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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MATERIAL AND METHOD OF TABLET COATING: ^(8-12)
Techniques used in tablet coating: Generally three method are used for tablet coating
1. Sugar coating
2. Film coating
3. Enteric coating

1. Sugar coating:Traditionally sugar coating form the bulk of the coated tablet, and it is a multistage process involves six separate operations:

Seal tablet core: Application of water impermeable polymer such as Shellac, Cellulose acetate phthalate and Polyvinyl acetate phthalate, which protect the core tablet from moisture, harden the tablet surface and increased its shelf life.

Sub coating: It causes a rapid build up to round off the tablet edges by adding the building agents such as Calcium carbonate or talc in combination with sucrose solution.

Smoothing: Smoothes out the sub-coated surface with the application of sucrose syrup and increases the tablet size to the predetermine size.
Colouring: Gives the tablet its colour and finished size, often titanium based pigments are used.

Polishing: Effectively polished to give characteristic shine, commonly used beeswax, carnauba wax.

Printing: Indelible link for characterisation.

2. Film Coating: It is a modern approach to coating tablets, capsules or pellets by surrounding them with a thin layer of polymeric material. It a single stage process, which involves spraying a coating solution containing the following:

Polymer
Plasticizer
Colourant
Solvent

The solution is sprayed onto a rotating tablet bed followed by drying which facilitates the removal of solvent leaving behind the deposition of thin film of coating materials around each tablet.

* Polymer used in film coating:
1. Enteric polymers: Cellulose acetate phthalate, Polyvinyl acetate phthalate, Shellac, Polymethacrylates.

2. Non-enteric polymers: Hypromellose, Hydroxyethyl cellulose, Carboxymethyl cellulose, Polyethylene glycol.

* Plasticizer used in film coating:
1. Polyos type: Glycerol, Propylene glycol, Polyethylene glycol

2. Organic esters: Phthalate and citrate esters

3. Oil/Glycerides: Castor oil, Fractionated oil.

* Colorant used in film coating:
1. Organic dyes and their lakes: Sunset yellow, Tartazine, Erythrosine

2. Inorganic colours: Iron oxide yellow, Titanium oxide

3. Natural Colours: Riboflavin, Carmine

* Solvents used in film coating:
1. Organic solvents: Methylene chloride, Isopropyl alcohol

2. Inorganic solvents: Water

Ideal Properties of Film Coating Material:

Solubility in solvent of choice for coating preparation
Solubility requirement for the intended use e.g. free water solubility, slow water solubility or pH- dependent solubility.
Capacity to produce an elegant looking product.
High stability against heat, light, moisture, air and the substrate being coated.
No inherent colour, taste or odor.
High compatibility with other coating solution additives.
Non-toxic with no pharmacological activity.
High resistence to cracking.
Film former should not give give bridging or filling of the debossed tablet.
Compatible to printing procedure.

* Merits of film coating over sugar coating:

Film Coating	Sugar Coating
Tablet appearance:
· Retain shape of original core	· Rounded with high degree or polish
· Small weight increase of 2-3% due to coating material	· Larger weight increase 30-50% due to coating material
· Logo or break lines possible	· Logo or break lines are possible
Process:
· Can be automated e.g. Accela cota	· Difficult to automated e.g.Taditional coating pan
· Easy training operation	· Cosiderable training operation required
· Single stage process	· Multistage process
· Easily adaptable for controlled release allows for functional coating.	· Not able to be used for controlled release apart from enteric coating.

3. Enteric coating: An enteric coating is a barrier that controls the location of oral medication in the digestive system where it is absorbed. The word “enteric” indicates the small intestine, therefore enteric coating prevent the release of medication before it reaches the small intestine.The enteric coated polymers remain unionise at low pH and therefore remain insoluble. But as the pH increases in the GIT, the acidic functional groups are capable of ionisation, and the polymer swells or becomes soluble in the intestinal fluid.

Material used for enteric coatings include: Cellulose acetate phthalate, Polyvinyl acetate phthalate, Hydroxypropyl methyl cellulose phthalate.

* Reasons for enteric coating:
I. Protection of active pharmaceutical ingredients from the acidic environment of the stomach e.g. enzymes and certain antibiotics.

