advances in healing-on-demand polymers and polymer

7/25/2019 Advances in Healing-On-Demand Polymers and Polymer

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P. Zhang, G. Li / Progress in Polymer Science xxx (2016) xxxxxx 3

1. Introduction

With the popular use of polymer and polymer compos-itesinindustry,damageandfracturewithinthesematerialsare inevitable [15]. Both expected loading such as fatigueload, and incidental loading such as foreign object impact,canlead toserious shortening of service life.Various effortshavebeenmadeover thepastdecades to improve thedura-bility of polymer and polymer composites by designingnewmaterialsordevelopingcrack-healingtechniques.Bio-inspirationhasplayed animportantrole indevelopingnewcrack-healing strategies [6].

The history and evolution of bio-inspired materialsand damage-healing techniques have been examined in anumber of published reviews, patents, and books [650].Pioneer work mainly focused on the self-healing conceptin thermoplastic and thermoset polymers and/or polymercomposites through extrinsic or intrinsic resources, aswell as the design and generic principles for self-healingsystems. These systems include thermosetting polymers,thermoplastic polymers, composite materials, metallicsystems, ceramics and ceramic coatings [51], andconcrete[5254]. Self-healing materials by nanotechnology werealsodeveloped[5558]. There isnodoubt that thepreviousendeavors have greatly advanced understanding of theself-healingabilities of polymers andpolymer composites,or at least created new knowledge on what-to-do andwhat-not-to-do when designing a crack healing system.However, due to the fast development in this emergingfield, there is a need to review these new developmentswithin the framework of healing-on-demand polymersandpolymer composites.

Here, we define a healing-on-demand material as onewithinwhich a crack, due tomechanical damage or degra-dation, may be closedand healed in timeand in situ, whenthe crack is detected or sensed by internal or externalmeans.Inotherwords,whenneeded,ahealing-on-demandmaterial exhibits thecapability topermit crack closure andhealing under in-service conditions, and to recover func-tionalityusingintrinsic orextrinsicresources. In this sense,healing-on-demand doesnot necessarilymeancompletelyautonomous healing or self-healing. It means that withsome intervention such as bringing fracture surfaces incontact, heating, etc., healing canbe triggered andproceedwithout additional intervention. For example, fracturedpolymerpanelsmaynotbe able toheal themselveswithoutexternalhelptobringfracturesurfacesintocontact,regard-less of intrinsichealing (withouthealing agent)or extrinsichealing (with healing agent). Within this definition, somehealing schemes, usuallynot reviewed in-depthwithin theself-healing literature, belong to the broader frameworkof healing-on-demand. Actually, as indicated by Li [47],healing-on-demandmay bemoreappropriateindescribingin-service healable materials, because in real world struc-tures, for example a panel under fixed boundary conditionand/or under external loading, some help or intervention,althoughperhapsminimal, isalmostalwaysneededto healwide-opened cracks or large damage volumes.

Fig. 1 showsa concept of healing-on-demandmaterials,through which inspection and maintenance techniqueshave been developed to prolong service life of engineering

materials. We believe that there is a need for distinctionbetween materials scientists and engineers with respectto self-healing. Five stages of crack healing have beenproposed for healing through physical molecular entan-glement by Wool and OConnor [59]. They are (i) surfacerearrangement, (ii) surface approach, (iii) wetting, (iv)diffusion, and (v) randomization. For healing throughchemical bond interaction, there are four stages, whichare (i) surface rearrangement, (ii) surface approach,(iii) chemical reaction, and (iv) dynamic equilibrium. Ingeneral, materials scientists are mainly concerned withsurface rearrangement, wetting, diffusion or chemicalreaction, and randomization or dynamic equilibrium, i.e.,reestablishment of physical entanglements or chemicalbonds. The stage of surface approach does not causeenough attention. In the lab, fractured specimens are usu-ally brought into contact manually before healing starts.As indicated by Wool [7], and echoed by Binder [48] andLi [47], this manual operation represents the largest chal-lenge in the realworld applications. Fromthe point ofviewof engineers, one could not bring a fractured skin paneltogether by hand in a Boeing aircraft and may not bringa fractured specimen together manually if the boundaryof the specimen is fixed [47]. However, crack healingcannot occur without external help to bring the fracturesurfaces in contact. Therefore, engineering applicationsface additional challenge to self-healing materials. Webelieve that it is time to consider the practical constraintof how to bring the fracture surfaces in contact.

In this review, we discuss the potential challenges andopportunities from the point of view of engineering appli-cations. We focus mainly on the crack closing and healingprinciples for polymers and polymer composites thathave emerged over the past decade. Especially, healing-on-demand load-carrying polymer composites and shapememory polymer based crack healing structural compos-ites, which have not been a focus in previous reviews,are discussed in more detail and depth in this review.The close-then-heal strategy, which has been demon-strated by shape memory polymer matrix, shape memorypolymer fiber, and polymeric artificial muscle in healinglarge volume damage for functionality restorations, willbe reviewed. The associated healing theories and healingefficiency evaluationswill also be reviewed.

2. Healing of polymers

Polymers used in industrial applications are designedwith a specific service life. Loss of structural capacity orfunctionality can occur because of incidental damage ordegradation over time. It will be a tragedy if polymersfail within their designed lifespan. In order to maintaintheir service life even after structural damage, polymericmaterials have been designed to have the ability to healthemselves on-demand. The study of healing-on-demandpolymers, in general, is continuously progressing. Moreexciting discoveries in this area are expected as numerousstudies are evolving.

According to the published literatures, themethods fortriggeringhealing-on-demand in polymers include extrin-sic stimuli suchaspH [6064], salt [65], thermal treatment
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4 P. Zhang, G. Li / Progress in Polymer Science xxx (2016) xxxxxx

Fig. 1. Conceptual healing-on-demandin prolongingmaterial service life. As damage occurs, engineering polymers/polymercomposites suffer fromfunc-

tional andstructuraldegradationoverservicelife.Crackclosing andhealingare triggeredby theresponseto thelowerbound (function limit)of thefunction,i.e., on-demand, and thus recovers its functionality to initial status repeatedly for prolonging service life.

[6674], water [75,76], light [7783], sonication [84], andelectrical treatment [85]. The intrinsic healing mecha-nisms include labile bonds [8691], fusion [92], reversibledissociationassociation [93,94], hostguest interaction[95,96], metallo/ligand complexation [97], and dynamiccovalent bonds [98,99]. In some cases [62,6670,72,73,82,89], both extrinsic and intrinsic stimuli are involved.

Crack healing polymers have been well reported andsummarized by pioneer works; however, in this review,we will focus primarily on the crack approaching meth-ods as well as the damage healing mechanisms. As givenin Table 1, the healing-on-demand polymers publishedsince 2007 are summarized based on damage type, crackapproaching method, healing mechanism, healing mea-surement, efficiency, repeatability, healing condition, andhealing time. Not all reported researchworks are includedin the table.Weonly include those that have clear descrip-tions on crack approaching method and damage healingmechanism so that comparisons can be made. The crackmodes, crack approaching methods, and healing mecha-nismsunderon-demandconditionswillbediscussedbasedon the summarization in Table 1.

2.1. Crack initiation and surface approaching

As shown in Fig. 1, the need for healing is triggered bydamage or cracking. Cracking inmaterials can be initiatedby various internal and external means. Fig. 2 shows thetypesofcrackinitiationmodesbasedonthesummarizationin Table 1. For example, ballistic impact could create shearplug through thethickness if theimpact energyis sufficientor random cracks on the back face if the energy is insuffi-cient. The hole or cracks can be patched due to the elasticspring back such as ionomers [100104]. Razor cut couldinitiateadeepcutorevencutmaterialsintotwohalves.Thecrack is usually closed by bringing the two halves togethermanually [83,107,119]. Sawing is another type of cut sim-ilar to razor cut [100,101]. An accidental damage might

result in a random microcrack or structural-scale wide-opened crack depending on the impact energy or damagelocation [62]. There is no need to bring fractured surfacestogether for the scratch damage since the name impliesthat only scratch marks are created on polymer materialsurfaces. These scratches may close due to swelling whenbeing submersed into a solution, or close with the shapememory effect when exposed to triggering factors (e.g.,UV light, thermal treatment) [81,123125]. The polymermaterials under compression suffer from bucklingandcanultimately fracture. A crackmight propagate to a large sizeif a precrack is introduced to the body prior to buckling[71]. It can be closed through elastic recovery but if a frac-tureoccurs, it is a challenge toheal it. Tensile stretchmightresult in different damage modes [126]. The mode inves-tigated includes breaking the specimen into two halves[83,99,110]. The fracture surfaces were brought into con-tact to march toward each other manually prior to thehealing process. The crack initiated by bending can beclosedduetoelastic recovery; forwider cracks, they canbeclosedwith thehelp of compression topush thecracksintocontact after the removal of external bending load [67,68].

