Freeze drying has been extensively used to prepare porous structures with many different polymers, including silk proteins [213, 214], PGA, PL .LA, PLGA and PLGA/PPF blends, for tissue engineering and biological applications. Freeze drying is a slow batch process based on the principle of sublimation to extract dry product from an aqueous solution. This technique first dissolves the polymer in a solvent to form a solution of desired concentration. Then the solution is usually frozen, and the solvent is removed by lyophilization under the high vacuum to prepare a the scaffold with high porosity and interconnectivity . Freeze drying technique is also a common and effective method that is widely used in a suitable reinforced scaffold for successful transplantation therapy. Kane et al.  studied the freeze-dried HA reinforced collagen scaffolds to evaluate the effects of HA reinforcement on the architecture and mechanical properties of freeze-dried collagen scaffolds. Mobini et al.  fabricated 3D scaffolds by embedding of natural degummed silk fibers in a matrix of regenerated fibroin, followed by freeze-drying. Hiraoka et al.  obtained PGA fiber-reinforced collagen sponges by freezedrying a collagen solution with PGA fibers and the dehydrothermal cross-linking technique.
When compared to the other methods , the main advantage of this technique is that it doesn’t require a high temperature and is conducive to maintaining the activity of biomacromolecules and pharmaceuticals [220, 221]. The freezedrying process doesn’t bring any impurities into the samples, so that a separate leaching step is not required. The drawback of this technique is smaller pore size and long processing time . Despite some of the shortcomings of this technique, it is often applied for the manufacture of tissue-engineered scaffolds, even in the present study.