Vol.2, No.8, 873-901 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.28110
Self-repairing material systems―a dream or a reality?
Hartmut Fischer1,2
1TNO Science and Industry, Eindhoven, The Netherlands;
2CSIRO Material Science and Engineering, Clayton South, Australia;
Received 21 April 2010; revised 22 June 2010; accepted 27 June 2010.
ABSTRACT
Currently, most industrial materials rely entirely
on passive protection mechanisms; such me-
chanisms are readily applicable and universal
for many different materials systems. However,
they will always stay passive, and therefore their
lifetime and functionality is limited and related
to the amount of protective additives and the
intensity of their consumption. Therefore, better,
and preferentially active process for the protec-
tion/repair of damaged materials―self-repairing-
processes―were developed and need to be
developed further. Although it sounds futuristic
or like a fiction in the modern, trendy times,
which in many ways affects also directions of
research; self healing of material systems exists
already for a long time in all sorts of systems of
materials or functionalities. The aim of this work
is to go beyond the scope of a classical review
the ones published recently in this field which
almost entirely focused only onto polymeric
systems. In this work, an analysis of the under-
lying functional and constructional principles of
existing natural and synthetically self-healing
systems spanning over a range of classes of
materials is given leading to general rules and
principles for the design of new and application
tailored self-healing material systems.
Keywords: Self Healing; Systems; Sensors
1. INTRODUCTION
Self-healing is an intrinsic property of living organisms,
enabling them to cope with all sorts of damage or injury
they experience during their lifetime. This repair occurs
with essentially no external intervention. Thus, wounds
heal, broken bones heal, and even lost parts of living
bodies (lizard tails etc.) can be replaced in some cases.
Some natural self-healing composite systems such as
bones go beyond simple healing to the extent that they
remodel themselves continuously. Damaged material is
removed and replaced by new material and over-de-
signed material is also removed and structures under
stress are enhanced by additional material. To enable this
process, specific cells entrapped in the bone tissues act
as strain sensors and feel large deformations. Subse-
quently, signals are sent to cells responsible for remov-
ing or forming material (bone) [1].
In order to deal with the phenomenon and the con-
nected principles of self-healing in general, and even
further to design and construct artificial self-healing
functionalities for systems and devices, it is important to
understand the nature and the consequences of damage
and of degradation processes first.
Damage in general terms can be described as changes
introduced to a system that affect its function and/or
performance. That means that damage is not meaningful
without a comparison of two different states of the sys-
tem in question, one of which is assumed to be the initial
(and often un-damaged) state. The term damage does not
necessarily imply a total loss of system functionality, but
rather a departure in level of system operation from op-
timal. A fundamental challenge is the fact, that damage
is typically a local phenomenon and may be therefore
difficult to detect (in time) [2].
Most materials (systems) lose their integrity, their
value, operation or usefulness over time due to degrada-
tion processes where defects grow and coalesce to cause
component and finally system level damage (e.g. fatigue
or corrosion damage accumulation). On a relatively short
time scale, damage can also result from scheduled dis-
crete events such as aircraft landing and from unsched-
uled discrete events such as an impact. As damage grows,
it will reach a point where it affects the system operation
to a level, which is not any longer acceptable for a user;
this point is referred to failure.
The main causes of deterioration of materials may be
summarised as:
1) Harmful materials, substances and agents such as
oxygen, oxidizing agents, water, salts, poisons, active
materials, and living bodies such as virus, bacteria, fungi,

H. Fischer / Natural Science 2 (2010) 873-901
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insects, animals, human beings and others,
2) Factors from the surroundings such as heat, visible
light, external mechanical force e.g. strong wind, radia-
tion, pressure, rain, collapse of the adjacent structure, a
sudden impact and others.
Such deterioration (or damage) are in general irre-
versible, they occur progressively and exponentially in
time until a certain threshold is reached where either
significantly high costs for repairs are necessary or a
(fatal) failure is going to occur. Increased lifetime and
reliability of materials systems and devices can be criti-
cal requirements in traffic, construction, information
transfer, medicine, military, space missions as well as in
ordinary daily life.
In order to prevent such detoriation the following
response mechanisms are known:
PASSIVE (Built-in damage prevention): by protec-
tion agents, that are directly attacked by the deterioration
and the concentration of the agent is directly related to
the amount of protection and that provide no repairabil-
ity and hence no memory of the inherent structure.
ACTIVE (Autonomous or self-repair): here the dete-
rioration attacks directly atoms and molecules of the
material itself instead of protection agents. The amount
of repairing agents needed depends on the concentration
of damaged sites and the accumulation of damage in
time and at certain locations as well as on the repair rate;
a transfer of material is needed and a memory of the
original structure is preferable.
The pictures in Figure 1 show schematically the de-
scribed mechanisms applied to the case of polymers ma-
trices as damaged material.
Currently, most industrial materials rely entirely on
passive protection mechanisms; such mechanisms are
readily applicable and universal for many different ma-
terials systems. However, they will always stay passive,
and therefore their lifetime and functionality is limited
and related to the amount of protective additives and the
intensity of their consumption.
Therefore, better, and preferentially active processes
for the protection/repair of damaged materials―self-
repairing-processes―were developed and need to be
developed further. Although it sounds futuristic or like a
fiction in the modern, trendy times, which in many ways
affects also directions of research; self healing of mate-
rial systems exists already for a long time in all sorts of
systems of materials or functionalities.
The aim of this work is to go beyond the scope of a
classical review which almost entirely focused only onto
polymeric systems like the ones published recently in
this field [4-6]. In this work, an analysis of the underly-
ing functional and constructional principles of existing
natural and synthetically self-healing systems spanning
over a range of classes of materials is given leading to
general rules and principles for the design of new and
application tailored self-healing material systems.
Therefore, specific choices from the existing literature
Figure 1. Passively protective mechanism versus active protective mechanism. Reference [3]: 1) Degradation factors are
prevented from interacting with polymer chains, 2) Concentration of stabilizer deteriorates, 3) Degradation factors in-
teract directly with polymer chains, 4) Degradation of polymeric chains, 5) Degradation factors directly interact with
polymer chains and degradation occurs temporarily, 6) Cleavage of main chain, 7) Catalytic re-bonding reaction occurs,
8) Catalyst becomes reactivated.
 
 

 

H. Fischer / Natural Science 2 (2010) 873-901
875
have been made for this analysis, which either show new
principles or contribute to this aim without repeating
already existing systems in a similar or slightly different
way.
In the next section, a discussion of the phenomenon of
repair and especially self-repair is presented, together
with the principal design features and requirements that
enable a self-repair to take place.
Examples of self-repairing systems in material and
life science starting with assisted (stimulated) repairing
and of self repairing systems already known or currently
under development are discussed in Section 3. This is
followed by a discussion of the sensors and triggers em-
ployed or potentially employable in self-healing systems
in Sections 4 and 5. Finally, conclusions are drawn with
respect to requirements for the design of self-healing
systems and working areas necessary to concentrate on
are presented (in the Sections 6 and 7).
2. WHAT IS SELF REPAIR?
Repair itself is a process which can be initiated after
damage on a local or global scale of a given functional
system with the aim, to reduce the local or global level
of damage and to extend or to renew the functionality
and life-time of the damaged part, system or device.
Self-repair is then in principle an autonomically initi-
ated response to damage or failure. In order to perform
this repair, any self-repair system must be capable of 1)
identifying and 2) repairing failures. If failure is classi-
fied as ‘any occurrence, which results in the system de-
viating from its original task or not being any more able
to fulfil its function’, what is than ultimately classified as
‘repair’? (Self) repair can be classified under two sig-
nificant approaches:
Attributive repair and
Functional repair
Attributive repair are attempts to restore the attrib-
utes of the system to their original state, the full capacity
of the system.
Functional repair are attempts to restore the function
of the system. If full functionality cannot be restored,
this strategy attempts to focus the remaining available
resources to maximise the available functionality.
Attributive repair is the optimal solution. If the at-
tempt to restore the system to its complete original con-
dition fails, there is still significant benefit to be gained
in most instances if the system continues to operate,
even with reduced functionality.
Attributive self-repair is an intrinsic property of nature.
Figure 2 shows a schematic example of the steps in a
self-repair cycle in nature, in this case bone failure due
to fracture. This autonomic healing is initiated by the
(a) (b) (c) (d) (e)
Figure 2. Schematic picture of the healing stages of bone: a)
internal bleeding (sensor), forming of a fibroin cloth; b) unor-
ganized fiber mesh develops (first step of (primitive) repair); c)
calcinations of the fibrocartilage (second step of repair); d)
calcification converted into fibrous bone (structural repair); e)
transformation into lamellar bone (complete recovery of intrin-
sic structure and functionality) [7].
local damage of the vascular blood system. Healing a
bone requires energy in the form of nutrition together
with the delivery of cell material to the fracture site by
the network of blood vessels; for healing a sufficient
healing time must elapse. The healing process consists of
multiple stages of deposition and assembly of material.
Mimicking nature is the ultimate goal for materials
designers and developers.
There are essentially two design approaches to realize
self-repairing systems:
1) a conventional design modifying existent design
philosophies by incorporating intelligent techniques at
the design stage to obtain a system with sufficient
autonomous capabilities to initiate and effect a func-
tional and in ideal cases an attributive repair.
2) a radically new design philosophy, where the sys-
tem is designed in a cellular structure such that each cell
contains a (genetic) code, which defines and determines
its functionality. This term is called embryonics and it
attempts to replicate the cellular structure of organisms
in nature and their inherent self-replicating and thus
self-healing capabilities resulting mostly in attributive
repair.
Although both strategies will certainly lead to new
self-repairing systems, the conventional process will be
easier to implement in the short term.
Generally, self-repair processes in smart systems
should not only replace any passive protection mecha-
nism but should finally lead to structures, which are able
to sense its internal state, to monitor the healthiness of
the material/functionality/system/device at any time and
at any location and respond in a manner that fulfils its
functional requirements based on the gained informa-
tion―sensing/detection, diagnosis of failure and activat-
ing of repair!
In order to perform properly, a smart system must
have incorporated (embedded or surface mounted) ac-

