Generally, materials will degrade over time due to fatigue, environmental conditions, or damage incurred during operation. Repair of certain materials during their lifecycle (especially those with structural functions such as concrete) can be very expensive and labour intensive. Therefore Scientists began developing self-healing materials , those artificial (synthetic) substances that automatically repair themselves without any overt diagnosis of the problem or intervention by a human being. Self-healing materials could have applications from bridges and buildings that repair their own cracks to car fenders made from shape memory polymers that automatically flex back to shape after low-speed collisions.
Polymers and fiber-reinforced polymers (FRPs) are common as structural materials due to lightweight, easy processability, and constancy against adverse environmental impacts. However, mechanical properties are associated with many variables including service time, operating temperature and pressure, molecular weight, and constitution of matrix. Long-term durability, high performance and reliability are major challenges for polymeric architecture. Limiting factors of polymer composite is relatively poor performance under impact loading due to lack of plastic deformation, which is a most prominent aspect of any vehicle component design. To resist these failures, the new-generation materials having autonomic healing capability to damage repair are needed to develop.
Similarly, in thermoplastics which are known for their mechanical performances, service temperature, and solvent resistance, fusion bonding techniques are used to recover mechanical integrity. However, these repair strategies are very costly, time-consuming, followed by complicated procedure, and assisted only by expert technicians. Self-healing of cracks is an eminent and efficient possible solution of these issues. In healing systems, a damage incident triggers the internal processes that generate the healing response which cured the damages.
The first self-healing materials we’re likely to see in mass production will be paints and coatings that can better survive the weather and other kinds of surface wear-and-tear. More advanced self-repairing materials are likely to follow on, including things like self-repairing seals and gaskets for pipelines.
Researcher Cai Liheng is part of a team at Harvard University that has just patented a new kind of self-healing rubber. Rather than cracking when excess force is applied, the material, which incorporates reversible polymer bonds, will return to its original form when the stress is released, he says. The breakthrough could eventually lead to tyres that last forever, says Cai, but the material has a wide range of other potential uses too. Rubber is also used for medical implants and in automotive supplies, among other applications. “Think about it – anywhere we use rubber, it could still have the mechanical properties of old rubber but can also self-heal. That would result in huge environmental benefits.”
Self Healing Materials
Although the most common types of self-healing materials are polymers or elastomers, self-healing covers all classes of materials, including metals, ceramics, and cementitious materials.
Healing mechanisms vary from an intrinsic repair of the material to the addition of a repair agent contained in a microscopic vessel. First are materials with embedded “healing agents,” or have built-in microcapsules (tiny embedded pockets) filled with a glue-like chemical that can repair damage. If the material cracks inside, the capsules break open, the repair material “wicks” out, and the crack seals up.
In a slightly different approach, the main body of the material is a solid polymer, while the capsules contain a liquid monomer (one of the basic, endlessly repeated units that makes up the polymer). When the material fails and the capsules break, the monomer mixes with the polymer, more polymerization occurs, and the damage is healed effectively by creating more of the original material to replace the damaged area. Typically, a powdered chemical catalyst has to be embedded as well so the polymerization will happen at a relatively low, everyday temperature and pressure.
The main drawback with the encapsulation method is that the capsules have to be very small indeed or they weaken the material in which they’re embedded; that limits the amount of damage they can fix (the size of the cracks they can fill). Another problem is that the capsules can only heal damage once: if the material fails again (more likely since it is almost certainly weaker after repair) it cannot heal itself twice.
The bio-systems are a source of innovation to design such functionality as they have builtin such damage detection and subsequent prevention techniques. Mimicking biosystems, in synthetic materials, damage repair is performed by three steps including actuation of healing, later transport of healing precursors to the damage site, and finally chemical repair process occurring with the contact of catalyst or curing species at an angstrom level in which matrix is restructured by bonding of cleaved bonds at the damage site.
This kind of Material has a kind of internal “vascular” circulation analogous to blood built into them that can pump healing agents (adhesives, or whatever else is needed) to the point of failure only when they need to do so. Materials scientists have been trying to design self-healing materials that work the same way. Some have networks of extremely thin vascular tubes (around 100 microns thick—a little thicker than an average human hair) built into them that can pump healing agents (adhesives, or whatever else is needed) to the point of failure only when they need to do so. The tubes lead into pressurized reservoirs (think of syringes that are already pushed in slightly). When a failure occurs, the pressure is released at one end of the tube causing the healing agent to pump in to the place where it’s needed.
