Biodegradable PHBV Polymer-Based Scaffolds for Bone Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 46

Fig. 3.3 SEM micrographs of PHBV scaffolds fabricated from: a 5 %, b 7.5 %, c 10 %, and d
12.5 % PHBV emulsion (Sultana and Wang 2012b )
order to decrease the interfacial energy, which ultimately results in larger pores.
Nevertheless, the physical mechanism of convection is related to the heat conduc-
tion through the thin layer of fluid adjacent to the heat-transfer surface. Fourier's
law is applicable in both conduction and convection. Besides, fluid mechanics
must be brought into play in the convection problem in order to establish the tem-
perature gradient (Holman 1997 ). The time lag allows different microstructures to
be formed at the surface and the centre of a sample.
The characteristic internal ladder-like structure observed in the current inves-
tigation was also observed by other researchers (Schugens et al. 1996 ; Holman
1997 ; Ma and Zhang 2001 ). It seems that the polymers were rejected from the
solvent crystal front with the formation of radial sheets of an internal ladder-like
structure. High-concentration emulsion and relatively fast freezing were known to
be favorable for this particular inner morphology for scaffolds. The temperature
gradient along the solidification direction from the sample surface to the sample
center may also lead to anisotropic pore structure. It was reported (Ma and Zhang
2001 ) that the direction of temperature gradient had a great influence on produc-
ing ladder-like structures. It was also reported that at high polymer concentrations,
the architecture of oriented ladder-like (parallel microtubules with thin partitions)
structures could be achieved if the solvent type and the temperature gradient were
maintained the same. Directional freezing and subsequent freeze-drying can be