Quantification of microparticle coating quality by confocal laser scanning microscopy (CLSM)

In this work, a novel protocol was developed for determining film coating thickness and coating quality of microparticles, based on the use of confocal laser scanning microscopy (CLSM). CLSM was found to be an adequate non-destructive technique for the quantification of the coating thickness and coating quality of individual thin-coated small particles. Combined with image analysis, it was possible to derive with high accuracy the coating thickness distribution of a representative number of microparticles. The performance of the novel methodology was assessed by the quantification of the coating thickness and coating quality of protein-coated microparticles produced by fluidized bed coating. It was found that the CLSM data on coating layer thickness were generally in good agreement with the results from chemical analysis, down to a thickness of 1–1.5 μm. Using CLSM the importance of setting up the appropriate distance between the coating nozzle and the powder bed with respect to microparticle coating quality in fluidized bed processing was illustrated. Coating quality was found to decrease with increasing distance the coating droplets have to travel before impinging onto the core particles as a result of spray-drying of the coating droplets. Also, coating quality decreased with increasing viscosity of the coating droplets, resulting in reduced spreading on the cores.