Hierarchical fibrous structures for muscle-inspired soft-actuators: A review

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University of Groningen

Hierarchical fibrous structures for muscle-inspired soft-actuators

Gotti, Carlo; Sensini, Alberto; Zucchelli, Andrea; Carloni, Raffaella; Focarete , Maria Letizia

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Applied Materials Today

DOI:

10.1016/j.apmt.2020.100772

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Gotti, C., Sensini, A., Zucchelli, A., Carloni, R., & Focarete , M. L. (2020). Hierarchical fibrous structures for

muscle-inspired soft-actuators: A review. Applied Materials Today, 20, [100772].

https://doi.org/10.1016/j.apmt.2020.100772

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ContentslistsavailableatScienceDirect

Applied

Materials

Today

journalhomepage:www.elsevier.com/locate/apmt

Hierarchical

ﬁbrous

structures

for

muscle-inspired

soft-actuators:

A

review

Carlo

Gotti

a

_,

_Alberto

_Sensini

b

_,

_Andrea

_Zucchelli

a,b,∗

_,

_Raffaella

_Carloni

c

_,

Maria

Letizia

Focarete

d,e

a Department of Industrial Engineering, Alma Mater Studiorum—University of Bologna, I-40131 Bologna, Italy

b Advanced Mechanics and Materials – Interdepartmental Center for Industrial Research (CIRI-MAM), Alma Mater Studiorum—University of Bologna, I-40123

Bologna, Italy

c Faculty of Science and Engineering – Bernoulli Institute for Mathematics, Computer Science and Artiﬁcial Intelligence, University of Groningen, Nijenborgh

9 9747 AG, Groningen, The Netherlands

d Department of Chemistry ‘Giacomo Ciamician’ and National Consortium of Materials Science and Technology (INSTM, Bologna RU), Alma Mater

Studiorum—University of Bologna, I-40126 Bologna, Italy

e Health Sciences and Technologies—Interdepartmental Center for Industrial Research (CIRI-HST), Alma Mater Studiorum—University of Bologna, I-40064

Ozzano dell’Emilia, Bologna, Italy

a

r

t

i

c

l

e

i

n

f

o

Article history: Received 24 March 2020 Revised 12 June 2020 Accepted 19 July 2020 Keywords: Soft robotics Nanostructured materials Artiﬁcial muscles Bioinspired structure Linear ﬁbrous actuators

a

b

s

t

r

a

c

t

InspiredbyNature,oneofthemostambitiouschallengeinsoftroboticsistodesignactuatorscapableof reachingperformancescomparabletotheskeletalmuscles.Consideringtheperfectlybalancedfeaturesof naturalmusculartissueintermsoflinearcontraction,force-to-weightratio,scalabilityandmorphology, scientistshavebeenworkingformanyyearsonmimickingthisstructure.Focusingonthe biomimicry, thisreviewinvestigatesthe state-of-the-artofsyntheticfibrous,muscle-inspiredactuatorsthat,aiming to enhancetheir mechanical performances, arehierarchically designedfrom the nanoscale up to the macroscale.Inparticular, thisreviewfocusesonthosehierarchicalfibrousactuatorsthatenhancetheir biomimicryemployingalinearcontraction strategy,closelyresembling the skeletal musclesactuation system.The literatureanalysis showsthatbioinspired artificialmuscles,developedupto now,onlyin partcomplywithskeletalones.Themanipulationandcontrolofthematteratthenanoscaleallowsto realizeorderedstructures,suchasnanofibers,usedaselementalactuatorscharacterizedbyhighstrains butmoderateforcelevels.Moreover,itcanbeforeseenthatscalingupthenanostructuredmaterialsinto micro-andmacroscale hierarchicalstructures,it ispossibletorealizelinearactuatorscharacterizedby suitablelevelsofforceanddisplacement.

Abbreviation: ATP, adenosine triphosphate; PVC, polyvinyl chloride; PVA, polyvinyl alcohol; DC, direct current; NFES, near field electrospinning; CVD, chemical vapour deposition; CNTs, carbon nanotubes; PANi, Polyaniline; PAN, Polyacry- lonitrile; PPy, polypyrrole; PEDOT, Poly(3,4-ethylenedioxythiophene); PES, polyester; PVDF, polyvinylidene fluoride; MM, molecular motors; PAA, polyacrylic acid; PET, polyethylene; PU, polyurethane; GO, graphene oxide; Nb, niobium; PEVA, poly(ethylene-co-vinyl-acetate); MWCNT, multi walled carbon nanotube; Pt, platinum; Ag, silver; Au, gold; C, carbon; Sn, tin; Al, aluminium; Cu, copper; p- TSA, para-toluene sulfonic acid; NaDBS, dodecylbenzene sulfonate; PDDA, Poly (diallyl dimethylammonium chloride); TPU, thermoplastic polyurethane; PVDF-co- HFP + TEA BF4, Poly (vinylidene fluoride-co-hexafluoropropylene) + tetraethyl ammonium tetrafluoroborate; PVA + H2SO4, polyvinyl alcohol + sulfuric acid; TBA PF6, tetrabutylammonium hexafluorophosphate; AFM, atomic force microscope; TEM, transmission electron microscopy; UV, ultra-violet light; CCD, charge-coupled device; CaCl 2 , calcium chloride; 2-ply, two-ply; 4-ply, four-ply; SMP, shape memory

1. Introduction

Bioinspirationisgettingan increasingattentioninthe produc-tionofinnovativeroboticactuatingsystems[1,2].Inparticular,the lastdecadehasseentheoutstandingariseofbioinspiredstructures in theﬁeld of softrobotics [3]. The main aim that distinguishes

alloy; Inox, stainless steel; SMC, shape memory ceramics; TCP, twisted and coiled polymers; UHMWPE, ultra-high molecular weight polyethylene.

∗ _{Corresponding author at: Department of Industrial Engineering, School of En-} gineering and Architecture, University of Bologna, Viale Risorgimento, 2 40136 Bologna, Italy

E-mail addresses: carlo.gotti@unibo.it (C. Gotti), alberto.sensini2@unibo.it (A. Sensini), a.zucchelli@unibo.it (A. Zucchelli), r.carloni@rug.nl (R. Carloni), marialetizia.focarete@unibo.it (M.L. Focarete).

https://doi.org/10.1016/j.apmt.2020.100772

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thisbranchofroboticsistodesignanewgenerationofcompliant actuatorsabletomimictheskeletalmuscleperformances.

Skeletal muscles properties mainly rely on their hierarchical structure,composedoffibrousactuatorsorganizedindifferent lev-elsofaggregation.Thisstructure can produceforce bythe short-eningoftheirelementarycontractileunits,thesarcomeres,making musclesclassifiableaslinearactuators.Thiscomplexorganization alsopermitsafinecontrolontheseparate activationoftheir sub-units,conferringthemahighscalabilityandmodularity.Thislinear shortening,matchedwiththeskeletalleversystem,allowsawide rangeof motions with a controllable stiffness, also reducing the muscle clutter.Consideringall theseproperties andthe perfectly balancedmechanicalperformances,the skeletalmuscleis consid-eredthebestactuatorinNature [4].

Aimingtomimicskeletalmusclesandthefeatureslistedabove, several innovative strategies were investigated to obtain syn-theticsoft-actuators,suchassmarttextiles,additivemanufactured devices, shape memory alloys, pneumatic, hydrogel composites, origami/kirigamistructures andmanyothers,revolutionizingthe fieldoftheclassicrobotics[3,5–8].Amongthese,someworkstried tofaithfullyreproducethehierarchicalmorphologyofmusclesand theirlinearactuation,byusingscalableandmodularfibrous struc-tures [173].Thisapproachwillbeofextremeinterestforallthose applicationsinwhichabiomimeticloadtransfermatchedwith re-ducedweight,clutterandhighsoftnessarerequired,suchasin ad-vancedsurgicalinstrumentations, orthosis, prostheticdevices, ex-oskeletons and biologically-inspired robots [9–17]. In this review we purposely focus only on linear actuators with fibrous struc-tureandnano ormicro dimensions, considered eligible tobe ar-rangedina bioinspiredhierarchicalfashion,enhancingtheir force andstraincapabilitieswiththesamestrategieschosenbythe evo-lutioninthebiologicalskeletalmuscles.

The paper isorganized asfollows.A briefreview of the mor-phology,mechanicalpropertiesandphysiologyofskeletalmuscles is given in chapter 1. In chapter 2, the most common features that are required for a muscle-inspired soft-actuator are investi-gated. The main technologies for nano- and microfibers produc-tionarepresentedinchapter3alongwith,inchapter4,themost frequentlyemployedmaterials forartificialmusclesproduction.In chapter5,ananalysisofthestate-of-the-artofmuscle-inspired ac-tuatorsispresented.Specifically,chapter5is dividedinfour sec-tions,dependingonthehierarchicallevelofcomplexityofthe ac-tuator:singlefibers,flat mats offibers,bundles/yarns andfinally coiledand/or pliedstructures.Foreachworkcitedinthischapter, thestructureproposedbyauthors,thematerialused,the manufac-turingprocessandtheprincipaloutcomesaresummarized.Finally, concludingremarksaredrawninchapter6.

