This study demonstrates a new in-line measurement technique for monitoring coating processes. Process monitoring is performed by an optical coherence tomography (OCT) sensor providing cross-sectional images of film-coatings in a non-destructive and contactless manner. Typically tablets or pellets are coated. Tablets are mostly coated in (perforated) pans or drums, whereas pellet coating is typically carried out in a fluid-bed. However, the monitoring of the coating growth as a function of process time is a key issue for both processes. This study shows results of coating thickness measurements from in-line OCT data for pellets and tablets coated in a fluid-bed and pan coater, respectively.
The coating thickness can be determined directly and automatically from OCT images and without the need of chemometric calibration models. Results of the in-line OCT measurements were validated by analyzing samples that were periodically removed from the process during the production. These samples were characterized off-line by OCT and dynamic image analysis in case of pellets or dimension measurements in case of tablets. Moreover, this methodology enables a multiple direct measurement of the coating thickness on individual pellets/tablets rather than providing the temporal and spatial average of a large number of pellets/tablets. This gives substantially more information about the coating quality, i.e., intra- and inter-particle coating variability, than standard quality control methods. The information about those parameters specifically emphasizes the high capability of the OCT technology to improve process understanding and to assure a high product quality. Specifically, the additional information as intra- and inter-particle coating variability as captured with the OCT sensor could be a valuable critical quality attribute (CQA). An in-line OCT system could act as a key tool for the development and control of coating processes within the process analytical technology (PAT) and quality by design (QbD) framework.
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