II. To prevent gastric distress or nausea from a drug due to irritation e.g. sodium salicylate.

III. For the delivery of drugs to their primary absorption site in their most concentrated form.

IV. To provide a delayed release component for repeat action.

V. Required for minimizing first pass metabolism of drug.

* Polymers used for enteric coating:

Shellac ( esters of aleurtic acid)
Cellulose acetate phthalate
Poly(methacrylic acid-co-methyl methacrylate)
Cellulose acetate trimellitate
Poly(vinyl acetate phthalate)
Hydroxypropyl methyl cellulose phthalate

Ideal Properties of Enteric Coating Material:

Resistance to gastric fluid
Susceptible/permeable to intestinal fluid
Formation of continuous film
Non-toxic, cheap and ease of application
Compatible with most coating solution components in the drug substrate
Ability to be readily printed

ADVANCEMENT IN TABLET COATING MATERIAL: ⁽¹³⁾

Aqueous based coating materials like colloidal dispersion (30%) of polymer Ethyl Cellulose ( aqua coat and aquateric) are now replacing organic solvent based coating materials to meet EPA & OSHA regulations and to reduce the cost & hazards associated with it, paving the way to produce totally water based enteric and sustained release coatings.
Water based controlled tablet coatings by Latex and Pseudo latex of acrylate polymers and ethyl cellulose have been reported.
Cross-linked product of hydroxyl-end blocked polydimethyl-siloxane has been developed as water based controlled release tablet coating system.
The most common colorants used are F, D & C or D & C colorants , but recently a variety of colour concentrates like Opalux, Opaspray, Opadry have been introduced to achieve less lot to lot variation.

ADVANCEMENT IN TABLET COATING TECHNIQUE: ^(13-26)

1. Fluid bed or Air suspension coating:

It involve the spray coating of pellets, beds, granules, powders or tablets held in the suspension by column of air.
The fluid bed processing equipment is multifunctional and may be used in preparing tablet granulation as well.
Depending upon the mode of applying solutions the system could be top spray, bottom spray r tangential fluidized bed system.
The top spray coating method is recommended for taste masking, coating of enteric release and barrier films on particles or tablets. The method is of special significance when coatings are being applied from aqueous solutions, latexes or hot melts. The bottom spray coating is recommended for sustained release and enteric release products. The tangential spray technique is recommended for layering coatings and also for sustained release and enteric release products.

2. Compression coating:

It is a technique where a large tablet either completely or partially surrounds a smaller tablet.
Smaller tablet is compressed first and is then surrounded by powder, which undergoes compression.
The technique is safely employed in the coating of tablets having a solvent sensitive or moisture sensitive drug or any provide delayed or enteric release product.

3. Electrostatic dry coating:

It is an efficient technique of applying coating to tablets having conductive tablet core.
The coated tablets have smooth surface, good coating uniformity and release profile that are comparable to that of the tablet core.
This process is an alternative to aqueous or solvent based coating process for pharmaceutical products.
Principle: Spraying of mixture of finely grounded particles and polymers on the substrate surface without using the solvent and then heating the substrate for curing on oven until the powder mixture is fused into the film.

4. Magnetically assisted impaction coating ( MAIC):

It is used for the coating of the organic and moderately soft which are very sensitive to heat and are easily deformed by the mechanical forces.

This coating attaches the coating material ( guest particles) on to the material to be coated (host particles) with least degradation of particle size, shape and composition caused by heat and mechanical forces.

It is developed to improve the effectiveness of mixing powders with nano-sized particles without the use of solvent or heat.

Mechanism involved in coating:

Excitation of magnetic particles.
De-agglomeration of the coating material.
Shearing and spreading of coating material on the surface of material to be coated.
Magnetic-host-host particle interaction.
Magnetic-host wall interaction.
Formation of coated products.

5. Gelcap (Gelatin coated capsule shaped tablets) coating:

It provides reduction in product size and gelatin coating facilitates ease of swallowing.
GELCAPS are more temper evident and resistant.

Advancement in coating equipment technology: ^(13-27)

Most coating processes use one of the three general coating systems like standard coating pan, perforated coating pan and the fluidized bed coater.
A significant landmark in the performance of standard coating pan systems have been introduced with the development of Pelligrini pan, Immersion sword and immersion tube systems as in these systems coating solutions are applied by an atomized spray system directed to the surface of the rotating tablet bed.
Similarly, tablet coating technology has been improved with the advent of perforated pan coaters (Dria-coater, Hi-coater and Accelacoater). The Glatt-coater (cap. 25-1000 kg) is the latest perforated pan coater to be introduced in the market.
Auto coaters (Granscoater) are available with software programmes and parameter setting profiles like temperature maintenance, spray pattern or spray rate, compressed air pressure etc.
NeoCota is the recent introduction of fully automated sugar and film coating system of tablets and pellets.