1.1.Hierarchicalmorphologyofskeletalmuscles

Askeletalmuscleisanorganmainlycomposedofstriated mus-culartissue.Itisthe majoractuatorofhuman body,providingits movementcapabilities,beingcomposedofcontractiblecellscalled myocytes. A skeletal muscle has three other main functions: to supportthebody,togenerateheatduringcontraction,andto pro-tectbonesandorgans.Itistheonlytypeofmuscletissueclassiﬁed asvoluntary,whosecontractionisregulatedbyourconscious cere-bralactivity. Itisalsocalledstriated musclebecauseit alternates light anddark bands,which are visible under theoptical micro-scope.Skeletalmusclesformabout40%oftheentirebodyweight andthey are madeup of water (75%), proteins (20%) and other substances,suchassalts,minerals,fat,andcarbohydrates(5%) [18]. Theproteincontentmainlyconsistsofcollagen(TypeI,III,IVand V)forthetissuesheaths [19],andmyosin,actin,titinandnebulin fortheinnerparts [18].Itisacompositestructure,madeby con-tractilematerials and, in minoramount,by blood vessels, nerves

andconnective tissue[20,21].Theskeletalmuscleshowsa hierar-chicalarchitecture of alignedstructures like other connective tis-sues,suchastendonsandligaments.

The fundamentalbuilding-block of the skeletal muscle is the sarcomere, which is composed of different filamentous proteins, called myofilaments (Fig. 1A) [22]. Sarcomeres are connected through a plate-shaped region of dense protein material, called theZ-line.Themyofilamentsaredividedinthick,thin, andelastic ones. Thinandthick filamentsoverlaponeach other witha pro-cesscalledslidingmechanism [22].Thisisthemechanismthatthe muscle employs toshorten itself andproduce acontractile force. The amount ofoverlapping dependson the muscle level of con-traction.Thickfilamentsaremadeofabout200myosinmolecules and have a diameter of about 16 nm [18]. Thin filaments are placed ontheside ofthethick onesandextendfromtheZ-lines to the beginningof the H-zone,the region inwhich no overlaps withthick filamentsarepresent.Thinfilamentsaremadeofactin moleculesjoinedtogetherinahelixpattern,withasmallamount oftwo proteinscalledtropomyosin andtroponin.Theelastic fila-mentiscomposedofthetitinproteinandlinksthethickfilaments totheZ-line.Thetitinfunctionistostabilizeandconferanelastic recoverytothemyosin [23].Thisoverlappingofmyofilaments pro-videsthetypicalstriatedappearance toskeletalmuscles(Fig.1B). Sarcomeres are connected each other both in a parallel and lon-gitudinalway.Thousandstomillionsofthesesarcomeresforman upper levelstructure called myofibril (Fig. 1B). Myofibrils have a mean diameterof 1-2 μm. Severalmyofibrils, parallelly arranged andwrappedinsidea sheathofconnectivetissue called endomy-sium,producethe musclefiber(Fig.1C). Endomysiumis primally composedbycollagenousfibers(TypeI,III,IV,V) [19].Eachmuscle fiberhasa meandiameterof10-100μm withatypical lengthof 1cm [18]andcanbe considered thebiological structuralunit of skeletal muscles,consistingin asingle, multinucleated cellcalled myocyte. Moreover, the whole size of the muscle is determined bythenumberanddimensionofitsindividual musclefibers [18]. Thecellmembraneofthesefibersisnamedsarcolemmaand sur-rounds its cytoplasm or sarcoplasm. Excluding their water con-tent, muscle fibers are mainly (80%) composed of proteins (clas-sified by specificfunction such as contractile,regulatory, and cy-toskeletalones)andsarcoplasm(8%) [18].Beingacell,themuscle fiberisirroratedbycapillariesandconnectedwithnervesthrough neuromuscular junctions. Inside and around the muscle fiber a membrane-bound structure, the sarcoplasmic reticulum, is found

[24]. Its main function is to store calcium ions (Ca2+₎ _which _are essentialforcontraction.Sarcoplasmicreticulumpresentsalso en-largedareasnamedterminalcisternae,thatsurroundintroflexions ofthecellularmembranes(i.e.T-tubules)insidethefiber.Bundles ofmusclefibersformamusclefascicle (Fig.1D).Thisstructureis surrounded byanother connective tissuesheath,the perimysium, composed ofTypeI,III andVcollagenfibers [19]. Groupsof fas-cicles,bundledtogether,generatethewholemusclebelly(Fig.1E). Thewholemuscleiscoatedby alayerofdense,irregular connec-tive tissue sheath named epimysium. This membrane isprimally madeoflargecollagenfilaments(TypeIandIII).Theterminalside of the muscle progressively changes composition, increasing the collagenamountanddecreasingthecellularpart,becominga ten-donthatconnectsthemuscletoabone [25].

1.2. Skeletalmusclesmechanicalpropertiesandcontraction mechanism

Themechanicalpropertiesofskeletalmusclescanbedividedin twocategories:thepassiveones,whichdependonlyonthetissue mechanicalresponse while stretched;andthe active ones, which arederivedfromtheactivationofthecontractileelements.The

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in-Fig. 1. Hierarchical structure of skeletal muscle. A) Sarcomere morphology and sliding mechanism (Scalebar 0.5 nm): Actin (red), Myosin (blue) and Titin (yellow) filaments are shown in the relaxed state (I) and during the contraction (II). The jagged sides represent the Z-lines. The central space without actin filaments is the H zone. B) Transmission Electron Microscopy (TEM) image of myofibrils (scalebar = 1 nm. Reproduced under CC0 1.0 Universal Public Domain Dedication. Author: Louisa Howard). C) Phase Contrast Microscope (PCM) image of skeletal muscle fibers. Dark violet elliptical elements are the myocytes nuclei (scalebar = 50 μm. Reproduced under CC0 1.0 Universal Public Domain Dedication. Author: Berkshire Community College). D) Histological image of a fascicle cross-section. Larger white bands are the perimysium membranes. Circular structures are the muscle fibers, while the darker violet dots are the myocytes nuclei (scalebar = 100 μm. Reproduced under Attribution-ShareAlike 3.0 Unported. Author: Ganimedes). E) Histological image of a portion of muscle cross-section. In the upper part, the epimysium membrane is visible (scalebar = 0.5 mm. adapted from [170] , reproduced under permission. Copyright 2008, Elsevier B.V.).

ternal biological structures that provide those propertiesare not alwaysanatomicallydistinct [26].

Passive properties are often obtained imposing an artiﬁcial stretch to the muscle, avoiding the activation of the contractile elements. Due to thehighvariability ofmuscle tissue,caused by the differences between the species tested and the intra-species variability, the mechanical properties reported in literature vary widely.Furthermore,therearenotconventionaltechniquestotest thepassivemechanicalpropertiesofthesetissues,andthiscauses additional variability [27]. The passive properties of the muscle bellydependontheintramuscularconnectivetissues(epimysium, perimysium,endomysium) [22].However,recentevidencessuggest thatmusclecellsarealsoresponsiblefortheir passivemechanical properties[26,28].Moreover,thetitin,themostabundant compo-nentofelasticﬁlaments,isnowadaysconsideredthestructurethat mainlybearsthepassive stressinsidea muscle[20,23,27,29].The twomainpassivemechanicalfeaturesofaskeletalmuscleare:

i) Extensibility: the muscles ability to be macroscopically stretcheduntilacertainlimitwithoutbeingdamaged [22]. ii) Elasticity: the musclesabilityto recover the initial shape and

lengthafterbeingstretched [22].

Themostpracticalwaytocomparevaluesfromdifferenttypes of muscles is to normalize their produced force to their cross-sectionalarea.Thisindexisknownasspeciﬁcstress [30].Whenan isolated muscleisstretched invitroatdifferentstrains,thestress response can be plottedina passive stress–straincurve, that ex-hibits the typical non-linear behaviour of soft connective tissues

(Fig.2A)causedbytheprogressivealignmentoftheﬁbers[31,32]. After atoe-region,themusclestartstodevelop stresslinearly(i.e. linear region)(Table 1) [23,26,27]. The slope ofthiscurve region determines the module of elasticity of the muscle. The relaxed muscleshowsalsoatimedependentviscoelasticbehaviour[31,33]. Viscoelasticity is due to the mechanical properties of intracellu-lar and extracellular proteins, like collagen. When stretched and

heldatconstantlength,thestressdecreasesovertimetoaplateau (stressrelaxation) [26]. Otherwise, ifstretched at constant stress overtime, the muscle lengthens toa newvalue (creep).Muscles alsoshowdifferentstress-straincurvesifstretchedandrelaxed re-peatedly [26].

Theactivemechanicalpropertiesoftheskeletalmuscledepend onitscontractilebehaviour.Thetwoprincipalactivepropertiesof skeletalmusclesare:

i) Excitability:themusclesabilitytorespondtoachemical stim-ulus,deliveredby thereleaseofaneurotransmitterby anerve orahormone,generatinganactionpotential [22].

ii)Contractility:themusclesabilitytogenerateaforceandto pro-duceaworkbyshortening.Musclescontractinresponsetoone ormoreactionpotentials [22].

