An oral tablet dosage form is usually the most preferred dosage form because of patient convenience and acceptance

Tablet properties and coating

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Tablet is a solid dosage form, comprising a mixture of active substances and excipients, commonly in powder form. Coating is a technique in which outer layer of coating material is surrounded to the surface of a dosage shape to achieve precise benefits.

 

Tablet is a solid dosage form, comprising a mixture of active substances and excipients, commonly in powder form

 

Tablets are  among  the most  convenient and preferred oral dosage forms because of  their many advantages, including ease of administration, high patient compliance, and cost-effectiveness. Among the multiple steps in pharmaceutical manufacturing of tablets, coating is a critical process that is often used for functional and aesthetic reasons.

Tablet properties 

Tablets can be made in virtually any shape, although requirements of patients and tableting machines mean that most are round, oval or capsule shaped. More unusual shapes have been manufactured but patients find these harder to swallow, and they are more vulnerable to chipping or manufacturing problems. 

Tablet diameter and shape are determined by the machine tooling used to produce them – a die plus an upper and a lower punch are required. This is called a station of tooling. The thickness is determined by the amount of tablet material and the position of the punches in relation to each other during compression. Once this is done, we can measure the
corresponding pressure applied during compression. The shorter the distance between the punches, thickness, and the greater the pressure applied during compression, and sometimes the harder the tablet. Tablets need to be hard enough that they don’t break up in the bottle, yet friable enough that they disintegrate in the gastric tract.

The tablet is composed of the Active Pharmaceutical Ingredient (that is the active drug) together with various excipients. These are biologically inert ingredients which either enhance the therapeutic effect or are necessary to construct the tablet.

The filler or diluent (e.g. lactose or sorbitol) is a bulking agent, providing a quantity of material which can accurately be formed into a tablet.

Binders (e.g. methyl cellulose or gelatin) hold the ingredients together so that they can form a tablet.

Lubricants (e.g. magnesium stearate or polyethylene glycol) are added to reduce the friction between the tablet and the punches and dies so that the tablet compression and ejection processes are smooth.

Disintegrants (e.g. starch or cellulose) are used to promote wetting and swelling of the tablet so that it breaks up in the gastro intestinal tract; this is necessary to ensure dissolution of the API. Super disintegrants are sometimes used to greatly speed up the disintegration of the tablet.

Additional ingredients may also be added such as coloring agents, flavoring agents, and coating agents. Formulations are designed using small quantities in a laboratory machine called a Powder Compaction Simulator. This can prove the manufacturing process and provide information for the regulatory authorities.

Tablet coating

Coated tablets are defined as tablets covered with one or more layers of mixture of various substances such as natural or synthetic resins ,gums ,inactive and insoluble filler, sugar, plasticizer, polyhydric alcohol, waxes, authorized colouring material and sometimes
flavoring material . Coating may also contain active ingredient. Substances used for coating are usually applied as solution or suspension under conditions where vehicle evaporates.

Many tablets today are coated after being pressed. Although sugar-coating was popular in the past, the process has many drawbacks. Modern tablet coatings are polymer and polysaccharide based, with plasticizers and pigments included. Tablet coatings must be stable and strong enough to survive the handling of the tablet, must not make tablets stick together during the coating process, and must follow the fine contours of embossed characters or logos on tablets.

Coatings can also facilitate printing on tablets, if required. Coatings are necessary for tablets that have an unpleasant taste, and a smoother finish makes large tablets easier to swallow. Tablet coatings are also useful to extend the shelf-life of components that are sensitive to moisture or oxidation. Opaque materials like titanium dioxide can protect
light-sensitive actives from photo-degradation. Special coatings (for example with pearlescent effects) can enhance brand recognition.

If the active ingredient of a tablet is sensitive to acid, or is irritant to the stomach lining, an enteric coating can be used, which is resistant to stomach acid and dissolves in the high pH of the intestines. Enteric coatings are also used for medicines that can be negatively
affected by taking a long time to reach the small intestine where they are absorbed. Coatings are often chosen to control the rate of dissolution of the drug in the gastro-intestinal tract.

Some drugs will be absorbed better at different points in the digestive system. If the highest percentage of absorption of a drug takes place in the stomach, a coating that dissolves quickly and easily in acid will be selected. If the rate of absorption is best in the large intestine or colon, then a coating that is acid resistant and dissolves slowly would be
used to ensure it reached that point before dispersing. The area of the gastro-intestinal tract with the best absorption for any particular drug is usually determined by clinical trials.

The most common forms of tablet coating are sugar coating and film coating.

