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 pore sizes in tissue engineering scaffolds are sufficiently small enough for <strong>the</strong> fibre strand to bridge across without additional support structures. Figure 2.12 shows this system. Figure 2.12: Schematic diagram <strong>of</strong> <strong>the</strong> fused deposition modelling (FDM) system. [Sachlos and Czernuszka, 2003; Scott, 1992] 2.16.1 Three Dimensional Printing (3 DP) This system, developed by researchers at <strong>the</strong> University <strong>of</strong> Freiburg, [Landers and Mülhaupt, 2000] involves a moving extruder head (x-, y- and z-axis control) and uses compressed air to force out a liquid or paste-like plotting medium. The extruder head can be heated to <strong>the</strong> required temperature. The medium solidifies when it comes in contact with <strong>the</strong> substrate or previous layer. Figure 2.13-A shows <strong>the</strong> general principle <strong>of</strong> this system. Figure 2.13: Schematic diagram <strong>of</strong> <strong>the</strong> 3D Bioplotter TM system. [Sachlos and Czernuszka, 2003] In Figure 2.13-B, 3DP incorporates conventional ink jet printing technology (x- and y-axis control) to eject a binder from a jet head, which moves in accordance to <strong>the</strong> CAD cross-sectional data, onto a polymer powder surface [Sachs et al., 1998]. The binder dissolves and joins adjacent powder particles. The piston chamber is lowered (z-axis control) and refilled with ano<strong>the</strong>r layer <strong>of</strong> powder and <strong>the</strong> process is repeated. The unbound powder acts to support overhanging or unconnected features and needs to be removed after component completion. 2.16.2 Stereolithography (SLA) The process involves selective polymerisation <strong>of</strong> a liquid photo-curable monomer by an ultraviolet laser beam [Hull, 1990]. The UV beam is guided (x- and y-axis control) onto <strong>the</strong> liquid monomer surface in accordance to <strong>the</strong> CAD cross-sectional data. After <strong>the</strong> first layer is built, <strong>the</strong> elevator holding <strong>the</strong> model is lowered into <strong>the</strong> vat (z-axis control) so as to allow <strong>the</strong> liquid photopolymer to cover <strong>the</strong> surface. A wiper - 40 -
Chapter 2. <strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface arm is <strong>the</strong>n displaced over <strong>the</strong> liquid to flatten <strong>the</strong> surface. The procedure is repeated until <strong>the</strong> model is completed. This system requires support structures to be added to <strong>the</strong> model, to prevent any overhanging or unconnected features from falling to <strong>the</strong> bottom <strong>of</strong> <strong>the</strong> liquid-filled vat. After completion, <strong>the</strong> model is raised and any support structures are removed manually. Figure 2.14 shows <strong>the</strong> principle <strong>of</strong> this system. Figure 2.14: Schematic diagram <strong>of</strong> <strong>the</strong> Stereolithography (SLA) system. [Sachlos and Czernuszka, 2003] 2.17 O<strong>the</strong>r Derivated Techniques Each <strong>of</strong> <strong>the</strong> techniques presented has its advantages, but none can be considered as ideal processing method for a scaffold to be employed for all tissues [Murphy and Mikos, 2007]. Macro-porous polymeric foams have been produced by dispersion <strong>of</strong> a gaseous phase in a fluid polymer phase, leaching <strong>of</strong> water-soluble inorganic fugitive phase, phase separation, polymer precipitation, particle sintering, extrusion, and injection moulding [Gibson and Ashby, 1999; Suh and Webb, 1988; Frisch and Saunders, 1972]. Air drying phase inversion has been also tested [Park et al., 1997]. However, <strong>the</strong>se processes do not generally <strong>of</strong>fer optimal control over pore structure (cell diameter and pore interconnectivity) and bulk characteristics (density, void volume, mechanical and electrical properties). Since P L LA is partially crystalline, it is not possible to obtain uniform porous structures with <strong>the</strong> gas foaming technique at temperatures lower than <strong>the</strong> melting point [Quirk et al., 2004]. 2.17.1 Combination <strong>of</strong> Leaching <strong>of</strong> a Fugitive Phase and Polymer Precipitation By combining two distinct foaming processes, (i) leaching <strong>of</strong> a fugitive phase with (ii) polymer precipitation, one could attain enhanced control over both porosity and bulk properties <strong>of</strong> polymer foam. This was achieved by using a non-water-soluble particulate hydrocarbon fugitive phase derived from waxes, which allowed for <strong>the</strong> formation <strong>of</strong> pores with concomitant precipitation <strong>of</strong> <strong>the</strong> polymer phase. The macroporosity <strong>of</strong> <strong>the</strong> polymer foam was determined by <strong>the</strong> hydrocarbon fugitive phase (porogen), which also functioned as a template for <strong>the</strong> rapid precipitation <strong>of</strong> <strong>the</strong> polymer. Bulk properties <strong>of</strong> <strong>the</strong> foam could be manipulated independently <strong>of</strong> <strong>the</strong> macro-porosity and pore size by incorporation <strong>of</strong> inorganic and organic fillers into <strong>the</strong> highly viscous polymer phase. The process is applicable to a wide range <strong>of</strong> polymer systems including water-soluble polymers, as long as <strong>the</strong> following conditions are satisfied [Shastri et al., 2000]: - The hydrocarbon porogen is extracted below <strong>the</strong> melting temperature <strong>of</strong> <strong>the</strong> polymer, to ensure isotropy in <strong>the</strong> properties <strong>of</strong> <strong>the</strong> resulting foam. - The polymer has good solubility (at least 100 mg/ml) in a solvent that is a poor solvent for <strong>the</strong> porogen, to obtain a viscous polymer solution wherein <strong>the</strong> porogen can be distributed uniformly. - 41 -
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