This may be the author’s version of a work that was submitted/accepted for publication in the following source: Huynh, Tan-Phat, Sonar, Prashant, & Haick, Hossam (2017) Advanced materials for use in soft self-healing devices. Advanced Materials, 29 (19), Article number: 1 1-14. This file was downloaded from: https://eprints.qut.edu.au/104632/ c Consult author(s) regarding copyright matters This work is covered by copyright. Unless the document is being made available under a Creative Commons Licence, you must assume that re-use is limited to personal use and that permission from the copyright owner must be obtained for all other uses. If the docu- ment is available under a Creative Commons License (or other specified license) then refer to the Licence for details of permitted re-use. It is a condition of access that users recog- nise and abide by the legal requirements associated with these rights. If you believe that this work infringes copyright please provide details by email to [email protected]Notice: Please note that this document may not be the Version of Record (i.e. published version) of the work. Author manuscript versions (as Sub- mitted for peer review or as Accepted for publication after peer review) can be identified by an absence of publisher branding and/or typeset appear- ance. If there is any doubt, please refer to the published source. https://doi.org/10.1002/adma.201604973
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This may be the author’s version of a work that was submitted/acceptedfor publication in the following source:
Huynh, Tan-Phat, Sonar, Prashant, & Haick, Hossam(2017)Advanced materials for use in soft self-healing devices.Advanced Materials, 29(19), Article number: 1 1-14.
This file was downloaded from: https://eprints.qut.edu.au/104632/
This work is covered by copyright. Unless the document is being made available under aCreative Commons Licence, you must assume that re-use is limited to personal use andthat permission from the copyright owner must be obtained for all other uses. If the docu-ment is available under a Creative Commons License (or other specified license) then referto the Licence for details of permitted re-use. It is a condition of access that users recog-nise and abide by the legal requirements associated with these rights. If you believe thatthis work infringes copyright please provide details by email to [email protected]
Notice: Please note that this document may not be the Version of Record(i.e. published version) of the work. Author manuscript versions (as Sub-mitted for peer review or as Accepted for publication after peer review) canbe identified by an absence of publisher branding and/or typeset appear-ance. If there is any doubt, please refer to the published source.
DOI: 10.1002/((please add manuscript number)) Article type: Progress report Advanced Materials for Use in Soft Self-healing Devices
Tan-Phat Huynh*, Prashant Sonar, and Hossam Haick* Dr. T. -P. Huynh The Department of Chemical Engineering Technion – Israel Institute of Technology Haifa 3200003, Israel E-mail: [email protected] Prof. P. Sonar School of Chemistry, Physics and Mechanical Engineering Queensland University of Technology (QUT) 2 George Street, Brisbane, QLD-4001, Australia Prof. H. Haick The Department of Chemical Engineering and The Russell Berrie Nanotechnology Institute Technion – Israel Institute of Technology Haifa 3200003, Israel E-mail: [email protected] & Max Planck Institute for Polymer Research Ackermannweg 10 D-55128 Mainz, Germany Keywords: self-healing, polymer, sensor, transistor, solar cell
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The next generation of „smart living“ is based on the advanced applications of smart wearable
devices,[1] e.g., from the Apple Watch to Google Glass and Microsoft’s HoloLens. Such
wearable devices are becoming an inseparable part of our lives and bring about a new
revolution in flexible and printed electronics. One could imagine, for instance, how
uncomfortable it is to go out of your house without a bluetooth headset, Apple watch, smart
phone or a similar device. In due course, more and more wearable devices will be developed
that can improve the convenience and security of our lives; for instance, wearable earrings
(Ear-O-Smart) for tracking body temperature or replacing a bluetooth headset, a (wearable)
shirt that monitors your body physiology,[2] wearable shoes that act as energy generators
and/or a weight tracker,[3] contact lens that measure intraocular pressure,[4] and many others.
Many start-up companies are bringing smart prototype-based wearable devices for advanced
healthcare, energy harvesting and display technologies. Wearable devices should preferably
be cost-effective, soft, flexible, lightweight, easy-to-make, biocompatible and versatile
sensing systems. Much attention is now focused on how to endow wearable devices with as
many new functions as possible.[5] However, apart from this effort, their durability and
flexibility are also of concern because these devices may break down due to mechanical
fracture during deformation with time or accidental damage in their practical applications.
With regard to this matter, wearable devices based on self-healing materials are being
thoroughly explored; devices made of these materials will be in great demand in future.
3
Figure 1. Present and future applications of self-healing materials for different wearable devices.
(adapted from ref. [6]).
