Biomedical Engineering Reference
In-Depth Information
hard material, the mineral and much softer material, the collagen fi brils
(level 1). Mineralized collagen fi brils are always present in bundles or
arrays aligned along their length (level 3). These fi bril arrays organize into
four common patterns: arrays of parallel fi brils, woven fi ber structure,
plywood-like structure, and radial fi bril arrays (level 4). At a higher level
of organization, the initially deposited primary bone undergoes internal
remodeling and forms the secondary bone with a central canal for blood
vessels and nerves, which is called “Haversian system” (level 5). The levels
6 and 7 refer to solid versus spongy bone and whole bones, respectively.
2.2.2
Self-Assembly of Mineralized Collagen Fibrils in Nature
2.2.2.1
Collagen and Collagen Fibrils Array
Collagen is the most abundant fi bril-forming protein in mammalian tissues,
accounting for up to one-third of all proteins. Collagen fi bers are the main
components of the extracellular matrix in various tissues [15, 16]. Variations
in the amino acid sequence generate the different types of collagen: type I, II,
III and so on. The main function of collagen is as an integrity and mechani-
cal reinforcement of both soft and hard connective tissue [14, 17, 18]. Type I
collagen molecule, as the major organic component of bone, is made up
of three polypeptide strands called alpha peptides forming a triple helical
assembly with approximately 300 nm long and 1.5 nm in diameter. A distinct
feature of each alpha peptide chain of collagen is the repeated arrangement
of amino acids Gly-X-Y, where X and Y can be any amino acid, but are fre-
quently the amino acids proline and hydroxyproline, respectively [19]. The
side chains of residues in X- and Y-positions point out of the helix, and play
an important role in fi bril formation through interactions between oppo-
sitely charged residues and through hydrophobic interactions between resi-
dues of different molecules. The three chains are arranged in parallel, and
supercoiled along a common axis to form a right-handed triple helix [20].
The triple helical collagen molecules self-assemble with their long axes
in parallel into a staggered arrangement in which each molecule is shifted
with respect to its neighboring molecules forming characteristic D-periodic
cross-striated pattern (where D
67 nm, the characteristic axial periodicity
of collagen) [21]. The successive molecules in same axial dimension are
40 nm apart, named “Hole” or “Gap” zone. The typical 67 nm period of
cross-striation pattern of collagen assembly is widely observed by trans-
mission electron microscope (TEM), atomic force microscope (AFM), and
X-ray diffraction (XRD) investigation. The most widely accepted model
for packing of collagen molecules is that fi ve triple helices align hexago-
nally in cross section and longitudinally with approximately a quarter of
the molecular length of staggered arrangement to form the fi ve-stranded
microfi brils. The diameter of the collagen fi brils in the fi ve-stranded
=
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