May/June_AMP_Digital

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 6 (a) (b) (c) W ith the growing need for elec- tronics integrated into daily lives, electronic devices that were formerly bulky and heavy nowadays are lightweight, compact, and portable. In addition, increased longevity is expect- ed for emerging applications generally re- ferred to as soft electronics, which may include artificial skin, projection screens or monitors, health monitoring devices, sensors, solar cells, dielectric actuators, and bioelectric implants. To meet these requirements, new materials design and fabrication processes are necessary, and a property that has arisen as a promis- ing approach is the ability of autonomous self-healing. The concept of self healing is de- fined as the ability of a material to recover from physical or chemical dam- age, and is typically achieved by in- corporating active chemical and/or physical components into existing ma- terials. This may or may not require en- ergy input or external stimuli. Over the last two decades, the field of self-heal- ing polymers has grown tremendously and recent review articles provide in- depth insights into these processes [1-2] . may enhance the toughness of a mate- rial. One of the drawbacks of reversible covalent rebonding is that it requires high activation energy, which is typical- ly achieved by elevated temperatures. In contrast, supramolecular inter- actions involve H-bonding, π-π stack- ing, guest-host chemistry, metal-ligand coordination, and ionic interactions that hold networks back together with non-covalent bonds that can be re- modeled reversibly. Physical self-heal- ing of polymers is often accomplished by built-in shape-memory effect (SME), which relies on the ability of a poly- mer to close a wound using the ener- gy stored during mechanical damage. Another approach disperses micro- spheres into a polymer matrix, which upon mechanical damage spills a heal- ing agent to fill a crack. An extension of this concept uses the same princi- ples by introducing networks connect- ed with hollow fibers filled with healing agents. An additional way of producing self-healing polymers incorporates su- perparamagnetic Fe 2 O 3 nanoparticles into a thermoplastic polymer network. Applying a remote oscillating magnetic In general, depending upon the nature of the process, there are two distinct and overlapping approach- es, physical and chemical self-healing. Physical self-healing (Fig. 1a) usually re- fers to a flow of macromolecular chains at or near a damaged area resulting in the closure of the wound, whereas chemical mechanisms involve rebond- ing of cleaved bonds (Fig. 1b). These events may occur concurrently and the overlap between these mechanisms is common and most of the time unavoid- able (Fig. 1c). Chemical self-healing mechan- isms can be classified into four catego- ries: incorporating covalent rebonding, supramolecular dynamicbonds, self-as- sembly induced self-healing, and intro- ducing living organisms. Cleavage and reformation of specific bonds is usual- ly achieved by reformation of covalent bonds or supramolecular interactions. Bothmechanisms usually require an ex- ternal energy input. Reversible covalent bonds that bring reactive chain ends back together are good candidates for developing self-healing polymers due to their high bonding strength, which Fig. 1 — Self-healing mechanisms. (a) Physical processes to realize self-healing include inter-diffusion of polymer chains, shape-memory effects and the introduction of active nanoparticles into a polymer matrix. (b) Chemical processes to facilitate self-healing involve either introducing reactive chain ends or supramolecular chemistries. (c) Physical and chemical processes can be combined to realize self-healing. Self-healing is achieved by incorporating enhanced van der Waals interactions, or encapsulating nanocapsules or microcapsules containing reactive liquids to heal a wound.