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W Tablets & Capsules April 2017 29 coating Assessing the options: Fluid-bed processor, mixer, and drum coater Reiner Lemperle Lödige There are many reasons to coat tablets, granules, and particles, and a variety of factors determine which coating process to use. This article summarizes the characteristics of three types of coat- ing machines: fluid-bed processors, mixers, and drum coaters. hen applying a coating, the goal is to modify the sur- face properties of tablets or particles. Pharmaceuticals are often coated to protect them from the effects of exposure to water or moisture. In other cases, coatings protect the formulation from the effects of gastric juices in order to extend the release of the active ingredient. Coatings also add color and lubricity and mask unpleasant tastes and odors. The basic steps in applying a coating are mixing, spraying, homogenizing, forming (in the case of sugar coating), drying, and polishing. Depending on the prod- uct and process, the steps may repeat several times, and they can be performed in a variety of machines, including fluid-bed processors, mixers, and drum coaters. The size of the core strongly influences which coating process to use. Fluid-bed processors and mixers are best suited to coating powders and other products whose median parti- cle size is less than 1 millimeter. Drum coaters are usually the best choice for tablets. Fluid-bed processors For complex applications and/or sensitive products— such as coating an active pharmaceutical ingredient (API)—fluid-bed processing is the best choice. In a fluid- bed processor, warm air flowing through a distributor plate at the base of the process vessel moves through the product bed. As it does, a mist of coating is sprayed. As the liquid coating contacts the particles, the fluidizing air dries it, and the vapor is exhausted from the vessel. The process continues until the particles are coated as desired. The Wurster method is the most widely used fluid-bed coating technique. It uses one or more cylindrical inserts in the process vessel that cause the particles to flow in a cyclic pattern as the coating is sprayed from below. The fluid-bed process is adaptable to suit a variety of applications. For example, it can apply the coating from the top or bottom of the process vessel and, in some cases, tangentially [1]. With tangential application, the coating enters the product bed directly. In so doing, the process provides the same advantages of the Wurster technique while minimizing the amount of coating that must be applied. Using a tangential spray instead of a standard Wurster technique also eliminates the need for inserts in the process vessel and allows the distributor plate to remain in place between batches. This reduces the size of the product-contact area, which simplifies cleaning. Furthermore, processes that use a tangential spray typically require only one vessel, whereas many commercial installations based on the Wurster principle require a second process vessel. Nonetheless, many prod- ucts are still made using the Wurster process because that was the equipment used for validation. Plowshare mixers The need to coat particles isn't limited to the pharma- ceutical industry. Many dietary supplement ingredients also require encapsulation in a coating to prevent them from losing flavor from exposure to moisture and/or oxy- gen. The practice, however, is only slowly gaining LFP fluid-bed processor