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