Self-Healing Materials Systems: Overview of Major Approaches and Recent Developed Technologies

Abstract

The development of self-healing materials is now being considered for real engineering applications. Over the past few decades, there has been a huge interest in materials that can self-heal, as this property can increase materials lifetime, reduce replacement costs, and improve product safety. Self-healing systems can be made from a variety of polymers and metallic materials. This paper reviews the main technologies currently being developed, particularly on the thermosetting composite polymeric systems. An overview of various self-healing concepts over the past decade is then presented. Finally, a perspective on future self-healing approaches using this biomimetic technique is offered. The intention is to stimulate debate and reinforce the importance of a multidisciplinary approach in this exciting field.

Full text

Hindawi Publishing Corporation
Advances in Materials Science and Engineering
Volume 2012, Article ID 854203, 17 pages
doi:10.1155/2012/854203
Review Article
Self-Healing Materials Systems: Overview of Major
Approaches and Recent Developed Technologies
B. A¨
ıssa,1, 2 D. Therriault,2E. Haddad,1and W. Jamroz1
1Department of Smart Materials and Sensors for Space Missions, MPB Technologies Inc., 151 Hymus Boulevard Pointe Claire,
Montreal QC, Canada H9R 1E9
2Department of Mechanical Engineering, and Composites Center for Applied Research on Polymers QC (CREPEC),
´
Ecole Polytechnique de Montr´
eal, P.O. Box 6079, Montreal, Canada H3C 3A7
CorrespondenceshouldbeaddressedtoB.A
¨
ıssa, brahim.aissa@mpbc.ca
Received 2 September 2011; Accepted 25 November 2011
Academic Editor: S. Miyazaki
Copyright © 2012 B. A¨
ıssa et al. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The development of self-healing materials is now being considered for real engineering applications. Over the past few decades,
there has been a huge interest in materials that can self-heal, as this property can increase materials lifetime, reduce replacement
costs, and improve product safety. Self-healing systems can be made from a variety of polymers and metallic materials. This
paper reviews the main technologies currently being developed, particularly on the thermosetting composite polymeric systems.
An overview of various self-healing concepts over the past decade is then presented. Finally, a perspective on future self-healing
approaches using this biomimetic technique is offered. The intention is to stimulate debate and reinforce the importance of a
multidisciplinary approach in this exciting field.
1. Introduction
Polymers and structural composites are used in a variety
of applications. However, these materials are susceptible to
damage induced by mechanical, chemical, thermal, UV radi-
ation, or a combination of these factors [1]. When polymer
composites used as structural materials become damaged,
there are only a few methods available to attempt to extend
their functional lifetime. Ideal repair methods are ones that
can be executed quickly and effectively directly on damaged
site, eliminating thereby the need to remove a component for
repair. However, the mode of damage must also be taken into
consideration as repair strategies that work well for one mode
might be completely useless for another. For example, matrix
cracking can be repaired by sealing the crack with resin,
wherefibrebreakagewouldrequirenewfibresreplacement
or a fabric patch to achieve recovery of strength.
One of the earlier healing methods for fractured surfaces
was “hot plate” welding, where polymer pieces were brought
into contact above the glass transition temperature of the
material, and this contact was maintained long enough for
interdiffusion across the crack face to occur and restore
strength to the material. It has been shown, however, that the
location of the weld remains the weakest point in the material
and thus the favourable site for future damage to occur [2].
For laminate composites, resin injection is often employed
to repair damage in the form of delamination. This can be
problematic, however, if the crack is not easily accessible for
such an injection. For fibre breakage in a laminate com-
posite, a reinforcing patch is often used to restore some of the
strength to the material. Often, a reinforcing patch is used
in conjunction with resin injection to restore the greatest
amount of strength possible [3]. None of these methods of
repair is an ideal solution to damage in a structural compos-
ite material. These methods are temporary solutions to pro-
long the lifetime of the material, and each of these repair
strategies requires monitoring of the damage and manual
intervention to enact the repair. This greatly increases the
cost of the material by requiring regular maintenance and
service.
Alternative healing strategies are therefore of great inter-
est. Moreover, with polymers and composites being increas-
ingly used in structural applications space, automobile, de-
fence, and construction industries, several techniques have

2 Advances in Materials Science and Engineering
0
20
40
60
80
100
120
140
160
180
200
220
Number of publications
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
Year
(a)
Composite materials
13.06%
Protective coating
9.9%
Mechanical
properties
9.76%
Corrosion
protection
7.6% Coatings
7.75% Repair
7.6%
Encapsulation
6.31%
Catalyst
5.74%
Polymer
15.64%
Cracks
16.64%
Key vocabullary
(b)
Figure 1: (a) Recent refereed publications related to the field of self healing materials, together with (b) their corresponding distribution of
the employed key words vocabulary. All published languages were included. All document types, including journal and conference articles,
report paper, conference proceeding, and monograph published chapters were recorded. Statistics are available from 2000 to August 2010
inclusively. Data were collected from Engineering Village web-based information service.
been developed and adopted by industries for repairing
visible or detectable damages on the polymeric structures.
However, these conventional repair methods are not effec-
tive, for example, for healing invisible microcracks within the
structure during its service life. In response, the concept of
“self-healing” polymeric materials was proposed in the 1980s
[4] as a means of healing invisible microcracks for extending
the working life and safety of the polymeric components.
The publications in the topic by Dry and Sottos [5] in 1993
and then White et al. [6] in 2001 further inspired world
interests in these materials [7]. Examples of such interests
were demonstrated through US Air force [8]andEuropean
Space Agency [9] investments in self-healing polymers.
Conceptually, self-healing materials have the built-in
capability to substantially recover their mechanical proper-
ties after damage. Such recovery can occur autonomously
and/or be activated after an application of a specific stimulus
(e.g., heat, radiation, pressure, etc.). As such, these materials
are expected to contribute greatly to the safety and durability
of polymeric components without the high costs of active
monitoring or external repair. Throughout the development
of this new range of smart materials, the mimicking of
biological systems has been used as a source of inspiration
(since most materials in nature are themselves self-healing
composite materials) [10].
The number of publications dealing with various aspects
of self-healing materials has increased markedly in recent
years. Figure 1 shows how the number of refereed various
articles in the self healing field has steadily increased since
2001, based on data collected from the Engineering Village
web-based information service. Along with the increase in
the number of publications in this area comes a need for a
comprehensive review article, and the objective of this paper
is to address this need.
In addition, the vast majority of the surveyed articles
deal with polymer composites. Due to the large number of
articles involved and the lack of electronic access to many
conference proceedings, the emphasis of this paper is on the
more accessible refereed journal articles. It was not practical
to cover all of these articles, and, since a lot of articles had
already been covered by previous related paper articles, an
attempt was made to select representative articles in each of
the relevant categories.
This paper briefly describes the traditional methods of
repairing damage in the polymeric materials during the last
decade. Tabl e 1 provides summary of some developments
and achieved performances. It can be seen that both thermo-
plastic and thermosetting materials were investigated for self
healing, where the research interests have been more shifted
to thermosetting-composite-based systems in recent years.
We start by describing the methods for evaluating self healing
efficiencies. We will then describe briefly some examples
of different approaches proposed to heal the thermoplastic
systems, and we follow by emphasising the preparation and
characterization of the self healing of the thermosetting
ones. We will take a short view on the self-healing coating
for metallic structures systems, and we conclude by future
research outlooks.
2. Quantification of Healing Efficiency
Healing of a polymeric material can refer to the recovery of
properties such as fracture toughness, tensile strength, and
surface smoothness. Due to the range of properties that are
healed in these materials, it can be difficult to compare the
extent of healing. Wool and O’Connor [32] proposed a basic
method for describing the extent of healing in polymeric
systems for a range of properties. This approach has been

