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Tablets & Capsules May/June 2021 15 50 pilot-scale coater and achieved RSD values as low as 2.7 percent. Out of 19 trials conducted as part of a statistical design of experiments, only four trials failed to meet the compendial requirements of not more than 6.25 percent for drug content uniformity. It is not unrealistic to expect that the Bohle Koco coater could achieve similar results. As previously mentioned, intra-tablet coating uniformity is often of equal or greater importance as inter- tablet coat- ing uniformity, such as when applying a modified- release coating. Interestingly, Cunningham, et al. [9] conducted a study using a ConsiGma coating process in which they were able to demonstrate substantial improvement in intra-tablet coating uniformity compared to a traditional batch coater, as shown in Figure 6. The coating being deposited was a delayed-release coating based on Acryl-Eze (Colorcon), where improvements in intra-tablet coating uniformity are certainly a benefit. Some of their data are summarized in Table 5. Perhaps these results are not surprising, since tablet motion in the ConsiGma coater is totally different to that of a traditional batch coater. In the ConsiGma coater, all tablet surfaces are likely to be equally exposed to the spray, whereas in a batch coater, because of the way tablets slide down the surface of the tablet bed, the tablets' upper and lower faces get more exposure than the edges and ends. Measuring coating uniformity has been of increasing interest in recent years, but traditional assessment tech- niques often involve rescuing and weighing marked tablets at the end of the coating process. Such methods are tedious and cannot provide immediate feedback during the coating process, which has encouraged interest in using various in-line process analytical technology (PAT) tools, including near-infrared spectroscopy (NIR), Raman spectroscopy, terahertz analysis, and optical coherence tomography. Knop and Kleinebudde [10] have provided an interesting overview of the application of many of these techniques as they relate to the film-coating process. Möltgen, et al. Some equipment designs, such as the Bohle coater, min- imize this problem by using elongated coating pans that effectively reduce the tablet bed depth, more spray guns to increase the area of coverage in the spray zone, and unique baffle systems to assist in mixing the tablet bed. A potential advantage of continuous coaters is that, by reducing the bed depth and controlling (or possibly eliminating) axial movement and relying primarily on radial movement, they may achieve more frequent and consis- tent tablet exposure to the spray, resulting in improved coating uniformity. This may be true to some extent, but there are caveats. For a fully continuous coating process, Marjoram [2] suggested that coating uniformity can be improved by: • Increasing the tablet feed rate, since this reduces the chances of axial mixing; and • Reducing the tablet bed depth, since this increases the area of the tablet bed exposed to the spray. The possibility that fully continuous processes can achieve improved coating uniformity is supported by the data shown in Table 4, where the continuous process achieves a better result, and that result is further enhanced when the feed rate is increased. Cunningham, et al. [7] provide data illustrating that, when using a novel high-solids film coating, a Driaconti-T coater achieved better mass uniformity than a traditional batch coater. At 25 percent w/w coating solids and a 3 percent target weight gain, the Driaconti-T coater attained a 15.2 percent RSD, while a 48-inch batch coater attained a 23.9 percent RSD. Little data has been published on coating uniformity in Bohle Koco rapid-turnaround batch coaters. However, a plethora of information is available relating to the Bohle BFC coater (on which the Koco coater is based) suggest- ing that the coater can achieve remarkable improvements in coating uniformity. For example, Just, et al. [8] have conducted coating optimization trials using a Bohle BFC Table 5 Data comparing intra-tablet coating uniformity for tablets coated in a ConsiGma coater with those coated in a traditional batch coater (data courtesy of Colorcon) Location on tablet surface Coating thickness (µm) ConsiGma coater Traditional batch pan coater Face 104.0 88.1 Land 117.0 122.0 Edge 94.7 61.1 End 101.0 62.9 Average 104.2 83.5 Standard deviation 9.4 28.5 % RSD 9.0 34.1 Figure 6 Photomicrographs comparing intra-tablet coating uniformity for tablets coated in a ConsiGma coater with tablets coated in a traditional batch coater (images courtesy of Colorcon) ConsiGma 290x magnification Band Edge Land Face Traditional batch coater 290x magnification Band Edge Land Face