Page 377 - Handbook of Properties of Textile and Technical Fibres

350 Handbook of Properties of Textile and Technical Fibres

For instance, in tendon, collagen fibril diameters are between 20 and 280 nm and
collagen fibers have diameters between 1 and 300 mm(Silver et al., 2003). While
collagen fibrils form from lateral addition of smaller fibrils (see Fig. 11.3), they
form bundles of fibrils and larger bundles termed fascicles as shown in Fig. 11.4.
Groups of fibril bundles form fascicles that in turn make up the cross-section of a
tendon bundle (Fig. 11.4). These structural elements acting in concert give rise to
the mechanical properties of tendon.
The ultrastructure of tendon has been studied extensively as a function of matura-
tion (Torp et al., 1974; Parry et al., 1978). Collagen microfibrils appeared to be held
together by an interfibrillar matrix containing proteoglycans (Scott, 1992). These
collagen fibrils were observed to have a planar waveform, termed crimp, and be the
load-bearing units (Diamant et al., 1972)(Fig. 11.3(k)). Upon deformation, the stresse
strain curve of tendon curves upward after the crimp is removed (Diamant et al., 1972).
After birth, the distribution of fibril diameters from rat tail tendons is fairly flat support-
ing the concept that fibril diameter and length increases occur by the lateral fusion of
fibril bundles (Torp et al., 1974; McBride et al., 1985, 1988; Birk et al., 1991). The
volume fraction of collagen fibrils increases during maturation until it reaches about
0.5; in fibril cross-sections of tendon small collagen fibrils fill the space between larger
ones (Torp et al., 1974).

11.4 Collagen crosslinking

The ability of fibril-forming collagens to store, transmit, and dissipate energy requires
crosslink formation between molecules within a microfibril and between microfibrils
and other structural units. These crosslinks include lysine and hydroxylysine deriva-
tives and other amino acid residues including histidine (Silver et al., 1992). Crosslink-
ing also occurs during aging and involves glucose molecules (Ricard-Blum, 2011).
The stiffening and poor energy dissipation of collagen fibers associated with aging
probably involves collagen fiber fragmentation by ultraviolet light and glucose-
derived crosslinking. This leads to loss of the energy dissipating ability of ECM and
is associated with large energy losses during pulsatile blood flow in the cardiovascular
system (Horvath et al., 2005).

11.5 Collagen self-assembly

Collagen molecules self-assemble under physiological conditions into a quarter-
staggered structure, the microfibril, that in turn grows linearly and laterally into
collagen fibrils (Silver et al., 2003). Collagen triple helices and microfibrils are very
long and thin and are therefore quite flexible. Solution studies suggest that the collagen
molecule can be modeled as a semiflexible rod with bend angles up to about 120 de-
grees with respect to the molecular axis (Silver et al., 2003). This flexibility allows
collagen molecules to undergo conformational changes required for self-assembly
into long thin fibrils via end-overlap of several molecules followed by lateral assembly