Development and Application of Naturally Renewable Scaffold Materials for Bone Tissue Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 296

Inorganic materials include demineralized bone matrix (DBM), hydroxyapaptite, nacre,
and coral. DBM can come from allograft and xenograft sources and is also known
as decalcified cortical bone. To reduce host rejection and foreign body response, it is
processed until it only retains a highly porous collagenous structure (10). Hydroxyapatite
is the natural inorganic component of bone and is typically incorporated as a filler
material in composite systems. Nacre is a calcified structure that forms the inner layer
of some sea shells. Coral is a marine invertebrate that consists of CaCO 3 . Coral has
a porous structure with an interconnected network of pores. To be used in vivo it
undergoes a partial hydrothermal exchange process that converts carbonate to phosphate
(27). These materials resemble the natural architecture and porosity of bone and are
adequate scaffold materials based on this striking similarity.
The remainder of this chapter will present a brief background on the anatomy and
function of bone, highlighting the extracellular matrix components, physical properties,
overall architecture, and the osteodifferentiation of progenitor cell populations featuring
mesenchymal stem cells. This will be followed by specific examples of investigators
implementing naturally renewable materials for bone tissue engineering, discussing the
successes and limitations with each example.

11.3 Bone Background

Bone is involved in many diverse roles within the body such as: (1) the protection of
vital organs, (2) support and attachment to muscles for locomotion, (3) the generation
of red and white blood cells for immunoprotection and oxygenation or other tissues,
and, (4) mineral storage and ion homeostasis (28-31). The architecture of bone is
representative of the many functions it serves in the body. There are two types of bone
that make up the adult skeleton, cortical bone (80%) and cancellous (or trabecular) bone
(20%) (30). Cortical bone provides mechanical stability and protection to vital organs
and is therefore almost completely solid, having a very low porosity (10%) (28, 30). In
comparison, trabecular bone is loosely organized and very porous (50-90%) in order to
provide a proper environment for metabolic activity (28, 31). In bone, entire collagen
triple helices lie in a parallel, staggered array. 40 nm gaps between the ends of the
tropocollagen subunits probably serve as nucleation sites for the deposition of long,
hard, fine crystals of the mineral component, which is (approximately) hydroxyapatite,
Ca 5 (PO 4 ) 3 (OH), with some phosphate (mineralization during endochondral ossification
of articular cartilage occurs in a similar fashion). Collagen gives bone its elasticity and
contributes to fracture resistance.
Bone replacement has become an important area in tissue engineering. The previously
described limitations of autografts and allografts have led researchers to investigate the
use of natural scaffold materials, in combination with human mesenchymal stem cells
(hMSCs), to provide biocompatible, biodegradable, and mechanically stable bone grafts
for critical size defects. The success of bone grafts relies heavily on the architecture of
the scaffold, specifically the pore size and porosity, and should be designed to mimic
the physical properties of native bone (32, 33). For design purposes, the porosity of
native trabecular bone is estimated at > 75% (34) and typical pore sizes are approxi-
mately 1 mm in diameter (35). Investigators have recently shown a direct correlation