Althoughallthe cracksin Table1 arecreatedbyexternalmeans, they may not be predictable in real world struc-tures such as those under impact load; on the other hand,the crack propagation is somewhat predictable. This givesus an opportunity to close or fill in the cracks by bringingfracture surfaces into contact. Because manually bringingfracture surfaces into contact is very difficult in real worldload-bearing structures, the true challenge in self-healingof load carrying structures is how to bring the fracturedmaterials in contact, regardless of visible crack on the sur-face or nonvisible crack within the body of the material.

2.2. Crack healing mechanisms

For allpolymers, crack healing follows a two-stage pro-cess. On-demand fracture surface approaching is the first


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Fig. 2. Types of crackmodes initiatedby various loadings.

stage. In the second stage, healing through physical orchemical means is performed on the surfaces in proxim-ity or in-contact. The crack healing mechanisms have beenwell assessed by recent review papers and books on self-healingpolymers [21,47,50,127,128]. Toavoidoverlaps,wewillbriefly reviewandsummarizethehealingmechanismsbased on the categorization in Fig. 3. Themechanisms arecategorized into two groups, physical molecular interdif-fusion and chemical bond interaction, which is further

subdivided into covalent and non-covalent bond interac-tions, and intermolecular force.

2.2.1. Molecular interdiffusion

Polymers can regain their mechanical functionalitiesthrough physical interaction (e.g., molecular interdiffu-sion). Continued interdiffusion, randomization, and long-term relaxation of thepolymer chainsarerequired inorderto achieve optimal healing performance. There are several

Fig. 3. Types of crack healingmechanisms of smart polymers including physical interaction and chemical bond interactions.


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Fig. 4. Proceduresof preparing, cutting, healing, and stretching of the compact polyelectrolyte (all at roomtemperature). [65], Copyright 2014.Reproduced with permission from JohnWiley & Sons Inc.

factors that can affect the crack healing performance,such asmolecular reptation time, healing temperature, co-surfactant effect, andconcentration. For example, sodium-chloride (NaCl) has been used as an initiator for increasing

themobility of the chain segmentswith the aim atacceler-atingdamage healing process. In order tomake thehealingfaster and more complete, compact polyelectrolyte com-plexes (CoPECs)were prepared bymixingsolutions of PAAand PAH, and compacting by ultracentrifugation in thepresenceofNaCl [65,129]. ItshowedthattheCoPECshealedthe fracture surfaces when they were brought into con-tact for 15min in a salt concentration of 2.5M NaCl, aspresented in Fig. 4. The healing behaviors of the com-pounds indicate that the healed sample strength dependson the contact time and NaCl concentration.With increas-ing salt concentration, the mobility of the chain segmentsincreases; and the more the mobile segments, the faster

the chains diffuse and the faster the fracture surfaces heal.

2.2.2. Reversible covalent bond

Makingcovalent links reversible is oneof the strategiesto heal cracks and prolong service life of polymers. Thereversible bonds allow dynamic bond reactions betweencovalent links by maintaining constant the total numberof network links and average functionality of polymers.Some chemical covalent bond reactions for crack heal-ing applications are presented in Table 2. The reversiblecovalent bonds have thepotential to heal damage-inducedcracks inthermosetbyrestoringmechanicalfunctionalitiesunder on-demandthermal orwater stimuli [130132]. Forexample, Montarnal et al. reported a concept for healingcross-linkedpolymer network by dynamic bond exchangereactions [133]. A cross-linked sample broken into pieceswas reheated over 180 C and reprocessed in an injectionmolding machine. It was found that the initial geome-try and properties have been recovered due to the healedpolymer chains at a high temperature. Transesterificationreaction occurs between two -hydroxyl-esters, leadingto rearrangement of network topology while preservingthe total number of links and integrity of the networkfunctionality.

In contrast to the dynamic bond exchange, reversibleDielsAlder (rDA) and hetero DielsAlder (HAD) reac-tions could make the cross-linked polymer network

atemperature-sensitivepolymerization-depolymerizationequilibrium[136138]. Forexample,Zhangetal.reportedathermallyhealing-on-demand thermosetpolymer in orderto resolve the issue on recycling of thermosetting mate-

rials at the end of life cycle [68]. The thermoset polymerwas prepared by the Paal-Knorr reaction of the PK withfurfurylamine, where the PK was used as precursor forDA reactions.When the fractured samplewas heated over110 C, which was above its glass-transition temperature,the thermoset polymer became soft because of the open-ing of the DA adduct, then reactions occurred between thePK-furanandbis-maleimide, leading to regenerationof theDAadduct. Upon cooling, thesample recovered itsoriginalshape, whichwas repeatable without any loss inmechan-ical properties. Theuniqueness in this healing-on-demandbehavior persists in its ultrafast healing response, such as5minuponheating (among110150 C)duringthehealing

event.Repeatable crack healing through photostimuli and

simultaneously macroscopic fusion of separate piecescan be achieved by photo-reversible reshuffling reac-tion [79,82,83,135]. For example, Ling et al. reported amendomer synthesized by cross-linked polyurethanecontaining dihydroxyl coumarin derivative 5,7-bis(2-hydroxyethoxy)-4-methylcoumarin (DHEOMC). Uponphoto stimulation, the fracture surfaces were successfullyhealed within two hours. The dynamically reversibleC ON bond and S S bond show an interesting behavior offrequent cleaving but immediate rebonding when undercertain hemolysis temperatures [71]. Upon the thermal

treatment at 130 C, covalent C ON bonds fission andradical recombination synchronously took place amongalkoxyamine moieties. Eventually, the fracture surfacescould be completely healed after 2.5h. It was pointedout that the alkoxyamine could be used for healingstructural applications on-demand based on dynamicallyreversible C ON bonds nomatter they appear in linear orcross-linked polymers.

2.2.3. Noncovalent bond interaction

As classified in Fig. 3, the noncovalent bond interac-tion includes reversible non-covalent bonds (ionic bond[100,101], metallic bond [66,74,77]), intermolecular force(e.g., hydrogen bond [139,140], and Van der Waals bond


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Table 2

Chemical covalentbond reactions for crack healing application.

Entry Covalent bond Molecular chain reactions Ref.

1 Dynamic bond exchange [133]

2 Reversible DielsAlder reaction [134]

3 Hetero DielsAlder reaction [116]

4 Reversible C ON bonds [71]


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Table 2 (Continued)

Entry Covalent bond Molecular chain reactions Ref.

5 Photo-reversible r eshufflingreaction [135]

6 Disulfide interchange reaction [72]

[141]). In terms of pure ionomer of EMAA or in the formof ionomer blends of EMAA and functionalized elastomersENR and PISP, the healing of puncture-induced damageis due to the ionic bond interactions in their molecularstructure. Below theorder-disorder transition (Ti) temper-ature, ionomersaresolid.Whileupon temperature change,the structure of ionomer rearranges itself over time dueto the ionic interactions or aggregations. Metallic bond inpolymer is designed for the crack healing applications inelectric field, magnetic field, or optical field. One exam-ple is that, due to the endowed conductive properties, thestructure status of the polymer can be monitored real-time through its electrical feedback. The structure statusincludesmicrocrackinitiation,stresshistory,andsoon.Theunique features can be lost if used improperly, like inter-nal damage. An organometallic polymer was developedby compoundingbetween N-heterocyclic carbenes (NHCs)and transition metals at molecular level [66]. As shownin Fig. 5a, the chemical dynamic equilibrium betweenmolecules, like amonomer species (1)andanorganometal-lic polymer (2), is controlled by an external stimulus. Thesynthesis of sucha polymer was a challenge aspointed out

in their work because of the requirement for the synthesisof appropriately functionalizedmultitopic NHCspoised forpolymerization. Owing to the transition metals and struc-turally dynamic equilibrium, the synthetic organometallicpolymer was an electrical conductive and crack-healingmaterial.Thehealing-on-demandmechanismiselucidatedin Fig. 5b. The created microcrack results in inherent elec-trical resistance change (i.e., high-resistance/low-current).As a result, the voltage bias generates localized heat atthe microcrack site. In turn, local heat overcomes the frac-ture surface kinetic barriers, leading to reformation ofthe broken NHCs-metal bonds. Consequently, the systemis electrically driven back to its original state (i.e., low-resistance/high-current) and themicrocrack is healed.

