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