The concept of self-healing is attracting a lot of attention in material science, particularly in polymeric materials which can mend and repair themselves if scratched or bent. Airplanes that can complete their journey safely even if damaged, bridges that can continue standing through and after an earthquake, boats that can keep afloat in high seas despite damages, oil rigs and pipelines that can withstand damages caused by nature� these are the contributions of self repairing polymer composites. The future demand of next-generation products for enhancing durability, safety and aesthetics will be partially met with these novel materials.
An novel development in this field comes from a research team at Eindhoven University of Technology in the Netherlands. The research team led by Rint Sijbesma has engineered homogenous, latent polymerization catalysts which get activated once subjected to mechanical force or increased stress. This marks the first time that any catalyst gets triggered by mechanical stress, which occurs when any material is bended, torn or scratched. Previously, there have been catalysts which are activated by thermal energy, light energy or healing agents. The catalyst developed consists of a metal (Silver or Ruthenium) core complexed with a pair of organic ligands, which in turn, are attached to the polymer chains. The two ligands also play a role of catalyst in absence of metal. The material, thus formed, was dissolved by the team in a solution. As per the research team, the mechanical force supplied by ultrasound pulses through the liquid rips apart the link (weakest point in the chain) between the metal complex and one of the organic ligands attached to the polymer chains. As a result, the core metal center is released which acts as a catalysts for polymerization. However, under normal circumstances (when mechanical stress is not applied), these novel latent polymerization catalysts remain absolutely dormant and inactive. Thus, the presence of such latent catalyst in a material or a coating can enable mechanical forces that result in cracks and bends to mend them precisely when required. The research group�s aim is to develop these polymerization catalysts for use in self-healing materials where increased mechanical stress will initiate the repair process there and then. In the experiments conducted, the stress supplied by ultrasonic liquid breaks the polymer tail attached to metal ion freeing the ion (catalyst) for polymerization (repair) of the material. This first-of-a-kind molecular engineering could enable two distinct types of reactions- polymerization as well as transesterification reactions.
The research team used ultrasound method only for testing purpose. The group feels the ultrasonic activation of the catalyst in the liquid solution poses some shortcomings including �lack of control over the mechanochemical forces� and �irreversibility of activation.� The researchers expected that the time when ultrasound pulses are removed, the polymer chain-based ligand and the metal would rebond, forming the dormant catalyst again for use as and when required. However, when incorporated in a crosslinked network of materials or coatings, the impact of these limitations will decrease for these novel catalysts. The next step will be to demonstrate the catalytic activity of this stress-responsive metal-ligand bond when in a crosslinked network. Though this work shows immense promise, the team aims to further work to increase the stability of the catalyst and its ability to be activated on demand.

Other recent studies done on self-healing materials include the one by University of Southern Mississippi material scientists who created a unique polymer coating comprising of three main chemicals - polyurethane, chitosan and oxetane � exhibiting self-mending mechanism under the influence of ultraviolet light from the Sun. 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. 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. In addition, University of Illinois (Urbana-Champaign, USA) researchers 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 within the network to interact with the catalysts for the fixing the cracks on the surface.
A polymer system based on weak, reversible bonds that can heal it self when heated has been created by UK and US chemists. The project is a deliberate effort to design polymers which behave in this way, with reversible assembly characteristics. The team designed a polymeric structure which holds together using aromatic electronic interactions and can easily repair damage to itself when heated to a modest temperature. Previous self-healing polymers have relied on stronger interactions and often require additives to facilitate the healing process. The system uses two polymers, one larger than the other. The larger polymer naturally folds itself up to maximise attractive interactions, leaving tweezer-shaped electron-deficient receptor units. A smaller, linear polymer with an aromatic end group inserts into the folds of the larger polymer, forming interactions known as - stacking. The faces of the aromatic components stack together to form relatively weakly bonded systems, but if you introduce enough of them overall you build up a significant interaction. At room temperature, the polymer mix forms a flexible, self-supporting material. When the temperature is raised, the interactions holding the structure together are weakened, which allows the polymers to flow into the damaged area. In one experiment, the team found that a broken film could be re-healed by simply pressing the broken ends gently together and heating briefly at 80�C.
The team deduces that the smaller polymer acts as a plasticiser or solvent for the larger polymer, allowing the mix to flow as a liquid. When the mix is cooled the interactions reform and the original appearance and strength of the material is regained, a change that can be seen by observing the colour of the material. At room temperature the material has a blood red colour as a result of the aromatic stacking. The colour is lost when the material is heated to 60 - 65�C, indicating the material has a lower viscosity. When cooled, the red colour returns. Though a lot is desired to achieve the mechanical properties of conventional materials of, for example, carbon fibre composites, the team views this material as a starting point.