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

Chapter 5.
Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering
Actually, this swelling is <strong>the</strong> result <strong>of</strong> <strong>the</strong> pore growth. When a slow depressurization happens, <strong>the</strong>
polymer swells slowly and since <strong>the</strong> temperature drop is also slow, <strong>the</strong> vitrification happens later. Thus,
when <strong>the</strong> polymer swells enough to touch <strong>the</strong> edges <strong>of</strong> <strong>the</strong> pressure chamber, <strong>the</strong> edges are behaving like a
volume constraint and <strong>the</strong> pores which are closer to <strong>the</strong> edges is growing more than <strong>the</strong> pore which are in <strong>the</strong>
centre <strong>of</strong> <strong>the</strong> scaffold, indeed, due to <strong>the</strong> blocking <strong>of</strong> desorption <strong>of</strong> <strong>the</strong> CO 2 by <strong>the</strong> wall edges (cf. Figure
5.26). The CO 2 inside <strong>the</strong> pores expands as <strong>the</strong> pressure and <strong>the</strong> temperature decreases which results in <strong>the</strong>
growth <strong>of</strong> <strong>the</strong>se pores. This behaviour is mostly related to <strong>the</strong> much plasticized state <strong>of</strong> <strong>the</strong> polymer.
For this reason, we have changed our experimental setup. We have removed <strong>the</strong> Teflon isolation
which was restricting <strong>the</strong> diameter <strong>of</strong> <strong>the</strong> working area inside <strong>the</strong> chamber. We have replaced <strong>the</strong> volume by
small glass balls with 3 mm <strong>of</strong> diameter, and we have placed a metal grill with holes on <strong>the</strong>se glass balls. As
a result, we have gained approximately 50% more area which was sufficiently enough to prevent <strong>the</strong> volume
constraint.
4.8 Interconnectivity and Coalescence Behaviour <strong>of</strong> <strong>the</strong> Scaffolds
Our image analysis on micrographs and calculations shows <strong>the</strong> decrease <strong>of</strong> <strong>the</strong> average pore
density per unit volume, with <strong>the</strong> increasing pore size. This trend is in agreement with <strong>the</strong> literature work
[Barry et al., 2006]. The coalescence phenomenon is <strong>the</strong> master bone <strong>of</strong> <strong>the</strong> interconnectivity <strong>of</strong> <strong>the</strong> pores.
During <strong>the</strong> growth, two pores join to create one, and reduce <strong>the</strong> pore density. This decrease in <strong>the</strong> pore
density confirms <strong>the</strong> bubble coalescence <strong>the</strong>ory proposed in <strong>the</strong> literature [Rodeheaver and Colton, 2001].
Since <strong>the</strong> pore density is decreasing with <strong>the</strong> increasing depressurization time (foaming time), longer
foaming times results in more pores to coalesce. An example to this phenomenon is presented in Figure
5.27-(A). A dP/dt <strong>of</strong> 0.056 bar/s has been applied to create this scaffold and it has been observed that <strong>the</strong>re is
only one giant pore. An o<strong>the</strong>r example in Figure 5.27-(B), dP/dt <strong>of</strong> 1 bar/s was applied to create scaffold but
a collapse <strong>of</strong> large pores was observed in SEM micrograph. On <strong>the</strong> o<strong>the</strong>r hand, we must admit, it is not easy
to interpret <strong>the</strong> interconnectivity behaviour only by SEM micrographs. Thus, fur<strong>the</strong>r investigation is
necessary in order to quantify <strong>the</strong> degree <strong>of</strong> interconnectivity. Procedures like mercury porosimetry or µ-
tomography can be considered for such analysis.
(A)- PLGA 50:50 (B)- PLGA 85:15
P sat = 100 bar, T sat = 36.5°C,
t sat = 60 min. and dP/dt = 0.056 bar /s.
P sat = 200 bar, T sat = 45°C,
t sat = 20 min. and dP/dt = 1 bar /s.
Figure 5.27: Micrographs depicting coalescence and collapse <strong>of</strong> pores.
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