self-healing materialsa review of advances in materials

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Review

Self-healing materials: A review of advances in materials, evaluation,characterization and monitoring techniques

D.G. Bekas, K. Tsirka, D. Baltzis, A.S. Paipetis*

Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece

a r t i c l e i n f o

Article history:

Received 1 July 2015

Received in revised form

3 September 2015

Accepted 27 September 2015

Available online 31 October 2015

Keywords:

Self-healing materials

A. Polymerematrix composites (PMCs)

B. Mechanical properties

C. Mechanical testing

a b s t r a c t

Self-healing materials are attracting increasing interest of the research community, over the last decades,

due to their efciency in detecting and autonomically healing damage. Numerous attempts are being

presented every year focusing on the development of different self-healing systems as well as their

integration to large scale production with the best possible propertyecost relationship. The current work

aims to present the most recent breakthroughs in these attempts from many different research groups

published during the last ve years. The current review focuses in polymeric systems and their com-

posites. The reviewed literature is presented in three distinct categories, based on three different scopes

of interest. These categories are (i) the materials and systems employed, (ii) the experimental techniques

for the evaluation of materials properties and self-healing efciency of the materials/structures and (iii)

the characterization techniques utilized in order to evaluate (off-line) and monitor (on-line) the healing

efciency of the proposed systems. Published works are presented separately in all the different cate-

gories, thus the interested reader is advised to follow the structure of the review and refer to the chapter

of interest.

2015 Elsevier Ltd. All rights reserved.

1. Introduction

Self-healing materials are a relatively new class of smart mate-

rials that possess the ability to fully or partially recover a func-

tionality that is mediated by operational use. Local functionality

loss can be dened as the situation when a section of a material or

structure exhibits degraded performance when compared with the

rest of the material/structure. Global functionality loss can be

dened as the situation when the material or structure exhibits

degraded performance when compared to its properties prior to

any exposure to operational loads.

This work focuses on self-healing polymers and their com-posites. The incorporation of healing agents in polymeric mate-

rials inadvertently leads in a new material with altered

properties when compared to the material that does not possess

the healing functionality. The performance and life-time of the

new composite in conjunction with the efciency of the self-

healing functionality are of primary importance as they are

often competing with each other.

The scope of the rst section of this work is to describe the three

primary self-healing approaches (intrinsic, capsule based and

vascular) as well as the critical issues and challenges associated

with each approach. A review of the literature on the materials that

have been used as healing agents over the last ve years is

presented.

The aim of the second section of this review is to present the

most frequently used testing procedures and specimen geometries

found in research publications during the last ve years. Special

consideration is given to the ones that provide, either qualitatively

or quantitatively, insight on the self-healing performance of the

composites. Associated ASTM standards are also presented.Finally, in order to gain an overall insight into the behavior of

self-healing materials, their structure, performance and self-

healing effectiveness is evaluated via the use of various character-

ization and monitoring techniques. The combination of mechanical

testing and materials characterization techniques can exploit the

actual capabilities as well as restraints associated with these ma-

terials. The ongoing development of the microscopic, spectroscopic

and other characterization methods during the last decades, ren-

ders them invaluable tools, which canprovide knowledge about the

structure of materials, their chemical composition, as well as the

way the react. In the last section of the current review, an extensive* Corresponding author.

E-mail address:[email protected] (A.S. Paipetis).

Contents lists available at ScienceDirect

Composites Part B

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http://dx.doi.org/10.1016/j.compositesb.2015.09.057

1359-8368/

2015 Elsevier Ltd. All rights reserved.

Composites Part B 87 (2016) 92e119
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overview of the most common monitoring methods is presented.

Typical microscopic methods such as optical and scanning electron

microscopy as well as analytical and spectroscopic methods like

NMR, AFM, FTIR and Raman spectroscopy employed for the eval-

uation and monitoring of self-healing are presented. The present

review covers the literature published during the last ve years,

with more extensive information and paradigms in each different

case.

2. Materials

The ability of self-healing materials to regain autonomously or

externally assisted, their initial properties is primarily affected by

the selection of the healing agents. Inspired by the biological sys-

tems, intrinsic, capsule-based and vascular methods, are the main

approaches used in order to impart self-healing functionalities to

materials or structures. A variety of self-healing agents have been

extensively studied to meet the requirements of the new highly

demanding applications of smart materials. This section is dedi-

cated to the review of the research on the aforementioned three

approaches as well as the materials that have been proposed as

healing agents over the last ve years.

2.1. Intrinsic self-healing materials

In the recent past, polymer science has reached at a point where

it is possible to synthesize smart polymers that possess the

remarkable, bio-inspired ability of regaining their initial properties

completely, ideally without external input. These polymers

constitute one of the most important categories of self-healing

materials, that of the intrinsic or remendable healing polymers. In

this case, repair is achieved through the inherent reversibility of

bonding in the matrix phase, which acts as a healing agent.

Despite the good healing performance that was achieved in therst generation of intrinsic self-healing epoxy systems [1], the

incorporation of dicyclopentadiene (DCPD)/Grubbs' catalyst within

the matrixe

an expensive and unstable in the hostile environmentGrubbs' catalyst-limits its applications[2]. Within the aim of the

research community is to maximize the healing efciency and

minimize the cost. Therefore, several other material and techniques

have been developed in order to satisfy these criteria.

Thermally reversible reactions, especially the DielseAlder (DA)

reaction, for cross-linking linear polymers have been extensively

studied by many researchers. Their main advantage is the theo-

retically innite number of repetitions of the healing process

without any further addition of chemical or healing agents[3e6].

Hermosilla et al.[7] presented a novel reversible thermoset poly-

mer based on chemical modication of aliphatic polyketones into

the corresponding derivatives containing furan and/or amine

groups along the backbone. The furan moieties allow for the ther-

mal setting of the polymer by the Dielse

Alder (DA) and retro-DAsequence (bis-maleimide), while amine moieties allow for the

tuning of the hydrogen bonding density. This new class of polymer

material showed improved Tgvalues with respect to the respective

counterparts containing only furan groups. Via this modication,

these materials recover their mechanical properties after three

thermal cycles. In another study, Joost Brancart et al. [8] investi-

gated the ability of furan-maleimide building blocks to create

reversible covenant networks in an epoxy based coating. Furan-

ctionalized precursors were synthesized via reaction of amines

with furfuryl glycidyl ether (FGE). The reversible cross-linking of

the furan-precursors with a bis-maleimide was achieved in a two-

step procedure. Thermal analysis of these composites showed that

modication of the polymer network structure allows for the

tailoring of the temperature for the self-healing process. Jenifer Ax

and Gerhard Wenz [9] created a processible, remendable and

highly oriented polymeric material with pending furane sub-

stituents, esterifying hydroxyethylcellulose with both furoyl chlo-

ride and acetic anhydride. In order to achieve crosslinking (DA

reaction), 1,6-bis(N-maleimido)hexane was used. They have shown

that both constituents can be mixed without premature formation

of gels due to the low rate DA reaction under 70 C. Yoshifumi

Amamoto et al. [10] have successfully produced a cross-linked

polymer based on reshufing of thiuram disulde (TDS) units.

Stimulation of the self-healing process occurred under ambient

conditions (visible light, air, room temperature) in the absence of a

solvent. To carry out the self-healing reaction in a bulk material at

room temperature, the reactive TDS units, capable of re-shufing,

were incorporated in the main chain of a low Tg polyurethane. In

a more recent work, ClaudioToncelli and co-workers [11] presented

the successful synthesis and crosslinking of functionalized (varying

amounts of furan groups) polyketones with (methylene-di-p-phe-

nylene)bis-maleimide. In addition, they managed to modify ther-

mal and mechanical properties of the material by controlling the

furan reactions. This self-healable polymer exhibited an almost full

recovery of thermal and mechanical properties for seven consec-

utive self-healing cycles, independently of the furan intake. Gua-

dalupe Rivero et al. [12] managed to produce polyurethanenetworks with healing capability, based on PCL and furan-

maleimide chemistry, at mild temperature conditions via one-pot

synthesis. A combination of a quick shape memory effect (contact

of the free furan and maleimidemoieties) followed by a progressive

DielseAlder reaction (reformation of the covenant bonds) allows

the remendable process to take place at 50 C, resulting in a com-

plete recovery of the structural integrity without complete melting

of the polymer. A schematic representation of the DielseAlder

based shape memory assisted self-healing process is depicted in

Fig. 1.

A new approach for the development of self-healing nano-

composites was proposed by Sandra Schafer and Guido Kickelbick

[13]. In their study surface-functionalized silica nanoparticles were

used as cross-linking agents in thermally triggered self-healing

Fig. 1. Schematic depiction of the DielseAlder based shape memory assisted self-

healing process in a polyurethane material based on PCL and furan-maleimide

chemistry[12].