Although this method can seal cracks up to ten times the size that the microcapsule method can manage, it works more slowly because the repair material has further to travel; that could pose a problem if a crack is spreading faster than it’s being repaired. But in something like a skyscraper or a bridge, where a failure might appear and creep (spread slowly) over months or years, a system of built-in repair tubes could certainly work well.
Self-healing shape-memory materials have some sort of mechanism for delivering heat to the place where damage has occurred. In practice, that might be an embedded network of fiber-optic cables similar to the vascular networks used in other self-healing materials except that, instead of pumping up a polymer or adhesive, these tubes are used to feed laser light and heat energy to the point of failure.
Reversible Polymers don’t always need sophisticated internal systems, such as embedded capsules or vascular tubes, to repair internal damage. Some of them break apart to reveal what we might think of as highly “reactive” ends or fragments that naturally try to join up again. Energized by either light or heat, these stray fragments naturally try to rebond themselves to other nearby molecules, effectively reversing the damage and repairing the material. Some break to expose electrically charged ends, which give the broken fragments a built-in electrostatic attraction. When damage occurs, electrostatic forces pull the fragments together, enabling the material to self-repair.
Plastics come in two main kinds. Some (known as thermoplastics) are relatively easy to melt down, recycle, and mold into new forms; PVC (polyvinyl chloride), polyethylene, and polypropylene are typical examples. Others (known as thermosets or thermosetting plastics) work a different way: if you heat them, they degrade before they melt so you can’t heat them to reshape them; melamine and bakelite are good examples. This suggests that we might be able to use thermoplastics (but not thermosets) as self-healing materials. We’d simply need them to melt under stress so the long polymer chains inside could rearrange themselves back into a strong, new form.
Self-healing material can build itself from carbon in the air
In 2018, MIT chemical engineers reported to have developed a material designed that couls react with carbon dioxide from the air, to grow, strengthen, and even repair itself. The polymer, which might someday be used as construction or repair material or for protective coatings, continuously converts the greenhouse gas into a carbon-based material that reinforces itself.
The current version of the new material is a synthetic gel-like substance that performs a chemical process similar to the way plants incorporate carbon dioxide from the air into their growing tissues. The material might, for example, be made into panels of a lightweight matrix that could be shipped to a construction site, where they would harden and solidify just from exposure to air and sunlight, thereby saving on the energy and cost of transportation.
“This is a completely new concept in materials science,” says Strano, the Carbon C. Dubbs Professor of Chemical Engineering. “What we call carbon-fixing materials don’t exist yet today” outside of the biological realm, he says, describing materials that can transform carbon dioxide in the ambient air into a solid, stable form, using only the power of sunlight, just as plants do.
Developing a synthetic material that not only avoids the use of fossil fuels for its creation, but actually consumes carbon dioxide from the air, has obvious benefits for the environment and climate, the researchers point out. “Imagine a synthetic material that could grow like trees, taking the carbon from the carbon dioxide and incorporating it into the material’s backbone,” Strano says.
The material the team used in these initial proof-of-concept experiments did make use of one biological component — chloroplasts, the light-harnessing components within plant cells, which the researchers obtained from spinach leaves. The chloroplasts are not alive but catalyze the reaction of carbon dioxide to glucose. Isolated chloroplasts are quite unstable, meaning that they tend to stop functioning after a few hours when removed from the plant. In their paper, Strano and his co-workers demonstrate methods to significantly increase the catalytic lifetime of extracted chloroplasts. In ongoing and future work, the chloroplast is being replaced by catalysts that are nonbiological in origin, Strano explains.
The material the researchers used, a gel matrix composed of a polymer made from aminopropyl methacrylamide (APMA) and glucose, an enzyme called glucose oxidase, and the chloroplasts, becomes stronger as it incorporates the carbon. It is not yet strong enough to be used as a building material, though it might function as a crack filling or coating material, the researchers say.
The team has worked out methods to produce materials of this type by the ton, and is now focusing on optimizing the material’s properties. Commercial applications such as self-healing coatings and crack filling are realizable in the near term, they say, whereas additional advances in backbone chemistry and materials science are needed before construction materials and composites can be developed.
One key advantage of such materials is they would be self-repairing upon exposure to sunlight or some indoor lighting, Strano says. 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.
