Biomimetic Synthesis of Self-Assembled Mineralized Collagen-Based Composites for Bone Tissue Engineering - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 28

(Gly-X-Y)n

2 α 1(I) + α 2(I)

Collagen molecule

1.5 nm

300 nm

D=67 nm

1

2

1

Five-stranded
collagen microfibrils

2
3

3

5 4 nm

4

4

5

Overlap zone

Hole
zone

Collagen fibrils

Figure 2.2 Structural assembly of collagen fi brils.

packing model should be about 3.6 nm according to 1.5 nm of diameter of
single collagen molecule, which has been verifi ed by TEM examination of
native collagen fi brils. The collagen microfi brils are then assembled into
collage fi brils in diameter from 35-500 nm that are further combined, ori-
ented and laid up to form ordered structures with particular morpholo-
gies for tissues. The overwhelming consideration in the arrangement of
collagen fi brils to form connective tissues is the resulting tissue function.
The structural assembly of collagen fi brils is shown in Figure 2.2.

2.2.2.2 Structural Organization of Mineralized Collagen Fibrils

The basic building blocks of bone materials are mineralized collagen
fi brils. The 67 nm periodic cross-striated pattern has been observed in
both stained unmineralized collagen fi brils and mineralized collagen
fi brils, which implies that the stain and minerals should deposit at the
same location within the fi brils. The schematic diagram of the structural
organization of mineralized collagen fi brils is shown in Figure 2.3. The
fi rst-formed minerals nucleate initially in the “hole” zones of assembled
collagen fi brils with continuous growth, and then penetrate into the over-
lap regions of the fi brils. The tiny crystals, carbonated nano-HA with
hexagonal crystal symmetry, are plate-shaped and array in parallel with
preferred crystallographic orientation that is their c-axes co-align with the
long axes direction of collagen fi brils.
The crystals are extremely small-in fact they are probably the smallest
biologically formed crystals. In addition, the crystals may change in size and
composition with age. Therefore, it is very diffi cult to determine the exact
crystal dimensions in native occurring mineralized collagen fi brils. Although