Tablet coating is a process by which an essentially dry, outer layer of coating material is applied to the surface of a dosage form in order to confer specific benefits over uncoated variety. Coatings may be applied to various oral dosage forms such as particles, powders, granules, crystals, pellets and tablets.
Over the course of time, coating processes have developed from the art of earlier years to those that are more technologically advanced and controlled such that compliance with good manufacturing practices (GMPs) is facilitated. The development and availability of new coating materials, the recognition of the impact of applied coatings on subsequent release of drug(s) from dosage forms and the advancement in equipment design have all contributed to improved products.
Tablets that do not meet these requirements create a lot of problems to the formulation scientist engaged in coating of the dosage form.
Read Also: Manufacture of Pharmaceutical Tablets
There are essentially four major techniques for applying coatings to pharmaceutical solid dosage forms. They include:
The process of sugar coating, which has its origins in the confectionery industry is perhaps one of the oldest pharmaceutical processes still in existence. The process involves the deposition of aqueous sugar solutions onto the surface of the core tablet.
Sugar coating involves many steps hence it requires experience, and in some cases elaborate equipment. Though the stages differ from one manufacturer to another, a typical sugar-coating process involves the following steps:
Sugar coats are often shiny and highly coloured.
Film coating involves the deposition of a thin layer of film-forming polymeric material on the tablet core. The process was introduced in the 1950s as improvement on the traditionally sugar-coating process. Film coating has proved successful as a result of the many advantages offered, including:
Film coating can also be used to coat other formulations including capsules, beads, granules, drug powders, and crystals. A spray-atomization technique is more commonly utilized.
Compression coating also referred to as press coating or dry coating is the process by which a fine dry granulation is compressed onto a tablet core of drug. This is usually achieved using a specially designed tablet press such as Drycota (Manesty) and Prescota (Killian).
Compression coating is essentially a dry process and thus may be suitable for coating tablets containing heat and moisture liable drug(s) such as aspirin and penicillins. This coating process has also been used to separate two incompatible active pharmaceutical ingredients; one contained in the tablet core and the other in the coating. Repeat action and sustained action tablets are produced by this coating method. Although traditionally a less popular process, compression coating has gained increased interest in recent years as a means of creating specialized modified-release products.
The process involves the application of relatively thin coating to a small particle of solids, liquids or even gases in a micron dimension. The microcapsules thus formed range dimensionally between 3 – 800 µm in diameter with about 10 – 90 % w/w core.
Microencapsulation differs from film coating only in the size of the particles to be coated and the methods by which this is accomplished. Hundreds of drugs including Aspirin (Arthritis Bayer), Potassium chloride (Mico-K Extecaps), Phenylpropanolamine (Dexatrim) and Dimetapp Elixir (Brompheniramine and Pseudoephedrine Elixir) have been microencapsulated and used as controlled – release systems.
There are several types of tablet coating machines currently in the market. Depending on the working principles, these machines may be classified into:
After coating, tablets should be inspected and tested. According to British Pharmacopeia, 2009, when a tablet is presented as coated formulations, where justified and authorized, it may be necessary to remove the coating before performing a test described in the monograph. Removal of coating is not permitted where it affects the functionality of the product, for example in dissolution or disintegration tests. Where justified, uniformity of mass or uniformity of content of coated tablets other than film-coated tablets may be ensured by control of the core. The following quality control tests could be performed on coated tablets.
The coating of pharmaceutical solid-dosage forms, especially tablets, has been practiced for over a century. Although such a process is often applied to a dosage form that is functionally complete and thus may cause us to reflect on the need for incurring the additional expense, it is evident that the continued use of coating processes in pharmaceutical production remains very popular.
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