A group of polymers perform their crack healingvia intermolecular force, such as hostguest interaction[85,142], dynamic polar group [70], weak hydrogen bondof stacking [141], hydrogen bond through dynamicolefin metathesis [122], and hydrogen bond interac-tion between terminal-carboxyl groups [63], UPy groups[90,139], PA-amidegroups [109,111], supramolecularclus-ters [112], and hydroxyl groups [140]. The on-demand


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Fig. 5. (a) Compounds formed between NHCs and transition metals in polymeric materials. The structurally dynamic equilibrium between a monomerspecies (left)andan organometallic polymer(right)is controlledviaa thermaltreatment.M =Ni,Pd andPt; R=alkyl,benzyl andaryl. (b)Self-healingschemeofan electrically conductive, self-healing organometallic polymer.When used as an electrical wire or incorporated into a device, upon the formation of amicrocrack, the total number of electronpercolation pathwayswithin thematerial should decrease, resulting in increase in inherent electrical resistance,which leads to the generation of heat localizedat themicrocrack due to the voltage bias. Thegenerated thermal energythus overcomesthe kinetic energybarriers, driving the system back to itsoriginal state. A: amperes; V: volts. [66], Copyright 2007.Reproduced with permission from theRoyal Society of Chemistry.

conditions for driving crack healing can be pH aque-ous solution, water, heat treatment, UV light, electricalfield, and magnetic field. For example, the acryloyl-6-aminocaproic acid hydrogel exhibited repeatable damage

healing ability at low pH, at which the terminal-carboxylgroupswere protonated to generate hydrogen bonds withother terminal-carboxyl groupsor amidegroupsacrosstheinterface [63]. Fig. 6 shows the performance of hydrogel

Fig. 6. Wound healing behavior of polymeric hydrogel. At low pH (i.e., less than 3.0), molecular network was regenerated at the fracture surface dueto the protonation of terminal-carboxyl groups, leading to the recovery of fracture surface. However, in the case of high pH (i.e., larger than 9.0), the two

healed hydrogels separateddue to thedeprotonation of hydrogel carboxyl groups. And it rehealed againwhen exposed to lowpH. [64], Copyright 2013.Reproducedwithpermission fromthe AmericanChemical Society.


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healing via change in pH. The weld of the fracture surfacetook only two seconds, but it took 24h for complete crackhealing. Another pH-responsive repeatable healing-on-demandhydrogelwas investigatedbyKrogsgaard et al., byincorporating additional features of mussel adhesive pro-teins [64]. The 3,4-dihydroxyphenylalanine was attachedto amine-functionalized polymeric hydrogel. The healingwas a resultof theformationofnetworks through thereac-tionwith ion atpHof8.0. The healing eventwas completedwithin 45min.

3. Healing of polymer composites

The repair of damage-induced crack in polymer com-posites has been well discussed, including both intrinsicand extrinsic strategies [6,8,12,14,15,20,47,127,128,143,144]. Like inSection2, herewefocusonseveral specificitiessuch as composite type, damage type, crack approachingmethod, healing mechanism, healing measurement, heal-ingefficiency, repeatability, healing condition and time, assummarized in Table 3. The healing-on-demand polymercomposites can be categorized into five types, which willbe discussed in the following subsections.

3.1. Capsulated polymer composites

These polymer composites incorporate capsules ofvarious sizes uniformly distributed within the matrixmaterials. Based on the size, capsules can be categorizedintomicro size [145150] andnanosize [57,151,152]. Theyare filled with liquid healant or liquid crosslinking harde-ners (e.g., catalyst). Healant can be a crack healing agent,which is used for mechanical property restoration; or canbe a function restoration agent for conductivity restora-tion [153155], anticorrosion [156], and water resistance[157]. Upon fatigue loading or incidental loading, the cap-sules are ruptured by propagating micro-cracks in thematrix. The loaded healant is released from the capsules,filling in the propagated micro-cracks via capillary action,and comes into contact with crosslinking hardeners; orreleased liquid crosslinking hardener comes into contactwith embedded healant to initiate polymerization at themicro-cracksite, leading to healing-on-demand healing, inautonomous manner. As shown in Fig. 7, crack healing bycapsules includes bothsingle capsulatedpolymer compos-ites and dual-capsulated polymer composites. In Fig. 7(a),thequestionmark indicates that thecapsule canbefilledwith either liquid healant [158165] or crosslinking agent[166,167], dependingon thedesign of thematerial system.Fig. 7(b) indicates that two types of capsules areembeddedin the polymer matrix, which are encapsulated with heal-ingagentand crosslinkinghardener individually [168,169],or healing agent and liquid initiator individually [170].

Sinceamicrocapsulecrackhealingsystemwiththe abil-ity to heal cracks for mechanical property restoration wasreportedonstructural polymer composites[171],microen-capsulation hasmotivated studies frommultiple academicdisciplines to a wide variety of industrial applications.Healant was encapsulated in a microcapsule and the cata-lyst waswell dispersedwithin thecompositematrix in thefirst reported single-microcapsulepolymer composite.The

catalyst deactivation, aswell as thepotentialside reactionsbetween catalyst andpolymer matrix after embedding thecatalysts in the matrix, was a major challenge facing thistype of healing-on-demand composites. Faced with thesechallenges, two solutions have been proposed: (1) no cat-alyst and (2) catalyst protection.

Yang et al. reported a catalyst-free crack healingpolymer composites by incorporating IPDI-loaded micro-capsules [159]. Thecrackhealingbehaviorwastriggeredbyhumid orwet environment, inwhich the ruptured healantIPDIwas polymerized. Photo-induced healing through theuse of ruptured healant methacryloxypropyl-terminatedPDMSwas reported by Songs group,who prepared a poly-mer compositewith crackhealing abilitywithinfour hoursupon exposure to sunlight [190]. The advantage of thissystem persists in its repeatability in damage healing dueto the reversible polymerization between the healant andhostmatrix.

The agent for catalyst protection could be wax if solidcatalyst is used [176178,183,245], and polyester shell orglass shell if liquid catalyst is used [191,192,195]. Sinceboth healant and catalyst are capsulized, it is called dual-microcapsule polymer composite. Obvious advantages ofthis systemare: (1)better thermal tolerance; (2)better cor-rosionresistance if shelled byglass;(3)fast polymerizationspeed (i.e., fast crackhealing rate)dueto two liquidphases.

3.2. Hollow-fiber reinforced polymer composites

Crack healing in hollow-fiber reinforced polymer com-posites was investigated through (1) one-part liquidhealing agent, (2) two-part liquid healing agent and liq-uid hardener, and (3) two-part liquid healing agent andencapsulated catalyst [208].

The hollow glass fiber (HGF) used in this system is anideal medium for storing healing agents due to its goodmechanical properties as structural reinforcement, whichprovides impact protection to some extent. Microcrackdamage within structural polymer composites leads tofracture of the hollow glass fiber, which releases liquidagents tofill in themicrocracksby capillaryactionandhealthem through healing agent polymerization. Compared tothe HGF preparation technique, the preparation of hollowpolymer fiber (HPF) is much more effective. The healingagents or catalyst-solvent were filled into polymer fibersduring the processing of hollow polymer fiber, forming acore-shell bead-on-string morphology. The advantage ofthis technique is the capability in controlling thediameterof HPF frommicro- to nanometer scales [246]. In contrastto the functionalized capsule-based coatings, the HPF-based healing-on-demand coatings could solve a numberof issues, withchemical incompatibilities between variousmatrices andhealing agentsassociatedwith thedispersingcapsules in thematrix precursors.

3.3. Vascular networks based healing-on-demand

polymer composites

The microcapsule and hollow fiber serve the functionof liquid storage and delivery. By implementing vascularnetworks, the healant from a reservoir is delivered to a


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Fig. 7. Microcapsulated polymer composites: (a) single-microcapsule; and (b) dualmicrocapsule.

damage site via a network of vessels. The advantage ofvascular basedhealing-on-demand polymer composites isthat it provides thehealing ability forlargedamagevolumeand multiple healing events as well as the replenishmentof the healant. The branchingmacrovascular network wasinspired by human circulatory system [226] and plantvasculature system [247,248], as shown in Fig. 8. This

branchingnetworkistoavoidthelossofhealingrepeatabil-ity due to bleeding clots. Additionally, 3D vascular systemgives higher potential for fast healing response [230].