Reprinted with permission from Rivero G, Nguyen L-TT, Hillewaere XKD, Du Prez FE.

One-Pot Thermo-Remendable Shape Memory Polyurethanes. Macromolecules.

47(6):2010e

8. Copyright (2014) American Chemical Society.

D.G. Bekas et al. / Composites Part B 87 (2016) 92e119 93


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nanocomposites based on Diels-Alder (DA) chemistry of poly (butyl

methacrylates) and structurally varied polysiloxanes. It has been

observed that the healing properties of the nanocomposite are

highlyaffected by the molecular structure of the crosslinker (spacer

length),the length of the polymeric chain but also bythe type of the

polymer. DA reaction seemed to be favored by the use of modied

polymers (high mobility) and by the presence of particles that have

long spacer groups along with lower molar amount of coupling

agent.

Apart from the DielseAlder reaction, different healing chemis-

tries have been explored to meet the application requirements of

self-healing polymers in different occasions. Healing functionality

was successfully incorporated in a polyurethane (PU) elastomer by

crosslinking the tri-functional homopolymer of hexamethylene

diisocyanate (tri-HDI) and polyethylene glycol (PEG) with

alkoxyamine-based diol. It has been shown that the design of the

polyurethane molecules can be used to optimize not only the me-

chanical properties but also the healing performance. Moreover,

the healing process is completed only by a single step dynamic

equilibrium of CeON bonding, unlike to self-healing based on

reversible DA bonds, which has to be heated up to a certain tem-

perature for disconnecting the intermonomer linkages and then

cooled down for reconnection [14]. Additional efforts have beenmade towards the production of autonomously self-healing mate-

rials using non-covalently bonded systems where the polymeri-

zation and/or the crosslinking occur by intermolecular interactions

of the monomer units and/or the side chains. Compared to chem-

ically cross-linked hydrogels, supramolecular hydrogels demon-

strate better reproducibility of the healing procedure. Takahiro

Kakuta et al.[15]used the hosteguest interaction to produce self-

healing materials that can recover their initial strength even after

being sectioned in the middle. The aforementioned self-healable

supramolecular materials consisted of cyclodextrins (CD) - guest

gel crosslinked between poly(acrylamide) chains with inclusion

complexes. The obtained CDeguest gels exhibit a self-standing

property without chemical crosslinking reagents, indicating that

the newly formed hoste

guest interactions between the CD and theguest units stabilize the conformation of the CDeguest gels. In

another study,[16], a promising non-covalent thermal-switchable

self-healing hydrogel was developed by mixing hydrophobically

modied chitosan (hm-chitosan) with thermal-responsive vesicle

composed of 5-methyl salicylic acid (5 mS) and dodecyl-

trimethylammonium bromide (DTAB). By altering the temperature,

the hydrogel can be switched from sol to gel state (Fig. 2). These

transitions can be reversibly performed for several cycles in a

similar way to a supramolecular gel. The gelation temperature in

particular, can be easily controlled by varying the ratio of DTAB to

5 mS.

In a more recent work, Lafont and his team [17], created a

multifunctional self-healing composite capable of multiple healing

by mixing an uncured thermoset rubber with reversible disulphidebonds, loading it with inert, thermally conductive graphite and

hexagonal boron nitride (hBN) as llers. They proved that higher

the healing temperature the better was the cohesion recovery even

for highly loaded composites. A very promising concept that com-

bines reversible covalent linkages through imine bond formation

with non-covalent interactions through hydrogen bonds between

urea-type groups inside the same polymer structure was presented

by Nabarun Roy and his team [18]. Through polycondensation re-

action between siloxane-based dialdehyde and carbohydrazide

they managed to address reversibility to carbinol (hydroxyl) e

terminated polydimethylsiloxane (PDMS) via the formation of bis-

iminourea type subunits. Acylhydrazone units and lateral hydrogen

bonding interactions impart to the polymer structure reversible

covalent and non-covalent linkages respectively, resulting to a soft

dynamic polymer lm capable of autonomous healing. In addition,

alterations to mechanical properties of the polymer can be ach-

ieved by modifying the length of the siloxane spacer units. In

another study, So Young An et al. [19]reported a novel dual sul-

deedisulde crosslinked networks which exhibited a rapid

(30 se30 min) and effective self-healing ability at room tempera-

ture without external stimuli. The method that has been used for

the synthesis of dual-suldeedisulde crosslinked network, pro-

duced a sufcient density of disulde crosslinkages which was

necessary for the completion of the self-healing process at room

temperature.

2.2. Capsule-based self-healing materials

An alternative approach to achieve self-repair polymeric mate-

rials is the incorporation of capsules within the polymer. Inside

these microcapsules lies the healing agent which will be delivered

to the damaged area upon rupture of the capsule. The rst capsule-

based self-healing concept was proposed by White et al. [2]. They

embedded microcapsules containing healing agent and catalyst

particles into a matrix material achieving a very promising self-

healing efciency. Since then, microcapsules were extensively

studied by many researchers due to their ease of applicability andtheir potential for mass production. Several epoxy monomers have

been easily encapsulated using various methods [20e25]. However,

the encapsulation of suitable hardeners remains an issue. Recently,

Li Yuan et al.[26]demonstrated the self-healing ability of a cyanate

ester (CE) resin by the addition of microcapsules within the volume

of the material. The capsules consisted of a poly(urea-

formaldehyde) shell lled with an bisphenol A epoxy (EP) as

curing agent. Diaminodiphenylsulfone (DDS) catalyst was also used

in the CE formulation to decrease the polymerization reaction

temperature. Specimens exhibited an 85% self-healing efciency

proving the effectiveness of the microcapsule approach for the

development of self-healing polymer materials, as well as for ber-

reinforced CE composites. Henghua Jin et al. demonstrated a self-

healing epoxy adhesive suitable for bonding steel substrates us-ing DCPD lled microcapsules and Grubbs'rst generation catalyst

[27]. It was noteworthy that the addition of both components to the

neat resin epoxy (EPON 828) increased the virgin fracture tough-

ness by 26% and a recovery of 56% of fracture toughness was

reported.

Capsule-based self-healing coatings have been studied by many

researchers over the last three years to due to the increased

importance of maintaining the potential of protection of the un-

derlying substrate[28e33]. In detail, Xiuxiu Liu et al. [33]prepared

a smart self-healing coating consisted of an epoxy resin (diglycidyl

ether of bisphenol A) as matrix and microcapsules lled with the

same polymer as curing agent. Capsules were synthesized by

interfacial polymerization of epoxy droplets with ethylenediamine

(EDA). These microcapsules exhibited high shell strength whilethey could rupture under external force, releasing the healing agent

to the damaged area. It should be noted that the complete absence

of catalyst along with the high level of healing efciency, make

epoxy-capsule loaded polymers excellent candidates for the

development of self-healing lms. In another study, Erica Manfredi

and co-workers [34] produced glass ber reinforced polymer

(CFRP) containing a solvent (ethyl phenylacetate e EPA) capsule-

based healing system using vacuum assisted resin infusion mold-

ing technique. Capsules were manually dispersed into the com-

posite and the maximum pressure threshold, in order to avoid

premature capsule rupture was 0.3 bar. It must be pointed out that

the healing process is based on the swelling mechanism of the

polymeric matrix (Epon 828/DETA) in the presence of EPA solvent,

and

lling the defects that have been created due to static loading

D.G. Bekas et al. / Composites Part B 87 (2016) 92e11994


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in Mode I and II. Using a single capsule, resin-solvent self-healing

chemistry, Jones and co-workers[35]managed to obtain a full re-

covery of interfacial shear strength (IFSS) for a glass/epoxy com-

posite. Microcapsules contained EPON 862 (diglycidyl ether of

bisphenol-F) dissolved in ethyl phenylacetate (EPA) while the

shell material consisted of poly(urea-formaldehyde)e

pUF. More-over, several parameters that can affect the healing efciency of the

system, like the resin-solvent ratio, the capsule coverage and the

capsules size were also examined. Results indicated that the critical

resin-solvent ratio in order to obtain submicron capsules (0.6 lm

diameter) was 30:70 in which a total 83% recovery of IFSS was re-

ported. Fig. 3depicts SEM images of bers with varying capsule

coverage that have been used for the IFSS experiment.

