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 />
arm is <str<strong>on</strong>g>the</str<strong>on</strong>g>n displaced over <str<strong>on</strong>g>the</str<strong>on</strong>g> liquid to flatten <str<strong>on</strong>g>the</str<strong>on</strong>g> surface. <strong>The</strong> procedure is repeated until <str<strong>on</strong>g>the</str<strong>on</strong>g> model is<br />
completed. This system requires support structures to be added to <str<strong>on</strong>g>the</str<strong>on</strong>g> model, to prevent any overhanging or<br />
unc<strong>on</strong>nected features from falling to <str<strong>on</strong>g>the</str<strong>on</strong>g> bottom <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> liquid-filled vat. After completi<strong>on</strong>, <str<strong>on</strong>g>the</str<strong>on</strong>g> model is raised<br />
and any support structures are removed manually. Figure 2.14 shows <str<strong>on</strong>g>the</str<strong>on</strong>g> principle <str<strong>on</strong>g>of</str<strong>on</strong>g> this system.<br />
Figure 2.14: Schematic diagram <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Stereolithography (SLA) system.<br />
[Sachlos and Czernuszka, 2003]<br />
2.17 O<str<strong>on</strong>g>the</str<strong>on</strong>g>r Derivated Techniques<br />
Each <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> techniques presented has its advantages, but n<strong>on</strong>e can be c<strong>on</strong>sidered as ideal<br />
processing method for a scaffold to be employed for all tissues [Murphy and Mikos, 2007]. Macro-porous<br />
polymeric foams have been produced by dispersi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a gaseous phase in a fluid polymer phase, leaching <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
water-soluble inorganic fugitive phase, phase separati<strong>on</strong>, polymer precipitati<strong>on</strong>, particle sintering, extrusi<strong>on</strong>,<br />
and injecti<strong>on</strong> moulding [Gibs<strong>on</strong> and Ashby, 1999; Suh and Webb, 1988; Frisch and Saunders, 1972]. Air<br />
drying phase inversi<strong>on</strong> has been also tested [Park et al., 1997]. However, <str<strong>on</strong>g>the</str<strong>on</strong>g>se processes do not generally<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g>fer optimal c<strong>on</strong>trol over pore structure (cell diameter and pore interc<strong>on</strong>nectivity) and bulk characteristics<br />
(density, void volume, mechanical and electrical properties). Since P L LA is partially crystalline, it is not<br />
possible to obtain uniform porous structures with <str<strong>on</strong>g>the</str<strong>on</strong>g> gas foaming technique at temperatures lower than <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
melting point [Quirk et al., 2004].<br />
2.17.1 Combinati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Leaching <str<strong>on</strong>g>of</str<strong>on</strong>g> a Fugitive Phase and Polymer Precipitati<strong>on</strong><br />
By combining two distinct foaming processes, (i) leaching <str<strong>on</strong>g>of</str<strong>on</strong>g> a fugitive phase with (ii) polymer<br />
precipitati<strong>on</strong>, <strong>on</strong>e could attain enhanced c<strong>on</strong>trol over both porosity and bulk properties <str<strong>on</strong>g>of</str<strong>on</strong>g> polymer foam.<br />
This was achieved by using a n<strong>on</strong>-water-soluble particulate hydrocarb<strong>on</strong> fugitive phase derived from waxes,<br />
which allowed for <str<strong>on</strong>g>the</str<strong>on</strong>g> formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> pores with c<strong>on</strong>comitant precipitati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer phase. <strong>The</strong> macroporosity<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer foam was determined by <str<strong>on</strong>g>the</str<strong>on</strong>g> hydrocarb<strong>on</strong> fugitive phase (porogen), which also<br />
functi<strong>on</strong>ed as a template for <str<strong>on</strong>g>the</str<strong>on</strong>g> rapid precipitati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer. Bulk properties <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> foam could be<br />
manipulated independently <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> macro-porosity and pore size by incorporati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> inorganic and organic<br />
fillers into <str<strong>on</strong>g>the</str<strong>on</strong>g> highly viscous polymer phase.<br />
<strong>The</strong> process is applicable to a wide range <str<strong>on</strong>g>of</str<strong>on</strong>g> polymer systems including water-soluble polymers, as<br />
l<strong>on</strong>g as <str<strong>on</strong>g>the</str<strong>on</strong>g> following c<strong>on</strong>diti<strong>on</strong>s are satisfied [Shastri et al., 2000]:<br />
- <strong>The</strong> hydrocarb<strong>on</strong> porogen is extracted below <str<strong>on</strong>g>the</str<strong>on</strong>g> melting temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer, to ensure<br />
isotropy in <str<strong>on</strong>g>the</str<strong>on</strong>g> properties <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> resulting foam.<br />
- <strong>The</strong> polymer has good solubility (at least 100 mg/ml) in a solvent that is a poor solvent for <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
porogen, to obtain a viscous polymer soluti<strong>on</strong> wherein <str<strong>on</strong>g>the</str<strong>on</strong>g> porogen can be distributed uniformly.<br />
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