All the living (biological) materials a part of nature are gifted with properties of self healing/repairing, growing and sensing. Man made materials have been away from such qualities which were limited to only biological materials but now scientists are coming with the development of new materials which have all these qualities of biological materials i.e., self healing, growing and sensing. Not only construction of bridges, tunnels and roads with structural materials like concrete but any electronic devices, utensils, or anything man made, we need the structural elements to make which need repairs, leading to loss of time, traffic jams, inconvenience to the user.  Smart materials (manmade living materials) with qualities of self healing and growing might be the answer for sustainable infrastructure. Like natural biological building systems these living materials are inexpensive, and self-form through interaction of the materials. They sense and self-repair, respond to changes in the environment. This article aims to bring to point the developments made in this direction

Self-healing mechanisms

There has been widespread effort to develop self-healing materials that could mimic the self healing ability of biological organisms.   Heating, UV light, mechanical stress, or chemical treatment was needed to activate the process. One key advantage of such materials is they would be self-repairing upon exposure to sunlight or some indoor lighting. If the surface is scratched or cracked, the affected area grows to fill in the gaps and repair the damage, without requiring any external action. The scientists are examining following three different self-healing mechanisms.

Constructing bacteria: Certain bacteria produce calcium carbonate as a metabolic product. The scientists soak balls of clay with the spores of these bacteria and mix the balls into concrete. Once water penetrates the concrete, the microorganisms become active and release calcium carbonate, one of the main components of concrete. The bacteria can close cracks of up to a few millimeters in width in a matter of a few days.

Hydrogels as gap fillers: Hydrogels are polymers that absorb moisture. They are used in diapers, among other things. Materials containing hydrogels can expand to ten or even 100 times their original size. Cracks that form in concrete can be healed by a hydrogel that expands when it comes into contact with moisture, thus preventing the water from penetrating further without expanding the cracks.

Greater strength with resin: Epoxy resins or polyurethane can be encapsulated and mixed into the concrete. When the concrete cracks, the capsules break open and the polymer is released. It forms a hard mass that seals the crack. It also has a positive side-effect: It increases structural stability.

Self-growing mechanism

A team in Japan has succeeded in developing self-growing materials that respond to mechanical stress by regenerating themselves like human muscle tissue. The team placed a double-network polymer hydrogel in a supply of monomers. When the network in the hydrogel was fractured under stress, the monomers bonded with the broken ends of the existing polymers to form new network material. A hydrogel is a solid, jelly-like material composed of long polymer chains. A double-network hydrogel is a cross-woven structure of polymers with opposing properties, one set rigid and brittle and the other soft and stretchy. When the hydrogel is subjected to mechanical stress, the brittle network strands break and attract monomers to heal them, while the stretchy ones maintain the structural integrity of the gel. The key to make the strategy successful is how to bridge the gap between the molecular mechanism of mechanochemistry and material science. It is well-known that a broken polymer chain generates mechanoradicals, free radicals that trigger a chemical reaction. These mechanoradicals attract monomers to initiate new polymerisation. But doing this in practice is challenging. The team overcame this difficulty by using double-network hydrogels, which generate more mechanoradicals than conventional polymer gels. The team is now looking to develop new self-growing materials based on this approach. They hope that, by using a range of monomers and network materials, they can generate materials that will change structure and demonstrate new properties and functions over time, just like a living biological system.

Brief of developments

• The materials form as bone does from the innate attributes of the material without much labor. They sense the environment, respond to it, and repair any damage. This composite bridge is designed from a self-forming polymer and concrete system. Internal release of chemicals, their properties and location account for responsiveness to change and for repair. The choice of matrix additives also allow for the responsiveness. Bridge frames were fabricated for dynamic testing. The results showed that self repair and response to loads could be accomplished by careful placement of chemicals for later release and by use of chemicals which could alter such attributes as stiffness, flexure and permanent deformation. Internal viewing sensors could determine the state of the frames after testing.
• Scientists have taken a page from green plants and created a gel-like polymer that uses carbon dioxide from the air to grow, strengthen, and even mend itself. The researchers developed the material with future applications of the polymer in construction, repair or protective coatings in mind. It is known that carbon-fixing materials don’t exist yet today outside the biological realm.  The material developed is a synthetic material that could grow on trees, take carbon from carbon dioxide and add them to its material backbone to grow. The material doesn’t just avoid fossil fuels in its creation, but eats up carbon dioxide from the air and transforms it into a solid, stable form using sunlight, akin to plants. The polymer is made up of three primary components — aminopropyl methacrylamide (APMA), glucose and chloroplasts — along with an enzyme called glucose oxidase that allows it to add carbon molecules to its own chemical backbone. The properties of the polymer itself can be optimized further as, it isn’t strong enough for use in building material but can function as a filling or coating on surfaces. The life spans of chloroplasts, which, by virtue of being biological elements with a shelf-life, are one of the areas of future work for the team. Once the chloroplasts die, they can no longer carry out the reaction that fixes carbon dioxide — a crucial aspect of the material’s intended purpose. In ongoing and future work, the chloroplast is being replaced by catalysts that are nonbiological in origin.

Applications

• Commercial applications such as self-healing coatings and crack filling are realizable in the near term, whereas additional advances in backbone chemistry and materials science are needed before construction materials and composites can be developed.
• The researchers say the development of self-growing gel materials will help in applications as flexible exosuits for patients with skeletal injuries; these suits would potentially become stronger and more functional the more they are used. Since many types of such gels have similar mechanical features, this process could be applied to a wide range of gels, expanding the range of potential applications.