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 processing parameters are: saturation pressure P sat = 150 bars, saturation time t sat = 20 minutes, saturation temperature T sat = 36.5 o C and rapid depressurization rate dP/dt (dP/dt time less than 3 sec). The saturation time for PLGA 85:15 has been reduced from 60 minutes to 20 minutes because we have increased <strong>the</strong> LA/GA ratio. Pini et al. [2008] have proved that <strong>the</strong> concentration <strong>of</strong> CO 2 inside PLGA 85:15 is greater than inside PLGA 50:50 during <strong>the</strong> foaming process. 3.1.4 Discussion on <strong>the</strong> Blends <strong>of</strong> P L,DL LA and PLGA 85:15 Foaming experiments on blends <strong>of</strong> P L,DL LA and PLGA 85:15 revealed that <strong>the</strong> pore diameter is first increased and <strong>the</strong>n decreased with increasing amount <strong>of</strong> PLGA 85:15 in <strong>the</strong> blend. As shown in Figure 5.12, <strong>the</strong>re is one composition (50% P L,DL LA + 50% PLGA 85:15 ), at which <strong>the</strong> average pore size is maximum. Moreover, <strong>the</strong> average pore size is lower than with <strong>the</strong> previous blends with <strong>the</strong> PLGA 50:50 . Average Pore Diameter (m) 16 Psat = 150 P bars L,DL LA and PLGA 85:15 Blend 14 T sat = 36.5 °C 12 10 8 6 4 2 t sat = 20 min dP/dt = Rapid 0 0 20 40 60 80 100 Fraction (%) Figure 5.12: Average pore diameter <strong>of</strong> polymer blends as a function <strong>of</strong> P L,DL LA ratio. These preliminary experiments on (PLGA 50:50 + P L,DL LA) blends and (PLGA 85:15 + P L,DL LA) blends confirm <strong>the</strong> literature works [Goel and Beckman, 1994b] concerning <strong>the</strong> influence <strong>of</strong> <strong>the</strong> scCO 2 parameters on <strong>the</strong> pore size. Various process parameters (i.e. <strong>the</strong> pressure, <strong>the</strong> temperature and <strong>the</strong> time <strong>of</strong> <strong>the</strong> saturation, but also <strong>the</strong> rate <strong>of</strong> depressurization) have an influence on <strong>the</strong> final pore size. The drop <strong>of</strong> <strong>the</strong> temperature acts differently on <strong>the</strong> polymers foaming behaviours. For <strong>the</strong>se reasons, we have carried on different experimental designs. Our experimental study showed that <strong>the</strong> solubility <strong>of</strong> CO 2 into <strong>the</strong> polymer increases while <strong>the</strong> LA content increases in a PLGA co-polymer. Also, <strong>the</strong> solubility <strong>of</strong> CO 2 into P D,L LA is higher than in PLGA, whatever its composition. This behaviour has been explained by <strong>the</strong> existence <strong>of</strong> an extra apolar CH 3 methyl group in LA than GA [Liu and Tomasko, 2007b], which, according to <strong>the</strong> authors, can drive to two opposites phenomena: firstly, it decreases <strong>the</strong> effect <strong>of</strong> <strong>the</strong> CO 2 ’s interaction with <strong>the</strong> carbonyl group <strong>of</strong> <strong>the</strong> polymer and secondly, it creates more available free volume for CO 2 to solubilise. Besides, Kazarian et al. [1996b] have found that <strong>the</strong> interaction <strong>of</strong> CO 2 with polymers can also be explained by chemical interactions and CO 2 can behave like a Lewis acid. 3.2 <strong>Influence</strong> <strong>of</strong> <strong>the</strong> <strong>Parameters</strong> <strong>of</strong> <strong>the</strong> scCO 2 Process 3.2.1 Model with a 2 4 Complete Design We have started with <strong>the</strong> most basic experimental design, a complete plan with 4 parameters and 2 levels. As listed in Table 5.12, PLGA 100/0 or 0/100 represents <strong>the</strong> pure PLGA 50:50 or PLGA 85:15 respectively and on <strong>the</strong> Table 5.13. 