How cracks close themselves
Self-healing materials used to be part of the science fiction world, but today they’re a reality. Due to smart processes, they can repair themselves – regardless of the type of damage: cuts, cracks, or fractures. These materials contain living biological cells providing them with properties that just exist in nature. The human skin serves as a role model. Essentially, living materials like these consist of two components: of organisms such as yeasts or bacteria that are supposed to fulfill a specific function and have been programmed accordingly, and of a carrier material into which the living organisms are enclosed.
These organisms have specific metabolic properties and can produce a variety of substances, ranging from inorganic salts to metal oxides and bi-polymers to highly effective active ingredients in medical drugs. This ability can be used for producing technical and medical materials with novel functions that non-living materials don’t have. Besides self-regeneration of the material after having been damaged, that includes flexible adjustment to environmental stimuli or extremely long life.
The construction materials industry has been investigating self-repairing materials for a long time, and successfully so. There’s concrete that heals itself, enabled by bacteria that in the form of spores are cast into the concrete. Spores can survive for decades and centuries. When a crack emerges in the concrete water entering there revives the spores. They start producing calcium carbonate – lime. That lime seals the crack from the inside. Because the bacteria “heal” the
cracks that have emerged the concrete lasts longer, a building doesn’t have to be torn down. That saves material resources and energy, and reduces greenhouse gases.
The University of Cambridge pursues the self-healing capacities of concrete as well. The researchers there have 3D-printed a supporting structure made of concrete. Not only is it less bulky than comparable cast parts and so saves more materials but it’s also equipped with sensors that can independently monitor the construction for decades and initiate auto-repairs. The self-healing effect works with paints and varnishes as well. Initially, they were intended for self-repairing car paints. However, what already works with microscopically small scratches that may, for instance, be caused by a carwash leaves a disturbing crater landscape in the case of paint scratches that are visible by the naked eye.
Medical device technology is another field in which interesting applications could emerge, for instance in the area of implants or wearables for monitoring diseases, or regarding low-cost sensors for environmental monitoring. An application that’s already on the horizon today is actuators for soft robotics. That involves materials changing their shape or volume when exposed to light, external impulses, moisture, or specific substances. Another interesting product is coatings that renew themselves. In the case of its “Corrotect” specialty coating, a corrosion protection for rolling bearings and precision parts, Schaeffler works with ultra-thin layers of silicon oxide nanoparticles that can heal themselves due to contact with oxygen in the event of damage.
With her research, Professor Aránzazu del Campo, Scientific Director of the Leibnitz Institute for New Materials, wants to imbue materials with new vigor, but also names issues, particularly with a view toward recycling. “It’s important,” she says, “to clarify the question of how to ensure that the utilization of living materials does not pose any risks to the environment – i.e., biocontainment, in other words, the biosafety of laboratories. For instance, as early as in the material development stage you must include in your plans the possibility that the cells contained cannot survive under certain conditions.” Despite all the strides that have been made in bionics – not everything seems to have been decrypted yet.