While there has been widespread effort to develop self-healing materials that could mimic this ability of biological organisms, the researchers say, these have all required an active outside input to function. Heating, UV light, mechanical stress, or chemical treatment were needed to activate the process. By contrast, these materials need nothing but ambient light, and they incorporate mass from carbon in the atmosphere, which is ubiquitous. The material starts out as a liquid, Kwak says, adding, “it is exciting to watch it as it starts to grow and cluster” into a solid form.
Self-healing, flexible electronics
In 2016, The Penn State Researchers reported to have developed a flexible electronic material that self-heals to restore many functions, even after multiple breaks. Electronic materials have been a major stumbling block for the advance of flexible electronics because existing materials do not function well after breaking and healing.
“Wearable and bendable electronics are subject to mechanical deformation over time, which could destroy or break them,” said Qing Wang, professor of materials science and engineering, Penn State. “We wanted to find an electronic material that would repair itself to restore all of its functionality, and do so after multiple breaks.”
Researchers added moisture-impermeable boron nitride nanosheets to a base layer of plastic polymer. The material is more rugged than earlier self-healable materials, which often were soft or even “gum-like,” according to Qing Wang, the Penn State professor of materials science and engineering who led the research team.”This material is not going to repair itself forever, but we damaged our material at least 10 times and most of the key properties were maintained,” he said.
According to Wang, the new material is able to self-heal due because boron nitride nanosheets can link to each other via hydrogen bonding groups found on their surfaces. When two sheets are placed in close proximity, the naturally occurring electrostatic attraction draws them together. When the hydrogen bond is restored, the two pieces are healed.
In the U.S., researchers are working to combine self-healing properties with protection from chemical and biological attack. Inspired by squid ring teeth proteins, the Materials Research Institute at Penn State University have developed self-healing polyelectrolyte coatings comprised of positively and negatively charged polymers. The coating is applied in a series of layers that increase the strength of the fibers, and it is self-healing in wet conditions, so just washing the fabric can repair small defects in the coating.
Gamache, with Charles Roland, Daniel Fragiadakis, Carl Giller and Roshdy G S Barsoum, patented “Polymer coatings with embedded hollow spheres for armor for blast and ballistic mitigation” in a project with the U.S. Navy. The goal is a lightweight armor system providing blast protection and ballistic protection against small arms fire and suitable for use in helmets, vehicle protection, and other armor systems.
A hard substrate is coated on the front surface with a thin, elastomeric polymer layer, in which hollow ceramic or metal spheres are encapsulated. The coating layer, having a thin elastomeric polymer layer with encapsulated metal or ceramic hollow spheres, can be stand-alone blast protection, or it can be added to an underlying structure.
Materials may lead to self-healing smartphones
In 2017, researchers reported that they have developed a self-healing polymeric material with an eye toward electronics and soft robotics that can repair themselves. The material is stretchable and transparent, conducts ions to generate current and could one day help your broken smartphone go back together again.
The new material not only heals itself, but it also stretches up to 50 times its usual size. The key to self-repair is in the chemical bonding. Two types of bonds exist in materials, Wang explains. There are covalent bonds, which are strong and don’t readily reform once broken; and noncovalent bonds, which are weaker and more dynamic. For example, the hydrogen bonds that connect water molecules to one another are non-covalent, breaking and reforming constantly to give rise to the fluid properties of water. “Most self-healing polymers form hydrogen bonds or metal-ligand coordination, but these aren’t suitable for ionic conductors,” Wang says.
Wang’s team at the University of California, Riverside, turned instead to a different type of non-covalent bond called an ion-dipole interaction, a force between charged ions and polar molecules. “Ion-dipole interactions have never been used for designing a self-healing polymer, but it turns out that they’re particularly suitable for ionic conductors,” Wang says. The key design idea in the development of the material was to use a polar, stretchable polymer, poly(vinylidene fluoride-co-hexafluoropropylene), plus a mobile, ionic salt. The polymer chains are linked to each other by ion-dipole interactions between the polar groups in the polymer and the ionic salt. The resulting material could stretch up to 50 times its usual size. After being torn in two, the material automatically stitched itself back together completely within one day.