Dual-component microcapsules also drew the attention of theresearch community. The approach lies in fabricating a self-healing

epoxy composite by embedding a healing agent consisting of epoxy

and its hardener inside separate capsules. He Zhang and co-worker

[36]created two types of healing agent carriers, i.e. microcapsules

containing epoxy solution (Epolam 5015 and hardener 5015) and

etched hollow glass bubbles (HGBs) loaded with amine solution

(diethylenetriamine and ethyl phenylacetate) which they incor-

porated in self-healing epoxy system (Epolam 5015 and hardener

5015). Using TGA, SEM and optical microscopy they managed to

characterize both capsules and bubbles. The results indicate that

the amine in the etched HGBs shows high thermal stability during

the curing stage. A mathematical model has been also formulated

in order to calculate the available healants and the diffusion dis-

tance on the crack plane of a two-part epoxy-amine. Based on thesimple cubic array model, the diffusion distance of the released

healing agent was calculated to be inversely proportional to the

cubic root of the concentration of the healing agent carrier. In a

more recent study[37], Jin and his team focused on the encapsu-

lation of epoxy and amine reactants in separate polymeric micro-

capsules. In the case of the epoxy resin, a polyurethane (PU)-

poly(urea-formaldehyde) (PUF) double shell wall was used. The

core consisted of disphenol-A epoxy resin diluted with a low vis-

cosity reactive diluent (o-cresyl glycidyl ether). As for the amine

capsules, they were produced following a method of vacuum

inltration of polyoxypropylenetriamine (POPTA) into polymeric

hollow (PUF walled) microcapsules, demonstrating thus a simple

approach for the encapsulation of a highly reactive core material.

Both epoxy and amine microcapsules can be seen in Fig. 4.

Fig. 2. Thermal-switching of the vesicle-based gel. Photographs of a sample in aqueous solution: (a) before and (b) after heating. (c) Schematic illustration of the solegel transition

[16].

Reprinted from Colloid and Polymer Science, Vol. 291(7), 2013, pp. 1749e58, Thermal-responsive self-healing hydrogel based on hydrophobically modied chitosan and vesicle. Hao

X, Liu H, Xie Y, Fang C, Yang H., Figure 2, Original caption: Thermal-switching of the vesicle-based gel. Photographs of a sample of hm-chitosan (0.4%) and DTAB/5 mS (16 mM/

20 mM) in aqueous solution: a before and b after heating. Before heating, the sample is strongly viscoelastic and holds its weight in the inverted vial. After heating at 55 C for

10 min, the sample is transformed into a low viscosity uid that ows easily.c Schematic illustration of the solegel transition., Copyright Springer-Verlag Berlin Heidelberg 2013,

with kind permission from Springer Science and Business Media.

Fig. 3. SEM micrographs of glass bers with varying capsule[35].

Reprinted from Composites Science and Technology, Vol. 79, Jones AR, Blaiszik BJ,

White SR, Sottos NR., Full recovery of ber/matrix interfacial bond strength using a

microencapsulated solvent-based healing system. pp. 1e7, Copyright (2013), with

permission from Elsevier.



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Afterward, the capsules were embedded into an epoxy matrix

system (Araldite/Aradur 8615) while taking into account the

required stoichiometry. Maintaining the total capsule concentra-

tion at 10 wt% while varying the ratio of epoxy to amine capsules,they managed to obtain the highest average healing efciency

which was at an equal mass ratio of amine:epoxy capsules (5:5). It

was demonstrated, higher exposure temperature caused more loss

of core contents for both types of capsules leading to a poor mixing

of the reactants in the damaged area.

Apart from the well-known poly(urea-formaldehyde)-shell mi-

crocapsules, a generalized silica coating scheme was developed by

Jackson et al. in order to functionalize and protect sub-micron and

micron size dicyclopentadiene monomer-lled capsules and

Grubbs' catalyst particles[38]. Fluoride-catalyzed silica condensa-

tion chemistry was used for the construction of the protective and

functional silica coatings resulting to an improvement of the

dispersion of the capsules and catalyst particles inside the epoxy

matrix. Unlike many other studies, a successful incorporation ofboth capsules into the epoxy was achieved without signicant loss

of healing agent. In Fig. 5, a TEM image of a silica coated DCPD-lled

capsule is presented.

In an effort to improve the self-healing efciency of epoxy

resin, Qi Li and his co-workers [39]prepared a dual-component

microcapsule of diglycidyl ether of bisphenol A epoxy (DGEBA)

(resin) and polyether amine (hardener) using a water-in-oil-in-

water emulsion solvent evaporation technique with polymethyl

methacrylate (PMMA) as shell material. They have shown that

the healing efciency of epoxy was affected by the content and

ratio of the dual-component microcapsules. Self-healing was

carried out successfully at room temperature, but as was indi-

cated, increase in temperature led to higher levels of the self-

healing ef

ciency.

A very interesting concept developed by Dong Yu Zhu and his

team [40] constitutes the construction and development of a

multilayered microcapsules used for self-healing thermoplastics

(Fig. 6). By optimizing the synthesis conditions, robustpoly(melamine-formaldehyde) (PMF)-walled microcapsules con-

taininguidic glycidyl methacrylate (GMA) monomer with proper

size and core content were produced. Second and third (outer/

protective) layers consisted of living poly(methyl methacrylate)

(PMMA-Br) and wax respectively. Results concerning the perfor-

mance and stability (thermal and chemical) indicate that the

multilayered microcapsules might be applicable for manufacturing

not only self-healing thermoplastics but also self-healing

thermosets.

2.3. Vascular self-healing materials

Similar to blood vessels in biology, vascular self-healing systems

incorporate healing agents into a polymer matrix through micro-channels. The original idea as proposed by Toohey et al. [41], con-

cerned the incorporation of a microchannel network containing

dicyclopentadiene (DCPD) in the material. Microchannels delivered

DCPD to an epoxy surface coating containing Grubbs' catalyst. Over

the years vascular self-healing materials were extensively studied

due to the variety of healing that can be used and the large scale of

damage that can be healed [42e47]. Patrick et al. [48] demon-

strated that in situ self-healing can be achieved in structural ber-

composites via microvascular delivery of isolated, reactive healing

reactants. Diglycidyl ether of bisphenol A (DGEBA) based epoxy

resin (EPON 8132) and aliphatic triethylenetetramine (TETA) based

hardener (EPIKURE 3046) were used as healants due to their re-

action kinetics and their post-polymerized mechanical properties.

In order to create the microvascular network, pre-vascularized

Fig. 4. (a) Epoxy capsules consist of a polyurethane e poly(UF) double shell wall and a DGEBA/o-CGE core. (b) Amine capsules contain a poly(UF) shell wall and a POPTA core [37].

Reprinted from Jin H, Mangun CL, Grifn AS, Moore JS, Sottos NR, White SR. Thermally stable autonomic healing in epoxy using a dual-microcapsule system. Advanced Materials.

26(2):282e7, Copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.



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composite textile reinforcement was produced by stitching

catalyst-infusion, in a precise pattern of aerospace-grade woven

fabric. The ber composite preform was then consolidated into a

structural laminate via vacuum assisted resin transfer molding

(VARTM) of a thermoset epoxy matrix. It is noteworthy that after

the nal thermal PLA evacuation step (three-dimensional (3D)

microvasculature), no signicant change to fracture properties was

observed. They have also shown that vascular architectures not

only provide efcient and repetitive delivery of healing agents, but

they also contribute to increased resistance to delamination initi-

ation and propagation. The self-healing cycle of the aforemen-tioned microvascular system is depicted inFig. 7.

The effect of microvascular channels on the in-plane tensile

properties and damage propagation in a 3D orthogonally woven/

glass epoxy has been successfully described by Coppola et al. [49].

Using Vaporization of Sacricial Components (VoSC) process they

managed to produce composites consisted of two part epoxy ma-

trix (EPON 862 epoxy/EPIKURE W curing agent) with straight and

wave shaped channels (Fig. 8). Sacricialbers (SF) were prepared

using poly(lactic acid) (PLA) monolament bers treated with

tin(II) oxalate (SnOx) catalyst so as to decrease their thermal

degradation temperature. SF removed during the post-curing pro-

cess leading to an insignicant alteration on the tensile properties,

strength and modulus of the composite material. In another work,

A. R. Hamilton et al.[95]reported the use of active pumps that can

deliver a two-part healing system (Epon 8132/Epikure 3046) inside

a material through microvascular networks. This technique allows a

small vascular system to deliver large volumes of healing agent to

the damaged area. Moreover, dynamic pumping leads to an

enhancement of component's mixing in the target region,

improving with that way the self-healing ef

ciency.The construction of self-healing materials with embedded

ternary interpenetrating microvascular networks by direct-write

assembly of fugitive inks has been reported by Hansen and his

team[50]. The matrix of the material consisted of a two part epoxy

system diglycidyl ether of bisphenol-A resin (EPON 8132) and an

aliphatic amidoamine (Epikure 3046) as hardener. It was note-

worthy that they managed to accelerate the recovery of mechanical

properties of the resin by exploring the effect of temperature on the

healing reaction kinetics of the healing agent. They report a

Fig. 5. A representative TEM image of a microtomed cross-section of a silica coated DCPD-lled capsule. The DCPD core is removed during the microtoming process [38].

