Biominerals--hierarchical nanocomposites: the example of bone

Abstract

Many organisms incorporate inorganic solids in their tissues to enhance their functional, primarily mechanical, properties. These mineralized tissues, also called biominerals, are unique organo-mineral nanocomposites, organized at several hierarchical levels, from nano- to macroscale. Unlike man-made composite materials, which often are simple physical blends of their components, the organic and inorganic phases in biominerals interface at the molecular level. Although these tissues are made of relatively weak components under ambient conditions, their hierarchical structural organization and intimate interactions between different elements lead to superior mechanical properties. Understanding basic principles of formation, structure, and functional properties of these tissues might lead to novel bioinspired strategies for material design and better treatments for diseases of the mineralized tissues. This review focuses on general principles of structural organization, formation, and functional properties of biominerals on the example the bone tissues.

Figure 1

Hierarchical organization of bone from macro to nanoscale. a) Organ level- femoral bone. b) Tissue level- haversian (osteonal) compact bone; red ellipse outlines an individual osteon. c) Microscopic level- bone lamellae are the structural elements of lamellar bone tissues; red parallel lines outline one lamella. d) Mesoscopic level arrays (buldles of mineralized collagen fibrils. e) nanoscale level- mineralized collagen fibrils. f) Molecular level- arrangements of collagen molecules and mineral crystallites in the mineralized collagen fibril. The figure is reprinted with permission from reference .

Figure 2

A. Triple-helical structural motif of collagen molecules. B. Organization of collagen fibril. Three hundred nm long and 1.5 nm wide triple-helical collagen molecules assemble into the fibril in a staggered arrangement, where each molecule is shifted in respect to its neighbor. The neighboring molecules in the axial dimension are 40 nm apart. This structural organization of the fibril gives rise to the 67 nm periodic pattern- D-period (D), which consists of Gap (G) and overlap (O) region.

Figure 3

Structural organization of the mineralized collagen fibril. A. Schematic representation of collagen fibril showing channels in which crystals start to form (left side); on the right, arrays of plate like mineral crystals sandwiched between layers of collagen triple-helices (depicted as cylinders), note that the structure of the mineralized collagen fibril is orthogonally anisotropic. Reprinted with permission from 4 B. Face-on and Edge-on projections of the crystals in the mineralized fibril. C. The drawing of two mineralized collagen fibrils obtained from the electron tomographic reconstruction of the normally calcified mineralized avian tendon. Note that the density of the crystals is higher in the gap regions, leading to a periodic mineral density profile with 67 nm spacing. Also note that the periodic patterns in the neighboring fibrils are in register. Reprinted with permission from . D. TEM micrograph of an isolated mineralized collagen fibril from human dentin. The fibril is twisted and both edge-on (left) and face-on (right) crystals present in the same fibril. Note the periodic pattern of mineral density (D-period). E. HRTEM micrograph of two edge-on crystals (M) in the mineralized fibril from human dentin. The narrow space between them is filled with collagen molecules (C). F. An array of the mineralized fibrils from human dentin with crystals in face-on orientation, with the exception of the fibril marked with asterisk with crystals in edge-on orientation. Note the periodic changes in mineral density, corresponding to D-period. Also note that the periodic patterns in neighboring fibrils are in register.

Figure 4

Different types of fibril array patterns. A. Parallel fiber array, found in mineralized turkey tendons and parallel fiber bone. B. Woven array. Found in woven bone. C. Lamellae. Found in lamellar bone. D. Radial array. Found in dentin. The figure is reprinted with permission from reference .

Figure 5

A. Forming bone. OB-osteoblasts, OS-osteoid (nonmineralized matrix), MM-mineralized matrix. B. Forming dentin. OD-odontoblasts. PD-predentin, D-dentin. The figure is reprinted with permission from reference .

Figure 6

Schematic representation of changes in organic matrix composition in predentin prior to mineralization. Green arrows indicate the secretion of dentin macromolecules and Ca²⁺ ions by the odontoblast processes; red arrows indicate removal of processesed predentin molecules by odontoblast processes. Purple arrows indicate the secretion of collagen, predentin macromolecules and phosphate ions by odontoblast cell bodies.