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tuation and sensor elements which offer the capacity of
health monitoring (to be able to diagnose, interpret and
correct structural faults in situ as they occur) by meas-
urement of physical quantities such as vibration, perme-
ability, current flow, strain, acoustic emission, imped-
ance, pH-changes, colour tracers, etc., that are informa-
tive with respect to the state of structural health. Meth-
ods and instruments are X-ray tomography, ultrasonic
methods and eddy current methods and strain- gauges,
optical fibres, piezos and in general “smart tagged”
composites.
Specifically, information relating to the severity, sig-
nificance and location of damage is required. Therefore,
a detection of:
1) Damage occurrence,
2) Damage location,
3) Damage type
4) Damage force magnitude and consequently
5) The remaining lifetime of the structure is necessary
for maintaining the functionality of the material/struc-
ture/device. The information obtained has to be fed into
the recovery cycle as sketched in Figure 3.
The interpretation and operational evaluation of the
obtained data may include on top of the data acquisition
a normalisation and filtering process, a feature selection
and information condensation and finally a statistical
model development for feature discrimination. This di-
agnosis will lead to an eventual initiation of the repair
action.
Finally, to perform a repair action, mobile parts are
needed to be transported to the location of the damage.
This can be realised using a liquid either locally avail-
able or available via a vascular network system like the
blood system in living organisms but also a transport via
the gas phase is possible, in case material for the repair
action is needed.
Self-healing or self-repair can therefore in summary
be defined as “the ability to substantially return to an
initial, proper operating state or condition prior exposure
to a dynamic environment by making the necessary ad-
justments to restore to normality and/or the ability to
resist the formation of irregularities and/or defects”. It is
a much more effective damage management decreasing
damage level during certain stages of life time than pas-
sive prevention, since it is only applied in cases, times
and locations of real need―damage which jeopardizes
the functionality of a given system. Ideally, a self-heal-
ing system will therefore display an unlimited lifetime
since the rate of damage formation is negative or zero.
Figure 3. Health monitoring and repair cycle for an active
smart self-repairing system [8]. The system monitor must be
able to analyse system performance against predefined per-
formance benchmarks in order to continuously verify correct
operation of the process. The testing process also must have
bi-directional communication to activate specific test se-
quences and receive results. This process should exploit any
built -in self-test ability, and the testing results are forwarded to
the diagnostic process. Automating the fault diagnosis process
inherently demands the application of intelligent techniques.
Finally, the self-repair process must be able to affect a repair
based on the reduction in resources available, following isola-
tion of the faulty components. If the original level of perform-
ance cannot be restored, the monitoring system must be modi-
fied to reflect the changes in system characteristics. This fea-
ture demands an optimisation capability. Reprinted from Engi-
neering Applications of Artificial Intelligence, Self-repair of
embedded systems, 17, E. A. Coyle, L. P. Maguire, T. M.
McGinnity, 1-9, Copyright (2004), with permission from El-
sevier.
3. EXAMPLES OF SELF REPAIRING
SYSTEMS IN MATERIAL AND LIFE
SCIENCE
In this section a number of existing self-repair systems is
critically analysed with respect to its design, functional
units/modules and their action/performance.
Within all classes of materials, the self-repair potential
of polymers is probably the largest of all materials, since
the polymeric nature offers more possibilities of devel-
oping and displaying molecular mobility than any other
material. However, and as discussed below, this does not
exclude other materials from a participation in functional
self-healing systems.
The different options for the design of the repair ac-
tion of a polymeric based system connected with exam-
ples of how this can be realized are illustrated in Figure

H. Fischer / Natural Science 2 (2010) 873-901
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4 [9] and will be discussed in the following paragraphs.
Essentially a differentiation between nature-analogous
true autonomous self-repairing systems and other sys-
tems, which may be only able to react onto damage and
to make the necessary adjustments to restore itself to
normality with human intervention, and may be there-
fore better called assisted healing systems, can be made.
Assisted (local) healing can be achieved, e.g. by an in-
crease of temperature, by irradiation or by other stimuli.
3.1. Thermally Stimulated Repair
A repair of accumulated damage may be triggered by
thermal stimulation where the system allows the flow of
material, wetting of the crack surfaces and gluing them
together in a heating-cooling cycle.
This can be most easily achieved in a system contain-
ing polymeric chains, which during the transition from
the glassy to the rubbery state experience a huge in-
crease in mobility and hence the possibility to re-entan-
gle after physical separation.
The model to explain the movement of a polymer
molecule in a snake-like fashion inside a cross-linked
polymeric gel, also known as the reptation model as
proposed by de Gennes [10] implies also a possibility of
polymeric chains to repair damage. Prager and Tirell
[11], Jud and Kausch [12] and later Wool and O’Connor
[13] used the same reptation model to determine the time
required for healing caused by the inter-diffusion of
molecules between crack faces and thus re-gaining
strength by annealing just above the Tg. Kim and Wool
[14] predicted theoretically the recovery process as a
function of healing time confirming existing experimen-
taldata [15]. Also, it has been shown that, if amorphous
bulk samples of high-molecular-weight polystyrene (PS)
were brought into contact with themselves in a lap-shear
joint geometry below the bulk glass transition tempera-
ture, an increase in lap-shear strength and fracture oc-
curred [16,17]. This can be attributed to the time–tem-
perature shift of the glass transition, as well as to the
possible enhanced surface molecular mobility of poly-
mer chains leading to a lower Tg at free polymer surfaces
compared to the bulk Tg which enables inter-diffusion of
chains and self-healing even below Tg! At the present
time, a decrease in density and a decrease in the entan-
glement density close to the polymer surface can be con-
sidered as the factors contributing to the increased mo-
lecular mobility at polymeric surfaces in comparison with
that in the bulk [18]. In addition to these factors, a "jump"
in the conformational entropy of the chain segments lo-
cated at the surface (these segments have a decreased
entropy imposed by the polymer-air interface [19] upon
the contact of the surfaces) may be considered as one of
the driving forces of inter-diffusion across the interface.
In principle, self-repairing polymers can therefore be
realized essentially attributed to molecular inter-diffu-
sion. Polymer networks with dangling chains can there-
Figure 4. Organization of self-healing polymer based material systems according to the different principles employed adapted from [9].

H. Fischer / Natural Science 2 (2010) 873-901
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fore be employed to heal cleavages at room temperature
without any manual intervention [20].
However, self-repair needs time and rest. In a later
study, the concept of re-entanglement of polymeric chains
by reptation has been applied in thermoset composite
materials where a linear thermoplastic polymer material
was inserted into an epoxy matrix leading to a recovery
of up 70% of the fracture toughness [21]. In this case as
in the previous cases, multiple repairs are possible [22].
Alternatively, Zako and Tanako embedded small grain-
particle adhesives in a glass epoxy composite laminate
[23] an Meure, Wu and Furman polyethylene-co-me-
thacrylic acid particles in an epoxy resin [24] to act as
repair agents. The 50 micron sized particles consisted of
thermosetting type epoxy; the matrix was a cold-setting
epoxy. Damage can be repaired by the particles acting as
repairing actuator when melted by heat. A similar mecha-
nism can be used for the thermally stimulated self-repair
of glass-ceramic composites, here heals the flow of the
viscoelastic glass matrix at higher temperatures aging
induced micro-voids in the composite material [25].
A very efficient and fast assisted repairing system by
external stimulation consist just of one material capable
of fulfilling all functions (structural properties, sensor
and providing of mobility) by reversible formation of
covalent or non-covalent bonds (ionic or H-bonds).
Thermally controlled covalent bond formation is already
long known in organic chemistry and may be applied to
the formation of linear polymers as well as to the forma-
tion of polymer networks. Besides thermally reversible
urea [26], alkoxyamine units incorporated in the main
chain [27], nitroso-dimerisation and ester formation un-
der presence of cyclic anhydrides [28], the (4 + 2) Di-
els-Alder reaction is by far the most important reaction.
It was already discovered in 1928 [29] and honoured
with the Nobel Prize in 1950 and generally considered
the "Mona Lisa" of reactions in organic chemistry since
it requires very little energy for the formation of a cova-
lently connected ring-structure. This reaction enables
step polymerizations and cross-linking reactions together
with a thermally reversibility and is therefore ideally
suited for thermally stimulated polymer repair and po-
tentially, recycling. Mostly, but not exclusively, the reac-
tion between furane and maleimide derivatives is used
for the generation of a thermally reversible bond. This
has been applied already in 1969 for the synthesis of
reversibly cross-linked networks, which yielded a tough
rubbery film at 100ºC and returned to a re-mouldable
polymer at 140ºC [30]. Subsequently, this principle has
been further developed and the suitability of the system
for self-healing bulk materials demonstrated [31-33].
These thermally reversible bond (cross-link) formations
leading to self-healing properties upon thermal stimula-
tion using the Diels-Alder reaction (see Scheme 1) can
be used in application areas like coating systems [34].
Using this concept, powder coatings can be applied in
their cross-linked state, which has distinct advantages
with respect to their storage stability. When heated above
the threshold temperature flow should be sufficient to
enable proper film formation. As soon as the surface is
of sufficient quality, cross-linking can be started―sim-
ply by cooling down! In a fully cross-linked state of a
powder coating system, i.e., below a certain threshold
temperature, the coating will exhibit its essential proper-
ties with respect to mechanical strength, etc., but in its
(partly) de-cross-linked state, above the threshold tem-
perature, it will show a certain level of plasticity and
flow. This plasticity will decrease problems in the area
of film formation of powder coatings, will enable repair
of the coating using their self-healing properties (see
Figure 5).
Scheme 1. Schematic representation of the Diels-Alder reac-
tion used for thermally stimulated repair.
Figure 5. Visualisation of the self-healing of a cross-linked powder coating based on the acrylate copolymers: a) crosslinked powder,
b) molten powder, c) damaged coating, d) re-flow, e) repaired coating [34], scale bar = 5 mm.