The healant or crosslinking hardener was protected bya shell from directly contacting the host matrix in themicrocapsule system as well as the hollow fiber system.However, in the vascular network system, the healant orcrosslinkinghardenerwasin directcontactwith thematrixwhile in delivery. This is one of the concerns when imple-menting vascular network for crack healing purposes. Inthe case of large volume damage, it requires overcomingthe interplay between mass transport, environmental fac-tors, intrinsic forces(suchas surface tension),andextrinsic

forces (such as gravity) that act on the liquid reagents[222,223]. How to heal large-scale or wide-opened crackwithout mass loss is a challenge. To date, there are twoapproaches to deal with the challenge. One is the close-then-heal (CTH) method proposed by Lis group (to bediscussed in the next section), and the other is the methodproposed byWhites group [222]. By using a vascular sys-temof microchannels,Whiteet al.proposedtwostages: (1)gel-stage (liquid togel) toplug theholequickly andpreventbleed-out of the healing agent; (2) polymerization-stage(gel to polymer) to undergo polymerization of the healing

agent and heal the large volume crack. By evaluating theimpact damage on the healed sample, about 62% of theimpact energy (on a time scale of 3h room temperaturecuring) was recovered. The challenge is that the surfacetension would become insufficient to retain theunreactedfluid when the damage size exceeds a certain threshold,leading to bleeding-out of healant.

3.4. Healing in layer-by-layer coating/film

A skinmaterialwithflexibility, healing-on-demand anddamage sensing was studied and fabricated by using alayer-by-layer technique with copperclad imidized poly-imide sheets and Loctite Impruv 365 UV-curable epoxy,which was used as structural adhesive as well as crack-healing fill material [232]. After an intentional puncture,the damaged skin can heal itself through the flow of UV-curable epoxy under UV exposure in a matter of minutes.This fast healing behavior is one of the advantages exhib-ited by healing-on-demand skin.

Andreeva et al. designed a healing-on-demand anti-corrosion coating system on aluminum alloy basedon pH-sensitive polyelectrolyte/inhibitor sandwich-likenanostructures [249]. ThePEI, PSSand8-hydroxyquinolinenanolayers were deposited on the pretreated aluminumalloy under the layer-by-layer deposition procedures. Thenanometer-thick novel coating protected the aluminumalloy effectively from corrosion. The healing behavior ofthe coating prevented the propagation of corrosion onmetal surfaces based on the suppression of accompa-nying physico-chemical reactions. The mechanisms for

Fig. 8. Branchingvasculature networks in animal (e.g., human) body andplant.


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healing-on-demand of the corrosion damaged areasincluded pH neutralization, passivation by inhibitor, andhealing by mobile polymer layers. Once the corrosionoccurred, the inhibitor was released to the damage site onthesubstratesurfacetopreventadsorptionofchlorideions.Due to thechanges of localpH,thepolyelectrolytecomplexwas swollen andpolymer chainsdiffused through the rup-ture surfaces. The gradual evolution of ionic bonds resultsin damage healing from the healing-on-demand coatingsystem.

Water-induced healing syntheticmaterials areused forapplications from food packaging to sub-water coating[250,251]. A healing-on-demand polyvinyl based coatingblend was reported by Ensslin et al., who used the blendsofPVAcandPVA-PEGinthefieldofpelletcoating [252]. ThePVAc/PVA-PEG coating in the damage site started to swellunder theexposuretowater, leading to theclosingofholes,craters, andclefts, whichpreventa burst release. However,the crack-healing efficiency needs further investigation.

3.5. Solid-state healant embedded in polymer composites

Theintrinsicself-healingpolymers described in Section2 could be fabricated into small pellets anddispersedinto apolymer composite matrix as a healant. In this system thecrack is healedonce theembeddedhealableparticlesare incontact, through molecular interdiffusion [253], dynamiccovalent bonding [254256], non-covalent bonding [239],intermolecular force [257,258], and so on. However, crackapproaching is a prerequisite and is a challenge when thecrack opening iswide.

4. Close-then-heal strategy for polymer composites

From the above reviews, it is seen that some healingschemes need external help, i.e., bringing fracture surfacesinto contact manually before healing occurs. While thisis legitimate in lab scale specimens, it represents one ofthe greatest challenges in real world structures. This isbecause in large scale structures, fractured structural ele-ments cannot be brought in contact manually. If they areforced together, newdamages may form [47].

It is a straightforward idea to utilize the shape mem-ory effect for crack closure because cracks can be treatedas a type of reversible plastic deformation, and the shapememory effect can restore the original shape upon exter-nalstimuli.However, theability to close cracksdepends on(1) the level of programming and (2) the constraint duringshape recovery [47]. For some shape memory effect basedapplications such as coatings, the system uses scratching,indentation, or cracking itself as programming. Clearly, ifthere is no significant barrier to resist the shape recovery,suchas a free-standing specimenorpanel, the crack can beclosedif theshaperecovery ratio is close to100%.However,if there is a significant barrier to resist free shape recovery,such as a specimen or panel with fixed boundary or underexternal tensile load, thecrack cannotbe closed. Therefore,energy storage by programming is usually required and isa betterway for crack closure as it can bedesigned to con-sider the level of barrier to resist shape recovery and theshape recovery ratio [47].

In their review paper, Hu et al. clearly defined twotypes of crack healing schemes based on shape memoryeffect [259]. One is free shape recovery and the other isconstrained shape recovery. Most recently, Yougoubareand Pang compared the two popular shape memory effectbased crack closure schemes [260]. One is shape memoryassisted self-healing(SMASH) proposedbyRodriguezet al.[261] and Luo and Mather [262], and the other is close-then-heal (CTH, firstly close the crack through confinedexpansion of the SMP matrix, and then heal it by embed-ded thermoplastic particles), which was proposed by Liet al. [242,244] based on their previous test results of heal-able SMP syntactic foam [263]. According to YougoubareandPang, thefundamental difference between SMASHandCTH is that SMASH targets non-load carrying materials,uses no constraint during shape recovery, does not con-duct programming, and is usually suitable formicrocracksand small indentations; on the other hand, CTH focuses onload carryingmaterial, considers constrained shape recov-ery, needs programming depending on the width of crackto be closed, and is suitable for wide-opened crack andlarge indentation. Itmust be emphasized that shapemem-ory effect does not necessarily heal the material. It onlynarrows or closes the crack. In order to heal the crack, itmust be combinedwith other physical or chemical healingschemes such as shapememory polymer with self-healingcapability ora combinationofshapememorypolymerwithexternalhealingagent[47]. Inthefollowing,wewillreviewthe shape memory effect based self-healing schemes, par-ticularly SMASHandCTH.After thediscussionon thetopicsof SMASH and CTH, the crack healing mechanisms, withinthehealing-on-demandpolymermaterialsframework,willbe covered via healing theories.

4.1. Shape memory assisted crack healing

Over the past decade, shape memory materials havebeen used to improve the healing-on-demand processby providing functionality to partially or fully closecracks. A poly(-caprolactone) (PCL) based compositesystem is one of the examples used to explain thisapproach [261]. The advantages of shape memory poly-mer materials are the ability to sustain high strain (upto 500800%), low response temperature, tunable elasticmodulus, and low density [264]. Rodriguez et al. reportedSMASH of polyurethane blends with varying composi-tions. The blends were synthesized via the incorporationof a covalently cross-linked network by end-linking end-functionalized n-PCL as a thermoset for shape-memoryand a linear l-PCL thermoplastic for healing. Upon the on-demand thermal treatment at 80 C, the fractured surfaceswere driven back for contact by the n-PCL shape-memoryability; and then, surface wetting and chain diffusion byl-PCL occurred at the crack surface. The equilibrium andrandomization of PCL networks were achieved over timeand thus the fracture surface was healed, as shown inFig. 9. This damage-healing application overcomes theabove mentioned limitation, i.e., crack closure withoutmanually pushing the fractured parts together.

Luo and Mather proposed a new SMASH strategy tail-ored for coating/corrosion-inhibition applications [262].


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Fig. 9. Crack closure due to the n-PCL shape-memory ability along with temperature and spontaneously crack rebonding due to themelt andmoleculardiffusionof linear l-PCL at thecrack site (stereo micrographs scale bar: 500m). [261], Copyright 2011.Reproducedwithpermission fromthe AmericanChemical Society.

Nonwoven nano- andmicro-PCLfiberswere distributed ina shape memory thermoset matrix by an electrospinningprocess. Both fibers and matrix served as coating agentson a steel substrate. The randomly oriented and evenlydistributed PCL fibers can heal larger cracks and defectsdue to the more significant flow of the liquefied PCL fiberscompared to the previous SMASH example. The workingprincipals during the healing-on-demand process includetwo steps, which take place simultaneously: (1) crack clo-sure due to the shapememorybehavior of the SMPmatrix,which releases the stored strain energy in the plastic zone

after thermal treatment, and (2) crack rebinding (i.e., heal-ing) due to the melting and flow of the PCL fibers. It wasreported that thecrack-healing performance could be con-ducted by heating at 80 C for 10min, leading to almostcompletely restored corrosion resistance.

