Biodegradable PHBV Polymer-Based Scaffolds for Bone Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 43

was maintained at that temperature for overnight. The frozen emulsion was then
transferred into a freeze-drying vessel at a preset temperature of − 10 °C. The
samples were freeze-dried for at least 46 h to remove the solvent and water phase
completely. The properties of scaffolds prepared from the freezing temperature
of − 35 or − 18 °C were studied. Freeze-drying was performed at − 10 °C. The
finished scaffolds were stored in a vacuum dessiccator at room temperature for
storage and further removal of any residual solvent until characterization (Sultana
and Wang 2012a ).
Figure 3.1 a shows the physical appearance of the tissue engineering scaf-
folds by emulsion freezing/freeze-drying technique using PHBV polymers. All
the scaffolds were relatively large sized, homogeneous, could be handled easily
and found to have less voids (pore sizes >1 mm). The scaffolds also had nonpo-
rous outer layer of skin. Figure 3.1 b is one of the SEM micrograph of the inner
side of the scaffolds. The scaffolds had pores which were highly interconnected
with large pore size distribution. It was found that the emulsion viscosity rap-
idly increased with the increased in concentration of the PHBV. The viscosity of
PHBV emulsion increased rapidly over 7.5 % (w/v) PHBV emulsion and formed
the pore sizes of several microns more than three hundred microns and stable
pore interconnectivity. The ratio of water phase volume with respect to solvent
volume was found to have direct influence on emulsion stability and resulted
scaffold volume. After trying several times, 1:1 ratio of solvent to acetic acid or
the volume fraction of dispersed water phase,  = 0.5 was found to be appropri-
ate for the scaffolds.
Figure 3.2 a shows some of the defects which were observed during the scaffold
fabrication process. If the solubility of the PHBV polymers was not high enough
(using another solvent, dioxane), scaffolds with broken structures were formed
(Fig. 3.2 a) whereas if the solvent removal rate was too fast at high PHBV concen-
trations, nonporous structures were observed (Fig. 3.2 b).
The effect of polymer weight fraction or the emulsion concentration on scaf-
fold porosity and physical quality was evaluated. Scaffolds produced from PHBV
concentration with 2.5 % (w/v) PHBV had physically weak structure. Emulsion
concentrations of more than 12.5 % (w/v) showed high viscosity of the organic
phase which ultimately prevented homogenization adequately. Scaffolds produced
from PHBV concentrations 5, 7.5, 10 and 12.5 % (w/v) had better physical qual-
ity. Scaffolds produced from polymer concentration more than 12.5 % (w/v) were
considered to be too dense, whereas less than 2.5 % (w/v) were considered to be
structurally inadequate. Higher homogenization speed was also found to provide
better physical properties.
PHBV scaffolds from different emulsion concentration of high porosity (low
density) were fabricated by emulsion freezing/freeze-drying technique. At the
same quenching temperature, the density increased with emulsion concentration
whereas porosity decreased with increasing emulsion concentration. Scaffold
porosity dropped from 85 to 71 % and the scaffold density increased from 0.0540
to 0.2926 g/cm 3 when the emulsion concentration increased from 2.5 to 12.5 %
(w/v) (Table 3.1 ).