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