The “self-healing” materials are 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.
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.
This 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.
Self-healing materials come in four main kinds. 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. The second kind are Materials with 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.
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.
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.”
Materials may lead to self-healing smartphones
Researchers report that they have developed a self-healing polymeric material with an eye toward electronics and soft robotics that can repair themselves. 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.
Self-healing Electronic Skin
Researchers in the Department of Chemical Engineering at the Technion – Israel Institute of Technology in Haifa (Israel), have developed a self-healing device based on new kind of synthetic polymer (a polymer is a large molecule composed of many repeated smaller molecules).
“The vulnerability of flexible sensors used in real-world applications calls for the development of self-healing properties similar to how human skins heals,” said self-healing sensor co-developer Prof. Hossam Haick. “Accordingly, we have developed a complete, self-healing device in the form of a bendable and stretchable chemiresistor where every part – no matter where the device is cut or scratched – is self-healing.”
Once healed, the polymer substrate of the self-healing sensor demonstrates sensitivity to volatile organic compounds (VOCs), with detection capability down to tens of parts per billion. It also demonstrates superior heal ability at the extreme temperatures of -20 degrees C to 40 degrees C. This property, said the researchers, can extend applications of the self-healing sensor to areas of the world with extreme climates. From sub-freezing cold to equatorial heat, the self-healing sensor is environment-stable.
The new sensor is comprised of a self-healing substrate, high conductivity electrodes, and molecularly modified gold nanoparticles. “The gold particles on top of the substrate and between the self-healing electrodes are able to “heal” cracks that could completely disconnect electrical connectivity,” said Prof. Haick. The researchers are currently experimenting with carbon-based self-healing composites and self-healing transistors.
“The self-healing sensor raises expectations that flexible devices might someday be self-administered, which increases their reliability,” explained co-developer Dr. Tan-Phat Huynh, also of the Technion, whose work focuses on the development of self-healing electronic skin. “One day, the self-healing sensor could serve as a platform for biosensors that monitor human health using electronic skin.” Future possible applications could include the creation of ‘electronic skin’ and prosthetic limbs that allow wearers to ‘feel’ changes in their environments.
A research team based at Pennsylvania State University reports that it has created a way to produce more durable flexible wearable systems embedding circuits that can heal themselves after breaking. The technology could allow troops operating in the harshest and stressful environments to access communications, information and delivery tasks without the need to carry bulky external devices.
The Penn State technology adds 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.
Global Markets for Self Healing Materials: 2017-2024 – Reversible Polymers Dominate the Market with 37% of Market Share
Research and Markets has announced the addition of the “Markets for Self Healing Materials: 2017 – 2024” report to their offering. The types of materials covered 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 automotive sector already uses self-healing aftermarket coatings, but has begun to sample much higher performing self-healing materials that can comply with the industry’s demanding coatings requirements. The rapidly growing use of relatively fragile composites in the automotive sector is a strong driver for the development of self-healing composites. There are similar trends in the construction sector, with self healing concrete undergoing commercialization. Although, it will take until 2019 to take off, we expect sales of self-healing materials for construction to reach $475 million by 2022.
Composites are not the only materials that are becoming self-healing. 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.
References and Resources also include: