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
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.
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