1 Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, UK
2 Institute of Pharmaceutics and Biopharmaceutics, University of Duesseldorf, Duesseldorf, Germany
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ConclusionWe conclude that TPI is a robust technique, and, that due to its very simple measurement principle, it is an ideal measurement technique to quantify the coating thickness in process control and quality monitoring applications. The method introduced in this paper will be useful to explore whether there is an influence on the refractive index of a given coating formulation with changes in process conditions during coating or curing. It furthermore opens the possibility, in pharmaceutical applications and beyond, to develop metrology standards for coatings that can be used to calibrate TPI measurements for absolute thickness.
Beyond its use as a calibration technique for TPI the XµCT method has provided some intriguing insight into the coating properties at extremes of the tablet geometry, at the edges of tablets where high curvature was thought to typically prevent quantitative TPI measurements due to strong scattering. The main limitations of the XµCT technique are the measurement, and in particular, processing time together with the high demands in computational power and data storage as well as the limited contrast that can be resolved between coating and core. We therefore envisage that the main impact of this technique is likely to be in research and development and to support techniques such as TPI rather than in routine quality control or process measurements.
...Samples were measured on a TPI imaga 2000 (Teraview Ltd., Cambridge, UK), using a point spacing grid of 200 x 200 µm and a penetration depth of 1 mm in air...