Microparticle Purification and Freeze-Drying

MLC were purified by a gradient centrifugation method using a sucrose solution. The centrifugation process was carried out in a centrifuge model SL8R (Thermo Fisher Scientific, Monza, Italy) equipped with a fixed angle microtube rotor model MicroClick 18 × 5, at 14,000 rpm, 8 °C for 40 min. At the end of the process, the microparticles on the top of the tube were collected, mixed with a trehalose solution added to the final microparticle suspension as a cryoprotectant. The freeze-drying process of samples was carried out by placing the suspensions in −80 °C for 2 days and then in a Modulyo freeze-dryer (Edwards, Cinquepascal, Milano, Italy) at −40 °C, 10⁻¹ mbar for 4 days [59].

Several batches of MLC were prepared using different concentrations of trehalose added to microparticles before lyophilization, in order to determine the best cryoprotectant concentration. Table 4 reports the percentage composition of all batches of microstructured lipid carriers prepared. All batches were prepared in triplicate.

Percentage composition (p/p) of the microstructured lipid carrier (MLC) suspension (for all batches, the following substances were kept at the same concentration: POE-POP: 1.2%, glycerol dibehenate 3.2%, caprylic/capric triglycerides 1.2%, polygliceryl-4 sorbitan olivate phosphate 0.4%).

Figure 10 shows the schematic procedure used to obtain microstructured lipid carrier powder.

MLC preparation schematic procedure.