Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 21

Fig. 2.1 Schematic equilibrium temperature-composition phase diagram for a polymer solution
system
polymer solution is cooled to a temperature-composition point below the binodal
envelope. Characterized by an upper critical solution temperature, phase separa-
tion usually occurs in these systems by nucleation and growth (NG) or spinodal
decomposition (SD) manners. It was also described that the region is thermo-
dynamically metastable between binodal and spinodal envelops (Chen and Ma
2005 ). In this region, solutions are stable with respect to small fluctuations in
the composition and the liquid-liquid phase separation occurs by nucleation
and growth. In the metastable region of low polymer concentration (Area A in
Fig. 2.1 ), the resulting structure is a powder like structure whereas of high pol-
ymer concentration, the resulting foam structure is a closed-pore one (Area C
in Fig. 2.1 ). The region is thermodynamically unstable under the spinodal enve-
lope. Any fluctuation in composition results in a decrease in free energy and
triggers a wave of fluctuation throughout the solution and the phase separation
occurs via spinodal decomposition. In this unstable region (Area B in Fig. 2.1 ),
a bicontinuous pattern of interconnected polymer-rich and polymer-lean phases
are formed. The resulting scaffold exhibits a foam structure with a continuous
pore network.
2.1.4 Rapid Prototyping
In recent years, rapid prototyping (RP) or solid free form (SFF) provide more
options to fabricate scaffolds. These techniques have overcome certain problems
faced in traditional techniques. These RP techniques enable the fabrication of
objects with complex architectures according to the computer aided design (CAD)
without the preparation of moulds. The virtual object is converted by the computer