The Present and Future of Self-Healing Composite Material


How is self healing technology being used in materials today, and does it have a promising place in the future?

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By Sam Tiler

The concept of composite materials has existed since the ancient periods of human existence, they are intended to create an optimum material from an amalgamation of primary components that contain desirable characteristics. They first took form in mixtures of straw and mud to build huts and great structures in Ancient Egypt, through to the present day in complex composites used in space shuttles and probes. However, all of these materials experience, to different degrees, the irreversible effects of exposure damage and ageing which compromise the structural integrity of the material, leading to a reduced functionality.

In recent years, the field of self-healing materials has grown considerably in order to combat this problem. Not only do they reduce the need for constant repairs, but they also have a positive environmental impact as they limit the need for more, new materials to be manufactured as replacements, reducing pollution and overstripping. But how useful and realistic is this concept, now and in the future?

Concrete is the second most used substance on Earth behind water. It is made of sand, water and cement; its viability in most building projects is due to its hardness and chemically inert characteristics. The material’s drawback, however, is its predisposition to cracking with age and changing environmental conditions, which can cause structural failures and safety hazards. As a result, an autogenous (healing using the material’s components), self-healing derivative of concrete has been researched to mitigate these issues. The healing process is facilitated by dehydrated cement particles within the composite mixture that, when hydrated, dissolve into ions that diffuse into the gaps of the damaged material. This diffusion leads to the precipitation of calcium carbonate which is the key to completely repairing the cracks within the material.Although this sounds promising, this complete self-healing is only viable for cracks of 300 micrometres or smaller, so larger cracks are only partially repaired. Therefore, to minimise this issue, fibre-reinforced composites are used to control the size of cracks formed within the concrete, so they are more easily repaired by this autogenous process.

Another method of self-healing concrete is an autonomous mechanism, meaning the material makes use of foreign particles to facilitate the healing process rather than local ones of the same material as in the aforementioned method. In the context of concrete, specific bacterial cultures contained within silica particles can be sprayed or injected into the cracks and left to biologically heal the fractures. The Tittelboom Research Group of Ghent University used Bacillus Sphaericus to convert urea into ammonium and carbonate thus creating the calcium carbonate precipitate used to fill the cracks. Furthermore, the metabolic process of the bacteria produces carbon dioxide which reacts with the calcium hydroxide that is leached from the cement mixture, forming further calcium carbonate crystals to fill the cracks. However, the challenge presented by this method is the survival of the bacteria cultures within the concrete over extended periods of time due to limited access to nutrients.

Realistic applications, as above, have already been put into practice in multiple industries around the world. But what about the potential of these materials, and where they could fit into our future?

The global space market is expected to reach a value of $1 trillion by 2030 and makes frequent appearances in global news with new high-resolution telescopes producing high quality pictures of distant celestial bodies, or further talk of the vision for Mars colonisation, making it a highly valuable and relevant field to the future of humanity. The harsh conditions of space require very specific materials that can withstand the challenges posed, they must be lightweight and durable and so are very specifically designed to meet these conditions. Nevertheless, the exposure to space causes damage, which must be fixed, consequently requiring astronauts to perform arduous spacewalks to identify and fix the damage, therefore the future of materials in this field must be in long lasting, reliable and low maintenance composites.

Fast travelling space debris in the form of micrometeoroids or pollution from artificial satellites, greater than one centimetre in diameter, pose a huge threat to hull integrity and can destroy entire spacecraft. Impact can lead to an effect named Kessler Syndrome where, due to the numerous particles of pollution in low earth orbit, each collision causes a chain reaction with increasing probability of further collisions, and exposure to UV radiation can worsen the damage. The insertion of self-healing materials, contained within dedicated containers inside the layers of the composite material, provide the necessary components to restore the structural integrity and extend the operating life of the hull. In 2012, Dr Brahim Aissa and his team proposed proposed a healing agent containing a blend of monomers, carbon nanotubes and epoxy resin encapsulated within a carbon fibre polymeric layer. They  bombarded these  materials with small debris-like particles to determine the best healing agent mixture. It was found that equal parts of each monomer and a small quantity of single-walled carbon nanotubes resulted in an 83 percent efficiency for restoring the material to its original mechanical strength, proving a promising application to hull impact shields and reducing the need for repairs on missions where time and resources are limited.

However, this idea is very much in its theoretical stages and the insertion of healing agent capsules within composites can affect the properties of the material itself and hence is extremely important to monitor these effects to determine suitability.

The field of self-healing materials poses an extremely valuable opportunity in the progression of man-made structures to make what is now considered irreversible damage become a more trivial concern. It aims to increase the lifespan of a large variety of products, making them more viable long term and reducing the need for constant human intervention, saving money and time. As of yet, many of these products are in early developmental stages and are far from being commercially available making them relatively fictitious irrespective of their promising impact.

Even though there are limitations evident, progress is being made towards implementing them into real world applications and once achieved, could signal the beginning of a new wave of materials.