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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iological performance. Biocompatibility <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> scaffold is critical and must not damage cells and alter <str<strong>on</strong>g>the</str<strong>on</strong>g>ir<br />
functi<strong>on</strong>s or lead to significant scarring.<br />
A number <str<strong>on</strong>g>of</str<strong>on</strong>g> biodegradable scaffolds are described in <str<strong>on</strong>g>the</str<strong>on</strong>g> literature. Most <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g>se scaffolds come<br />
from <str<strong>on</strong>g>the</str<strong>on</strong>g> family <str<strong>on</strong>g>of</str<strong>on</strong>g> polyesters. Poly(α-hydroxy acids) like poly(lactic acid) (PLA), poly(glycolic acid)<br />
(PGA), and <str<strong>on</strong>g>the</str<strong>on</strong>g> co-polymer, known as poly(lactic-co-glycolic acid) (PLGA) are a part <str<strong>on</strong>g>of</str<strong>on</strong>g> tissue engineering<br />
studies. <strong>The</strong> comm<strong>on</strong> use <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g>se polymers is basically related to <str<strong>on</strong>g>the</str<strong>on</strong>g>ir degradati<strong>on</strong> behaviour. Polymer<br />
degradati<strong>on</strong> takes place mostly through scissi<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> main chains or side-chains <str<strong>on</strong>g>of</str<strong>on</strong>g> polymer molecules,<br />
induced by <str<strong>on</strong>g>the</str<strong>on</strong>g>ir <str<strong>on</strong>g>the</str<strong>on</strong>g>rmal activati<strong>on</strong>, oxidati<strong>on</strong>, photolysis, radiolysis, or hydrolysis. Some polymers undergo<br />
degradati<strong>on</strong> in biological envir<strong>on</strong>ments when living cells or microorganisms are present around <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
polymers. PLA degrades into lactic acid, and PLGA degrades into lactic and glycolic acid. Also, for PLGA,<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> degradability rate can be c<strong>on</strong>trolled by changing <str<strong>on</strong>g>the</str<strong>on</strong>g> co-m<strong>on</strong>omer compositi<strong>on</strong>. Fur<str<strong>on</strong>g>the</str<strong>on</strong>g>rmore, PLA and<br />
PLGA are approved by United States Food and Drug Administrati<strong>on</strong> for biomedical uses [Ikada and Tsuji,<br />
1999; Steinbüchel, 2003].<br />
Supercritical CO 2 (scCO 2 ) foaming was first proposed by Mo<strong>on</strong>ey et al. [1996] to create porous<br />
PLGA and PLA scaffolds by <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure quench method, which was first proposed by Goel and Beckman<br />
[1994] to manufacture microcellular PMMA foams. <strong>The</strong>re have been a number <str<strong>on</strong>g>of</str<strong>on</strong>g> followers, which worked<br />
<strong>on</strong> foaming <str<strong>on</strong>g>of</str<strong>on</strong>g> biodegradable polymers to create porous scaffolds by this method [Reverch<strong>on</strong> and Cardea,<br />
2007a; Tsivintzelis et al., 2007a; Quirk et al., 2004a; Goel and Beckman, 1994a, 1995; Khang et al., 2007].<br />
Supercritical CO 2 is a green solvent and this method c<strong>on</strong>sists in using CO 2 as a blowing agent for<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> polymer to create porous material. CO 2 is used because it is relatively n<strong>on</strong>-toxic, relatively inert, and n<strong>on</strong><br />
combustible. Also, it has relatively reachable critical points (T c = 31°C and P c = 73.8 bars). Thus it can be<br />
used to prepare microcellular foams using supercritical fluids as foaming agents. It has many advantageous<br />
properties, which enable <str<strong>on</strong>g>the</str<strong>on</strong>g>ir use as foaming agents; <str<strong>on</strong>g>the</str<strong>on</strong>g>se include a tuneable solvent power, <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
plasticizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> glassy polymers (as a c<strong>on</strong>sequence <str<strong>on</strong>g>of</str<strong>on</strong>g> glass transiti<strong>on</strong> temperature depressi<strong>on</strong>) and enhanced<br />
diffusi<strong>on</strong> rates. <strong>The</strong> low critical temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 allows an easy and complete separati<strong>on</strong> from <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
polymer, without a vapour-liquid transiti<strong>on</strong> during <str<strong>on</strong>g>the</str<strong>on</strong>g> expansi<strong>on</strong>. General principle <str<strong>on</strong>g>of</str<strong>on</strong>g> foaming method is<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> following: saturati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> pellets with CO 2 at desired temperature and high pressure, followed by a rapid<br />
depressurizati<strong>on</strong> which causes <str<strong>on</strong>g>the</str<strong>on</strong>g> super saturati<strong>on</strong>. As a result <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> super saturati<strong>on</strong>, <str<strong>on</strong>g>the</str<strong>on</strong>g> creati<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><br />
nuclei occurs and <str<strong>on</strong>g>the</str<strong>on</strong>g> depressurizati<strong>on</strong> induced desorpti<strong>on</strong> from <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer matrix and <str<strong>on</strong>g>the</str<strong>on</strong>g> phase change <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
CO 2 provides <str<strong>on</strong>g>the</str<strong>on</strong>g> pore growth.<br />
<strong>The</strong> solubility <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 increases with pressure, which leads to work at supercritical pressures.<br />
Moreover, since <str<strong>on</strong>g>the</str<strong>on</strong>g> critical temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 is 31°C, it can be used to process <str<strong>on</strong>g>the</str<strong>on</strong>g>rmally sensitive<br />
materials. Sorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 into <str<strong>on</strong>g>the</str<strong>on</strong>g> polymers depresses <str<strong>on</strong>g>the</str<strong>on</strong>g>ir glass transiti<strong>on</strong> temperature which results in a<br />
polymer/gas soluti<strong>on</strong>.<br />
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, in <str<strong>on</strong>g>the</str<strong>on</strong>g> tissue engineering field, as CO 2 replaces <str<strong>on</strong>g>the</str<strong>on</strong>g> chemical solvents, it<br />
provides <str<strong>on</strong>g>the</str<strong>on</strong>g> complete disappearance <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> residual amounts <str<strong>on</strong>g>of</str<strong>on</strong>g> undesired substances in <str<strong>on</strong>g>the</str<strong>on</strong>g> scaffolds <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
biomedical use. <strong>The</strong> manufacturing methods are very important for <str<strong>on</strong>g>the</str<strong>on</strong>g> specific organs because <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
physicochemical properties <str<strong>on</strong>g>of</str<strong>on</strong>g> scaffold matrices — such as porosity, equivalent pore diameter, and specific<br />
area — are determined by <str<strong>on</strong>g>the</str<strong>on</strong>g> manufacturing methods.<br />
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