pH responsive chitosan-coated microemulsions as drug delivery systems

Oil-in-water microemulsions (o/w MEs) were coated with chitosan to obtain new curcumin delivery systems. First, ternary phase diagrams were constructed using mixtures of Tween 20, Tween 80, ethanol, and monopropyleneglycol (MPG) to determine the macroscopic phase region of MEs and their capacity to incorporate clove oil. Particularly, MEs with size less than 20 nm could be formed using low amount of the surfactant Tween 20. The prepared MEs were subjected to diverse stability tests and characterized by dynamic light scattering (DLS), zeta potential (Z potential), conductivity, pH, and transmission electron microscopy (TEM). Stable, spherical uncoated and polymer-coated MEs were obtained with low droplet size (Dp), <50 nm, for the water/Tween 20/clove oil/ and water/Tween80/MPG/clove oil systems using a chitosan solution of 1 mg/mL. For all polymer-coated MEs, an increase of Z potential (17–54 mV) was observed due to the presence of chitosan. The loading with curcumin did not affect the Dp of MEs (e.g., for ME with Tween 20, Dp unloaded 12.35 ± 1.86 nm and Dp loaded 11.13 ± 0.32 nm) but Z potential and conductivity values were increased (typically for coated ME prepared with Tween 20 17.00 ± 0.36 mV ≤Z potential ≤41.5 mV, 162.20 ± 8.95 μS/cm ≤ conductivity ≤183.8 ± 0.56 μS/cm). Drug delivery studies for the chitosan-coated MEs were carried out at different pH (7.4, 6.5, and 5.8). The release of curcumin from the chitosan-coated MEs, formed with Tween 20, showed a dependence of the pH with a maximum release at the acidic pH of 5.8 (58%). The release of curcumin from all MEs was best described by the Korsmeyer- Peppas model with a Fickian transport mechanism.