Composition of Bone Composed of mineralized calcium phosphate (specifically, the calcium phosphate phase called hydroxyapatite (HAP)  Ca10(PO4)6(OH)2) within a matrix of collagen fibrils (~1/3 dry weight, 50% of bone by volume, 90% of protein content) and 200 other proteins (e.g., acidic glycoproteins, proteoglycans & carboxylated proteins comprising the other 10% by weight) The HAP of bones and teeth is not compositionally pure it’s composition can actually be better represented as (Ca, Sr, Mg, Na, H2O, [])10(PO4, HPO4, CO3P2O7)6(OH, F, Cl, H2O, O, [])2 where [] represent crystal lattice defects HAP is a ‘living mineral’ that is continually grown, dissolved & remodeled in response to signals of internal (e.g., pregnancy) and external (e.g., gravity, exercise) origin

Composition of Bone Bone stiffness (i.e., Young’s modulus) is controlled by varying its mineral content Increasing stiffness causes increasing brittleness, therefore, natural selection favors the production of mineralized long bone structures that have maximal energy absorption (i.e., toughness) Specialized bones often have different degrees of mineralization that are adapted to their function (e.g., auditory bones with high mineral content)

Composition of Bone The integration of organic and inorganic materials, as well as their sculpting into unique microstructures, creates a final object that is tougher that either HAP or collagen alone

tropocollagen triple helix Glycine, Proline & Hydroxyproline Composition of Bone Collagen is a protein containing approximately 1000 amino acids A repeating sequence of Glycine-Proline-Hydroxyproline in Type I collagen produces a helical twisting that packs most tightly as a triple helical coiled coil (i.e., a superhelical cable called tropocollagen) tropocollagen triple helix Glycine, Proline & Hydroxyproline Amino Acid Structures The tropocollagen cables are lined up head-to-tail in rows that are staggered by 64 nm along their long axes and crosslinked at their ends to improve strength

Composition of Bone The mismatch created between the staggered tropocollagen cables creates a regular array of “hole zones” that are 40 nm long & 5 nm wide The hole zones are sites where HAP crystals are nucleated and grown Crystal growth is initially confined to the hole zone, producing HAP plates 45 nm long, 20 nm wide & 3 nm thick The HAP crystals grow in a parallel fashion across individual collagen fibrils and are oriented within the hole zone This long range ordering is believed to be responsible for the unusual fracture properties of bone

Composition of Teeth Teeth contain tooth enamel and dentine Enamel is 95% HAP and is consequently less tough than bone Enamel gains mechanical strength by interweaving HAP crystals Enamel initially starts with a high protein content, but these are removed and the voids backfilled with HAP as the tooth matures Dentine contains collagen and is similar in composition to bone

Other Functional Forms of Biominerals Navigation Aids Fe3O4 (magnetite) Crystal  Magnetosphere Sensor BaSO4 Crystal  Gravity Sensor

Other Functional Forms of Biominerals Structures for Mechanical Strength CaCO3  Armor (sea shell ‘mother-of-pearl’) CaCO3  Rigid Skeleton (deep sea enabled cuttlefish) Pillar-supported layer structure of cuttlebone Organic matrix Inorganic component Side view Top view

Structural Hierarchy in Biomineralized Materials SiO2-based deep sea sponge skeleton  a mechanically-rigid structure made of glass! J. Aizenberg et al., Science 2005 309, 275 1 cm 5 mm 100 m 20 m 25 m 10 m 5 m 1 m 500 nm