Emulsion Freeze Drying

Emulsion freeze-drying is mainly used for the preparation of soft tissue scaffolds [222, 223]. The basic principle of emulsion freeze drying is to form a water-in-oil emulsion by homogenisation of two immiscible phases, in which the continuous phase contains the polymer-rich solvent and the dispersed phase is water [224]. Next, the homogeneous emulsion is quenched in liquid nitrogen to maintain a liquid structure and then freeze-dried to produce a porous structure with a pore size of 20-200 pm and a porosity of more than 90% [221, 225]. The porosity and pore size depend on the concentration and viscosity of the polymer solution and the amount of aqueous phase dispersed in the emulsion. The average pore size also decreases as the volume of the aqueous phase decreases. However, the dispersed aqueous phase is exposed to higher shear forces when the polymer concentration increases in the continuous phase, which reduces the size of the small water domains, resulting in materials with lower porosity and smaller pores. More uniform porous scaffolds, including the reinforcing scaffold, can be manufactured using a constant cooling rate freezing techniques. For example, Shemesh et al. [226] developed and studied the bioresorbable hybrid structures that combined a synthetic porous drug-loaded top layer with a spongy collagen sublayer made by emulsion freeze drying. As for the reinforced composite materials, this technology provides a promising way, though there are few research reports.

The advantage of this technique is that the use of toxic solvents is reduced and it doesn’t eliminate the drying and leaching process of the porogen component. In order to maintain the stability of the emulsion, the method requires the addition of suitable surfactants, like PU etc.

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