Technically, a self-healing wearable device is the product of the merger of 2 fast
developing research areas, i.e., self-healing material and wearable devices. Self-healing
material has recently developed as a branch of smart materials, designed to self-heal
following (mostly) mechanical damage without using any external stimuli.[7] Therefore, one
should differentiate between materials that are healing (electrical[8] or thermal[9], with
triggering needed) and self-healing (triggering by itself at ambient conditions without an
external stimuli). Between them, self-healing materials are preferred because of advantages,
such as automation, and avoidance of damaging caused by triggering. A polymer with self-
healing properties was found within the last 25 years.[10] Thanks to some outstanding organic
4
and materials scientists, a range of self-healing materials have been developed and exploited
in different research areas, due to their flexibility, biocompatibility, and ease of
functionalization. Specifically in this review, we will introduce and discuss self-healing
polymers. and then provide an update in the current status of applications involving self-
healing polymers, including their composites in wearable devices (Figure 1).
1. Scope of self-healing polymers
Inspired by the wound healing properties of biological skin in nature,[11] self-healing polymers
are dramatically moving towards overcoming mechanical failure of in-use materials or
devices. Instead of mimicking the whole complex of the healing processes of human skin
(which can take weeks for 100% recovery), healing mechanism of this novel polymer is much
simpler than in nature.[12] Regarding this approach, 3 kinds of self-healing polymers have
been categorized (Figure 2a-c), i.e., capsule-based, vascular-based, and intrinsic self-healing
polymers.[10] Healing of the first 2 self-healing polymers (Figure 2a and 2b) is based on the
release of monomers and a catalyst stored inside capsules (Figure 2a) or vessels (Figure 2b)
dispersed in the polymer matrix immediately after damage. By mixing with each other they
start polymerizing to help heal the cut.[12] Even though large-volume self-healing can be
achieved with these self-healing polymers (Figure 2d), their disadvantages are the slow and
single-time (for capsule-based polymer) healing, as also their complicated fabrication
(encapsulation of monomer and catalyst, and then their dispersion inside polymer).
5
Figure 2. Demonstration of the self-healing process with (a) capsule-based, (b) vascular-based, and
(c) intrinsic polymers. (d) Performance map for self-healing materials. Each polymer has
demonstrated healing for different damage volumes. (e) Chemical formula of the self-healing
supramolecular rubber, L, derived from fatty acids and urea (adapted from ref. [12-13])
On the other hand, healing of intrinsic self-healing polymer (Figure 2c) based on
molecular interactions including hydrogen bonding, π-π stacking, and metal-ligand
coordination,[14] attracts many scientists because of its simple chemical design (by
functionalization of polymer with different self-healing groups[14-15]) and multi-time reversible
healing, which is difficult to achieve with capsule-based or vascular-based self-healing
polymers. Another benefit of using intrinsic polymer is its fast healing (Figure 2d) due to the
absence of diffusion and polymerization control steps. This is crucial factor in their
applications in wearable device where signal interruption because of damage has to be
avoided as quickly as possible for the convenience for users. Moreover, due to demand of
extending its potential uses, intrinsic self-healing polymers were diversely modified in order
to achieve high flexibility, fast self-healing ability, biocompatibility, and physical and
chemical properties.[16] Figure 2e shows the supramolecular rubber L[13] self-healing by
means of the hydrogen bonds formed among acid and urea group. Therefore, our discussion
6
will focus mainly on the importance of intrinsic self-healing polymers and composites with
special functions suitable for wearable devices.
2. Self-healing polymers/composites for wearable devices
Scientists have explored a wide range of wearable devices, including sensors, electronic skin
(e-skin), energy harvesting and storage systems, which has lead to novel applications in a
range of fields, from consumer and mobile appliances to biomedical systems, and to sports
and healthcare.[17] Therefore, different kinds of materials are being intensively explored for
the fabrication of flexible and wearable devices. However, the future of material sciences and
engineering calls for polymeric materials due to the their overwhelming importance in many
applications[18], including biomedicine (drug delivery),[19] bioadhesive uses,[20] chemical or
biosensors,[21] and e-skin,[22]. Indeed, these (organic or inorganic to hybrid) materials have a
great diversity of mechanical, electronic, electrochemical and optical properties. Among them,
the“self-healing” property is becoming a new trend in wearable devices that function in many
different ways. Ideally, the devices integrated with healing properties not only benefit the
maintenance of long-term use of devices, but their contribution reliability, maintenance,
durability, irrepairable capability (without assistance) and other similar circumstances. In this
section, we discuss different strategies that scientists used to incorporate self-healing
properties of polymers in potentially wearable devices, such as chemiresistors, field-effect
transistors (FET), solar cells and electrochemical sensor, as well as their application in
different fields, e.g. health, energy, and the environment.
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Table 1. Summary of recent self-healing materials including their composition, healing mechanism,
and potential applications in wearable devices.
Self-healing material Composition Healing mechanism Application Ref.