Advances in Materials Science and Engineering 3
Tab le 1: Nonexhaustive main developments in self-healing polymer composites.
Host material Healing system Stimulus Best efficiency achieved Healing condition Test method Ref.
Thermosetting
and/or
thermosetting
composites
Hollow glass fibre Mechanical 93% 24 hours at
ambient atm. Flexure Strength [11–20]
Microencapsulation
approach 80–93%
48 h at 80◦C
24 h at Ambient
then 24 h at 80◦C
Fatigue resistance
Fracture toughness
Tensile strength
[21,22]
Microvascular network 60–70%
7–30 cycles
12 hours at
ambient atm. Fracture toughness [23,24]
Thermoplastic additives 30–100% 10 min at 120◦C
1-2 h at 130–160◦C
Flexure strength
Tensile strength
Impact strength
[25,26]
Shape memory alloy Electrical 77% 24 hours at
ambient atm. Fracture toughness [27]
Carbon fibre 46% 1–20 minutes at
70–120◦CImpact strength [28]
Elastomeric Silicone rubber Mechanical 70–100% 48 hours at
ambient atm. Tear strength [29]
Thermoplastic Molecular diffusion 100% 5 min. at 60◦C. Fracture toughness [30]
Photo-induced healing Photo 26% 10 min. at 100◦C Flexure strength [31]
commonly adopted and has been used as the basis for
method of comparing “healing efficiency” of different self
healing polymeric systems.
There are different methods to effect healing that are
applicable for each individual mode of damage as well as
each unique damaged material. This makes quantifying the
extent of healing within the material and comparing it to
healing in other systems rather difficult. The susceptibility of
a given material to fracture can be expressed in terms of the
plane strain fracture toughness, KIC. It has become standard
practice to assess the healing ability of a particular material
by comparing the fracture toughness of the material both
before and after healing. The healing efficiency is η,
η(%)=Khealed
IC
Kvirgin
IC
×100, (1)
where Kvirgin
IC is the fracture toughness of the virgin specimen
and Khealed
IC is the fracture toughness of the healed specimen.
2.1. Self-Healing Efficiency Assessed by Fracture Test. For qua-
sistatic fracture conditions healing efficiency is defined in
terms of the recovery of fracture toughness. Healing evalua-
tion begins with a virgin fracture test of an undamaged taper-
ed double cantilever beam (TDCB) sample (Figure 2(a)). A
precrack is introduced to sharpen the crack-tip, and loading
of the specimen is increased until the crack propagates along
the centerline of the sample until failure. The crack is then
closed and allowed to heal at room temperature with no
external intervention. After healing, the sample is loaded
again until failure.
Crack healing efficiency, η, is defined as the ability of a
healed sample to recover fracture toughness [32]:
η=Khealed
IC
Kvirgin
IC
,(2)
where Kvirgin
IC and Khealed
IC represent the fracture toughness of
the virgin and healed samples, respectively.
2.2. Self-Healing Efficiency Assessed by Fatigue Test. For dy-
namic fracture conditions, healing efficiency based on static
fracture toughness recovery is no longer meaningful. Instead,
the fatigue crack propagation behaviour of the self-healing
epoxy was evaluated using the protocol outlined by Brown
et al. who defined healing in terms of the life extension factor
[33]:
ηd=Nhealed −Ncontrol
N,(3)
where Nhealed is the total number of cycles to failure for a self-
healing sample and Ncontrol is the total number of cycles to
failure for a similar sample without healing.
Characterization of fatigue response is more complex
than monotonic fracture because it depends on a number
of factors such as the applied stress intensity range, the load-
ing frequency, the ratio of applied stress intensity, the healing
kinetics, and the rest periods employed [34]. The investiga-
tion considered successful healing as the recovery of stiffness
lost due to damage induced by cyclic loading rather than
changes in crack growth rate or absolute fatigue life.

4 Advances in Materials Science and Engineering
(a) (b)
Figure 2: (a) Schematic of the TDCB-based fracture toughness and (b) tear protocols to evaluate healing performance.
2.3. Self-Healing Efficie ncy Ass ess ed by Tear Te st. For elas-
tomeric self-healing material, the TDCB-based fracture
toughness protocol to evaluate healing performance is
inappropriate. Instead, the recovery of tear strength using a
tear specimen is used to define healing efficiency, where
ηc =Thealed
TVirgi n
.(4)
A tear test utilizes a rectangular coupon of material with
a large axial precut that produces two loading arms. These
arms are loaded in tension until the tear propagates through
the rest of the specimen (Figure 2(b)). Healing evaluation
begins with a virgin tear test of an undamaged sample. After
failure, the sample loading arms are reregistered and healing
occurs at room temperature with no external intervention.
After healing, the tear sample is loaded again to failure. Using
this test protocol, more than 70% recovery of the original tear
strength was achieved in the PDMS (polydimethylsiloxane)
system [29].
3. Self-Healing of the Thermoplastic Materials
Crack healing of thermoplastic polymers has been the subject
of extensive research in the 1980s. The polymers investigated
cover amorphous, semicrystalline, block copolymers, and
fibre-reinforced composites. It has been discovered that when
two pieces of the same polymer are brought into contact at
a temperature above its glass transition (Tg), the interface
gradually disappears and the mechanical strength at the
polymer-polymer interface increases as the crack heals due
to molecular diffusion across the interface. For example, by
using thermoplastics chain mobility with a minimal appli-
cation of heat, Lin et al. [30] have studied crack healing
in PMMA (poly(methyl methacrylate)) by methanol treat-
ment from 40 to 60◦C. The authors have found that the
tensile strength of PMMA treated by methanol can be fully
recovered to that of the virgin material. On the other hand,
another example of photo-induced self-healing in PMMA
was reported by Chung et al. [31]. Mixture of photo link-
able TCE (1,1,1-tris-(cinnamoyloxymethyl) Ethane) with
UDME- (urethane-dimethacrylate-) and TEGDMA- (tri-
ethyleneglycol-dimethacrylate-) based monomers, blended
with visible light photo-initiator CQ (camphorquinone), was
polymerized into a hard and transparent film after its ir-
radiation for 10 min with a 280 nm light source. The healing
was shown to only occur upon exposure to the light of
the correct wavelength, proving that the healing was light
initiated. Healing efficiencies in flexural strength up to 14%
and 26% were reported using light or a combination of light
and heat (100◦C). However, healing was limited to the sur-
faces being exposed to light, meaning that internal cracks or
thick substrates are unlikely to heal. In summary, self-healing
of thermoplastic polymers can be achieved via a num-
ber of different mechanisms, including (i) recombination of
chain ends, (ii) self-healing via reversible bond formation,
(iii) living polymer approach, and (iv) self-healing by
nanoparticles, in addition to the (v) molecular interdiffusion
and (vi) photo-induced healing reported here. The processes
are well known and have been well reported. A detailed
description of these approaches can be found in [38].
4. Self-Healing of Thermosetting Materials
The search for self-healing thermosetting materials coincides
with these materials being more and more widely used in
numerous structural applications. These applications ge-
nerally require rigid materials with a thermal stability that
most thermoplastics do not possess. The rigidity and ther-
mal stability of thermosetting comes from their cross-linked
molecular structure, meaning that they do not possess
the chain mobility so heavily utilized in the self-healing
of thermoplastics. As a result of their different chemistry

