Tablet coating is a common pharmaceutical technique that involves the application of an essentially dry, outer layer of coating material to the surface of dosage forms (e.g., particles, powders, granules, crystals, pellets, and tablets) in order to confer specific benefits over uncoated variety.
A good number of tablets marketed today are coated for a number of reasons, the most important of which is controlling the release profiles. It is most desirable that the coating should be uniform and should not crack under stress. Hence, various techniques were designed for the application of the coating on the tablet surface.
This article focuses on the various techniques available for tablet coating.
Tablet coating techniques
The techniques available for tablet coating include but are not limited to
1. Tablet Wrapping or Enrobing
This technique uses a process similar to soft gelatin capsule manufacture, where tablets and polymer (e.g. gelatin) sheets are fed between die rollers that press the polymer around the tablet. The coated formulations are tamper-evident and can be designed with different colors for branding purposes.
Tablets coated with this technique have capsule-like appearance with high gloss. They are reported to be preferred by patients due to their ease of swallowing and superior taste – and odor – masking properties.
Enrobing requires specific equipment (e.g., Banner’s Soflet Gelcaps or Bioprogress’ Nrobe technology). An alternative is the Press-fit Geltabs system, which uses a high-gloss gelatin capsule shell to encapsulate a denser caplet formulation.
2. Hot-melt coating
Hot-melt coating is used for pellet coating to provide taste masking or modified release. They consist of waxy materials (such as beeswax, synthetic spermaceti, and other synthetic mono/ diglycerides) that have melting points in the range of 55–65 °C and exhibit melt viscosities that are typically less than 100 mPa s (in order to allow the formation of smooth coatings on the surfaces of particles).
Low-melting waxes can be sprayed using conventional equipment with heated lines to prevent the melted wax from congealing in the equipment.
3. Compression coating
This technique involves the use of standard tableting materials to compress a coating around a core, on a special tablet press. Compression coating is mostly used to provide a modified release profile of some sort e.g. a fast-release outer coat with a prolonged-release inner core.
4. Sugar coating
Sugar coating process involves building up layers of coating material on the tablet cores as they are tumbled in a revolving pan by repetitively applying a coating solution or suspension and drying off the solvent. Sugar coating involves many steps and in some cases elaborate equipment.
The steps involved in the sugar coating process may vary from manufacturer to manufacturer. An average coating process may involve the following steps: sealing, subcoating, smoothing, color coating, polishing, and printing.
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5. Film coating
Film coating involves the deposition of a thin layer of film-forming polymeric material on the tablet core. Film coating of tablets was introduced in the 1950s as an improvement on the traditional sugar coating process.
The popularity of film coating stems from the flexibility, reproducibility, and ease of control of the process together with the ready availability of equipment and materials.
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6. Electrostatic coating
Electrostatic coating is an efficient method of applying coating to conductive substrates. A strong electrostatic charge is applied to the substrate. The coating material containing conductive ionic species of opposite charge is sprayed onto the charged substrate.
Complete and uniform coating of corners and adaptability of this method to such relatively nonconductive substrates as pharmaceuticals are limited.
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