Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 24

2.1.8 Emulsion Freezing/Freeze-Drying Technique
In order to fabricate highly porous scaffolds, the emulsion freezing/freeze-drying
technique is called as the novel processing technique as these scaffolds have the
added benefit of being amenable to the incorporation of protein-based growth
and differentiation factors at the time of processing. More specifically, emul-
sion freezing/freeze-drying technique involves fabricating scaffolds with poros-
ity greater than 90 % and ability to control pore size. The method consisted of
creating an emulsion by homogenization of a polymer solvent solution and water,
rapidly cooling the emulsion to lock liquid state structure and removing the sol-
vent and water by freeze drying (Whang and Healy 2002 ). The development of
porous materials for use as scaffolds for the sustained 3D growth of tissue is a
fast growing field that has gained commercial interest to a large extent. To fab-
ricate both polymer scaffold and composite scaffold, emulsion freezing/freeze-
drying technique is a potential route. It was found that it was possible to produce
hard and tough scaffolds through this technique (Whang and Healy 2002 ). The
porosity can be controlled by controlling the polymer concentration and freeze-
drying parameters. It was reported previously that the porous structure of other
polymeric scaffolds could be controlled by varying the processing or formulation
parameters such as polymer, polymer solution concentration, solvent and water
phase concentration, quenching temperature, etc. (Hua et al. 2002 ; Whang and
Healy 2002 ). The careful selection of various processing parameters was crucial
in creating an emulsion from two immiscible phases. The fabrication and charac-
terization of highly porous and interconnected poly( α -hydroxy acid) foams were
performed using a phase separation multisolvent system, followed by a sublima-
tion process by optimizing several fabrication parameters (Hu et al. 2002 ). It was
reported that the selected polymer foams has pore size ranges of 100-350 μ m, a
porosity of more than 90 %, with an interconnecting open-pore morphology. It
was also reported that the scaffold degradation profiles varied according to the
type and molecular weight of the polymers and cytocompatibility assays demon-
strated that the scaffolds were non-toxic and osteoprecursor cells seeded into the
scaffolds exhibited the ability to attach, propagate and differentiate into a calcified
structure (Hu et al. 2002 ). Highly porous degradable PDLLA/Bioglass ® compos-
ites as potential scaffolds for bone tissue engineering were produced via thermally
induced solid-liquid phase separation and subsequent solvent sublimation (Lin
et al. 2002 ). It was reported that the scaffolds had a bimodal and anisotropic pore
structure, with tubular macro pores of ~100 μ m in diameter with interconnected
micro pores of ~10-50 μ m in diameter. It was also demonstrated that the mechani-
cal anisotropy concomitant with the direction of the macro-pores and the presence
of Bioglass ® did not significantly alter the porous architecture of the foams and
reflected the mechanical anisotropy.
Scaffolds with variable porosity and pore size utilize an emulsion freez-
ing/freeze-drying technique. According to this method, water was added
to a solution of PLGA in methylene chloride to create an emulsion (Whang