4.2. Close-then-heal

It is emphasized here that, the CTH strategy is differ-ent from SMASH. The healing-on-demand composite byCTH approach, as proposed, undergoes a process of crackclosure by constrained shape recovery, followed by crackhealing through healing agents. As compared to SMASH,theboundaryconditionofthespecimensorstructural com-ponents per the CTH schemedoes not need to be free, andthe shape recovery ratio does not need to be 100%. There-fore, the CTH scheme may be appropriate for real worldload-bearing structures because these structures are gen-erally constrained at the boundary and/or under externaltensile loading during the healing process, and the shapememory capacity of SMPs degrades with time.

It is worth mentioning that CTH is in line with thewidely accepted five-step healing theory proposed byWool and OConnor [59]. The first two-steps: (i) surfacerearrangement, and (ii) surface approach, correspond toCloseinCTH,andthelastthreesteps:(iii)wetting,(iv)dif-fusion, and(v)randomization, correspond to heal inCTH.

A concern with CTHmay be that one needs todesignormanually provide the external confinement. Actually, theexternal confinement or constraint needed in CTH is pro-vided naturally by thematerials and structures. AccordingtoCTH,onlylocalorinsituheatingsurroundingthecrackina specimen or panel is needed to close the crack and healit. The rest of the specimen or panel is still cold, whichprovides the external constraint. Furthermore, constraintcanbe provided by thearchitectural configurationof com-posite structures, such as sandwich face sheets [263], 3-Dwoven fabric [265], grid skeleton [266], etc.

Another concern with crack closure by shape recoveryis that the SMP could be very soft at high temperatures.

In several experimental studies, the healing temperatureis slightly above the glass transition temperature; how-ever, it canbewithintheglass transition temperature zone[263,265,266], aslongasonecanfindahealingagentwhichmelts and bonds at that healing temperature. When thehealing temperature is slightlyhigherthan theglasstransi-tiontemperaturebutstillwithintheglasstransitionzoneorslightlyabove thezone, theSMP isstill stiff enough.Also, asindicatedin severalpreviousstudies,oneonlyneedstoheatup locally surrounding thecrack according toCTH. Therestof the structure is still cold. Therefore, the overall struc-

ture is still very stiff[47,267]. Furthermore, research hasproved that time-temperature equivalent principle holdsforshaperecovery,whichsuggests that, as long as theheal-ing temperature is within the glass transition zone, theshape recovery can occur even at a temperature slightlybelowthe glass transitiontemperature [268].Whatneededisa longerhealing time.Again,as longas onecanfinda suit-able healing agent, CTH can proceed when the SMP is stillconsiderably stiff.

Currently, the CTH scheme has evolved into threesub-systems, i.e., SMP as a matrix, SMP as a dispersedreinforcing phase, and polymer artificial muscle as aninherent actuator. In other words, the crack closing can be

driven either by a compression programmed shape mem-orypolymermatrixor tension programmed shapememoryfibers or artificialmuscles. The followingthree subsectionswill discuss in detail how the CTH scheme works in thesethree sub-systems.

4.2.1. Shape memory polymer asmatrix

Shapememorypolymers are trained by a programmingprocess, which is necessary to create a nonequilibriumconfiguration and enables them to have shape recoverycapability [268276]. The driving force for shape recoveryis the conformational entropy of the molecular segmentsin terms ofmicro-Brownianthermalmotion. Upon heatingto the polymer glass transition temperature (Tg) for cross-linked polymer network or melting temperature (Tm) forsemi-crystallinepolymer, themolecularmobility increasesandtheorientation ofmolecularchains tend to be random,accompaniedbyan increase intheconformationalentropy.The increase in entropy creates the driving force for shaperecovery. It is an autonomous process for molecules torecover from nonequilibrium to equilibrium states.

Li and John developed a new shape memory poly-mer based syntactic foam and foam cored sandwichcomposite for the purpose of repeatedly healing impactdamage [263]. The syntactic foam was prepared by dis-persing 40% by volume of glass microballoons into a shapememory polystyrene matrix. The foam cored composite


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Fig. 10. Schematic of theclose thenheal (CTH) schemeof theproposedsmart foam. Compressionprogrammingprocess is requiredandnecessaryto enablethe smartfoam tohaveshapememory ability.Crackis narrowedorclosedat a temperatureabovetheTgsoftheSMPbased foamby the shaperecoveryof thefoam, whichis confined by surroundingmaterials. Narrowed crack is filled in bymolten thermoplastic at a temperature above theTgpof the thermoplasticparticle. Fracture surface would be bonded together after cooling down. Programming process: from (a) to (c); damage and crack narrowing process: (d)and (e); crackhealing process: from (f)to (h). Here theTgpof thermoplastic particlemust be higherthan theTgsof SMP basedfoam. [244], Copyright2010.Reproducedwithpermission fromElsevier.

sandwich plateswere fabricated by vacuumassisted resininfusion molding (VARIM) technology. In order to haveshape memory effect, the composite was programmedby a typical three-step programming process. In step 1,pre-deformation was initiated in a rubbery state at a tem-perature above the glass transition temperature of theshape memory polymer. In step 2, strain storage processwas conducted by maintaining the pre-deformation con-stantwhilecoolingdowntobelowTg.Instep3,theloadwasremoved at the temperature lower than Tg. This completesthe typical hot programming process (at a temperatureabovetheglass transition temperature). It isnotedthatpro-gramming does not necessarily need a temperature event.Cold programmingata temperaturebelowtheglass transi-tionzone(inglassystate)alsoworksaslongastheprestrainlevel is beyond the yielding point of the shape mem-ory polymer [268,270]. It is noted that, the compressionprogramming process was integrated with the fabricationprocess of the sandwich panel per the VARIM technologyin Li and Johns study. After impact damage, the damage-healing of the shape memory polymer based compositewas due to constrained shape recovery of the compres-sion programmed foam core. The partial confinement tothe damaged foam core was provided by materials sur-rounding thecrack inthein-planedirectionandbytheskinin the transverse direction during the shape recovery pro-cess. In this study,becausethecompositewasprogrammedby transverse compression, it tended to have a volumegrowth during shape recovery. Because of the constraint,such a tendency was not allowed. As a result, the foamwas pushed into any internal open space, leading to crack

narrowing or closing. This fast response healing-on-demand shape memory polymer based composite couldtake up to seven damage and healing cycles. In order toprovide better constraints during shape recovery process,a new architectural design was proposed, which led toinherent constraint to resist volume growth in the shapememory polymer matrix. The developed new compositearchitecture design included 3D woven fabric reinforcedshapememory polymer composite [265] andgrid stiffenedcomposite sandwich structure [266]. The 3D woven fab-ric or the fiber reinforced grid skeleton provides strongconstraint to the SMP foam core, in addition to its rein-forcement effect.

While the previous studies byLi et al. haveutilized con-strained shape recovery for closing impact induced cracks,the molecular length scale healing was not conducted. Liand Nettles firstly proposed a two-step healing scheme ofclose-then-heal (CTH), similar to the biological healing ofwounds in thehuman skin, formolecularly andrepeatedlyhealing structural length scale cracks [242]. This schemewas further elaborated by Li and Uppu [244]. Fig. 10 showsthe schematic of the CTH scheme of the proposed syntac-tic foam. The SMP based structure obtains its permanentshape first. Afterwards, the structure is programmed at atemperature above Tgs but bellow Tgp by compression toreduce structure volume. A temporary shape of the struc-tureisachievedforworkingaftercoolingdownbelowTgs. Ifsome internaldamagesuch asmatrixcracking is formedbyservice loading during lifetime, thematerial can be heatedabove Tgs and the material tends to expand in volume(shape memory effect). Due to the external confinement,


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however, the expansion of the structure is resisted; as aresult, the SMPmatrix is pushed toward any internal openspace, leading to narrowing or closure of the cracks. Theincorporated thermoplastic particles are then heated tomelt andthemoltenthermoplasticflows into thenarrowedcrackspacebycapillaryforce.The thermoplasticmoleculesthen diffuse into the fractured SMP matrix and establishphysical entanglementwith theSMPmolecules, which aredrivenby concentration gradient andshape recovery pres-sure at a temperature above Tgp. After cooling below Tgs,a solid thermoplastic wedge is formed, which glues thefracture surfaces together and prolongs service lifetime.Li and Uppu indicated that each constrained shape recov-ery (crack closing) represents a newround of compressionprogramming. Also, the thermoplastic healing agent canbe melted and solidified quite a few times. Therefore, thehealing is repeatable and only one time programming isneededbefore service.Another featureofthis systemis thatit depends on physical change only. No chemical reactionis involved in the system, which ensures repeatability. Itis noted that, while CTH is conceptually divided into twosteps, it is actually one step in practice, i.e., heating up allthe way to the bonding temperature of the thermoplastichealing agent.

