It ’ s tough to be strong : Advances in bioinspired structural ceramic - based materials

Biological materials science focuses on the structure–function–property–processing paradigm, a common theme in materials science. However, synthesis and growth of natural materials is quite different from that of synthetic materials. Almost all biological systems follow six fundamental design principles.1–3 • Water. Although essential for biological systems, synthetic materials typically avoid water. • Cyclic, green process. The life and decomposition cycles of biological systems occur at about standard temperature and pressure—300 K and 1 atm. Ceramic and metal processing involves high pressures and temperatures. • Local resources. Biological systems use available organic (soft) and inorganic (hard) building blocks, made of carbon, nitrogen, oxygen, hydrogen, calcium, phosphorus, and sulfur, resulting in a vast array of hybrid systems. Synthetic materials require acquisition of resources. • Self-assembly. A “bottom-up” process builds structural hierarchy across nanolength to macrolength scales. Engineers build structures from the top down. • Fitting form to function. Biological systems grow, selfrepair, and evolve as needed. Function dictates the organism’s shape, not vice versa. This allows identification of the properties that have been optimized for a certain function. Natural materials, such as nacre (top left), bone (top right), and narwhal tusk (bottom), suggest structures and processing approaches for new synthetic materials.