epitaxial growth mechanism may possibly be used to explain the assembly.
As mentioned previously, it has been shown that the negatively charged groups
of the collagen molecules are the nucleation sites for HA crystals. The positions of
the oxygen atoms in the hydroxyl groups of HA crystals have epitaxial relationships
with those in the carboxylate groups of collagen fibrils (Cui et al. 2007).
The nanoscale organization of the composites resembles that of HA crystals in
mineralized tissue in which the HA crystals also align their c-axes with the
longitudinal axes of the collagen fibrils. Such alignment is the most impressive
characteristic of bone minerals. Development of novel self-assembled structures
should therefore improve the understanding of collagen-mediated mineralization in
other calcified tissues, and point the way to the development of new functional
materials for biomimetic engineering. Moreover, these fundamental studies provide
the basic theoretical support for the fabrication of HA/collagen composites and
their application in bone regeneration.
6.5.2 Peptide–Amphiphilic Nanofibers-Induced Calcium
Phosphate Mineralization
It is a challenge that the preparation of any material with multiple levels of
hierarchical organization based on nano-building blocks. Fabrication of materials
that resemble bone, even at the lowest level of hierarchical organization, is even
more difficult because it involves two dissimilar organic and inorganic nanophases
each of which have a specific spatial relation with respect to each another. One way
to accomplish this in an artificial system is to prepare an organic nanophase
designed to exert control over crystal nucleation and growth of the inorganic
component, as has been widely investigated for a long time. Studies on such
template crystal growth methods have suggested that nucleation occurs on surfaces
which expose repetitive patterns of anionic groups. These anionic groups tend to
concentrate the inorganic cations creating a local supersaturation followed by
oriented nucleation of the inorganic crystal phase. At present, there is increasing
interest in the fabrication of HA/peptide composites using designed self-assembling
systems (Cui et al. 2007).
In this context, Stupp et al. (1997) have reported several studies on the use of
self-assembly and mineralization to prepare a nanostructured composite material
((Zubarev et al. 1999b); (Silva et al. 2004)); the structural assembly of collagen and
HA in bone has been recreated (Hartgerink et al. 2001). In their work, the
composites are prepared by self-assembly, covalent capture, and mineralization
of a peptide–amphiphile, which is synthesized by standard solid phase chemistry
ending with alkylation of the NH 2 terminus of the peptide. The peptide–amphiphile
consists of five key structural segments. The first region is a long alkyl tail that
conveys hydrophobic character and makes the molecules amphiphilic. Four consecutive cysteine residues are also incorporated in the sequence following the alkyl
6 Principles of Calcium-Based Biomineralization
191
As mentioned previously, it has been shown that the negatively charged groups
of the collagen molecules are the nucleation sites for HA crystals. The positions of
the oxygen atoms in the hydroxyl groups of HA crystals have epitaxial relationships
with those in the carboxylate groups of collagen fibrils (Cui et al. 2007).
The nanoscale organization of the composites resembles that of HA crystals in
mineralized tissue in which the HA crystals also align their c-axes with the
longitudinal axes of the collagen fibrils. Such alignment is the most impressive
characteristic of bone minerals. Development of novel self-assembled structures
should therefore improve the understanding of collagen-mediated mineralization in
other calcified tissues, and point the way to the development of new functional
materials for biomimetic engineering. Moreover, these fundamental studies provide
the basic theoretical support for the fabrication of HA/collagen composites and
their application in bone regeneration.
6.5.2 Peptide–Amphiphilic Nanofibers-Induced Calcium
Phosphate Mineralization
It is a challenge that the preparation of any material with multiple levels of
hierarchical organization based on nano-building blocks. Fabrication of materials
that resemble bone, even at the lowest level of hierarchical organization, is even
more difficult because it involves two dissimilar organic and inorganic nanophases
each of which have a specific spatial relation with respect to each another. One way
to accomplish this in an artificial system is to prepare an organic nanophase
designed to exert control over crystal nucleation and growth of the inorganic
component, as has been widely investigated for a long time. Studies on such
template crystal growth methods have suggested that nucleation occurs on surfaces
which expose repetitive patterns of anionic groups. These anionic groups tend to
concentrate the inorganic cations creating a local supersaturation followed by
oriented nucleation of the inorganic crystal phase. At present, there is increasing
interest in the fabrication of HA/peptide composites using designed self-assembling
systems (Cui et al. 2007).
In this context, Stupp et al. (1997) have reported several studies on the use of
self-assembly and mineralization to prepare a nanostructured composite material
((Zubarev et al. 1999b); (Silva et al. 2004)); the structural assembly of collagen and
HA in bone has been recreated (Hartgerink et al. 2001). In their work, the
composites are prepared by self-assembly, covalent capture, and mineralization
of a peptide–amphiphile, which is synthesized by standard solid phase chemistry
ending with alkylation of the NH 2 terminus of the peptide. The peptide–amphiphile
consists of five key structural segments. The first region is a long alkyl tail that
conveys hydrophobic character and makes the molecules amphiphilic. Four consecutive cysteine residues are also incorporated in the sequence following the alkyl
6 Principles of Calcium-Based Biomineralization
191