H. Fischer / Natural Science 2 (2010) 873-901
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Besides application in coatings [35-38], reversibly
cross-linkable materials are of interest for several areas
of applications including thermally reversible bonding
[39], thermally reversible solubility of polymers [40],
processes connected with the recycling of plastics [37],
encapsulates which open at elevated temperatures, re-
versible data storage medium [41] and also as matrix
material in composites [42,43]. Here, polymeric materi-
als with dicyclopentadiene units in the polymer back-
bone were used as a matrix for re-mending composites
reinforced with graphite fibers [42]. The graphite fibers
are used as electrical conductors to provide the necessary
heat to the polymer. Microcracks, introduced by bending
the substrate were healed by applying electric currents.
Alternatively, arrays of conductive, electromagnetic
scattering elements such as copper wires and coils were
incorporated together with reinforcing fibres into a ma-
trix consisting of a thermally reversible crosslinked net-
worked base on a retro-Diels-Alder mechanism [43].
Since the wires are uniformly distributed within the ma-
trix they can be used to heat the composite quick and
uniformly by absorption of electromagnetic radiation.
Furthermore, the reinforcing fibres are also contributing
to the healing mechanisms, since they display a negative
coefficient of thermal expansion (Kevlar: –2 to –6 ppm/K)
and ensure therefore a mechanical closure of cracks
while raising the temperature and increasing the healing
efficiency in such a way to a great deal.
In a similar way, embedded shape-memory alloy wires
[44] as well as a syntactic shape memory polystyrene
foam [45] have been used to improve the performance of
an existing self healing system. The improvement in
performance due to crack closure enables multiple self-
healing actions and increases crack fill factors during the
repair action.
Non-covalently bonded polymeric systems are also
known to heal upon thermal treatment. For example a
supramolecular network can be established by thermally
reversible H-bonds leading to rearrangements of the
molecules in the non-bonded, heated state and reforma-
tion of the network in the “cold” state [46,47].
A reversible formation of ionomeric “cross-links”―
clusters, which are formed by phase separation of the
charged parts of macromolecules from the neutral chain
segments can be used also as a self-repair mechanism
being repeatable many times depending primarily on the
stability of the polymer. Although it is not an autonomic
process, it is activated when thermal energy is trans-
ferred to the polymer. During high-speed impact, energy
is absorbed which is elastically stored and dissipated as
heat. This increases the local temperature of the im-
pacted polymer above the melting point (disrupting the
physical cross-links) while having little effect upon the
temperature of the surrounding matrix. The ionic do-
mains persist in the melt so that the polymer can be
elongated to high levels of strain and rebound elastically
when the stored energy is released at failure. The level of
stretching enables the polymer to seal the cavity as it
returns to its original position, but it is the viscoelastic
properties and the capacity of physical cross-links to
reform that determines the final level and strength of
healing. The structural integrity of the polymer provides
sufficient strength in the melt to prevent other deleteri-
ous effects from polymer flow [48,49].
3.2. Non-Thermal Stimulation of Self Repair
In a similar manner to the retro-Diels-Alder compounds,
reversible cross-links can be established by photo in-
duced cross-linking, which can be reversed by irradia-
tion with a different wavelength to heal cracks, such as
with cinnamic acid derivatives [50].
A covalently cross-linked network containing ally sul-
fides is able to undergo photomediated, reversible
cleavage of its backbone to allow chain rearrangement
for rapid stress relief at ambient conditions without me-
chanical property degradation [51]. The key to this re-
versible backbone cleavage is addition-fragmentation
chain transfer. A reaction diffusion of radicals through
the cross-linked matrix occurs initially by reaction of a
radical with an in-chain functionality, forming an inter-
mediate, which in turn fragments, reforming the initial
functionality and radical. This addition-fragmentation
process alters the topology of the network, but the poly-
mer chemistry and network connectivity remain un-
changed.
Alternatively, also an electrical current run through a
conductive material system may be used to assist healing
[52]. By imparting conductive properties into these ma-
terials, one may obtain ‘real-time’ status of a material’s
structural integrity through electric feedback mecha-
nisms. This feature could lead to new approaches for
detecting and quantifying microcracks which further
could lead to materials capable of recording their
stress/load histories. Other possibilities include using
electric fields or currents as healing function. Upon the
formation of a microcrack, the total number of electron
percolation pathways within the material should de-
crease. As a result, its inherent electrical resistance
should increase accordingly. If the material is integrated
into a circuit, the drop in conductivity could be used to
trigger a simultaneous increase in the applied electric
field. Considering that the microcrack is the source of
the increased resistance, this voltage bias should result in
the generation of heat localized at the microcrack. By
harnessing the generated thermal energy to overcome
kinetic barriers, the system may be electrically driven

H. Fischer / Natural Science 2 (2010) 873-901
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back to its original (i.e. a low resistance/high-current)
state. This has been shown using N-heterocyclic carbines
and transition metals [52].
It is well known that “crack healing” of polymers is
observed via stimulation using a specific solvent. This
phenomenon is called “solvent healing”. In this healing
process, the solvent is applied onto the fractured poly-
mer until healing has occurred and then removed. The
observed crack healing is a superposition of two differ-
ent mechanisms: firstly, the formation of a close contact
of the crack surfaces initiated by internal compression
induced by surface swelling, and secondly, the self-dif-
fusion of polymer chains across the interface forming the
physical links (entanglements) between polymer chains
coming from different sides of the interface promoted by
the action of the solvent lowering locally the Tg [53,54].
This concept can also be used for a built-in self-healing
system, as described below.
3.3. Synthetically Designed Autonomously
Self-Repairing Systems
3.3.1. Ceramic Systems
An existing self-healing material or system can be found
in the layers applied in barrier systems (liners) to ensure
security of deposits of dangerous waste. Here, geo-syn-
thetic clay liners (GCLs), which contain natural sodium
bentonite encapsulated between geo-textile components,
are the sealing elements [55]. Desiccation cracks, which
cause a significant increase of the permeability, can be
repaired by the self-sealing properties of calcium and
sodium bentonite.
Alternatively, self-healing sealing elements based on
the principle that two or more parent materials (for ex-
ample pozzolanic material and lime containing material)
placed in vertical or horizontal layers, are described.
These materials will react at their interfaces to form in-
soluble, barrier reaction products and heal themselves
after being fractured (see Figure 6 [56]).
Curiously, the same strategy has been operating in the
construction for centuries. Many ancient Roman con-
structions made from limestone survived only because of
the self-healing capacity of limestone in interaction with
moisture/water. The phenomenon of self-healing in con-
crete has been known for many years [57-62]. Cracks in
reinforced concrete are unavoidable and the corrosion of
reinforcing steel due to de-icing salts or sea water is a
major course of deterioration of reinforced concrete
structures while disrupting the naturally formed corro-
sion protective passivation film. It has been observed
that some cracks in old concrete structures are lined with
white crystalline material suggesting the ability of con-
crete to self-seal the cracks with chemical products
(CaCO3), most likely with the aid of rainwater and car-
bon dioxide (see Figure 7). The healing of cement re-
sults in the voids being filled with hydration products
which have a bulk volume approximately 2.1 times the
original volume [63], an effect which is discussed below.
However, efficient self-healing occurs only for cracks
not exceeding a certain width (ca 135 microns) [64].
Unfortunately, this mechanism not always work in
normal concrete because the width of the tensile cracks
cannot be easily controlled or tuned. Localized fracture
leads to continued increases in crack width under de-
creasing tensile load, and rapidly exhausts the amount of
chemicals available for crack sealing and composite
re-healing. Thus, it is critical that the tensile crack width
is controlled and must be limited to a few tens of mi-
crons. Again, the success of the repair action depends
Figure 6. Schematic illustration of the self-sealing/self-healing feature of a pozzolanic and lime containing barrier material system: a)
before fracture; b) after fracture. Reprinted from Waste Management, Laboratory development and field demonstration of
self-sealing/self-healing landfill liner, 25, C. Shi, R. Booth, 231-238, Copyright (2005), with permission from Elsevier.

H. Fischer / Natural Science 2 (2010) 873-901
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strongly on the damage degree accumulated. When the
damage degree is less than the threshold, the self-healing
ratio of concrete is increased with the increase in dam-
age degree; once the damage degree exceeds the thresh-
old, the self-healing ratio is decreased with the increase
in damage degree [65].
In recent years, fiber-reinforced cement-based com-
posites optimized for ultra high tensile ductility with a
minimized amount of fibres have been designed based
on micromechanics design tools [66]. Such materials,
defined as Engineered Cementitious Composites (ECC),
display “plastically yield” under excessive loading
through controlled micro-cracking while suppressing
brittle fracture localization and thus enabling self-repair
actions.
The potential of calcite-precipitating bacteria for con-
crete or limestone surface remediation or durability im-
provement has only recently been investigated and is a
possible alternative to repair cracks in concrete [67].
In the case of composite materials, it is also possible
to use an inorganic phase to perform the self-repairing
process, as demonstrated in the case of decalcifica-
tion-hydration reactions of hydraulic (CA) fillers in
poly-phenylenesulfide (PPS) coatings used in geother-
mal applications. The hydraulic fillers, which can heal
and repair micro-sized cracks appearing on the surfaces
of corrosion protection coatings, are embedded into the
matrix material (PPS) [68]. The decalcification-hydra-
tion reactions of the CaO-Al2O3 and CaO-2 Al2O3 reac-
tants present and exposed in the cracks lead to the rapid
growth of boehmite crystals, densely filling and sealing
the cracks, while the calcite leaches out of cracks be-
cause of the formation of water-soluble calcium bicar-
bonate (see Figure 8).
Healing and repairing of micro-sized cracks generated
on the surfaces of the PPS coating was observed after
exposure of the cleaved coatings to a simulated geo-
thermal environment (200°C, CO2-loaded brine). During
exposure for 24 h block-like boehmite crystals (ca. 4 m
in size) filled and sealed the open cracks. This was re-
flected in an increase in pore resistance up to two orders
of its magnitude compared with that of cleaved coatings
without fillers [68].
3.3.2. Viscoelastic Recovery and Healing
Micro-damage self-healing also exists in asphalt mix-
tures. After the removal of external load, two processes
occur: the first is viscoelastic recovery in the bulk of the
material and the second is healing in a fracture process
zone [69]. Viscoelastic recovery occurs in the bulk of the
material only after a stress or strain is induced which is
sufficiently large to generate damage. The phenomenol-
ogical difference between viscoelastic recovery and
healing is that the former is due to the rearrangement of
molecules within the bulk of the material, whereas the
latter is due to the wetting and inter-diffusion of material
between the two faces of a micro-crack to achieve prop-
erties of the original material. The three primary steps in
the healing process are:
1) Wetting of the two faces of a micro-crack
2) Diffusion of molecules from one face to the other
3) Randomization of the diffused molecules to reach
the level of strength of the original material
Again, especially rest periods introduced at certain
damage levels increase the fatigue life of asphalt binders
[69].
A very recent example of self-healing via reformation
of weak, fracture interrupted bonds has been demon-
strated on a supramolecular rubber employing molecules
that associate together to form both chains and cross-
links via hydrogen bonds [70] (see Figure 9).
By mixing ditopic and multitopic molecules, which
are able to associate with more than two other molecules,
a network could be formed. The system shows recover-
able extensibility up to several hundred per cent and
very little creep under load. In contrast to conventional
cross-linked or thermo-reversible rubbers, these systems,
when broken or cut, can be simply repaired by bringing
together fractured surfaces in contact―an example of
autonomous self-healing at room temperature. The
process of breaking and healing can be repeated many
Figure 7. Crack blockage in mortar specimen by CaCO3 after
NaCl solution exposure, crack width = 49 microns. Reprinted
from Cement and Concrete Research, Permeability and
self-healing of cracked concrete as a function of temperature
and crack width, 33, H.-W. Reinhardt, M. Jooss, 9131-9136,
Copyright (2003), with permission from Elsevier.

