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

iological performance. Biocompatibility <strong>of</strong> <strong>the</strong> scaffold is critical and must not damage cells and alter <strong>the</strong>ir
functions or lead to significant scarring.
A number <strong>of</strong> biodegradable scaffolds are described in <strong>the</strong> literature. Most <strong>of</strong> <strong>the</strong>se scaffolds come
from <strong>the</strong> family <strong>of</strong> polyesters. Poly(α-hydroxy acids) like poly(lactic acid) (PLA), poly(glycolic acid)
(PGA), and <strong>the</strong> co-polymer, known as poly(lactic-co-glycolic acid) (PLGA) are a part <strong>of</strong> tissue engineering
studies. The common use <strong>of</strong> <strong>the</strong>se polymers is basically related to <strong>the</strong>ir degradation behaviour. Polymer
degradation takes place mostly through scission <strong>of</strong> <strong>the</strong> main chains or side-chains <strong>of</strong> polymer molecules,
induced by <strong>the</strong>ir <strong>the</strong>rmal activation, oxidation, photolysis, radiolysis, or hydrolysis. Some polymers undergo
degradation in biological environments when living cells or microorganisms are present around <strong>the</strong>
polymers. PLA degrades into lactic acid, and PLGA degrades into lactic and glycolic acid. Also, for PLGA,
<strong>the</strong> degradability rate can be controlled by changing <strong>the</strong> co-monomer composition. Fur<strong>the</strong>rmore, PLA and
PLGA are approved by United States Food and Drug Administration for biomedical uses [Ikada and Tsuji,
1999; Steinbüchel, 2003].
Supercritical CO 2 (scCO 2 ) foaming was first proposed by Mooney et al. [1996] to create porous
PLGA and PLA scaffolds by <strong>the</strong> pressure quench method, which was first proposed by Goel and Beckman
[1994] to manufacture microcellular PMMA foams. There have been a number <strong>of</strong> followers, which worked
on foaming <strong>of</strong> biodegradable polymers to create porous scaffolds by this method [Reverchon and Cardea,
2007a; Tsivintzelis et al., 2007a; Quirk et al., 2004a; Goel and Beckman, 1994a, 1995; Khang et al., 2007].
Supercritical CO 2 is a green solvent and this method consists in using CO 2 as a blowing agent for
<strong>the</strong> polymer to create porous material. CO 2 is used because it is relatively non-toxic, relatively inert, and non
combustible. Also, it has relatively reachable critical points (T c = 31°C and P c = 73.8 bars). Thus it can be
used to prepare microcellular foams using supercritical fluids as foaming agents. It has many advantageous
properties, which enable <strong>the</strong>ir use as foaming agents; <strong>the</strong>se include a tuneable solvent power, <strong>the</strong>
plasticization <strong>of</strong> glassy polymers (as a consequence <strong>of</strong> glass transition temperature depression) and enhanced
diffusion rates. The low critical temperature <strong>of</strong> CO 2 allows an easy and complete separation from <strong>the</strong>
polymer, without a vapour-liquid transition during <strong>the</strong> expansion. General principle <strong>of</strong> foaming method is
<strong>the</strong> following: saturation <strong>of</strong> pellets with CO 2 at desired temperature and high pressure, followed by a rapid
depressurization which causes <strong>the</strong> super saturation. As a result <strong>of</strong> <strong>the</strong> super saturation, <strong>the</strong> creation <strong>of</strong> <strong>the</strong>
nuclei occurs and <strong>the</strong> depressurization induced desorption from <strong>the</strong> polymer matrix and <strong>the</strong> phase change <strong>of</strong>
CO 2 provides <strong>the</strong> pore growth.
The solubility <strong>of</strong> CO 2 increases with pressure, which leads to work at supercritical pressures.
Moreover, since <strong>the</strong> critical temperature <strong>of</strong> CO 2 is 31°C, it can be used to process <strong>the</strong>rmally sensitive
materials. Sorption <strong>of</strong> CO 2 into <strong>the</strong> polymers depresses <strong>the</strong>ir glass transition temperature which results in a
polymer/gas solution.
On <strong>the</strong> o<strong>the</strong>r hand, in <strong>the</strong> tissue engineering field, as CO 2 replaces <strong>the</strong> chemical solvents, it
provides <strong>the</strong> complete disappearance <strong>of</strong> <strong>the</strong> residual amounts <strong>of</strong> undesired substances in <strong>the</strong> scaffolds <strong>of</strong>
biomedical use. The manufacturing methods are very important for <strong>the</strong> specific organs because <strong>the</strong>
physicochemical properties <strong>of</strong> scaffold matrices — such as porosity, equivalent pore diameter, and specific
area — are determined by <strong>the</strong> manufacturing methods.
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