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 concentration <strong>of</strong> <strong>the</strong> solution. Structural integrity <strong>of</strong> <strong>the</strong> porous sponge-like structure should also be maintained, even after wetting in <strong>the</strong> interstitial fluid or culture medium. If <strong>the</strong> scaffold is made from watersoluble materials without cross-linkage or if only <strong>the</strong> 3D frame is maintained by <strong>the</strong> ionic interaction, <strong>the</strong>n <strong>the</strong> 3D sponge structure can be easily resolved or turned into a gel-like structure in <strong>the</strong> aqueous environment. This structural integrity in <strong>the</strong> aqueous environment is determined mainly by <strong>the</strong> degree <strong>of</strong> water accessibility within <strong>the</strong> frames <strong>of</strong> <strong>the</strong> scaffold and <strong>the</strong> presence <strong>of</strong> degradation enzymes, which is more importantly regulated by <strong>the</strong> ionic status, water solubility, and innate property <strong>of</strong> <strong>the</strong> biomaterial itself. Therefore, <strong>the</strong> concentration <strong>of</strong> solution, ionic status <strong>of</strong> biomaterials, solvents, and freezing temperature are important factors to be considered in <strong>the</strong> manufacturing <strong>of</strong> suitable tissue engineering scaffolds. The procedure for freezing-drying is (cf. Figure 2.7): (a) (b) The polymer is solubilized in a good solvent. The insoluble materials are removed by filtration through sintered glass filter. The filtered solution is stored overnight at room temperature to remove entrapped air bubbles. The solution is neutralised. The solution is poured into a Teflon-coated mould. The solution is freeze at – 70°C or – 196°C, for 12 h. (c) The solution is freeze-dried by lowering <strong>the</strong> pressure below 7 mTorr, for 48 h. (d) (e) The solution is neutralized by removing excess acid within <strong>the</strong> scaffold by washing with absolute ethanol for 1 h. and with water for 3 h. The scaffold is soaked in <strong>the</strong> solution containing growth factors or matrix proteins. The scaffold is freeze at − 70°C and lyophilised in order to maintain <strong>the</strong> original form <strong>of</strong> <strong>the</strong> scaffold. (a) (b) (c) (d) Introduc Figure 2.7: Schematic preparation processing <strong>of</strong> scaffold by <strong>the</strong> freeze-drying method. [Kim et al., 2007a] 2.8 Thermally Induced Phase Separation (TIPS) Technique Freeze-drying via TIPS has received much attention in industrial applications for <strong>the</strong> production <strong>of</strong> isotropic, highly interconnected, and porosity-designed membranes. Figure 2.8.a represents a schematic temperature–composition phase diagram for a binary polymer/solvent system. Above <strong>the</strong> binodal curve, a single polymer solution phase is formed; and if cooling below <strong>the</strong> curve, polymer-rich and polymer-poor phases are separated in a <strong>the</strong>rmodynamic equilibrium state. The spinodal curve is defined as <strong>the</strong> line at which <strong>the</strong> second derivative Gibbs free energy <strong>of</strong> mixing is equal to zero, and it divides <strong>the</strong> two-phase region into unstable and metastable regions. If <strong>the</strong> system is quenched into <strong>the</strong> metastable region, phase separation occurs in a nucleation and growth mechanism, leading to a bead-like isolated cellular structure. On <strong>the</strong> o<strong>the</strong>r hand, if <strong>the</strong> system temperature is quenched into <strong>the</strong> unstable region, phase separation takes place in a spinodal decomposition mechanism (liquid–liquid phase separation), resulting in a micro-porous interconnected structure. The phase separation and freeze-drying method appears as a versatile technique to prepare highly porous three-dimensional polymer scaffolds that fulfil all <strong>of</strong> <strong>the</strong> requirements for cell transplantation. Porosity can be controlled in terms <strong>of</strong> pore size and morphology by a suitable choice <strong>of</strong> processing conditions and by a strict control <strong>of</strong> phase separation conditions such as quenching temperature, - 34 -
Chapter 2. <strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface quenching depth, ageing time, polymer concentration, molecular weight, solvent/non solvent composition, and additives. The schematic procedure for <strong>the</strong>rmally induced phase separation is given in Figure 2.8. Polylactide solution with a mixture <strong>of</strong> good and bad solvent is prepared [Figure a]. The sample is reheated to 15°C above <strong>the</strong> measured cloud point temperature, and <strong>the</strong>n placed in a water bath preheated to <strong>the</strong> quenching temperature. It is kept for 2, 10, 30, 60 or 120 min, at <strong>the</strong> quenching temperature. The annealed sample is directly immersed in liquid nitrogen to be fast-frozen for 1 h, and <strong>the</strong>n a small hole is cut in <strong>the</strong> vial cap to release <strong>the</strong> solvent [Figure b]. Freeze-drying is performed in a freeze-dryer at −77°C and 7 mTorr for 3 days in order to remove <strong>the</strong> solvent and obtain <strong>the</strong> macroporous scaffolds [Figure c]. The dry scaffolds are cut into cubes with a surgical blade. Prior to cell seeding, 3D scaffolds are pre-wetted with 70% ethanol for 3 h to sterilise <strong>the</strong>m and enhance <strong>the</strong>ir water uptake. The ethanol is removed by soaking with agitation for 1 h in six changes <strong>of</strong> PBS, and <strong>the</strong>n <strong>the</strong> scaffolds are left overnight in <strong>the</strong> culture media. Figure 2.8: Schematic preparation processing <strong>of</strong> <strong>the</strong>rmally induced phase separation method. [Kim and Doo, 2007a] 2.9 Centrifugation Method Recently, a centrifugation method has been introduced as an effective method to fabricate scaffolds that have various shapes with a uniform surface and inside pore structures. The scaffolds can be fabricated in various shapes from many different natural and syn<strong>the</strong>tic polymers by <strong>the</strong> centrifugation method (cf. Figure 2.9). - 35 -
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