Synthetic fascia for stiff and tough 4D printed multifunctional structures that detect and tolerate damage

Domain: physics
Published: 2025-11-24
Editorial outcome: minor_revision

Abstract
Creatingshape-morphingstructureswithtime-responsivematerialsisakeygoalof4Dprinting,butcombininghighstiffness(E)andtoughness(K)inasinglematerialremainsdifficult.Softmaterialsstretchbutlackstrength,whilestiffmaterialsresistdeformationbutarebrittle.Natureovercomesthistrade-offinskeletalmusclebysurroundingstrongfiberswithsoft,toughtissueforprotectionandsupport.Inspiredbythisstrategy,wedevelopamulti-materialprintingmethodthatcombinesstiffsyntheticmusclewithasoft,stretchableadhesivetoformacompositestructure.Here,weshowthatthisdesigngreatlyimprovestoughnesswithoutsacrificingstiffness,enablescontrolledactuation,andmaintainsfunctionaftermultiplefractures.Wedemonstratethesepropertiesinadamage-tolerantactuator,aliftingrobotwithrecordperformance,andalatticethatdetectsandwithstandsextremeloadswhileremainingoperational.

Introduction
4Dprintingreferstoanadvancedformofadditivemanufacturinginwhicha 3D printed object isdesigned to undergo controlled, time-dependentchangesinshape,properties,orfunctioninresponsetoexternalstimulisuchastemperature,light,humidity,magneticfields.Thisstimuli-responsivebehaviordistinguishes4Dprintingfromconventional static 3D structures, enabling dynamic and functional transformations. While many 4D printed systems are designed for reversible shape changes, certain applications exploit irreversible transformationstoachievestable,deployedconfigurations,expandingthefunctionalscopeof4Dprintedarchitectures1,2.Previousstudieshavedemonstratedthatadoptingthismanufacturingapproachleadsto a substantial reduction in both printing time and materials consumption1.Furthermore,itfacilitatestheachievementofgreaterlevels of geometric complexity and responsiveness in the resulting structures3–5,a capabilitythatremainsbeyondthereach ofconventionalmanufacturingmethods6.Thesemorphingstructuredsystemscouldpotentiallybeusedinarangeofapplications,fromdeployablesystems3,7,8 and dynamic optics9,10, to robotics5,11,12 and frequency-shifting antennae3,13. Commonly used materials for 4D printing are soft composites (elastic modulus range 10−4−10MPa), such as hydrogels2,4,14,15, poly(dimethylsiloxane) (PDMS)3,16,17, liquid crystal elastomers (LCEs)12,18–21, and shape memory polymers (SMPs)22–24. However,thelowelasticmodulus(E≤10)ofthesematerials(TableS1),when undergoing shape change, limits the scalability3, actuation stress14,andload-bearingcapabilitiesofcurrentapplications1.

Recently,weaddressedthischallengebyintroducingmultiscaleheterogeneousepoxycomposites(MHECs)5,achievingaremarkable104 increasein E compared to previous efforts (see TableS1),whiledemonstratingreversibleshapechangescomparabletostate-of-the-art materials3,4. The mechanism of these actuators is governed by Timoshenko’s theory for bimetallic thermostats, wherein a bilayer composedoftwostiffmaterialswithdifferingcoefficientsofthermalexpansion(CTEs)undergoescurvaturechange duetothermalmismatch. This CTE mismatch induces bending upon Joule heating, resulting in a predictable and programmable shape transformation. Depending on the application, individual bilayers can operate as standalone actuators or be arranged into systematic lattice configurations to enable more complex, multi-curvature shape changes. Notably, this study showcased added functionalities, including addressable and localized Joule heating actuation, alongside self-sensing capabilities. These capabilities stem from the increase in electricalconductivity(σ)oftheutilizedmaterialscomposites,marking a notable advancement in the multifunctionality compared to others5. While the high E contributes to increased actuation stress, load-bearing capabilities, and scalability, these materials are predominantlyconstrained by their inherent brittleness. Consequently, structuresconstructedfromthesematerialsexhibitsusceptibilitytomechanical failure even under minor strain perturbations, underscoringtheirinherentlimitationintermsofdamagetolerance.

Onemethodtoquantifythisisbycharacterizingthetoughness(K)ofthesematerials,definedastheenergyperunitvolumeneededtoinduce mechanical failure25. A summary of E and K values for commonlyused4DprintablematerialsisreportedinTableS1.Current4Dprintable materials exhibit a trade-off between failure strain and E. MaterialswithahighfailurestrainareflexibleandstretchablebuttendtohavealowE,whilethosewithahighEusuallyhavealowfailurestrain,makingthemmorebrittle.Consequently,anexistingchallengein4Dprintingisthedevelopmentofmaterialscapableofmaintainingalarge,predictable,andaddressablemorphingmechanismforintricateshape transformations while enhancing both E and K for high-performanceapplications.

To address these challenges, we introduce a synthetic muscle compositetoenhancetheEandKofour4Dprintedstructures,allwhilemaintainingalarge,predictable,andaddressableactuationmechanism.Inspired by the skeletal muscle system26,27, we combine PDMS and MHEC to develop a synthetic analog. In the skeletal muscle system, fasciatissueplaysacrucialroleinmaintainingmusclefiberintegrity,akin to an exoskeleton27. Additionally, fascia tissue facilitates neural connections involved in sensing mechanical changes and pain perception28. The synthetic muscle composite exhibits significant similaritiestonaturalskeletalmuscles,utilizinghighelasticmodulusMHECinks5asasyntheticmuscleformechanicalforcegeneration,andPDMS acting as synthetic fascia (SF), binding the composite and enabling electrical connections even after mechanical failure (see Fig.1a).TheMHECinksconstitutingthesyntheticmusclecompositearedeveloped in our previous work5. The synthetic fascia inks are composedofaPDMSmatrixwithadjustablecross-linkdensity,displaying strongadhesiontoMHECs.Withinthisframework,wedevelopedfiveSFinksforDIW,allowingfurthertuningoftheinterfacialadhesivestrength(P)(10to300Nm−1)andK(4to6MJm−3).Throughtheintegrationofthesesoftadhesivematerialswithhigh-stiffnessthermallyresponsiveMHECsviaDIW,weachievedanimprovementofaboutthreeordersofmagnitudeintheKoftheresultingsyntheticmusclecomposite(~27MJm−3)comparedtoMHECs(~0.3MJm−3),allwhilemaintaininghighstiffness(~22GPa)andmorphingmechanism.Utilizingthisfabricationmethod,weprintedanelectricallycontrollablebilayerexhibitingdamage tolerance and detection, enduring up to 7 fractures while continuingtofunctionasprogrammed.Furthermore,weintegratedthesyntheticmusclecompositeintoapreviouslydesignedliftingrobot5,settingrecordsinliftingcapabilities(~1230timesitsownweight)andactuationstress(5.8MPa)whencomparedtoother3Dprintedactuators.Notably,evenafterfailure,theactuatormaintaineditsoperationalintegrity and high performance due to the high K imparted by the syntheticfascia.Ultimately,wepresenta4Dprintedlatticestructurefeaturingtheincorporationofthesyntheticmusclecomposite,showcasingasensitiveelectricallyresponsivesurfacewithfracturedetectioncapabilities.Toemphasizethis,wesubjectedoneofthese4Dprintedlatticestoextremeconditions,drivingacaroverit.Notably,thelatticestructure detected fractures and exhibited high resilience, enduring externalcompressivedamageequivalentto ~3×105itsownweight.

Results
Syntheticfasciaenablestough,stiffmusclecomposites
Theformulationoftheinkscommenceswithapoly(dimethylsiloxane)(PDMS)resinexhibitinganoperationaltemperaturerangeof−54to290∘C, and a high elongation (150–250%)29. This PDMS was chosen sinceitcanbepolymerizedathightemperatures(upto290∘C),andcanbondtoavarietyofsubstrates29.Todevelopinkssuitablefordirectinkwriting(DIW)(i.e.,ashear-yieldingstress,shear-thinningresponse,andplateaustoragemodulus30),weincludedfumedsilica(FS)intothePDMSformulations,servingasrheologicalmodifiers3,5(seeFig.S3).AsmeansfortuningP,E,andK,wevarythebase-to-cross-linkerweightratiowithinthePDMSmatrix.

Based on prior workon high toughness adhesives31, we reason thatthehighadhesion(i.e.,peelstrengthP)ofourSFstronglycorrelateswithhightoughnesssyntheticmusclecomposites.Inthisregard,wesystematicallycharacterizePandKbetweenoneofourMHECinksand various base:crosslinker formulations ofour SF inks (Fig.1b,c). Indeed,weobserveanincreaseinneckingoftheinterfacialSFwithincreasedbase:crosslinker(Fig.1b)andcorrespondingincreaseinP,whichhasastrongpositivecorrelationwithK.

