Biomimetic Synthesis of Self-Assembled Mineralized Collagen-Based Composites for Bone Tissue Engineering - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 25

synthesis of self-assembling mineralized collagen-based composites for
bone tissue engineering.

2.2 Hierarchical Assembly of Mineralized Collagen

Fibrils in Natural Bone

2.2.1 Panorama of Natural Bone

2.2.1.1 Chemical Composition of Bone

Bone is a type of specialized mineralized connective tissue with highly
complex hierarchical structure composed primarily of 34% organic matrix,
65% inorganic minerals, and 1% water by weight [19]. It is a precisely
organized natural hybrid nanomaterial with unique mechanical proper-
ties determined by its chemical composition and microstructure.
The organic matrix consists of around 90% type I collagen and 10%
other so-called noncollagenous macromolecules, including acidic proteins,
glycosaminoglycans, and proteoglycans. Collagen is a long, fi brous struc-
tural protein that provides fl exibility of bone and structural templates for
mineral deposition. The noncollagenous proteins contain over 200 differ-
ent functional proteins, such as osteonectin, osteocalcin, bone morpho-
genetic proteins, bone proteoglycan, and bone sialoprotein [23]. And the
inorganic part of bone is composed of nonstoichiometric carbonated HA
with poor crystallinity [9]. The total carbonate content in HA is about 4-7%
with replacing both PO 3− (B-type substitution) and OH − (A-type substitu-
tion) in various lattice sites. The biological HA crystals are only 2-6 nm
thick, 30-50 nm wide, and 60-100 nm long, which are directly related to
the stiffness of bone. During the processes of HA biomineralization, several
other calcium phosphate phases have been identifi ed as intermediates [10-
12]. There is evidence of the formation of an amorphous calcium phosphate
(ACP) or Dicalcium Phosphate (DCPD) in the early stage of bone and car-
tilage mineralization, and then ACP transforms to octacalcium phosphate
(OCP). OCP or DCPD will stabilize readily in the form of HA at last.
In bone tissues there are three types of bone cells, osteoblasts, osteo-
cytes, and osteoclasts, entrapped in the homogeneous bone matrix, which
are primarily responsible for bone formation, remodeling, and mainte-
nance of osteoarchitecture. Osteoblasts are bone-forming cells and are
generally considered to differentiate from preosteoblast, mesenchymal
progenitor cells, which secrete bone matrix to form bone with further
mineralization. Osteocytes are also bone-forming cells, which maintain
bone as differentiated osteoblasts by secreting growth factors. Osteoclasts
are bone-resorbing cells that are large with multinuclei. Osteoblasts and
osteoclasts work simultaneously contributing to bone remodeling, which
is the replacement of the old bone tissue by new bone tissue.