25/75, 50/50 and 75/25 denominations are representative <strong>of</strong> <strong>the</strong> blends. - 126 -
Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering Table 5.12: Variation domain <strong>of</strong> <strong>the</strong> various factors. Level T sat (°C) P sat dP/dt (bars) (bar/s) t sat (min) PLGA 50:50 /PLGA 85:15 - 1 36.5 100 1 20 100/0 + 1 60 250 20 20 0/100 Table 5.13: 2 4 design <strong>of</strong> experiments: levels <strong>of</strong> factors and average size <strong>of</strong> pores. Experiment X 1 X 2 X 3 X 4 T (°C) P (bar) dP/dt (bar/s) PLGA 50:50/85:15 Pore Diameter (μm) 1 -1 -1 -1 -1 36.5 100 1 100/0 153 2 -1 -1 -1 1 36.5 100 1 0/100 264 3 1 -1 -1 -1 60 100 1 100/0 130 4 1 -1 -1 1 60 100 1 0/100 187.5 5 -1 1 -1 -1 36.5 250 1 100/0 30.3 6 -1 1 -1 1 36.5 250 1 0/100 72.5 7 1 1 -1 -1 60 250 1 100/0 27 8 1 1 -1 1 60 250 1 0/100 59 9 -1 -1 1 -1 36.5 100 20 100/0 367.5 10 -1 -1 1 1 36.5 100 20 0/100 71 11 1 -1 1 -1 60 100 20 100/0 151.5 12 1 -1 1 1 60 100 20 0/100 108 13 -1 1 1 -1 36.5 250 20 100/0 4 14 -1 1 1 1 36.5 250 20 0/100 25 15 1 1 -1 -1 60 250 1 100/0 23 16 1 1 -1 1 60 250 1 0/100 36 17 -1 1 -1 -1 36.5 250 1 100/0 47 18 1 1 -1 1 60 250 1 0/100 61 19 -1 -1 -1 0 36.5 100 1 50/50 258 20 1 -1 -1 0 60 100 1 50/50 60 21 -1 -1 1 0.5 36.5 100 20 25/75 93.5 22 1 -1 1 0.5 60 100 20 25/75 110 23 -1 1 1 -0.5 36.5 250 20 75/25 3 24 1 1 -1 -0.5 60 250 1 75/25 23 25 -1 1 -1 0.5 36.5 250 1 25/75 30 26 -1 1 -1 0 36.5 250 1 50/50 36 27 -1 1 -1 -0.5 36.5 250 1 75/25 22.5 28 -1 -1 1 0 36.5 100 20 50/50 222 29 -1 -1 1 -0.5 36.5 100 20 75/25 275 The first 2 4 = 16 values allow us to calculate <strong>the</strong> coefficients <strong>of</strong> <strong>the</strong> two first order models reported on <strong>the</strong> Table 5.14, <strong>the</strong> o<strong>the</strong>r values play a role to valid <strong>the</strong> first order model: y = a 0 + a 1 X 1 + a 2 X 2 + a 3 X 3 + a 4 X 4 + a 12 X 1 X 2 + a 13 X 1 X 3 + a 23 X 2 X 3 For both blends, it is important to note that <strong>the</strong> most influent factor is <strong>the</strong> pressure (a 2 ). For PLGA 50:50 <strong>the</strong> saturation temperature (a 1 ) is also an influent effect while it is non influent for <strong>the</strong> PLGA 85:15 . It is <strong>the</strong> contrary for factor 3, as <strong>the</strong> detail given in Table 5.14. The variation <strong>of</strong> average pore diameter as a function <strong>of</strong> PLGA 50:50 fractions in <strong>the</strong> blends (cf. Figure 5.13), for experiments performed in <strong>the</strong> same operating conditions, shows that with increasing amount <strong>of</strong> PLGA 50:50 , <strong>the</strong> average pore size increases when high depressurization rate (20 bar/s) is applied. Analysis <strong>of</strong> <strong>the</strong> average diameter <strong>of</strong> pores presented in Table 5.13, showed us that to have a trend for bigger - 127 -
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