Self-healing electrode EMIMTCB and tannic acid reversible
electrochemical
reaction
FET [43]
Self-healing ionic
conductor
Supramolecular ionic polymers
based on (di-/tri-) carboxylic
acids and (di-/tri-) alkyl
amines
Hydrogen bonding Electrolyte for
electrochemical
devices
[44]
Self-healing proton
conductor
Oxalic-based metallogel Coordination
bonding
Electrolyte for
electrochemical
devices
[45]
Self-healing coating Bi-layered PPy, inner layer is
doped with heteropolyanions
of PMo12O403; the outer layer
is doped with dodecylsulfate
Release of available
MoO42- ions to the
defect zone
Corrosion protection [46]
2.1 Self-healing materials and electronic devices
Electronic devices focused on herein are mainly organic electronics due to their promising
application in wearable devices.[47] Design, synthesis, characterization and application of
organic semi-conducting small molecules or polymers showing desirable electronic properties
can be used as an active layer in a range of organic electronic devices. Therefore, in order to
create self-healing and electronic properties in one layer, composites of these organic semi-
9
conducting materials with self-healing polymers have to be prepared because most of the self-
healing polymers are insulating. There are different ways of preparing self-healing
composites and to integrate them into electronic devices requiring advanced fabrication
methods, explained in details below.
The most straightforward application of self-healing polymer in electronics is as a
protective coating or a layer of an inner electrical wire or a metallic circuit through sandwich
architecture, i.e., a self-healing polymer on both sides of the circuit.[23] A self-healing
polymer was synthesized following on from Leibler’s work[13]. Leibler’s self-healing
polymer, L, derived from fatty acids and urea, is marketed now under the name ReverlinkTM.
This polymer, with recoverable extensibility up to several hundred percent and little creep
under load, can be easily processed, re-used and recycled. Moreover, the simplicity of its
synthesis, availability from renewable resources and the low cost of raw ingredients bode well
for future applications. It is noteworthy that the sandwich architecture for electronic circuits
only self-repair when one can connect exactly 2 parts of the cut pieces. Therefore, these
healing concepts and design are good for demonstration, but impossible for micro- or nano-
devices that we know have to connect exactly 2 cut pieces.
As mentioned above, there is strong urge to develop electronically conductive and
self-healing composites. An intrinsic self-healing conductive composite is an important
requirement for a number of organic electronic device, including organic light-emitting
diodes (OLEDs), organic field effect transistors (OFETs) and organic photovoltaics (OPVs).
Bao et al.[24] provided the first example of a self-healing conductive composite where they
used host polymer L, which has a urea group for hydrogen networking and nickel
microparticles (µNi) as filler (Figure 3a). Hydrogen bonding can be help the thin oxide layer
covered with a µNi polymer network; the 31% volume nickel can be added without any
agglomeration. Conductivity of 40 S cm-1 can be reached by adding 15%+ volume fraction of
10
the µNi and composite, which display both electrical and mechanical self-healing abilities at
room temperature. The composite is highly responsive in terms of the healing process and this
can occur to 90% to its original conductivity within 15 sec at room temperature. The use of a
self-healing conductor for LED lighting has been successfully demonstrated (Figure 3b). One
of the most important aspects of this self-healing conductive nanocomposite and its
performance can be attributed to the flower-like nanostructure of Ni microparticles. This
nanostructure filler provides good wetting behavior and an appropriate surface area to assist
self-healing, whereas nanostructure greatly enhances quantum tunneling for high conductivity.
Figure 3. (a) Self-healing conductor based on a composite of self-healing branched polymers with
urea groups at the end and nickel particle; (b) Demonstration of the healing process using a self-
healing electrical conductor based on a composite with an LED in series with: 1 an undamaged
conductor; 2 a completely severed conductor (open circuit); 3 an electrical healing, and 4 a healed
film being flexed with original mechanical strength (adapted from ref. [24]).
Carbon nanotube (CNT) material is a favorite of chemists who want to develop a
hybrid CNT/polymer material due to its unique properties, e.g., high electron conductivity,[48]
well-developed routes of functionalization,[49] and low percolation threshold of CNT-based
composite[50]. Unsurprisingly, self-healing polymer could also incorporate CNT to form a
composite for humidity and touch sensing.[25] First, pyrene-modified β-cyclodextrin (β-CD)
is attached to the surface of CNT by π-π stacking (step 1, Figure 4). Second, a self-healing
a b
11
conductive composite can be formed through inclusion of poly(2-hydroxyethyl methacrylate)
(PHEMA) and β-CD (step 2, Figure 4), and finally followed by polymerization (step 3,
Figure 4). This host-guest interaction is also effective under water because of its
hydrophobicity. Even though this composite has a higher conductive (~60 S cm-1) than µNi/L
composite and has a wide linear range of humidity, its percolation threshold (7 to 11 wt%) is
quite high, causing an increase in a glass transition temperature of the composite. As a result,
material becominge stiffer makes self-healing inefficient because the polymer chains cannot
move freely.
Figure 4. Schematic preparation of conductive self-healing composite, PHEMA-CNT-β-CD, using
inclusion chemistry between β-CD and HEMA (adapted from ref. [25]).
