Tablets are the most important oral dosage form in the pharmaceutical industry. Pharmaceutical film coating, a thin film made from one or more polymers (and usually other functional excipients such as color-pigments or surfactants) that can fulfill a variety of functions is applied to the tablet, represents nowadays an important process step in the pharmaceutical industry.
Derived originally from dragee pan processing, coating technology has developed continuously in the last decades. Most of the coating processes are performed for drug release modification, drug stability improvement against light or moisture and taste masking. Furthermore, patient compliance issues play an important role, as swallowability improvement or a simpler identification due to a different color.
The quality requirements for coated tablets can also be versatile and different for each coating purpose. The simplest coatings only require a small minimum thickness. These are, for example, swallowing aids or simple taste and odor masking and the coating is often colorless. The coating uniformity within a batch only plays a subordinate role.
Cosmetic coating, where the tablet is colored with a pigment, or a dye is the next highest level of difficulty. The main arguments for a cosmetic coating are compliance, prevention of medication errors and marketing oriented. The quality requirement is that the tablets must be uniform within a batch as well as between produced batches and should have low surface roughness and sometimes high gloss (depending on the market).
When we are talking about tablets with a delayed release time, in addition, the thickness of the tablets must be higher than the coatings for swallowing aids. Furthermore, the tablets must be coated with a high degree of uniformity, as disintegration and release tests in various media must be passed within narrow limits according to pharmacopeia requirements.
Coated tablets, used to control the delayed release of active ingredients and tablets that are coated with medicinal substances have the highest demands on the coating quality. Very high uniformities must absolutely be achieved because otherwise the complex release profiles cannot be reproduced.
The introduction to the importance shows that the coating process is a very complex process in which the individual process operations must be precisely coordinated.
API coating is getting more and more important in the last few years because it enables fixed-dose combinations or the combination of incompatible drugs. Furthermore, different drug release characteristics can be realised by applying for example sustained-release coatings in addition to immediate-release coating layers. These formulations can consist of up to four coating layers, which results in a long processing time. To be able to develop and produce such formulations successfully, coating uniformity is a prerequisite and a quality attribute, because coated tablets must pass the test on uniformity of dosage units according to the pharmacopeias.