In order to validate the CTH scheme, a particulate com-posite with shape memory polystyrene matrix dispersedwith copolyester healing agent was prepared and tested.By studying the copolyester-polystyrene shape memorypolymer composite and the SMP based syntactic foam, itis shown that the close-then-heal scheme works, whichleads to healing of structural-length scale damage repeat-edly, efficiently, molecularly, and timely [243,244,265]. Aspointed out by Nji and Li, the combination of thermoplas-tic particles and close-then-healing healing process wasable to heal wide-opened cracks with a small amount ofhealing agent (as low as 3% by volume) [329]. The do-it-yourself manner [277] in crack closing and healing inpolymer matrix has been applied to various applicationsincluding syntactic foam [278281], sealant [282284],fiber reinforced polymer composites [285], and healablecomposite joint [286].

4.2.2. Shape memory fibers as dispersed suture

While healing of cracks in SMP matrix has been suc-cessful, the challenge is howto heal cracks in conventionalthermosetting polymerswhichdonothave shapememorycapability.Onewaytodothisistoaddshapememoryfibersto the matrix, similar to embedded sutures when doctorsstitch wounds in human skin. For shape memory fibersbased polymer composites, the self-healing mechanismis similar to the two-step close-then-heal (CTH) scheme.The created matrix crack is narrowed or closed by embed-ded pre-tension programmed shape memory fibers firstthrough constrained shrinkage, followed by crack healingby healing agents, like liquid healing agent/hardener ormolten thermoplastic [47]. From recent publications, theshape memory fibers embedded in polymer compositesinclude shape memory alloy (SMA) wires and shape mem-ory polymer (SMP) fibers. This approach, utilizing shapememory alloy wires or shape memory polymer fibers topull cracked surfaces closer by thermal-activated shape

recovery forces, was mainly studied by Whites group,Huangs group, and Lis group.

The difference between shape memory polymer mate-rial basedandshapememoryalloymaterial basedpolymercomposites is the level of cracks that can be closed. SMAwiresare featured as having large recovery force but smallrecovery strain, which is opposite to SMP fibers.

SMA has the capability to memorize its original shapeafter a large pseudoplastic deformation when subjectedto external force [287291]. It tends to contract if it isheated above the austenite transformation temperature.Such shape recovery behavior of SMA has been appliedto intrigue crack narrowing in the self-healing applica-tions [292297]. Kirkby et al. investigated the influenceof SMA wires on the self-healing properties by combin-ing SMA wires with a self-healing polymer [292,293]. Intheir study, the microcapsulated liquid healing agent andwax-protected Grubbs catalyst microspheres were mixedand embedded in the polymermatrix. The SMAwireswereembedded in the polymer composite as well. After a crackwas openedin a tapered doublecantilever beam specimen,theSMAwiresexhibited shapememoryfunctionality,lead-ing to the crack closure by passing a current through thewire. The 0.5A current provided to each SMA wire gen-erated heat and increased the temperature to above 80C,whichin turn improvedthedegree ofpolymerizationof thehealing agent during the healing process. The healing-on-demand composite showed that significant improvementshave been achieved in the polymer composite by incor-porating SMA wires that bridge over the opened cracks.Furthermore, SMA spring was embedded in a cylindricalsilicone melting glue (SMG) sample to pull fracture sur-faces into contact [275]. As comparedwith SMA wires, theadvantage ofSMAspringis theefficientstress transfer fromshape recovery and larger recovery strain, under thermalstimulus. Hence, it generates high compressive force dur-ing the crack closing process, leading to fast and efficientcrack healing, which is about 5min to heal a cylindricalsample with a diameter of 7.56mm. Li indicated that theefficiency for a shape memory fiber to narrow or closecracks depends not only on recovery force, but also onrecovery strain, or on recovery energydensity [47]. He fur-ther indicated that, due to the very limited recovery strainof SMA wires, SMP fibers may be more appropriate forclosing cracks with wider opening. He also indicated that,in order to overcome the constraint during crack narrow-ing process, increasing the recovery stress of SMP fibers isneeded to further enhance the crack closing efficiency ofSMP fibers because SMPfibers have already had consider-able recovery strain.

The healing-on-demand polymer composite by SMPfibers was firstly reported by Li and Shojaei [298], whoincorporated pre-stretched shape memory polyurethane(SMPU)fibers inpolymer compositeasa grid skeleton (ribsand z-pins). Macroscopic crack was introduced in a localsite on the composite.When local heating was applied, thecrackwas closedor narrowedas a result of the constrainedrecovery of the SMPU fiber ribs and z-pins (constrainedshrinkage). Following closure of the crack, the dispersedthermoplastic particles were melted and flowed into thenarrowed or closed crack by capillary force, and diffused


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Fig. 11. Two-step healing-on-demand composite by continuous shape memory polyurethane fibers. After the macroscopic crack was created in thecomposite, themacrocrack was closed firstly by the stimuli-responsive contraction of SMPU fibers, followed by the second step when the thermoplasticparticlesmelt andflow into the narrowed crack to bond the crack. When the composite specimen was cooled down to room temperature (below Tg), itsservice strengthwas recovered. [299], Copyright 2012.Reproduced with permission from theRoyal Society of Chemistry.

into the fractured surface by concentration gradient andshape recover pressure.Bycooling down tobelowtheglasstransition temperature of the SMP fibers, the solid wedgewas formed, leading to crack healing. Fig. 11 shows thetwo-step self-healing behavior by continuousSMPUfibers,i.e., close-then-heal mechanism. Li et al. investigated thehealing-on-demand application further in polymer com-posite bypre-tensionedcontinuous SMPUfibers[267,299].It was shown that the cold-drawing programming pro-cess was necessary for healing efficiency enhancement. Itisnoted that theyusedfixedboundary condition in the on-demand healing test, i.e., free shape recovery of the beamspecimens was not allowed. This suggests that the SMPfiberbasedCTHschemecanbeusedinrealworldstructuresor structure components, which are usually constrained atthe boundary.

Unlike SMAwires, SMP fibers have low recovery force.In order to increase its recovery force, SMP fibers needto be heavily programmed. The reported technique forSMPfiberprogrammingis the cold-drawingprogramming,i.e., pre-tensioning at a temperature below the glass tran-sition zone. The programmed SMP fibers exhibit higherstress recovery than thenon-programmed ones [300]. Thestress recovery is necessary for crack closure; otherwisethe shape recovery would not be able to overcome thebarriers and to bring the fracture surfaces together intocontact. The greatest challenge in the use of SMP fibers isits structural relaxation after cold-drawing programming,whichpotentially hinders theutilization of shapememorypolyurethane fibers for healing-on-demand applications.Zhang and Li studied the structural relaxation behavior

of tension programmed SMPU fibers based on molecular-level force model, reptation model, and thermodynamicstheory [301]. They pointed outthat theprogrammed SMPUfibers canrecoverenoughforce forcrack closing even aftera time scale of 13 years of structure relaxation, which pro-vides a theoretical backgroundthattheprogrammedSMPUfibers,afteryearsof hibernation,stillpossessthecrackclos-ing ability when triggered by thermal stimulus.

Li andZhangextended theinvestigation on thehealing-on-demand polymer composite from the programmedcontinuous SMPU fiber to programmed short SMPU fiber[302]. Continuous SMPU fibers were programmed firstlyand then cut into short fibers. Short SMPUfibers and ther-moplastic particles were embedded into a conventionalthermosetpolymermatrix.Afterthree-pointbendingdam-age of notched beam specimens, the specimens wereheated to80C, followingtheCTHscheme, andresultingincrack healing. It again proved that this subsystem can healmillimeter-wide cracks repeatedly, efficiently, and timely.