Along with CNT, reduced graphene oxide (rGO) is also another promising conductive
material for preparation of composite.[26] rGO/polyborosiloxane (rGO/PBS) composite was
cross-linked in-situ by heating the infiltrated networks of rGO at 200°C by vacuum casting.
PBS is a supramolecular polymer of an intrinsic self-healing character due to its dynamic
12
dative bonds between boron and oxygen in the Si-O groups and hydrogen bonds between
residual -OH groups at the end of some unreacted polymer chains. The resulting composites
comprise an rGO continuous network confining PBS (Figure 5a). The first highlight of this
hybrid material is its very high electron conductivity of ~8×103 S cm-1. This is probably the
highest conductive self-healing composite ever developed, which is due to its high density
and the uniform honeycomb structure of rGO (Figure 5a). Second is its low percolation
threshold, i.e., 0.5 wt% rGO used in this composite, which is difficult to achieve by normal
dispersion techniques. Therefore, self-healing of this polymer is efficient (Figure 5b) and the
composite film is manifested as a sensitive flexion sensor (Figure 5c). In contrast, another
self-healing composite prepared by mixing of graphite and polyethylenimine (PEI), which is
then ground, contains several challenges[27] - a very high percolation threshold (65 wt%
graphite), and a lower conductivity of 1.98 S cm-1(compared to above structure).
13
Figure 5. (a) rGO/PBS networks contain microscopic channels separated by thin walls and are
packed to form a honeycomb cross-section with residual porosity in the composite of <1%. Scale bar
= 20 μm. (b) Optical images of deeply scratched composites obtained with high and low molecular-
weight polymer right after cutting followed by 24 h healing. Scale bar = 1 mm. (c) Dependence of
relative resistance change of self-healing rGO/PBS film on flexion change. The film recovers its initial
resistivity after relaxation (adapted from ref. [26]).
The work described so far has been only about preparing self-healing for a part of a
device. But here, a first fully self-healing device has been reported.[28a], which is a platform
that is very different from other reported self-healing devices because of its whole self-healing
structures (substrate, electrodes and sensing layer; Figure 6). Its central self-healing material
is a new synthetic polyurethane, and its self-healing mechanism is based on reformation of
hydrogen bonds between polymer chains at 2 sides of the cut (Figure 6b). Healing time and
14
efficiency of this polyurethane depends on the density of hydrogen bonds created by the urea
groups of polyurethane and the flexibility of the polymer chains. The polymer is used as a
self-healing substrate and its composite with silver particles (in micron size) works as self-
healing electrode of chemiresistor. The role of the electrode is to pass electrical current
through the sensing layer, which is made of organic-capped gold nanoparticles that has an
electrical resistance in the range of hundreds of kΩ to a few MΩ. The resistance of this layer
varies with change of the surrounding environment (pressure, temperature, presence of
gaseous compounds, etc.). Healing of the polyurethane substrate of the chemiresistor induces
healing of this layer. To demonstrate the device‘s functions, the self-healing chemiresistor
was exposed to different environments (Figure 6c), e.g., pressure or strain (Figure 6c1-2), and
volatile organic compounds (VOCs) (Figure 6c3).[28] For particular clinical applications, a
flexible nanoparticle-based sensor array is useful in the early diagnosis if an individual’s
health state is being continuously monitored. Sampling of 11 VOCs in human breath or skin,
or monitoring abrupt changes in heart-beat and breath rate, should be a non-invasive way of
monitoring diseases at an early stage. These results presage a new type of smart-sensing
device, with a desirable performance in the possible detection and clinic applications in a
“primary prevention” strategy.[28b] However, one disadvantage of the sensing layer made
from organic-capped gold nanoparticle is “induced” self-healing, i.e., this self-healing of the
polymeric substrat on its own results in a pronounced scar. Even though the sensor can still
function, this phenomena creates several problems, including low mechanical properties and a
drop of electrical conductivity of the healed film, both of which are harmful for the sensor.
Regarding “induced” self-healing, a better approach was found with a self-healing
conductive wire based CNT.[29] When the broken parts are brought back into contact,
hydrogen bonds are formed between the 2 sections of fiber of the self-healing polymer L[13] to
connect them together. At the same time, the aligned CNTs between the two cross-sections
15
are reconnected by van der Waals forces[51] during the self-healing process of the fiber. We
had found that a high adhesive force is produced between aligned CNT arrays and different
substrates by van der Waals interactions; although these forces are weak, the collective effect
of many CNTs with a density range of 1010-1011 cm2 produces a high adhesive force.