Advances in Materials Science and Engineering 5
and molecular structure, the development of self-healing
thermosetting has followed distinctly different routes.
The most common approaches for autonomic self-
healing of thermosetting-based materials involve incorpora-
tion of self-healing agents within a brittle vessel prior to addi-
tion of the vessels into the polymeric matrix. These vessels
fracture upon loading of the polymer, releasing the low-vis-
cosity self-healing agents to the damaged sites for subsequent
curing and filling of the microcracks. The exact nature of the
self-healing approach depends on (i) the nature and location
of the damage, (ii) the type of self-healing resins, and (iii) the
influence of the operational environment.
4.1. Hollow Glass Fibres Systems. The development of ad-
vanced fibre-reinforced polymers (FRPs) to achieve perfor-
mance improvements in engineering structures focuses on
the exploitation of the excellent specific strength and stiffness
that they offer. However, the planar nature of an FRPs micro-
structure results in relatively poor performance under impact
loading. This is an indication of their susceptibility to dam-
age, which manifests mainly in the form of delamination.
Hollow glass fibres have already been shown to improve
structural performance of materials without creating sites of
weakness within the composite [39,40]. These hollow fibres
offer increased flexural rigidity and allow for greater custom
tailoring of performance, by adjusting, for example, both the
thickness of the walls and degree of hollowness [41,42]. By
using hollow glass fibres in these composites—alone or in
conjunction with other reinforcing fibres it would be possible
to not only gain the desired structural improvements, but to
also introduce a reservoir suitable for the containment of a
healing agent [40–43]. Upon mechanical stimulus (damage
inducing fracture of the fibres), this agent would “bleed” into
the damage site to initiate repair, not unlike biological self-
healing mechanisms [11,12].
The first systems that have been investigated in 1996
and 1998 by Dry [13]andLietal.[14], respectively, have
validated that the proposed architecture for releasing chem-
icals from repair fibres was totally possible and then have
used cyanoacrylate [13,14], ethyl cyanoacrylate [14], and
methyl methacrylate [15,16] as healing agents to heal
cracks in concrete. This methodology was then transferred
to polymer composite materials by Motuku et al. [17] in the
late 1999. The healing agents contained within the glass fibres
have been either a one-part adhesive, such as cyanoacrylate,
or a two-part epoxy system, containing both a resin and a
hardener, where either both are loaded in perpendicular fib-
res or one embedded into the matrix and the other inside
fibres [18].
One of the initial challenges encountered when creating
this type of self-healing systems is the development of a
practical technique for filling the hollow glass fibres with
repair agent. When approaching this problem, the dimen-
sions of the glass fibre itself must be considered, including
diameter, wall thickness, and fibre hollowness, as well as the
viscosity and healing kinetics of the repair agent. Bleay et al.
[18] were among the first to develop and implement a fibre
filling method involving “capillary action” that is assisted by
vacuum, which is now the main commonly used process. The
chosen glass fibre should be also evaluated for its capacity
to survive to the composite manufacturing process without
breakage, while still possessing its ability to rupture during a
damage event in order to release the required healing agent.
Motuku et al. [17] have clearly determined that hollow glass
fibres were best suited for this kind of application, as opposed
to polymer tubes or those made of metal, which often did not
provide controlled fracture upon impact damage.
In 2003, Hucker et al. [42] have shown that hollow glass
fibresofalargerdiameteroffered an increased compressive
strength, while giving larger volume of healing agent to
be stored. The second important parameter to investigate
was the capacity of the healing agent to adequately reach
the site of damage and subsequently undergo healing. This
mechanism will obviously depend upon the viscosity of the
healing material, as well as the kinetics of the repair process.
For example, the cyanoacrylate system studied by Bleay
et al. [18] was shown, indeed, to restore mechanical strength
to damaged specimens but also caused significant pro-
blems by curing upon contact with the opening of the fibre,
which prevented the healing agent from reaching the site of
damage in the sample. Various groups [11,12,17,40]have
then used liquid dyes inside the composites in order to serve
as a damage detection mechanism, providing hence a visible
indication of the damage site, while allowing a clear evalua-
tion of the flow of healing agents to those sites.
Finally, the third parameter to optimize is the concen-
tration of healing fibres within the matrix, their spacial
distribution, and the final dimensions of the specimen,
which have direct effects on the mechanical properties of the
resulting composite material. As early demonstrated by Jang
et al. [19] in 1990, the stacking sequence of the fibres within
the composite plays a role in inhibiting plastic deformation
and delamination and will also affect the response to an
impact damage event. In order to maintain high mechanical
properties, repair fibres need to be adequately spaced within
the composite. Motuku et al. [17] have shown that thicker
composites have shown better performances in healing
studies. These parameters, however, will depend upon the
dimension choice of fibre and chemical choice of the healing
agent employed, and so optimization will depend on the
specificities of the system being studied.
Until recently, the majority of the works done on self-
healing hollow fibre composites have focused on demon-
strating the feasibility of such concept for self-repair and have
reported qualitatively on the healing capacity of the studied
systems. Recently, numerous works have reported quanti-
tatively on mechanical properties associated with healing of
the materials. The inclusion of hollow glass fibres into a com-
posite system was shown by Jang et al. [19] and Trask and
Bond [20] to give an initial reduction in the strength of the
material, either by 16% in glass fibre reinforced polymer
(GFRP) composites and by 8% in carbon-fibre reinforced
polymer (CFRP) composites. These “self-repairing” compo-
sites were shown to recover 100% of the virgin strength
for GFRP and 97% of the virgin strength for CFRP, but
in both cases the composite materials were subjected to
a heat treatment to aid in delivery of the resin to the
damaged area as well as in curing of the healing agent. More

