Once lipid particles have been formed, maintaining the physical properties of the particles can be difficult. Size distribution can change on storage due to degradation of the components. Permeabilization of the membrane can lead to leakage of encapsulated material. Stability issues due to hydrolytic degradation is a general problem with lipid products. Aqueous formulations of drug products tend to be less stable since the presence of excess or bulk water leads to rapid hydrolytic degradation in lipid preparations.
After the sizing process is complete, lipid suspensions should be stored at close to pH 7 as possible. Lipids containing ester-linked hydrocarbon chains are susceptible to acid and base hydrolysis. Hydrolysis rate is dramatically affected by temperature, therefore lipid suspensions should be kept refrigerated during storage. Lipid suspensions should not be frozen as the freezing process could fracture or rupture the vesicles leading to a change in size distribution and loss of internal contents. The use of cryoprotectants such as dextrose, sucrose, and trehalose may increase stability from hydrolysis. Also, samples may experience oxidation upon storage. The addition of small amounts of antioxidants during processing may stabilize the suspension and limit oxidation of the product.
SUV should be stored above their transition temperature for no longer than ~24 hours. LUV may be store for a longer period of time if stored at 4-8°C when not in use. Hydrolysis of the lipid begins to occur immediately resulting in monoacyl derivatives (Lyso lipids) which act as detergents and disrupt the membrane, thus permeabilizing the membrane. After ~5-7 days at 4-8°C the internal contents will begin to leak indicating hydrolytic degradation of the lipid. If membrane structure is not a critical parameter in your experiments, vesicles may be stored for 1-2 months with minimal (<10%) hydrolytic degradation.