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

Chapter 5.
Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering
mention that when <strong>the</strong> number <strong>of</strong> pore increases with <strong>the</strong> increasing pressure, <strong>the</strong> pore size will absolutely
decrease (cf. Figure 5.22).
(A)-P sat = 100 bar/s
(B)-P sat = 300 bar/s
PLGA 85:15 , d e = 264 μm
PLGA 85:15 ,. d e = 89μm
Scaffold Manufactured at, T sat = 35°C ; t sat = 60 min ; dP/dt= 1 bar/s
Figure 5.22: Micrographs revealing <strong>the</strong> effect <strong>of</strong> P sat on pore size.
The number <strong>of</strong> <strong>the</strong> pores increases with <strong>the</strong> augmentation <strong>of</strong> <strong>the</strong> P sat as at higher pressure more
scCO 2 is saturated in <strong>the</strong> polymer thus size <strong>of</strong> <strong>the</strong> pores decreases.
4.4 Effect <strong>of</strong> Saturation Temperature (T sat )
In first experiments concerning <strong>the</strong> complete 2 4 design, we observed a decrease <strong>of</strong> pore diameter
as <strong>the</strong> temperature is increasing from 36.5 to 60°C. This trend was unexpected because <strong>of</strong> <strong>the</strong> decreasing
density <strong>of</strong> CO 2 at higher temperatures which must have lead to lower nucleation rates, consequently greater
pore diameter. It is also not in agreement with experimental results <strong>of</strong> literature [Tsivintzelis et al., 2007b].
According to <strong>the</strong>se authors, <strong>the</strong> pore sizes <strong>of</strong> <strong>the</strong> scaffolds increase with increasing saturation temperature.
They relate it to <strong>the</strong> decrease <strong>of</strong> <strong>the</strong> energy barrier <strong>of</strong> nucleation. On <strong>the</strong> o<strong>the</strong>r hand, we have observed
ano<strong>the</strong>r behaviour in Taguchi design experiments. Pore diameter <strong>of</strong> <strong>the</strong> PLGA 50:50 scaffolds decreased from
36.5 to 45°C, where it reached a minimum and increased until 60°C. We believe that, since <strong>the</strong> diffusion
coefficient is related to <strong>the</strong> temperature, <strong>the</strong>re must be a competition between <strong>the</strong> decreasing solubility and
increasing diffusion <strong>of</strong> CO 2 with temperature to yield <strong>the</strong> nucleation rate.
As mentioned by Krause et al. [2001], it exists an optimal foaming region which describes <strong>the</strong>
number <strong>of</strong> nuclei which starts with a T lower and exponentially increases with T, <strong>the</strong>n, reaches a maximum in
T max and shows a decreasing behaviour until T upper . Between T lower and T max <strong>the</strong> increasing effect <strong>of</strong> sorptiondiffusion
is dominant on <strong>the</strong> decreasing solubility <strong>of</strong> CO 2 . Hence, <strong>the</strong> number <strong>of</strong> nuclei is increasing as <strong>the</strong>
temperature increased. Once T max is reached, temperature is so high that <strong>the</strong> decreasing solubility effect is
becoming high enough to decrease <strong>the</strong> nucleation rate and consequently <strong>the</strong> generated number <strong>of</strong> nuclei
decreases which means that <strong>the</strong> pore size decreases. An experimental result is presented in Figure 5.23.
(A)-T sat = 35 o C
(B)-T sat = 60 o C
PLGA 85:15 , d e = 264μm
PLGA 85:15 , d e = 187 μm
Scaffold processed at, P sat = 100 bar ; t sat = 60 min ; dP/dt = 1 bar/s
Figure 5.23: Micrographs revealing <strong>the</strong> effect <strong>of</strong> T sat on pore size.
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