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

Chapter 5.
Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering
The primary approximation <strong>of</strong> this model is that <strong>the</strong> pressure difference between <strong>the</strong> two sides <strong>of</strong>
<strong>the</strong> bubble interface is equal to <strong>the</strong> pressure initial and final pressures <strong>of</strong> <strong>the</strong> chamber before and after
depressurization. On <strong>the</strong> o<strong>the</strong>r hand <strong>the</strong> energy barrier to create nuclei decreases with <strong>the</strong> increasing pressure
difference and it is represented in Figure 5.19-B. The exponentially decrease <strong>of</strong> <strong>the</strong> energy barrier with <strong>the</strong>
pressure means that more nuclei can be generated at high pressures. Since <strong>the</strong> interfacial tension is an
affecting parameter <strong>of</strong> <strong>the</strong> ΔG, it has a great influence on <strong>the</strong> nuclei density <strong>of</strong> <strong>the</strong> polymer. The plateau after
150 bars can be explained by <strong>the</strong> smaller pore size (great number <strong>of</strong> pore) at higher pressure.
As shown in Figure 5.19, <strong>the</strong> model is in agreement with <strong>the</strong> experimental data for <strong>the</strong> pressures
greater than 100 bars and diverges significantly for 80 bars where <strong>the</strong> number <strong>of</strong> nuclei is smaller than that
<strong>of</strong> <strong>the</strong> greater pressures. One must remember that this model is considering <strong>the</strong> homogenous nucleation
<strong>the</strong>ory. Thus, this divergence for low pressures can be attributed to <strong>the</strong> heterogeneous nucleation and <strong>the</strong>
coalescence <strong>of</strong> <strong>the</strong> growing pores at lower pressures. Indeed, at low pressures, <strong>the</strong> sorption <strong>of</strong> CO 2 into <strong>the</strong>
PLGA 50:50 is also lower and CO 2 is not completely distributed across <strong>the</strong> pellet by <strong>the</strong> sorption-diffusion.
(A)
(B)
Figure 5.19: (A) Variation <strong>of</strong> pore diameter <strong>of</strong> <strong>the</strong> PLGA 50:50 scaffolds as <strong>the</strong> function <strong>of</strong> P sat ;
(B) Variation <strong>of</strong> <strong>the</strong> energy barrier for PLGA 50:50 -CO 2 system.
By using <strong>the</strong> matrix formula: X s = - 0.5 A -1 a k where a k is <strong>the</strong> vector <strong>of</strong> <strong>the</strong> first order coefficients and
A <strong>the</strong> matrix <strong>of</strong> <strong>the</strong> second order coefficients, reported on <strong>the</strong> Table 5.23, we obtain <strong>the</strong> following values: As
both coefficients a 11 and a 22 are positive, <strong>the</strong> stationary point is a minimum. The pressure P sat = 100 bars and
<strong>the</strong> depressurization rate dP/dt = 0.625 bar/s have been chosen as <strong>the</strong> parameters which give <strong>the</strong> maximum.
The diameter <strong>of</strong> pores, at this point, is equal to 250 m.
Table 5.23: Analysis <strong>of</strong> <strong>the</strong> Doehlert design: research <strong>of</strong> <strong>the</strong> optimum <strong>of</strong> <strong>the</strong> pore dimension.
Reduced
natural
a k = - 74.4 A = 87.0 10.25 X 1 - 1 0.625 dP/dt
- 44.3 10.25 32.3 X 2 - 0.87 100 P sat
Results obtained from our experimentations has shown that when pore size is large higher porosity
is observed while pores with small pore diameter reflects small porosity. Beckman, 2004 have also shown
similar behaviours in his experimentations . As per our understanding , it is impossible to create pores <strong>of</strong>
large diameter with small polymer volume or small pores with important polymer swelling.
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