Biodegradable PHBV Polymer-Based Scaffolds for Bone Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 45

To study the effect of polymer/emulsion concentration on the scaffold struc-
ture, a series of PHBV scaffolds were prepared from PHBV/chloroform/acetic
acid emulsion. The PHBV concentration ranged from 2.5 to 12.5 % (w/v). The
scaffolds made from 5 % PHBV emulsion showed low polymer interconnectiv-
ity (Fig. 3.3 a). The scaffolds fabricated from 7.5, 10 and 12.5 % (w/v) emulsion
concentration were very hard and tough. The observation from SEM showed that
the pore structure of the scaffold with 7.5 % (w/v) PHBV solution was almost
same as the scaffold from 10 and 12.5 % PHBV solutions (Fig. 3.3 b-d). The pore
walls became thicker as the polymer concentration increased and the total porosity
decreased with the increasing polymer/emulsion concentration. The pore structure
was found to be more uniform which had the pore sizes ranging from 60-70 to
300-600 microns. The average diameter was 297 μ m.
Polymer scaffolds prepared from the optimum parameters exhibited more than
70 % porosity and better handling properties. The pore size ranged from several
microns to few hundred microns and they change with the concentration of the pol-
ymer. Figure 3.3 shows the morphology of PHBV scaffolds of different concentra-
tions. Anisotropic pore morphology with elongated pores and internal ladder-like
microstructures in the pores of scaffolds were observed. In this investigation, the
solvent solidification front proceeded mainly from the bottom to the top of the
emulsion and from the side walls to the centre within a few hours. As a result a con-
tinuous polymer-rich phase was formed which is in fact the aggregation of excluded
polymer from every single liquid crystal. After the sublimation of the solvent and
water phase, scaffolds with pores of the similar geometry of solvent and water
phase crystals was formed. As semicrystalline PHBV polymer was used to fabricate
the scaffolds, the phase separation became significantly more complicated because
of the potential for the PHBV polymer to crystallize. When the temperature of the
polymer solution is low enough, both liquid-liquid phase separation and polymer
crystallization can occur. Kinetic phenomena are important in this situation. It was
reported that the polymer solution firstly underwent liquid-liquid phase separation
and when the solution was cooled enough, polymer rich phase was able to crystal-
lize and the resulted morphology was largely dependent on the liquid-liquid phase
separation (Chen and Ma 2005 ). Generally liquid-liquid phase separation produces
a fine scaled (size range 0.1-1.0 μ m), early-stage structure which can be “frozen
in” if the polymer solution gels or the solvent freezes. The system phase morphol-
ogy can also be affected by coarsening of liquid phases which can occur during the
later stage of phase separation. Coarsening phenomena can be observed in systems
which exhibit a phase transition when the temperature is decreased below a critical
temperature. In the diffusive process of solutions, droplets that are well separated
and have well-defined interfaces can coarsen due to the fact that smaller droplets
have higher solubility and hence preferentially dissolve while larger droplets grow.
It was also demonstrated that the quenching rate of the original homogeneous
polymer solution had intense effect on the scaffolds prepared by freeze-drying
(Schugens et al. 1996 ). A faster quenching usually resulted in a decrease of the
average pore size due to the rapidly frozen two-phase structure leaving small pores
upon solvent sublimation. When the cooling is slow, phase coalescence occurs in