This high adhesion that we observed between our SF and our MHECisuncommonforPDMS.CommercialPDMSmanufacturerscanincreasetheadhesionofnon-polarPDMSpolymersusingacombinationoffillers32andsilanecouplingagents33(seeFig.S1).ToinvestigatetheoriginofthishighPweperformasystematicstudycomparingSFtoothercommercialPDMSformulations,includingEcoflex00–30andSE1700.WefoundthatthePforSF(~300N.m−1)is3ordersofmagnitude higher than both Ecoflex 00–30 (~0.30N.m−1) and SE1700 (~0.46N.m−1)(Fig.S6).WethenconductchemicalanalysistofurtherunderstandthedifferencebetweenourSFandtheothercommerciallyavailablePDMS. When compared to other commercial PDMS formulations,theSFwasfoundtohavenearlyundetectablequantitiesofnon-polar Si-H groups (Fig.S2), which are known to significantly reduceadhesionbetweenPDMSandavarietyofsubstrates34.WealsofindaningredientthatisuniquetoourSF,namely2-ethyl-1-hexanol(Fig.S2), which can perform oxirane ring opening reactions35–37, resulting in strong chemical bonds between the SF and the MHEC. Notably,theremaybeotherfillersandcouplingagentsintheSFthathelpcontributetothehighP,whicharenotdetectedinouranalysis(Fig.S6).However,theabsenceoftheSi-Handthepresenceof2-ethyl-1-hexanolarethestrongestknowncontributorstothehighdegreeofadhesion that we observe between the SF and MHECs. We also observed that the printing process further increases the adhesion between the SFandtheMHEC.Compared tocastandco-curedSF-MHEClaminates,theonesthatwereco-printedandco-curedexhibitan ~8 times higher peel strength. Thisis likely due to a significant increaseinpolymerchainmixingandinter-layerdiffusion38,39,intheprintedsamplesastheprintedinterfacialfilamentsoverlapby ~5%.

Figure1dshowsthestress-strainrelationshipfortheneatMHEC,the neat SF, and the synthetic muscle composite to illustrate the synergisticmechanicaleffectwhenwrappingtheMHECwiththeSF.TheMHECaloneexhibitsahighE(~31GPa),butaverylowstraintofailure(~0.3%).Conversely,theSFbyitselfexhibitsalowE(~2.5MPa)andarelativelyhighfailurestrain(~344%).However,whentheMHECiswrappedwiththeSF,theresultingsyntheticmusclecompositepreserves a high E (~22.41GPa) that is comparable to the underlyingMHEC,withalargeyieldregion,andahighfailurestrain(~187%)onthesameorderofmagnitudeastheSF.Thissyntheticmusclecomposite representsaclassof3DprintedactuatorsthatexhibitahighE(similartoMHEC)andaKthatishigherthananyother3Dprintedactuatortodate(~3ordersofmagnitudehigherthanMHECsaloneand~3xhigherthanthetoughestsoftactuator)(Fig.1e).

WecanfurthertuneEandKofthesesyntheticmusclecompositesthrough variations in the MHEC-to-SF ratio within the samples (see Fig.S7).AsillustratedinFig.2a,aschematicoutlinesthemanufacturingapproachemployedtoadjustEandKbasedonthevolumefractionoftheconstituentmaterials.RepresentativephotographsofdistinctcrosssectionscorrespondingtovaryingproportionsofMHECandSFarepresentedinFig.2b.Figure2c,dreportstheeffectiveEandKasfunctionsofthedifferentvolumefractionsbetweenMHECandSF(seeFig.S8).Asexpected,EexhibitsadiscernibletrendwhereinitsvaluesincreasewithahighercontentofMHECinthecomposite(seeFig.2c).ThecontributionsoftheSFandMHECstotheresultingeffectiveEare slightlylowerthanpredictedbytheruleofmixturemodel.Thiscanbeattributedtotheinterfacialbondingbetweenphases,whichmaybeinfluenced by the roughness of the materials. The roughness can generate stress concentrations, leading to a reduction in E40,41. Furthermore, the results for K reveal an interesting observation: the optimalproportionsformaximizingtoughnessareapproximately80%v/vMHECand20%v/vSF(seeFig.2d).Beyondthiscomposition,specifically exceeding 80 % v/v of MHEC, the composite begins to deviatefromitsductile-likeresponsesandtransitionstowardamorebrittle behavior,aligning closely with typical characteristicsof pure MHECs.Thisoptimalcombination,resultinginthehighestK,canbeattributedtothemitigationofcrackpropagationduetothecombinationoftheSF“soft"phaseandtheMHEC“stiff”phase.Thesefindingsarefurthersupportedbyprior related workon non-responsive stiff and tough printable composites42.

Self-sensing,damage-tolerantbilayeractuators
To showcase the practicality of our responsive synthetic muscle composite, we designed and 4D-printed an electrically controllable MHECbilayer,encapsulatedwithinanSF(Fig.3a,top)(seeMovieM1).The SF functions as soft “tissue” that will maintain the geometrical integrityofthebilayer,hence,actingasasoftectoskeletonFig.1a26–28. For the core MHEC bilayer, we selected a combination of two stiff epoxy-basedinkswithdistinctCTEs.ThemismatchinCTEsenablescurvaturechangesuponheating,asdescribedbyTimoshenko’stheoryforbimetallicthermostats.ToenableactivecontrolviaJouleheating,atleastoneoftheinksmustbeelectricallyconductive.Thisallowsustolocallyandpreciselyregulatetemperaturethroughelectricalinput.ForadditionaldetailsonthedesignandcompositionoftheelectricallyresponsiveMHECbilayer,wereferthereadertoourpreviouswork5. FortheSFink,weselectedonethatdemonstratessuperioradhesiontoMHEC(MS1:1+2.8%v/vFS),translatingtoahigherK(seeFig.1).To evaluate the performance of the synthetic muscle composite, we inducedequidistantfracturesalongthearclengthofthebilayer,asdepicted in Fig.3a (bottom). Figure3b presents the change inelectrical resistance (R/R o) across the bilayer composite over time, revealing discernible fractures and highlighting its inherent self-fracturedetectionproperties.Thesefracturesaremanuallyinducedonasinglebilayersampleinasequentialmanner.Eachsetofpeaksin thesignalcorrespondstoanewfractureevent.Atlowerpeaks(fromN=1andN=3),weobserveasinglepeakforeachnewfracture,withamagnitudeincreasingwithN.FromN=4toN=5weseeasinglehighmagnitudeprimarypeak,likelyresultingfromthenewfracture,andthepresenceofadditionalsecondarypeaks,whichislikelyaresultofsmallmotionsattheinterfacesoftheexistingfractures.AboveN=5weseemultiplepeaksofsimilarmagnitude,whichislikelyaresultofthe additional fracture and collective interfacial slipping across the multiple existing fractures of the sample. Moreover, we sought to assess whether the bilayer composite could sustain actuation responses post-fracture. We subjected the bilayer composite to actuation, transitioning from room temperature (0 W) to approximately110∘C(~6.57W).InFig.3c,theactuationresponseisillustratedfor scenarios with no fractures (N=0) (top), three fractures (N=3) (middle), and seven fractures (N=7) (bottom), demonstrating consistent modulated actuation even after multiple fractures (see MovieM2).Figure3dfurthersupportsthisbyrevealinglocalizedheatgenerationnearfracturepoints(seeMovieM3),attributedtoslightlyhigherelectricalresistanceinthoseareasduetothechangeincontactarea (e.g., reduction in contact area and increase in contact roughness43,44). Inspired by previous work5,12, we implemented an electro-thermomechanicalmodel,takingintoaccountthedifferencesinmaterialsanddimensionsinthissystem,whichmainlywillaffectthethermalresponsetime45(seeSIMethods:Electro-Thermo-MechanicalModelingDetails).Figure3e,fdemonstratesgoodagreementbetweenthe model and experimental electrical (ΔR/R o) and curvature (δκ) responses,respectively,asafunctionofinducedpower,affirmingthatthe bilayer composite retains key features such as self-sensing, repeatability, reversibility, and modulated actuation through Joule heating, a feat attributed to the addition of the synthetic fascia. Moreover,basedonourpreviousworkdemonstratingstableperformance over more than 20,000 actuation cycles for the underlying MHEC system, and considering the electrically and mechanically decoupled natureofthe syntheticfascia,weexpectcomparable or improvedlong-termdurabilityfortheintegratedstructure.Notably,it also exhibits additional capabilities such as fracture detection and tolerance.

