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

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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 - 46 -
Chapter 2. <strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface (t sat ). An increase <strong>of</strong> temperature (T sat ) above T c enhances <strong>the</strong> free volume and <strong>the</strong> increase <strong>of</strong> <strong>the</strong> pressure (P sat ) above P c causes <strong>the</strong> sorption <strong>of</strong> scCO 2 . The decrease <strong>of</strong> <strong>the</strong> temperature is at <strong>the</strong> origin <strong>of</strong> <strong>the</strong> departure <strong>of</strong> CO 2 molecules and <strong>the</strong> depressurization <strong>of</strong> <strong>the</strong> chamber causes <strong>the</strong> pore formation inside <strong>the</strong> polymer.Evolution <strong>of</strong> this phenomenon can be fur<strong>the</strong>r explained as under. Period I: The CO 2 is compressed to a pressure vessel where a polymer sample had already been placed (Part I <strong>of</strong> <strong>the</strong> Figure 2.20). Generally, <strong>the</strong> process is carried out until a value above <strong>the</strong> critical pressure (P c ). Pressure and temperature increase with <strong>the</strong> compression until desired values called saturation pressure (P sat ) and saturation temperature (T sat ). The sorption-diffusion <strong>of</strong> CO 2 begins but since <strong>the</strong> time <strong>of</strong> this period takes maximum 2 minutes, it can be neglected. Period II: The sorption-diffusion <strong>of</strong> CO 2 takes places. The solubility (phase equilibrium) is <strong>the</strong> limiting factor <strong>of</strong> <strong>the</strong> sorption-diffusion <strong>of</strong> CO 2 into <strong>the</strong> polymer since <strong>the</strong> polymer is not soluble in CO 2 . Moreover, under high pressure, <strong>the</strong> sorption <strong>of</strong> CO 2 into <strong>the</strong> polymer brings <strong>the</strong> structural phase transition <strong>of</strong> polymer. Polymer swells as <strong>the</strong> CO 2 sorbed and <strong>the</strong> transition occurs from glassy to plasticized (rubbery) state, by lowering <strong>the</strong> glass transition temperature <strong>of</strong> <strong>the</strong> polymer. This period is called <strong>the</strong> saturation time (t sat ) (Part II <strong>of</strong> <strong>the</strong> Figure 2.20). Figure 2.19: Schematic presentation for scaffold generation during scCO 2 foaming. Modified from [Cooper, 2003] Period III: Period II is followed by <strong>the</strong> depressurization <strong>of</strong> <strong>the</strong> pressure chamber (III). The rate <strong>of</strong> <strong>the</strong> depressurization (dP/dt) can be controlled. With <strong>the</strong> pressure drop <strong>of</strong> <strong>the</strong> pressure chamber comes <strong>the</strong> temperature drop (dT/dt). The temperature drop is proportional to <strong>the</strong> pressure drop. When <strong>the</strong> pressure is reduced from <strong>the</strong> equilibrium state, <strong>the</strong> formation <strong>of</strong> nuclei occurs as a result <strong>of</strong> supersaturation. These nuclei grow by <strong>the</strong> desorption-diffusion <strong>of</strong> <strong>the</strong> gas from <strong>the</strong> polymer matrix. We can state that <strong>the</strong> pressure difference between <strong>the</strong> two sides <strong>of</strong> <strong>the</strong> pore interface is <strong>the</strong> driving force <strong>of</strong> <strong>the</strong> pore growth. One must remember that, during <strong>the</strong> depressurization, <strong>the</strong> CO 2 is not supercritical anymore, and its molecular volume is greater than that <strong>of</strong> <strong>the</strong> supercritical state. Actually, <strong>the</strong> pore growth is provided by <strong>the</strong> expansion <strong>of</strong> CO 2 (Part III <strong>of</strong> <strong>the</strong> Figure 2.20). - 47 -
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Institut National Polytechnique de
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iii Dedicated to My Fathe</
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pleased I was to work with all memb
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Publications and Conferences The wo
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Nomenclature and Abbreviations Abbr
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ρ p Pellet density Splitting (Bra
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Chapter 2 .........................
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