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Terahertz Pulsed Imaging as a PAT Tool for Evaluating the Effect of Coating Methods and Application of Stress Conditions on Applied Film Quality
* 1 , 2 , 3 , 4 , 1 , 4 , 3, 5 , 6 , 3, 4, 5
1  Department of Pharmaceutics and Analytical Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Copenhagen, Denmark
2  Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdom
3  TeraView Ltd., St. John’s Innovation Park, Cambridge, United Kingdom
4  School of Pharmacy, University of Otago, Dunedin, New Zealand
5  Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
6  Department of Chemistry, MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, Dunedin, New Zealand

Abstract: Film coating process development and applied film quality have been in the focus of the pharmaceutical industry for decades. Many techniques have been used to investigate and characterise complex film coating structures. Recently, terahertz pulsed imaging (TPI) has been introduced to study and understand the quality of film coatings. In this study, the effect of two different film-coaters , a fluid bed (Combi Coata, Model CC1/LAB, Niro Atomizer, Denmark) and a drum coater (Hi-Coater, HCT 20, Lödige Mascinenbau GmbH, Germany) on the applied film quality was determined by TPI (TPI imaga 2000, TeraView Ltd, Cambridge, UK). In addition to the coating application under recommended conditions, stress conditions were applied and investigated; e.g. high spray rate, simulated nozzle block during the coating process, and varying curing times after coating. From the terahertz time-domain waveforms, maps of the terahertz electricfield peak strength (TEFPS), interface index (TII) and coating thickness (CT) were successfully derived. SEM was used as a complimentary technique to increase the insight into the various coating characteristics. The results showed lower TEFPS values for samples coated by the fluid bed device, indicating a rougher coating surface than samples coating by the drum coater. Although the amount of coating applied was found to be the same, the mean CT for the fluid bed coated samples was higher. Furthermore, it was possible to distinguish an interface between the subsequent coating layers in the CT maps (generated by a coating liquid application stop), which couldn't be resolved by SEM for all samples. The different process conditions in both pieces of equipment are considered to be reason for this observation, as well as the difference found in the TII values (interface coating/core) for tablets coated by the two methods, respectively. In good agreement with previous studies, the overall thickness of the coating layer was systematically thinner around the centre band compared to the top and bottom surfaces. TPI can be considered to be a sophisticated PAT tool for gathering information, and increasing the understanding of complex film coating structures.
Keywords: Terahertz Pulsed Imaging (TPI), film quality, coating