6 Advances in Materials Science and Engineering
Catalyst
Microencapsulated healing agent
Crack initiation
(i)
Healing agent
Crack evolution
(ii)
(iii)
Polymerized healing agent
(iv)
200 µm
(a)
One part resin Polymer matrix
Resin material Hardener material
Resin material Hardener microcapsules
25 µm
(b)
Epidermis
Dermis
Capillaries
Cut
Larger blood vessels
(i)
(ii)
Coating with catalyst particles
Microvascular substrate
(iii)
250 µm
(c)
Figure 3: (a) Autonomic healing concept incorporating encapsulated healing agent and embedded catalyst particles in a polymer matrix; (i)
damage event causes crack formation in the matrix; (ii) crack ruptures the microcapsules, releasing liquid healing agent into crack plane; (iii)
healing agent polymerizes upon contact with embedded catalyst, bonding crack closed; (iv) typical SEM image of the urea-formaldehyde
microcapsules containing dicyclopentadiene prepared by emulsion in situ microencapsulation, adapted from [35]. (b) (i) Schematic diagram
of smart repair concepts considered for hollow glass fiber polymer matrix composites. Use single-part, two-part resin and hardener, or resin
with a catalyst/hardener. (ii) Typical scanning electron microscopy (SEM) image of the hollow glass fiber, adapted from [20]. (c) Biomimetic
microvascular self-healing. (i) Schematic diagram of a capillary network in the dermis layer of skin with a cut in the epidermis layer (ii)
Schematic diagram of the self-healing structure composed of a microvascular substrate and a brittle epoxy coating containing embedded
catalyst, adapted from [23]. (iii) Optical image of the 3D microvascular network embedded in an epoxy matrix (Credit Photo D. Therriault).
recently, the work of Williams et al., [44] has considered
the development of autonomic self-healing within a carbon
fibre-reinforced polymer (CFRP), and has demonstrated the
significant strength recovery (>90%), which was possible
when a resin filled hollow glass fibre system was distributed
at specific interfaces within a laminate, minimising thereby
the reduction in mechanical properties whilst maximising
the efficiency of the healing event.
4.2. Systems Based on Microencapsulated Healing Agents. As
above mentioned, and since the first report of the self-
repairing composites systems in the literature [6], conven-
tional strategy was achieved by embedding a microencap-
sulated liquid healing agent and solid catalytic chemical
materials within a polymer matrix. Hence, upon damage-
induced cracking in the matrix, microcapsules are supposed
to release their encapsulated liquid healing agent into the
crack planes (Figure 3(a)). All the involved materials must be
carefully engineered. For examples, encapsulation procedure
must be chemically compatible with the reactive healing
agent, and the liquid healing agent must not diffuse out of
the capsule shell during its potentially long shelf-life. At the
same time, the microcapsule walls must be resistant enough
to processing conditions of the host composite, while main-
taining excellent adhesion with the cured polymer matrix to
ensure that the capsules rupture upon composite fracture.
Polymeric microcapsules are most often prepared via
a miniemulsion polymerization technique, as described in

Advances in Materials Science and Engineering 7
the work of Asua [45]. The procedure involves the well-
known oil-in-water dispersions mechanism of the polymeric
material. In the majority of self-healing composite systems
that have been studied, the microcapsules are made by a urea-
formaldehyde polymer, encapsulating DCPD as the liquid
healing agent [6,33,45–50] and/or epoxy resin [48,51–
54]. In the case of DCPD, during the in situ polymerization
process, urea and formaldehyde react in the water phase to
form a low-molecular-weight prepolymer; when the weight
of this prepolymer increased, it deposited at the DCPD-water
interface. This urea-formaldehyde polymer becomes highly
cross-linked and forms the microcapsule shell wall. The urea-
formaldehyde prepolymer particles are then deposited on
the surface of the microcapsules, providing a rough surface
morphology that aids in the adhesion of the microcap-
sules with the polymer matrix during composite process-
ing [55]. Moreover, composites using DCPD-filled urea-
formaldehyde microcapsules have shown concrete healing
ability in monotonic fracture and fatigue [6,33,45–50].
4.2.1. Size and Material Microcapsules Effects. In 2003, Willi-
ams et al. [44] have reported that the microcapsules made in
this oil-in-water in situ process have an average size of 10–
1000 μm in diameter, with a smooth inner shell in the 160–
220 nm thick range, and fill content up to 83–92% liquid
healing agent. The mechanical rupture of the microcapsule
is the sine qua non condition event for the healing process.
Hence, it is obviously important, therefore, to fabricate
microcapsules with optimal mechanical properties and wall
thickness. The relationship between the stiffness of the
capsule and the one of the polymer matrix determines how
the crack will propagate in the sample. In 2006, Keller and
Sottos [56] have described how a capsule that has higher
elastic modulus than the one of the polymer matrix material
should create a stress field that tends to deflect cracks away
from the capsule; a more compliant shell wall, on the other
hand, will produce a stress field that attracts the crack to-
wards the microcapsule.
The influence of microcapsule diameter and crack size
on the performance of self-healing materials was also inves-
tigated in 2007 by Rule et al. [57]. They have used an epoxy-
based material containing embedded Grubbs’ catalyst par-
ticles and microencapsulated DCPD. The amount of liquid
that microcapsules could deliver to a crack face was shown to
scale linearly with microcapsule diameter (and hence to the
volume1/3), for a given weight fraction of capsules. Moreover,
the size of the microcapsule also plays a role in the perfor-
mance of the system, in terms of the effect on toughness of
the composite, and the nature of interface between micro-
capsule and polymer matrix. Based on these relationships,
the size and weight fraction of microcapsules can be ra-
tionally chosen to give optimal healing of a given crack size.
However, as noted by Williams et al. [44], the shell
wall thickness is largely independent of manufacturing para-
meters and is typically between 160 and 220 nm thick; never-
theless, slight adjustments can be made during the encap-
sulation procedure to alter the resulting microcapsules. The
microcapsule size is controlled mainly via the rate of agita-
tion during the encapsulation process; typical agitation rates
reported by Williams et al. [44] range from 200 to 2000 rpm,
with finer emulsions and therefore smaller diameter capsules
being produced with increasing rates.
In 2004, Brown et al. have noted [46] that smaller micro-
capsules exhibit maximum toughening at lower concentra-
tions; on the other hand, Rule et al. [57] have reported, in
2007, that specimens that contain larger microcapsules per-
form better than those with smaller microcapsules at the
same weight fraction, presumably due to the amount of
healing agent present in the samples. In the latter study, the
best healing achieved was on a specimen containing 10 wt.%
of 386 μm capsules, which corresponds to 4.5 mg of heal-
ing agent being delivered per unit crack area (assuming all
capsules in the crack plane rupture). The amount of heal-
ing agent available for delivery to the crack plane was cal-
culated based on the microcapsule size and weight fraction
incorporated into the composite and was verified by com-
paring the data from these autonomously healing samples
with that of samples, in which a known volume of healing
agent was manually injected into the crack plane to initiate
the healing process.
In the light of synthesizing smaller microcapsules that ex-
hibit maximum toughening at lower concentrations, Blaiszik
et al. [58] have reported in 2008 an in situ encapsulation
method demonstrating over an order of magnitude size
reduction for the preparation of urea-formaldehyde (UF)
capsules filled with a DCPD healing agent, where capsules
with diameters down to 220 nm were successfully achieved,
using sonication techniques and an ultrahydrophobe solu-
tion to stabilize the DCPD droplets. The capsules were found
to possess a uniform UF shell wall (77 nm average thick-
nesses) and display good thermal stability. However, there
are drawbacks with UF microcapsules: first, (i) the formation
of agglomerated nanoparticles debris that could act as crack
initiation sites within the host matrix, second, (ii) rough and
porous wall surfaces formed by agglomerated nanoparticles
that may reduce the adhesion between the microcapsules and
matrix, and, finally, (iii) rubbery and thin capsule walls (160–
220 nm [59]) that lead to the loss of core material during
storage and cause handling difficulties during processing of
the composites. In addition to UF microcapsules, melamine-
formaldehyde [60,61] and polyurethane [62] shell wall
materials were successfully used to prepare microcapsules of
various healing materials. We note also the works of Liu et al.,
in 2009 [36], which have produced microcapsules for self-
healing applications with a melamine-urea-formaldehyde
(MUF) polymer shell containing two different healing agent
candidates, 5-ethylidene-2-norbornene (ENB) and ENB with
10 wt.% of a norbornene-based cross-linking agent (CL),
by in situ polymerization in an oil-in-water emulsion
(Figure 4). The microcapsules were found to be thermally
stable up to 300◦C and exhibited a 10 to 15% weight loss
when isothermally held at 150◦C for 2 h. Overall, these MUF
microcapsules exhibited superior properties compared to the
urea-formaldehyde (UF) microcapsules used extensively for
self-healing composites to date, and their manufacturing
process is simpler than that made from UF. On the other
hand, it is worthy reported at this level the innovative
work of Mookhoek et al. [63], where microcapsules of

