Composite materials consist of two or more distinct materials with a recognizable interface between them. The term composite is usually reserved for materials in which the distinct phases are separated on a scale larger than atomic, and in which the composite's mechanical properties have been significantly altered from those of the constituent materials.

Nature has been creating composites for millions of years. Natural composites include wood and bone. Wood is a composite of cellulose and lignin. Cellulose fibers are strong in tension and are flexible. Lignin cements these fibers together to make them stiff. The toughness and strength of bone arises from the dispersal of hard plate-like crystals of hydroxyapatite (mineral) in a soft matrix of collagen fibers (protein). As a composite, the resulting microstructure yields physical properties that even synthetic materials have been unable to match.

For the past several decades, scientists, taking their cue from nature...