Damage-tolerantheavyliftingrobot
To illustrate the capabilities of our synthetic muscle composite, we constructed a shape-shifting weightlifting robot using 4D multi-material printing (Fig. 4a). The design is based on insights derived fromourpreviouswork,5distinguishedbytheincorporationofSF27,whichenvelopstheMHECstructure(seeMovieM4).Thisintegration results in the creation of a stiff and tough lifting robot. The SF ink employedisMS1:1+2.8%v/vFS,selectedforitshighK(seeFig.1).To optimizetheactuationperformance,wefocusedonenhancingbothK andEforthiscompositestructure,essentiallystrivingforaMHEC-to-SFvolumetricratioclosetotheoptimalconfiguration(80to20)asdepictedinFig.2.Consideringtherequirementsoftheweightlifting robot’sdesigndepictedinpreviouswork5,weachievedaMHEC-to-SFvolumetricratioof7oto30.

Figure4billustratestheevaluationofthestiffandtoughlifting robot’s performance.We initiated the assessmentby subjecting the robottomodulatedactuationusingJouleheating.Theactuationcycle commencedwithafixedvoltageinputof30V,withthecurrentlimited toamaximumof0.2A,resultinginanapproximatepoweroutputof 6Wsustainedforapproximately25s(approximately1timeconstant, refer to SI Methods: Electro-Thermo-Mechanical Modeling Details). This was succeeded by a power cut-off and a cooldown interval of about ~120s(approximately5timeconstants,refertoSIMethods:Electro-Thermo-MechanicalModelingDetails).Subsequently,externalweight(acombinationofaglassslideand/orcalibratedweights)was incrementally added to the robot, and performance metrics were tracked until reaching mechanical failure. The monitored actuation metricsincludedspecificoutputwork,liftedspecificmass,andstroke. TherepresentativeimageinthetoppanelofFig.4bdepictsthetesting setupbeforereachingfailure.ThecenterpanelofFig.4billustratesthe pointoffailure,markedbyoneoftheactuatorlegssnappingintwo underthehighexternalloadsbeingcarriedwhileundergoingactuation.However,theincorporationoftheSFenabledtheretentionof somestructuralintegrity,allowingtherobottoremainfunctionalto someextent(Fig.4bbottompanel).Thisindicatesthatwenowhavea structurecapableofoperatingunderextremeconditions(Fig.4btop panelandMovieM5),experiencingmechanicalfailure(Fig.4bcenter panel),andstillbeingabletoactuateafterwards(Fig.4bbottompanel and Movie M6). In Fig.4c, d, the measured performance of these actuatorsisshownwithrespecttoliftedmassfordifferenttestings, whereblue,yellow,andblackdotsrepresenttheliftingrobotbefore, after,andrightatthemechanicalfailure,respectively.Thestiffand tough lifting robot demonstrated a maximum specific work of 14.64Jkg−1 and an actuation stroke of 18.52%, before reaching mechanicalfailure.Itwasabletoliftamaximumof1231timesitsown weight,whichisapproximately7.89%higherthanthestrongestnatural example, the dung beetle, capable of lifting 1141 times its own weight46,47. Conversely, after reaching mechanical failure, the lifting robot continued to function, yieldingamaximumspecificworkand actuationstrokeof6.31Jkg−1and15.67%,respectively,whileliftinga maximumof564timesitsownweight.Incontrasttopriorwork,these structuresexhibitedhighperformanceinaself-standingconfiguration and maintained comparable performance even when structural integritywascompromised.

Figure4epresentsactuationstressandspecificmassforrelevant previousworksonprintableresponsiveactuators4,12,16,20,22,48–50.Actuationstressisdefinedastheliftedweightdividedbythecross-sectional areaoftheactuator.Followingaclassificationsimilartopriorstudies5, wecategorizedtheactuatorsintotwogroupsbasedontheiractuation mechanisms:viachangeinenvironmentconditionsthroughadditional components (passive) (e.g.’s, hot plates, ovens, external magnets, waterbath)orusinginternalstimuli(active)(e.g.,voltage).Observing theresults,thestiffandtoughliftingrobotbeforemechanicalfailure (indicatedbyabluestarmarker)surpassesourpreviouslysetrecords (MHECs),showcasingimprovementsof ~4%and ~39%inactuation stress and specific mass, respectively. In contrast, post-mechanical failure(indicatedbytheyellowstarmarker),thestiffandtoughlifting robotexperiencesadecreaseinactuationstressandspecificmassof ~52%and~36%,respectively,comparedtoourpreviouswork(MHECs). Nevertheless,comparedtootheractuators,itcontinuestooutperform bothactiveandpassivepreviousworksintermsofgeneratedactuationstress,whileachievingthehighestliftingcapacityamongactive actuators. Furthermore, a comparison of the E and K of various materialsusedinprintableactuatorsrevealsthatthesyntheticmuscle composite employed inthis stiffand toughlifting actuator exhibits enhancements ~71%onK,whilemaintainingcomparablehighEtoour previous work (MHEC)5 (see Fig.S9 and TableS2). Similarly, when compared to commercially available options, our synthetic muscle compositeranksasthethird-highestperformingmaterial(intermsofE andK)among10differentoptions(seeFigs.S10,11andTableS3).

3DSensingSurfacethatDetectsandToleratesDamage
Inspired bythe improvements demonstrated by the stiffand tough lifting robot, we aimedtoincorporatetheSFintointricateelectrically responsiveshape-shiftinglattices,thusconstructingastiffandtough electricallyresponsive lattice(seeMovieM7).Thegeneraldesignof thelatticeisdetailedinourpriorwork5,andthespecificsofthedesign usesinthisworkisdetailedinsection.TheintegrationoftheSFlayer wasseamlesslyembeddedwithinthemulti-material4Dprintingmanufacturing method, as depicted in Fig.5a. The SF ink used was MS 1:1+2.8%v/vFS,chosenforitshighestK(seeFig.1).TheMHECinks usedtoconstructthismorphingelectricallycontrolledlatticewhere developedinpreviousresearch5.Consideringtherequirementsofthe latticesdesigndepictedinpreviouswork5,weachievedaMHEC-to-SF volumetricratio51to49.Toassesstheresilienceoftheseelectrically controlledmorphinglattices,wedevisedanexperimentexposingthe lattice structure to external perturbations, including compressive cycling loadings(i.e.,single-axis mechanical tester)andhigh-impact loadings (i.e., sledgehammer impacts). Figure5b provides a visual representationofacompletecycleinthecharacterizationprocessof these lattices. The cycle commenced with the application of compressivecyclingforces(10compressivecycles)ontothelatticesurface. Simultaneously,wemonitoredthechangeinresistance(R/R o)across thelatticeovertime.Subsequenttothecompressivecycling,thelattice underwent external perturbations (e.g., hits with a sledgehammer),whilestilltrackingthechangeinelectricalresistances(R/R o). This entire characterization cycle was repeated once for the MHEC lattice(Fig.S12)andsixtimesforthesyntheticmusclecompositelattice(Fig.5b).

Starting with the MHEC lattice characterization (Fig.S12 and MovieM8), it is noteworthy that the force generated in the initial compressivecyclewasapproximatelythreetimeshigherthaninsubsequentcycles.Thisnotablespikecanbeattributedtoasudden fractureinthelatticestructureduringfirstcycle(seeMovieM8), adverselyimpactingitsstructuralintegrityandsubsequentlyreducing therequiredforceforcompression.Additionally,asexternalperturbationswereapplied,thestructuralintegrityofthelatticewasvisibly compromised, leading to its inability to maintain its hemispherical form. This observation is further supported by the results of the electricalresponse,revealingasignificantjumpinthechangeinthe electricalresistance(R/R o)duringexternalperturbations.Thisabrupt increase suggests that the ribs of the lattice were entirely disconnected,andtherecordedresistancevalueindicatedanopen-loop condition.ThesefindingsarereinforcedbysupplementaryMovieM8, where the induced fractures in the structure during testing can be visuallyobserved.

Conversely, the synthetic muscle composite lattice (Fig.5c and MovieM9)demonstratesremarkablestructuralresilience,sustaining uptosixcharacterizationcycles.Notably,itisobservedthattheforce required to compress the lattice decreases with each subsequent externalperturbation(e.g.,hitswithasledgehammer),indicatingthe introduction of fractures in the lattice structure. This observation aligns with the spikes in the change in electrical resistance (R/R o), coincidingwiththeapplicationofexternalperturbations,showcasing an inherent fracture detection capability. An intriguing behavior is evidentintheelectricalresponsesofthelattice:asadditionalfractures areinduced,thesensitivityoftheresistanceinresponsetodeformationincreases.Thisphenomenonisattributedtotheamplifiedchange inresistanceresultingfromslightrelativemovementsbetweensegments of the fractured lattice ribs. These movements reduce the contact area withinthe fractured ribs, thus increasing the electrical resistancewithrespecttoanexternalforce.However,sincethesynthetic fascia surrounding the electrically conductive ribs is hyperelasticandadherestotheMHECribs,afterthecompressiveloadsare released, the MHEC ribs can revert to their initial configuration, increasingthecontactareaoftheribsandtherebyreducingtheelectricalresistanceofthelatticeclosetoitsnominalvalue.Priorresearch on soft sensors composed of conductive fillers provides additional supportforthesefindings51.

