Passive acoustic emission monitoring of pellet coat thickness in a fluidized bed

Abstract

Pharmaceutical pellets are often packed into capsules as a multiple unit dosage form. The small, spherical particles contain the active ingredient or drug and are film coated for modified drug release. Evaluation of pellet film coatings requires invasive sample acquisition and extensive off-line testing leading to inaccurate results non-representative of the pellets, identifying the need for a more reliable monitoring and control method. Passive acoustic emissions have been proven as a useful non-invasive monitoring tool with the ability to provide on-line and real-time measurements. The goal of this study was to assess the potential of passive acoustic emissions to monitor the pellet coat thickness of sugar coated glass pellets in a fluidized bed. A microphone attached to the exterior of a fluidized bed reflected changes in the signal amplitude corresponding to increases in pellet film coat thickness. Additional experiments showed that an increase in pellet mass produced larger acoustic emission amplitudes. Simulated pellet–wall collisions identified the sensitivity of the acoustic emission measurements to detect differences in pellet collisions, necessary for monitoring thin film coatings. Overall, changes in the acoustic emission amplitude were shown to reflect the film coating, provided the pellets were sufficiently fluidized, indicating the potential for applying passive acoustic emission monitoring for the determination of a desired coating end-point. Additional research to continue the development of passive acoustic emission monitoring is necessary for industrial fluid bed coating applications, with the potential to improve overall product quality and process cost-effectiveness.