16
Figure 6. (a) Schematic and photograph of the self-healing chemiresistor consisting of a transparent
(yellowish) self-healing substrate, a jelly-like self-healing electrode, and a pliable induced self-healing
AuNP film. (b) Optical images from top to bottom row of the self-healing substrate, the self-healing
electrode, and the ethoxyphenyl-capped AuNP film coated on the self-healing substrate before and
after cutting, and before healing at 20°C. Scale bar is 200 µm for the self-healing substrate, and
100 µm for the self-healing electrode and induced sh-AuNP film. (c) Resistance responses and
calibration plots of relative resistance response of the ethoxyphenyl-capped AuNP chemiresistor in
(c1) bending experiments (1) before and after (2) electrode-cut and (3) electrode&AuNP-cut; (c2)
stretching experiments (1) before and after (2) electrode-cut and (3) AuNP-cut. (c1) Exposure to n-
octanol at different concentrations (1) before and after (2) electrode-cut, second electrode-cut at (3) the
same and (4) a different position, and (5) an additional AuNP-cut.[28a]
Meeting more challenging tasks, researchers have partially inserted a self-healing
layer in a FET. Briefly, a FET includes a source-drain electrode to conduct electron or hole
current generated from a semiconducting layer between them. A gate of FET regulates current
density through its applied voltage together with a dielectric layer beneath.[52] For soft
electronic devices, a self-healing capability is highly desirable and among these devices,
17
organic FET (OFET) is one of the most important electronic devices for various applications
including RFID tags, chemical sensors and many more.[53] Up to now, development of self-
healing property for organic FET (OFET) is limited in dielectric (insulator) layer because of
its single-component chemistry and ease of construction. A poly(2-hydroxypropyl
methacrylate)/poly(ethyleneimine) (PHPMA/PEI) blend as a dielectric materials based self-
healable first low-voltage-operable, printable, and flexible OFETs has been produced by Katz
et al.[30] PHPMA has been successfully used as a dielectric material for OFET devices, but
this polymer has no self-healing ability due to lack of dynamic bonding; thus a new dielectric
material with dynamic hydrogen bonding is required. PHPMA and PEI polymers (Figure 7a)
have been selected due to interaction between both polymer chains through numerous
hydrogen bonds (Figure 7b). PEI polymer additives with PHPMA also reduce the glass
transition temperature of the blend, which is an efficient way of inducing hydrogen bonding at
room temperature. The blend system is also soluble in inexpensive environment-friendly
ethanol, which is an orthogonal solvent of many organic semiconductors. The PHPMA/PEI
(1:1, w:w) blend film 10-11 μm thickness scratch has excellent self-healing properties after
slight heating, which has been monitored by optical microscopy (Figure 7c). A self-healing
polymer blend with a carbon paint gate electrode was used in a flexible PET-based top gate
OFET devices using poly(3-hexylthiophene) as an active semiconductors. Before cutting, on
cutting and after cut healing, the top gate OFET device had excellent transistor behavior at a
low gate voltage (Figure 7d). On cutting, the mobility and drain current decreased, but after
healing at room temperature in air for 10 h, these parameters recovered to near the original
values. This automatically is one of the first reports showing that the multiple-layer OFET can
self-heal.
18
Figure 7. (a) Chemical structure of self-healing polymers; (b) image of self-healing process due to
hydrogen bonding between 2 polymer blends; (c) optical tracking of PHPMA/PEI blend film healing
process upon cracks made with a blade; (d) self-healing organic field-effect transistors (adapted from
ref. [30]).
Using a different approach, surface functionalized boron nitride nanosheets (BNNSs)
could become incorporated in the self-healing polymer L[13], yielding stable dielectric and
mechanical properties relative to the pristine polymer (Figure 8a-c).[31] BNNs are the
structural analogs of graphenes, are not conductive and have a good dielectric property.[54]
This polymer nanocomposite can retain various properties, such as larger electrical resistivity,
improved thermal conductivity, greater mechanical strength, a higher breakdown strength, and
very stabilized dielectric properties after mechanical damage. A unique feature of this
nanocomposite is that the recovery condition stays the same after sequential cycles of cutting
and healing. This clearly suggests that no aging of the nanocomposite occurs with mechanical
breakdown. Images of the cutting and healing process of this nanocomposite are shown in
19
Figure 8b. Repeatable healing is clearly desirable with microelectronic devices and their
applications. The integration of self-healingbased material significantly increases its
functional longevity under harsh conditions, such as abrupt temperature change, repeated
mechanical distortion and high voltage arcing. This research indicates the enormous scope of
self-healable supramolecular materials for flexible electronic devices, expanding from
conductive units to insulation regime.
Figure 8. (a) BNNSs-CONH2 being introduced into a polymer network to form hydrogen bonded
supramolecular polymer nanocomposites; photograph of the nanocomposites. (b&c) Developed of a
flexible electronic material that self-heals and restores many functions, even after multiple breaks. The
material is shown cut in half. The healed material can still be stretched and hold weight. (b) Sample
cut in the middle, and then the 2 portions put in contact with each other; upon heating they healed
completely; (c) large stretching of healed composite (adapted from ref. [31]).
