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Advanced Materials for Use in Soft Self-Healing Devices Tan-Phat Huynh,* Prashant Sonar, and Hossam Haick* acting as energy generators and/or weight trackers,[6,7] contact lenses that measure intraocular pressure,[8] amongst many others. Wearable devices should preferably be inexpensive, flexible, lightweight, easyto-make, biocompatible and versatile sensing systems.[1,9–11] Much attention is being focused on how to endow wearable devices with as many new functions as possible.[1,3,9,10,12–14] Attention is also being paid to their durability and flexibility, because these devices may break down due to mechanical fracture during deformation with time or accidental damage in their practical applications. Wearable devices that can correct the detrimental effects of scratching and/or mechanical damage and restore the mechanical/electrical/chemical properties are therefore of particular interest for real-world applications. This property being referred to as the “self-healing” capability. Technically, a self-healing wearable device is a merger of two fast developing research areas, i.e., self-healing materials and wearable devices. Self-healing materials have recently developed as a branch of smart materials, designed to self-heal mostly mechanical damage without using external stimuli.[15,16] Therefore, one should differentiate between materials that are healing with the help of electrical[17] or thermal[18] trigger and self-healing at ambient conditions without the need for any trigger or external stimuli.[19] Largely due to several outstanding organic and material scientists, a range of self-healing materials have been developed and exploited in different research areas, because of their flexibility, biocompatibility and ease of functionalization, see ref. [20–24]. This Progress Report provides an update on the current status of advanced materials for use in soft self-healing devices, with the main focus on wearable devices (Figure 1). It introduces the first self-healing materials, including the main composites and their healing mechanisms. It then presents and discusses recent advances in devices having self-healing characteristics, such as chemical sensors and e-skins – important components for medical and environmental control. Self-healing supercapacitors, batteries and solar cells as cost-effective and convenience-effective devices are also demonstrated and discussed.
Devices integrated with self-healing ability can benefit from long-term use as well as enhanced reliability, maintenance and durability. This progress report reviews the developments in the field of self-healing polymers/composites and wearable devices thereof. One part of the progress report presents and discusses several aspects of the self-healing materials chemistry (from noncovalent to reversible covalent-based mechanisms), as well as the required main approaches used for functionalizing the composites to enhance their electrical conductivity, magnetic, dielectric, electroactive and/or photoactive properties. The second and complementary part of the progress report links the self-healing materials with partially or fully self-healing device technologies, including wearable sensors, supercapacitors, solar cells and fabrics. Some of the strong and weak points in the development of each self-healing device are clearly highlighted and criticized, respectively. Several ideas regarding further improvement of soft self-healing devices are proposed.
1. Introduction The next generation of “smart living” is based on advanced applications of smart wearable devices,[1–3] 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 hail a new revolution in flexible and printed electronics. One could imagine, for instance, how uncomfortable it is to go out of house without a Bluetooth headset, Apple watch, smart phone or a similar device. More and more wearable devices are being developed to improve the convenience and security of our lives; wearable sensors on earrings (Ear-O-Smart) for tracking body temperature or replacing a Bluetooth headset, a shirt that monitors the body’s physiology,[4,5] devices on shoes Dr. T.-P. Huynh Department of Chemical Engineering Technion – Israel Institute of Technology Haifa 3200003, Israel E-mail:
[email protected] Dr. T.-P. Huynh Department of Chemistry and iNANO Aarhus University Gustav Wieds Vej 14, 8000 Aarhus C, Denmark 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]
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2. Self-Healing Polymers Inspired by the wound healing properties of natural skin,[27] selfhealing polymers are dramatically moving towards overcoming mechanical failure of materials or devices due to wear and tear.
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3. Self-Healing Polymers/Composites for Wearable Devices This section discusses the different strategies that scientists have used to incorporate self-healing properties of polymers in potentially wearable devices, such as chemiresistors, field-effect transistors (FETs), solar cells and electrochemical sensors, and their applications in different fields, e.g., health, energy, and the environment. Table 1 summarizes recent self-healing materials, specifying their composition, healing mechanism, and potential applications in wearable devices. For the sake of convenient presentation and discussion, the devices have been categorized under two main sub-categories – electronic and electrochemical devices. Some of the examples are detailed and further discussed below.
