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

Chapter 5.
Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering
chamber. Since <strong>the</strong> polymer swells during desorption <strong>of</strong> CO 2 , when <strong>the</strong> polymer has swelled approximately
by 50%, <strong>the</strong> edges <strong>of</strong> <strong>the</strong> polymer contact <strong>the</strong> wall <strong>of</strong> <strong>the</strong> chamber. Since <strong>the</strong> polymer is very s<strong>of</strong>t, due to <strong>the</strong>
depression <strong>of</strong> <strong>the</strong> glass transition, <strong>the</strong> chamber walls block desorption <strong>of</strong> CO 2 from <strong>the</strong> edges <strong>of</strong> polymer,
which results in <strong>the</strong> expanding <strong>of</strong> <strong>the</strong> pores in that region. Indeed, this expansion corresponds to <strong>the</strong> increase
<strong>of</strong> molecular volume <strong>of</strong> CO 2 molecules which are no longer in supercritical state.
(A) Sample 1-C (dP/dt = 2.5 bar/s).
(B) Sample 5-B (dP/dt = 0.625 bar/s).
Figure 5.16: Micrographs <strong>of</strong> <strong>the</strong> cross-section <strong>of</strong> <strong>the</strong> foams processed at P = 125 bars.
One can say that <strong>the</strong>re is a significant difference <strong>of</strong> pore size depending on where <strong>the</strong> pellet is
positioned. In <strong>the</strong> same Doehlert’ experiment (Nr 7), we have obtained a pore diameter ranging between 128
and 223 μm. If <strong>the</strong> pellet is positioned in <strong>the</strong> low position, <strong>the</strong> average pore diameter and <strong>the</strong> porosity are
found smaller than in <strong>the</strong> upper position. It can be explained by <strong>the</strong> faster vitrification <strong>of</strong> <strong>the</strong> polymer in <strong>the</strong>
low position which stops <strong>the</strong> growth <strong>of</strong> <strong>the</strong> pores. As explained earlier, <strong>the</strong> bottom part <strong>of</strong> <strong>the</strong> pressure
chamber is always colder than <strong>the</strong> upper part after depressurization.
Since, we have noticed that two supplementary factors affect <strong>the</strong> pore size (<strong>the</strong> constraint <strong>of</strong>
volume <strong>of</strong> <strong>the</strong> experimental setup and <strong>the</strong> position <strong>of</strong> <strong>the</strong> pellet in <strong>the</strong> chamber), we have decided to remove
<strong>the</strong> circle <strong>of</strong> Teflon which was placed in <strong>the</strong> pressure chamber. After that, we have filled <strong>the</strong> pressure
chamber with small glass marbles as described in chaptet 4, Figure 4.12, setup-02. Then, a pellet <strong>of</strong>
PLGA 50:50 has been placed in <strong>the</strong> chamber. The carried out repetitions experiments and corresponding results
are reported in Table 5.19. For <strong>the</strong> repetitions, we have obtained 52.4 and 26.5 μm, as <strong>the</strong> average diameter
<strong>of</strong> pores.
Table 5.19: Repetitions experiments <strong>of</strong> Doehlert’ design (P sat = 100 bars and dP/dt = 5 bar/s).
Height <strong>of</strong> <strong>the</strong> Glass
Balls in <strong>the</strong> Chamber
Pore Diameter
(µm)
Average Diameter <strong>of</strong> Pores
(µm)
~ 1/3
24.3
28.7
26.5
~ 2/3
50.7
54.0
52.4
The variation <strong>of</strong> <strong>the</strong> pore diameter with dP/dt is reported in Table 5.20 and illustrated by <strong>the</strong> series <strong>of</strong>
micrographs given in Figure 5.18. We have found that <strong>the</strong> pore size is decreasing with <strong>the</strong> increasing rate <strong>of</strong>
depressurization.
The variation to <strong>the</strong> pore size estimated by image analysis, is ranging from 18.9 µm, for dP/dt = 10
bar/s to 255.7 µm, for dP/dt = 0.625 bar/s. These experiments were carried out at high depressurization rate
(10 bar/s). Thus, we expect to find smaller pores. However, we have observed small (10−20 µm) and big
pores [~ 150 − 200 µm] toge<strong>the</strong>r. So that, we can state that <strong>the</strong>re is a coalescence phenomenon occurring
during <strong>the</strong> pore growth.
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