COATING TECHNOLOGY AND ITS RECENT ADVANCES – A REVIEW

ABOUT AUTHORS:
Vandna Dhiman*, Swati Modgil, Meenu Nagpal, Uprendra K Jain
Deptt. of Pharmaceutics, Chandigarh college of pharmacy,
Landran, Mohali-160065
*dhimanvandna27@gmail.com

ABSTRACT
Tablet coating is one of the oldest pharmaceutical process still in existence. It is a process of applying a thin polymer based film to a tablet or granule containing the active pharmaceutical ingredient. Solid dosage forms are coated for a number of reason, e.g. to control the drug release profile, to mask the bitter taste of the drug. There are various techniques for tablet coating such as sugar coating, film coating and enteric coating. Earlier, sugar coating was adopted for pharmaceutical as from the confectionary industry. But it was a tedious process and required skilled manipulation, therefore film coating was started to prefer over sugar coating. Film coating was mainly based on the solution of different polymers in organic and aqueous solution.

REFERENCE ID: PHARMATUTOR-ART-1932

INTRODUCTION
Tablet as a solid dosage form

A tablet is a pharmaceutical dosage form comprising a mixture of active substances and excipients, usually in powder form, pressed or compacted from a powder into a solid dose. The excipients can include glidants (flow aids), diluents, binders or granulating agents and lubricants to ensure efficient tableting; disintegrants to promote tablet break-up in the digestive tract; sweeteners or flavors to enhance taste; and pigments to make the tablets visually attractive. A polymer coating is often applied to enhance the tablet's appearance or to make the tablet smoother and easier to swallow and to control the release rate of the active ingredient, to make it more resistant to the environment (extending its shelf life). “Caplets” are those tablets which are in the shape of capsules. Medicinal tablets and capsules are often called pills. One can administer 0.01 mg to 1gm of a drug dose by oral route of administration by formulating as a tablet.(1-2)

Tablet coating: (2-7)
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 that broadly ranges from facilitating product identification to modifying drug release from the dosage form. After making a good tablet, one must often coat it. Coating may be applied to multiple range of oral solid dosage form, including tablets, capsules, multiparticulates and drug crystals. When coating composition is applied to a batch of tablets in a coating pan, the tablet surfaces become covered with a tacky polymeric film. Before the tablet surface dries, the applied coating changes from a sticky liquid to tacky semisolid and eventually to a non-sticky dry surface pans. The entire coating process is conducted in a series of mechanically operated horizontal coating pans of galvanized iron stainless steel or copper. The smaller pans are used for experimental, developmental and pilot plant operations, the larger pans for industrial production.

Currently, the most common technology for coating solid dosage forms is the liquid coating technology (aqueous based and organic based polymer solutions). In liquid coating, a mixture of polymers, pigments and excipients is dissolved in an organic solvent (for water insoluble polymers) or water (for water soluble polymers) to form a solution, or dispersed in water to form a dispersion, and then sprayed onto the dosage forms in a pan coater (for tablets) and dried by continuously providing heat, typically using hot air, until a dry coating film is formed.(2-3)

Organic solvent based coating provides a variety of useful polymer alternatives, as most of the polymers are soluble in the wide range of organic solvents. But there are several disadvantages associated with its use.

1. They are flammable and toxic
2. Their vapour causes hazards to coating equipment operator
3. High cost of solvent
4. Solvent residue in formulation (7)

RATIONALE FOR COATING A SOLID DOSAGE FORM: (8)
Coating of a solid dosage form is designed to perform a specific function. For example; protection against moisture, taste masking pH, time controlled.

Tablets can be easily coated and a variety of products are available in the market. Generally the coating process gives rise to:
1. Increased bioavailability
2. Improved patient acceptance
3. Formulation stability

The rationale for coating s solid dosage form such as tablet can be categorized into three main headings:
1. Therapy
2. Technology
3. Marketing

Therapy:
1. To minimise irritation of the oesophagus and stomach.
2. Minimise inactivation of active ingredient in the stomach due to acidic environment.
3. Control the duration and site of drug release.
4. Improve patient compliance e.g. easier to swallow, masks unpleasant taste.

Technology:
1. Separate the reactive component in the tablet.
2. Minimise dust formation and contamination with respect to tablets.
3. Easily identify a branded product.
4. Facilitates their handling on packaging equipment and on high speed automated filling.
5. Improve drug stability e.g. protect the drug from oxygen, moisture and light, the three key causes of drug degradation.

Marketing:
1. Mask the unattractive and mottled tablet core.
2. Mask unpleasant taste.
3. Improve product stability.

Primary components involved in tablet coating: (9-11)

  • Tablet properties
  • Coating process
  • Coating equipments
  • Parameters of the coating process
  • Facility and ancillary equipments
  • Automation in coating processes

FUNDAMENTALS OF FILM FORMATION: (10-12)
In the pharmaceutical industry, polymeric films are generally applied to solid dosage forms using a spray-atomization technique. The polymer is dissolved or dispersed in aqueous or organic solvents prior to spraying. The solid cores are often preheated in the coating equipment prior to initiation of the coating process. This pre-warming stage is especially important in the coating of soft gelatine capsules. The coating solution or dispersion is atomized with air into small droplets, which are then delivered to the surface of the substrate.

Upon contact, the atomized droplets spread across the substrate surface. The solvent may penetrate into the core, causing surface dissolution and physical mixing at the film–tablet interface. As the solvent begins to evaporate, the polymer particles densely pack on the surface of the solid. Upon further solvent evaporation, the particles flow together due to the cohesive forces between the polymer spheres, a process known as coalescence. Heat is generally added to the coating equipment to facilitate solvent evaporation and film formation. Immediately following the completion of the coating process, coated solids are generally stored at temperatures above the glass transition temperature of the polymer to further promote coalescence of the film and ensure a homogeneous distribution of the plasticizer.

Advantages of tablet coating: (11-12)

  • Tablet coatings must not make tablets stick together during the coating process must follow the fine contours of embossed characters or logos on tablets and must be stable and strong enough to survive the handling of the tablet.
  • Printing on tablets can also be done by coatings, if required. Coatings are necessary for tablets giving a smoother finish, makes large tablets easier to swallow and also to mask the unpleasant taste.

 Disadvantages of tablet coating

  • Limitations of sugar coating such as relatively high cost, long coating time and high bulk have led to the use of other coating materials.
  • However the process of coating is tedious and time-consuming and it requires the expertise of highly skilled technician.

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