Reprinted from Aaron C. Jackson, Jonathan A. Bartelt, Kamil Marczewski, Nancy R. Sottos, Paul V. Braun. Silica Protected Micron and Sub-Micron Capsules and Particles for Self-

Healing at the Microscale. Macromolecular Rapid Communications. Copyright (2010), Wiley Periodicals. Inc.

Fig. 6. Prole of multilayered microcapsule[40].

Reprinted from Polymer, Vol. 54 (16), Zhu DY, Rong MZ, Zhang MQ. Preparation and characterization of multilayered microcapsule-like microreactor for self-healing polymers. pp.

4227e

36, Copyright (2013), with permission from Elsevier.



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Fig. 7. Life-cycle of a self-healing microvascular ber-composite. Pristine woven composite laminate showing stacked textile reinforcement with dual-channel (red/blue), liquid

lled vascular network[48]. (For interpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)

Reprinted from Patrick JF, Hart KR, Krull BP, Diesendruck CE, Moore JS, White SR, et al. Continuous self-healing life cycle in vascularized structural composites. Advanced materials.


Fig. 8. (aec) Schematics of the unit cell of the preforms. Optical micrographs show surfaces (def) normal to the warp direction and surfaces (gei) normal to the weft direction. Scale

bars represent 1 mm[49].

Reprinted from Composites Part A: Applied Science and Manufacturing, Vol. 59, Coppola AM, Thakre PR, Sottos NR, White SR. Tensile properties and damage evolution in vascular

3D woven glass/epoxy composites. pp. 9e




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reduction in healing times by over an order of magnitude. In

attempting to improve the performance of self-healing materials,

Richard S. Trask and co-workers[51]presented the construction of

a combined sensing and healing vascular network within an

advanced ber-reinforced composite. Poly(tetrauoroethylene)

(PTFE)-coated steel wires used in order to create the microvascular

network inside a ber reinforced polymer composite. A low-

pressure sensor was directly connected within the perceived

damage zone, while the output signal of the sensor was monitored

via open-source microprocessors. The laminates were subjected to

a 10-J energy impact and the healing agent was delivered through a

pump from an external reservoir. Two different healing chemistries

were tested, a commercial system ResinTech RT151 and the well-

known epoxy based system of diglycidyl ether of bisphenol-A

(DGEBA), ethyl phenylacetate (EPA) and diethylenetriamine

(DETA) resulting to a recovery of 91% and 94% in post-impact

compression strength respectively.

The use of Hollow Glass Fibers (HGFs) lled with a single

component epoxy resin (Envirez 70301) in e-glass/epoxy compos-

ites has been reported by S. Zainuddin et al. [52]. The matrix con-

sisted of a two part epoxy system. Part-A was a blend of diglycidyl

ether of bisphenol-A (DGEBA), aliphatic deglycidylether and epoxy

terminated polyether polyol. The curing agent (Part-B) was amixture of 70e90% cycloaliphatic amine and 10e30% poly-

oxylalkylamine. A commercially available woven fabric oriented in

two directions (warp at 0 and ll at 90) was used as reinforce-

ment.Fig. 9 depicts the fabrication of composite embedded with

HGFs. Using this methodology the managed to achieve signicant

regaining of the mechanical properties after multiple Low Velocity

Impact (LVI).

Koralagundi Matt et al. [53]produced a vascular network within

a conventional glass ber reinforced polymer matrix composites

(PMC) in order to address self-healing capability to a composite

structure that can be used in wind turbine blades. Via dynamic

mechanical analysis (DMA), they proved that the vascular network

does not affect the dynamic mechanical properties of the nal

composite. Moreover, it has been shown that the most effective

way to produce vascular self-healing structures is to arrange the

tube network parallel to the resinow direction during the vacuum

infusion process.

3. Material properties as a means to self-healing evaluation

The challenge in the design of self-healing materials is to create

a new composite material with an autonomous or externally

stimulated damage healing capability in order to extend the per-

formance life time of the newly developed material or product. The

presence of local regions in the material with lower or degraded

performance than that of the surrounding areas can be dened as

damage[38]. Thermal or electrochemical degradation can also be

included under this denition.

The incorporation of self-healing agents(SHA) in a material such

as a typical polymer matrix would certainly alter its properties.

Hence it is crucial to monitor those changes in order to assess the

performance of the new composite material. These changes can

also be used as a mean to characterize qualitatively or quantita-

tively the healing performance. In the most favorable scenario, the

new material properties will be equal or better to that of the un-

modied one. The researcher or engineer should study the modi-ed self-healing material as compared to the unmodied, virgin

material in order to assess its performance.

The following sections are dedicated to the presentation of

several methods, techniques, and specimen geometries to describe,

either qualitatively or quantitatively, the unmodied and modied

self-healing material properties and damage focusing on those that

can be used to characterize the healing performance.

Fig. 9. Filling of HGFs and fabrication of e-glass/epoxy composite [52].

Reprinted from Composite Structures, Vol. 108, Zainuddin S, Aren T, Fahim A, Hosur MV, Tyson JD, Kumar A, et al. Recovery and improvement in low-velocity impact properties of

e-glass/epoxy composites through novel self-healing technique. pp. 277e




9/28

3.1. Self-healing modied and neat material's physical properties

Prior to the evaluation of the self-healing properties and per-

formance of the modied material, comparative tests are typically

employed in order to assess the modied and unmodied material

properties. Most of the self-healing systems that are reported in the

literature consist of a polymeric matrix and the self-healing agents.

These consist of microcapsules, vascular networks or other poly-

mers in the form of additives. The most frequently used charac-

terization techniques for these materials are Dynamic Mechanical

Analysis (DMA), Differential Scanning Calorimetry (DSC) and

Thermogravimetric Analysis (TGA).

DMA is a widely used technique for materials characterization

and it is mostly used to determine the glass transition temperature

of the constituent materials [44], the viscoelastic properties in

terms of storage and loss moduli or shear storage modulus[54]and

measure the coefcient of thermal expansion (CTE)[55].

DSC is also used to measure the glass transition temperature for

the matrix and the self-healing agents (SHA)[56], monitor the self-

healing process[39]and the curing process.

TGA is commonly used to determine selected characteristics of

materials that exhibit either mass gain or loss. In self-healing

studies, TGA is employed for the SHA thermal stability evaluation[57]and the evaluation of the amount of encapsulated SHA. It is

also useful for decomposition and maximum weight loss temper-

ature measurements[58].

Other techniques that are employed in self-healing materials

characterization include Fourier Transform Infrared Spectroscopy

(FTIR) [58], Nuclear Magnetic Resonance (NMR) [59,60] and

RAMAN spectroscopy [61]. These techniques are widely used for

monitoring the self-healing process and will be further discussed in

later sections.

3.2. Mechanical properties

The following sections are dedicated to the presentation of

mechanical performance evaluation and the respective techniquesemployed. It should be noted that apart from the information that

these properties and techniques provide regarding the healing ef-

ciency, they can offer quantitative and qualitative means for

comparing the modied and unmodied materials. It is also com-

mon that the mentioned techniques are complimented with other

qualitative techniques like scanning electron, transmission elec-

tron, acoustical, and/or optical microscopy.

3.2.1. Static damage

Static damage can occur in structural materials in the form of

cracks anywhere in the 3D structure and depending on the appli-

cation, loading conditions and type of damage, can occur over the

span of multiple length scales. For instance cracks in a ber com-

posite structure can initiate on the bere

matrix interface, propa-gate to the matrix phase and result in the failure of the structure

through ber eruption, pull out etc. The mechanical properties,

damage initiation and propagation and healing performance have

been extensively studied under universal testing machines under

various loading conditions and scenarios.

Tensile testing is one of those loading conditions and has been

used extensively for measuring stressestrain relations, ultimate

tensile strength and Young modulus [39]. Specimen geometries

for such tests include rectangular shaped specimens (Fig. 10) and

dog bone specimens[62,63]where ASTM D 3039[64]and D 638

[65] are the respective standards that describe these

two geometries along with all the variables included in the

testing procedures, while a representative graph obtained

from such tests can be seen inFig. 11.Coppola et al.[49], refers to

both of these standards in order to study the effect of vascular

channels on the in-plane tensile properties and damage pro-

gression of three-dimensional woven textile composites. Com-

posites specimens were prepared and tested according to

ASTM D3039 while epoxy dog bone shaped specimens wereprepared and tested according to ASTM D 638. A similar standard

is the ASTM D 1078 [66] which describes the microtensile dog

bone specimen geometry but has the limitation that it cannot

provide data for the determination of modulus of elasticity

(Fig. 12)[67].