20
Not only hydrogen bi(o?)nding, but also metal-ligand coordination, i.e., between 2,2’-
bipyridine-5,5’-dicarboxylic amide of functionalized PDMS and Fe2+ or Zn2+ of dispersed
metal salts, has been used to prepare a self-healing dielectric elastomer of OFET.[32] The self-
healing ability of the polymer is primarily determined by the kinetic lability of the
(Fe2+/Zn2+)-pyridine coordination bonds. Metal salts also enhanced the dielectric constant of
the self-healing polymer, while maintaining the stable capacitance without introducing
undesirable ionic effects. This dielectric has stable transfer characteristics and low gate
leakage current after 1000 cycles at 100% strain. Another example is PDMS functionalized
with 2,6-pyridinedicarboxamide, forming a coordination complex with Fe3+.[33] These
reversible coordination bonds have a self-healing ability even at low temperature (-20°C).
More interestingly, this elastomer has been applied to a dielectric actuator, i.e., a device that
transforms electric energy into mechanical work. When a high electric field of 17.2 MV m-1 is
applied, an area expansion of 3.6% was recorded without damage.
A solar cell is an electrical device that converts the energy of light directly into
electricity by the photovoltaic effect. There are many factors affecting the efficiency of solar
cells; an important one is the transparency of the substrate, which directly influences incident
photon-to-current efficiency,[55] and is therefore required to have a self-healing coating to
reserve this transparency. Triarm polyisobutylene (PIB) proved to be an excellent coating for
solar cell due to its high flexibility, strong adhesion to the substrate, good barrier properties,
optical transparency, thermal stability and and chemical resistance. Furthermore, it was
functionalized with coumarin to perform photo-assisted self-healing, based on reversibly
cross-linked reaction between 2 groups of coumarin at a wavelength of 365-nm (Figure 9).[34]
This coating is self-healing under ambient conditions because the wavelength for reversible
bonding is in theUV-visible range at 356 nm.
21
Figure 9. (a) Reversibly cross-linked network formation by photodimerization/photocleavage of
coumarin functionalized PIB. (b) Possible self-healing mechanism.
Referring back to mechanical damage, humidity is another harmful factor for solar
cells, especially perovskite solar cell (PSC), performance decreasing rapidly in a moist
environment due to degradation of perovskite CH3NH3PbX3.[56] This problem was overcame
for the perovskite solar cell (PSC) using polyethylene glycol (PEG) as a moisture absorber.[35]
Perovskite film without PEG decomposed into PbI2 and irreversibly turned yellow. In contrast,
the PEG scaffold perovskite film showed yellow at first and recovered to black in 45 s after
removal from the spray (Figure 10a). Self-healing was also observed by measuring the J-V
curve of PSC (Figure 10b), their self-healing effect can be ascribed to the excellent
hygroscopicity of PEG molecules and their strong interaction with the perovskite. The
omnipresent PEG molecules can also efficiently absorb water to form a compact moist barrier
around perovskite crystal grains, with little water penetrating into the film (Figure 10c).
22
Figure 10. (a) Photographs of perovskite films with and without PEG, showing the colour changes
after water-spraying for 60 s and in ambient air over 45 s. (b) J-V curves of PSCs before and after
water spray, showing complete recovery of the cells within 1 min when put back to ambient air. (c)
Schematic diagram to show mechanisms of the self-healing properties in PSC: (1) Absorption of water
on perovskite, (2) hydrolysis of perovskite into PbI2 and CH3NH3I·H2O, (3) restraint of CH3NH3I by
reacting PEG with nearby PbI2 to form perovskite once again after water has evaporated. PEG has a
strong interaction with CH3NH3I, preventing it from evaporating, subsequently MAI and PbI2 react in
situ to form perovskite after the film had been removed from the vapour source (adapted from [35]).
2.2 Self-healing electrochemical devices
We will now explore different types of electrochemical devices, which can generate electrical
energy either from chemical reactions (electrochemical solar cell and capacitor, fuel cell, etc.)
or by facilitating chemical reactions from applied electrical energy (electrochemical
sensor).[57] Therefore, self-healing polymers or composites in these devices require different
23
functions from electronic devices, such as electroactivity, ion or redox conductivity, or
double-layer capacity.