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Tablet coating defect: ^(1-30)

Sr. No.	Tablet Defect	Definition	Reason	Remedies
1	Blistering	Local detachment of film from the substrate forming blister.	Entrapment of gases in or underneath the film due to overheating either during spraying or at the end of the coating run	Use mild drying condition
2	Chipping	The film becomes chipped and dented, usually at the edges of the tablet	Decrease in fluidizing air or speed of rotation of the drum in pan coating	Not to over dry the tablet in preheating stage.
3	Cratering	Volcanic-like craters appears exposing the tablet surface	The coating solution penetrates the surface of the tablet, often at the crown where the surface is more porous, causing localized disintegration of the core and disruption of the coating.	Use efficient and optimum drying conditions. Increase viscosity of coating solution to decrease spray application rate
4	Picking	Isolated areas of film are pulled away from the surface when the tablet sticks together and then part	Over wet tablet bed where adjacent tablets can stick together and then break apart.	Use optimum and efficient drying conditions or increase the inlet air temperature
5	Pitting	Pits occur in the surface of a tablet core without any visible disruption of the film coating	Temperature of the tablet core is greater than the melting point of the materials used in the tablet formulation.	Modifying the drying (inlet air) temperature such that the temperature of the tablet core is not greater than the melting point of the batch of additives used.
6	Blooming	coating becomes dull immediately or after prolonged storage at high temperatures	It is due to collection on the surface of low molecular weight ingredients included in the coating formulation	Decrease plasticizer concentration and increase molecular weight of plasticizer.
7	Blushing	whitish specks or haziness in the film	It is thought to be due to precipitated polymer exacerbated by the use of high coating temperature at or above the thermal gelation temperature of the polymers.	Decrease the drying air temperature Avoid use of sorbitol with Hydroxy Propyl Cellulose, Hydroxy Propyl Methyl Cellulose, Methyl Cellulose and Cellulose ethers
8	Twinning	Two tablets stick together	Common problem with capsule shaped tablets	By balancing the pan speed and spray rate.
9	Colour variation	Variation in the colour of the film	Alteration of the frequency and duration of appearance of tablets in the spray zone or the size/shape of the spray zone	Geometric mixing, reformulation with different plasticizers and additives or used mild drying conditions
10	Orange peel effect	Film becomes rough and non glossy. Appearance is similar to that of orange	Inadequate spreading of the coating solution before drying	Use mild drying conditions. Use additional solvents to decrease viscosity of solution
11	Mottling	Uneven distribution of colour on the surface of tablet with dark and light patches.	Due to different colouration of the excipient or the degradation product of the tablet is coloured.	Coating solution is prepare properly in sufficient quantity.
12	Cracking or Splitting	Film either cracks across the crown of the tablet(cracking), or splits around the edges of the tablet (splitting).	Internal stress in the film exceeds tensile strength of the film.	Use of high molecular weight polymer increase the tensile strength of the film.

EVALUATION PARAMETER FOR COATED TABLETS: ^(1-4)
1. Water vapour permeability:If the coating is used as seal coat or to provide some physical protection for a tablet containing a water unstable drug, then the water vapour permeability test should be carried out.

2. Film tensile strength: This test is carried out when the effect of varying concentration of plasticizers or additives is being evaluated. Tensile strength testing is one of the better ways to optimize the level of additives in the formulation. Strips of the film are tested on a tensile strength tester by applying a known force at a constant rate. The elasticity and the tensile strength/ breaking stress of the films are evaluated.

3. Coated tablet evaluations:
* Adhesion test with tensile strength tester
* Diametric crushing strength of coated tablet:
i. Temperature and humidity may cause film defects
ii. Quantification of film surface roughness, hardness & colour uniformity.
iii. Mass variance of the coated tablets and variance of the tablet API content.

4. Stability studies must be conducted on the coated tablets to determine if temperature and humidity changes will cause film defects. Exposure of coated tablet to elevated humidity and measurement of tablet weight gain provide relative information on the protection provided by the film.

5. In process quality control:
a) Weight of tablet
b) Crushing strength
c) Tablet thickness
d) Disintegration time

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