H. Fischer / Natural Science 2 (2010) 873-901
882
Figure 8. Self-repair action of a decalcification-hydration reaction on a polymer composite surface [68].
Figure 9. Schematic view of the Supramolecular reversible
network formed by mixtures of ditopic (blue) and tritopic (red)
molecules associated by directional H-bonds (represented by
dotted lines) as employed for the self-healing rubber. Reprinted
by permission from Macmillan Publishers Ltd: [Nature]
(Self-healing and thermoreversible rubber from supramolecular
assembly, P. Cordier , F. Tournilhac , C. Soulie-Ziakov, L.
Leibler, 451, 977-980), copyright (2008).
times. However, the time needed to establish a contact of
the fractured surfaces determines the healing efficiency.
Longer healing times lead to better healing, but even
when contact time is as short as fifteen minutes a re-
paired sample can be deformed up to about 200% with-
out breaking.
In order to be self-mending, the supramolecular rub-
ber has to be made from small molecules and the su-
pramolecular associations have to be strong and long-
lived so that at equilibrium, the fraction of non-associ-
ated groups in the network is low. However, the strength
of the associations has to be lower than that of covalent
bonds so that, when broken, many non-associated groups
are present near the fracture surface. Self-healing is effi-
cient because a large number of groups ‘eager’ to link is
available. At shorter healing times, fewer bridges across
the interface are formed and the elongation at break is
lower. When the sample is not mended immediately after
being broken, the number of non-associated groups
available for healing progressively decreases as they find
other partners for H-bonds and become unavailable for a
re-formation of the entangled network.
3.3.3. Self-Healing by Phase Changes and
Volume Expansion
The self-healing potential in metals and metal alloys is
connected with a dynamic precipitation of atoms and the
consequent potential to delay crack initiation and crack
propagation as observed in Al alloys similar to a plastic-
ity-induced crack closure. When a dislocation interacts
with a precipitate, solute atoms will be transferred to the
precipitate, causing Ostwald ripening, if their binding
energy to the precipitate exceeds that to the dislocation.
Conversely, solute atoms will be removed from the pre-
cipitate if their binding energy to the dislocation is
greater. Dynamic precipitation tends to compensate for
micro structural damage, with the result that a higher
integrity structure is maintained for an extended lifetime
[71]. Alternatively, the precipitation of dissolved atoms
like B and N under formation of BN on creep cavity
surfaces (in for example austenitic steel) leads to a
self-healing of the creep cavitations together with an
increase in creep rupture strength and ductility [72].
10

m
10 m

H. Fischer / Natural Science 2 (2010) 873-901
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Oxidative self-recovery of surface damaged ceramics
and ceramic composites may also be a useful self-repair
mechanism (Figure 10). Oxidation of NiAl/α-Al2O3
composites provides the metamorphic surface layers
consisting of NiAl2O4 and Al2O3 with compressive stress
to improve the mechanical properties [73]. The peculiar-
ity of this material is the dominant formation of NiAl2O4
along with the grain boundary of the matrix Al2O3 by the
reaction,
232 24
2NiAlAlO(boundary)5/2O2NiAlO
The rapid diffusion of the reaction product through the
grain boundary results in the repair of surface defects
(see Figure 10).
This concept has been further developed for thermal
barrier coatings with a composition of NiCrAlY, again
with Al as reactive element. This enables a reaction with
oxygen to form a protective barrier layer under volume
expansion in response to damage due to thermal mis-
match during thermal cycling. Here, an addition of 0.1-
0.2% Zr leads to an optimisation of the self-repair proc-
ess [74]. Similar concepts were reported for the systems
ZrB2/SiC [75], SiC/Al2O3 [76], SiC/Si3N4 [77], BN/SiC,
B4C, SiC/B4C, MoSiC2 containing ceramics and Ti3AlC2
[78], where the surface crack oxidation leads to an effec-
tive volume expansion and, thus, to a filling of the crack
opening.
Volume-expanding phases are of course most suited to
be applied in self-healing systems, since they offer the
possibility to fill the empty crack volume completely and
to re-generate the structural integrity of the system. This
requires a material identically or similar in properties to
the damaged matrix material. Examples include phase
transition tetragonal-monoclinic (e.g. in ZrO2) phase
transitions,      phase transitions as seen in sulphur,
TRIP transformation, piezo-effects, curie-transitions and
others which induce a volume change of 1-4%, sufficient
to fill cracks and to perform a healing operation. Those
changes are mostly triggered by a critical strain
 or/and
by temperature. More simply is the healing of zirconia
and especially yttria-stabilized zirconia (YSZ) where an
efficient damage recovery in large-scale molecular dy-
namics simulations has been observed [79]. Dynamic
annealing is highly effective in zirconias during the first
5 ps of damage evolution, especially in and due to the
presence of oxygen structural vacancies. The number of
anion displacements in YSZ keeps increasing with time
instead of reaching a steady value as in ZrO2. This dem-
onstrates the role played by structural vacancies in the
migration of O2-. Defect diffusion plays an importantrole
in the dynamic recovery of radiation damage. Espe-
Result of the crack healing:
Damaged structure
Repaired structure
Figure 10. Crack healing of ceramics due to oxidative self-recovery of ceramics [73]. Flexural strength (a) without defect, (b) with
defects, (c) annealed, (d) repaired, (e) removal of defect, (f) oxidized without defect, and (g) oxidized after removal and fracture sur-
face of (A) with defects and (B) repaired materials, the symbol o corresponds to the right axis.
30

m
200

m
2 m
30

m
2 m

H. Fischer / Natural Science 2 (2010) 873-901
884
cially in YSZ, the anion interstitials almost completely
annihilate with structural vacancies, resulting in no re-
sidual radiation-induced defects on the anion sublattice
(see Figure 11).
This results in near-complete recovery of damage.
Damage recovery on the cation sublattice is assisted by
the anion sublattice recovery, which explains the re-
markable radiation tolerance of stabilized zirconia.
If those concepts are not applicable, a possible reac-
tion with components provided by the environment us-
ing transport via the gas phase might be an option for a
self-repair action. This principle is especially useful in
coatings where dimension constraints prevent the use of
other options for self-repairing design (see below).
Coatings should not only protect a surface from a possi-
ble damage and/or corrosion but should also provide
additional properties like anti-reflection, gas barrier etc.
However, if coatings are damaged and/or pinholes
formed, serious damage could occur due to corrosion on
and in the substrate. To fill such damage areas and/or
pinholes and to restore the passive corrosion protection
or other functionalities, the coating can be loaded with
nanoparticles, which react upon volume expansion with
moisture or oxygen. In a practical example, raw smectic
clay has been used as the phase being able to absorb
moisture under volume expansion [80]. In a later study,
this concept has been developed further leading to a true
recovery of damaged surfaces [81]. There, the option of
healing surface scratches in multi layer coatings consist-
ing of a polysiloxane film on top of a thin montmorillo-
nite layer as expandable phase was studied (see Figure
12). Filling of cracks in the polymeric topcoat occurred
primarily within the first two hours of exposure to mois-
ture-saturated air and resulted in a good restoration of
the surface flatness [81].
3.3.4. Mimicking Nature
Mimicking nature has resulted in a variety of self-heal-
ing systems. One of the first synthetically design sys-
tems developed, tested and used in ceramics (concrete)
and subsequently in polymeric matrices compromising
all three functionalities (damage sensor, healing mobile
materials, structurally integer functioning system) are
resin filled glass capillaries incorporated in the matrix
material [82-90]. Upon damage or fracture, the glass
capillaries act as a strain sensor, releasing the encapsu-
lated adhesive components (epoxy and hardener,
cyanoacrylate etc.) into the composite structure to per-
form the repair action while (partially) filling the crack.
The damage can be sensed/visualized by adding a fluo-
rescent dye to the components (see Figure 13); the cap-
sules (glass capillaries) also serve as part of the (rein-
forcing) composite structure.
This ‘bleeding composites’ approach thus combines
damage detection with a high sensitivity due to the con-
tinuity of the hollow fibres and self-repair, however, also
with two draw-backs: 1) they will release an excess of
healing liquid since the volume of the compartments is
often much larger than the damage volume and 2) they
will display a tendency to clotting since the propagation
of the repairing reaction of the stored agent within the
hollow fibre can still occur thus preventing the chance of
multiple repair.
Systems following a similar idea were developed for
an application in thermosets (epoxy); the approach has
been applied to other matrix materials as well [91-94].
Here, the resin containing containers are hollow poly-
meric capsules (see Figures 14,15).
The fragile capsules act as sensor and as reservoir of
the resin. Upon fracture the released monomer reacts
with the catalyst distributed within the matrix forming a
cross-linked polymer network and repairing the damage.
Figure 11. Evolution of the number of interstitials in YSZ and
pure ZrO2 following 30-keV Zr recoils. The numbers of cation
and anion interstitials in ZrO2 and YSZ peak at about 0.2 ps
and then decline sharply. Subsequently, in YSZ, the anion in-
terstitials almost completely annihilate with structural vacan-
cies, resulting in no residual radiation-induced defects on the
anion sublattice. The difference in anion sublattice damage
between ZrO2 and YSZ is due to the presence of structural
vacancies in YSZ. (squares, triangles, and diamonds represent
[001], [110], and [111] recoils, respectively) and pure ZrO2
(circles) [79].
(a) (b)
Figure 12. Crack-healing of multilayer coatings due to phase
expansion upon moisture absorption before (a) and after 21 h
of healing (b) in wet atmospheric conditions [81]. Scale bar =
10 microns.