The first example is a supercapacitor with self-healing ability; sometimes called an
ultracapacitor or electric double-layer capacitor, it is an electrochemical capacitor with
capacitance values much higher than other capacitors (but lower voltage limits).[57b] A
supercapacitor usually has double-layer capacitance, a non-fadaraic process. Therefore,
supercapacitors are sometimes classified as electronic devices. The first attempt was a self-
healing substrate for a supercapacitor prepared from a self-healing polymer L[13] filled with
hierarchical TiO2 nanoflower (~400 nm) by simply vigorous stirring followed by thermal
crosslinking with urea.[36] A layer of CNT deposited on the top of substrate as the property of
induced self-healing (as mentioned before). The experiments showed a high recovery (87.5%)
of specific capacitance after a 5th healing from 35 to 30 F g-1. A more efficient approach with
a yarn-based supercapacitor (Figure 11) can self-heal under 2 processes; magnetic assistance
from yarn to attract 2 cutting parts, and then the outside layer is made by self-healable
polyurethane (Figure 11c). Self-healing is based on hydrogen bonds between carboxylate
groups of the polymer chains.[37] This technique is an improvement for precise self-healing
for nano- or micro-scale circuit (see Section 2.1) because 2 cutting pieces could find each
other by magnetic attraction instead of being connected by hand. In a similar way to
magnetic-assisted self-healing, magnet-polymer composite consists of a magnetic Mn-Zn
ferrite nanoparticle filler in a commercial poly(ethylene-co-vinyl acetate) (PEVA)
thermoplastic matrix. The magnetic filler can trigger healing by locally heating the composite
by means of an external alternating magnetic field.[38] The thermoplastic matrix ensures
multiple strain sensing cycles and self-healing through a memeory-shape mechanism. The
novelty of this work lies in the development of damage sensing and healing functionalities in
24
commercial polymers. The magnetic filler can trigger actuation, self-healing and multiple
cycle damage sensing.
By using different fillers, i.e., silicon microparticles (µSi) and self-healing polymer L,
self-healing rechargeable batteries have been developed.[39] A µSi anode in this case provides
high capacity, but it usually suffers from fractures due to expansion and contraction of µSi
during lithiation and delithiation processes, respectively. Therefore, L plays an important role
in self-healing composite by increasing the lifetime of the high-capacity anode through its
limiting of mechanical fractures generated during the cycling process. The same self-healing
polymer has also been used to protect the structure of the graphite/Si anode of a battery.[40]
Although a very high specific capacity was achieved (~584 mAh g-1) and retention (81%) for
a 1:4 volume ratio, this hybrid material suffered under low mechanical strength (only 50%
recovery of tensile strength and strain after healing), which was due to the high percentage of
(graphite/Si)/polymer (>25 %) in the composite.
Figure 11. (a) Schematic illustration of the self-healing process of a yarn-based supercapacitor.
Magnetic alignment can assist the reconnection of the fibers in the broken yarn electrodes when they
are brought together, see inset. (b) SEM images of the electrode. (c) Chemical formula of self-healing
polyurethane (adapted from ref. [37]).
25
A film of capsule-based self-healing polymer has been used in electrochemical sensors
as self-healing working electrodes (Figure 12a) for voltammetric determination of sodium
using 10 mM ferricyanide in 1 M phosphate buffer (pH 7.0) as the redox probe.[41] After
mechanical damage, the capsules in the polymer film are ruptured, releasing hexyl-acetate
healing agent in the crack. This agent dissolves locally the acrylic binder, which leads to
redistribution of the filler particles and restoration of the conductive pathway. Even though
this approach has been successful in producing self-healing conductive ink,[58] its main
drawback, as also capsule-based self-healing materials, is single-time self-healing. This
means if the cut appeared again in the same location, healing will be inefficient or fail.
Moreover, this technique cannot apply in flow conditions because the carrier solution dilutes
down the healing agents.
In a glucose biosensor,[42] reversibility of cross-linking between gelatin and glucose-
oxidase (GOx) functionalized gelatin can be healed at low temperature because is 37°C can
break the physical bonds between gelatin and GOx/gelation. There are 2 benefits of this
approach, i.e., self-healing occurs at low temperature (which is a limitation of most self-
healing materials), and is highly reversible. Furthermore, because of its high volume swelling,
one could imagine its use as a humidity sensor.
26
Figure 12. (a) (top) Schematic diagram showing the preparation of the self-healing carbon ink and
screen-printing procedure, and (bottom) the self-repairable process occurring when a self-healing
printed electrode is mechanically damaged, along with a typical voltammetric response at the different
stages (adapted from ref. [41]). (b) Layout of the field effect device, and the chemical structure of ionic
liquid and polyphenol. EMIMTCB: redox of the electrolyte, α-ZnO: solution processed amorphous
zinc oxide, PDMS: stick-on cavity (adapted from ref. [43]).
A self-healing composite with electroactive species can be used as electrochemical
gate of a FET to manipulate the source-drain current by electrochemical processes. An ionic
liquid gate containing 1-ethyl-3-methylimidazolium tetracyanoborate (EMIMTCB) and tannic
acid can self-heal by using a reversible electrochemical reaction with an oxygen-deficient α-
ZnO thin film (Figure 12b).[43] During operation of the transistor by appling a voltage to the
liquid gate, α-ZnO is degraded, based on the cathodic reduction of the thin film and the
production of oxygen species, such as superoxide. Tannic acid is an oxygen scavenger that
traps radicals and at the same time acts as a source of oxygen to heal the highly conductive
reduced α-ZnO surface.
