Emulsion Freeze Drying

f) Emulsion Freeze Drying:
This technique is based on immiscibility, like with water and oil, among some solvents, which can be called solvent non-solvent pairs. In short, the polymer is dissolved into a solvent and then a non-solvent is added. The solu-tion is then mixed well to form an emulsion mixture. This mixture is poured into a mold and quenched under low temperature or by using liquid nitrogen. The frozen mixture undergoes a freeze-drying process to remove both the sol-vent and non-solvent. The advantages of this process are that greater than 90% porosity with pore sizes ranging from 15 —200 µm can be obtained. The pores are highly interconnected which is good for nutrient supply, metabolic waste clearance, cellular ingrowth, and vascularization. However, this technique is user-, equipment-, and technique-sensitive and the processing parameters have to be well controlled.

g) Phase Separation or Phase Inversion:
As opposed to the emulsion freeze-drying technique, phase separation is based on the miscibility of the solvent and non-solvent pair of the polymer. Briefly, the polymer is dissolved into a solvent, and then the polymer solution is delivered to a non-solvent bath and the polymer is precipitated to form membranes, microspheres, or porous scaffolds. This technique has been exten-sively applied to fabricate hollow fiber membranes and microspheres, which allows for the incorporation of pharmaceutical and biological agents and even cells and tissues into the membranes and microspheres. By varying several fab-rication parameters, such as the solvent non-solvent pair, polymer concentra-tion, temperature, small molecular additives, and flow rate, different micro-and macro-structures can be obtained. However, similar to the emulsion freeze-drying technique, the phase separation technique is user-, equipment-and technique-sensitive and the processing parameters must be well controlled. Combining this technique with gelatin, solvent exchange, and freeze-drying techniques, highly porous ( up to 98% ), three dimensional, nano-size fibrous matrices can be produced. Another interesting variation of this technique is using water-soluble filaments, such as sugar filaments, to fabricate three-dimensional scaffolds with a nano-scale fibril structure and interconnected macroporous channels, which may be useful for eliciting cell migration deep into the matrix and providing for better transport properties of the scaffold .