What are self-healing materials? Self-healing materials are materials that can detect when they have been broken and then repair themselves without any human intervention. In essence, our skin and bones are self-healing materials because they know when they have been damaged and can repair themselves when cut without needing any external support.
The majority of inanimate materials are not self-healing. If you break a window, you don’t expect the cracks to heal themselves in the same way our skin does when we’re cut. There are a number of self-healing materials around today and some of them work in a similar way to our body. If you cut yourself, chemicals are released by the tissue damage which alerts the body that there is an injury. This is the first step that self-healing materials need to have. These chemicals make the nearby blood vessels constrict to reduce the blood flow and they attract platelets. The platelets clump together in the cut, forming a clot, and stopping the flow of blood. The area around the cut then becomes inflamed as the immune response starts. The immune system directs white blood cells to the area and they clear away any debris that has been left after the cut and any bacteria that might have got in. While they are cleaning, a type of cell called fibroblasts turn up and they build a scaffold that holds the wound together and gives the new tissue a frame to grow on. The new tissue grows and the cut is healed. This is our self-healing material called skin.
The types of self-healing materials that exist at the moment are improving, but they are not on the level where they could repair something as well as our skin does. They are more to cope with degradation caused by aging or weathering. Quite often structures can fail because there are a lot of tiny microcracks inside them that are impossible to see and very difficult to detect. The goal of self-healing materials would be to fix these cracks as and when they appear and stop them from becoming big enough to bring down the structure.
There are several different ways of making materials that can heal themselves and many of them take some of the ideas that our body already has. One method is to implant sealing materials inside the material. Small capsules filled with an adhesive can be inserted throughout the material and when the material breaks, these capsules also break, releasing the adhesive. The problem with this system is that they capsules themselves weaken the structure, so they are not a great solution. The capsules can also only be used once, so it is not a long-term solution.
A way to get past this is to use a vascular system similar to our body. The adhesive is not stored within the material, but pumped to it along microscopic tubes when it is necessary. Pressurized spaces are built into the material and when the material breaks, the pressure drops, pulling the adhesive along the tube. This system can fix very large cracks, but it does take a long time for the adhesive to reach the damaged area. It would be a good system for buildings where the cracks appear slowly over a long period of time.
There is another method that uses heat to seal cuts. Some materials, such as several plastics, melt when you heat them and the polymers reconnect. To do this, they need a lot of heat energy and that wouldn’t be available in places like skyscrapers that are slowly weathering. However, the military is looking into this because the heat energy supplied by a bullet is enough to reseal the hole it makes as it passes through the material. That means bullets could pass right through a plane and the plane could carry on flying.
We are a long way from being able to make a car fix itself after a crash, but the goal for most of these self-healing materials is to help deal with weathering and aging. There are several paints on the market today that combine some of the methods we have looked at and provide a layer of protection for buildings and structures. They can be used on vehicles as well. They protect the material underneath from the weather, but they also have the capacity to fix and small cracks that might appear. They are not an ultimate cure, but they can help to make the building or vehicle last longer. And this is what I learned today.
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Sources
https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/how-wounds-heal
https://en.wikipedia.org/wiki/Self-healing_material
https://www.explainthatstuff.com/self-healing-materials.html
Photo by Brett Sayles: https://www.pexels.com/photo/green-bridge-over-the-river-11704829/