What do sea urchins and cement have in common? More than you'd think! The sea urchin spines are mostly made of calcite, usually a very brittle and fragile material. In the case of the sea urchin, however, the spines are much more durable than the raw material alone. The reason for its strength is the way that nature optimises materials using a brick wall-style architecture. The research team Physical Chemistry, headed by Professor Helmut Cölfen, successfully synthesised cement at the nano-level according to this "brick and mortar 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, with the blocks assembling themselves in an ordered manner. The aim is to make cement more durable.
In nanoscience, brick wall-style architecture can be compared to the work of a mason: each layer of brick that is laid is held in place by mortar. The guiding principle is to layer hard, then soft, hard, then soft materials. This is exactly the principle nature uses to make sea urchin spines so resilient. When force is applied to the brittle calcite, its crystalline block does crack, however, the energy is then transferred to a soft disordered layer. Since this material has no cleavage planes to tear, it prevents 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.
In collaboration with the University of Stuttgart, the research 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 megapascals. By comparison: Mussel shells, which are the gold standard in fracture-resistance, reach a value of 210 megapascals, which is only slightly higher. The concrete commonly used today has a value of two to five megapascals.
See urchin spines and mussel shells are made of calcite, because large quantities of calcium are available in water. Helmut Cölfen explains: "People have much better construction materials than calcite. 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."