Biomedical Engineering Reference
In-Depth Information
stress to the strong fibers. However, while a bond among the matrix
and reinforcements must exist for the purpose of stress transfer, it
should not be so strong that it prevents toughening mechanisms,
such as debonding and fiber pullout [217].
There is still doubt as to the exact bonding mechanism among
bone minerals (biological apatite) and bioorganics (collagen), which
undoubtedly plays a critical role in determining the mechanical
properties of bones. Namely, bone minerals are not directly bonded to
collagen but through non-collagenous proteins that make up ~3% of
bones (Table 6.1) and provide with active sites for biomineralization
and for cellular attachment [36]. In bones, the interfacial bonding
forces are mainly ionic bonds, hydrogen bonds and hydrophobic
interactions, which give the bones the unique composite behavior
[53]. There is an opinion that, opposite to bones, there is no sign
of chemical bonding among the phases in conventional calcium
orthophosphate/collagen biocomposites, probably due to a lack of
suitable interfacial bonding during mixing [39]. However, this is not
the case for phosphorylated collagens [753]. The interested readers
are forwarded to a density functional theory study of the interaction
of collagen peptides with hydroxyapatite surfaces [1129].
Anyway, Fourier-transformed infrared (FTIR) spectra of some
calcium orthophosphate-based composites and collagen films
were measured and transformed into absorption spectra using
the Kramers-Kronig equation to demonstrate energy shifts of
residues on the apatite/collagen interface. After comparing FTIR
spectra of biocomposites and collagen films in detail, red shifts of
the absorption bands for C-O bonds were observed in the spectra
of the biocomposites. These red shifts were described as a decrease
of bonding energies of C-O bonds and assumed to be caused by an
interaction to Ca
2+
ions located on the surfaces of apatite nano-sized
crystals, as shown in Fig. 6.6 [747]. Another proof of a chemical
interaction between apatite and collagen was also evaluated in
FTIR spectra of CDHA/collagen biocomposites, in which a shift of
the band corresponding to -COO
−
stretching from 1340 to 1337
−1
cm
was observed [708, 709]. More to the point, nucleation of
apatite crystals onto collagen through a chemical interaction with
carboxylate groups of collagen macromolecules has been reported
[1130-1132].
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