Bone as a material: Lessons from nature

Bone is a remarkable material that provides structure to the human body, protects internal organs and transmits movement. These functions are facilitated by this unique composite material consisting of an organic phase, largely comprised of collagen that provides shape and structure, and a mineral hydroxyapatite phase, which imparts bone strength. The quantity and mechanical integrity of each component phase, their structural organization and interactions at various hierarchical length scales create a material that is both lightweight and strong. Unlike engineering composite materials, bone can adapt itself and constantly renew and repair throughout life to replace aged or damaged bone with healthy bone tissue, which is facilitated by the activity of bone cells. This renewal and adaptive nature also enables redesign of the material and structure to optimize bone function in response to the ever changing demands of external loads. Disease and ageing impair the tissue leading to fractures, pain and immobility. Alternative materials have been explored to replace degenerative bones as part of medical devices or to encourage bone repair for tissue engineering applications.