AsummaryofthesefindingsispresentedinFig.5d.Thesynthetic musclecompositelatticeexhibitsvariedforceandresistanceresponses,dependingonthenumberofinducedfractures(i.e.,thenumberof externalperturbationsfromthesledgehammer).Atalownumberof externalperturbations(e.g.,threeorless),thesurfaceexhibitsarelativelyhighcompressiveforceandlowelectricalresponsiveness.Ata highernumberofexternalperturbations(e.g.,fourormore),thesurface exhibits a low compressive force and high electrical response. Furthermore, this synthetic muscle composite demonstrates high resiliencetovariousexternalperturbations,showcasing3Dsurfaces capableofsensinganddamagedetection.

Tofurtherevaluatetheresiliency,sensing,anddamagedetection capabilities,wesubjectedthesyntheticmusclecompositelatticesto anextremecondition-drivingacaroverit(seeFig.5etopandMovie M10). Figure5e (bottom) displays the change in the electrical resistances (R/R o) measured during the testing, accompanied by representativephotographsillustratingdifferentstagesoftheexperiment. Notably,itcanbeobservedthatwhenthecarispositionedcompletely overthelattice,thereisaspikeintheR/R o,signifyingtheinductionof fractures.Subsequently,thereisadeclineinR/R o,accompaniedbya permanentdriftinthenominalreadingofapproximately ~15%,indicating a lasting change. This may be attributed to the permanent reduction in the contact area between electrically conductive ribs causedbytheextremeloads43,44,51.Itiscrucialtoemphasizethatthe latticedemonstratedremarkableresiliencyduringthisextremetesting,withstandingcompressionunderanexternalloadequivalentto 3.3×105timesitsownweight,andevenaftersuchsignificantstress,it retaineditsshapeandactuationcapabilities(seeMovieM11).

Methods
Materials
AllSFinkswerecreatedbymixing(DAC150.1FVX-K,FlackTek,60sat2000rmin−1)appropriateamountsofbaseandcross-linkerofPDMS(MasterSIL323,MasterBond).Next,weaddedtheappropriateamountoffumedsilica(FS)(Aerosil106,Evonik),rangingfrom2.8–3%v/vandpre-mixing(DAC150.1FVX-K,FlackTek)for360sat2000rmin−1withcoolingdownstepsof60s,every120s.After,wefurthermix(DAC150.1FVX-K,FlackTek)theinksfor90sat3500rmin−1withcoolingdownstepsof30s,every30s.Thepotlifeofalltheseelastomerinksisabout3.5h.AlltheinksareloadedintoLuer-Locksyringes(3ccor10cc,Nordson,EFD),thencentrifuged(300sat4000rmin−1)toremoveairbubbles beforeprinting. A summary of ink formulations is listed in TableS4.ElectricallyconductiveMHECinkusedinthisworkwascreated byadding the properamountofcorresponding basetocross-linker(SUP121AO,MasterBond),followingwiththeadditionofcorresponding carbon fibers (K223HM, K223HM, Mitsubishi Chemical, diameter ~11μm,length ~200μm),andcorrespondingcarbonblack(100%compressed,99.9%,ThermoScientificChemicals),andmixing(DAC 150.1 FVX-K, FlackTek) for 180s at 3500 rmin−1 with cooling downstepsof60s,every60s5.

Electrically insulated MHEC ink was created by mixing the requiredamountofsurfactant(24%v/v)at3500RPMfor30s(DAC150.1FVX-K,FlackTek),following bytheadditionof(8%v/v)fumed silica(AerosilR106,Evonik)andmixing(DAC150.1FVX-K,FlackTek)for90sat3500rmin−1withcoolingdownstepsof30s,every30s.Ecoflex 00–30 ink used in this work was created by separately mixing(DAC150.1FVX-K,FlackTek)fumedsilica(5%v/v)(Aerosil150,Evonik)withpartsAandB,followingamixingscheduleof420sat3500rmin−1withcoolingstepsof60s.ThenmixingpartAandBfor60sat1750rmin−152. SE1700 ink used in this work was created by mixing the correspondingbasewithcross-linker(10:1),followingtheadditionof3.0%offumedsilica(Aerosil150,Evonik),andmixingfor420sat3500rmin−1withcoolingstepsof60s3,53.

Inkrheology
Rheological characterization is carried out using DHR-2 rheometer(TAinstruments), equipped with a 20mm diameter plate geometry andgapsizeof1.6mm.Materialsareequilibratedatroomtemperaturefor 30s before flow and amplitude sweeps are conducted. In flow sweeps,thematerialsareshearedat0.01[s−1]to10[s−1].Shearstorage(G’)andlossstorage(G”)moduliaremeasuredasafunctionofshear stress at a frequency of 1Hz during amplitude sweeps. Unlike the controlformulations,eachinkdisplaysclearplateau,yieldstress,andshearthinningbehavior,attributedtotheadditionoffumedsilica.A summary of the rheological characterization of the inks isresented inFig.S3.

Adhesivefasciachemicalcharacterization
UncuredPDMS(MasterSIL323(SF),EcoFlex00–30andSE1700)were preparedusingtheirsetmixingratios(seeTableS4),andmixed(DAC150.1FVX-K,FlackTek,60sat3500rmin−1)andimmediatelytakenforFTIRanalysis.CuredPDMSsampleswerepreparedthesamewayandcured(250∘C,2h).FTIRwastakenfromaneatversionoftheepoxy(Supreme 112SP, Masterbond), to prevent interference from carbon blackandotherfillers.ThemixeduncuredandcuredsamplesofPDMS(MasterSIL 323, Eco flex 00–30 and SE1700 were analyzed using a Nicolet4700ATR-FTIR.Absorbancemeasurementsweretakenfrom500–4000cm−1withastepsizeof1cm−1and128scans.

Uncured PDMS components (1g, MasterSIL 323 A, MasterSIL 323B,EcoFlex00–30A,EcoFlex00–30B,andSE1700AandSE1700B) wereeachsonicatedinacetone(10mL)for20min,andthesolution was filtered (Thermo Scientific, Nalgene Syringe Filter 0.45μm) to extract any polymer additives. Gas Chromatography Mass Spectrometry(GC-MS)wasrunonthesesolutionsusinganAgilent6890NGC-MSintriplicate,andcomparedagainsttheNationalInstituteofStandardsandTechnologyMassSpectralDatabase(NIST11)54toidentify compounds.

Multimaterial4Dprintingofsyntheticmusclecomposite
Samplesaremanufacturedfollowingmethodsandproceduresstated inpreviouswork5,withthemaindifferencesthatweincludedthenew PDMSlayerthatencapsulated theMHECsample.Extrusionprinting pressures(7x3ccHPxHigh-PressureDispensingTool,NordsonEFD) andspeedsrangedfrom160–495psiand12–16mms−1,respectively.TableS5presentprintingparametersforeachink.Theintegrationof thePDMSlayerwasachievedthroughacustomprintpath,combining MHEC inks and PDMS ink in a co-printing approach. During the incorporationofthePDMSlayer,westrategicallypredefinedexposed areasoftheMHECbilayer.Thiswasdonetofacilitateelectricalwiring connectionsforsubsequentdemonstrations.

Peelstrength,elasticmodulus,andtoughnessmeasurements
Samples (8mm wide, 50mm long, and 3.3mm tall) were printed to characterizetheadhesivestrengthforeachofourPDMS(SF)inksto MHEC (Fig.S4), following the standardized testing method ASTM D3330/D3330M-045555(seeMovieM12).Eachsamplewastestedina single-axismechanicaltester(Instron5944Micro-tester)usingapullingspeedof0.2mms−1(Fig.S4).Theresultingpeelforceversuspeel lengthdataispresentedinFig.S4.Thepeel/adhesivestrengthiscalculatedbytakingtheaveragepeelforceandthendividingthisforceby the width (~8mm) ofthe adhesive sample. The average of the peel forceistakenfrom10mm(peellength)tofailure.Asummaryofthe resultingpeel/adhesivestrengthisgiveninFig.1b,c,witherrorbars representing the standard deviation of the measurements. Three samplesweretestedperinkformulation.

Testsamples(gaugelength9.50mm,wide3.20mm,andthickness1.60mm)wereprintedtocharacterizetheelasticmodulusofeach PDMS (SF) ink and synthetic muscle composite, following standardizedmethodsASTMD0638-1456(seeMovieM13).Eachsamplewas testedunderuniaxialtensioninasingle-axismechanicaltester(Instron 5944Micro-tester).PDMS(SF)andsyntheticmusclecompositesamples were tested at engineering strain rates of 2.8×10−2s−1 and 1.6×10−2s−1, respectively (Fig.S5).The E for eachsample was determinedbyfittinglow-straindata(0≤ϵ≤0.005)57tothelinearrelationshipσ=Eϵviathe‘polyfit’functioninMATLAB.TheKforeachsample wasdeterminedbycalculatingtheareaunderthecurveofthestress versus strain data (Fig.S5). This was achieved using the numerical integration‘trapz’functioninMATLAB.AsummaryoftheresultingE and K for each sample is given in Fig.2c, d and TTableS6. Three samplesweretestedperinkformulation.