Beinganactuator,the mostimportantattribute ofamuscle is theforcethatitcangenerate.Themajordeterminantofthe maxi-mumpotentialforceisthemusclesize [47].Humanskeletal mus-cles show maximum active stress values in the range of 60-200 kPa [30,47]. These stresses are normally achieved during an iso-metric contraction, that occurs when the muscle generate force withoutchanging its length. An importantinformationabout the muscle contraction is the relationship between the active stress andthe sarcomere length (Fig. 2B) [22]. The skeletal muscle ex-ertsstress whenmyosin (from thick filaments) connects toactin (fromthinfilaments) through cross-bridges.The muscle develops itsmaximumforcewhenthereisanoptimaloverlapbetweenthin andthick filaments.When sarcomerelengthisabout2.2μm (op-timallengthinhumans, Lo)theforce itcan provideismaximum [22,27]. Furtherstretchingthemuscleresultsina decreaseofthe contractionforceand, whenthesarcomerereachesabout175%of its optimallength, noforce can be developed.The exerted stress is expressed aspercentage of maximum active stress, achievable during a tetanic contraction. Muscle stress increases along with thestimulusfrequency, untilreaching a plateauthat corresponds

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Fig. 2. Passive and active mechanical behaviours of skeletal muscles. A) Typical passive stress-strain curve. B) Typical sarcomere active stress (expressed as a percentage of the maximum) compared to the sarcomere length (expressed as a percentage of its optimal length). The active stress is maximum in correspondence of the sarcomere optimal length Lo. The pictures show the different levels of sarcomere overlap. C) Total stress-length behaviour during a contraction. Length is normalized by the resting length (R L ) of the muscle. The total curve (Tot) is the composition of the active curve (Act) and the passive one (Pas). The active curve ﬁnds its maximum in correspondence of the optimal length of the sarcomere (R L ) . D) Typical force (expressed as a percentage of the maximum) compared to the speed of contraction (normalized by the optimal length of the sarcomere).

Table 1

Typical ranges of animal skeletal muscle passive mechanical properties.

Elastic Modulus [kPa] Failure Strain [%] Failure Stress [kPa]

Myoﬁbril 25-40 [ 34 , 35 ] 30-60 [36–38] -

Fiber 20-100 [ 23 , 39–41 ] 30-60 [36–38] 430-1973 [42]

Whole Muscle 30-8000 [ 20 , 37 , 42–45 ] 30-60 [36–38] 70-800 [ 38 , 42 , 46 ]

tothemaximumtetanicforce [27].Thecombinationbetweenthe passive and the active mechanical properties generates the total stress-lengthrelationship(Fig.2C) [48].

Skeletal muscles are also able to regulate the force to be ad-equate to the load that they bear during the shortening. More-over,aforce-velocityrelationshipcandescribethelimitstomuscle speed and force output [32]: for fast movements, the force pro-ducedby the contractile system is low, and vice versa (Fig. 2D) [49,50].

Skeletal muscles produce work only by contraction, reducing orholdingthe jointanglebetween thetwo bonesto whichthey areconnectedthroughconnectivetissues(usuallytendons).Being onlyabletocontract,skeletalmusclesusuallyworkinpairsatthe opposite side of a joint (in an antagonist setting) [2]. Each mo-torsignal, originatinginto the motorneurons inthe central ner-voussystem, stimulates up to 1000 muscle ﬁbers. The combina-tion between a motor neuron and its connected ﬁbers is called motorunit [24].When theaction potentialhas reachedtheaxon terminal of the motor neuron, it promotes the release of chem-ical neurotransmitters at the neuromuscular junction. This com-plexiscalledsynapsis [51].Whentheneurotransmitterreachesthe receptors locatedon the muscle side, they bind together causing theopeningofsodiumionschannels(Na+).Therelatedchangein

thefibers restingmembranepotential triggers an electrochemical depolarization (i.e. action potential)that travelsthrough the sar-colemma.Theactionpotentialpassesintothemusclecellthrough T-tubules of the sarcoplasmic reticulum that stores calcium ions (Ca2+_)._Then,_{voltage-gated}_channels_located_in_T-tubules_open, al-lowinganinwardcurrentofCa2+_._This_current_further_opens_other channels,releasingalargeamountofCa2+_from_sarcoplasmic retic-ulumcisternae.The Ca2+ _high_{concentration} _removes_the_protein tropomyosin that inhibits the linking between the thin and the thick filaments. This process startsthe filaments to slide one on each other, shorting the sarcomere [22]. The muscle contraction also requiresenergyfor thesliding, obtainedwiththe hydrolysis of the organic complex adenosine triphosphate (ATP),that binds tothemyosinheads. Thesarcomereshortening drawstheZ-discs closertoeachother,leavingthelengthofthefilamentsunchanged. When thecontraction reachesthe peak value,the sarcomere can decreaseitslengthtoahalf.Therelaxationoccurswhenthemotor neuron stopstostimulatethefibers.The neurotransmitteris bro-kendownandthemuscularactionpotentialisnotgenerated any-more onthe sarcolemma.Cell membraneiontransporters, called Ca2+_pumps,_rapidly_remove_calcium_and_restore_the_original con-centration,leavingtropomyosinabletobindagainandinhibitthe sliding.

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The fibers contraction is an all-or-noneactivation [22]. When thefibersarereachedbytheactionpotential,theycontracttotheir fullestextent.Musclescanperformgradedcontractions,with vari-ableforce,regulatingthenumberofcontractedfibersandtheir fre-quencyofstimulationbythemotorneurons.Relaxedskeletal mus-clesalwayspresentafewfibersinthecontractedstatetomaintain themuscle firmness,which isnamedtone.Ifa skeletalmuscleis overstimulated,itwillrapidlylosestrength.Forthisreason,during a contraction, not all fibers work. The patternof firing of motor neurons changes, maintaining the contraction for longer periods by alternating the fibersinvolved. Thispreventsmuscular fatigue atbest,anditisalsousedtoproducesmoothmovementsinstead ofjerks [48].

Consideringthis,itisclearhowtheevolutionaimedtoenhance aspects likeforce, contractionratio,activation controlandfatigue resistancebyexploitingthesuperpositionconceptinits hierarchi-cal fibrous structures. Working in parallel, myofibrils, fibers and fasciclesscaleuptheforceoutputofthemuscle,whilesarcomeres, seriallyarranged,enhancetheshorteningduringcontraction.

2. Requirementsforamuscle-inspiredsoft-actuator

The contractile mechanism and the hierarchical structure of skeletalmusclesaredrawinganincreasingattentionintherobotic researchﬁeld,especiallyintheproductionofactuators.

A roboticactuatoris definedasadevice capable to transform energy,comingfromanyphysicaldomain,toamotionbyapplying mechanicalforcesonaroboticjoint,anobject,oronthe surround-ing environment [52]. Nowadays, the actuation systems mainly relyoncombustion,electricmotors,orpneumatic/hydraulic.These technologiesarewell establishedandhadbeenrefinedsincetheir early discoveries around the middle of XIX century. However, to date, an established technology with properties similar to the skeletal muscle is still missing. This paragraph will focus on the process that broughtresearchers to investigatea newgeneration ofartificialsoft-actuators,trying tomimicthebiologicalstructure andthelinearactuationsystemofskeletalmuscles.

Actually,musclesdonotovercomeartiﬁcialactuatorsinany as-pect [53].Forinstance,bothcombustionandelectricmotorshave higherspeciﬁc power [54].It isfortheir incrediblywell balanced performanceandfeatures,andnotforanysingledominating char-acteristic, that muscles are considered the best existing actuator

[4].Force, forexample,can be finely tuned withtherecruitment mechanism, optimizing energy consumption, and refining move-ments.Muscleshavealsotheabilitytochangetheir stiffness [53]. Theyconvertchemicalenergytomechanicalone“combusting” ATP molecules,obtainingafuelenergydensityuptotwoorderof mag-nitude higher than artificial batteries [53]. Moreover, thanks to thehierarchicalstructure,skeletalmusclesamplifythemicroscopic movements of the actin-myosin complex, obtaining macroscopic displacements [55].

Today, most of artiﬁcial actuator technologies are hard, non-compliant,heavyandnoisy [52].Theyhavetheadvantageofbeing usually precise,fastandpowerfulwhen usedintheir speciﬁc ap-plicationdomain,butlackofadaptabilitywhentheenvironmentof operationisnotwellknown [56].They,therefore,requirecomplex transmissionsystemstobeusableinnon-repetitivetasks.

Nevertheless, in the last years, Nature has become more and moreasourceofinspirationforroboticsandmachinesingeneral. Softness,compliance,andreducedcomplexityarekeyfeatures of-ten exploited in livingbeings. These features are also chased by roboticresearch toproduceactuatorscapabletobe usedinto un-knownenvironments,overcomingthecurrentdrawbacksof classi-calones[2,3].Thisresearchculminatedinthebirthofanewclass ofsystems,currentlygrowing,referredassoftrobots [57].Forthe purpose of soft robotics application, a variety of emerging

actu-ator technologies hadbeen exploited[58,59]. Soft-actuatorsand, more specifically, artificial muscles, share some common generic requirements.Adequatestress andstrain values shouldbe exhib-itedtobeofpracticaluse.Thestrain rate(i.e.theaveragechange instrain per unit time duringan actuatorstroke) should be suf-ficienttoensurea rapidresponsiveness [53].Otherkey-figures of merit are considered to evaluate artificial muscles, like thework density (i.e. the amount of work generated in an actuator cycle normalizedby itsvolume),thespecificpower(i.e.thepower out-putper unit mass) andthe efficiency [53]. High reversibilityand controllabilitywouldbealsosuitable [60],alongwithdurability.