Tan-Phat Huynh received his PhD in Physical Chemistry in 2014 from Institute of Physical Chemistry, Polish Academy of Sciences. He has been a postdoctoral fellow in the Laboratory of NanomaterialBased Devices from 2014 to 2016, focusing on the development of self-healing chemical sensors. His interests include supramolecular polymers and their applications in chemical sensing. He continues in his second postdoc position to work on mussel-inspired materials at Department of Chemistry, Aarhus University, Denmark. Prashant Sonar did his doctoral work at Max-Planck Institute of Polymer Research (Mainz, Germany) and awarded PhD in 2004. After a postdoctoral period at ETH in 2006, A/Prof Sonar moved to the Institute of Materials Research and Engineering (IMRE), Agency of Science, Technology and Research (A*STAR), where he served as a Research Scientist. Dr Sonar was recently appointed as Associate Professor in July 2014 at Queensland University of Technology (QUT), Brisbane, Australia. His research interests include the design and synthesis of novel functional materials for printed electronics, (OFETs, OLEDs, OPVs, OLETs, OPDs, and Sensors), bioelectronics and supramoleculecular electronic applications. Hossam Haick, Prof. at the Technion – Israel Institute of Technology, is an expert in the field of nanotechnology and smart sensors. He is the founder and leader of several European consortiums for the development of advanced generations of nanosensors for disease diagnosis. His research interests include nanomaterial-based chemical (flexible) sensors, electronic skin, nanoarray devices for screening, diagnosis, and monitoring of disease, breath analysis, volatile biomarkers, and molecular electronic devices.
3.1. Self-Healing Materials for Electronic Devices To create self-healing and electronic properties in one layer, composites of these organic semi-conducting materials with
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self-healing polymers have to be prepared because most of them are insulating. The most straightforward application of selfhealing polymers in electronics is as a protective coating or layer
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Instead of mimicking the complexity of the healing processes in human skin (which can take weeks for full recovery), the healing mechanism of this novel polymer is simpler than in nature.[28] Three kinds of self-healing polymers have been categorized (Figure 2a–c), i.e., capsule-based, vascular-based and intrinsic self-healing polymers.[19] Self-healing of the first two polymers is based on the release of monomers and a catalyst stored inside capsules (Figure 2a) or vessels (Figure 2b) and present inside the polymer matrix, which immediately release after damage. By mixing, they start polymerizing to help heal the cut.[28] Even though large-volume self-healing can be achieved (Figure 2d), the disadvantages are the slow and single-time (for capsule-based polymer) healing, and their complicated fabrication processes (encapsulation of monomer and catalyst, followed by their dispersion inside the polymer). In contrast to the capsule-based or vascular-based selfhealing polymers, intrinsic self-healing polymer (Figure 2c) based on molecular interactions (e.g., hydrogen bonding, π–π stacking, and metal–ligand coordination)[30] is characterized by functionalization of the polymer with different self-healing groups[30–32] and multi-time reversible healing. Figure 2e shows the supramolecular rubber, L,[29] self-healing by means of hydrogen bonds formed among acid and urea groups. Another benefit of using an intrinsic polymer is the fast healing (Figure 2d) because of the absence of diffusion and polymerization control steps, which is a crucial factor in their applications in wearable devices, where signal interruption because of damage has to be avoided as much as possible. Moreover, due to the demand of extending the potential uses, intrinsic self-healing polymers have been variously modified to achieve high flexibility, fast self-healing, biocompatibility, and many physical (e.g., electrical, electronic, and thermal) and chemical (e.g., electrochemical and photochemical) properties.[33] Due to these advantages, this Progress Report focuses mainly on the importance of intrinsic self-healing polymers and composites with special functions suitable for wearable devices.