Optimizingvolumefractionsforsyntheticmusclecomposites
FortheMHECink,wechoselowα-1:0.2+15%v/vCF+8%v/vCB5,due toitshighelectricalconductivitywhichallowuscontrolofthestructureusingJouleheating,whilealsoprovidingadditionalresistivesensingfunctionalities.FortheSFinkwechoseMS1:1+2.8%v/vFS,which istheinkthatexhibitsthehighestadherencetotheMHECinks(Fig.1c). Syntheticmusclesamplesaredevelopedfollowingmethodsandproceduresdetailedinthesection,withtheconsiderationofvaryingthe proportionsoftheSFandMHECaccordingly.Thevolumefractionof eachconstituentmaterialinthecompositewascharacterizedbytakingcross-sectionalpicturesofeachsampleusingacalibratedminiature microscope lens system (InfiniTube FM-100, Infinity) and an 18mm/2.00xlens(PL-series,Infinity).Dataforthecross-sectionwas extracted fromthe images using a custom MATLAB script. E and K werecharacterizedfollowingthemethodsandproceduresstatedin the section. Three samples were tested for different MHEC-to-SF proportions.

Characterizingdamage-tolerantelectromechanicalbilayers
Simplebilayersareconstructedfollowingmethodsandmaterials(low α-1:0.2+15%v/vCF+8%v/vCBandhighα-1:1.8+24%v/vtr-x+8%v/v FS)developedinpreviouswork5,withthemaindifferenceofintegration the SF layer (MS 1:1+2.8% v/v FS) thatencapsulates the MHEC bilayer (see section). The samples each had overall printed width, length,andtotalthicknessof3mm,42mm,and1.2mm,respectively. Thesamplesconstitute~50%v/voftheMHECbilayerand~50%ofthe SFlayer.Weinduceequidistantfracturesalongthearclength,ranging from0,1,2,3,7fracturesperspecimen,respectively.Followingthis, weproceededtotesttheelectrothermomechanicalresponseofthese viaJouleheating.Thebilayerswereelectricallyconnectedtoapower supply (E36233a, Keysight), where we limit the voltage at 30V and modulate the currentfrom 150–300mAwith incrementsof50mA, allowing60soneachsteptoreachasteadystatetemperature.Characterizationofthethermalandelectromechanicalresponsesiscarried outfollowingthesamemethodspresentedinpreviouswork5.AsummaryofthesestudiesisshowninFig.3.

Designandtestingofstiff,tough,electricallyresponsivelattices
Lifting robot is constructed using the same materials (low α- 1:0.2+15%v/vCF+8%v/vCBandhighα-1:1.8+24%v/vtr-x+8%v/v FS) and procedures to previous work5, with the main difference of integrating the SF layer (MS 1:1+2.8% v/v FS) to encapsulate the structure (see section). The samples constitute ~70% v/v of MHEC bilayerand ~30%ofSFlayer.Weconnectedthelegstoapowersupply (E36233a, Keysight), where we limited the voltage to 30 Volts and modulated the electrical current from 0 mA to 200 mA. Structures wereactivatedatmaximumstroke(~6W)for ~25s(~1timeconstant, seeSIMethods),followedbyacutofffrompowertocooldownto room temperature for 120s ( ~5 time constant, see SI Methods). Characterization of the performance is carried out following proceduresdescribedinpreviouswork5.TheSFlayerisconsiderasexternal weightandnotasanactuatorsincethisdoesnotcontributeonthe generation of actuation. External weights are normalized by the actuatormass(~65mg,totalmassof4legs).Asummaryoftheresults fromthischaracterizationispresentedinFig.4.

