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By Kevin Bailey, Technical Analyst at Jumpstart

In the early 2000s, smart materials began to be developed which have the inherent ability to repair damage to themselves without intervention, just like skin, bone and living tissue would do. These materials were termed as self-healing materials, and created a body of research which lead to the first international conference of self-healing materials being held in the Netherlands in 2007.

Self-healing materials include polymers, metals, ceramics and cementitious materials, and have physical properties that can combat ageing, wear and defects and also protect form spontaneous failure without resorting to routine inspection and non-destructive testing.

Early healers
The first examples of self-healing materials consisted of polymers embedded with an internal adhesive, or healing agent, in microcapsule form capable of repairing the material following crack formation. However, the limitation associated with these materials is that the microcapsules need to be small and dispersed throughout the bulk of the material, which can weaken the overall structure and also limit the amount of damage they can fix. An example of this material would be a coating impregnated with microcapsules containing water-borne polyurethane paint.

Other categories of self-healing materials include materials with an internal network of tubing with micron thickness used to transport healing agents is response to a pressure change, and shape memory alloys, such as nickel-titanium, that respond to heat and light input to rearrange into a preferred shape following deformation.

Relight my fire
More recently, reversible thermoplastic polymers have been developed that, upon fragmentation, result in the exposure of reactive or electrically charged sites that can reform, again by activation by heat or light.

Scratches be gone
Initial applications for self-healing materials have focussed on paints and coatings, such as the development of a self-healing paint from the car manufacturer Nissan, capable of healing 80% of surface abrasion by itself, employing a mechanism that allows its polymer chains to freely slide through cross linked molecules and form a supramolecular network. This technology uses a polyrotaxane, a type of mechanically interlocked molecule in which multiple cyclic molecules are threaded around a linear molecule and capped by two blocking molecules.

The use of polyrotaxanes have been reported more recently in a patent awarded to Samsung in August 2018 that details the composition of a self-restorative protective film that has a low coefficient of friction, water repellency and self-healing properties that could potentially be used to create a thin, light, flexible, protective display material for a mobile phone, or associative technology that is both scratch and fingerprint.

Self-healing materials comprising of liquid metal droplets in a soft deformable elastomer have recently been reported that can alter their internal configuration upon damage to the material, suggesting potential applicability in human machine interaction devices and wearable computing, as well as power and data transmission.

This is an exciting time for materials science, and the potential uses of self-healing materials are extremely diverse as new material swith new properties are being developed.

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