Tosatisfytheserequirementswithdesiredperformances,many effortshavebeenmaderesearchingtheemployedmaterials. How-ever, learning from Nature, it is clear how the geometrical and morphological structure of these materials is also fundamental when designing an artiﬁcial muscle. Working in the nanoscale it is possible to assemble nano-actuators and to scale up forces andcontraction,mimickingthe sameapproachofbiological mus-cles [61]. Moreover,since mostofresearched actuatorsactivation mechanismsrelyontheinwardelectrolyteorheatperfusion,their poweroutputtendstoincreaseatsmallerdimensionduetofaster mass,electronic andheat transport,orhighersurfaceareato vol-umeratio.Thisisthechoice “made” bytheevolutionfor biologi-calmusclesandforthisreason,solutionsinvolvingsmall-diameter actuatingﬁbers hadbeen proposed [62,63]. This approach is of-tenlimitedbythelaboriousfabricationandsometimesbylacksin technology.

Anotheradvantagebroughtbythemimickingofmusclescould bethefinetuningofactuationforceandstroke,inasimilarwayof therecruitment phenomenon. This mechanismis currentlybeing investigated,through the exploiting ofmany textile architectures obtainedusingknitting,weaving,andbraidingtechniques [62].For obviousmorphologicalsimilarities,alignedfibers-structured artifi-cial muscles wouldbe favourable. High surface-to-volume ratios, alongwithhighdegreeofporositybroughtbythenano-or micro-metricsize gaps of fibrous hierarchicalstructures, could enhance theiractuationperformanceandresponse[60,63,64].

Todate,an unsolvedtopicinthe softroboticsfield is to real-izeflexibleactuatorscapabletodevelophighforces,performances, and functionality by mimicking the hierarchical morphology of skeletalmusclesintheanimal body.Thesekindsofactuatorsare speciallyneededforassistiveandrehabilitationdevicesthat must safelyphysicallyinteractwithhumanbeings [65],forexample ar-tificiallimbs, prostheticdevices,exoskeletons,orevensurgical in-struments[3,66].

3. Technologiesfornano-andmicroﬁbersproduction

In orderto achieve ﬁbrous hierarchicallystructured actuators, few manufacturing processes were chosen by researchers, which willbeoverviewedinthisSection.

3.1. Spinning

Spinningisamanufacturingprocessinvolvedintheproduction ofpolymerfibers.Thethreemajortechniques(meltspinning,dry spinning andwet spinning) are used for the productionof most ofcommercialsyntheticfibers,usuallywithadiameterinthe mi-crometricormillimetricscale [67].Inordertoproducefiberswith diameters in the nanoscale, electrospinning or near-field electro-spinningareusuallyemployed.

3.1.1. Meltspinning(Extrusion)

Themeltspinningisawidelyusedandwell-establishedprocess inthe manufactory industry (Fig. 3A). Abulk material ispushed through a die, creating objects with ﬁxed cross-sectional proﬁle.

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Fig. 3. Typical manufacturing processes employed in the production of microfibers and nanofibers: A) polymer melt spinning; B) electrospinning (the inset shows the Taylor cone); C) near field electrospinning; D) chemical vapour deposition.

Theend-platediedetermines theshapeoftheextruded material. When polymers are extruded, the raw material is stored into a hopperintheformofpelletandconveyedthroughafeedingscrew to the die. During this process, the material is also heated with acontrolledtemperature. Extra-heatresultsalsofromtheintense pressure andfriction takingplace between the screw, the mate-rialandthebarrel.Thecontinuouspolymericﬁlamentproducedis thencooled by airorliquid [68].Melt spinningprocess outcome dependsonseveralparameterssuchasthepolymerviscositywith itsvariations(duetoshearstressandtemperature),andits elastic-ity.Processparametersthatshouldbemonitored,are [68]:

i) Barreltemperature:itisusuallychosenequaltotheglass tran-sitionorthemeltingtemperatureofthepolymer.

ii)Feedrate:itshouldbeconstantandlinkedtothescrewspeed. Thesetwoparametersarecriticaltoobtainahomogeneous in-ﬁlloftheextrusion.

iii) Motor load andmelt pressure: these parameters also depend onthefeedrateandthescrewspeed,alongwiththemolecular weightofthepolymer.

Like inthemicrofabrication,themicro-scalemeltspinning fol-lowsthe same process [69]. Fishing line, forexample, is usually madewithanextrudedpolymerthathadbeenmelted,before be-ingforcedthroughamicro-scalehole.

3.1.2. Dryspinning

Thismethodologyisemployedforthosepolymersthatneedto bedissolvedinasolvent,duetotheirthermaldegradation temper-aturelower than the meltingone. Inthe dry spinning,a volatile solventis firstused to dissolvethe polymer intoa solution. This solutionisthenpurifiedbyafilterandpushedthroughaspinneret into a warm air chamber to force the solvent evaporation. The polymerthensolidifiesintoathinfilamentthat iswoundaround drums.Thekeyvariablesofthedryspinningaretheheattransfer, themasstransferandthefilament stress [67].Commondryspun fibersincludeacrylics,polyvinylchloride(PVC)andpolyvinyl alco-hol(PVA).

3.1.3. Wetspinning

Thismethodology isused,likedryspinning,forpolymersthat needtobedissolvedtobespun.Thesolutionisextrudedthrough a spinneret directly into a chemical liquid bath. This causes the

fiberprecipitationandsolidification.The solventisthen removed, andthefilamentiswoundaroundadrum.Therateofextrusionis crucialtoavoidmicro-voidformationintothefiber.Commonwet spunfibersincludeacrylics,rayon,spandex,lyocell [70].

3.1.4. Electrospinning

Theelectrospinningprocesswasdevelopedintheearly twenti-ethcentury [71]but, onlyinthelast threedecades, ithasdrawn increasing attention dueto the exponentialdevelopment of nan-otechnologies [72]. This technique has proved to produce size-tuneable ﬁbers in the nano- or micrometric scale andis widely employed to mimic the morphology of many biological tissues, suchastendons,ligaments,andmusclesinthetissueengineering ﬁeld [72–78].

Theprocess isbasedonthe stretchingofapolymeric solution througha highelectrostaticfield (Fig.3B). The systemconsistsof a syringe witha thinmetallic needle,loaded withthepolymeric solution,a syringe pumpto control the flow rate, anda metallic collectorpositioned ata certain distance.The needleis linked to thepositiveterminalofadirectcurrent(DC)powersupply,while thecollectorisgrounded.Thevoltageappliedisofseveralkilovolts (5–30kV) [79].Thisproduceahighelectrostaticfieldbetweenthe needleandthecollector.Then,thesolutiondropletflowingoutof theneedleisstretched bytheelectrostatic fieldtoformaconical shape,calledTaylorcone(Fig.3B).Ajetstreamemanatesfromthe cone apex, firstly with straight trajectory, then with a whipping movement towards the collector. This motion induces the poly-mericchainstostretchandthestreamto shrink.Thiselongation, along with the evaporationof the solvent,produce a continuous fiberwitha tuneablediameterfromthenano to themacroscale

[80–82]. The outcome ofthe electrospinning process dependson threefamiliesofparameters[72,80]:

- Solutionparameters:thepolymer(s)andthesolvent(s)chosen, theirconcentration,conductivity,andviscosity [83].

- Processparameters:theappliedelectrostatic ﬁeld,theneedle– collectordistance, the collectorshape, the collector rotational speed,theﬂowrateandtheneedlediameter [84].

- Environmentalparameters:therelativehumidityandthe tem-perature.

Tuning these parameters, different ﬁber morphologies, cross-sectionalareasandporositiesareattainable.Bychangingtheshape

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ofthecollector,itisalsopossibletointroduceapreferential align-ment of the ﬁbers [71], reproducing the parallel arrangement of the skeletalmuscle ones.This allowedto producescaffolds (even resorbable) formuscle regeneration, able to guide the growthof differenttissuecells [85–88].Forallthesereasons,electrospinning isoneofthemostpromisingtechniquesalsoemployed inthe de-velopmentofbioinspiredmuscle-likeactuators.

3.1.5. Nearﬁeldelectrospinning

The near field electrospinning (NFES) is a spinning process where solid nanofibersare deposited in a direct,continuous and controllableway(Fig.3C) [89].“Nearfield” meansthattheprocess aims to depositthe nanofiber on the collectorwhen it isstill in thestableliquidjetzone.Converselytotheclassicelectrospinning, the whippingzone isabsent. The typical tip–collectordistanceis intherange0.5–3mm [89].Becauseofsuchaclosedistance,the requiredelectrostaticvoltageisreduced [90],maintaininganyway the typical value of the electrical field in the region (about 107 V/m) [89].With thisprocess single, thin(even lessthan 100 nm diameter)andseveralcentimetreslongnanofiberscanbeobtained [90,91].Thefibersaredepositedinasimilarwayofthe3Dprinting andmovingthecollectorwithadequatespeedallowstodrawthe fibersinacontrollablemanner [92].Themorphologyofthefibers can be tuned changing the solutionconcentration, the electrode-collectordistance,theappliedvoltage,thetipdiameter,thesizeof thedroplet[89,90,92].Theenvironmentalparametersarethesame oftheconventionalelectrospinninglistedabove.