H. Fischer / Natural Science 2 (2010) 873-901
885
The healing efficiency of this concept is high (see Fig-
ure 16) [95-97].
However, since a significant load of microcapsules is
required to get efficient self-healing due to the small
Figure 13. Fracture of a hybrid solid/hollow fibre reinforced
plastic showing bleeding of UV fluorescent dye along crack
paths(x 45 magnification) [88].
Figure 14. Schematic picture of the self-healing action of a
in spherical capsules embedded resin upon damage/fracture
of the matrix material. Reprinted by permission from Mac-
millan Publishers Ltd: [Nature] (Autonomic healing of
polymer composites, S. R. White, N. R. Sottos, P. H. Geu-
belle, J. S. Moore, M. R. Kessler, 409, 794-797), copyright
(2001).
Figure 15. SEM picture of a broken capsule. Reprinted by
permission from Macmillan Publishers Ltd: [Nature] (Auto-
nomic healing of polymer composites, S. R. White, N. R.
Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, 409, 794-
797), copyright (2001).
(a)
(b)
Figure 16. a) Healing efficiency of the capsule strategy in a
single experiment and b) in time [95].
amount of liquid depleted upon fracture and the limited
probability of fracture during damage, the mechanical
(m)
η=90.3%
20(m)

H. Fischer / Natural Science 2 (2010) 873-901
886
properties of the matrix material are substantially weak-
ened and limit the application of this concept. Further-
more several material processing aspects are also not
favourable. Anyway, these systems have been further
developed and optimized for coating applications
[98-101] since they offer an autonomic self-repair for
anti-corrosion protective coatings.
The latest developments of autonomous systems com-
bine the concept of the incorporation of a self-healing
liquid with the increase in mobility of macromolecules
via solvent stimulation [102-104]. Here, solvent-con-
taining micro-capsules were either embedded in an ep-
oxy matrix or in a matrix of thermoplastic polymers. In
the case of the thermoset, the presence of the solvent
increases the mobility in the system and healing effi-
ciencies of up to 82% was observed due to molecular
diffusion and reaction of residual functionality [102,
103]. The healing performance appears to be inversely
related to cross-link density. Healing with epoxy-solvent
microcapsules is superior to capsules that contain sol-
vent alone (up to 120% healing efficiency), and data
showing multiple healing events are reported for this
system. In the case of the thermoplastic matrix, the mo-
bility of the macromolecules increases due to the lower-
ing of the glass transition temperature via solvent chain
interactions leading to the possibility of re-entanglement
formation and complete crack closure (see Figure 17)
[104].
In order to combine the advantages of the two existing
concepts for the storage of a self-healing liquid (the hol-
low fibre container and the spherical capsule concept),
experiments have been carried out to develop anisotropic
capsules [105]. Such particles offer a better probability
of being fractured upon damage than spheres. Thus a
substantial lower load of capsules (up to five times less)
is needed for a realisation of self-healing properties and
the initial mechanical properties of the system will be
preserved [105]. On the other hand, they offer the same
advantages with respect to processing and preparation as
the spherical capsules compared to the hollow fibres.
Ultimately, a mimic of the blood vessel system capa-
ble of a repeated, autonomic repair of damaging events
seems a perfect self-healing material system. Healing in
such systems is accomplished by a vascular network
system supplying the necessary components at the dam-
age site. A vascular system permits a continuously and
repeatable repair of all types of failure modes at any
point in the structure since it renews the supply of the
healing part of the system during the lifetime of the
structure at any location and any time.
A skin structure exhibiting flexibility, self-healing and
damage sensing is the simplest system realizing a net-
work-like supply of an active self-healing liquid [106].
The skin is fabricated on a substrate of copper-clad
polyimide sheets in a layer-by-layer technique using
polyimide sheets and an ultraviolet (UV)-curable epoxy.
The UV-curable epoxy is used as both a structural adhe-
sive and as the self-healing fill material (see Figure 18).
The skin structure is integrated with an array of induc-
tor-capacitor (LC) circuits, where each circuit is charac-
terized by a unique resonant frequency. If the skin is
damaged, the UV-curable epoxy is released and is cured
by ambient sunlight. Further, damage affects one or
more of the LC circuits, altering its resonant frequency.
An integrated antenna coil is used to detect and locate
the damaged portion of the skin; so far tests indicated a
good performance with respect to self-healing of the skin
and fault isolation.
Other examples of such a system have been described
by Toohey et al. [107,108] and by Williams et al. [109,
110]. Here, a 3-dimensional micro-vascular network
system, either manufactured by a direct write assembly
procedure using a fugitive ink or by connection of glass
capillaries via risers has been embedded in an epoxy
Figure 17. MRI tomogram of a healed thermoplastic contain-
ing solvent filled capsules. The green capsules are empty. The
majority of the empty capsules are located at the fracture plane
as intended, healing is obtained, and no empty space of the
original fracture plain is observable [104].
Figure 18. Basic diagram of self-healing skin, top view with
cutaway and cross-section. Reprinted by permission from IOP
Publishing Ltd: [Smart Mater. Struct.] (A flexible, self-healing
sensor skin, J. A. Carlson et al, 15, N129), copyright (2006).

H. Fischer / Natural Science 2 (2010) 873-901
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matrix system and tested upon bending. These systems
show repeatedly crack healing upon damage.
Alternatively, 3D microvasular networks can be real-
ized simply by loading the hollow micro-fibres of a
free-standing paper manufactured by electro-spinning
with a healing agent displaying a rapid reactivity, vola-
tility and ability to propagate repair triggered e.g. by the
influx of moisture [111]. The self-healing agent used is
Ti Cl4 blocking the pores of the hollow micro-fibres as
well as sealing possible cracks by the formation of ce-
ramic particles of TiO2 after hydrolytic decomposition in
contact with water vapour.
The natural blood vessel system works, however, in a
slightly different way [112]. In this case, a temporary
repair as response of a physical damage is achieved in
the form of a clot that plugs the defect. During subse-
quent days steps to regenerate the missing parts are initi-
ated. The healing of a skin wound is a complex process
requiring the collaborative efforts of many different tis-
sues and cell lineages. Inflammatory cells and then fi-
broblasts and capillaries invade the clot to form a con-
tractile granulation tissue that draws the wound margins
together; meanwhile, the cut epidermal edges migrate
forward to cover the denuded wound surface. The for-
mation of a clot then serves as a temporary shield pro-
tecting the denuded wound tissues and provides a provi-
sional matrix over and through which cells can migrate
during the repair process. The clot consists of platelets
embedded in a mesh of crosslinked fibrin fibers derived
by thrombin cleavage of fibrinogen, together with smal-
ler amounts of plasma fibronectin, vitronectin, and throm-
bospondin.
An interesting analogy to the process uses nanoparti-
cles dispersed in polymer films and shows in simulations
and experiments a preferred coagulation of nanoparticles
at areas of stress concentration similar to the clotting of
blood platelets at zones of damage [113-116]. This is
because the mobility of the filler particles at the length
scales present in nanocomposites is controlled by the
conformational entropy of polymer chains. Polymer
chains close to nanoparticles are stretched and extended,
which results in an entropy penalty. This decrease in
polymer conformational entropy is greater than the de-
crease in nanoparticle translational entropy; thus,
nanoparticle-polymer interactions are minimized by
segregation of the nanoparticles in areas of stress con-
centration such as a crack tip. For particles comparable
to the radius of gyration of the polymeric chain, the de-
crease in conformational entropy of the respective
polymer sub-chains upon particle sequestration is domi-
nant; the chains gain conformational entropy while re-
pelling the particles because they do not have to stretch
around particles [113-115]. The system can relieve the
entropic cost of chain stretching by allowing the parti-
cles to self-assemble to the solid walls. In the presence
of a notch, the nanoparticles are driven to localize in the
notch. The driving force for this localization is a poly-
mer-induced depletion attraction; the confined polymers
in the melt gain conformational entropy by “pushing”
the fillers to the surfaces and into the notch (Figure 19).
The time required for the particles to migrate to the
notch is comparable to the time needed for the chains to
move by approximately four radii of gyrations. The
morphology obtained from the simulation allowed a de-
termination of the mechanical properties of the nano-
composite-coated surface. The calculations show that the
nanoparticle fillers significantly reduce the stress con-
centration at the notch tip relative to the case where the
notched surface is just coated with a pure polymer layer
(see Figure 20).
As a consequence, load transfer from the matrix to the
agglomerated nanoparticles is predicted and the me-
chanical properties can recover to 75% of their original
value [114]. The calculations on the crack tip opening
displacement indicate that the presence of the nanocom-
posite in the notch would inhibit the system from un-
dergoing further damage (crack propagation from this
notch) when an external load is applied to the system. It
is important to realize that some fractions of the polymer
are also localized in this damaged region. If the macro-
molecules and the particles are chemically compatible,
the chains provide cohesion between the fillers and the
polymer coating.
The application of such nanocomposite coatings could
thus constitute an important step in the production of
components with defect-free surfaces. Upon appearance
of a defect, the coating effectively senses its presence
and then causes the repair of the damaged area.
In a practical example, nanoparticles are added to a
high viscous liquid polymer that is sandwiched between
two brittle, glassy layers (see schematically in Figure
21).
This architecture is common in multi-layer composites
that are used in optical and anticorrosion coatings, mi-
Figure 19. An instantaneous molecular configuration showing
a surface notch filled with nanoparticles due to a depletion
attraction between the particles and the surface. Particle beads
are shown as spheres and polymer chains as lines [115].

H. Fischer / Natural Science 2 (2010) 873-901
888
Figure 20. Results of the calculation of the stress distribution
using a lattice-spring model for (a) a polymer coating not con-
taining nanoparticles. The stress reaches a maximum at the
notch tip. (b) The polymer coating contains nanoparticles and
the stress at the notch tip is significantly reduced. The colour
bar indicates the magnitude of the stress [114].
croelectronics packaging, and solid-state devices. Such
films are susceptible to failure through the formation of
cracks, which propagate vertically to the polymer layer.
Since the polymer is fluid-like it expels the nanoparticles
to the brittle surfaces, where some of the particles pack
into the cracks, effectively mending the brittle surface.
In the experimental study [117], the particles were fluo-
rescent and could readily be visualized in the cracks (see
Figure 22).
In this example, the polymers and particles provide
the healing mechanism without any external intervention.
Figure 21. Scheme of a multilayer composite (coating). A
polymer layer (represented by the black chains) containing
nanoparticles is sandwiched between two intact, brittle layers.
Upon appearance of, a crack in the top surface and due to the
particle-polymer interactions, the particles become localized in
the crack and effectively mend the damage.
Figure 22. Fluorescence microscope image of a crack in a 60
nm SiOx layer on a mixture of PMMA with PEO-covered
3.8-nm-diameter CdSe/ZnS nanoparticles. The cracked film is
viewed with a fluorescence microscope, in which the segrega-
tion of the CdSe nanoparticles to the cracks is highlighted by
the fluorescence of the nanoparticle. [117], scale bar = 50 mi-
crons. Reprinted by permission from Macmillan Publishers Ltd:
[Nature Materials] (Entropy-driven segregation of nanoparti-
cles to cracks in multilayered composite polymer structures, S.
Gupta , Q. Zhang , T. Emrick, A. C. Balazs, T. P. Russell, 5,
229-233), copyright (2006).

