Scaffolds for Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 4

Fig. 1.1 Schematic diagram showing the mechanical behavior of apatite , collagen , and
compact bone
The structure and properties of cancellous (or spongy) bone is well documented
(Gibson and Ashby 1997 ). The cancellous bone is made up of an interconnected
network of rods or plates. Low density, open cells are produced by a network of
rods while closed cells are produced when the rods progressively spread and flat-
ten as the density increases. The relative density of cancellous bone varies from
0.05 to 0.7. The compressive stress-strain curve of cancellous bone possesses the
characteristics of a cellular solid. Under compression, the scaffolds exhibited lin-
ear elasticity at low stresses followed by a long plateau of cell wall collapse and
then a regime of densification in which the stress rose steeply.
The linear elasticity is controlled by cell wall bending, the plateau is associ-
ated with collapse of the cells (of the “cellular structure”) and when the cells have
almost completely collapsed, opposing cell walls touch, with further strain com-
pressing the solid itself, giving the final region of rapidly increasing stress (Gibson
and Ashby 1997 ). As the relative density increases, the cell walls thicken and the
pore space shrinks. Increasing the relative density of the scaffold increases the
compressive modulus, raises the plateau stress and reduces the strain at which den-
sification starts.
1.2.3 Existing Approaches for Bone Replacement and
Regeneration
The requirement for bone is a major clinical and socioeconomic need. It has
been reported that the treatment of bone fracture costs over £900 million annu-
ally in the UK (Rose and Oreffo 2002 ). The conventional reconstruction for