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Self-healing materials are a dream concept in many biological and mechanical applications, where fatigue and machine or structural failure due to wear, aging-related changes and tiny defects is a constant threat to the safety and efficiency of the application.
These materials act somewhat like the human body: they sense a failure, for example, a hairline crack in an internal component. They then arrest further extension of the crack and simultaneously repair it, all without external intervention.
Thus a self-healing material is briefly described as a synthetic material that autonomically self-repairs without the problem being detected, diagnosed or addressed by a human being. Many materials produce microcracks which inevitably lead to microcracking, as a result of multiple physicochemical processes. Self-healing materials help to address this most crucial of problems in the use of polymers.
What are the Types of Self-Healing Materials?
The earliest self-healing materials were made of extremely long molecules called polymers which comprise repeating units called monomers, combined with a specific type of embedded adhesive. This gave rise to many other types of self-healing materials, which can be classified into four categories:
- Those with embedded adhesive or other healing agents
- Those with an internal circulating system and fluid analogous to blood
- Those with shape memory
- Reversible polymers
Embedded Self-Healing Materials
The first and most common self-healing materials contain structurally embedded microscale pockets (microcapsules) that are filled with an adhesive. Any crack causes these microcapsules to break open and releases the glue to seal the crack simultaneously.
Sometimes the adhesive just fills the crack and holds the material together. Another way is to make the component itself of polymer while filling the capsules with a liquid monomer so that in the event of breakage the polymer itself is newly formed to repair the component.
These materials have the drawback of requiring extremely small microcapsules to avoid structural weakness. Secondly, they can heal only once, and the final component will be slightly weaker than the original, which makes repetition of the failure more likely, but unrepairable.
Vascular Materials
These overcome the flaw inherent in embedded self-healing materials, of structural weakness caused by inserting the microcapsules. They are based on the human vascular system which carries repair materials to all sites of injury as and when they are needed.
Various types of vascular systems are available, some with networks of hair-thin vascular tubes maintained throughout the structure, and leading out of reservoirs kept under pressure. any failure will release the pressure at the other end of the tube, and this allows the repair fluid to be pushed to the failure site. This can tackle cracks ten times larger than those envisaged with the microcapsule materials, yet it is therefore also a slower process. This could be a potential issue in materials that face rapid crack extension, but not in large structures like bridges or multi-storeyed edifices which typically face slow creep over months or even years.
Unlike the nanoscale repair of microcapsule-based self-healing materials, vascular composites mimic macroscopic healing in the body. Another advantage is the ability to refill the healing agent as the reservoir empties, so that repeated healing can occur.
Shape Memory Materials
These materials are made from alloys that return to their original shape even after they are bent out of shape, typically when exposed to heat. Fiber-optic cables, for instance, could be embedded in a fine network through the structure, carrying energy to the failure site, causing the material to snap back to shape and reversing the failure. The light or other energy forms automatically reaches the failure site because the tube cracks with the failure, allowing the energy to leak out.
These materials were the brainchild of Henry Sodano, the iconic materials engineer, and are also called autonomous adaptive structures.
Reversible Polymers
Some polymers are extremely chemically active at their ends, and if they develop a crack, the reactive ends try their hardest to rejoin by chemical bonding. This is promoted by light or heat, which will result in effective arrest of the damage and repair of the material. Another mechanism of rebonding is electrostatic attraction between the broken pieces.
Thermoplastic materials are plastics that can be melted down and made into new forms. These could be made to break down into their monomers under the influence of heat so that once cooled the original polymer would re-form. For instance, fighter aircraft could conveniently seal bullet holes in their fuselages rather than decompress and crash!
Self-Healing Thin Films
Many self-healing materials are suitable only for thin film applications, such as UV-initiated reversible polymerization, either because the initiating agent needs to have access to the scratch or failure, or because of the low mechanical strength of the material.
Vanadium boride/oxide glass deposited in alternate layers formed a self-healing high-temperature glassy thin-film coating. Another type is the thin hydrogel film which is made of layers of polyelectrolyte. While these are sensitive to mechanical trauma, they also heal easily within seconds by the hydrogel interacting coulombically with the linear polymer.
A self-healing strong stretchable pressure-sensing membrane that might potentially lead to the development of artificial skin has been synthesized with a nanoscale structure of alternating piezoelectric and electrically conductive layers, such as polymer and porous graphene oxide layers.
What are the Applications of Self-Healing Thin Films?
Self-healing materials can be used for a huge range of applications like buildings, bridges, car components, paints and coatings which revert to their original state if exposed to adverse weather conditions or rough wear. Aerospace applications, automobile components, and energy production are other common applications. Pipeline gaskets and biological replacements are among the further potential applications for these materials, including joint prostheses.
However, self-healing thin films are optically transparent, and these have become still more attractive because of their ability to transmit more than 99% of light, even after ten cycles of fault-repair. Thus these are extremely promising in the construction of optically transparent devices. Another application is in the formation of antifogging films which have high hydrophilic and hygroscopic properties, besides healing any scratches by their abundant hydroxyl groups at the broken edges.
Yet another use could be to devise protective self-healing coatings on suits to protect farmers against pesticides like organophosphorus compounds that are absorbed through the skin, or factory workers exposed to harsh chemicals. Made of polyelectrolyte layers, these may also contain enzymes to degrade the specific toxin encountered before it reaches the skin. When washed, the exposure to water produces self-healing of any defects to keep the coating reusable.
Self-healing touch screen sensors are now available which heal themselves rapidly when heated. Thermosets, elastomers and powder coatings with microcapsule-based structures are now being developed. Anti-corrosion inhibitors loaded into halloysite nanotubes have been shown to be suitable for self-healing corrosion-resistant coatings.
DNA is envisaged as usable data storage on optical media, instead of computer tape, with molecular-scale carbon nanotubes used as transistors and intelligent implants to ensure self-healing begins as soon as required and ends once the sensors show the threat has been reversed.
Sources and Further Reading
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