Self-Healing ‘Living Materials’
A growing field of research dedicated to “living materials” could one day enable impressive functionalities like bridges patching up their own cracks or houses responding to a rupture in one of its structural beams by repairing itself. The latest example comes from an Imperial College London (ICL) study that developed tiny bacteria-based 3D building blocks to repair their own damage.
Dr. Patrick Rose of the US Office of Naval Research Global London, which helped fund the study, explained:
“The challenge is to mimic and combine the distinct features biology has to offer. We are not only trying to emulate those systems, but engineer biology to have additional features that are more amenable to the needs we seek without direct intervention.
Ultimately, we want to increase the lifetime of a product, prevent failures of systems before the problem is visible to the naked eye and have the material think for itself.”
Building on earlier work from the group, the ICL team set out to produce engineered living materials that can repair their own damage via a biologically inspired sense and response system.
Professor Tom Ellis, an author of the study, said:
“In the past, we’ve created living materials with inbuilt sensors that can detect environmental cues and changes. Now we’ve created living materials that can detect damage and respond to it by healing themselves.”
To achieve this, the scientists genetically engineered a bacteria called Komagataeibacter rhaeticus to produce fluorescent cell cultures shaped like a sphere, dubbed spheroids. Similar to building blocks, these 3D spheroids can then be arranged into shapes and patterns. Next, the ICL team tested their self-repairing abilities in a structural material known as bacterial cellulose.
Bacterial cellulose is a naturally occurring scaffold-like material synthesized by some bacteria. It has vast potential in several industries, finding use in medical care as wound dressing, high-strength paper, and filters in speakers.
To test the efficiency of the material, the scientists punched holes in a thick bacterial cellulose layer and then planted their spheroids in the cavities. After only three days of incubation, the spheroids repaired the damage and restored the original material’s consistency and appearance. “By placing the spheroids into the damaged area and incubating the cultures, the blocks were able to both sense the damage and regrow the material to repair it,” added Ellis.
Dr. Joaquin Caro-Astorga, the study’s first author, said:
“Our discovery opens a new approach where grown materials can be used as modules with different functions like construction. We are currently working on hosting other living organisms within the spheroids that can live together with the cellulose-producing bacteria.
The possible living materials that can come from this are diverse: for example, with yeast cells that secrete medically relevant proteins, we could generate wound-healing films where hormones and enzymes are produced by a bandage to improve skin repair.”
The team envisions integrating the spheroids into building materials to give them the ability to detect and repair their own damage. The work, published in Nature Communications, could lead to potholes that seal themselves in the road, windshields that fix their own cracks, and aircraft that repair their own fuselages.
Such a future is far away, but the ICL scientists are working toward it by fusing their spheroids with materials like graphite, wood, cotton, sponges, and gelatins to create more complex designs. This could pave the way for new applications like implantable electronics, medical biosensor patches, and biological filters.
Self-Healing Ceramics
Ceramic materials can withstand extremely high temperatures but are notoriously fragile. However, Texas A&M scientists have uncovered a previously unknown self-healing mechanism in a particular type of ceramic.
Ceramics are prone to cracking when put under mechanical stress, leading to complete failure as the material shatters. Some ceramics can self-repair these cracks, but it typically requires chemical reactions at very high temperatures.
In the study, the team discovered that a specific type of ceramic, chromium aluminum carbide, could slow down the crack spread and heal them, all at room temperature, making them extremely useful in real-world applications.
These ceramics are called MAX phases, and they comprise alternating layers of material. It is precisely this structure that gives them their healing ability. For example, when a crack begins, defects known as kink bands form between the different layers. As the pressure is applied, crystals in the kink bands rotate, preventing the cracks from spreading further.
The best part of all, these rotated crystals actively heal the crack so that once the force is relieved from the material, they’re almost untraceable. “What’s really exciting is that this kinking or self-healing mechanism can occur over and over, closing the newly formed cracks, thus delaying the failure of the material,” noted Hemant Rathod, the study’s lead author.
The scientists envision this self-healing ceramic being used to repair cracks that form in materials subject to high-stress situations, such as nuclear reactors, hypersonic craft, and jet engines. The research was published on Aug 11 in Science Advances. The video below shows an animation of the kink-band defects.
Self-Healing Concrete Consumes CO2 To Fill Its Own Cracks
In April, Scientists at the Worcester Polytechnic Institute (WPI) developed self-healing concrete that can repair its own cracks by consuming CO2.
Tiny cracks in concrete don’t pose an immediate threat to the structure of a building, but after time, water seeps in, and the rupture spreads. This compromises its strength and lifespan. Self-healing concrete is designed to intervene in this process while the cracks are still small, sealing up the material to prevent anything from a catastrophic collapse to expensive maintenance or an entire structure replacement.
The WPI team turned to the human body for inspiration, specifically, the enzyme in red blood cells known as carbonic anhydrase (CA) and how it can rapidly transfer CO2 from the cells into the bloodstream.
The scientists added CA enzyme to concrete powder before the material was mixed and poured. When the concrete forms a crack, the enzyme interacts with CO2 to create calcium carbonate crystals, which imitate the characteristics of concrete and promptly fill in the gap.
In lab tests, the team demonstrated that the concrete containing the enzyme could repair its millimeter-scale cracks within 24 hours.
Self-Healing Material Regains Function After Being Cut
Last year, a team at Carnegie Mellon University developed a self-healing material that could regain function after being cut. The material has several impressive applications, some of which include:
- Automatically sealing itself around a broken arm as a smart polymer cast,
- A membrane that can detect where it has been cut,
- And pneumatic actuators that can be dismembered and reconfigured into different shapes.
The unique characteristics of the composite material allow it to self-heal and enable devices made from it to regain their functionality or gain new functionality after being cut or punctured. Impressively, when the cut ends of the material are positioned together, the pieces reconnect, and the seam between them slowly disappears.
Self-Healing Stretchable Electronic Material That Glows
Also, last year, a team of National University of Singapore researchers produced a self-healing stretchable electronic material for next-gen electronic wearables and soft robots. The material also emits light as bright as a smartphone screen! As a result, things like light-emitting robots can be made that quickly locate survivors in the dark, and electronics could have unbreakable displays.
Beyond its robot-enhancing benefits, the material can be used in automotive applications, safety lighting, and specialty packaging.