Stiff,toughandelectricallyresponsivelatticedesign,testing, andcharacterization
Samplesarepreparedusingmaterials(lowα-1:0.2+15%v/vCF+8%v/v CBandhighα-1:1.8+24%v/vtr-x+8%v/vFS)andproceduresdetailed in previous work5, with additional integration of the SF layer (MS 1:1+2.8%v/vFS).Thestructureisprintedasaflatsquarelatticewithrib spacing (L~ =15mm) and number of cells along the x and y axis (N =N =N=6),programmedviastereographicprojection3totransx y formintoasphericalcapwithanopeningangleof135∘.Moredetailson thelatticedesigncanbefoundinourpreviousworkonshapeshifting structures3,5.Thesamplesconstitute ~51%v/vofMHECbilayerand ~ 49%oftheSFlayer.Wegluedthincopperwires(awg38)withcommerciallyavailablesilverMHEC(MGChemicals8331D)toeachcorner ofthelattice,servingasanelectricalinterfaceforresistivesensing.A digitalmultimeter(34460A,Keysight)isusedtomeasuretheequivalentelectricalresistanceofthelatticewhiletesting.After,wemounted itonauniaxialcompressionsingle-axismechanicaltester(Instron5944 Micro-tester), where compression cycles are induced on the lattice surface. Ten cycles were programmed at a speed of 0.5mms−1, followedbyexternalimpactloadsinducedwithasledgehammer.Acalibratedopticalcamera(D850,Nikon)wasusedforimagingthefront viewofthetestsample.Thisprocessisrepeatedsixtimes.Asummary of this characterization is shown inFig.5b–d. Furthermore, a similar characterizationiscarriedout,whereinsteadofauniaxialcompression test,wedriveoverthelatticewhilemeasuringtheequivalentelectrical resistance.Forthistesting,weplacethelatticeontopofawhitepaper (forbettercontrast),anduseacalibratedopticalcamera(D850,Nikon) toimagethecompressionofthelattice.Asummaryoftheresultsfrom thistestingispresentedinFig.5e.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present a continuation of their previous work on multiscale heterogeneous epoxy composites (MHECs) by introducing a co-printed PDMS-based “synthetic fascia” to enhance toughness and damage tolerance in 4D printed structures. The manuscript is well-organized and the experimental work is carefully executed, demonstrating improved mechanical resilience, self-sensing, and partial retention of functionality after mechanical failure.\nHowever, the contribution appears incremental. The core materials platform, fabrication method, and actuation mechanism (Joule heating) are largely unchanged from the prior study. The central novelty—the addition of a soft, co-printed fascia layer—is effective and biomimetic but follows well-established strategies in soft material toughening. While the added damage tolerance is useful, it represents a performance optimization rather than a conceptual or methodological advance. As such, I do not believe the manuscript meets the novelty threshold required for publication in Nature Communications, even if framed as a platform extension.\nComments for the authors:\n- The manuscript repeatedly refers to “constant power” actuation using a Keysight E36233A power supply. However, this instrument does not support constant power mode; it operates in either constant voltage or constant current. Based on the methods described, the authors likely performed constant current actuation by manually stepping current at a fixed voltage. This is a critical distinction: in thermally responsive systems with dynamic resistance, true constant power is required to avoid uncontrolled self-heating and thermal runaway. A brief survey of the literature (e.g., 10.1002/admt.202301769) shows that others have used dedicated power-regulated systems to address this challenge. I strongly recommend the authors either (1) revise their terminology throughout the manuscript to reflect constant current operation, or (2) repeat the relevant experiments using a power source that supports true constant power output. The current description is technically inaccurate and may misrepresent the thermal behavior of the actuators.\n- The manuscript claims record-setting lifting performance and actuation stress (e.g., lifting 1230× the actuator's own weight). However, there is no benchmarking table or standardized comparison with other reported actuators or artificial muscles. Quantitative comparisons of energy density, power density, and cycle life against state-of-the-art actuators are essential to support these claims.\n- The authors should explicitly assess long-term actuation performance under realistic conditions. I recommend conducting at least 1000 continuous actuation cycles using true constant power input, with tracking of curvature, response time, and degradation over time. Many Joule-heated polymer actuators degrade significantly after tens to hundreds of cycles, and without this data, the practical utility of the proposed system remains uncertain.\n- Finally, the terminology around \"damage tolerance\" vs. \"self-healing\" should be used with precision. The system does not demonstrate self-healing in the material science sense (e.g., polymer network reformation). Rather, the \"fascia\" structure enables structural containment and temporary mechanical function post-fracture. Clarifying this distinction will help avoid overstating the material’s capabilities.\nOverall, the work is technically sound and addresses an important limitation of the previous study. However, to warrant publication in a high-impact journal, the manuscript requires greater differentiation, clearer terminology, and more rigorous performance evaluation.", "reviewer_2": "This increased toughness of this composite of composites is very interesting and an excellent application of multi-material printing capabilities of DIW. The material engineering presented from toughening of the matrix materials to the adhesion of the composite is elegant and well described.\nThe importance of the work is sufficient for publication in Nat. Comm., as it is a very tough, switchable composite and the biomimetic comparisons are intriguing.\nThe description of the work, however, is confusing. If I understand correctly, there is not active control of motion; rather, there is active control of stiffness. Also, the term “4D” I find is hyperbolic and unnecessary. Particularly if I am correct in understanding that the motion comes from equilibrating with the environment rather than using energy to go out of equilibrium. I think the science and engineering would stand out more with better clarity in the contribution, and reduction in hype words.\nIf I am incorrect and these actuators cause motion, please make that more clear in the writing – and distinguish it as an out of equilibrium actuation mode.\nThis clarity could come quite easily:\n(i) The performance of prior MHECs are described in the introduction, but the mechanism is not clearly described. Referencing the publication is sometimes OK for efficiency, but I think it is important enough to briefly describe in the main text.\n(ii) Page 6, line 156 – please explain the mechanism a little bit in the text instead of just a reference.\n(iii) Page 3, line 85 – what does the pronoun “this” refer to?\nWith these changes, I would be delighted to re-review.\nThank you", "reviewer_3": "Boley and coworkers reported a strategy to print PDMS-epoxy composite structures. These structures exhibit high stiffness and high toughness at the same time. The authors characterized the mechanical properties of the composite through tensile test and measured the adhesion strength between the two component materials through peeling test. The authors printed a bilayer structure that can bend when a voltage is applied and designed an actuator that can lift weight. The second demonstration was a lattice structure that can be used as an impact sensor. The manuscript is technically sound and well-organized. The proposed strategy also showed novelty to some extent. The reviewer recommends the publication of this manuscript if the following concerns can be properly addressed:\nFig 1c error bar color is too faint.\nFig 3b, the reviewer is confused about how to understand the pattern shown in the plot. The authors should properly clarify what does the number of peaks mean and how to interpret the magnitude of the peaks.\nFig 4e, how is the actuation stress calculated? The calculation of actuation stress should depend on the type of the actuator, e.g. contraction-type or bending type. Actuation stress needs a clearer definition before comparisons can be made.\nFig 4c,d the black points seem redundant.\nFig 5c, the authors claimed to make a force sensor. How is the R/R0 correlated with the compressive force? In the first cycle, the R/R0 change is very small, after fifth impact, the R/R0 change becomes much larger, thought the compressive force becomes even smaller. Is this feature desired for a force sensor?\nFig 5d is not clear as all data points overlap."}, "author_response": "Dear Editors and Reviewers,\nBelow is a point-by-point list of how we addressed each reviewer comment in our revised submission. We\nwant to thank the reviewers for their important feedback, which we believe has significantly improved the\nimpact and quality of our work.\nSincerely, on behalf of the authors\n-Will Boley\nA guide to our color convention:\n• Original feedback from the Editor and Reviewers are in black,\n• Responses to the Editor and Reviewers are in blue,\n• New/modified text that we incorporated into the manuscript and Supplementary Information (SI) in\nresponse to the Editor and Reviewers are italicized and in teal,\n• Additional text incorporated into the manuscript and SI are italicized and in red.\nResponses to Reviewer #1\nThe authors present a continuation of their previous work on multiscale heterogeneous epoxy composites\n(MHECs) by introducing a co-printed PDMS-based “synthetic fascia” to enhance toughness and damage\ntolerance in 4D printed structures. The manuscript is well-organized and the experimental work is carefully\nexecuted, demonstrating improved mechanical resilience, self-sensing, and partial retention of functionality\nafter mechanical failure.\nHowever, the contribution appears incremental. The core materials platform, fabrication method, and\nactuation mechanism (Joule heating) are largely unchanged from the prior study. The central novelty—the\naddition of a soft, co-printed fascia layer—is effective and biomimetic but follows well-established strategies\nin soft material toughening. While the added damage tolerance is useful, it represents a performance op-\ntimization rather than a conceptual or methodological advance. As such, I do not believe the manuscript\nmeetsthenoveltythresholdrequiredforpublicationinNatureCommunications,evenifframedasaplatform\nextension.\nWe thank the reviewer for their thoughtful and detailed evaluation. We appreciate the recognition of\nthe quality and careful execution of the experimental work, as well as the acknowledgment of the improved\nmechanical resilience, self-sensing, and functional retention demonstrated by the system.\nWhile we agree that the current study builds upon our previously established MHEC platform, we re-\nspectfully emphasize that the co-printed synthetic fascia introduces a meaningful advance in the field of\ndamage-tolerant actuators. Specifically, the novelty lies not solely in the addition of a soft layer, but in\nits strategic co-design and integration with the underlying heterogeneous actuator, enabling simultaneous\nimprovements in mechanical toughness, damage tolerance, and post-fracture functionality — all without\ncompromising actuation stiffness, performance, or sensing capability. A key distinction of this work is that\nsuchtoughnessenhancementviasoftadhesiveinterlayersistypicallydemonstratedinnon-functionalorpas-\nsivematerials,whereasoursystemachievesthiswhilepreservingfullactuationandself-sensingfunctionality.\nFurthermore, the approach is generalizable and can be extended to a wide range of responsive actuator ma-\nterials. This positions the work as a platform-level enhancement rather than a simple material substitution\nor marginal optimization.