3.2. Chemicalvapourdeposition

Thechemicalvapour deposition(CVD)isasynthesis technique usedtoobtainasolidsupportfromamolecularprecursor(Fig.3D). Itconsistsinthethermaldecompositionofahydrocarbonvaporin presenceofametalcatalyst.Itisthemostemployedmethodology toproducecarbonnanotubes(CNTs),thatarecarbon-based nanos-tructured tubular elements. Since mid-twentieth century, CVD is an establishedmethodformicrofibersproductionandin1952 re-searchers obtained50-100 nm diameter CNTs. The CVDfor CNTs manufacturing consistsinahydrocarbonvapor flowingthrougha cylindrical reactor, inwhich a metal catalyst is present. The cat-alyst, heated at a proper temperature (600-1200°C), decomposes thehydrocarbonfromthevaporintocarbonandhydrogenspecies. Hydrogenfliesawayandcarbongetsdissolvedintothemetal [93]. Oncethesolubilitythresholdofthecarboninthemetalisreached, itprecipitatesandcrystallizesinanetwork-shapedstructure ener-getically stable, the CNT. Upon cooling the system, the CNTs are collected. However, the CNT growth mechanism is still not well known [93].TheCVDiswidelyusedbecauseofitsreasonable pro-cessingcost,highproductionyield,theabilitytocontrolthecrystal structureandthedepositionrate [94].Manyparameterscanbeset totunetheCNTsstructure:thehydrocarbontype,thecatalyst,the temperature, the pressure, the gas-flow rate, the deposition time andthereactorgeometry [93].

4. Materialsemployedinmuscle-inspiredﬁbersproduction

In orderto produce fibrous actuators,differentmaterials with specificpeculiaritieshadbeenused.Aclassofmaterialsfrequently employed is represented by conjugated polymers: organic semi-conductors characterized by single or double bonds alternated along the polymeric backbone [59]. This configuration confers themelectrical conductivity:onceananion approachesthechain, itforcesthepolymertoreconfigureitself,slidingitsdoublebonds alongthechain.Thiscreatesafluxofelectronsand,consequently, an electroniccurrent. Applyingapotential tothepolymerends,a volumetricchange occurs,a phenomenon mainlycausedby mass

transport. The applied potential creates a charge mismatch, bal-anced by the entrance of anions or cations (often solvated) into the interstices of the polymeric chain. The polymer thus con-tracts or expands by changing its volume. Polyaniline, polyacry-lonitrile, polypyrrole andpoly(3,4ethylenedioxythiophene) belong to this class. These materials can be used both as electro-active coatingandtoproduceﬁbersthemselves.

Carbon is also frequently used. Speciﬁcally, CNTs are a class ofallotropes of carbonwith cylindricalnanostructure. They have appealing propertieslike low density, hightensile strength, high conductors ofelectricity and/or heat. CNTs are oftenobtained in form of multi walled carbon nanotubes (MWCNT), consisting in several concentric graphene tubes [95]. They can be grouped in yarnsandtwisted to make spring-shaped structures ableto con-tractaxially and to rotate in response ofvarious kindof stimuli

[96–98].

Thesame principle isshared withsome non-conductive poly-mers or natural ﬁbers that, when over-twisted, rely on their anisotropic thermal expansion coeﬃcients to produce actuation

[98].Otherpassivefibers,likesilk,canbeusedasbulkmaterialsto producenanofibers.Thesefibersare oftenresponsivetohumidity changes,making themsuitable forsoft-actuatorsproduction [99]. Othermaterials, usuallyconjugated polymers,arefrequently used ascoatingonthefibers,toachievearesponsetothestimuli [100]. Recently,molecularmotors(MM)hadbeendevelopedstarting di-rectlywiththeartificialassemblyofamoleculecapabletochange itslengthinresponsetoastimulus [101–105](Tables 2and3).

5. Analysisofstate-of-the-artofmuscle-inspiredactuators

InthisSection,thestate-of-the-artofmuscle-inspiredactuators is analysed. Specifically, theliterature will be classified consider-ingthehierarchicallevelreachedandtested, i.e.singlefibers,flat mats,bundles,yarns,coiledand/orpliedstructures(Table4).

5.1. Singleﬁberactuators

Up to now, only a few works involved actuators made of in-dividual fibers. In [113], hydrogel nanofibers had been produced throughtheelectrospinningtechnique.Themain aimofthework was to investigate the actuation performance of a single hydro-gel nanofiber made of PAA. The atomic force microscope (AFM) wasused to measure forces in the nanonewton scale [131]. The PAA nanofibers were directly electrospun across two parallel 40 μmthick bars ofa transmission electron microscopy (TEM) stub, 50μmspacedfromeachother(Fig.4).

After an ultra-violet light (UV) crosslinking the fiberwas me-chanicallytestedhookingtheAFMcantilevertiptransverselytoits axialdirection.Ina firsttest,thenanofibersweredepositedslack across the bars (Fig. 4AI) andlater activatedby pH variationsof the bathing solution. Moving frompH 8 to 3, a swelling of the nanofiberwas observed,witha consequent axial contraction (re-sponse time = 1-4 s) until its complete stretching between the bars(Fig.4AII).Inasecondtest,thenanofiberswerepre-stretched withaconstantstrainby thecantilevertipmovement.Witha se-riesoftiposcillationandpHvariations,thenanofibersmechanical properties were determined: a 20–24% axial strain was reported duringthefreestrokeinthefirsttest,whileduringthesecondone (comparabletoanisometriccontraction)anelasticmodulusof8.5 MPa(pH=3)andof5.1MPa(pH=8)wasdetermined.The contrac-tileforcewas0.5μN, correspondingto ameanstress of230kPa, comparabletoskeletalmuscles[30,47].

In [132], theNFES techniquehadbeen used to producePVDF ﬁbers (diameter = 2.6 μm, length = 500 μm) studying their in-dividual actuation (Fig. 4B). The PVDF ﬁbers were spun across two aluminium contact pads suspended over an insulating glass

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Table 2

Materials used to produce muscle-inspired actuators

Acronym Extended Name Manufacturing technique References PANi Polyaniline Electrospinning [ 4 , 61 , 106 ] PAN Polyacrylonitrile Electrospinning [ 107 , 108 ] CNT Carbon Nanotube Chemical Vapour Deposition [ 12 , 14 , 60 , 109 ]

PPy Polypyrrole Coating [ 65 , 100 ]

PEDOT Poly(3,4ethylenedioxythiophene) Coating [ 65 , 100 , 110 ]

PES Polyester - [65]

- Nylon 6.6 Dry / wet / Melt spinning [111]

- Silk Electrospinning [100]

Natural harvesting

PVDF Polyvinylidene ﬂuoride Electrospinning [112]

MM Molecular Motors Solvent Casting [105]

PAA Polyacrylic acid Electrospinning [113]

- Cellulose Electrospinning [ 106 , 112 , 114 ] PET Polyethylene Melt Spinning [ 10 , 115 , 116 ]

PU Polyurethane Electrospinning [61]

ZrO 2 Zirconia Electrospinning [117]

- Spandex Melt Spinning [118]

UHMWPE Ultra-molecular weight polyethylene Melt Spinning [116]

GO Graphene Oxide Wet spinning [119]

Nb Niobium Etching [9]

PEVA Poly(ethylene-co-vinyl-acetate) Melt Spinning [120]

- Wool Natural harvesting [121]

- Cotton Natural harvesting [121]

- Flax Natural harvesting [121]

- Cotton Natural harvesting [122]

- Viscose Melt Spinning [123]

Table 3

Coating, electrolytes, and additives used to enhance muscle-inspired actuators contraction

Acronym Extended Name Usage References

PANi Polyaniline Coating / Particles inclusion [ 4 , 61 , 106 ]

PPy Polypyrrole Coating [ 61 , 100 , 124 ]

MWCNT Multi Walled Carbon Nanotubes sheet Coating [111]

PEDOT Poly(3,4ethylenedioxythiophene) Coating [ 65 , 100 ]

- Graphene Coating [125]

Pt Platinum Coating [126]

Ag Silver Coating [ 10 , 65 ]

Au Gold Coating [65]

C Carbon Particles inclusion [65]

Sn Tin Coating [65]

Al Aluminium Coating [120]

Cu Copper Coating [65]

p-TSA Para-toluene sulfonic acid Coating [100]

NaDBS Dodecylbenzenesulfonate Coating [100]

- Wax CNT-structures guest material [127]

PDDA Poly(diallyl dimethylammonium chloride) CNT-structures guest material [128]

TPU Thermoplastic Polyurethane CNT-structures guest material [96]

PVDF-co-HFP + TEA •BF4 Poly(vinylidene fluoride-co-hexafluoropropylene) + tetraethyl ammonium tetrafluoroborate Gel electrolyte [ 12 , 13 ]

PVA + H 2 SO 4 Polyvinyl alcohol + Sulfuric acid Gel electrolyte [13]

TBA •PF 6 Tetrabutylammonium hexaﬂuorophosphate Organic electrolyte [ 12 , 97 , 129 ]

PEO-SO 3 Sulfonated Poly(ethylene Oxide) CNT-structures guest material [130]

TBA •PF 6 /PC Tetrabutylammonium hexaﬂuorophosphate + Propylene carbonate Organic electrolyte [130]

substrate, fixing their ends with conductive epoxy on the pads (Fig.4BI).PVDFhasanegativepiezoelectriccoefficient,sopositive ornegativeelectricfieldscauserespectivelyashrinkageor elonga-tion [132].Thesuspended fibersshowedan initialdownward de-formation due to gravity force, measured with a charge-coupled device (CCD) sensoras the variation ofdownward flexion inthe central part of the fiber (Fig. 4BII). The authors reported a bias inthe data causedby the electrostatic attraction ofthe fiber to-wardthepad.Thisside-effectwasminimizedbyplacingtwo iden-ticalelectrodessymmetricallyoverthemainones(Fig.4BIII).The PVDF electrospun fibers showed about a two-time higher piezo-electriccoefficientcomparedtocommerciallyavailablePVDFfilms. This coefficient is directly proportional to the induced mechani-calstrain.Itwasconcludedthatthisimprovementwasduetothe fewerdefectsandsmallerdomainwallmotionbarrierinthePVDF electrospunfibers.