4.2.3. Polymer artificialmuscle as inherent actuator

In all animals and humans, muscle is a soft tissue thatconstitutes of a part of the musculoskeletal system. Itis the only component of the system that enables ourbody tomove through contracting and even fast contract-ing at higher speed upon stimuli. Analogous to naturalmuscle, polymer artificial muscle could contract fast anddeliver large strokes from inexpensive high strength poly-mer fibers, such as commercial fishing lines [303305].Since repeatable contraction is based on anisotropic ther-mal expansion in fishing line axial and radial direction,


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Fig. 12. (a)Structureof naturemuscle. (b)Schematic of on-demandhealingprocess:(i) a polymercomposite sample reinforcedbypolymerartificialmuscle(light golden coiled fiber) and thermoplastic particle (light golden spheres) in a matrix (blue); (ii) crack initiated by external load during service life; (iii)crack closed by thermally activated artificialmuscle andhealed by thehealing agent; (iv) solid wedge formed after cooled down, establishing continuitybetween the healing agent andthe matrix. (c)Crackbeforeclosing and after healed. [307], Copyright 2015. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)Reproducedwithpermission fromElsevier.

it contracts even after many actuation events withoutconsidering its structural relaxation and chemical stabil-ity [306]. Structure relaxation has been a challenge topicfor shape memory materials when used for crack heal-ing applications at a long-term time scale. Therefore, itis worth studying polymeric artificialmuscles when usingthe CTH scheme in healing damage induced cracks. Zhangand Li have reported embedding a uniaxial polymer artifi-cial muscle into a thermoset polymer composite beam toclosewide-opened cracks [307]. Three-pointbendingdam-age to the notched beam specimens can be healed evenat a constrained boundary condition upon local heating,undergoing the close-then-heal procedure. The fracturedbeams were heated locally for 10min. The healing effi-ciency was investigated at both free boundary conditionand fixed boundary condition, by measuring fracture peakload. The fast contraction of artificial muscle brings thefractured surfaces in spatial proximity; following crackclosure, the molten healing agent fills in the crack viacapillaryactionandbonds the twofracture surfaces by dif-fusion and randomization. Fig. 12(a) shows an illustrationof natural muscle contraction. Physically, the contractionofmuscle generatestension onboth connections.Fig. 12(b)schematically presents the crack closing or narrowingdueto on-demand artificial muscle contraction and healing ofthehealing-on-demandpolymer composite.With60%pre-strainof thereinforcingpolymer artificialmuscle, over 60%of healing efficiency was achieved at free boundary condi-tion and 54% at fixed boundary condition after repeateddamage-healing cycles. Fig. 12(c) shows the crack closingand healing performance.

4.3. Healing theories and healing efficiency evaluation

4.3.1. Healing theory within the continuum damage

mechanics framework

In the continuum damagemechanics framework, dam-age is represented by a damage variable. Healing canbe treated as the opposite process of damage. There-fore, healing variables can be defined the same way asdamage variables. Polymer damage usually involves finitedeformation and thus it is highly nonlinear, and bothviscoelasticity and viscoplasticity need to be considered.In damage-healing studies, damage inside the materialat the micro-scale level could be represented by mea-suring changes in elastic modulus via damage variables[308310]. Voyiadjis et al. [311313], Shojaei and Li [314],and Shojaei et al. [315] used this idea to model theviscoplastic-viscodamage-viscohealing behavior of shapememory polymer matrix duringdamage andhealing.

By introducing internal variables, the Helmholtz freeenergy (HFE) function is obtained and decomposed as fol-lows [311]:

ij,

pij, ij,p,dij, d

Kij , d

I, hij, hKij , h

I

= W

ij,

pij, dij, hij

+H

ij,p,d

Kij , d

I, hKij , hI

+Gdhdij, hij

(1)

where HFE is a function of thermodynamic fluxes andis decomposed into elastic part W(ij,

p

ij

, dij, hij), harden-

ing function H(ij,p,dKij

, dI, hKij

, hI), and damage-healing


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Fig. 13. The overall mapping procedure between the real damaged and fictitious healed effective configurations: (a) damaged configuration, and (b)fictitiouseffective configuration after the healing process. [311], Copyright 2012.Reproduced with permission from theRoyal Society of Chemistry.

functionGdh(dij,hij).Thedamage-healingfunctiontookintoaccount the microcrack as well as microsurface propaga-tion and recovery. The idea is that part of the energy ina damaged material is converted to surface energy andthe remaining energy is converted to heat. When a heal-ing event occurs, the surface energy reduces due to thepolymer chain diffusion process of thehealing agent.

By conjugating thermodynamic forces for each flux,the healing criterion for the generalized healing surface isdefined as [312]:

fhyh, yhK, yhl,,ypk, ypI, yd, ydK, ydI

=fh1yh yhK

fh2

yhl

fh3

, ypK, ypI

fh4yd, ydK, ydI

wh0 0 (2)

where wh0 is the initial size of the healing surfaces. The

first two terms fh1 and fh2 show the respective kinematicand isotropic hardening/softening because of the healingprocess and fh3 and fh4 represent the effect of the plas-tic deformation. The derived healing criterion is a functionof the relevant healing mechanism. It is argued that the

healing of damage is activated whenfh =0 and

fh = 0.The overall mapping procedure between the real dam-

agedandfictitioushealed effectiveconfigurationsis shownin Fig. 13.Within thecontinuumdamagemechanics (CDM)framework, the Cauchy stress tensor ij shows the stresscondition in the real damaged configuration, while theeffective stress tensor ijrepresents thestress condition inthe effective healed configuration. The vector ofdAnirep-resents therealdamaged area,andthe vectorofd Aniservesas theeffectivefictitiousarea. Theeffectivearea,where theloads are carried, increases during thehealing process dueto healedmicroscale damages. Hence, it indicates that theeffective area grows as the damage heals.

It has been argued to use an indirect measurementmethod to calibrate damage and healing based on elasticmodulus changes during the damage and healing process[308]. The fourth-order anisotropic healed elasticmodulusEh

ijklafter accomplishing the damage healing process was

expressed in term of a new fourth rank healing variabletensor h

ijkl[311]:

Ehijmn =Eijmn+ Eklmn

h(1)ijkl k

(1)ijkl k

(1)pqkl

h(1)ijpq

(3)

Ehijmn =Eijmn+ Eijpq

h(2)pqmn k

(2)pqmn k

(2)pqkl

h(2)klmn

(4)

where kijkl is a fourth-order anisotropic damage variabletensor. Ifh

ijkl= 0ijkl, it indicates thatnohealing at all. In the

case that theeffectiveareaafterhealing isequal to theorig-inal area, a full recovery event is obtained, and the elasticmodulus hmaxijkl = k

maxijkl .

4.3.2. Chain diffusion theoryAs discussed above, solid healing agent such as ther-moplastic has been used in healing cracks in polymercomposites, including the CTH strategy. During the heal-ing process, physical molecular interdiffusion has beeninvolved. Thehealing theory is discussed as follows.

Crack healing in polymeric materials follows a five-stage healing theory [59,315317], which includes (i) sur-face rearrangement, (ii) surface approach, (iii)wetting, (iv)diffusion, and (v) randomization. Surface rearrangementand surface approach experience a process by bringingtwo similar polymeric surfaces into good contact at a tem-perature above the glass transition. Brownian motion is

highly active above this temperature on the interfaces;healing is achieved by the high mobility molecules whena mass of polymer chainsmove across the interface due tothe reduction in thermodynamic barriers (diffusion), andentanglement of molecules (randomization).

The wetting phenomenon in crack healing process wasdiscussedbyWool andOConnor, concerning thelinemodehealing for cracks, crazes, andvoids [59]. Thewettedpoolsin a domain propagated over the entire domain and inter-face until they reached an impingement and coalescenceof wetted areas. The wetting rate depends on differentstages of wetting, which are instant wetting, constant ratewetting, and Gaussianwetting. Among these five stages of

healing, thestagesofwetting, diffusionandrandomizationare very important because the healed polymer material


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Fig. 14. Reptationmodel formolecular diffusion and randomization behavior across the interface during healing. [321], Copyright 1998.Reproducedwithpermission fromthe AmericanChemical Society.

mechanical property is determined in these stages by theintrinsic healing function. However, the first two stages,especially stage 2 (surface approach), are also the sameimportant. As indicated byWool [7], and echoedby Li [47]andBinder [48], bringing thefracturedparts incontact rep-resents oneof thegrandchallenges inrealworldstructures.This is alsowhy closing the crack by shape memory effectis necessary, as discussed above.