Figure 7

Acidic amino acids prominent in noncollagenous acidic proteins: A. Aspartic acid. B. Glutamic acid, C. Phosphoserine

Figure 8

The classical models of regulation of mineralization by acidic proteins. A. Randomly organized acidic macromolecules (folded ribbon) can induce crystal nucleation by attracting metal ions and increasing a local supersaturation. This mechanism, however, does not provide means to control the crystals orientation. Irregular blocks represent crystals. B. Acidic proteins adopting regular conformation can promote an oriented crystal nucleation and growth via epitaxy. A nucleating macromolecule is represented as a ribbon with periodic structure; epitaxially growing crystals depicted as blocks forming a “brick wall” structure. C. Shape modifying macromolecules are thought to adopt regular conformation, matching lattice parameters of certain crystallographic planes, which results in the preferred binding of the macromolecules to specific crystalline faces. As a result, the crystal growth is inhibited in a direction normal to the affected crystalline face. G is the rate of growth in different crystallographic directions. It is slower in the along Z-axis. Unit cell (smallest repeating unit) of a crystal is represented by blocks forming a “brick wall”. A periodic ribbon represents a shape-modifying macromolecule.

Figure 9

A. TEM micrograph of collagen fibril mineralized in vitro in the presence of polyaspartic acid. Note that the mineral particles are aligned along the fibril axis. B. Electron diffraction pattern of the mineralized fibril in Figure 9A.; c-axes of the crystals are co-aligned with the fibril axis. The figure is reprinted with permission from reference .

Figure 10

Models of collagen mineralization. A. Templating of mineral crystals, represented by gray blocks by acidic noncollagenous macromolecules (green wavy ribbons) bound to collagen molecules in the gap region. B. Size exclusion model. Large acidic proteins (green lightening bolts) prevent mineral nucleation (nuclei represented by grey spheres) outside of the fibrils. However, the proteins are too big to fit into the gap regions of the fibrils, where mineral crystals start to grow. C. Protein assemblies (green circles) stabilize the mineral nuclei outside of the fibrils. Upon binding to collagen molecules in the fibril proteins change their conformation, leading to the templated nucleation of mineral crystals in the gap regions.

Figure 11

Stress-strain curve of parallel arrays of collagen fibrils as found in tendons can be divided into several regions, corresponding to different strain mechanisms. (a) In the toe region, the strain is due to the straightening of the macroscopic crimp in the tissue. (b(i)) In the hill region the strain is due to the lateral alignment of the collagen molecules inside the fibrils. (b(ii)_) In the linear region the strain is due to the sliding of the collagen molecules along each other, in this region the D-spacing of collagen fibrils increases by up to 10%. The figure is reprinted with permission from reference .

Figure 12

Mechanical anisotropy of the mineralized collagen fibrils is determined by their structural anisotropy. Reprinted with permission from reference .

Figure 13

Models of mechanical behavior of bone tissues at several hierarchical levels. A. The stress in the mineralized collagen fibrils is transferred from stiff mineral particles in tension to the softer organic matrix in shear mode. Reprinted with permission from reference B. Model of bone deformation in tension at several hierarchical levels. In the fibrils the load is transferred between stiff mineral platelets deforming in tension and shearing collagen molecules. The load between adjacent stiff mineralized fibrils is transferred by shearing in the extrafibrillar matrix, containing noncollagenous macromolecules and extrafibrillar mineral. Reprinted with permission from reference .

Figure 14

Extrafibrillar organic matrix in bone tissues forms supramolecular networks based on the reversible sacrificial ionic bonds with calcium ions. A. SEM micrograph of a crack in the bone tissue, showing filaments formed by the extrafibrillar macromolecules. B. The experimental set up for measuring the forces in the extrafibrillar matrix. A piece of bone attached to the AFM cantilever is pressed against another piece of bone. When pulled apart the forces holding these two pieces of bone together can be measured. C. The representative pulling curve; large amounts of energy (the area under the curve) required to break two bone pieces apart. D. In the medium containing only monovalent ions (Na⁺) the amount of energy dissipated is much lower than in the medium containing polyvalent ions (Ca²⁺) E. The model showing different types of sacrificial bonds in the extrafibrillar matrix: 1) intramolecular bonds; 2) intermolecular bonds 3) protein-mineral bonds. Reprinted with permission from .

Figure 15

In the lamellar bone the energy required for crack to propagate along the lamellae (left side) is much lower than across the lamellae (right side). Reprinted with permission from .

Figure 16

Crack propagation in the osteonal bone under compression a) Epi-fluorescence image showing four groups of arc-shaped circumferential microcracks (bright green) arranged in the quasiorthogonal pattern; b) Epi-fluorescence image showing the crack propagation across neighboring osteons. The cracking of the central osteon is transfered to the osteons to the lower left and upper right; c) SEM micrograph showing arc-shaped microcracks. d) Closer observations of (c) (asterisks) showing the short micro-radial cracks in the thick lamellae and a circumferential microcrack. The figure is reprinted with permission from reference .