Electronic products that were formerly large and bulky have now become small and portable. Electronic devices that are flexible and stretchy are also evolving, with potential uses in artificial skins, health monitoring and implantable bioelectronics. These devices, which may enable electronics to be smoothly incorporated into our daily lives, are designed using a variety of techniques including ultrathin nanomembranes, strain engineering and inherently soft polymers. Flexible components, however, are not immune to unforeseen mechanical damage induced by recurrent wear and tear, as well as inadvertent scratching or cutting, often cited as primary reasons for hardware failures. Therefore, experts have sought to develop self-healing electronic systems that, similar to human skin, are capable of repairing unanticipated internal or external damage and resuming key operations.
Creation of self-healing electronics
In electronically active polymeric materials, the self-healing mechanism is because of the dynamic equilibrium achieved in percolation pathways and cross-linking networks in the polymers. The pace of healing is governed by polymer chain mobility, the activation energies for dynamic bond exchange, and the concentration of accessible broken dynamic bonds. In other instances, a solvent vapor is also required to initiate the reorganization of polymer chains. Because polymeric materials have poor conductivity, design techniques for high-performance and self-healing electronic materials have primarily focused on integrating electrically active fillers into a dynamic polymer matrix. The surface and size characteristics of electrically active fillers in a polymer matrix can impact both the dynamics of self-healing and electrical responses.
It is critical to differentiate mechanically robust self-healing materials from fluid viscoelastic materials, as extremely flowable viscoelastic materials can also be utilized as the polymer matrix for self-healing electronics. In flowable viscoelastic materials, healing occurs by material intermixing at the 'reconnected interface,' but in self-healing materials, healing transpires through the repair of broken cross-linking connections besides intermixing. The disadvantage of flowable viscoelastic materials is that they exhibit almost persistent plastic deformation even when mechanical force is exerted. Even if external forces or inherent flowability can reconnect interfaces, these viscoelastic materials cannot revert to their previous characteristics. In comparison, merging both weak and strong dynamic bonding can produce elastic but durable self-healing composites.
Elasticity can aid in both independent and effective self-healing recovery methods. An example of elasticity aiding in self-healing is the process of self-healing in leaves, which may swiftly repair wounds due to stored elastic tension in their internal structure. In this sense, mechanically robust self-healing composites will be preferable to extremely flowable viscoelastic materials for manufacturing robust electronics. Moreover, human skin's high resilience and high interfacial resilience of cartilage and tendons to bone are all a result of the body's self-recoverable heat removal mechanisms. Therefore, hydrogel materials with high stretchability and toughness, as well as robust interfaces with varied surfaces such as metals, glass and polymers, were formerly produced using hydrated self-healing polymer networks that disperse energy.
Examples of self-healing electronic devices
Electronic materials that self-heal can be incorporated into functioning electrochemical and electronic devices such as field-effect transistors, sensors and energy storage devices.
Field-effect transistors
Organic field-effect transistors have been incorporated effectively in memory devices, sensors, and driving circuits for LCDs. Numerous polymer and organic semiconductors now have field-effect mobilities greater than one, a performance comparable to that of amorphous silicon thin-film transistors. Now, organic field-effect transistors are being used to create flexible, printable and even stretchy electronics but these are more prone to wear and tear from scratching and rubbing.
Three self-healing parts must be manufactured concurrently to accomplish self-healing organic field-effect transistors: dielectrics, semiconductors, and gates, drain and source. Individual self-healing bulk components have already been created, and their electrical properties are adequate for integration into organic field-effect transistors. For instance, self-healing and electrically stable dielectric layers have been produced by cross-linking metal-ligand linkages into a non-polar polymer matrix. By selecting appropriate counter anions, such layers avoided the double-layer capacitive changes produced by mobile ions, resulting in hysteresis-free curves of such field-effect transistors.
Sensors
Sensors such as heat, light, strain, pressure and humidity are frequently used in daily life to identify physical or chemical signals. Self-healing sensors have the potential to significantly extend the life of such gadgets, both functionally (for example, sensing capability) and aesthetically. The majority of known self-healing electronic sensors operate by sensing changes in their conducting paths caused by external stimuli.
[Learn more about sensors on GlobalSpec]
Energy storage devices
Energy storage technologies are needed for electronic gadgets to operate adequately. A self-healing supercapacitor has been reported to be made simply by laminating acid-treated carbon nanotubes sheets onto a self-healing substrate consisting of a self-healing polymer and titanium dioxide nanoflowers. Both the separator components and self-healing electrolyte were made of a polyvinyl pyrrolidone-sulfuric acid gel. The flexible supercapacitors as produced demonstrated excellent electrochemical performance and self-healing capabilities even after being sliced for the fifth time.
In another example, the same self-healing polymer was used to construct a wire-shaped self-healing supercapacitor. Additionally, new conductive inks incorporating ferromagnetic particles have been created and utilized to construct electrochemical devices that are magnetically self-healing. Another example is a self-healing perovskite solar cell that was developed using the integration of chains of polyethylene glycol in the active layer.
Conclusion
Biological systems possess an extraordinary capacity for self-healing. However, human-made electronic equipment deteriorates with time as a result of corrosion, fatigue or injury sustained during operation, eventually failing. Self-healing chemistry has evolved as a potential approach for developing mechanically strong and self-repairing soft electronic materials in recent years. This article reviews their creation methods and discusses some of their examples.