H. Fischer / Natural Science 2 (2010) 873-901
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The approach has the advantage of being “re-usable”;
when new cracks appear, the polymers again drive the
particles to the damaged site and thereby continue the
repair. In principle, this process can continue until essen-
tially all the particles in the polymer layer are expelled
and localized in the cracks.
Finally, since the particles can potentially be made out
of the same materials as the brittle walls, the system will
resemble the initial, undamaged materials. Additionally,
the fluorescing particles provide a useful diagnostic tool,
pinpointing the location of the cracks and revealing the
mechanical state of the material.
A model has been published describing the rolling
motion of a fluid-driven, particle-filled microcapsule
along a heterogeneous, adhesive substrate to show how
the release of the encapsulated nanoparticles can be har-
nessed to repair damage on the underlying surface [118].
The microcapsules that could act as ‘artificial leuko-
cytes’ are driven by an imposed flow to move along an
adhesive surface, which represents the wall of a micro-
channel (either in the synthetic microvasculature, or
more generally, in a microfluidic device). The micro-
capsules enclose a solution of nanoparticles and these
nanoparticles can diffuse from the interior of the capsule
into the host fluid. When the capsule is trapped at the
leading edge of the damaged region, a relatively high
fraction of the released particles are now localized near
this region and can more effectively cover this damaged
site. Once the damage is repaired, the capsule can again
be driven by the imposed flow to move along the surface
where they could potentially sense and perform the re-
pair action at another damage site.
So far, all self healing mechanisms and discussed sys-
tems tried to mimic nature by filling or re-filling cavities
of damaged materials just like it is the case in nature.
However, nature has more than one strategy to deal with
threads. Continuously re-shaping, metabolic cycles as
well as isolation of sub-critical thread concentrations
(bacteria) are also healing strategies in nature.
Such metabolic reactions can also be used in poly-
meric systems to accompany a self-healing process as
described for polycarbonate (PC, polyetherketone (PEK)
and poly-p 2.6 dimehylphenylenether (PPE) [119]. In the
last case e.g., the polymeric chains cut by heat, light,
oxygen and external mechanical force will produce a
radical on the end of the scission chain in the first step.
Subsequently, a hydrogen donor stabilizes the radical.
The Cu (II) catalyst added beforehand forms a complex
with each end of the two different chains and withdraws
two electrons from them. The chains combine, eliminat-
ing two protons from the ends and reducing the copper
Cu (II) to Cu (I). The Cu (I) migrates in the polymer and
reacts with an oxygen molecule in an oxidation step to
Cu (II). The oxygen ion then reacts with two protons to
form a water molecule which leaves the system. Key
factors for the speed of the self-repair action are the
concentration of the chain ends and the mobility of the
chains (the recovery rate increases with decreasing ini-
tial molecular weight and increasing amount of dimeth-
ylphthalate as plasticizer), the oxygen partial pressure in
the surroundings and the speed of emission of water (see
Figure 23) [119].
The isolation of subcritical damage and the stop of
further growth is the strategy applied while introducing
Figure 23. Scheme of the self-repairing metabolism in PPE.
Reprinted by permission from IOP Publishing Ltd: [Sci. Tech-
nol. Adv. Mater.] (Self-repairing mechanism of plastics, K.
Takeda et al, 4, 435), copyright (2003).
Figure 24. An illustration of S segregation and BN precipita-
tion on creep cavity surface [120].
Grain boundaryCreep cavity
S segregation
(MP:112.8℃)
Grain boundaryCreep cavity
BN
B
N
BN precipitation
(MP:3000℃)
S
S

H. Fischer / Natural Science 2 (2010) 873-901
890
self-repair to austenic stainless steel [120] (see Figure
24). High temperatures during use of austenic steels
leads to low ductility creep fracture in long timescales
caused by the nucleation, growth and coalescence of
grain boundary cavities. Dissolved sulphur segregates on
cavity surfaces easily, and enhances the creep cavitation
remarkably most likely due to a lowering the creep cav-
ity surface energy and increasing the surface diffusion
rate. The additions of Ce (0.016 wt%) is highly effective
in removing the traces of soluble S in the steel through
the formation of Ce2O2S. In the absence of S segregation,
B (0.07 wt%) and N (0.007 wt%) can segregate due to
their small atomic diameter efficiently to the creep cav-
ity surface and form boron nitride (BN) compound on
the surface during creep. As BN is very stable at high
temperatures, the precipitation of BN film on the creep
cavity surface suppresses the creep cavitation in steel by
reducing the creep cavity growth rate. The BN precipita-
tion continues during creep exposure and heals the creep
damage. The creep cavities are self-healed by a con-
tinuous precipitation of BN on creep cavity surface dur-
ing creep exposure, and the growth of the self healed
creep cavities is suppressed almost completely. The
function of self-healing for creep cavitation provides the
steel with a longer rupture life and higher rupture ductility.
4. SENSORS AND TRIGGERS FOR
SELF-HEALING SYSTEMS
Fundamental to the understanding and implementation
of self-healing systems is the detection of damage and a
trigger mechanism which initiates the repair action. This
is done in nature efficiently e.g. in the case of wound-
healing by growth factors and matrix components that
are available to provide these “start” signals, triggering
relatively sedentary cell lineages at the wound margin to
proliferate, to become invasive, and then to lay down a
new matrix in the wound gap [112].
However, the tricky part is really the distinction be-
tween the healthy and the damage status of a given sys-
tem. Here, a decision between acceptable damage and
unacceptably changed behaviour of the system has to be
made, or in other words an indication of the time of the
usual transition from a normal state to a degraded state
when attacks become successful and/or faults begin to
take effect. In the end, any system should detect failure
in a timely manner and must be smart enough to com-
pute the degree of malfunction in the system and to asses
finally whether the system actually needs the interven-
tion by a “recovery program”.
As stated above, sensor elements are essential for the
design and functionality of self-healing systems. In the
case of the hollow glass capillaries or the urea-formal-
dehyde capsules used for the storage and delivery of a
liquid performing the self-healing action, the shell of the
capsule or the glass capillary itself act as strain sensor.
They are tuned by their mechanical properties to break
and to release the fluid in the event of damage.
The self-healing system based on re-formation of co-
valent bonds during a heating-cooling cycle fulfils in
itself the sensor function. The low viscous system
bridges the gaps of crack or damage due to the surface
tension properties of the liquid.
However, it is certainly preferable to get access to
autonomic self-healing systems with a more sophisticated
embedded structural health monitoring sensor unit ena-
bling decision making about the place and time of the
required repair action to be taken. There are a number of
possible damage sensors available, which are discussed
with respect to their advantages and performance in
[121]. Most important is the size and/or the location of
the damage that can be resolved by a certain sensor sys-
tem (see Figure 25), the size of the sensor itself and the
requirements to get a certain sensor functioning (power
consumption).
A combination of vibration and wave propagation data
has been used to determine the location and degree of
damage in structural components in an automated dam-
age identification technique requiring minimal operator
intervention [122]. To build such a detection system, a
structure had to be instrumented with an array of actua-
tors and sensors to excite and record its dynamic re-
sponse. In order to determine structural damage, a dam-
age index, calculated from the measured dynamic re-
sponse of the structure in a reference state (baseline) and
the current state, was introduced.
While the vibration-based analysis was used to iden-
tify widespread damage within the structure, the analysis
of the waveform signals provides detailed information
on the location and nature of smaller defects. The unified
computer-assisted automatic data analysis procedure can
improve the reliability of the defects detection capability
and aid in the development of in-situ health monitoring
systems for defects-critical structures. In a similar way,
electrical resistance measurements are applied to moni-
tor the health situation of a certain system using a (per-
colated) conductive network or carbon fibres incorpo-
rated in a structure. The resistance changes irreversibly
upon damage, as shown for damage inflicted by flexure,
tension, fatigue, and impact. The oblique resistance, as
measured at an angle between the longitudinal and
through-thickness directions, is particularly sensitive.
This enables the real-time monitoring of damage in form
of fire breakage and delamination in the case of compos-
ites tested in tension fatigue, compression and impact
and provides an estimate of the remaining fatigue

H. Fischer / Natural Science 2 (2010) 873-901
891
Figure 25. Sensor selection space comparing size of detectable damage with sensor size for various sensing methods [121].
life-time [123].
Also, carbon nanotube networks have been employed
for sensing of distributed strain and damage and conse-
quently for lifetime prediction and initiation of healing
[124,125]. Under static load, changes in resistance with
deformation and the initiation of microcracking during
loading of the composite laminate can be monitored (see
Figure 26).
On unloading, the resistance decreases to nearly the
original value as the transverse cracks were closed by
the stiff outer plies of the composite system. Upon re-
loading, the specimen show a sharp increase in resis-
tance at much lower levels of deformation corresponding
to reopening of the microcracks, indicative of permanent
damage to the composite. This approach may be useful
in self-healing systems.
In a recent study, the sensor possibilities of carbon
nanotube networks incorporated in composite systems
with respect to fatigue-induced damage was investigated
with regard to impact on in-situ health monitoring,
damage prognosis and the success of self-healing [126].
Monitoring of the volume and through-thickness resis-
tance enabled a determination of the extent and propaga-
tion of fatigue-induced damage such as crack and de-
lamination growth in the vicinity of stress concentrations.
The conductive nanotube network also provides oppor-
tunities to repair damage by enabling fast heating of the
crack interfaces; up to 70% recovery of the strength of
the undamaged composite has been achieved. Again, the
repair action has been taken by an enhanced mobility of
a resin at temperatures above Tg.
Anyway, the first step to autonomic self-repair is cer-
tainly structural health monitoring. Since all materials
contain inherent defects, the detection of existence and
location of damage requires a comparison between two
system states. Sensors cannot measure damage directly,
features need to be extracted through data processing
and in a learning process type of damage and severity of
damage can be identified.
Therefore, e.g. machine learning for structural health
monitoring has to be applied. In general, there are two
approaches to damage identification [127]. Model-driven
methods establish a high-fidelity physical model of the
structure, usually by finite element analysis, and then
establish a comparison metric between the model and the
measured data from the real structure. If the model is for
a system or structure in normal (i.e. undamaged) condi-
tion, any departures indicate that the structure has devi-
ated from normal condition and damage is inferred.
Data-driven approaches also establish a model, but this
is usually a statistical representation of the system, e.g. a
probability density function of the normal condition.
Any departures from normality are then signalled by
measured data appearing in regions of very low density.
The algorithms that have been developed over time for
data-driven approaches are mainly drawn from the disci-

