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