Hot air coating technique as a novel method to produce microparticles.

In this work a new technology to produce microparticles, as well as the equipment suitable for its application, are described. This technique, called hot air coating (HAC), was developed to overcome the drawbacks of the conventional spray-congealing technique and consists in a special venturimeter, deliberately designed to prevent any hindrance along the axial path through which the powder is conveyed. In HAC technique the raw material is a solid, generally small granules, which is aspirated through the “Venturi effect” and accelerated in a flux of hot air to soften and then to melt the excipient, especially on the particle surface. The microparticles then solidify during falling in air at room temperature. Model formulations, containing acetaminophen or theophylline as drugs and glycerilmonostearate, stearic acid or carnauba wax as coating waxes, were tested. The choice of the optimal operating parameters was found to be a function of the formulation and of the particle size of the starting material. A pressure of 3 atm and a temperature of 20°-60°C above the melting point of the excipient were found generally to be the optimal parameters for the coating process. The morphology, the in vitro dissolution profile and the possible drug/excipient interactions of formulations containing different percentages (30%, 50% and 70% w/w) of acetaminophen were evaluated. The results showed that the morphology and the dissolution profiles of the microparticles were quite different from those of the starting material; in particular the best coating was achieved by microparticles lower than 500 mm. Therefore, the HAC process could be a viable alternative to the conventional spray-congealing technique to produce microparticles with a high drug content.