Preparation and Characterization of Usnic Acid Nanocrystals with the Wet Grinding Method Using a Planetary Ball Mill

Abstract

Usnic acid is a potential active pharmaceutical ingredient derived from Usnea sp. It has a low bioavailability because of poor aqueous solubility. Particle size reduction by nanosizing has been applied to improve the solubility of active pharmaceutical ingredients which is achieved by preparation of nanocrystals. This study aimed to prepare usnic acid nanocrystals by employing a wet grinding method with a planetary ball mill and using Poloxamer 188 as a stabilizer. The solid state properties of the resulting usnic acid nanocrystals were characterized with a particle size analyzer, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and dissolution rate profile. Usnic acid nanocrystals had a particle size of 358.77 ± 21.18 nm, evenly distributed with a polydispersity index of 0.591±0.02 and a zeta potential of −24.4 ± 0.1. The DSC thermogram showed a sharp endothermic peak at 193.413°C. The XRD pattern contained a high-intensity crystalline peak at 2Ɵ=10.213°. The dissolution rate of the usnic acid nanocrystals was 2.7 fold faster than that of the intact usnic acid. This study concluded that nanocrystals of usnic acid have improved physicochemical properties and a significantly increased dissolution rate.