The behavior of polymer chain diffusion and random-ization has been widely studied over the past decades. DeGennes [318] has discussed the molecular chain diffusionand randomization by a tube model [319,320], throughwhich the molecular chain was allowed to reptate ran-domlythrough one-dimensionalback-and-forthBrownianmotion, along the randomly coiled conformational tubeduring a certain time. The diffusion mechanism was dis-cussed by Bousmina et al., who described the diffusion

process by Ficks law[321]. Fig. 14 shows thediffusionandrandomization behavior in the polymer/polymer interfacebased on the reptation theory. At t=0, two pieces of poly-meric surfaces are brought into contact at a temperatureabove the glass transition temperature. At the momentt1 >0, the molecular chain starts to slip out the initial tubeand into a new random-shape conformational tube (i.e.,, , ). At t2 > t1, the chain reptates completely into thenew tube. Due to step reptational relaxations, the tuberenewal process will continue until reaching a dynamicequilibriumconformation. InFig.14c, thereptationtimeforchains A and B, located on both sides of the interface withcompletely reptational diffusion to an equilibrium state, isobtained as follows:

rep =N3b4

2e2kBT (5)


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where is the friction coefficient, N is the number of monomers, b is the effective bond length, e is the segmen-tal chain length, kB is Boltzmanns constant, and T is theabsolute temperature. Obviously, the diffusion timebeforereaching the equilibrium state depends on the polymericmaterial properties and temperature.

As discussed above, the interpenetration distance isthe average distance of the segments from the interfacebetween chain A and chain B,which is given by [316]:

l1/2 (6)

where l is the average of chain motion l(t).Because the wetting and diffusion stages as well as

randomization stage determine the mechanical propertydevelopment, thehealed fracture stress is as follows:

= 0+ d (7)

where 0is the stress due to wetting, and dis the stressdueto thereptationof randomlycoiledchainsat a distance

normal to the interface. Assuming that the randomlycoiled tubes have Gaussian conformations, the stress ispostulated as:

t

M

1/4(8)

whereMis themolecularweightof thepolymericmaterial.It indicatesthat fora certain polymericmaterial, thehealedfracturestressdependson thediffusionandrandomizationprocess.

Shojaei et al. [313] further considered the contributionof shape recovery pressure applied on the narrowed frac-ture interface to thehealing efficiency. They found that the

diffusion depth or the healing efficiency increases asymp-totically as the healing temperature or shape recoverypressure increases.

4.3.3. Healing efficiency evaluation

The performance of healing is determined based on acertain criteria. Wool and OConnor studied the recoveryratio of stress R(, t) and energy R(E, t) in the case of linemode healing. The general expressions are:

R(, t) =R0 +

K

t1/4

(t)

(t) (9)

R(E, t) =R0+

Kt1/4 + Gt1/2

(t)

(t) (10)

where R0 is the wetting parameter, is the fracturestrengthof thevirgin polymericmaterial,KandG aremate-

rial parameters,

(t) is the diffusion rate according to the

diffusion initial function (t), and

(t) is the wetting rateaccording to the wetting distribution function (t). Thetheoretical healing efficiency in strength and energy arecalculated via Eqs. (9) and (10), which can be compared toexperimental data.

Thedimensionless recovery ratios R additionallycanbeexamined andcomparedbased on elongation strain, ten-sile modulus Y, fatigue life N, and general spectroscopy of

molecular microstructural parameters via infrared beforebeinghealed(i.e., inthevirgin state)andafterbeinghealed:

R() =

(11a)

R(Y) =Y

Y(11b)

R(N) = NN (11c)

R(I) =I

I(11d)

where thesubscribe denotes theoriginalmaterial prop-erty before damage.

Actually, healing efficiency canbe determined bymanyparameters, depending on the property that is to berecovered. These properties, and thus the healing effi-ciency measurements, can be physical, mechanical, orother functional properties. So far, themajority of healing-on-demand studies are focused on recovery of mechanicalproperties. Therefore, mechanical measurements such asstrength, stiffness, ductility, toughness, etc. have beendominating. As discussed by Li [47], even for fracturetoughness measurement, the fracture modes need to beclearly defined, such as Mode I, Mode II, and Mixed ModeI & II, because different fracturemodes will yield differenthealing efficiencies. The dependence of the strength andtoughness on the thickness of the healing agent layer hasbeen studied experimentally [126,322326].

5. Conclusions and futureperspectives

In the past several decades, the desire for lighter,tougher, stronger, and smarter materials in transporta-tion vehicles, energy production, storage, and transport,military equipment and vehicles, infrastructure, chemi-cal processing equipment, offshore oil and gas equipment,and consumer goods, has driven the use of polymersand polymer composite materials. Polymers or polymercomposite materials, while have high specific strength,stiffness, corrosion resistance, and design tailorability, areprone to damage due to the various weak interfaces andother inherent properties such as brittleness of thermosetpolymers. Therefore, healing-on-demand polymers andpolymer composites have been growing at an unprece-dented speed,which have emerged as an interdisciplinaryclass of materials that need collaboration from variousscience and engineering fields such as mechanical, chem-ical, biological, electrical, and civil engineering, as wellasmechanics, mathematics, physics, and chemistry. Manypolymers canheal themselves as long as they aremanuallybrought into contact, which may be the biggest obstaclefor real world applications, particularly in load bearingengineering structures,wheremanuallybringing fracturedstructural components in contact is prohibited.

As observed from Tables 1 and 3, the development ofcrackhealing inmaterialsundergoes two stages. In thefirstdevelopment stage, almost all attention was paid to howto heal damage on bulk material, film, or coating for solofunctionality restoration. Repeatability in healing was notconcerned. In the seconddevelopment stage, repeatability


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as well as multiple functionalities restoration has becomea main topic when conducting crack healing. Physical orchemical healing mechanism between fracture surfaceshas been well explored by previous researchers. However,fracture surface approachingwas intentionally or uninten-tionally neglected.

All thehealingmechanismsusedinhealing-on-demandpolymers and polymer composites share an essentialconcept: close-then-heal (CTH). For healing-on-demandpolymers thecracks areclosedeitherby theirelasticbehav-iors (such as ionomer) or by manual relocation (mostintrinsichealingpolymers), followedby crack-healingpro-cess through chemical and/or physical interactions acrossthe fracture surfaces under thermal treatment, or chemi-cal treatment, or photo treatment, or electrical treatment,or none treatments under ambient condition. When theCTH scheme is integrated with shape memory polymers,the crack closing step is achieved by constrained shaperecovery, which is triggered by heating or by other meansdepending on the functional groups in the SMP. Such anapproach may be more realistic for real world structuresbecause a crack in a large engineering structure cannot beclosed manually.

SMP matrix based or SMP fiber based or polymericartificial muscle based system can close and heal wide-openedcracks (up tomillimeter scale) per the CTH schemerepeatedly, efficiently, molecularly, and timely. However,minimalhumaninterventionisrequiredtotriggertheheal-ing mechanism, such asproviding local heating.One of themajor advantages of the SMP fiber is that it has good inter-facial bonding between the functional fibers and the hostmatrix. If thefiber is givenmulti-functionality, like electri-calconductivity, itcangeneratelocalheatby electricityandthus may lead to fast, repeatable, and more autonomoushealing-on-demand effect because the electrical conduc-tivity can be used as a damage sensing device, making thehealing more toward an autonomous fashion [47]. Addingdamage sensing capability to the CTH system opens upnew opportunities to make autonomous healing. In addi-tion, the crack closure in CTH depends on both recoverystressandrecovery strain [47]. While SMPs have consider-able recovery strain, their recovery stress is comparativelylow. Further endeavors should be toward increasing therecovery stress of SMPs and artificial muscles. Both physi-cal means (e.g., cyclic programming) and chemical means(e.g., changingthecomposition or controlling thesequenceof polymerization bymimickingbiopolymers, for instance,proteins andDNAs) deserve investigation.

Further development in the CTH scheme may alsoinclude a combination of shape memory and other intrin-sichealing schemes such as shapememory ionomer, shapememory supramolecular, thermoset polymers with cova-lent adaptable network (or dynamic covalent network),etc. [47], or use intrinsic healable polymers as heal-ing agents. For example, ionomer particles, which haveweak shape memory capability, may be compression pro-grammed before embedding into conventional polymermatrix.When triggeredby heating, theembedded ionomerparticleswill expand, whichwill push thefracture surfacesmarching towardeach other, and themolten ionomer par-ticles can heal the crack. In otherwords, ionomer can serve

as both a crackling closing device and a healing agent. Stillanother alternative may be the two-way SMP. This typeof SMP expands when cooling down and shrinks whenheating up. This unique behavior, which is opposite to thecommonphysical behavior ofmaterials, canbe very usefulin healing-on-demand applications.

In real world structures, on-demand healing includesstructures that are under in-service conditions. Sev-eral preliminary explorations have been conducted, suchas healing of continuous SMP fiber reinforced polymerbeam specimens under fixed boundary conditions [267],polymericartificialmuscle reinforced polymer beam spec-imens under clamped boundary conditions [307], andshort SMP fiber r

advances in healing-on-demand polymers and polymer

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