Chapter 5 Biomineralization Introduction 5.1 Nucleation 5.2 Growth and morphology of crystals 5.3 Structures 5.4 Origins and structures Summary

Homogeneous and Heterogeneous Nucleations

Critical level of saturation for homogeneous and heterogeneous nucleation

Biomineralization Biomineralization is the process by which living organisms produce minerals, often to harden or stiffen existing tissues Biominerals perform a variety of roles in organisms, the most important are support, defense and protection

Principal components of biological materials Calcium phosphate (hydroxyapatite Ca10(PO4)6(OH)2 + collagen Calcium carbonate (CaCO3) + chitin + protein Turtle shell, toucan beak, armadillo shell have thin layer of keratin on top. Glass sponge has outstanding flexural toughness, diatoms. Ability to create sophisticated composites from relatively weak constituent materials. Amorphous Silica (SiO2 )(H2 O)n 5 Meyers et al., 2008 5

Basic Biomineralization Principles Starts with an amorphous mineral phase The main processes are nucleation and crystal growth; they depend on: - level of supersaturation of the medium - nature of the molecular interactions between biomineral and organic macromolecules Organic molecules control nucleation Proteins (organic matrix) play the most important role in biomineralization Aragonite tiles Aragonite crystals Biomineralization begins when the supersaturation of this amorphous state is occur. Organized surfaces of organic molecules act as catalysts and molecular template for oriented crystallization. 6 6

Minerals in Biological System 7

Biominerals: types and functions The most fascinating functions of mineral in the living tissues: Reinforcement Gravity sensors Inertia sensors Sensors of the Earth’s magnetic field Optical systems Single crystals of calcite were also used in the compound eyes of trilobites. But, single crystals of calcite are renown for their ability to doubly refract white light, suggesting that the trilobites suffered a life of continual double vision! 8 Mann, 1991 8

Biomineralization: Thermodynamics & Kinetics Representation of activation energies of nucleation in the presence and absence of an organic matrix for two nonspecific polymorphs phase transformation is dictated by the solubilities of amorphous precursor into the crystalline intermediates AFM images; (a) pure calcite growth hillock. (b-d) growth hillocks after the addition of supersaturated solutions of (b) glycine, an achiral amino acid, (c and d) aspartic acid enantiomers. (From Orme CA) Mann S. Biomineralization. Oxford, 2001. Orme CA, Noy A, Wierzbickl A, McBride MT, Grantham M, TengHH, et al. Nature 2001;411:775-78.

Mineral Crystals TEM X-ray Diffraction antler non-stoichiometric 40-100 nm antler bovine femur X-ray Diffraction 2- 4 nm (2θ) non-stoichiometric hydroxyapatite Ca10(PO4)6(OH)2 Scherrer Formula: λ = 0.15 nm B = 0.035 radian 2θB = 32.2° Thickness ~ 4 nm

Nanocrystalline minerals = failure strength of the mineral γs = surface energy Em = elastic modulus of the mineral h = critical dimension of the mineral The critical length scale can be determined using the Griffith fracture criterion Bone and teeth have nanocrystalline minerals as the reinforcement Platelets Abalone ~ 500 nm x 10 µm Bone ~ 4 nm x 40 nm H. Gao, et al., PNAS, 100[10], 5597-5600 (2003). Critical crack length in the mineral phase > size of the mineral Ceramic phase does not fracture 25

Aspect ratio of mineral phase The aspect ratio of the mineral phase is large (20-100) The elastic modulus is stiffer with higher aspect ratio minerals Spherical minerals not observed in most biological materials β = volume fraction of mineral Gp = shear modulus of the protein α = aspect ratio of the mineral Em = elastic modulus of the mineral H. Gao, et al., PNAS, 100[10], 5597-5600 (2003). E α = 1 α 26

Biomineral-related Diseases Study diseased, aged, and injured tissues using the Materials Science/Mechanical approaches Kinetic studies on osteoporosis, kidney stone, metabolic arthritis, and other mineral related diseases The use of nano-mechanic methodologies applied to regenerative medicine AFM studies on calcium oxalate monohydrate (COM), the major inorganic component of kidney stone Osteoporosis Calcium oxalate monohydrate (COM), which plays a functional role in plant physiology, is a source of chronic human disease, forming the major inorganic component of kidney stones. Qui et al. PNAS 2004;101:1811