Fig. 10. Schematic of the tensile test specimen [49].

Reprinted from Composites Part A: Applied Science and Manufacturing, Vol. 59, Cop-

pola AM, Thakre PR, Sottos NR, White SR. Tensile properties and damage evolution in

vascular 3D woven glass/epoxy composites. pp. 9e17, Copyright (2014), with permis-

sion from Elsevier.

Fig. 11. Representative stressestrain curves of virgin and repeatedly repaired

IDHPEG800-0.5 specimen[63].

Reprinted from Polymer, Vol. 53(13), Ling J, Rong MZ, Zhang MQ. Photo-stimulated

self-healing polyurethane containing dihydroxyl coumarin derivatives. pp. 2691e8,

Copyright (2012), with permission from Elsevier.



10/28

Ultimate tensile strength can be used in order to compare the

neat material properties with the modied ones [68] orto provide a

metric of the healing performance as described in the works of

Yuan et al. [14]. In their work, self-healing is characterized by quasi-

static tensile tests performed on rectangle shaped specimens. The

specimens are tested till fracture, then put together to heal and re-

tested. The ratio of tensile strength of the healed specimen to that

of the virgin one provides the healing efciency. The improper

alignment of the fractured surfaces and surface roughness effects

are challenges for the tensile tests because they can lead to porosity

of the healed specimens. In addition, tensile tests are inherently

designed to characterize bulk continuous deformation leading up

to failure and the tensile stress and strain values might be

misleading as the material necks prior to failure. Elongation at

break, yield point stress [69] and force displacement curves [70]

have also been used to quantify and characterize the self-healing

performance but due to the aforementioned challenges, these

metrics do not fully reect the healing quality.

Tensile loading conditions can be used to study the adhesion

behavior of adhesives or composites via lap shear tests. In such a

conguration, a thin slice of a self-healing adhesive is sandwiched

between two plates and the sample is tested under tension till the

lap joint fails and the maximum shear strength can be used as aself-healing efciency in multiple cycles (Fig. 13)[71e73]. Another

approach is according to ASTM D 897[74] where shear testing is

performed under compressive loading conditions. ASTM D 3846

[75]refers to reinforced plastics and is concerned with the deter-

mination of in plane shear strength of reinforced thermosetting

plastics in at sheet form in thicknesses ranging from 2.54 to

6.60 mm and is adopted by Hondred et al.[76]in order to examine

the adhesive properties of thermosetting polymers modied with

rare earth triates.

A major advantage of the lap shear tests is that fractured sur-

faces can be brought into contact in a more controlled manner

compared to tensile tests and the alignment and clamping condi-

tions are easily reproducible and less sensitive to topological

effects. In addition for self-healing systems with reversible chem-istries, the lap shear tests can be designed to study experimental

parameters of controlled force, curing temperature, and multiple

healing cycles. However the distinction between adhesive and

cohesive failure is of paramount importance.

The cohesion recovery is related to the ability of the material to

exhibit temperature-activated mending within its volume. To

investigate the cohesive healing ability, Lafont et al. [17]cut their

self-healing samples into four pieces using a sharp razor blade. The

pieces of material were put back together until visual contact and

were placedbetween twoglass slides. The initial cut width and area

was recorded under an optical microscope. During the healing

procedure, the samples were visually inspected at various intervals

in order to measure the evolution of the cut width and area for the

healing efciency quantication.

Bending and compressive loading can also be used to provide

insight on the healing efciency and the structural integrity

restoration recovery. Wu et al. [77] used for the healing perfor-

mance evaluation of a self-healing carbon ber/epoxy composite

system (Fig. 14), the stiffness recovery ratio (SRR%) dened as:

SRR% healed flexural stiffness

Initial flexural stiffness;

The SSR was derived from rectangular shaped specimens under

3 point bending testing according to the ASTM D 790[78]standard.

In addition load-deection curves are used in order to evaluate thecore shell nano-bers effect on the mechanical properties of the

laminates. Li et al. [79] adopt the 3 point bending xture with single

edge notched beam (SENB) specimens, that are described in the

ASTM D 5045[80]standard, to evaluate the healing behavior of a

modied DGEBA epoxy resin. Healing is presented as a function of

load recovery for the modied and unmodied resins.

The SENB specimen geometry is also used by Meure et al.[81].

The healing efciency is determined by comparing fracture

toughness KIC values of fractured specimens after healing with

those of pre-fractured specimens. The primary advantage of using

the fracture toughness values for healing performance evaluation is

that it yields a quantitative measure of healing efciency that is tied

to the recovery of an inherent material property. However, the

calculations of KIC with the SENB geometry require accurateknowledge of the initial crack length and the crack length after

healing.

The tapered double cantilever beam (TDCB) specimen geometry

can be used in order to overcome the crack length measurement

difculty mentioned for the SENB specimen geometry [24]. The

primary feature of this geometry is that it exhibits a linear rela-

tionship between critical load P and fracture toughness KIC inde-

pendent of crack length (Fig. 15). In addition, the short groove of the

geometry requires small amounts of self-healing material and the

crack initiates and propagates in a more controlled manner. In

addition, if it is required, the test can be stopped in a desired crack

length. A detailed comparison between the SENB and TDCB ge-

ometries is presented in the works of Brown[82]. The TDCB spec-

imens are widely used in self-healing applications for healingperformance evaluation either by comparing K IC values [59] or

critical peak loads values[83,84].

The TDCB specimens can also be used in fatigue loading sce-

narios like in the work of Neuser and Michaud[85]. In their work,

epoxy TDCB specimens with microcapsules and shape memory

alloy wires were subjected to tensionetension fatigue and tension

testing. Fatigue testing was conducted in order to compare the

behavior of pure epoxy, epoxy with microcapsules, and epoxy with

SMA wires while tension testing provided the virgin and healed

peak load data for the healing efciency evaluation.

However, a major disadvantage of the TDCB geometry is that the

fracture behavior of the specimens, show an important dispersion

and unstable fracture behavior that must be taken into account to

obtain accurate results[86].

Fig. 12. Microtensile test specimens[67].

Reprinted from Grande AM, Castelnovo L, Landro LD, Giacomuzzo C, Francesconi A,

Rahman MA. Rate-Dependent Self-Healing Behavior of an Ethylene-co-Methacrylic

Acid Ionomer Under High-Energy Impact Conditions. Journal of Applied Polymer Sci-

ence. Copyright (2013), Wiley Periodicals. Inc.



11/28

An alternative geometry to the TDCB geometry is the width-

tapered double cantilever beam (WTDCB) (Fig. 16). The WTDCB

provides a crack length independent measurement of mode I

fracture toughness like the TDCB geometry. Jin et al. [27]uses the

WTDCB geometry under quasi static fracture and fatigue testing.

Specimens of steel adherents bonded with self-healing epoxy ad-

hesive were prepared and tested on a universal testing machine

under quasi static loading and cyclic loading conditions. The

Fig. 13. Adhesion recovery as function of the healing temperature and cross-linker type at 65 C using (a) 4-SH or (b) 3-SH[73].

Reprinted from Lafont U, van Zeijl H, van der Zwaag S. In uence of cross-linkers on the cohesive and adhesive self-healing ability of polysulde-based thermosets. ACS applied

materials & interfaces. 4(11):6280e8. Copyright (2012) American Chemical Society.

Fig. 14. (a) Three-point bending test set up, (b) three-point bending specimens [77].

Reprinted from Wu X-F, Rahman A, Zhou Z, Pelot DD, Sinha-Ray S, Chen B, et al. Electrospinning Core-Shell Nano bers for Interfacial Toughening and Self-Healing of Carbon-Fiber/

Epoxy Composites. Journal of Applied Polymer Science. Copyright (2012), Wiley Periodicals. Inc.

Fig. 15. TDCB specimen geometry and dimensions in mm [86].

Reprinted from International Journal of Solids and Structures, Vol. 64e65, Garoz Gomez D, Gilabert FA, Tsangouri E, Van Hemelrijck D, Hillewaere XKD, Du Prez FE, et al. In-depth

numerical analysis of the TDCB specimen for characterization of self-healing polymers. pp. 145e




12/28

healing efciency is assessed by the ratio of the healed fracture

toughness to the virgin fracture toughness while the fatigue per-

formance of the self-healing adhesive was investigated under cyclic

loading.

Another specimen geometry that can be used for fracture

toughness calculations in the form of opening mode I interlaminar

fracture toughness GIC[87,88], is the double cantilever beam (DCB)[89]as described in ASTM 5528[90]standard for ber reinforced

composite materials and depicted in Fig. 17. It is widely used in self-

healing vascularized materials research because it has the advan-

tage that the test can be stopped at any prescribed crack length

[91]. However, mode I critical strain energy release rate calculations

require that applied load, crack opening displacement and crack

length values are recorded [92]. The accurate crack length mea-

surement is a disadvantage like in the SENB geometry. DCB speci-

mens can be tested under different testing geometries and loading

scenarios like mode II endloaded split (ELS) [43] as seen in Fig.18 or

end notched exure under three point bending (Fig. 19)[34,93,94]

providing information for mode II strain release energy rate GIIcalculations.

