Scaffolds for Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 2

The tissue construct in tissue engineering is classified into two major types,
closed systems and open systems (Langer and Vacanti 1993 ). The implant which
is used as external organ support is termed as the closed system whereas the scaf-
fold with attached cells implanted into the body is termed as open system. The
scaffold is three-dimensional (3D), highly porous with an interconnected pore
network which provides as a intermediary template/model for tissue regenera-
tion. In the general procedure of tissue engineering, the cells can be isolated from
the biopsy taken from the patient, expanded in vitro, and seeded into a scaffold
(Schultz et al. 2000 ). Incorporated with signalling molecules in some strategies,
this cell-scaffold construct can be cultured in the bioreactor until an appropri-
ate and developed graft is formed. This final 3D cell-scaffold construct can be
implanted into the patient (Temenoff and Mikos 2000 ). It has been demonstrated
that bone has the highest possibility for regeneration among many other tissues in
the body (Chen et al. 2006 ).
1.2 Replacement and Regeneration of Bones
1.2.1 Bone Structure and Composition
At the ultra-structural level, bone is a composite with mechanical properties which
can be matched by man-made composites (Wang 2004 ). In order to develop bone
replacement materials, bone serves as the template. Human body is supported by
bones which are the substantial unit of human skeletal system. As a natural tissue,
bone has a complex structure where several macroscopic to microscopic levels
of organization can be identified (Park 1979 ). Bone possesses an intricate struc-
ture. The basic unit of bone is the Haversian system (also known as “osteons”),
which is a hollow, laminated rod of collagen and calcium phosphate. The hollow
core is a nutrient channel, the Haversian canal. Many of these Haversian systems
within the shaft of a long bone are bundled together in parallel and form a kind
of bone called cortical or compact bone, which is optimized to handle mechani-
cal forces. Near the ends of the bones, where the stresses become more complex,
the Haversian systems play out and branch to form a meshwork of cancellous, or
spongy bone.
Human bone contains the mineral crystallites which are structurally calcium-
deficient, carbonate-substituted hydroxyapatite which are generally referred to
as bone apaptite. The normal dimension of bone apatite is 5 × 5 × 50 nm with
a rod-like (or sometimes plate-like) microstructure and is embedded in collagen
fibers. Bone apatite occupies about 50 % of the total volume in mature bone.
The particular microstructural organization of bone is a function of age and it
varies between different bones and between different locations of the same bone
(Wang 2004 ).