Only a few works investigated the properties of single fibers mainlybecauseofthechallengingtestingproceduresaccordingto thedimensionsofthefibersandtheinfinitesimalforcestobe mea-sured(fromnNtoafewμN [113]).

5.2. FlatMatactuators

Trying to obtain a scaled up hierarchical complexity of their actuators, other researchers focused on theproduction offibrous mats. Preliminary works investigated commercially available wet anddryspunPANfibers[126,133–135].In [133],anartificial mus-clemadeofPANfiberswasactivatedthroughanelectricfield.This activationwasobtainedbathingthestructureinanaqueous solu-tionofsodiumchlorideincontactwithoneofthetwoelectrodes. Thelocalacidityofthesolutionchangedneartheelectrode, imply-ing the contractionor expansionof the artificialmuscle

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depend-Fig. 4. Single fibers actuation. A) Optical microscope images of PAA nanofibers UV treated for 5 minutes and suspended between bars of a TEM grid at different pH values (adapted from [113] and reproduced with permission. Copyright 2012, The Royal Society of Chemistry): I) swollen fibers (pH = 8; scale bar = 20 μm); II) contracted fibers (pH = 3; scale bar = 20 μm). B) Piezoelectric actuation of PVDF nanofibers fixed between two aluminium contact pads (adapted from [132] and reproduced with permission. Copyright 2010, Elsevier B.V.): I) scheme of the electrostatic attraction forces between the nanofiber and aluminium pads; II) deformation of a PVDF fiber under positive ( + E) or negative ( −E) electrical field as a result of (a) piezoelectric and (b) electrostatic effects; III) adding two aluminium electrodes upside, the electrostatic force can be reduced.

ing on the polarity of the electric field. In a later study the au-thorsaddedtwostepsofannealingandchemicaltreatmenttothe PANfibers,reachinga100%strainduringtheactivation [134].The forcedominatingthecontraction-expansionofPANfiberswere re-ported to be mainly intermolecular Coulomb ones. Trying to in-creasetheir conductivity,themuscle-inspiredPANfibershadalso beencoatedwithplatinum [126].However, thecontractionspeed was quite low, reaching a 40% contraction in 10 minutes. To in-creasetheions/solventsdiffusionalprocessesthatgovernsthePAN contraction,thecontraction speedandtheresponsetime aswell, the electrospinning technique was adopted to produce mats of PAN nanofibers [108]. The same investigation was carried on in [107,136],wheretheshapechangesofPANnanofibersinresponse toapHvariationhadbeenobserved.Thechangesofthenanofibers diameterswerealsomeasuredin-situwithanenvironmental scan-ningelectronmicroscope(E-SEM)andanAFM.Avariationofmore than 100% wasobserved [134] withan increase ofthe mean di-ameter of PAN nanofibers from 250 to 750 nm, after the con-traction [107,136]. In [4], to improve the overall actuator perfor-mances,abiomimeticperspectivewasinvestigated,exploitingthe nanoscalerangeandthe highermolecularalignment within indi-vidual fibers to increase the Young’s modulus [137,138]. Electro-spunPANnanofiberswerecollectedinrandomlyalignedmatsand then annealed. Several stripes of the PAN mat were cut and di-vided in three groupsto be passively mechanically characterized indifferentstates:annealed,fullycontracted(inalowpHsolution) andfullyexpanded(inahighpHsolution).Eachcategoryshowed differentvaluesofultimatestressandstrain(i) annealedsamples 0.07MPaat8%;(ii)fullyexpanded3.5MPaat42%;(iii)fully con-tracted80MPaat125%.Acompositeactuatorwasthenproduced, embeddingagraphiteelectrodesheetbetweentwolayersof

elec-trospunPANnanoﬁbers.A25%linearcontractionin1minutewas observed when electrically activated, with a maximum strain of 58.8%,an ultimatestress of77.1 MPa, and a Young’s modulus of 0.21MPa [4].

Cellulose acetate combined with synthetic polymers is an alternative strategy when prototyping contractile flat mats of nanofibers.Anelectrospuncelluloseacetate-polyaniline(PANi) bio-composite membrane was proposed as soft-actuator in [106], wherethe cellulose acetate fibers,embeddedwithchopped PANi particles, were directly electrospun. Just a low amount of this conductive polymer was sufficient to enhance the performance of this cellulose-based biopolymer actuators [106]. The obtained nanofiberswere collected in randomly orientedmats andplaced between thin gold electrodes. The devices were electromechan-ically tested with harmonic responses and current-voltage tests, showinganelectrically drivenbendingdeformation. Theentityof thedisplacement wascorrelated withthe amountof PANiinside thecellulose-acetatenanofiber [106].Thecontractionwasnot lin-earprobablyduetotherandomarrangementofthenanofibers.

5.3.Bundleandyarnstructuredartiﬁcialmuscles

Following the hierarchical structure exhibited by biological skeletal muscles, researchers investigated parallel (bundle) or twisted(yarn)fibroustopologies,toenhancetheactuation perfor-mances.Abundle of50μmPETfibers,platinumcoated, was pro-ducedthrough melt spinning [124]. The whole bundle wasthen coatedwithanadditionallayerofPPy.Sincethiskindofactuator needs aliquidelectrolyte, thebundle shape waschosen to facili-tatetheiondiffusionkineticsandtodecreasetheelectrical resis-tance,avoidingthelimitationspreviouslyobservedinPPy-film

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ac-tuators [139].Thedeviceweightfraction,constitutedbytheactive componentwasthen calculated,resultinginanincrease from0.3 ofthedouble-sidedlaminatedfilm(thickness=100μm;PPy coat-ingthickness₌20μm),to0.64ofabundleofPETplatinumcoated fibersthatoccupiestheequivalentareaofthefilm.Themechanical testsreportedamaximumstressof6.8MPa.

Bundles of PANmicroﬁbers, coatedwithconductive platinum, wereproducedin [140].Thesebundleswereused insidea chem-icalelectrolyticcellasanelectrode andactivatedwitha10 VDC powersupply.ApHchangeinthevicinityofthePANbundle elec-trodewasthenobserved,leadingtoacontractionofupto100%of thebundlewithinafewseconds.

Inanotherstudy [65],theperformancesofsevendifferentkinds ofactuators,producedstartingfromcommercialyarns,were eval-uated: spun yarns (of polyamide, silicon or cellulose) with car-bon particles inclusion; polyamide silver coated; polyester yarn woundedbyastainless-steelwire;polyesteryarndoublewounded by a copper wire; gold coated polyester yarn. Finally, all these yarnswere coatedwith a conductive PPy layer. The actuation of the yarns showed a linear contraction in a range of 0.01 – 0.1%

[65].

Other groupsinstead worked on the developmentof artificial musclesbased on CNTs, which is one of the mostprolific fields. Firstly,anovelmethodtoobtainyarnsofmultiwalledcarbon nan-otubes (MWCNTs)was introduced, inserting a twist during their drawing fromthe, so called, nanotube forest [141] (Fig. 5AI- II). Theseyarns haveshowntoachieve ultimatestressesgreaterthan 460 MPa with about9% strain. In [142], MWCNTs were used to developasoftactuator.Thedependenceoftheactuationstrainon theappliedvoltagewerequantified.Alinearstrainof0.5%was ob-tainedinresponsetoa 2.5Velectricalstimulus. Thisdependence wasfound tobequadratic,whereas theYoung’s moduluswas in-dependent from the applied load or voltage. A similar structure wasproposed in [129], where a muscle-inspired electromechan-ical actuator wasproduced witha twisted CNT yarn, filled with an electrolyte (i.e. 0.2 M TBA_•PF6 in acetonitrile). The yarn was firstlyobtainedby harvesting andtwisting MWCNTsfroma nan-otubeforest.Thenitwasput,togetherwithacounterelectrode,in theelectrolytebathwithanappliedvoltagebetweenthetwo elec-trodes.Forthe electrochemical double-layer charge injection, the yarnpartiallyuntwisted.Theactuationperformancesshoweda re-versiblemaximumrotationof41fullturns,correspondingto3.4% of the twist inserted in the actuating yarn length. However, the linearmaximumcontraction wasabout1%, far fromthe one ob-servedinskeletalmuscles(i.e.20-40%duringphysiological activi-ties[143,144]).Aslightlybettercontractionwasobtainedin [145], byusingleftandright-handedfibrousyarnsofsimilartwistedand alignedMWCNTs,(diameter=20μm)(Fig.5AII).Theseassemblies were firstly actuated in air, obtaining reversible lengthwise con-tractionsupto2%androtatorytorsionslargerthan360°.These re-sultswereequivalentforbothleft- andright-handedyarns.Their rotationsandcontractionswerequitefast(_<0.4s)andthe devel-opedstress reached6MPaduringan isometrictestonto a table-toptestinginstrument.

In [127],a hybridelectrothermally poweredMWCNTsyarnhad beenproduced, by exploitingthe same strategy that spiders use in Nature to eliminate uncontrolled spinning at the end of the dragline. To damp unwanted dynamic oscillations, the yarn was infiltrated with paraffin wax. Moreover, the wax, mixed with a triblock copolymer, was characterized by a large thermally in-ducedvolumechange that can alsoenhance theyarn lengthwise contractionwhen activated [146]. Ina later work, the conductiv-ity of MWCNTs was increased by depositing graphene flakes on the MWCNTs sheets,during yarns production [125]. In this way, theconductivityof the device increasedto 9_×104 _S/m_(from _the 22×104_S/m_of_the_pristine_MWCNTs).