The double cleavage drilled compression (DCDC) specimen or

open hole specimen (OHS) under compressive load is selected from

Hamilton et al.[95]. They study vascular epoxy specimens in order

to evaluate the healing performance of pumping protocols as

manifested by the recovery of fracture toughness after each healing

cycle. The DCDC geometry is more appropriate for studying the

fracture toughness of brittle materials like the epoxy matrices and

their ber reinforced composites. Under a uniform axial compres-

sion load, the Poison effect produces a tensile stress concentrated

around the central hole which induces the initiation of two sym-

metric mode I cracks at each crown of the hole propagating along

the mid-plane of the sample.

3.2.2. Impact damage

Impact damage is more difcult to describe compared to static

and fatigue damage. The impact damage is a dynamic response of

the impacted material, the impacting material, as well as the sup-

porting jig. Impact events in ber reinforced composites can cause

signicant reduction in mechanical performance whilst leaving

little visual evidence of the impact event. Self-healing of impact

damage in ber-reinforced composites is regarded as one of themost difcult areas of ongoing research because of the large

damage volume and multiple failure modes. In such cases the

testing procedures involve secondary testing for determining the

mechanical properties after the impact event like compression af-

ter impact (CAI) testing [96] and/or non-destructive techniques,

like ultrasonic C-scan, in order to assess the damage and the

healing process as well.

The impact event can be generated with different testing

apparatus like impact drop tower devices on e-glass/epoxy samples

(Fig. 21) [97] or glass ber composites with microcapsules [98],

falling weight impact test machine[99]or ballistic pendulum set-

up on ionomeric polymers[100]covering a wide range of impact

energies and projectile velocities[67,98]. These types of tests can

provide data for peak load, energy to maximum load and absorbedenergy which can be used for the healing performance evaluation

(Fig. 20). The impact testing apparatus and procedure are described

in standards like ASTM D 7136[101]while CAI testing can be found

in ASTM D 7137[102].

Norris et al.[96,79]employ the previous mentioned standards

(D 7136, D 7137), in order to investigate the impact behavior of a

vasculature design in a ber reinforced composites and the post

healing compressive strength of the proposed system is used for

the healing performance evaluation. Ultrasonic C-scanning pro-

vided information on the delamination location and helped to

assess the different proposed vascular networks.

Haase et al. [100] investigates the behavior of a self-healing

ionomer under dynamic puncture testing. An impactor with a

similar shape to the ballistic impact tests projectiles is pushedthrough a self-healing polymer sheet at a constant speed. The main

focus of this research is the temperature rise caused by the

impactor which was recorded by three thermocouples embedded

in the polymer sheet.

3.3. Corrosion resistance and protection

Most metals in natural environments exist in their oxidized

form, which means that metals tend to corrode, leading to loss of

mechanical and esthetic properties. The easiest way to protect

metals from undesired corrosion is to apply protective coatings that

offer activeprotection, passive protection, or both. The failure of the

protective layer leads unavoidably to corrosion of the underlying

metal.The self-restoration of this protective coating is a typical self-

Fig. 16. (a) Geometry of WTDCB specimen consisting of adhesively bonded A36 steel

adherents. (b) Optical microscopy of cross section of a self-healing adhesive incorpo-

rated with Grubbs' catalyst and DCPD microcapsules [27].

Reprinted from Polymer, Vol. 52(7), Jin H, Miller GM, Sottos NR, White SR. Fracture and

fatigue response of a self-healing epoxy adhesive. pp. 1628e34, Copyright (2011), with

permission from Elsevier.

Fig. 17. Schematic showing the interplay locations of the EMAA bers and the

delamination fracture plane in the carbon bereepoxy laminates[92].

Reprinted from Composites Part A: Applied Science and Manufacturing, Vol. 43(8),

Pingkarawat K, Wang CH, Varley RJ, Mouritz AP. Self-healing of delamination cracks in

mendable epoxy matrix laminates using poly[ethylene-co-(methacrylic acid)] ther-

moplastic. pp. 1301e




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healing functionality. In order to explore if a system shows self-

healing functionalities the creation of an articial defect in the

coating system, and evaluation of the ability of the system to sup-

press or decrease corrosion to desired levels and restore the pro-

tective functionality has been broadly used. A detailed review

regarding the employed self-healing corrosion protection meth-

odologies can be found in Ref. [103].One of the most used techniques in corrosion science to

monitor local corrosion damage is the Electrochemical Imped-

ance Spectroscopy (EIS) via bod plots (Fig. 22) [104e106]. In or-

der to access the charge transfer resistance or polarization

resistance that is proportional to the corrosion rate at the

monitored interface, EIS results have to be interpreted with the

help of a model of the interface. An important advantage of EIS

over other laboratory techniques is the possibility of using very

small amplitude signals without signicantly disturbing the

properties being measured. To make an EIS measurement, a small

amplitude signal is applied to a specimen over a range of fre-

quencies. The EIS instrument records the real (resistance) and

imaginary (capacitance) components of the impedance response

of the system. Depending upon the shape of the EIS spectrum, acircuit model or circuit description code and initial circuit pa-

rameters are assumed and input by the operator. EIS can provide

quantitative information about the electrochemical state of a

coating. The EIS set up is used by Garcia et al. [29] in order to

assess the healing performance of their proposed self-healing

anticorrosive organic coating. Scanning Vibrating Electrode

Technique (SVET) is also employed in order to provide further

verication on the EIS results.

Corrosion activity maps can be obtained by using the SVE

Technique[107].SVET uses a single wire to measure the voltage

drop in a solution. This voltage drop is a result of local current at

the surface of a sample. Measuring this voltage in the solution,

the current at the sample surface is mapped. Current can be

naturally occurring from a corrosion or biological process, or the

current can be externally controlled using a galvanostat. A key

application of SVET is to study corrosion process of bare or

coated metals. Hollamby et al. [30] employ SVET in order to

evaluate the anticorrosive and self-healing behavior of their

proposed hybrid polyester coating. Control and coated specimenswere scratched and immersed in NaClaqsolution and the current

density maps from SVET measurements were charted as seen in

Fig. 23.

Vimalanandan et al.[108]employed the Scanning-Kelvin-Probe

(SKP) technique in order to investigate the self-healing perfor-

mance and the corrosion-driven catholic delamination progress of

a conductive polymer (CP) based nano-capsule system. SKP is a

scanning probe method where the potential offset between a probe

tip and a surface can be measured using the same principle as a

macroscopic Kelvin probe (Fig. 24).

For the SKP measurements, a scratch was introduced to the

CP-coating, this defect was then covered with KCl and introduced

to the SKP chamber. The behavior of the corrosion potential and

the progress of the delamination were studied. The corrosionpotential in the electrolyte defect was monitored by positioning

the SKP tip close to the electrolyte drop serving as a reference

electrode. The cathodic delamination progress was monitored by

scanning the coatings from the defects to reect the potential

Fig. 18. Mode II ELS specimen geometry[43].

Reprinted from Composites Science and Technology, Vol. 71(6), Norris CJ, Bond IP, Trask RS. Interactions between propagating cracks and bioinspired self-healing vascules

embedded in glass ber reinforced composites. pp. 847e53, Copyright (2011), with permission from Elsevier.

Fig. 19. End notched exure specimen geometry as adopted by Ref. [94].

Reprinted from International Journal of Solids and Structures, Vol. 46(13), Ouyang Z, Li

G. Nonlinear interface shear fracture of end notched exure specimens. pp. 2659e68,


Fig. 20. Energy vs. time curves for microcapsules contained glass ber reinforced

composites[98].

Reprinted from Chowdhury RA, Hosur MV, Nuruddin M, Tcherbi-Narteh A, Kumar A,

Boddu V, et al. Self-healing epoxy composites: preparation, characterization and

healing performance. Journal of Materials Research and Technology. 2015;4(1):33e43.




14/28

distribution as a function of the distance from the defect and

time.Scanning Electrochemical Microscopy (SECM), can provide in-

formation about the redox activity, redox mode and topography,

feedback mode of liquid/gas, liquid/solid and liquid/liquid in-

terfaces. SECM measurements can be used in order to yield topo-

graphic information and to probe the surface reactivity of solid-

state and electro-catalyst materials. The long term anticorrosive

efciency of a damaged epoxy coating containing silyl-ester mi-

crocapsules on an aluminum substrate is studied via SECM testing

in the works of Gonzalez-Garcia et al. [28]. Combining redox and

feedback modes, the long term healing of the coating was

demonstrated (Fig. 25).