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processed, reused and recycled. 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 conductively self-heal after reconnecting two parts of the cut circuits to recreate electron flow. Therefore, while these healing concepts and design are good for demonstration, they are extremely challenging for micro- or nano-devices, because of the lack of compatible manipulators that could be used together with them. Bao et al.[37] provided the first example of a self-healing conductive composite using the host polymer, L, which has a urea group for hydrogen networking and nickel microparticles (µNi) as filler (Figure 3a). Hydrogen bonding was enhanced within a µNi polymer network by a thin native oxide layer covered µNi; the 31% by volume nickel can be added without any agglomeration. Conductivity of –1 Figure 1. Present and future applications of self-healing materials for different wearable 40 S cm could be reached by adding +15% volume fraction of the µNi and composite, devices. They include self-healing chemical sensors and e-skin that are important for medical and environmental control; for instance, surgery for healing or replacing a medical sensor is which have both electrical and mechanical eliminated with the help of self-healing. In energy applications, self-healing supercapacitors, self-healing abilities at room temperature. batteries and solar cells are cost-effective and convenience-effective, thereby attracting invest- The composite is highly responsive to the ment. Last but not least, self-healing coatings for devices are straightforward in preparation, healing process, and this can occur to 90% but crucial for long-term elegance of devices. (Image “Supercapacitor” reproduced with perof its original conductivity within 15 sec mission.[25] Copyright 2011 ACS Publications and image “Electronic skin (e-skin)” reproduced [26] at room temperature. The use of this selfwith permission. Copyright 2010 Nature Publishing Group). healing conductor for Light Emitting Diodes (LEDs) has been successfully demonstrated (Figure 3b). One of an inner electrical wire or a metallic circuit through sandwich of the most important aspects of this self-healing conductive architecture, where the self-healing polymer is on both sides of nanocomposite and its performance is due to the flower-like the circuit.[34–36] A self-healing polymer was synthesized by the nanostructure of µNi. The thin oxide layer of this nanostrucmethod of Leibler.[29] Leibler’s self-healing polymer, L, derived from fatty acids and urea, is now marketed under the name tured filler provides good wetting behavior and an appropriate ReverlinkTM. This polymer, with recoverable extensibility up to surface area to assist self-healing, whereas the nanostructure greatly enhances quantum tunneling for high conductivity. several hundred percent and little creep under load, can be easily
Figure 2. Demonstration of self-healing 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 volumes of damage. e) Chemical formula of the self-healing supramolecular rubber, L, derived from fatty acids and urea. (Figure 2a–d reproduced with permission.[28] Copyright 2010 by Annual Reviews and Figure 2e reproduced with permission.[29] Copyright 2008 Nature Publishing Group).
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Self-healing material
Composition
Healing mechanism
Application
ref.
Supramolecular rubber L[29]
Hydrogen bonding
Protective layer for circuit board
[34–36]
Self-healing conductor
µNi/L composite
Hydrogen bonding
E-skin
[37]
Self-healing conductor
PHEMA-CNT-(β-CD)
Inclusion chemistry between β-CD and HEMA
E-skin
[38]
Self-healing conductor
rGO/PBS composite
Dynamic dative bonds between boron and the oxygen in the Si-O groups
Flexion sensor
[39]
Self-healing materials for electronic devices Self-healing polymer
Self-healing conductor
Graphite/PEI
Hydrogen bonding
Strain sensor
[40]
Self-healing polyurethane
Hydrogen and reversible covalent disulfide bonding of substrate and electrode, and “induced” self-healing of AuNP film
Pressure/strain, VOC, and temperature sensors
[2,41]
Self-healing conductor
L fiber
Hydrogen bonding and “induced” self-healing of CNT film
Capacitor
[42]
Self-healing dielectric
PPMA/PEI
Hydrogen bonding
OFET
[43]
Self-healing dielectric
BNNs/L
Hydrogen bonding
OFET
[44]
Fully self-healing chemiresistor
2+
Zn2+)
Coordination bonding
OFET
[45]
Fe-Hpdca-PDMS
Coordination bonding
Dielectric actuator
[46]
Self-healing coating
Coumarin-functionalized triarm PIB
Photo-assisted reversibly cross-linked reaction
Solar cell
[47]
Self-healing sealant
Perovskite/PEG
Humid absorption of PEG
Perovskite solar cell
[48]
Hydrogen bonding for substrate and “induced” self-healing of CNT film
Supercapacitor
[49]
Self-healing dielectric
Pyridine-functionalized PDMS/(Fe or
Self-healing dielectric
Self-healing materials for electrochemical devices TiO2/L composite
Self-healing subtrate Self-healing electrode