8 Advances in Materials Science and Engineering
220 µm
(a) ENB-Filled MUF microcapsules
220 µm
(b) ENB+CL-Filled MUF microcapsules
Figure 4: Optical microscopic images of ENB- and ENB + CL-filled MUF microcapsules with no debris formed (M/U/F =3 : 1 : 8.5, reaction
temperature =86◦C, rpm =500). Copyright Macromol. Mater. Eng. [36].
size around 1.4 μm dibutylphthalate-(DBP-) filled urea-for-
maldehyde (UF) were used as pickering stabilizers to create
larger (∼140 μm) microcapsules containing a second liquid
phaseofDCPD.Thebinarymicrocapsulesweremadeby
encapsulating the dispersed DCPD liquid (stabilized with the
UF (DBP) microcapsules in water), via an isocyanate-alcohol
interfacial polymerization reaction.
Various applications have been attempted with more or
less success. Microcapsules have been used in the paper in-
dustry for a range of different purposes, for example, in
self-copying carbonless copy paper [64], and in the food
and packaging industries for applications such as control of
aroma release and temperature or humidity indicators [38].
Other possible applications might include encapsulation of
antimicrobial agents or scavengers in active packaging. Re-
cently, Andersson et al. [65] have developed microcapsules
with a hydrophobic core surrounded by a hydrophobically
modified polysaccharide membrane in aqueous suspension,
to obtain capsules fulfilling both the criteria of small cap-
sule size and reasonably high solids content to match the
requirements set on surface treatment of paperboard for en-
hancement of packaging functionality, and they have shown
a reduced tendency for deteriorated barrier properties and
local termination of cracks formed upon creasing.
4.2.2. Fatigue Cracks Retardation. To retard the growth of
fatigue cracks, shape-memory alloy (SMA) wires are well
suited to this application since they exhibit a thermoelastic
martensitic phase transformation, contracting above their
transformation temperature and exerting large recovery
stresses of up to 800 MPa, when constrained at both ends
[66–68]. Moreover, Rogers et al. [69] have shown that, when
an SMA wire is embedded within an epoxy matrix, the
full recovery force acts at the free edges of the component.
Therefore, an SMA wire bridging a crack should induce a
large closure force on the crack. Indeed, Kirkby et al. [27]
have reported on the self-healing polymers with embedded
shape-memory alloy (SMA) wires, where the addition of
SMA wires shows improvements of healed peak fracture
loads by up to 160% (comparatively with specimen without
SMA), approaching the performance of the virgin material.
Moreover, the repairs can be achieved with reduced amounts
of healing agent. The improvements in performance were
attributed mainly to the crack closure, which reduces the
total crack volume and increases the crack fill factor for a
given amount of healing agent and the heating of the healing
agent during polymerization, which increases the degree of
cure of the polymerized healing agent.
4.2.3. Delaminating Substrate. Because of their excellent in-
plane properties and high specific strength, fibre-reinforced
composites with polymeric matrices have found many uses
in structural applications. Despite this success, they are
particularly prone to damage from out-of-plane impact
events. Although fibre damage is usually localized at the site
of impact, matrix damage in the form of delaminations and
transverse cracks can be more widespread. Delaminations, in
particular, pose a serious issue because they can significantly
reduce compressive strength [70–73] and grow in response
to fatigue loading [70,74,75]. In addition to this problem,
impact damage can be subsurface or barely visible, necessi-
tating the use of expensive and time-consuming nondestruc-
tive inspection [70]. Once damage is located, there are many
repair techniques that have been proposed and/or are cur-
rently practiced [76–79]. As we have mentioned, most solu-
tions rely on resin infiltration of delaminations or composite
patches, to provide load transfer across the damaged region.
In cases of severe damage, damaged regions are removed
and replaced with new composite material that is bonded
or cocured to the original one [76]. These repair techniques
are generally time-consuming complicated and require
unhindered access. Recently, Patel et al. [80] have studied
the autonomic self-healing of impact damage in compos-
ite materials by using a microencapsulated healing agent
(DCPD liquid healing agent and paraffinwaxmicrospheres
containing 10 wt.% Grubbs’ catalyst), which has been suc-
cessfully incorporated in a woven S2-glass-reinforced epoxy
composite. Low velocity impact tests reveal that the self-
healing composite panels are able to autonomically repair
impact damage. Fluorescent labelling of damage combined
with image processing shows that total crack length per
imaged cross-section is reduced by 51% after self-healing.