3.4. Electrical conductivity

The self-healing concept has also been implemented and ach-

ieved for materials with electrical functionality. Such materials are

able to recover conduction paths at different scales and most in-

vestigations of conductivity recovery in the literature deal with the

healing of such conductive paths.

A qualitative way to monitor the recovery of conductivity in

self-healing materials can be found in the works of Palleau et al.

[109]. Here a simple electronic circuit consisting of a LED, a voltage

source and a self-healing stretchable (SHS) wire in series is

monitored and captured in video (Fig. 26). The SHS wires are a

combination of a self-healing polymer structured with micro-

channels lled with EGaIn. Scissors are used to cut the wire so

that the circuit continuity is lost. When the wires are aligned, theliquid metal components merge together forming a continuous

and conductive wire.

For studies where a very small change in resistivity is to be

monitored, a Wheatstone bridge set-up is preferred. This technique

measures an unknown resistance by using an electrical circuit. The

Fig. 22. (a, b) Bode plot and phase angle, (c) of specimens coated with nanocapsules loaded with various types of corrosion inhibitors [106].

Reprinted from Progress in Organic Coatings, Vol. 76(10), Choi H, Kim KY, Park JM. Encapsulation of aliphatic amines into nanoparticles for self-healing corrosion protection of steel

sheets. pp. 1316e


Fig. 21. Target mounting in the impact chamber [97].

Reprinted from Advances in Space Research, Vol. 51(5), Francesconi A, Giacomuzzo C,

Grande AM, Mudric T, Zaccariotto M, Etemadi E, et al. Comparison of self-healing

ionomer to aluminum-alloy bumpers for protecting spacecraft equipment from

space debris impacts. pp. 930e40, Copyright (2013), with permission from Elsevier.



15/28

set up consists of an electrical source with a known voltage, threeresistors with known values, and a galvanometer. In self-healing

studies, the Wheatstone bridge set up allows monitoring of both

the creation and the healing of damage since when the damage

occurs the system's resistivity will increase[110].

Blaiszik et al. [111] employ the Wheatstone bridge set up in

order to in situ monitor a four point bending test conducted on

specimens of microencapsulated metal dispersed in a dielectric

material. The specimen acts as one resistor on the Wheatstone

bridge circuit. The circuit is monitored throughout the four-point

bend test using a Wheatstone bridge with the specimen as onebridge arm. The performance of the circuit is evaluated by

measuring the normalized bridge voltage:

Vnorm VhV=VoV;

where Vo is the bridge voltage before damage, V is the bridge

voltage measured for a fully broken circuit, and Vhis the instan-

taneous bridge voltage of the circuit. The value of Vnorm ranges

from zero for a specimen with no electrical conductance to one for

Fig. 23. (A) SVET current density map and (inset) visual appearance of the scratched control sample. (B) SVETcurrent density map and (inset) appearance of the scratched NPs_BTA-

a sample[30].

Reprinted from Hollamby MJ, Fix D, Donch I, Borisova D, Mohwald H, Shchukin D. Hybrid polyester coating incorporating functionalized mesoporous carriers for the holistic

protection of steel surfaces. Advanced materials. 23(11):1361e5, Copyright (2011) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 24. (A) Scheme depicting the model-coating system and the experimental set-up used to evaluate the self-healing performance of the coating system. (B) Corrosion potential

monitored by SKP in the defect. (C) Delamination proles recorded by SKP[108].

Reprinted from Vimalanandan A, Lv LP, Tran TH, Landfester K, Crespy D, Rohwerder M. Redox-responsive self-healing for corrosion protection. Advanced materials. 25(48):6980e4,

Copyright (2013) Wiley-VCH Verlag GmbH&

Co. KGaA, Weinheim.



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a fully conductive specimen. The efciency of conductivity

restoration, hc, is dened for each specimen asVnormafter fracture

(Fig. 27).

Another interesting approach where mechanical and electricalproperties are simultaneously recorded and analyzed can be

found in the works of Bailey et al. [112]. Here apart from the EIS

technique, the authors employ an in-situ electro-tensile tech-

nique in order to assess the degree of mechanical and electrical

self-healing efciency of a composite coating. This technique

involves the controlled introduction of a crack by pulling the

coating in tension while measuring the changes in electrical

conductivity on-line. Complementing the EIS results, it was

demonstrated that when microcapsules possessing an EPA:ECNT

(ethyl phenylacetate: epoxy with carbon nano-tubes) core were

incorporated into the coating, electrical conductivity and me-

chanical properties were restored to 64% (23) and 81% (39)

respectively (Fig. 28). Furthermore, sequential cracking and

healing events were noticed while the coating was pulled in

tension and both EIS and in situ tensile loading and electrical

conductivity test revealed a 24 h restoration of this coating

analogous to pure ECNT.

4. Characterization of self-healing systems and monitoring of

their healing efciency

Next to the technical challenge of realizing a self-healing sys-

tem, there is an inevitable need both for characterizing the func-

tional components that constitute it and monitor the whole process

of self-healing. A variety of characterization techniques can be

found in the literature. However, the methods for monitoring the

self-healing process are limited.

More specically, in the area of characterization the most

common techniques are Fourier Transform Infrared Spectroscopy

(FTIR), Nuclear Magnetic Resonance Spectroscopy, Optical and

Scanning Electron Microscopy (OM and SEM), Transmission elec-

tron microscopy (TEM), Atomic Force Microscopy (AFM), X-ray

Fig. 25. (a) Optical micrograph of AA2024-T3 sample with bare and silyl-treated surface. (b) SECM image of the transition area on (a) using the electroreduction of oxygen. (c)

Approaching-curves performed on the bare metal (black line) and on the silyl-covered area (red line). (d) Overlapped approaching-curves corresponding to measurements using

electrochemical mediator (red-dashed line) and oxygen reduction (black-solid line) [28]. (For interpretation of the references to color in this gure legend, the reader is referred to

the web version of this article.)

Reprinted from Electrochemistry Communications, Vol. 13(10), Gonzalez-Garca Y, Garca SJ, Hughes AE, Mol JMC. A combined redox-competition and negative-feedback SECM

study of self-healing anticorrosive coatings. pp. 1094e7, Copyright (2011), with permission from Elsevier.



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diffraction analysis and rheological studies. In the eld of moni-

toring, the reported techniques include Raman spectroscopy,

acoustic emission and ultrasonics.

The following section presents an overview of the research

conducted by several groups, with specic examples for eachtechnique.

4.1. Characterization techniques

4.1.1. Fourier transform infrared spectroscopy (FTIR)

FTIR is a well-established technique based on molecular in-

teractions. In the eld of self-healing materials, this technique is

by far the mostly employed methodology in order to conrm

the healing functionality, compare the virgin and healed mate-

rials, as well as to monitor the process of self-healing reactions.

It has been employed both for the characterization of micro-

capsules [26,39,113e115] and intrinsic self-healing systems

based on DielseAlder cycloadditions[11,116e118]. FTIR has also

been used in other self-healing systems such as self-healinggels [118], intrinsic reversible crosslinked networks healed via

photocyclization or on disulde links [119], mendable epoxy

networks and 3D braided composites with vascular channels, or

polyurethane/graphene self-healable nanocomposites[120].

An interesting example of the use of FTIR can be found in the

work of Araya-Hermosilla et al.[7]who presented a novel revers-

ible thermoset with tunable Tg based on chemical modication of

aliphatic polyketones and furan and/or amine groups. In this ma-

terial system they monitored the cycloaddition through the spec-

tral band of CeO stretching around 1000e1300 cm1. As the molar

ratio between furan and maleimide groups increased, the intensity

of the band centered around 1180 cm1 (corresponding to CeOeC

ether peak) also increased, thus testifying the occurrence of the

Dielse

Alder reaction.

In another case concerning anti-corrosive self-healing organic

coatings Szabo et al. [121] investigated the application of linseed oil

e a lm former healing materiale and octadecylamine (ODA) e a

corrosion inhibitor in the core of microcapsules which were added

in a self-healing paint using FTIR. Moreover, they tried to specifythe inuence of Co-octoate, used as a drier in order to reduce so-

lidication time and thus improve the self-healing ability of the

paint. They found out that seven days were needed by the linseed

oil lm in order to drycompletely and additionally that this amount

of time decreased to several hours with the addition of Co-octoate.

Regarding the self-healing functionality they encountered some

difculties with the addition of ODA which weakened the healing

process. This difculties were overcame by increasing the Co-

octoate concentration.