Self-healing polyurethane
Hydrogen bonding
Supercapacitor
[50]
Self-healing insulator
Composite of PEVA and Mn-Zn ferrite nanoparticle filler
Magnetic attraction
Coating
[51]
Self-healing electrode
µSi/L composite
Hydrogen bonding
Battery
[52] [53]
Self-healing anode
(Graphite/Si)/L composite
Hydrogen bonding
Battery
Self-healing electrode
Hexyl-acetate healing agent
Release of healing agent from a fracture capsule
Electrochemical sensor
[54]
Self-healing electrode
GOx/gelation
Reversible bonding between gelatin and GOx/ gelatin
Biosensor
[55]
Self-healing electrode
EMIMTCB and tannic acid
Reversible electrochemical reaction
FET
[56]
Supramolecular ionic polymers based on (di-/tri-) carboxylic acids and (di-/tri-) alkyl amines
Hydrogen bonding
Electrolyte for electrochemical devices
[57]
Self-healing proton conductor
Oxalic-based metallogel
Coordination bonding
Electrolyte for electrochemical devices
[58]
Self-healing coating
Bilayered 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
[59]
Self-healing ionic conductor
Carbon nanotube (CNT) material is a favorite amongst chemists trying to develop a hybrid CNT/polymer material due to the unique properties, e.g., high electron conductivity,[60] well-developed routes of functionalization,[61,62] and low percolation threshold of CNT-based composites.[63] Unsurprisingly, the self-healing polymer could also incorporate CNTs to form a composite for humidity and touch sensing.[38] First, pyrene-modified β-cyclodextrin (β-CD) is attached to the surface of CNTs by π–π stacking (step 1, Figure 4). Second, a self-healing conductive composite can be formed through inclusion (host-guest) chemistry
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of poly(2-hydroxyethyl methacrylate) (PHEMA) and β-CD (step 2, Figure 4), and this is 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 (approx. 60 S cm–1) than a µNi/L composite and a wide linear range of response to humidity (from 30–90%), its percolation threshold (7–11 wt%) is quite high, causing an increase in the glass transition temperature of the composite. As a result, the material becomes stiffer, making self-healing inefficient because of restricted movement of the polymer chains.
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Table 1. Summary of recent self-healing materials including their composition, healing mechanism, and potential applications in wearable devices.
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Figure 3. a) Self-healing conductor based on a composite of self-healing branched polymers with urea groups at the end and nickel microparticle; 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. Reproduced with permission.[37] Copyright 2012, Nature Publishing Group.
Along with CNTs, reduced graphene oxide (rGO) is another promising conductive material for preparation of composite.[39] Networks of rGO were infiltrated with a solution of polydimethylsiloxane (PDMS) and boron oxide nanoparticles by vacuum casting. The rGO/polyborosiloxane (rGO/PBS) composite was formed by in situ cross-linking at 200 °C.[64] PBS is a supramolecular polymer of an intrinsic self-healing character because of the dynamic 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 the very high electron conductivity of ≈8 × 103 S cm–1, probably the highest conductive self-healing composite ever developed because of its high density and the uniform honeycomb structure of rGO (Figure 5a). Second is the low percolation threshold (0.5 wt%) of rGO used in this composite, which is difficult to achieve by normal dispersion techniques. Therefore, self-healing of this polymer is efficient with
almost 100% recovery (Figure 5b), and the composite film is manifested as a sensitive flexion sensor (Figure 5c). In contrast, another self-healing composite prepared by mixing and grinding graphite and polyethylenimine (PEI) creates several challenges[40] – a very high percolation threshold (65 wt% graphite), and a lower conductivity (1.98 S cm–1) compared to the rGO/PBS composite. A multifunctional self-healing sensing device has been reported by Huynh et al.[41] (Figure 6). The substrate and electrodes were made from self-healing materials, the main selfhealing component being a newly synthesized polyurethane derivative. The self-healing mechanism is based on reformation of hydrogen and disulfide bonds between polymer chains at two sides of the cut (Figure 6b). Healing time and efficiency of this polyurethane depends on the density of hydrogen bonds created by the urea groups of polyurethane, and on disulfide bonds and the flexibility of the polymer chains. The polymer has been used as a self-healing substrate; the composite with silver particles (in micrometer size) works as self-healing
Figure 4. Schematic preparation of conductive self-healing composite, PHEMA-CNT-(β-CD), using inclusion chemistry between β-CD and HEMA. Reproduced with permission.[38] Copyright 2015, Wiley–VCH.