Advances in Materials Science and Engineering 9
Tab le 2: Literature summary of self-healing chemicals investigated for the microencapsulation approach. Adapted from [34].
Self-healing agent Catalyst Self-healing reaction Reference
Dicyclopentadiene (DCPD) Bis(tricyclohexylphosphene) benzylidine
ruthenium (IV) dichloride (Grubbs’ catalyst)
Ring-opening metathesis
polymerization
[6,21,22,34,46,
47,49,83–90]
5-Ethylidene-2-norbornene (ENB) Bis(tricyclohexylphosphene) benzylidine
ruthenium (IV) dichloride (Grubbs’ catalyst)
Ring-opening metathesis
polymerization [86]
DCPD/ENB blends Bis(tricyclohexylphosphene) benzylidine
ruthenium (IV) dichloride (Grubbs’ catalyst)
Ring-opening metathesis
polymerization [87]
Mixture of hydroxyl end functionalised
Polydimethylsiloxane (HOPMDS) and
Polydiethoxysiloxane (PDES)
Di-n-butyltin dilaurate Polycondensation [91]
Epoxy Amine Polycondensation [48,51,92]
Styrene-based system Cobalt naphthenate, dimethylaniline Radical polymerization [38]
On the other hand, flexible, laminated, self-healing blad-
der material was investigated to mediate the impact of
small tears and punctures. Previous attempts at healing pun-
cture damage have focused on ionomers [81]. A self-healing
response in ionomers initiates through the transfer of energy
from a fast moving projectile, which is typically a few milli-
metres in diameter. Frictional heating of the material from
the passage of the projectile leads to a reorientation of the
polymer chains in the ionomer. This rearrangement can,
under some conditions, seal the hole generated by the pro-
jectile. However, this healing occurs only when the damaged
area is heated to near the melt temperature of the material
[81]. In 2009, Beiermann et al. [82]havemanufactureda
three-layer flexible self-healing materials, capable of repair-
ing puncture damage. The used material consisted of three
layers: a poly(dimethyl siloxane) (PDMS) composite, embed-
ded with a self-healing microcapsule system, sandwiched
between two layers of poly(urethane)-coated nylon. A pro-
tocol was established in which samples were damaged using
a hypodermic needle or a razor blade, and a successful heal
was defined as the ability to reseal the damage to with-
stand a pressure differential across the laminate of 103kPa (at
1 atm.). Healing was shown to var y significantly with micro-
capsule size, with the maximum healing success rate (100%
successfully healed) occurring in samples with 220 μmin
diameter microcapsules. Additionally, healing was found to
increase with composite layer thickness, and decrease with
increasing puncture size.
Finally, fracture testing, in the form of single-edge
notched bending tests, has shown a healing efficiency of
111%, when the concentration of microcapsules and latent
hardener were optimized. Some preliminary tests on epoxy-
based fabric laminates containing this self-healing system
demonstrated a 68% recovery of virgin inter-laminar frac-
ture toughness. Yuan et al. [52] have reported another pro-
mising combination of healing agent and catalyst for self-
healing polymer composites. The healing agent, consisting
of a mixture of diglycidyl ether of bisphenol A (DGEBPA)
along with a catalyst made from 1-butyl glycidyl ether
(BGE), was stored in poly(urea-formaldehyde) (PUF) micro-
capsules, which were prepared by the conventional oil in-
water emulsion process. This process of preparing the PUF
microcapsules has promoted long shelf-life and good chem-
ical stability at temperatures below 238◦C. This system is still
in the early developmental stages, and its self healing effici-
ency within a composite material is yet to be tested.
In sum, the microencapsulation approach is by far the
most studied self-healing concept in recent years. Tab le 2
summarizes the type of self-healing systems investigated
in the literature, and it is noticed that the self-healing
system based on living ring-opening metathesis polymeriza-
tion (ROMP) has attracted most of the research attention.
There are some obvious similarities between the micro-
encapsulation and hollow fibre approaches, but the use of
microcapsules alleviates the manufacturing problems experi-
enced in the hollow fibre approach. The microencapsulation
approach is also potentially applicable to other brittle mate-
rial systems such as ceramics and glasses [93]. On the other
hand, the most successful and extensively investigated self-
healing system comprises the ROMP of dicyclopentadiene
(DCPD) with Grubbs’ catalyst. The synthesis and character-
ization of the DCPD/Grubbs catalyst system have recently
been papered [94], and their use as a self-healing agent has
been widely reported as we mentioned here above. This
system supposedly provides a number of advantages such
as long shelf-life, low monomer viscosity and volatility, and
completion of polymerization at ambient conditions in sev-
eral minute. Further attempts were made to improve the per-
formance of the self-healing system by replacing DCPD
with 5-ethylidene-2-norbornene (ENB) [86] or blending
ENB with DCPD [87]. Microencapsulation of ENB was also
achieved by in situ polymerization of urea and formaldehyde.
This system was supposed to overcome some of the limi-
tations of the DCPD including the low melting point and
the need to use a large amount of catalysts. It is recogniz-
ed that DCPD is capable of forming a cross-linked structure
with high toughness and strength [38,95,96] whilst ENB po-
lymerizes to a linear chain structure and may possess inferior
mechanical properties. However, ENB is known to react
faster in the presence of a lower amount of Grubbs’ catalyst,
has no melting point, and produces a resin with a higher Tg
[38,86]. Hence, a blend of DCPD with ENB was believed
to provide a more reactive healing system with acceptable
mechanical properties, making it more suitable for practical

