Complex Composites Built through Freezing

Using a limited selection of ordinary components and at ambient temperature, nature has managed to produce a wide range of incredibly diverse materials with astonishingly elegant and complex architectures. Probably the most famous example is nacre, or mother-of-pearl, the inner lining of the shells of abalone and certain other mollusks. Nacre is 95% aragonite, a hard but brittle calcium carbonate mineral, that exhibits fracture toughness exceedingly greater than that of pure aragonite, when tested in the direction perpendicular to the platelets. No human-made composite outperforms its constituent materials by such a wide margin. Nature's unique ability to combine the desirable properties of components into a material that performs significantly better than the sum of its parts has sparked strong interest in bioinspired materials design.Inspired by this complex hierarchical architecture, many processing routes have been proposed to replicate one or several of these features. New processing techniques point to a number of laboratory successes that hold promise in mimicking nacre. We pioneered one of them, ice templating, in 2006. When a suspension of particles is frozen, particles are rejected by the growing ice crystals and concentrate in the space between the crystals. After the ice is freeze-dried, the resulting scaffold is a porous body that can eventually be pressed to increase the density and then be infiltrated with a second phase, providing multilayered, lamellar complex composites with a microstructure reminiscent of nacre. The composites exhibit a marked crack deflection during crack propagation, enhancing the damage resistance of the materials, offering an interesting trade-off of strength and toughness.Freezing as a path to build complex composites has turned out to be a rich line of research and development. Understanding and controlling the freezing routes and associated phenomena has become a multidisciplinary endeavor. A step forward in the complexity was achieved with the use of anisotropic particles. Ice-induced segregation and alignment of platelets can yield dense, inorganic composites (nacre-like alumina) with a complex architecture and microstructure, replicating several of the morphological features of nacre. Now, a different class of complex composites is quickly arising: engineered living materials, developed in the soft matter and biology communities. The material-agnostic nature of the freezing routes, the use of an aqueous system, the absence of organic solvents, and the low temperatures being used are all strong assets for the development of such complex composites. More complex building blocks, such as cells or bacteria, can be frozen. Understanding the fundamental mechanisms controlling the interactions between the ice crystals and the objects as well as the interactions between the soft objects themselves and their fate is essential in this context.In this Account, we highlight our efforts over the past decade to achieve the controlled synthesis of nacre-like composites and understand the associated processes and properties. We describe the unique hierarchical and chemical structure of nacre and the fabrication strategies for processing nacre-like composites. We also try to explain why natural materials work so well and see how we can implement these lessons in synthetic composites. Finally, we provide an outlook on the new trends and ongoing challenges in this field. We hope that this Account will inspire future developments in the field of ice templating and bioinspired materials.