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

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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, - 38 -
Chapter 2. <strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface 1984; Bruck, 1982; Lindenauer et al., 1976].A schematic representation <strong>of</strong> <strong>the</strong> steps involved in <strong>the</strong> preparation <strong>of</strong> polymeric foams is given in Figure 2.11. Step 1: The polymer is dissolved in a suitable solvent and <strong>the</strong>n mixed with <strong>the</strong> hydrocarbon porogen (e.g. paraffin, beeswax, bonewax) to yield a mouldable mixture. Step 2: This mixture is <strong>the</strong>n compacted in a Teflon ® mould. Step 3: The polymer/porogen mixture in <strong>the</strong> mould is <strong>the</strong>n immersed in an aliphatic hydrocarbon solvent (pentane, hexane), which is a nonsolvent for <strong>the</strong> polymer. During this step, <strong>the</strong> porogen and polymer solvent are extracted with concurrent precipitation <strong>of</strong> <strong>the</strong> polymer phase. To improve <strong>the</strong> efficiency <strong>of</strong> solvent penetration, <strong>the</strong> mould is equipped with small openings on all faces. Residual porogen is removed by repeating <strong>the</strong> last step. The foam obtained is <strong>the</strong>n dried under vacuum to remove any trace <strong>of</strong> solvents. Figure 2.11: Schematic stepwise representation <strong>of</strong> <strong>the</strong> polymeric foaming using hydrocarbon porogen. [Shastri et al., 2000] 2.15 Microspheres Bonding Technique PLGA has been also sintered with microspheres <strong>of</strong> varying sizes [Borden et al., 2003]. These authors manufactured foams with porosity ranging between 30 and 40%, and an elastic modulus ranging beween 135 and 300 MPa. They presented <strong>the</strong>ir scaffold as a reverse template <strong>of</strong> trabecular bone, since scaffold resorption would leave a porosity <strong>of</strong> about 70%, corresponding to bone void volume. 2.16 Rapid Prototyping Techniques Rapid prototyping technologies aim at producing complex free-form parts directly from a computer aided design model. 2D printing or 3D prototyping and fused deposition modelling were tested to obtain porous structures in <strong>the</strong> biomedical field. 3D prototyping consists <strong>of</strong> printing a binder through a print head nozzle onto a powder bead [Cima et al., 1991]. Removing <strong>the</strong> excess powder leads to <strong>the</strong> porous structure. The part is built sequentially in layers, at room temperature. The main problem with bioresorbable polymers is <strong>the</strong> use <strong>of</strong> organic solvents. In <strong>the</strong> fused deposition modelling process, parts are also fabricated in layers, where a layer is built by extruding <strong>the</strong> material in a particular lay-down pattern, directly defined from a computer aided design (CAD) file [Hutmacher, 2000]. A drawback <strong>of</strong> <strong>the</strong>se techniques is that <strong>the</strong>y are cost-effective, and require <strong>the</strong> use <strong>of</strong> complex and specific equipment. Fused deposition modelling uses a moving nozzle to extrude a fibre <strong>of</strong> polymeric material (x- and y-axis control) from which <strong>the</strong> physical model is built layer-by-layer. The model is lowered (z-axis control) and <strong>the</strong> procedure repeated. Although <strong>the</strong> fibre must also produce external structures to support overhanging or unconnected features that need to be manually removed, <strong>the</strong> - 39 -
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