Unlike electronic conductivity, ionic conductivity[59] is important in electrochemical
processes. Ion conductivity of composite of polymer and an ionic conductor (usually called
electrolyte) is based on the migration of cations and anions ions in a polymer network. Indeed,
ions in a polymer are good for self-healing processes because of the electrostatic interaction
between cations and anions. A new family of supramolecular ionic polymers has been
synthesized by a straightforward method using commercially available (di-/tri-)carboxylic
acids and (di-/tri-)alkyl amines.[44] Apart from the self-healing ability, these supramolecular
ionic polymers have unique rheological properties, such as the sharp transition between a
viscoelastic gel and a viscous liquid, resulting in acceptable ionic conductivity (10-5 S cm-1).
This chemistry has the potential to be used in developing a self-healing electrolyte for
wearable electrochemical devices. Similarly, a unique proton conductive oxalic-based
27
metallogel, made by mixing from solutions of copper(II)acetate hydrate Cu(OAc)2·H2O and
oxalic acid dehydrate at room temperature, proved to have self-healing properties
(Figure 13).[45] The self-healing mechanism of this system remains unclear, but it suggests
that the rapid desolvation/resolvation of (copper oxalate)-based 1D coordination oligomers at
the interface is induced. Due to physical stress and subsequent relaxation, this allows rapid
restoration of the multicomponent supramolecular network without necessarily disassembling
the coordination complexes. Remarkably, the system could also impart induced self-healing
ability to other gel networks lacking this capacity.
Figure 13. A “LEGO-car” fabricated by using (1) oxalic-based metallogel and with (2) rhodamine B,
(3) with lanasol, (5) MWNTs (1 wt %); (4) is diaminocyclohexane bis(amine) gel, and (5) MWNTs (1
wt %) (adapted from ref. [45]).
A self-healing ability shows its usefulness in coating for protection against corrosion.
An ion-permselective conducting membrane made from a 4.2mm thick conducting
polypyrrole coating with a bipolar structure on carbon steel without any additional barrier-
type top-coat possesses a self-healing ability in aggressive 3.5 wt% NaCl solution.[46] The
ability of the coating to self-repair is controlled by the oxidizing properties of polypyrrole and
28
the amount of available MoO42- ions, which can be directly released from the inner layer to
the defective zone. Dissolved iron then reacts with MoO42- ions to form iron molybdate
inside the defect, which has a blocking effect on iron dissolution. The general concept of the
controlled release of healing ions to the defect zone during the self-repair event may also be
suitable for other active metals.
Conclusions and perspectives
We have summarized developments in advanced self-healing polymers and composites for
wearable devices over the last 5 years. This research topic is state-of-the-art, demonstrating
fast growth by the number of publications and the quality of the research. More innovations in
self-healing materials will definitely be introduced in the near future. In most applications,
self-healing polymers must be functionalized or modified to find composites with increasing
functions, such as electrical conductivity, magnetic property, dielectric, electroactive and
photoactive properties. As a result, a range of devices (supercapacitors, sensors, solar cells,
and fabrics that are wearable) can be fabricated. However, there are still long way to go to
have commercial self-healing wearable devices because of several drawbacks:
There is not as yet a trult self-healing device; the most recently developed
multifunctional sensor of Huynh et al.[28a] still cannot satisfy its practical application
because of its “induced” self-healing sensing layer.
Integrating self-healing devices into circuit board is also questionable, as to how to
make them comparable in size and architecture with board. Not especially with self-
healing FET, there are up to 3 electrical wires from source, drain, and gate connecting
to the board.
In some applications, e.g., chemical or biological sensing, flexibility of substrate
should be avoided to reduce parasitic response. Therefore, rigid self-healing materials
need to be developed.
29
It is difficult to find self-healing polymers with comparable physical functions
(electrical conductivity, chemical or biological sensitivity, or photochemistry) to non-
healing materials.
Self-healing polymers are usually more expensive compared to commercial polymers
because self-healing requires more synthetic steps and number of chemical
modifications.
To partially solve these problems, new technologies need to be developed that should
address some important characteristics:
High-resolution 3D-printing for auto-construction of small-scale multilayer self-
healing devices.[60] This would help to solve the first and second problems above.
With the third problem, self-healing ceramic[61] or metal[62] might be a better choice.
Proposing synthesis of new conducting polymers with self-healing ability that could
overcome the fourth problem.
The last problem relates to using new bio-inspired self-healing materials.[63]
In the near future, our dream is to use a wearable device without having to keep mind whether
or not “is it still working?”, since the self-healing device is capable of auto-repair.
Acknowledgements
This research received funding from the Phase-II Grand Challenges Explorations award of the
Bill and Melinda Gates Foundation (grant ID: OPP1109493). HH thanks the Alexander von
Humboldt Foundation for a senior research fellowship in the Max-Planck Institute for
Polymer Research (Mainz, Germany). PS thanks the Australian Research Council (ARC) for a
sponsored Future Fellowship (FT130101337) and Queensland University of Technology
(QUT). The authors thank Drs. Yunfeng Deng and Weiwei Wu for reviewing and making
comments on the manuscript. We also thank BioMedES Ltd UK for editorially improving the
final manuscript.
30
Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
31
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