\n1. Themanuscriptrepeatedlyrefersto“constantpower”actuationusingaKeysightE36233Apowersup-\nply. However, this instrument does not support constant power mode; it operates in either constant\nvoltage or constant current. Based on the methods described, the authors likely performed constant\ncurrentactuationbymanuallysteppingcurrentatafixedvoltage. Thisisacriticaldistinction: inther-\nmallyresponsivesystemswithdynamicresistance,trueconstantpowerisrequiredtoavoiduncontrolled\n2\nself-heatingandthermalrunaway. Abriefsurveyoftheliterature(e.g.,10.1002/admt.202301769)shows\nthat others have used dedicated power-regulated systems to address this challenge. I strongly recom-\nmend the authors either (1) revise their terminology throughout the manuscript to reflect constant\ncurrent operation, or (2) repeat the relevant experiments using a power source that supports true\nconstant power output. The current description is technically inaccurate and may misrepresent the\nthermal behavior of the actuators.\nWethankthereviewerforthisimportantandtechnicallyinsightfulcomment. Weacknowledgethatour\nuse of the term ”constant power” to describe our actuation conditions was imprecise and potentially\nmisleading. To clarify, we operated the system using the Keysight E36233A power supply in constant\nvoltagemode,settingthevoltageto30Vandlimitingthemaximumcurrenttoasetpointtargetbased\non the specific demonstration (i.e., bilayers, lifting robot), thereby ensuring that the power output\nremained approximately constant during operation. We did not use a true constant power control\nmode, and we agree that this distinction is critical, particularly for thermally responsive systems\nwith dynamic resistance characteristics. We have thoroughly revised the manuscript to correct the\nterminology.\nSpecifically, in the ”Damage-tolerant heavy lifting robot” section, we corrected the statement:\nOriginal: “The actuation cycle commenced with a constant power input (6 W) sustained for approxi-\nmately 25 seconds...”\nRevised: “The actuation cycle commenced with a fixed voltage input of 30 V, with the current limited\nto a maximum of 0.2 A, resulting in an approximate power output of 6 W sustained for approximately\n25 seconds...”\nWe also include the following statement in the Methods section, specifically within the Characterizing\nDamage-Tolerant Electromechanical Bilayers subsection, to clarify the voltage and current parameters\nused during bilayer testing.\nThe bilayers were electrically connected to a power supply (E36233a, Keysight), where we limit the\nvoltage at 30 V and modulate the current from 150 – 300 mA with increments of 50 mA, allowing 60\nseconds on each step to reach a steady state temperature.\nWe believe these revisions improve the technical accuracy and clarity of the manuscript, and we ap-\npreciate the reviewer bringing this to our attention.\n2. The authors should explicitly assess long-term actuation performance under realistic conditions. I\nrecommend conducting at least 1000 continuous actuation cycles using true constant power input,\nwith tracking of curvature, response time, and degradation over time. Many Joule-heated polymer\nactuators degrade significantly after tens to hundreds of cycles, and without this data, the practical\nutility of the proposed system remains uncertain.\nWe thank the reviewer for this important suggestion regarding the assessment of long-term actuation\nperformance, which is indeed a critical consideration for the practical deployment of Joule-heated\nactuators. We would like to note that in our previous work, we conducted extensive cyclic testing of\nthe underlying multiscale heterogeneous epoxy composite (MHEC) system without the integration of\nthe synthetic fascia.\nInthatstudy,wedemonstratedstableactuationperformanceovermorethan20,000continuouscycles,\nhighlighting the repeatable and reliable actuation and self-sensing mechanisms of these actuators,\nthereby confirming the durability of the core material platform under Joule heating.\nInthecurrentstudy,ourfocuswasontheintroductionoftheco-printedPDMS-basedsyntheticfascia,\ndesignedtoenhancetoughness,damagedetectionandtolerance,andfunctionalretentionunderextreme\nmechanical deformation. While the synthetic fascia significantly improves mechanical robustness, it is\nmechanically integrated yet electrically isolated from the Joule-heated regions. Therefore, it does not\ndirectlyinfluencetheJouleheatingresponse,thermalactuationmechanism,oractuationrepeatability.\nBasedontheactuator’sperformance, wedonotexpectthecyclinglifetobenegativelyaffectedbythe\ninclusion of the fascia.\nactuationresponse. ThisisconsistentwiththefactthattheYoung’smodulusofthesyntheticfasciais\nsignificantly lower than that of the epoxy actuator, and therefore, it does not constrain the actuation.\nFurthermore, the damage-tolerant characteristics provided by the fascia are expected to mitigate the\neffects of thermal fatigue and mechanical degradation, potentially improving long-term operational\nreliability.\nWe have added the following statement on section Self-Sensing, Damage-Tolerant Bilayer Actuators\nfor clarity:\nMoreover, based on our previous work demonstrating stable performance over more than 20,000 ac-\ntuation cycles for the underlying MHEC system, and considering the electrically and mechanically\ndecoupled nature of the synthetic fascia, we expect comparable or improved long-term durability for the\nintegrated structure.\n3. Finally, the terminology around ”damage tolerance” vs. ”self-healing” should be used with precision.\nThe system does not demonstrate self-healing in the material science sense (e.g., polymer network\nreformation). Rather,the”fascia”structureenablesstructuralcontainmentandtemporarymechanical\nfunctionpost-fracture. Clarifyingthisdistinctionwillhelpavoidoverstatingthematerial’scapabilities.\nWe thank the reviewer for this important comment regarding the terminology around “damage tol-\nerance” and “self-healing.” We have carefully reviewed the manuscript and confirm that we do not\nuse the term “self-healing” to describe our system. Instead, we consistently refer to the role of the\nsynthetic fascia as providing damage tolerance, emphasizing structural containment and the retention\nof partial mechanical functionality post-fracture without implying polymer network reformation or\nintrinsic material self-repair.\nTo ensure clarity and avoid any potential misunderstanding, we have double-checked the manuscript\nand ensure that the term self-healing is not use in any of the sections.\nOverall, the work is technically sound and addresses an important limitation of the previous study.\nHowever, to warrant publication in a high-impact journal, the manuscript requires greater differentiation,\nclearer terminology, and more rigorous performance evaluation.\nWe sincerely thank the reviewer for their constructive feedback and thoughtful assessment of our work.\nWe appreciate the recognition of the technical soundness of the study and its relevance in addressing an\nimportant limitation of our previous work. In response to the reviewer’s suggestions, we have carefully\nrevised the manuscript to improve clarity, strengthen the differentiation from our prior work, and ensure\npreciseandconsistentterminologythroughout. Wehavealsoclarifiedthescopeandpracticalimplicationsof\ntheperformanceevaluationpresented. Webelievetheserevisionshavesignificantlyimprovedthequalityand\nclarity of the manuscript, and we thank the reviewer for their valuable input, which has helped us enhance\nthe presentation and impact of this work.\nResponses to Reviewer #2\nThis increased toughness of this composite of composites is very interesting and an excellent application\nof multi-material printing capabilities of DIW. The material engineering presented from toughening of the\nmatrixmaterialstotheadhesionofthecompositeiselegantandwelldescribed. Theimportanceofthework\nis sufficient for publication in Nat. Comm., as it is a very tough, switchable composite and the biomimetic\ncomparisons are intriguing.\nWe thank the reviewer for their supportive and thoughtful comments. We appreciate the recognition\nof the composite’s enhanced toughness, the effective use of multi-material DIW, and the elegance of the\nmaterial integration strategy. We are pleased that the biomimetic comparisons and the multifunctional\nperformance were found compelling and that the significance of the work aligns with the scope of Nature\nCommunications.\n4\n1. Thedescriptionofthework,however,isconfusing. IfIunderstandcorrectly,thereisnotactivecontrol\nof motion; rather, there is active control of stiffness. Also, the term “4D” I find is hyperbolic and\nunnecessary. Particularly if I am correct in understanding that the motion comes from equilibrating\nwith the environment rather than using energy to go out of equilibrium. I think the science and\nengineeringwouldstandoutmorewithbetterclarityinthecontribution,andreductioninhypewords.\nIf I am incorrect and these actuators cause motion, please make that more clear in the writing – and\ndistinguish it as an out of equilibrium actuation mode.\nWe thank the reviewer for the opportunity to clarify these important points.\nTo clarify, our actuators do indeed exhibit active motion, rather than solely stiffness modulation.\nThe motion is driven by Joule heating, which induces a controlled temperature increase within the\ncomposite. This electrically activated thermal input causes bending due to anisotropic expansion\nor contraction due to the mismatch in coefficients of thermal expansion (CTEs) across the bilayer\nstructure that composes the actuators. Based on Timoshenko’s theory for bimetallic thermostats, we\ncan accurately model and predict the resulting curvature as a function of temperature and time.\nRegarding the term “4D printing,” we respectfully note that its use is well-established in the active\nmaterials and additive manufacturing community. It describes 3D printed structures designed to\nundergo time-dependent transformations in response to external stimuli such as temperature, light,\nhumidity,ormagneticfields. Oursystemfitsthisdefinition,astheprintedcompositeexhibitsreversible,\nstimulus-responsive shape change upon electrical activation.\nTofurtherimproveclarity,wehaveincludedthefollowingstatementsintherevisedmanuscript,\nspecifically in the introduction section:\nActuation mechanism: The mechanism of these actuators is governed by Timoshenko’s theory for\nbimetallic thermostats, wherein a bilayer composed of two stiff materials with differing coefficients of\nthermalexpansion(CTEs)undergoescurvaturechangeduetothermalmismatch. ThisCTEmismatch\ninducesbendinguponJouleheating,resultinginapredictableandprogrammableshapetransformation.\nDepending on the application, individual bilayers can operate as standalone actuators or be arranged\ninto systematic lattice configurations to enable more complex, multi-curvature shape changes.\n4D printing: 4D printing refers to an advanced form of additive manufacturing in which a 3D printed\nobject is designed to undergo controlled, time-dependent changes in shape, properties, or function in\nresponse to external stimuli such as temperature, light, humidity, magnetic fields, or other environ-\nmental factors. This stimulus-responsive behavior distinguishes 4D printing from conventional static\n3D structures, enabling dynamic and functional transformations. While many 4D printed systems\nare designed for reversible shape changes, certain applications exploit irreversible transformations to\nachieve stable, deployed configurations, expanding the functional scope of 4D printed architectures.\n2. The performance of prior MHECs are described in the introduction, but the mechanism is not clearly\ndescribed. Referencing the publication is sometimes OK for efficiency, but I think it is important\nenough to briefly describe in the main text.