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
Liquid-liquid phase separation gives rise to scaffolds with porosity up to 90% and an average pore
size <strong>of</strong> 15 to 35 μm depending on <strong>the</strong> processing parameters and <strong>the</strong> <strong>the</strong>rmodynamics <strong>of</strong> <strong>the</strong> polymer/solvent
system. In comparison to <strong>the</strong> previous technique, this method leads to scaffolds with a much larger surface
area. However, <strong>the</strong> overall pore size is smaller, and organic solvents are still required. Both limit <strong>the</strong> use <strong>of</strong>
liquid-liquid phase separation in <strong>the</strong> field <strong>of</strong> bone tissue engineering [van de Witte et al., 1996].
2.12 Solid-Liquid Phase Separation Technique
A polylactide-solvent solution is quenched below <strong>the</strong> melting point <strong>of</strong> <strong>the</strong> solvent and dried under
vacuum to remove <strong>the</strong> solvent by sublimation. Solid–liquid phase separation, with solvent crystallization,
leads to ladder or sheet-like anisotropic morphologies, which strongly depend on <strong>the</strong> quenching rate [Ma
and Choi, 2001; Lo et al., 1995]. The ladder-like structure results from <strong>the</strong> forward progress <strong>of</strong> solvent
crystallization front [Schugens et al., 1996]. When <strong>the</strong> polymer concentration increases, pore diameter and
porosity tend to decrease. Porosity <strong>of</strong> 80−95%, with a pore size mainly between 20 and 100 μm and a
compressive modulus up to 20 MPa in <strong>the</strong> longitudinal direction could be obtained.
This technique was also used to manufacture composite scaffolds, ei<strong>the</strong>r with hydroxyapatite
[Zhang and Ma, 1999b]or Bioglass ® particles [Maquet et al., 2004; Boccaccini and Maquet, 2003]. Fillers
were added to <strong>the</strong> polymer solution before quenching and solvent removal. Similar ladder-like anisotropic
morphology was obtained, becoming more heterogeneous as filler content was increased.
2.13 Fibre Mesh/Fibre Bondong Technique
Fibres, produced by textile technology, have been used to make non-woven scaffolds from PGA
and P L LA [Cima et al., 1991]. The lack <strong>of</strong> structural stability <strong>of</strong> <strong>the</strong>se nonwoven scaffolds, <strong>of</strong>ten resulted in
significant deformation due to contractile forces <strong>of</strong> <strong>the</strong> cells that have been seeded on <strong>the</strong> scaffold. This led
to <strong>the</strong> development <strong>of</strong> a fibre bonding technique to increase <strong>the</strong> mechanical properties <strong>of</strong> <strong>the</strong> scaffolds
[Mikos et al., 1993]. This is achieved by dissolving polylactide in methylene chloride and casting over <strong>the</strong>
polygycolide mesh. The solvent is allowed to evaporate and <strong>the</strong> construct is <strong>the</strong>n heated above <strong>the</strong> melting
point <strong>of</strong> PGA. Once <strong>the</strong> PGA-P L LA construct has cooled, <strong>the</strong> P L LA is removed by dissolving in methylene
chloride again. This treatment results in a mesh <strong>of</strong> PGA fibres joined at <strong>the</strong> cross-points [Sachlos and
Czernuszka, 2003].
Bonded PGA fibre structures with high and open porosity, a high area-to-volume ratio and pore
diameters up to 500 μm were thus produced. These biocompatible matrices, with structural integrity, are
suitable as scaffolds for organ regeneration. In addition, <strong>the</strong> technique does not lend itself to easy and
independent control <strong>of</strong> porosity and pore size. Finally <strong>the</strong> combination <strong>of</strong> toxic chemicals and high
temperature presents difficulties if cells or bioactive molecules, such as growth factors or proteins, are to be
included in <strong>the</strong> scaffold during processing.
2.14 Hydrocarbon Templating Technique
By using a hydrocarbon particulate phase as a template it is also possible to form pore for a wide
range <strong>of</strong> polymers. The use <strong>of</strong> hydrocarbon template allows for enhanced control over pore structure,
porosity, and o<strong>the</strong>r structural and bulk characteristics <strong>of</strong> <strong>the</strong> polymer foam. Polymer foams have been
produced with densities as low as 0.120, porosity as high as 87% and high surface areas (20 m 2 /g). Foams <strong>of</strong>
polylactides produced by this process have been used to engineer a variety <strong>of</strong> different structures, including
tissues with complex geometries such as in <strong>the</strong> likeness <strong>of</strong> a human nose [Gibson and Ashby, 1999; Yoda,
1998; Szycher and Lee, 1992; Guidoin et al., 1988; Suh and Webb, 1988; Alsbjörn, 1984; Pruitt and Levine,
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