Tablet Coating Process: Concepts and Advancements

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.

Objectives of tablet coating

1. To mask the bitter taste and unpleasant odour of some drugs
2. To improve product appearance for aesthetic or commercial purposes (aiding in brand identification)
3. To prevent drug-induced irritation at a specific site within the gastrointestinal tract, e.g. the stomach for non-steroidal anti-inflammatory drugs (NSAIDs).
4. To protect the drug from the external environment (particularly air, moisture, and light) in order to improve stability.
5. To enhance ease of swallowing large dosage forms
6. To facilitate handling, particularly in high-speed packaging/filling lines, and automated counters in pharmacies, where the coating minimizes cross-contamination due to dust elimination.
7. To facilitate rapid identification of a product by the manufacturer, the dispensing pharmacist and the patient.
8. To reduce the risk of interaction between incompatible materials
9. To retard loss of volatile ingredients
10. To modify and/or control the rate of drug release as in repeat-action, delayed-release (enteric-coated) and sustained-release products.
11. To control the site of action of drugs e.g., colon delivery
12. To avoid inactivation of drug in the stomach e.g., enteric coating

Core requirements for successful tablet coating

1. Tablets to be coated should normally be compressed with deep concave punches and die sets. This allows easy coverage of the tablet by the applied coating material.
2. The core tablet should be compressed harder than uncoated tablets because they must withstand the additional processing operations.
3. They must be free from dust to ensure uniformly smooth coating.

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

 Techniques of Tablet Coating

There are essentially four major techniques for applying coatings to pharmaceutical solid dosage forms. They include:

1. Sugar coating

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:

1. Sealing
2. Subcoating
3. Smoothing (or Grossing)
4. Colouring/ Colour coating
5. Polishing (or Glossing)
6. Printing

Sugar coats are often shiny and highly coloured.

Read more on sugar coating of pharmaceutical tablets

2. Film coating

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:

1. Minimal weight increase (typically 2–3% of tablet core weight)
2. Significant reduction in processing times
3. Capacity to include organic solvents if required
4. Increased process efficiency and output
5. Increased flexibility in the choice of film-forming polymers
6. Improved resistance to chipping of the coating

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.

Read more on film coating of pharmaceutical tablets

3. Compression coating

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.

4. Microencapsulation

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.

Classification and Types of Tablet Coating Equipment

There are several types of tablet coating machines currently in the market. Depending on the working principles, these machines may be classified into:

1. Standard Coating Pan/ conventional pan system
   1. Pear-shaped
   2. Hexagonal
   3. Spherical
2. Perforated Coating Pan
   1. Accela-Cota/Hi-coater Systems
   2. Driacoater
   3. Glatt coater
3. Fluidized Bed Coater

Quality Control and Evaluation Parameters for Coated Tablets

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.

1. Appearance characteristics – colour, size, physical defect and imprinting
2. Disintegration test
3. Dissolution tests
4. Drug content
5. Uniformity of tablet weight
6. Uniformity of content
7. Hardness/ crushing strength test
8. Friability test
9. Stability studies
10. Water vapour stabilities
11. Adhesion test
12. In vivo drug release in different experimental animals

Read more on quality control tests for tablets

Recent Advancement in Tablet Coating Technology

1. Electrostatic coating
2. Magnetically assisted impaction coating (MAIC)
3. Vacuum film coating (Fluid Bed coating)
4. Compression coating
5. Dip coating
6. Aqueous film coating technology
7. Supercell coating technology
8. Polymer blend
9. A novel method of manufacturing One-Step Dry-Coated (OSDrC) tablets
10. Top spray coating process
11. Bottom spray coating process
12. Tangential spray coating process
13. Time-dependent and pH-dependent polymer for colon targeted

Disadvantages of Tablet Coating

• Tablet coating process is tedious and time-consuming
• The process increases the cost of formulation
• Tablet coating may interfere in pharmacodynamic properties of drug formulation
• The process may sometimes result in various coating defects like chipping, cracking etc.
• The process remained complicated and thus requires the expertise of highly skilled technician.

Conclusion

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.

References

• Allen L. V and Ansel H. C. (2014). Ansel’s Pharmaceutical Dosage Forms and Drug Delivery Systems. Philadelphia: Lippincott Williams and Wilkins.
• Banker, G. S. and Rhodes, C. T. (n.d.). Modern Pharmaceutics (4^th ed.) New York: Marcel Dekker, Inc.
• Dash, A. K., Singh, S. and Tolman, J. (2014). Pharmaceutics – Basic Principles and Application to Pharmacy Practice. USA: Academic Press.
• Felton, L.A. (2012). Remington Essentials of Pharmaceutics. UK: Pharmaceutical Press.
• Ghosh, T. K. and Jasti, B. R. (2005). Theory and Practice of Contemporary Pharmaceutics. USA: CRC Press LLC.
• Lieberman, H. A., Lachman, L. and Schwartz, J. B. (1990). Pharmaceutical Dosage Forms: Tablets. New York: Marcel Dekker, Inc.
• Ofoefule, S. I. (2002). Textbook of Pharmaceutical Technology and Industrial Pharmacy. Nigeria: Samakin (Nig.) Enterprise.
• Shayne, C. G. (2008). Pharmaceutical Manufacturing Handbook: Production and Processes. New Jersey: John Wiley & Sons, Inc.

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