10 Advances in Materials Science and Engineering
use. Cho et al. [91] chose to develop a completely different
healing system using di-n-butyltin dilaurate (DBTL) as the
catalyst and a mixture of HOPDMS (hydroxyl end-function-
alized polydimethyl-siloxane) and PDES (polydiethoxysilox-
ane) as the healing agent. The polycondensation of
HOPDMS with PDES is alleged to occur rapidly at room
temperature in the presence of the organotin catalyst even
in open air [97,98].
4.3. Three-Dimensional Microchannel Structure Systems. As
reported by the paper of Murphy and Wudl [55], complex
microvascular networks are widely observed in biological
systems, such as leaf venation [99–102] and blood vascu-
larisation [103–105]. Indeed, in the latter case, the human
circulatory system is comprised of vessels of varying diameter
and length: arteries, veins, and capillaries. These vessels
function together in a branched system to supply blood to
all points in the body simultaneously. However, due to their
complex architecture, replication of these microvascular
systems remains a significant challenge for those pursuing
synthetic analogs. As outlined in 2006 by Stroock and Cabodi
[106], these microvascular networks can be created via
soft lithographic methods [107–109], in which all micro-
channels can be fabricated at the same time, laser ablation
[110,111] or direct write methods [112], which are more
suited for building three-dimensional (3D) micro-channel
structures (Figure 3(c)). One of the main advantages of
those systems comparatively to both the hollow fibre and
microcapsule systems is their ability to heal the same location
in the material more than once. Indeed, often, a second
fracture event will occur along the plane of the initial crack.
By providing a material with a quasicontinuous flow of
healing agent, numerous healing cycles can be achieved.
In 2007, Toohey et al. [23] have published one of the first
of these types of composite materials. Authors have reported
self-healing systems that are capable of autonomously repair-
ing repeated damage events. The reported system which
bio-inspired coating-substrate design delivered healing agent
to cracks in a polymer coating via a three-dimensional
microvascular network [112] that was first embedded into
the substrate. This system utilized the healing combination
of liquid DCPD as the healing agent and solid Grubbs’
catalyst to initiate ROMP polymerization of the DCPD.
In the reported work, the catalyst was incorporated into
a 700 μm thick epoxy coating that was applied to the top
surface of the microvascular substrate, and the 200 μmwide
channels were successfully filled with DCPD and then sealed.
This system achieved a peak healing efficiency up to 70%
with 10 wt.% catalyst in the top coating and was able to
demonstrate healing for up to seven cycles. It is important to
mention that the amount of catalyst in the top epoxy layer
did not affect the average healing efficiency per cycle, but
rather limited how many cycles of testing and healing could
be performed successfully. Indeed, once all of the catalyst has
been used, healing ceased due to depletion of catalyst in the
crack plane, even with a continuous supply of monomer.
To overcome this limitation, in 2009, Toohey et al.
[113] have modified their design by photolithographically
patterning four isolated regions within the embedded
microvascular network. Authors have reported the repeated
healing of crack damage in a polymeric coating through
delivery of two-part epoxy, healing chemistry via multiple
microvascular networks embedded in isolation within a
polymeric substrate. They first have created a continuous,
interconnected microvascular network using the direct-write
method. Second, they then have isolated multiple networks
by infilling the network with a phot-ocurable resin and
selectively photopolymerizing thin parallel sections of these
resin-filled microchannels. Epoxy resin and amine-based
curing agents were transported to the crack plane through
two sets of independent vascular networks embedded within
a ductile polymer substrate beneath the coating. The two
reactive components remain isolated and stable in the
vascular networks until crack formation occurs in the coating
under a mechanical load. Both healing components were
wicked by capillary forces into the crack plane, where they
react and effectively bond the crack faces closed. Several
epoxy and curing agent combinations were evaluated for
their suitability in microvascular-based autonomic systems,
and healing efficiencies of over 60% for up to 16 intermittent
healing events out of 23 cycles were successfully achieved.
In a related effort, Williams et al. have published their
version of a microvascular network containing mechanically
stimulated healable material, in the form of sandwich struc-
ture composite configurations that contain either single [24]
or dual [114] fluidic networks. In the single network design,
sandwich structures use high-performing skin materials,
such as glass or carbon fibre composites, separated by a
lightweight core to obtain a material with very high specific
flexural stiffness. A vascular network incorporated into a
sandwich structure would address the larger damage volume
expected of these systems, as well as allowing for multiple
healing events to occur. Samples were fabricated with chan-
nels containing a healing agent, which had a negligible effect
on the mechanical properties of the composite. Rupture of
the vessels released the healing fluid, filling the void that
formed as a result of impact damage on the sample. Initial
tests were run on samples containing premixed resin and
hardener, to demonstrate the healing capability of the system.
Indeed, these samples have shown consistent and complete
recovery of compressive stress at failure after impact damage.
In their dual network design, significant recovery was also
observed when samples were infiltrated with pressurized
unmixed dual fluids [114].
5. Self-Healing Coating Systems for
Metallic Structures
The large economic impact of corrosion of metallic struc-
tures is a very important issue all over the world. Generally,
rapid field-specific testing is done when material failure
is observed. Despite intense research and developments
in corrosion protection coatings of metals and alloys, the
real-world performance results are not always satisfactory.
Furthermore, development of all around coatings to protect
and prolong service life of the infrastructure is still a
big challenge, owing to wide variations in environmental

