**7.3 Freeze drying**

Freeze drying is a new method for preparation of submicron sized liposomes which are sterile and pyrogen-free [56]. Freeze drying method depends on dispersion having lipids with water-soluble carriers like sucrose and mannitol. Sucrose and mannitol are dissolved in cosolvent system containing tert-butyl alcohol and water. Lyophilizer can be used for freeze drying. The conditions for freeze drying are first freezing to be maintained at 40° C for eight hours followed by 1° drying maintained at 40°C for 48 hours and then secondary drying maintained at 25°C for 10 hours. Mainly 20 pascal pressure is to be maintained in freeze drying chamber at drying process. After reconstitution with aqueous phase the prepared freeze dried product forms liposomal dispersion. The lipid/carrier ratio plays vital role in size and polydispersity of the liposomal dispersion [56].
