Tablets are the most important oral dosage form in the pharmaceutical industry. The advantages of tablets are numerous. First and foremost is the ability to precisely control dosage and the high stability of active ingredients. In addition, tablets can be produced quickly and in large quantities and offer flexible design freedom in terms of size, shape, and color.
Tablets can be effervescent, dissolving, or dispersible, dissolving tablets, lozenges, or enteric-coated and retarding tablets, as well as uncoated and coated tablets.
Tablets are often coated. This involves applying a thin film of one or more polymers and other functional excipients (colorants or humectants) to the tablet, which can perform a variety of functions.
Tablets are coated to modify the release of the active ingredient, such as in entericcoated or extended-release dosage forms, to protect the active ingredient from light or moisture, or to mask a bitter taste in the tablet formulation. In addition, tablets are coated to improve swallowability or to mask with colour (for unique identification or marketing purposes).
Coating of active ingredients is becoming increasingly important. This includes combination products as well as the combination of two incompatible active ingredients in one dosage form. In addition, different release profiles of the same active ingredient can be combined. In this case, the core contains the slow-release component, and the tablet coating contains the fastrelease initial dose. Formulation approaches sometimes consist of up to four different film types. This results in long process times. In order to successfully develop and produce such formulations, coating uniformity is a mandatory requirement.
Coating uniformity is indicated by the Relative Standard Deviation (RSD) and has a very high analytical significance. The smaller the RSD, the more uniform the coating on the tablet. However, the exact determination of the RSD requires a high analytical effort, since the same individual tablets have to be analysed before and after coating, or components of the coating have to be quantified by content determination methods.
Not to be confused with RSD uniformity is the determination of color difference (?E) for coloured coatings. High ?E values describe a noticeable color difference. The dimensionless quantity ?E is often mistakenly equated with coating uniformity in % RSD, but this is a gross error and can completely distort the validity of a process.
Mixing, Spraying and Drying
The interaction of mixing, spraying, and drying is critical in tablet coating. Mixing, spraying and drying must be performed simultaneously and with the correct settings to achieve optimum coating uniformity.