Biodegradable PHBV Polymer-Based Scaffolds for Bone Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 48

that the typical compressive stress-strain curve exhibited linear elasticity at low
stresses (Region I) followed by a long collapse plateau (Region II) and a regime
of densification (Region III) in which the stress rises steeply. It was reported that
linear elasticity could be controlled by cell wall bending, the plateau was associ-
ated with collapse of the cells and when the cells had almost completely collapsed
opposing cell walls touch, further strain compresses the solid itself, giving the
final region of rapidly increasing stress (Gibson and Ashby 1997 ). Compressive
modulus is the initial slope of the stress-strain curve. As the relative density
increases, the cell walls thicken and the pore space shrinks. Increasing the relative
density of the foam increases the modulus, raises the plateau stress and reduces
the strain at which densification starts. For these reasons, it was found that com-
pressive modulus increases with increasing polymer/emulsion concentration or
decreasing porosity.
Based on the research conducted, the optimum processing conditions for the
fabrication of scaffolds by using emulsion freezing/freeze drying technique are as
follows:
• Polymer concentration: 10 % (w/v)
• Solvent to water phase ratio: 1:1
• The volume fraction of water phase: 0.5
• Homogenizer speed: 17,500 rpm
• Freezer temperature: − 35 °C
3.2 Polymer/Ceramic Composite Scaffolds and Characteristics
of the Scaffolds
Various amounts of nano-sized HA (5, 10, 15 and 20 % by weight) were incorpo-
rated into PHBV or PHBV/PLLA scaffolds. The spherical HA nanoparticles used
for composite scaffolds were produced in-house through a nanoemulsion process
as shown in Fig. 3.6 a (Zhou et al. 2008 ). The freeze-dried HA powders consisted
of tiny agglomerates of HA nanocrystallites. The particle size of the HA powders
was found to be in the range of 20-30 nm. The nanoparticles were basically HA
crystallites precipitated from the chemical reaction in the nanoemulsion process.
The XRD pattern of HA nanoparticles indicated a nearly amorphous structure or a
very low degree of crystallinity. FTIR spectra exhibited the presence of the bands
for OH − and PO 4 3 − . The existence of a small amount of occluded water was
found at 1,647 cm − 1 , bands for the CO 3 2 − group were also observed at 870 cm − 1
and 1,467-1,412 cm − 1 (Zhou et al. 2008 ).
The fabrication technique of nanocomposite scaffolds involved making
an emulsion by homogenization of PHBV or PHBV/PLLA-chloroform solu-
tion, addition of nano-sized HA powder and acetic acid, freezing at a pre-set
temperature to induce solid-liquid phase separation, and freeze-drying with