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 />
4 Factors Affecting <strong>on</strong> Pores Size and Porosity<br />
4.1 Effect <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Polymer Compositi<strong>on</strong><br />
<strong>The</strong> Hildebrand’s solubility parameters can be separated into three Hansen’ comp<strong>on</strong>ents by <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
relati<strong>on</strong>ship given in following equati<strong>on</strong> 5.1.<br />
δ t 2 = δ d 2 +δ p 2 +δ H<br />
2<br />
(5. 1)<br />
where δ t is <str<strong>on</strong>g>the</str<strong>on</strong>g> Hildebrand’s parameter (cf.Table 5.24), δ d is <str<strong>on</strong>g>the</str<strong>on</strong>g> dispersive comp<strong>on</strong>ent, δ p is <str<strong>on</strong>g>the</str<strong>on</strong>g> polar<br />
comp<strong>on</strong>ent and δ H is <str<strong>on</strong>g>the</str<strong>on</strong>g> hydrogen b<strong>on</strong>d comp<strong>on</strong>ent [Risanen, 2010; Schenderlein et al., 2004].<br />
In <str<strong>on</strong>g>the</str<strong>on</strong>g> case <str<strong>on</strong>g>of</str<strong>on</strong>g> supercritical fluids <str<strong>on</strong>g>the</str<strong>on</strong>g> relati<strong>on</strong>ship linking <str<strong>on</strong>g>the</str<strong>on</strong>g> Hildebrand’ parameter to <str<strong>on</strong>g>the</str<strong>on</strong>g> P c<br />
critical pressure is <str<strong>on</strong>g>the</str<strong>on</strong>g> following [Li. and Perrut, 1991].<br />
= 1,25 Pc / l (5.2)<br />
where and l are corresp<strong>on</strong>ding to <str<strong>on</strong>g>the</str<strong>on</strong>g> density to <str<strong>on</strong>g>the</str<strong>on</strong>g> fluid in <str<strong>on</strong>g>the</str<strong>on</strong>g>ir supercritical and liquid state<br />
respectively. ScCO 2 is a apolar fluid (14,3 < < 18,4).<br />
Table 5.24: Hidebrand’ and Hansen’ parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> PLA and <str<strong>on</strong>g>the</str<strong>on</strong>g> PGA in (MPa) 1/2 .<br />
d p H t t *<br />
PLA 18.5 9.7 6.0 21.7 20.2<br />
PGA 11.7 6.21 12.5 18.2 24.8<br />
*Calculati<strong>on</strong>s with <str<strong>on</strong>g>the</str<strong>on</strong>g> Small’ group c<strong>on</strong>tributi<strong>on</strong> method.<br />
Normally both amorphous PLAs and PLGAs produce scaffolds <str<strong>on</strong>g>of</str<strong>on</strong>g> higher porosity and large pore<br />
diameters. Increase in <str<strong>on</strong>g>the</str<strong>on</strong>g> LA c<strong>on</strong>tent in <str<strong>on</strong>g>the</str<strong>on</strong>g> PLGA co-polymer increases <str<strong>on</strong>g>the</str<strong>on</strong>g> solubility <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 in <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
polymers. <strong>The</strong> solubility <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 is always higher in all poly D,L-lactides than poly(lactides co-glycolide).<br />
<strong>The</strong> extra apolar group in polylactide acid is resp<strong>on</strong>sible for higher solubility in <str<strong>on</strong>g>the</str<strong>on</strong>g> polylactide which<br />
eventually generates highly porous foams depending up<strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> process c<strong>on</strong>diti<strong>on</strong>s. According to Liu and<br />
Tomasko [2007b] <str<strong>on</strong>g>the</str<strong>on</strong>g> extra apolar group which is not present in glycolic acid, leades to two opposite and<br />
different phenomen<strong>on</strong>. <strong>The</strong> effect <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 interacti<strong>on</strong> with <str<strong>on</strong>g>the</str<strong>on</strong>g> carb<strong>on</strong>yl group <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer is decreased<br />
due to <str<strong>on</strong>g>the</str<strong>on</strong>g> apolar group first and sec<strong>on</strong>dly, more available free volume for CO 2 to solubilise is created.<br />
Kazarian et al. [1996b] has also found that <str<strong>on</strong>g>the</str<strong>on</strong>g> interacti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 with polymers can also be explained by<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> CO 2 behaviour as a Lewis acid, an electr<strong>on</strong> pair acceptor.<br />
LA/GA ratio <str<strong>on</strong>g>of</str<strong>on</strong>g> a PLGA co-polymer is an important parameter to c<strong>on</strong>trol <str<strong>on</strong>g>the</str<strong>on</strong>g> pore diameter in a<br />
foaming process. Foaming <str<strong>on</strong>g>of</str<strong>on</strong>g> PLGA with different LA and GA c<strong>on</strong>tents, has given different results. In our<br />
study, with <str<strong>on</strong>g>the</str<strong>on</strong>g> same foaming c<strong>on</strong>diti<strong>on</strong>s (P sat , T sat , t sat , dT/dt), we have experienced different pore size<br />
behaviours when processed with rapid or slow depressurizati<strong>on</strong>s. Sec<strong>on</strong>dly, low pores size can be due to<br />
high saturati<strong>on</strong> pressure and low saturati<strong>on</strong> temperature with comparis<strong>on</strong> to <str<strong>on</strong>g>the</str<strong>on</strong>g> T g <str<strong>on</strong>g>of</str<strong>on</strong>g> both polymers (cf.<br />
Table 5.5).<br />
4.2 Effect <str<strong>on</strong>g>of</str<strong>on</strong>g> Depressurizati<strong>on</strong> Rates<br />
For low depressurizati<strong>on</strong> rates (0.625 to 1.25 bar/s), when <str<strong>on</strong>g>the</str<strong>on</strong>g> lactic acid c<strong>on</strong>tent increases in<br />
PLGA, <str<strong>on</strong>g>the</str<strong>on</strong>g> pore size increases as well. This behaviour can be attributed to <str<strong>on</strong>g>the</str<strong>on</strong>g> greater capacity <str<strong>on</strong>g>of</str<strong>on</strong>g> solubilized<br />
CO 2 inside <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer with <str<strong>on</strong>g>the</str<strong>on</strong>g> increasing amount <str<strong>on</strong>g>of</str<strong>on</strong>g> LA. Actually, <strong>on</strong>e can expect that since <str<strong>on</strong>g>the</str<strong>on</strong>g> CO 2<br />
c<strong>on</strong>centrati<strong>on</strong> is greater in a high lactic acid c<strong>on</strong>taining PLGA, <str<strong>on</strong>g>the</str<strong>on</strong>g> nucleati<strong>on</strong> rate must be greater (which<br />
means lower pore size). However, even if <str<strong>on</strong>g>the</str<strong>on</strong>g> number <str<strong>on</strong>g>of</str<strong>on</strong>g> pores is determined by <str<strong>on</strong>g>the</str<strong>on</strong>g> saturati<strong>on</strong> period and <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
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