H. Fischer / Natural Science 2 (2010) 873-901
892
Figure 26. Load-displacement and resistance curves for a
composite system with incorporated carbon nanotubes acting
as strain sensors [124]. A linear increase in resistance with load
is clearly seen. Upon the initiation of microcracking there is a
sharp change in the resistance. In the progression from the first
initiation of cracking to ultimate failure of the composite
laminate the resistance changes drastically. Thostenson, E.T.,
Chou, T.-W., “Carbon nanotube networks: Sensing of distrib-
uted strain and damage for life prediction and self healing”,
Advanced Materials, (2006) 18 (21), pp. 2837-2841 Copyright
Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with
permission.
pline of pattern recognition, or more broadly, machine
learning. Apart from the first step to structural health
monitoring, the detection of damage including data cap-
ture, feature selection as a process of amplification and
discarding of redundant information and novelty selec-
tion, the damage location and the damage assessment
which need as additional part a network of novelty de-
tectors the information is finally computed via a neu-
ronal network prediction of damage severity. However,
without a detailed knowledge of the underlying fatigue
and e.g. fracture properties of the system it will not be
possible to extrapolate to failure.
5. REQUIREMENTS AND
CONSEQUENCES OF SELF-REPAIR
Self-healing is, in a sense, opposite to a degradation
process, which as a dissipative process is connected with
the production of entropy. In 1877, Boltzmann suggested
a definition of entropy using the statistical thermody-
namics approach and the concept of microstates as
lnSk
with k as the Boltzmannis constant and  as the number
of microstates corresponding to a given macroscopic
state of a system. Microstates are the arrangements of
energy and matter in the system that are distinguishable
at the atomic or molecular level, but are indistinguish-
able at the macroscopic level. Any system tends to
evolve into a less-ordered and thus more random macro-
state that has a larger number of corresponding micro-
states, and thus the “configurational” entropy S grows.
All processes that lead to degradation (wear, corrosion,
fatigue, etc.) often involve interactions with different
characteristic length scales. For example, friction and
wear involve interactions of microscale and nanoscale
asperities and wear particles, capillary interactions, ad-
hesion, chemical molecular bonding. In most cases, these
interactions lead to an irreversible energy dissipation and,
therefore, to the production of entropy. In many cases, a
system can be divided naturally into several scale levels
with a limited interaction between hierarchical scales.
The entropy production at the macroscale can be there-
fore compensated by the entropy consumption at another
level. Since the entropy is an additive function and the
levels of the hierarchy are separated, the net entropy can
be presented as the sum of entropies associated with the
structures and process at corresponding scale levels as
netmacro meso nano
SSS S

 (1)
where the indices “net”, “macro”, “meso” and “nano”
correspond to the net entropy, macroscale, microscale
(mesoscale), and nanoscale (atomic scale) components
[128]. For most applications, is the integrity of the mac-
roscale structure (e.g., the absence of cracks, appearance
etc.) of predominant interest whereas the mesoscale and
nanoscale structure is of lower interest. The integrity of
the macroscale structure may be therefore repaired or
restored at the expense of the micro- and atomic scale
structures.
In the case of the autonomous self-healing using fluid
systems as applied while using capsules or capillaries
filed with a glue [82-104] excess entropy, macro
S asso-
ciated with the macroscale defects, such as cracks or
voids is compensated by affecting the mesoscale structure,
e.g., by fracture of microcapsules and the release of the
fluid, which decreases the degree of order of the micro-
structure and thus increases the entropy for meso S. Crack
propagation is an irreversible process, because when
intermolecular bonds are broken, the energy

is re-
leased irreversibly, so the entropy amount Scrack =

KA/T is produced to create a crack with area A. The co-
efficient 0 < K < 1 is the fraction of the dissipated energy
Q that is consumed for the creation of the crack, whereas
the rest of the energy is dissipated. The ideal state without
cracks corresponds to the minimum number of micro-
states and thus to the lowest possible configurational
entropy. The crack can be formed in many different ways
and the cracked macrostate corresponds to a number of
microstates producing excess configurational entropy,
macr o
S. In a similar manner, when a capsule ruptures and

H. Fischer / Natural Science 2 (2010) 873-901
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its content is released, the configurational entropy grows
because mixing occurs. The configurational entropy
growth of mixing of two substances is given by

mixing1 122
ln lnSRnXnX  (2)
where n1 and n2 are the amounts in moles of two pure
substances, X1 and X2 are mole fraction in the solution,
and R is the gas constant. Part of this excess entropy can
be consumed for healing the bonds at the crack. The net
configurational entropy grows, however the growth is
not due to the cracking but due to microcapsule rupture
and an irreversible decrease of their number.
If N capsules are ruptured to heal the crack with the
area A, the net entropy production is given by the meso-
scale entropy of mixing minus the macroscale entropy of
crack healing
-0
netmeso macromixing
L
SSS NSK
T

  (3)
In casemacro
S < meso S, the healing is done by de-
creasing the macroscale component of entropy at the
expense of the mesoscale component [128]. Crack heal-
ing decreases then disorder (and entropy) as observed at
the macroscale, while fracture of the microcapsules in-
creases disorder (and entropy) when observed at the
mesoscale. Self healing occurs thus autonomically if the
net entropy of the system is increasing and if the rate of
healing is higher than the rate of degradation; the effec-
tiveness of the healing mechanism can be influenced or
perhaps measured using relating microstructure parame-
ters to the entropy.
It is important to stress that self-repair is not simply a
material property, but rather a system property. There-
fore, it is not realistic to develop universal self-repairing
systems being applicable in all sorts of applications.
Consequently, it is important to develop new concepts of
self-repairing systems with additional values compared
to the existing one, which are focused on a well-defined
area/field of applications. However, the self-repair
mechanisms used in most current systems and devices
are not yet as developed as in the complex functional
systems as existing in nature.
Existing autonomic self-healing processes need time
(rest periods) and energy. However, the end product is
neither aesthetically nor functionally perfect. Defects on
a nanoscale size are randomly distributed in any material,
mechanical loads during the use cause the formation of
cracks initiated by the nanoscaled defects, which cause,
in time, degradation of the material up to a possible
catastrophic failure.
A self-healing mechanism already initiated at a nano-
scale offer many advantages for a more effective pre-
vention of further propagation and growth of mi-
cro-cracks as has been shown in a recent modelling
study [129]. There, the self-healing process of materials
with embedded “glue”-carrying cells, in the regime of
the onset of the initial fatigue has been studied by
three-dimensional numerical simulations within the
percolation- model approach. The onset of material
fatigue is delayed in such a system by development of a
plateaulike time dependence of the material quality. In
this low-damage regime, the changes in the conduc-
tance and thus in similar transport and response proper-
ties of the material can be used as measures of the ma-
terial quality degradation. A feature found for three
dimensions being much more profound than in ear-
lier-studied two-dimensional systems, is the competi-
tion between the healing cells. Even for low initial
densities of the healing cells, they interfere with each
other and reduce each other’s effective healing effi-
ciency. Short-range healing means that cells affect their
neighbourhood approximately within a distance equal
to their size. Thus, in order not to interfere literally, not
to waste glue, other healing cells should not be healing
this whole neighbourhood; thus they must be about two
“shells of influence” away. The exclusion radius is thus
at least three times the cell radius, in terms of the cen-
ter-to-center separation, it is likely even larger, de-
pending on the specific geometry.
Any architecture for self-healing systems should sat-
isfy essential properties like adaptability, dynamicity,
awareness, autonomy, robustness and distribution. From
this analysis and discussions of the currently active and
explored principles of self-repair the following require-
ments for the design of an ultimate self-healing struc-
ture/material can be formulated:
 A reflection mechanism to detect internal or exter-
nal conditions for which the system should respond
to sensing function of damage
 A reasoning mechanism to determine which ac-
tions should be taken in order to response to an in-
put from the reflection mechanism―the feedback
to repair mechanism, signal transport
 A configuration mechanism to perform necessary
changes to repair or optimize the system as directed
by reasoning mechanism and activation of the repair
mechanism (repair-on-demand)―transport of en-
ergy and/or material, repair with the ability to
“heal/fill” damage (e.g. volume increase!)
 Repair has to be either attributive or functional,
may be single repair (only one time) or repeating
repair
 Detection of success and of the status of the
structure, recovery of the initial status
 No reduction of the performance of the matrix ma-
terial by the self-repair functionality; good adhesion
and bonding and sufficient thermo-mechanical

H. Fischer / Natural Science 2 (2010) 873-901
894
properties of the repair material
 Sufficient high stability of the self-repair functional-
ity during processing and storage
Such systems have been already constructed and will
be discussed at two examples:
5.1. Self-Repairing Strategies and
Structures Used in Current Electronic
Devices
One method of self-healing is to design a multiply re-
dundant system that can reconfigure itself when dam-
aged. Re-configurable control systems require three
separate functions: failure detection and isolation to de-
termine which components are no longer useful; pa-
rameter identification to provide a model of the damaged
structure, and online control design that uses information
from the other two to re-establish control of the modified
structure. This approach has been applied for some time
already in re-configurable circuit hardware. This is par-
ticularly the case for DRAM. As a typical industry case,
the original yield before a built-in self repair (BISR)
using redundant elements for repair for a 256-Mbit com-
modity DRAM in a 0.11-micron CMOS process is al-
most 0, yet the yield of the same product after repair can
increase to more than 60%, and even to 80%. Here, the
BISR consists of three main blocks: built-in self test
(BIST), built-in redundancy analysis (BIRA), and ad-
dress reconfiguration (AR) [130]. Typically, a BIST cir-
cuit consists of a controller, a test pattern generator
(TPG), a data comparator, and interface logic. The con-
troller executes the test algorithm and issues commands
for the TPG. One of the algorithms used is testing is the
Syndrome identification algorithm. This method identi-
fies fail patterns during the test process to increase spare
allocation efficiency. After analysis, the AR circuit re-
pairs the memory; e.g. it replaces the faulty memory
elements with the fault-free, spare ones. This typically
involves address remapping or address decoder recon-
figuration. When the memory size increases, the total
benefit as a combination of early-market entry benefit,
test benefit, development cost and cost for the redundant
memory grows quickly, because the yield decreases ex-
ponentially when the memory size increases, and thus
the BISR design shows its effectiveness in enhancing the
yield. Development costs become negligible when pro-
duction volume is high, because they are constant for the
product. Self-repair systems are currently well known
and in use in several electronic structures to maintain
Figure 27. Flow diagram typical for a self-repairing structure used in electronic devices. Reprinted from Engineering Applications of
Artificial Intelligence, Self-repair of embedded systems, 17, E. A. Coyle, L. P. Maguire, T. M. McGinnity, 1-9, Copyright (2004),
with permission from Elsevier.

