Evaluation of critical process parameters for intra-tablet coating uniformity using terahertz pulsed imaging

The purpose of this study was to evaluate the intra-tablet coating uniformity and the identification of critical process parameters in an active pan coating process using terahertz pulsed imaging (TPI). A design of experiments (DoE) was performed with drum load, drum speed, spray rate, run duration and spray pressure as factors. Different measures of intra-tablet uniformity were investigated: the average thickness on the individual tablet faces, spatial variation in layer thickness over the tablet surface, and the coefficient of variation (CV(intra)). Data analysis revealed that the process parameters in the investigated parameter space had hardly any influence on the difference in layer thickness of the tablet faces and centre band. No increase or decrease in layer thickness--as described in the literature--was found towards the edges of the tablet face. In overwetted process conditions a higher layer thickness at the centre band edges could be observed. Still, the highest variability in coating thickness was found along the circumference of the centre band rather than the height. In general, higher CV(intra) of layer thickness were found on the centre bands in comparison with the tablet faces. The analysis of the DoE model revealed that the run duration had the highest influence on the CV(intra) on the tablet faces. TPI showed high potential in the assessment of intra-tablet uniformity and layer thickness distributions over the whole tablet surface. It was successfully used to identify critical process parameters regarding intra-tablet coating uniformity.