In another encapsulation attempt, Garcia et al.[29], utilized FTIR

in order to conrm the hydrolysis of a water reactive silyl ester

which had been encapsulated in a self-healing anticorrosive organic

coating. Through the use of FTIR and contact angle measurements

they showed that this silyl ester, after its hydrolysis, had the abilityto completely coat a metallic surface and form a hydrophobic pro-

tective layer, which actually became denser with time.

Yuan et al. [122] produced a self-healing system based on

cyanate ester resins (CE) with the addition of low molecular weight

poly(phenylene oxide) resins (PPO). This CE/PPO system was

studied via FTIR in order to quantitatively estimate the extent of

conversion, a, of the cyanate ester groups (eOCN) and the amount,

x, of unreacted eOCN groups according to the FTIR spectra of un-

cured and cured CE resin/PPO resin, as is shown in Fig. 29. They

used as a reference peak the vibration band of the phenyl ring at

1510 cm1 and chose the vibration bands of eOCN at 2280/

2238 cm1 to calculate x and a. Moreover, they attributed the

improved exural strength of CE/PPO systems to the higher con-

version (a) ofe

OCN detected by FTIR spectroscopy.

Fig. 26. a) Schematics illustrating the disconnection and reconnection of a simple electronic circuit using a self-healing wire. b) Variation of the resistance of SHS wires during

connection/disconnection/reconnection experiments[109].

Reprinted from Palleau E, Reece S, Desai SC, Smith ME, Dickey MD. Self-healing stretchable wires for recon gurable circuit wiring and 3D microuidics. Advanced materials.




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4.1.2. Near-infrared spectroscopy (NIR)

Near infrared spectroscopy has been utilized by Varley et al. [56]

as a convenient technique to compare the concentration of

different functional groups and determine whether there was any

impact on a mendable epoxy network as a result of thermoplastic

addition, which may affect the healing process.

Using this technique the aforementioned group demonstrated

that themodication of their mendable epoxy system, withdifferent

healing agents, had a negligible impact upon the network formation,

or, the chemistry of polymerization after curing had occurred. This

was assumed as NIR did not reveal any changes neither between the

spectra of the unmodied and the modied material, nor between

the spectra acquired before and after healingof the modied epoxies.

In conclusion, they conrmed that healing was more likely via

physical processes namely diffusion through free volume and repu-

tation across a crack plane during thermal activation.

4.1.3. Nuclear magnetic resonance spectroscopy (NMR)

This experimental technique is typically used in order to exploit

the magnetic properties of certain atomic nuclei. Relying on the

phenomenon of nuclear magnetic resonance it can provide detailed

information about the structure, dynamics, reaction state andchemical environment of the molecules.

In the case of self-healing materials numerousstudies have used

NMR in order to identify interactions between atoms and to

conrm the formation of self-healing systems through various

chemical reactions. These studies include not only intrinsic chem-

istries such as the nano-composite self-healing gel produced by

Sharma[118]et al., the supramolecular self-healing gels exhibited

by Zhang et al.[123].

An example of NMR utilization in microcapsule based self-

healing systems is the work of Zhu et al. [40]who, through the

use of NMR, conrmed that capsule rapture led to polymerization

achievement, thus conrming that the produced multilayered

capsules enclosed the monomer needed for self-healing reaction to

occur.

Furthermore, Kakuta et al.[15]employed NMR to trace a reason

why the formation of inclusion complexes plays such an important

role in the formation of their supramolecular preorganized

hydrogel system. This system was based on non-covalent hoste-

guest interactions between polymers and was produced by radical

copolymerization of monomers of a complex of a cyclodextrin (CD)

host and aliphatic guest in aqueous solution. The group of Kakuta

showed that the inclusion complexes undergo a dissolving effect

between the CD and guest monomers causing homogeneous

radical copolymerization which results to the production of a su-

pramolecular self-healing hydrogel system.

Jinhui et al.[124]also, used NMR in order to conrm the suc-

cessful modication of a commercial epoxy resin with furan groups

as well as the occurrence of the self-healing DielseAlder reaction

between the modied epoxy and bismaleimide.

4.1.4. Optical microscopy (OM), scanning electron microscopy

(SEM) and transmission electron microscopy (TEM)

Microscopy is widely used to conrm the self-healing compo-

nents and structures, after the production step, optical microscopy

Fig. 27. Evolution of the normalized bridge voltage and force during four-point bend tests of a self-healing specimen (a) and a control specimen (b). (c) The percentage of samples

where healing was observed[111].

Reprinted from Blaiszik BJ, Kramer SL, Grady ME, McIlroy DA, Moore JS, Sottos NR, et al. Autonomic restoration of electrical conductivity. Advanced materials. 24(3):398e401,

Copyright (2012) Wiley-VCH Verlag GmbH&

Co. KGaA, Weinheim.



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(OM), scanning electron microscopy (SEM) or transmittance elec-

tron microscopy (TEM) are employed depending on the size of the

studied morphology.

Numerous researchers employed SEM to study various features

including epoxy/hardener containing microcapsules[23,33], frac-

ture surfaces of microcapsules[114], mendable epoxy resins[125],

unhealed and healed CFRPs [126], polyurethane/graphene self-

healing nanocomposites [95], solvent-lled microcapsules incor-porated into a polyurethane layer which is deposited atop a silver

ink line for restoring electrical conductivity of the ink [88], shape

memory polymers[104]as well as healing agent containing micro/

nanocapsules embedded in anticorrosive coatings[105].

A very interesting analytical study using SEM in order to view a

three-dimensional image of both the inner and outer surface and

morphology of various capsules ranging from several tens of mi-

crons to below 100 nm in size, has been published by Hodoroaba

et al.[127]. In this study SEM was used in the Transmission Mode

and the samples were prepared on thin supporting foils (on TEM

grids). Fig. 30 shows SEM micrographs with corresponding EDX

analyses presented in their study.

Li et al.[128]produced a cement based system containing self-

healing microcapsules. Then, they studied those microcapsules

Fig. 28. Stress and normalized electrical resistance of an ECNT coating (a) without capsules, with (b) hexyl acetate capsules, (c) EPA:EPON microcapsules, and (d) EPA:ECNT [112].

Reprinted with permission from Bailey BM, Leterrier Y, Garcia SJ, van der Zwaag S, Michaud V. Electrically conductive self-healing polymer composite coatings. Progress in Organic

Coatings. 2015;85:189e98, Copyright (2015), with permission from Elsevier.

Fig. 29. Fourier transform infrared spectra of poly(phenylene oxide) (PPO) and the

uncured and cured cyanate ester (CE)/poly(phenylene oxide) systems[122].

Reprinted from Yuan L, Huang S, Hu Y, Zhang Y, Gu A, Liang G, et al. Poly(phenylene

oxide) modied cyanate resin for self-healing. Polymers for Advanced Technologies.

Copyright (2014) John Wiley&

Sons Ltd.



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composed of diglycidyl ether of bisphenol A epoxy resin as the core

material and polystyreneedivinylbenzene as the shell material and

the fracture surfaces of them via OM and SEM. SEM was also

employed by the group of Jinhui et al. [124]in order to assess the

self-healing process of the produced self-healing epoxy system.

TEM is mostly used in self-healing systems which incorporate

carbon allotrope nanoinclusions as well as in some capsule based

self-healing systems with sub-micron sized capsules.

Specically, TEMwas used in a study published by Leterrieret al.

[112]who synthesized an electrically conductive partially cured

epoxy coating incorporating a microcapsule based healing mech-

anism. The microcapsules contained a mixture of ethyl phenyl-

acetate and a nanoreinforced epoxy resin, the matrix was also

reinforced with nanoinclusions. TEM facilitated to the visualization

of the carbon nanotube distributions into the core of the

microcapsules.

4.1.5. AFM (Atomic Force Microscopy)

Atomic force microscopy is generally utilized in self-healing

systems in order to assess their healing performance in terms of

temperature, time and local mobility of the atoms of the studied

materials.

Brancart et al. [8] performed an extensive study in order toassess self-healing coatings based on reversible polymer networks

using AFM. They studied the self-healing ability of the coatings

through the healing of well-dened and reproducible nanosized

scratches and other defects applied by nanolithography.

The group of Dikic [129] produced a self-replenishing hydro-

phobic coating based on peruoroalkyl dangling chains covalently

bonded to a cross-linked polymer network through a polymeric

spacer. They used AFM to assess the self-replenishing process by

comparing the uorine end groups concentration at the virgin and

healed specimen. The decrease in the Force Displacement mean

value measured by AFM depicting an increase in uorine content in

the healed materials showed that the prerequisite for replenishing

is the mobility of the species within the coating network. This

mobility was triggered by annealing.

In a another study, Faghihnejad et al. [130] employed AFM to

characterize self-healing lms with multiple hydrogen bonding

groups with respect to changes in the Relative Humidity (RH) of the

lm

self-healing materialsa review of advances in materials

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