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 4.9 <strong>Influence</strong> <strong>of</strong> Pellet Thickness on Foaming To study <strong>the</strong> effect <strong>of</strong> polymer mass and o<strong>the</strong>r parameters pellets <strong>of</strong> different thickness ranging between (0.2 − 1.8 mm) were made (cf. Table 5.25). Table 5.25: Pellets <strong>of</strong> variable thickness and <strong>the</strong>ir foam data. Pellet Dimensions Foam Dimensions Porosity Diameter Thickness Mass Diameter Thickness Mass d p (mm) ø p (mm) m p (mg) d f (mm) ø f (mm) m (mg) (%) 12.93 0.25 28.60 23.63 0.92 28.90 91.8 12.85 0.43 63.30 25.05 1.49 63.10 92.4 12.90 0.62 96.00 25.08 2.06 96.10 92.0 12.94 0.83 126.70 32.13 2.12 126.10 93.7 12.92 1.10 165.00 24.59 4.72 164.90 93.6 13.00 1.23 203.00 23.00 5.52 202.80 92.9 12.94 1.37 230.70 20.81 5.76 230.70 90.8 12.90 1.62 273.10 20.23 6.14 272.20 89.3 12.95 1.82 298.60 20.79 6.33 297.60 88.9 The pellets were prepared at P = 150 bars, T = 60 o C and t = 20 min as per procedure described in section 4.3 and illustrated in Figure 4.9-(B). Before and after scCO 2 treatment, <strong>the</strong> pellets and foams diameter and thickness was measured by using a digital vernier caliper at eight different points (cf. Table 5.25). The mean values were considered during <strong>the</strong> calculation and analysis. Pellets were treated at following supercritical CO 2 conditions: P sat = 120 bars, t sat = 20 min, T sat = 35°C and dP/dt = 3 bar/s. Experiments were carried out in <strong>the</strong> SEPAREX ® SF200 pilot plant by adopting <strong>the</strong> setup-02 as described in (cf. chapter 4, Figure 4.12-B). 4.9.1 Porosity and Cell Density By using <strong>the</strong> dimensions <strong>of</strong> pellets and foams, geometric porosity <strong>of</strong> <strong>the</strong> foam was calculated. It was above 88% for all <strong>the</strong> samples. Average porosity was calculated from <strong>the</strong> diameter, thickness and mass <strong>of</strong> pellets and foams, <strong>the</strong>n data was considered for <strong>the</strong> calculation <strong>of</strong> cell density. (cf. Table 5.25 and Figure 5.28). Cell densities <strong>of</strong> <strong>the</strong> foams are presented in Figure 5.29 and corresponding images <strong>of</strong> obtained foams are represented in Figure 5.30. Foam Porosity ( %) 100 95 90 85 Foams Porosity Variation Pore Density--Pores /(cm) 3 10 6 Cell Density <strong>of</strong> Pores Micro Pores Meso Pores Macro Pores 10 5 10 4 10 3 80 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 PLGA 50:50 Pellets Thicknesses (mm) 10 2 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pellet Thickness (mm) Figure 5.28: Porosity <strong>of</strong> PLGA 50:50 foams for pellets with different initial thickness. Figure 5.29: Pore density <strong>of</strong> PLGA 50:50 foams with different initial pellet thickness. - 142 -
Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering Figure 5.30: PLGA 50:50 foams obtained with different initial pellet thicknesses. Best foams have thickness ranging between 0.6 and 1.2 mm. They are characterized by: Porosity <strong>of</strong> foam greater than 90%. Cell density around 25 ×10 3 and 12 ×10 6 pores /cm 3 for macro and micro pores. Macro pores surface area ranging between 80 and 96%. Pellet <strong>of</strong> 0.6 mm thickness seems to be <strong>the</strong> best trade-<strong>of</strong>f between quality and price <strong>of</strong> <strong>the</strong> foam. 4.9.2 Pores Size Distribution We have used <strong>the</strong> measured porosity data to estimate <strong>the</strong> number <strong>of</strong> nuclei to <strong>the</strong> average pore diameter. Average pore diameter for <strong>the</strong> 9 samples was calculated by using <strong>the</strong> SCION ® image analysis. Figure 5.31-(A) depicts <strong>the</strong> micro, meso and macro pores diameter for <strong>the</strong> 9 previous different pellets <strong>of</strong> PLGA 50:50 foamed at P sat = 120 bars, t sat = 20 min, T sat = 35°C and dP/dt = 3 bar/s. Average Pore Diameter ( d e ) m 300 250 200 150 100 50 0 Pore Variation in Foam 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pellet Thickness (mm) (A)-Pore variation diameter. Micro Pores Meso Pores Macro Pores % Pore Surface Area 100 Micro Pores 80 60 40 20 0 Area Distribution Meso Pores<strong>of</strong> Pores Macro Pores 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pellet Thickness (mm) (B)-Area distribution <strong>of</strong> pores. Figure 5.31: Different distribution <strong>of</strong> pores in PLGA 50:50 foams. As shown on Figure 5.31-(B), in <strong>the</strong>se experimental foaming conditions, <strong>the</strong> relative ratio <strong>of</strong> pores surface area seems to be stable: around 0.2 to 0.3% for <strong>the</strong> micro-pores, between 4 and 20 % for <strong>the</strong> mesopores and <strong>the</strong> complement for <strong>the</strong> macro-pores. 4.9.3 Correlation Between Effects <strong>of</strong> Pellet Thickness and Process <strong>Parameters</strong> Earlier, we concluded that pellets <strong>of</strong> thickness <strong>of</strong> 0.6 mm produced foams with porosity <strong>of</strong> 93% and maximum surface area <strong>of</strong> macro pores along micro and meso pores, thus a foam <strong>of</strong> better quality as it - 143 -
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