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Sufficient ophthalmic drug delivery is still challenging for pharmaceutical technologists, despite various scientific efforts. Several ocular drug carriers have been designed to enhance bioavailability by prolonging the drug retention time. One of the current encouraging approaches is the utilization of colloidal carriers with the characteristic submicron-nanometer size. Microemulsions (MEs) are such colloid systems that present sizes between 5 and 200 nm with significant thermodynamic stability and low surface tension. In addition, MEs as topical ocular carriers can lead to great ocular drug adsorption due to their enhanced retention time. Furthermore, considering that MEs are stable for long time and various temperatures, their ocular application is of great interest. The aim of this study is to cover basic physicochemical principals of ocular MEs such as their possible size, stability, and therapeutic efficacy against various eye disorders. Thus, a comprehensive review for ocular drug delivery systems in the form of MEs that show promising characteristics as their stability and therapeutic efficiency is performed.

The purpose of the current research was to prepare and evaluate the potential use of microemulsion-based hydrogel (MBH) formulations for dermal delivery of benzocaine (BZN). The pseudoternary-phase diagrams were constructed for various microemulsions composed of isopropyl myristate (IPM) as oil phase, Span 20, Tween 20, Tween 80, cremophor EL and cremophor RH40 as surfactants, ethanol as cosurfactant and distilled water as aqueous phase. Finally, concentration of BZN in microemulsions was 2% (w/w). The physicochemical properties, such as conductivity, viscosity, pH, droplet size, polydispersity index and zeta potential of microemulsions, were measured. Carbopol 940 was used to convert BZN-loaded microemulsions into gel form without affecting their structure. Furthermore, excised rat abdominal skin was used to compare permeation and penetration properties of BZN loaded M3 and M3BHs with BZN solution. According to ex vivo study results, BZN-loaded M3BH1 showed highest flux values and high release rate values, and furthermore, this gel formulation had low surfactant content. Finally, in order to learn the localization of formulations within the dermal penetration, formulations and BZN solution were labeled with red oil O and subjected to fluorescence observation. In conclusion, BZN-loaded MBHs could be offered as a promising strategy for dermal drug delivery.

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