Influence of the Processes Parameters on the Properties of The ...

Chapter 2.
<strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface
3.4 Scaffolds Prepared by scCO2 Foaming
Mooney and co workers [Harris et al., 1998; Mooney et al., 1996] were <strong>the</strong> first to describe <strong>the</strong>
use <strong>of</strong> supercritical foaming for <strong>the</strong> preparation <strong>of</strong> macr-oporous scaffolds for tissue engineering
applications. Interconnected porous structures <strong>of</strong> P D,L LGA were successfully produced [Tai et al., 2007a;
Singh et al., 2004]. Bioresorbable ceramic–polymer composites were also prepared and are described by
Mathieu et al., [2005] and Georgiou et al., [2007]. The ability to process composite matrixes <strong>of</strong> ceramics
and polymers or blends <strong>of</strong> different polymers demonstrates <strong>the</strong> versatility <strong>of</strong> this technology and shows <strong>the</strong>
potential to develop materials with <strong>the</strong> desired morphological and mechanical properties. The gas foaming
process has also proven to be a very promising technique for <strong>the</strong> preparation <strong>of</strong> scaffolds loaded with growth
factors and cells. Howdle et al., [2001] have encapsulated proteins in biocompatible and biodegradable
polymers, such as PLA, PLGA, and Polycaprolactone (PCL), at relatively low temperatures and moderate
pressures.
CO 2 exhibits unique features and benefits, such as appreciable solubility in polymer melt and fast
diffusivity that ensures an efficient mixing process, as well as environmental advantages and low cost. On
<strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> challenges <strong>of</strong> CO 2 as a foaming agent are mainly associated with <strong>the</strong> higher-pressure
operation, dimensional instability during <strong>the</strong> foam shaping process, and paradoxically <strong>the</strong> high diffusivity <strong>of</strong>
CO 2 out <strong>of</strong> <strong>the</strong> foam resulting in a quick loss <strong>of</strong> R-value (resistance to heat flow).
The volumic variation between <strong>the</strong> pellets and <strong>the</strong> foams are illustrated in Figure 2.18. This
technique has <strong>the</strong> potential to be used to prepare 3D materials having high porosity and interconnected pores
with a wide range <strong>of</strong> applications in <strong>the</strong> field <strong>of</strong> tissue engineering and regenerative medicine.
Figure 2.18: Schematic representation <strong>of</strong> <strong>the</strong> supercritical fluid foaming process.
[Duarte et al., 2009b]
A foaming process can be carried out in a batch system where <strong>the</strong> pre-shaped samples are placed
in a pressurized autoclave to be saturated with CO 2 [Kumar and Weller, 1994a, 1994b; Park et al., 1994].
Nucleation and cell growth are controlled by <strong>the</strong> pressure-release rate and foaming temperature. The
introduction <strong>of</strong> nano-particles to foams provides a novel solution to fur<strong>the</strong>r sharpen <strong>the</strong> operation window
and <strong>the</strong> product performance in weight, mechanics, insulation and barrier. Because <strong>of</strong> <strong>the</strong> similarity <strong>of</strong>
foamed porous structure to some human tissues, CO 2 is also proposed to foam biodegradable or
biocompatible polymers to produce porous scaffolds or o<strong>the</strong>r medical devices. Since CO 2 can easily escape,
<strong>the</strong> prepared products are always assured to be solvent-free and non-toxic.
The principles <strong>of</strong> <strong>the</strong> scCO 2 foaming by pressure quench method by supercritical carbon dioxide
(scCO 2 ) were first described by Goel and Beckman [1994a]. Schematically, <strong>the</strong> foaming effect can be
separated in five stages (cf. Figure 2.19). The pellets were exposed to carbon dioxide for few minutes
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