If the recent developments in �self-healing' coatings are anything to go by, blemishes and mars occurring on the surfaces of popular electronic handhelds, polished furniture and luxurious cars could repair on their own. A certain number of research studies have largely made strides in this field which boasts of innumerable applications. The first comes from University of Southern Mississippi (USA) where researchers have developed a special polyurethane coating which can fix its scratch under the presence of natural sunlight. The credit for another promising development goes to University of Illinois (Urbana-Champaign , USA) researchers who have developed a 3D microvascular network that closely resembles the human circulatory system which advances self-healing mechanism. Just as a skin blemish prompts blood flow to advance healing, the scratches occurring in such novel polymeric materials prompts the healing agent to interact with the catalysts for the fixing the cracks.
University of Southern Mississippi material scientists created a unique polymer coating comprising of three main chemicals that exhibited self-mending mechanism under the influence of ultraviolet light from the Sun. The polymer comprised of polyurethane, chitosan and oxetane. Accordingly, the inherent high scratch resistance of polyurethane is further strengthened and given special properties by the other two chemicals which render the novel self-healing property to the polymer. Chitosan (CHI) is the tough, hard UV sensitive material which makes the exoskeletons (outer protection cover) for crustaceans and insects. On the other hand, oxetane (OXE) is characterized by a four member unstable ring structure. Since the OXE ring contains three carbon atoms and a single oxygen atom, it is highly prone to split open and react. Just when the excessive stress leads to a scratch on the polyurethane, the OXE rings breaks open and shows high affinity to bond to something again owing to its unstable state. Simultaneously, the UV light from the sun triggers the CHI to create new links with the OXE's open reactive rings so as to heal the blemish and end up as a smooth surface again in 30 minutes as proposed by the scientists. The material scientists observed the cracked film under a 120 W fluorescent UV lamp, and noted that the splits disappear in about half an hour. The UV lamps were said to generate about 0.3 W/m 2 per nm power density - only a touch more than the power density emitted by the sun. Even though the three-component material exhibits clear advantages with countless applications in mobile phones, PDAs, Laptops, car bodies, furniture, music handhelds; some questions till need to be answered related to the shell life of the material. Also, one important aspect to delve into involves the possibility of the same position marred by subsequent scratches and the risk of intersecting scratches.
Another fascinating development in this field came from the researchers at University of Illinois in 2007 who invented the self-repairing materials which resembled the process by which a human skin is healed over time. The new materials rely upon embedded, 3-D scaffold-like microvascular networks like those found in circulatory systems of humans. These self-healing materials were found to be just like human skin in the sense that they could be repaired up to seven healing cycles. The researchers developed a scaffold using a robotic deposition process called direct-write assembly by using a concentrated polymeric ink, dispensed as a continuous filament, to create a 3-D structure, layer by layer. Subsequently, this structure was bounded by an epoxy resin which after curing, is heated. Once heated, the polymeric ink liquefies and is extracted leaving behind a network of intertwined micro-channels . Then, a brittle epoxy coating is applied on substrate while network of interlocking channels is filled with a liquid healing agent. Any crack on the epoxy coating triggers the healing agent filled in the interlocking channels to interact with the encapsulated catalysts thereby making the crack disappear. Even if the same position on the coating experiences crack, the healing cycle repeats itself. This process builds upon the original approach from the same group which developed self-healing materials comprising of a microencapsulated healing agent (dicyclopentadiene � DCPD) and a catalyst (Grubbs ruthenium catalyst) distributed throughout a composite matrix. When the substrate gets cracked, microcapsules would break and release healing agent which interacted with the embedded catalyst to repair the damage. However, this approach has limitations and could not work if the cracks appear at the same position again due to exhaustion of the healing agent. The circulation-based approach addressed this difficulty and ensured continuous supply of healing agent, so the material could heal itself a number of times. To further advance, the scientists felt that limited number of healing cycles might be overcome by implementing a new microvascular design based on dual networks for unlimited healing feats.
The next advancement in this regard also came from University of Illinois where the researcher team first encapsulated a catalyst into spheres less than 100 microns in diameter. Then the healing agent was similarly encapsulated into another set of microcapsules. The microcapsules of both the healing agent as well as the catalyst were then dispersed in the coating material and applied to the substrate. This formed what they say �dual healing system' which repair on their own and inhibit the corrosion of the underlying substrate materials. According to the scientists the dual healing systems can be applied to any liquid coating material. The new coatings are designed to better protect materials from the effects of environmental exposure.
There two notable self-repairing technologies in polymer materials: adhesives and thermal encapsulation.
As the name suggests, the first of these involves a series of 'stores' of adhesive found distributed in the most homogenous manner possible throughout the material, so that when the crack reaches one of these nodes the adhesive is secreted, together with a catalyst, and the crack is closed and the material polymerised. There are two variants within this line of technology, depending on whether adhesive-containing microcapsules or tubes filled with adhesive are employed.
INASMET-Tecnalia has worked on this line in a project undertaken for the AIRBUS, having managed to produce a series of microcapsules and distribute them in a polymeric resin. This was a fundamental step to finding out the difficulties that might arise in the encapsulation process.
The second method, developed by Bristol University , is a project for the ESA, is very similar. The difference lies in the use of tubes rather than microcapsules filled with adhesive.
The thermal method uses a different repair methodology. The material, developed by the University of Sheffield , is a polymeric matrix compound, reinforced with carbon fibres. The polymer matrix, in turn, is made of a solid solution of a thermoplastic polymer and another thermostable polymer.
The only restriction of the thermostable material is that it has to be suitable for incorporating these reinforcement fibres into it. The thermoplastic material has greater limitations, limiting it chances of being chosen for use, being highly dependant on the thermostable material used. In this case, when damage is detected, repair is carried out by heating the material with some device incorporated into it. This heating is capable of raising the temperature above that of the fusion of the thermoplastic material which, as a result, melts and flows into the damaged areas so that the cracks are sealed and the component restored to its former condition.
To sum up, the self-repairing materials have enormous potential as far as end uses are concerned but yet there remains additional scope to delve in to before commercialization. The scope of such self-healing materials has no bounds with applications spanning from automotive paints and marine varnishes to the thick on expensive furniture and housing items. The industry which will welcome such marvelous materials with wide open arms will be consumer electronics to safeguards products like tiny music players, laptop screens, mobile phones, PDAs and the like.