A Supercritical Fluid-Based Process for the Production of Fluorescein-Loaded Liposomes

Fluorescein is a synthetic dye used as a marker to monitor the absorption of liposomes into tissues, but its efficient entrapment is still challenging. In this work, we apply a supercritical fluid process that allows the production of liposomes with a high drug encapsulation efficiency. The effects of operating parameters such as the diameter of the injector, the concentration of phospholipids, and the concentration of fluorescein were studied. Liposomes with mean diameters down to ∼300 nm were obtained with fluorescein encapsulation efficiencies of ≤96%. In detail, the increase in the concentration of phospholipids produced a relative increase in liposome mean diameter and particle size distribution width. With an increase in fluorescein theoretical loading, instead, a reduction in liposome mean diameters and standard deviation was obtained. Smaller liposomes were produced using smaller injector diameters; however, the reduction of the injector diameter from 80 to 40 μm caused a reduction in the fluorescein encapsulation efficiency from 96 to 58%. Suspensions remained stable for more than four months, without drug leakage.