\nWe thank the reviewer for this important suggestion. To improve clarity and provide a more self-\ncontained description, we have expanded the introduction to briefly summarize the actuation mecha-\nnism of the prior MHECs. Specifically, we now include the following explanation: The mechanism of\nthese actuators is governed by Timoshenko’s theory for bimetallic thermostats, wherein a bilayer com-\nposedoftwostiffmaterialswithdifferingcoefficientsofthermalexpansion(CTEs)undergoescurvature\nchange due to thermal mismatch. This CTE mismatch induces bending upon Joule heating, resulting\nin a predictable and programmable shape transformation. Depending on the application, individual\nbilayers can operate as standalone actuators or be arranged into systematic lattice configurations to\nenable more complex, multi-curvature shape changes.\nThis addition ensures the manuscript is more accessible and clear to readers unfamiliar with our prior\nwork, while maintaining appropriate referencing.\n3. Page 6, line 156 – please explain the mechanism a little bit in the text instead of just a reference.\nmain text rather than relying solely on a citation. In response, we have revised the manuscript (Page\n6, Line 156) to include the following statement:\nFor the core MHEC bilayer, we selected a combination of two stiff epoxy-based inks with distinct\nCTEs. ThemismatchinCTEsenablescurvaturechangesuponheating, asdescribedbyTimoshenko’s\ntheory for bimetallic thermostats. To enable active control via Joule heating, at least one of the inks\nmust be electrically conductive. This allows us to locally and precisely regulate temperature through\nelectrical input. For additional details on the design and composition of the electrically responsive\nMHEC bilayer, we refer the reader to our previous work.\nThis addition provides a clear, concise explanation of the actuation mechanism while still directing\nreaders to our prior publication for more in-depth technical details.\nWe sincerely thank the reviewer for their constructive and thoughtful feedback. We have carefully ad-\ndressedallcomments,clarifyingterminology,improvingdescriptionsoftheactuationmechanism,andrefining\nour presentation of the novelty and technical significance of the work. These suggestions have helped us sig-\nnificantly strengthen the manuscript, and we appreciate the opportunity to improve the clarity, rigor, and\nimpact of our study.\nResponses to Reviewer #3\nBoleyandcoworkersreportedastrategytoprintPDMS-epoxycompositestructures. Thesestructuresexhibit\nhigh stiffness and high toughness at the same time. The authors characterized the mechanical properties of\nthecompositethroughtensiletestandmeasuredtheadhesionstrengthbetweenthetwocomponentmaterials\nthrough peeling test. The authors printed a bilayer structure that can bend when a voltage is applied and\ndesignedanactuatorthatcanliftweight. Theseconddemonstrationwasalatticestructurethatcanbeused\nas an impact sensor. The manuscript is technically sound and well-organized. The proposed strategy also\nshowednoveltytosomeextent. Thereviewerrecommendsthepublicationofthismanuscriptifthefollowing\nconcerns can be properly addressed:\nWe sincerely thank the reviewer for their positive evaluation and thoughtful comments on our work.\nWe appreciate the recognition of the novelty and technical merit of our approach. In response to the\nreviewer’s concerns, we have provided detailed point-by-point responses and made corresponding revisions\nin the manuscript to address each comment.\n1. Fig 3b, the reviewer is confused about how to understand the pattern shown in the plot. The authors\nshould properly clarify what does the number of peaks mean and how to interpret the magnitude of\nthe peaks.\nWe thank the reviewer for pointing out the need for clarification. In Figure 3b, each isolated set of\npeaks corresponds to a fracture event induced manually on a single bilayer sample. At lower peaks\n(from N =1 and N =3), we observe a single peak for each new fracture, with a magnitude increasing\nwith N. From N =4 to N =5 we see a single high magnitude primary peak, likely resulting from the\nnew fracture, and the presence of additional secondary peaks, which is likely a result of small motions\nat the interfaces of the existing fractures. Above N = 5 we see multiple peaks of similar magnitude,\nwhich is likely a result of the additional fracture and collective interfacial slipping across the multiple\nexisting fractures of the sample. We have revised the corresponding section in the manuscript to\nexplicitly state this interpretation for improved clarity. This is the statement that we added:\nThese fractures are manually induced on a single bilayer sample in a sequential manner. Each set of\npeaks in the signal corresponds to a new fracture event. At lower peaks (from N = 1 and N = 3),\nwe observe a single peak for each new fracture, with a magnitude increasing with N. From N = 4 to\nN = 5 we see a single high magnitude primary peak, likely resulting from the new fracture, and the\npresenceofadditionalsecondarypeaks,whichislikelyaresultofsmallmotionsattheinterfacesofthe\nexisting fractures. Above N =5 we see multiple peaks of similar magnitude, which is likely a result of\n6\nthe additional fracture and collective interfacial slipping across the multiple existing fractures of the\nsample.\n2. Fig4e,howistheactuationstresscalculated? Thecalculationofactuationstressshoulddependonthe\ntype of the actuator, e.g. contraction-type or bending type. Actuation stress needs a clearer definition\nbefore comparisons can be made.\nWe thank the reviewer for raising this important point regarding the definition of actuation stress.\nIn our study, actuation stress is defined as the lifted mass divided by the cross-sectional area of\nthe actuator. We have clarified this definition in the revised manuscript to ensure transparency and\nconsistency when comparing actuation performance. This is the statement that was added to the\nmanuscript:\nActuation stress is defined as the lifted weight divided by the cross-sectional area of the actuator.\n3. Fig 4c,d the black points seem redundant.\nWe thank the reviewer for the observation. The black points in Figures 4c and 4d represent failure\npoints and are intentionally included to highlight the resilience and damage tolerance of our approach.\nShowingthesepointsemphasizestheactuator’sperformanceevenaftermechanicalfailureevents,which\nis a key aspect of our study. We have included the following statement in the manuscript to explicitly\nhighlight this:\nIn Figure 4c-d, the measured performance of these actuators is shown with respect to lifted mass for\ndifferent testings, where blue, yellow, and black dots represent the lifting robot before, after, and right\nat the mechanical failure, respectively.\n4. Fig 5c, the authors claimed to make a force sensor. How is the R/R0 correlated with the compressive\nforce? In the first cycle, the R/R0 change is very small, after fifth impact, the R/R0 change becomes\nmuch larger, thought the compressive force becomes even smaller. Is this feature desired for a force\nsensor?\nWe thank the reviewer for this insightful question. The normalized resistance change (R/R0) in our\n3D sensing surface correlates with the compressive force through deformation-induced changes in the\nconductive network within the actuator. Initially, small resistance changes correspond to minor struc-\ntural damages or fractures under impact. As the number of impact cycles increases, microstructural\ndamageaccumulates, leadingtolargerresistancechangesevenunderreducedcompressiveforcedueto\nchanges in the contact area between the conductive ribs composing the lattice.\nThisbehaviorreflectsaformofdamagesensingratherthanapurelylinearforce-resistancerelationship.\nWhilethesensor’ssensitivityincreaseswithrepeatedimpactsduetodamageaccumulation,thisfeature\nis beneficial for applications requiring detection of cumulative mechanical stress or fatigue rather than\ninstantaneous force measurement alone.\nImportantly, the main goal of this structure is to increase toughness and resilience. By incorporating\nan electrically conductive core, we also enable multifunctionality: a structure with high stiffness,\nmechanical resilience, and the capability to sense damage. This technology could potentially be used\nfor surface damage mapping or cumulative damage detection in real-time.\nWe refer the reviewer to Section 3D Sensing Surface That Detects and Tolerates Damage, where these\nstatements are found:\nNotably, it is observed that the force required to compress the lattice decreases with each subsequent\nexternal perturbation (e.g., hits with a sledgehammer), indicating the introduction of fractures in the\nlattice structure. This observation aligns with the spikes in the change in electrical resistance (R/R0),\ncoinciding with the application of external perturbations, showcasing an inherent fracture detection\ncapability. An intriguing behavior is evident in the electrical responses of the lattice: as additional\nfractures are induced, the sensitivity of the resistance in response to deformation increases. This\nphenomenonisattributedtotheamplifiedchangeinresistanceresultingfromslightrelativemovements\nbetween segments of the fractured lattice ribs. These movements reduce the contact area within the\nfractured ribs, thus increasing the electrical resistance with respect to an external force. However,\n7\nsince the synthetic fascia surrounding the electrically conductive ribs is hyperelastic and adheres to\nthe MHEC ribs, after the compressive loads are released, the MHEC ribs can revert to their initial\nconfiguration, increasing the contact area of the ribs and thereby reducing the electrical resistance of\nthe lattice close to its nominal value. Prior research on soft sensors composed of conductive fillers\nprovides additional support for these findings [52].\n5. Fig 5d is not clear as all data points overlap.\nWe thank the reviewer for this observation. The overlapping data in Fig. 5d is intentional and serves\nto highlight the evolution of electrical response as damage accumulates in the 3D surface. Specifically,\nthis presentation allows for a direct visual comparison of performance across different damage states.\nBy overlaying the curves, we aim to demonstrate how the 3D surface can exhibit different sensing\nresponses depending on the degree of induced damage — a feature that can be tailored based on the\napplication.\nTo further clarify this, we have edited part of the relevant section titled 3D Sensing Surface That\nDetects and Tolerates Damage, where we state:\nAsummaryofthesefindingsispresentedinFigure5d. Thesyntheticmusclecompositelatticeexhibits\nvariedforceandresistanceresponses,dependingonthenumberofinducedfractures(i.e.,thenumberof\nexternalperturbationsfromthesledgehammer). Atalownumberofexternalperturbations(e.g.,three\nor less), the surface exhibits a relatively high compressive force and low electrical responsiveness. At\na higher number of external perturbations (e.g., four or more), the surface exhibits a low compressive\nforce and high electrical response. Furthermore, this synthetic muscle composite demonstrates high\nresilience to various external perturbations, showcasing 3D surfaces capable of sensing and damage\ndetection.\nWe sincerely thank the reviewer for their thoughtful and constructive feedback. We greatly appreciate\nthetimeandefforttakentoevaluateourworkandprovidedetailedcomments. Inresponse,wehavecarefully\naddressed each point raised and revised the manuscript accordingly to improve clarity, technical accuracy,\nand overall presentation. Specific responses to each comment, along with the corresponding changes in the\nmanuscript, are provided below. We believe these revisions have significantly strengthened the manuscript\nand we hope it now meets the expectations for publication.\n8"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed my earlier concerns regarding technical execution and clarification of experimental methods. However, I remain unconvinced about the novelty of the contribution. While the integration of the fascia layer improves performance, the core concepts and mechanisms remain largely consistent with the authors’ previous work. As such, the advance feels incremental rather than foundational.", "reviewer_3": "Authors have addressed my concerns. Publication is suggested."}, "author_response": ""}]

Decision letter