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

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bulletin c o v e r s t o r y It’s tough to be strong: Advances in bioinspired structural ceramicbased materials By Michael M. Porter and Joanna McKittrick Adapting biological processes to synthesize ceramic materials yields microstructures remarkably similar to natural materials with mechanical properties at least as good. 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. Natural materials, such as nacre (top left), bone (top right), and narwhal tusk (bottom), suggest structures and processing approaches for new synthetic • 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. materials. • Hierarchical structures. Efficiency and multifunctionality are organized over a range of scale levels (nanoscale to macroscale). Structure confers distinct and translatable properties from one level to the next and may be optimized for more than one function. For example, bone supports the body, stores ions, and produces marrow. The idea behind bioinspired materials is to adapt the apparent effortlessness of biological systems to produce complex, multifunctional materials to make synthetic materials. Biological systems adapt to changing ambient conditions, continually refining and adjusting shape through chemical, cellular, and mechanical signaling. This requires a systems approach with the expertise of engineers as well as life scientists to develop materials with complex, hierarchical structures. Two model structural materials—abalone nacre and bone— have exceptional mechanical properties designed for body support, as well as impact resistance (nacre) or blood flow and joint movement (bone). These properties result from highly ordered, structural alignment in multiple directions across several length scales. Bioinspired design seeks to mimic the nanostructural and microstructural features of natural materials to fabricate high-performance, multifunctional materials. This review focuses on the development of cellular solids, tough ceramics, and hybrid composites inspired by bone and abalone nacre. These materials may be useful for a variety of applications ranging from load-bearing bone implants and lightweight structural composites to separation filters and catalyst supports. Learning from bone and nacre Bone and nacre are natural ceramic-based materials, containing organic matter, with extraordinary mechanical properties given their lightweight composition from locally available elements—calcium, phosphorous, carbon, oxygen, and hydrogen. They are stiff, strong, and tough—mechanical properties usually considered mutually exclusive4. The Ashby plot in Figure 1 compares the stiffness and toughness of bone and 18 www.ceramics.org | American Ceramic Society Bulletin, Vol. 93, No. 5


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