As an alternative to CNTs, niobium nanowires were pro-posed as high strength yarn-based artiﬁcial muscles [9]. To ex-tracthighly alignedniobium nanowires, copper hadbeen etched fromcopper-niobiumwires(diameter₌100nm).Theseﬁlaments were later twisted into yarns of different diameters and electro-mechanically characterized. The devices showed strong values of ultimate strength (from0.4 to 1.1GPa), higherconductivity than CNTs (3×106 _S/m_versus ₃_×104 _S/m) _and_mean _tensile_moduli_of about19GPa.Theelectricalactivation,withapulseof4.8V, gen-eratedamaximumvalue of1800rpm,despiteatensileactuation ofonly0.24%,evenlowerthanCNTsactuators[142,145].

Theelectrospinningtechniquewasalsoproposed asasuitable tooltoproducebundlesresemblingtheskeletalmusclemyofibrils andmyofibers.PUnanofiberscoatedwithPANi wereproposed in

[61].PU waschosen becauseofitsrubbery andflexible naturein ordertofollowthecontractionofPANiwithoutblockingits actu-ation. An axially alignedbundle wasobtained witha gap collec-tor duringtheelectrospinning process. Thenanofibers were then coatedwitha260nmthicklayerofPANi,usinganin-situchemical polymerizationof aniline.Thedevice wasthen testedwithcyclic voltammetry,showinga highelectroactivity. Current-voltage(I-V) plots were obtained, reporting 50 S/m of conductivity, that was considerablyhighercompared tothepure PUnanofibrous bundle (1.76×10−6 _S/m)._The _{electromechanical}_{characterization} _revealed alinearcontractionof1.65%atanappliedstressof1.03MPa.The bundle could stably be actuated without significant creep, atan applied stress up to 2.263MPa (linear strain ofabout 0.6%). The workper cyclewasreportedwithan efficiencyabove 75%during the actuation,even beyond 100 cycles. The device was also pas-sivelytested,reportinganultimatestrengthof35MPa,higherthan theultimatestrengthofapurePUnanofibrousbundle(25MPa).

In [100], the electromechanical actuation of electrospun silk fiberbundleswasproposedtomimicthemusclefiber.Silkfibroin waschosen due to its biocompatibility, asdemonstrated in drug delivery [147] and tissue engineering of similar hierarchical bio-logical structures [148]. Silk nanofiberswere first electrospun on a high-speedrotating drumcollector inan aligned pattern;mats werelaterrolledupdirectlyonthedrum(Fig.5BI)toobtain bun-dleswithan averagediameterof200-500μm(Fig.5BII-III),the sameorderofmagnitudeofmusclefibers.The fibersthen under-went sequential chemical and electrochemical polymerization to increase the hydrophilicityof thefibers and theincorporation of the conductivepolymer. These processesalso changedthe colour ofthebundles(Fig.5BII).Then,toformasilk-PPyinterpenetrating network,aPPydepositionwascarriedoninsitu,soakingthe bun-dles in a polymerization solution containingPPy andFeCl3. This stepturnedthebundlessufficientlyconductivetobeusedas work-ing electrodesforan additionalconductive coating step, using p-TSAorNaDBSasdopants(orPEDOTasanalternativetoPPy).The resulting nanofibers exhibited conductivities in the order of 103 S/m.Usingasquarewaveof±2Vtoactuatethedevices,the bun-dlescoatedwithPPyanddopedwithp-TSAachievedamaximum stress of 400 kPa, in the same range of skeletal muscles. While devices employingPEDOT showed higherstresses andrelaxation rates(0.12%/s),thePPycoatedanddopedwithNaDBSshowedthe longestlifetime,beingfullyoperationalafter24hours,butwitha stress generation capacitydropped to a fifth ofthe initial value. However, the maximum strain was still low when compared to naturalmuscles,reachingmaximumvaluesof1-2%.

In [105], a hierarchical self-assembly of photo-responsive am-phiphilic MM [102,149–151] was proposed. This work relies on an unpreceded ampliﬁcation of motion,startingfrom the molec-ular level over various length scales, up to a macroscopic con-tractile movement of a hierarchical ﬁbrous structure [105]. Ro-tary MM were synthetized in such a way as they automatically self-assemble in a supramolecular unidirectional aligned system

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Fig. 5. Applications of fibrous bundles and yarns in the production of muscle-inspired soft-actuators. A) Scanning electron microscopy (SEM) image showing the production of a MWCNTs yarn obtained by twisting during drawing from a nanotube forest (adapted from [171] , reproduced with permission. Copyright 2010, Elsevier B.V.): I) overview (scale bar = 500 μm); II) zoom into the carbon nanotube yarn (scale bar = 250 μm). B) Electrospun silk fibroin electromechanically actuated bundles (adapted from [100] , reproduced with permission. Copyright 2017 The Royal Society of Chemistry): I) production by rolling up nanofibrous mats on a drum collector; II) different chemically treated bundles (scale bar = 50 μm); III) SEM image of a bundle cross-section (scale bar = 200 μm). C) Nanofibrous bundle actuated by means of photo responsive MM (adapted from [105] , reproduced with permission. Copyright 2017 Springer Nature): I) Picture of a photo responsive molecular machine; II) self-assembly of molecules into nanofibers; III) assembly of nanofibers in axially aligned bundles; IV) image of a bundle before the application of the UV light (axial alignment = 0 °; scale bar = 5 mm); V) bundles nanofibers bending after the UV light application; VI) image of a bended bundle after the application of the UV light (radial bending = 90 °; scale bar = 5 mm).

to form a nanoﬁber (Fig. 5CI). Then, the solution containingthe MM was matched into an aqueous solution of calcium chloride (CaCl2)withapipettetoautomaticallyobtainanoodle-likebundle ofalignednanoﬁbers(Fig.5CII-IV).Thisstring,about10mmlong andcomprising95%ofwater,wasactivatedwithalightsourcethat induced the rotationof the rotary MM and, thus,the bendingof

thestringtowardthelightsource(Fig.5CIII– VI).Themacroscopic bendingofthe stringwascausedby the orientationalchanges of thebundles [105].A90° stringbendingwasobtainedinwater af-ter60sofirradiation;torestorethestringoriginalconformation, itwasnecessarytoheatthewaterfor3hours.Asimilartestwas carriedoninair,witha0.4mgpieceofpaperattachedtotheend

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Fig. 6. Yarn configurations for linear and torsional actuators. A) Different methods of twist insertion during spinning of a MWCNT sheet. The cross section scheme of the yarn is shown in the inset (adapted [154] , reproduced with permission. Copyright 2002 Science Magazine): I) Fermat-type twist insertion (scale bar = 75 μm); II) Archimedean- type twist insertion (scale bar = 250 μm); III) Dual-Archimedean-type twist insertion (scale bar = 300 μm). B) Different muscle configuration using twisting, coiling and plying techniques (adapted from [14] , reproduced with permission. Copyright 2012 Science Magazine): I) two-end-tethered, fully coiled wax infiltrated homochiral muscle; II) two-end-tethered, half wax infiltrated, noncoiled homochiral muscle. A paddle, free to rotate, separates in the middle the infiltrated and the neat part of the device; III) one-end-tethered, fully wax infiltrated, 4-ply homochiral muscle; IV) two-end-tethered, fully wax infiltrated, 2-ply heterochiral muscle. The paddle, free to rotate, separates in the middle the two opposite-twisted part of the device. Red and green attachments are tethers that prohibit end rotation. Red ones prohibit also translational displacement.

ofthestring.Afterirradiation,thedevicewasabletobendof65° in60s,liftingthepaper.Eveniftheproposed actuatorinvolvesa bendingmotion,whichisfarfromoneofthetopicsofthisreview, itisinteresting tonote thatinthiscasethebasicactuatingunits are the molecules themselves. These MM firstly self-assemble in alignedsystems toform nanofibersandfinally were grouped to-getherproducingabundle-likehierarchicalstructure.

5.4.Coiledand/orpliedstructuredartiﬁcialmuscles

The amount ofcontractive strains andactuatingforcesis par-ticularly critic for muscle-inspired actuators. To overcome these problems, researchers adopted techniques such as twisting, coil-ing, and plying [16,152]. A coiled structure is obtained by over-twistingasingleyarn,whiletwisting severalyarnstogether leads to a plied assembly. These types of actuators can produce force throughtorsionalmotion [153]orlinearcontraction.Inthisreview, forthereasonspresentedabove,onlythestudiesbelongingtothe latter category will be described. In [14], a guest-ﬁlled MWCNT yarn actuator was designed. The need of an electrolyte (with a counter-electrodeanda packaging)isanother commondrawback

ofthesedevices[129,142].In [14],theactuatingguest(i.e.,paraffin wax)betweenMWCNTswasincorporatedinbothsolidandmolten state to avoid this limitation. The volume expansion of the wax drove the actuation (about 20%, while the temperatureincreases from30°Cto 90°C). Waxwasnot extrudedfromtheporousyarn dueto thehigh interfacialenergies arising atthenanoscale [14]. ThisguestconfinementinsidethenanoscaleporesoftheMWCNT yarn was intended to avoid hydraulic and external heating sys-tems. These hybrid yarns contained up to 99% in weight of the guestsubstancewithoutlosingtheflexibilityandthepropertiesof the CNThost structure [13]. Furthermore,in [14], manydifferent structures hadbeenproposed, startingfromtheharvestingofthe MWCNTsheetsfromthe CNTforest. Duringthetwistinsertionof thesheetstoformtheyarn(Fig.5AI),theparaffinwaxwas even-tually inserted with the biscrollingtechnique. Toincorporate the wax into the yarns, the sheets were twisted in different-shaped structures depending on thedesired application [14]. The typical yarn-gueststructuresareknownasFermat,Archimedeanand dual-Archimedeanscrollsandoriginatesbythetwistmovementthatis impartedtothesheetsduringtheyarnproduction [154](Fig.6 AI-III).Furthermore,homochiralandheterochiralstructures hadbeen

