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A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 0 1 7 field makes Fe 2 O 3 nanoparticles oscil- late at the frequency of the magnet- ic field, resulting in localized melt and repair of physical damage. More re- cently, self-healing has been achieved in commodity copolymers that can self-heal autonomously under ambient conditions due to favorable interchain van der Waals interactions. Although there are many attempts to utilize these breakthroughs in physics and chemis- try of self-healable polymers and incor- porate them into energy harvesting and storage devices, there are significant challenges and opportunities in this emerging field. ENERGY HARVESTING AND STORAGE DEVICES With this introduction in mind, the remainder of this brief overview will focus on self-healing materials in energy harvesting and storage devic- es represented by insulators, electrical conductors, semiconductors, and ion- ic conductors, followed by selected ap- plications in photovoltaic solar cells, supercapacitors, and lithium batter- ies. Because the majority of self-heal- ing polymers are insulators, they can be used directly as a dielectric layer or as supporting substrates for electronic devices. One example is a polymer pre- pared by crosslinking fatty dimer ac- ids and urea, in which the self-healing mechanism is achieved by H-bonding [3] . Behaving like conventional thermo- responsive rubber, this polymer can self-heal multiple times under am- bient conditions. Self-healable elec- trodes for supercapacitors can be also fabricated by depositing carbon nano- tubes on top of this insulating poly- mer substrate. In addition, protective coatings for electronic devices can be prepared using self-healing insulators, in which coatings are composed of en- capsulated self-healable layers. Fib- rous supercapacitors have been fab- ricated using self-healable graphene oxide in which self-healing is achieved by a polyurethane shell. This stretch- able and self-healable supercapacitor may offer a platform for designing and fabricating multi-functional electronic devices [4] . Electrical conductors are essential for electronic applications ranging from interconnects, contacts, electrodes, to current collectors in sensors, displays, actuators, circuits, and energy storage devices. Introducing self-healing con- ductors significantly extends the life- time of these devices, and early studies utilized microcapsule/microchannels strategy to achieve self-healing. More recently, built-in self-healable poly- mers with instantaneous healing of conductive pathways upon physical disturbance have been developed. Con- ductive self-healing composites are also of interest, consisting of dynam- ically cross-linked polymer networks with conductive nanofillers, such as metallic nanoparticles or nanowires, carbon nanotubes or graphene [5] . An- other strategy to fabricate multiphase, multi-strength H-bonds is to intro- duce thiourea into microphase-sepa- rated polyurea networks. The resulting self-healing polymer exhibits ultra- strechability and notch-insensitive- ness, which makes it a promising ma- terial for underwater conductors and protective films for S cathode in Li-S batteries [6] . Notably, fabrication pro- cesses also largely affect the properties of electronic conductors. For exam- ple, by adjusting the prestrain ratio of a self-healable polymeric substrate, tun- able conductivity, self-healing efficien- cy, and stretchability can be obtained [7] . Although there are several promising conductive polymers, such as polyacet- ylene, polyaniline, polypyrrole, poly- thiophene, poly(p-phenylene), and poly(phenylene vinylene), one of the challenges is processing. The family of conjugated polyaniline is of particular interest due to the conduction mecha- nism and environment stability. Self-healing ionic conductors take advantage of hydrogels containing elec- trolytes and polymer gels swollen with ionic liquids. Using polyelectrolytes and reversible crosslinkers, self-healing in ionic conductors prepared by proton ion containing self-healable polyacryl- ic acid (PAA) hydrogel electrolytes and vinyl hybrid silica nanoparticles were developed [8] . High stretchability, fast self-healing, and high conductivity are facilitated by the dual crosslinked hy- drogel via hydrogen bonding. Stretch- able and transparent ionic conductors consisting of poly(vinylidene fluo- ride-co-hexafluoropropylene) and ionic liquids were also utilized, and self-heal- ing of these networks was attributed to strong ion-dipole interactions between stretchable polymer chains and an ion- ic liquid. The uniqueness of this system is that self-healable by touch, pressure, and strain sensors can be fabricated [9] . The main challenges in fabricating self-healing semiconductors are thick- ness limitations (<100 nm) and high T g of conventional polymeric semicon- ductors. To alleviate these problems, stretchable nanostructured self-heal- able semiconductors were developed, which are capable of recovering nano- size cracks resulting from fatigue by ap- plying thermal or solvent annealing. Self-healing is achieved by incorporat- ing dynamic hydrogen bonding capable of reconstructing the network [11] . More recently, a strain-sensitive, stretchable, and autonomously self-healable semi- conducting filmwas prepared by blend- ing a polymeric semiconductor and a self-healable elastomer crosslinked by dynamic metal-ligand coordination [10] . Supercapacitors are also known as electrochemical capacitors, which pos- sess a high power density, rapid charge and discharge rates, and a long cycle lifetime. A conventional solid-state su- percapacitor consists of two electrodes and a gel electrolyte. To obtain recovery of electrical and ionic conductivities, it is desirable to introduce self-healing moieties into both components. Ear- ly studies on self-healable supercapac- itors utilized laminating acid-treated carbon nanotube (CNT) films embed- ded into self-healing substrate forming BECAUSE THE MAJORITY OF SELF-HEALING POLYMERS ARE INSULATORS, THEY CAN BE USED DIRECTLY AS A DIELECTRIC LAYER OR AS SUPPORTING SUBSTRATES FOR ELECTRONIC DEVICES.