Composites are ubiquitous in nature. The properties of those materials are adjusted and graded by subtle vari- ations in its structure. By judiciously adjusting the pro- portion and orientation of the reinforcing phase, being frequently fibrous in natural composites, and the continu- ous phase (usually referred to as the matrix), it is possible to obtain materials with tunable properties. Nature has used this elegant solution to develop the range of mate- rial properties that we can find in bones, trees, or sea shells. The combination of material properties, following a rationale similar to the one followed in nature, allows developing and using a wealth of new synthetic mate- rials in many engineering applications. Composites allow obtaining combinations of properties that are not available in the range of initially existing materials. An illustra- tive example is the combination of low density with high stiffness, which are properties required for many prod- ucts associated with transportation or with mobile device industries.

Natural-based structural composites are the result of the combination of compliant matrices and rigid rein- forcements that may be either organic or ceramic. Nature designs systems that adapt their structure and, conse- quently, their properties to the stresses and strains that are locally supported. The variation of properties in those structures is continuous and allows for the structural development and growth required in biologically derived systems. Examples may be found in the properties of the wood found in different zones of trees or of the different bones existing in the skeletal systems of vertebrates.