Centrisart tube

The entrapment efficiency was determined by the ultrafiltration method [40 (link)] using a Centrisart tube with a molecular weight cut-off of 10 kDa (Sartorius, Gottingen, Germany). Centrifugation was performed on the device containing 2 mL of the sample in the donor chamber for 30 min at 6000 rpm to separate the unloaded DTX or CCM in the aqueous phase into the inner floater tube. The drug solution in both the recovery and donor chambers was collected and quantified using the HPLC method as outlined in our previous report [30 (link)]. The following equation was used to calculate the percentage of drug that was entrapped.
$$\mathrm{Entrapment} \mathrm{efficiency} (\%)=\frac{\mathrm{Winitial} - \mathrm{Wobtained} }{\mathrm{Winitial}} \times 100$$
where “ $\mathrm{Winitial}$ ” is the amount of drug loaded in the formulation, while “W_obtained” is the amount of unloaded drug in the aqueous phase of the formulation.

The average sizes of the primary emulsification product (W₁/O) and also the lipid vesicles formed were determined using a dynamic light scattering machine (DLS, Zetasizer Nano ZS, Malvern Instruments, Worcester, UK). The intensity versus size distributions of the particles were presented. The entrapment yield (EY) of metformin hydrochloride in the prepared lipid vesicles was determined using UV–Vis spectrophotometric detection of the drug. Equal volumes of methanol and lipid vesicle suspension were mixed together in a vial, a treatment that leads to the destruction of the vesicle membranes, releasing their contents into the medium. The solution was then subjected to centrifugal filtration for 25 min at 4000 rpm using a Centrisart tube (Sartorius, Germany) with 10 kDa molecular weight cut-off. Analysis of the clear permeate obtained was performed using UV–Vis spectrophotometer (Jenway, model 6504, UK) at 236 nm to give total concentration (C_total) of the free drug and the entrapped ones. For the concentration of the free/unentrapped drug (C_out), a mixture of the vesicle suspension and phosphate buffer was made and subjected to the above filtration condition. The lipid vesicles were precipitated and the permeate analyzed at 236 nm using a UV–Vis spectrophotometer.
The entrapment yield, EY, was therefore calculated using the formula: $\mathbf{Entrapment} \mathbf{yield} (\%) = \frac{C_{total}-C_{out}}{C_{total}}\times 100$