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

Chapter 2.
<strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface
The gas-foaming/salt-leaching method is based on <strong>the</strong> idea that sieved salt particles <strong>of</strong> ammonium
bicarbonate dispersed within a polymer–solvent mixture can generate ammonia and carbon dioxide gases
within solidifying matrices upon contact with hot water or aqueous acidic solution, <strong>the</strong>reby producing highly
porous structures (cf. Figure 2.3). These scaffolds show macro-pore structures over 200 μm with no visible
surface skin layers, thus permitting sufficient cell seeding within <strong>the</strong> scaffolds. In addition, porosities can be
controlled by <strong>the</strong> amount <strong>of</strong> ammonium bicarbonate incorporated into <strong>the</strong> polymer solution. It is possible to
make various scaffolds with different geometries and sizes by a hand-shaping or moulding process because
<strong>the</strong> polymer–salt mixture becomes a gel paste after partial evaporation <strong>of</strong> <strong>the</strong> solvent.
Polylactides must be completely dissolved in chlor<strong>of</strong>orm. An excess volume <strong>of</strong> cold ethanol is
<strong>the</strong>n added to <strong>the</strong> polymer solution. The whole is <strong>the</strong>n mixed homogeneously. A gel-like slurry precipitates
immediately in <strong>the</strong> solvent/non solvent mixture. The turbid solution is removed and <strong>the</strong> gel slurry is
recovered. Ammonium bicarbonate is added to <strong>the</strong> solution which is mixed to make a homogeneous gel
paste mixture <strong>of</strong> polymer/salt. A small volume <strong>of</strong> chlor<strong>of</strong>orm can be added to <strong>the</strong> slurry as a plasticizer. The
paste mixture is <strong>the</strong>n casted into a disc–shaped Teflon ® mould or manipulated to <strong>the</strong> desired shape. The gel
paste mixture is dried by partial evaporation <strong>of</strong> ethanol, under atmospheric pressure, to obtain <strong>the</strong> semisolidified
mixture. A polymer/salt complex is removed from <strong>the</strong> mould and wetted with cold ethanol. The
matrix is immersed into supersaturated citric acid solution to effervescence from embedded salt particles.
After complete effervescence, <strong>the</strong> scaffolds are washed with H 2 O <strong>the</strong>n freeze-dried and finally stored at
−80°C with desiccant until use.
Polymer Gel Prepared by
nonsolvent precipitation
Solvent
Semi solidified
Polymer / Salt complex
Polymer
gel paste
Sieved
Ammonium bicarbonate
salt particles
Freeze dry
Gas Forming
In acidic aqueous solution
or hot water
Macroporous
scaffold
2.4 Gel-Pressing Technique
Figure 2.3: Procedure <strong>of</strong> gas foaming/salt-leaching method.
[Park, 2007b]
The particulate-leaching process dissolves <strong>the</strong> polylactide in chlor<strong>of</strong>orm, and <strong>the</strong>n casts it onto a
dish filled with <strong>the</strong> porogen. After evaporation <strong>of</strong> <strong>the</strong> solvent, <strong>the</strong> polymer/salt composite is leached in water
to remove <strong>the</strong> porogen. The process is easy to carry out. The pore size can be controlled by <strong>the</strong> size <strong>of</strong> <strong>the</strong>
salt crystals, and <strong>the</strong> porosity by <strong>the</strong> salt/polymer ratio. However, certain critical variables such as pore
shape, limited membrane thickness (3 mm), plastic operation, and interpore openings are not controllable.
To overcome <strong>the</strong>se shortcomings, a method to fabricate porous, biodegradable scaffolds using a combined
gel-pressing method and salt-leaching technique has been developed (cf. Figure 2.4):
(a)
(b)
(c)
(d)
(e)
A polymer/salt composite is firstly prepared by dissolution process in a solvent.
The polymer is dissolved in a solvent and <strong>the</strong>n mixed with salts.
The solvent is evaporated under air condition to form gels.
Gels are put inside moulds.
Polymer gels are pressed.
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