German researchers have made a stronger cement inspired by the spines of sea urchins, which is significantly more fracture-resistant.
Sea urchin spines are mostly made of calcite, usually very brittle and fragile. However, their structure is optimised with a ‘brick wall style’ architecture making them much more durable.
A team from the University of Konstanz in southern Germany successfully synthesised cement at the nano-level using the same principle.
During this process, macro-molecules were identified that take on the function of mortar, affixing the crystalline blocks to each other on the nano-scale.
The guiding principle is to layer hard, then soft materials, the same principle nature uses to make sea urchin spines so resilient. When force is applied to the brittle calcite, its crystalline block cracks, however, the energy is then transferred to a soft disordered layer with no cleavage planes to tear, preventing further cracking. A thin section of sea urchin spine reveals this structural principle: crystalline blocks in an orderly structure are surrounded by a softer amorphous area. In the sea urchin’s case, this material is calcium carbonate.
“Our cement, which is significantly more fracture-resistant than anything that has been developed thus far, provides us with completely new construction possibilities,” said team leader Professor Helmut Cölfen.
In collaboration with the University of Stuttgart, the team was able to use an ion beam under an electron microscope to cut a bar-shaped micro-structure out of the nanostructured cement that was three micrometers in size. This micro-structure was then bent using a micro-manipulator.
As soon as it was released, the micro-structure returned to its original position. Mechanical values could be calculated based on the elastic deformation of the micro-structure. Based on these calculations, the optimised cement achieved a value of 200 MPa. By comparison, mussel shells, which have a similar structure to urchin spines and are considered the gold standard in fracture-resistance, reach a value of 210 MPa. The concrete commonly used today has a value of 2-5 MPa.
“Humans have much better construction materials than calcite,” said Cölfen. “If we succeed in designing the structures of materials and reproduce nature’s blueprints, we will also be able to produce much more fracture-resistant materials – high-performance materials inspired by nature.”