Advances in Materials Science and Engineering 11
conditions. Therefore, in order to improve the equip-
ment service prediction capabilities of infrastructure, it is
indispensable to develop new state-of-the-art smart/self-
healing coating formulations for corrosion inhibition. In
this context, autonomic healing materials respond without
external intervention to environmental stimuli and have
great potential for advanced engineering systems [6,11,
12,23,29,46,53,85,91,115–126]. Self-healing coatings,
which autonomically repair and prevent corrosion of the
underlying substrate, are of particular interest. Notably,
the worldwide cost of corrosion has been estimated to
be nearly $300 billion per year [127]. Recent studies on
self-healing polymers have demonstrated repair of bulk
mechanical damage as well as dramatic increases in the
fatigue life. The majority of these systems, however, have
serious chemical and mechanical limitations, preventing
their use as coatings. Polymer coating systems are classically
applied on a metal surface to provide a dense barrier against
the corrosive species. Cathodic protection is also used for
many applications, in addition to coatings, to protect the
metal structures from corrosive attack when the coating is
damaged. Hence, self-healing coatings are considered as an
alternative route for efficient anticorrosion protection while
maintaining a low demand in cathodic protection.
Cho et al. [128] have explored two self-healing coat-
ing approaches, starting from the siloxane-based materials
system. In the first approach, the catalyst was microencap-
sulated and the siloxanes were present as phase-separated
droplets. On the second process, the siloxanes were also
encapsulated and dispersed in the coating matrix. Encap-
sulation of both phases (the catalyst and the healing agent)
is advantageous in cases where the matrix can react with
the healing agent. In the other hand, Aramaki [129,130]
has prepared a highly protective and self-healing film of
organosiloxane polymer containing sodium silicate and
cerium nitrate, on a zinc electrode previously treated in
aCe(NO
3)3solution. Self-healing mechanism of the film
was investigated after it was scratched and immersed in the
NaCl solution for several hours, where a passive film has
been found to be formed on the scratched surface, resulting
in suppression of pitting corrosion at the scratch. More
recently, the same group [131,132] has prepared an ultrathin
2D polymer coating, on a passivated iron electrode, which
was subsequently healed in NaNO3. Thus done, localized
corrosion was markedly prevented by coverage with the
polymer coating and the healing treatment in 0.1 M-NaNO3.
Indeed, prominent protection of iron from corrosion in
0.1 M-NaCl was observed. The protective efficiencies were
found to be extremely high in certain cases, where more than
99.9% before the passive film was broken down.
The development of effective corrosioninhibitor coat-
ings for prevention of corrosioninitiation and suppres-
sion of galvanic activity of metals and alloys has always
been a challenging problem. Recent concerted efforts of
researchers at US Army Engineering Research and Devel-
opment Center at the Construction Engineering Research
Laboratory (ERDC-CERL) and at other facilities [133,
134] have led to development of self-healing corrosion
inhibitors, to reduce and/or prevent corrosion of metal
hardware. Previously, heavy metal-based epoxy primer pre-
treatment systems [135], including quaternary ammonium
salt-based and multifunctional microcapsulated corrosion-
inhibitor system [133–135], have demonstrated corrosion
protection performance of metals and alloys. These studies
have demonstrated that the scribe or damaged film area
on otherwise corroded panels experienced little lifting and
blisters, among others, of the film because of the presence of
microcapsules at the scribes. Mehta and Bogere [136]have
evaluated the smart/self-healing microcapsulated inhibitor
incorporated in epoxy primer before painting on a steel
surface, for its corrosion protection effectiveness on exposure
to ASTM (American Society for Testing and Materials)
D 5894 electrolyte in laboratory and natural tropical sea-
shore environment. The “healant” inhibitor was industrial
custom made. Their results have indicated that the active
components in ruptured embedded inhibitor microcapsules
were released into an inflicted scribe primer and topcoat
film on steel surface on exposure to inhibit development
of an electrochemical cell. Undamaged surface film of the
test and control specimens exposed in the environments
demonstrated excellent corrosion-inhibition performance
as reflected by both visual inspection and electrochemical
impedance spectroscopy experimental data.
All of those reported results should provide an under-
standing of the fundamental material-property relationships
of smart inhibitor coatings and, thus, should facilitate the
development of optimized paint compositions in order to ex-
tend the useful service life of steel infrastructure applications.
6. Futures Outlooks
In summary, we finally know that the material degradation
can occur for a wide variety of reasons, such as fatigue
loadings, thermal effects, and corrosion, or, more in general,
for environmental effects of all kinds. However, the materials
durability is probably one of the main challenges encoun-
tered today for structural as well as coating applications. As
the materials failure normally starts at the nanoscale level and
is then amplified to the micro- up to the macro-scale level
until catastrophic failure occurs, the ideal solution would
be to block and/or eliminate damage as it occurs at the
nano/microscale and restore the original material properties.
We have seen that the healing process can be initiated
by means of an external source of energy (stimuli), as it
was shown in the case of a bullet penetration [137]where
the ballistic impact caused local heating of the material
allowing self-healing of ionomers, or in the case of self-heal-
ing paintings used in the automotive industry. In the latter
case, small scratches can be restored by solar heating [138].
Single cracks formed in PMMA specimens at room tempera-
ture were also shown to be completely restored above the
glass transition temperature [4,139,140]. The presence
of noncovalent hydrogen bonds [141] in mechanosensitive
polymers can allow a rearrangement of principal chemical
bonds so that they can be used for self-healing. Numerical
studies have also shown that nanoscopic gel particles, which
are interconnected in a macroscopic network by means of

12 Advances in Materials Science and Engineering
Matrix
Crack
Catalyst
(a)
CNT filled healing agent
(b)
Released healing agent
(c)
Sealed crack
Sealed CNT
(d)
Figure 5: Concept of the self-healing process using carbon nanotubes. Adapted from [37].
stable and labile bonds, have the potential to be used in self-
healing applications.
To date, all the employed techniques are, however, limited
by the container size. Containers should be in the nanoscale
range since larger ones could lead to large hollow cavities,
that could compromise the mechanical properties of the
hosting structural material, and/or the passive protective
properties of the coating material [25]. Moreover, up to date,
advanced materials are designed to be either tough or self-
healing, but typically not both. It would be ideal to have
a material which could be at the same time tougher and
self-repairable, and this is still not possible with current
technologies.
Carbon nanotubes (CNTs) are considered to be an ideal
filler material for mechanical reinforcement as well as ideal
molecular storage devices. This is due to the fact that CNTs
are very small, thus they have an extremely large interfacial
area. CNTs have interesting mechanical and chemical prop-
erties and have a hollow tubular structure. Polymer/CNTs
composites (e.g., [142]) have already shown many promising
results, and various materials, such as hydrogen (H2)[143],
metal and/or metal carbide [144], C60 [145], CH4[146]
and DNA [147], have been successfully inserted inside CNT.
Although a great deal of work has been done with CNTs as
self-storage devices, CNTs have not been yet investigated as
nanoreservoirs for self-healing applications.
The main challenges related with this application are
how to insert molecules into the carbon nanotubes, whether
crack can form on the sidewall of a carbon nanotube during
its propagation, and if the healing agent will come out of
the carbon nanotube when the crack forms. In this avenue,
recently, Lanzara et al. [37]haveinvestigatedtheuseof
CNTs as nanoreservoirs for automatic repairing applications,
through a molecular dynamics (MD) study with particular
focus on the CNTs capacity of delivering a healing agent.
Authors have shown, interestedly, that the CNTs were not
only able to carry the catalytic healing agent for local repair
but also can simultaneously play the role of filler material for
mechanical reinforcement prior and after the delivery of the
active material (Figure 5).
7. Conclusions
In conclusion, we have briefly presented a series of recent
results related to the various self-healing concepts and sys-
tems. Research into self-healing materials is an active and
exciting field, with an increasing number of research papers
being published every year. From the studies on healing in
concrete structures via embedded glass fibres to the more
recent work on healing using shape memory alloy wires in
a polymer composite, and/or the use of multidimensional
microvascular network for the healing applications, the
different avenues being explored to achieve the common
end goal of prolonged functional lifetimes for composite
structural materials are astounding. Beyond a strong interest
of both academic and commercial researchers in the hollow
fibre and microencapsulation approaches to self-healing
polymer development, new types of self healing technology
have been emerging at an increasing rate over the last decade.
Indeed, in recent years, interesting perspectives have opened
for the design of innovative self-healing nanosystems. Com-
puter simulations have provided useful indications for
directing the efforts of scientists toward the fabrication of
repairing systems.
Acknowledgments
The authors would like to gratefully acknowledge the finan-
cial assistance of the Canadian Space Agency for this work,
the Natural Science and Engineering Research Council
(NSERC) of Canada, and the Fonds Qu´
eb´
ecois de la Recher-
che sur la Nature et les Technologies (FQRNT).

Advances in Materials Science and Engineering 13
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