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proposed (Fig.6BI-IV) [14]. The latterwere obtainedby inserting a paddle in the middle of the yarns: in this way, one half was twistedintheoppositedirectionofthesecondhalf.Acoiled dual-Archimedeanneat(withoutwax)yarnwasfirstlytestedbyheating, showing a 7.3% contractionunder 3.8 MPaof applied stress. The contractile work capability (scaled on the yarn weight) was0.16 kJ/kg. Atwo-end-tetheredcoiledFermatwax-filledyarnwas ther-moelectricallyactivatedwithasquarewavestimulusof18.3V/cm at20Hz.The devicesshowedfor1.4million cyclesatensile con-tractionof3% ata rateof1200cycles/minute,lifting17700times their own weight.A fastpassive coolingof 25ms wasnecessary betweencycles.The samedevice wasthen optimizedby increas-ingtheelectricalstimulationat32V/cm,reachingthecapabilityto lift175000timesitsownweight,acontractileworkof0.836kJ/kg, andapoweroutput of27.9kW/kg,whichwas85timesthepeak value formammalian skeletalmuscles(0.323kW/kg).As a draw-back, a decreased life cycle ofthe device had been reported. To optimizethelevelofcoilingoftheactuators,aninvestigationwas carried onyarns having differentlevels ofinserted twists [14].A coiledFermatyarnwithanintermediatetwist(3990turns/minute) showeda maximumcontractionof5.6% wasobservedwitha 5.7 MPaofappliedstress.Increasingthetwistinsertionby6.8%(4263 turns/minute)resultedalsoinanincrementofthemaximum con-traction(16.4MPaat5.1%strain)andofthecontractilework(1.36 kJ/kgat84 MPa).Conversely, thereduction ofthetwist insertion by 41%eliminated the coiling, thus reducing the maximum con-tractionat0.7%andthecontractileworkto0.31kJ/kg.Apartially coileddual-Archimedeanyarnproduceda10%ofcontractionat5.5 MPaofappliedstress.Thisimprovementwasachievedby increas-ingtheyarndiameterofabout13times;asdrawbacksthecooling timeincreasedto2.5sand,thus,thecontractilepowerloweredto 0.12 kW/kg.A two-handed-tethered homochiralFermatyarn, half waximpregnatedandhalfnot(separatedinthemiddlebyapad) was also tested. When thermoelectrically activated, the actuator wasabletorotatethepaddleat11500rpm,firstinone direction theninreverse.Thedeviceshowedahighlifecycle,duetothe si-multaneous twistingoftheunactuatedhalfduringtheuntwisting ofthe actuatinghalf (theunactuated halfactedasatorsional re-turnspring [127])(Fig.6BIV).Furthermore,thesameworkshowed howthewaxinfiltrationandthecoilingincreasedthetensile con-traction ofyarnsaswellashow thereversibilityoftheactuation wasgreatlyenhancedby havinga sufficienttwisttocausecoiling

[14].

A similar guest-filled hybrid CNT torsional and linear yarns were proposed in [13].The peculiarityoftheseactuators wasthe solid gel electrolyte (PVDF-co-HFP based TEA_•BF4) in which they were embedded.Theseactuators wereelectrochemically driven,a method used to increase the efficiency compared to electrother-malactivation [13].AMWCNTsheetwaspulledfromaforestand a twist was inserted to produce yarns with a high oriented fi-brous structure (Fig. 7AI and II). To produce torsional actuators, two yarns were firstly embeddedinthe gel electrolyte and, sub-sequently, were plied together in a two-ply (2-ply)mechanically coupled assembly, using opposite directions of twist (Fig. 7AIII). The two yarnswere then used asanode andcathode. A square-wave stimulation of 2.5 to 5 V resulted in a reversible torsional strokeof14-53°/mmwithaminorhysteresis.Conversely,thelinear actuators were obtainedusing two fully coiled yarns (Fig. 7AIV). Thesetwo coiled yarns(i.e.,anodeandcathode) were simultane-ously infiltrated andcoatedwithan aqueoussolid gel electrolyte (PVAinsulfuric acid)andpliedtogether ina2-plyartificial mus-cle(Fig.7AV).Alinearcontractionof0.52%wasobtained,applying asquare waveof1V(atan appliedstressof11MPa).Othertrials inthesamework [13]obtained1.3%contractionat10.1MPawith a stimulation of2.5V.While itwasquite fast(about 1s)to ob-tain thefirst1%ofcontraction,thestimulationneededtoachieve

itsmaximumcontractionwasquitelonger(about20s).Unlikethe electrothermallypowered artiﬁcialmuscles, thesedevicesdidnot requireanyfurtherenergyinputtomaintainthecontraction.

Anotheractuationmethod,withsolventsandvapours,was em-ployed in [109] to drive helical CNTs artificial muscles (Fig. 7B). Using the dry-spinning technique, MWCNTs were drawn from a forest to form a yarn (named primary fiber;diameter ₌ 15 μm) (Fig.7BI).Twentyyarnswere thenpliedtogetherina2-levels hi-erarchicalstructure (Fig. 7BII).,later over-twistedto becamefully coiled(3-levelshierarchicalstructure)(Fig.7BIII).Thesestructures were made with different helical angles: 0°, 8°,16°, and 32°, us-ing a left-handedtwisting. The 3-levels structures were then ex-posed to a polar solvent, an ethanol droplet, showing a rapid reversible contraction (about 0.5 s) able to last for 30 cycles (Fig. 7BIV and VI). The contractive peak stress of 1.5 MPa was reachedwiththe32° helicalangle3-levelsstructures.This actua-torrotatedat6361rpm,resultingcapableof738turns/mmofthe muscle length. The maximum linear contraction was about 10%. Thetotalwork densitycalculated,includingrotatoryand contrac-tionoutputs, was26.7 J/kg.The responserate wasfurther inves-tigated,reportingacontractive strainrateof340%/s [109].The 2-levelsstructurewasalsotested(withoutcoiling)usingtheethanol droplet, showing an excessive irreversible untwisting, confirming thatthecoilingprocessenhancedthereversibilityoftheactuation (Fig.7BIVandV).The3-levelsstructure,instead,wasshapedintoa springwithaleft-handedchirality,showinguptoacontractionof 59% whenactivated through thenarrowing to aliquid surfaceof dichloromethane(Fig.7BVII to7BIX).Finally,many3-levels struc-tureswere wovenina netthat,when sprayedwithethanol, was capableto liftaball 100times heavierthanthenet itselffor4.5 mmwithinmilliseconds.Forcomparison,asingle-plyhelicalfiber (SHF)actuatorwasproducedover-twisting(plying)directly20 lay-ers of MWCNT sheets [109]. Even if the generated stress, once activated with ethanol, was similar to the 3-level structure, the stressratewasthreetimeslowerandtheefficiencyoftheactuator markedlydecreasedafter15cyclesduetopartialuntwistingofthe coils.The rotation speedwas of760 rpm,with223 turns/mm of themusclelength.Theresponsivenessandthestrainratewere6.7 and11.3slowerthanthe3-levelsstructure.Thesedifferenceswere explainedwiththehighercapacityofthesolventtoinfiltrateinthe microscalegapsinthe3-levelsstructure,whichwereabsentinthe SHFactuators.Bothtypesinsteadshowednanoscalegapsbetween CNTs [109].

Inalaterstudy,a2-plystructure(Fig.7CIandII)wasinﬁltrated byanorganicelectrolyte (TBA•PF6dissolved inpropylene carbon-ate) [12].When activated witha pulseof-3.5 V (20 s long), re-peatedevery 40s, theobtainedcontractionwas16.5%(Fig.7CIII) (maximumworkdensity=2.2KJ/Kg).Moreover,toavoidtheir im-mersionin an electrolytebath,two parallel identical2-plycoiled cathode-anode yarns were coatedwith a gel electrolyte made of TEA•BF4 inPVDF-co-HFP (Fig. 7CIVandV). Actuatingthis device, the maximum contraction (11.6%) was obtained with an applied stressrangingfrom12to30MPa.Themaximumworkdensitywas 1.12 kJ/kg, obtainedat 45MPa. Finally, to increase the hierarchi-cal complexity, an all-solid-state artiﬁcial muscle with a braided coreanodeanda braidedsheathcathode,both wovenusingfour strands ofthe 2-ply coiled yarns, wasfabricated (Fig. 7CVI) [12]. The anodeandcathodewere then embeddedin agel electrolyte (PVDF-co-HFPcontainingTEA_•BF4).Whendrivenbyasquarewave of 5V at 0.1Hz square wave, the muscle showed a 5% contrac-tionwithan applied stress of24MPa. Sincethe contractionwas partiallyinhibitedby thecoils contact,the tensilecontraction in-creasedwiththeloaduntilthecoilwere separated [12].Inorder toanalysethemechanicalbehaviourofthehierarchicalhelical ac-tuators,atheoreticalmodel,focusedontheCNTs,wasproposedin