Smartphone company Motorola had filed a patent for a self-healing phone display. The design includes a “shape memory polymer”, which the patent application says would at least partly reverse damage when exposed to heat. Before long, we’ll see self-healing paints and coatings—maybe even self-healing cars, bridges, and buildings. The hit film featured a Terminator made of liquid metal that could form its arms into stabbing weapons and heal itself after being shot with everything from a 12-gauge shotgun to a 40mm grenade launcher.
Self-healing materials for soft machines, reported in July 2020
Self-healing materials are indispensable for soft actuators and robots that operate in dynamic and real-world environments, as these machines are vulnerable to mechanical damage. Repeated activity wears on soft robotic actuators, but these machine’s moving parts need to be reliable and easily fixed. Now a team of researchers has a biosynthetic polymer, patterned after squid ring teeth, that is self-healing and biodegradable, creating a material not only good for actuators, but also for hazmat suits and other applications where tiny holes could cause a danger.
“Current self-healing materials have shortcomings that limit their practical application, such as low healing strength and long healing times (hours),” the researcher report in today’s issue of Nature Materials. The researchers produced high-strength synthetic proteins that mimic those found in nature. Like the creatures they are patterned on, the proteins can self-heal both minute and visible damage. “Our goal is to create self-healing programmable materials with unprecedented control over their physical properties using synthetic biology,” said Melik Demirel, professor of engineering science and mechanics and holder of the Lloyd and Dorothy Foehr Huck Chair in Biomimetic Materials.
Demirel’s team creates the self-healing polymer by using a series of DNA tandem repeats made up of amino acids produced by gene duplication. Tandem repeats are usually short series of molecules arranged to repeat themselves any number of times. The researchers manufacture the polymer in standard bacterial bioreactors. By adjusting the number of tandem repeats, Demirel’s team created a soft polymer that healed rapidly and retained its original strength, but they also created a polymer that is 100% biodegradable and 100% recyclable into the same, original polymer.
“We were able to reduce a typical 24-hour healing period to one second so our protein-based soft robots can now repair themselves immediately,” said Abdon Pena-Francelsch, lead author of the paper and a former doctoral student in Demirel’s lab. “In nature, self-healing takes a long time. In this sense, our technology outsmarts nature.” The self-healing polymer heals with the application of water and heat, although Demirel said that it could also heal using light. “If you cut this polymer in half, when it heals it gains back 100 percent of its strength,” said Demirel.
Demirel explained that while many petroleum-based polymers can be recycled, they are recycled into something different. For example, polyester t-shirts can be recycled into bottles, but not into polyester fibers again. Just as the squid the polymer mimics biodegrades in the ocean, the biomimetic polymer will biodegrade. With the addition of an acid like vinegar, the polymer will also recycle into a powder that is again manufacturable into the same, soft, self-healing polymer.
New materials may soon make it possible for damaged electronic components, such as in spacecrafts, to mend themselves, reported in 2021
New materials may soon make it possible for damaged electronic components, such as in space crafts, to mend themselves. The materials recently developed by scientists can repair their own mechanical damages with the electrical charges generated by the mechanical impact on them. Devices that we use daily often break down due to mechanical damage, forcing us either to repair or replace them. This decreases the life of the equipment and increases maintenance costs. In many cases, like in space crafts, human intervention for restoration is not possible.
Keeping such necessities in mind, researchers from the Indian Institute of Science Education and Research (IISER) Kolkata, teaming up with IIT Kharagpur, have developed piezoelectric molecular crystals that repair themselves from mechanical damages without need for any external intervention. Piezoelectric crystals are a class of materials that generate electricity when it undergoes a mechanical impact.
The piezoelectric molecules developed by the scientists called bipyrazole organic crystals recombine following mechanical fracture without any external intervention, autonomously self-healing in milliseconds with crystallographic precision. In these molecular solids, due to the unique property of generating electrical charges on mechanical impact, the broken pieces acquire electrical charges at the crack junction, leading to attraction by damaged parts and precise autonomous repair. This research supported by the Department of Science and Technology, GoI via Swarnajayanti Fellowship to CM Reddy and Science and Engineering Research Board (SERB) research grants has been published in the journal ‘Science’ recently.
This methodology was initially developed by the IISER Kolkata team led by Prof. C Malla Reddy, a recipient of Swarnajayanti fellowship (2015) given by the Department of Science & Technology, GoI. Prof. Nirmalya Ghosh of IISER Kolkata, a laureate of the Society of Photo-Optical Instrumentation Engineers (SPIE) G.G. Stokes Award in Optical polarization 2021, used a custom-designed state-of-the-art polarization microscopic system to probe and quantify the perfection of the piezoelectric organic crystals. These materials with perfect internal arrangement of molecules or ions are called ‘crystals’, which are abundant in nature.
