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

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Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering The primary approximation <strong>of</strong> this model is that <strong>the</strong> pressure difference between <strong>the</strong> two sides <strong>of</strong> <strong>the</strong> bubble interface is equal to <strong>the</strong> pressure initial and final pressures <strong>of</strong> <strong>the</strong> chamber before and after depressurization. On <strong>the</strong> o<strong>the</strong>r hand <strong>the</strong> energy barrier to create nuclei decreases with <strong>the</strong> increasing pressure difference and it is represented in Figure 5.19-B. The exponentially decrease <strong>of</strong> <strong>the</strong> energy barrier with <strong>the</strong> pressure means that more nuclei can be generated at high pressures. Since <strong>the</strong> interfacial tension is an affecting parameter <strong>of</strong> <strong>the</strong> ΔG, it has a great influence on <strong>the</strong> nuclei density <strong>of</strong> <strong>the</strong> polymer. The plateau after 150 bars can be explained by <strong>the</strong> smaller pore size (great number <strong>of</strong> pore) at higher pressure. As shown in Figure 5.19, <strong>the</strong> model is in agreement with <strong>the</strong> experimental data for <strong>the</strong> pressures greater than 100 bars and diverges significantly for 80 bars where <strong>the</strong> number <strong>of</strong> nuclei is smaller than that <strong>of</strong> <strong>the</strong> greater pressures. One must remember that this model is considering <strong>the</strong> homogenous nucleation <strong>the</strong>ory. Thus, this divergence for low pressures can be attributed to <strong>the</strong> heterogeneous nucleation and <strong>the</strong> coalescence <strong>of</strong> <strong>the</strong> growing pores at lower pressures. Indeed, at low pressures, <strong>the</strong> sorption <strong>of</strong> CO 2 into <strong>the</strong> PLGA 50:50 is also lower and CO 2 is not completely distributed across <strong>the</strong> pellet by <strong>the</strong> sorption-diffusion. (A) (B) Figure 5.19: (A) Variation <strong>of</strong> pore diameter <strong>of</strong> <strong>the</strong> PLGA 50:50 scaffolds as <strong>the</strong> function <strong>of</strong> P sat ; (B) Variation <strong>of</strong> <strong>the</strong> energy barrier for PLGA 50:50 -CO 2 system. By using <strong>the</strong> matrix formula: X s = - 0.5 A -1 a k where a k is <strong>the</strong> vector <strong>of</strong> <strong>the</strong> first order coefficients and A <strong>the</strong> matrix <strong>of</strong> <strong>the</strong> second order coefficients, reported on <strong>the</strong> Table 5.23, we obtain <strong>the</strong> following values: As both coefficients a 11 and a 22 are positive, <strong>the</strong> stationary point is a minimum. The pressure P sat = 100 bars and <strong>the</strong> depressurization rate dP/dt = 0.625 bar/s have been chosen as <strong>the</strong> parameters which give <strong>the</strong> maximum. The diameter <strong>of</strong> pores, at this point, is equal to 250 m. Table 5.23: Analysis <strong>of</strong> <strong>the</strong> Doehlert design: research <strong>of</strong> <strong>the</strong> optimum <strong>of</strong> <strong>the</strong> pore dimension. Reduced natural a k = - 74.4 A = 87.0 10.25 X 1 - 1 0.625 dP/dt - 44.3 10.25 32.3 X 2 - 0.87 100 P sat Results obtained from our experimentations has shown that when pore size is large higher porosity is observed while pores with small pore diameter reflects small porosity. Beckman, 2004 have also shown similar behaviours in his experimentations . As per our understanding , it is impossible to create pores <strong>of</strong> large diameter with small polymer volume or small pores with important polymer swelling. - 134 -
Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering 4 Factors Affecting on Pores Size and Porosity 4.1 Effect <strong>of</strong> <strong>the</strong> Polymer Composition The Hildebrand’s solubility parameters can be separated into three Hansen’ components by <strong>the</strong> relationship given in following equation 5.1. δ t 2 = δ d 2 +δ p 2 +δ H 2 (5. 1) where δ t is <strong>the</strong> Hildebrand’s parameter (cf.Table 5.24), δ d is <strong>the</strong> dispersive component, δ p is <strong>the</strong> polar component and δ H is <strong>the</strong> hydrogen bond component [Risanen, 2010; Schenderlein et al., 2004]. In <strong>the</strong> case <strong>of</strong> supercritical fluids <strong>the</strong> relationship linking <strong>the</strong> Hildebrand’ parameter to <strong>the</strong> P c critical pressure is <strong>the</strong> following [Li. and Perrut, 1991]. = 1,25 Pc / l (5.2) where and l are corresponding to <strong>the</strong> density to <strong>the</strong> fluid in <strong>the</strong>ir supercritical and liquid state respectively. ScCO 2 is a apolar fluid (14,3 < < 18,4). Table 5.24: Hidebrand’ and Hansen’ parameters <strong>of</strong> <strong>the</strong> PLA and <strong>the</strong> PGA in (MPa) 1/2 . d p H t t * PLA 18.5 9.7 6.0 21.7 20.2 PGA 11.7 6.21 12.5 18.2 24.8 *Calculations with <strong>the</strong> Small’ group contribution method. Normally both amorphous PLAs and PLGAs produce scaffolds <strong>of</strong> higher porosity and large pore diameters. Increase in <strong>the</strong> LA content in <strong>the</strong> PLGA co-polymer increases <strong>the</strong> solubility <strong>of</strong> CO 2 in <strong>the</strong> polymers. The solubility <strong>of</strong> CO 2 is always higher in all poly D,L-lactides than poly(lactides co-glycolide). The extra apolar group in polylactide acid is responsible for higher solubility in <strong>the</strong> polylactide which eventually generates highly porous foams depending upon <strong>the</strong> process conditions. According to Liu and Tomasko [2007b] <strong>the</strong> extra apolar group which is not present in glycolic acid, leades to two opposite and different phenomenon. The effect <strong>of</strong> CO 2 interaction with <strong>the</strong> carbonyl group <strong>of</strong> <strong>the</strong> polymer is decreased due to <strong>the</strong> apolar group first and secondly, more available free volume for CO 2 to solubilise is created. Kazarian et al. [1996b] has also found that <strong>the</strong> interaction <strong>of</strong> CO 2 with polymers can also be explained by <strong>the</strong> CO 2 behaviour as a Lewis acid, an electron pair acceptor. LA/GA ratio <strong>of</strong> a PLGA co-polymer is an important parameter to control <strong>the</strong> pore diameter in a foaming process. Foaming <strong>of</strong> PLGA with different LA and GA contents, has given different results. In our study, with <strong>the</strong> same foaming conditions (P sat , T sat , t sat , dT/dt), we have experienced different pore size behaviours when processed with rapid or slow depressurizations. Secondly, low pores size can be due to high saturation pressure and low saturation temperature with comparison to <strong>the</strong> T g <strong>of</strong> both polymers (cf. Table 5.5). 4.2 Effect <strong>of</strong> Depressurization Rates For low depressurization rates (0.625 to 1.25 bar/s), when <strong>the</strong> lactic acid content increases in PLGA, <strong>the</strong> pore size increases as well. This behaviour can be attributed to <strong>the</strong> greater capacity <strong>of</strong> solubilized CO 2 inside <strong>the</strong> polymer with <strong>the</strong> increasing amount <strong>of</strong> LA. Actually, one can expect that since <strong>the</strong> CO 2 concentration is greater in a high lactic acid containing PLGA, <strong>the</strong> nucleation rate must be greater (which means lower pore size). However, even if <strong>the</strong> number <strong>of</strong> pores is determined by <strong>the</strong> saturation period and <strong>the</strong> - 135 -
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Institut National Polytechnique de
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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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