is evidence of increasing metabolic activity, prominently
Figure9.CytocompatibilityofSMPs.Thereisevidenceofincreasingmetabolicactivity,prominently
72 h after cell introduction, but some samples show evidence of metabolic activity increasing just 48
72haftercellintroduction,butsomesamplesshowevidenceofmetabolicactivityincreasingjust48h
h after cell seeding. Samples that are cytotoxic have little metabolic activity as compared to samples
aftercellseeding.Samplesthatarecytotoxichavelittlemetabolicactivityascomparedtosamplesthat
that are cytocompatible, confirming a lack of HUVEC presence. Significance was determined using
arecytocompatible,confirmingalackofHUVECpresence.Significancewasdeterminedusingone-way
one-way ANOVA to determine significant differences between samples of a given wt % ratio and
ANOVAtodeterminesignificantdifferencesbetweensamplesofagivenwt%ratioandcrosslinker
crosslinker combination, in addition to the Tukey’s Honest Significance Difference Procedure
combination,inadditiontotheTukey’sHonestSignificanceDifferenceProcedurebetweenindividual
between individual samples. * corresponds to p < 0.05, ** corresponds to p < 0.01, *** corresponds to p
samples.*correspondstop<0.05,**correspondstop<0.01,***correspondstop<0.001.
< 0.001.
4. Discussion
Current stent technologies have found broad clinical utility, but continue to encounter issues,
such as restenosis and/or thrombosis, both of which may require subsequent reintervention to
prevent further complications. While there has been extensive work on fine-tuning the bulk
mechanical properties of SMPs and some work on cytotoxicity and biocompatibility for applications
such as hernia meshes, embolic coils, and stent grafts from our group as well as others, little work
has focused on surfaces, and more specifically, the cytocompatibility and hemocompatibility of these
materials [29,35,46,54,55].
Previous work from our group has addressed bulk mechanical properties, such as
thermomechanical behavior, the shape memory effect, partially constrained and free recovery,
biocompatibility, and cytotoxicity of these tBA:PEGDMA SMPs. However, previous studies have not
reported on surface properties or endothelial cell attachment on the surface of these materials
[25,29,45,46,48]. Endothelialization of implanted biomedical devices increases the likelihood of device
integration due to improvements in hemocompatibilty and a reduced risk of device rejection, which
necessitates optimization of surfaces to encourage HUVEC attachment [23–25].
This study evaluated the ability of select acrylate-based shape memory polymers to attach and
retain endothelial cells. Numerous studies have shown that interactions between a material’s surface
and its surroundings play a notable role in dictating the success of an implanted biomedical device
[3,56,57]. While previous studies from our lab have assessed the bulk mechanical properties of
acrylate-based SMPs for stent use, no studies have evaluated the surface characteristics of these SMPs
in detail.
Our group has previously examined the activation temperature for the SMP formulations used
in this study [25,29,48]. Briefly, the 80:20 wt % tBA:PEGDMA1000 and the 50:50 wt %
tBA:PEGDMA550 samples have glass transition temperatures closest to body temperature, with Tg’s