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PROGRESS REPORT Figure 5. a) rGO/PBS networks contain microscopic channels (PBS) separated by thin walls (rGO), which are packed to form a honeycomb crosssection with residual porosity in the composite of 25%) in the composite. A film of a capsule-based self-healing polymer has been used in electrochemical sensors as self-healing working electrodes (Figure 12a) for voltammetric determinaFigure 11. a) Schematic illustration of the self-healing process of a yarn-based supercapacitor. tion of sodium using 10 mM ferricyanide in Magnetic alignment can assist the reconnection of the fibers in the broken yarn electrodes when 1 M phosphate buffer (pH 7.0) as the redox they are brought together; see inset. b) SEM image of the electrode. c) Chemical formula of probe.[54] After mechanical damage, the self-healing polyurethane. Reproduced with permission.[50] Copyright 2015, ACS Publications.
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 with an external alternating magnetic field.[51] The thermoplastic matrix ensures multiple strain sensing cycles and self-healing through a memory-shape mechanism. The novelty here lies in the development of damage sensing and
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Figure 12. a) Schematic diagram showing the preparation of the self-healing carbon ink and screen-printing procedure (top), and the self-healable process occurring when a self-healing printed electrode is mechanically damaged (bottom), along with a typical voltammetric response at the different stages. Reproduced with permission.[54] Copyright 2015, Wiley–VCH. 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. Reproduced with permission.[56] Copyright 2015, Wiley–VCH.
capsules in the polymer film are ruptured, releasing the hexylacetate healing agent in the crack. This agent dissolves locally the acrylic binder, leading to redistribution of the filler particles and restoration of the conductive pathway. Even though this approach has been successful in producing a self-healing conductive ink,[80,81] the main drawback, as also of capsule-based self-healing materials, is single-time self-healing. This means that if the cut reappeared in the same location, healing would be inefficient or fail. Moreover, this technique cannot be applied under flow conditions because the carrier solution dilutes down the healing agents. In a glucose biosensor,[55] reversibility of cross-linking between gelatin and glucose-oxidase (GOx) functionalized gelatin can be healed at low temperature, because temperatures >37 °C can break the physical bonds between gelatin and GOx/gelation. There are two benefits of this approach, i.e., self-healing occurs at low temperature (a limitation of most self-healing materials), and is highly reversible. Furthermore, one could imagine the use of this approach as a humidity sensor because of its high volume swelling. A self-healing composite with electroactive species can be used as electrochemical gate of a FET to manipulate the sourcedrain 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).[56] During operation of the transistor by applying 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, acting at the same time as a source of oxygen to heal the highly conductive reduced α-ZnO surface. Unlike electronic conductivity, ionic conductivity[82,83] is important in electrochemical processes. Ion conductivity of a composite polymer and an ionic conductor (usually called an 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 1604973 (11 of 14)
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method using commercially available (di-/tri-) carboxylic acids and (di-/tri-)alkyl amines.[57] Apart from their self-healing ability, these supramolecular ionic polymers have unique rheological properties, such as a sharp transition between a viscoelastic gel and a viscous liquid, resulting in acceptable ionic conductivity (10–5 S cm–1). This chemistry could potentially be used to develop a self-healing electrolyte for wearable electrochemical devices. Similarly, a unique proton conductive oxalic-based metallogel made by mixing solutions of copper(II) acetate hydrate Cu(OAc)2·H2O and oxalic acid dehydrated at room temperature proved to have self-healing properties (Figure 13).[58] The self-healing mechanism of this system remains unclear, but rapid desolvation/resolvation of (copper oxalate)-based 1D coordination oligomers at the interface might be induced. Due to physical stress and subsequent relaxation, this allows rapid restoration of the multicomponent supramolecular network without necessarily disassembling the coordination complexes. In addition, the high-conductivity composite could readily be prepared by blending the metallogel with CNTs (as in Figure 13a). Remarkably, the system could also impart the induced self-healing ability to other gel networks lacking this capacity (Figure 13b). For these reasons, this metallogel
Figure 13. a) Demonstration of the restoration of bulk conductivity of the oxalic-based metallogel/multi-wall CNTs composite gel used to bridge an electrical circuit. b) A “LEGO-car” fabricated by using 1) oxalic-based metallogel and with 2) rhodamine B, 3) with lanasol, and 5) with multiwall CNTs (1 wt%); 4) is diaminocyclohexane bis(amine) gel. Reproduced with permission.[58] Copyright 2016, ACS Publications.