H. Fischer / Natural Science 2 (2010) 873-901
895
their reliability for a long time. In fact, embedded sys-
tems are increasingly entrusted with critical tasks and it
becomes crucial that these systems exhibit a high level
of reliability. The increasing complexity of such systems
makes production of defect-free systems extremely dif-
ficult. Therefore, self-repair methodologies were intro-
duced employing a unified modelling language and ge-
netic algorithms and increasing the flexibility and reli-
ability of such systems by enabling the transfer of func-
tionalities between hardware and software. A flow dia-
gram of the processes in such a system is shown in Fig-
ure 27.
In contrast to the majority of the already covered sys-
tems, this system is an example where the functionality
is related to information flow and not to mechanical or
aesthetic or other main functionalities. Hence, also the
flow of information is used as the signal in the diagnos-
tic process and as the mobile part of the system, able to
perform the repair action. The diagnostic circle ad-
dresses the functionality during operation of the target
system and decides whether the current configuration is
able to deal with the isolated fault and to update the sys-
tem resource value on demand resulting in new parti-
tions, compilation and upload of the new partitions onto
the system information. Industrial leaders like Microsoft,
Sun Microsystems and IBM are carrying out research on
autonomic self-healing systems, grid computing, soft-
ware agents and middleware computing are typically
strategies and software systems and hardware architec-
ture are being developed with self-healing properties. A
summary of the developments in this area is given re-
cently by Gosh (2007) [131].
5.2. Self-Repairing Bolted Joints
Bolted joint, as one of the most common mechanical
components in all types of engineering structures, are
critical to the function of the structure and their failure
could have huge costs or endanger lives. Unfortunately,
bolted joints are subject to a variety of common modes
of failure. These include self-loosening, shaking apart,
breaking because of corrosion, stress cracking or fa-
tigue, slippage (which can change the way a structure
absorbs load, leakage of corrosive substances in the
joint), and separation leading to rapid fatigue. One of
the most frequent modes of failure for bolted joints is
self-loosening. To reduce this mode of failure a concept
of a self-sensing and self-healing bolted joint has been
developed, consisting of structural members joined
together by bolt and nut combinations equipped with
piezoceramic and shape memory alloy elements [132]
(see Figure 28(a)). This concept combines an imped-
ance-based health- monitoring technique (monitoring
of bolt tension and connection damage) together with
actuators to restore tension in the system. The actuators
are included in the joint as shape memory alloy (SMA)
washers. The most common SMA is nickel-titanium,
often referred to NiTi or Nitinol, a material that has the
ability to convert heat to mechanical energy through a
phase transition.
The actuation ability of smart materials should also
provide force to a smart structure to counteract damage
once it is detected, introducing the possibility of self-
healing structures as demonstrated on this example. The
impedance method detects and inspects whether the
damage threshold value has been reached or not, and
provides a signal to activate the SMA actuator if needed.
When damage occurs, temporary adjustments of the bolt
tension can be achieved actively and remotely in order to
restore lost torque for continued operation (see Figure
28(b)). Thus, this is an example for the construction of a
self-monitoring and self-healing system that could be
added to existing structures, and provides both condition
monitoring and self repair.
(a)
(b)
Figure 28. a) Bolted Joint Configuration with SMA washer
and PZT impedance sensor, b) Part of the impedance spectrum
of a tight, loose and by SMA actuation fastened bolt connec-
tion. Reprinted by permission from IOP Publishing Ltd: [Smart
Mater. Struct.] (Practical issues of activating self-repairing
bolted joints, D. M. Peairs et al, 13, 1414), copyright (2004).

H. Fischer / Natural Science 2 (2010) 873-901
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6. TOWARDS A NEW GENERATION OF
SELF-HEALING SYSTEMS
In order to develop a new generation of self-healing de-
vices-smart structures, the following working areas need
particular effort:
1) Development/design of sensing elements to be in-
corporated within structures/surfaces which are able to
trigger the self-repair process (signal transport, activa-
tion of the repair mechanism). This means that, contrary
to most of the model systems, the sensor function has to
be further developed and extended with an active learn-
ing functionality, able to differentiate and to detect
damage, to interpret the obtained information and to
trigger/stimulate the repair action on demand. These
sensoric elements should be ideally structural parts of
the system and should not detoriate the general function-
ality of the system.
2) Development of transport/repair mechanisms for
different activation energies/materials and different cir-
cumstances (speed, temperature, amount of recovery).
This is especially needed to widen the area of applica-
tions of self-healing systems to more materials and sys-
tems. Up to now, only a limited number of solutions are
existing (encapsulated glues etc.), however, new princi-
ples have to be developed and employed to cover a
broader range of performance demands.
3) Development of multiple repair processes applica-
ble and sufficient for the purpose. This relates especially
to the speed of the repair action, which has to be ad-
justed to the needs and rest time periods available. An-
other need is the development of the action-on-de-
mand-only, strongly coupled with the development of
sensor/trigger mechanism. If an action-on-demand-only
can be realized, then it will be easier to ensure the capa-
bility of multiple repair actions. Such an action could be
triggered by a concentration of stress which initiates the
repair action while activating an initiator. Materials that
anticipate damage and increase their strength where
damage probability is largest like the system developed
by nature for the maintenance of our skeleton are virtu-
ally damage free, if enough energy and time for the con-
stant maintenance actions is supplied and given. Alterna-
tively, continuously active materials like present in a
living polymerization will realize a perfect restoration of
the undamaged state if more reactive monomers are sup-
plied after damage, e.g. via capsules [133].
4) Finally, methods must be developed to test and
characterise the structures/devices and to quantify the
success of the self-repair action. Up to now, there is only
a minor understanding of quantification of the success of
self-healing, mostly by measurement of mechanical per-
formance, however, since self-healing is not necessarily
only connected to mechanical damage, some measure of
the quality of the self-repair has to be developed.
If these requirements are met, then it will be possible
to create truly smart structures, which sense their inter-
nal state and external environment and based on the in-
formation gained respond in a manner that fulfils their
functional requirements. The primary advantage of mov-
ing towards smart structures technology is the potential
cost benefit of condition-based maintenance strategies
and the prospective life extension that may be achieved
through in-situ health monitoring.
7. CONCLUDING REMARKS
Implementation of self-healing is not intended to deal
with poor or inadequate application design, development
flaws, and problems with the quality of materials or op-
erational errors of systems and devices. However, it
should respond to damage caused by external deteriora-
tion factors. Thus, self-healing should offer great oppor-
tunities for increases in durability and reliability, reduced
maintenance and overall costs. This includes reduced
material resources, since the usual over-design of mate-
rials is no longer required. Repair will be addressed at
the very position of first appearance of damage, mini-
mizing the need to have self-repairing functionality
throughout the whole system.
Autonomic self-repair should be intrinsically con-
nected with a minimization of the free energy of the
system after experiencing damage (e.g. fracture would
create additional surfaces and hence enhance the overall
energy level of the system). However, this is not as easy
and autonomic as it may seem, damage may only lead to
another local minimum of the overall free energy and
this would be in some cases difficult to detect for a given
system. It is for example extremely difficult to detect the
energy changes connected with a change in pure ap-
pearance of a coating without having scratches and even
more difficult to react with a repair action on such
changes, however the changes would account for a
damage of the esthetical function of the coating. As for
now, most of the introduced synthetic self-healing sys-
tems have still substantial short-comes, such as the liq-
uid based capsule system, which requires a large amount
of capsules to fulfil the function thereby reducing the
mechanics of the system in a way that it can only be
treated as a model system for fundamental studies [91]
or the difficulty in filling and sealing of the liquid based
capillary systems which limits applicability [82-90].
Fully autonomous systems need to provide currently a
level of mobility which makes them rather unattractive
as constructive materials [70]. However, the latest de-
velopments show ways for possible solutions such as the
application of liquid filled capsules in coating systems or
flexible laminates [134,135], here enabling a functional

H. Fischer / Natural Science 2 (2010) 873-901
897
repair of a system as corrosion protection which has not
the same constraints as a structural material or the use of
the same strategy while deploying conductive particles
in liquids and as such repairing conductive pathways
[136]. The design principles of a self-healing system are
universal, however the synthetic system solutions de-
veloped so far are mostly special for the very circum-
stances of the material system and the application (con-
ditions), no universal solution exists applicable for all
material systems.
At the most basic level, a self-repairing system re-
quires redundant capacity and the ability to use it effec-
tively. The system must be capable of modifying the
configuration of the target system in order to affect a
repair. New design principles and strategies will be ap-
plied, and finally self-repairing potential will only be
incorporated where it is needed or where damage of the
material, system or device is going to start. Self-repair
will be universal in terms of materials and will follow
universal design principles and hence can be applied
anywhere. The overall capabilities required for a self-
repairing system extend the application functionality
specified by the system designers. The additional ele-
ments required include monitoring, test, diagnosis, and
repair capabilities. The system monitor must be able to
analyse high-level system performance of the system
against predefined performance benchmarks, continu-
ously verifying the correctness of the operation of the
process and bi-directionally communicating with the
target system. The testing process also must have bi-
directional communication to activate specific test se-
quences and receive results. This process should exploit
any built-in self-test (BIST) ability, and the testing re-
sults are forwarded to the diagnostic process. Automat-
ing the fault diagnosis process inherently demands the
application of intelligent techniques. Finally, the self-
repair process must be able to affect a repair based on
the reduction in resources available, following isolation
of the faulty components. If the original level of per-
formance cannot be restored, the monitoring system
must be modified to reflect the changes in system char-
acteristics. This feature demands an optimisation capa-
bility. Providing these additional capabilities impose a
significant burden on the system, there is a trade-off
between the additional cost of the system to facilitate
self-repair and the potential cost of failure.
The purpose of this reflection is not to provide a re-
view of the knowledge on self-healing systems; there-
fore, it is by far not covering all work on this subject.
The analysis of the systems as discussed in this work
shows that the creation of self-healing systems is and is
becoming reality. Although a number of synthetically
designed self-healing systems are developed and still
under development, they mostly fulfil only partially all
requirements of an ideal self-healing system as produced
by nature.
The knowledge of the system complexity enables the
design of a self-healing system for a given application
and possible damage. The introduction of self-healing
properties into materials systems is with this knowledge
and an interdisciplinary approach of design combining
all disciplines of science is a growing reality, which has
been already demonstrated for a number of synthetic
model systems and will have a great future in materials
technology especially for systems used in maintenance
critical applications such as areas with limited accessi-
bility, high demands of reliability, a guaranteed long life
or in areas where repairs cause a lot of hindrance/ an-
noyance/costs.
8. AKNOWLEDGEMENTS
The author acknowledges numerous stimulating discussions within the
Delft Centre for Materials (DCMat) and the Innovative Research Pro-
gram “Self-Healing Materials” of the Dutch Government and espe-
cially within the group Fundamentals of Advanced Materials during
the weekly stays as Visiting Scientist at the Faculty of Aerospace En-
gineering at the TU Delft, Netherlands. CSIRO Materials Science and
Engineering enabled during a stay as Senior Research Fellow the fi-
nalization of the manuscript. Special thanks go to O. Adan TNO/TU
Eindhoven and to Tim Harvey, “the butcher”, CSIRO for critically
reading the manuscript.
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