The IIT Kharagpur’s team, Prof. Bhanu Bhusan Khatua and Dr. Sumanta Karan studied the performance of the new materials for fabricating mechanical energy harvesting devices. The material may find application in high-end micro-chips, high precision mechanical sensors, actuators, micro-robotics, and so on. Further research into such materials may eventually lead to the development of smart gadgets that self-repair cracks or scratches.
Global Markets for Self Healing Materials
The global self-healing concrete market size was valued at USD 24.60 billion in 2019 and is expected to expand at a compound annual growth rate (CAGR) of 37.0% from 2020 to 2027.
Rising demand for reliable and durable constructions, such as infrastructure, commercial, industrial, and residential, is expected to drive the demand for self-healing concrete over the forecast period. Growing demand for advanced self-repairing polymers from end-use industries and the ability of materials to repair damages caused due to mechanical friction are the factors estimated to drive overall market growth in the forecast period.
The common types of materials comprise reversible polymers, shape-memory materials, vascular systems, capsule-based systems and biologically based materials. Reversible polymers dominate the self-healing materials market at present, and will still account for 37 percent of the market by 2022. However, the big growth opportunities will come from self-healing systems based on microencapsulation or vascular systems. Self-healing materials using microencapsulation systems will generate revenues of $1.1 billion in revenues in 2022.
The polymers sector is estimated to grow at a CAGR of 46.4% from 2019 to 2025, on account of its rising use in plastic industry and engineering activities. Polymers are extensively used owing to their ability to repair micro-scale damages and restore the passive state of substrate thereby resulting in increased adoption.
Fiber-reinforced composites are exclusively used owing to their durability and superior corrosion resistance. The global fiber-reinforced composites sector accounted for a notable revenue share on account of the growing use of the products in aerospace industry to store liquid oxygen and hydrogen.
The coatings segment accounted for 27.3% of the total market share in 2018 and is estimated to reach 28.3% market share by 2025 on account of the rising demand of the product from automotive and aerospace industries. Increase in the operation lifetime of paints and coatings coupled with the reduction in the maintenance cost are expected to emerge as the major factors driving growth.
The self-healing concrete is estimated to grow at the fastest pace with a CAGR of 48.4% over the forecast period owing to its durability & strength coupled with the high use of the product in critical applications in the building & construction industry. In addition, rising demand from construction sectors mainly in Europe and Asia Pacific is estimated to boost concrete demand in future.
The self-healing materials produced from hydroxyl end-functionalized polydimethylsiloxane (HOPDMS) and polydiethoxysiloxane (PDES) are used to prevent the substrate from external environmental impact. In addition, the introduction of self-healing microcapsules containing rejuvenator has resulted in the decrease in crack formation in asphalt formulations.
Interesting work is being done on self-healing metals – although at an early stage, a market for self-healing metals can hardly be doubted. We are also witnessing a concerted effort to create self-healing ceramics – these will have an important role to play for engines and electrical generators. Carbon nanotubes have been envisioned for sensors that can detect cracks and initiate a self-healing process. CNTs could also be used as thermal guides to heal reversible polymers.
Biological material systems are exclusively used in medical implants with the sector estimated to witness a CAGR of 47.5% in the forecast period. In addition, growing geriatric population is anticipated to increase the demand for biological self-healing material which is further expected to benefit the industry growth over the forecast period.
Europe dominated the self-healing materials market accounting for 31.1% of total market share in 2018. High use of self-repairing specialty polymers in several countries like France, UK, and Germany coupled with process innovation, advanced R&D, and expansion of automobile production across the region are estimated to fuel the industry growth over the forecast period.
North America is the second largest consumer of self-healing materials, followed by Asia Pacific, on account of rising demand from end-user industries. The U.S. dominates North American market, in terms of consumption, owing to the high demand from residential sector, extensively developed manufacturing industry, and government initiatives towards infrastructure development.
Some prominent players in the global self-healing concrete market include: Basilisk, PENETRON, Kryton, Xypex Chemical Corporation, Sika AG, BASF SE, Hycrete, Inc., Cemex, Oscrete, GCP Applied Technologies, and RPM International
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