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4. Conclusions and Perspectives This Progress Report reviews the developments in the field of self-healing polymers/composites and the wearable devices thereof. One part of the report presents and discusses many aspects of the chemistry of self-healing materials (from noncovalent to reversible covalent based mechanisms), and the approaches used for functionalization of the composites to enhance their electrical conductivity, magnetic, dielectric, electroactive or photoactive properties. The second and complementary part of the report links the self-healing materials with partially or fully self-healing device technologies, including wearable sensors, supercapacitors, solar cells and fabrics. Although very promising advances have been made so far, there is still a long way to go to implement self-healing wearable devices in real-world conditions or in commercial avenues. This is because of one or a combination of the following challenges: • Integration of self-healing devices into circuit board remains challenging, mainly as to how making them comparable in size so they can fit the architecture of the board; and in how to provide metallization to ultrathin organic films (compare ref. [84,85]). • Flexibility of the self-healing substrates might introduce parasitic responses in some applications, e.g., chemical or biological sensing. Rigid self-healing materials, therefore, have to be developed. • The electrical conductivity, chemical or biological sensitivity, or photochemical properties of the self-healing polymers are lagging behind comparable non-healing materials. • Self-healing polymers are usually more expensive than commercial polymers, mainly because they require more synthetic steps and chemical modification processes. To overcome these challenges, important aspects have to be addressed and developed further. These include, but are not confined to: • Development of high-resolution 3D-printing approach for the auto-construction of small-scale multilayer self-healing devices.[86–88] This would help to solve the first and second problems above. • Development of self-healing ceramics[89] or metals[90] to obtain rigid self-healing materials or devices with minimal
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parasitic side effects. • Synthesis of self-healing polymers that are intrinsically conductive, for example, by a polymer having electron/hole conductivity in its backbone and self-healing in its functional side-chains. • Integration of self-healing polymers with deliberately controlled (semi)conducting inorganic nanomaterials, such as molecularly modified Si nanowires, without carrier donating or carrier withdrawing side-groups,[91–94] or TiO2.[95] More innovations in self-healing materials are anticipated in the near future. Towards this end, self-healing materials should come down to three main features, namely fast healing, biocompatibility and cost-efficiency, so that one could see selfhealing devices in real-world conditions or the market within a short time. With this prospect, one can imagine self-healing devices or sensors that are implanted in human body or tattooed on the skin to allow continuous health monitoring, with only once-in-a-lifetime surgery for implantation/installation of the device/sensor.
Acknowledgements This research received funding from the Phase-II Grand Challenges Explorations award of the Bill and Melinda Gates Foundation (grant ID: OPP1109493). H.H. thanks the Alexander von Humboldt Foundation for a senior research fellowship in the Max-Planck Institute for Polymer Research (Mainz, Germany). P.S. 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. Received: September 15, 2016 Revised: November 21, 2016 Published online: February 23, 2017
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and the derivatives will be of interest in the development of fully self-healing devices. An ion-permselective conducting membrane made from a 4.2 mm thick conducting polypyrrole (PPy) 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.[59] The ability of the coating to self-heal is controlled by the oxidizing properties of PPy and the amount of available MoO42– ions, which can be released directly from the inner layer to the defective zone. Dissolved iron then reacts with MoO42– ions to form iron molybdate inside the defect, thereby blocking iron dissolution. The general concept of the controlled release of healing ions to the defect zone during the self-healing event might also be suitable for other active metals.
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