Coating is also performed for the following reasons:

  • Controlling site of drug release 
  • Providing controlled, continuous release or reduce the frequency of drug dosing 
  • Maintaining physical or chemical drug integrity 
  • Enhancing product acceptance and appearance 
  • Protects the tablet (or the capsule contents) from stomach acids 
  • Protects the stomach lining from aggressive drugs such as enteric coated aspirin 
  • Provides a delayed release of the medication 
  • Maintains shape of the tablet

Aspects of tablet coating

  1. Therapy
  • Avoid irritation of oesophagus and stomach
  • Avoid bad taste
  • Avoid inactivation of drug in the stomach
  • Improve drug effectiveness
  • Prolong dosing interval
  • Improve dosing interval
  • Improve patient compliance

2. Technology

  • Reduce influence of moisture
  • Avoid dust formation
  • Reduce influence of atmosphere
  • Improve drug stability
  • Prolong shelve life

3. Marketing

  • Avoid bad taste
  • Improve product identity
  • Improve appearance and acceptability

Coating process, design and control

In maximum coating methods, the coating solutions are sprayed onto the capsules because the tablets are being agitated in a pan, fluid mattress, and so forth. As the solution is being sprayed, a thin film is formed that adheres directly to every tablet. The coating may be shaped with the aid of an unmatched application or can be constructed up in layers through the use of more than one spraying cycles. Rotating coating pans are regularly
used within the pharmaceutical industry.

Uncoated tablets are located inside the pan and the liquid coating answer is brought into the pan even as the tablets are tumbling. The liquid part of the coating answer is then evaporated by using passing air over the surface of the tumbling drugs. In assessment, a fluid bed coater operates via passing air through a bed of tablets at a velocity sufficient to
assist and separate the drugs as person gadgets. As soon as separated, the drugs are sprayed with the coating composition.

The coating method is normally such as the subsequent steps:

a. Batch identification and Recipe selection

b. Loading/Dispensing

c. Warming

d. Spraying

e. Drying

f. Cooling

g. Unloading

Type of tablet coating process

  1. Sugar coating

Compressed tablets may be coated with coloured or uncoloured sugar layer. The coating is water soluble and quickly dissolves after swallowing. The sugar coat protects the enclosed drug from the environment and provides a barrier to objectionable taste or order. The sugar coat also enhances the appearance of the compressed tablet and permit imprinting manufacturing’s information. Sugar coating provides a combination of insulation, taste masking, smoothing the tablet core, colouring and modified release.

The disadvantages of sugar coating are the time and expertise required in the coating process and thus increases size, weight and shipping costs.

Sugar coating process involves five separate operations:

  • Sealing/Water proofing: provides a moisture barrier and harden the tablet surface.
  • Subcoating: causes a rapid buildup to round off the tablet edges.
  •  Grossing/Smoothing: smoothes out the subcoated surface and increases the tablet size to predetermine dimension.
  • Coloring: gives the tablet its color and finished size.
  • Polishing: produces the characteristics gloss.

2. Film Coating

Film coating is more favored over sugar coating. Film coating is deposition of a thin film of polymer surrounding the tablet core. Conventional pan equipment may be used but now a day’s more sophisticated equipment are employed to have a high degree of automation and coating time. The polymer is solubilized into solvent. Other additives like plasticizers and pigments are added. Resulting solution is sprayed onto a rotated tablet bed. The drying conditions cause removal of the solvent, giving thin deposition of coating material around each tablet core.

Specialized coating

  1. Compressed coating

This type of coating requires a specialization tablet machine. Compression coating is not widely used but it has advantages in some cases in which the tablet core cannot tolerate organic solvent or water and yet needs to be coated for taste masking or to provide delayed or enteric properties to the finished product and also to avoid incompatibility by
separating incompatible ingredients. 

2. 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 substrate as pharmaceutical is limited. 

3. Dip coating

Coating is applied to the tablet cores by dipping them into the coating liquid. The wet tablets are dried in a conventional manner in coating pan. Alternative dipping and drying steps may be repeated several times to obtain the desired coating. This process lacks the speed, versatility, and reliability of spray-coating techniques. Specialized equipment
has been developed to dip-coat tablets, but no commercial pharmaceutical application has been obtained.

4. Vacuum film coating

Vacuum film coating is a new coating procedure that employs a specially designed baffled pan. The pan is hot water jacketed, and it can be sealed to achieve a vacuum system. The tablets are placed in the sealed pan, and the air in the pan is displaced by nitrogen before the desired vacuum level is obtained. The coating solution is then applied with airless spray system. The evaporation is caused by the heated pan, and the vapour is removed
by the vacuum system. Because there is no high-velocity heated air, the energy requirement is low and coating efficiency is high. Organic solvent can be effectively used with this coating system with minimum environmental or safety concerns

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